Polyester and Alkyd Resins: Technical Basics and Applications 9783748602835

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Ulrich Poth

Polyester and Alkyd Resins Technical Basics and Applications

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Ulrich Poth Polyester and Alkyd Resins: Technical Basics and Applications Hanover: Vincentz Network, 2020 European Coatings Library

© 2020 Vincentz Network GmbH & Co. KG, Hanover Vincentz Network, Plathnerstr. 4c, 30175 Hanover, Germany This work is copyrighted, including the individual contributions and figures. Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punishable by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems. Discover further books from European Coatings Library at: www.european-coatings.com/shop Layout: Vincentz Network, Hanover, Germany Printed by: Qubus Media GmbH, Hanover, Germany

European Coatings Library

Ulrich Poth

Polyester and Alkyd Resins Technical Basics and Applications

Foreword

Foreword Polyesters and alkyd resins have been the most varied class of binders for coating applications for quite some time. They are the subject of many reports and publications. So, is another one necessary? Perusal of the various publications gave the impression that the calculation proposals provided there were not precise enough to allow the development of systematic trial plans. Therefore, we developed our own calculation procedures. These formed the basis for the development of numerous polyesters and alkyds, some of which became large-scale products which were produced for long periods or are still in production. In addition, there was a desire to commit all my experience to paper. However, the next question was: If polyesters and alkyd resins are well established on the coatings market would there even be a need to develop new products using the latest calculation principles? The answer is: We believe that developments are still needed. Finally, friends and colleagues asked me to write down the knowledge that I have gained over the years. The determining factor was that even current publications state that the feasibility limit for polyesters and alkyd resins lies somewhere between the definitions of Carothers [26] and Flory [32]. As the two definitions diverge significantly, there is a conception that the formulation of polyesters and alkyd resins still follows empirical approaches. Chapter 3 presents theoretical approaches which not only contain evaluations of molecular weights, and functionalities of resins and conclusions about molecular weight distributions and feasibility limits, but in addition can be used to formulate systematic trial plans. The goal is to explain the basis for the underlying systematic structure/property relationship. The calculations have been designed to cover all grades of polyesters and alkyd resins, initially without regard for the influence of individual building blocks. There then follows a description of the influences exerted by building blocks (Chapter 4). Descriptions of the different grades of polyesters and alkyd resins are provided. There are some binders which would not spontaneously be deemed polyesters. Therefore, besides the primary categorisation of product classes by chemical compositions, the products are also described from the application point of view. To clearly reveal the structure/property relationship, formulation examples are pro­ vided in the form of tables, including molar composition, composition by weight, and char­acteristic values resulting from the calculation methods. The examples are based on patent information and model binders from basic trial programmes which did not materialise as large-scale products. While patent examples (and also the commercial products) can have rather complex compositions, the model examples are simpler, and therefore go a long way towards representing the principles behind the various product classes and could well prove to be the optimal starting point for further developments.

4

Foreword Some typical examples of commercial products and their suppliers are listed. The application areas mentioned are derived from the resins’ different properties. For more information on the binders, the reader should refer to the data sheets issued by the resin manufacturers. The third question is: What is the future of polyesters and alkyd resins? Alkyd resins now play a lesser role than in the past. The main reason is that alkyd resins are relatively more susceptible to saponification and therefore are not the first choice for water-borne coating formulations, which are mandated by environmental considerations. On the other hand, alkyd resins for oxidative-cure contain renewable raw materials and curing by atmospheric oxygen is essentially non-hazardous. Unfortunately, the curing process is too slow for it to be implemented in industrial coating processes. However, in the future, a concept may be found which could supply a missing property. Saturated polyesters are enjoying substantial market growth, even in the case of solvent-borne systems. However, saturated polyesters are important for powder coatings and for the UV-curable coatings segment. Saturated polyesters will also play an important role in the future for high-solids (including 100 % systems) and water-borne coatings. This book is mainly directed at persons working in resin development and production, and in the development and manufacture of resin raw materials – not just experts but also students and newcomers. However, it is also bound to appeal to anybody else in the coat­ ings industry who deals with the application of polyesters and alkyd resins. Ulrich Poth

Editor's note: Ulrich Poth passed away in July 2018 and could no longer complete the work on the English manuscript of this present book. Dr. i.R. Toine Biemans thankfully fulfilled the work on the basis of Mr Poth's original German publication.Ulrich Poth

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Contents

Contents Foreword............................................................................................................4 1

Definitions....................................................................................................... 11

2

History of polyester resins.......................................................................... 13

3 Formation and structure of polyesters and alkyd resins..................... 17 3.1 Reactions that produce polyesters.....................................................................17 3.1.1 Fundamental reactions.........................................................................................17 3.1.2 Structure of polyesters.........................................................................................23 3.2 Determination of and limitations on the size of polyester molecules.......26 3.2.1 Dependencies regarding molecular weight.....................................................26 3.2.2 Derivation of gel-point equations.......................................................................29 3.3 Methods of calculating average molecular weights of polyesters..............32 3.3.1 Factors that influence molecular weights........................................................32 3.3.2 Influence of molar ratios of polyol and poly­carboxylic acid molecules on molecular weight.............................................................................................32 3.3.3 Calculating the influence of the degree of condensation on molecular weight...................................................................................................39 3.3.4 Sample calculations of molecular weights and related characteristics.....43 3.4 Molecular weight distribution of polyesters ..................................................48 3.4.1 Definitions of average molecular weights .......................................................48 3.4.2 GPC analysis...........................................................................................................51 3.4.3 Influences on the molecular weight distribution...........................................55 3.5 Formation and structure of alkyd resins..........................................................70 3.5.1 Special aspects of the preparation of alkyd resins.............................................70 3.5.2 Calculation of molecular weights of alkyd resins...........................................71 3.5.3 Molecular weight distribution of alkyd resins................................................74 3.6 Functionality of polyesters and alkyd resins...................................................78 3.7 Exceptions and their influence on the molecular weight distribution.......81 3.8 Explanation of symbols in definitions and equations...................................82 3.9 Index of equations.................................................................................................83 4

 Influence of building blocks on properties of polyesters and alkyd resins..................................................................................................... 91 4.1 Selection criteria for the different building blocks........................................91 7

Contents 4.2 Influences on solubility and compatibility.......................................................93 4.3 Influences on film properties..............................................................................94 4.4 Classification of polyesters and alkyd resins...................................................96 5 Saturated polyesters..................................................................................... 97 5.1 High-molecular weight, saturated polyesters..................................................97 5.2 Polyesters as plasticisers................................................................................... 100 5.3 Polyester hard resins.......................................................................................... 101 5.4 Polyester segments for other resins............................................................... 102 5.4.1 Polyester segments for polyurethane elastomers....................................... 102 5.4.2 Polyester polyurethanes for crosslinking in the presence of atmospheric moisture........................................................................................ 105 5.4.3 Polyester acrylates.............................................................................................. 106 5.5 Saturated polyesters containing OH groups for crosslinkable, solvent-borne coatings....................................................................................... 107 5.5.1 Structure and composition of OH polyesters for solvent-borne coatings....................................................................................... 107 5.5.2 OH polyesters for crosslinking with amino resins.......................................... 109 5.5.3 OH polyesters for crosslinking by polyisocyanates.................................... 113 5.5.4 OH polyesters for crosslinking with blocked polyisocyanates................. 117 5.5.5 OH polyesters for high-solid coatings............................................................ 118 5.6 Polyesters for water-borne systems................................................................ 125 5.7 Polyesters for powder coatings....................................................................... 134 5.7.1 Thermoplastic polyesters.................................................................................. 135 5.7.2 COOH polyesters................................................................................................ 135 5.7.3 OH polyesters...................................................................................................... 141 5.8 Self-crosslinkable polyesters............................................................................ 144 5.9 Silicone polyesters.............................................................................................. 147 6 Unsaturated polyesters.............................................................................. 151 6.1 Crosslinking of unsaturated polyesters......................................................... 151 6.2 Unmodified unsaturated polyesters – “wax polyesters”............................ 155 6.3 “Gloss polyesters”............................................................................................... 158 6.4 UV crosslinking of unsaturated polyesters................................................... 161 6.5 Other unsaturated polyesters.......................................................................... 162 7 Alkyd resins.................................................................................................. 165 7.1 Classification of alkyd resins............................................................................ 165 7.2 Alkyd resins for oxidative crosslinking.......................................................... 166 8

Contents 7.2.1 Crosslinking reactions....................................................................................... 166 7.2.2 Long-oil alkyd resins for oxidative crosslinking........................................... 172 7.2.3 Medium- and short-oil alkyd resins for oxidative crosslinking................. 176 7.2.4 Anti-corrosive alkyd resins............................................................................... 180 7.2.5 High-solid alkyds for oxidative crosslinking................................................. 184 7.2.6 Styrenated and acrylated alkyd resins........................................................... 185 7.2.7 Urethane-modified alkyd resins...................................................................... 187 7.2.8 Thixotropic alkyd resins.................................................................................... 189 7.2.9 Other modified alkyd resins for oxidative crosslinking............................. 191 7.3 Alkyd resins for co-crosslinking....................................................................... 195 7.3.1 Alkyd resins for stoving enamels.................................................................... 195 7.3.2 Alkyd resins for acid-cure systems................................................................. 202 7.3.3 Alkyd resins for polyisocyanate crosslinking............................................... 203 7.3.4 Alkyd resins for high-solid reactive coatings................................................ 205 7.3.5 Alkyd resins for water-borne reactive coatings........................................... 207 7.3.6 Other alkyd resins for reactive coatings........................................................ 209 7.4 Comparison of OH alkyd resins and OH polyesters with other resins.....210 7.5 OH alkyd resins – combination partners with physically drying binders.................................................................................................................. 214 8 Special polyesters........................................................................................ 217 8.1 Polycarbonates.................................................................................................... 217 8.2 Polycaprolactones............................................................................................... 218 8.3 Polyesters based on diene adducts................................................................. 219 8.4 Stand oils.............................................................................................................. 221 9 Literature....................................................................................................... 225 9.1 General literature................................................................................................ 225 9.2 References............................................................................................................ 225

Author............................................................................................................ 231 Index............................................................................................................... 233

9

Definitions

1

Definitions

In the chemical sense, polyesters are a class of polymer in which the repeating units are linked together by ester groups. Polyesters used in coatings are also referred to as resins, by analogy with natural resins (rosins) which were the first products to be used to form coating films. In DIN 55958 and DIN 55183 polyesters are defined as: “synthetic resins which are based on polyesters and whose structural units in the polymer chain consist of ester groups”. Resins for coatings must be capable of transformation into a form ready for application (solutions in organic solvents, solutions or dispersions in water, non-aqueous dispersions, and aerosols). The applied coating subsequently dries (initially by the evaporation of solvents or water) or coalesces by melting (in the case of powder coatings) to yield a film with a specific performance profile. Chemical reactions may also take place during this film-forming and drying process and which contributes to an enhanced performance profile. The term “polyester” has had various meanings in the past. The first polyesters pro­ duced industrially (glyptal resins) played a secondary role in the coatings industry because they had poor solubility in common solvents and poor compatibility with other coating components. In 1927, Kienle founded the chemistry of oil-modified polyesters, by introducing the terms alkyd which is a contraction of the words alcohol and acid. By “alkyd” is meant the product of the reaction of an alcohol and an acid. By that definition, it should be applied to all polyesters. However, it is mostly reserved for oil- or fatty-acid-modified polyesters because they were the first polyesters to find widespread use in the coatings industry. So, until the 1960s, the term “alkyd” stood for polyesters composed almost exclusively of phthalic anhydride (contributing the polycarboxylic acid component) and polyalcohols, and which were modified either with the fatty acids of natural oils or with the fats or oils themselves. However, the current definition of alkyds is: “polyester resins prepared by the polycondensation of polyfunctional carboxylic acids, polyfunctional alcohols and oils or fatty acids.” Therefore, the terms often found in the literature, namely “oil-modified alkyds” or “fatty-acid-modified alkyds” are incorrect because they are redundant. In line with this definition, polyesters modified with synthetic, aliphatic monocarboxylic acids (synthetic fatty acids) are also alkyds. In this book, it is assumed that, because fatty acids are struc-

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

11

Definitions tural units which are integral to and have a fundamental impact on all polyesters, any polyester modified with any kind of monocarboxylic acid will be considered an alkyd resin. The term alkyd is therefore extended to include resins which contain benzoic acids or monocarboxylic acids from natural rosins. By “modified alkyds” are meant only those alkyds which contain components other than polycarboxylic acids, polyalcohols and monocarboxylic acid. These other components may be styrene, acrylics, polyamides, urethanes, epoxies, or siloxanes. Next to alkyd resins, unsaturated polyester resins played a dominant role in the coat­ ing resins market (particularly for furniture) for a considerable period of time. Their name was shortened to polyester resins and the coatings were called polyester coatings. But, of course, the correct term is unsaturated polyester resin, the definition of which is given in DIN 53184: “Unsaturated polyester resins (UP resins) are polyester resins, in which at least one of the polyfunctional components (polycarboxylic acid or polyalcohol) is unsaturated (olefinic unsaturation) and is capable of reacting by copolymerisation with polymerisable monomers”. This definition does not include the alkyd resins, which are modified with unsaturated monocarboxylic acids. It was only when other, more complex polyalcohols and new carboxylic acids became available on the raw materials market that polyesters consisting only of polycarboxylic acids and polyalcohols (saturated), i.e. polyester resins in the original meaning of the term, became important to the coatings industry. As a way of distinguishing this class of resins from the other polyesters, they were called “oil-free alkyds” or “saturated polyesters”. While the term “oil-free alkyds” should be avoided, the term “saturated polyesters” is extremely common and is therefore also used in this book. Saturated polyesters are defined as “polyester resins whose polyfunctional components (polycarboxylic acids and polyalcohols) do not contain double bonds capable of reacting by polymerisation.” There are some special coating resins which would not intuitively be considered polyester resins, namely polycarbonates, polycaprolactones, resins of diene adducts of natural rosins (maleic resins, acrylic resins), oils modified with maleic anhydride, and stand oils. However, in the chemical sense, all these resins contain ester groups or are obtained by esterification and all the rules governing the other polyester resins apply to these classes of resin as well. They are therefore covered in this book as well. The title of this book is “Polyesters and Alkyd Resins”. Strictly speaking it is needlessly redundant, because alkyds are but a specific class of polyester resins. The thinking behind the title is to draw attention to the importance of alkyd resins as a class.

12

History of polyester resins

2 History of polyester resins Since the early Middle Ages, if not before, drying oils (e.g. linseed oil) have served as binders for paints and coatings and as “solvents” for waxes, rosins and bitumen in various decorative coatings. Even back then, there were attempts to speed up the rate of drying (which today we would call a crosslinking reaction with atmospheric oxygen). One observation was that this could be achieved by the addition of certain metal oxides, one of them being lead(II)oxide or litharge, which was used as a pigment. Another was that drying oils stored in glass containers became more and more viscous when standing in sunlight. The drying properties of such stand oils, as they were called, proved to be far superior to those of the original oils (in this book, stand oils are deemed to be a special type of polyesters). It transpired that the entire drying process could be accelerated by combining oils or stand oils with natural rosins (which increased the extent of physical drying). The best mixing results were obtained by employing a heating process known as “cooking”. One particular combination of cooked drying oils, stand oils, rosins and certain amounts of metal oxides yielded varnish, whose properties satisfied most requirements at that time. In the 17th century, an influx of coating knowledge from the Far East and the first throes of industrialisation gave rise to coatings manufacturers which brought further developments in coating systems. It was discovered that, besides bituminous paints, oilbased systems could be improved by combining “drying oils” with high-melting natural rosin products. Typical products were amber and the copals later found in colonial countries. These were combined with drying oils in an elaborate process known as hot blend. The next stage of industrialisation in the second half of the 19th century required larger quantities of raw materials for coatings. However, these were unavailable, especially in those countries which had few colonies (like the German states), or when military conflicts severely hindered the exchange of goods. As this period coincided with the founding of the chemical industry, efforts were undertaken there to find alternatives to natural products, mainly on the basis of coal tar. In 1846, J. J. Berzelius [1] was the first to describe the product which is formed by the reaction between tartaric acid and glycerol and which is considered by all modern authors to be the first polyester. Somewhat later, in 1853 and 1854, P.E M. Berthelot [2, 3] characterised products of glycerol with sebacic acid and with camphoric acid. In 1856, J. M. van Bemmelen [4] described the products of the reaction of glycerol with succinic acid, with citric acid and with mixtures of succinic acid and benzoic acid. He also mentioned the reaction of succinic acid with mannitol. He was the first to observe that, by varying the mi-

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

13

History of polyester resins xing ratios of these raw materials and the degree of condensation, it was possible to obtain products which were no longer soluble or meltable. In 1901, W. Smith [5] was the first to prepare polyesters of phthalic anhydride and glycerol. He suggested using such products for moulding compounds. However, it was not until 1914 that the first technical application of such “glyptal resins” for plastic parts was described by M. J. Callahan [6] and L. Weisberg [7]. These resins of phthalic anhydride and glycerol, for the most part with rather high acid values, were and still are used as raw materials for special coating applications. They are soluble only in ketones and lower alcohols and were incompatible with most other coating raw materials known at that time. After several empirical attempts to make mixtures of glyptal and drying oils (hot blend) in the manner of the process involving copals and oils [8], R. H. Kienle succeeded in preparing polyesters from phthalic anhydride, polyols (mainly glycerol) and natural oils [9]. He named these products alkyds (a contraction of the words alcohol and acid). Alkyds became a very important class of binders for coatings and found widespread application and distribution. Initially, alkyd resins mainly gained importance due to their ability to oxidatively crosslink as a result of the presence of unsaturated fatty acids having two or more double bonds. Later alkyd resins were also used as a plasticising component in combination with fast, physically drying binders, e.g. cellulose nitrate. These alkyds contain fatty acids of “non-drying” oils and fats (castor oil, peanut oil, coconut fat). When stoving enamels based on phenol resins and amino resins were introduced into the coatings industry in the 1930s, they were also combined with alkyd resins for plasticising. At a surprisingly late stage – in the 1950s – it was reported that short-oil and medium-oil alkyd resins containing free OH functionalities crosslink by reacting with the functional groups of the amino resins. The same applies to the combination of alkyd resins with amino resins in “acid-curable” lacquers, which were developed and introduced onto the market at around the same time. Initially, alkyd resin formulations, which had small quantities of fatty acids, contained free OH groups more or less by accident. However, from the early 1930s onwards, they were introduced systematically for detailed theoretical studies [10–12]. All of these started by calculating stoichiometric ratios of the three classes of building blocks for alkyd resins. During this period, efforts focused on formulations that would yield the highest-possible molecular weights, and thus deliver the optimal film properties. An additional goal was to create materials which were suitable for moulding compounds. The idea was to prepare soluble and meltable precursors which, after application (e.g. by filling of the moulds), would then continue to react to yield crosslinked and therefore rugged polymers (comparable to phenol and amino resins). Surprisingly, it was thought for a long time that, during stoving, polyesters and alkyds would crosslink by themselves in a continuation of the es-

14

History of polyester resins terification reaction. However, it should have been known that it was impossible for this reaction to go to completion under such conditions. Wherever short- and medium-oil alkyds for stoving enamels were formulated stoichiometrically, the condensation reaction had to be interrupted at lower degrees of condensation to avoid gelation. As a result, studies and publications from 1930 to 1965 focused on determining gel points. Subsequently, lower degrees of condensation and combination with a stoichiometric excess of OH groups [13, 14] led to alkyd resins containing substantial quantities of residual free OH groups and also free carboxyl groups. Some time elapsed before it was realised that lower-molecular weight alkyd resins – containing free OH groups – were suitable for crosslinking reactions. Combination with crosslinkers followed by application and the subsequent film forming process yielded coatings with optimal properties. In the USA from the late 1950s on, attention shifted more to acrylic resins containing OH groups thereby replacing alkyd resins, but in Europe the optimisation of alkyd resins continued. Up to the 1980s, such optimised alkyd resins were the preferred binders for industrial coatings (primer surfacers, topcoats, one-coat-enamels for vehicles, apparatus, and machines) in Europe and the rest of the world, apart from the USA and Japan. Only clearcoats were formulated worldwide with acrylic resins, and only since the 1970s. With the advent of water-borne coating systems during that period, usage of alkyd resins declined, in Europe too. The decline stemmed from the fact that alkyd resins are not ideal for water-borne coatings as they have limited resistance to saponification. However, there are some specialty products which are suitable for water-borne formulations. For example, linseed alkyds, the oldest class of alkyd resins, are still used in the form of aqueous emulsions. After the introduction of alkyd resins, it was not long [15] before patents relating to the formulation of unsaturated polyesters (UPs) were granted (1930). However, UP resins only became widespread in the coatings industry after 1950, mainly for furniture coatings. Until the late 1960s, closed-pore clearcoats and also coloured coatings (UP varnishes, flatting varnishes) were fashionable. However, tastes changed and open-pore, more natural-looking wood coatings became popular. For most other applications, less expensive systems were used. As a result, market volumes of UP resins in the furniture segment declined. Now, more or less the only uses for UP resins there are, are for coating pianos and automotive wooden dashboards. Outside the traditional coatings segment, UP resins are mainly used for automotive repair systems (putties) and for plastic parts and sheets (gel coats). The rise of unmodified saturated polyesters began with the invention of two-pack coatings based on binders containing OH groups and polyisocyanates at Bayer in the 1930s [16–18]. These OH group containing binders were mostly saturated polyesters and alkyd resins initially, but later polyethers were introduced as well. The resins needed to

15

History of polyester resins have sufficient quantities of OH groups for effective crosslinking with polyisocyanates. Although the formulations were calculated stoichiometrically, the polyester and alkyd resins actually had relatively low molecular weights, because the high excess of OH groups ultimately curbed the average molecular weights. Saturated polyesters did not become important until the raw materials currently employed for saturated polyesters, e.g. isophthalic acid, dimethyl terephthalate, neopentyl glycol, hexanediol, and trimethylol propane, became available in industrial quantities at more affordable prices. As mentioned earlier, polyesters were initially used in two-pack coatings but, in the 1970s, their range was extended to other industrial coating systems. Because of these developments, saturated polyesters consisting solely of polyols and polycarboxylic acids, came to prominence. The use of common diol combinations led to adequate solubility in common solvents for coatings and better compatibility with other raw materials, properties which are essential for widespread application. Saturated polyesters in the coatings industry continue to enjoy volume growth. Also, two-pack coatings for plastic coatings based on saturated polyesters are rising continuously in volume. Unlike classic alkyd resins, saturated polyesters are more suitable for water-borne coating systems. However, it must be remembered that these polyesters, too, naturally need to have good resistance to saponification. Next to solid epoxy resins, saturated polyesters are the most important class of materials for powder coatings. And the market share of powder coatings, too, is still growing. Furthermore, saturated polyesters play a key role as building blocks in other resin classes. For example, there are polyester acrylates for UV crosslinking and polyester soft segments for aqueous polyurethane dispersions. The latest books and other publications still refer to formulation rules published in the 1960s. These references often include the caveat that, although the calculation rules are suitable, empirical values still need to be taken into consideration. As polyesters are considered to be an exhaustively defined class of substance, there have been no new theoretical calculations in the intervening period.

16

Reactions that produce polyesters

3 Formation and structure of polyesters and alkyd resins In the literature, polyesters and particularly alkyds are usually described in terms of their building blocks. However, to facilitate an evaluation of the different factors that govern the properties of polyesters and alkyd resins, this chapter deals with general structural influences, as opposed to those exerted by the choice of building blocks. Although there exist overlapping interrelations, first the basic molecular structure of polyesters and alkyd resins and the relevant influencing variables are discussed, which generally apply to the most different material compositions. Building block selection is discussed in Chapter 4.

3.1 Reactions that produce polyesters 3.1.1 Fundamental reactions 3.1.1.1

Esterification of alcohols and carboxylic acids

Esterification of alcohols with carboxylic acids is a classic example of a condensation reaction and useful for describing chemical equilibrium reactions. It is usually the reaction chosen to explain the law of mass action. In the conventional view, which provides a good model of how polyesters are prepared, esterification consists in addition of the electrophilic H atom of the alcoholic OH group to the nucleophilic O atom of the carboxyl group to

Figure  3.1:  Conventional model of the mechanisms behind esterification and saponification Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

17

Formation and structure of polyesters and alkyd resins form an intermediate (see Figure 3.1). The steric effect of this intermediate structure with its three oxygen atoms causes the adduct to decompose either into the starting substances or into the ester and water. The ratio of reactants to products in the equilibrated reaction mixture depends in part on the “R” groups of the reactants. Other factors are the concentrations of reactants and the temperature. The chemical equilibrium stems from the fact that the ester on the product side can polarise itself to the extent that it can revert back to the intermediate by reacting with the water. This reaction is defined as saponification. Evidence for this has been provided by radioactively doping the oxygen atoms in the alcohol, which are subsequently found only in the ester molecule. When a carboxyl group oxygen is radioactively doped, it is found in both the ester and the water molecules, because the two OH groups of the “ortho structure” of the adduct are wholly equivalent [19]. The Figure 3.2 shows the overall equilibrium equation for esterification and saponification. The reaction rates for esterification (vforward) and saponification (vback) at equilibrium (vback = vforward) are governed by the law of mass action [20]. Equation 3.1

In other words, for quantitative ester preparation, the equilibrium must be shifted to the product side. This is usually accomplished by removing the water of reaction through distillation. The standard production processes for polyesters are based on this approach. From the point of view of the distillation process, this is an example of “residue recovery”.

O R2

C

O OH + HO

carboxylic acid

R1

alcohol

R2

C ester

O

R1 + H OH water

Figure  3.2:  Overall equilibrium equation for esterification and saponification

18

Reactions that produce polyesters

3.1.1.2

Transesterification

As the saponification reaction shows, the ester group can be polarised by water. But it can also be polarised by alcohols. Thus, ester groups can react with mobile hydrogen atoms from alcohols to form an intermediate structure. This intermediate structure can decompose into its reactants, but it can also decompose into the ester formed with alcohol 2 and into the free alcohol of ester 1 (see Figure 3.3 and overall equation in Figure 3.4). The chemical equilibrium involved in transesterification is also subject to the law of mass action. Equation 3.2

Here, again, the equilibrium state is influenced by the “R”-groups in the carboxylic acid as well as the alcohol. Naturally, it is also influenced by the concentration of reactants and by the temperature. If the goal is to prepare one of the esters in high yield, the reaction equilibrium needs to be shifted to the correct side. This is usually accomplished by removing one of the products through distillation. This means that polyesters can be pro­ duced from polycarboxylic esters of lower alcohols. Full transesterification with polyols is then achieved by distilling off all the low boiling alcohol from the starting product. Transesterification also plays a role in the preparation of alkyd resins. Starting products are then natural oils or fats (triglycerides) which are transesterified with polyols. Because of the excess of hydroxyl groups, reaction equilibrium is reached when the fatty acids are distributed on all polyol molecules

–

O C R

O C +

O

R

R1

ester 1

O

R1

+ O

H

R2

O

OH R2

O

O –

O C R

alcohol 2

R2

R1

C R

– O

H +

C + O

R

R2 ester 2

O

O

+ R2

R1

H alcohol 1

Figure  3.3:  Mechanism of transesterification

Figure  3.4:  Overall equation for transesterification

19

Formation and structure of polyesters and alkyd resins (partial glycerol esters). These serve as intermediates for a second step in which dicarboxylic acids or anhydrides (e.g. phthalic anhydride) are added to continue the polycondensation and form the final alkyd resin. Although transesterification in an industrially accepted production process has been described in publications, it is usually ignored when it comes to theoretical descriptions regarding the preparation of polyester molecules. The methods for determining molecular weights and molecular weight distributions or defining gel points assume that only esterification reactions occur during the preparation of polyesters. However, transesterification happens throughout the production process and is not restricted to the primary products. Transesterification mainly affects the molecular weight distribution.

3.1.1.3 Reaction catalysis

Many authors believe that the order of the reaction for esterification, saponification and transesterification (mostly 2nd order reactions) requires the use of catalysts. The addition of such acid catalysts or Lewis acids causes polarisation of the carboxyl group on acids and esters by protonation, and that subsequently the alcohol adds to an oxonium complex or a carbonium ion (see Figure 3.5). However, carboxylic acids themselves can liberate protons and so are able to polarise carboxyl groups by themselves. Support for this is provided by the formation of dimers of carboxylic acids (see Figure 3.6). Thus, the reactions of carboxyl groups or ester groups with alcohols + OH are quite complex, especially those R C between carboxyl groups and tertiaOH ry alcohols or phenols – which are O completely different from those with + R C + H OH primary and secondary OH groups. OH For this reason, common polyesters C + R are not usually prepared with the aid OH of tertiary alcohols and phenols. Figure  3.5:  Acid catalysis at the start of esterification

O R

H

O C

C O

H

R

O

Figure  3.6:  Dimerisation of carboxylic Figure 3.6: Carboxylic acids acids form dimers

20

3.1.1.4 Anhydride addition Due to ease of handling, more often anhydrides (e.g. o-phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, pyromellitic anhydride,

Reactions that produce polyesters succinic anhydride, maleic anhydride) are used in industrial processes than their corresponding diacids. Owing to the molecular stress in the anhydride ring and the exposed position of the nucleophilic oxygen atom, anhydrides readily react with electrophilic active hydrogen atoms, such as those of alcohols. The addition reaction yields an ester group and a free carboxyl group (see Figure 3.7). The latter is then able to form a further ester group. The reaction rate for the formation of the second ester group – especially in the case of aromatic polycarboxylic acids – is lower than that for isolated carboxyl groups due to steric hindrance. Higher temperatures can also cause reversion to the anhydride. This mainly happens in the case of aromatic 1,2-carboxyl groups (e.g. phthalic, trimellitic, and pyromellitic esters).

3.1.1.5

Epoxy addition

Formally, 1,2-epoxies are anhydrides of 1,2-diols. Ring stress in epoxies makes them readily polarisable to yield nucleophilic oxygen and electrophilic carbonium ions. The electrophilic carbonium ion can add nucleophilic oxygen from carboxyl groups to form ester groups and secondary OH groups (see Figure 3.8). Under the influence of strong acids, epoxies can react with themselves to form polyethers in a secondary reaction.

O

+ O –

H O

+

C

R

H

+

– O

R

alcohol

O O R

C

O

C

OH

-carboxy ester

O

Figure  3.7:  Anhydride addition

R1

3.1.1.6 Other reactions

Several other reactions are known in organic synthesis that can yield polyesters. Two particular reactions are described to prepare a polyester and a polyester-like product. Polycarbonates are prepared by reaction of phosgene or other derivatives of carbonic acid with alkaline alcoholates. Alkaline phenolates may also be used for this reaction. The high enthalpy of formation of alkaline halogenates ensures that

anhydride

C

CH

O

R1

CH2 +

O R2

C

R2

OH

C

– O

+ O

CH

O

–

CH2 +

epoxide

+

R2

H

–

C

OH O

carboxylic acid

O R2

C

O

CH2

CH OH

R1 ß-hydroxy ester

Figure  3.8:  Epoxy addition to form esters

21

Formation and structure of polyesters and alkyd resins

_ O Na+ + Cl

C

O

Cl

_ + Na+ Cl

O

O arylcarbonate

O phosgene

sodium phenolate

C

sodium chloride

Figure  3.9:3.9:   Preparation ofaryl aryl carbonates Figure Preparation of carbonates

O

C

O

+ H

O

C

R

R

-caprolactone

O H

O ester

O alcohol

Figure  3.10:  Ring-opening reaction of ε-caprolactone

O HO

R1 OH + HO

O

O R2

C

C

OH

– H 2O

dicarboxylic acid O O

diol HO R1

O

C R2

C OH

R1 OH

HO

+

O O C R2 C

R1 O

HO

OH

O O monoester HO

O

C R2 C

C R2 C O R1

O

OH

O

O

C R2

C

– H 2O

O

C R2

C

O

O OH + HO

R1 O

C R2 C

OH

monoester O

R1 O

OH

O

O monoester

HO

– H 2O

dicarboxylic acid

O

HO R1 O

+ HO

O

C R2 C

O O R1 O

C

O R2

C

OH

Figure  3.11:  Initiation and formation of linear polyester molecules

22

O

C R2 C

O R1 OH O

C R2 C O R1

O

monoester

diol

monoester

HO

R1

HO

– H 2O

OH

Reactions that produce polyesters there is a very high yield of carbonates, which are otherwise difficult to prepare. The carbonates formed (see Figure 3.9) are surprisingly stable. Cyclic esters (lactones) can, to an extent depending on the number of atoms in the ring, undergo ring-opening reactions with carboxylic acids or alcohols to form chain esters. While the chemical equilibrium of cyclic esters of lactic acid, γ-butyrolactone, and δ-valerolactone is on the side of the cyclic ester (five- and six-membered rings), the equilibrium of ε-caprolactone (seven-membered ring) favours the formation of polyester chains (see Figure 3.10). O O n HO

3.1.2 Structure of polyesters

OH

n HO

+

C

R2

C

OH

dicarboxylic acid – n H2O

n HO

3.1.2.1 Formation of linear polyesters The formation of polyester resins requires the presence of polyfunctional building blocks, i.e. with at least two functional groups. If a diol reacts with a dicarboxylic acid, the first entity formed is a single ester, as de­ scribed above (see Chapter 3.1.1). This single ester still contains both reactive groups. The OH group of the single ester can react with the carboxyl group of a dicarboxylic acid or of another single ester, while the carboxyl group of the single ester can react with the OH group of a diol or of another single ester. This “head-totail” reaction yields chains of oligomers and finally polymers, i.e. linear polyesters (see Figure 3.11). If n moles of diol react with n moles of dicarboxylic acid, theoretically, linear polyester molecules with an average number of (n-1) structural units (monoesters) are

R1

diol

R1

O

O

O

C R2

C

OH

monoester – (n–1) H2O O HO

R1

O

O

C R2 C

O O R1

O

C

O R2

C

OH

n–1

polyester

Figure  3.12:  Formation of polyester molecules containing n moles diols and n moles dicarboxylic acids

n HO

OH + n HOOC

COOH

– n H2O n

HO

COOH – (n – 1) H2O

COOH

HO n–1

Figure  3.13:  Simplified way of depicting the formation of linear polyesters

23

Formation and structure of polyesters and alkyd resins formed, all of which are terminated with both types of functional groups (see Figure 3.12). There is the further possibility that there will be a mixture of molecules bearing two terminal OH groups and molecules bearing two carboxyl groups. In the past, a simplified way of representing such structures was found and this is now encountered in the literature as well. Polycarboxylic acids are symbolised by rings and lines, because most polycarboxylic acids in polyesters intended for coatings are aromatic compounds. By contrast, the usually aliphatic polyols are represented simply by lines. The number of line ends represents the functionality of the building blocks. The simplified representation for the formation of linear polyester molecules is shown in Figure 3.13.

3.1.2.2 Formation of branched polyesters

Whenever building blocks contain more than two functional groups, branched polyesters will be formed (see Figure 3.14). While there is a possibility during the formation of linear polyesters that molecules of different structure will be formed, the likelihood of this is all the greater during the formation of branched polyesters. A multitude of structural isomers are formed. Their number increases even further because some of the polyester molecules formed are terminated solely with OH groups while others are terminated solely with carboxyl groups. In addition, the number of structural isomers increases exponentially as the molecular weight of the polyesters inn HO OH COOH + n HOOC creases. It is theoretically possible OH for linear molecules to result from – n H2 O the preparation of polyesters based on building blocks bearing more n HO COOH OH than two functional groups. How­ – 6 H2 O ever, the quantity of linear molecules will decline as the polyester moOH OH lecules become larger. The idea that COOH HO polyesters based on building blocks OH OH bearing more than two functional groups of different reactivity will OH OH HO primarily yield linear molecules OH OH needs to be viewed critically. A classic example of such an idea is the COOH preparation of polyesters with the aid of glycerol as building block. This postulates that linear polyesFigure  3.14:  Incipient formation of branched polyesters

24

Reactions that produce polyesters ters are the first to form and that branched polyesters form only at the end of the reaction, if at all. Of course, the primary OH groups of glycerol will react faster than the secondary ones and, what is more, there are two of them. However, as soon as some of the primary OH groups have formed esters, the concentration of secondary OH groups grow and will be available for reactions that yield branched polyesters. The use of building blocks of different functionalities (two and more) increases the potential number of structural isomers of polyester molecules.

3.1.2.3 Ring closure during formation of polyesters?

Publications [21–23] deal in great detail with the theory behind the formation of cyclic structures during polyester formation. Well known is that five- or six-membered rings are formed easily, because of the resulting bonding angles. The suggestion is made that rings will have stressed bonding angles until the number of atoms exceeds fourteen. The conclusion is then drawn that the formation of ring structures by polyester molecules may explain why observed molecular weights and molecular weight distributions deviate from theoretical predictions. However, it must be remembered that polyester chains have numerous potential tertiary structural isomers that would make “head-to-tail” arrangement rather improbable. Furthermore, esterification requires certain kinetic conditions and successful collisions between functional groups (this is described by the reaction rate constant k' for esterification.) Thus, the formation of large rings during the production of polyesters is improbable. In this book, it is assumed rings are not formed during the preparation of polyesters. However, there are building blocks which tend to enter into ring-forming reactions. For example, oxalic acid and ethylene glycol form dioxolandione in high yield; and 4-hydroxybutyric acid react to yield an internal ester, γ-butyrolactone. Such building blocks are not used in the production of polyesters. It is reported [24] that GC analysis confirms that the preparation of polyethylene terephthalate by transesterification of dimethyl terephthalate with ethylglycol in an intermediate step gives rise to cyclic molecules consisting of two molecules terephthalic acid and two molecules of ethylene glycol. The amount of such molecules allegedly worsens the properties of the otherwise high molecular weight polyester, which serves as a fibre raw material. Preparing a model of this molecule shows that it is implausible or nearly impossible to assemble the atoms to form such a structure (due to the planar structure of the terephthaloyl group and the repulsion effect of π-electrons on aromatic rings).

3.1.2.4 Crosslinking during polyester formation

Under certain stoichiometric conditions, branched polyesters can consist of very large molecules whose molecular branches bear terminal OH groups and carboxyl groups. If such molecules react even further, the branched molecules can become crosslinked. Crosslinked

25

Formation and structure of polyesters and alkyd resins molecules are also formed when large branched molecules become joined together by multiple reactions among small molecules. The crosslinked networks can contain smaller molecules within them. When crosslinked polyester molecules are forming, the viscosity of the reaction melt suddenly changes. The melt reverts to a gel of very high viscosity and a marked yield point. Crosslinked polyesters cannot be melted and are not soluble in any solvents although they might be swellable. Thus, they cannot be transformed into a usable condition. Since work on the development of polyesters focused initially on producing the high­ est-possible molecular weights, a great deal of time was spent on elucidating the conditions for crosslinking (gelation), with numerous experiments aimed at predicting the degree of condensation which would induce gelling (gel point).

3.2 Determination of and limitations on the size of polyester molecules 3.2.1 Dependencies regarding molecular weight In the early days, the first polyesters and especially alkyd resins [25] were formulated according to empirical rules. Subsequently, in the early 1930s, they attracted the attention of theoretical chemists. Carothers and Flory played key roles in establishing the theoretical conditions needed for the preparation of polyesters. Studies back then focused on the development of high molecular weight polymers that would be suitable for plastics and fibres. However, initial attempts to prepare such high molecular weight polyesters for such applications failed. It was realised at an early stage that the molecular weight of polyesters based on bifunctional building blocks depends on the molar ratio of the components and on the degree of condensation. This led to the development of what are now well-established equations for the formation of A—B linear polymers. The molecular weight (Mn) of polyesters or the number of structural units (q) exhibits a dependence on the quotient of the number of functional groups at a given time during the reaction (νCOOH or νOH) and the number of functional groups at the start of the reaction (nCOOH or nOH) for a polyester consisting of equal moles of diol (n1) and dicarboxylic acid (n2). By structural unit here is meant a combination of one molecule of dicarboxylic acid and one molecule of diol or – by extension – a combination of one mole of polycarboxylic acid and the attached polyol molecules. For bifunctional building blocks, the number of structural units exhibits a dependency on the quotient of the initial number of moles of building blocks (n1 or n2) and the number of functional groups at a given time during the reaction (νOH or νCOOH), i.e. the degree of polycondensation, as shown in the following equations:

26

Determination of and limitations on the size of polyester molecules Equation 3.3

This dependency is presented in Figure 3.15. The second dependency is given by the use of a molar excess of one building block. Normally, polyesters contain an excess of polyols. In that case, the number of structural units in a polyester molecule (q) exhibits a dependence on the quotient of the number of moles of diol (n1) and the number of moles of dicarboxylic acid (n2), as soon as the number of residual free carboxylic (νCOOH) groups tends towards zero and the degree of condensation tends towards 1.

Figure  3.15:  Number of structural units as a function of degree of polycondensation in linear polyesters

27

Formation and structure of polyesters and alkyd resins Equation 3.4

This dependency is presented in Figure 3.16: A corresponding result is obtained when there is an excess of carboxylic acid or COOH groups, as shown in the following equation: Equation 3.5

The attempts to achieve the highest-possible molecular weights led to the study of polyesters containing building blocks which have more than two functional groups. The most commonly observed effect then was gelation or the formation of infinitely large molecules,

Figure  3.16:  Number of structural units of linear polyesters as a function of molar excess of diols

28

Determination of and limitations on the size of polyester molecules i.e. molecules which grow until they occupy the full volume of the reaction mixture. Such molecules are crosslinked. Consequently, many tests were then focused on establishing the degree of condensation at which gels form, i.e. the gel point.

3.2.2 Derivation of gel-point equations In 1935, Carothers [26] proposed an equation which describes the degree of polycondensation (p) as being dependent on the difference between the initial number of molecules of reactants (n1,2) and the number of polyester molecules formed (np) at a given time during the reaction in proportion to the total number of initial functional groups: Equation 3.6

Other authors also use this equation [27–31]. In the event that the quotient of the number of building blocks and the number of polyester molecules formed tends towards infinity, i.e. the numerous molecular building blocks have formed a few, very large polyester molecules, the degree of polycondensation (p) becomes critical (pc). The critical degree of polycondensation is defined as the quotient of twice the number of all building block molecules (n1,2) and the sum of all functional groups (n1,2 · F1,2), as shown in the following equation: Equation 3.7

This Carothers equation means that, at the gel point (p=pc=1), the number-average molecular weight of polyesters tends towards infinity. Equation 3.8

By contrast, Flory [32 and as described in 27, 28, 30, 31, and 36] found that gelation takes place at much lower degrees of condensation than is assumed in the Carothers equation. He proposed that, at the gel point, the weight-average molecular weight tends towards infinity. In other words, only some of the polyester molecules strive to occupy the full reaction volume, with a great many molecules remaining smaller. He defined a statistical term (α) to describe the

29

Formation and structure of polyesters and alkyd resins probability of two polyester molecules reacting together to form bridges. At the gel point, this term becomes critical (αc). The term (αc) is that value at which there is a better-than-even chance of more than one bridge being built between two chains of polyester molecules, leading to a fractional quantity of infinite network molecules (see next equation): Equation 3.9

Later authors tried to quantify the value of Flory’s critical degree of condensation (αc) by introducing further statistical terms and others ran trial series to study the influence exert­ ed by building block functionality. Thus, Stockmayer [34] developed an extension of the Flory equation by adding the statistical effect of building block functionality, as shown in the following equation: Equation 3.10

Jonason [33] included the modification of polyesters with monocarboxylic acids (alkyd resins) and obtained comparable results. His critical degree of polycondensation (pc) varies with the square root of the quotient of the functional groups (nOH/nCOOH), divided by a term including the ratio of the number of carboxyl groups of monomer carboxylic acids (n3) to the number of carboxyl groups of polycarboxylic acids (n2 · F2), as shown in the following equation: Equation 3.11

Kilb [35] tried to explain the differences between his test results and the predictions of Flory and Stockmayer in terms of intermolecular reactions that vary with the length and stiffness of polyester chains and the instantaneous concentration of polymer and functional groups in a given reaction volume. Bernardo [27] provided a good overview of the development of all these models and equations. He compared the theoretical gel point yielded by the various equations with the results he obtained in a large test programme for producing alkyd resins. The biggest difficulty with the calculation of gel points of branched polyesters was the fact that – and this is the basis of Flory’s theory – only a small number of the growing polyester

30

Determination of and limitations on the size of polyester molecules molecules strived to reach an infinite size at the gel point. The reason was the different molecular weight distributions and the reactivity of the monomers and polyester units in the reaction. Flory’s theory, in which he compared the growth of polyester resins (polycondensation) with the polymerisation reaction undergone by unsaturated monomers, predicted that the curves for the molecular weight distribution of polyesters would be relatively flat. However, Kilb [35], Korschak [37] und Bresler [38] found that the polycondensation products, contrary to Flory’s prediction, had relatively steep Gaussian molecular weight distributions. They postulated that the gel points of polyester and alkyd resin formulations were intermediate between the values postulated by Flory and by Carothers [40]. This view is still found in current literature [41]. The reason is – as was subsequently discovered – that during the process of polycondensation, an equilibrium exists not only between the acids and alcohols and the esters and water, but also between the various polyester molecules, which can undergo transesterification. The bulk of the polyesters synthesised in the various trial programmes exhibited a tendency towards average molecular size, a tendency which is influenced primarily by the functionality of the reactants, but also by the reactivity of the functional groups and by the molecular weight of the polyesters, as would be expected. Surprisingly, the reaction conditions are of minor importance. If it is difficult to calculate the molecular weights of branched polyesters, it is even harder to do the same calculation for alkyds under the additional influence of the monocarboxylic acids. Patton [39] modified Carothers’ gel equation (Equation 3.7) to include the number of moles of monocarboxylic acids (n3). By performing a great many calculations on the alkyds produced, he found that the ratio of the number of moles of reactants to the number of acid equivalents must be 1.000 to 1.005 if an alkyd formulation is not to lead to gelation. This ratio is called the Patton (or alkyd) constant (KPatton). Equation 3.12

Patton’s constant remains the basis of several published trial programmes and is still used by many developers and producers of alkyd resins to formulate alkyds. As this equation is based on stoichiometric ratios, its scope is not wide enough to cover all alkyd resins, especially those of relatively low molecular weight and high numbers of residual OH groups Patton himself recommended some restrictions and extensions to his equation and Sunderland [36] tried to improve it. Dyck [29] gives an overview of the development of gel-point equations from Carothers and Flory to Patton to Sunderland.

31

Formation and structure of polyesters and alkyd resins

3.3 Methods of calculating average molecular weights of polyesters 3.3.1

Factors that influence molecular weights

With regard to the question as to which of the aforementioned equations are suitable for describing polyesters and which of them might lend themselves to systematic trial programmes, a few critical observations need to be made. Although the gel point is an important limit in any attempt to prepare different polyesters or alkyd resins, it is even more interesting to be able to calculate the molecular weights of any of the polyesters or alkyd resins being considered. Naturally, it is important for air-drying alkyds to attain the highest-possible molecular weight, because the crosslinking reaction with atmospheric oxygen is relatively slow. Consequently, the step from resin polymers to polymers with coating properties necessarily has to be small. In most cases, the resin chemist must bear two things in mind. First, the soluble resin which he has to put together must contribute to good application properties in the coating system. Second, the resin must be able to form sufficiently (ideally: infinitely) large molecules in the coating film. In recent decades especially, much development work has focused on high-solid systems. One way to obtain a higher solids content in a coating system is to lower the molecular weight of the vehicle resins. Previously the goal was to maximise the molecular weight of polyester or alkyd resins (as a way of quickly achieving sufficiently high crosslinking and high performance) which ran up against the difficult task of avoiding gelation and defining gel points. The goal for high-solid systems would be to create polyesters and alkyd resins that have good crosslinking properties but the smallest-possible molecular weight.

3.3.2

I nfluence of molar ratios of polyol and poly­ carboxylic acid molecules on molecular weight

Key to the average size of polyester molecules are the molar ratio of the building blocks in the polyester formulation and the degree of polycondensation (see Figures 3.15 and 3.16). Figure 3.17 shows that polyesters formulated with diols and dicarboxylic acids have different possible structures than those formulated with triols and dicarboxylic acids. However, there is no difference in the size of the polyester molecules, i.e. the number of structural units (q), provided that the same molar ratios of polyol to dicarboxylic acid are used. Figure 3.17 can be extended to polyols of higher functionality, polyol mixtures of different functionality, and to alkyd resins as well, without change to the definition that the number of structural units depends on molar ratios.

32

Methods of calculating average molecular weights of polyesters Such a molar approach is described by Kraft [42], who used it to calculate and formulate alkyd resins produced with different reactants using equal molar quantities of polycarboxylic acid and polyol but did not derive a calculation method for molecular weights. Finally, U. and H. Holfort [43] arrived at an equation for calculating the number-average molecular weight of polyesters from the molar ratio of polyol (n1) to polycarboxylic acid (n2). Most real polyesters and alkyds are made with a molar excess of polyol. If it is assumed that polyesters are made only by esterification and transesterification, and no additional

models of polyester molecules q diols + dicarboxylic acids

triols + dicarboxylic acids

n1/n2

1

2.00

2

1.50

3

1.33

4

1.25

5

1.20

1.00

Figure  3.17:  Number of structural units (q) of polyesters as a function of the molar ratio of polyols to dicarboxylic acid (n1/n2), regardless of the functionality of the polyols

33

Formation and structure of polyesters and alkyd resins intermolecular reaction occurs during the process, the equation for determining the number of structural units (q) can be extended as a function of the molar ratio of bifunctional reactants to polyols of all functionalities (F1 ≥ 2), provided that there is an excess of polyol (n1 ≥ n2). The molar ratio of polyol to dicarboxylic acid is defined – in contrast to Patton’s constant – as a constant (kM) for obtaining the number of structural units (q) and ultimately calculating the molecular weight (MP) of polyesters (see the following equation): Equation 3.13

On the assumption that the preparation of polyesters involves esterification and transesterification only and that the formation of ring molecules is negligible, the number of

Figure  3.18:  Number of structural units (q) as a function of the polycondensation constant (kM) and the number of polyol functional groups (F1)

34

Methods of calculating average molecular weights of polyesters structural units of a polyester (q) can be defined as a function of the molar ratio of polyols (n1; regardless of their functionality F1 ≥ 2) to – initially – dicarboxylic acids (n2, F2 = 2): Equation 3.14

Under these conditions, the curve of the function of the number of structural units (q) is congruent for all polyols, regardless of their functionality, as shown in Figure 3.18. Where the dicarboxylic acid is in excess (n2 > n1), the number of structural units (q) is dependent on the value of the polycondensation constant (kM) and the functionality of the polyols. In the case of diols (F1 = 2), an excess of dicarboxylic acids is just as limiting as an excess of polyols. The curve of the number of structural units for (kM) values between 0 and 1 is the mirror-image of the curve for (kM) values between 1.00 and 2.00. At (kM) = 1.00, both sections of the curve tend towards infinity. For polyols of functionality higher than 2 (F1 > 2), there are different ranges for (kM) below 1.00 where the number of structural units (q) is not defined, or they tend towards infinity: crosslinked polyester molecules are present. Only when the dicarboxylic acid is in high excess is the number of structural units limited. These conditions are described in the following equation. Equation 3.15

The model of the polyester segment in Figure 3.19 shows the crosslinked structure of a polyester comprising three moles of triol and four moles of dicarboxylic acid (kM = 0.75). The repetition segment which is depicted has two pendant carboxylic acid groups and three pendant hydroxy groups. These allow this structure to become fully crosslinked which is indicated by the arrows.

Figure  3.19:  Polyester segment of triol and dicarboxylic acid and (kM) = 0.75

35

Formation and structure of polyesters and alkyd resins The conditions for crosslinked molecules are shown in the following equation. Equation 3.16

The range of values for constant (kM) over which the number of structural units (q) of polyesters tends towards infinity lies between 0.50 and 1.00 for triols and dicarboxylic acids and between 0.33 and 1.00 for tetrols and dicarboxylic acids. Usable polyesters will result only if all their molecules have (kM) values outside these ranges. To include the possibility of branching not only by triols, tetrols etc, but also by polycarboxylic acids, the definition of constant kM needs to be expanded to include a term to account for the functionality of polycarboxylic acids (F2 ≥ 2). Polycarboxylic acids with F2 ≥ 2 will require one more mole of polyol for every functionality over 2. So, the additional moles of polyol can be calculated by multiplying the moles of carboxlic acids (n2) by the functionality of these carboxylic acids (F2) but subtracted by 2 (moles of polyol Figure  3.20: Molecular models of polyesters of dicarboxylic acid and tricarboxylic acid having the already accounted for in linear polysame number of structural units (q = 4). merisation). This number is subtract­ ed from the total amount of polyols (n1) when calculating KM (Equation 3.17). In other words, a given number of moles of polyols (n1) must be

Figure  3.21:  A model representing all possible polyester molecules

36

Figure  3.22:  Simplified model of all possible polyester molecules

Methods of calculating average molecular weights of polyesters reduced by the additional number of COOH groups (over two) on the polycarboxylic acids if the latter have more than two such groups per molecule. To illustrate, Figure 3.20 shows two polyester molecules with equal numbers of structural units (q = 4). The first consists of four moles of dicarboxylic acid and five moles of diol. The second consists of four moles of tricarboxylic acid and nine moles of diol, as each of the four moles of tricarboxylic acid requires a mole of diol for limiting the molecular weight and maintain a value of q = 4. If it did not have these additional polyol molecules, further polymerisation could take place and q would be greater than 4. The extension of the definition of the polycondensation constant is presented in the following equations: Equation 3.17

For the constant (kM), the numerator is extended by the term {- (F2 - 2)} per mole of polycarboxylic acid (n2). For dicarboxylic acids, of course, the term is zero (F2 = 2). In the case of mixtures of polycarboxylic acids, the average of the functionality can be used for the calculation, or the term can be disaggregated into factors. The definition of the constant (kM) can now be used for all polyester building blocks (F1 ≥ 2, F2 ≥ 2). Where hydroxycarboxylic acids are employed, they are considered to be a combination of polyol and polycarboxylic acid without ester group. Hydroxycarboxylic acids are counted both as polyols and polycarboxylic acids, and the equation given above (Equation 3.17) applies. If all polyesters consist of molecules without rings or crosslinks, they can be considered “open branched” and it is possible to draw linearised molecular models. All polyesters consist of structural units of polycarboxylic acid and attached polyol plus the excess of attached polyol. Such a model is shown in Figure 3.21. Figure 3.22 shows the simplified version of the model, without the functional groups. This general model, which serves for all types of polyesters, makes it very easy to calculate molecular weights. The molecular weight of a polyester (MP) is the molecular weight of the structural unit (MS) times the number of structural units (q) plus the molecular weight of one molecule of polyol (M1), so as to block the last carboxylic acid chain end also by a polyol molecule. (see the following equation): Equation 3.18

37

Formation and structure of polyesters and alkyd resins Since generating structural units as well as coupling them to a polymer will liberate water this needs to be corrected for. This is done by defining a corrected mass (M’2) for the carboxylic acid containing moiety (Equation 3.19). The molecular weight of the structural unit (MS) is then as the corrected mass of the carboxylic acid containing moiety (M’2) and of the associated moles of polyol ([F2 - 1] · M1). Equation 3.19

Substitution of equations 3.17 (q) and 3.19 (MS) in 3.18 yields the following equation for determining the molecular weight of polyester molecules. Equation 3.20

If the second term (M1) is extended by the denominator (kM – 1), the equation can be rearranged as a fraction: Equation 3.21

The term (kM + F2 – 2) is, after rearrangement of the equation for the polycondensation constant (kM; Equation 3.17), equivalent to the molar ratio of polyol to polycarboxylic acid (n1/n2). The result is an equation (3.22) for calculating the molecular weights of polyesters based on the number of moles of building blocks (n1, n2), their molecular weights (M1, M'2) and the polycondensation constant (kM). Equation 3.22

Rewriting 3.22 yields an equation whose numerator represents the yield of a polyester composed of a given number of moles of reactants (n1, n2). Equation 3.23

38

Methods of calculating average molecular weights of polyesters The yield mass (A) of any polyester composed of n1 and n2 moles of building blocks can thus be used to calculate the molecular weight of the polyester, as shown in the following equation. Equation 3.24

All terms in the equations are average values. In the case of the usually complex polyester compositions, however, it makes sense to disaggregate the terms n1 · M1 and n2 · M’2 into factors, as shown in the following equations: Equation 3.25

The calculation of the molecular weights of polyesters in the manner just presented returns average molecular weights, in this case the number-average molecular weight (Ṁn).

3.3.3

 alculating the influence of the degree C of condensation on the molecular weight

All definitions and equations relating to the calculation of the molecular weights of polyesters prepared with an excess of polyols presuppose that the degree of condensation has a value of one, meaning that the number of instantaneous residual carboxyl groups (νCOOH) is approaching zero. In reality, polyesters and alkyd resins rarely have exhausted every possibility for forming ester groups, with the result HO OH that usually some carboxyl groups remain. Each residual carboxyl q=6 group forms a chain-end in the polyOH ester molecule. For all polyesters, there are two limiting effects on HO COOH chain length: the excess of polyol and the residual carboxyl groups. q=6 Figure 3.23 compares two polyesOH ters having the same number of structural units (q = 6), the first of Figure  3.23:  Excess of polyol or residual carboxyl which was prepared by reaction of groups – an alternative limitation on molecular size

39

Formation and structure of polyesters and alkyd resins 6 moles of dicarboxylic acid with 7 moles of polyol, while the second was prepared from 6 moles of dicarboxylic acid and 6 moles of polyol, in this case resulting in one residual carboxyl group, which forms an alternative chain-end. If carboxyl groups in polyesters act to form a chain-end in a similar manner as an excess of polyol, the equation of the constant (kM) can be extended by the number of residual carboxyl groups to arrive at the constant (k’M), as shown in these equations: Equation 3.26

As residual carboxyl groups (limited degree of condensation) can act as an alternative to an excess of polyol, this equation also has validity for polyesters that do not contain an excess of polyol (n1 ≤ n2). Equation 3.27

Figure  3.24: General model of linearised polyesters

Figure  3.25:  Simplified form of a general linearised polyester

40

Methods of calculating average molecular weights of polyesters This equation is therefore valid for all polyesters containing n1 moles of polyols and n2 moles of polycarboxylic acids and any number of residual carboxyl groups νCOOH ≥ 0. The general linearised polyester model is shown in Figure 3.24. Or, in a more simplified form shown in Figure 3.25. The molecular weight calculation is similar to that calculation which ignores the degree of polycondensation (described earlier). There are only minor differences, concerning the residual carboxyl groups, as shown in the following equations: Equation 3.28

The term f2 is the quantity of residual carboxyl groups (νCOOH), per mole of polycarboxylic acid. Equation 3.29

The term k'M + F2 - f2 - 2 in the numerator is replaced by the quotient of the number of moles of polyol and the number of moles of polycarboxylic acids (n1/n2) resulting from the rearrangement of the equation for k'M (using Equations 3.26 and 3.29). Equation 3.30

The residual carboxyl groups (νCOOH) can be calculated with the aid of the acid value (AV) for the polyester, as shown in the following equation. The acid value is expressed in terms of the current yield mass of polyester (AAV). Equation 3.31

The resulting equation for the molecular weight of the polyester (by combining Equation 3.28 and 3.30) applies to all polyesters consisting of n1 moles of polyol of molecular weight M1 and n2 moles of polycarboxylic acid of molecular weight M’2.

41

Formation and structure of polyesters and alkyd resins Equation 3.32

This equation also applies when there is no excess of polyol (n1 ≤ n2), i.e. the limitation on the molecular weight is due solely to the degree of condensation. The yield mass for a given acid value (AAV) is higher than the theoretical yield mass (A0) of the condensation reaction that went to completion by an amount of water corresponding to the number of carboxyl groups that are still free: Equation 3.33

Replacing the number of residual carboxyl groups (νCOOH) by Equation 3.31, gives us: Equation 3.34

Rewriting Equation 3.34 and solving for AAV results in: Equation 3.35

Substituting Equation 3.35 in 3.32 gives us the following expression for the molecular weight: Equation 3.36

By introducing the known constant terms this leads to: Equation 3.37

42

Methods of calculating average molecular weights of polyesters Table 3.1: Calculation of a model polyester with a limited degree of condensation n•F

n

–1.40

0.70

–0.60 +1.00

building element

M [g/mole]

n • M [g]

wt.%

isophthalic acid

166

116.2

49.72

0.30

adipic acid

146

43.8

18.74

0.50

neopentyl glycol

104

52.0

22.25

+0.60

0.30

propylene glycol

76

22.8

9.76

+0.75

0.25

trimethylol propane

134

sum –0.08

33.5

14.33

268.3

114.81

34.6

14.81

233.7

100.00

νCOOH

+0.43

νOH 1.92

water yield

18

What can easily be seen is that if the acid values are quite low, the second bracketed term in the denominator can be reduced to “1”. This provides enough precision.

3.3.4 Sample calculations of molecular weights and related characteristics The following examples show how the calculation methods are used. a) A polyester is to be prepared by making 0.70 moles of isophthalic acid, 0.30 moles of adipic acid, 0.50 moles of neopentyl glycol, 0.30 moles of propylene glycol and 0.25 moles of trimethylol propane react until 96 % of the carboxyl groups have been converted into ester groups. The number of functional groups (n · F) for carboxyl groups are assigned negative values while the number of OH groups are assigned positive values. This is only done to indicate that these numbers need to be subtracted due to reactions taking place. The acid value and the OH value are computed as follows: Equation 3.38

43

Formation and structure of polyesters and alkyd resins Table 3.2:  Calculation of a model polyester of given acid value n•F

n

–1.50

0.75

building elements isophthalic acid

M [g/mol]

n • M [g]

wt.%

166

124.5

46.10

–0.75

0.25

trimellitic anhydride

192

48.0

17.97

+1.30

0.65

neopentyl glycol

104

67.6

25.31

+0.70

0.35

hexane diol-1,6

118

41.3

15.64

+0.70

035

ethylene glycol

62

sum +0.45

21.7

8.12

303.8

113.48

νOH 2.00

36.0

13.48

yield A0

water

18

267.1

100.00

yield AAV

268.4

The average molecular weight is: Equation 3.38

a) A polyester consisting of 0.75 moles of isophthalic acid, 0.25 moles of trimellitic anhydride, 0.65 moles of neopentyl glycol, 0.35 moles of 1,6-hexanediol and 0.35 moles of ethylene glycol is condensed until the acid value is 15 mg KOH/g. The yield mass is then: Equation 3.40

The quantity of residual free carboxyl groups is: Equation 3.41

The number of residual OH groups increases accordingly: Equation 3.42

44

Methods of calculating average molecular weights of polyesters Table 3.3:  Systematic trial plan for studying the variation in molecular weights and degree of branching in polyesters trials building elements [mole] isophthalic acid

a

b

c

d

e

0.65

0.65

0.65

0.65

0.65

adipic acid

0.35

0.35

0.35

0.35

0.35

neopentyl glycol

0.80

0.85

0.90

0.90

0.80

trimethylol propane

0.30

0.30

0.30

0.20

0.40

KM (AV = 0)

1.10

1.15

1.20

1.10

1.20

10

10

10

10

10

1716

1295

1049

1701

1058

acid value [mg KOH/g] molecular weight [g/mol] AV = 10

The resulting OH value is: Equation 3.43

The polycondensation constant (k’M) is: Equation 3.44

The value of 0.25 in the numerator is the molar quantity of trimellitic anhydride and is obtained by factorising the term n2 · (F2 - 2). The (number) average molecular weight then computes to: Equation 3.45

Or, based on the yield mass for an acid value of 0 mg KOH/g, the average molecular weight computes to: Equation 3.46

45

Formation and structure of polyesters and alkyd resins Table 3.4:  Calculation of the characteristics of a polyester based on analytical data wt.%

building elements

M [g/mol]

m/M = n

n•F

0.3061

–0.6122

45.3

phthalic anhydride

148

11.2

adipic acid

146

n2

0.0767

–0.1534

0.3828

–0.7656

10.6

glycerol

92

0.1152

+0.3456

9.0

hexane diol-1,6

118

0.0763

+0.1526

23.9

neopentyl glycol

104

0.2298

+0.4596

0.4213

+0.9578

n1 100.0

sum

8.3

water

91.7

yield (AV = 0)

+0.1912 18

0.4595

Equation 3.47

If the third term in the denominator is ignored, the molecular weight is 1571 g/mol, indicating as mentioned previously that for small acid values, this term can be set to “1”. The number of moles of polycarboxylic acid was deliberately taken to be 1.00 in both calculations. This simplifies the calculations and makes them easier to illustrate. It makes sense to adopt the same approach in trial plans, as shown in the following example: a) Trial plan to study the variation in molecular weight and degree of branching of polyesters prepared from isophthalic acid, adipic acid, neopentyl glycol and trimethylol propane: In practice, trials a to c will show the influence of differences in molecular weight on the properties of the polyesters (viscosity, application characteristics). Comparisons of trial a with trial d, and trial c with trial e will show the influence exerted by the degree of branch­ ing on these properties. The calculations presented above can also be used for polyesters described by formulations based on percentage mass, e.g. analytical data. The calculation process is shown in the following example. a) An analysis of a polyester yields 45.3 wt.% phthalic anhydride, 11.2 wt.% adipic acid, 10.6 wt.% glycerol, 9.0 wt.% 1,6-hexanediol, and 23.9 wt.% neopentyl glycol. The total is 100 wt.% which is the initial weight – not the yield mass of the polyester. The acid

46

Methods of calculating average molecular weights of polyesters number of the polyester is found to be 18 mg KOH/g. The calculation of the molar composition and quantities of functional groups is presented in Table 3.4. 0.3061 moles of phthalic anhydride and 0.0767 moles of adipic acid can cleave at most 0.45596 moles of water by condensation, equivalent to 8.3 parts by weight. For an acid value of 18 mg KOH/g, the residual quantity of carboxyl groups (νCOOH) computes to: Equation 3.48

The polycondensation constant (k’M) is then calculated as follows: Equation 3.49

The yield mass at the given acid value is 0.53 parts by weight (0.0296 · 18) larger than at the acid value of 0 mg KOH/g; i.e. the mass is 92.23 parts by weight. The average molecular weight of this polyester is then calculated as follows: Equation 3.50

And the OH value is given by: Equation 3.51

Both the calculated average molecular weight and the calculated OH value can be correlat­ ed with corresponding analytical results (GPC, OH value determination). Naturally, it must be borne in mind that the analytical data may contain systematic errors and deviations arising from the analytical process. The results have been presented here with greater precision than is necessary – for illustration purposes.

47

Formation and structure of polyesters and alkyd resins

3.4 Molecular weight distribution of polyesters 3.4.1

Definitions of average molecular weights

The definitions and calculations described above (k'M, q', MP,AV) deal with average values. However, polyesters produced industrially with the usual process always consist of mix­ tures of molecules of different size and are hence polydisperse. Now, it might be thought that, at the start of polyester preparation, the building blocks are consumed via the formation of individual esters in a reaction that subsides over time. However, smaller polyester molecules (M1) are built up before the individual esters are formed. These smaller molecules are consumed over time through the formation of larger polyester molecules (M2, M3). Very large molecules (M4) are formed slowly, but continuously. The tendency is for polyester molecules of average size (M2) to form. All studies show that molecules of average size are formed preferentially due to transesterification reactions in which smaller molecules react with larger molecules to form average-size molecules. The overall reaction involves a plethora of dynamic equilibria between polyester molecules of different sizes. At the planned degree of condensation (acid value), these equilibria are frozen, yielding a mixture of polyester molecules of different sizes. The reaction sequence is shown in Figure 3.26.

Figure 3.26:  Model of the formation of polyester molecules of different molecular weights over time

48

Molecular weight distribution of polyesters The mixture of polyester molecules of different size – the molecular weight distribution – can serve as the basis for deriving average molecular weights. There are different definitions of average molecular weight. The number-average molecular weight (Ṁn) is the quotient of the sum of the molecular weights of the individual number fractions of molecules (ni · Mi) and the total number of polyester molecules (ni). Equation 3.52

The weight-average molecular weight (Ṁw) is the quotient of the sum of the molecular weights of the individual weight fractions of molecules (mi · Mi) and the total mass of the polyester (mi). And since the weight of the polyester molecules (mi) is the product of the number of molecules and their molecular weights (ni · Mi), this leads to the following equation: Equation 3.53

Since higher molecular weight fractions contribute more to the weight-average molecular weight (MW), the MW is always larger than the number-average molecular weight (MN). The larger the difference between the two averages, the broader is the molecular weight distribution. The quotient of both values for the average molecular weights is defined as the dispersity (DM) of the molecules. The larger the value for dispersity, the broader is the molecular weight distribution. Equation 3.54

A hypothetical example serves to illustrate the various terms. Consider a model polyester which consists of structural units prepared from 0.75 moles of isophthalic acid, 0.25 moles of adipic acid, 0.75 moles of neopentyl glycol, and 0.25 moles of trimethylol propane and which has a molecular weight of 236.5 g/mol. The excess of polyol in the mixture has a molecular weight of 111.5 g/mol. The limitation on

49

Formation and structure of polyesters and alkyd resins Table 3.5:  Molecular weight distribution of a polyester model q

ni

Mi

ni • Mi = mi

ni • Mi²

1

0

347

0

0

2

1

585

585

342,225

3

6

821

4,926

4,044,246

4

10

1,058

10,580

11,193,640

5

12

1,294

15,528

20,093,232

6

14

1,531

21,434

32,815,454

7

15

1,767

26,505

46,834,335

8

15

2,004

30,060

60,240,240

9

14

2,240

31,360

70,246,400

10

13

2,477

32,201

79,761,877

11

12

2,713

32,556

88,324,428

12

11

2,950

32,450

95,727,500

13

10

3,186

31,800

101,505,960

14

9

3,423

30,807

105,452,361

15

8

3,659

29,272

107,106,248

16

7

3,896

27,272

106,251,712

17

6

4,132

24,792

102,440,544

18

5

4,369

21,845

95,440,805

19

4

4,605

18,420

84,824,100

20

3

4,842

14,526

70,334,892

21

3

5,079

15,237

77,388,723

22

2

5,315

10,630

56,498,450

23

2

5,552

11,104

61,649,408

24

1

5,788

5,788

33,500,944

25

1

6,025

sum

184

6,025

36,300,625

485,763

1,548,318,349

number-average molecular weight = 485763/184 = 2640 g/mol, weight average molecular weight = 1548318349/485763 = 3187 g/mol, dispersity = 3187/2640 = 1.21

the molecular weight arising from the degree of condensation is neglected in this exercise (AV = 0). Table 3.5 shows the number of structural elements (column 1), the number of molecules (arbitrary, column 2), their molecular weights (column 3), the mass fraction

50

Molecular weight distribution of polyesters (column 4) and the mass fraction times the molecular weight (column 5). The quotient of the sum of column 4 and the sum of column 2 yields a number-average molecular weight (Ṁn) of 2640 g/mol. The quotient of the sum of column 5 and the sum of column 4 yields the weight-average molecular weight (Ṁw) of 3187 g/mol. The dispersity is therefore 1.21. The measured dispersity of polyesters produced according to this recipe is actually higher, at around 3. The reason is that the formation of polyesters by conventional preparation methods leads to some molecules which have molecular weights above 10,000 g/mol. The values in Table 3.5 are plotted in Figure 3.27.

3.4.2 GPC analysis Currently, gel permeation chromatography (GPC) is the most widely employed method for analysing the molecular weight distributions of polymers, including polyesters and alkyds. To assist with interpreting the results of GPC analysis of polyesters and alkyd resins, the method is briefly described below. The results quoted were obtained on an “Agilent 1000” [formerly Hewlett Packard] and software from PSS (Polymer Standards Service). GPC analysis works on the principle that polyester molecules of different molecular weight in solutions of very low concentration have different hydrodynamic volumes. The stationary

Figure 3.27:  Plot of molecular weight distribution of a model polyester

51

Formation and structure of polyesters and alkyd resins phase is a gel of a crosslinked copolymer of styrene and some divinyl benzene that contain pores with different sizes. The polyester molecular coils have different retention times that vary with their hydrodynamic volume. Smaller molecules enter the pores more easily and therefore remain longer on the column than larger molecules. The solvent (mobile phase) is usually tetrahydrofuran, but other solvents like dimethylformamide may sometimes be used in the case of sparingly soluble polymers. The polymer test solution has a concentration of about 5 g/l and is metered automatically from a loop onto the columns. A flow rate of typically 1 ml/min is maintained and the temperature is kept strictly constant, e.g. at 35 °C. After passage through the columns, the concentration of polymer in solution is determined continuously, commonly with a refractometer, which determines the refractive index (RI) as a function of concentration. However, UV detectors may also be used. The refractometer quantifies the changes in refractive index at short time intervals and interprets the changes as mass concentrations. The detected mass concentrations are then expressed in terms of the total mass. Each molecular weight has a different elution time. The relationship between elution time and molecular weight is established by calibration against standards, usually polystyrene standards are used. These standards contain polystyrene

Figure  3.28:  Calibration curve showing the dependence of the molecular weight (mass fraction) of polystyrene standards on elution time

52

Molecular weight distribution of polyesters Table 3.6:  Composition of the model polyesters, theoretical molecular weights and analytical results building elements

polyester

/1

/2

/3

/4

/5

phthalic anhydride

0.500

0.500

0.500

0.500

0.500

adipic acid

0.500

0.500

0.500

0.500

0.500

neopentyl glycol

0.750

0.650

0.600

0.550

0.525

hexane diol-1,6

0.750

0.650

0.600

0.550

0.525

1.500

1.300

1.200

1.100

1.050

acid value (measured)

4.6

5.0

6.2

4.5

4.9

OH value (calculated)

200.2

132.2

94.7

50.8

28.6

molecular weight, calculated

548

818

1113

2030

3353

molecular weight, osmometric

680

984

1150

1903

2570

molecular weight, GPC

744

944

1274

1488

2089

characteristic values: kM (AV = 0)

average molecular weights:

samples of well-defined molecular weight and a particularly narrow molecular weight distribution. A typical calibration curve is shown in the Figure 3.28. The equation that is obtained by fitting a curve through the datapoints is: Equation 3.55

Thus, the dependence of the decimal logarithm of the molecular weight (mass fraction) on the elution time follows a polynomial of degree three. Further standards are available for other polymers (e.g. polymethyl methacrylates). Comparisons of analytical results with theoretical values need to make allowance for the analytical conditions used which may cause variations between the two. For instance, the elution times may have been calibrated with polystyrene standards. However, the coils formed by polyesters in tetrahydrofuran will very likely differ from those formed by polystyrene and therefore have different hydrodynamic volumes. Hence the elution times may therefore differ for the same molecular weights. This applies especially to branched polyesters since branched molecules will have different coiling beha­ viour than linear molecules. For this reason, it is the polystyrene equivalent of the molecular weight which is measured for polyester molecules, not the actual molecular weight.

53

Formation and structure of polyesters and alkyd resins Another deviation may be software-related. The algorithm has to determine the median concentration at every time interval, not at the edge of the interval. Otherwise, the calculations of the high molecular weight fractions could lead to lower values for these fractions which in turn would lead to lower average molecular weights. To minimise this effect is important to maintain a high sampling frequency. The use history of the actual GPC columns also plays a role, as they are often used for all kinds of analyses. This has the effect of lowering the reproducibility of GPC determinations. It therefore makes sense to perform comparative measurements in series. To illustrate the scope for deviation, model polyesters of different molecular weight were prepared, and the theoretical values were compared with the analytical results. Be­ sides GPC analysis, osmotic determinations were carried out. The composition of the model polyesters, the theoretical molecular weights and the analytical results are presented in Table 3.6 and illustrated in Figure 3.29. The figures in Table 3.6 and the curves in Figure 3.29 show very good correlation between theory and analytical results at low molecular weights. At higher theoretical molecular weights, the data diverge, increasingly so at higher molecular weights. The GPC

Figure  3.29:  Comparison of theoretical molecular weights with analytical results

54

Molecular weight distribution of polyesters curve deviates much more substantially than the osmometric curve. From this deviation it cannot be concluded that the calculated values are wrong, but rather the conditions of the analytical methods have to be examined.

3.4.3

Influences on the molecular weight distribution

Ever since Flory [32] postulated that, at the gel point, only a fraction of polyester molecules strive to become infinitely large and Stockmayer [34] tried to quantify this, there have been numerous trials aimed at predicting the molecular weight distribution or the weightaverage molecular weight of polyesters. Korshak [37] and Bresler [38] found that the molecular weight distribution of polyesters is always narrower than statistically calculated by Flory and Stockmayer.

Figure  3.30:  Integral distribution curve of linear polyesters of different number-average molecular weights

55

Formation and structure of polyesters and alkyd resins Again, there were many attempts to extend the above-mentioned definitions and the equations that were subsequently formulated or to at least calculate the weight-average molecular weight. The first step, of course, was to analyse the factors that influence the molecular weight distribution. This was done by examining the results of numerous polyester syntheses. The key influencing factors were found to be the: – targeted number-average molecular weight – degree of branching in the polyester – reactivity of the functional groups of the building blocks Other factors, chief among them the reaction conditions, play a very minor role. Thus, the reaction temperature has no particular influence and the polycondensation process can be interrupted, without changing the molecular weight or the molecular weight distribution at a given degree of condensation. There are only a few exceptions to this.

Influence of the targeted number-average molecular weight If the targeted number-average molecular weight increases, the dispersity of the polyester molecules also increases, and the molecular weight distribution becomes broader. The

Figure  3.31:  Comparative scope for esterification and transesterification of growing polyester molecules as a function of degree of branching.

56

Molecular weight distribution of polyesters reason is that growth of polyester molecules takes place not in distinct steps, but continuously, as shown in Figure 3.26. The larger molecules, too, grow continuously, but transesterification has the effect of regulating for average molecular weights. Figure 3.30 shows the integral distribution curves for linear model polyesters of different number-average molecular weight which were prepared as described in Table 3.6 before. The diagram clearly indicates that the higher the average molecular weight, the broader is the molecular weight distribution. While the polyester of polycondensation constant (kM) 1.50 contains molecules having molecular weights of up to 5,000 g/mol, the polyester with a (kM) of 1.30 has molecular weights of up to 7,000 g/mol, with a (kM) of 1.20 extending to 9,000 g/mol, a (kM) of 1.10 reaching 10,000 g/mol and a (kM) 1.05 attaining 20,000 g/mol.

Influence exerted by the degree of branching The degree of branching in polyesters has a pronounced effect on the molecular weight distribution. The more extensive the branching, the broader is the molecular weight distribution. The reason is that the regulatory effect of transesterification declines as polyester molecules become more branched. The scope for molecular growth outweighs the probability of transesterification. This is illustrated in Figure 3.31. The degree of branching, which is so crucial to the molecular weight distribution, is calculated as follows: Equation 3.56

Figure  3.32:  Model polyesters of different degrees of branching

57

Formation and structure of polyesters and alkyd resins Table 3.7:  Polyesters of equal number-average molecular weights but different degrees of branching polyester building elements (mol) /6 /7 /8 /9 phthalic anhydride

0.700

0.700

0.700

0.700

adipic acid

0.300

0.300

0.300

0.300

MPPD-1,3

1.150

1.000

0.850

0.700

trimethylol propane

0.000

0.150

0.300

0.450

kM (AV = 0)

1.150

1.150

1.150

1.150

acid value (mg KOH/g, measured)

13.6

13.8

15.2

13.0

OH value (mg KOH/g, calculated)

74.4

104.8

136.4

164.5

degree of branching (mol/kg)

0.00

0.54

1.08

1.62

characteristic values

average molecular weight (g/mol) number-average, calculated

1276

1271

1233

1296

number-average, GPC

1152

1200

1251

1275

weight average, GPC

2245

2856

3529

3938

dispersity

1.95

2.38

2.82

3.09

viscosity (ICI-P+C-visc., 23 °C) 60 % in MPA (mPa·s)

180

235

345

370

65 % in MPA (mPa·s)

375

520

710

850

This equation factors in all the branching permutations for the building blocks, i.e. the sum of all building blocks having a functionality greater than 2. Like other characteristic values, the degree of branching (v) is expressed in terms of unit mass and indicates the number of potential branching points in 1000 g of polyester. This is illustrated in Figure 3.32 for two polyester models. If the first consists of 5 moles of phthalic anhydride, 5 moles of neopentyl glycol, and 1 mole of trimethylol propane, and if the acid value approaches 0, the number-average molecular weight is 1304 g/mol and the potential degree of branching is 0.77 mol/kg. In contrast, the second polyester, consisting of 5 moles of phthalic anhydride, 4 moles of neopentyl glycol, and 2 moles of trimethylol propane, has a number-average molecular weight of 1334 g/mol, and a degree of branching of 1.50 mol/kg. To demonstrate the influence of the degree of branching (v) on the molecular weight distribution, polyesters of different branching but the same number-average molecular weight were prepared and analysed. The polyesters consisted of phthalic anhydride, adipic acid, and different quantities of methylpropyl 1,3-propanediol (1,3-MPPD) and trimethylol

58

Molecular weight distribution of polyesters propane (TMP), but with an equal excess of polyols. 1,3-MPPD was chosen because its molecular weight of 132 g/mol is close to the molecular weight of TMP at 134 g/mol. Thus, the different structural units have nearly the same weight. The formulations for the four polyesters (series 1) are presented in Table 3.7. GPC analysis shows that the number-average molecular weights match the theoretical values with sufficient precision. The molecular weight distribution curves for the polyesters are shown in Figure 3.33. From Table 3.7 it can be seen that a higher degree of branching leads to a higher weight average molecular weight. In Figure 3.33 this effect is visualised by broader molecular weight distributions, increased fractions of higher molecular weight polyester and lower curve maxima. The use of polyols of very similar molecular weights yields structural units of very similar molecular weights. Thus, it is possible to match the individual peaks for polyester molecules of lower molecular weight in the GPC diagram to the theoretical values.

Figure  3.33:  Molecular weight distribution of polyesters of equal number-average molecular weights but different degrees of branching

59

Formation and structure of polyesters and alkyd resins Table 3.8:  Polyesters of the same average molecular weight but different degrees of branching building element

polyester

/10

/11

/12

/13

phthalic anhydride

1.000

1.000

1.000

1.000

adipic acid

0.540

0.480

0.430

0.380

MPPD-1,3

0.540

0.480

0.430

0.380

trimethylol propane

0.120

0.240

0.340

0.440

kM (AV = 0)

1.200

1.200

1.200

1.200

acid value (mg KOH/g, measured)

11.8

11.2

10.3

11.0

OH value (mg KOH/g, calculated)

121.1

144.4

163.0

182.8

degree of branching (mol/kg)

0.45

0.89

1.25

1.60

1042

1062

1088

1081

characteristic values

average molecular weight (g/mol) number-average, calculated number-average, GPC

940

982

998

1037

weight average, GPC

1629

1847

1991

2228

dispersity

1.733

1.881

1.995

2.149

160

205

285

315

viscosity (60 % in MPA) (mPa·s, 23 °C)

The logarithmic value for the first peak is 2.58, corresponding to 380 g/mol. This corre­lates well with the theoretical value of 390 g/mol for the molecular weight of one mole of a mixture of polycarboxylic acid and two moles of polyol – the smallest possible molecule in the distribution. When polyesters are being prepared in practice, it is important to monitor the growth in size of the polyester molecules. The degree of condensation is usually determined by measuring the acid values. It would be very time consuming to follow the molecular weight development by GPC. Instead, a faster indication can be obtained by measuring the viscosity of a specific test solution or of the reaction melt. Both allow the weight-average molecular weight to be quantified. The viscosity of polyester solutions correlates with the weight-average molecular weight. The theoretical background for this was laid down in the equations of Einstein and Staudinger [45] these state that the viscosity of solutions of very low concentration is dependent on the coil volume of diluted polymer. And the coil volume is dependent on the weight-average molecular weight of the polymer.

60

Molecular weight distribution of polyesters To eliminate the interactions of the molecular coils amongst themselves, the viscosity is taken to be the intrinsic viscosity ([η], Staudinger index). The intrinsic viscosity is the extrapolation of the specific viscosity as a function of polymer concentration for a concentration of 0. The specific viscosity is the difference between the solution viscosity and the solvent viscosity divided by the solvent viscosity. The intrinsic viscosity is then the weight-average molecular weight with exponent (α) multiplied by a specific viscosity constant k[η] (see Mark-Houwink equation [46]): Equation 3.57



η = viscosity of solution η0 = viscosity of solvent ηsp = specific viscosity c = solution concentration [η] = intrinsic viscosity k[η] = viscosity constant Ṁw = weight-average molecular weight α = exponent of intrinsic viscosity The exponent α as well as the viscosity constant k[η] are dependent on the interaction between polymer and solvent. The values are therefore substance- and temperature-specific. For polyester solutions in typical analytical solvents (e.g. tetrahydrofuran), the vis­ cosity constant (k[η]) assumes values between 0.01 and 0.04. The exponent (α) has values between 0.70 and 0.90 (at 20 °C). It is not possible to derive the two terms theoretically; they must be determined experimentally for various polymers and solvents. However, for practical polyester preparation, it is sufficient to monitor the viscosity of a test solution and to set a value for the end of the polycondensation process. This value is then used for continuous, reproducible production of that polyester. A second series of model polyesters illustrates the relationship between molecular weight distribution, degree of branching and viscosity. To this end, polyesters were synthesised from phthalic anhydride, neopentyl glycol, 1,6‑hexanediol, and trimethylol propane, with the same excess of polyol and the same degree of condensation, and therefore with approximately the same molecular weight (number-average molecular weight) but different degrees of branching (see Table 3.8).

61

Formation and structure of polyesters and alkyd resins The table shows that the number-average molecular weights obtained by GPC analysis are somewhat lower than the theoretical values. Obviously, the result of GPC is influenced by the building blocks that compose the polyesters. Figure 3.34 shows how the dispersity and viscosity vary with the degree of branching in both series of polyesters. A nearly linear relationship exists between dispersity and degree of branching for both series and it increases substantially with increase in the degree of branching. A higher number-average molecular weight clearly leads to higher dispersity (series 6–9: number-average molecular weight about 1280 g/mol; series 10–13: number-average molecular weight about 1070 g/mol). The viscosity of the polyester solutions increases exponentially as a function of the degree of branching. The difference in the viscosity curves of both series is explained by the different molecular weights of both polyesters. A plot of viscosity – representing the weight-average molecular weight – against the polycondensation constant (k'M ≥ 1.00) and the degree of polyester branching – shown

Figure  3.34:  Dispersity and viscosity of polyesters as a function of degree of branching

62

Molecular weight distribution of polyesters here as the average functionality of the polyols – reveals a region where the viscosity tends towards infinity. The polyesters there have gelled. The higher the functionality of the polyols and thus the higher the degree of branching, the larger is this region. Although the number-average molecular weight is still of finite dimension (k'M ≥ 1.00), the polyester has gelled. This substantiates Flory’s postulate that gelation takes place if the weight-average molecular weight, but not the number-average molecular weight, tends towards infinity. This universal statement is illustrated in Figure 3.35. This condition is also illustrated in Figure 3.18. The curves there for the polycondensation constants (kM) represent average values. If the curve is extended to include polyesters made from triol and dicarboxylic acid, the molecular weight distribution of such polyesters has the effect of more or less broadening the region for the constant. If, as a result of the broad molecular weight distribution, these regions extend into the region where the polycondensation constant is below 1.00 (kM < 1.00), the outcome is some molecules

Figure  3.35:  Viscosity of polyester solutions as a function of polycondensation constant and polyol functionality

63

Formation and structure of polyesters and alkyd resins which are tending towards infinite molecular weight, i.e. the polyester has gelled (in accordance with Flory's postulate). Polyesters can naturally then be obtained only if the values of the polycondensation constant for all molecules exceed 1.00 (kM > 1.00). In other words, it is necessary to increase the planned average number of structural units and thus also the expected number-average molecular weight, so that all molecules fulfil the condition kM > 1.00. That is the reason why all practical highly branched polyesters (if they are prepared by the conventional polycondensation process) have lower number-average molecular weights, as shown in Figure 3.36.

Figure  3.36:  Molecular weight distributions as regions of the polycondensation constant; preconditions for the feasibility of branched polyesters

64

Molecular weight distribution of polyesters Table 3.9:  Composition, characteristic values and solution viscosities of model polyesters of different molecular weights and different degrees of branching polyester building element /6

/7

/8

/9

/14

/15

/16

/17

/18

/19

phthalic anhydride

[mol]

0.70

0.70

0.70

0.70

0.70

0.70

0.70

0.70

0.70

0.70

adipic acid

0.30

0.30

0.30

0.30

0.30

0.30

0.30

0.30

0.30

0.30

MPPD-1,3

1.15

1.00

0.85

0.70

0.72

0.53

0.28

0.20

–

0.11

trimethylol propane

–

0.15

0.30

0.45

0.30

0.52

0.82

0.95

1.20

1.02

characteristic values: K'M

1.22

1.22

1.23

1.21

1.09

1.12

1.17

1.22

1.27

1.20

q'

4.6

4.6

4.4

4.7

11.3

8.1

6.0

4.5

3.7

5.1

acid value [mg KOH/g]

13.6

13.8

15.2

13.0

14.8

15.6

13.5

14.6

13.7

13.5

OH value [mg KOH/g]

74

105

136

165

88

147

224

266

328

273

0.00

0.54

1.08

1.62

1.15

1.96

3.01

3.40

4.19

3.69

2936

2143

1644

1253

1060

1407

1883

v [mol/kg]

molecular weights: (g/mol) Mn (calculated)

1276

1271

1233

1296

Mn (GPC)

1152

1200

1251

1275

2257

1693

1121

1408

1619

Mw (GPC)

2245

2856

3529

3938

30829 14154 10527

2927

5317

8746

dispersity

1.95

2.38

2.82

3.09

13.70

7.52

6.22

2.61

3.78

5.40

180

235

345

370

2100

1600

1800

920

1600

1900

viscosity [mPa·s] 60 % in MPA

To quantify the general statement of Figure 3.35 showing the dependence of the molecular weight distribution of different polyesters on the polycondensation constant , a fourth series of model polyesters complementing the polyesters of the second series was prepared. This series is presented in Table 3.9. The viscosities of solutions of these model polyesters are illustrated in Figure 3.37, in which the number-average molecular weight, as expressed by the polycondensation constant (kM),

65

Formation and structure of polyesters and alkyd resins and the average number of structural units (q) are plotted against the degree of branching (v). Any attempt to draw lines of equal viscosity, based on the viscosity values of the polyesters, reveals a skewed exponential dependence of the viscosity on the two values. As a result, it is possible to estimate the feasibility of polyesters due to viscosity limitations.

Influence of the reactivity of the polyester building blocks Next to the targeted number-average molecular weight and the degree of branching, the reactivity of the functional groups of the building blocks is the third important influence on the molecular weight distribution of polyesters. For example, a comparison of the molecular weight distributions of polyesters based on either adipic acid or phthalic anhydride but of otherwise identical composition shows that the adipic polyesters always have a broader molecular weight distribution than their phthalic anhydride counterparts. This difference is not necessarily discernible from viscosity values due to the huge difference in solubility of the products. Adipic acid polyesters have much more mobile aliphatic polyester chains than phthalic anhydride polyesters, with their relatively stiff,

Figure  3.37:  Viscosities of model polyesters as a function of polycondensation constant and degree of branching

66

Molecular weight distribution of polyesters aromatics containing chains. However, GPC does reveal the difference. The reason for the narrower molecular weight distribution of phthalic anhydride polyesters is the differential reactivity of the two potential carboxyl groups of phthalic anhydride. The anhydride addition takes place very rapidly, even at low temperatures, to yield individual esters. The second carboxyl group, which is the outcome of the anhydride addition, reacts slowly due to steric hindrance of the ortho-structure. In addition, the vicinal ester groups readily support alcoholysis, which forms the basis for a transesterification reaction, which is the reason that average molecular weights are formed. By contrast, adipic acid has two carboxyl groups of similarly high reactivity. The esters formed are not so readily amenable to transesterification. As another example, consider polyesters composed of trimellitic anhydride and of trimesic acid but of otherwise comparable composition. The trimellitic anhydride polyesters always have a narrower molecular weight distribution than their trimesic acid counterparts. Also, the viscosity of trimellitic anhydride polyester solutions is much lower than that of trimesic acid polyester solutions of otherwise identical composition and the same number-average molecular weight. This also applies if the two compounds constitute only a fraction of the polycarboxylic acids in a polyester formulation. These differences also become noticeable when polyesters based on isophthalic acid are compared with those based on phthalic anhydride. As isophthalic acids have melting temperatures greater than 300 °C, during polyester production at conventional temperatures of 180 to 240 °C, they react only at the particle interfaces. Once isophthalic acid molecules available at the interface go into solution, both carboxyl groups may react at the same time by esterification. This boosts the growth of polyester molecules. By contrast, phthalic anhydride reacts in two steps, as described above, to yield polyester molecules of narrower molecular weight distribution. A further example is the production of polyesters from terephthalic acid compared with that of polyesters from dimethyl terephthalate by transesterification. Dimethyl terephthalate polyesters initially have a much narrower molecular weight distribution than their free terephthalic acid counterparts, because terephthalic acid is less reactive and has a higher melting point than isophthalic acid, as described above. As dimethyl terephthalate is readily available in the molten phase at common reaction temperatures, the two ester groups can react at different times. Again, as both polyesters consist of the same building blocks, the differences in molecular weight distributions can be recorded by measuring the viscosity of solutions. However, it must be remembered that the degree of transesterification of dimethyl terephthalate is difficult to establish because acid values cannot be measured. It may be assumed that, after sufficient reaction time, the molecular weight distributions of both polyesters approach one another due to the transesterification equilibrium. Given that the width of the molecular weight distributions is dependent on the specific reactivity of the building blocks, there is no way, based on the information described above, to incorporate the parameters governing the molecular distribution into a universal

67

Formation and structure of polyesters and alkyd resins equation. The limitation of the area of polyesters (see Figures 3.35 and 3.37) which are not feasible since they would gel, even though their number-average molecular weight is finite, is variable for different polyester formulations. Therefore, what remains is only the definition of the number-average molecular weight, calculated from the excess of polyol and the degree of condensation and the estimate of the molecular distribution curves which are influenced by the value of the expected number-average molecular weight, the degree of branching, and the reactivity of functional groups of building blocks. Clearly, the reaction conditions for preparing polyesters play only a minor role as far as the molecular weight distribution is concerned. The reason is that the gradients of the reaction rate for esterification and transesterification are nearly the same under the common reaction conditions (180 to 240 °C) of polyester production. If a polyester is prepared one time at 200 °C and another at 240 °C under the same degree of condensation, the outcome will be a solution of the same viscosity and the same molecular weight distribution. However, it takes much longer to achieve the projected degree of condensation at 200 °C than at 240 °C. It seems that only at very high temperatures does the relative transesterification reaction rate surpass the relative esterification reaction rate. However, that favours the formation of average polyester molecules. These facts explain why polyester preparation can be interrupted in the laboratory without impairing the reproducibility of molecular weight and molecular weight distribution, followed by viscosity at the same degree of condensation. To emphasise the importance of the transesterification reaction in the control over the molecular weight distribution – which has been neglected by previous authors – consider the following examples.

Example a) Variations on the preparation of branched polyesters of terephthalic acid Since the 1950s, relatively low-molecular weight, branched polyesters prepared from dimethyl terephthalate, glycerol and ethylene glycol have been used for heat-stable electrical insulation coatings (wire enamels). Usually the polyesters are prepared by transesterification, starting with the aforementioned raw materials and adding transesterification catalysts (e.g. tetrabutyl titanate) at temperatures of about 240 °C. It is possible to synthesise the same polyesters from polyethylene terephthalate (PET). Polyethylene terephthalate is an important raw material for fibres and film and has a molecular weight of more than 20,000 g/mol. It consists of esters of terephthalic acid and ethylene glycol in the precise ratio of 1 : 1 (i.e. kM −> 1.00). If the polyethylene terephthalate is made to react with an appropriate excess of polyol, the same polyester as described earlier can be produced. Here, too, it is necessary to use transesterification catalysts and to carry out the reaction at temperatures of about 240 °C for a long time. The two poly-

68

Molecular weight distribution of polyesters Table 3.10:  Comparison of the preparation of polyester from raw materials and from polyethylene terephthalate polyester 1 building element dimethyl terephthalate PET

M 194 (192)x

ethylene glycol

62

glycerol

92

n

n•M

m-%

n

n•M

m-%

1.00

194.0

85.9

–

–

–

–

–

–

1.00

192.0

85.0

1.10

68.2

30.2

0.10

6.2

2.8

0.30

0.30

sum methanol yield

32

polyester 2

2.00

27.6

12.2

289.8

128.3

64.0

28.3

225.8

100.0

–

27.6

12.2

225.8

100.0

–

–

225.8

100.0

esters have nearly the same viscosity, a fact which points to the same molecular weight and molecular weight distribution. This is evidence that polyesters can be prepared not only from monomeric building blocks, but also from high molecular weight polyesters. It is also a clear indication of the effect that transesterification has on regulating average molecular weights. At one time, it seemed a good idea to use polyester film scrap in the preparation of polyesters (several patents describing this method exist). However, the process failed to find acceptance. The reason was that the bulky scrap was difficult to handle, and it was not readily absorbed into the relatively small quantity of polyol. The two formulations are shown in Table 3.10 for comparison.

Example b) “Repairing” of polyesters undergoing gelation Gelation of polyesters may happen in the laboratory if the polyol is not present in sufficient excess or if the degree of branching is too high. More problematic is gelation that takes place in production due to metering errors. In such cases, the viscosity rises rapidly and a gel forms. According to Flory’s theory, the gel consists of only a few molecules striving to become infinitely large. The bulk consists of small molecules. If the gel is sufficiently mobile (due to adequate agitation energy), it is possible to add polyol and so to destroy the gel state. This works because the added quantities of polyols can react by transesterification and can so reduce the size of the very large molecules. To achieve this, the temperature needs to be raised. In production, it takes courage to heat the reaction mixture of such gelling polyesters to raise the temperature once excess quantities of polyol have already

69

Formation and structure of polyesters and alkyd resins been added. However, this is the only way to “repair” the polyester. If the added polyol is already contained in the polyester, this approach will often “save” the batch of polyester being prepared and produce a suitable batch that meets the specifications. At the very least, this will prevent the cold, solidified gel from having to be “dug out”. Here, too, the transesterification reaction is an important part of the regulating effect.

3.5 Formation and structure of alkyd resins 3.5.1 Special aspects of the preparation of alkyd resins As mentioned earlier, alkyd resins are prepared by reaction of polycarboxylic acids, polyols and monocarboxylic acids. Common alkyd resins contain phthalic anhydride as the polycarboxylic component, polyols bearing more than two (average value in the case of mixtures) hydroxyl groups (both primary and secondary), and monocarboxylic acids containing primary and, rarely, secondary carboxyl groups. There is a particular reason for this selection. When alkyd resins are produced from a reaction mixture of these three ingredients, the phthalic anhydride reacts first with OH groups to form an ester and a vicinal carboxyl group. This carboxyl group reacts relatively slowly with further OH groups due to steric hindrance of the vicinal ester group and the π-electron system of the aromatic ring. Therefore, the remaining OH groups react much faster with the carboxyl groups of the monocarboxylic acids (especially if they are primary types). Only then does the second carboxyl group of phthalic anhydride react with OH groups, yielding larger molecules, the polyester backbone of alkyd resins. The process is shown in Figure 3.38. Naturally, the preparation of alkyds is not a genuine step-wise process. The steps shown are merely intended to indicate the different reaction velocities of the building blocks. The reactions take place in parallel but with markedly different velocity gradients. Of particular importance is the fact that the polyols form esters with the monocarboxylic acid faster than they participate in polyester chain growth of the alkyd resins. If, at the end of alkyd resin production, the measured acid values are relatively low, the remaining carboxyl groups stem from the partial phthalic esters and not the monocarboxylic acids. This has been confirmed by GC analysis, which found only partial esters and free phthalic anhydride after preparation of alkyd resins. This fact has consequences for the molecular weight distribution of alkyd resins. If they are prepared from dicarboxylic acids other than phthalic anhydride, different molar approaches are required. For example, the polycondensation constant needs to be increased if isophthalic acid is used or if adipic acid is employed in addition to phthalic anhydride.

70

Formation and structure of alkyd resins The reason is that both carboxyl groups of these dicarboxylic acids have nearly the same reaction velocity, in contrast to the two potential functional groups on phthalic anhydride. The use of tertiary monocarboxylic acids, too, requires specific measures. The reason is that the intermediate reduction in OH group functionality through the formation of monocarboxylic esters is not as effective as when phthalic anhydride is used. Such differences become noticeable when primary carboxyl groups are compared with secondary carboxyl groups, e.g. caprylic acid with 2-ethylhexanoic acid.

3.5.2 Calculation of molecular weights of alkyd resins Particularly in American literature [44], monocarboxylic acids acting as building blocks for alkyd resins are known as chain stoppers. This term is mainly used in connection with benzoic acid. However, if the term is understood as implying a limitation on molecular size, it is not really appropriate. Alkyd resins are always formulated in such a way that the number of hydroxyl groups (nOH) from the polyols (n1) is larger than the number of carboxyl groups

Figure  3.38:  Step-wise formation of alkyd resins

71

Formation and structure of polyesters and alkyd resins (nCOOH) from the polycarboxylic acids (n2) and the monocarboxylic acids (n3), as shown in the following equation. Equation 3.58

Alkyd resins always contain an excess of hydroxyl groups (νOH), one of the reasons being to avoid residual free monocarboxylic acids, which would impair the resin properties. If alkyd resin molecules contain not only free hydroxyl groups (νOH) but also free carboxyl groups (νCOOH), they can keep growing. This will occur totally independently of the quantity of monocarboxylic acid. Therefore, the same rules, definitions and calculations apply to alkyd resins as to polyesters. Thus, the same equations for calculating the polycondensation constant (kM) and the number of structural units of polyester can also be used for alkyd resins, as shown in the following equation: Equation 3.59

However, the molecular weight of the structural units (M'S) of alkyd resins may differ considerably from that of the structural units of unmodified polyesters, because allowance needs to be made for the quantity of monocarboxylic acid (n3). For example, the structural unit of an alkyd resin consisting of the residue of phthalic acid, pentaerythritol and 1.6 moles of a monocarboxylic acid (molecular weight 280 g/mol) has a molecular weight of 685 g/mol. It is thus three times as large as a structural element of a polyester prepared from isophthalic acid and neopentyl glycol (234 g/mol). For the molecular weight of a structural element of alkyd molecules, the following applies. Equation 3.60

The number-average molecular weight of alkyd resins is calculated as follows: Equation 3.61

72

Formation and structure of alkyd resins Table 3.11:  Formulation of sample alkyd resin and calculated characteristic values n•F

n

building elements

M [g/mol]

n•M [g]

wt.%

–2.00

1.00

phthalic anhydride

148

148.0

41.02

+3.09

1.03

trimethylol propane

134

138.0

38.26

–0.65

0.65

isononanoic acid

158

102.7

28.47

388.7

107.75

sum +0.44

νOH 1.65

29.7

7.75

yield A0

water

18

359.0

100.00

yield AAV

360.7

which can be re-written as: Equation 3.62

The numerator of the quotient then represents the yield mass of an alkyd resin consisting of n1 moles of polyol, n2 moles of polycarboxylic acid and n3 moles of monocarboxylic acid. The equation also applies if there is no excess of polyol (n1 ≤ n2). In such cases, limiting of the molecular weight is accomplished by restricting the degree of condensation. However, in such cases, the number of all hydroxyl groups then needs to be higher than the number of carboxyl groups (nOH > nCOOH). From the yield mass and with allowance for the actual acid value (AAV), the number-average molecular weight and the related data are calculated as follows. Equation 3.63

For alkyd resins, too, the calculation methods will now be illustrated with an example. An alkyd resin is to consist of 1.00 mole of phthalic anhydride, 1.03 moles of trimethylol propane, and 0.65 moles of isononanoic acid. The condensation reaction is carried out until the acid value is 15 mg KOH/g. The formulation and the calculated characteristic values are presented in Table 3.11.

73

Formation and structure of polyesters and alkyd resins The yield mass at a given acid value is calculated as follows: Equation 3.64

The number of unreacted carboxyl groups is given by the following equation Equation 3.65

That value increases the quantity of residual OH groups as follows: Equation 3.66

The OH value is then given by: Equation 3.67

The polycondensation constant (k'M) is: Equation 3.68

The number-average molecular weight then computes to:

Equation 3.69

3.5.3 Molecular weight distribution of alkyd resins Theoretically, the same factors which influence the molecular weight distribution of polyesters apply to alkyd resins. These are mainly the targeted number-average molecular weight and the reactivity of the different functional groups of the building blocks. However,

74

Formation and structure of alkyd resins unlike in the case for unmodified polyesters, the quantity of monocarboxylic acid plays an important role in regulating the molecular weight distribution. As described in Chapter 3.6.1, the monocarboxylic acids act in situ, as it were, to reduce the functionality of polyols. The extent of this effect, i.e. the quantity of the monocarboxylic acid, has a significant bearing on the molecular weight distribution. When a greater excess of polyol OH groups (νOH) in the alkyd resin molecule has reacted with monocarboxylic acid, the possibility of disproportionate alkyd molecule growth has been reduced. The degree of branching of alkyd resins may be derived from the degree of branching for the polyester backbone by subtracting the quantity of monocarboxylic acid in the numerator of the equation: Equation 3.70

The degree to which an excess of polyol has reacted with monocarboxylic acid is equivalent to the degree of branching in unmodified polyesters. Thus, the degree of branching (v) in polyesters is replaced by the degree to which an excess of polyol has reacted with monocarboxylic acid (b), as defined in the following equation: Equation 3.71

(b) is the fraction of monocarboxylic acid (n3) on the excess residual OH groups (νOH). And the excess residual OH groups are the balance of all the functional groups of polyols and the polycarboxylic acids in the alkyd backbone. Equation 3.72

If the degree of condensation is incorporated into the definition (b'), the number of excess OH groups (νOH) increases by the number of residual carboxyl groups (νCOOH), giving rise to the following equation for the degree to which an excess of polyol has reacted with monocarboxylic acid: Equation 3.73

75

Formation and structure of polyesters and alkyd resins The influence of b' on the molecular weight distribution can be illustrated with viscosity curves for alkyd resins. Figure 3.39 shows how the viscosity of triol alkyd resins varies with the polycondensation constant (k'M; representing the number-average molecular weight) and the number of moles of monocarboxylic acid per mole of triol (n3/n1; representing b'). These curves are equivalent to the curves showing the dependency of the viscosity of unmodified polyesters on the polycondensation constant and the average functionality of polyols (representing the degree of branching), as shown in Figure 3.35. The higher the degree of reaction (b`), the narrower is the molecular weight distribution and the lower is the solution viscosity. For alkyd resins, too, there is a region where the alkyds still have finite number-average molecular weights but are gelled, because some molecules tend towards infinity. In the description of the preparation of alkyd resins (see Chapter 3.6.1), it was stressed that the difference in reactivity of the two potential carboxyl groups of phthalic anhydride ensures that the in situ reduction in functionality of polyols through occupation with mo-

Figure 3.39:  Viscosities of triol alkyd resins as a function of polycondensation constant and degree of reaction of triols with monocarboxylic acid

76

Formation and structure of alkyd resins nocarboxylic acids is very efficient. This influence exerted by the building blocks is very important. If alkyd resins are formulated not with the usual phthalic anhydride but rather with isophthalic acid, it is necessary to lower the expected number-average molecular weight, i.e. to increase the polyol excess (k'M) or to lower the degree of condensation by raising the acid value. Otherwise the feasibility limit might be reached, i.e. the alkyd resin would gel at comparable degree of condensation. This is all the more true for alkyds prepared from adipic acid instead of phthalic anhydride. Thus, it is practically impossible to prepare an alkyd resin from adipic acid, pentaerythritol and monocarboxylic acid at low values for the polycondensation constant (k'M) – not even at the maximum-possible degree of reaction of the monocarboxylic acid with the excess of polyol. The competition between the two reactive carboxyl groups on the adipic acid and the carboxyl groups on the monocarboxylic acid gives rise to gelled – partially crosslinked – polyesters of pentaerythritol and adipic acid, along with residual free monocarboxylic acid. If the monocarboxylic acid is esterified first with pentaerythritol and the adipic acid is added in a second stage, the changes of obtaining usable alkyd resins is increased. Also, the reactivity of the monocarboxylic acids is important for the molecular weight distribution of alkyd resins. For example, comparison of an alkyd containing phthalic anhydride, polyol and isononanoic acid (3,5,5-trimethyl hexanoic acid, with primary carboxyl group), with another containing 2-ethylhexanoic acid (secondary carboxyl group) and with a third containing neodecanoic acid (mainly 2,2,3,5-tetramethylhexanoic acid, tertiary carboxyl group) shows that the alkyd made with 2-ethylhexanoic acid has a broader molecular weight distribution than its isononanoic counterpart. Whether or not an alkyd resin containing neodecanoic acid is feasible at all depends on the size of the polycondensation constant (k'M). The reason is that the in situ decrease in polyol functionality varies with the differential reactivity of the carboxyl groups of monocarboxylic acids. Naturally, the reaction of excess functional groups of the polyols with monocarboxylic acids does not necessarily lead to linear alkyd molecules. Reaction of monocarboxylic acids may give rise to side chains as well as end groups. The possible structures which can occur in alkyd resin molecules are presented in Figure 3.40. Therefore, if conventional methods are used in their preparation, alkyd resins with a high degree of reaction with excess polyol are the only way to achieve polyester molecules which are highly branched and have relatively high molecular weights. Figure  3.40:  Possible structures in alkyd resin molecules

77

Formation and structure of polyesters and alkyd resins Figure 3.41 shows a typical molecular distribution curve for an alkyd resin consisting of phthalic anhydride, pentaerythritol and a fatty acid with a high degree of reaction with excess polyol (b'). At high average molecular weight, the distribution curve is relatively broad. The mass fraction of very large molecules falls away, because the functional groups on the large molecules have lower reactivity for kinetic reasons.

3.6 Functionality of polyesters and alkyd resins The most important application of polyesters and alkyd resins lies in coating systems which form films by crosslinking. Crosslinking takes place by reaction of the functional groups of polyesters and alkyd resins with themselves or with crosslinking partners. The crosslinking reactions mainly occur via the OH groups, although some occur via the carboxyl groups.

Figure  3.41:  Typical molecular distribution curve for an alkyd resin of phthalic anhydride, pentaerythritol and fatty acid, with high degree of reaction with excess polyol

78

Functionality of polyesters and alkyd resins The content of functional groups in polyesters and alkyds is found via the acid and OH values or the OH-% content. The corresponding calculation equations are expressed in terms of unit mass of polyesters and alkyd resins. Equation 3.74

Equation 3.75

Equation 3.76

When it comes to the estimation of crosslinking efficiency, it is not the mass concentration of functional groups (e.g OH groups and the OHV) which is decisive, but rather the func-

Figure  3.42:  Correlation between OH value, degree of branching and the number-average molecular weight and functionality of polyester or alkyd resin molecules

79

Formation and structure of polyesters and alkyd resins tionalities (F) of the reacting partners: crosslinking molecules as well as the resins used. For example, a linear polyester with two terminal OH groups may lead to totally different molecular networks than a branched polyester containing four OH groups per molecule. Nonetheless, the two can have the same OH value. It is therefore instructive to review the relationships between OH value or acid value and the functionality of polyesters (FP,OH and FP,COOH), the degree of branching and the number-average molecular weight. Calculation equation is provided below: Equation 3.77

Figure 3.42 describes the relationship between OH value (OHV), the degree of branching (ν) and the number-average molecular weight and functionality of the polyester or alkyd resin molecules. Accordingly, a polyester with an OH value of 112 mg KOH/g and a number-average molecular weight of 1000 g/mol contains two OH groups per mole and is linear, and its degree of branching is 0 mol/kg. If the molecular weight were 2000 g/mol and the OH value

Figure  3.43:  Dependence of functionality of polyester molecules on the OH value and the number-average molecular weight

80

Exceptions and their influence on the molecular weight distribution remained unchanged, the molecule would contain four OH groups and the degree of branch­ ing would be 1.00 mol/kg. For a number-average molecular weight of 3000 g/mol, the polyester contains six OH groups and the degree of branching is 1.50 mol/kg. This relationship can be plotted as the dependence of the functionality of polyester molecules on the OH value and the number-average molecular weight (see Figure 3.43).

3.7

 xceptions and their influence E on the molecular weight distribution

All the definitions and calculations presented so far apply to polyesters and alkyd resins prepared by esterification and transesterification at elevated temperatures and by distillation of a low-molecular reaction product. It was pointed out that the number-average molecular weight and the degree of branching or reaction of excess OH groups deter­ mine the molecular weight distribution, in addition to the influence exerted by specific building blocks. Accordingly, it is impossible for highly branched polyesters or alkyd resins with a low degree of reaction of excess polyol with monocarboxylic acid to achieve high number-average molecular weights, because the molecular distribution func­ tions are so broad that some of the molecules will strive to become infinitely large, i.e. become crosslinked. However, there are special processes which give rise to polyesters by means of step-wise addition. The goal here is to achieve polyesters of high functionality but narrow molecular weight distribution. These processes are time consuming and there­ fore very expensive. Several accounts describe the preparation of dendrimer polyesters [47]. For example, one reports the esterification of polyfunctional compounds by biosynthesis [48] at low temperatures. The resulting polyesters are claimed to have narrower molecular weight distributions than polyesters of the same composition that have been prepared in the conventional way. Another describes the preparation of polyesters from polyols or polyol derivatives and dimethylol propanoic acid in a step-wise process to form spherical molecules of high functionality and narrow molecular weight distribution [49]. That is impossible by conventional esterification. For one thing, the tertiary carboxyl group is very difficult to esterify. Fur­ thermore, polyesters cannot be formed in separate steps. Dimethylol propanoic acid can also form esters with itself and there is also the possibility of transesterification. The result will be a polyester with a random distribution of different molecules. Such polyesters can be synthesised by a series of addition reactions at low temperatures. One example is an anhydride addition followed by addition of epoxy compounds [50].

81

Formation and structure of polyesters and alkyd resins Finally, there is the possibility of a step-by-step reaction. This starts by masking the OH groups of dimethylol propanoic acid with benzaldehyde. The resulting aldol adduct is then made to react with pentaerythritol in the presence of a special catalyst to yield a tetra-ester. The tetra-ester is unmasked, yielding the first generation of dendrimer polyesters. The three reaction steps are repeated to produce further generations of dendrimer polyesters, with larger molecules of higher functionalities [51]. These were initially prepared with high-solid products in mind. However, such polyesters are very expensive to prepare, and the future will tell whether the preparation of high-solid coatings in this way is worth the effort.

3.8

 xplanation of symbols E in definitions and equations

Symbols n ν F M

number of moles of building blocks or of functional groups (type indicated by an index) number of current or residual functional groups (type indicated by an index) functionality of a building block molecule (average value in the case of mixtures molecular weight of building blocks or polyesters or alkyd resins (type indicated by an index) A yield mass of a polyester or an alkyd resin q  number of structural units in a polyester or an alkyd resin, expressed in terms of the number of moles polycarboxylic acid and the associated polyols and, where indicated, of monocarboxylic acid p degree of polycondensation (see Equation 3.4) K  calculation constants (KM polycondensation constant for molar ratios) KPatton Patton or alkyd constant) v degree of branching – number of potential branches in a polyester or an alkyd resin, expressed in terms of 1000 g b occupation of the excess OH groups on the polyester backbone of an alkyd resin by monocarboxylic acidsw D dispersity – measure of the molecular distribution AV acid value OHV OH value OH-% OH-% content Indices 1 expressed in terms of polyols and their derivatives

82

Index of equations 2 expressed in terms of polycarboxylic acids and their derivatives 3 expressed in terms of monocarboxylic acids and their derivatives OH expressed in terms of OH groups COOH expressed in terms of carboxyl groups n calculated on the number of moles m calculated on the mass M calculated on the molecular weight AV calculated on the acid value s expressed in terms of the structural unit of polyesters and alkyd resins c critical value of a term Apostrophe as exponent: K'M KM with allowance for the degree of condensation M'2 M2 without condensation product (residue of polycarboxylic acid) M'3 M3 without condensation product (residue of monocarboxylic acid) M'P MP, with allowance for the degree of condensation b' b, with allowance for the degree of condensation

3.9

Index of equations

Equation 3.78



page 27

Dependence of number of structural units of linear polyesters on the degree of condensation Equation 3.79 

page 28

Dependence of number of structural units of linear polyester on the molar ratio of polyol and polycarboxylic acid Equation 3.80

page 

page 28

Dependence of number of structural units of linear polyester on the degree of condensation

83

Formation and structure of polyesters and alkyd resins Equation 3.81



page 29

Equation 3.81: Degree of polycondensation (Carothers) Equation 3.82



page 29

Critical degree of polycondensation (Carothers) Equation 3.83

 page 30

Critical degree of branching (Flory)

Equation 3.84

 page 30

Critical degree of polycondensation (Stockmayer)

Equation 3.85

 page 30

Critical degree of polycondensation (Jonason) Equation 3.86



page 31

Polycondensation constant (Patton) Equation 3.87



Molar ratios as polycondensation constant

84

page 34

Index of equations Equation 3.88



page 35

Dependence of number of structural units on the molar ratio of polyol and polycarboxylic acid, irrespective of polyol functionality Equation 3.89



page 35

Dependence of number of structural units on the molar ratio of poly carboxylic acids and polyol, irrespective of polycarboxylic acid functionality Equation 3.90



page 37

Polycondensation constant for all polyols and polycarboxylic acids Equation 3.91



page 37

Molecular weights for all polyesters Equation 3.92



page 38

Calculation of molecular weights of polyesters of n1 and n2 building blocks Equation 3.93



page 38

Conversion of calculation of molecular weights of polyesters of n1 and n2 building blocks Equation 3.94



page 38

Conversion of calculation of molecular weights of polyesters of n1 and n2 building blocks

85

Formation and structure of polyesters and alkyd resins Equation 3.95



page 39

Calculation of molecular weight of polyesters based on the mass yield Equation 3.96



page 39

Development of mean values for moles of polyols Equation 3.97



page 40

Polycondensation constant after allowance for degree of condensation Equation 3.98



page 41

Polycondensation constant after allowance for degree of condensationdegree Equation 3.99



page 42

Calculation of molecular weight of polyesters by given acid value Equation 3.100



page 42

Yield mass as function of acid value Equation 3.101



Yield mass as function of acid value

86

page 44

Index of equations Equation 3.102



page 47

Calculation of molecular weight of polyesters based on yield mass Equation 3.103



page 49

Number-average molecular weight

Equation 3.104



page 49

Weight-average molecular weight

Equation 3.105



page 49

Value of molecular inconsistency (dispersity)

Equation 3.106



page 57

Degree of branching of polyesters Equation 3.107



page 72

Preconditions for the composition of alkyd resins Equation 3.108



page 72

Molecular weight of the structural units of alkyd resins

87

Formation and structure of polyesters and alkyd resins Equation 3.109



page 72

Calculation of molecular weights of alkyd resins Equation 3.110



page 73

Calculation of molecular weights of alkyd resins

Equation 3.111



page 73

Calculation of molecular weights of alkyd resins based on the yield mass

Equation 3.112



page 75

Degree of branching of alkyd resins

Equation 3.113



page 75

Occupation of excess hydroxyl groups on alkyd backbone

Equation 3.114



Occupation after allowance for the acid value

88

page 75

Index of equations Equation 3.115



page 79

Calculation of acid values Equation 3.116



page 79

Calculation of OH values

Equation 3.117



page 79

Calculation of OH-% content

Equation 3.118



page 80

OH functionality of polyester molecules

89

Selection criteria for the different building blocks

4

I nfluence of building blocks on properties of polyesters and alkyd resins

4.1

 election criteria for the different S building blocks

So far, we have looked at influences on the properties of polyesters and alkyd resins, namely influences on molecular weight, molecular weight distribution, degree of branching, functionality, which apply generally to the various polyesters. To these must be added the influences of the choosen building blocks of polyesters and alkyd resins. These influences must be considered in the context of purely structural influences. Here, too, there are principles which must be respected. Two basic properties of polyesters arise from the structure of the building blocks. First, there is the mobility of polyester chain segments. Some molecules contribute various degrees of rigidity while others provide high mobility. The different types of building blocks can be ranked as follows, starting with the most rigid: aromatic compounds, cycloaliphatic compounds, short-chain aliphatic compounds, long-chain aliphatic compounds, and ether groups containing chains. Second is the ability of polyester chains to associate. Aromatic ring structures can associate with the methylene groups of aliphatic chains. Long, linear aliphatic chains, too, can associate with each other. Association is supported by neighbouring ester groups. In special instances, the tendency to associate may lead to crystallinity. As crystallinity can boost toughness in the case of polyesters for fibre materials (e.g. polyethylene terephthalate), this property is undesirable for coating binders. Crystalline polyesters can lead to opacity, inferior flow and levelling, as well as loss of gloss. The tendency for molecular association to occur increases with increase in the linearity of the polyester molecules, in molecular weight, and in uniformity of the polyester chain segments. Crystallinity can be prevented in different ways. The simplest consists of choosing mixtures of different building blocks. For example, if linear polyesters contain three different diols of whatever type, crystallinity of polyester molecules is suppressed. Another way is to select building blocks which contain aliphatic side-chains. The most important example of

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

91

Influence of building blocks on properties of polyesters and alkyd resins Table 4.1:  Experimental values for glass-transition temperatures and melting temperatures of high molecular, linear polyesters polyester containing: dicarboxylic acid

diol

TG

Source

MT

Source

terephthalic acid

ethylene glycol

+ 69 °C

[53]

+265 °C

[53]

terephthalic acid

butane diol-1,4

+15–50 °C

[54]

+230 °C

[58]

isophthalic acid

ethylene glycol

+ 51 °C

[58]

+240 °C

[58]

isophthalic acid

butane diol-1,4

+ 24 °C

[55]

+152 °C

[53]

adipic acid

butane diol-1,4

- 68 °C

[56]

+ 57 °C

[57]

adipic acid

neopentyl glycol

- 48 °C

[56]

+ 46 °C

[57]

adipic acid

hexane diol-1,6

- 73 °C

[56]

+ 61 °C

[57]

- 60 °C

[57]

+ 60 °C

[58]

ε-hydroxy caproic acid

this is neopentyl glycol, whose side-chains act as molecular spacers. Crystallinity may also be avoided by branching of polyesters. The mobility of polyester molecules is a measure of flexibility and hardness. It is possible to obtain figures to describe the balance of hardness and flexibility, the definition of glass-transition temperatures. For example, the glass-transition temperature can be determined from the value of the loss modulus for elasticity as a function of temperature (by differential scanning calorimetry or DSC). It is defined as the value at which the loss modulus falls from the glass state to the elastic state. The higher the level of the loss modulus and the higher the glass-transition temperature, the greater is the physical hardness of the polyester. Crosslinking too raises the glass-transition temperature. Its value is also affected by the mobility of building-block molecules as well as the tendency to associate. All these complex influences therefore make it difficult to predict the level of the glass-transition temperature. It is possible to increment the glass-transition temperatures via individual structural units [52]. However, there is no practical method for calculating glass-transition temperatures for polyesters containing mixtures of different building blocks – in contrast to the case for acrylic resins (Fox’s equation). Values for the glass-transition temperatures and melting temperatures of high molecular, linear polyesters of uniform structure are available, and are substantially influenced by the crystallinity of the products. The values are listed in Table 4.1. These values are of no consequence for the characterisation of polyesters for coatings as they are not crystalline and contain mixtures of different building blocks or have branch­ ed structures. However, they do provide some idea of the effect of the building blocks on glass-transition temperatures. The following trends can be identified:

92

Influences on solubility and compatibility – Glass-transition temperatures decrease most clearly for the transition from aromatic to aliphatic building blocks via cycloaliphatics. – The longer the linear chains of the aliphatic building blocks, the lower the glass-transition temperature becomes. The chain lengths of dicarboxylic acids have a more pronounced effect than those of diols. – Side-chains lower the glass-transition temperatures but not by as much as corresponding linear products (e.g. 1,4-butanediol produces lower glass-transition temperatures than 2-methyl-1,3-propanediol). – Ether groups in aliphatic chains (e.g. of polyether polyols) have a stronger suppressant effect than would CH2 groups at the same position.

4.2

I nfluences on solubility and compatibility

By solubility, in the context of polyesters and alkyds, is meant the ability of molecules to physically interact or associate with solvent molecules to form solutions known as solvates. Such organic solutions are for the most part totally homogeneous. Solubility of polyesters is achieved mainly with building blocks that bear aliphatic side chains. The latter not only act as spacers which prevent molecular association; they also render the molecules less polar. This makes it possible to use highly non-polar solvents to prepare stable solutions of binders. For coating applications, non-polar solutions are preferred, because lower polarity is linked to lower surface tension, which in turn leads to better wetting properties and better levelling. This insight led to the development of alkyd resins which are polyesters that contain non-polar side chains of fatty acids. As early as the 16th century, the famous physician Paracelsus declared that “similia similibus solvuntur” (“like dissolves like”), because he observed that hydrophilic substances are readily soluble in water and hydrophobic compounds are soluble in oils. Trials have since been conducted to quantify solubility. This has spawned definitions of solubility parameters[59] for solvents and polymers, figures for evaporation enthalpy as a measure of molecular association, as well as definitions of dispersion forces, dipolar forces and hydrogen bonds as factors contributing to solubility [60]. Solvent solubility parameters are intended to aid with formulation and determination of the scope for replacing solvents. How­ ever, the solubility parameters of binders are very difficult to determine. The conclusion to be drawn is that solvents that will form the most molecular solvates with binder molecules will lead to solutions with relatively high viscosities. However, there are other requirements when it comes to the formulation of binder solutions for coating applications. Good solvents for coating formulations yield solutions which are stable in storage (no separation, no turbidity) but also feature a combination of

93

Influence of building blocks on properties of polyesters and alkyd resins low viscosity and high-solid content. Good solvents will possess the right evaporation properties under the corresponding application conditions as a precondition for forming smooth, homogeneous films without film defects. Combinations of different binders are compatible if they yield homogeneous mixtures in solution and in films. A precondition for this is a good interdiffusion of the different binder molecules. Physical phenomena occur which are comparable to the formation of solvates in solutions. Polyesters and alkyd resins contain functional groups which can be used in crosslinking reactions. The most important functional groups are hydroxyl, followed by carboxyl. However, the double bonds of unsaturated polyesters and of fatty acids in alkyd resins also act as functional groups for crosslinking reactions. Furthermore, the building blocks for polyesters and alkyd resins are chosen on the basis of availability and cost. The raw materials must also be selected on the basis of their environmental impact. This criterion therefore restricts the use of some raw materials. Characteristic data of raw materials, material safety data sheets, manufacturing technologies and general application instructions are described in chemical dictionaries [61] and in handbooks of industrial chemistry [62].

4.3

Influences on film properties

Usually, the objective of coatings is to combine high film hardness and durability with the greatest possible flexibility. However, it often proves difficult to combine these two properties and improve them independently. Most coating films are therefore a compromise between hardness and flexibility that reflects the requirements of the intended application. High hardness generally results from less-mobile molecular structures. In addition, such components form a barrier in diffusion processes. Thus, films containing such building blocks, are more resistant to solvents and chemicals. Hardness and resistance to diffusion are mainly conferred by aromatic building blocks. However, they reduce solubility and flexibility of the polymer chains. Cycloaliphatic building blocks yield much better solubility and flexibility, without substantial decrease in hardness. Building blocks bearing aliphatic chains can reduce hardness and resistance properties and confer a degree of flexibility that varies with the chain length of the products. The aliphatic monocarboxylic acids in alkyd resins lower the hardness of films, but substantially boost solubility, wetting properties and flexibility. The various types of polyesters are formulated by combining components of different effectiveness in a way that meets the requirements of the different applications. Experience shows that it is better to combine raw materials which possess different properties than to choose building blocks of average properties. Naturally, the choice of raw materials is influenced by availability and cost.

94

Influences on film properties The aromatic building blocks employed in polyesters are in most cases polycarboxylic acids and their derivatives (phthalic anhydride, isophthalic acid, terephthalic acid and dimethyl terephthalate) which have long been available in large-scale quantities and at affordable prices. Aromatic polycarboxylic acids and their derivatives contribute hardness and resistance. There are virtually no polyols that contain aromatic structures. Phenols and methylol-aromatics do not react as other polyols in esterification and are hence unsuitable for standard polyester preparation. The flexibility of films containing polyester resins is usually achieved with aliphatic polyols. A number of polyols serve as raw materials for polyesters. These include ethylene glycol, 1,2- and 1,3-propanediols, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol and other neo-diols, hydroxy pivalic neopentyl esters (HPN), 2,2,4-trimethyl-1,3-pentanediol, and polyether polyols. Further improvements in flexibility of polyester chains can be achieved with polycarboxylic acids having aliphatic components. Examples of such components are adipic acid, azelaic acid, sebacic acid, and fatty acid dimers. Cycloaliphatic components confer a balanced range of properties. Suitable polycarboxylic acids and derivatives are hexahydrophthalic anhydride and other isomers of cyclohexane dicarboxylic acids; tetrahydrophthalic anhydride; endomethylene tetrahydrophthalic anhydride; and tricyclodecane dicarboxylic acid (TCD-dicarboxylic acid). Suitable diols are perhydro bisphenol A, dimethylol cyclohexane, tricyclodecane dimethanol (TCD-alcohol DM). Cycloaliphatic building blocks confer excellent solubility and compatibility and in films they contribute flexibility without any significant reduction in hardness. Some cycloaliphatic components are considerably more expensive than the other raw materials. Modification of polyesters by aliphatic monocarboxylic acid – the basic principle behind the alkyd resin structure – not only boosts solubility and compatibility. It also enhances the wetting and levelling properties of the coatings. Naturally though, film hardness and durability are then reduced to an extent in line with the chain length of the monocarboxylic acid. Hardness can be improved by replacing some of the fatty acids with aromatic monocarboxylic acids (e.g. benzoic acid) or rosin acids (e.g. abietic acid) without departing from the underlying structural principle of alkyd resins. However, rosin acids generate other properties which preclude general usage (yellowing tendency). A number of building blocks enable polyesters and alkyd resins to be crosslinked. For example, they may contain OH groups in excess for combination with different crosslinkers. The OH groups are usually introduced via higher-functional polyols (e.g. glycerol, trimethylolpropane, pentaerythritol). The crosslinking efficiency is determined by the quantity of functional groups as a function of molecular weight and degree of branching (see Chapter 3.7). However, functional groups can vary in their own reactivity. For example, primary OH groups are more reactive than secondary OH groups, while terminal primary OH groups on longer aliphatic chains, e.g. 1,6-hexanediol or ε-caprolactone, are particularly reactive.

95

Influence of building blocks on properties of polyesters and alkyd resins Crosslinking can improve film hardness and flexibility. Hardness of coating films is understood to be resistance to mechanical influences. It is measured as surface hardness or indentation hardness. Flexibility in the physical sense is the sum of the plasticity and elasticity components of films. Plasticity results from the mobility of molecular segments. Elasticity mainly stems from crosslinking. High hardness and adequate flexibility are possible, although this is commonly disputed in the literature, where it is stated that molecular networks of high crosslinking density are always brittle. However, it is possible to achieve hard and flexible films. An optimum combination of both properties is achieved with binders having hard building blocks and which are crosslinked effectively to form extended networks. However, that is only possible in special cases (see Chapter 5.8). For most systems that require high flexibility, a compromise needs to be found between adequate flexibility and hardness. In that case, building blocks which confer plasticity and relatively low crosslinking density are chosen so as to balance the required properties.

4.4

 lassification of polyesters C and alkyd resins

Polyesters and alkyd resins can be classified by their type of building block. Saturated polyesters are binders containing exclusively polycarboxylic acids and polyols and do not contain olefinic double bonds. They are suitable for a diverse range of application areas (Chapter 5.1 to 5.9). They include some modifications. Unsaturated polyesters contain partly olefinic, unsaturated components (Chapter 6). Alkyd resins are polyesters containing polycarboxylic acids or their derivatives, higher functional polyols and monocarboxylic acids in the form of fatty acids, which occupy some of the excess hydroxyl groups. There are alkyd resin types which form films by oxidative-curing (Chapter 7.2). Different modifications exist. The second group contains alkyd resins which are combined with different crosslinkers (Chapter 7.3). The third group is alkyd resins which are suitable for combination with binders which form films by purely physical drying. Here, the alkyd resins act as plasticisers (Chapter 7.5). Some special binders belong to a group of polyesters containing unique building blocks. These are polycarbonates (Chapter 8.1), polycaprolactones (Chapter 8.2), polyesters based on diene adducts (Chapter 8.3), and stand oils (Chapter 8.4).

96

High-molecular weight, saturated polyesters

5 Saturated polyesters As mentioned above, saturated polyesters contain only polyols and polycarboxylic acids without any olefinic double bonds. The term saturated serves to differentiate them from unsaturated polyesters, which contain polymerisable double bonds and which played a more dominant role in the past. Saturated polyesters come in different types, varying in the nature of the building blocks, structure, and application fields.

5.1 High-molecular weight, saturated polyesters High-molecular weight, linear, saturated polyesters for coating applications are related to the fibre raw materials polyethylene terephthalate and polybutylene terephthalate. These fibre polyesters are semi-crystalline, a fact which benefits the fibre properties of tensile strength and elasticity. Such semi-crystallinity is unsuitable for coatings applications. Like the fibre polyesters, high-molecular weight, linear, saturated polyesters contain aromatic dicarboxylic acids and short-chained diols but, to avoid crystallinity, are made from mixtures of such dicarboxylic acids and diols. For example, they contain mixtures of terephthalic acid and isophthalic acid instead of just terephthalic acid, and mixtures of ethylene glycol and propylene glycol instead of just ethylene glycol or 1,4-butanediol. Although the crystallinity of such polyesters is largely suppressed, they still have fairly high glass-transition temperatures. The specific properties of such polyesters result from the high molecular weights (number-average molecular weight 15,000 to 22,000 g/mol) and the relatively narrow molecular weight distributions (D = 2.5 to 3.0). These are achieved by a special preparation process, which is comparable to that of the aforementioned fibre polyesters. It starts with the esterification or transesterification of polycarboxylic acids or their derivatives (isophthalic acid, terephthalic acid or dimethyl terephthalate) with high molar excesses of diols (ethylene glycol, propylene glycol) to yield very low-molecular products. The products are formed at elevated temperatures (more than 240 °C) and very low pressure (about 1 hPa) and then undergo transesterification catalysed by Lewis acids (e.g. dibutyltin oxide) into high molecular weight polyester molecules, with liberation of low-boiling diols. This is mainly performed as a batch process and, after the final characteristic values (mainly viscosity) have been attained, the resulting resins are discharged as a resin melt, cooled and then granulated. These polyesters are soluble in ester solvents, ketones, glycol ethers, glycol ether esters, dimethylformamide and N-methylpyrrolidone. These solutions may contain alcohols and aromatic hydrocarbons acting as thinner. Owing to the high molecular weights of the polyesters, the solutions have very high viscosity. Therefore, solutions of such polyesters for making coatings usually contain only 25 to 40 wt.% solids.

97

Saturated polyesters Low-molecular weight variants of such polyesters can be produced by interrupting the reaction at a lower degree of transesterification. The resulting products will have lower viscosity and better compatibility but will lack some of the typical properties (namely, forming elastic films by physical drying only). Initially, the high molecular, linear aromatic polyesters served as the sole binder for varnishes, powder coatings and electrical insulation coatings. Apart from these applications, these polyesters are currently used for hot-melt adhesives, sealings, foil coatings and lamination. Films of these polyesters offer good resistance to non-polar solvents and water. Their high glass-transition temperatures render them resistant to heat. The films are relatively flexible on account of their high molecular weights. The main reason for these film properties is the fact that, although the semi-crystallinity is suppressed by mixing building blocks for the molecular structure, there is still a high tendency for the association between aromatic rings Table 5.1:  Sample formulation for a high-molecular, linear polyester characteristic factors

symbol building elements

n-1

n-2

isophthalic acid

0.500

0.500

terephthalic acid

0.500

0.500

ethylene glycol

0.825

0.506

neopentyl glycol

0.500

0.500

m-‰-1

m-‰-2

isophthalic acid

355.4

388.8

terephthalic acid

355.4

388.8

ethylene glycol

219.0

147.0

neopentyl glycol

222.7

243.6

sum

1152.6

(1168.2)

– water

152.6

(168.2)

yield (AV)

1000.0

1000.0

mol/n2

0.6708

0.0177

– ethylene glycol

93.3

excess of OH groups

νOH

remaining carboxyl groups

νCOOH

mol/n2

0.0208

0.0057

acid value

AV

mg KOH/g

5.0

1.5

g/mol

675

18235

2.9

85.4

161.2

4.7

number-average molecular weight Mn number of structural elements

q'

n/mol

OH value

OHV

mg KOH/g

98

High-molecular weight, saturated polyesters and the methylene groups of polyols. This tendency also boosts rapid release of solvents during physical drying, as well as resistance properties and flexibility. Particular advantages can be obtained when these polyesters are combined with melamine resins for can-coating and coil-coating systems. The requirements imposed on such coatings are excellent deformation properties and chemical resistance (mainly for can coatings). In the past, various theories were proposed to explain the positive properties of such combinations. Polyesters having molecular weights of up to 20,000 g/mol and having only terminal OH groups have little opportunity to engage in effective crosslinking. Reaction of these OH groups with functional groups of melamine resins is relatively unlikely. A previous theory that the polyesters react with melamine resins by transesterification has now been rejected. The current theory is as follows: the HMMM resins (hexamethoxymethyl melamine, a low-molecular, fully etherified melamine resin) which are used for can coatings and coil-coatings crosslink with each other at the high temperatures in the presence of strong acid catalysts (sulfonic acids). The network, which by itself is very brittle, is interrupted by embedded polyester molecules. The latter play a role similar to that of steel mesh reinforcement in concrete. (However, the volume fraction of polyesters is substantially larger). In addition, it is possible that distinct molecular domains form. This in principle heterogeneous film structure is the reason for excellent deformation properties on one hand, resulting from thermoplastic behaviour of the linear polyester molecules, and very good chemical and solvent resistance on the other, resulting from the crosslinked melamine domains. This combination of properties ideally meets the requirements imposed on can-coating and coil-coating films. For example, a combination of 85 to 90 wt.% of such polyesters with 10 to 15 wt.% of HMMM resin and sufficient acid catalysts (e.g. 0.25 wt.% p-toluenesulfonic acid, calculated on solid resin) stoved at 20 min. at 180 °C or at 220 °C PMT (peak metal temperature) yields films which have excellent thermoforming properties and are highly resistant in sterilisation processes. Resistance to acids is also fairly good. This suggests that the melamine crosslinking bridges consist mainly of methylene groups and the polyester content wards off diffusion by acids. For most applications, the films afforded by such binder combinations have adequate weathering resistance. Typical commercial products: Dynapol L 205, 206 (Evonik [63]) Table 5.1 shows the formulation for a high-molecular, linear polyester resin which in the first production step consists of 0.825 moles of ethylene glycol, 0.50 moles of neopentyl glycol, 0.50 moles of terephthalic acid and 0.50 moles of isophthalic acid [64]. The fourth column lists the molar quantities of dicarboxylic acid and diols (standardised to a total of 1.00 moles) and the mass quantities (expressed in terms of 1000 mass units) in the starting formulation. The fifth column lists the corresponding composition of the finished polyester product, after transesterification and removal of excess polyol. For convenience, it has been assumed that only ethylene glycol is removed and ultimately that the polyester

99

Saturated polyesters contains ethylene glycol and neopentyl glycol in a molar ratio of 1 : 1. The molecular weight of the polyester is 18,000 g/mol. Table 5.1 also lists the typical values of such polyesters.

5.2 Polyesters as plasticisers Relatively low-molecular weight polyesters consisting of aliphatic dicarboxylic acids and aliphatic diols form especially soft resins. The most commonly employed dicarboxylic acid for such polyesters is adipic acid. Azelaic acid and sebacic acid are used rarely. The most suitable diols are propylene glycol, 1,4-butanediol, and 1,5-pentanediol. The molecular weights are limited by adding appropriate quantities of excess polyol. The polyesters are highly condensed and have low acid values (e.g. below 2 mg KOH/g). As the polyesters consist solely of one dicarboxylic acid and one diol, they tend to be crystalline at lower temperatures. Such plasticising polyesters are soluble in ester solvents, glycol ethers, glycol ether esters, and in most alcohols and ketones. The solutions are compatible with aromatic hydrocarbons acting as thinners. The polyesters are insoluble in aliphatic hydrocarbons and terpene solvents. There are also polyesters available which are made from dicarboxylic acids, diols and mono-alcohols bearing longer aliphatic chains. The mono-alcohols act as chain stoppers (see Figure 5.1). Other plasticising polyesters contain mixtures of diols, small quantities of triols and, apart from aliphatic dicarboxylic acids, smaller quantities of aromatic polycarboxylic acids. Such polyesters have a much lower tendency to crystallise. The polyesters are used to plasticise cellulose nitrate, other cellulose esters, and PVC and its copolymers. In the case of cellulose nitrate, they act as plasticisers that have a gelling effect. This type of polyester is colour-fast, adequately resistant to weathering, and resistant to low temperatures and atmospheric moisture. Compared with other ester plasticisers (diesters of phthalic acid, adipic diesters or triesters of trimellitic acid with higher mono-alcohols, esters of phosphoric acid, and mono-esters of fatty acids), they are much more stable to migration, even at elevated temperatures. Naturally, however, ester plasticisers are not totally resistant to saponification. Plasticising polyesters are less compatible

Figure  5.1:  Model of a polyester plasticiser containing adipic acid, 1,4-butanediol and 2-ethylhexanol

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Polyester hard resins Table 5.2:  Sample composition of a polyester hard resin [70] n

building element

M

n·M=m

wt. ‰

0.500

pentaerythritol

136

68.00

321.4

1.000

phthalic anhydride

148

148.00

699.5

216.00

1020.9

0.250

water

4.43

20.9

211.57

1000.0

sum 18

yield (AV) νOH = 0.7541 mol νCOOH = 0.7541 mol polyester constant number-average molecular weight [g/mol] number of structural units acid value [mg KOH/g] OH value [mg KOH/g]

k'M Mn q' AV OHV

1.2541 833 3.9 200 200

with PVC than with lower-molecular weight ester plasticisers. As such polyesters are incompatible with polystyrene, they are suitable for formulating coatings which dry physically, and which do not interact with polystyrene substrates; such coatings have excellent long-term flexibility because migration of plasticiser into the substrate does not occur. Unlike some diesters of phthalic acid and the esters of phosphoric acid, polyester plasticisers are completely non-hazardous. Some types have FDA approval and are suitable for food packaging. Typical commercial products: Palamoll 632 (BASF [65]), Paraplex G 50 (Hall-Co), Edenol 1208 [66] (Emery), Uraplast RA 25 (DSM [67]), Santhicizer 433 [68] (Ferro)

5.3 Polyester hard resins Saturated polyester as hard resins belong to the “first” polyesters produced in large-scale quantities. These are phthalic polyesters contain mainly stoichiometric quantities of phthalic anhydride and glycerol or pentaerythritol. Of course, stoichiometric combinations allow reactions only to proceed to low condensation degrees, giving rise to higher acid values. Those glyptal resins are only soluble in lower alcohols, glycol ethers and ketones. The solutions can be thinned with ester solvents and glycol ether esters. The polyesters made with pentaerythritol have softening temperatures of 75 to 85 °C (Kofler measurement), while the corresponding figures for polyesters with glycerol are 65 to 75 °C. The products are used for special metal varnishes and solvent-borne tinctures. In the past, they were combined with cellulose esters to improve gloss and raise application solids. As they have high acid values, the resins are suitable – after neutralisation – for water-soluble inks. Typical commercial products: Phthalopal PP (BASF, [65]), Rokrapol 7160 (Krämer, [69])

101

Saturated polyesters

5.4 Polyester segments for other resins 5.4.1 Polyester segments for polyurethane elastomers Polyurethane elastomers contain polyisocyanate hard segments and various, mainly linear, soft segments. Typical soft segments are polyethers, polyesters, OH-terminated polysiloxanes, and special OH-terminated polyacrylates. The polyesters are polycarbonates (see Chapter 8.1), polycaprolactones (Chapter 8.2) and, more commonly, polyesters based on dicarboxylic acids and diols (in excess). Given the vast range of available building blocks for common polyesters, the properties of polyurethane elastomers can be varied over a wide range. Thus, it is possible to suppress the plasticity of the entire polymer by selecting high quantities of aromatic dicarboxylic acids, yielding coating films of high hardness and relatively good resistance to solvents and chemicals. In contrast, dicarboxylic acids and diols with longer aliphatic chains confer high flexibility. Of the dicarboxylic acids, fatty acid dimers contribute the greatest plasticising effect. In diols, polyether diols and the dimer diols confer the most plasticity. An optimum balance between hardness and plasticity is obtained with cycloaliphatic dicarboxylic acids (e.g. hexahydrophthalic acid) or diols (e.g. dimethylol cyclohexane). A further possibility is to employ polyester soft segments that vary in their molecular weight. Varying their molecular weights between 800 and 5000 g/ mol increases their influence on the entire polyurethane and, in addition, lowers the amount of polyisocyanate hard segments and decreases its influence considerably. Polyurethane elastomers are ideal for water-soluble, anionically stabilised coating systems. At pH values of 7.5 to 9.0, urethane groups are more resistant to saponification than are ester groups. The anionic carrier groups are usually introduced by reaction with isocyanate groups to form urethane groups or urea groups, which are even more resistant to saponification. The most important anionic carrier group for polyurethanes as coating binders is formed by an addition reaction involving dimethylol propanoic acid (DMPA). The tertiary carboxyl group of DMPA does not react with NCO or OH groups and is at least partially neutralised by amines to form carboxylate anions, the carrier groups responsible for solubility in water. As the molecules of polyurethanes are more coiled than, for example, the polyester molecules, better protection is afforded against saponification. Thus, it is not necessary to choose particularly saponification-resistant building blocks to act as the soft segments of polyurethanes. However, if the anionic carrier groups are incorporated into the polyester soft segment, saponification resistance is worsened substantially. Polyesters for producing polyurethane elastomers lend themselves to both the polymer melting process and the ketone process. The latter affords a much greater variety of polyurethane compositions (especially with regard to the selection of the type and molecular weight of polyester soft segments). Slightly branched polyesters can also serve as polyester soft segments, in which event a greater excess of polyisocyanate is required. The

102

Polyester segments for other resins Table 5.3:  Example of a polyester soft segment for a polyurethane dispersion n = m/M

building element

M

m

wt.  ‰

2.626

hexane diol-1,6

118

309.87

318.9

1.000

isophthalic acid

166

166.00

170.9

1.000

dimer fatty acid (≥ 98 %)

567

sum 3.961

water

18

yield (AV) polyester constant number-average molecular weight [g/mol] number of structural elements acid value [mg KOH/g] OH value [mg KOH/g]

k'M Mn q' AV OHV

567.00

583.9

1042.87

1073.4

71.29

73.4

3266.6

1000.0

1.3327 1460 3.0 2.3 74.6

molecules of the end products will then not be as large as in the case of linear molecules. If a chain extension with amine to yield the end polymer is performed in aqueous media, polyurethane microgels (containing crosslinked particles) in stable aqueous dispersions can be obtained. Polyester soft segments are more expensive than polyethers, but they offer a greater variety of building blocks and substantially better weatherability. The better saponification resistance of polyethers relative to polyesters has little bearing here. From this point of view, the use of OH-terminated acrylic resins, which possess excellent saponification resistance and weatherability, is not absolutely necessary (if such products are true macro-diols, they are expensive). Tests should be conducted to establish if it is really necessary to choose such special building blocks. However, the use of the above-mentioned polycarbonates, polycaprolactones and the terminally functionalised polysiloxanes do introduce very particular properties. Table 5.3 describes an example of a polyester soft segment [71], which is based on dimer fatty acid, isophthalic acid and 1,6-hexanediol and is suitable for an anionically stabilised polyurethane dispersion. Its production is conducted by the ketone process. The polyester (569.0 g) is diluted in methyl ethyl ketone (MEK) and made to react at relatively low temperatures (80 °C) with isophorone diisocyanate (213.0 g, in excess), with dimethylol propanoic acid (DMPA, 46.0 g) and neopentyl glycol (NPG, 7.0 g), yielding a polyurethane prepolymer. The average molecule of the prepolymer consists of 2.56 moles of polyester, 0.35 moles of NPG, 2.25 moles of an adduct of 2 moles of IPDI and 1 mole of DMPA and a further 1.79 moles of IPDI. The average molecular weight is 5252 g/mol corresponding to the NCO equivalent weight of 2626 g/mol. The measured values are somewhat higher. The acid value is 25 mg KOH/g. Figure 5.2 shows a model of this prepolymer.

103

Saturated polyesters The described pre-polymer (835.0 g, 67 wt.% in 420.0 g MEK) is chain-extended by adding trimethylolpropane (24.0 g TMP). The resulting polyurethane is relatively high molecular and has a theoretical molecular weight of more than 40,000 g/mol. The polyurethane contains terminal OH groups and has OH groups in the side chain. The acid groups of the polyurethane are neutralised by N,N-dimethylethanolamine (25.8 g DMEA), corresponding to a degree of neutralisation of 85 %. The polyurethane is dispersed in deionised water (2552 g). The process solvent (MEK) is distilled off under vacuum. The result is a solvent-free dispersion with fine particles. The product of this example serves as a binder for water-borne basecoats for automotive OEM applications [71]. Polyurethane dispersions containing polyester soft segments may also contain other diisocyanates and other anionic carrier groups. Besides water-borne basecoats for automotive coating systems (OEM and repair applications), they are suitable for water-borne primer surfacers, wood and foil coatings, plastic coatings, and paper and leather coatings. For application as physical drying systems, the polyurethanes are prepared with fairly high molecular weights. These products distinguish themselves by rapid initial drying, high flexibility and – to an extent – depending on the type of building block – good weather­ ability. Such polyurethane dispersions can also combined with acrylic dispersions. However, there are in addition polyurethanes containing polyester soft segments which have lower molecular weights and bear functional groups (terminal and in the side chains) and can react with other binders and crosslinkers during film forming. The result­ ing films are distinguished by particular flexibility and are recommended, for example, for water-borne two-pack systems. Polyester soft segments are also used for polyurethane prepolymers which are not chain-extended to high-molecular weight polyurethanes. The terminal NCO groups of such prepolymers are made to react with blocking agents. The resulting blocked polyurethanes are combined with different binders containing OH groups (e.g. polyesters) for highly flexible water-borne stoving enamels.

Figure  5.2: Molecular model of a typical polyurethane prepolymer

104

Polyester segments for other resins

5.4.2 Polyester polyurethanes for crosslinking in the presence of atmospheric moisture Branched polyurethane prepolymers terminated with NCO groups can crosslink in the presence of atmospheric moisture. To this end, an excess of diisocyanate is made to react with low-molecular weight branched compounds bearing free OH groups, yielding prepolymers with terminal NCO groups. The OH compounds employed are mainly polyesters, although polyols and polyethers are also used. The diisocyanates are toluene diisocyanate (TDI), 4,4'-diisocyanato diphenylmethane (MDI) and its isomers, hexamethylene diisocyanate (HDI), hydrogenated 4,4'-diisocyanato diphenylmethane or 4,4’-diisocyanato methylendicyclohexan (HMDI) and isophorone diisocyanate (IPDI). The excess free diisocyanate is removed by an evaporation under vacuum. An alternative process is based on oligomer polyisocyanate adducts (commonly used crosslinkers for two-pack paints) which are made to react in excess with polyesters containing OH groups. If higher-functional polyisocyanate adducts are used, they can be combined with linear polyesters. The preferred polyisocyanate adducts are trimers of hexamethylene diisocyanates or of isophorone diisocyanates. As such adducts contain only very low quantities of free diisocyanates, no distillation process is required. The polyesters selected for polyurethane prepolymers contain building blocks that exert a plasticising effect (aliphatic dicarboxylic acids, diols with longer chains) and also those that bear side chains. The resulting prepolymers, bearing free NCO groups, are soluble in aromatic hydrocarbons, in esters and ketones. They can be pigmented. Naturally, to prevent premature crosslinking, the product needs to be protected against atmospheric moisture. Crosslinking consists in the reaction of the NCO group with water to form a derivative of carbamic acid, which is unstable and decomposes immediately into a primary amine, with liberation of carbon dioxide. The amine groups react with the remaining NCO groups to form urea linkages and thus form crosslinked films. Crosslinking occurs at a minimum humidity of 30 vol.%. It takes a relatively long time and cannot be accelerated by raising the temperature. One of the reasons for this is that relative humidity decreases as the temperature rises. Prepolymers containing aromatic NCO groups react much faster than those containing aliphatic or cycloaliphatic NCO groups. The reaction rate can be accelerated with catalysts, the most commonly employed being dibutyltin dilaurate (DBTL). Owing to the need to avoid the use of tin compounds, the search is on for alternatives. Polyurethane prepolymer films have a decidedly elastomeric character. The polyisocyanates, linked by urea groups, constitute the hard segments while the polyester fractions form the soft segments. The films are tough and flexible. They are resistant to most chemicals and to water. Products containing aliphatic or cycloaliphatic polyisocyanates form weatherable films.

105

Saturated polyesters In view of the long reaction time of such binders, industrial applications are limited. They are mostly employed for corrosion protection. This application has the advantage that moisture present in the substrate is utilised during film formation. Other applications are concrete sealers and roof coatings. Solvent-free systems are available which are suitable for sealants (automotive glazing sealants).

5.4.3 Polyester acrylates Here, polyester acrylates are defined as the product of the reactions of low-molecular polyesters bearing terminal or side-chain OH groups with acrylic acid. These products are suitable for UV coatings and can be crosslinked by UV light. Acrylic esters are much more reactive than methacrylic esters. The reaction starts with decomposition of the UV initiator by UV light to form free radicals. These initiate typical free radical chain polymerisations, forming C-C bridges between the building blocks and producing highly extended networks. Film properties are greatly influenced by the size and composition of the polyester segment. Naturally, the density of the film network increases with increase in the degree of branching and with decrease in the molecular weight of the polyesters. Film flexibility rises with increase in both the number and length of the aliphatic chains in the polyester segment. Of course, hard films result from polyester acrylates made with low-molecular weight polyester segments which contain aromatic or short chain aliphatic building blocks and may be branched. The esterification of acrylic acid with the OH groups of a polyester is very difficult because the carboxyl group of acrylic acid is influenced by the molecular structure and so the reaction rate is rather low. It is therefore necessary to use esterification catalysts (acids, e.g. p-toluenesulfonic acid or Lewis acids). On the other hand, it is important to avoid premature polymerisation of acrylic acid and Michael addition of acidic protons across double bonds. Consequently, the reaction is performed at relatively low temperatures (at 120 °C max.) and from the outset in the presence of polymerisation inhibitors (e.g. methyl hydroquinone). To shift the chemical equilibrium to the product side, the water needs to be distilled off as effectively as possible. This is achieved with a low-boiling azeotropic reflux solvent (e.g. toluene), which is removed at the end of the reaction by vacuum distillation. Any remaining quantities of free acrylic acid can also be distilled off. Products in competition with polyester acrylics are as follows. Polyether acrylics: produced by reaction of polyethylene glycols, polypropylene glycols, and their copolymers or of branched polyesters with acrylic acid. They have lower viscosities and lower costs, but they also generally have inadequate weathering resistance. Polyurethane acrylics: produced by reaction of polyurethane prepolymers (see Chapter 5.4.1) with hydroxyalkyl acrylates. These are easier to prepare. On average they are more viscous than polyester acrylates and are distinguished by toughness and high flexibility.

106

Saturated polyesters containing OH groups Epoxy acrylics: produced by reaction of aromatic epoxy resins with acrylic acid. On aver­ age, these have a higher viscosity. They are not weatherable but offer excellent adhesion and chemical resistance.

5.5 Saturated polyesters containing OH groups for crosslinkable, solvent-borne coatings Saturated polyesters containing OH groups (OH polyesters) for reaction with crosslinkers are by far the most important class of polyesters for solvent-borne coating systems, not only with regards to volumes but also in terms of the multiplicity of application fields and attainable properties. Although all of these polyesters contain OH groups capable of reacting with the functional groups of different crosslinkers, a great many different types exist for all kinds of applications. Hardly any other class of binder contains as many different products. This diversity is the outcome of the targeted development of dedicated products for different coating applications. Industrial paint applications are the main driver behind ever-stricter demands, mostly with regard to systems for automotive coatings and for the electrical and electronics industry. This is one of the reasons that the larger coating companies make their own polyester resins and only for captive use. Then the resin composition constitutes an important part of a company’s specific know-how. A corollary of this is that major resin producers – for the most part associated with large chemical companies – have lost market share. They are focusing their activities on smaller paint manufacturers that do not prepare their own resins. In so doing, they are competing against smaller resin producers which manufacture solely for the coatings industry. However, despite the plethora of commercial products on the market, many types offered by the various suppliers are comparable in composition, structure and properties. This means that the attainable coating properties are also more or less comparable. The resulting competition is advantageous for the paint manufacturers, especially as there is so much global production capacity.

5.5.1 Structure and composition of OH polyesters for solvent-borne coatings The structures of saturated polyesters containing OH groups for co-crosslinking are characterised by their average molecular weight, molecular weight distribution, degree of branching and number of free (residual) functional groups (OH and carboxyl groups). The conditions and effects described in Chapter 3, such as the calculation of molecular weights,

107

Saturated polyesters definition of degree of branching, and the theoretical and measured contents of functional groups, apply of course to all these products. The number-average molecular weights of co-crosslinkable polyesters used in industry mainly lie between 800 and 4000 g/mol. Naturally, the choice of molecular weight depends on the degree of branching which varies from 0.4 to 2.0 mol/kg. The OH values range, as a function of molecular weight and degree of branching, from 50 to 150 mg KOH/g. These OH groups are intended for crosslinking reactions. There are, however, some linear, relatively low-molecular weight polyesters which are recommended for co-crosslinking and can react with higher-functional crosslinkers. The residual carboxyl groups resulting from the degree of condensation usually yield acid values between ≤ 1.0 and 25.0 mg KOH/g. These carboxyl groups are not just used for limiting the degree of condensation but may also play a catalytic role. They exert an appreciable influence mainly wherever amino resins serve as crosslinkers, although they normally do not take part directly in the crosslinking reaction. And, of course, these polyesters are differentiated by the choice of building blocks. Despite the foregoing diversity, preferred raw materials exist for this class of binder. They are available on a large scale and they are relatively inexpensive. The most important polycarboxylic acid for such polyesters is isophthalic acid, which is the main component of numerous polyesters. Isophthalic acid is widely combined with adipic acid to lower the glass-transition temperature and to improve solubility and flexibility. The molar combination ratios of isophthalic acid to adipic acid are usually between 60:40 and 85:15. On account of the lower rate at which esterification occurs, terephthalic acid does not play an important role in this polyester class. In addition, there are polyesters which contain phthalic anhydride as building block instead of isophthalic acid. Those polyesters are less expensive and lower in viscosity, but they do not confer the same toughness and flexibility as polyesters based on isophthalic acid. To avoid crystallinity in the case of less highly branched polyesters that have a high content of aromatic dicarboxylic acids, it is recommended that isophthalic acid be combined with some phthalic anhydride. The desired effect can be achieved with just a small quantity of phthalic anhydride. Cycloaliphatic polycarboxylic acids deliver an optimum balance of solubility, compatibility, hardness and flexibility. They are mainly employed in binders for clearcoats which must provide excellent weatherability. Cycloaliphatic polycarboxylic acids or their derivatives are relatively expensive. Tetrahydrophthalic anhydride as a building block of polyesters does not contribute much weatherability, but does provide optimum intercoat adhesion, and so it is mainly used for primers and primer surfacers. Long-chain aliphatic dicarboxylic acids are more expensive than adipic acid. However, they yield coating films of high flexibility with no loss of hardness (better toughness). Very high plasticity and good solubility are achieved by combining aromatic dicarboxylic acids (e.g. isophthalic acid) with various quantities of fatty acid dimers.

108

Saturated polyesters containing OH groups The most important diol is neopentyl glycol, which confers excellent solubility and compatibility by virtue of the methyl side chains (without reduction in hardness thanks to the short C3 chain). Neopentyl glycol is often combined with smaller quantities of other diols, for example with 1,6-hexanediol or hydroxypivalic neopentyl glycol ester (HPN). As ethylene glycol does not contribute adequate solubility, it is only rarely used, compared with propylene glycol which is used more often. The less expensive ether diols and polyether diols (EO and PO polyethers) exert a substantial plasticisation effect, but polyesters containing significant quantities of these polyols fail to provide adequate weatherability. They are unsuitable for topcoats or clearcoats. There are diols available which have a neo-structure (2,2-dialkylpropanediols) and have longer side chains than neopentyl glycol, e.g. 2-ethyl-2-butyl-propanediol. Polyesters containing such diols are completely soluble in aromatic hydrocarbons and show broad compatibility with many other resins. The goal of eliminating aromatic dicarboxylic acids without reduction in hardness is achieved by combining cycloaliphatic polycarboxylic acids or their derivatives (e.g. hexahydrophthalic anhydride) with dimethylol cyclohexane (DMC, or cyclohexane dimethanol CDM). Such combinations are relatively expensive, however. The most important building block for preparing branched saturated polyesters is trimethylolpropane (TMP). It is preferred over glycerol. TMP has three active primary-OH groups and resists discolouration, unlike glycerol. Other polyols bearing more than two functional groups do not find application as they provide no special advantage. Branched saturated polyesters can also be prepared with polyfunctional carboxylic acids or their derivatives, e.g. trimellitic anhydride. In polyesters for solvent-borne systems there is no particular advantage to be gained from using trimellitic anhydride and so it is only used in special cases. Added to which, it is more expensive. Saturated polyesters containing OH groups are readily soluble in esters, glycol ethers, glycol ether esters, ketones, and alcohols. Some of them are soluble in aromatic hydrocarbons, but they are insoluble in aliphatic hydrocarbons and in terpenes. For cost reasons, most solutions contain aromatic hydrocarbons in combination with a smaller quantity of more polar solvents, e.g. 60 wt.% solids in xylene and n-butyl acetate 30:10, or 60 wt.% in Aromatic 100 and methoxypropyl acetate 35:5. In principal all saturated OH polyesters can be combined with a corresponding crosslinkers. However, certain polyesters are matched with certain crosslinkers so as to fully exploit the properties of the latter. The most important crosslinkers are amino resins and polyisocyanates (free isocyanate groups and blocked polyisocyanates).

5.5.2 OH polyesters for crosslinking with amino resins The OH groups of the saturated polyesters described here can react with the functional groups of amino resins, namely methylol groups and methylol groups etherified with mono-­

109

Saturated polyesters alcohols. When the polyester OH groups react with the methylol groups, water is liberated. When the OH groups react with etherified methylol groups by transetherification, mono-alcohols are liberated. The possible reactions between OH groups and functional groups of amino resins (e.g. with melamine resins) are presented in Figure 5.3. Suitable amino resins are urea resins, melamine resins, and benzoguanamine resins. The reactions take place either at elevated temperatures (stoving enamels) or at ambient temperatures under the catalytic influence of strong acids (hydrochloric acid, phosphoric acid, p-toluenesulfonic acid). However, the amino resins can also crosslink by themselves under the described conditions (elevated temperatures, acid catalysis). In extreme instances, molecular clusters of self-crosslinked amino resin are formed. The ratio of co-crosslinking to self-crosslinking depends not only on the concentration and reactivity of partner groups but also on the film forming conditions. It therefore makes no sense to use stoichiometric calculations to determine the optimum ratios of polyesters to amino resins. As the ratio of co-crosslinking to self-crosslinking substantially influences the film properties, the optimum needs to be determined experimentally under the intended film-forming conditions. While co-crosslinking supports flexibility, adhesion, weathering and chemical resistance, self-crosslinking supports hardness and solvent resistance. The commonly chosen mixing ratios of OH polyesters to amino resins are between 60:40 and 85:15 parts by weight. Co-crosslinking can be boosted by increasing the polyester content, higher OH values, and more reactive OH groups. Lower stoving temperatures and lower contents of acid catalysts or weak acids also support co-crosslinking, as does the use of less reactive amino resins. More extensive self-crosslinking is achieved with higher quantities of amino resins, amino resins of high reactivity, high stov­ ing temperatures, and a higher content of strong acids. OH polyesters for crosslinking with amino resins mostly contain isophthalic acid; more rarely they contain phthalic anhydride, tetrahydrophthalic anhydride or terephthalic acid. The aromatic dicarboxylic acid is often combined with aliphaFigure 5.3:  Possible reactions between OH groups of tic dicarboxylic acid, e.g. with adipic polyesters and functional groups of melamine resin

110

Saturated polyesters containing OH groups acid (15 to 40 mol- % of polycarboxylic acid). The polyols chosen are mainly neopentyl glycol in combination with propylene glycol, 1,4-butanediol, 1,6-hexanediol and hydroxypivalic neopentyl glycol ester (HPN). In special cases they may contain neo-diols with longer side chains. In nearly all cases, the component responsible for branching is trimethylolpropane (TMP). The number-average molecular weights are mainly between 1500 and 4000 g/mol, while the OH values are 70 to 130 mg KOH/g. The acid values of the polyesters are usually between 15 and 20 mg KOH/g. It must be remembered that higher acid values can catalyse the crosslinking reaction. This makes it possible to eliminate additional acid catalysts from most stoving enamels. However, acid catalysts are needed for achieving optimum crosslinking at low stoving temperatures (e.g. 80 °C) and, of course, even more so at ambient temperatures. In such cases, a reduction in storage time (and in addition pot-life) needs to be accepted after the catalyst is added. As amino resins always have higher functionality, they can be combined with linear polyesters which contain only terminal OH groups. With the aid of such polyesters or with high-molecular weight low-branched polyesters, it is possible to formulate systems which impart particularly high flexibility (can-coating, coil-coating systems). Coating systems featuring combinations of OH polyesters and amino resins are suitable for primer surfacers, effect basecoats and topcoats for automotive OEMs, industrial stoving enamels for machines and other equipment. They are used for coil-coating and can-coating systems. They are also suitable for acid-curing paints which are applied to wood surfaces, foils and plastics to yield crosslinked films at ambient temperatures. They offer some advantages over other binder types containing OH groups which are also crosslinked by amino resins (see Chapter 7.4). A combination of OH polyesters and amino resins in automotive OEM basecoats has a plasticising effect on the binders used in physically drying formulations (e.g. cellulose acetobutyrate). This flexibilization needs to be retained when the basecoats are stoved together with the wet-on-wet clearcoat. Combinations of OH polyesters with melamine resins in appropriate mixing ratios yield pigmented stoving enamels that have adequate weathering resistance. Their chemical resistance, too, is adequate for most applications. However, the melamine resin content in such films is still the weak link in the film as far as acid resistance is concerned. Contact with acid can lead to reversible liberation of methylol ethers. The polyester content of such combinations is more stable towards acid. Combinations of OH polyesters with urea resins or with benzoguanamine resins are not weatherable due to the structure of these crosslinkers. What was said before about the good weatherability of stoving enamels containing a combination of OH polyesters and melamine resins does not apply to clearcoats. However, the reason is not – as has been supposed in the past – the low saponification resistance of the polyester chains. The ester groups are relatively stable under acid conditions, such as

111

Saturated polyesters Table 5.4:  Example of a branched saturated OH polyester used in combination with melamine resin and polyurethane for solvent-borne effect basecoats n = m/M

building element

M

m

m-‰

1.829

ethylene glycol

62

113.4

94.3

1.371

neopentyl glycol

104

142.6

118.6

1.372

HPN

206

279.8

232.7

0.686

trimethylol propane

134

91.9

76.5

1.826

isophthalic acid

166

303.5

252.5

1.354

HHPA

154

208.5

173.4

adipic acid

146

1.371

sum 7.646

water

18

yield (AV) polyester constant number-average molecular weight [g/mol] degree of branching [mol/kg] acid value [mg KOH/g] OH value [mg KOH/g]

k'M Mn v' AV OHV

200.2

166.5

1339.9

1114.4

137.6

114.4

1202.3

1000.0

1.1782 1482 0.57 5.0 102.5

they exist during acid-rain weathering. They are not stable under basic conditions, but such conditions do not really occur in natural weathering processes. The objective reason for the limited weatherability of polyester-melamine resin combinations is the common use of aromatic compounds in the polyesters (mainly isophthalic acid). Clearcoat films with an adequate content of aromatic building blocks absorb UV light and are physically more weatherable than films without aromatic compounds, but – unfortunately – they form cracks. Clearcoats with no aromatic content are less stable to UV light: they are degraded, and loss of film thickness occurs, but they do not crack. If OH polyesters are formulated solely with non-aromatic building blocks (e.g. with hexahydrophthalic acid instead of isophthalic acid), they do not show cracks upon weathering. They have similar weatherability to common combinations of OH acrylic resins and melamine resins in clearcoats. Most commercial OH polyesters are compatible with commonly employed amino resins. However, if the polarities of the two binders are very different, miscibility gaps may exist. That can lead to incompatibility in solution (turbidity) and to flaws during film formation. In such cases, the binder combinations are heated to induce partial condensation (hot blends that form pre-condensates). This usually resolves the compatibility issues. Naturally, the pre-condensation must be controlled, mainly by controlling the increase in viscosity.

112

Saturated polyesters containing OH groups Table 5.4 presents an example of a branched saturated OH polyester which is used in combination with melamine resin and polyurethane to prepare solvent-borne effect basecoats for an automotive OEM [72]. Typical commercial products: Desmophen TO 1665 (Covestro [73]), Uralac SN 820 (DSM [67]), Vialkyd AN 950 (Allnex [75]), Dynapol H 700 (Evonik [63]), Setal 186 (Allnex-Nuplex [76]), Synolac E 21015 (Arkema [77]), WorléePol 1181 (Worlée [78]), Crodapol O 85 (Croda [74]). There is currently a discussion on totally eliminating emissions of formaldehyde, the toxicity classification of which has been increased. Consequently, amino resins (melamine resins) are being chosen that have a very low content of free formaldehyde. However, it needs to be remembered that formaldehyde is also formed during the crosslinking reaction.

5.5.3 OH polyesters for crosslinking by polyisocyanates The OH groups of saturated polyesters react with isocyanates at ambient temperatures to form urethane groups. The reaction is suitable for crosslinking polyesters. In view of this reactivity at low temperatures, the two components of the coating need to be supplied separately. Two-pack paints contain an A-component (basecoat) which consists of polyester, solvents, and perhaps pigments and most additives, while the B-component (hardener) consists of polyisocyanates, solvents and, if necessary, some of the additives. The polyisocyanates employed as hardeners are exclusively oligomeric polyisocyanate adducts, because the lower molecular weight polyisocyanates (diisocyanates) are physiologically hazardous (toxicity of the vapours). Commercially available are oligomer adducts of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), 4,4'-diisocyanato diphenyl methane (methylene phenyl isocyanate, MDI) and its hydrogenated variant, 4,4'-diisocyanato dicyclohexyl methane (HMDI). Oligomerisation with polyols yields urethanes while trimerisation yields biurets, isocyanurates, allophanates and uretdiones. The reaction of OH groups with isocyanate groups is very clear. Figure 5.4 shows the crosslinking reaction of an OH polyester with a polyisocyanate adduct, namely the isocyanurate adduct of HDI. The presence of atmospheric moisture is the only source of a competitive reaction of the isocyanate groups with OH groups. The outcome in that case is the formation of a carbamic acid derivative. This is unstable and decomposes into a primary amine and carbon dioxide. The primary amino group immediately reacts with residual isocyanate groups to yield urea linkages which will be part of the film network and so do not impair the film properties. The crosslinking reaction can be accelerated by adding catalysts. Potential

113

Saturated polyesters catalysts are heavy metal cations and tertiary amines (including heterocyclic compounds, e.g. triethylene diamine, diaza bicyclo octane [DABCO]). For a long time, the most efficient catalyst was dibutyltin dilaurate (DBTL). However, organic tin compounds have been reclassified as hazardous and must be replaced. Recommendations have been made to use organic bismuth salts or a combination of zinc, calcium and lithium salts with tertiary amine, to take advantage of the synergistic effect of both components. The crosslinking reactions can also be accelerated by elevated temperatures. Naturally, catalysed coating mixtures have a shorter pot-life (application time) than uncatalyzed systems. The target pot-life is 3 to 6 hours. For industrial spray-application processes, two-pack spray guns are used. The two components are not mixed until they have been charged into the spray gun in defined volumetric ratios, and they are then mixed in the gun by a static mixer. The practical effect of this is that pot-life is no longer a constraint. Unlike the crosslinking of the above-mentioned combinations of OH polyesters with amino resins, the crosslinking reactions of polyisocyanates are well defined. They form homogeneous, more extensive molecular networks. Generally, the films thus formed, given appropriate formulation and efficient curing, offer better flexibility and chemical resistance

Figure  5.4:  Crosslinking reaction of an OH polyester with a polyisocyanate adduct

114

Saturated polyesters containing OH groups than films containing melamine resins. However, due to the size of the network with its larger molecular meshes, the films allow more diffusion of solvents and chemicals and can be swollen more easily. Aromatic polyisocyanates react much faster with polyester OH groups than with aliphatic and cycloaliphatic isocyanate groups. However, films containing products of the reactions of aromatic polyisocyanates are generally not weatherable. Such crosslinkers are mainly used for primers and coatings for indoor application. They are distinguished by excellent hardness, good adhesion and adequate flexibility. Films containing the product of the reactions of aliphatic and cycloaliphatic polyisocyanate hardeners are theoretically more weatherable than films of OH polyesters and melamine resins as crosslinkers. However, here again, systems (polyester content) containing significant quantities of aromatic building blocks are more physically resistant to UV light, but will suffer film damage in the form of cracks. Films containing polyester resins which are free of aromatic building blocks and aliphatic or cycloaliphatic polyisocyanate adducts show at least the same weatherability (without cracking) as films made with suitable OH acrylic resins and polyisocyanate adducts. As far as weatherability is concerned, both products are superior to the corresponding products containing OH polyesters and melamine resins. Theoretically, the same OH polyesters can be used for crosslinking with polyisocyanates as with amino resins. However, there exist polyesters which have been specifically developed for isocyanate crosslinking and which are preferred for those combinations. As there is a better balance of hardness and flexibility in films with polyisocyanate crosslinkers, it is better to choose polyesters based on phthalic anhydride. The difference in flexibility between isophthalic and o-phthalic polyesters is not as great as in melamine resin-crosslinked polyester systems. Such polyesters are less costly, a fact which compensates for the higher cost of the polyisocyanate crosslinker. If high flexibility is the main goal for films of OH polyesters and polyisocyanate crosslinkers, the polyester may contain a combination of aromatic polycarboxylic acids and aliphatic dicarboxylic acids (e.g. adipic acid), with a higher content of the latter. There are polyesters which contain just aliphatic dicarboxylic acids and yield highly flexible films (e.g. for plastic coatings). The adequate resistance of these films stems from the very efficient crosslinking behaviour of polyisocyanates to form large, extended molecular networks. In general, that is the reason that polyesters for polyisocyanate crosslinking can have somewhat lower number-average molecular weights (800 to 2500 g/mol), higher OH values (90 to 160 mg KOH/g) but lower acid values (from less than 1 to 10 mg KOH/g). The acid values should be low because their catalytic effect is not necessary and even undesired. Higher acid values can lead to yellowing and may shorten the pot-life. When the oligomeric polyisocyanate adducts have functionalities of three and higher, it is possible to use relatively low-molecular weight, linear polyesters which contain only terminal OH groups, but will nevertheless form efficiently crosslinked film networks.

115

Saturated polyesters Table 5.5:  Example of a saturated OH polyester for crosslinking with free polyisocyanates n = m/M

building element

M

m

wt. ‰

2.115

phthalic anhydride

148

313.0

96.6

5.892

isophthalic acid

166

978.0

301.7

3.767

adipic acid

146

550.0

169.7

11.780

hexane diol-1,6

118

1390.0

428.8

2.948

trimethylol propane

134

395.0

121.9

3626.0

1118.7

21.374

water

384.7

118.7

3241.3

1000.0

sume 18

yield (AV) polyester constant number-average molecular weight [g/mol] acid value [mg KOH/g] degree of branching [mol/kg] OH value [mg KOH/g] OH content [%]

k'M Mn AV v' OHV OH-%

1.2558 1076 1.0 0.91 154.3 4.7

Coating systems containing OH polyesters and polyisocyanate adducts are preferred for automotive OEM coatings, automotive repair coatings, coatings for large vehicles (trucks, buses, agricultural equipment, locomotives, rail wagons and rail cars), for plastic and foil coatings, for ship paints, and other high-quality coating systems. Two-pack wood coatings are formulated with OH polyesters and aromatic polyisocyanate adducts. There are OH polyesters recommended for two-pack systems which have lower OH values. If such polyesters are combined stoichiometrically, less polyisocyanate adduct is required. The entire combination is therefore less expensive. Such combinations are used for coating systems which must provide moderate properties. OH polyester and polyisocyanate combinations are also used for paper coating, foil lamination, sealing applications and adhesives. Low-molecular weight polyesters are combined with MDI derivatives for casting compounds (100 % systems). Table 5.5 shows an example of a saturated OH polyester for crosslinking with free aliphatic and cycloaliphatic polyisocyanates [79] which is suitable for plastic parts and highly flexible primer surfacers. The polyester in the example has a particularly high content of 1,6-hexanediol as diol and a low content of trimethylolpropane. Such formulations yield exceptionally tough and flexible films. However, in this case the isophthalic acid must be combined with smaller quantities of other dicarboxylic acid (here: phthalic anhydride in addition to adipic acid) to achieve molecular structures that have no tendency to crystallise. Typical commercial products: Desmodur 670 (Covestro [73]), Setal 168 (Allnex-Nuplex [76]), Uralac SN 831 (DSM [67]), Rokrapol 2135 (Krämer [69]), Synolac 1529 (Arkema [77]), Synthoester 1130 (Synthopol-Chemie [80]), Vialkyd AN 950 (Allnex [75]).

116

Saturated polyesters containing OH groups

5.5.4 OH polyesters for crosslinking with blocked polyisocyanates The pot-life effect of two-pack systems based on OH polyesters and polyisocyanate adducts can be circumvented, and the benefits of properties of urethane networks still achieved, by using blocked polyisocyanates. The latter are produced by reactions of polyisocyanates with blocking agents, which are released again at higher temperatures. Such blocking agents are H-active compounds of relatively high polarity. Typical blocking agents (the temperatures in parentheses are the optimum reaction temperatures for the compounds in combination with OH polyesters) are: phenols (~180 °C), ε-caprolactam (~ 170 °C), 1,2,4-triazole (~155 °C), methyl ethyl ketoxime (~ 155 °C), ethyl acetoacetate (~140 °C), 2,3-dimethyl pyrazole-1,2 (~ 140 °C), diethyl malonate (~ 130 °C). The same polyisocyanate adducts can be used in the blocking reactions as are employed for regular two-pack coating systems. Here it has been found that the reaction of a blocked polyisocyanate with the OH groups is only a two-step decomposition reaction at higher temperatures. At the before mentioned effective reaction temperature, the reaction is a trans-urethanisation, analogous to a trans-esterification. Most of these exchange reactions generate new urethane linkages with the OH groups of polyesters, combined with the release of blocking agent. However, there are some exceptions. For example, due to the molecular directing effect of the amide-β-diketo structure, the reaction of polyisocyanates, blocked with diethyl malonate, is a transesterification with liberation of ethanol. Naturally, the quantity and the reactivity of polyester OH groups exert an important influence on the crosslinking efficiency. The OH polyesters chosen do not differ principally from those for two component systems. Consequently, they have mainly high OH values, low acid values, and relatively low number-average molecular weights. However, the polyesters need to be selected with the application conditions in mind. For example, the use of polyesters containing heat-sensitive building blocks (e.g. higher quantities of long chain aliphatic polycarboxylic acids) should be avoided. The described polyester combinations are mainly suitable for highly flexible stoving enamels for automotive OEM application, and for can-coating and coil-coating systems. For such systems, it is possible to combine the blocked polyisocyanates with melamine resins to achieve a particularly good balance of properties (flexibility and durability). Branched OH polyesters with a high content of aromatic dicarboxylic acids and crosslinked by aromatic polyisocyanate adducts blocked with phenols serve as the basis for direct-solder wire enamels. Table 5.6 [81] shows a polyester made from 10 moles of dimethyl terephthalate, 8 moles of trimethyl-1,6-hexanediol, and 3 moles of trimethylolpropane in a transesterification process. The stated number-average molecular weight of 1600 to 1700 g/mol and an OH value of 93 to 95 mg KOH/g suggest less than total conversion of about 96 mol-%. The

117

Saturated polyesters Table 5.6:  Example of a saturated OH polyester for crosslinking with blocked polyisocyanates n

building element

M

10.0

dimethyl terephthalate

194

1940.0

645.0

8.0

trimethyl hexane diol-1,6

160

1280.0

425.6

trimethylol propane

134

3.0

sum 19.2

methanol

32

yield polyester constant number-average molecular weight [g/mol] degree of branching [mol/kg] acid value [mg KOH/g] OH value [mg KOH/g]

m=n·M

wt. ‰

402.0

133.7

3622.0

1204.3

614.4

204.3

3007.6

1000.0

k'M 1.18 Mn 1671 v 1.0 AV 1.0 OHV 108.2

acid value is of course very low and provides no indication of the degree of transesterification. This polyester is intended to be combined with IPDI which is blocked by ε-caprolactam and ultimately destined for coil-coating systems. Typical commercial products: Setal 1600 (Allnex-Nuplex [76]), Uralac SN 810 (DSM [67]), Dynapol H 905 (Evonik [63]). For optimum compatibility between polyesters and crosslinkers, partially blocked diisocyanates can be made to pre-react with some of the OH groups of polyesters. The resin modified in this way contains free residual OH groups and blocked polyisocyanates immobilised on the polyester chain. The result is a self-crosslinkable binder for stoving enamels (can-coating and coil-coating systems).

5.5.5 OH polyesters for high-solid coatings Since the 1970s, it has been necessary to cut solvent emissions. The initial goal was to conserve oil resources. The second, more important issue is the protection of both the environment and of persons handling materials containing solvents against noxious effects. Finally, regulations were passed on the reduction of volatile organic compounds (VOC) in coatings. For coatings applied in industrial processes, limits apply to the emission of solvents to work places and waste air. The limits are to be achieved by cleaning waste air and reducing the solvent content of coating materials in the application state. Where coating materials are applied by skilled manual workers (do-it-yourself enthusiasts, tradesmen), it is essential to lower the solvent content of paint materials (including thinners and washing materials). For optimum application behaviour, paints must have an application-specific viscosity. Compliance with VOC regulations requires in practice that the viscosity of the polymers themselves

118

Saturated polyesters containing OH groups be reduced as a result of which the solids content at application viscosity be increased. Paints with elevated solids content are defined as “high-solids”. The term “high-solids” is not an absolute value. It is used for systems which have an elevated solids content relative to conventional materials and meet the different regulatory values. Thus, a white stoving enamel (topcoat) is deemed “high-solids” if the application solids are increased from 55 wt.% to 70 wt.%. In the past, conventional solvent-borne OEM basecoats started with a solids content of 13 wt.%. Basecoats with more than 24 wt. % are called “medium solids” and with more than 35 wt.% are known as “high-solid” basecoats. The most important influence on viscosity at a given temperature is exerted by the binders and solvents. Besides the solids content, molecular weight, molecular weight distribution, and interaction between binders and solvents are the factors that determine viscosity. Viscosity can be reduced by raising the temperature. There are paint materials which are applied at 80 °C by hot-spray processes (mainly airless application). Such materials have to be stable at this temperature. This process is therefore used for paint materials which form films solely by physical drying. To produce lower viscosities in saturated OH polyesters, it would seem obvious to lower the number-average molecular weights of the polyesters. However, this must not adversely affect the properties of the coating films. On the contrary, modern, new coating systems are expected to possess even better properties. A reduction in number-average molecular weight can be achieved by increasing the polyol excess and/or by decreasing the degree of condensation as described before (see Chapter 3.4.2 and 3.4.3). Both methods lead to higher contents of functional groups (OH values, acid values). While the acid values are kept at the usual levels, polyesters for high-solid coating systems often have higher OH values, up to 200 mg KOH/g. Higher OH values increase the polarity of the polyesters, and greater polarity leads to greater viscosity. Thus, there is a constraint in effect. In addition, the route to film formation, when starting from low molecular weight, is longer than when starting from high molecular weights. This means that measures must be taken to improve crosslinking efficiency: namely increase the content of functional groups and increase the degree of branching, both of which will increase the viscosity and produce the desired outcome. As most crosslinkers (amino resins and polyisocyanate adducts) contain a minimum of three reactive functional groups, linear polyesters bearing terminal OH groups can be used to produce crosslinking coating systems. The advantage to this is that linear polyesters have a smaller molecular weight distribution and therefore relatively lower solution viscosities. Of course, such combinations also have a lower crosslinking density. Another way is to choose building blocks and solvents which interact such that lower viscosities result. In addition, lower polyester viscosities have an impact on application behaviour. The lower molecular weight slows down physical solidification during film formation, because

119

Saturated polyesters a certain degree of crosslinking is required before the film viscosity will increase sufficiently. In stoving enamels, especially, the viscosity of such products can pass through longer and deeper minima, leading to sagging (tears and runs). Figure 5.5 illustrates the dependence of the number-average molecular weight, the OH value, and the degree of branching of a polyester consisting of isophthalic acid (0.70 moles), adipic acid (0.30 moles), trimethylolpropane (0.25 moles) and neopentyl glycol on the polycondensation constant (kM), which is varied by continually raising the fraction of neopentyl glycol (from 0.80 to 1.15 moles). All the polyesters are esterified until the acid values reach about 15 mg KOH/g. In the examples chosen, the number-average molecular weight decreases from 2100 to 570 g/mol and the degree of branching from 1.03 to 0.90 mol/kg while the OH value increases from 95 to 230 mg KOH/g. At the same time, the solution viscosity (solution 65 wt.% in xylene and methoxypropyl acetate) falls from 1500 to 450 mPa s.

Figure  5.5:  Dependence of number-average molecular weight, OH value, degree of branching on increase in polycondensation constant

120

Saturated polyesters containing OH groups Trials have been conducted to avoid the disadvantage of decreasing number-average molecular weight. To improve the crosslinking reactions, the number of functional groups was increased by increasing the degree of branching and thus the OH value. However, these measures were accompanied by increases in both the dispersity of molecular weights and the solution viscosity. In addition, the solubility of binders with increasing OH values in common solvents for coatings is limited. The sagging tendency of coating materials based on low-molecular polyesters can be offset by adding rheological additives, such as colloidal silica, bentonite, polyurethanes, polyamides, and waxes. These products are occasionally termed thickeners as, again, they increase the viscosity. The optimum additives are therefore those which do not increase the viscosity of coating materials in the application state, or, if so, only slightly. However, during film forming – solvent evaporation – they should start unfolding their effect. Examples of such optimum additives are dispersions of low-molecular crystalline ureas [82] or colloidal silica whose surfaces have been pre-treated with organic compounds [83]. When the limitations on the effect of decreasing molecular weights on film properties became known, other ways of lowering the solution viscosities of polyesters were sought. An additional starting point was to exploit the different solubility of polyesters bearing different types of building blocks in different solvents. The viscosity of polyesters of same number-average molecular weight, same content of functional groups (OH value, acid value) dissolved in solvent mixtures (aromatic hydrocarbons, esters) decreases significantly from aromatic dicarboxylic acid (e.g. isophthalic acid) to cycloaliphatic dicarboxylic acid (e.g. hexahydrophthalic anhydride) to aliphatic dicarboxylic acid (e.g. adipic acid). The same effect can be observed when diols with shorter aliphatic chains are replaced by diols with longer aliphatic main chains or longer side chains. However, there are limits to the positive effects of viscosity reduction. Films based on polyesters containing longer aliphatic chains are very soft and less resistant to solvents and chemicals, because they are open to diffusion processes. In addition, such polyesters containing longer aliphatic chains tend to yellow, especially at higher film-forming temperatures. Therefore, cycloaliphatic compounds are used by way of compromise with respect to properties. They confer lower viscosity without substantially lowering the hardness and diffusion density. Suitable cycloaliphatic building blocks are hexahydrophthalic anhydride (in the case of polycarboxylic acid and derivatives) and dimethylol cyclohexane (DMC, or cyclohexane dimethanol DMC, in the case of the diols). If isophthalic acid is replaced by hexahydrophthalic anhydride, the polyester is distinguished by a much lower solution viscosity, without a major reduction in the hardness of the resulting films. The effect of solubility is supported by the effect of anhydride addition which narrows the molecular weight distribution. Figure 5.6 shows the differences in viscosities of two polyesters of comparable molecular size and structure and compares the influence of replacing isophthalic acid by the same molecular quantity of hexahydrophthalic anhydride as a function of solution concentration. It is

121

Saturated polyesters striking that the viscosity of the polyester containing hexahydrophthalic anhydride decreases more rapidly with decrease in concentration, when compared to the isophthalic acid polyester. When cycloaliphatic building blocks are used, it is unnecessary to decrease the molecular weight to the extent that film formation is impaired. Since polyesters based on cycloaliphatic building blocks do not suffer from cracks in clearcoat films, their use is particularly recommended. However, cycloaliphatic building blocks are more expensive than the comparable aromatic compounds. Crosslinkers for the described polyesters for higher-solid coatings are amino resins and polyisocyanate adducts bearing free isocyanate groups (two-pack coatings). In the case of amino resins, low-molecular weight, fully etherified melamine resins (HMMM resins, hexamethoxymethyl melamine resins) are preferred which, on account of their low-molecular weight and broad solubility, contribute to an increase in application solids. For optimum

Figure  5.6: Comparison of the viscosity of polyester solutions as a function of concentration, with isophthalic acid and hexahydrophthalic anhydride as building blocks

122

Saturated polyesters containing OH groups Table 5.7:  Example of a polyester based on hexahydrophthalic anhydride for a high-solid two-pack topcoat n = m/M

building elements

M

m

m- ‰

3.538

neopentyl glycol

104

368.0

400.5

0.963

trimethylol propane

134

129.0

140.4

1.295

adipic acid

146

189.0

205.7

2.026

hexahydro phthalic anhydride

154

312.0

339.5

998.0

1086.1

sum 4.394

water

18

yield (AV) polyester constant number-average molecular weight [g/mol] number of structural units degree of branching [mol/kg] acid value [mg KOH/g] OH value [mg KOH/g]

79.1

86.1

918.9

1000.0

k'M 1.4221 656 Mn q 2.4 v 1.05 AV 13.5 OHV 216.5

crosslinking efficiency, these fully etherified melamine resins require strong acid catalysts (e.g. p-toluenesulfonic acid [84]) or the amine salts thereof. They can then be used in the usual industrial application conditions. The low-molecular weights of the aliphatic polyisocyanate adducts mean that the viscosities are low. As such combinations are capable of reacting at ambient temperatures, a lower molecular weight on the part of polyesters does not exert as much influence as it does in the case of crosslinking by melamine resins. Table 5.7 shows an example [85, example 1] of a saturated polyester based on hexahydrophthalic anhydride which is destined to be crosslinked with aliphatic polyisocyanate adducts for use in high-solid topcoats. This polyester makes full use of the lowering of the molecular weight, cycloaliphatic building blocks and solvent with thinning potential to yield high-solid formulations. In the example presented in the patent, the polyester solution is pigmented with titanium dioxide and completed with solvent (mainly n-butyl acetate) and additives. A portion of 100 parts by weight of the base material is combined with 23 parts by weight hardener (HDI trimer, 90 wt.% in n-butyl acetate). The solids content of the mixture is about 70 wt.% at application viscosity (21 s; DIN 53211). The PVC of the coating film is only about 14 vol.%. The other properties of the paint are presented in detail in the patent example [85, example 2]. Saturated polyesters for high-solid coating materials play a role in the formulation of stoving enamels, such as primer surfacers (application solids ≥ 65 wt.%), OEM topcoats and one-layer coatings (application solids ≥ 65 wt.% for white; spray application), effect

123

Saturated polyesters basecoats (application solids ≥ 25 wt.%), and can-coating and coil-coating formulations (application solids ≥ 70 wt.%; roller application). They are suitable for two-pack coatings for topcoats (application solids ≥ 70 wt.%, spray application) for OEM clearcoats (application solids ≥ 55 wt.%; spray application), for repair clearcoats (application solids ≥ 70 wt.%; spray application) and for various plastic coatings. Given the relatively high cost, they are not often used for wood coatings – despite their optimum properties – as it is hard to sell such high-cost products in that market. Typical commercial products: Dynapol HS 706 (Evonik (Synthopol-Chemie [80]), Uralac SY 941 (DSM [67])

[63]),

Synthalat OF 831

A method mentioned above (see Chapter 3.8) yields polyesters of very narrow molecular weight distributions which have relatively low solution viscosities and lead to the formation of dendrimer polyesters. True dendrimer polyesters are formed only when the polyester is prepared step by step, including blocking all excess functional groups at all steps to avoid random growth of molecules. In one literature example [51], the first step consists in blocking the hydroxyl group of dimethylol propanoic acid (DMPA) by reaction with benzaldehyde to form an aldol condensate (with PTSA catalyst, at 40 °C under vacuum). In the second step, the remain­

Figure  5.7:  Example of the step-by-step preparation of a dendrimer polyester

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Polyesters for water-borne systems ing tertiary carboxyl group of DMPA is esterified with pentaerythritol to form a tetra-ester (Steglich esterification, under the influence of dicyclohexyl carbodiimide and 4-dimethyl aminopyridine). The blocked tetra-ester is hydrogenated (in the presence of Pd/C catalysts) to form free hydroxyl groups – the first dendrimer generation, which has a theoretical molecular weight of 600 g/mol and eight peripheral OH groups. The second generation is formed by starting with the first generation and repeating the three preparation steps. The second generation then contains 16 peripherals OH groups and has a theoretical molecular weight of 1528 g/mol. The step-by-step preparation of that dendrimer polyester is shown in Figure 5.7. The molecular weight distribution of the product described is very narrow. The solution viscosity is much lower than that of conventionally prepared polyesters of same molecular size and same functionality. However, the complex production process renders the product very expensive. It must be decided from case to case if the effort expended on achieving the target properties is worthwhile. In addition, the supplier of the product suggests using the dendrimer polyester for the preparation of polyester acrylates for UV coatings (see Chapter 5.4.3) or preparing special alkyd resins by esterification of some of the peripheral OH groups with fatty acids. Typical commercial products: Boltorn H 311, H 2004 (Perstorp) Polyesters for high-solid coating systems are in competition with high-solid alkyd and acrylic resins. Alternatively, there are water-borne polyester systems (see Chapter 5.8) or polyesters for powder coatings (see Chapter 5.9), which completely eliminate solvents, as well as so-called 100 % systems.

5.6 Polyesters for water-borne systems There are only a few types of coating binders which are genuinely soluble in water. Among them are polyethylene oxides including copolymers, polypropylene oxides (up to a specific molecular weight, 600 g/mol for linear types); amino resins and phenol resins, both etherified with methanol and non-etherified; polyvinyl alcohols; cellulose ethers; and special polyesters. Polyesters, mainly those based on adipic acid as polycarboxylic acid, containing significant quantities of the special triol trishydroxyethyl isocyanurate (THEIC) are soluble in water. These binders form colloidal solutions by interaction of their molecules with water molecules. Optimum film properties are achieved by necessarily suppressing the water solubility, e.g. by effective crosslinking to avoid sensitivity to atmospheric moisture. All other polyester binders are hydrophobic and not readily transferred into the aqueous phase. However, there are several ways to modify polyesters so that they possess a certain water tolerance. These employ different methods to introduce hydrophilic molecule parts that act as solubilising groups and help distributing the polymers in water.

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Saturated polyesters The first method is the incorporation of polyethylene oxide chains (mainly side chains). Such modification yields hydrophilic groups which offer the possibility of generating a stable distribution of polyesters in water. With such polyesters, it is again necessary to suppress the water tolerance to obtain an optimum film resistance. A more important method introduces ionic groups into the polyester molecules, which form solvates with water molecules and can carry the hydrophobic polyester molecules in aqueous phase (in a similar manner as surfactants). It is possible to introduce either cationic or anionic carrier groups. In practice, usually anionic groups are chosen for polyesters for aqueous systems. Such anionic carrier groups can be free carboxylic acid groups which are already contained in all polyesters. Above a specific minimum acid value, polyesters can be transferred into aqueous phase. Transfer usually takes place by partial neutralization of carboxyl groups with amines (rarely with alkali metal cations). The carboxyl groups themselves are not sufficiently hydrophilic to act as carrier groups. The resulting product is a colloidal solution with open shells of particles, which is a different structure from that of dispersions, which have defined particle surfaces. The minimum acid value for producing such a colloidal solution varies with the size and structure of the polyesters while, additionally, the degree of neutralisation is important for water tolerance. As polyesters modified in this way are not truly soluble in water, the definition of water tolerance is “water-thinnable”. Above all, this applies to aqueous systems which contain a water soluble co-solvent in addition to water. Such co-solvents are organic solvents which can form solvates with the more hydrophobic moieties of the polyester molecules but are simultaneously miscible with water. The most important co-solvent is butyl glycol (butyl cellu­ solve, ethylene glycol mono n-butyl ether). To an extent, depending on their water tolerance and degree of interaction with polyester molecular coils, the co-solvents become distributed between the water and the polymer phase. In the polymer phase, the function of the co-solvents is to lower the viscosity and to support a type of uncoiling for better orientation of anionic carrier groups on the particle surfaces. There are then only minor differences between water-borne polyester systems and polyesters in organic solutions. They can have even the same particle size. The open-loop particle surfaces present an opportunity for intense interdiffusion of particles during film formation. That is a major advantage over true dispersions. If the particle surfaces do not change the optical density, the aqueous system is transparent. Figure 5.8 is an attempt to describe the differences between an organic solution, an aqueous colloidal solution and an aqueous dispersion; the particles are intentionally all the same size. (The molecular dimensions in the diagram are not to scale.) Doping of the polyester molecules with carboxyl groups can take place in various ways: – addition of anhydrides to polyester OH groups – limiting the degree of condensation at high acid values.

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Polyesters for water-borne systems Anhydrides for such addition reactions are phthalic anhydride, maleic anhydride, trimellitic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, succinic anhydride and alkylated succinic anhydride (e.g. dodecenyl succinic anhydride). The attainment of high acid values by limiting the degree of condensation needs to take into consideration that this goes hand in hand with a limitation on molecular weight. To achieve comparable molecular weights, it is necessary to decrease the molar excess of polyols. Polyesters based on isophthalic or terephthalic acid with high acid values can suffer from the problem that some unreacted acid can remain due to the high melting temperatures of these building blocks. If such polyesters contain mixtures of these dicarboxylic acids, it makes sense to esterify the acids separately. The same considerations apply to the use of dimethyl terephthalate, which needs to be transesterified in a first step because it is difficult to monitor the degree of transesterification. Polyesters for aqueous systems always have sufficient residual OH groups for crosslinking reactions. The acid values are usually between 25 and 65 mg KOH/g. The most important neutralisation agents are triethylamine (TEA), N,N-dimethylethanolamine (DMEA), diethanolamine, diisopropylamine (DIPA), 2-amino-2-methyl propanol (AMP) and, more rarely, ammonia. The degree of neutralisation is between 60 to 100 mol-%, expressed in terms of the content of carboxyl groups (acid value). As the amines are strong bases and the carboxyl groups are weak acids, the pH values at stoichiometric neutralisation are substantially higher than 8. The pH values of commercial products are commonly between 7.2 and 8.5. To prepare aqueous systems, it is necessary to use distilled or deionised water, because traces of dissolved polyfunctional cations would lead to instability.

Figure  5.8:  Models illustrating the differences between an organic solution, an aqueous colloidal solution and an aqueous dispersion

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Saturated polyesters The pH level is a reason for problems with aqueous polyester systems. The ester groups of polyesters are not particularly resistant to saponification. When aqueous polyester solutions are in storage, the partially reacted anhydride undergoes saponification, with liberation of the acids and decomposition of the ester chains. This can be accompanied by a loss of water tolerance and a substantial decrease in the molecular weights of the polyesters. It is therefore essential to take measures to avoid saponification reactions in aqueous polyester systems or, at the very least, to minimise the reaction. The added anhydrides can be classified by the rate of the saponification reaction. This has been determined by adding different anhydrides to the OH groups of a polyester resin to produce equal acid values. The polyesters doped in this way were neutralised stoichiometrically with N,N-dimethylethanolamine (DMEA), thinned with deionised water to 30 wt.% solids. The resulting aqueous solutions were stored at 40 °C and the rise in acid values and fall in pH were tracked during storage. In order of decreasing saponification rate, the classification is as follows: – phthalic anhydride – trimellitic anhydride – maleic anhydride – tetrahydrophthalic anhydride – hexahydrophthalic anhydride – succinic anhydride – dodecenyl succinic anhydride The lower stability of adducts made with aromatic polycarboxylic anhydrides relative to those made with cycloaliphatic and aliphatic adducts can be explained by the directional effect of the aromatic π-electron system and by the vicinal structure of the two carboxyl groups (anchimeric assistance). It is also possible to rank the esters of polycarboxylic acids inside polyester chains with respect to their saponification resistance. Terephthalic and isophthalic esters are much more resistant than tetrahydrophthalic or hexahydrophthalic esters, which are more resistant than esters of phthalic acid and adipic acid. The most stable esters are formed by fatty acid dimers. These statements also apply when the polycarboxylic acids constitute only part of the composition. In addition, it is possible to define an order of saponification sensitivity. Diols with short and linear chain can be saponified very easily. Diols with longer chains and side chains and cycloaliphatic diols saponify much more slowly. The most stable diols are 2-ethyl-2-butyl 1,3-propanediol; hydroxypivalic neopentyl glycol ester (HPN), dimethylol cyclohexane (CHDM), perhydro bisphenol A, and the dimer diols. In the case of higher-functional polyols, trimethylolpropane is more stable than glycerol or pentaerythritol. The reason for the better saponification resistance is undoubtedly the hydrophobic effect of nonpolar building blocks which protect ester groups against contact with water molecules.

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Polyesters for water-borne systems Table 5.8:  Example of a polyester for water-borne OEM basecoats n = m/M

building elements

M

m

m-‰

3.000

hexahydro phthalic anhydride

154

462.0

109.2

3.016

dimer fatty acid (98 % dimer)

567

1710.0

404.2

7.010

neopentyl glycol

104

729.0

172.3

7.008

hexane diol-1,6

118

827.0

195.9

4.000

trimellitic anhydride

192

768.0

181.6

4496.0

1062.8

265.9

62.8

4230.1

1000.0

sum water

18

yield (AV) polyester constant k'M 1.2260 number-average molecular weight [g/mol] Mn 1869 acid value [mg KOH/g] AV 30.0 OH value [mg KOH/g] OHV 83.1 degree of branching [mol/kg] v' 0.95 statistically the 4 mol TMS generate 1.7 mol branching units and 2.3 Mol COOH groups

Co-solvents, too, can render polyesters in aqueous phase resistant to saponification. If, within the complex solution equilibrium, co-solvents tend to orient themselves into the polymer phase (inner phase) instead of the outer phase (water), they can protect the ester groups. That is the reason why butyl glycol, which has a molecular structure resembling a surfactant, acts more efficiently than the more polar and therefore more hydrophilic butyl diglycol. Finally, it is possible to use solvents which are not totally water-tolerant, e.g. n-butanol or even small quantities of aromatic solvents (e.g. xylene), as co-solvents. Thus, in aqueous coating systems, co-solvents do not act solely by improving compatibility, wetting and levelling but also provide resistance to saponification, and conferring much better storage stability. How­ever, co-solvents containing OH or ester groups can lead to transesterification reactions. Polyesters for water-thinnable coating formulations therefore preferably consist of isophthalic acid, hexahydrophthalic anhydride or tetrahydrophthalic anhydride which constitute the main part of the polycarboxylic acids and, if more plasticisation is required, they may contain fatty acid dimer fractions. Besides neopentyl glycol, other preferred neo-diols are hydroxy pivalic neopentyl glycol ester (HPN) or dimethylol cyclohexane. The most commonly employed polyol is trimethylolpropane. In summary, polyesters for water-thinnable coating systems need to contain, in addition to the essential hydrophilic carrier groups, e.g. primarily carboxylate anions, as many hydrophobic building blocks as possible. This statement is only seemingly paradoxical. There are test results which show that the saponification resistance of adducts of trimellitic anhydride and OH polyesters can be compensated by esterifying the second car-

129

Saturated polyesters boxylic group of the trimellitic partial ester, so that trimellitic acid is randomly incorporated into the polyester with two ester groups, leaving just one free carboxyl group available for neutralisation. Such structures are much more stable than the simple addition product. In addition, there is the advantage that the necessary carboxyl groups are better distributed across more polyester molecules. Of course, to achieve the equivalent acid value, it is necessary to use twice the quantity of trimellitic anhydride and to lower the molar fraction of the dicarboxylic by the quantity of trimellitic anhydride to achieve comparable molecular weights [86]. In such cases, it is obvious to use trimellitic anhydride in higher quantity so as to achieve the branched polyesters. Table 5.8 [87] describes such a polyester, which is used for water-borne basecoats for automotive OEM application. When the conditions for termination have been satisfied, the polyester is cooled and dilut­ed with n-butanol (80 wt.% solids). Then the remaining free carboxyl groups are neutralised with N,N-dimethylethanolamine (neutralisation degree: 80 mol-%) and thinned with deionised water to 60 wt.% solids. The solution has a pH of 7.8. Statistically, all molecules of the polyester in this example contain a carboxyl group. However, polyesters can still be sufficiently thinnable with water even if not all molecules bear carboxyl groups, due to the carrier effect exerted on all molecules by such groups. There are alternative ways to introduce carboxyl groups into polyesters with the aim to prepare water-thinnable binders. The polycarboxylic acids or anhydrides may be replaced by hydroxycarboxylic acids, e.g. hydroxypivalic acid and dimethylol propanoic acid. The OH groups of these compounds can react with carboxyl groups of other building blocks in polyester composition. The tertiary carboxyl groups of these compounds do not easily form esters under common reaction conditions; they remain and, neutralised, form carrier groups for water-borne systems. Of course, the hydroxypivalic acid forms a chain end and dimethylolpropanoic acid is incorporated into the polyester chain. This segment is relatively resistant to saponification. There is another possibility of incorporating dimethylol propanoic acid for the purpose of greater saponification resistance, via urethanes instead of esters. When dimethylol propanoic acid is made to react with two moles of diisocyanates, an adduct bearing two residual isocyanate groups is formed, which is then incorporated into polyester chains at relatively low temperatures (polyurethane modified polyesters for water-borne coatings). Further alternative ways of preparing water-thinnable polyesters include the incorporation of building blocks containing sulfonic acid or their anions (salts with alkali cations or amines). One example is the lithium salt of sulfoisophthalic acid [89]. Another approach utilises polyesters containing unsaturated building blocks, such as maleic anhydride. The double bonds (fumaric esters) incorporated can add bisulfite anions to yield sulfonic anion side-chains. As sulfonic anions are more hydrophilic than carboxylate anions, only small molar quantities are necessary to form stable aqueous systems.

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Polyesters for water-borne systems When aqueous solutions of polyesters are being thinned, abnormal viscosity behaviour is often observed. Adding organic solvent to a polyester solution will lower the viscosity of that solution in an exponentially fading curve. In contrast, adding water to an aqueous solution (including co-solvent) will give rise to a significant increase in viscosity, up to a maximum value. Addition of more water then causes the viscosity to drop rapidly. Figure 5.9 shows the viscosity curves for an organic polyester solution, an aqueous polyester solution, and a true aqueous dispersion. The phenomenon is commonly called as the “water hill” of viscosity. Different interpretations of this abnormal viscosity behaviour exist. One assumes that the concentrated solution contains water in an inner phase (water-in-oil distribution). Adding water then causes phase inversion into an oil-in-water distribution. The point of inversion is interpreted as the viscosity maximum. (This is comparable to what happens in the production of creams.) However, the presumption here is that the reason for the anomaly lies in the complex solution equilibrium between polymer, water and co-solvent. When the water is first added, it will diffuse into the particles, which will then grow – the viscosity increases. Once a specific ratio of water to co-solvent is achieved, the ability to form solvates is lost. The particles contract – the viscosity decreases. At very low concentration, the distribution of polyester in the aqueous

Figure  5.9:  Viscosity curves for an organic polyester solution, an aqueous polyester solution and a true aqueous dispersion

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Saturated polyesters phase tends to assume more or less the state of a true dispersion. It is possible for the polyester to precipitate. The position and shape of the “water hill” of viscosity vary with the molecular weight and molecular weight distribution of the polyester and its type of building blocks, the acid value, the degree of neutralisation, the type of neutralisation agent, and the type and quantity of co-solvent. For example, alkanolamines have a positive effect on application behaviour when serving as neutralisation agent, because they additionally act as co-solvents. The viscosity anomaly must be taken into consideration when water-thinnable coatings are prepared from polyesters in their delivery form. Stirrers capable of generating adequate shear forces are required. The anomaly can also impact the application behaviour and is reduced by adding crosslinkers and more co-solvents. However, of course, the content of co-solvent may not exceed regulatory limits. Aqueous stoving enamels may not contain more than approx. 10 wt.% co-solvent in the application state. Water-thinnable polyesters containing OH groups as well as carboxyl groups are combined with water-soluble melamine resins for use in stoving enamels. The melamine resins chosen are mainly the low-molecular types which are highly methylolated and fully etherified with methanol (HMMM resins). For many applications, the high content of carboxyl groups (high acid values) acts sufficiently as acid catalyst to effect adequate crosslinking. However, this catalytic effect can be intensified with additions of amine salts of sulfonic acids. More reactive melamine resins may therefore suffer from problems with stability. If such melamine resins manage to diffuse into the colloidal particles, they might react prematurely, due to the high concentration of functional groups and of catalytic carboxyl groups inside the particles. Whereas in organic solutions the viscosity would be increased by premature reaction, in water-borne systems the viscosity appears to be unchanged. Due to contraction of the particles, it is even possible for the viscosity to decrease. If the water-borne system is defective, problems may occur during application (no wetting, no levelling). In extreme cases, the colloidal phase will precipitate. Uses for combinations of water-thinnable polyesters with water-soluble melamine resins are in primer surfacers and basecoats for automotive OEM application, water-borne paints for industrial application, and can-coating and coil-coating systems. In addition, there are water-borne stoving enamels which contain polyesters and blocked polyisocyanate adducts. Blocked polyisocyanate adducts for water-borne systems can be prepared by partial reaction of isocyanate groups with hydroxycarboxylic acids which, after neutralisation, confer solubility in water. There is also the possibility of combining blocked polyisocyanate adducts in organic phase with polyesters which contain a sufficient quantity of carboxyl groups to provide water-solubility for the entire combination. Typical commercial water-thinnable polyesters: Setal 6306 (Nuplex [76]), Dynapol HW 112 (Evonik [63]), Plusaqua V494 (Mäder [88]), Synthoester WP 181 (Synthopol-Chemie [80]), WorléePol 191 (Worlée [78])

132

Polyesters for water-borne systems Surprisingly, polyisocyanates bearing free isocyanate groups can be used to make water-borne systems, even though isocyanate groups react with water to form derivatives of carbamic acid which decompose spontaneously into primary amines and carbon dioxide. There are two different ways to render polyisocyanates suitable for water-borne systems. Polyisocyanate adducts for coatings with free isocyanate groups are relatively nonpolar and hydrophobic. Adding such polyisocyanates to a water-borne polyester system leads to the formation of large particles only. The biggest problem is the efficiency of inter-diffusion of the high content of water-thinnable polyesters composed of very small colloidal particles, and the low content of polyisocyanate composed of quite large particles. However, optimum inter-diffusion is essential for optimum film forming. The first method utilises polyisocyanate adducts of very low viscosity. Such products are much easier to distribute in aqueous phase. They form small particles and therefore inter-diffuse more efficiently with polyester particles. Low-viscosity polyisocyanates are aliphatic polyisocyanate adducts of narrow molecular weight distribution (isocyanurate trimers of hexamethylene diisocyanates) or allophanate adducts. Alternatively, solutions of polyisocyanate adducts in polar non-protic solvents (e.g. in methoxy propyl acetate) are chosen. Such solutions are distributed efficiently in the aqueous phase with little mixing effort and are accompanied by more efficient crosslinking. The second method consists in doping polyisocyanate adducts with hydrophilic chains, such as by partial reaction of polyisocyanate adducts with polyethylene oxides (methoxy ether of polyethylene oxide). The resulting more hydrophilic products are easier to distribute in the aqueous phase, and this is followed by more efficient interdiffusion with polyester particles and improved crosslinking. However, with this method, some of the hydrophilic polyisocyanate adducts may react with water. It is therefore necessary, in contrast to the first method, to increase the polyisocyanate content. To this end, molar ratios of isocyanate to OH groups of 1.3 to 1.5:1.0 are chosen. The parallel formation of urea groups is not a disadvantage; they participate in crosslinking and support the development of optimum film properties. The film properties of combinations of water-borne polyesters with polyisocyanate adducts are nearly comparable to those of combinations in organic solutions. Naturally, a twopack effect also occurs in aqueous phase. There is no possibility to determine the pot-life by measuring an increase in viscosity, but only by application results. The products are problematic during application, as they tend to undergo popping (of gas bubbles) and poor levelling. These tendencies must be compensated by adding certain co-solvents and additives (de-aeration additives). Although OH polyesters for water-borne systems are at a disadvantage relative to OH acrylic resins, because they are less resistant to saponification, they also have an advantage. Due to the colloidal particles of polyesters in aqueous phase they generally have a lower coil density than acrylic resins in the aqueous phase. This favours inter-diffusion of polyester and polyisocyanate and more efficient crosslinking. In addition, water-borne polyester combinations with polyisocyanate adducts give rise to fewer problems during application.

133

Saturated polyesters

5.7 Polyesters for powder coatings Polymers for coatings (binders) usually need to be transferred into a liquid phase before they can be applied. To this end, most are diluted with or dispersed in an organic solvent or water (with or without additional organic solvent) to a viscosity appropriate for the application method. Some binders are already liquids. An alternative application form is an aerosol, for which purpose the binder particles are distributed in air. Stable aerosols are similar to liquids but need to be mobile enough to allow conveying and support application. Powder coatings generated by this method have the advantage of having virtually no volatile organic content. Powder coatings form films by melting processes. To be suitable for powder coatings, the binders (here: polyesters) have to meet special requirements. They must be crushable to fine particles by common methods and the resulting powders must be stable in storage under the usual conditions. They must be easy to transform into aerosols, i.e. very easy to fluidise. The aerosols must lend themselves to application by currently the most common method, which is electrostatic spraying. This requires them to be perfect dielectrics. In addition, they must possess special melt behaviour (melt temperature, melt viscosity) so as to yield optimum flow and levelling during film formation. Suitable polyesters for preparing powder coatings have glass-transition temperatures above 55 °C and softening temperatures – the values of which vary with the measurement method – above 75 °C. The required temperature also depends on the temperature at which potential crosslinkers can be added to the main binder without the need for pre-reaction. The glass-transition temperature or the softening temperature determines whether the binders or binder combinations can be crushed by common milling processes, because the milling process naturally generates heat. The crushed powder is sieved and classified to yield a specific particle size distribution. During the preparation process and storage, the specified temperatures must be observed. Saturated polyesters of suitably high glass-transition temperatures mainly contain aromatic or cycloaliphatic polycarboxylic acids and short-chain aliphatic diols or cycloaliphatic diols. Aliphatic dicarboxylic acids or long-chain aliphatic diols chains are only used to such an extent that they do not substantially lower the glass-transition temperatures of the final material. These smaller quantities are added to control the application behaviour (melt viscosity) and flexibility of the films. Trimethylolpropane, the branching component of the other polyesters, is mainly replaced by trimellitic anhydride (TMA), which does not act as a plasticising element. Optimum levelling of powder coating is ensured with polyesters which have steep temperature-viscosity curves. Higher molecular weight polyesters re­ quire elevated temperatures to achieve this. As optimum solubility is not essential for polyesters for powder coatings, semi-crystalline polyesters may be used. Although crystallinity leads to comparatively high melting temperatures, it also generates rather steep

134

Polyesters for powder coatings temperature-viscosity curves during melting – and this is advantageous for the flow and levelling of powder coating films. The melting temperatures and melt viscosities must be matched to the crosslinking conditions. The powder coating film must be melted sufficiently before crosslinking takes place to yield smooth films.

5.7.1 Thermoplastic polyesters Some of the high-molecular weight polyesters already described (see Chapter 5.1) are also suitable for powder coatings. As they contain practically no functional groups, they cannot be crosslinked. They can only generate films by a physical process (melting). Their high glass transition makes these polyesters easy to convert into powder form. Unlike solvent-borne coatings, the polyesters for powder coatings can be semi-crystalline. The powder coatings prepared in this way require relatively high film forming temperatures (240 to 400 °C). The resulting films show good adhesion, are flexible, resistant to atmospheric moisture and fairly weatherable. Typical commercial product: Dynapol P 1500 (Evonik [63])

5.7.2 COOH polyesters The most important class of polyesters for powder coatings in terms of volume are those containing carboxyl groups. These polyesters are initially crosslinked with epoxy compounds in an efficient reaction at elevated temperatures. This addition reaction has the advantage of not liberating any compounds, which makes it possible to produce high film thicknesses without any trouble. Carboxyl polyesters for powder coatings are constructed from aromatic polycarboxylic acids such as terephthalic acid, isophthalic acid, trimellitic anhydride and occasionally small amounts of phthalic anhydride. They contain short-chain aliphatic diols, such as ethylene glycol, propylene glycol and neopentyl glycol, but also cycloaliphatic diols such as dimethylol cyclohexane. They can contain smaller amounts of long-chain aliphatic compounds, such as adipic acid or 1,6-hexanediol. The amounts are limited to quantities which, together with the other components, do not lower the glass-transition temperature to below 55 to 60 °C. The carboxyl groups for crosslinking are generated in the same way as for the preparation of water-thinnable polyesters. The acid values of such polyesters are mainly between 30 and 80 mg KOH/g. The OH values are normally low. COOH polyesters are usually prepared in two steps. First, an OH polyester containing terephthalic acid or isophthalic acid and polyols in molar excess is prepared in a reactor. Then trimellitic anhydride is added and, if necessary, partially esterified. The resulting polyesters mostly have number-average molecular weights of between 1500 and 3000 g/mol.

135

Saturated polyesters The finished polyester is discharged, cooled, and crushed to particles of defined size. Then the crosslinker is added. After transfer to a solid mixer, the other ingredients of the powder coating are incorporated: pigments, additives (flux, catalyst, levelling agent). The mixture is then intimately homogenised in an extruder at temperatures chosen to avoid premature crosslinking. Having then been discharged, cooled and crushed, the product is milled and sieved to yield a powder coating. The particle size distribution is crucial to the attainable film thickness and optimum application. Suitable powder coatings have average particle sizes of about 20 µm. In that event, the largest particle in the particle distribution will be about 50 µm. Smooth, glossy films are achieved by applying such powder coatings in thicknesses of not less than 65 µm. The most commonly employed crosslinkers are solid aromatic epoxy resins and must have the appropriate glass-transition temperatures. Suitable are epoxy resins based on bisphenol A and epichlorohydrin that have average molecular weights of 1300 to 1800 g/ mol and corresponding epoxy equivalent weights. COOH polyesters and epoxy resins are commonly mixed in stoichiometric ratios. If, for example, an epoxy resin of epoxy equivalent weight 740 g/mol is combined with a polyester of acid value 30 mg KOH/g, the mixing ratio is 70 polyester to 30 epoxy resin. If the polyester has an acid value of 80 mg KOH/g, the mixing ratio is 50 : 50. Figure 5.10 shows the mass ratios of polyester and epoxy resin (EEW 740 g/mol) as a function of the acid value of the polyesters. There are powder coatings which have a higher polyester content (70/30 systems) and those which have equal contents of both resins (50/50 systems). A higher content of polyester supports flexibility and weathering resistance, while a higher epoxy resin content supports adhesion, and resistance to corrosion, solvents and chemicals. Nevertheless, all powder systems containing aromatic epoxy resins have limited weatherability. The crosslinking reaction of carboxyl groups of polyesters and epoxy groups can be catalysed by Figure  5.10:  Mass ratios of polyester and epoxy resin acids and Lewis acids. In the past, (EEW 740 g/mol) as a function of the acid value of polyesters the most common catalysts were

136

Polyesters for powder coatings organo-tin compounds. As such tin compounds have now been defined as critical, they must be replaced by alternatives (e.g. bismuth compounds). Aromatic epoxy resins based on bisphenol A contain only two epoxy groups per mole. Therefore, the combination polyesters would have to have a minimum of three carboxyl groups for effective crosslinking. If this is to be achieved, polyesters of number-average molecular weight 2500 g/mol need to have acid values of at least 67 mg KOH/g. Fig­ ure 5.11 shows the dependence of average polyester functionality on the number-average molecular weight and acid value. Sample polyesters (taken from data sheets and patent examples) with number-average molecular weights below 2500 g/mol and acid values of 45 mg KOH/g do not even contain two carboxyl groups per molecule. However, because optimally crosslinked powder coat­ ing films are formed, it has to be assumed that further reactions occur which are involved in crosslinking. These are surely also addition reactions between epoxy groups and OH groups of the epoxy resins themselves. The example of a powder-coating polyester in Table 5.9 [90] contains propylene glycol (9 moles), dipropylene glycol (2 moles) and terephthalic acid (10 moles). The OH value, cited in the example as being 28 mg KOH/g, indicates a roughly 7.0 wt.% loss of propylene glycol during the reaction, and so the content in the polyester is re­ duced to 8.37 moles propylene glycol. In the second step, trimellitic anhydride (TMA) is added (0.5 moles to 1000 g from the first step). The resulting binder is a weakly branched polyester with an acid value of 57.0 mg KOH/g, an OH value of practically 0, and a calculated number-average molecular weight Figure  5.11:  Dependence of average polyester of 3913 g/mol. Such polyesters are functionality on the number-average molecular suitable for crosslinking with aro- weight and acid value

137

Saturated polyesters Table 5.9:  Example of a COOH polyester for powder coatings with added TMA 1st step

2nd step

propylene glycol (start 9,000 mol)

8.370

8.370

dipropylene glycol

2.000

2.000

10.000

10.000

0.000

1.105

propylene glycol (start 309.5 g)

287.8

262.8

dipropylene glycol

121.2

110.7

terephthalic acid

750.9

685.7

building elements [mol]

terephthalic acid trimellitic anhydride building elemnts [wt. ‰]

trimellitic anhydride sum water yield (AV)

0.0

87.7

1159.9

1146.9

159.9

146.9

1000.0

1000.0

9.2

57.0

characteristic values acid value [mg KOH/g]

1.0732

1.0557

number-average molecular weight

polyester constant k'

3018

3913

number of structural units

13.7

18.0

0.0

0.46

degree of branching [mol/kg] OH value [mg KOH/g] softening temperature [°C]

28.0

- 2.7

96–113 °C

108–120 °C

matic epoxy resins, as well as with other crosslinkers (see examples described in the patent). Besides the aforementioned catalysts, the powder coatings can contain further additives: fluxing and de-aeration additives (e.g. benzoin), surface additives (waxes, silicones) and levelling agents (nonpolar acrylic polymers). As the acrylic polymers are liquids, it is difficult to incorporate the commonly employed small quantities via an extruder. An intermediate is therefore prepared in the form of a mixed melt of binder and additive (e.g. with 10 wt.% additive) and this “master batch” is added as a solid to the extruder. Powder coatings based on COOH polyesters and aromatic epoxy resins have a large range of applications in different industrial coating fields: washing machines, refrigerators and other household appliance, heating elements, structural elements of machines, metal

138

Polyesters for powder coatings racks, grids, containers, and metal garden furniture. The pigmented 70/30 systems are said to possess adequate weathering resistance. Typical commercial products: Crylcoat 327 (Allnex [91]), Uralac P 2450 (DSM [67]) Alternative crosslinking partners may yield more defined crosslinking reactions than the epoxy resins. First of all, consider triglycidyl isocyanurate (TGIC, chemical structure shown in Figure 5.12). The technical product [92] has an epoxy equivalent weight of 100 to 108 g/mol and softening temperatures of between 88 and 98 °C. Powder polyesters made for crosslinking with TGIC have comparatively low acid values (25 to 50 mg KOH/g) and tend to have higher number-average molecular weights (2000 to 4000 g/mol). Such polyesters are prepared not so much by anhydride addition as by incomplete esterification of the aromatic polycarboxylic acids and interruption of the polycondensation process at higher acid values. Some of these resins are explicitly labelled as polyesters which are free of trimellitic anhydride. The polyesters and TGIC are usually combined in stoichiometric ratios. For example, for a polyester with an acid value of 35 mg KOH/g (corresponding to an equivalent weight of 1603 g/mol), the mixing ratio is 93 wt.% polyester and 7 wt.% TGIC. If polyesters with higher acid values and more TGIC are chosen, the glass transition temperature of the entire system is lowered. Overall, the glass-transition temperatures of polyesters to be crosslinked by TGIC and its alternatives should be somewhat higher (e.g. greater than 60 °C) than for those crosslinked by epoxy resins. Table 5.10 describes a polyester for powder coatings, crosslinked by TGIC and containing terephthalic acid, and small quantities of adipic acid, neopentyl glycol and isophthalic acid [93]. The polyester is prepared in two steps. First, the terephthalic acid and the adipic acid are esterified with neopentyl glycol to form a linear polyester bearing terminal OH groups. Second, the isophthalic acid is added and esterified until the acid value is 35 mg KOH/g. This consumes all of the OH groups remaining from the first step. The resulting number-average molecular weight agrees well with the calculated value. Powder coating films containing COOH polyesters and TGIC as crosslinker are very resistant to weathering and yellowing. They are mostly used for outdoor coatings. Typical commercial product: Crylcoat 630 (Allnex [91]) Crosslinker TGIC is now classified as toxic [92]. TGIC is irritant to mucous membranes, sensitising, and mutagenic (group

Figure  5.12: Structure of triglycidyl isocyanurate

139

Saturated polyesters Table 5.10:  Example of a COOH polyester for powder coatings, crosslinked by TGIC and its alternatives n=m/M

building elements

M

m

wt. ‰

8.668

terephthalic acid

166

1438.90

574.7

0.519

adipic acid

146

75.73

30.2

10.194

neopentyl glycol

104

1060.20

423.4

1.782

isophthalic acid

166

sum water

18

yield (AV) polyester constant number-average molecular weight [g/mol] (specified in patent:3205) acid value [mg KOH/g] OH value [mg KOH/g]

295.89

118.2

2870.72

1146.5

366.78

146.5

2503.94

1000.0

k'M 1.0717 Mn 3182 AV OHV

35 0.3

2 of products which are harmful to health). TGIC should therefore be replaced in new powder coating formulations. Among the alternative crosslinkers to TGIC for COOH polyesters are the glycidic esters of terephthalic acid and trimellitic acid. A commercial product containing mixtures of both compounds exists [94]. The mixture has an epoxy equivalent weight of 141 to 154 g/mol, an average functionality of 3.3 per mole and a softening temperature of 90 to 102 °C. As the triglycidyl ester of trimellitic acid is a liquid and diglycidyl terephthalate is crystalline, the combination of both forms an eutectic system. The melt viscosity is thus lower than that of other products (11.5 mPa s at 100 °C; 320 mPa s in the case of TGIC). On account of the higher equivalent weight of the alternative crosslinker, it is necessary to use greater amounts, e.g. 92 wt.% polyester and 8 wt.% crosslinker in the case of a polyester with an acid value of 35 mg KOH/g. As the crosslinker will lower the glass-transition temperatures, the chosen polyesters should have relatively high softening temperatures. The glycidic ester groups are less reactive than the glycidyl groups of TGIC. Powder coatings containing glycidic esters as crosslinkers have adequate weathering resistance. Typical commercial product: Crylcoat 804 (Allnex) [91]) Further crosslinkers for COOH polyester in powder coatings are β-hydroxy alkylamides [95]. The inductive effect of the amide group renders their β-hydroxyl groups much more reactive than common OH groups. Commercial products [91] include N,N,N',N'-(tetra-2-hydroxyethyl)-adipic acid diamide and N,N,N',N'-(tetra-2-hydroxypropyl)-adipic acid diamide (structure shown in Figure 5.13). It must be taken into consideration that water is formed

140

Polyesters for powder coatings during crosslinking that will evaporate during film formation. Special additives are necessary to avoid pinholes in the film. In principle, these crosslinkers can be combined with the same polyesters already described in connection with crosslinking with glycidyl esters. Films containing N,N,N',N'(tetra-2-hydroxyethyl)-adipic acid diamide crosslinkers have excellent weathering resistance, but tend to yellow. Typical commercial product: Crylcoat 810 (Allnex [91]) Cycloaliphatic and aliphatic polyepoxies have been recommended as crosslinkers for COOH polyesters [96]. These epoxy compounds are liquids and are combined already into the polyester melt. Some pre-reactions must be expected. As cycloaliphatic and aliphatic epoxies react relatively slowly, it is necessary to use effective catalysts (e.g. tertiary ammonium salts) which have to be added via master batch. Commercial products already containing such combinations are available. The softening temperatures are lower, a fact which can impair the storage stability. Also, acrylic resins containing epoxy groups make suitable crosslinkers for COOH polyesters. It is important to coordinate the softening temperatures of the two products. (Acrylic resins containing epoxy groups are more often combined with polycarboxylic acids or their derivatives (e.g. dodecanedicarboxylic acid), which then are defined as crosslinkers.) Both binder/crosslinker combinations are suitable for preparing weatherable powder coatings.

5.7.3 OH polyesters Combinations of OH polyesters and blocked polyisocyanates are also suitable for powder coatings, provided that the glass-transition temperatures of the products are sufficiently high. The softening temperatures of the polyesters for such combinations need to be between 85 and 120 °C. This is achieved with polyesters consisting of aromatic polycarboxylic acids and shortchain aliphatic diols or cycloaliphatic diols. These mostly have small frac­ tions of long-chain aliphatic compounds. A commonly used branching element is trimethylolpropane. The OH values of the polyesters are between 35 and 100 mg KOH/g, and the acid values are low. A patent (see Table 5.11) [97] describes the example of a polyester Figure  5.13:  Structure of N,N,N',N'-(tetra-2-hydroxycontaining a mixture of terephthalic alkyl)-adipic acid diamide

141

Saturated polyesters Table 5.11:  OH polyester for powder coatings n

building elements

M

m=n·M

m-‰

12.00

terephthalic acid

166

1992.0

319.1

12.00

dimethyl terephthalate

194

2328.0

372.9

11.50

neopentyl glycol

104

1196.0

191.6

6.75

hexane diol-1,6

118

796.5

127.6

5.00

dimethylol cyclohexane

144

720.0

115.3

3.00

trimethylol propane

134

sum methanol/water

18

yield (AV)

402.0

64.4

7434.5

1190.9

1192.0

190.9

6242.5

1000.0

polyester constant k'M 1.1123 2316 (in patent) number-average molecular weight [g/mol] Mn acid value [mg KOH/g] AV 4 (3 to 4) OH value [mg KOH/g] OHV 71.4 (56 to 61) glass transtion temperature [°C] 55 softening temperature [°C] 80 viscosity (160 °C) [mPa·s] η 1600

acid and dimethyl terephthalate. The polyester is condensed until the acid value is low, and the resulting OH value is also rather low. (There are differences in the values expressed in the patent and those obtained by calculation. The reason may be a partial loss of polyol excess.) With regard to the loss of polyol, the calculated OH value is 59.5 mg KOH/g and the number-average molecular weight is 3101 g/mol. The polyester is designed to be crosslinked by a blocked polyisocyanate (IPDI, blocked with diketo piperazine in the patent example). The blocked polyisocyanates chosen for powder coatings must have sufficiently high glass-transition temperatures. As powder coatings containing blocked polyisocyanates are intended for films of excellent weatherability, mostly cycloaliphatic polyisocyanate adducts are chosen, e.g. adducts of isophorone diisocyanates (IPDI) and 4,4'-diisocyanato dicyclohexyl methane (HMDI). The blocking agents are ε-caprolactam, dimethyl pyrazole,

Figure  5.14:  Structure of a polyuretdione made from IPDI

142

Polyesters for powder coatings and the aforementioned diketo piperazines. The effective crosslinking temperatures are between 160 and 220 °C. It was found that some of the blocking agents remain in the film during the stoving process and may act as levelling agent or plasticiser. Powder coating films formed from a combination of OH polyesters and blocked polyisocyanates are weatherable and resist discolouration. Most of the films are very flexible and resistant to chemicals. The systems are mainly used for durable, high-end, outdoor coatings. The reason that the market share is not especially large is the higher costs of powder coatings based on polyesters and blocked polyisocyanates. Typical commercial product: Crylcoat 820 (Allnex [91]) Uretdiones can serve as alternative crosslinkers for OH polyesters in powder coatings. These are dimers of diisocyanates which decompose into free isocyanates. The crosslinkers chosen for powder coatings are polyuretdiones whose terminal NCO groups are made to react with diols, which can be incorporated into the molecular network after crosslinking reactions. The structure of such a product is presented in Figure 5.14. A combination of such polyuretdiones with OH polyesters is stable in storage – the crosslinkers will not decompose to form free diisocyanates until elevated temperatures are reached, whereupon they will act as crosslinkers for the OH polyesters. The advantage is that the products do not contain any blocking agent that needs to be removed. The resulting powder coat­ ing films combine high flexibility, good chemical resistance and very good weatherability. Other crosslinkers for OH polyesters for powder coatings are glycolurils. Tetramethoxy methyl glycoluril is a fully etherified amino resin [98], with an equivalent weight of 90 to 125 g/mol, and a softening temperature of 90 to 110 °C. Its structure is presented in Figure 5.15. The glycoluril is formed by making urea react with acetylene. It is methylolated with formaldehyde and etherified with methanol. The crosslinking reaction with OH polyesters is a transetherification reaction. It is necessary to catalyse the reaction with strong acids. During crosslinking, methanol is liberated, a fact which must be taken into consideration in applications with high film thickness. The resulting films are reasonably chemically resistant and weatherable. Suitable polyesters are the same as those chosen for combination with blocked polyisocyanate adducts.

Figure  5.15:  Structure of tetramethoxy methyl glycoluril

143

Saturated polyesters

5.8 Self-crosslinkable polyesters Until the 1950s, electrical insulation coatings mostly contained natural oils which dried by oxidative crosslinking (linseed oil, tung oil), and their derivatives, often in combination with phenol resins (resols). Demands for better heat stability and lasting flexibility led to the development of terephthalic polyesters capable of crosslinking by themselves. These polyesters have relatively low number-average molecular weights and contain terephthalic acid – which is more heat stable than isophthalic acid – and polyol combinations of glycerol and ethylene glycol. They contain a high excess of polyol. Owing to the limited solubility of such polyesters, the solvents chosen are cresols and xylenols. The solutions may contain high-boiling aromatic hydrocarbons by way of thinner. For the self-crosslinking reaction, highly effective transesterification catalysts are added (e.g. tetrabutyl titanate). The solutions prepared in this way serve as wire enamels. They are applied to mainly copper wires of different diameters by feeding through a dipping-trough (the excess material is stripped off by felts or nozzles) and the coated wires are fed for several times (up to 25 times) through a circulating air oven (air temperature greater than 400 °C). The self-crosslinking takes the form of a transesterification reaction. The equilibrium reaction liberates excess ethylene glycol. The composition tends to have the same content of carboxyl groups and OH groups in an ester group network. The result is a dense and extended molecular network consisting of terephthalic esters with glycerol and possibly residual ethylene glycol. The films, consisting of several thin layers, are resistant to chemicals, solvents and heat. They combine excellent flexibility with high hardness. In the 1960s, there were two developments in this resin class. While the other common triols failed to provide any benefits relative to glycerol, the use of the special triol trishydroxyethyl isocyanurate (THEIC) did confer significant advantages. The low volatility and the heat resistance (the isocyanurate cycle is as stable as an aromatic cycle) of this building block boosted the effective crosslinking and stability much more clearly than glycerol. The introduction of THEIC led to better heat resistance, hardness and solvent resistance. It became possible to use these coated wires in transformers which were cooled with fluorohydrocarbons (Frigen). Although the efficiently crosslinked films consist solely of terephthalic esters of trishydroxyethyl isocyanurate they are outstandingly flexible. It is possible to pull apart the 0.8 mm wire and then wind the ends about their own diameter, without any defects occurring in the coating layer. The reason for this property is the large extension of the molecular networks. The structure of the building blocks confers almost exclusively ideal elasticity, with virtually no plasticity. This fact is an example that highly crosslinked networks are not necessarily brittle. Differential scanning calorimetry (DSC) of the films reveals high glass-transition temperatures, a flat transition into the region of elastic behaviour, and high modulus levels in this region.

144

Self-crosslinkable polyesters Table 5.12:  Example of a polyesterimide for heat-resistant wire enamels n

building elements

M

m=n·M

wt. ‰

0.60

dimethyl terephthalate

194

116.4

233.2

0.67

trishydroxyethyl isocyanurate

262

174.87

350.2

0.67

ethylene glycol

62

41.54

83.2

0.80

trimellitic anhydride

192

153.6

307.6

0.40

diamino diphenyl methane

198

79.2

158.7

sum

565.61

1132.9

methanol

32

38.39

76.5

water

18

28.01

56.4

499.21

1000.0

yield (AV) polyester constant number-average molecular weight [g/mol] degree of branching [mol/kg] acid value [mg KOH/g] OH value [mg KOH/g]

k'M 1.3845 Mn 1298 v 1.34 AV 5.0 OHV 156.7

The second innovation was the introduction of building blocks possessing heterocyclic structures. The most important building block here is the product of the reaction of one mole of diaminodiphenyl methane and two moles of trimellitic anhydride to yield a diimide dicarboxylic acid intermediate (see Figure 5.16 for structure). The diimide dicarboxylic acid is formed in situ with the other resin components (terephthalic acid, THEIC and ethylene glycol). The trivial name of the product is polyesterimide (but imide polyester would be more correct). The polyols employed are THEIC and ethylene glycol. The imide-modified polyesters are applied in the above-mentioned way and form films by self-crosslinking under the influence of catalysts. The introduction of diimide dicarboxylic acid gives a substantial boost to heat stability. Table 5.12 shows a polyester which contains terephthalic acid and the aforementioned diimide dicarboxylic acid in a molar ratio of 6:4, and the corresponding stoichiometric quantity of trishydroxyethyl cyanurate (6.6 moles). Moreover, the polyester has the same content of ethylene glycol, yielding a total polyol excess of 1.34:1.00 [99]. For the preparation of this polyester example, dimethyl terephthalate is transesterified together with all the polyol in the presence of 0.1 wt.% tetrabutyl titanate as catalyst at 240 °C. The product of the reaction is then cooled and trimellitic anhydride and diaminodiphenyl methane are added in the corresponding molar ratios. At about 160 °C, spontaneous formation of diimide dicarboxylic acid occurs (addition reaction and cyclisation, with liberation of water). The non-soluble and unmelted diimide dicarboxylic acid reacts

145

Saturated polyesters at 240 °C at the particle surfaces to form the polyesterimide until the acid value reaches 5 mg KOH/g. The resulting imide-polyester has a number-average molecular weight of 1298 g/mol, a degree of branching of 1.34 and an OH value of 157 mg KOH/g. The binder is dissolved in cresol (isomeric mixture) to yield a 60 wt.% solution. The solution is thinned with xylenol and aromatic 150, and then a levelling additive (silicone oil) and further catalyst are added. The catalyst is responsible for the golden-brown colour of wire enamels. This colour is welcome, as it is deemed to be a sign of quality. The wire enamel just mentioned is applied repeatedly by the process mentioned earlier to wires of 0.8 mm diameter at a conveyor speed of 14 m/min. through a circulating air oven at 410 °C air temperature until the film thickness is 50 µm. The excess ethylene glycol is liberated, a fact which is confirmed by spectrometric analysis. The resulting network consists solely of dicarboxylic esters of trishydroxyethyl isocyanurate. The films show outstanding resistance combined with excellent flexibility (tear-and-wind stability after elongation of 25 length-%, equivalent overall to an elongation of 60 length-%). Naturally, the fact that the entire coating on wire consists of several thin layers makes a definite contribution to the outstanding flexibility. Other heterocyclic compounds can serve as building blocks for modifying this type of binder: diimide dicarboxylic acid from TMA and diaminodiphenyl oxide; imide dicarboxylic acid from TMA and p-amino benzoic acid; diimide dicarboxylic acid from pyromellitic anhydride and p-amino benzoic acid; dipyrazole dicarboxylic acid from TMA and terephthalic dihydrazide; and pyrrolidone dicarboxylic acid from itaconic acid and diamino diphenyl methane. As phenolic solvents are classified as toxic, trials are underway to find substitutes, e.g. dimethylformamide, n‑methyl pyrrolidone, and high-boiling glycol ethers. However, the alternatives are also physiologically dubious or are more expensive. Since the solvents of wire enamels are extracted in the circulating air oven and are combusted virtually to completion, producing added oven energy, there is no great pressure to replace the phenolic solvents (with the exception of the handling of paint materials). The imide-polyesters described still represent the state of the art in wire enamels. Currently, they are combined (the upper layers) with coating layers formed from polyimi-

Figure  5.16:  Structure of diimide dicarboxylic acid

146

Silicone polyesters des, polyamide imides or polyhydantoins (pseudoplastic compounds with very high softening temperatures). All the products mentioned are prepared by the wire-enamel manufacturers themselves [100] and are not available as separate products.

5.9 Silicone polyesters The reaction of highly pure silicon with alkyl and aryl chlorides (at 280 °C) over metal catalysts (zinc, tin) yields alkyl and aryl chlorosilanes which contain different degrees of alkyl or aryl substituents (mono-, di-, and tri-alkyl(aryl)chlorosilanes [Müller-Rochow synthesis]). The most important product is dimethyldichlorosilane, formed from silicon and two moles of methyl chloride. The various alkyl and aryl chlorosilanes are separated by distillation processes. Polymeric building blocks are prepared by hydrolysing the alkyl and aryl chlorosilanes to yield alkyl and aryl silanols containing different quantities of OH groups. Reaction with alcohols (e.g. methanol) generates the corresponding alkoxysilanes. On account of their amphoteric nature, silanols can react with themselves in a condensation reaction which produces polysiloxanes. By contrast, alkoxysilanes are stable when stored at ambient temperatures. However, in the presence of catalysts and water, polysiloxanes also

Figure  5.17:  Reactions in the preparation of polydimethylsiloxanes

147

Saturated polyesters form. Figure 5.17 shows an overview of the formation of polydimethylsiloxanes by the reactions just described. Hydrolysis of trichlorosilanes or tetrachlorosilanes leads to three-dimensional polymer networks. Unlike polymer compounds formed from carbon atoms, polysiloxanes form cage-like molecular networks akin to the structure of silica. These networks are resistant to acids, water, and weathering and very high temperatures. However, they are sensitive to strong alkalis and are not flexible. Before these compounds are utilised, they are combined with other binder components. The choice falls on low-molecular siloxane intermediates, which serve as modification products for other resins. Since the methyl siloxanes are incompatible with nearly all other binders, siloxanes bearing phenyl side chains are preferred. The structure of one such suitable oligosiloxane is presented in Figure 5.18, and which is formed from two moles of dimethoxydimethyl silane and one mole of trimethoxyphenyl silane. Such siloxane intermediates are combined with saturated polyesters. To compensate for the remaining compatibility problems and the effect of the low molecular weights, the intermediates are made to pre-react with OH groups of polyesters, yielding products known as silicone polyesters. Suitable polyesters preferably contain aromatic polycarboxylic acids (isophthalic acid and terephthalic acid), aliphatic diols, mainly those with alkyl side chains but also Figure 5.18:  Structure of siloxane intermediate from dimethoxydimethyl silane and trimethoxyphenyl silane cycloaliphatic diols. Branching is

Figure  5.19:  Crosslinking reactions of silicone polyesters

148

Silicone polyesters achieved with trimethylolpropane. They are relatively highly branched and have relatively low molecular weights. The modification takes place (180 °C) in the presence of catalysts (Lewis acids, such as organo-tin compounds or titanium esters). The content of siloxane intermediate is between 30 and 50 wt.% of total binder. The reaction takes place between the OH groups of the polyesters and the methoxy groups of the intermediates, with liberation of methanol. Naturally, molecular growth takes place. The reaction is stopped by cooling once a predetermined viscosity has been reached. Silicone polyesters are very light in colour; they are soluble in mixtures of esters, glycol ethers, and glycol ether esters with aromatic hydrocarbons. They are employed in stoving enamels. The crosslinking reaction requires elevated temperatures and the addition of further quantities of catalysts. Besides the reaction of OH groups with methoxy groups, the methoxy groups react with themselves to form Si-O-Si bridges, which are more stable than the Si-O-C linkages. Figure 5.19 shows the possible crosslinking reactions. Silicone polyesters are distinguished by excellent pigment-wetting and levelling properties. The films are resistant to discolouration and weathering, are hard and, to an extent depending on crosslinking density, also flexible. If they contain high quantities of siloxanes, they are very heat resistant (up to 250 °C). The siloxane moiety introduces hydrophobicity. The products can be expensive, in accordance with their siloxane content. Silicone polyesters are used for outdoor coil-coating systems for façade panels, for electronic equipment and heat-resistant coatings in industrial applications. Typical commercial products: Uralac SQ 871 (DSM [67]), Plusodur 261 (Mäder AG [88]).

149

Crosslinking of unsaturated polyesters

6 Unsaturated polyesters 6.1 Crosslinking of unsaturated polyesters As mentioned before, unsaturated polyesters are polyesters which have a certain content of unsaturated compounds bearing olefinic double bonds. This definition excludes polyesters containing aromatic double bonds or alkyd resins containing unsaturated fatty acids. In nearly all products, the olefinic component is a polycarboxylic acid or a derivative. And nearly all unsaturated polyesters contain maleic anhydride (MA). During preparation of unsaturated polyesters, most maleic anhydride is incorporated by forming fumaric esters. Fumaric acid is the trans-isomer of maleic acid (cis-ethylene dicarboxylic acid). Fumaric esters are formed predominantly because the molecule contains less energy than the cis-structure. Fumaric esters are susceptible to free-radical polymerisation and are polymerised together with co-monomers. The latter are reactive vinyl compounds, the most important of which is styrene. Combinations of unsaturated polyesters with other co-monomers (e.g. vinyl toluene or methacrylic esters) are employed but are of minor importance. The polymerisation reaction must be started by initiators: peroxide compounds. The initiator reaction consists in the decomposition of the peroxy group to form free-radicals. The most suitable peroxides are described in Figure 6.1. The reactivity of initiators is improved by redox reactions. The redox compounds, called accelerators, are either organic cobalt(II)salts or tertiary aromatic amines. As a rule, the cobalt compounds are combined with ketone peroxides (e.g. cyclohexanone peroxide,

Figure  6.1:  Peroxide initiators for crosslinking of unsaturated polyesters

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

151

Unsaturated polyesters methyl ethyl ketone peroxide). The reaction of organic cobalt(II)salts with peroxides is shown in Figure 6.2. The tertiary aromatic amines (e.g. N,N-dimethylaniline) are combined with acyl peroxides (e.g. benzoyl peroxide). The reaction of aromatic tertiary amines with acyl peroxides is shown in Figure 6.3. Where cobalt(II)salts are combined with ketone peroxides, the initiator reaction consists of the reduction of peroxide group to a hydroxyl anion and an alkoxy free-radical. The alkoxy free-radical adds across the double bond of the fumaric ester, producing an ether group and a new free-radical. This initiator reaction is shown in Figure 6.4. The new free-radical reacts preferentially with the co-monomer (e.g. styrene), and free-radical chain propagation commences. However, the reaction swiftly leads to recombination with other free-radical chains or free-radicals of fumaric esters. This reaction sequence is the most probable, because of the different polymerisation parameters for fumaric esters and styrene. The values in the e/Q chart of the polymerisation

Figure  6.2:  Reaction of organic cobalt (II) salts with peroxides

Figure  6.3:  Reaction of aromatic tertiary amines with acyl peroxides

152

Crosslinking of unsaturated polyesters (parameters for styrene are e = - 0.80 and Q = + 1.00 (these are the standard values for both) and for fumaric ester e = + 1.50 and Q = + 0.75. Q stands for the activation energy of the free-radical and e is the polarisation tendency of double bonds. The values shown are highly conducive to alternating co-polymerisation. No homopolymers are formed dur­ ing the crosslinking of unsaturated polyesters and styrene. Co-polymerisation leads to relatively dense but extended molecular networks. These are necessary for good chemical and solvent resistance. The extension and density of the molecular networks vary with the free-radical concentration and content of fumaric esters and the size or molecular weight of the unsaturated polyester. The free-radical concentration depends on the content of initiator, the content of accelerator and the temperature.

Figure  6.4:  Initiator reaction with unsaturated polyester

Figure  6.5:  Copolymerisation reaction

153

Unsaturated polyesters A particular advantage is that styrene can act as a solvent for the unsaturated polyester resin. However, the styrene is incorporated into the film matrix during co-polymerisation, so it acts as a reactive solvent. Thus, the content of volatile organic compounds is low, and the entire system is environmentally friendly as a result. This is furthermore the reason that the system can be applied to form high film thickness without problems (up to 500 µm). And, besides coating applications, the unsaturated polyesters are suitable for plastic parts and laminates (glass-fibre-reinforced plastic, assembly parts, and ships’ hulls). Unsaturated polyester systems containing a combination of acyl peroxides and tertiary aromatic amines can react at lower temperatures but suffer from the disadvantage of possible discolouration. These products are mainly used for putties and fillers. It is also possible to crosslink unsaturated polyesters with reactive monomers. In such cases, initiation can take place by adding cumene hydroperoxide and organic cobalt(II)salts or peresters (e.g. tert.-perisononanate) without accelerators. Products initiated in this way are relatively stable when stored at ambient temperatures. As a rule, the peroxides are not added until just before the system is ready to be applied. The accelerators are already incorporated during manufacture of the coating materials. The aforementioned aromatic tertiary amines may already have been added to the delivery form in the case of unsaturated polyesters. The pot-life of the finished material varies with the type of unsaturated polyester, the initiator and the accelerator. If large quantities of fully initiated unsaturated polyester remain after the application process, there is a risk that large amounts of heat will be generated (reaction enthalpy) which could lead to combustion. The pot-life of activated industrial quantities of UP systems can be compensated for in different ways. The first employs two-head curtain coating equipment, with two storage tanks. One contains UP resin and initiator, while the other contains UP resin and accelerator. The reaction does not start until the two components have been cast one on top of the other, whereupon both materials inter-diffuse. The second, known as the reactive-primer technique, entails applying a primer containing a binder which forms films by physical drying only and in addition a necessary quantity of peroxide. This is followed by the unsaturated polyester containing the accelerator. Here, too, crosslinking starts after a diffusion process has occurred. The crosslinking of unsaturated polyesters by co-polymerisation with styrene leads to a decrease in volume. This shrinkage is advantageous in the production of moulded parts, as they are easy to remove from the mould after crosslinking. However, it is a disadvantage in coatings application, because the shrinkage may generate loss of adhesion (particularly on bare metal). Unsaturated polyesters can be crosslinked by UV light (see Chapter 6.4). Inhalation of styrene may lead to irritation of skin and mucous membranes. It can impair procreative capacity and styrene is a presumed to be carcinogenic. The occupational

154

Unmodified unsaturated polyesters – “wax polyesters” exposure limit is 20 ml/m3. Replacements for it are being sought. Alternatives to styrene as a reactive solvent are vinyl toluene, tert.-butyl styrene, α-methyl styrene, maleic diesters of lower monoalcohols, and methacrylates. On account of their polymerisation parameters, vinyl esters react only at elevated temperatures. If polyfunctional reactive solvents serve as co-monomers, e.g. divinyl benzene or 1,4-butanediol dimethacrylate, networks of high crosslinking density are formed. However, owing to the poor performance of the alternatives, styrene is still in use, but naturally the necessary precautions must be adopted. Crosslinking of unsaturated polyesters by co-polymerisation is inhibited by atmospheric oxygen. It is believed that atmospheric oxygen, by virtue of its ability to form a bi-radical, becomes attached to the free-radicals on the growing polyester chains, as a result of which peroxide formation occurs, followed by recombination. The reaction constant for oxygen addition is much higher than for the co-polymerisation step, as presented in Figure 6.6. Crosslinking is then incomplete. The upper layers of the coating films become soft and tacky. However, there are different ways to avoid these effects.

6.2

 nmodified unsaturated polyesters – U “wax polyesters”

Unmodified unsaturated polyesters are prepared from combinations of maleic anhydride with other polycarboxylic acids, e.g. phthalic anhydride. Mainly they contain short-chain

Figure  6.6:  Oxygen inhibition of polymerisation

155

Unsaturated polyesters aliphatic diols, e.g. ethylene glycol, propylene glycol, 1,3-butanediol, or neopentyl glycol. The ratio of maleic anhydride to other polycarboxylic acids determines the solubility and crosslinking efficiency. Typically it is between 4 : 6 and 7 : 3. For more flexible coating films, the maleic anhydride is combined with adipic acid, or the polyesters contain longchain aliphatic diols, e.g. 1,4 butanediol, trimethyl 1,3-pentanediol, or ether diols, e.g. diethylene glycol, dipropylene glycol or higher polyether diols. Higher functional polyols are unsuitable or, if so, only in small amounts. The components for preparing unsaturated polyesters are esterified at relatively low temperatures (e.g. at 180 °C). The products are usually not formulated with an excess of polyols; the limitation on the number-average molecular weight is imposed by lower degrees of condensation. The acid values are high at 30 to 50 mg KOH/g. The reason for not using an excess of diols and higher functional polyols and for choosing relatively low esterification temperatures is to avoid secondary reactions. These secondary reactions take the form of premature polymerisation of double bonds and Michael addition by OH groups. The Michael reaction is the addition of hydrogen atoms of terminal OH groups to double bonds of fumaric esters. This gives rise to side-chains and the molecular weights increase significantly, as shown in Figure 6.7. Furthermore, there is the risk that polymerisation may start during dissolution of unsaturated polyesters in reactive solvents (styrene). GPC analysis always reveals higher molecular weights than those calculated. To prevent premature polymerisation, small quantities of inhibitors are added (hydroquinone, methyl hydroquinone). Some formulations (see patent examples) specify fumaric acid instead of maleic anhydride. The disadvantages here are that fumaric acid has a very high melting temperature

Figure  6.7:  Secondary reaction during production of unsaturated polyesters

156

Unmodified unsaturated polyesters – “wax polyesters” Table 6.1: Example of an unmodified, unsaturated polyester resin n = m/M

building element

M

m

m-‰

propylene glycol

76

790.0

437.3

3.797

pthalic anhydride

148

562.0

311.1

6.204

maleic anhydride

98

608.0

336.5

1960.0

1084.9

10.395

sum water

18

yield (AV = 46) polyester constant average molar mass [g/mol] acid value [mg KOH/g] OH value [mg KOH/g] viscosity (65 % in styrene) [mPa·s]

153.4

84.9

1806.6

1000.0

k'M 1.1874 964 Mn AV 46 OHV 70.4 η 1486

(287 °C, in a closed tube) and tends to undergo sublimation. Like isophthalic acid, fumaric acid is esterified only by reaction at particle interfaces. This leads to broader molecular weight distributions and higher solution viscosities. Given that maleic anhydride is converted into fumaric ester during esterification, there would appear to be no advantage to be gained from the use of fumaric acid. Commonly, unsaturated polyesters are dissolved at low temperatures, with addition of inhibitors. The quantity of styrene in the delivery form is between 30 and 50 wt.%. If an unsaturated polyester resin has an equivalent weight of about 300 g/mol, expressed in terms of the double bonds (see example in the following) and it is diluted 60 : 40 in styrene (molecular weight 104 g/mol), the molar ratio of double bonds to styrene is almost 1 : 2. This means there is one double bond in the polyester for every two molecules of styrene (see Figure 6.5). The following example [101] describes an unsaturated polyester resin (see Table 6.1) containing phthalic anhydride and maleic anhydride in the molar ratio of 62 : 38, made to react with propylene glycol in small molar excess, calculated on dicarboxylic acids (1.04 : 1.00). The esterification is carried out until the acid value is 48 mg KOH/g. The resin then has a number-average molecular weight of 964 g/mol (theoretical). As the polyester molecules contain 3.31 moles of fumaric ester on average, the corresponding equivalent weight is 291 g/mol. The unsaturated polyester resin of the example is dissolved in styrene to a concentration of 55 wt.% solids in solution, and 0.012 wt.% methyl hydroquinone is added by way of stabiliser. Such resins are suitable for high-quality coatings. However, ways are needed to prevent the inhibition by atmospheric oxygen to achieve optimum film properties on the surface.

157

Unsaturated polyesters Initially, building blocks were chosen for the polyester that would yield higher glass-transition temperatures (e.g. higher than 50 °C) by physical drying. The surface then appeared solid but had no adequate resistance. A more efficient method consists in protecting the surface against the influence of atmospheric oxygen by adding paraffin waxes or stearin. The additions vary from 0.1 to 2.0 wt.%, in accordance with the melt temperatures of the additives of 40 to 70 °C. Products that have higher melting temperatures require smaller additions. Some unsaturated polyesters already contain waxes in their delivery form. In other cases, the waxes are added during preparation of the coating material. The binders in this class are therefore known as “wax polyesters”. During curing of unsaturated polyesters and styrene, the waxes become incompatible with the film matrix (gel), they separate out and float on the coating surface. The wax film on the surface protects the upper film layer against penetration by air and so prevents inhibition by oxygen. Thus, the upper film layers, too, can crosslink without failure. There is another advantage of wax film: it reduces the evaporation losses of reactive solvent (styrene). However, the wax on the surface generates irregular, matte films. The coating layers must be re-treated by sanding and polishing. This yields films which are very hard and smooth, have high gloss, and with a bright appearance (filling power). The films have excellent resistance to chemicals, including household agents, such as fruit juices, dressings and drinks containing alcohol, and water. The UP resin films are not sufficiently weatherable. In the past, such systems were very popular for wooden furniture treated with closed-pore clearcoats and pigmented coatings, and flatting varnishes. However, such coatings then fell out of style and were gradually replaced by open-pore systems which highlight the natural texture of the wood. Such UP resin systems are now only used for special applications, among them coatings for pianos. The physical properties of UP resin coatings intensify the acoustic timbre of these instruments. The systems are also used for automotive dashboards. However, dashboards are now mainly coated with aqueous polyurethane dispersions. UP resins of this type are also used for highly-filled putties, mainly for automotive repair coating systems, but in that event without wax, because here the inhibition effect of atmospheric oxygen plays only a minor role. The UP resins in the class below are also suitable for such putties. Typical commercial products: Palatal P 6 (BASF [65]), Roskydal E 65 (Allnex-Nuplex [76]), Sacopal L 100 w (Krems-Chemie [102])

6.3

“Gloss polyesters”

There is also a way to modify the molecular structure of unsaturated polyester resins with the purpose of avoiding or minimising the inhibitory effect of atmospheric oxygen. This approach utilises special compounds which compete against the free-radicals for the oxygen

158

“Gloss polyesters” needed for polymerisation. Such compounds are described in numerous patents dating from the 1960s. They include cycloaliphatic compounds which mainly contain olefinic double bonds and, more important, compounds which contain allyl ethers. Suitable cycloaliphatic compounds for the polyols are perhydrobisphenol A (2,2-[4,4'-bishydroxy-cyclohexyl]-propane), and tricyclododecane dimethanol (TCD alcohol DM) and, for the polycarboxylic acids, tetrahydrophthalic anhydride (THPA), tricyclodecane dicarboxylic acid (TCD dicarboxylic acid) and endomethylene tetrahydrophthalic anhydride. Suitable allyl compounds are monoallyl ethers of glycerol and trimethylolpropane and diallyl ether of glycerol, trimethylolpropane and pentaerythritol. Naturally, the allyl compounds, containing only one OH group, form the terminal groups of the polyester chains. The structures of the most important compounds are given in Figure 6.8. It is believed that the activated CH2 groups in the vicinity of the double bond and, in the case of allyl ethers, in the vicinity of ether oxygen (the allyl position) react with atmospheric oxygen to form hydroperoxides. These hydroperoxides can form free-radicals which can then take part in polymerisation or enter into recombination reactions. Both reactions prevent the oxygen from inhibiting the actual co-polymerisation reaction. The formation of free-radicals of oxygen adducts requires the use of redox accelerators, which must be added in sufficient quantity, e.g. organic cobalt(II)salts. The possible reactions are described in Figure 6.9. The process is comparable to the oxidative curing of alkyd resins containing multiple unsaturated fatty acids (e.g. linoleic acid; see Chapter 7.2.1) Unsaturated polyesters containing such building blocks co-crosslink with styrene (or alternative co-monomers) to immediately yield hard and glossy film surfaces. To differentiate these systems from “wax polyesters”, they are called “gloss polyesters”. The measured gloss values, among others, are high, due to the high refractive index of styrene. There is another building block which can suppress the inhibitory effect of oxygen, namely 2,2,4-trimethyl-1,3-pentanediol. For the pre- Figure  6.8:  Building blocks for avoiding inhibition paration of saturated polyesters this by oxygen

159

Unsaturated polyesters

Table 6.2: Example of a "gloss polyester” n

building element

M

n·M=m

wt. ‰

0.45

propylene glykol

76

34.20

140.6

0.40

neopentyl glykol

104

41.60

171.1

0.60

maleic anhydride

98

58.80

241.8

0.40

tetrahydrophthalic anhydride

152

60.80

250.0

0.30

trimethylolpropane dially ether

214

64.20

264.0

259.60

1067.6

16.44

67.6

243.16

1000.0

sum water yield (AV = 20)

18

polyester constant k’M 1.2367 average molar mass [g/mol] Mn 1027 acid value [mg KOH/g] AV 20 OH value [mg KOH/g] OHV 20

diol has the disadvantage of eliminating water to form an unsaturated mono-alcohol (as a chain-end). However, the diol is advantageous for unsaturated polyesters, because it can help to avoid inhibition by oxygen, in a manner akin to that of allyl ether compounds. Table 6.2 [103] describes an unsaturated polyester resin containing maleic anhydride (0.60 moles), tetrahydrophthalic anhydride (0.40 moles), propylene glycol (0.45 moles), neopentyl glycol (0.40 moles), and trimethylolpropane diallyl ether (0.30 moles) to form the termination of the polyester molecules (on average 1.3 moles per mole of polyester). With an acid value of 20 mg KOH/g, the polyester has a number-average molecular weight of 1027 g/mol. The equivalent weight, expressed in terms of double bonds, is 253.3 g/mol. Such binders are suitable for wood coatings which are notable for their high gloss, high hardness, and excellent resistance to chemicals and solvents. They are also used for pigmented coatings and putties.

Figure  6.9:  Reaction for avoiding inhibition by oxygen

160

UV crosslinking of unsaturated polyesters Typical commercial products: Roskydal 500 grades (Allnex-Nuplex [76]), Sacoval L 300 T (Krems-Chemie [102]) Unsaturated polyesters which, besides maleic anhydride and tetrahydrophthalic anhydride and other cycloaliphatic polycarboxylic acids, contain significant quantities of long-chain aliphatic diols, e.g. 1,4-butanediol, 1,5-pentanediol as well as 2,2,4-trimethyl 1,3-pentanediol, and ether diols such as diethylene and propylene glycol, and higher polyether diols, are ideal for putties and fillers. Such materials are mainly employed in automotive repair systems. They cure with­ out problems in thicker film layers and can be sanded and reworked after a short time. The cycloaliphatic compounds generate good adhesion properties. These unsaturated polyester resins also cure well in thick layers. The films are adequately flexible and can be sanded and re-worked after a short time. They impart excellent adhesion properties. Particularly good adhesion is provided by alkoxylated bisphenol A as polyol component. In addition, such products are also relatively resistant to heat. This an important condition for the formulation of primers which are combined with stoving primer surfacers and stoving topcoats. Primers for use at ambient temperatures contain a combination of acyl peroxides and tertiary aromatic amines as this combination confers reactivity at low temperatures (around 0 °C). While the unsaturated esters already contain the amines in the delivery form, the acyl peroxides are added mainly in the form of dispersions in plasticisers (pastes in collapsible tubes) just before the putty is used. As the pot-life of the finished product is very short, in manual application only small amounts are prepared, by mixing the putty materials with peroxide hardener. Typical commercial products: Roskydal E grades (Allnex-Nuplex [62])

6.4 UV crosslinking of unsaturated polyesters Back in 1955, trials commenced on the crosslinking of unsaturated polyesters by UV light [104]. As here also atmospheric oxygen was observed to inhibit surface curing, the process was not taken up again until the development of gloss polyesters containing the above-mentioned building blocks (see Chapter 6.3). The UV crosslinking is started by UV initiators. UV light has wavelengths between 280 and 400 nm. The initiators absorb UV light of different wavelengths to form free-radicals. These initiate free-radical chain polymerisation at the double bonds of fumaric esters, as well as at the double bonds of co-monomers. UV initiators can decompose into free-radicals in the presence of high-energy light, due to steric effects in their molecules. The most suitable products are benzophenone, benzoin ethers, benzilketals, β-hydroxyalkyl phenyl ketones, β-aminoalkyl phenol ketones

161

Unsaturated polyesters and benzoyl phosphine oxides [105]. For UV clearcoats, the use of between 2.5 and 3.5 wt.% (calculated on polyester resin) of a β-hydroxyalkyl phenyl ketone has been recommended, with between 2.5 and 3.5 wt.% (calculated on polyester resin) of a benzoyl phosphine oxide recommended for pigmented systems [106]. Benzoyl phosphine oxides are suitable for pigmented systems because they absorb UV wavelengths up to 450 nm, a region in which, e.g., titanium dioxide is still partly transparent to this light. Films of unsaturated polyesters (gloss polyesters) cured by UV light contain extended molecular networks, which lead to high hardness, good flexibility, high gloss values, and excellent resistance to chemicals and humidity. They are not particularly weatherable. Unsaturated polyesters for UV curing can be combined with alternative co-monomers, instead of styrene. These are primarily acrylic esters (monofunctional and polyfunctional esters), e.g. hexanediol diacrylate or trimethylolpropane triacrylate. If unsaturated polyesters contain sufficient numbers of double bonds, it is possible to cure them with UV light, without co-monomers. Such resins are delivered as 100 % products or diluted with solvents (mainly esters, e.g. butyl acetate). Example of a commercial product: Roskydal 502 BA (Allnex-Nuplex [78]) As UV coatings are deemed to be environmentally friendly and the films are of premium quality, the systems’ range of applications is being greatly expanded. Their market share is projected to keep growing in the future. However, in this market the unsaturated polyester resins play only a minor role. The reason is that the double bonds of acrylic esters are much more reactive than the double bonds of fumaric esters. Consequently, UV crosslinking is dominated by binders containing acrylic esters, e.g. polyester acrylates (see Chapter 5.4.3). However, there are polyester acrylates which contain an unsaturated polyester resin segment and terminal acrylic esters.

6.5 Other unsaturated polyesters If unsaturated polyesters (UPs) prepared from maleic anhydride and other dicarboxylic acids contain significant quantities of polyethylene glycol, they can be made soluble in water. Such water-tolerant binders can also be used for UV curing. It is possible to prepare high-molecular weight polyesters which are thinned with water to convert them into a form suitable for application. However, unlike other UV systems, the water has to be evaporated (which consumes energy). As UV crosslinking is highly efficient, the hydrophilic behaviour of some of the building blocks is not a major problem. If UP resins are processed without the use of styrene, they offer excellent adhesion properties. This applies particularly to UP resins containing cycloaliphatic building blocks. These building blocks were first used for formulating polyesters which surface-cure without inhibition by atmospheric oxygen. Thereafter, unsaturated polyesters containing such build­

162

Other unsaturated polyesters ing blocks (tetrahydrophthalic anhydride, endomethylene tetrahydrophthalic anhydride) were formulated for putties and fillers. Finally, it was discovered that polyesters of such composition also support adhesion in other coating systems that cannot be cured by polymerisation of double bonds. These are expressly called adhesive resins. They consist of maleic anhydride in combination with phthalic anhydride (hard adhesive resins) or with adipic acid (soft adhesive resins) and of tricyclodecane dimethanol (TCD alcohol DM). They serve as additives for can-coating and coil-coating systems, where they improve adhesion. Typical commercial products: Tego Add-Bond LTH, LTW (Evonik [107])

163

Classification of alkyd resins

7 Alkyd resins 7.1

Classification of alkyd resins

Traditionally, alkyd resins have been classified on the basis of their oil content, which is known as oil length. This applies not only to alkyd resins which are prepared direct from oil by transesterification, but also to those made from fatty acids and to those which do not even contain glycerol by way of polyol. In the latter cases, the fatty acid content may be quoted. The factor for converting oil content into fatty acid content in a system comprising C18 fatty acids is: Equation 7.1

This conversion factor is so small that it makes little difference whether such alkyd resins are classified by their oil length or their fatty acid content. The classification given in DIN 55945 is [108]: – long-oil alkyd resins: ≥ 60 wt.% oil – medium-oil alkyd resins: 40 – 60 wt.% oil – short-oil alkyd resins: ≤ 40 wt.% oil Although the content of oil or fatty acids is important, especially for oxidative-cure alkyd resins, the classification given below will be based on the molecular structure of the alkyd resins and their film-forming mechanism. Thus, it makes little sense to define the oil content for alkyd resins that contain synthetic fatty acids or benzoic acids. The classification adopted here is: – alkyd resins which crosslink/cure oxidatively (and several modifications) – alkyd resins for co-crosslinking (with different crosslinkers) – alkyd resins as plasticiser resins (combinations with polymers which yield films by physical drying) The various types are presented in the form of molecular structures and are illustrated with model formulations.

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

165

Alkyd resins

7.2 Alkyd resins for oxidative crosslinking 7.2.1 Crosslinking reactions Alkyd resins which cure oxidatively mainly contain fatty acid chains bearing two or more double bonds per chain. These are primarily fatty acids containing isolated double bonds, i.e. 9,12-linoleic acid and 9,12,15-linolenic acid, the molecular structures of which are shown in Figure 7.1. The most important natural sources of 9,12-linoleic acid are cottonseed oil (33 to 45 wt.%), soybean oil (35 to 58 wt.%), sunflower oil (53 to 58 wt.%) safflower oil (58 to 68 wt.%) and tall oil fatty acids (58 to 68 wt.%). 9,12,15-linolenic acid is the main component of linseed oil (35 to 60 wt.% plus 17 to 24 wt.% 9,12-linoleic acid). Some fish oils, such as herring, menhaden, and sardines, also contain fatty acids bearing two or more isolated double bonds; the fatty acids contain 20 to 22 C atoms. Historically, it was observed that the curing of drying oils (e.g. linseed oil) involved oxidation and solidification of the liquid oils. Hence, the process was called oxidative drying. However, this is not a physical drying process – it is a chemical crosslinking reaction which involves atmospheric oxygen, and so it is more correct to use the term oxidative curing or crosslinking. Starting reaction Oxidative curing starts with the addition of atmospheric oxygen to the CH2 group in the vicinity of the two isolated double bonds (position 11 in linoleic acid and 11 and 14 in linolenic acid). The hydrogen atoms of these CH2 groups are activated by the double bonds (double allyl position). As molecular oxygen is capable of forming bi-radicals, it is added, with the result that hydroperoxide groups are generated (see also Chapter 6.1 and Figure 6.9). The reaction takes place spontaneously and can be supported by complex formers. Elevated temperatures provide only minor acceleration.

Figure 7.1:  Molecular structures of 9,12-linoleic acid and 9,12,15-linolenic acid

166

Alkyd resins for oxidative crosslinking Free-radical formation Hydroperoxides can decompose into free-radicals. This reaction is accelerated by adding catalysts called siccatives or driers. Siccatives Siccatives or driers are organic salts of metals in several oxidation states. The best-known metals are cobalt (2+, 3+), manganese (1+, 2+, 3+, 4+, 6+, 7+) and lead (2+, 4+). The multivalent metal cations induce redox reactions (see Figure 6.2). Lead has long been classified as toxic and has already been replaced by alternatives, chief among them zirconium (2+, 4+). Cobalt is contained in vitamin B12, which is essential for humans, who have a daily cobalt requirement is 0.1 µg, but it is actually also classified as harmful to health. The lethal dose for cobalt LD50 (rat, oral) is 6170 mg/kg, a figure which is very high. However, cobalt can induce cardiomyopathy and large quantities (25 mg/d) can give rise to diseases of other organs and also cancer. For handling of cobalt siccatives, the occupational exposure limit has been set at 0.1 mg/m³ [109]. However, as cobalt is the most efficient siccative it is very difficult to find a replacement. Some approaches use manganese (1+, 2+, 3+, 4+, 6+), iron (2+, 3+), vanadium (2+, 3+, 4+, 5+) and cerium (3+, 4+). However, unfortunately the cations of manganese and iron are strongly coloured so that it is difficult to prepare white topcoats and clearcoats without discolouration. The solution may be to choose quantities of these metal cations which are small enough to not generate colour and to combine them with accelerators. (Cobalt, too, can lead to discolouration [Co2+ is magenta, Co3+ is green], but one of the advantages of cobalt is that it is highly efficient in very small quantities). Thus far, it is permissible to add cobalt in quantities below the labelling limit, mainly in combination with other cations (e.g. of zirconium). It has also been observed that those metal cations which can have only one oxidation state provide substantial support for curing. These include organic salts of zinc, calcium, magnesium, and barium which act as accelerators (co-siccatives). They support the metals via their redox potential but are suitable only in small quantities. The reason is that these cations can form complexes with a system containing two isolated double bonds and hydroperoxides. The overall effect of all metal cations is dependent on their ability to form complexes with the double bonds and hydroperoxide structures (synergy effect). As the various metal cations act differently, they are combined to ensure that curing proceeds uniformly through the full coating layer (efficient through-drying). The co-siccatives can further act as wetting agents (for pigments and substrates), mainly organic salts of zinc and calcium. Organic complex formers too can support the formation of free-radicals [110], a fact which allows the metal content to be lowered. It is important to use precise quantities of siccatives and co-siccatives, because the number of free-radicals generated plays a key role in the subsequent free-radical polymerisation

167

Alkyd resins process. Too much siccative should be avoided, as high concentrations of free-radicals can trigger premature recombination reactions and lead to a lower crosslinking density. In addition, too much siccative may induce drying in the upper film layer only, preventing penetration of oxygen into the lower layers (less through-drying). However, this effect can be put to good use. The upper layers of coatings (mainly those with alkyd resins containing fatty acids bearing conjugated double bonds) which undergo oxidative curing can crosslink very rapidly if high quantities of siccative are added. This may be accompanied by variations in density to yield larger volumes. This creates scope for forming textured surfaces of the kind found in wrinkle varnishes. The optimum amount of siccative to add is specified in the technical data sheets issued by the resin producer and the siccative manufacturer [111]. For example, consider a long-oil, oxidative-cure alkyd [112] which is recommended for a white topcoat that does not require labelling [113]. The amount to add (calculated on solid resin) might be about 0.80 wt.% of a siccative containing small quantities of cobalt in combination with zirconium, 1.30 wt.% of a zirconium siccative (containing 12 % Zr), including about 1.30 wt.% of an organic calcium salt (10 % Ca) or, for a cobalt-free version, 1.30 wt.% of a zirconium siccative (12 % Zr), and 0.40 wt.% of a manganese siccative (12.0 % Mn, which additionally contains about 15 wt.% of an organic drying accelerator [114]) including about 1.30 wt.% of organic calcium salt (10 % Ca). In earlier times, the siccative salts contained 9,12-linoleic acid (linolates), e.g. of tall oil fatty acids, soybean fatty acids, and linseed fatty acids (linoleates) and the acids of natural rosin (resinates). In the interim, naphthenic acids are used. These acids are mixtures of alkylated cyclopentane and cyclohexane compounds. Currently still in use are siccatives containing 2-ethyl hexanoic acid as anion (iso-octoates). Now that 2-ethylhexanoic acid has been classified as harmful to health, it has been partly replaced by isononanoic acid and neodecanoic acid. These synthetic fatty acids yield siccatives of more homogeneous composition and no discolouration. The main issue for the organic salts is effective distribution in oil phase – i.e. their oleophilic behaviour. Free-radical chain polymerisation Oxo free-radicals, formed from hydroperoxides in the presence of redox accelerators (siccatives), can initiate free-radical chain polymerisation with the double bonds of fatty acids. The chain propagates by means of free-radical transfer from one double bond to the next. The outcome is molecular networks containing C–O–C and C–C bridges. Chain propagation is interrupted when two free-radicals recombine. The extent of the molecular networks formed in this way depends on the concentration of free-radicals. High concentrations lead to shorter polymer chains and faster recombination: the reaction time is short. Lower concentrations lead to fewer, but longer polymer chains, later recombination and a much longer overall reaction time. Thus, the quantity of added siccatives must be matched to the

168

Alkyd resins for oxidative crosslinking

Figure 7.2:  Possible reactions in the oxidative-cure process

169

Alkyd resins type of resin (quantity of fatty acids and their double bonds) and the application requirements. There is not only a lower limit but also an upper limit on the dosage of siccatives. In the past, numerous trials sought to elucidate the various reaction pathways in oxidative curing [115]. But this is unnecessary for our purposes. It is sufficient to know that molecular networks of different extents are formed which contain C–C and C–O–C bridges. How­ ever, it is highly likely that some double bonds remain in the film network. Possible reactions steps in the oxidative-cure process are shown in Figure 7.2. Influence of the type of double bond Initiation of oxidative curing is linked to the CH2 group in the vicinity of isolated double bonds. However, during chain propagation, conjugated double bonds may also be involved. It is also possible that during polymerisation the isolated double bonds are transformed into isomeric conjugated double bonds [116]. Those double bonds undergo 1,4-polymerisation, a reaction which leads to energy gain (negative reaction enthalpy). A combination of specific quantities of isolated and conjugated double bonds therefore represents the optimum for oxidative curing. The resulting networks are distinguished by greater extension, which is essential for high flexibility and sufficient durability. Alkyd resins with a substantial quantity of fatty acids bearing conjugated double bonds show less oxygen absorption than those containing isolated double bonds only. The reason is the preferred 1,4-polymerisation reaction. Empirical studies in the past looked at combining isolated and conjugated double bonds in alkyd resins. These resins contained fatty acids from linseed oil in combination with tung oil (the latter contains the conjugated 9,11,13-octadecatriene acid [α-eleostearic acid]). Such alkyds were notable for having the best drying properties (short through-drying times) relative to other alkyd resins. Nowadays medium-oil alkyds are prepared from combinations of fatty acids rich in 9,12-linoleic acids and specific quantities of 9,11-linoleic acids [117], which confer efficient drying properties. Besides resorting to a natural base of fatty acids containing conjugated double bonds in tung oil (α-eleostearic acid) and oiticica oil (containing licanic acid, 4-keto-9,11,

Figure 7.3:  Structures of 9,11-linoleic acid

170

Alkyd resins for oxidative crosslinking 13-octadecatrienoic acid), conjugated double bonds are prepared by isomerisation of 9,12-linoleic acid and by dehydration of 12-hydroxy-octadeca-9-ene acid (ricinoleic acid from castor oil). The isomers of the resulting 9,11-linoleic acid are shown in Figure 7.3. Normally there is little scope for accelerating oxidative curing by employing elevated temperatures. However, these do boost the polymerisation of conjugated double bonds. Alkyd resins exist which contain fatty acids bearing conjugated double bonds and which are intended for coating systems which cure at elevated temperatures (“80 °C enamels”, see Chapter 7.3.1). Anti-skinning agents When coating systems containing oxidative-cure alkyds are stored, air can diffuse into the upper layers of the coating material and the atmospheric oxygen can trigger the curing reaction. This leads to the formation of a more or less thick skin which is no longer soluble. Great effort is required to remove the skin, some of the binder is lost, and residual specks can cause film blemishes during application. This can be avoided by adding anti-skinning agents. There are two classes of anti-skinning agents: phenols and ketoximes. Phenols act as reducing agents for the hydroperoxides which are formed by absorption of atmospheric oxygen. As they are not volatile, they delay the crosslinking reaction until their reduction potential is exhausted. Ketoximes, too, can react as reducing agents, but their primary effect is complexation of siccatives, thereby suppressing their redox potential. As ketoximes (e.g. methyl ethyl ketoxime) are volatile, curing reactions start just after application. However, phenols as well as ketoximes are now classified as harmful to health and substitutes must be found. Various recommendations have been made on replacements (e.g. the use of hydroxylamine components) [118]. Decomposition reactions Residual double bonds in films formed by oxidative-cure alkyd resins and the active CHand CH2- groups still present are the reason that the aforementioned reaction can continue during aging, primarily under the influence of sunlight. In particular, the C–O–C bridges can react to form carbonyl and carboxyl groups. Films of alkyd resins are therefore not particularly weatherable. The failure modes are yellowing and embrittlement. It was once believed that aging is accompanied by post-curing, but the embrittlement and yellowing are induced solely by decomposition. Such decomposition reactions are observed especially when the fatty acids of alkyd resin have a high content of active CH2- groups, a condition satisfied by resins containing 9,12,15-linolenic acid (based on linseed oil). Films of alkyd resins which do not contain linolenic acid, but rather 9,12-linoleic acid or 9,11-linoleic acid, are much more weatherable. However, they do not achieve the weatherability of alkyds co-crosslinked with amino resins or polyisocyanates (see Chapter 7.3.1 and 7.3.3).

171

Alkyd resins

7.2.2

Long-oil alkyd resins for oxidative crosslinking

Given that oxidative curing is a quite slow reaction, maximising the number of double bonds will make for more efficient crosslinking. High contents of double bonds in unsaturated fatty acids are attainable if the polyester backbone of the alkyd resins contain a high excess of OH groups which can be occupied by fatty acids. Accordingly, long-oil alkyds, with a high fatty acid content, are prepared with substantial quantities of pentaerythritol by way of polyol. Alkyd resins based on glycerol, with the same general structure and the same OH-modification percentage, contain only 53.7 wt.% fatty acid compared with 66.7 wt.% for the pentaerythritol types. In addition, the content of 9,12-linoleic acid in industrially available fatty acids is crucial to crosslinking efficiency and increases in the following order: cottonseed fatty acid, soybean fatty acid, sunflower fatty acid, tall oil fatty acid, safflower fatty acid. The sources of these fatty acids are semi-drying oils (exception: tall oil is a by-product of the preparation of cellulose from wood). Semi-drying oils are incapable of forming durable films by themselves. However, if the fatty acids are incorporated into alkyd molecules, the products will cure efficiently (high molecular weights, more double bonds per binder molecule). Much better curing properties are obtained with fatty acids containing 9,12,15-linolenic acid sourced from linseed oil, because two active CH2groups are present which are capable of adding oxygen. However, this advantage is also a disadvantage. Oxygen absorption continues during aging, and decomposition reactions occur, mainly in sunlight. The outcome Figure 7.4:  Structure of a molecular segment of is yellowing and embrittlement. a long-oil alkyd resin based on fatty acid

172

Alkyd resins for oxidative crosslinking Table 7.1:  Model example of a long-oil alkyd resin based on fatty acids pos.

n

building element

M

1

0.970

pentaerythritol

136

131.92

193.1

2

1.000

phthalic anhydride

148

148.00

216.6

3

1.600

technical linoleic acid

280

448.00

655.6

727.92

1065.3

sum 2.478

water

18

yield (AV) acid value polycondensation constant number-average molecular weight (calc.) average number of structural units degree of branching occupation of excess OH groups OH value

AV k' M q v b' OHV

n·M

wt.‰

– 44.61

– 65.3

683.31

1000.0

10.0 mg KOH/g 1.0918 7445 g/mol 10.9 0.50 mol/kg 0.80 33.0 mg KOH/g

When long-oil alkyds are being prepared, the residual OH groups on the polyester backbone of the alkyd resin can be occupied by up to 80 mole-% and more (b' values). However, high modification values harbour the risk that some free fatty acid will remain, which leads to film defects. Figure 7.4 shows a molecular segment of a long-oil alkyd prepared from phthalic anhydride, pentaerythritol and fatty acid. As stated previously (Chapter 3.5.3), a high modification percentage of the excess OH groups offers the possibility of high number-average molecular weights. This is an important advantage of the oxidative-cure reaction. Starting with high molecular weights, it is only a small step to achieving optimum crosslinking and film properties. This compensates again for the slow curing by atmospheric oxygen. Oxidative-cure alkyds have number-average molecular weights of 5,000 to 9,000 g/mol. The limitation on molecular weights for long-oil alkyds is not determined by the molar excess of polyol, but by the degree of condensation (residual acid groups). A typical example of a long-oil alkyd resin, based on fatty acids, is given in Table 7.1. Such alkyd resins are produced in a one-step process at 220 to 240 °C. To avoid sublimation of phthalic anhydride, a reflux solvent (e.g. 3 wt.% of xylene, calculated on resin yield) is added. Reaction water is distilled off and isolated in a water separator. Progress is monitored by measuring the acid values and viscosity (of the melt or of a test solution). When the target values are achieved, the process is stopped by cooling. Then the resin is converted into a delivery form (solution). An alternative production method is transesterification. Preparation starts with transesterification of oil (triglyceride) and polyol in excess. At chemical equilibrium, the fatty acids are distributed across all polyol molecules, yielding mono- and di-esters. The transesterification

173

Alkyd resins Table 7.2:  Model formulation of a long-oil alkyd resin based on soybean oil pos.

n

1

0.400

building element soybean oil

M

n·M

wt. ‰

878

351.20

631.7

2

0.530

pentaerythritol

136

72.08

129.7

3

1.000

phthalic anhydride

148

148.00

266.2

571.28

1027.6

sum 2.478

water

18

yield (AV) acid value polycondensation constant number-average molecular weight (calc.) average number of structural units degree of branching occupation of excess OH groups OH value

AV k' M q v b' OHV

– 18.00

– 32.4

555.96

1000.0

15.0 mg KOH/g 1.0786 7071 g/mol 12.7 0.47 mol/kg 0.82 27.1 mg KOH/g

reaction is carried out at 240 to 260 °C in the presence of a catalyst, the most common of which are organic salts of lithium (stearate, octoate solution). Lithium is not only an effective catalyst, but the salts have the additionally benefit of possessing excellent solubility in alkyd solutions – no separation or turbidity occurs. Sufficient distribution of the fatty acids is checked by solubility tests in alcohol (methanol, ethanol). Once a sample demonstrates sufficient solubility in alcohol, the remaining phthalic anhydride of the alkyd resin formulation is added, and production of alkyd resin is finished under the same conditions as in the first process. A typical structural building block of a long-oil soybean alkyd prepared by transesterification is presented in Figure 7.5 and a corresponding model formulation is described in Table 7.2. By virtue of the high content of aliphatic fatty acid chains, the long-oil alkyd resins are non-polar and therefore readily soluble in non-polar solvents. All kinds of hydrocarbons are suitable (aliphatic hydrocarbons, aromatic hydrocarbons, and terpene hydrocarbons). As aromatic hydrocarbons and also some terpene hydrocarbons are considered harmful to health, aliphatic hydrocarbons are favoured. There are aliphatic hydrocarbons (white spirits, based on mineral oil) which are free of aromatic compounds, iso-paraffins, which are prepared synthetically, and hydrogenated hydrocarbons, which are prepared from white spirits containing aromatic compounds which are transformed into cyclohexane derivatives. Although the thinning effect of pure aliphatic hydrocarbons is relatively low, the viscosities of the long-oil alkyd solutions are not high. The reason is that, although the alkyd resins have high molecular weights, they are still liquids, a fact which leads to relatively low solution viscosities. Another advantage is that the pure aliphatic hydrocarbons have only a mild odour.

174

Alkyd resins for oxidative crosslinking These alkyd resins are naturally also soluble in esters, glycol ethers, glycol ether esters and higher alcohols (and in ketones and halogenated hydrocarbons). If polar solvents are added to the solutions of the alkyd resins in aliphatic hydrocarbons (e.g. a few wt.% of n-butanol), the viscosity of solution drops rapidly. The reason is that the small quantities of polar solvents extract hydrocarbon solvent molecules from colloidal coils (solvates) into the outer phase. The solvates shrink, the solvent content in the outer phase increases, and then the viscosity drops rapidly. The most important application fields for long-oil oxidative-cure alkyd resins are house paints, which are mainly applied manually (by do-it-yourself enthusiasts and tradesmen). Mostly wooden substrates are painted (e.g. windows, furniture). Pigmented systems and also glazes are available. The alkyd resins are distinguished by excellent wetting properties for pigments and for surfaces (high penetration rate on wooden surfaces). The initial drying time is relatively

Figure 7.5:  Structure of a model alkyd prepared by transesterification

175

Alkyd resins long. However, that can prove an advantage in this application field. The long open time offers good brushability, the possibility of reworking coating layers and confers good de-aeration properties. Long-oil alkyd resins have excellent through-drying. With appropriate siccatives, the drying time is about 16 hours (“overnight”). As the alkyd resins contain solely 9,12-linoleic acid or 9,11-linoleic acid – but no 9,12,15-linolenic acid – they are sufficiently weatherable and have no tendency to yellow. A white house paint based on a long-oil alkyd resin usually has a solids content of 65 wt.% and more at its application viscosity. Efforts are underway to lower the solvent content, mainly for do-it-yourself systems, and corresponding regulations have been put in place to reduce the content of volatile organic compounds (VOC regulations). To achieve higher application solids, so called high-solids binders have been developed (see Chapter 7.2.5). In addition, water-borne alkyd systems are available which make do with a lower solvent content (see Chapter 7.2.10). In these cases, the binders for water-borne house paints contain primary acrylic dispersions, which are distinguished by high saponification resistance. Extensive development work has yielded dense, glossy films with special primary acrylic dispersions. However, the solvent-borne alkyd resins are still the best choice for these application fields, as they have excellent penetrating power and workability. Overall, though, the market share for this type of house paint is declining. For example, wooden window frames are increasingly being replaced by plastic (PVC) window frames. Typical commercial products: Setal AF 681, Setal 62 (Allnex-Nuplex [76]), Synolac 6166 (Arkema-Cray Valley [77]), Synthalat SF 653 (Synthopol [80]), Uralac AD 96 (DSM [67]), Worléekyd B 870 (Worlée [78]).

7.2.3 Medium- and short-oil alkyd resins for oxidative crosslinking Even though the long “open-time” of long-oil alkyds is an advantage for house paint, it is a disadvantage for other application fields that require faster initial drying. The reason for the low softening point temperatures of long-oil alkyds is of course the high content of fatty acids with their long aliphatic side-chains. Lowering the content of fatty acid therefore seems to be an obvious step. However, to do so would decrease the modification percentage of the excess OH groups (b' value). Lower b' values widen the molecular weight distribution and harbour the risk of gelling. It would then be necessary to increase the molar polyol excess or to decrease the degree of condensation – and thus to lower the molecular weight – to avoid premature gelling. Both lowering the fatty acid content (lower quantity of reactive double bonds) and reducing the molecular weight impair the film forming properties. There is a way to compensate for this. If some of the fatty acids are replaced by aromatic monocarboxylic

176

Alkyd resins for oxidative crosslinking acids (benzoic acids [119]) the content of long aliphatic chains is decreased and the content of aromatic building blocks, which confer hardness and initial drying properties, is increased. It is then possible to retain the molecular structure of alkyds, i.e. the degree to which the excess OH groups are modified remains the same, and high number-average molecular weights can be obtained without risk of gelation. Initial drying is then excellent and only the through-drying is retarded because fewer double bonds are available. Some manufacturers of alkyd resins refer to the benzoic acid as a chain stopper [120]. However, that is not strictly true. The content of benzoic acid does not limit the molecular weight. On the contrary, it contributes to the modification of the excess OH groups, which allows for relatively high number-average molecular weights. The content of benzoic acids reduces the dispersity (width of the molecular weight distribution). If half of the fatty acid in pentaerythritol alkyd resins is replaced by benzoic acid, the content of fatty acid is decreased from 68.7 wt.% to 41.1 wt.% (in alkyds having the same structure and the same degree of occupation [b' value]) and the quantity of benzoic acid is increased to 17.9 wt.%. However, the incorporation of such high quantities of benzoic acid can impair the solubility of resins, and the films resulting from such resins are matt. In that event, the benzoic acid must be replaced by p-tert.-butyl benzoic acid [121], at least in part. Due to the bulky aliphatic substituent, p-tert.-butyl benzoic acid confers better solubility and optimum gloss properties. p-tert.-Butyl benzoic acid is classified as teratogenic, but that need only be taken into account when the raw material as such is being handled. And p-tert.-butyl benzoic is more expensive than benzoic acid. Figure 7.6 shows a typical structural segment of a medium-oil alkyd resin containing benzoic acid, including the calculation of mass fractions (wt.% of segment). The molar ratio of Figure 7.6:  Structure of a medium-oil model alkyd fatty acid to benzoic acid is 2 : 1. containing benzoic acid

177

Alkyd resins Table 7.3:  Model formulation for a medium-oil alkyd resin containing benzoic acid pos. 1

n

building element 1.000

pentaerythritol

M

n·M

wt. ‰

136

136.00

233.9

2

1.000

phthalic anhydride

148

148.00

254.5

3

0.700

technical linoleic acid

280

196.00

337.1

4

0.300

conjugated linoleic acid

280

84.00

144.4

4

0.500

benzoic acid

122

61.00

104.9

625.00

1074.8

18

– 43.51

– 74.8

581.49

1000.0

sum 2.417

water yield (AV)

acid value polycondensation constant number-average molecular weight (calc.) average number of structural units degree of branching occupation of excess OH groups OH value

AV k' M q v b' OHV

8.0 mg KOH/g 1.0829 7014 g/mol 12.1 0.86 mol/kg 0.72 56.2 mg KOH/g

As stated previously, a combination of isolated and conjugated double bonds in the fatty acids of alkyd resins improves curing efficiency. Given that alkyd resins made with benzoic acid have a lower fatty acid content and consequently crosslink less, it makes sense to compensate for this with a mixture of fatty acids containing isolated and conjugated double bonds. An example of such an alkyd resin is presented in Table 7.3 [122]. A corollary of the change of the longer aliphatic chains to shorter aromatic units is that medium-oil alkyd resins are in general insoluble in purely aliphatic hydrocarbons. Aliphatic hydrocarbons often have to be mixed with aromatic hydrocarbons (e.g. xylene, Aromatic 100) or with other, more polar solvents (e.g. butyl acetate, methoxy propyl acetate, butanols) to yield stable solutions. Alkyd resins containing benzoic acid and having a fatty acid content of between 38 and 52 wt.% are used for coatings in a range of industrial applications, which has narrow­ ed considerably over the years. They were employed in automotive repair coatings, but have now been mostly superseded by two-pack coatings based on acrylic resins and polyisocyanate adducts. Radiators used to be coated with alkyd resin paints containing benzoic acid, but this is now mostly done with powder coatings based on COOH polyesters (see Chapter 5.7.2) in combination with epoxy resins. Nonetheless, major application areas remain, e.g. agricultural machines, large vehicles, and wooden furniture. Alkyd resins containing fatty acids rich in 9,12-linoleic acid and fractions of 9,11-linoleic acid, with little or no 9,12,15-linolenic acid, are fairly weatherable and the films do not

178

Alkyd resins for oxidative crosslinking Table 7.4:  Model formulation for a short-oil alkyd resin containing benzoic acid pos.

n

building element

M

m=n·M

wt. ‰

1

1.000

trimethylol propane

134

134.00

340.7

2

1.000

phthalic anhydride

148

148.00

376.2

3

0.350

technical linoleic acid

280

98.00

249.1

4

0.350

benzoic acid

122

42.70

108.6

422.70

1074.8

18

– 29.34

– 74.6

393.36

1000.0

sum 1.377

water yield (AV = 10)

acid value polycondensation constant number-average molecular weight (calc.) average number of structural units degree of branching occupation of excess OH groups OH value

AV k' M q v b' OHV

10.0 mg KOH/g 1.0701 5611 g/mol 14.3 0.76 mol/kg 0.65 52.8 mg KOH/g

yellow. Containing a combination of fatty acids bearing isolated double bonds and conjugated double bonds, they are primarily used for forced drying. Formulations for coatings containing such alkyd resins are treated with siccatives and anti-skinning agents in the same way as those based on long-oil alkyd resins. One example is a white topcoat [123] containing a medium-oil alkyd [124], an addition (calculated on solid resin) of about 0.70 wt.% cobalt siccative (containing 12 % Co), 0.50 wt.% manganese siccative (6 % Mn), 0.40 wt.% zirconium siccative (12 % Zr), including about 2.50 wt.% of organic calcium salt (10 % Ca), or for a cobalt-free version with 0.60 wt.% zirconium siccative (12 % Zr), and 0.40 wt.% of a manganese siccative (6.0 % Mn, which also contains about 15 wt.% of an organic drying accelerator [bipyridyl]) including about 2.50 wt.% of organic calcium salt (10 % Ca). Typical commercial products: Setal AF 48, Setal 196 (Allnex-Nuplex [76]), Synolac 3132 (Arkema-Cray Valley [77]), Synthalat F 477 (Synthopol [80]), WorléeKyd V 543 (Worlée [78]) There are alkyd resins which have a particularly low content of fatty acid (short-oil alkyd resins) and benzoic acid. They contain a triol instead of pentaerythritol. The formulations for such alkyd resins follow the same concept as for the medium-alkyd resins: high occupation of excess OH groups (b' value), high number-average molecular weights. An example of the structure of such an alkyd resin is shown in Figure 7.7; the triol is trimethylolpropane. Table 7.4 presents an example of an alkyd based on trimethylolpropane and with a content of 25 wt.% fatty acid and 10.9 wt.% benzoic acid [125].

179

Alkyd resins These alkyd resins are used for industrial coatings which are notable for their particularly fast initial drying. The low concentration of double bonds leads to the formation of wide­-meshed molecular networks which are fairly flexible, although the films are hard. The binders are used for machinery coatings (one-coat paints) as well as for primers, putties and fillers which cure oxidatively. There are also types recommended for forced drying. Typical commercial products: Setal AF 26 (Allnex-Nuplex [76]) Synolac 272 (Arkema-Cray Valley [77]), WorléeKyd SM 426 (Worlée [78])

7.2.4 Anti-corrosive alkyd resins Optimum corrosion resistance is achieved with binders that have optimum wetting behaviour on metals and high hydrophobicity. Alkyds containing linseed fatty acids readily provide such properties, because the linolenic acid, with its three isolated double bonds, confers good wetting on metal surfaces. An intensive reaction also occurs with atmospheric oxygen, which initiates efficient curing that supports high film density and provides resistance to diffusion by moisture – a necessity for optimum corrosion resistance. Film

Figure 7.7:  Structure of a short-oil model alkyd containing benzoic acid

180

Alkyd resins for oxidative crosslinking solidification occurs much faster than in alkyds containing 9,12-linoleic acids. The good wetting properties are also helpful for dispersing pigments, as anti-corrosive paints can have a high inorganic pigment content (PVC ≥ 20 vol.%). The high reactivity with respect to oxidative curing allows comparatively low number-average molecular weights to be chosen, which opens up possibilities to achieve high application solids. A model of a structural building block of a linseed oil alkyd based on glycerol is shown in Figure 7.8. Table 7.5 shows a formulation for a typical medium-oil alkyd based on linseed oil [126]. Production consists in transesterifying linseed oil and glycerol at 250 °C in the presence of a catalyst. Phthalic anhydride is then added and the polycondensation reaction carried out until the target acid value and viscosity are achieved. Such linseed alkyds have limited weatherability and so are used in primers and primer surfacers, but not in outdoor applications. In the past, such alkyd resins served as the base binder for anti-corrosive primers containing red lead (lead(II),(IV)oxide) pigment, and provided excellent corrosion resistance. How­ ever, the classification of lead compounds as harmful to health has meant that the products have been abandoned for some years now (with a few exceptions). Also, in the past, zinc oxides served as anti-corrosive pigments. Zinc oxide forms insoluble salts with the carboxyl groups of resins (phthalic acid and fatty acids) and so the acid value was limited to ensure “zinc oxide compatibility”. Otherwise, problems arose with the storage stability of the paints. This property is no longer important because modern active anti-corrosive pigments are acidic (e.g. acidic zinc phosphates). Typical commercial products: Setal 16 (Allnex-Nuplex [76]), Synthalat L 492 (Synthopol [80]), Uralac AD 143 (DSM [67]) There are also alkyd resins in this product class that offer improved initial drying properties. This is achieved by replacing fatty acid by benzoic acid in a 1 : 1 ratio in the resin formulation, in the manner described earlier. However, it is also possible to replace the fatty acid

Figure 7.8:  Structure of a model linseed oil alkyd

181

Alkyd resins Table 7.5:  Model formulation for a medium-oil linseed alkyd resin pos. 1

n

building element 0.320

linseed oil

M

n·M

wt. ‰

872

134.00

584.1

2

0.730

glycerol

92

148.00

140.6

3

1.000

phthalic anhydride

148

98.00

309.8

494.20

1034.5

sum 0.915

water

18

yield (AV = 10) acid value polycondensation constant number-average molecular weight (calc.) average number of structural units degree of branching occupation of excess OH groups OH value

AV k' M q v b' OHV

– 16.47

– 34.5

477.73

1000.0

10.0 mg KOH/g 1.1351 3535 g/mol 7.4 0.19 mol/kg 0.78 32.3 mg KOH/g

with other monocarboxylic acids, e.g. rosin acids. These are diterpenes which contain carboxyl groups and occur naturally in various types of conifers. They chiefly consist of abietic acid and levopimaric acid, the structures of which are shown in Figure 7.9, with smaller quantities of other derivatives of diterpene acids. Table 7.6 shows an example of a linseed alkyd [127] with rosin acid. The linseed fatty acid content is 39.1 wt.% and the rosin acid content is 11.7 wt.%; the molar ratio of the two is 4 : 1. The structure of a segment of a linseed alkyd containing rosin acid is shown in Figure 7.10. These cycloaliphatic monocarboxylic acids raise the glass-transition temperature of the binders and boost the hardness of the resultant films. They improve solubility and wetting properties, and are soluble in all kinds of hydrocarbons (including pure aliphatic hydrocarbons). The tertiary carboxyl group of rosin acid is not very reactive. To ensure that the rosin acid is esterified completely, the enough OH groups need to be available and the process is carried out until low acid values are achieved. Not only do the rosin acids improve initial drying, it has also been found that such acids, which contain isolated double bonds, support through-drying. Such binders are used for fast-curing primers, primer surfacers, fillers, and putties. They are not very weatherable and tend to yellow outdoors. The alkyd resins described are Figure 7.9:  Molecular structures of abietic acid and also used for printing inks. levopimaric acid

182

Alkyd resins for oxidative crosslinking Table 7.6:  Model formulation for a short-oil linseed alkyd resin that contains rosin acid pos.

n

1

0.600

building element linseed fatty acid

M

m=n·M

wt. ‰

278

166.80

390.9

2

1.000

glycerol

92

92.00

215.6

3

1.000

phthalic anhydride

148

148.00

346.8

4

0.150

technical rosin acid

333

49.95

117.0

456.75

1070.3

1.667

water

18

– 30.01

– 70.3

426.74

1000.0

sum yield (AV = 11) acid value polycondensation constant number-average molecular weight (calc.) average number of structural units degree of branching occupation of excess OH groups OH value

AV 11.0 mg KOH/g k' 1.0837 M 5101 g/mol q 12.0 v 0.59 mol/kg b' 0.60 OHV 43.9 mg KOH/g

Figure 7.10:  Structure of a model linseed oil alkyd that contains rosin acid

183

Alkyd resins Also available are alkyd resins which contain phenol resins, modified with rosin acids, which can act as polycarboxylic acids when incorporated into alkyd molecules. Such binders contribute to a better chemical resistance. Typical commercial products: Synthalat HL 30 (Synthopol [80]), Uralac AM 352 (DSM [67]), WorléeKyd L 138 (Worlée [78])

7.2.5

High-solid alkyds for oxidative crosslinking

Experience has shown that the higher the content of fatty acids, the lower is the solution viscosity of alkyd resins. However, this effect is hardly noticed when a restriction to pure aliphatic solvents is in place (e.g. for health reasons), because these solvents have only little thinning effect. Consequently, additional efforts have been made to raise the application solids so as to reduce emissions of volatile organic compounds (VOC). The first step consists of combining long-oil alkyds with stand oils (see Chapter 8.4), which are formed by oligomerising drying and semi-drying oils. For optimum compatibility, the combinations are heated (hot-blend process). During this process, some transesterification may occur. In practice, the resulting combinations consist of lower molecular weight alkyd resins with a very high content of fatty acids, which give rise to very low solution viscosities. Such resins can be prepared directly with a sufficient molar excess polyol (e.g. polycondensation constant k' = 1,25) and a significant amount of modification of OH groups (high b' value) by fatty acids (high-solid alkyds). The content of the fatty acids increases substantially above 70 wt.% and the resulting lower viscosity allows formulation to higher solid contents in the application state. The products have longer initial drying times and slower through-drying. There are high-solid alkyd resins in which the phthalic anhydride is replaced by isophthalic acid. At first sight this seems counterproductive, as replacing phthalic anhydride by isophthalic acid leads to a broader molecular distribution, followed by higher viscosity. To obtain comparable viscosities, the polyol excess must be increased, and that leads to lower number-average molecular weights. However, there are some advantages. Evaporation of solvents from alkyds containing isophthalic acid is faster. The large molecules in alkyds containing isophthalic acid build a matrix for effective film formation as the crosslinking step is shortened. These alkyds can have a fatty acid content of up to 80 wt.%. This type of alkyd is prepared by transesterification, which is usually performed between the oils and polyols. Alternatively, some formulations start with oil and isophthalic acid, in which event the transesterification is an acidolysis reaction in the presence of a catalyst and is followed by adding polyols and finishing the process by esterification of the carboxyl groups. Furthermore, there is the possibility of replacing trimethylolpropane and pentaerythritol by higher-functional polyols, using di-trimethylolpropane (4 OH groups) and di-pentaerythritol

184

Alkyd resins for oxidative crosslinking Table 7.7:  High-solids alkyd resin containing di-pentaerythritol n = m/M 2.689

building element fatty acid, rich on 9,12-linoleic acid

M

wt. ‰

280

753.0

0.768

di-pentaerythritol

254

195.0

0.770

phthalic anhydride

148

114.0

sum

1062.0

water

– 18

yield (AV = 4.0) polyester constant number-average molecular weight [g/mol] acid value [mg KOH/g] occupation rate OH value [mg KOH/g] viscosity [dPa·s] (80 % in white spirit)

– 62.0 1000.0

k'M 1.0893 Mn 14548 AV 4.0 b' 0.86 OHV 25.1 η 10

(6 OH groups). The higher functionalities allow more fatty acids to be incorporated, which leads to low viscosities. Also, the molecular mobility of the main chain is increased by the ether groups of these polyols, a fact which further lowers the viscosity. Table 7.7 [128] describes such an alkyd resin consisting of a fatty acid with a high content of 9,12-linoleic acid, phthalic anhydride and di-pentaerythritol. In spite of the high molecular weight, the solution viscosity of this alkyd is surprisingly low, due to the high content of fatty acids and the effect of di-pentaerythritol. The resins are used for high-solids house paints and for printing inks. Typical commercial products: Synthalat HS 80 (Synthopol [80]), WorléeKyd SB 990 (Worlée [78])

7.2.6 Styrenated and acrylated alkyd resins The most important goal of modifying alkyd resins is the improvement of initial drying properties – or, preferably, the optimisation of initial drying and through-drying. Besides the previously described method of replacing some of the fatty acid by benzoic acid, there is another method of incorporating other building blocks. They employ combinations with styrene and vinyl toluene, with methacrylates (methyl methacrylate, isobutyl methacrylate, tert.-butyl methacrylate, n-butyl methacrylate). The modification is achieved by co-polymerising the alkyd resins with the aforementioned monomers. Such modified alkyd resins are called styrenated or acrylated alkyd resins. The modification process [129] is comparable to the commonly employed process for preparing acrylic resins in solution [130]. The chosen alkyd resins have a medium to high modification ratio of the OH groups with fatty acids, and relatively low molecular weights.

185

Alkyd resins The alkyd resins contain fatty acids which are rich in linolenic acid (source: linseed oil), or rich in 9,12-linoleic acid (sources: soybean oil, sunflower oil, safflower oil). The fatty acids can contain 9,11-linoleic acid. The co-polymerisation is carried out in the melt or in aromatic solvents, and is initiated with peroxide compounds (initiators), e.g. di-tert.-butyl peroxide (1.0 to 4.0 wt.% calculated on resulting solids). The ratio of alkyd to monomers is between 60 : 40 and 85 : 15. The polymerisation temperature is 130 to 170 °C. Monomers and initiators are added in a feed process over the course of 1 to 5 hours. In the examples given, shots of initiator are added at the end of the feed to ensure that all monomers are consumed. Given the different polymerisation parameters of the unsaturated fatty acids and the combination monomers, alternating co-polymerisation is the most likely reaction. All together it is believed that only short polymer chains are formed between fatty acid molecules, and branched side-chains of polystyrene or polymethacrylates are formed by free-radical transfer processes. The molecular weight of the finished product increase substantially. Particularly reactive are fatty acids containing 9,11-linoleic acid. Overall, the reactions are complex. Parameters affecting the structures of co-polymers are the type and quantity of fatty acids, the ratio of alkyd to monomers, the type of monomer, the solution concentration, the type and quantity of initiator, and the reaction temperature. Unlike the polymerisation process for acrylic resins, raising the initiator concentration will also increase the viscosity of the product. The reason is that more molecular bridges are generated between fatty acid molecules. Styrenated alkyds are distinguished by very good initial drying. They offer the possibility for early remake operations; therefore, they are suitable for wet-on-wet applications. The styrene part improves the chemical resistance and the resistance to moisture. Styrenated alkyds with higher quantities of styrene are less compatible with other resins and the adhesion is worse. As styrene is replaced by vinyl toluene, the compatibility is improved. Also, acrylated alkyd resins have better initial drying properties. They show good resistance to chemicals and moisture, are reputed to offer better weatherability, and to exhibit better pigment wetting and better adhesion. Since the polymerisation process consumes some double bonds and the percentage of fatty acids in the entire resin is reduced by the addition of monomers, the through-drying of such resins is reduced. Alkyd resins modified in this way are used for anti-corrosive coatings, for ships, machinery equipment, and steel structures. It is possible to formulate primers and topcoats, and also single-coat paints. The last of these can be applied in relatively thick layers without problems. Special binders are also used in coatings for collapsible tubes. Typical commercial products: Styrenated alkyd resins: Synthalat SF 260 (Synthopol [80]), Uralac AS 381 (DSM [67]) / Acrylated alkyd resins: Setyrene 78 (Allnex-Nuplex [76]), Uralac AC 651 (DSM [67]), WorléeKyd AC 2550 (Worlée [78])

186

Alkyd resins for oxidative crosslinking

7.2.7

Urethane-modified alkyd resins

Binders possessing special properties are obtained when alkyd resins are modified with urethanes. Such binders are obtained when using diisocyanates as a substitute for some of the commonly employed phthalic anhydride. The first step is to produce a low-molecular weight alkyd resin that has less phthalic anhydride. The condensation reaction is carried out until low acid values are achieved. This precursor product is diluted with non-protic solvents and the diisocyanate is then added in a feed process. The reaction is performed at relatively low temperatures (80 to 120 °C). Where modification is done with aliphatic or cycloaliphatic diisocyanates, catalysts may be used (e.g. organic metal salts). The modification with diisocyanates is an addition reaction, which generates broader molecular weight distributions than the esterification with phthalic anhydride, because the regulatory effect of transesterification does not occur. To obtain comparable viscosities, the polyol excess must be increased. In addition, modification with diisocyanates leads to the formation of urethanes which, due to their solubility behaviour, also increases the viscosity. The most suitable urethane alkyd resins contain combinations of phthalic anhydride and toluene diisocyanates and have a high content of polyunsaturated fatty acids. The molar ratios of phthalic anhydride to toluene diisocyanates are 7 : 3 to 3 : 7. Table 7.8 compares a long-oil alkyd resin with a urethane alkyd containing toluene diisocyanates [131]. The two resins have roughly the same fatty acid content and OH-modification ratio. Although the number-average molecular weight of the urethane alkyd is lower, the viscosity of a solution (60 wt.% in white spirit) is substantially higher than the viscosity of the solution of unmodified alkyd. The same siccatives are added to urethane alkyd resins as to non-modified alkyd resins. They show much faster initial drying properties. Given the tendency of urethane groups to associate, films of such alkyd resins are hard, but flexible and possess excellent abrasion resistance. Furthermore, they confer better resistance to chemicals (including alkalis) and moisture. For this reason, urethane resins are often chosen for wood coatings which are subject to mechanical impact. A typical application field is parquet and similar wooden flooring. They are also used for wooden furniture (e.g. chairs and benches), fast-drying industrial coatings on metals, and spar varnishes. Both clearcoat and pigmented systems are available. Urethane alkyd resins containing urethanes made from aromatic diisocyanates (TDI) are not particularly weatherable. There are also alkyd resins available which contain aliphatic or cycloaliphatic diisocyanates which confer better weatherability. Isophorone diisocyanate (IPDI) is preferred for these, because the glass-transition temperature of IPDI alkyd resins is higher than that of alkyd resins containing urethanes from aliphatic diisocyanates, which leads to better initial drying properties. However, urethane alkyds made with aromatic diisocyanates dry substantially faster.

187

Alkyd resins Table 7.8:  Comparative compositions of a long-oil alkyd resin and a urethane alkyd building elements, moles

alkyd resin

urethane alkyd

pentaerythritol

1.000

1.100

fatty acid, rich on linoleic acid

1.700

1.921

phthalic anhydride

1.000

0.500

–

0.500

pentaerythritol

190.6

185.5

fatty acid, rich on linoleic acid

667.0

667.2

phthalic anhydride

207.4

91.8

–

107.9

1064.5

1052.5

toluene diisocyanate building elements, wt. ‰

toluene diisocyanate sum water

64.5

52.5

1000.0

1000.0

acid value [mg KOH/g]

10.0

5.0

polyester constant (k')

1.1272

1.1719

yield (AV = 10,0) characteristic values

number-average molecular weight [g/mol]

5611

4692

number of structural units (q')

7.9

5.8

degree of branching [mol/kg]

0.42

0.35

occupation value (b')

0.80

078

OH value [mg KOH/g]

33.6

38.3

Typical commercial products: Setal AU 601 (Allnex-Nuplex [76]), Unithane 1077, Unithane 6451 (Arkema-Cray Valley [77]), Uralac AL 202 (DSM [67]), WorléeKyd S 6003 (Worlée [78]) Replacing all of the phthalic anhydride in an alkyd resin by diisocyanates (e.g. by toluene diisocyanates) yields a fatty-acid-modified polyurethane resin. Such resins are called urethane oils and are prepared by transesterification. The oil (triglyceride) is transesterified with excess of polyol to form monoesters (as described above in Chapter 7.2.2), which react with the diisocyanate to form fatty-acid-modified polyurethanes. As the molecular weight distribution of such addition polymerisation products is substantially broader than that of polycondensation products, it is necessary to choose a higher polyol excess (higher value of constant k'). The urethane oils are suitable for fast-drying coating systems which are distinguished by resistance to mechanical impact, high flexibility and excellent chemical

188

Alkyd resins for oxidative crosslinking resistance. Naturally, urethane oils are not classified as polyester resins, but have similar properties to urethane alkyd resins.

7.2.8

Thixotropic alkyd resins

Low-molecular weight liquids have viscosities which are independent of the shear velocity gradient. Their viscosity, as the quotient of shear stress and shear rate, is constant: i.e. they are Newtonian fluids. In general, colloidal solutions of polymers display non-Newtonian behaviour. Their viscosity is dependent on the shear velocity gradient, which decreases with increase in shear stress and shear rate; such solutions are said to be pseudoplastic. Some products require a minimum of shear stress to become mobile. They are defined as pseudoplastic liquids with a yield value of viscosity (Bingham pseudoplastic). In addition, there can be a dependency of the viscosity on time. If the viscosity of liquids changes over time, this behaviour is called thixotropy. When, after an initial increase in shear velocity gradient of thixotropic liquids, the shear velocity is reduced, the viscosity values increase again, but more slowly compared to decrease in the first phase. The viscosity behaviour of thixotropic liquids is shown in Figure 7.11. The reasons for pseudoplastic behaviour are interactions of the colloidal particles in polymer solutions. The extent of the interactions can be reduced by increasing the shear velocity gradient. If the shear velocity gradient is reduced or the shear removed, the colloidal particles can start interacting again. The type of colloidal particles determines how much time is needed for this rebuilding of interactions. Such behaviour is exploited in coating formulations. The viscosity of paint materials containing thixotropic binder solutions is lowered during application (e.g. shearing due to brushing). After application – in the absence of further shear stresses – the viscosity will gradually increase again, but not before the paint layer has had time for flowing and levelling. This increase in viscosity prevents sagging and drips. Thixotropic behaviour in oxidative-cure alkyd resins is achieved by modification with polyamides. There are polyamides which are formed from fatty acid dimers and aliphatic polyamines. The polyamide content is between 5 and 14 wt. % of total solids. Although they contain non-polar building blocks (fatty acid dimers), the polyamides are not soluble in aliphatic hydrocarbons, nor are they compatible with long-oil alkyd resins. However, compatibility is improved by blending at elevated temperatures (180 to 200 °C). It is believed that such hot-blend processes involve trans-amidation reactions between alkyd molecules and polyamide, yielding alkyd resin molecules containing amide segments and polyamide containing fatty acid amides. The process is stopped once the reaction mixture shows sufficient compatibility. It is controlled by monitoring the viscosity of a test solution (at ambient temperature) and the appearance (clear point at ambient temperature) over time. The solution viscosity passes through a maximum; the clear point is reached shortly after

189

Alkyd resins the viscosity maximum. The process is then halted by cooling, and the products are diluted with aliphatic hydrocarbons. Small fractions of polyamide remain in the modified material which generate strong physical interactions between colloidal particles – this being manifested as thixotropic behaviour in non-polar solutions. The determination of thixotropy is a laborious process. As might be expected, the values are dependent on the history of the test sample. Thixotropic alkyd resins are used for house paints, mainly DIY. The products can be applied in relatively thick layers, without sagging or dripping. Application by brush or roller lowers the viscosity. After optimum flow and levelling, without sagging, the viscosity increases again. In the past, highly thixotropic, non-drip paints were extremely popular. Nowadays, the trend is to use paints of medium thixotropy for house paints and corrosion protective materials because they allow thick layers to be applied in just one working step.

Figure 7.11:  Viscosity behaviour of thixotropic liquids

190

Alkyd resins for oxidative crosslinking Typical commercial products: Gelkyd 3605 (Arkema-Cray Valley (Synres [132]), Worléethix S 6455 (Worlée [78])

[77]),

Urathix AT 415

There are alternative ways – through the use of additives – to influence the rheological behaviour of paints based on oxidative-cure alkyds. These employ phyllosilicates (bentonite, montmorillonite), fumed silica, crystalline ureas, low-molecular polyurethanes, and hydrogenated castor oil.

7.2.9

 ther modified alkyd resins O for oxidative crosslinking

Other ways to improve specific properties of oxidative-cure alkyd resins exist. Residual carboxyl groups on alkyd resins can be made to react with the epoxy groups of aromatic epoxy resins. Or, the epoxy resins can be incorporated at the beginning of alkyd resin preparation, where they act as an oligomeric polyol derivative. The molar fraction of epoxy resin must be factored into the calculation of the number-average molecular weight of the entire product. Such epoxy alkyd resins are distinguished by improved adhesion and optimum anti-corrosive properties. They are suitable for metal primers and for coatings for collapsible tubes. Typical commercial product: Resydrol VAX 6050 (Allnex [68]) The residual carboxyl groups of alkyd resins can be made to react with metal alcoholates to yield metal-reinforced alkyd resins. The most important metal alcoholate is aluminium iso-propylate. During film forming, the aluminium cations react with the carboxyl groups of the alkyd resin in a type of crosslinking. The reaction intensifies the initial drying of the alkyd resins modified in this way. In addition, the films become more resistant to chemicals, solvents, humidity and mechanical impact. Such modified alkyd resins may have a limited storage time. The binders are suitable for premium house paints, spar coat­ ings and anti-corrosive coatings. Similar to saturated polyesters (see Chapter 5.9), alkyd resins containing residual hydroxyl groups can be made to react with silicone intermediates. The resulting silicone alkyd resins confer improved resistance to chemicals, moisture and mechanical impact. Heat resistance is improved only slightly, due to the properties of the other building blocks. Silicone alkyd resins are used for premium house paints, industrial coatings and marine paints. Typical commercial products: Synolac 5600 (Arkema-Cray Valley 830 (Worlée [78])

[77]),

WorléeKyd BS

191

Alkyd resins

7.2.10 Water-thinnable alkyd resins and alkyd emulsions for oxidative crosslinking The necessary quest to reduce emissions of volatile organic compounds (VOCs) continues. As limitations on properties have emerged in the course of the development of high-solid alkyd resins (see Chapter 7.2.5), the next step in reducing or preventing VOC emissions has been the formulation of water-thinnable oxidative-cure alkyd resins. Preparation of water-thinnable alkyd resins initially adopts the same methods as for water-thinnable saturated polyesters: addition of anhydrides or partial esterification of polycarboxylic acids. The first method consists in preparing alkyd resins of low acid value from polycarboxylic acid, higher functional polyols and polyunsaturated fatty acids. The resins should have residual hydroxyl groups. To these hydroxyl groups are then added anhydrides to form esters and free carboxyl groups. The latter are subsequently partially neutralised by adding amines. The resulting carboxylate anions are capable of dissolving the alkyd resins into the aqueous phase. Sometimes with help of cosolvents. For the preparation of the water-thinnable polyesters and alkyd resins under discussion here, it is important to find ways to provide resistance to saponification. As the phthalic anhydride widely employed in alkyd resins is particularly susceptible to saponification, it is replaced by more stable polycarboxylic acids or their derivatives (e.g. isophthalic acid). It is also important that the added anhydrides form adducts that have the highest-possible saponification resistance (e.g. tetrahydrophthalic anhydride). The disadvantage of the resulting alkyd resins is that they have relatively low modification ratios of the excess hydroxyl groups, because a proportion of the free hydroxyl groups are consumed in the anhydride addition. Low modification ratios are associated with low number-average molecular weights; film forming by oxidative-cure is then less effective. The following example (Table 7.9) describes a water-thinnable alkyd resin [133]. First, an alkyd resin is prepared at 220 °C by making pentaerythritol, isophthalic acid and linseed fatty acid react until the acid value is low (1 mg KOH/g). Tetrahydrophthalic anhydride is then added at 100 °C. The resulting resin has an acid value of 75 mg KOH/g while the number-average molecular weight is 1825 g/mol (calculated). Such alkyd resins are mainly dissolved in co-solvents (e.g. in small quantities of butyl glycol) and are then transferred to the aqueous phase to form a homogeneous aqueous solution. A second method, described earlier, consists in interrupting the condensation reaction at a high acid value or low degree of condensation. Doing this with alkyd resins can be problematic. It has to be ensured that the residual acid groups stem solely from the polycarboxylic acids and not from free fatty acids. The best way to achieve this is transesterification: oils containing polyunsaturated fatty acids are transesterified with an excess of

192

Alkyd resins for oxidative crosslinking Table 7.9: Typical water-thinnable linseed alkyd resin by anhydride addition n = m/M 4.007

building element pentaerythritol

M

m

m‰

136

545.0

221.0

2.952

isophthalic acid

166

490.0

198.7

4.137

linseed fatty acid

278

1150.0

466.4

3.000

tetrahydrophthalic anhydride

152

456.0

184.9

2641.0

1071.1

175.4

71.1

2465.6

1000.0

sum water

18

yield (AV = 75) polyester constant number-average molecular weight [g/mol] acid value [mg KOH/g] occupation rate OH value [mg KOH/g]

k'M 1.2270 Mn 1825 AV 75 b' 0.56 OHV 74.8

polyol to form monoesters. The polycarboxylic acids or their derivatives are then added and polycondensation is carried out until the target acid value is reached. A third method is the aforementioned addition and partial esterification of trimellitic anhydride. A special, fourth method consists in introducing diisocyanate adducts into alkyd resin molecules. This process has already been described under the preparation of aqueous polyester-urethane dispersions (see Chapter 5.4.1). First, a precursor comprising a relatively low-molecular weight alkyd containing residual hydroxyl groups is prepared. An adduct is then formed with two moles of diisocyanate and one mole of dimethylol propanoic acid. This adduct is incorporated into the alkyd structure to form urethane linkages. The resulting urethane alkyds contain free carboxyl groups of the dimethylol propanoic acid, which can be neutralised with amines to form anions. The product is then transferred into aqueous phase [134]. Polyurethane alkyds are more resistant to saponification than are anhydride adducts. The disadvantage of these four methods is the need for a low degree of modification of the free OH groups with fatty acids (lower b' values) because the processes require free OH groups. This prerequisite requires low number-average molecular weights, which has a detrimental effect on film formation. The unsaturated fatty acids in alkyd resins afford scope for further preparation methods. Maleic anhydride or acrylic acid can add to the double bonds of fatty acids in alkyd resins (fifth method). Addition of such unsaturated acids or acid derivatives to fatty acids containing 9,11-linoleic acid is a diene addition. They can also add to 9,12-linoleic acid at the activated CH2 groups, comparable to the addition of oxygen, or through the diene addition described above, after isomerisation to conjugated double bonds. The anhydride

193

Alkyd resins ring of the addition product formed with maleic anhydride is opened by water or alcohol to form free carboxyl groups. These can be neutralised with amines for thinning with water. Although the addition reactions consume some double bonds, many still remain available for oxidative-cure reactions. Such products can contain high quantities of fatty acids and can have high number-average molecular weights. In addition, there is the advantage that the incorporated carboxyl groups are connected by C–C bonds to binder molecules. (There is no chance of decomposition by saponification as happens in the other linkages formed via ester groups.) Then there is a sixth method in which alkyd resins are combined with acrylic polymers containing free carboxyl groups. One portion of the combination is an alkyd resin, of low average molecular weight and containing polyunsaturated fatty acids and residual hydroxyl groups. The second portion is a copolymer of polyunsaturated fatty acid containing methacrylic esters and a high quantity of methacrylic acid. The copolymer is prepared by solution polymerisation (feed process, e.g. in xylene), with peroxides as initiator. The two portions are combined at about 180 °C. Some esterification reactions occur which confer better compatibility on both portions (hot-blend). Partial neutralisation with amines then renders the products water-thinnable [135]. It is believed that the aqueous dispersion consists of particles with a kind of core-shell structure, where the inner phase contains the alkyd and the outer phase the copolymer with carboxylate groups (protective colloid). The various types of water-thinnable alkyd resins have acid values of 30 to 75 mg KOH/g. The neutralisation degree is between 60 and 100 mol-% and the resulting pHs are 7.2 to 8.5. As the amines act as reducing agent, they can limit the curing process. It is therefore essential that they evaporate as soon as possible. Consequently, triethylamine and ammonia are preferred. The prepared aqueous solutions or dispersions usually contain between 35 and 50 wt.% solids. Some products are available without any solvents, but many contain co-solvents in quantities of 10 to 30 wt.%, expressed in terms of solids. The most common co-solvent is butyl glycol, followed by other glycol ethers. The quantity is kept low to conform with VOC regulations in the various application fields. To complete the formulations based on water-thinnable alkyd resins, mention must be made of siccatives. Manufacturers offer special siccatives for water-borne systems. The purpose of modification with siccatives is to incorporate the products into the aqueous phase. The siccatives themselves may remain hydrophobic, because the compounds must diffuse into the hydrophobic structure of unsaturated fatty acids in order to be able to act effectively. The water-thinnable alkyd resins described here are suitable for aqueous house paints (DIY and trade) and for industrial coating applications and anti-corrosive paints. Typical commercial products: Setal 146 (Allnex-Nuplex [76], Resydrol AY 241, AY 6150, AY 6173, AZ 6185 (Allnex [68], Synthalat W 46 (Synthopol-Chemie [80]), (WorléeSol 61 A (Worlée [78])

194

Alkyd resins for co-crosslinking Such alkyds are also combined with primary acrylic dispersions. The acrylic dispersion component supports initial drying properties and durability, whereas the alkyd resin component supports flow and levelling, gloss and flexibility, and is suitable for pigment dispersing. The seventh method consists in producing alkyd resins in aqueous emulsions. To this end, oxidative-cure alkyd resins are combined with surfactants and dispersed in deionised water. Dispersing starts with the melt or with concentrated solutions in non-polar solvents (e.g. hydrogenated hydrocarbons). Alkyd resins of higher number-average molecular weights than the different alkyds described above can be used, and this proves to be an advantage for optimum crosslinking. Anionic surfactants, mainly ammonium salts of alkyl sulfonic acids or ethoxylated sulfonic acids, exist. Anionic stabilisation can be supported by neutralising the carboxyl groups of alkyd resins. Non-ionic surfactants also exist and consist of polyethylene oxide modified higher alcohols (e.g. fatty alcohols). A further possibility consists in modifying the alkyd resins by incorporating polyethylene glycols into alkyd molecules [136]. The polyethylene glycol elements – which will not evaporate – contribute some hydrophilic behaviour in films, but their main advantage is that they do not require amine addition, and so there is no reduction reaction. Alkyd resin emulsions differ from colloidal aqueous solutions in their viscosity behav­ iour, offering lower viscosity at a lower solids content. They are used for house paints and anti-corrosive coatings [137]. Typical commercial products: Synthalat AEM 440 (Synthopol-Chemie [80]), WorléeSol E 440 W (Worlée [78])

7.3 Alkyd resins for co-crosslinking Alkyd resins which form films together with crosslinking agents constitute the largest group of alkyd resins by volume. These resins bear hydroxyl groups almost exclusively as functional groups for crosslinking reactions. The most important crosslinking agents are amino resins (urea resins, melamine resins, benzoguanamine resins) and polyisocyanates bearing free and blocked isocyanate groups. These alkyd resins are described below in terms of their application fields.

7.3.1 Alkyd resins for stoving enamels In the past, alkyd resins containing polyunsaturated fatty acids were used for stoving enamels. However, experience showed that elevated temperatures have very little accelerating effect on oxidative-cure, one exception being alkyd resins containing fatty acids rich in 9,11-linoleic acid. Alkyd resins were also chosen as plasticisers for self-crosslinking

195

Alkyd resins amino resins in order to compensate for the brittleness of the resulting films. Alkyd resins are compatible with amino resins only if they are sufficiently polar. Polar alkyds have a substantial hydroxyl content and a low fatty acid content (short-oil and medium-oil types). At a surprisingly late stage, it was discovered that co-crosslinking reactions occur between OH alkyds and the functional groups of amino resins [138] and have a major influence on film properties of the coatings based on the combination stated. Crosslinking between hydroxyl groups and functional groups of amino resins has already been described for saturated polyesters (see Chapter 5.5.2). The hydroxyl groups of the alkyd resins react in the same manner as the hydroxyl groups of saturated polyesters. Here, too, a certain amount of self-crosslinking of amino resins is to be expected. The ratio of co-crosslinking to self-crosslinking is influenced by the mass ratio of alkyd resin to amino resin, and the number and the reactivity of the hydroxyl groups and the functional groups on the amino resins. It is further influenced by the film forming conditions (time and temperature) and by catalytic effects. While the alkyd resins initially used for crosslinking with amino resins contained unsaturated fatty acids, preference was subsequently given to those containing saturated fatty acids. The latter lead to better weatherability and do not exhibit yellowing. In the past, the most important sources of saturated fatty acid were coconut fat and palm kernel oil. The fatty acid composition of these triglycerides contains an even number of C8 to C18 chain lengths (linear), with the main component being lauric acid (C12, dodecanoic acid). They have only a low content of unsaturated fatty acid, namely oleic acid (C18, octadecaene acid, 5 to 8 wt.%). Special fractions are prepared by distillation, for example, the fractionated fatty acids (C8 and C10 acid). A further member of this class of products is pelargonic acid (nonanoic acid), which is prepared by ozonolysis of oleic acid. The higher fractions of these fatty acids can also be hydrogenated. In the past, the various fatty acids were really low-cost products, especially the fractionated fatty acids, which were by-products. Now, though, the linear saturated fatty acids are important raw materials for surfactants and high-end lubricants. They are far too expensive for use in alkyd resins and have been almost totally replaced by synthetic fatty acids. The latter are prepared mainly by oxo-synthesis from olefins (e.g. propene, isobutylene). The key products are isononanoic acid [139] (3,3,5-trimethylhexanoic acid), 2-ethylhexanoic acid, and neodecanoic acid [140, 141] (which

Figure 7.12:  Molecular structures of synthetic fatty acids

196

Alkyd resins for co-crosslinking consists mainly of 2,2,3,5-tetramethylhexanoic acid). The molecular structures are shown in Figure 7.12). As isononanoic acid, the most preferred fatty acid, contains a primary carboxyl group, it reacts readily with the excess hydroxyl groups in alkyd molecules. The secondary carboxyl group of 2-ethylhexanoic acid is less reactive. For an alkyd resin of the same molar composition, the use of 2-ethylhexanoic acid generates a broader molecular weight distribution than isononanoic acid. Neodecanoic acid with its tertiary carboxyl group cannot be incorporated into alkyd resins by the conventional preparation process. It would be possible to esterify it with polyol first, but that is very complex. The solution is to form a glycid ester [142, 143]. Neodecanoic acid reacts with epichlorohydrin to form a chlorohydrin. The epoxy group is then re-formed by reaction with concentrated alkali. The glycid ester of neodecanoic acid readily adds to the carboxyl groups of polycarboxylic acid or fatty acids. The hydroxyl group which develops in the process is esterified by the conventional method. On account of the branched structure of the synthetic fatty acids, they are less sensitive to yellowing and weathering than are alkyds based on natural linear saturated fatty acids. They develop higher hardness and exhibit less plasticity. Fatty-acid based alkyd resins for stoving enamels also consist of phthalic anhydride and polyols in addition to the monocarboxylic acid already described. The most important polyol is trimethylolpropane. However, such alkyds may also be synthesised from mixtures of pentaerythritol and propylene glycol. Alkyd resins of higher OH values are prepared exclusively with pentaerythritol. There are also products which contain benzoic acid as the monocarboxylic acid in addition to the fatty acids. The alkyd resins are prepared in a one-step process at 200 to 240 °C. It is advisable to use a reflux solvent for the process. The process flow is monitored by measuring the acid values and viscosity of a test solution. The alkyds have substantially lower number-average molecular weights than alkyds for oxidative-cure, namely 1500 to 3000 g/mol. The fatty acid content is relatively low and so the OH modification ratio is low. OH values range from 80 to 140 mg KOH/g. The residual carboxyl groups lead to acid values of 10 to 25 mg KOH/g. The resins are soluble in aromatic hydrocarbons (e.g. Aromatic 100), in esters (e.g. butyl acetate), and glycol ether esters (e.g. methoxy propyl acetate). Minor quantities of alcohols (e.g. n-butanol) or glycol ethers (e.g. methoxy propanol) may also be used. Table 7.10 shows two model alkyd resins [144]. The first contains trimethylolpropane as polyol, and the second contains a mixture of pentaerythritol and propylene glycol. The OH modification ratio with monocarboxylic acid is 50 mol-%, leading to a content of synthetic fatty acid of about 30 wt.%. Acid values of 15 mg KOH/g yield OH values of about 100 mg KOH/g. The viscosity of the two resin solutions is nearly the same. The table shows the molar

197

Alkyd resins Table 7.10: Composition and characteristic values of alkyd resins with different polyols building element [mol]

triol-type

tetrol-diol-type

phthalic anhydride

1.000

1.000

trimethylol propane

1.050

0.000

pentaerythritol

0.000

0.500

propylene glycol

0.000

0.570

isononanoic acid

0.620

0.620

phthalic anhydride

412.0

448.9

trimethylol propane

building element, wt.‰ 140.7

0.0

pentaerythrol

0.0

206.2

propylene glycol

0.0

448.9

isononanoic acid

272.7

297.1

sum

1076.4

1083.6

water

– 76.4

– 83.6

yield (AV = 15.0)

1000.0

1000.0

characteristic values acid value (AV) [mg KOH/g] polyester constant (k') number-average molecular weight [g/mol]

15.0

15.0

1.1460

1.1581

2460

2085

number of structural units (q)

6.8

6.3

degree of branching [mol/kg]

1.20

1.15

occupation rate (b')

0.50

0.50

OH value [mg KOH/g]

97.8

103.5

Figure 7.13 shows the structural segments of the alkyd resins described. Alkyd resins for stoving enamels may also be formulated solely with pentaerythritol. The structure of such alkyds is given in Figure 7.14). The same OH modification rate with fatty acids leads to a higher content of fatty acids (about 40 wt.% and more) and higher OH values. Table 7.11 shows such a model alkyd resin [145], based on pentaerythritol and a 50 % modification rate of the OH groups with synthetic fatty acid. The three alkyd resins presented form the basis for numerous combinations.

198

Alkyd resins for co-crosslinking These include short-oil alkyds containing mixtures of triol and diol and only small quantities of fatty acids (about 20 wt.%). Although synthetic fatty acids confer good weatherability and resistance to discolouration, they have some disadvantages. In comparison to fatty acids with longer linear chains they contribute less wetting and less flexibility and the resulting alkyd resins exhibit inferior flow and levelling. For this reason, many alkyd resins for stoving enamels are formulated with small quantities of long-chain fatty acids. Where the emphasis is on excellent yellowing resistance, they may contain fractions of hydrogenated coconut fatty acids. There are also combinations of synthetic fatty acids with fatty acids that contain unsaturated C18 carboxylic acids. Such combinations improve solubility, compatibility, wetting properties, flow, levelling, and gloss. These offer less risk of blistering or pinholing

Figure 7.13:  Structural segments of alkyd resins with different polyols

Figure 7.14:  Structure of a molecular segment of a pentaerythritol alkyd

199

Alkyd resins Table 7.11:  Model alkyd based on pentaerythritol n

building element

M

m=n·M

m-‰

1.000

phthalic anhydride

148

148.00

327.0

1.080

pentaerythritol

136

146.88

324.5

1.240

isononanoic acid

158

195.92

432.8

490.80

1084.3

sum 2.119

water

18

yield (AV = 15.0) acid value [mg KOH/g] polyester constant number-average molecular weight [g/mol] number of structural units occupation rate degree of branching [mol/kg] OH value [mg KOH/g]

– 38.14

– 84.3

452.66

1000.0

AV 15.0 1.2010 k'M Mn 2252 q 5.0 b' 0.51 v 2.03 OHV 148.9

during application. Small quantities of unsaturated fatty acids containing 9,12- or 9,11-linoleic acid will not impair either the resistance to discolouration or weatherability. Here it has been found that these combinations of fatty acids can even support gloss retention during weathering. The various alkyd resins described here that contain synthetic fatty acids and, where required, combinations with unsaturated fatty acids, are combined with urea resins or melamine resins as crosslinking agents. Combinations with melamine resins remain the most important basis for stoving enamels, for solvent-borne automotive OEM topcoats, for tough industrial coatings, for can-coating and coil-coating systems. In addition, they form the basis for solvent-borne pigment pastes. Currently, the melamine resins chosen contain only tiny amounts of free formaldehyde, because it has been classified as mutagenic (category 2 mutagen) and carcinogenic (category 1B carcinogen) since 1 April 2015. The guide value for in-door application is an occupational exposure limit 0.1 mg/m³ formaldehyde. The combination of alkyd resins with melamine resins, described here, yields coatings which are notable for their optimum balance of hardness and flexibility and adequate resistance to yellowing and weathering. They bestow several advantages on alkyd resins over competing binders, namely saturated polyesters and acrylic resins, in the stated application fields: better solubility, better pigment and substrate wetting, and optimum application properties (see Chapter 7.4). The most important crosslinkers are butanol-etherified melamine resins. In accordance with the OH value, the reactivity of the OH groups and functional groups of melamine resins, and the molecular weight of melamine resins, the combination ratios are between 60 : 40 wt.% and 85 : 15 wt.%. The curing conditions are 30 min at 130 °C and up to 20

200

Alkyd resins for co-crosslinking min at 180 °C. Co-crosslinking occurs between alkyd resin and melamine resin while the melamine resin can undergo self-crosslinking. The curing temperature depends on the reactivity of the melamine resins. The crosslinking reactions are catalysed by residual carboxyl groups on the alkyd resins. However, if necessary, catalysts can be added: sulfonic acids, acidic phosphoric acid esters, maleic acid, and maleic acid half-esters. The balance between co-crosslinking and self-crosslinking of melamine resins is influenced by the mass ratio of alkyd resin to melamine resin, the reactivity of the functional partner groups, the temperature, and the efficiency of the catalysts. Co-crosslinking is supported by a high content of alkyd resin, a high OH value and highly reactive OH groups, and a low curing temperature (which mainly entails a long curing time). Self-crosslinking is supported by high quantities of melamine resin, highly reactive functional groups on the melamine resin, elevated temperatures, and strong acid catalysts. The co-crosslinking improves the flexibility, chemical resistance and weatherability of coating films. The self-crosslinking improves hardness and solvent resistance. Typical commercial products: Setal F 310 (Allnex-Nuplex [76], Synolac 4422 (Arkema-Cray Valley [77]), Uralac AN 633 (DSM [67]), WorléeKyd C 628 (Worlée [78]) Furthermore, there are alkyd resins for stoving enamels which, in addition to sufficient quantities of OH groups, contain only fatty acids rich in 9,12-linoleic acid. They are distinguished by good pigment wetting and optimum application behaviour. Naturally, they do not confer the same yellowing resistance as the alkyds containing synthetic fatty acids. They are used for industrial stoving enamels. Typical commercial product: Synolac 130 (Arkema-Cray Valley [77]) Of particular significance are alkyd resins for stoving enamels which contain conjugated fatty acids (9,11-linoleic acid). At elevated temperatures, 9,11-linoleic acid can undergo 1,4-polymerisation. The combination of crosslinking reactions by melamine resins and the polymerisation reactions yield optimum molecular networks at low temperatures, leading to good film properties. They show excellent pigment and surface wetting and improve flexibility. They have less tendency to yellow than products containing 9,12-linoleic acid. There are products which contain mixtures of synthetic fatty acids and fatty acids rich in 9,11-linoleic acid. Fatty acids containing 9,11-linoleic acids are based on fatty acids containing 9,12-linoleic acids obtained from soybean oil or sunflower oil. The isomerisation is carried out during saponification of the oils with excess alkali at 200 to 300 °C. The free fatty acids are obtained by adding mineral acids. The yield of conjugation is about 95 mol-% of diene components [146]. 9,11-Linoleic acid is also prepared by dehydration of ricinoleic acid (12-hydroxy-9-cis-octadecenoic acid), based on castor oil, with toluenesulfonic acid (dehydrating

201

Alkyd resins agent). The result is 9,11-linoleic acid in different isomers (DCO, dehydrated castor oil fatty acid), mainly trans-isomers. The dehydrating process can be performed simultaneously with the production of alkyd resins. The process starts with castor oil, the polyol and a portion of phthalic anhydride. The mixture is heated to 260 to 280 °C, and the oil is transesterified to mono- and di-esters. Simultaneously the phthalic anhydride adds to the 12-hydroxyl group, and this is followed by decomposition of the ester to generate a new double bond and phthalic acid. Later, the remaining portion of phthalic anhydride is added to complete the preparation of the alkyd resin. This type of alkyd resin is mainly used for stoving enamels which are cured at relatively low temperatures (e.g. in “80°” enamels). Such coating systems are used for substrates which are sensitive to elevated temperatures (e.g. plastics and hybrid constructions), for larger objects which cannot be heated to elevated temperatures (e.g. agricultural equipment, large vehicles) and for repair coatings. In these application fields, the systems are in competition with two-pack coatings, which crosslink very efficiently, but have a pot-life and are more expensive. Typical commercial products: Setal 118 (Allnex-Nuplex [76]), Synthalat RT 35 (Synthopol [80]), Uralac AD 543 (DSM [67], WorléeKyd RM 232 (Worlée [78])

7.3.2 Alkyd resins for acid-cure systems The alkyd resins described in Chapter 7.3.1 also lend themselves for acid-cure coatings. The binders for acid-cure coatings are principally the same as for stoving enamels. The alkyds contain sufficient quantities of hydroxyl groups for crosslinking. The crosslinking agents are amino resins, with urea resins preferred. Crosslinking is initiated by adding acid catalysts. Suitable catalysts are relatively strong acids, e.g. hydrochloric acid, phosphoric acid, acidic phosphoric esters, and sulfonic acids. The reaction takes place at ambient temperature. Of course, once the catalyst has been added, the application time is limited (pot-life). For acid-cure systems, alkyds with high OH values are preferred, which can be achieved by using pentaerythritol as the polyol. The monocarboxylic acids are synthetic fatty acids, as well as fatty acids with double bonds, e.g., tall oil fatty acids which contain 9,12-linoleic acid (55 to 62 wt.%) and substantial quantities of oleic acid (25 to 32 wt.%). The unsaturated fatty acids support the plasticising effect of alkyds in these combinations. The alkyd resins are combined with etherified urea resins (butanol ether) in ratios ranging from 50 : 50 to 70 : 30 wt.%. Under these conditions, self-crosslinking of urea resins predominates. Acid-cure systems are preferred for wood coatings and flat laminates. The resulting films are hard and are resistant to mechanical impact. They offer good resistance to solvents and chemicals.

202

Alkyd resins for co-crosslinking In the past, the domain of acid-cure paints was furniture coatings. However, they have since become less important for this application field. The reason is that the curing reaction, which occurs in the presence of a strong catalysts, involves release of formaldehyde. Residues of free formaldehyde can continue to escape from the furniture films for prolonged periods of time, a fact which is noticeable from the typical odour. Initially, attempts were made to add agents to absorb the formaldehyde, e.g. urea, but without any real success. As formaldehyde is now classified as mutagenic and carcinogenic, acid-cure coat­ ings for furniture have nearly all been replaced at this stage. Nowadays, the binders for most furniture coatings are water-borne primary acrylic dispersions, which form films by physical drying. The films are less resistant to solvents, chemicals and moisture. Typical commercial products: Setal 118 (Allnex-Nuplex [76]), Synolac 1035 (Arkrema-Cray Valley [77]), Synthalat R 43 (Synthopol-Chemie [80]), WorléeKyd SH 380 (Worlée [78])

7.3.3 Alkyd resins for polyisocyanate crosslinking Alkyd resins which are intended for crosslinking with polyisocyanate adducts which contain free isocyanate groups, consist mainly of phthalic anhydride, trimethylolpropane or pentaerythritol, and saturated fatty acids, especially synthetic fatty acids. There are products which contain some unsaturated fatty acids. In composition, these alkyd resins are essentially no different from the alkyd resins used for stoving enamels. However, there are products which are specifically recommended for crosslinking with polyisocyanates. The reaction of OH groups in alkyd resins with isocyanate groups is well established. A secondary reaction occurs with atmospheric humidity to yield urea linkages. To yield optimum film properties, the alkyd resins have high OH values – or low modification ratios of excess hydroxyl groups. Alkyd resin and polyisocyanate adducts are combined in more or less stoichiometric ratios, with the molar ratio of isocyanate groups to OH groups ranging from 1.2 : 1.0 to 0.9 : 1.0. Well-defined, extended molecular networks are formed. Since combinations of alkyd resins and polyisocyanate adducts are ideal for flexible coating systems, there is scope for incorporating building blocks which support flexibility. This is important for achieving high OH values if the content of monocarboxylic acids is low. Building blocks that boost flexibility are aliphatic dicarboxylic acids, e.g. adipic acid, which replace some of the phthalic anhydride. Greater flexibility is achieved with ether-diols, e.g. diethylene glycol or dipropylene glycol, or polyether polyols. Such products lead to saturated polyesters (see Chapter 5.5.3). As a result of the effective crosslinking reaction, it is possible to select number-average molecular weights for the alkyd resins which are lower than those for alkyd resins for stoving enamels. Unlike the case for crosslinking with amino resins, the carboxyl groups do not act as crosslinking catalysts in combinations with polyisocyanate adducts. Carboxyl

203

Alkyd resins Table 7.12:  Alkyd resin based on phthalic anhydride, adipic acid, trimethylolpropane and isononanoic acid for two-pack coatings n

building element

M

m=n·M

wt. ‰

1.120

trimethylol propane

134

150.08

416.9

0.750

phthalic anhydride

148

111.00

308.3

0.250

adipic acid

146

35.50

101.4

0.600

isononanoic acid

158

sum 1.799

water

18

yield (AV = 8.0) acid value [mg KOH/g] polyester constant number-average molecular weight [g/mol] number of structural units occupation degree degree of branching [mol/kg] OH value [mg KOH/g] viscosity (ICI, 23 °C) [mPa·s]

94.80

263.3

392.38

1089.9

32.38 360.00

89.9 1000.0

AV 8.0 1.1713 k'M Mn 2101 q 5.8 b' 0.43 v 1.44 OHV 126.5 η 4200

groups can actually give rise to problems here (e.g. yellowing). Thus, alkyd resins which are designed for isocyanate crosslinking have relatively low acid values (much lower than 10 mg KOH/g). Alkyd resins containing synthetic fatty acid, crosslinked with aliphatic or cycloaliphatic polyisocyanate adducts (HDI trimers, IPDI trimers), are preferred for two-pack topcoat formulations. This combination offers good weatherability and resistance to discolouration. The resulting films are high-gloss and flexible. Such combinations are also used for plastic coatings, especially if the alkyd resins contain building blocks which support flexibility. Alkyd resins with an unsaturated fatty acid content, which are crosslinked by aromatic polyisocyanate adducts (TDI adducts, MDI derivatives), are mainly used for wood coatings. They provide good wetting and penetration and confer excellent resistance to chemicals, solvents and moisture. Such alkyd resin and polyisocyanate adduct combinations are also suitable for anti-corrosive coatings (primers). A special class of alkyd resins for polyisocyanate crosslinking are those containing ricinoleic acid sourced from castor oil (content 85 to 90 wt.%). Ricinoleic acid is 12-hydroxy-9-cis-octadecenoic acid. The hydroxyl group can participate in the formation of the polyester molecule or can react with isocyanates in crosslinking processes. To determine the number-average molecular weight, ricinoleic acid must be calculated as hydroxycarboxylic acid, and must be considered to the molar contents of polyol and of polycarboxylic acid (see Chapter 3.3.2). Ricinoleic acid contributes outstanding flexibility and good wetting properties.

204

Alkyd resins for co-crosslinking Table 7.12 presents data for a model alkyd resin [147] which contains a mixture of phthalic anhydride and adipic acid and a substantial excess of trimethylolpropane. The monocarboxylic acid is isononanoic acid (low b' value). The resin has an acid value of 8.0 mg KOH/g, an OH value of 126.5 mg KOH/g, and an average molecular weight of 2101 g/mol. Typical commercial products: Setal D RD 181 (Allnex-Nuplex [76]), Synolac 1333 (Arkema-Cray Valley [77]), WorléePol 6631 (Worlée [78]) Alkyd resins containing hydroxyl groups can also be combined with blocked polyisocyanates as crosslinking agents. For effective crosslinking, elevated temperatures are needed, the precise value of which varies with the type of blocking agent. The alkyd resins for this application should contain appropriate building blocks. Combinations of alkyd resins and blocked polyisocyanates for stoving enamels are distinguished by their wetting behaviour, levelling and gloss. However, polyesters are preferred to the alkyd resins because their crosslinking with blocked polyisocyanates is more efficient.

7.3.4 Alkyd resins for high-solid reactive coatings To increase application solids via a reduction in solution viscosity, the number-average molecular weights can be lowered, or the dispersity of the molecules reduced (narrower molecular weight distribution). The number-average molecular weights can be lowered substantially if the alkyd resins are crosslinked efficiently. This offers a way of forming optimum films from low-molecular weights. There are alkyd resins for high-solids reactive coatings that have number-average molecular weights of about 800 g/mol, i.e. the resins contain just 2.5 to 3 structural units on average. For optimum crosslinking, the alkyd resins need to have sufficient hydroxyl groups. Thus, the modification ratio of excess hydroxyl groups is low. Such resins require relatively polar solvents. If the polyester back-bone of the alkyd resins is mostly composed of the common building blocks (phthalic anhydride, trimethylolpropane or pentaerythritol), the viscosity is relatively high. What is more, a higher degree of branching and higher OH values can also increase the viscosity. Specific measures are therefore needed when the goal is to produce high-solids alkyd resins. One possibility is to replace the aromatic phthalic anhydride by aliphatic or cycloaliphatic dicarboxylic acids. The benefit of replacing phthalic anhydride by hexahydrophthalic anhydride is greater than that of replacing isophthalic acid in the case of saturated polyesters (see Chapter 5.5.5). The reason is that the addition reaction of phthalic anhydride has the positive effect of reducing the dispersity of the molecular weight distribution. The introduction of aliphatic building blocks leads to lower viscosities at a comparable number-average molecular weight but contributes greater plasticity. Also, the use of ether diols reduces viscosity, but limits weatherability.

205

Alkyd resins Table 7.13:  Alkyd resin containing glycid ester, phthalic anhydride, trimethylolpropane and DCO fatty acid n = m/M

building element

M

m

wt. ‰

0.505

glycidester of neodecanoic acid

249

125.75

286.8

0.190

DCO fatty acid

280

53.20

121.3

0.911

trimethylol propane

134

122.07

278.4

1.000

phthalic anhydride

148

148.00

337.6

449.02

1024.1

sum 0.588

water yield (AV = 12,4)

acid value [mg KOH/g] polyester constant number-average molecular weight [g/mol] number of structural units occupation rate degree of branching [mol/kg] OH value (mg KOH/g) viscosity (80 % in xylene) [mPa·s]

18

– 10.56

24.1

438.43

1000.0

AV 12.4 1.5129 k'M Mn 855 q 1.90 b' 0.30 v 1.64 OHV 211.2 η 60 – 70

One particular method consists in using the glycid ester of neodecanoic acid [142, 143]. The ester acts as a diol-anhydride and is incorporated into the alkyd resin molecule by an addition reaction. This leads to a narrower molecular weight distribution and thus to lower solution viscosities. Table 7.13 shows an example [148] of an alkyd resin consisting of such a glycid ester, phthalic anhydride, trimethylolpropane, and DCO fatty acid (dehydrated castor oil). For stoving enamels, high-solids alkyd resins are combined with low-viscosity amino resins, mainly with the low-molecular melamine resins fully etherified with methanol (hexamethoxymethyl melamine resins [HMMM resins]). These combinations require the addition of acid catalysts (e.g. p-toluenesulfonic acid). The binder compositions allow the application solids to be raised to more than 55 wt.% for black topcoats and up to 85 wt.% for white ones. The coatings exhibit good levelling and flow, while the films are glossy and have adequate weatherability. They are used for industrial stoving enamels as well as for special automotive topcoats. If polyisocyanate adducts are used, the application solids can be relatively high due to the low viscosity of isocyanate hardeners. Such two-pack systems are suitable for high-solid topcoats, which are very flexible and contribute chemical resistance and solvent resistance.

206

Alkyd resins for co-crosslinking

7.3.5 Alkyd resins for water-borne reactive coatings Nearly all alkyd resins for water-borne reactive coatings contain carboxylate or sulfonate anions as carrier groups in aqueous media. The method for introducing carboxylates is the same as that described for saturated polyesters (see Chapter 5.6), namely: 1. Addition of anhydrides to OH alkyd resin precursors 2. Limiting the degree of condensation at higher acid values (requires pre-reaction of fatty acids or transesterification) 3. Addition and partial esterification of trimellitic anhydride 4. Making an adduct of diisocyanate and reacting it with dimethylol propanoic acid in alkyd molecules Resistance to discolouration and weathering of water-thinnable alkyd resins is provided by saturated fatty acids (synthetic fatty acids). However, some types contain unsaturated fatty acids. To facilitate crosslinking, the alkyd resins need a sufficient quantity of hydroxyl groups. The acid values of the alkyds are between 25 and 60 mg KOH/g. The neutralisation agents are amines (N,N-dimethylethanolamine [DMEA], diethanolamine [DEA], diisopropanolamine [DIPA], and 2-amino-2-methyl propanol [AMP]). The neutralisation degrees are between 70 and 100 mol-%, leading to pHs of 7.5 to 8.5. For stoving enamels, these water-thinnable alkyd resins are combined with water-thinnable amino resins, namely the low-molecular, fully methanol-etherified melamine resins, as well as benzoguanamine resins etherified with methanol. These combinations are suitable for industrial stoving enamels (primer surfacers, topcoats) and for automotive topcoats. Under many application conditions, the relatively high acid values are sufficient to catalyse the crosslinking reaction. However, there is also the possibility of adding external catalysts, e.g. amine salts of sulfonic acids. Water-thinnable alkyd resins of high OH value can also be combined with polyisocyanate adducts containing free isocyanate groups. In comparison to saturated OH polyesters, the reactivity of the hydroxyl groups of water-thinnable alkyds is diminished due to partial steric hindrance by the fatty acid side-chains. The low polarity of such alkyd resins supports inter-diffusion with the more apolar polyisocyanate adducts. In addition, the fatty acid content supports flow, levelling and appearance of films. Again, the biggest issue is the sensitivity of alkyd resins to saponification. To improve saponification resistance, it is necessary to choose special building blocks and structures. This can be achieved, for example, by incorporating an adduct of diisocyanate and dimethylol propanoic acid (urethane modification). Table 7.14 [149] shows an alkyd resin containing the hydrogenated fatty acid fraction of palm kernel oil [150], trimethylolpropane, pentaerythritol, phthalic anhydride, and an adduct of isophorone diisocyanate (IPDI) and dimethylol propanoic acid (DMPA). The alkyd resin pre-product has an acid value of 1.0 mg KOH/g, an OH value of 248.6 mg KOH/g,

207

Alkyd resins Table 7.14:  Water-thinnable urethane-alkyd with hydroxyl groups for crosslinking by amino resins n = m/M 0.043

building elements hydrogenated palm kernel fatty acid, C16–C18-fraction

M

m

wt. ‰

277

11.880

296.8

0.058

trimethylol propane

134

7.820

195.4

0.024

pentaerythritol

136

3.320

83.0

0.051

phthalic anhydride

148

7.570

189.2

adduct of 2 IPDI + 1 DMPA

578

10.790

269.6

41.38

1034.0

18

1.36

34.0

40.02

1000.0

0.019

sum 0.075

water yield (AV = 26,0)

acid value [mg KOH/g] polyester constant number-average molecular weight [g/mol] number of structural units occupation rate degree of branching [mol/kg] OH value [mg KOH/g]

AV 26.0 1.1838 k'M Mn 3119 q 5.4 b' 0.32 v 2.07 OHV 126.3

and a number-average molecular weight of 899 g/mol. The finished urethane alkyd has an acid value of 26.0 mg KOH/g, an OH value of 126.3 mg KOH/g, and a number-average molecular weight of 3119 g/mol. The resin in this example is neutralised by triethylamine; the degree of neutralisation is 0.80 moles per carboxyl group. The resin is dissolved in N-methylpyrrolidone, and the solids content is 88 wt.%. A surfactant is also added. The solution is diluted with deionised water to a solids content of 40 wt.%. This binder has been proposed for the formulation of water-borne automotive effect basecoats. To this end, it is combined with a polyurethane dispersion and a water-thinnable melamine resin. Typical commercial products: Synolac 8300 (Arkema-Cray Valley [77]), Synthalat KF 43 (Synthopol-Chemie [80]), Uradil AZ 3540 (DSM [67]), WorléeSol 84 C (Worlée [78]) Water-thinnable alkyd resins with cationic stabilisation also exist. Typical commercial product: Uradil AZ 515 (DSM [67]) On account of their better saponification resistance, the OH acrylic resins in secondary aqueous dispersions are preferred in the aforementioned application fields.

208

Alkyd resins for co-crosslinking

7.3.6 Other alkyd resins for reactive coatings Like oxidative-cure alkyd resins, alkyd resins for co-crosslinking can be modified with further building blocks. Urethane alkyd resins confer rapid initial drying to yield films which are distinguished by high hardness, good flexibility, and resistance to chemicals and moisture. If the modification is performed with aliphatic or cycloaliphatic diisocyanates, combinations with melamine resins or with aliphatic or cycloaliphatic polyisocyanate adducts have good weatherability. Such compositions are suitable for high-grade industrial coatings. Modification of alkyd resins for stoving enamels with siloxane intermediates is carried out by the same methods as described for modified saturated polyesters (see Chapter 5.9) and oxidative-cure alkyds (see Chapter 7.2.10). If they have a sufficient content of siloxane intermediates, the silicone alkyd resins can crosslink by themselves. Moreover, silicone alkyd resins with a low content of siloxane intermediates can be combined with melamine resins. Films containing silicone alkyd resins have good resistance to chemicals, water, weathering, mechanical impact and elevated temperatures (if they contain saturated fatty acids). Integration of epoxy resins into alkyd resins containing OH groups yields co-crosslinkable epoxy alkyd resins. For the molecular structure and the calculation of number-average molecular weights, the epoxy resin fraction is treated as a macro-polyol. Modifications with epoxy resins mainly improve the adhesion and corrosion resistance of the resulting coatings. Stoving enamels and two-pack systems can be formulated. The products are suitable for primers and primer surfacers, and especially for can-coating and coil-coating systems. Epoxy alkyd resins for water-borne systems are also available. Typical commercial products: Resydrol AX 246, AX 247, AX 250 (Allnex [68]) A special case is the use of acrylic resins to modify alkyd resins containing crosslinkable OH groups. The rationale behind this modification is to combine the advantages of their respective properties (see Chapter 7.4). In the past, trials aimed at combining the two resins in solution often revealed incompatibility. The answer was to polymerise acrylic monomers in alkyd resin solution. It is believed that the polymerisation process involves free-radical transfer between polyacrylate and alkyd resin, even though the alkyd resin does not contain double bonds for polymerisation. The resulting combinations are characterised on one hand by good wetting, flow and levelling (of the alkyd fraction) and on the other by fast initial drying and durability (acrylic fraction). Such binders are recommended for automotive repair coatings. Typical commercial products: Setalux C 1151, 1152, 1184 (Allnex-Nuplex [76])

209

Alkyd resins

7.4 Comparison of OH alkyd resins and OH polyesters with other resins The most important functional group for crosslinking reactions is the hydroxyl group. Resins containing hydroxyl groups include saturated polyesters, alkyd resins, acrylic resins, epoxy resins, polyethers, polyvinylchloride co-polymers, and cellulose esters. Of these, the most important are the first three. The crosslinking agents for the OH groups are amino resins, polyisocyanate adducts, phenol resins (resol types), siloxanes. Amino resins and polyisocyanate adducts are mostly combined with OH polyesters, OH alkyd resins, and OH acrylic resins. The amino resins are urea resins, melamine resins, and benzoguanamine resins. Polyisocyanate adducts can contain free isocyanate groups or blocked polyisocyanate groups. Solvent-borne and water-borne systems and special 100 % systems exist. OH polyester resins, alkyd resins and acrylic resins are available in large diversity. Within each of these groups, the products differ in molecular weight, molecular weight distribution, molecular structure, number of functional groups (especially OH groups), and the influence exerted by the building blocks. Nevertheless, it is possible to define properties which are typical of the group members and to compare such properties. The binders’ typical properties can be used to derive the resins’ main application fields. There is a correlation here between molecular composition and structure, behaviour in the application state, and the application and film properties. Saturated OH polyesters have molecular weights between 800 and 3000 g/mol. There are linear and branched products, with different degrees of branching in open chains. In organic solutions, they form colloidal coils containing several molecules. The particles are effectively flooded with solvents to form solvates. Due to the molecular structure of polyesters and the colloidal state in solution, the OH groups of polyester molecules are readily available for crosslinking reactions – more so than the other product classes. This is one condition for optimum co-crosslinking. The outcome is extended networks. Another condition is optimum compatibility between polyester and crosslinker, as crosslinking dur­ ing film formation entails diffusion of colloidal particles. OH polyesters confer an optimum balance of hardness and flexibility on films. The contribution by elasticity to overall flexibility is fairly high but depends also on the type of building block. Chemical resistance is high as well, due to the optimum crosslinking. Combinations of saturated polyesters and crosslinkers are therefore employed wherever there is a need for high flexibility combined with adequate hardness and good durability. Such combinations are suitable for primers and primer surfacers, basecoats in automotive OEM coatings and repair coatings, as well as for plastic coatings, for can-coatingand coil-coating systems, and for electrical insulation coatings.

210

Comparison of OH alkyd resins and OH polyesters with other resins Saturated polyesters do not exhibit the same breadth of solubility and compatibility as the two other product groups. They require careful choice of solvents and combination binders. Alkyd resins confer better pigment wetting and better flow and levelling. However, provided that the pigments are optimally wetted, the weatherability of topcoats containing saturated polyesters and melamine resins or aliphatic and cycloaliphatic polyisocyanates as crosslinkers is good. One reason for this is the effective, extended molecular networks in the crosslinked film. When films start to deteriorate due to weathering, the film properties are still stable. Loss of gloss, discolouration, chalking, embrittlement, cracking or blistering are delayed or avoided. As stated previously in connection with binders for clearcoats, mainly for automotive clearcoats in two-layer effect coatings, the salient factor is just a small content of aromatic building blocks or lack of such building blocks. They should also not possess overly high glass-transition temperatures. Otherwise, the clearcoat films are at risk of cracking after weathering. The weatherability of polyesters is not related to the saponification sensitivity of polyester chains. Polyesters are relatively resistant to saponification in moderate acidic media and will not react until the pH is really high. That being said, weathering conditions are in the main slightly acidic. The first points of attack are the crosslinking agents. Films of polyesters crosslinked with aromatic polyisocyanate adducts, urea resins or benzoguanamine resins are essentially non-weatherable. In films of polyesters crosslinked with melamine resins, the latter are moderately resistant to acids. They can be decomposed even by slightly acidic media. However, films which contain aliphatic or cycloaliphatic polyisocyanate crosslinkers are particularly stable. Water-thinnable polyesters generate open-coiled colloidal particles in aqueous phase, especially when the formulations contain co-solvents. Again, this is the reason that the hydroxyl groups of polyesters are more readily available for crosslinking reactions. This also predestines the resins for combination with polyisocyanate adducts. However, the saponification resistance is limited at high pH – acrylic resins confer better saponification resistance. OH alkyd resins mainly have number-average molecular weights between 1500 and 4000 g/mol. They are always branched and usually have OH values of 80 to 150 mg KOH/g. High-solid alkyd resins have low molecular weights and high OH values. Here, it is believed that alkyd resins in organic solution form colloidal particles of several binder molecules, in which the side-chains of monocarboxylic acids are oriented towards the particle surface (shell). These particle structures are similar to those of surfactant micelles. This idea stems from the fact that the crosslinking reactions of alkyd resins are less efficient than in the case of saturated polyesters. The oriented fatty acid side-chains can have a steric hindrance effect. The lower the content of fatty acid in alkyd resins, the less pronounced is this effect. Total hindrance of crosslinking does not occur, merely gradual differences in efficiency and reaction rate. On the other hand, this structure postulated for diluted alkyd resins is likely the reason for a particular advantage of alkyd resins. They are soluble in relatively nonpolar solvents –

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Alkyd resins much more so than all saturated polyesters – and are compatible with a wider range of binders. They are better at wetting pigments and substrates. They show good application behaviour and exhibit less tendency to form blisters and pinholes. Generally, they generate better levelling properties and optimum initial gloss. Consequently, the main application fields for OH alkyd resins are stoving enamels for industrial coatings (machinery), automotive topcoats (OEM, large vehicles, agricultural equipment) and coil-coating and can-coating systems. They also serve as two-component topcoats for industrial application and plastic coatings. Films yielded by OH alkyd resins crosslinked with melamine resins have comparatively low chemical resistance, yet the chemical resistance is sufficient for most applications. Good chemical resistance is obtained by combinations of OH alkyd resins with polyisocyanate adducts. On account of the optimum pigment wetting, weatherability is good, as there is good gloss retention. In highly pigmented topcoats, they are far superior to saturated polyesters in this regard. Loss of gloss and chalking is observed much later in weathering tests. For clearcoats, it also holds that products with a high content of aromatic building blocks must be avoided. Clearcoats of OH alkyd resins containing aliphatic and cycloaliphatic building blocks do not show cracking upon weathering. OH alkyd resins in aqueous media are less resistant to saponification than the products of the two other classes of binder. It is thus essential to use building blocks which improve saponification resistance. For water-borne alkyd resins, it is believed that, unlike the case for organic solutions, the non-polar side-chains of monocarboxylic acids are oriented towards the core of the colloidal particle while the polyester backbone bearing the anionic carrier groups point towards the shell. The optimum compatibility of alkyd resins with other binders also applies to aqueous systems, which afford plenty of opportunities for interdiffusion of alkyd resin and crosslinkers. The water-thinnable alkyd resins show good flow and levelling during film formation. OH acrylic resins consist of linear polymer chains which are intimately coiled. The chains bear OH groups and carboxyl groups as side-chains. The number-average molecular weights of polyacrylates are between 2500 and 7500 g/mol. The OH values are usually 60 to 150 mg KOH/g. High-solids acrylic resins have low number-average molecular weights (about 1500 g/mol) and high OH values. In organic solutions, the colloidal molecular coils have a higher density than polyesters and alkyd resins. The particles contain fewer solvent molecules (solvates), and this influences the solution viscosity. For example, a solution of an acrylic resin of number-average molecular weight of 5000 g/mol has the same viscosity as a solution of a polyester resin of 1800 g/mol, given the same solvent and concentration and the same polarity of resin molecules. This result is explainable only in terms of different numbers and size of the colloidal particles. Due to the higher particle density, the OH groups of acrylic resins are less amenable to crosslinking reactions. Diffusion by crosslinkers is also decreased. OH acrylic resins crosslink

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Comparison of OH alkyd resins and OH polyesters with other resins less effectively than OH alkyd resins, and far less so than saturated OH polyesters. Thus, the balance of hardness and flexibility needs to be provided via the building blocks. The plasticity component has to make a greater contribution to the overall flexibility. Given the colloidal particle structure stated previously, wetting properties on pigments and substrates are less pronounced but can be provided by adding wetting agents. There are ways to optimise the behaviour of OH acrylic resins. If monomers with long side-chains or an adduct of the glycid ester of neodecanoic acid [142, 143] added to carboxyl groups of OH acrylic resins, alkyd-like properties result. This is essential for obtaining the application behaviour. Spray coatings prepared from common OH acrylic resins exhibit a greater tendency to form blisters and pinholes than the other resins. A comparison of pigmented OH alkyd and OH acrylic stoving topcoats of the same pigment volume concentration and having the same melamine resin crosslinker shows that the acrylic topcoat has much less filling power and hold-out. OH acrylic resins show – to an extent depending on the type and content of crosslinker – particularly good hardness and resistance to chemicals and discolouration. They are as weatherable as the other binders. Acrylic resins for clearcoats should not have a high content of aromatic building blocks (e.g. styrene), as otherwise they will crack upon weathering. This tendency is supported by a high glass-transition temperature of resins. Nevertheless, OH acrylic resins are preferred to the other resins when it comes to automotive OEM and repair clearcoats. The objective reason is not the oft-quoted good weatherability, but rather the positive initial drying properties. Although the high density of colloidal particles is a disadvantage for effective crosslinking, it is an advantage for initial drying properties. The tendency of acrylic resin molecules to associate leads to much faster solvent evaporation. Thus, wherever fast solvent evaporation is required, acrylic resins are beneficial. This is crucial for clearcoats which are applied by the wet-on-wet process in which the clearcoats are applied direct to effect basecoats, which are dried only physically, without any re-dissolving. Furthermore, the viscosities of films containing acrylic resins decline less during the heating-up phase of stoving. Thus, the real cause for preferring acrylic resins to alkyd resins and polyester resins is not better weatherability – as there are products which offer comparable weathering behaviour – but rather their better application behaviour. On account of their good initial drying properties, OH acrylic resins are used if film forming has to take place at ambient temperatures. They therefore find application in twopack automotive repair coats (topcoats, clearcoats). For primer surfacers and for plastic coatings, polyester systems are preferred. As two-pack coatings containing OH acrylic resins possess good chemical resistance, they are used for high-grade coatings, generally for industrial applications. In water-borne coatings, OH acrylic resins are distinguished by their good saponification resistance, even though the side-chains are linked together by ester groups. However, the ester groups in side-chains are sterically protected. Given the tendency of acrylic resin

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Alkyd resins molecules in colloidal particles to associate in water, the crosslinking efficiency is also hindered. This applies to crosslinking with melamine resins, but obviously more so in the case of polyisocyanate adducts as crosslinkers. In summary, the main application fields of the different classes of binder are as follows: Saturated OH polyesters – Primer surfacers, basecoats, coil-coating, can-coating, plastic coatings OH alkyd resins – Stoving enamels (topcoats, one-layer coatings) plastic topcoats, pigment pastes OH acrylic resins – Repair coatings (topcoats, clearcoats), automotive OEM clearcoats, plastic coatings, industrial coatings

7.5

 H alkyd resins – O combination partners with physically drying binders

Coating systems which form films solely by physical drying are usually based on high-molecular weight polymers. Film formation takes place by rapid solvent evaporation and the tendency of polymer molecules to associate. The resulting films are often relatively brittle and so the polymers are combined with plasticisers. The latter are used in the case of cellulose ethers, cellulose esters (acetyl cellulose, cellulose acetobutyrate), cellulose nitrate, chlorinated rubber, polyvinyl chloride and its copolymers, thermoplastic acrylic resins, and polystyrene. There are different theories surrounding the plasticising effect. Plausibly, the molecules of gelling plasticisers associate in a manner akin to the formation of solvates in the case of solvents. Non-gelling plasticisers act like solvents or thinners. The polymer and the type and quantity of plasticiser need to be chosen carefully so that the films formed have sufficient mechanical resistance (subjectively solid surfaces) along with good adhesion and good flexibility. In addition, the plasticisers have to be aging-resistant, which means they must not evaporate or migrate through the film. This condition is met by the use of polymers as plasticisers. One such polymeric plasticiser is polyester (see Chapter 5.2), another is alkyd resin. As, for example, cellulose esters and polyvinyl chloride are relatively polar, the plasticisers too must be polar in order to be able to confer optimum compatibility and migration resistance. The OH alkyd resins described in Chapter 7.3 make suitable plasticiser resins. Although their molecular weights are not high, in combination with the physically drying polymers they are sufficiently resistant to migration. The content of monocarboxylic acids

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OH alkyd resins – combination partners with physically drying binders is usually low. Monocarboxylic acids are chosen which confer adequate and lasting flexibility. Synthetic fatty acids are not effective enough. Polyunsaturated fatty acid can age through oxidative-cure. The preferred products are fatty acids with a sufficient content of oleic acid (octadecenoic-9 acid) which generate optimum molecular mobility and do not react with atmospheric oxygen. The most important source of oleic acid is peanut oil (contains 40 to 65 wt.% oleic acid). However, alkyd resins with ricinoleic acid (12-hydroxy-9-cis-octadecenoic acid) are suitable and sourced from castor oil (contains 85 to 90 wt.% of ricinoleic acid). Also used as plasticisers are alkyd resins containing conjugated fatty acid, namely 9,11-linoleic acid, which is prepared by isomerisation of 9,12-linoleic acid or by dehydration of ricinoleic acid. The conjugated fatty acids can polymerise but do not lose their plasticisation effect. In the past, combinations of cellulose nitrate and alkyd resins played an important role as so-called combination enamels. Thanks to their somewhat better weatherability, they replaced polishing enamels, which contain mainly phthalic diesters (chiefly dibutyl phthalate) as plasticisers and confer adequate initial gloss without the need for polishing. The alkyd combinations are much more resistant to migration. Combination enamels were used until the 1960s for repair coatings and for application to large vehicles. The products are distinguished by ease of application, rapid initial drying, brilliant colours, and high gloss. However, they are not sufficiently resistant to solvents and chemicals. In addition, in the application state they contain too much solvent to meet current VOC regulations. They were replaced by coatings containing crosslinkable binder combinations. Such systems have greater weatherability, resistance to solvents, chemicals and mechanical impact and allow the application solids to be increased (see Chapter 7.3.5). Minor quantities of combinations of cellulose nitrate and alkyd resins are still used in low-cost furniture coatings for industrial application processes. Table 7.15 [151] presents a classic alkyd for plasticising cellulose nitrate containing glycerol, phthalic anhydride, and peanut fatty acid in the molar ratios of 1.1 : 1.0 : 0.6. The acid value is 15.0 mg KOH/g, the number-average molecular weight is 1916 g/mol and the OH value is 115.0 mg KOH/g. The content of fatty acid is 43.4 wt.%. The product has a relatively low viscosity. Such alkyd resins are combined with cellulose nitrate in ratios between 1 : 1 and 2 : 1 (solid alkyd : cellulose nitrate, dry). The alkyd resin can act as a dispersing medium for pigments. In addition, there are combinations of cellulose nitrate on one hand and amino resins on the other which are crosslinked by adding acid catalysts. Such systems are distinguished by fast initial drying but enhanced resistance to chemicals, solvents and moisture. A combination of polyvinyl chloride copolymers (with vinyl acetate, maleic esters, or acrylic esters) and alkyd resins is suitable for anti-corrosive coatings. Such systems can yield coatings of high film thickness. These coating films are characterised by good resistance to

215

Alkyd resins Table 7.15: Model formulation for a peanut oil fatty acid alkyd resin, a combination binder for cellulose nitrate n = m/M

building element

M

m

wt.‰

1.100

glycerol

92

101.20

257.7

1.000

phthalic anhydride

148

148.0

376.9

0.600

peanut oil fatty acid

284

170.40

433.9

419.60

1068.5

18

– 26.91

– 68.5

392.69

1000.0

sum 1.495

water yield (AV = 15.0)

acid value [mg KOH/g] polyester constant number-average molecular weight [g/mol] number of structural units occupation of excess OH groups degree of branching [mol/kg] OH value [mg KOH/g] viscosity [mPa·s] (60 % in xylene, ICI-Viscosity, 23 °C)

AV 15.0 k'M 1.2050 Mn 1916 q 4.9 b' 0.43 v 1.27 OHV 115.0 η 540

moisture. However, to provide saponification resistance, the polyvinyl chloride is com­ bined with other plasticisers. Typical commercial products: Setal AE 41 (Alnex-Nuplex [76], Synolac 926 (Arkema-Cray Valley [77]), Synthalat E 42 (Synthopol [80]), Uralac AN 582 (DSM [67]), WorléeKyd C 640 (Worlée [78])

216

Polycarbonates

8 Special polyesters 8.1 Polycarbonates Polycarbonates are essentially prepared by reacting alkali salts of bisphenols with phosgene or with diphenyl carbonate. Besides bisphenol A, other bisphenols with alkyl or aryl side chains are considered whenever particular properties are required. There are basically two production processes [62]. The first consists in making alkali bisphenolate react in aqueous phase (pH 9 to 11) with a solution of phosgene in halogenated hydrocarbon (e.g. carbon tetrachloride). The reaction takes place at the interface at 20 to 40 °C (see Figure 3.9). Oligomers are formed initially before the reaction is continued in the organic phase where, in the presence of a catalyst (tertiary amine), polycarbonates with molecular weights of 20,000 to 35,000 g/ mol are formed. The polymer is washed, precipitated and then either centrifuged or the solvent and remaining reactants are removed by distillation. The second process is the conversion of diphenyl carbonate and bisphenols in the melt at 190 to 320 °C in the presence of alkali catalysts and at reduced pressure. The resulting high-molecular aromatic polycarbonates are only soluble in halogenated hydrocarbons, pyridine and cresols and therefore unsuitable for common coating materials. The products have glass-transition temperatures of 150 °C and melting temperatures of more than 260 °C. They are highly transparent, and resistant to heat and mechanical impact. They are used for components in the electrical industry, for components, for automotive accessories, for household goods (tableware, food containers, bottles) and for electronic objects, especially CDs and DVDs. The polycarbonates intended for coating applications have much lower molecular weights and therefore different properties. They are prepared by replacing the bisphenols with long-chain aliphatic diols, e.g. 1,6-hexanediol. The production process is the transesterification of diphenyl carbonate and diol in the melt. The molecular weight is controlled by providing an excess of diol. The molecular weights, which are between 1000 and 4000 g/mol, determine whether the products are still liquids or have a wax-like consistency. They are soluble in esters, ketones, glycol ethers, glycol ether esters, and aromatic hydrocarbons. On account of their uniform molecular structure, they tend to crystallise. The products are very light, flexible and particularly resistant to saponification.

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

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Special polyesters If they are crosslinked with higher functional aliphatic or cycloaliphatic polyisocyanate adducts, the outcome are hard, tough films with excellent weatherability. A major application field for polycarbonate diols is as polyester segments for polyurethanes or polyester acrylates (see Chapter 5.4.1 and 5.4.3). Typical commercial product: Desmophen C 1200 (Covestro [73])

8.2 Polycaprolactones γ-Butyrolactone and δ-valerolactone are unsuitable for preparing polyesters, because the five-membered or six-membered rings are relatively stable and therefore re-form readily when conversion equilibrium is reached. However, the seven-membered ring of ε-caprolactone has such high ring tension that it can be incorporated quantitatively into polyester chains (the reaction is shown in Figure 3.10). Partners for the ring opening reactions are water, alcohols and carboxylic acids, the reaction with water or alcohols being preferred. The length of the resulting polyester chains depends on the molar ratios of water or alcohol to ε-caprolactone. Ring opening takes place by addition of the polarised hydrogen atom of alcohol or water to the oxygen bridge of the cyclic ester. The reaction is carried out at 120 to 180 °C and may be supported by catalysts, adding acids or Lewis acids, such as organic tin salts. It is possible to start the addition reaction with polyalcohols. Diols yield linear polyesters with OH end-groups, while higher functional polyester afford branched polyester polyols. For example, the reaction of trimethylolpropane with three moles of ε-caprolactone leads to a low-molecular polyester polyol with a number-average molecular weight of 476 g/mol, a degree of branching of 2.10 mol/kg, and an OH value of 353.6 mg KOH/g. Caprolactone polyesters are soluble in esters, ketones, glycol ethers, glycol ether esters, and to a limited extent in aromatic hydrocarbons. The high-molecular linear grades tend to crystallise. There are linear and branched caprolactone polyesters available which differ in molecular weight. Caprolactone polyester polyols are mainly used for high-solid, two-component coatings which contain polyisocyanate adducts as crosslinking agents. In that application field, they are in competition with polyether polyols. The latter are inexpensive and confer excellent saponification resistance. However, the caprolactone polyesters are tough and weatherable and therefore suitable for topcoats. Furthermore, caprolactone polyesters make suitable segments for incorporation into other polymers. The first binder formulations for automotive OEM cathodic electrodeposition primers contained caprolactone polyesters for the purpose of plasticising the aromatic epoxy resins [152]. Caprolactone polyesters serve as soft segments for polyurethane dis­persions (see Chapter 5.4.1). Typical commercial products: Tone polyol 0231, 0305 (Dow [153])

218

Polyesters based on diene adducts The ring-opening reaction can be used to perform polymer-like modifications. The most widely known is the conversion of the OH groups of acrylic resins with ε-caprolactone to form polyester side-chains bearing exposed OH groups [154]. The reaction takes place in solution at around 140 °C. It can be accelerated with catalysts, e.g. organic tin compounds (tin octoate). The degree of conversion can be checked by measuring the solids content, as free caprolactone is fairly volatile. Comparisons of such products with others prepared by alternative processes revealed that the addition of caprolactone takes place preferentially at exposed OH groups. Therefore, addition to already added caprolactone units predominates. To an extent, depending on the number of OH groups on the acrylic resin and the content of ε-caprolactone, the products contain molecules with long side-chains of caprolactone polyester and residual free OH groups on the main chain of acrylic resin. The outcome is a markedly segmented polymer. It seems obvious then to combine OH acrylic resins of relatively high glass-transition temperature, i.e. high hardness, with flexible polyester side-chains. If such resins are crosslinked with aliphatic or cycloaliphatic polyisocyanate adducts, they produce films which are distinguished by a combination of high hardness and toughness (viscoelasticity) and good weatherability. The exposed hydroxyl groups react faster than the OH groups of hydroxyalkyl acrylate monomers and they support effective crosslinking, yielding good resistance of the films to chemicals, solvents and mechanical impact. The higher reactivity is reflected in a shorter pot-life. Besides OH acrylic resins, other binder groups may be modified by polymer-like addition of ε-caprolactone, namely OH polyesters, OH alkyd resins, and epoxy resins.

8.3 Polyesters based on diene adducts Rosin acids (abietic acid and levopimaric acid) contain conjugated double bonds (see Figure 7.9). Conjugated double bonds react with further double bonds by diene addition to yield a cyclohexene ring. This reaction is also the basis for the preparation of tetrahydrophthalic anhydride from butadiene and maleic anhydride, of endomethylene tetrahydrophthalic anhydride, and of TCD dicarboxylic acid, which are suitable for water-thinnable alkyd resins (see Chapter 7.2.10). Rosin acids – primarily levopimaric acid at equilibrium – add unsaturated compounds in a 1,4-addition to form a six-membered ring containing only one double bond. For such reactions, gum resins with the highest-possible content of rosin acids are chosen. The most important partners for diene addition to rosin acids are maleic anhydride and acrylic acid. The reaction of levopimaric acid with maleic anhydride at more than 150 °C affords a tricarboxylic anhydride. Figure 8.1 shows the molecular structure of such adducts. The adduct is esterified (melt condensation) with polyfunctional alcohols (glycerol, pentaerythritol) to form polyesters known as maleic ester gum. The resins are hard and have relatively low molecular weights. The maleic ester gums are soluble in aromatic

219

Special polyesters hydrocarbons, esters, glycol ethers, and glycol ether esters. They are less soluble in aliphatic hydrocarbons and not in alcohols. The commercial products differ in their content of maleic anhydride, type of polyol and degree of condensation (acid values). A high content of maleic anhydride leads to polyesters of high hardness and high melting temperatures. A low content of maleic anhydride yields polyesters which contain rosin acid adduct and polyol, where the excess hydroxyl groups are partly occupied by rosin acid (as monocarboxylic acid). Such products conform principally to the basic structure of alkyd resins. They have lower melting temperatures and their softening temperatures are between 80 and 170 °C. Naturally, pentaerythritol esters have higher melting temperatures than equivalent glycerol esters. Products which contain high excess polyol have lower acid values (≤ 30 mg KOH/g). They serve as hard resins in combination with drying oils and alkyd resins, where they improve initial drying and hardness. The resins support pigment wetting and oxidative-cure. In addition, they have been combined with cellulose nitrate to improve hardness, gloss and filling power. They allow realisation of higher application solids in the coating formulation. The products are still used for wood finishing and furniture coatings and they were used in road-marking paints. There are maleic ester gums that have a lower excess of polyol, where the molecular weights are limited by a lower condensation degree and higher acid value (100 to 150 mg KOH/g). Such resins are soluble in alcohols. In addition, they can be transferred into aqueous media by at least partial neutralisation of the carboxyl groups with amines. Such resins are mainly suitable for printing inks (e.g. water-borne inks for flexographic printing). Typical commercial products: Erkamar 1065, 2300, 3300 (Krämer [65]) There are also adducts of rosin acids and unsaturated compounds. The reaction of levopimaric acid with acrylic acid affords a cycloaliphatic dicarboxylic acid. This di-acid is then esterified with polyols to yield polyesters which, unfortunately, are called acrylic resins. However, these should not be confused with the acrylic resins prepared by polymerisation of monomeric acrylic esters. Polyesters containing adducts of rosin acids and acrylic acid can have high molecular weights and thus higher melting temperatures. They are used for oil and alkyd resin combinations. The reaction of ortho-methylol phenols (in phenolic resins – resols) with rosin acids was once explained as the intermediate formation of quiFigure  8.1:  Adduct of levopimaric acid and maleic none methide (diene), with liberation anhydrid

220

Stand oils of water. However, here it is assumed that the adduct is formed by removing water molecules during addition of methylol groups across the double-bond system of rosin acids. This leads to the formation of polycarboxylic acids which can be at least partially esterified with polyols. Although the resulting products are polyesters, they are usually categorised as phenolic resins and described under this heading. The products were the first synthetic resins to be produced in commercial quantities (in the second decade of the 20th century). At that time, they were used in combination with drying oils, improving initial drying and hardness, where they replaced copals as natural products.

8.4 Stand oils As stated previously, in the late medieval period it was observed that drying oils become more viscous after standing in glass containers in sunlight under exclusion of air. The products exhibited improved initial drying and better through-drying than the oils themselves. Later, stand oils were produced by treating drying oils under exclusion of air. The substantial increase in viscosity stems from polymerisation of the double bonds of fatty acids to yield C–C bridges. These are highly complex reactions that lead to the formation of cyclic addition products which may be dehydrogenated to yield aromatic structures. However, open-chained branched structures are formed as well. The addition reactions may involve several fatty acid molecules so that the result is not only dimers but also high­ er oligomers. Of course, monocarboxylic acids remain as well. Formally, the stand oils have the structure of alkyd resins: the polyol is glycerol, the polycarboxylic acids are polymeric fatty acids and the residual monocarboxylic acids. To an extent, depending on the degree of conversion, more or less high-molecular weight products may be formed. Factors affecting the degree of conversion are temperature and time. Oils which contain isolated double bonds require high temperatures and more time. It is believed that some of the double bonds are isomerised during the reaction and thus become amenable to 1,4-polymerisation. In this way, linseed oil is converted into linseed stand oil in relatively large batches. The process is carried out under exclusion of atmospheric oxygen through the use of inert gas (carbon dioxide or nitrogen). Figure 8.2 [155] shows how the viscosity changes as a function of temperature and time during the preparation of linseed stand oil. Oils which contain fatty acids with conjugated double bonds, i.e. tung oil, oiticica oil and dehydrated castor oil (DCO), polymerise very rapidly. What follows is a personal account of how tung stand oil was produced at one paint maker [156]. Small quantities of crude tung oil were charged into an open varnish kettle (copper bottom with aluminium mantle) and heated rapidly on an open fireplace to 280 °C under manual stirring. Heating was then stopped but the temperature rose to 290 °C. After approximately 8 minutes, the viscosity was tested visually by dripping a sample onto a cold glass panel and dipping in

221

Special polyesters a finger and then withdrawing it. If a long thread was obtained, the polymerisation process was complete. To stop the polymerisation process, the same quantity of cold finished tung stand oil was added very quickly. Cooling was then continued on a “water hole” under manual stirring. Finally, several single batches were combined into a larger batch. Tung stand oil was used for coatings which undergo rapid oxidative drying. They are notable for their optimum wetting of pigments and surfaces. Stand oil coatings contain the same siccatives as the products containing alkyd resins (see Chapter 7.2.19). They show better initial drying and through-drying than the drying oils. However, stand oils dry more slowly than alkyd resins. Although the average molecular weights may be high, the viscosity is relatively low. They are soluble in all conventional solvents, except alcohols. They are compatible with alkyd resins, but in some cases hot-blending is required.

Figure  8.2:  Increase in viscosity during the preparation of linseed stand oil as a function of time and temperature

222

Stand oils Although stand oils are an old class of product, new applications could be found for them. Stand oils are based on renewable vegetable products and the crosslinking reaction itself is physiologically harmless. On account of their low viscosity, they can be used to formulate high-solid coatings. Stand oils are combined with alkyd resins to raise the application solids contents of house paints so that they meet VOC regulations. They are used for pigmented systems and for glazes. The only disadvantage is delayed through-drying. In addition, stand oils still play a role in artist’s colours. Linseed stand oils and dehydrated castor stand oils are commercially available. The products have been formally classified in a standard [157]. Manufacturers include the viscosity values of their stand oils in their products’ trade names (mainly in dPa s). Typical commercial products: Aco LS 6, 30, 45, 60, 90 (Abshagen [158])

223

Literature

9 Literature 9.1 General literature 1. 2. 3.

4.

Scheiber: Chemie und Technologie der künstlichen Harze Wissenschaftliche Verlagsanstalt m. b. H. Stuttgart 1943 Wagner-Sarx: Lackkunstharze, 5th Edition, Carl Hanser Verlag, Munich, 1971 Kittel: Lehrbuch der Lacke und Beschichtungen, Volume 2 and Volume 3, 2nd Edition, S. Hirzel Verlag, Stuttgart – Leipzig, 1998 and 2001 Don Sanders (Editor): Waterborne & Solvent Based Saturated Polyesters and their End User Applications, Volume IV, Surface Coatings Technology John Wiley

5.

6. 7.

& Sons, New York, with SITA Technology Ltd, London, 1999 N. Tuck: Waterborne & Solvent Based Alkyds and their End User Applications, Volume VI, Surface Coatings Technology John Wiley & Sons, New York, with SITA Technology Ltd, London,2000 D. Stoye, W. Freitag: Lackharze, Chemie, Eigenschaften und Anwendungen Carl Hanser Verlag, Munich–Vienna, 1996 U. Poth: Synthetische Bindemittel für Beschichtungssysteme, Vincentz Network, Hanover, 2016

9.2 References [1] J. Berzelius, Rapport annual (1847) 1847, 260 [2] M. M. Berthelot, Compt. rend. (1853) 37, 398 [3] M. M. Berthelot, Ann. chim. phys. (3) (1854) 41, 293 [4] J. M. van Bemmelen, Journ. pract. Chem. (1) (1856) 69, 84 [5] W. Smith, Journ. soc. chem. ind. (1901) 20, 1075 [6] M. J. Callahan, US 1 108 329 [7] L. Weisberg, Barrett Co., US 1 413 144, US 1 413 145, GB 173 225 [8] L. V. Adams, GB 273 290, GB 638 275, US 1 893 874 [9] R. H. Kienle, Ind. Engng. Chem. 21 (1929), US 1 803 174 (GE, 1925), DE 547 963 (AEG, 1926), GB 252 394 (1926)

[10] W. H. Carothers, Collected Papers on High Polymeric Substances Vol. 1 Interscience High Polymers Series, NY (1940) [11] R. H. Kienle et al, J. Am. Chem. Soc. (1929) 51, 509, (1930) 52, 3636 (1939) 61, 2258, (1940) 52, 1053, (1941) 63, 491 [12] R. Houwink, Physikalische Eigenschaften und Feinbau von Natur-und Kunstharzen, Kolloid Zeitschrift (1935) 70, 329 [13] R. Baker, Amer. Paint J. 49 (1965) 46, 12 [Ref. F & L 71 (1965)] [14] K. A. Earhart, J. Paint Techn. 41 (1969), 529, 104 [15] I.G. Farben, GB 376 479 (priority DE 540 101 [1930]), GB 376 481 (priority DE 544 326 [1930]) [16] I.G. Farben, DE 728 981 (1937) [17] O. Bayer, Angew. Chemie 59 (1947) 257

Ulrich Poth: Polyester and Alkyd Resins © Copyright 2020 by Vincentz Network, Hanover, Germany

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Literature [18] O. Bayer, Das Diisocyanat-PolyadditionsVerfahren (Historie und Grundlagen) C. Hanser Verlag (1963) [19] L. F. Fieser, M. Fieser, Organische Chemie, VCH (1965) Chap. 11, p 436 [20] K. H. Näser, Physikalische Chemie, VEB Verlag für Grundstoffindustrie (1966) Chap. 4, p 90 [21] W. H. Carothers, J. Amer. chem. Soc. 51 (1929) 2548 [22] K. Ziegler, H. Holl, Ann. 528 (1937), 143 [23] J. Scheiber, Chemie und Technologie der künstlichen Harze, Kapitel: Kondensationsharze, p 608, Wissenschaftliche Verlagsanstalt m. b. H. Stuttgart 1943 [24] I. Goodman, Angew. Chemie 74 (1962) 607 [25] R. H. Kienle, GB 284 349 (1927) [26] W. H. Carothers: Polymers and Poly­ functionality, Trans. Farad. Soc. (1936) 32, 39–49 [27] Joseph J. Bernardo: Gelation Prediction and Related Concepts, Journ. of Paint Techn. (Dec. 1968) 40 [28] Gunnar Christensen: Gelation of Alkyd Resins, Official Digest (Jan. 1964) [29] Manfred Dyck: Zur Berechnung der polyfunktionellen Esterkondensation unter spezieller Berücksichtigung ölmodifzierter Alkydharze, Farbe und Lack (Nov. 1965) 71, 11 [30] D. M. French: Functionality and Observed Versus Predicted Gel R. A. H. Strecker, Points. Journ. Macromol. Sci. Chem. (Aug. 1971) A 5,5 [31] Lawrence H. Brown: Condensation Polymer Formulation in the Age of Oligomers. Journ. of Coat. Techn. (Aug. 1980) 52, 667 [32] J. P. Flory: Principles of Condensation Polymerization, Journ. Am. Chem. Soc. (1936) 58, 1877, Journ. Am. Chem. Soc. (1941) 63, 3083 [33] M. Jonason: The Gelation Point of Alkyd Resins, Journ. Appl. Polym. Sci. (1960) 4, 11 [34] W. H. Stockmayer: Gelation Point of Polyesterresins, Journ. Appl. Polym. Sci. (1952) 9, 69 Journ. Appl. Polym. Sci. (1953) 11, 424 [35] R. W. Kilb: Gelation Point Equation, Phys. Chem. Review (1958) 62, 969

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[36] E. Sunderland: Alkyd Resins, a New Viewpoint Journ. of Paint Techn. (Jan. 1962) 26, 1 [37] W. W. Korschak: Grundgesetze der Polykondensation, VEB-Verlag Technik, Berlin, Band 36; (1953) [38] S. E. Bresler: Investigation of Polymerisation Mechanism by Means of Molecular Weight Distribution of Reaction Products. Journ. of Polym. Sci. 30,285 [39] Temple C. Patton: A Unique Alkyd Constant for Designing and Assessing Alkyd Formulations. Official Digest (Nov. 1960) [40] J. P. Flory: Molecular Size Distribution in Linear Condensation Polymers. Journ. Am. Chem. Soc. (1936) 58, 1877, Journ. Am. Chem. Soc. (1941) 83, 3091 [41] L. A. Tysall: Calculation Techniques in the Formulations of Alkyds and Related Resins; Toddington, England; Paint Res. Ass. (1982), 14 [42] William M. Kraft: Molecular Approach To Alkyd Structure Official Digest, (Aug. 1957) [43] U. und H. Holfort: Theoretische Betrachtungen über Aufbau und Einsatz von Alkydharzen. Farbe und Lack 68, (Aug. 1962) 513, Farbe und Lack 68, (Sept. 1962) 598 [44] N. Tuck, Waterborne and solvent based alkyds and their end user application. Chapter III,5 John Wiley & Sons, New York, with SITA Technology Ltd, London, 2000 [45] A. Einstein: Ann. Phys. IV. Sequence 19, 289-306 (1906) Ann. Phys. IV. Sequence 34, 591-592 (1911) and described in: [46] B. Vollmert: Grundriss der makromolekularen Chemie, publisher E. Vollmert, 1979 [47] E. Malmström, Hyperbranched Aliphatic Polyesters, Ph. D. Thesis Royal Inst. of Techn., Stockholm, Sweden, 1996 [48] Patent GB 2 272 304; Baxenden [49] Patent US 6,093,777; Perstorp [50] Patent US 6,515,192 (priority DE 19 840 605); BASF Coatings [51] Patent WO 1999-000440 (priority SE 1997 0 002 447); Perstorp

Literature [52] D. Porter, Group Interaction Modelling of Polymer Properties, Marcel Dekker, New York (1995) [53] I. Goodman: Polyesters in Encyclop. of Polym. Sc. and Eng. Wiley, New York, (1988) Vol 12, p 1 [54] T. G. Fox, P. J. Flory: J. of Appl. Physics (1950) 21, 581 [55] M. Gilbert, F. J. Hybart: Polymers (1972) 13, 327 [56] E. M. Wood, J. W. Barlow, D. R. Paul: J. Appl. Polym. Sc. (1987) 29, 197 [57] M. Ajzola, H. Suizo, C. Higuchi, T. Kashina: Polymer Degradation Stability (1998) 59, 137 [58] H. J. Kob, E. F. Izad: J. of Appl. Physics (1949) 20, 564 Elsevier Sc. Publ., New York, (1976) 2nd Ed. Ch. 6 [59] Solubility parameters – John Burke (1984). "Part 2. Hildebrand Solubility Parameter". Retr. 2013-12-04. [60] Solubility parameters – Charles M. Hansen: The three-dimensional solubility parameter and solvent diffusion coefficient: Their importance in surface coating formulation. Dissertation. Danish Technical Press, Århus 1967 [61] Römpp, Lexikon Chemie, 10th Edition, Georg Thieme Verlag, 1996 and newer on-line [62] Ullmann's Encyclopedia of Ind. Chemistry, 6th Edition, Wiley-VCH, Weinheim, 2001 H. Kittel, Lehrbuch der Lacke und Beschichtungen, Vol 1, Chap. 1.4 S. Hirzel Verlag, Stuttgart–Leipzig 1998 [63] Evonik: Dynapol – high-molecular and medium-molecular co-polyesters (formerly Degussa-Hüls) [64] Patent example 1 in US 3,684,565 (priority DE-AS 01 807 776 Dynamit Nobel (Degussa-Hüls) [65] BASF AG, Ludwigshafen [www.basf.de/basf/hmtl/d/produkte] [66] Emery Oleo Chemicals [67] DSM Resins BV, Zwolle, The Netherlands [www.dsm.com] [68] Allnex (formerly Monsanto) [69] Lackharzwerke Robert Krämer GmbH & Co., Bremen [www.rokra.de]

[70] Stoichiometric conversion of phthalic anhydride and pentaerythritol, condensation to acid value 200 mg KOH/g, calculated without regard for possible loss of phthalic anhydride. [71] Patent example C of US 5,502,101 (priority EP 0 521 919 BASF Lacke & Farben [BASF-Coatings]) [72] Patent example B 3 of US 4,576,868 (priority EP 0 137 256 BASF Lacke & Farben [BASF-Coatings]) [73] Covestro (formerly Bayer AG, Leverkusen, brochure) [74] Croda Resin Ltd., South Belvedere, GB [www.croda.com] [75] Allnex (formerly Cytec and Vianova) [76] Allnex (formerly Nuplex, Bergen op Zoom, The Netherlands) [77] Arkema (formerly Cray-Valley Ltd) [78] Worlée-Chemie GmbH, Hamburg [www.worlee.de] [79] Patent example B of EP 0 142 701 der Herberts AG (Axalta) [80] Synthopol Chemie, Buxtehude [www.synthopol.com] [81] Patent example B 1 of GB 1 487 563 (priority DE 2 346 818 der CWH [Evonik]) [82] Basic Patent US 4,311,622 (priority DE 27 51 761 of AKZO-Resins [now Allnex-Nuplex]) [83] See product information from Evonik (formerly Degussa) on hydrophobised silica (Aerosil) [84] H. Batzer: Verwendung von p-Toluolsulfonsäure, Makromol. Chem. 5 (1960), 11 [85] Patent example B 2 of US 5,552,184 (priority DE 42 04 611 [Bayer AG, now Covestro]) [86] Brochure of AMOCO Chemicals [87] Patent example C of US 5,502,101 (priority DE1990/4009857 [BASF Lacke & Farben [BASF-Coatings]) [88] Mäder AG Coatings technology [89] Data sheet Li-SIPA and 5-SSIPA (Eastman Chemicals) [90] Patent example 8 of GB 1970 0 061 107 (DSM) [91] Allnex (formerly UCB)

227

Literature [92] Data sheet and material safety data sheet of Araldit PT 810 (Huntsman) [93] Patent example 1 b of GB 1992 0 000 330 (UCB [now Allnex]) [94] Data sheet and material safety data sheet of Araldite PT 910 (Huntsman) [95] Primid XL-552 and Primid QM-1260 (Ems Chemie, data sheets) [96] Uranox-Types, DSM data sheets [97] Patent example 8 of DE 42 04 995 (Hüls [now Evonik]) [98] Powderlink C 1174, data sheet Cytec (now Allnex) [99] Model polyesters described in the series: Influence of the amount of diimide dicarboxylic acid on properties of THEIC polyesters for high-temperature stable wire enamels, process in Patent US 3,676,403 (priority DE 1 937 312 Dr. Kurt Herberts & Co [now Axalta]) [100] Manufacturer of wire enamels: DuPont, ICI, Altana, Allnex [101] Patent example A of US 3,794,695 (priority DE 1 927 320 [Bayer. now Covestro]) [102] KCCS, Linz, Österreich (Krems-Chemie) [www.kccs.at] [103] Model binder, investigation of influence of amount of trimethylol propane diallyether [104] C. M. McCloskey, J. Bond: Ind. Eng. Chem. 47 (1955) 2125 [105] P. G. Garratt: Strahlenhärtung, Curt. R. Vincentz Verlag, Hanover 1996 [106] Data sheet Roskydal 502 BA (Nuplex, now belonging to Allnex) [107] Evonik, Tego Additive Resins [108] Definition in DIN 55945 [109] Cobalt – Classification and Labelling Inventory of the European Chemicals Agency (ECHA), August 1st 2016 [110] For example, see MSDS of Borchers-dry 0410 [OMG Borchers] [111] OMG Borchers, brochure [112] Product data sheet: Setal AF 681 [Allnex-Nuplex, formerly Alkydal F 681 from Bayer (now Covestro)] [113] Borchers: Starting formulations SF 3.11 [114] Complex former: 2,2-bipyridyl [115] Educational advertising of oxidative crosslinking reactions: W. Francke: Neue Arbeiten über die autoxydativen Primär-

228

vorgänge bei der Öltrocknung, Farben – Lacke – Anstrichstoffe, 8, 4 (1950); W. Kern: Elementarvorgänge bei der Öltrocknung, Farben – Lacke – Anstrichstoffe, 4, 5 (1950); J. Scheiber: Zur Frage der Leinöl-Trocknung, Fette – Seifen – Anstrichmittel, 53, 1 (1951); H. P. Kaufmann: Oxydation und Verfilmung trockn. Öle, Fette – Seifen – Anstrichmittel, 59,3 (1957); E. Gulinski: Pflanzliche und tierische Fette und Öle, C. R. Vincentz-Verlag, Hanover, (1963); M. Dyck: Der Chemismus der Öltrocknung, Farbe und Lack, 67, 7 (1961); A. Rieche, M. Schulz, H. E. Seyfahrt, G. Gottschalk: Autoxydationsversuche an Modellsubstanzen, zur Frage der Fett-Autoxydation, Fette – Seifen [116] J. C. Cowan: „Isomerization and TransEsterification“, J. Am. Oil Chem. Soc., 1950, 492–499; P. L. Nichols, S. F. Herb, R. W. Riemenschneider: “Isomers of conjugated fatty acids. I. Alkali-isomerized linoleic acid”, J. Am. Chem. Soc., 1951, 73, 247-252; Bauer, P. Horlacher, P. Claus: “Direct Isomerization of Linoleic Acid to Conjugated Linoleic Acid (CLA) using Gold Catalysts”, Chem. Eng. Technol., 2009, 32, 2005–2010 [117] Patent example 2 of EP 00 01 400 (priority DE 1977 27 42 584, Bayer, now Covestro) [118] Borchers brochure: OMG Borchers’ Antioxidants and Antiskinning Agents [119] Data sheet Benzoic acid D (Elektrochemie) [120] Data sheets of DSM and Synres [121] Data sheet p-tert.-Butylbenzoic acid (Hexion) [122] Model medium oil alkyd resin containing 10.5 wt.% benzoic acid and a combination of fatty acids with isolated and conjugated double bonds (7 : 3, in sum 48.1 wt.%) [123] Borchers: Starting formulations SF 1.12 [124] Datasheet: Setal AF 48 [Allnex-Nuplex] (former Alkydal F 48 [Bayer, now Covestro]) [125] Model binder: Trimethylolpropane alkyd resin, modified with benzoic acid [126] Model binder: Low-viscosity linseed alkyd resin

Literature [127] Model binder: Linseed alkyd, modified with rosin acids [128] High-solids alkyd resin: Patent example 1 of WO 88/03153 (priority DE 1986 36 36 929, BASF Coatings) [129] One of the basic patents is GB 711 611 (Bayer, now Covestro) [130] Poth, Schwalm, Schwartz: Acrylic Resins, Vincentz Network, Hanover, 2011 [131] Model comparison of urethane alkyd resin and "longoil“-alkyd resin [132] Synres, founded in 1947, was part of DSM from 1970 on but became an independent resin producer and a part of Standard Investment Company in 2015 [133] Patent example 1 of DE 32 19 327 (Vianova, now Allnex) [134] Patent US 6,166,150 (priority DE 1996 13 93 25, BASF Coatings) [135] Patent EP 0267562 (priority AT 1986 000 30 29, Vianova, now Allnex) [136] Patent GB 2084165 (priority AT 1980 000 48 46, Vianova, now Allnex) [137] W. Weger: Waterborne Resins for the Paint Industry – Theory and Practice, ChinaCoat 98 Conference [138] R. Seidler, H. J. Graetz, Vorgänge beim Einbrennen von Alkyd-Melamin-Gemischen, Fette – Seifen – Anstrichmittel 64 (1962) 1135 [139] Data sheet Isononanoic acid (Celanese) [140] Data sheets Versatic acids (Hexion) [141] Data sheets Neo-acids (Exxon) [142] Data sheets Cardura E 10 P (Hexion)

[143] Data sheets Glydex (Exxon) [144] Model alkyd resins containing trimethylolpropane in comparison to a polyol mixture of pentaerythritol and propylene glycol for stoving enamels, influence on solution viscosity [145] Model alkyd resin consisting of phthalic anhydride, pentaerythritol and isononanoic acid [146] T. E. Bradley, D. Richardson, Ind. Eng. Chem. 34 (1942) 237–242 [147] Model alkyd resin consisting of phthalic anhydride, adipic acid, trimethylolpropane and isononanoic acid, influence of amount of adipic acid [148] Model formulation LR 2009 (Hexion) [149] Patent example 1 of US 4,594,374 (priority DE 34 09 080 (Bayer, now Covestro) [150] Hydrogenated palm kernel fatty acid fraction Kortacid PKGH (AKZO) [151] Model alkyd resin consisting of phthalic anhydride, glycerol and peanut fatty acid [152] Patent US 4,594,374 (PPG-Industries) [154] Placcel EPA 2250 (Daicel Chemical Industries) [155] E. Gulinski: Pflanzliche und tierische Fette und Öle, C. R. Vincentz-Verlag, Hanover, 1963 [156] Experience of my own apprenticeship in the varnish decoction plant in Dr. Kurt Herberts & Co. in Wuppertal (now Axalta) [157] DIN 55391: Stand oils [158] Abshagen, Hamburg (now part of Evonik)

229

Author

Author Ulrich Poth completed his apprenticeship at Herbert Corporation in Wuppertal (today Axalta). He then worked there on developing resins and coatings for electro-insulation, and powder coatings. He studied at the University of Applied Science Niederrhein at Krefeld/ Germany and graduated as a diploma engineer. In 1971 he joined BASF Coatings, Münster, where he worked in research and development of resins for industrial coatings applications. Since 1976 in BASF Coatings he focused the activities on resins for automotive coatings; and was finally in a leading position. In 1995 he moved to the automotive OEM coatings business unit of BASF Coatings in Münster. He was responsible for the operation of automotive OEM clearcoats and topcoats, including the development of resins and coating formulations, production and tests laboratories. He retired in 2002. Until his death in July 2018, he was still engaged in projects as a consultant, lecturer at the university and author.

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Index

Index Symboles 1,2-propanediol 95 1,3-propanediol 95 1,4-butanediol  95, 100, 111 1,5-pentanediol  95, 100 1,6-hexanediol  95, 103, 111, 116, 217 2,2,4-trimethyl-1,3-pentanediol 95 2-amino-2-methyl propanol  207 2-ethyl-2-butyl propanediol  109 2-ethylhexanoic acid  77, 168 9,11-linoleic acid  170, 171, 201, 215 9,12,15-linolenic acid  166, 172 9,12-linoleic acid  166, 168 ε-caprolactone  95, 218

A acid catalyst  99, 110, 111, 201, 202, 206 acid curing  202 acid value  14, 41, 43, 44, 47, 60, 70, 73, 79, 100, 101, 108, 115, 117, 126, 127, 137, 139, 194 acrylic acid  220 acrylic resin  15, 209 containing epoxide groups  141 modified with ε-caprolactone 219 acyl peroxides  154 adhesion  110, 115, 136, 163, 186 adipic acid  43, 46, 47, 49, 58, 66, 77, 95, 100, 108, 111, 120, 203, 205 alcohol as solvent  14, 100, 101, 109 aliphatic hydrocarbon  100, 109, 174, 182 alkoxysilane 147 alkyd resin  70, 71 calculation 31 emulsion 195 definition  11 long-oil 165 medium-oil 165 metal-reinforced 191 oxidative crosslinking  14 oxidative-cure 166

alkyd resin preparation 70 short-oil 165 aluminium iso-propylate  191 amino methyl propanol  127 amino resin  14, 108, 109, 196, 207, 210 crosslinker 110 anhydride addition  207 anti-corrosive coating (paint)  186, 194, 195, 204 anti-skinning agent  171 aromatic hydrocarbon  100, 109, 144, 174, 217, 220 as solvent  105 association of molecules  91 automotive OEM application  104, 117 coating  116, 210 automotive repair coating  116, 178, 209, 210, 213 azelaic acid  95

B basecoat  119, 124, 130, 132, 208, 210 benzoguanamine resin  110, 111 benzoic acid  13, 95, 177 bisphenol A  217 blocking agent  104, 117 brittleness 214 butane diol-1,4  97 butyl diglycol  129 butyl glycol  126, 129, 194

C calculation method  43 can-coating  99, 111, 117, 124, 209, 210, 212 caprolactam 117 caprolactone 23 carboxyl group  41 castor oil  14

233

Index catalyst  113, 136 for transesterification  68 preparation of polyesters  20 CED primer  218 cellulose ester  100, 101, 214 cellulose ether  214 cellulose nitrate  100, 214, 220 chain extension by amines  103 chlorinated rubber  214 clearcoat  15, 108, 111, 124, 158, 167, 187, 211, 214 cobalt siccative  167 coconut fat  14, 196 co-crosslinking  110, 201, 210 coil-coating  99, 111, 117, 124, 149, 209, 210, 212 colloidal silica  121 colloidal solution  93 compatibility  94, 95, 199 complex former  167 condensation degree  207 conjugated double bonds  170, 178 co-polymerisation 153 co-solvent  126, 129, 194 cresol 144 crosslinking 96 agent 195 by amino resin  14 by blocked polyisocyanates  117 by epoxide resin  137 by humidity of air  105 by polyisocyanates  16, 113, 115 crystalline urea  121 during polyester formation  25 oxidative  13, 166 crystallinity  91, 97, 134, 217 crystallisation 116 cyclic structure forming polyesters  25 cycloaliphatic compounds  161 cycloaliphatic polycarboxylic acid  108 cyclohexane dicarboxylic acid  95

234

D DCO stand oil  223 de-aeration additive  138 degree of branching  56–59, 61, 62, 66, 75, 80, 81, 107, 108, 120 degree of condensation  39, 48, 68, 75, 77 dehydrated castor oil  221 dehydrating process  202 deionized water  127 diene addition  219 diethanolamine 207 diethylene glycol  203 diethyl malonate  117 diimid dicarboxylic acid  145 diisocyanate 187 diisocyanato dicyclohexyl methane  142 diisopropanolamine 207 dimer diol  102, 128 dimer fatty acid  95, 102, 128, 189 dimethyl ethanol amine  127 dimethylformamide  52, 97, 146 dimethylol cyclohexane  95, 109, 121, 128, 135 dimethylol propanoic acid  103, 193, 207 dimethylol propionic acid  81, 102 dimethyl pyrazole  117 dimethyl terephthalate  25, 67, 68, 95, 97 di-pentaerythritol 184 diphenyl carbonate  217 dipropylene glycol  137, 203 discolouration 154 dispersity 49 di-trimethylolpropane 184 divinyl benzene  52 durability 94

E elasticity 96 electrical insulation coating  68, 98, 144, 210 embrittlement 171 enamel, combinated  215 environmental behaviour  94 epoxy acrylate 107

Index epoxy addition 21 alkyd resin  191, 209 resin 191 ester  217, 218, 220 as solvent  97, 100, 101, 109 esterification  157 ethyl acetoacetate  117 ethylene glycol  25, 44, 68, 95, 97, 99, 109, 135, 144 ethylhexanoic acid  196 evaporation enthalpy  93

F fatty acid  78, 93, 151, 166 cottonseed 172 filling power  220 film matrix  158 flexibility  92, 94–96, 108, 110, 115, 136, 144, 146, 204, 209, 210, 213 flow and levelling  134 fluorine hydrocarbon  144 forced drying  179 formaldehyde  200, 203 functional group  94 furniture coating  203, 220

G gel point  26, 29, 32 glass-transition temperature  92, 97, 108, 134, 140, 141, 144, 182, 217 gloss  101, 204, 205, 206, 212, 215 gloss polyester  158 glycerol  13, 24, 46, 68, 95, 101, 144, 165 glycidester 140 of neodecanoic acid  197, 206, 213 glycol ether  217, 218, 220 glycol ether ester  217, 218, 220 glycoluril 143 glyptal resin  14 GPC analysis  51, 54, 59

H hardness  92, 94, 110, 115, 144, 149, 182, 209, 210, 213, 219, 221 heat resistent  161 heterocyclic building element  145 hexahydrophthalic anhydride  20, 95, 109, 112, 121, 127, 128 hexamethylene diisocyanate  105, 113 hexane diol-1,6  44, 46, 61 high-solid  32, 82, 119, 123 coating  184, 218, 223 topcoat 206 history of polyesters, coatings raw material  13 HMMM resin  206 house paint  175, 190, 191, 194 hydrogenated hydrocarbon  174 hydroxy alkylamide  140 hydroxycarboxylic acid  37, 130 hydroxyl group  195, 210 hydroxypivalic neopentyl ester  95 hydroxypivalic neopentyl glycol ester  109, 111, 128

I inhibition by oxygen  160 initial drying  175–177, 180, 181, 185–187, 191, 195, 209, 213, 215, 221 isobutyl methacrylate  185 isononanoic acid  73, 77, 168, 196, 205 iso-paraffin  174 isophorone diisocyanate  103, 105, 113, 142, 187, 207 isophthalic acid  43, 44, 49, 67, 70, 77, 95, 97, 99, 103, 108, 110, 120, 127, 128, 135, 139, 184, 192

K ketone  14, 97, 100, 218 as solvent  101 process 102 ketoxime 171

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Index

L lauric acid  196 law of mass action  17, 19 levelling  205, 206, 209, 212 agent 138 property 149 levopimaric acid  219 linseed alkyd  15 linseed fatty acid  168, 180 linseed oil  144, 170, 172, 181 linseed stand oil  223 lithium catalyst  174

M machinery coating  180 maleic anhydride  21, 127, 128, 193, 219 maleic ester gum  219 manganese siccative  167 master batch  138 melamine resin  99, 110, 200 methoxy propyl acetate  133 methylene diisocyanate  105 methylene phenyl isocyanate  113 methyl ethyl ketoxime  117 methyl methacrylate  185 methylpropyl propanediol-1,3  58 mobility of molecules  91, 96 molecular weight  14, 20, 26, 28, 29, 31, 37, 38, 44, 54, 80 molecular weight distribution  20, 31, 48, 58, 61, 63, 66, 67, 74, 81, 97, 121, 187 monocarboxylic acid  12, 30, 31, 70, 71, 75, 77

N naphthenic acid  168 n-butanol  129, 175 n-butyl methacrylate  185 n-domethylene tetrahydrophthalic anhydride 95 neodecanoic acid  77, 168, 196 neo-diol 95

236

neopentyl glycol  43, 44, 46, 49, 58, 61, 92, 95, 99, 103, 109, 111, 120, 135 neutralisation  126, 194 agent 127 degree  207, 194, 208 N-methylpyrrolidone 208 N,N-dimethyl ethanolamine  207 neutralisation agent  104 number-average molecular  56, 58 number-average molecular weight  49, 56, 68, 80, 97, 111, 115, 117, 119, 120, 121, 177, 195, 197, 204, 205 number of structural units  23, 27, 32, 35, 36, 39, 50, 64, 66

O OEM basecoat 111 topcoat 111 OH acrylic resin  212 OH alkyd resin  211 OH-% content  79 OH group  72, 203 OH value  16, 43, 47, 79, 80, 108, 110, 115, 117, 120, 121, 141 oil content  165 oleic acid  215 one-layer coating  186 oxalic acid  25 oxidative curing  170

P palm kernel oil  196, 207 peanut fatty acid  215 peanut oil  14 pelargonic acid  196 pentaerythritol  77, 78, 95, 101, 197, 203 perhydro bisphenol A  95, 128 peroxide initiator  186 phenol 171 phosgene 217 phthalic anhydride  14, 20, 46, 47, 58, 61, 66, 70, 73, 77, 78, 95, 101, 108, 110, 127, 128, 135, 203, 205

Index pH value  102, 127, 194 pigment wetting  149, 181, 186, 201 plastic coating  204, 210, 212 plasticiser  111, 214 plasticity 96 polishing enamel  215 polyalcohol 12 polyamide 189 polycaprolactone  96, 102, 208 polycarbonate  21, 102 polycarboxylic acid  12, 16, 24, 37, 70, 97, 108, 156 polycondensation constant  35–37, 40, 47, 57, 62, 65, 70, 74, 76, 77 polyester acrylate 106 branched 24 definition  11 dendrimer  81, 124 polyesterimid 145 polyester initiator 153 crosslinking 156 crosslinking of unsaturated  151 segment 218 unsaturated 151 polyether polyol  95 polyethylene glycol  106, 195 oxide 133 terephthalate 68 polyisocyanate 102 adduct  113, 123, 133, 203, 204, 210 aromatic 105 blocked  141, 205 polymerisation 152 1,4 position  170 free-radical 151 polyol  16, 24, 32, 37, 70 polyolcarboxylic acid  32 polypropylene glycol  106 polysiloxane 147 polystyrene  52, 101 polyurethane elastomer  102 prepolymer 106 polyvinyl chloride  214 pot-life 114

powder coating  16, 134 primary acrylic dispersion  195, 203 primer  115, 180, 182, 209, 210 primer surfacer  108, 111, 132, 209, 210 printing ink  220 propylene glycol  43, 97, 111, 135, 137, 197 p-tert.-butyl benzoic acid  177 p-toluene sulfonic acid  99, 106 pyromellitic anhydride  20

R reactivity of functional groups  56, 66 resistance against atmospheric moisture  100 against chemicals  99, 102, 105, 110, 111, 114 136, 144, 184, 186, 187, 191, 202, 204, 206, 209, 212, 213 against corrosion  136, 180 against discoloration  109, 143, 149, 204, 213 against heat  68, 98, 144, 145, 149, 217 against mechanical impact  96, 187, 191, 209, 217 against migration  214 against moisture  180, 186, 187, 204, 209 against sagging  190 against saponification  15, 16, 102, 103, 128, 129, 213, 217 against solvents  98, 99, 110, 136, 144, 202, 204, 206 against weathering  100, 103, 105, 110, 111, 115, 136, 139, 141, 143, 149, 178, 186, 204, 206, 209, 213, 218, 219 against yellowing  139, 201 resol 220 rheological acting agent  121 ricinoleic acid  171, 204, 215 rosin acid  95, 168, 182, 219

237

Index

S safflower fatty acid  172 saponification  18, 192 sensivity 207 saturated fatty acid  196, 203 OH polyester  210 sebacic acid  13, 95 self-crosslinking  110, 201 siccative 168 side-chain, aliphatic  91 silicone alkyd resin  191, 209 polyester 148 siloxane intermediate  148, 209 single-coat paint  186 softening temperature  134, 139 solubility  16, 93, 95, 199 parameter 93 solvent  16, 26, 52, 61, 93, 94, 97–104, 107–115, 118, 125, 129, 131, 174, 184, 197, 200, 205, 210–217, 222 reactive  154, 155, 158 resistance  160, 201, 203, 204 soybean fatty acid  172 stand oils  13 stoving enamel  195, 201, 202, 206, 212 structural unit  26, 34, 37 styrene  52, 154, 185 succinic acid  13 succinic anhydride  21, 127, 128 sulfonic acid  130 sunflower fatty acid  172 surfactant 195 synthetic fatty acids  196

T tall oil fatty acid  168, 172, 202 terephthalic acid  25, 67, 95, 97, 99, 110, 127, 128, 135, 137, 139, 144 terpene hydrocarbon  174 terpene solvent  100, 109 tert.-butyl methacrylate  185 tertiary aromatic amines  154

238

tetrabutyl titanate  145 tetrahydrophthalic anhydride  20, 95, 108, 110, 127, 128, 192 thixotropy 189 through-drying  167, 168, 176, 182, 185, 221 toluene diisocyanate  105, 113, 187 topcoat  119, 123, 204, 212 toughness 219 trans-esterification  19, 25, 31, 67, 81, 97, 165, 173, 184, 188, 192 trans-urethanisation 117 triazole 117 tricyclodecane dicarboxylic acid  95 tricyclodecane dimethanol  95 triethyl amine  127 triglycidyl isocyanurate  139 trimellitic anhydride  20, 44, 67, 109, 127–129, 134, 135, 193, 207 trimesic acid  67 trimethylol propane  43, 49, 58, 59, 61, 73, 95, 109, 111, 120, 179, 197, 203, 205 trishydroxy ethyl isocyanurate  125, 144, 146 tung oil  144, 170, 221

U unsaturated polyester  15 definition  12 UP systems  154 urea resin  110, 111, 202 urethane alkyd resin  209 urethane oil  188 UV initiator  161

V vinyl toluene  185 viscosity  26, 60, 61, 76, 98, 118, 184, 205 VOC regulation  118, 184, 192, 194, 215

Index

W

X

water as solvent  101 as solvent and dispersing agent  125 water-borne  15, 125, 130, alkyd  176, 207 automotive 208 basecoat  104, 129, 208 polyester  125, 133 stoving enamel  132, systems  125, 132, 194, 209, 210 water hill of viscosity  131 wax polyesters  155 weight-average molecular weight  49, 61 wet-on-wet process  213 wetting  199, 204, 205, 209, 212 property  175, 182 wire enamel  68 wood coating  202 wood finishing  220

xylenol 144

Y yellowing  95, 139, 171, 196, 199, 204 yield mass  39, 47

Z zirconium siccative  167

239