High Solid Binders 9783748602019

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Mircea Manea

High Solid Binders

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Cover picture: EVONIK TEGO Chemie GmbH

Mircea Manea High Solid Binders Hannover: Vincentz Network, 2008 (European Coatings Tech Files) ISBN 978-3-7486-0201-9

© 2008 Vincentz Network GmbH & Co. KG, Hannover Vincentz Network, P.O. Box 6247, 30062 Hannover, 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. The information on formulations is based on testing performed to the best of our knowledge. Please ask for our book catalogue Vincentz Network, Plathnerstr. 4c, 30175 Hannover, Germany T (202) 684-6630, F (202) 380-9129 E-mail: [email protected], www.american-coatings.com Layout: Maxbauer & Maxbauer, Hannover, Germany ISBN 978-3-7486-0201-9

European Coatings Tech Files

Mircea Manea

High Solid Binders

Mircea Manea: High Solid Binders © Copyright 2008 by Vincentz Network, Hannover, Germany

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European Coatings Tech Files

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Contact: Vincentz Network Fon: (202) 684-6630 Fax: (202) 380-9129 email: [email protected]

Foreword



Foreword Water-borne, UV curing, powder coatings and high solids are modern coatings technologies complying with new regulations and standards. High solids represent an interesting group of coatings, as the environmental pressure and legislations are a daily concern. They impart the benefits of the solvent-borne coatings with low environmental impact. High solid coatings have been a central preoccupation of science engineers for a very long time and are in many cases the best option for applications where only solvent-borne coatings may be used. In these cases the employment of this technology should and will make a difference. Although several approaches have been made no solution to solve all problems generated by high solids has been found. A relatively long history and extensive literature is presently dealing with this subject, but only offering solutions for particular cases. With respect to water availability and secondary pollution in terms of disposal, energy costs, versatility and latitude of applications, high solid technology is by far more recommendable compared to any other emerging technology. Regarding the market and academic interest for certain issues related to the coatings industry, one may observe that some topics pop up from time to time. The present book gives an overview of binder categories and the possibilities of transferring them into high solid binders. It is meant to encourage the activity and development in high solid technology offering broad latitude of choices and angles to tackle the technological and environmental problems and to exhaust the subject without any ambition. This book addresses chemists involved in binder development, being merely a review of the instruments available in terms of chemistries and design of modern compliant film forming polymers. Also paint and coatings chemists are targeted for a better understanding of chemistries and processes involved in film forming. It is the author’s believe that an image can say more than a thousand words; therefore a large part of the present book is composed in the form of chemical reactions. The state of the art offers a multitude of possibilities and opportunities in binder formulation, exemplified by description and chemical reactions which hopefully will capture the reader’s interest. Malmö, Sweden, August 2008 Mircea Manea Mircea Manea: High Solid Binders © Copyright 2008 by Vincentz Network, Hannover, Germany

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Content



Content 1 1.1 1.2 1.3 1.4

General considerations...................................................... Introduction......................................................................... Coatings............................................................................... Quick guide to coatings....................................................... Coatings global market........................................................

12 12 12 13 15

2 2.1 2.2 2.2.1 2.2.2 2.2.2.1 2.2.2.2

Environmental awareness................................................. Environmental concerns...................................................... VOC regulations.................................................................. Estimation of VOC emissions............................................. Categories of industrial surface coatings operation............. Selected USA coating regulations by region....................... Environmental targets in Europe.........................................

17 17 18 19 20 20 20

3 3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2 3.1.3 3.1.3.1 3.1.3.2 3.1.3.3 3.1.4 3.2 3.2.1

General concepts in coatings............................................ Basics in coatings technology............................................. Film formation..................................................................... Molecular weight................................................................. Molecular weight................................................................. Molecular weight analysis methods.................................... Polymer structure................................................................. Crystalline and amorphous polymers.................................. Polymer tacticity.................................................................. Phase transitions in polymers.............................................. Viscosity.............................................................................. Solvents............................................................................... Solvents for specific resin types..........................................

23 23 23 25 25 27 33 34 34 35 36 40 41

4 4.1 4.1.1 4.1.1.1 4.1.1.2

New technologies................................................................ Emerging technologies........................................................ Powder coatings definitions................................................. Powder coatings................................................................... Radiation curing powder coatings.......................................

43 43 43 44 48

Mircea Manea: High Solid Binders © Copyright 2008 by Vincentz Network, Hannover, Germany

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 4.1.1.3

Content

4.1.2 4.1.3 4.1.4

Overview of chemistry of powder coatings versus performance.............................................................. Radiation curing coatings.................................................... Water-borne coatings........................................................... Comparison between emerging technologies......................

50 50 55 57

5 5.1 5.1.1 5.1.1.1 5.1.1.2 5.1.1.3 5.1.1.4 5.1.2 5.1.2.1 5.1.2.2 5.1.3 5.1.3.1 5.1.3.2 5.1.3.3 5.1.3.4 5.1.4 5.1.4.1 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.3.3 5.3.3.1 5.3.3.2 5.3.3.3 5.4 5.4.1 5.4.2 5.5 5.5.1 5.6 5.6.1

Polymers and resins........................................................... Alkyd resins......................................................................... Property-property relationship in alkyd resins.................... Drying characteristics.......................................................... Raw materials in alkyd resins formulation.......................... Concept of functionality and gelation................................. Thumb rule for performance estimation.............................. Alkyd manufacturing........................................................... Alcoholysis process............................................................. Acidolysis process............................................................... Modification of alkyds......................................................... Vinylated alkyds.................................................................. Combination of alkyds with silicon backbones................... Urethane alkyds................................................................... Other alkyd modifications................................................... Cross-linking of alkyds........................................................ Oxidative cross-linking of alkyds........................................ Polyesters............................................................................. Polyester synthesis............................................................... Polyester cross-linking........................................................ Polyethers............................................................................ Polyether synthesis.............................................................. Aromatic polyethers............................................................ Aliphatic polyethers............................................................. Polyethylene oxide.............................................................. Polypropylene oxide............................................................ Polytetrahydrofurane and polyoxetane................................ Amino resins........................................................................ Synthesis of amino resins.................................................... Methylol derivatives reactions............................................ Epoxy resins........................................................................ Reactions of the epoxy group.............................................. Acrylic resins....................................................................... Acrylic monomers...............................................................

61 63 65 65 67 73 74 75 75 77 77 77 78 79 80 81 81 85 85 87 88 88 89 90 90 91 92 93 93 95 97 99 100 101

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Content



5.6.2 5.6.3 5.6.4 5.6.4.1 5.6.4.2 5.7 5.7.1 5.7.1.1 5.7.1.2 5.7.1.3 5.7.2 5.7.3 5.7.4

Manufacturing of acrylic resins........................................... Thermoplastic acrylic resins................................................ Thermosetting acrylic resins................................................ Hydroxyl functional acrylic resins...................................... Acrylic resins having miscellaneous functionalities........... Polyurethane resins.............................................................. Chemical reactions of the isocyanate group........................ Insertion reactions............................................................... Cycloaddition reactions....................................................... Addition polymers............................................................... Catalysts.............................................................................. Water-borne polyurethane systems...................................... Isocyanate free polyurethane...............................................

103 103 103 103 103 107 109 109 110 113 115 116 117

6 6.1 6.2 6.3 6.4 6.4.1 6.4.1.1 6.4.1.2 6.4.1.3 6.4.2 6.4.3

High solids approach......................................................... Definitions of high solid coatings........................................ Drivers................................................................................. High solid: publication statistics......................................... Coatings with high non-volatile content.............................. Solventless epoxy coatings.................................................. Reactive solvents................................................................. Curing agents that reduce the viscosity of the formulation Inert solvents and plasticizers.............................................. Radiation curing coatings.................................................... Solventless or high solid systems based on bismaleimide..

118 118 118 119 119 123 124 124 124 125 129

7 7.1 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.2.7 7.2.8 7.2.9 7.2.10 7.3

High solids as modern binder systems............................. Film formation in high solid systems.................................. High solid binders design parameters.................................. Polymer architecture and free volume................................. Molecular weight and molecular weight distribution.......... Presence of colloidal particles............................................. Presence of hydrogen bonds................................................ Reactive groups with plasticizing effect.............................. Choice of monomers and building blocks........................... Two steps cross-linking and new chemistries..................... Solvents............................................................................... Reactive solvents................................................................. Pigments and additives........................................................ Free volume.........................................................................

132 132 135 136 136 136 137 137 137 137 137 137 138 138

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10 8 8.1 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.2.6 8.2.7 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.4.5 8.4.6 8.4.7 8.5 8.5.1 8.5.2 8.5.3 8.5.4 8.5.5 8.6 8.6.1 8.6.2 8.6.3 8.6.4 8.6.5 8.6.6 8.6.7 8.7 8.7.1

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Content

High solid strategies.......................................................... Polymer architecture............................................................ Molecular weight and molecular weight distribution.......... Alkyd resins......................................................................... Polyesters............................................................................. Polyethers............................................................................ Epoxy resins........................................................................ Acrylic resins....................................................................... Polyurethanes...................................................................... Amino resins........................................................................ Presence of colloidal particles............................................. Alkyd resins......................................................................... Polyesters............................................................................. Acrylic resins....................................................................... Polyurethanes...................................................................... Amino resins........................................................................ Hydrogen bond control........................................................ Alkyd resins......................................................................... Polyester resins.................................................................... Polyethers............................................................................ Epoxy resins........................................................................ Acrylic resins....................................................................... Polyurethane resins.............................................................. Amino resins........................................................................ Reactive groups with plasticizing effect.............................. Alkyd resins......................................................................... Polyesters............................................................................. Acrylic resins....................................................................... Epoxy resins........................................................................ Polyurethanes...................................................................... Choice of monomers in binder formulations....................... Alkyd resins......................................................................... Polyesters............................................................................. Epoxy resins........................................................................ Acrylic resins....................................................................... Polyurethanes...................................................................... Amino resins........................................................................ Miscellaneous...................................................................... Reactive diluents................................................................. Alkyd resins.........................................................................

141 141 149 150 151 153 154 154 158 158 158 159 160 161 161 161 162 162 166 166 168 168 173 176 176 176 180 182 185 185 188 189 190 192 192 193 194 195 197 197

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Content

11

8.7.2 8.7.3 8.7.4 8.7.5 8.7.6 8.7.7 8.8

Polyesters............................................................................. Polyethers............................................................................ Epoxy resins........................................................................ Acrylic resins....................................................................... Polyurethanes...................................................................... Amino resins........................................................................ Two steps cross-linking and new chemistries.....................

202 206 207 210 211 211 211

9 9.1 9.2 9.2.1

Examples of high solid formulations................................ Polymer architecture and free volume................................. Molecular weight................................................................. Acrylic hydroxyl functional binder for 2K polyurethane coatings................................................................................ Two component epoxy ester................................................ Hydrogen bond management and groups with plasticizing effect.................................................................................... Monomer choice..................................................................

216 216 217

219 220

Conclusions........................................................................

221

Abbreviations.....................................................................

222

Literature...........................................................................

223

Author.................................................................................

235

Index...................................................................................

236

9.2.2 9.3 9.4

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217 218

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12

General considerations

1

General considerations

1.1

Introduction

Coatings have being used since early dawn of human history. Defined as any liquefiable material designed for application to a substrate in a thin layer, coatings are further converted in solid film after application. The main drivers behind coating material development being their decorative/esthetical function along with later developed requirement of functional protective and functional signaling properties. History of coatings goes back to over 25,000 years ago to cave paintings. The Bible describes the use of pitch to coat and protect Noah’s Ark and findings in the Middle East confirm the use of lime, silica, copper and iron oxides to produce many colors. In Asia resins from insect secretions and sap from trees were used in clear coatings. Coatings represent added value to objects that without the protection conferred by them would have a much shorter life. Generally, coatings are systems produced from four groups of raw materials: binders, pigments, solvents and additives. When applied, the coating starts to loose the solvent and remaining materials (binders, pigments and additives) build a hard solid film having an esthetic and protective function. The present book is investigating the possible strategy of reducing, minimizing and eliminating the solvent from coating formulations in such way that the impact of the coating on environment is minimal.

1.2

Coatings

Coatings are ever-present in the modern society. They find applications for decorative, protective and functional purposes in the treatment of many kinds of surfaces. The role coatings may fulfill is a combination of different properties according to the area they are used. In modern society the environmental awareness has a strong impact on the coating composition and performance. Emissions from a coating system are the main issue. Therefore high attention is paid to the volatile content in a coating. Related to the presence of volatile organic compound (VOC) in coatings, there has been a continuous preoccupation to tackle this problem. Development and trends have brought on the market new systems complying with the new requirements. Mircea Manea: High Solid Binders © Copyright 2008 by Vincentz Network, Hannover, Germany

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Quick guide to coatings

13

Solventless coatings have been developed bringing the benefit of high build-up in a single application, minimal surface defects due to absence of solvents, excellent heat and chemical resistance and an overall lower application cost. Development in this segment had to deal with certain drawbacks such as poor impact resistance, poor flexibility and short pot-life problems that have been approached by new application techniques in which components are blended in proper proportions in the application process and respectively by development in the area of new curing agents providing better mechanical properties and longer pot-life. High solid coatings is a group of coatings that highly resemble to the solventless coatings but the composition contains less than 30 % solvent and may also contain reactive diluters, low molecular weight multifunctional resins or backbone structures containing moieties other than bisphenol A.

Powder coatings have been developed in middle 1950s [1, 2] and in spite of slow inroads in the market during 1960’ using as application system the fluidized bed process, are presently developing at a very high rate. Since the break through by the use of electrostatic spraying application method, powder coatings are regarded as a fusion coating process and bring a series of strong advantages to the coating technology: 100 % non-volatile content, no thinning or dilution required and no solvents or other pollutants are given off during the application or curing process. Further the application procedure is easier and no phenomena such as running, dripping, sagging may be encountered as in the case of liquid coatings. Other significant benefits are the low reject rate, tough and abrasion resistant film and high reclaim, by collecting and reutilizing the over spayed coating. Liquid low emission coatings are basically grouping two categories of coatings: • water-borne coatings and • UV curing coatings.

The water-borne coatings have strongly developed in recent years although technologically the basic principles have been established in late 1940’. Reviews of acrylic dispersions applications on different substrates list a series of properties that recommend these systems for coatings applications and explain the market growth. Further the UV curing systems are growing as a high speed/high performance and low energy cost application of coatings and inks especially for wood substrates.

1.3

Quick guide to coatings

A quick guide to coatings schematically categorization may be done as in Figure 1-1 (page 14). For some time now, the interest for clean coating technology lead to development and market share growth for water-borne, UV and powder coatings. The approach has yielded a less polluting technology. Although the solvent-borne coatings exhibit

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14

General considerations

Figure 1-1: Quick guide to coatings

excellent performance, the significant solvent content is a major concern due to environmental impact, toxicity, and flammability. Hence the development of other systems like powder coatings, high solids, radiation curable and water-borne as options for the commonly accepted solvent-borne coatings. However solvent-borne coatings coexist with the new emerging technologies and the development is illustrated by the increasing interest. A perfect coating must respond to a series of demands such as: • workable • good mechanical and chemical properties • storage stability and long pot-life • rapid curing (“curing on the whistle”) Recently these demands have increased by the ecological interest and constraints applied to this market segment: • low energy consuming • less pollutant • chemistry based on renewable raw materials These demands are leading constantly to a green chemistry and to a higher nonvolatile content.

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15

Coatings global market

Figure 1-2: Coatings production summary in 1997, 2000 and 2004 Solvent-borne second generation Solvent-borne first generation Water-borne architectural Water-borne industrial High solids Powder coatings Radiation curings Others Figure 1-3: Coatings market structure

1.4

Coatings global market

Coatings, as ubiquitous part of the modern society, are manufactured at a level higher than 30 million tons per year. In Figure 1-2, a summary of the geographical coatings production is presented [177, 319].

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General considerations

The coatings production volume is considered to be mature in areas such as Northern America, Japan, and Western Europe and generally correlates to the health of the economy. In other areas such as Asia Pacific, Eastern Europe and South America the coatings industry is still strongly growing, expected 10 to 15 % mostly due to the economical growth in China. The market structure at the level of 2004 is presented in Figure 1-3 (page 15) and has a strong geographical character. However it is clear that the trend for solvent borne coatings is declining while radiation curing, powder and water borne coatings are still pioneering. High solids coatings in spite the long history still present a strong potential being able to take shares in the solvent borne segment.

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Environmental concerns

2

Environmental awareness

2.1

Environmental concerns

17

Air and water pollution progressed over the last decades from an art to a science. This turn has evolved in both fundamental and applied research. Pollution today is regarded as a multidisciplinary approach that is directly related to the quality of life. While water pollution today mainly relates to secondary pollutants such as nutrients and refractory organics, air pollution is mainly dealing with harmful emission. Air pollution as a concept evolved in time. Historically three events had a considerable impact on the awareness or air polluting: • Week-long air stagnation in the Meuse Valley in Belgium in 1930, leading to the death of 60 people and respiratory problems for a large number of others. • Air stagnation in Donora, Pennsylvania in 1948, resulting in nearly 7000 illness cases and 20 deaths. • Four-day “killer fog” in London, England in 1952, resulting in 4000 deaths. Definition of air pollution Air pollution has been defined as being any atmospheric condition in which substances are present in high enough concentrations above the normal ambient level capable to generate measurable effects in humans, animals, vegetation or materials, or to produce an objectionable effect on the natural balance of any ecosystem. The definition would comprehend any substance: solid, liquid or gas, which is generated in an anthropogenic activity or by natural sources that may as well affect the global climate or global ecosystem by a greenhouse effect or by an ozone depleting property. Although dramatized, these episodes increased the awareness on the acute health effects of high concentrations of air pollutants and concerns over longer term chronic effects have been raised. This initiated a package of original six criteria pollutants, so named in the 1970s: • sulfur dioxide [7446-09-5], SO2 ; • nitrogen dioxide [10102-44-0], NO2 ; • ozone (qv) [10028-15-6], O3, also known as phytotoxins, • carbon monoxide (qv) [630-08-0], CO; • suspended particulates; and • non-methane hydrocarbons, NMHC. Mircea Manea: High Solid Binders © Copyright 2008 by Vincentz Network, Hannover, Germany

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Environmental awareness

Later on, a new concept has been introduced usually referred as to the Maximum Incremental Reactivity (MIR). Species of Volatile Organic Compounds that can combine with nitrogen oxides (NOx) and under the energy from sunlight are able to form ozone. The impact of a given VOC on formation of ground-level ozone is sometimes referred to as “reactivity”. Similarly, owing to increased public awareness of water pollution, stringent wastedisposal regulations have been introduced, and best available technology (BAT) had to be implemented to industrial wastewater treatment by the mid-1980s. The NMHC are presently referred to as Volatile Organic Compounds, VOC. A source of VOC is the coating industry. The environmental awareness and attention is presently mostly related to the release of carbon trapped for million of years as coal and oil in the Earth crust into the atmosphere as carbon dioxide. The mass balance is still the same, but the partition of components drastically changed for the last 500 years. The awareness of the impact of coatings on the environment has grown almost at the same speed with the market request and coatings production. The driver as finding less polluting solutions has increased along with the environmental concerns in general. This phenomenon has developed on both individual levels as well as through a serious commitment and governmental regulations [3–5]. Historically, it looks like the trend for higher non-volatile content in coatings has been initiated by the well-known “Rule 66” of California, on the emission level of photo chemically reactive solvents in paint formulations. These drivers, expressed as controlled emissions, environmental concerns, governmental regulations have inspired the development of coating industry in finding compliant solutions.

2.2

VOC regulations

Coatings are complex systems containing binders, pigments, additives, solvents, diluents, thinners. Binders, pigments and the largest number of additives are constituents of the solid matter of the coating system and are non-evaporating or nonvolatile. The volatile portion of the coatings is made up of water, solvents, diluents, reducers or thinners, and is removed during drying and application by an evaporation process. Most of the solvents, thinners, diluents contain VOC. By VOC it is understood any compound containing carbon that may participate in atmospheric photochemical reactions. For coatings, the general concern related to the concept of VOC is regarding any organic compound that does not remain in the coating film after the drying process, basically comprising the entire volatile part of the coating if not any specifically exempted compound is listed. Considering VOC defined as carbon containing compounds participating in atmospheric photochemical processes the list is of chemicals is not definitive and is constantly under review and being updated.

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VOC regulations

19

There are thousands of individual species of VOC chemicals that can combine with nitrogen oxides (NOx ) and the energy from sunlight to form ozone. The impact of a given VOC on formation of ground-level ozone is sometimes referred to as its “reactivity”. It is generally understood that not all VOCs are equal in their effects on ground-level ozone formation. Some VOCs react extremely slowly and changes in their emissions have limited effects on ozone pollution episodes. Some VOCs form ozone faster, or they may form more ozone than other VOCs. Others not only form ozone themselves, but also enhance ozone formation from other VOCs. By distinguishing between more reactive and less reactive VOCs, it should however be possible to decrease ozone concentrations further or more efficiently than by controlling all VOCs equally. Assigning a value to the reactivity of a compound is a complex undertaking. Reactivity is not simply a property of the compound itself; it is a property of both the compound and the environment in which the compound is found. The reactivity of a single compound varies with VOC-NOx ratios, meteorological conditions, the mix of other VOCs in the atmosphere, and the time interval of interest. Designing an effective regulation that takes account of these interactions is difficult and implementing and enforcing such a regulation carries the extra burden of characterizing and tracking the full chemical composition of VOC emissions. For a better understanding of VOCs an estimation scale has been implemented: MIR. Maximum Incremental Reactivity (MIR) scale Maximum Incremental Reactivity (MIR) scale is designed using certain assumptions about meteorological and environmental conditions where ozone production is most sensitive to changes in hydrocarbon emissions and, therefore, is intended to represent conditions where VOC emission controls will be most effective. The MIR scale is expressed as grams of ozone formed per gram of organic compound reacted. Each compound is assigned an individual MIR value, which enables reactivities of different compounds to be compared quantitatively. Individual MIR values now exist for many commonly used compounds, and a list of these individual values comprises a scale. 2.2.1

Estimation of VOC emissions

The determination of the VOC content in a coating is simply done by the mass balance approach. The VOC content is generally available in the manufacturer’s specification or material safety data sheet, in form of % by weight (bw) or volume (bV). In such case the VOC may be calculated for each ingredient in the coating as follows:   Equation 2-1

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Environmental awareness

where density is given in g/l, and % is given in parts by weight   Equation 2-2     gVOCi/lCoating = %bV × density where the density is given in g/l and the % of VOCi is given in % by volume for component i. Hence the total VOC is the sum of all VOC components:   Equation 2-3

The VOC regulations and targets differ from country to country. Bellow is given the EPA classification for coatings and the EPA target for United States of America [320]. 2.2.2

Categories of industrial surface coatings operation

An “in use” classification for coating categories for different application areas is presented by EPA (Table 2-1) for the United States. For the European Community, the product groups are listed in the Table 2-3 (page 22). 2.2.2.1 Selected USA coating regulations by region Table 2-2 presents the American regulation for different areas and coatings category. 2.2.2.2 Environmental targets in Europe In Europe the targets are as follows and the strategy is a two step process with toll gates scheduled for 2007 and 2010 (Table 2-3, page 22): Table 2-1: EPA classification of coatings Automobile and light duty truck coating/ manufacturing

Paper and other web (film and foil)

boat manufacturing

plastic parts and products

fabric coating, printing, and dyeing

reinforced plastic composites manufacturing

wood building products

aerospace coatings

large appliances

automobile refinishing

metal can

consumer products

metal coil

shipbuilding

metal furniture

wood furniture coatings

miscellaneous metal parts and products

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21

VOC regulations

Table 2-2: VOC targets for North America Product subcategory

Current DE,NY, target PA,VA 2005

SCAQMD 2002 July

SCAQMD 2008

Bay Area 2004

OTC 2005

SCAQMD 2006 July

CARB 2004

250

50

250

150

150

50

150

gloss,70+at60º

380

250

150

non-flat, 5–7at60º

380

150

150

flat, 15at85º, 5at60º

250

100

100

100

100

100

stain, does not conceal grain

550

250

250

250

250

250

quick dry enamel, 70+gloss, 8 hr dry hard

450

250

250

250

250

50

250

quick dry primer, dry like enamel

450

200

200

200

200

100

350

50

primer and undercoater

350

200

200

200

200

100

200

floor, opaque

400

250

100

250

250

50

250

varnish, clear wood finish

450

350

350

350

350

350

industrial maintenance, wood or metal, primer, mid-and top coat, industrial use only

450

340

250

250

340

250

rust preventive, metal only

400

400

400

400

dry gog

400

400

400

400

sanding sealer

550

350

350

350

350

–

350

specialty primer, stain block type

400

350

350

100

100

400

350

(Values are given in g/l)

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Environmental awareness

Table 2-3: VOC targets for Europe Product subcategory

Technology

Phase I from 1.1.2007

Phase II from 1.1.2010

interior matt walls and ceilings (gloss 25 at 60º)

WB SB

150 400

100 100

interior/exterior trim and cladding paints for wood and metal

WB SB

150 400

130 300

interior/exterior trim varnishes and wood stains, including opaque wood stains

WB SB

150 500

130 400

interior and exterior minimal build wood stains

WB SB

150 700

130 700

exterior walls of mineral substrate

WB SB

75 450

40 430

primers

WB SB

50 450

30 350

binding primers

WB SB

50 450

30 350

one-pack performance coatings

WB SB

140 600

140 500

two-pack reactive performance coatings for specific applications such as floors

WB SB

140 550

140 500

multicolored coatings

WB SB

150 400

100 100

decorative effect coatings

WB SB

300 500

200 200

SB= solvent-borne, WB= water-borne. (Values are given in g/l)

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Basics in coatings technology

3

General concepts in coatings

3.1

Basics in coatings technology

23

For a better understanding of the problems generated by high solids systems Chapter 3 will address some concepts related to coatings in general. These problems are related to the film formation capabilities, non-volatile content, viscosity, and solvents. Going deeper into the property generation of a coating parameters such as polymer structure, molecular weight, etc. must addressed. 3.1.1 Film formation Paints and in general any coating material are semi-finished products. They became coatings as soon as the material has been applied to a surface of an object and has been transformed into a continuous film, smooth with good adhesion to the substrate, and capable to meet the requirements it has been designed for. The transformation process from a liquid coating material to a covering film is usually referred to as the “drying process”. The coating process is depending on several parameters: • Good flow in order to seal any void in the film and level in order to give a smooth finish. • Good wetting on the surface in order to properly cover the whole surface. The good wetting is dependent on the surface tension of the coating material, generally lower than that of the substrate. • Low viscosity in order to let entrapped air bubbles to be released and to form a continuous film • Proper viscosity in order to secure a good leveling on horizontal surfaces and prevent flow on vertical surfaces. It must be noted that the surface tension endeavors in the same way on both vertical and horizontal surfaces, while the gravity is enhancing flow on horizontal surfaces and runs on the vertical ones. In general terms the transformation of a coating material from a liquid material to a solid film implies a series of changes, fundamental in terms of rheology. As times goes on, the liquid material applied on a surface solidifies and transforms to an elastic or durometric body and follows two different and simultaneously processes [155] : • solvent release from the coating material (the physical drying process) and • chemical drying process in which the polymer molecules are forming a dense network Mircea Manea: High Solid Binders © Copyright 2008 by Vincentz Network, Hannover, Germany

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In terms of viscosity the process may be described by Equation 3-1:   Equation 3-1 where is the viscosity of the coating material, and NV is the non-volatile content. The viscosity is as well a function of molecular weight, Mn and from this point of view, as the solvent goes off the concentration of polymer material is increasing and the durometric body is obtained as soon as the solvent has been completely released. The case of physical drying is generally important for the first steps of the film forming process. Physical forming coatings are no longer desired nowadays for environmental and performance reasons. To achieve a hard coating from a physical drying polymer that finally exhibits good properties it is necessary to use polymers having high molecular weights. This is the case of thermoplastic acrylates, cellulose ester, chlorinated rubber, nitrile- rubber, etc. On the other hand, polymers initially having low molecular weight but are functional, allow the use of higher non-volatile content and achieve better performance by the molecular growing process in the film formation process and network building through chemical cross-linking. The Figure 3-1 illustrates the better performance of reactive polymers in spite of lower molecular weight. However, while the physical drying is related to the physical change in the film due to solvent evaporation, the chemical drying is related to changes in molecular structure and creation of new chemical bonds.

Figure 3-1: Viscosity evolution during film formation process

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The new generated chemical bond in the curing process may be of different type: polyaddition, polycondensation or polymerization. In polycondensation reactions, the activation energy must be overcome and either catalyst or higher temperature, or both. In polyaddition reactions that build the cross-linked film, the drawback of higher temperature is not existent and consequently, the risk for over bake is of no importance. Therefore the risk for brittleness and discoloration is of no importance. However, there is another problem to overcome: the polyaddition reaction starts at the component mixing and these systems are generally presented as two pack systems. Further, as the curing takes place at ambient temperature, the film build up results into a polymer that has Tg higher than the curing temperature. In this case a number of reactive groups loose their mobility and remain unreacted and frozen in the polymer network constituting attack points for the coating in service. The polymerization curing, successfully used in UV curing coatings, unsaturated polyesters, etc., leads to very strong and chemical resistant coatings. No matter the film formation process, physical drying or chemical curing, an aspect of relevance must be noted. This aspect relates to the fact that during the drying process, low molecular products leave the coating film in formation: the solvent molecules in physical drying or low molecular weight condensation products. This leads to a volume reduction of the applied coating, leaving irregularities in the film or internal tensions. In polyaddition and polymerization curing processes, the shrinkage is a result of new chemical bonds building, resulting in a volume reduction of about 10 %. However the immediate result of the shrinkage phenomenon in coatings is the deterioration of the visual aspect of the coating. For sure, the low molecular weight condensation products and polyaddition or polymerization shrinkage may generate hollows at low solvent concentration in the film, which generate undesirable discontinuities. 3.1.2 Molecular weight Molecular weight is of great importance in polymer chemistry. It has a significant importance and influence of physical and chemical properties of the polymer affecting the high majority of properties of the coating system. Solubility, rheology, dry speed and ultimately performance are all affected by the molecular weight of the polymer involved in the system [156 –158]. 3.1.2.1 Molecular weight Definitions

For a linear polymer, the molecular weight is easy to define. The molecular weight may be calculated with the formula as in Equation 3-2.   Equation 3-2   

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MW = DP × MWi

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where DP=degree of polymerization and MWi is the molecular weight of the monomer in the chain. In the case of polycondensation reactions, the molecular weight of the polymer is more complicated to calculate. Some factors, such as functionality and conversion degree, must be considered in this case.

Polymers used for coatings applications are obtained by methods that lead to a population of polymers, (distributions or mixtures) having polymer chains with various weights. This is an important parameter for the polymers in the coatings industry, as the molecular weight composition of the polymer plays an extremely important role in the final properties. Low molecular weight polymer chains are acting as plasticizers while the high and very high molecular weight members of the polymer population affect parameters like viscosity, hardness, chemical properties, drying, etc., performance, in general. So, the key of binders manufacturing is among others, the reproducibility of the synthesized polymer to confer consistent properties in repeated synthesis. However, in spite the fact of dealing with a population of polymers, we refer in the daily language to the molecular weight as to a population of polymers having an average molecular weight. At this point there are defined two different molecular weights: • number-average molecular weight (Mn) defined by Equation 3-3:

  Equation 3-3 where, Mi is the molecular weight of species i compared to Hydrogen (MWH =1) Ni is the number of molecules of species i of molecular weight M i The weight-average molecular weight (Mw) is defined by Equation 3-4   Equation 3-4 where wi is the weight of molecules of species i and Mi is the molecular weight of molecules of molecular weight Mi (Equation 3-5):   Equation 3-5   

(wi = Ni × Mi )

According to Equation of Mw (Equation 3-4), each molecule contributes to Mw with the square of its mass, resulting in the fact that always Mw>Mn. Mw and Mn are theoretical concepts and the ratio between the two of them gives the polydispersity P, Equation 3-6 (the breadth of the distribution of molecular weight species in the polymer).

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  Equation 3-6 The polydispersity P is used as an indication of the distribution of the molecular species within the polymer. In the case that the polymer contains molecules having the same MWi , the Mw=Mn and the polymer is monodisperse (P=1). The reciprocal concept is the nonuniformity U, Equation 3-7:   Equation 3-7 The difference between the Mw and Mn is the fact that Mw gives an indication of the configuration of the polymer (straight, chain, branched, random-coil, etc) and is approximated through the analysis method presently used. 3.1.2.2 Molecular weight analysis methods The molecular weight of a polymer may be determined by some chemical and physical methods [11, 12, 159]. Methods like • functional group analysis • measurement of colligative properties • light scattering • ultracentrifugation are virtually absolute and the molecular weight can be calculated without reference to a calibration method. Other methods such as • dilute solution viscosity • GPC (Gel Permeation Chromatography) are techniques that respectively relate the viscosity to molecular weight and to a calibration polymer. End-Group Analysis According to this method, the polymer contains a known number of functional groups per molecule. The nature of the polymer limits the existence of the functional groups in terminal positions. Hence, the name of the method. The method counts the number average molecular weight (Mn) for the sample polymer and is depending on the accuracy of the analysis method. The methods differ and require different techniques when applied to addition polymers or condensation polymer. A typical case is presented by epoxy resins (Table 3-1, page 28).

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Table 3-1: Typical relation reactivity-molecular weight for epoxy resins Average n value

Epoxy equivalent weight

Molecular weight

Softening point (orientative values)

0.1

200

400

9 ºC

2.2

500

1000

64 ºC

5.5

900

1800

90 ºC

12 to 14

1800

3600

124 ºC

15 to 16

2500

5000

135 ºC

Other cases that make use of the method are polyesters and amides where the functional groups are easy to titrate with base and respectively acid. In case of hydroxyl group titration, the methods may use acetylation or infrared spectroscopy. The limitations of the method are presented by the limited solubility of the polymer in proper solvents for titration. In the case of addition polymers, things are different: the polymer may present a multitude of terminal groups as type and origin. In the case of polymerization of known kinetics, the analysis of terminal groups as initiator fragments may be successfully used. Groups resulting as from the chain transfer with the solvent, or the use of isotopes are also useful approaches. Unsaturated end groups in vinyl polymers may also be a good path by analyzing as in infrared spectra. Colligative property measurement Colligative properties are associated with the free energy of a solution. They are determined by measurements on dilute polymer solution. Such parameters that can be measured and relate to the molecular weight of polymers are: • vapor pressure lowering • boiling point (ebulliometry) • freezing point (cryoscopy) • osmotic pressure (osmometry) The background of colligative property measurement to determine the molecular weight of polymers resides in the fact that for infinitely low concentration of polymer solutions, the activity of a solute is equal to the mole fraction of the solute. The activity of the solvent is equal to its molar fraction, and the depression of the activity of the solvent is equal to the mole fraction of the polymer. Vapor pressure lowering Measurements are made at several polymer concentrations and extrapolated to the concentration equal to zero using low molecular standards to calibrate a diagram.

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The method is useful for molecular weights up to 40,000 and determines the number-average molecular weight Mn. The method uses a vapor phase osmometer measuring the small temperature variation resulting from different evaporation rates and solvent condensation in small droplets of pure solvent, maintaining the polymer solution in a solvent vapor atmosphere. The temperature differences are proportional to the lowering vapor pressure solution at equilibrium, and hence the dependence of the number-average molecular weight. Ebulliometry This method is comparing the boiling point of a polymer solution to the pure solvent in a vessel known as ebulliometer in isobaric or isothermal conditions (Equation 3-8).   Equation 3-8     

TB.P. = i · Kb · m

The equation relates the Van’t Hoff factor i (the number of dissolved particles) to Kb, the ebullioscopic constant unique for each and every solvent, and m is the molality of the solution. Customary the ebulliometer is calibrated with a known substance such as octacosane [(C28H58), MW=394.7662] or tristearin [(C57 H110O6), MW= 891.4924]. However the method is rarely used in molecular weight determination for polymers used in the coatings industry. Cryoscopy The cryoscopic method is the mirror method when compared to ebullioscopy. In this method the depression of the freezing point of a solution is compared to the freezing temperature of the pure solvent.   Equation 3-9a     

Tf = i · Kf · a

In the equation i is the Van’t Hoff factor (the number of dissolved particles), Kf is the cryoscopic constant unique for each and every solvent given by the ratio: Equation 3-9b

where R is the gas constant 8.314 J·K-1·mol-1 and Hf is the fusion heat of the solvent and a is the activity coefficient multiplied by the molality of given solution. The cryoscopic method is useful to determine polymer molecular weights up to 30,000. Membrane Osmometry According to the osmometric method, the osmotic pressure of a polymer solution is measured using a membrane that allows only solvent molecules to penetrate. The

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polymer solution is confined to one side of the osmometer and by this a difference in activity of the solvent is recorded, the thermodynamic driver towards equilibrium is pushing the solvent resulting in a difference in liquid level in the two compartment of the osmometer. The calculation of the number-average molecular weight (Mn) is performed by Equation 3-10:   Equation 3-10

where is the osmotic pressure R is the ideal gas constant T is the absolute temperature A2, A1 are the polymer solvent interaction terms c is the polymer concentration in solvent The values of π are determined versus concentration of the polymer solution and the diagram of /c vs. c. The interception point at c= 0 is the value of 1/Mn and the slope of the line is the A2 value. Light scattering Scattering is a general physical process whereby light (but also sound, moving particles) are forced to deviate from a straight trajectory or reflected by another angle than the one predicted by the laws of reflection, when they encounter any kind of non-uniformities (scattering centers). In this method the light scattered when passing through a polymer solution is measured by a photometer as a function of the observation angle and polymer solution concentration. When the solvent has a refractive index different of that of the polymer and is completely free from any mechanical impurities, several measurements at different angles and concentrations are possible, followed by light intensity extrapolation to the observation angle 0º. The intensity of the scattered light is proportional to the square of the mass of the particle encountered as a non-uniformity and the thus the weight-average mass Mw may be determined. The method yields good values in the range of 10,000 up to 10,000,000. Ultracentrifugation The method is very suitable for biological polymers such as proteins and less applicable for synthetic polymers that generally have a random configuration. Under the centrifugation process in constant conditions, a concentration gradient is developed as a function of the molecular weight as heavier molecules are spun out away from the center of the centrifuge. The concentration of polymer along the

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probe is determined by optical methods. The result is the weight-average molecular weight as function of concentration differences, location, angular velocity and partial specific volume of the polymer. Molecular weight determination by viscosity measurements Polymer solution viscosity is one of the most important parameters from all points of view. Viscosity is a strong indication of the way to handle and apply coatings and which is generally perceived as “thick” or “thin”. Basically the viscosity is a measure of the extension of polymer molecules into space and is understood as the resistance opposed by a fluid (polymer solution) or as the measure for the internal friction of the fluid. For practical reasons the viscosity is generally referred to as the efflux time and it is easy to compare viscosities of different polymers or polymer solution by this. The efflux time is the time necessary for a given volume of solution to flow through a capillary tube or a simple orifice and may be referred as flow time. It is obvious that at different concentrations of polymer, solutions present accordingly viscosities. Empirically the viscosity is related to the molecular weight of linear polymers and the meaning of viscosity is strongly implemented as such in the daily life. The determined molecular weight addressed as the viscosity average molecular weight (Mv) is dependant on the polymer nature, solvent, polymer/solvent interaction and the distribution of molecular weight. The values determined by this method are relatively lower than Mw by 10 to 20 % and the viscosity average molecular weight equals the Mw when a =1 (in the Mark-Houwink equation). By extrapolation at infinite dilution (concentration c= 0), the viscosity is a function of the solvent. This would be the intrinsic viscosity. Different expressions for viscosities are given in Table 3-2. Table 3-2: Expressions for viscosity Viscosity relative viscosity

r

= / 0=t/t0

specific viscosity

sp

reduced viscosity

= r-1= ( -

red=

inherent viscosity

inh

intrinsic viscosity

[ ]=(

)/

0

=(t-t0)/t0

0

sp /c

=(ln sp

)/c

sp

/c)c=0=[(ln

)/c]c=0

r

where: c=concentration (g/100ml solution) t=efflux time polymer solution (corresponds to ) t0 = efflux time solvent (corresponds to 0)

Viscosity is determined by rigorous measurements on dilute solutions in capillary viscometers such as Ostwald-Fenske or Ubbelohde in controlled temperature con-

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ditions for given solution concentrations. From the relative viscosity r, the Fikentscher constant k may be calculated, Equation 3-11:   Equation 3-11

This equation shows that it may be applied regardless the polymer, solvent or temperature. Further the method implies the extrapolation of data for concentration c= 0 by the use of Huggins equation (Equation 3-12):   Equation 3-12

Or respectively Kraemer equation (Equation 3-13):   Equation 3-13

where k’ and k’’ are constants for a series of polymers of different molecular weight in a given solvent and k’– k’’= 0.5. The intrinsic viscosity is an expression of molecular weight and is frequently quoted in the literature to be in the range of 0.3 to 0.6 for polymers used in the coatings industry. However certain acrylic polymers may have the intrinsic viscosity in the range of 1 to 2 and this values correspond to molecular weight in the range of million. This is the case of polymers obtained in emulsion. The expression to calculate the molecular weight is the Mark-Houwink equation (Equation 3-14)   Equation 3-14

where K’ and a are constants determined from the plotting of the intrinsic viscosity for model polymers in different solvents. For many polymers the constants range for a and K’ are: 0.5NBu4+ >Na+. Polypropyleneoxide with narrow polydispersity has been obtained at low temperature with high yield and short reaction time in hydrocarbon media.

Figure 8-16: Narrow polydispersity polyether synthesis

8.2.4 Epoxy resins Epoxy resins (Chapter 5.5) result in condensation reactions where an aromatic diol (such as bisphenol A) or an aliphatic diol is substrate for glycidylation. The reaction may occur in the presence of a basic catalyst from raw materials or from an intermediate. As shown before, the condensation process from epichlorhydrine and bisphenol A yields a number of repeating moieties of 0, 1, 2, 3, etc, while the advancement process yields only even number of repeating moieties in case of higher molecular weight. Homologues exceeding the molecular weight of 400 are higher in viscosity and homologues having molecular weights in the range of 1000 or higher are solids. 8.2.5 Acrylic resins Acrylic resins are obtained by radical polymerization. The benefit of the process stems in the tolerance for all kind of impurities, stabilizers, traces of water and oxygen. In order to obtain a manageable viscosity it is necessary to control the

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molecular weight. This may be done by several methods, generally accepted being the use of chain transfer agents, temperature control, pressure control, variable amounts of initiator. Commonly accepted the concept that higher molecular weight, better performance, the molecular weight must be limited in the range of 20,000 in order to have a reasonable viscosity. However new techniques recently developed allow a rigorous control of the molecular weight and polymer architecture. A polymerization technique has been presented in Chapter 8.1 (Figure 8-9) as allowing the polymer construction in a controlled way. Since 2005, new techniques have been developed and in most cases the approach is based on three principal directions: • Atom transfer radical polymerization. ATRP is a controlled/“living” polymerization system and is based on a reversible exchange between a low concentration of growing radicals and a dormant species [267]. Reactivation of the dormant species allows the polymer chains to grow and deactivate again. ATRP is the most powerful, versatile, simple, and inexpensive method able to polymerize a wide range of monomers including various styrenes, acrylates and methacrylates as well as other monomers such as acrylonitrile, vinyl pyridine, and dienes. ATRP commonly uses simple alkyl halides as initiators and simple transition metals (iron, copper) as the catalysts (Figure 8-17).

Figure 8-17: Atom transfer radical polymerization

• Reversible addition-fragmentation chain transfer, RAFT, (Figure 8-18, page 356) technology is a very sophisticated form of controlled free radical polymerization. referred to as ‘living polymerization’, it can be stopped and restarted at anytime, enabling the synthesis of tailored polymers with unprecedented control over composition, architecture, predetermined molecular weight and

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narrow molecular weight distributions, with reactive terminal groups that can be purposely manipulated, including further polymerization extended over a wide range of monomers and reaction conditions.

Figure 8-18: Reversible addition-fragmentation chain transfer polymerization

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• Stable free radical mediated polymerization, SFRP, (Figure 8-19) also addressed as nitroxide mediated polymerization, as discovered while using a nitroxide as radical scavenger. When the coupling of the stable free radical with the polymeric radical is sufficiently reversible, termination is reversible, and the propagating radical concentration can be limited to levels that allow controlled polymerization. Similar to atom transfer radical polymerization (ATPR), the equilibrium between dormant chains (those reversibly terminated with the stable free radical) and active chains (those with a radical capable of adding to monomer) may be designed to heavily favor the dormant state.

Figure 8-19: Stable free radical mediated polymerization

A polymerization method, although reported in 1983, that yet did not get too much attention in industrial applications is related to group transfer polymerization (GTP). The method is efficient for living polymerization of methyl methacrylate, but as well may be applied to other acrylates according to recent development. Nucleophiles (azides, cyanindes, etc.) catalyze the polymerization in the presence of silyl ketene acetal initiators. Recent development shows that the control in the polymerization of acrylates by group transfer polymerization is possible when steric hindered N-heterocyclic carbenes are involved (Figure 8-20, page 158) [109–113].

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Figure 8-20: Group transfer polymerization

8.2.6

Polyurethanes

The viscosity of polyurethanes is mainly referring to the viscosity of polyisocyanates. In general isocyanate monomers used in coatings are liquid but due to health hazards they are presented as adducts with polyols or macrodiols, when presented as prepolymers, or as polymerized diisocyanate in forms presented earlier (alophanate, uretdione, etc.). The viscosity is dictated by the hydrogen bonding and the molecular weight is limited to dimers and trimers. The alophanate form seems to give the lowest viscosity due to internal hydrogen binding. Other forms contain precursor to isocyanate groups, such as uretdione, carbodiimide. 8.2.7

Amino resins

As shown earlier the amino resins present a viscosity depending on the ratio between the formaldehyde and amino groups, as well as on the etherification degree. Higher these ratios lower the viscosity. The amino resin has in such cases a more monomer aspect, exhibiting a functionality of max. 6 methylol- or alkyl ether groups. An important role is also played by the condensation between methylolated amino groups bridging the amino substrates. In general, melamine based resins may reach a nonvolatile content as high as 100 % without risk for gelling.

8.3

Presence of colloidal particles

Coatings may commonly use microgel particles to control viscosity and sagging. Swollen acrylic microgels take a larger volume in the liquid coating and hence may confer some thixotropic effect without essential modification of the refractive index. The use of colloidal particles is mainly interesting for condensation binders.

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Colloidal silica particles are also used to control viscosity and other mechanical and chemical properties. Although silica particle reduce gloss and modify viscosity, surface modification results in interesting properties. A large effort is put in hybrids inorganic/organic in order to achieve the desired property. A typical approach is also related to the use of multifunctional polyhedral oligomeric silsesquioxane obtained by condensation of a triethoxysilane precursor (Figure 8-21):

Figure 8-21: Multifunctional polyhedral oligomeric silsesquioxane

8.3.1

Alkyd resins

In the case of alkyd resins the theory predicts that microgel formation precedes the alkyd gelation. The whole picture presents the microgel particles becoming more and more crowded ending in a coalescing gel, the gelation of the alkyd resin. The interesting part is that the microgel having a high molecular weight is in colloidal state and has a very little contribution to viscosity as not being present in the alkyd solution as a solute but as a microgel. This results in a higher nonvolatile content for a given viscosity. Further, the presence of microgels is reducing somehow the drying time in air drying systems and lowers gloss when cross-linking with amino resins is involved. This may be the result of the fact that microgels have lower number of hydroxyl functional groups to participate to the pigment wetting as well as to react with the cross-linker. In an experimental way it has been established that it is not the bodying of the fatty acids that generates the microgels, but the polyols and polyacids. In general some film performance, such as thermo mechanical properties, has been found to be superior to alkyds with current low content of microgels. An addition of microgels may be performed by loading to an alkyd a small amount of dendrimers of hyperbranched core alkyd resins that will behave like microgels inducing the expected benefits in terms of performance and correlation non-volatile content/viscosity.

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Figure 8-22: Atomic force microscopy for a concept alkyd

Microgels may be generated intentionally by the design of the alkyd molecule. The atomic force microscopy brings some evidence for this when the applied to concept alkyds such as explained and illustrated in Figure 8-22 (AFM section analysis of concept alkyd Figure 8-4. The area analyzed is 2x2 μm large, with permission from the editor [30]). The molecules may form agglomerates and by this they generate nanoscopic gels which due to the air drying moieties of low Tg exhibit good drying speed. 8.3.2

Polyesters

The case description as for alkyds (Chapter 8.2.1) is not valuable for polyesters also regarded as oil-free alkyds. The esterification process is a resulting in a large population of molecular weight species from raw materials to gelled material. However a microscope investigation did not reveal any microgel formation and the experimental hydroxyl values are in the range with the calculated ones provided glycol loss is controlled.

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8.3.3 Acrylic resins Microgel formation is possible in acrylic binders. This may be the resultant of more head-to-head chain defects having a lower solubility compared to head-to-tail polymer chains. As well a source of microgels in the polymerization process is the grafting and chain transfer which results in branching and thus reduces the solubility of the polymer backbone. A draw back of the presence of microgels in acrylic resins is the fact that they reduce the gloss and bring haze in the liquid binder as well as in the film. Microgel particles may be produced from N-vinyl caprolactame and poly(ethylene glycol) diacrylate or respectively N,N’-methylenbisacrylamide. The type and concentration of cross-linker influence the polymerization kinetics and colloidal characteristics of the particles. The properties and behavior are attributed to the capability of polyethylene glycol diacrylate having higher solubility and stabilizing effect [116] . Core cross-linked star clusters are also possible by conventional free radical polymerization. The employed technique is using the “core first” approach building crosslinked cores from divinyl benzene followed by linear polymethyl methacrylate linear segments surrounding multiple cores and bonding them covalently [300]. The clusters have a radius in the range of 20 to 50 nm. 8.3.4 Polyurethanes The polyurethane reaction is an addition reaction. As a difference from the esterification reaction we may note that from the very beginning the monomer species disappear and high molecular weight is obtained as the functional groups is consumed. This reaction goes to a conversion of the functional groups of 100 % while in the esterification reaction the best conversion does not exceed 75 to 80 %. Even in the case of very high molecular weight the polyurethane backbone may preserve the solubility in certain solvents. 8.3.5 Amino resins Some microgels may result in the amino resins synthesis due to a local condensation of the polymer in formation as result of a local stronger heating, local higher catalyst concentration.

Figure 8-23: Formolysis reaction

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In general, microgels from urea-formaldehyde resins are undesirable as giving a milky aspect to the binder and reduced transparency. The size and content of microgels in melamine resins may be controlled by formolysis [35] in the presence of a soft acid (Figure 8-23).

8.4

Hydrogen bond control

The binders used in coating formulations contain polar groups always capable to give hydrogen bonds. The hydrogen bonds are generally provided by the functional groups and have an important roll in cross-linking as well as in pigment wetting, substrate anchoring, etc. The negative part is that the hydrogen bonds give associations that increase viscosity. In order to achieve binders that have less hydrogen bonds, but still undiminished reactivity and properties it is necessary to introduce a certain management of the groups that are capable to give hydrogen bonds by blocking, steric hindrance, etc. 8.4.1

Alkyd resins

As described in an earlier chapter, alkyds are modified polyesters. The modification is performed by saturated or unsaturated fatty acids. Considering the possible approaches to target high solid alkyd binders it is necessary to differentiate the curing type of the alkyd system. In case the alkyd is curing by the use of a crosslinker, things must be considered from another point of view when compared to alkyds that have oxidative drying. Alkyds, in general, must be considered as type b polymers. In other words, they have low molecular weight but high functionality. Alkyds contain at a certain extent unreacted hydroxyl and carboxyl groups. These groups are necessary and important as polar groups in the process of coating manufacturing, pigment wetting, etc. and as well play an important roll in the cross-linking process when thermo-setting or two component systems are employed. In t he final coating they contribute to the adhesion of the coating on the substrate. Further, as shown before they control the molecular weight of the alkyd. However, in order to reduce the viscosity it is possible to block these groups, especially the hydroxyl groups as hydrogen bond donors by different reactive groups that may deblock in the curing process or contribute to the cross-linking. The blocking agent will act as a plasticizing group and build in solvent and as well by higher weight will contribute to an increased non-volatile content. A well preferred blocking agent is the acetic anhydride. This will convert the hydroxyl group to an ester group consuming the possible hydrogen bonds. Any other mono-functional acid is capable to block all or a part of hydroxyl groups. The fact that acetic acid is preferred is related to the lower contribution to decreasing of the Tg of the alkyd in general (Figure 8-24).

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Figure 8-24: Acetylated backbone

Another method for blocking the OH groups is by using dihydropyrane in acid environment, leading to an addition to the double bond and yielding a cyclic ether which strongly cuts the viscosity (Figure 8-25):

Figure 8-25: Ether protection of hydroxyl groups

In general the acetalization method has been long used to consume the hydroxyl groups into cyclic acetals. The acetal formation is very convenient due to the fact that in an alkyd the vast majority of the hydroxyl groups are primary groups (from pentaerythritol, trimethylolpropane, etc.), resulting in six membered acetals. Formaldehyde has been used as such or as paraformaldehyde to result in cyclic inner formals. Anhydrides present in the alkyd formulation are active catalysts for the acetalization process, probably due to the fact that they accept the reaction water from the acetal formation, later entering the esterification process as diacids. This may explain the fact that anhydride sublimation is under control. Further it may be noted that general properties of the alkyd are improved (through-dry time, alkali resistance, etc.) and in case the modification is limited then the water resistance of the final coating is not jeopardized (Figure 8-26). In general any aldehyde may be used resulting in cyclic acetals. The use of formaldehyde for alkyd modification is long time known (Figure 5-23) introduced as such (paraformaldehyde) or present as impurity in the polyol.

Figure 8-26: Acetal formation

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An interesting modification is using acrolein resulting in vinyl dioxolanes for 1,2 dihydroxyl moieties which exhibit an increased oxidative reactivity, useful in the oxidative drying process and as well as second curing mechanism when 2-pack systems are involved (Figure 8-27):

Figure 8-27: Unsaturated acetals

Blocking the hydroxyl groups by unsaturated esters is an interesting method to reduce viscosity. The use of acrylic acid to block the hydroxyl groups results in an increased unsaturation. The combination of acrylic moieties with vinyl dioxalane or vinyl dioxane yields better oxidative drying for alkyd systems. This blocking may be done by the use of acrolein which in the oxidation process yields an acrylic group and a free radical (Figure 8-28):

Figure 8-28: Dioxalane autoxidation

The hydroxyl groups may be blocked by esters derived from dicyclopentadiene. This reaction has been known for a long time. The dicyclopentadiene is first reacted with a diacid resulting in an ester which is later esterified on the alkyd. At a ratio of 2 to 5 % modification the alkyd presents a higher nonvolatile content and improved drying properties due to the autoxidation of the dicyclopentene moiety (Figure 8-29). The hydroxyl groups may be also blocked by acetoacetate groups. This modification is generally performed by a transesterification reaction with tert-butyl acetoacetate (Figure 8-30). The acetoacetate groups confer lower viscosity and also a combination with a small quantity of acrylic modification results in dual cure mechanism based on Michael reaction.

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Figure 8-29: Autoxidation of dicyclopentene

Figure 8-30: Acetoacetate modification

The silylation method applied on alkyds is an approach that consumes both hydroxyl and carboxyl groups (Figure 8-31). The reaction yields a new reactive group capable of cross-linking and building a silica matrix which consequently improves chemical and mechanical properties.

Figure 8-31: Silylation of a functional backbone

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Polyester resins

The case of polyester resins is more or less similar to the case of alkyds. However saturated polyesters are used as plasticizers or as thermo-setting/2-component reactants. When used as plasticizers it is desired that the polyester backbone should not content free hydroxyl groups. As reactive binder, a certain amount of free hydroxyl groups are required to achieve the desired reactivity, whatever the co-reactant binder may be: amino binders, polyisocyanates, etc. The use of the same kind of modifications on the hydroxyl groups is therefore limited and requires deblocking conditions. Consequently, the increased non-volatile content for polyesters should be addressed in another way: steric hindrance of the functional group (Figure 8-32).

Figure 8-32: Functional group modification by reaction with a glycidyl group

The volume of the R and respectively R’ radicals strongly influence the viscosity of the resulting modified polyester backbone. However the resulting hydroxyl group is a secondary group and therefore less reactive than a primary hydroxyl group, but the difference may be surpassed by the use of appropriate catalysts. Another factor influencing the reactivity of the resulting hydroxyl group is the position in the polyester chain, in other words how exposed the group is. Considering the fact that in general polyesters have as building polyols having primary hydroxyl groups, the allover reactivity is not substantially affected. The use of βketoesters as modifying agents in the polyester backbone is introducing a factor of viscosity reduction, in the same time offering a substantial improvement in properties of two component polyurethane coatings [252].

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Figure 8-33: -ketoester modification

The transformation of the hydroxyl or carboxyl groups to ester groups resulting in the displacement of the functional groups out of the main backbone may be preformed at a certain extent with ε-caprolactone. Small quantities of ε-caprolactone or lactone as chain extender will increase the reactivity and reduce the viscosity [98]. Exceeding a certain level may result in increased crystallinity and thus in higher viscosity at ambient temperature (Figure 8-34).

Figure 8-34: Increased mobility by pushing the functional group out of the backbone

8.4.3 Polyethers As previously described (Chapter 5.3), polyethers are best defined by the hydroxyl number which is directly related to the molecular weight of the polyether. Polyethers are generally (when a mono- or di- alcohol is used) linear but when further modification is targeted and a higher functionality is required, then a polyol may be used as starter. The viscosity is depending on the functionality of the used polyol starter, higher hydroxyl functionality resulting in higher viscosity. For lower molecular weight polyethers, the viscosity may be influenced by the catalyst used in the manufacturing process. The viscosity of polyethers follows the Mark-Houwink equation (Equation 3-14), where K and a are constants in for the polyether in respective solvent. The viscosity of water solutions may be stabilized by the use of isopropyl alcohol, ethanol, low molecular weight diols or Mn 2+ solution at a level of 10 –5 to 10 –2 weight percent on solution. As the hydroxyl groups are the only reactive groups they can not be blocked, but may be modified by using a substituted glycidyl ether or ester as earlier presented.

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Epoxy resins

Epoxy resins contain a number of hydroxyl groups that is determined by the condensation degree. The epoxy groups have a plasticizing effect as such and do not contribute to viscosity increase before the cross-linking reaction starts, but the hydroxyl groups have a strong influence on viscosity. In general, the hydroxyl groups are blocked by esterification reaction using a monocarboxylic acid. Fatty acids from vegetable oils are used to block the hydroxyl groups, but they also open the epoxy group as well. In general this modification is done at levels of 40 to 70 % of the total number of hydroxyl groups resulting in viscous epoxy esters used in oven or respectively air drying coating systems. The remaining free hydroxyl groups are important as usual for pigment dispersion, substrate wetting and adhesion. Modification of epoxy resins with methyl ricinoleate is leading to hydroxyl group preservation and lower viscosity due to the shielding of the OH-group on the fatty acid moiety. Such polyols are enhancing the array of polyols for polyurethane coatings [258, 262]. Low molecular epoxy resins are used as such for formulation of solventless coatings in combinations with amino or amino/amido functional cross-linkers but as well may be used as cheap acrylation substrate for UV curing coatings. 8.4.5

Acrylic resins

Acrylic resins may be formulated as thermoplastic binders in which case the hydroxyl or carboxyl functionality is limited to low values mainly contributing to pigment wetting and substrate adhesion. However in the case of thermosetting acrylic resins there are present hydrogen donor groups. The viscosity of an acrylic resin is influenced by the Tg of the polymer and its molecular weight. In general the carboxyl groups are provided by the use of acrylic or methacrylic acid and the hydroxyl groups are introduced by monomers such as 2-hydroxy-ethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, or by precursors to hydroxyl groups such as glycidyl methacrylate (Figure 8-35). Other functionalities may be present for alternative curing, such as acetoacetate groups (Figure 8-36). Other monomers such as acrylamide or methacrylamide may also introduce hydroxyl groups when they are methylolated. For a lower viscosity, methylated or butylated groups are preferred (Figure 8-36). Reactive hydroxyl groups may be introduced in the backbone as acetals directly as follows (Figure 8-37, page 170), or by reacting an acrylic resin with 4,4-dimethoxybutane-1-amine on the moieties of ethyl acrylate in the polymer chain (Figure 8-38, page 170). Blocking the hydroxyl functionality with aldehydes and glycidylated derivatives as explained in Chapter 8.3.1 is a largely used approach [36–43]. Cardura as glycidylated NEO acid is proven a good and reasonable modification for the hydroxyl

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Figure 8-35: Functional acrylic monomers

Figure 8-36: Methylolation and etherification of acrylamide

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Figure 8-37: Acetal functionalization of monomer or polymer backbone

Figure 8-38: Polymer functionalization with acid curing groups

groups still preserving the hydroxyl functionality [93]. Hydroxyl acrylate may also be obtained by polymerization at 122 to 126 ºC in the presence of small quantities of 2-mercaptoethanol and treating the product with Cardura at 150 ºC [94]. Oxazoline modified acrylates and methacrylates for high solid coatings may be obtained by the use of an oxazoline monomer (2-alkyl-5,5-dimethylol-2-oxazoline methacrylate) (Figure 8-39) [95].

Figure 8-39: Oxazoline functional monomer

Another way to contain the hydroxyl groups of an acrylic binder that makes strong inroads in the market is using a cyclocarbonate as a precursor for the functional groups.

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Figure 8-40: Acrylic monomers with cyclic carbonate functional group

The generation of the cyclic carbonate group is presented in the Figure 8-49 and the reactions involving the cyclic carbonate are presented in Figure 8-47. The acrylic monomers containing the cyclic carbonate group may be generated by the following reactions [315] as shown in Figure 8-40.

Figure 8-41: Catalytic formation of unsaturated catalytic carbonate

An interesting approach to generate cyclic carbonates is using propargyl alcohols. The method is interesting for the fact that it first generates a cyclic carbonate which has a double bond that may be further reacted to generate interesting substrates (Figure 8-41). Acetalization of acrylic resins is also an employed technology for reduction of hydrogen bonds. Mainly employed for oven drying compositions the method makes

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use of an aldehyde is targets improved solubility properties. The oldest approach known is related to acetalization of polyvinyl alcohol in order to improve solubility in organic solvents and compatibility with other solvents. As well new polymers are targeted by this approach, replacing the traditional butyraldehyde with other aldehydes, eventually containing electron donor groups (Figure 8-42) [314].

Figure 8-42: Polyvinyl acetals containing electron-donor groups

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173

Polyurethane resins

Polyurethane resins are by excellence rich in hydrogen bonds. This is the key of the high performance of the polyurethane binders. Basically there is very little that can be done to block the hydrogen bonds donors in a polyurethane binder. However a reasonable approach is to introduce steric hindrance in order to reduce the access to the polar groups and to limit the hydrogen bond formation [72]. It is claimed that isocyanates derivatives from substituted amines give by reaction with phosgene polyisocyanate of lower viscosity (Figure 8-43).

Figure 8-43: Low viscosity isocyanate

When addressing the polyurethane resins a difference must be made: 1) is it a polyurethane backbone? or 2) it is a precursor to polyurethane, which is a polyisocyanate? As an example for the first case it is claimed that ester bond free polyurethane polyols are obtained from 1,2 or 1,3 diols 1 mol /1 NCO group. The reaction product of said diols and the trisisocyanurate from hexamethylene diisocyanate has very low polydispersity and give high quality melamine baking coatings for OEM [74]. In general in coatings the interesting part is the polyisocyanate component which we refer to. For a two component polyurethane coating it is important that the hydroxyl functional polymer has a low viscosity and is the high solids component. An alternative to hydroxyl groups is the use of thiol functional derivatives. The thiol groups is reacting like wise the hydroxyl group, but the thiol functional backbones exhibit a lower viscosity [73]. Polyisocyanates in general are liquids of relatively high viscosity mainly due to the moiety resulting from the polymerization of the isocyanate group that can be of isocyanurate, asymmetric trimer iminooxadiazindione, uretdione, etc. In Figure 5-78 the structure of allophanate having an intramolecular hydrogen bond exhibits lower viscosity compared to other homologues. Typical viscosities for trimers from hexamethylene diisocyanate are presented in Figure 8-44 [55] (page 174). New non-isocyanate curatives for two component solvent-borne urethane coatings are building on uretdiones [268] just because the viscosity is lower [69], and allophanates [70]. The blocking of isocyanate groups having shielded moieties with plasticizing effect is possible by the use of diethyl malonate (Figure 8-45, page 174).

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Figure 8-44: Typical viscosity for hexamethylene diisocyanate trimers

Figure 8-45: Blocked isocyanate group

Starting from this concept, a new approach has been developed, the new intermediate giving off no low molecular weight de-blocking moiety (Figures 8-46 and 8-47) [64].

Figure 8-46: Blocked isocyante

Figure 8-47: Cross-linking of blocked isocyanate

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Further the cross-linking reaction is catalyzed by a Zn based catalyst (Figure 8-47) [64]. In a similar way in the polyurethane chemistry, the shielding of hydrogen bonds may be performed by using secondary hydroxyl groups such as from castor oil [247] or by completely blocking them, using derivatives as precursors for the hydroxyl groups necessary for cross-linking. Such an approach is described as using bicyclic orthoesters [249, 251], Figure 8-98, that in acid catalysis is releasing the blocking acid and release the primary hydroxyl groups capable to react with the isocyanate groups. A particular group of binders that generates polyurethane backbones is not using isocyanates. The reaction cross-linking reaction described in Figure 5-85 involves a cyclic carbonate. The importance of the cyclic carbonate must be considered as the group is a precursor for two hydroxyl groups which potentially may react to generate different types of backbones (Figure 8-48) [315–317].

Figure 8-48: Cross-linking of cyclic carbonate functionality

In both cases the intramolecular hydrogen bond between the hydroxyl group and the -oxygen in the urethane or ester group is improving the hydrolysis stability, adhesion and other mechanical properties. The cyclic carbonate group may be generated by different methods as described in Figure 8-49.

Figure 8-49: Generation of cyclic carbonate group

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Amino resins

In melamine, glycoluril, benzoguanamine the hard core is the major high viscosity source. In amino resins the hydroxyl groups are etherified with aliphatic alcohols fact that reduces the number of hydrogen bonds and increases the compatibility of the resin with other binders. The higher the ratio formaldehyde/amino, and higher the etherification degree, lower the viscosity [96]. Other tailored core types may be produced from OH- or glycidyl functional substrates modified further with methyl carbamate and further methylolated and etherified with aliphatic alcohol (Figure 8-109) [272–274].

8.5

Reactive groups with plasticizing effect

Reactive groups in polymers have a contradictory effect. On one side they are building hydrogen bonds increasing viscosity and on the other side they are acting like plasticizing groups and build in solvents reducing viscosity. Which of these effects is stronger is difficult to establish as the effect on viscosity is related to the exposure of the functional groups and their ability to generate hydrogen bonds. As well the functional groups may be pendent on the polymer chain as end groups for moieties that may increase the crystallinity and therefore the glass transition temperature for the binder. Such moieties may also have a positive influence in the case of reflow, improving the mechanical properties of the coating and the scratch resistance. However a judgment on the contribution to viscosity increase or decrease may be done from this point of view. 8.5.1

Alkyd resins

In the case of alkyd resins the functionality is unsaturation when the alkyd is targeting the air drying applications or it may be hydroxyl functional in case the alkyd is intended to be used in thermo-settings and two component applications. The fatty acid in the alkyd is contributing to viscosity decrease and in the same time is increasing functionality for oxidative drying systems. More fatty acid in the alkyd formulation, higher chances to shield hard segments in the polyester backbone of the alkyd. Of even higher importance is the case when the alkyd is of urethane type. In this case the inter-chain hydrogen bonding is very strong and the contribution of the isocyanate moiety is strongly contributing to the physical drying of the alkyd. Further reactions between the isocyanate group and the hydroxyl functional intermediate are simultaneously occurring with side reactions between the isocyanate group and hydroxyl or carboxyl groups on the fatty acid resulted in the oxidation process of the oil. Situations in which the oil is bodied also result in higher viscosity. Introduction of acrylic groups is reducing viscosity and improving the oxidative drying of the alkyd. The acrylic group as ester of acrylic acid may be introduced through direct esterification or by transesterification (Figure 8-50).

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Figure 8-50: Etylenic group modification of an alkyd

Other approaches use acetoacetate groups for both improved drying and lower viscosity (Figure 8-51):

Figure 8-51: Acetoacetate modification of a backbone for secondary cross-linking

An approach that is making inroads in the market is the modification of the alkyd by converting the hydroxyl groups to thiol groups (Figure 8-52, page 178). As free radicals are generated in the drying process, they initiate as well the addition of the thiol group to the fatty acids double bonds as explained earlier. An even better and stronger cross-linking may be obtained when some of the diacid (phthalic anhydride, adipic acid, etc.) is replaced by a small quantity of 0 to 25 % tetrahydrophthalic anhydride or endomethylene tetrahydrophthalic anhydride. The system may be stabilized for shelve life by a corresponding amount of anti-skin additive (oxime or similar) generally used in all air draying alkyd systems.

The alkyd itself may preserve the composition, but in order to reduce the viscosity may be modified with respect to the architecture. The branching of the alkyd will reduce the viscosity. This is possible by using the succinate groups generated on the fatty acid moieties by maleic anhydride or Diels-Alder adducts of the fatty acids,

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Figure 8-52: Thiol modification for secondary thiol-ene cross-linking

yielding isomers of tetrahydrophtalic anhydride which will open and react by ester bonding alkyd molecules of lower molecular weight or any other hydroxyl functional polymer intended to modify the alkyd (Figure 8-53). Hydroxyl groups on an alkyd may be used to generate unsaturation and cut off the number of hydrogen bonds, but as well the modification is acting as a plasticizing moiety. When the modification by acetalization is done by a saturated aldehyde, then the moiety will by active at higher temperature and in acid catalysis (Figure 8-54). Other benefits of the acetalization of alkyd resins have been presented in Chapter 8.4.1. For alkyds that are thermosetting or targeting two component systems, introduction by oxidation of epoxy groups on the fatty acid or the use of vernonia oil will yield products with lower viscosity. Likewise the use of epoxy groups as hydroxyl group precursors will yield products of lower viscosity. The epoxy group may also be generated by oxidation of double bonds on tetrahydrophthalic anhydride or similar unsaturated monomers. Epoxy groups may be seen as precursors for hydroxyl groups but they may participate in cross-linking reactions. In this sense it is quite widely known a system building on such chemistry, consisting in two components, an epoxydated oil and an acid polyester (Figure 8-55, page 180).

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Figure 8-53: Allyl and Diels Alder modification of an alkyd

Figure 8-54: Acetalization of 1,3-hydroxyl groups

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Figure 8-55: Epoxyester cross-linking

The recent preoccupation of using renewable raw materials combined with the interest of capturing the carbon dioxide resulted in new development in generation of cyclic carbonate groups. The approach is following the typical case of epoxidation of the alkyd at the level of the double bonds on the fatty acids present in composition. Further the newly generated epoxy group is capable to incorporate carbon dioxide to yield a cyclic carbonate group (Figure 8-56) [321]. The catalyst used in this reaction is tetramethyl ammonium hydrogen carbonate.

Figure 8-56: Modification of fatty acids to cyclo carbonates functionality

Figure 8-57: Salen aluminum catalyst

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Recently a new developed in aluminum catalyst has been brought to light. The new aluminum salen catalyst allows reaction of carbon dioxide on a substrate containing an epoxy group at room temperature and atmospheric pressure (Figure 8-57) [322]. 8.5.2 Polyesters For polyesters the approach is in some sense similar to the path adopted for the alkyds. In general polyesters must contain hydroxyl groups or carboxyl groups which are participating in cross-linking reactions. In this case the functional groups must be preserved or contained as precursors. The acetal formation may be a suitable approach to contain two hydroxyl groups which will be later released to react at higher temperature and acid catalysis. A similar approach may be done by using unsaturated bulky acids that through oxidation will yield epoxy groups which will react as such or generate hydroxyl groups (Figure 8-58).

Figure 8-58: Functionalization by double bond oxidation

Along with dicyclopentadiene, substrates for epoxidation quoted in literature are emerging from nadic anhydride, tetrahydrophthalic anhydride, etc. (Figure 8-59).

Figure 8-59: Epoxydation of an unsaturated backbone

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An interesting approach is introducing a spacer between the polyester backbone and the functional hydroxyl group. This may be done by several methods; the spacer may be of polyether or polyester nature (Figure 8-60 and figure 8-61).

Figure 8-60: Introducing a spacer between the functional group and the backbone

Figure 8-61: Plasticizing effect from a “Cardura E10”

As well other modifications as discussed for alkyds are also available for polyesters. 8.5.3

Acrylic resins

Acrylic resins contain functional groups from functional monomers meant to react in two pack systems, by self-cross-linking, or in thermo-setting conditions. In general the functionality is hydroxyl, but a small amount of acrylic acid is used to improve the adhesion and wetting. Monomers have been presented also in Chapter 5.6.1. In Figure 8-62 are presented a range of functional acrylic monomers which along with those described in Figure 8-67 increase the latitude of formulations for acrylic binders:

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Figure 8-62: Cationic and other specialty monomers

The modification of the hydroxyl group may be done by the methods explained for the polyesters. Another suitable approach is reacting maleic anhydride in the acrylic backbone instead of acrylic acid or hydroxyl functional monomer. The succinic moiety will be later able to react with a polyol, preferably substituted 1,3-propane diol or higher, such as 1,4-butane diol, 1,6 hexane diol, etc. generating a hydroxyl group (Figure 8-63, page 184).

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Figure 8-63: Hydroxyl functionalization of a styrene-maleic anhydride backbone

Hydroxyl functionality may be introduced in the polymer backbone by using vinyl alcohol or trimethylolpropane monoallyl ether (Figure 8-64).

Figure 8-64: Allyl functional monomers for hydroxyl functional acrylic polymers

Other modifications as such using acetoacetate group are also possible in order to reduce viscosity and to contribute to dual curing. Methylene active monomers in this group are acetoacetate methacrylate (Figure 8-35) and diacetone acrylamide (Figure 8-65).

Figure 8-65: Diacetone acrylamide

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Acrylic resins having pendant cyclic carbonate functional groups (Figure 8-66) may be obtained from monomers prepared as explained in Figure 8-40 or by transformation of the glycidyl group already present in the polymer (from glycidyl acrylate or methacrylate) followed by modification with carbon dioxide in the presence of quaternary ammonium catalysts (Figure 8-66):

Figure 8-66: Cyclic carbonate functional acrylic resins

8.5.4

Epoxy resins

Epoxy resins and epoxydated novolacs are also used as substrates for advanced cyclic carbonate functional oligomers. The transformation of the epoxy groups into cyclic is possible using carbon dioxide in the presence of quaternary ammonium catalysts (Figure 8-40). The oligomers have structures as shown in Figure 8-67 (page 186) [316] 8.5.5

Polyurethanes

Considering a polyurethane backbone that has other functionality than isocyanate the reactive groups may be of any choice. Usually hydroxyl groups, amino, acetoacetate, acrylic, oxazolane and oxazoline functionalities are present [50]. Aldimine groups are also quoted as for lowering the viscosity of the curing system [71, 101]. An interesting modification is using the blocking of amino groups with ketones, yielding ketimines. The ketimine is participating in the cross-linking process first undergoing a hydrolysis process which will regenerate the amino group (Figure 8-68, page 187) which is further participating to the cross-linking reaction. In the process is generally given off methyl-ethyl ketone. This approach is used for work-shop primers, due to insufficient color retention (Figure 8-69, page 187). In the case one component is bearing an acrylic functionality, then the cross-linking reaction is yielding networks according to the Figure 8-70 undergoing a vinylog cross-linking reaction. As methylene active group other modifications are possible using levulinic acid (Figure 8-70, page 188) or diacetone alcohol (Figure 8-76), mainly used for dual cure in polyurethane dispersions.

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Figure 8-67: Advanced polycyclocarbonate oligomers from glycidylated substrates

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Figure 8-68: Cross-linking via ketimine-acetoacetate chemistry

Figure 8-69: Cross-linking via Michael addition

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Figure 8-70: Levulinic acid

In the conditions in which the functional groups are oxazolane and oxazoline, then the reaction is also humidity controlled (Figure 8-71) [44–53].

Figure 8-71: Cross-linking of oxazoline and oxazolone groups (carboxyl groups contained as anhydride).

Some papers dealing with the oxazolidine-aldimine concepts promote these functionalities for high solid polyurethane systems where the reactive moieties fulfill the roll of water scavenging and cutting down the viscosity of the system [254-257].

8.6

Choice of monomers in binder formulations

The choice of monomers as building blocks for binders intended for high solid applications is of high importance. Not only is the monomer choice, but as well the ratio used in the combination with the other co-reactants of importance. This allows more or less free rotation and mobility, may generate free volume and as well expose more or less the functional groups enabling more or less reactivity.

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Alkyd resins

In alkyd resins the use of oils is reducing the viscosity and it is well known the fact that shorter the oil length, higher the viscosity. This is the result of the fact that the oil content in short and medium oil long alkyds is not sufficient to shield the high Tg moieties derived from the polyester backbone contributing to the performing physical drying. A higher oil content is also reducing the density and from this it is expected a larger free volume, thus a lower viscosity. The polyester backbone also plays a role in viscosity build up. The higher the Tg of the backbone, the higher the viscosity! The ratio of aromatic or cyclo-aliphatic components is contributing to the viscosity increase or decrease. This has to do with the packaging level that may result in the final product. Higher the aromatic content, higher the viscosity. Thus it is expected that alkyds containing a modification with styrene to present a higher viscosity compared to the core, unmodified alkyd. The nature of the aromatic diacid is also important from the point of view chain packaging and crystallinity. Alkyd containing increasing ratio of isophthalic acid will have higher viscosity compared to phthalic based homologues while terephthalic acid may even give spontaneous crystallization. The choice of polyols also contributes or the disturbance of the packaging level of the alkyd. When −alkyl substituted polyols are used, the length of the alkyl group is extremely important. As an example, the viscosity is increasing in a series trimethylolethane to trimethylolpropane. This is an expression of how susceptible is the ester moiety to build crystalline segment of higher Tg and higher viscosity. With respect to the hydroxyl functionality things are related to the presence of polyols as providers of hydroxyl groups. Lower viscosity in the alkyd binders may be obtained by using polymer moieties having relatively high molecular weight, but being of low Tg. An example is the use of macrodiols that are liquid at room temperature, taking precaution with respect to the synthesis conditions not to break down this moiety. Such would be the case of using polyethers as building blocks. Other bulky monomers such as rosin are also used to reduce the viscosity when a small amount of solvent is used. In other applications the alkyd is further reacted with poly-allyl ether and tricarboxylic anhydride (such as trimellitic anhydride) to yield a lower viscosity modified alkyd [89]. The use of hyperbranched polyols based on dimethylol propionic acid and t-Bu benzoic acid is claimed to reduce VOC by 25 % in alkyd resins [97]. As well the use of at least 20 % dimethylol propionic acid in alkyd resins, even by redesign is a way to reduce the viscosity [99] shown in Figure 8-72).

Figure 8-72: Branching centers introduced by dimethylol propionic acid

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Polyesters

In the case of polyesters the situation is similar and the polyol choice if of major importance. The loading order of raw materials also has a strong impact on final viscosity especially in the case that the reaction time is relatively short and does not allow the transesterification reactions to redistribute the monomers. Bulky monomers contribute to a lower viscosity, while higher aromatic content contribute to viscosity increase. An interesting group of diols is generated from -olephines. The -olephines are in a range form C7 to C10, so the latitude of formulation is quite large offering both lower viscosity alternatives as well as higher hydrofobicity and lower surface tension (Figure 8-73).

Figure 8-73: Functionalization of -olefins

A typical case is again the choice of polyols. Neopentyl glycol is giving crystalline ester moieties, while the use of 2-methyl-1,3-propane diol or 2-ethyl,-2-butyl-1,3propane diol is resulting in lower viscosity. A remarkable difference must be mentioned when di-methylol cyclohexane is used. The 1,4 isomer is giving hard, xylene insoluble polyester while the use of a mixture of 1,4-dimethylol cyclohexane with 1,3-dimethylol cyclohexane in equal parts yield a polyester having good xylene solubility and practical similar properties in terms of reactivity and mechanical properties. In a similar way, the use of pentaerythritol will give a higher viscosity compared to the use of di-trimethylolpropane, having as well a functionality of four hydroxyl groups. In this case the -ethyl groups are responsible for viscosity reduction. Polyesters based on combinations of 1,4-cyclohexane diol and 1,3-propane diol are reported as yielding high solid polyurethane coatings with excellent mechanical properties [248, 253]. The effect of the difunctional dibasic acid in polyesters is a lower viscosity when the nature of the diacid is cycloaliphatic compared to linear diacids [248, 250, 259]. Interesting cycloalkane non aromatic polyesters based on are quoted as using a novel diacid as presented in Figure 8-74 [87].

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Figure 8-74: Non-aromatic building block for hyperbranching

Diols for aromatic polyesters are described as in Figure 8-75 and actually are oligo­ ester diols [88] or oligoethers diols [318]:

Figure 8-75: Aromatic diols, polyesters and polyethers

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High solid strategies

Epoxy resins

Epoxy resins of low molecular weight are known to be liquid at room temperature. Glycidylated phenol and cresol give low viscosity epoxy resins and are mainly used as reactive solvents. As well glycidylated polyols such as trimethylolpropane are used as reactive solvents due to very low viscosity. Bisphenol F is also known for lower viscosity and crystallization tendency compared to glycidylated bisphenol A. 8.6.4

Acrylic resins

In the synthesis of acrylic resins the use of monomers emerging from aliphatic alcohol esters of acrylic acid are known to reduce the viscosity and the Tg of the polymer. In the case of thermosetting acrylics a possible approach is to modify the functional group with monomers that preserve the functionality but introduce plasticizing moieties that also hinder hydrogen bond formation and in case they are bulky increase the solubility and the free volume in the polymer solution. As discussed earlier the use of block copolymers of the type AB will lead to lower viscosity. Branched acrylic polymer obtained from methacrylate, 2-hydroxyl acrylate and mercapto-acetic acid further reacted with glycidyl methacrylate and copolymerized with styrene 45:25:30 is claimed to exhibit lower viscosity suitable for high solid formulations [90]. The use of combinations of alkyl (C1, C2, C4) acrylates as monomers and at least one derivative from 2-hydroxylalkyl methacrylate modified with caprolactone yield acrylic resins with high non-volatile content [266]. Of high importance in acrylic manufacturing is the use of initiators and temperature in order to limit the molecular weight growth, as well as the chain transfer agents. An example is given in the use of tert-amyl peroxides and initiator combinations proven to be superior to azonitriles and conventional peroxides in order to meet VOC requirements and to yield high solid acrylics [91, 92, 270]. The synthesis of the acrylic binder at higher temperature and eventually under higher pressure than atmospheric pressure will yield a polymer having a lower molecular weight [271]. The process design and the monomer choice must be done in accordance to the solvent selection for the synthesis. The use of C6-C8 branched alcohol acetates in combination with 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol) shown in Figure 8-76 yield low viscosity acrylic resins [269].

Figure 8-76: Branched alcohol with cutting power

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8.6.5 Polyurethanes Polyurethanes present interest in this approach when they are presented as adducts of di-isocyanates with polyols. The considerations are similar related to the choice of polyol. As well using prepolymers derived from low Tg macrodiols end capped with diisocyanates or trimers from diisocyanates that are reacted at one isocyanate group with a monofunctional alcohol, the result is a di-functional isocyanate of lower viscosity (Figure 8-77).

Figure 8-77: Modified isocyanates for lower viscosity

The polyol choice from the range of renewable raw materials is a recent preoccupation. Polyols produced from cardanol (from the distillation of cashew nut shell liquid) or from polyesters resulted from polycondensation of the C7 cut of cardanol with other diols bringing the benefit of liquid crystallinity are discussed [295, 296].

Figure 8-78: Cardanol and derivatives

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Considerations related to the polyol component in the case of two component polyurethane coatings are the same as for polyesters or polyacrylates having hydro­ xyl functionality. 8.6.6

Amino resins

The relation viscosity/non-volatile content for amino resins is dependent on the molar ratio between components as explained earlier. However the nature of the etherification alcohol plays an important role. Higher the alcohol, lower the viscosity. Further melamine resins present lower viscosity compared to urea resins. The use of a combination urea/melamine will yield a lower viscosity urea resin when melamine is used as amino substrate. Also of interest, the use of plasticizers such as toluene sulfonamide (Figure 8-79) is reducing the viscosity in different combinations.

Figure 8-79: Plasticizing monomer for amino resins

A case that may be considered as both melamine resin and urethane resin is the case of carbamates. Carbamates may be considered as formaldehyde free cross-linkers for hydroxyl functional binders as well as epoxy resins, etc. [104 –108]. A case presently marketed is tris(alkoxycarbonylamino) triazine, Figure 8-80.

Figure 8-80: Carbamates

The reaction is catalyzed by acids and starts at 110 ºC with hydroxyl functional binders (Figure 8-81) and lower temperatures (80 ºC) for epoxy groups. The higher temperature (130 ºC) in reaction with epoxy functional binders is leading to oxazolidone rings (Figure 8-82).

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Figure 8-81: Cross-linking of carbamates with hydroxyl functional polymers

Figure 8-82: Thermal curing of carbamates with epoxies

8.6.7

Miscellaneous

Finally, there are novel modification possibilities for a larger application area comprising several binder types. Such an approach quoted in the literature [66] describes a reactive all-around solvent that is using as a building block a dimer/trimer diacid or diol obtained from dimer acids by hydrogenation. The dimer fatty acids or diols constitute a substrate that may be further esterified with a multitude of moieties that makes it suitable for a broad range of applications. When the substrate is reacted to an air drying moiety, such as a fatty acid, then it becomes suitable to be used as reactive solvent in high solid alkyds. In case the unsaturated moiety used to obtain the ester is further epoxydated to a glycidyl moiety, then it is suitable as reactive solvent for high solid epoxy formulations. The dimer fatty acids may be esterified with polyols to obtain moieties that can be further used as building blocks. Like wise the use of maleic anhydride is also a suitable modification for reactive solvent formulations (Figure 8-83, page 196). Such an approach allows the user to build polyester backbones having crystalline sequences and amorphous sequences [292]. The reaction of epoxidated oils and fatty acids or esters thereof with acids is leading to heavy secondary diols of lower viscosity and strong hydrophobicity. Likewise the Diels-Alder reaction or linseed oil fatty acid with acrylic acid is leading to diacids with strong hydrophobic character (Figure 8-84, page 196).

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Figure 8-83: Modification of dimmer fatty acids

Figure 8-84: Diols and diacids derivatives from fatty acids

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197

Reactive diluents

In order to comply with the new regulations and market pressure the first attempt to increase the non-volatile content is to look for new solvent having good cutting power and being compliant with the governmental requirements, so called exempt solvents. This approach enables the use of existing binders in solvent that are not listed and make possible a higher non-volatile content. One of these solvents having lower reactivity and good cutting power is para-chlorobenzotrifluoride (Figure 8-85) [65] or tert-butyl acetate (Figure 8-86) [260].

Figure 8-85: p-chlorobenzotrifluoride

Figure 8-86: t-butyl acetate

In many cases the shortest way to market is to find for existing binder system, a reactive solvent. These diluents have the same reactive groups as the binder, the system resulting in a homogenous way with no compatibility or phase separation problems, but are of low molecular weight, more like model molecules, being capable to act like solvent for the binder and react in the same condition as the binder participating in the film formation. 8.7.1

Alkyd resins

In alkyd resins the attempt to use reactive diluents is an old story. For oxidative drying alkyds, oils have been used from the very beginning with the draw back of slower drying properties and lower hardness. A better approach have been the use of modified oils with maleic anhydride, urethane oils, blown oils but the success has never been complete. Linseed oil and tung oil are still in use but they lost their importance as reactive solvents. Since oils contain unsaturated fatty acids chains they will polymerize with the unsaturated moieties in the alkyd binder.

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Reactive diluents synthesized to be compatible with air drying alkyds have been done by different approaches. One approach is using allyl ethers as such derived from trimethylolpropane or pentaerythritol (Figure 8-87). They have very good cutting power and also generate hydroperoxides in the drying process. However after a short time use of such kind of reactive solvents, this path has been forgotten due to potential acrolein release from the coating during the film formation process. A new attempt has been made in late 1990s when the use of methallyl ethers has been tested. Another interesting approach described in the literature invokes the use of melamine condensates which are acrylic functionalized. The modification is done using hexamethoxymethyl melamine functionalized with acrylamide with or without the presence of allyl alcohol in the presence of strong acids. The ratios employed in the process are 1 to 3 mols acrylamide per melamine ring or when allyl alcohol is present, the sum of unsaturated moieties from allyl alcohol or acrylamide does not exceed 3 mols per mol melamine [59].

Figure 8-87: Allyl and methallyl polyol ethers

Another method to bring reactable unsaturation to the alkyd system is the use of ethers and acetals. Esters of unsaturated fatty acids with heavier polyols have also been tested, however, without too much market interest. The use of acetals derived from dienes and aldehydes. A very simple acetal that has been employed in this approach is derived from butadiene and acetaldehyde (Figure 8-88) [55].

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Figure 8-88: Acetals from butadiene

This acetal shows a set to dry time of 1 to 4 hours depending on temperature [56, 57]. An acetal that may be of interest as a concept reactive solvent, emerging from glyoxal is not quoted in literature as a reactive solvent (Figure 8-89).

Figure 8-89: Acetal from glyoxal

Other acetals of interest are vinyl oxolanes. In these derivatives the allyl group has high oxygen reactivity. The ring structure is providing higher reactivity when compared to linear homologues. Further, the presence of functional free groups enables the vinyl oxolanes to be incorporated in low molecular weight polymers used as reactive diluents (Figure 8-90, page 200). The diallylidenacetal of pentaerythritol is less interesting, being a solid substance with a melting point of about 42 ºC. A recent invention claims reactive solvent for high solid alkyd resins building on an alditol, submitted to esterification reaction with unsaturated vegetable fatty acid, preferable using linoleic acid and sorbitol at a level of at least one mol acid per mol sorbitol [60]. An interesting reactive solvent is dicyclopentenyloxyethenyl methacrylate (Figure 8-91, page 200) [55, 56].

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Figure 8-90: Cyclic unsaturated acetals

Figure 8-91: Dicyclopentenyloxyethenyl methacrylate

This monomer can be cured at ambient temperature by oxidative polymerization, probably initiated in the allyl position in the five membered ring. Other interesting acrylic esters as reactive solvents potential candidates for oxidative drying alkyds are polyol acrylates and respectively alkoxylated polyol acrylic esters. The later group of derivatives is recorded as polymers and further the presence of the ether groups on the polyols arms from ethylene oxide reduce the oxygen inhibition which is a competing process in the oxidative drying. In the group of liquid unsaturated derivatives that may be used as reactive solvent for oxidative drying is the family of Diels-Alder adducts from polyol acrylic monomers and cyclopentadiene (Figure 8-92) [297]. An interesting reactive solvent quoted in the literature is thiol functional and has as target to reduce the viscosity of the alkyd [61], (Figure 8-93), and in the same time to combine the oxidative drying with a thiol addition to the double bond of the fatty acid or other unsaturation in the backbone alkyd composition. The chemistry of the combined curing process has been explained in Chapter 8.6 and earlier.

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Figure 8-92: Diels Alder modification of acrylic monomers

Figure 8-93: Mercaptoesters

Hydrolytically stable thiol functional monomers may be produced as in Figure 8-94 from polyol allyl ethers. They present the advantage of higher Tg when the core is 4,4’-(propane-2,2-diyl)bis(2-allyl-1-(allyloxy)benzene) [323] and as well high functionality (Figure 8-94, page 202). Alkyds targeting two component systems may be easily formulated to low molecular weight and higher functionality will compensate for the low physical drying. However it should be mentioned that alkoxylated polyols may be used as reactive diluents when the prepared alkyd is the component in reaction with isocyanates or amino resins.

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Figure 8-94: Hydrolytically stable thiols

8.7.2

Polyesters

Polyesters must be considered from the point of view of reactivity. In the case of unsaturated polyesters, the application area related to coatings is related to gel-coat and high gloss coatings for wood. Unsaturated polyesters generally comprise a reactive solvent which is the vinyl monomer. Styrene is widely used as reactive solvent and unsaturated polyesters may be regarded as having 100 % reactive matter contributing to the film building. The unsaturation of the polyester is provided by moieties loaded as maleic anhydride or fumaric acid. Other monomers may be used, such as (Figure 8-95) itaconic and mesaconic anhydride (however of lower reactivity).

Figure 8-95: Unsaturated anhydrides

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However unsaturation contributing to oxygen depletion and higher reactivity is introduced as allyl ethers from polyols such as trimethylolpropane and pentaerythritol, where at least a hydroxyl group is left unreacted. Another option is the use of esters from dicyclopentadiene and a diacid which is further grafted to the polyester backbone. The vinyl monomer is actually bridging the polyester chains connecting the unsaturated moieties in the backbone between them (Figure 8-96):

Figure 8-96: Cross-linked unsaturated polyester

An important aspect in the reactivity of unsaturated polyesters is the ratio of unsaturation in the binder composition as well as the ratio maleic/fumaric. The fumaric groups are by far more reactive compared to maleic moieties. The manufacturing process is designed in such a way that it allows the isomerization of maleic anhydride to fumaric moieties. This reaction is spontaneous and is catalyzed, but as well the temperature range and the choice of diol play an important roll. The best isomerization temperature is in the range of 190 to 195 ºC and diols must be branched or vicinal to make the process easier. As well it has been observed that phthalic anhydride yields more fumaric moieties than isophthalic acid (Figure 8-97, page 204). Other reactive monomers that can be used are combinations of styrene and acrylic monomers. Acrylic monomers such as triethyleneglycol-diacrylate or dimethacrylate may be used. There are reactivity differences between monomers as reactive

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Figure 8-97: Initiating species for unsaturated polyesters polymerization

solvents. It is possible to use monomers that will preferentially homopolymerize combined with another vinyl monomer that will copolymerize with the polyester backbone. This will end in two different networks, yielding so call interpenetrating networks. The approach may also be used in so called dual cure, where a radical polymerization is followed by a cationic curing process (Chapter 8.4). The radical polymerization of unsaturated polyester systems are initiated by red-ox reactions involving peroxide and a transitional metal ion. Reactions will occur as in the case of alkyd oxidative drying process. In putties and other coatings, a red-ox initiation is performed by using as a reducing agent an amine (Figure 8-98). Saturated polyesters are as explained intended for either catalyzed systems or for 2-component systems. In such cases the design and formulation will dictate the relation viscosity/non-volatile content. However the functionality of the polyester may reduce the non-volatile content due to the increasing number of hydrogen bonds for lower molecular weight. In such cases a reactive solvent may be needed. It may generate acid or hydroxyl groups when the catalyst is added or it may simply act like a solvent increasing the space between the polyester molecules. Such solvent may be polyhydroxyl functional alkoxylates from polyols or glycidylated polyols, the glycidyl group being a precursor for two hydroxyl groups. Other reactive solvents used as hydroxyl group donors are very effective in the case of polyurethane systems. As precursors for hydroxyl groups of interest are spiroorthoesters and oxazolidines. Ortho-spiroesters open the ring in acid conditions to generate hydroxyl groups (Figure 8-98):

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Figure 8-98: Hydroxyl group release from orthospiroesters

As well the chemistry of oxazolidines implies water scavenging to yield reactive NH- and OH-groups (Figure 8-99) [254 –257].

Figure 8-99: Hydroxyl group release in oxazolidines

A very interesting approach to tackle the relation viscosity/non-volatile content for two component polyurethane systems is the rage of aspartic esters as reactive solvents. The intra-molecular formed hydrogen bond between the carbonyl group and the amino group keeps the viscosity low and the pot-life in place (Figure 8-100). Humidity will disturb this intra-molecular hydrogen bond and release free the hydrogen atom, capable to react with isocyanate groups [55, 67]. Polyaspartic esters are different from other amine functional polymers in reaction to isocyanate group, the internal hydrogen bond offering longer pot-life in absence of humidity [75].

Figure 8-100: Polyaspartic esters

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Other possible approaches may make available both hydroxyl groups and amino groups (Figure 8-101).

Figure 8-101: Mix functionality for isocyanate curing

In other cases the amino functional group may be contained as a ketimine which will release the active hydrogen atoms in contact with humidity, giving off the blocking ketone (Figure 8-102):

Figure 8-102: Ketimine functionality

The literature also quotes the use of fatty acid methyl esters as reactive diluents in coil coatings. Although the coatings exhibited good curing and properties, no evidence of the incorporation of the rape oil methyl ester has been brought [272]. 8.7.3

Polyethers

Polyethers are themselves used as reactive solvents. It is important that they comprise different building blocks from different alkylene oxides in combination. As explained, polyalcoxylates from propylene oxide or from butylenes oxides yield lower viscosity when compared to pure polymerized ethylene oxide.

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8.7.4 Epoxy resins Epoxy resins as explained in earlier chapters (Chapter 5.5) are well defined and of low molecular weight when intended for solventless coatings (Chapter 8). However a whole range of glycidylated products is available to reduce the viscosity of the epoxy component.

Figure 8-103: Glycidyl functional solvents

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These reactive diluents are generally glycidylated phenols, polyols having functionality ranging from 1 to 6 and meant to act as reactive solvent but as well at network controllers when the glycidyl functionality is higher than two. The glycidylation substrate may be as described a polyol, but as well a natural product such as castor oil. Along with glycidyl ethers, there are other glycidyl esters such as derived from neo- or branched acids or dimer fatty acids (Figure 8-103). Oxetanes behave as epoxy resins, but they only cure in acid conditions. Hyper branched polyesters based on vinyl ether and oxetane modification give slow reaction [80]. Vernonia oil as a solvent has made strong inroads in the market. It contains mainly vernolic acid (Figure 8-104) and derivatives based on it are quoted as good reactive solvents [81– 84].

Figure 8-104: Vernolic acid

Figure 8-105: Acrylic esters for Michel addition in epoxy resins

Another group of reactive solvent for epoxy binders is selected from acrylated derivatives. Generally, this group of solvents may be acrylated polyols (such as propylene glycol di acrylate or dimethacrylate, Figure 8-105, trimethylolpropane triacrylate, Figure 6-14) or acrylated polymeric substrates of lower viscosity such as alkoxylated polyols (figure 6-15), acrylated polyesters or even polyurethanes [306–308].

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The cross-linking mechanism in this case is Michael addition of the amine functional cross-linker (pseudo Michael addition) competing with the cross-linking of the epoxy group. Along with the benefit of lower viscosities the combined curing mechanisms allows hardening at lower temperatures (Figure 8-106).

Figure 8-106: (Pseudo) Michael addition

The calculation of the curing agent (amine) is done as in the case of using only epoxy binder at the level of stoichiometric quantity according with Equation 8-2. The equivalent weight of amine (EWNH) is calculated according to Equation 8-1 where Mwam is the molecular weight of the amine and NH is the number of active hydrogen atoms:   Equation 8-1

 

Equation 8-2

High solid epoxy binders may be produced from amphiphilic di- or tri- block copolymer from propylene oxide or butylene-oxide further functionalized with epoxy groups or from polymer containing alicyclic polyepoxide and polyalkoxysilane curable with amino functional derivatives [78, 79]. In the case of epoxy resins the cross-linker plays an important roll as a solvent as well. Amines are used and they have very good cutting power, but as well high reactivity. New cross-linkers have been developed to both overcome the reactivity, indulging a longer pot-life but as well better performance by the fact that they are of polymer type. Polyaminoamides are generally of high viscosity due to the presence of hydrogen bonds, but the use of certain amines adducts from an epoxy resin, a liquid polyamidoamine from a long chain carboxylic acid, a liquid polyamine, and a monoglycidyl ester yields a low viscosity curing agent [85]. Other high solid amine adducts are obtained by reaction of diethylene glycol diglycidyl ether with m-xylene diamine in presence of vinyl acetate. Reaction with vinyl acetate is also reported [86, 261, 264]. 

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The use of amine blocked acids may also yield high solid epoxy systems. As an example the use of trifluoromethane sulfonic (Figure 8-107) acid blocked with an amine are found to allow higher solid content and to catalyze both epoxy ring opening and condensation reactions [263]. As well stibate amino salts are strong acids capable to catalyze the ring opening polymerization (Figure 8-108).

Figure 8-107: Superacids

The acid catalyst may be released due to the volatility of the amine or by first the amine undergoing a Sommerlet-Hauser (Figure 8-108).

Figure 8-108: Sommerlet Hauser transposition

8.7.5

Acrylic resins

Considering acrylic resins as functional resins intended for curing with amino resins or isocyanate resins, the approach is similar with the case discussed earlier regarding polyesters. The reactive solvent will bring as in the polyesters case the necessary hydroxyl groups to compensate for the loss in Tg by building hardness through cross-linking. For the thermoplastic acrylic resins there is no case of reactive solvent and the relation viscosity/non-volatile content is controlled by the adjustment of molecular weight. A special case may be however considered when the concept of acrylic resins refers to UV curing resins. In such case the acrylic is denomination of the functionality of a certain backbone optionally having a polyether, polyester or polyurethane structure. This backbone may exhibit a high molecular weight, especially in the case of polyurethanes where the hydrogen bonding is difficult to hinder, the acrylic moieties generally end-capping the polymer backbone. These systems then require reactive solvent selected from the low molecular weight acrylic esters having at

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least a functionality of two. Such monomers, generally addressed as stenomers typically are: dipropyleneglycol diacrylate, triethyleneglycol diacrylate, trimethylolpropane triacrylate, pentaerythritol tetraacrylate, etc. 8.7.6

Polyurethanes

In polyurethanes the case of reactive solvent is less interesting. The meaning of polyurethanes is the repeatable urethane bond. This is only formed during reaction of an isocyanate group. Urethane prepolymers are used in fewer cases and then the understanding is that a polyurethane backbone has terminal isocyanate groups. The prepolymer is in this case a building stone for larger molecular weight polymers. For this case the use of diisocyanate or isocyanates dimers or trimers as discuss earlier makes sense in terms of viscosity cut with certain limitations. However, the use of hydroxyl group precursors such as orthospiroesters is a good choice for a reactive solvent (Figure 8-98) and water depletion approach. 8.7.7

Amino resins

Amino resins may be produced at high non-volatile content and therefore they do not need reactive solvents. As previously discussed the viscosity of the amino resin is controlled by the ratio formaldehyde/amino, the etherification degree and the nature of the etherifying alcohol. However the use of carbamate intermediates from epoxydated oils or castor oil, as well as other core than melamine such as acrylic is possible and may be achieved as in Figure 8-109 (page 212) lead to lower viscosity, plasticizing effect and higher reactivity [272–274].

8.8

Two steps cross-linking and new chemistries

A combination of two chemistries that may be involved in the curing reaction at different moments has a positive impact on the relation viscosity/non-volatile content as well as the performance of the coating. The dual cure system may solve problems discussed in earlier chapters as drawback for high solid coatings. Historically the first approaches on this path have been presented as a combination of UV curing systems and isocyanate curing. Some proposed binders also contained blocked isocyanate, in which case the primary UV curing was just helping to set the coatings immediately after application, avoiding coating run due to low viscosity and making possible handling of the coated objects prior to the final cure [287–291]. Other applications make use of silane modified binders. These systems allow a primary curing by siloxane bridges followed by a slower cure of typical chemistry such as oxidative drying or isocyanate curing (Figure 8-110, page 213). In most cases double cure systems have a UV cross-linkable moiety (ethylene type unsaturation) and any other functionality. An example given in Figure 4-13 [305] describes a dual UV cure, based on the fact that the generated cation is continuing

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Figure 8-109: Functionalization of fatty acid with etherified methylol groups

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Figure 8-110: Primary reactivity through silane modification

the process even after the irradiation has been interrupted. A similar monomer is glycidyl methacrylate (Figure 8-35). Another example is the use of acetoacetate chemistry combined with another type of cross-linking. Using acid curing formulation, the binder is functionalized with acetoacetate moieties. The amino functional cross-linker is releasing formaldehyde during the network formation process. The released formaldehyde is scavenged by the acetoacetate moieties contributing to a denser cross-linked network (Figure 8-111).

Figure 8-111: Formaldehyde scavenging by acetoacetate groups

Further, a combination of two cross-linking chemistries is relating the oxidative slow drying to a quick reaction which is controlled either by a deblocking or by solvent release. An approach has been explained in alkyd cross-linking when the thiol-ene reaction combines with the oxidative drying (Chapter 8.5.1), but another possibility is combining the oxidative drying with azomethine chemistry (Figure 8-112) [293].

Figure 8-112: Azomethine cross-linking

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Also preferred in water-borne systems, this chemistry may be extrapolated to solvent-borne and high solids, especially in the case of polyurethane backbones, in which case the formulator is using diols having groups from the unsaturated fatty acids for oxidative drying [294]. Recently attention has been paid to bismaleimide systems combining the Alder-ene chemistry with a chemistry allowing lower viscosity by the use of a reactive solvent or by containing the functional hydroxyl group. This is the case when along with the Alder-ene monomers there is present an epoxy functional monomer such as bisphenol A diglycidyl ether (Figure 8-113) [298] or when the ene component has primarily undergone a modification to a benoxazine, allowing the bismaleimide to react at a lower temperature. The benoxazine is also ring opening allowing a dual cure of the system (Figure 8-114) [299].

Figure 8-113: Alder-ene reaction in the presence of glycidyl groups

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Figure 8-114: Benoxazine based bismaleimide system

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9

Examples of high solid formulations

Examples of high solid formulations

In this chapter are given some examples to illustrate the High solids approaches as described earlier in Chapter 8.

9.1

Polymer architecture and free volume

Reactive diluents The polymer architecture strategy as explained in Chapter 8.1 allows the binder chemist to choose positioning a core in a polymer that has a higher glass transition temperature. However shielded by moieties having a lower glass transition temperature, the polymer will exhibit a lower viscosity. Due to the fact that by choice the lower glass transition moieties have a lower density and are hyperbranched or star type, the free volume will increase, thus contributing to a lower viscosity. The example is describing an alkyd that has the architecture as presented in the Figure 8-4 where the polyester spacer is shielded by flexible arms in a branched moiety having a dipentaerythritol core, bearing unsaturation from tall oil fatty acids. The example given in Table 9-1 is a high solid air drying alkyd. Table 9-1: Example of a high solid air drying alkyd Raw materials

part by weight

TOFA

751.76

Di pentaerythritol

166.26

1,6-hexane diol

38.64

Phthalic anhydride

96.92

Maleic anhydride

6.54

Yield

1000

Final acid number