Additives for Waterborne Coatings 9783748602187
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Wernfried Heilen et al.
Additives for Waterborne Coatings
English text reviewed by John Haim and David Hyatt
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Cover: Evonik Tego Chemie GmbH, Essen, Germany
Bibliographische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar.
Wernfried Heilen et al. Additives for Waterborne Coatings Hannover: Vincentz Network, 2009 (European Coatings Tech Files) ISBN 978-3-7486-0218-7 © 2009 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 Tel. +49 511 9910-033, Fax +49 511 9910-029 E-mail: [email protected], www.european-coatings.com Layout: Maxbauer & Maxbauer, Hannover, Germany ISBN 978-3-7486-0218-7
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European Coatings Tech Files
Wernfried Heilen et al.
Additives for Waterborne Coatings
English text reviewed by John Haim and David Hyatt
Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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4
Foreword
Foreword The impulse to produce this book together with my colleagues in the raw materials industry stems from the fact that I have been involved with waterborne coatings technology for nearly 30 years, both as a formulator and as a user of additives. I have often missed having a standard volume at hand which explains the background to additive technology and at the same time would give me the choice of the right additives to use. I confess, as I developed my first waterborne paint for metal window frames, that this endeavour was not a crowning success. On the one hand, this had to do with the relative small array of binders and additives, but also on the lack of literature that would have made possible a deeper understanding of the subject. The present state of development of waterborne coatings technology allows the conversion of conventional solvent-based products to waterborne products in everincreasing areas. Waterborne coatings today represent approx. 8 %, of the Heavy Duty segment, 37 % of Can Coatings and Plastic Coatings, 23 % of Wood Coatings, 36 % of general Industrial Coatings, 68 % of Automotive Coatings (OEM) and 90 % of the Architectural Coatings sector. In the automotive industry the change partially took place in the early seventies with the introduction of electro-deposition coatings. Fillers and basecoats followed. Waterborne clearcoats have been tried in mass production but have not yet been fully accepted. All things considered, waterborne coatings for this sector as well as architectural coatings are state of the art. In metal finishing they are used predominantly as heat-curing systems for coating household appliances, agricultural machinery, etc. Waterborne coatings are being increasingly used in industrial wood coating applications (window frames, parquet flooring etc.).
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Foreword
5
As in my first book “Silicone resins and their combinations,” which was published in 2005, by Vincentz Verlag, I believe that this work also treads a path between practice-oriented clarity and scientific depth. For those who are approaching the subject for the first time, this book offers an overview of the most important aspects and applications of additives for waterborne systems in diverse market segments. Many thanks to the colleagues who contributed their efforts in bringing this book to fruition, as well as to Evonik Tego Chemie GmbH and Nuplex Resins for making available literature and illustrative material. Wernfried Heilen Essen/Germany, May 2009
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Master Class For Additives And Resins
The Tego brand is our promise to coatings and printing inks manufacturers all over the world: whatever ideas and solutions you require for your products, we are there for you. In our Technical Competence Centers, we offer regional support and specialist advice. Our global logistics network and numerous production facilities guarantee prompt deliveries and the renowned Tego quality. In our laboratories, we carry out research and develop solutions to your formulation problems. Our formula for the future: Chemistry with Imagination. Formulate your success. More information at www.tego.de
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38 Uhr
Contents
7
Contents 1
Introduction.......................................................................
18
2 2.1 2.1.1 2.1.2 2.1.3 2.1.3.1 2.1.3.2 2.1.3.3 2.1.4 2.1.4.1 2.1.4.2 2.1.4.3 2.1.4.4 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.3
Wetting- and dispersing additives.................................... Modes of action................................................................... Pigment wetting................................................................... Grinding............................................................................... Stabilisation......................................................................... Electrostatic stabilisation..................................................... Steric stabilisation............................................................... Electrosteric stabilisation..................................................... Influences on formulation.................................................... Viscosity.............................................................................. Colour strength.................................................................... Compatibility....................................................................... Stability................................................................................ Chemical structures............................................................. Polyacrylate salts................................................................. Fatty acid and fatty alcohol derivatives............................... Acrylic-copolymers............................................................. Maleic anhydride copolymers............................................. Alkyl phenol ethoxylates..................................................... Alkyl phenol ethoxylate replacements................................ Wetting and dispersing additives in different market segments.............................................................................. Architectural coatings.......................................................... Direct-grind......................................................................... Pigment concentrates........................................................... Wood and furniture coatings................................................
20 20 20 22 22 23 23 24 25 25 25 27 27 27 27 28 28 29 29 30
2.3.1 2.3.1.1 2.3.1.2 2.3.2
30 30 30 31 31
Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Contents
9
2.3.2.1
Direct grind .........................................................................
31
2.3.3
Industrial coatings ..............................................................
32
2.3.2.2 2.3.3.1
2.3.3.2 2.3.4
2.3.4.1
2.3.4.2 2.4 2.5
2.5.1
Pigment concentrates ..........................................................
Direct grind .........................................................................
Pigment concentrates ..........................................................
Printing inks........................................................................ Direct grind .........................................................................
Pigment concentrates .......................................................... Tips and Tricks ...................................................................
Test methods .......................................................................
3
Zeta potential ......................................................................
Summary............................................................................. Literature ............................................................................
3.2.2
3.2.2.1
3.2.2.2
3.2.2.3 3.2.2.4 3.2.2.5
3.2.2.6
3.2.2.7 3.2.2.8
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34 35
35 38 38
39
Causes of foam ...................................................................
3.2.1
33
Foam ...................................................................................
3.1.1.1 3.2
32
39
Defoaming mechanisms .....................................................
3.1.1.2
32
Defoaming of coating systems..........................................
3.1
3.1.1
32
34
Viscosity .............................................................................
2.7
32
Rub-out ...............................................................................
2.5.4 2.6
32
33
Colour strength ...................................................................
2.5.5
32
Particle size .........................................................................
2.5.2
2.5.3
31
Types of foam ..................................................................... Defoamers........................................................................... Composition of defoamers ................................................. Defoaming mechanisms .....................................................
Defoaming by drainage/slow defoaming............................
Entry barrier/entry coefficient ............................................ Bridging mechanism ........................................................... Spreading mechanism .........................................................
Bridging stretching mechanism ..........................................
Bridging dewetting mechanism ..........................................
Spreading fluid mechanism ................................................ Spreading wave mechanism ...............................................
39 40 40 42 42 42
42
43 44 45
46
46 47 47
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10
Contents
3.2.2.9
Effect of fillers on the performance of defoamers...............
48
3.3.1
Active ingredients in defoamers..........................................
49
Mineral oils..........................................................................
49
3.3
Chemistry and formulation of defoamers............................
49
3.3.1.1
Silicone oils (polysiloxanes)................................................
3.3.1.3
Vegetable oils.......................................................................
49
Molecular defoamers (gemini surfactants)..........................
50
3.3.1.2
3.3.1.4
Polar oils..............................................................................
3.3.1.6
Hydrophobic particles.........................................................
3.3.1.5 3.3.1.7
Emulsifiers...........................................................................
3.3.2
Defoamer formulations........................................................
3.3.1.8 3.3.3
3.4
3.4.1
Solvents............................................................................... Suppliers of defoamers........................................................
Product recommendations for different binders..................
Acrylic emulsions................................................................
49
50 50 51 51 51
51
52
52
3.4.2
Styrene acrylic emulsions....................................................
3.4.4
Polyurethane dispersions.....................................................
53
Influence of the pigment volume concentration (PVC).......
53
Application of shear forces during application...................
54
3.4.3
Vinyl acetate based emulsions.............................................
3.5
Product choice according to field of application.................
3.5.2
Method of incorporating the defoamer................................
3.5.4
Surfactant content of the formulation..................................
3.5.1
3.5.3 3.6 3.7 3.8 4
Tips and tricks..................................................................... Summary.............................................................................. Literature.............................................................................
53 53
53
54 54 54 55 55
Rheology modifiers............................................................
56
Market overview..................................................................
56
4.1
General assessment of rheology modifiers..........................
4.1.2
Basic characteristics of the different rheological additives...............................................................................
57
Rheology..............................................................................
58
4.1.1
4.2
4.2.1
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Requirements for rheology modifiers..................................
56
58
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Contents
4.2.2
4.3
Example of application........................................................
62
Associative properties of HEUR additives..........................
63
Synthesis of HEUR..............................................................
4.3.3
From self association to associative behaviour...................
4.3.4 4.3.5
Hydrophobic/hydrophilic equilibrium of waterborne coatings................................................................................
69
ASE......................................................................................
4.4.2
66
Synthesis..............................................................................
4.4.1.1 4.4.1.3
65
67
Alkali swellable emulsions: ASE and HASE......................
4.4.1.2
62
Improved colour acceptance with HEUR............................
4.4
4.4.1
59
Ethoxylated and hydrophobically modified urethanes........
4.3.1 4.3.2
11
HASE................................................................................... Interaction with binders....................................................... Thixotropy and HASE.........................................................
69 70 71 73 76
4.5
Outlook................................................................................ Literature.............................................................................
78
5
Substrate wetting additives...............................................
79
Water as a solvent................................................................
79
4.6 5.1
5.1.1
5.1.2
5.1.3
5.1.4
Mechanism of action........................................................... Surface tension....................................................................
Reason of the surface tension..............................................
Mode of action of substrate wetting additives.....................
83
5.1.7
Further general properties of substrate wetting additives/side effects............................................................
5.2.1
5.2.2
5.2.2.1
5.2.2.2 5.2.2.3
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79
80
81
Substrate wetting additives are surfactants..........................
5.2
79
Effect of the high surface tension of water..........................
5.1.5 5.1.6
77
Chemical structure of substrate wetting additives............... Basic properties of substrate wetting additives...................
Chemical structure of substrate wetting additives important in coatings...........................................................
82
83 84
84
84
Polyethersiloxanes...............................................................
84
Fluoro surfactants................................................................
85
Gemini surfactants...............................................................
85
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12 5.2.2.4 5.2.2.5 5.2.2.6 5.2.2.7 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 5.3.6 5.3.6.1 5.3.6.2 5.3.6.3 5.3.6.4 5.3.6.5 5.4 5.5 5.5.1 5.5.2 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.4.1 5.6.4.2 5.6.4.3 5.6.4.4 5.6.4.5 5.6.5 5.7
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Contents
Acetylenediols and modifications........................................ Sulfosuccinate...................................................................... Alkoxylated fatty alcohols................................................... Alkylphenol ethoxylates (APEO)........................................ Application of substrate wetting additives.......................... Basic properties of various chemical classes....................... Reduction of static surface tension...................................... Possible foam stabilisation.................................................. Effective reduction in static surface tension versus flow.... Reduction of dynamic surface tension................................ Which property correlates with which practical application?.......................................................................... Craters.................................................................................. Wetting and atomisation of spray coatings.......................... Rewettability, reprintability, recoatability........................... Flow..................................................................................... Spray mist uptake................................................................ Use of substrate wetting additives in different market sectors.................................................................................. Tips und tricks..................................................................... Successful use of substrate wetting additives in coatings... Metallic shades.................................................................... Test methods for measuring surface tension........................ Static surface tension........................................................... Dynamic surface tension..................................................... Dynamic versus static.......................................................... Further practical test methods............................................. Wedge spray application...................................................... One spray path..................................................................... Crater test............................................................................. Draw down.......................................................................... Spray drop uptake................................................................ Analytical test methods....................................................... Literature.............................................................................
86 86 87 87 88 88 88 89 89 89 90 90 91 91 92 92 92 93 93 94 95 95 95 95 96 96 97 97 98 98 99 99
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Contents
6
6.1
6.1.1
6.1.1.1
6.1.1.2
Improving performance with co-binders........................ Preparation of co-binders.................................................... Secondary dispersions.........................................................
Polyester dispersions...........................................................
13 100 101
101
103
Polyurethane dispersions.....................................................
104
Co-binders for better property profiles................................
106
6.2
Applications of co-binders..................................................
6.2.1.1
Drying time..........................................................................
106
Hardness-flexibility balance................................................
110
6.2.1
6.2.1.2 6.2.1.3
6.2.1.4
Adhesion.............................................................................. Gloss....................................................................................
106
108 111
6.2.2
Co-binders for pigment pastes.............................................
112
6.4
Literature.............................................................................
114
6.3 7
Summary.............................................................................. Deaerators..........................................................................
114 115
7.1
Mode of action of deaerators...............................................
116
7.1.2
Rise of microfoam bubbles in the coating film...................
118
7.1.1
7.1.3
7.1.4
7.1.4.1 7.1.4.2
Dissolution of microfoam.................................................... How to prevent microfoam in coating films........................ How deaerators combat microfoam.....................................
Deaerators promote the dissolution or formation of small microfoam bubbles.................................................... How deaerators promote the dissolution of microfoam bubbles.................................................................................
7.2
Chemical composition of deaerators...................................
7.4
Main applications according to market segments...............
7.3
7.5 7.6
7.6.1
7.6.2
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Main applications according to binder systems................... Tips and tricks.....................................................................
Evaluating the effectiveness of deaerators..........................
Test method for low to medium viscosity coating formulations.........................................................................
Test method for medium to high viscosity coating formulations.........................................................................
116
119
120
120 121 122 124 124 125
125
126 126
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14 7.6.3
Contents
Further test methods for microfoam....................................
126
Literature.............................................................................
128
7.7
Conclusion...........................................................................
8
Flow additives....................................................................
8.1.1
Mode of action in waterborne systems without co-solvents.
7.7 8.1
127 129
Mode of action.....................................................................
129
8.1.2
Sagging................................................................................
130
8.1.4
Mode of action in waterborne systems with co-solvents.....
8.1.3
Total film flow.....................................................................
8.1.5
Mode of action in an example of a thermosetting waterborne system with co-solvents....................................
8.1.7
Summary..............................................................................
8.1.6 8.2
8.2.1
8.2.2
Surface tension gradients..................................................... Chemistry of active ingredients........................................... Polyether siloxanes..............................................................
8.4.1.2
Main applications by market segment................................. Industrial metal coating.......................................................
Electrophoretic coating........................................................
140 140
Test methods........................................................................
143
8.6.1
Measurement of flow...........................................................
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140
142
Conclusion...........................................................................
8.7
139
High gloss emulsion paints..................................................
8.5
8.6.3
138
142
Flat and semi-gloss emulsion paints....................................
8.6.2
135
Architectural coatings..........................................................
8.4.3.1
8.6
135
141
Industrial coatings...............................................................
8.4.3.2
135
Waterborne coatings............................................................
8.4.2 8.4.3
134
138
Film formation.....................................................................
8.4.1.1
132
Slip.......................................................................................
8.3
8.4.1
132
137
Side effects of polyether siloxanes......................................
8.4
131
Polyacrylates........................................................................
8.2.3 8.2.4
129
Measuring flow and sagging by DMA................................. Measuring the surface slip properties.................................. Literature.............................................................................
141
142
142 143 144 144
145
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Contents
15
9
Wax additives.....................................................................
9.1.1
Natural waxes......................................................................
9.1.1.2
Waxes from fossilised sources.............................................
9.1.2.1
Semi-synthetic waxes..........................................................
9.2
From wax to wax additives..................................................
151
Wax emulsions.....................................................................
151
9.1
9.1.1.1
9.1.2
9.1.2.2
Raw material wax................................................................
146
Waxes from renewable raw materials..................................
147
Semi-synthetic and synthetic waxes....................................
148
Synthetic waxes...................................................................
149
9.2.1
Wax and water.....................................................................
9.2.1.2
Wax dispersions...................................................................
9.2.1.1 9.2.3 9.3
Micronized wax additives....................................................
Gloss reduction.................................................................... Texture and structure........................................................... Rheology control.................................................................
9.4
Summary..............................................................................
10
Light stabilizers for waterborne coatings........................
10.2
Light and photo-oxidative degradation...............................
10.3.1
UV absorbers.......................................................................
10.3.2.1
Antioxidants........................................................................
10.1
10.3
10.3.2
151 152 152
155
Surface protection................................................................
9.3.2.4
148
Coating properties...............................................................
9.3.2.1
9.3.2.3
147
153
Acting mechanism...............................................................
9.3.2.2
146
Wax additives for the coating industry................................
9.3.1 9.3.2
146
153 155 159 160
160 161 162
Introduction.........................................................................
162
Stabilization options for polymers.......................................
165
Radical scavengers..............................................................
170
162
166 170
10.3.2.2
Sterically hindered amines..................................................
10.4.1
Market overview..................................................................
174
Application specific product selection.................................
178
10.4
10.4.2
10.4.2.1
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Light stabilizers for waterborne coatings............................ Application fields and market segments..............................
171
173
174
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16
Contents
10.5
Conclusions.........................................................................
10.6.1
UV absorbers.......................................................................
10.6.3
Weathering methods and evaluation criteria.......................
10.6
10.6.2 10.6.3.1 10.6.3.2 10.7 11
179
Test methods and analytical determination.........................
180
HALS...................................................................................
180
Accelerated exposure tests.................................................. Further evaluation criteria................................................... Literature.............................................................................
180 180 180 180 181
In-can and dry film preservation.....................................
183
In-can preservation..............................................................
185
11.2.2
Selection of active ingredients for the preservation system
188
11.3
Dry film preservation...........................................................
11.1
11.2
11.2.1
11.2.3
11.3.1
Sustainable and effective in-can and dry film preservation. Types of active ingredients..................................................
Plant hygiene.......................................................................
183
185 188
188
Conventional dry film preservatives....................................
189
11.3.3
Improvements in the ecotoxicological properties................
193
11.5
Prospect...............................................................................
12
Hydrophobing agents........................................................
12.1.1
Capillary water-absorption..................................................
12.1.3
How hydrophobing agents work.........................................
11.3.2
11.4
11.6 12.1
12.1.2
New, “old” actives...............................................................
External determining factors...............................................
Literature.............................................................................
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197
198
Silicone resins/silicone resin emulsions..............................
12.2.5
196
Hydrophobicity....................................................................
12.2.2
12.2.4
195
197
Chemical structures.............................................................
12.2.3
195
Mode of action.....................................................................
12.2
12.2.1
192
Linear polysiloxanes and organofunctional polysiloxanes...
197 199 201
202
203
Other hydrophobing agents.................................................
204
Production of silicone resin emulsions................................
205
Production of linear polysiloxanes......................................
204
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Contents
12.2.5.1 12.2.5.2 12.3 12.3.1 12.3.2 12.3.3 12.3.4 12.3.5 12.4 12.5 12.5.1 12.5.2 12.5.3
12.5.4 12.5.5 12.6
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Secondary emulsification process........................................ Primary emulsification process............................................ Waterborne architectural paints........................................... Synthetic emulsion paints.................................................... Silicate emulsion paints....................................................... Emulsion paints with silicate character (SIL-paints)........... Siloxane architectural paints with strong water-beading effect.................................................................................... Silicone resin emulsion paints............................................. Conclusions......................................................................... Appendix............................................................................. Facade protection theory according to Künzel.................... Measurement of capillary water-absorption (w-value)....... Water vapour diffusion (sd-value)........................................
17 205 205 206 206 206 207 207 207 209 210 210 210
Simulated dirt pick-up......................................................... Pigment-volume concentration (PVC):............................... Literature.............................................................................
211 212 213 214
Authors...............................................................................
215
Index...................................................................................
218
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18
Introduction
1
Introduction Wernfried Heilen
Waterborne coatings materials have very different properties from those of conventional solvent-based systems. The reason for this lies in the physical properties of water. The heat of evaporation of water is very high compared to that of many other solvents [1]. Consequently air-drying systems dry more slowly at lower temperatures and/or higher relative humidity. Significantly more energy must be utilized in heat-curing systems. Numerous solvents with different heats of evaporation and boiling points can be used to optimize drying and film-forming of solvent-based systems. In contrast, formulators of waterborne coatings have only a limited choice of solvents which can be used as water-soluble co-solvents. As a strongly polar solvent, water has a comparatively high surface tension. Because of this and the make-up of the binder, which consists of incompletely dissociated polyelectrolytes or colloidal systems or emulsions based on various polymers, characteristic problems can occur during manufacture and application. This necessitates the development of specialist additives. Essential for the manufacture of waterborne – as well as for solvent-based coatings systems are: •
wetting and dispersing agents
Especially important in waterborne formulations are •
defoamers
as well as •
rheology-modifying additives
In contrast to solvent-based coatings materials, in which mostly polymeric wetting and dispersing agents are currently used, waterborne systems continue to be formulated with polyphosphates and polyacrylic acid salts as well as alkyl phenol ethoxylates (although their eco-toxicity is controversial). Fatty acids, fatty alcohol derivatives and acrylate-copolymers are being increasingly used to stabilise organic pigments. Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Introduction
19
Foam-forming substances include emulsifiers used in the manufacture of waterbased binders but also the wetting and dispersing agents mentioned above. Nonassociative thickeners, such as those derived from cellulose, which have many hydrophilic segments in the molecules can also cause foam formation. Modern defoamers comprise a complex mixture of active substances, including mineral oils, polyether siloxanes, waxes, precipitated silicas, etc. Synthetic as well as inorganic thickeners are used to control viscosity in all shear conditions, as well as properties such as flow, sagging, settling and storage stability. The polyurethane thickeners described in this book belong to the class of associative thickeners. The thickening function of these products is dependent on the system and is strongly influenced by certain constituents in the formulation. The high surface tension of water can cause surface defects and inadequate adhesion on poorly-cleaned surfaces. Therefore, depending on the surface it is important to use •
substrate wetting agents
and •
adhesion promoters
as additives or as co-binders in waterborne coatings systems. Deaerators are also indispensible in many formulations and particularly useful during airless application. Flow additives based on polyether siloxanes or polyacrylates, which are also utilised in solvent-based coating systems, are only used in waterborne systems such as stoving enamels which contain large amounts of co-solvents. Such additives are essential in many cases where surface tension gradients occur. Polyether siloxanes and waxes are also used because of positive characteristics such as reduction of friction. Film-formers have already been discussed extensively in the literature and will therefore not be covered in any detail in this book, although their importance in water-based emulsions is undisputed. To protect the applied waterborne coating from degradation the use of light absorbers, as well as of film preservatives, is absolutely essential. This book considers the various topics in the order presented above and concludes with a chapter on hydrophobing agents, which are used primarily in facade coatings. Literature [1]
Kittel, “Lehrbuch der Lacke und Beschichtungen”, Volume 3, Hirzel Verlag, Stuttgart– Leipzig 2001
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20
Wetting- and dispersing additives
2
Wetting- and dispersing additives Frank Kleinsteinberg
Dispersion of pigments is indisputably one of the most demanding steps in the manufacture of coatings but preventing reflocculation of the dispersed particles is even more difficult. A further problem occurs during formulation of waterborne coatings as the high surface tension of water makes wetting of low energy surfaces difficult. Although wetting agents with anionic character are sufficient for easily formulated emulsion paints containing inorganic pigments, organic pigments are needed to satisfy the demand for brilliant colours, high colour strength, and transparency. Because organic pigments have a very low surface energy, additives with greatly improved wetting properties are required. Finely divided organic pigments also have a high surface area. It is not possible to stabilise such pigments by simple means. Modern wetting and dispersing additives are copolymers and have outstanding performance. The combination of pigment wetting and pigment stabilisation makes these products unique and detailed explanation of every phenomenon observed sometimes difficult. The mode of action of wetting and dispersing additives is explained in the following and their relevance to different phenomena is discussed. Various chemical concepts are elucidated and their significance for specific market segments considered.
2.1
Modes of action
The function of wetting and dispersing additives can be considered under three headings: • • •
pigment wetting grinding of the pigment particles stabilisation of the pigment particles
2.1.1
Pigment wetting
The process of wetting a solid by a liquid is summarised by Young’s equation: s
=
sl
+
l
· cos
s
–
sl
/
l
= cos
or
Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany ISBN: 978-3-86630-850-3
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Modes of action
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where γs : surface tension of the solid γsl : interfacial tension solid/liquid γl : surface tension of the liquid θ : contact angle solid/liquid see Figure 1.1. A contact angle of 0 indicates spontaneous wetting or spreading. The cosine of 0 is 1 and in this case the equation becomes: γl = γs – γsl To achieve wetting the surface tension of the liquid must be lower than the surface tension of the solid. A liquid with low surface tension wets a pigment surface better than a liquid with high surface tension. An additive which helps wetting must, as a first step, lower the surface tension. During wetting, the additive adsorbs on the surface of the pigment particles and forms an envelope around them. At this stage the pigment particles are still large. The interactions between these particles are lowered and the viscosity of the grind is reduced. Reduction in grinding viscosity is a first indication of pigment wetting: optimal grinding of agglomerates can only be achieved with very good pigment wetting. In this context, optimal grinding means achieving the largest surface area possible. The larger the surface area the more light can be absorbed and the higher the resulting colour strength.
Figure 1.1: Equilibrium of forces according to Young
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Wetting- and dispersing additives
The particle size also determines transparency and hiding power. While organic pigments show a higher transparency at lower particle size, inorganic pigments have a maximum hiding power at a particle size of λ / 2 [1]. 2.1.2
Grinding
During grinding the pigment agglomerates are broken down mechanically using a variety of equipment. The simplest device is the dissolver. Normal inorganic pigments such as titanium dioxide can be ground with good results using an appropriate blade. The dissolver can only be used for premixing when organic pigments, which are more difficult to disperse, have to be ground. A bead-mill is recommended for achieving the required fine grind. Because wetting and dispersing additives accelerate the wetting of the newly created surface, they improve the grinding process and reduce the dispersing time. During grinding, additive molecules adsorb on the new interfaces. They minimise the interaction between the increasingly smaller pigment particles and maintain a constant viscosity. At the same time the pigment particles are stabilised against flocculation. Without stabilisation the primary pigment particles would re-agglomerate and release the energy which was introduced into the system during the grinding process. The necessary work for grinding and breakdown of agglomerates can be calculated from the following equation: dW = γ · dA where W : work to change the interface γ : surface tension A : surface area This equation shows that the energy required to increase the surface area during dispersion, dW, is proportional to the surface tension γ. The smaller the surface tension the greater is the change of surface area for a given amount of dispersing energy. Wetting and dispersing additives reduce the surface tension. In other words to achieve a certain change of surface area using a wetting and dispersing additive a smaller amount of work is necessary. Wetting and dispersing additives thus perform some of the most important functions during the grinding process. They shorten the grinding time by reducing the contact angle, they reduce the amount of work necessary for dispersion and they prevent re-agglomeration of the pigment particles during the grinding process [2]. 2.1.3
Stabilisation
The basic requirement for stabilising the finely ground pigment particles is the adsorption of the additive molecules on the pigment surface. The additive mol-
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Modes of action
23
ecules must have groups or segments that interact very strongly with the pigment surface. Possible interactions are ionic bonding, dipole-dipole forces and hydrogen bonding. Stabilisation is thought to involve several mechanisms, which will be discussed below. 2. 1. 3. 1
Electrostatic stabilisation
Electrostatic repulsion is a very important mechanism for stabilising pigment particles in waterborne formulations. Figure 2a: Electrostatic stabilisation This makes use of the Coulombic interactions between similarly charged particles. These interactions can be described by DLVO-theory (named after Derjagin, Landau, Verwey and Overbeek). The wetting and dispersing additive, adsorbed on the pigment surface, dissociates into a polymeric segment, which is anionic, and cationic counter ions. These counter ions are not adsorbed and form a mobile diffuse cloud at the outer edge of the polymeric shell. An electrostatic double layer is created. This leads to repulsion and the particles are stabilised against flocculation. Addition of electrolytes, especially multivalent cations, destabilises the electrostatic double layer disrupting the balance between anionic polymer and cationic cloud and removing the stabilisation. The zeta potential ζ describes the electrostatic interaction within the polymeric shell. The smaller the numerical value of ζ, the lower is the electrostatic stabilisation. The zeta potential gives no information about steric stabilisation because steric stabilisation does not involve the creation of ions and so no potential can be measured (Figure 2.2a). 2. 1. 3. 2
Steric stabilisation
In aqueous environments, steric stabilisation is another mechanism which frequently occurs. The adsorbed additive molecules form a polymeric shell around the pigment particle. This polymeric shell consists of the anchoring groups of the additives and a diffuse layer of polymeric chains. To achieve optimal stabilisation the polymeric chains must be very soluble in the surrounding water/binder mixture. They form an outer shell around the pigment particle. As particles come closer, the polymeric shells start to overlap leading to steric hindrance. A simple model would be two wooden balls that carry wire springs. If the balls approach each other the springs prevent contact between the wooden surfaces. In thermo-
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Wetting- and dispersing additives
dynamic terms the degree of freedom of movement of the polymeric chains is reduced when the chains overlap leading to a reduction in entropy. To compensate for this reduction and to reinstate the mobility, the separation of the pigment particles must increase. The change in free energy is given by ΔG = ΔH – TΔS where ΔH : change of enthalpy ΔS : change of entropy T : absolute temperature Important factors influencing the efficiency of stabilisation are the degree of adsorption of the polymers on the surface, the integrity of the polymeric shell and its thickness. The thickness of the polymeric shell and the degree of stabilisation are increased if the additive chains interact with binder molecules (Figure 2.2b) [3]. 2.1. 3. 3
Electrosteric stabilisation
The complex demands made on wetting and dispersing additives in waterborne coatings make it necessary to combine electrostatic repulsion and steric hindrance. This is called electrosteric stabilisation and modern wetting and dispersing additives for waterborne systems work on this principle. Only such additives can fulfil the high demands made on pigment stabilisation and long term storage stability.
Figure 2.2b: Steric stabilisation
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Figure 2.2c: Electrosteric stabilisation
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Modes of action
2.1.4
25
Influences on formulation
The different mechanisms of stabilisation affect various properties which are very important when developing formulations for grinds and pigment concentrates. 2.1. 4. 1
Viscosity
One important effect is the way in which viscosity is reduced when additives are used in different quantities. An additive which predominantly stabilises by electrostatic repulsion is adsorbed on the pigment surface when added to the grind and immediately decreases the interaction between the pigment particles resulting in a strong reduction in viscosity. At higher addition levels the effect does not continue and, in fact, a small increase in viscosity can be observed arising from the higher concentration of polymer and the resulting higher solids content. High-polymer additives which stabilise by steric or electrosteric effects exhibit a different behaviour. At a particular level of addition there is a maximum reduction in viscosity. Amounts below this are not sufficient to stabilise the pigment particles which can interact with each other, leading to a high viscosity. However amounts above the optimum also lead to high viscosity of the pigment concentrate. This increase cannot be explained by the higher amount of polymer in the formulation. Furthermore the additive molecules in the outer polymer shell are not fully orientated and these can also interact with the polymer shells of other pigment particles. This bridging leads to reduced mobility of the pigment particles and, in consequence, to higher viscosity (Figure 2.4, page 26). 2.1. 4. 2
Colour strength
Consideration of colour strength and amount of additive shows a different behaviour. Increased concentration of additive gives higher colour strength but the curve
Figure 2.3: Viscosity behaviour and additive dosage in direct grind
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Wetting- and dispersing additives
Figure 2.4: Viscosity behaviour and additive dosage in pigment concentrates
Figure 2. 5: Colour strength and additive dosage (schematic)
(Figure 2.5) ends in a plateau where an additional amount of additiv produces only a small effect. Initially, increased addition of additive improves the pigment wetting and hence the colour strength but very high concentrations lead to double layers on the pigment particle at which stage the pigment wetting cannot be improved and the colour strength no longer increases.
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Chemical structures
2.1. 4. 3
27
Compatibility
At higher concentrations of wetting and dispersing additives, compatibility between pigment concentrate and base paint (pigment paste absorption) is also improved. The better colour acceptance leads to lower rub-out values. The additives used contain hydrophilic structures such as hydrophilic side chains or saltlike groups. These hydrophilic structures orientate to the aqueous medium. The more additive used, the more hydrophilic the pigment particle becomes, leading to increasingly greater compatibility of pigment concentrate and waterborne paint. 2.1. 4. 4
Stability
As already described, stabilisation depends on the thickness of the polymer shell around the pigment particle and is thus also related to additive concentration. A higher concentration of stabilising additive leads to a more stable dispersion.
2.2
Chemical structures
To stabilise pigment particles two different molecule segments are necessary: anchor groups with an affinity for the pigment, which adsorb on the pigment surface, and water soluble side chains which produce the steric hindrance. Because the groups with affinity for the pigment are mostly hydrophobic and the soluble side chains hydrophilic, wetting and dispersing additives are called amphiphilic structures. The simplest amphiphilic structure is a surfactant. Because of their low molecular weight, surfactants are not suitable for stabilising pigment particles. Commercially available products are mostly polymeric. The polymers can contain various functional groups with high pigment affinity (anchor groups). An aromatic ring forms a suitable anchor group for organic pigments with the adsorption being caused by van-der-Waals forces. Adsorption on inorganic, oxidic pigment particles involves hydrogen bonding and induced dipoles; hydroxyl or carbonyl groups are suitable here. Additives containing both of these groups in the form of the carboxyl group, also show a strong affinity to inorganic pigments. Additives with nitrogen containing groups (e.g. amines or imines) exhibit good adsorption on carbon black surfaces. Additives without nitrogen groups are of only limited suitability for carbon black pigments. 2.2.1
Polyacrylate salts
Polyacrylate salts are simple polymers which stabilise pigments in waterborne paints. They are characterised by their high acid value which promotes good
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Wetting- and dispersing additives
Figure 2. 6a: Structural example polyacrylate salt
2.2.2
anchoring to inorganic pigments. The highly ionic character means that there is a strong tendency to dissociate in water and this allows good electrostatic repulsion. Common counter cations are sodium and ammonium. Coatings containing ammonium polyacrylates are more resistant to water.
Fatty acid and fatty alcohol derivatives
Polyacrylate salts are not suitable for stabilising organic pigments. Fatty acid derivatives are a group of simple compounds which stabilise pigments. Fatty acids have structures which will also anchor to organic and carbon black pigments. The hydrophilic part consists of a polyether chain. Fatty acid ethoxylates are excellent emulsifiers and allow the production of very compatible pigment grinds. As with polyacrylate salts, the water resistance of coatings containing fatty acid derivatives is limited. Fatty acid derivatives differ from polyacrylate salts in that they can be used to produce pigment Figure 2. 6 b: Structural example glycerol fatty acid ethoxylate concentrates. 2.2.3
Acrylic-copolymers
Acrylic-copolymers are also suitable for formulating pigment concentrates. The broad variety of monomers available allows the development of wetting and dispersing additives which are suitable for all kinds of pigments and compatible with many different binders. Acrylic copolymers can be modified, so that coatings
Figure 2. 6c: Structural example methacrylic-polyether copolymer
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29
containing them become very water resistant. Hydrophilic polyether chains also provide steric stabilisation of the pigment particles. A-B copolymers or comb like structures are possible. 2.2.4
Maleic anhydride copolymers
These copolymers contain maleic anhydride instead of acrylic acid. The copolymers are mostly comb-like structures. Ethoxylate chains are also used here to sterically stabilise the pigments. Maleic anhydride chemistry does not allow the broad variety of structures available with acrylates but nevertheless enables development of additives for all kinds of pigments. These additives are very resistant to water.
Figure 2. 6d: Structural example maleic anhydride-polyether-copolymer
2.2.5
Alkyl phenol ethoxylates
Alkyl phenol ethoxylates (APE) have very good pigment dispersing properties and are very low cost. Because of their broad compatibility and strong emulsifying performance they can be used for the production of universal colorants suitable for tinting waterborne and solvent-based base paints. APEs can create nonyl phenol by hydrolysis. Nonyl phenol is very similar in structure to the female hormone oestrogen and can produce the same effects. If waters become polluted with this, aquatic animals will only bear female descendants and so die out within a few generations. The use of APEs is therefore controversial but they are still widely used in the NAFTA region.
Figure 2. 6e: Structural example non-ionic APE
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Wetting- and dispersing additives
Alkyl phenol ethoxylate replacements
Among alternatives to APEs are the so called Guerbet-derivatives (modified fatty acid ethoxylates) and modified polyethers. The modified polyethers contain only polyether bonds and are thus very stable against hydrolysis and are very suitable for exterior applications.
Figure 2.6f: Guerbet-derivative (R= fatty acid or phosphate)
Figure 2.6g: Modified polyether
2.3
Wetting and dispersing additives in different market segments
Wetting and dispersing additives are used in the grinding stage of paints and coatings as well as in pigment concentrates. The different applications make very distinct demands on the additives which must be taken into account when discussing market segments. 2.3.1
Architectural coatings
2. 3.1.1
Direct-grind
During production of waterborne emulsion paints, extenders such as calcium carbonate and titanium dioxide are used as a millbase. Grinding these materials is not particularly demanding and, because of their excellent viscosity reduction, poly-
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31
acrylate salts are widely used. Electrostatic repulsion is sufficient in this case and the cost-performance ratio of these additives is appropriate to the application. 2. 3.1.2
Pigment concentrates
Pigment concentrates, which are used in architectural coatings, can be divided into several groups. Firstly there are mass-tones and tinting colorants. These are coloured emulsion paints which can be used alone as a full shade or used to tint white base paints and have a relatively low pigment content. They contain binder and small amounts of fatty acid ethoxylates as wetting and dispersing additives. The second group comprises the binder-free tinters. They contain larger amounts of fatty acid derivatives and are used to tint white base paints or for colour corrections. This market segment also includes high performance pigment concentrates which contain acrylic- or maleic-anhydride- copolymers and which, because of their high price, are used only when essential, for example, in facade coatings with strong beading effects or silicate paints because they have a high pH. So called universal colorants are widely used in architectural coatings. These aqueous pigment concentrates can be used in waterborne emulsion paints as well as in solvent-based alkyd lacquers. Alkyl phenol ethoxylates can be used to formulate universal colorants. Because of environmental concerns, alternatives such as fatty acid derivatives (Guerbet-derivatives) and modified polyethers are used. 2.3.2
Wood and furniture coatings
2.3.2.1
Direct grind
As with architectural coatings, polyacrylate-salts are used in the direct grind of titanium dioxide and iron oxide pigments in wood coatings. To stabilise transparent inorganic pigments, higher quality additives such as acrylic- and maleic-anhydride- copolymers have to be used. 2.3.2.2
Pigment concentrates
The use of very fine, transparent pigments and required resistance are the reason for using high performance acrylic- and maleic-anhydride copolymers with high chemical resistance. The manufacture of flatting agent pastes involves considerable dispersion input and thus requires the use of high quality additives.
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Wetting- and dispersing additives
2.3.3
Industrial coatings
2.3.3.1
Direct grind
2.3.3.2
Pigment concentrates
2.3.4
Printing inks
2.3.4.1
Direct grind
2.3.4.2
Pigment concentrates
2.4
Tips and Tricks
The high demands made on resistance and weatherability necessitate high quality additives such as acrylic- and maleic-anhydride copolymers. The use of very finely divided organic pigments also necessitates the use of additives with good performance. In industrial coatings the difference between direct grind and pigment concentrates is not significant as far as wetting and dispersing additives are concerned. Pigment concentrates in industrial coatings mostly contain binder or grinding resins. Because of this, compatibility between binder/grinding resin and wetting and dispersing additive is very important.
The classical way of producing printing inks involves a resin solution based on an acidic styrene-acrylic or pure-acrylic resin neutralised with amines to make it water soluble. The resin solution is able to stabilise pigments very well, but pigment wetting is sometimes not acceptable. This is apparent particularly with carbon black pigments where the viscosity is very high and the colour strength quite poor so that use of suitable additives with good wetting is advantageous. Resin-free concentrates have also been developed. Avoiding the use of resin solution results in greatly improved water resistance. High levels of very fine, intensely coloured pigments pose special demands which can be met by the use of acrylicand maleic-anhydride copolymers.
When selecting a wetting and dispersing additive, the suitability of its chemical structure for the particular pigment and its compatibility with the surrounding binder are of prime importance. The suitability of a wetting and dispersing additive for a particular pigment is described in Chapter 2.3 “Chemical structures”. To summarise: an additive which contains acid groups is adequate for inorganic pigments; an additive with nitrogen groups is very effective on carbon black surfaces.
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Test methods
33
The compatibility of wetting and dispersing additives for water-borne applications with the binder matrix can only be tested in conjunction with a pigment. The surfactant structure – hydrophobic anchor groups and hydrophilic side chains – of some additives makes it impossible for some surfactants to be water-soluble in pure form. As soon as pigment particles are present, the hydrophobic portions of the additive molecules orientate themselves to the pigment surface and the hydrophilic segment protrudes into the water phase. The pigment-particle/additive combination then becomes water-soluble. The pH is also of great importance. To avoid pigment shock, the pH of the pigment grind/pigment concentrate and the let down resin or the base paint has to be the same. In many cases, and this is especially true for inorganic and carbon black pigments, the pH of the pigment concentrate needs to be adjusted. Amines and alkali hydroxides are among the suitable compounds for the neutralisation process. Neutralisation should be carried out after the pigments have been wetted by the wetting and dispersing additive. For many inorganic pigments a free acid group can be more conducive to adsorption of the additive than a neutralised acid group.
2.5
Test methods
2.5.1
Particle size
The primary criterion for the quality of dispersion is the particle size. Monitoring the particle size allows a decision to be made as to when the grinding process can be terminated. The simplest method of measuring the particle size of inorganic pigments is the Grindometer draw down. A sample of the millbase is poured into the deep end of a groove, and scraped towards the shallow end with a flat metal scraper. At the point where the depth of the groove equals the largest particles in the suspension, irregularities (for example stripes in the draw down) will become visible. The depth of the groove is marked on a graduated scale next to it. With some practice, use of a Grindometer allows the maximum particle size of the mill-base to be determined quickly and simply but cannot be used to measure pigment particle size distribution. When grinding binder-free pigments, which dry very rapidly and have particle sizes smaller than 5 µm, the Grindometer can easily give a false value. More sophisticated measurements such as laser diffraction or ultrasound give a more precise result in terms of particle size and particle size distribution. Due to their high cost, such methods are not suitable for routine use. Achievement of the desired particle size distribution can be detected by reliable secondary indications.
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Wetting- and dispersing additives
Colour strength development of organic pigments is dependent on pigment particle size. Determining colour strength at different stages of the grinding process allows the final point of the grinding process to be detected. 2.5.2
Colour strength
For colour strength determination a mill base sample is let down in an appropriate coating formulation and applied. Evaluation is carried out by optical examination or by a spectro-photometer and compared with that of a standard grind using the same amount of the mill base sample. The amount of the mill base sample under evaluation is then adjusted. When both samples give the same optical appearance, the relative colour strength of the new sample in % of the standard can be calculated. Relative colour strength determination is very complex but provides meaningful comparative data. This test method is used mainly by pigment manufacturers. Determination of absolute values is based on Kubelka-Munck theory. This involves the relationship between reflection and transmission of light. Summation of the reflection over the entire spectrum gives a value for the colour strength. In practice, this method suffers from a systematic error since it is based on the assumption of an infinite film thickness and a constant degree of reflection. It is therefore not suitable for pigment development and pigment manufacturers prefer the method of relative colour strength determination. Kubelka-Munck equation CS = K/S = (1–R)²/2R where, CS = colour strength K = absorption coefficient S = scattering coefficient R = reflectance at infinite film thickness (hence no change in degree of reflection) 2.5.3
Rub-out
The Rub-Out test is used to check the stabilisation of pigment particles. It can be used to assess the compatibility of pigment concentrates, the tendency of pigment particles to flocculate or pigment flooding phenomena. An area of the moist but partially dry paint film is rubbed with a finger or a brush. If the pigments have separated or are strongly flocculated, this mechanical procedure of rubbing reestablishes a homogeneous pigment distribution. The viscosity of the dry film will already have increased strongly. The homogeneous distribution of pigment particles is stabilised this way. The colour difference relative to the unrubbed film is
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Test methods
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an indication of pigment separation or flocculation. The colour difference is usually quoted as the “separation” of the chromaticity ΔE (ΔE is dimensionless). For ΔE less than 0.5, no colour difference is visible. The automotive sector requires ΔE < 0.3. Between 0.5 and 1.0 the colour difference is only slightly visible. For architectural paints, ΔE of < 1.0 is still adequate but ΔE values greater than 1 are not acceptable. 2.5.4
Viscosity
The viscosity of a mill-base must be adjusted to suit the dispersing unit. If the viscosity of the mill-base is excessive, the unit may be damaged. If it is too low, shear forces will be inadequate to break down the pigment agglomerates. The viscosity is also an important indicator of the stability of a pigment concentrate. Any change in rheology during storage indicates inadequate pigment stabilisation. An easy method to determine viscosity is by measuring the efflux time. For millbases, however, the viscosity is usually simply too high to use a flow cup. A rotational viscometer is often used to determine the viscosity more precisely. The resultant complete flow curves provide information on flow characteristics of the particular material, from the manufacturing process through transport to the final application. During development of a pigment concentrate, its flow characteristics over the entire shear rate spectrum are of great importance. For quality control purposes, measurement at two points, e.g. at low and medium shear rate, is usually sufficient. 2.5.5
Zeta potential
Electrostatic stabilisation can be characterised by measuring the zeta (ζ) potential which assumes formation of an electrical double layer. In a solution of electrolyte, particles with a charged surface such as metal oxide pigments adsorb counter ions which form an immobile film known as a Stern layer. The diffuse cloud of ions, consisting of similarly charged ions and counter ions lies outside this layer. If a particle moves, part of the loosely-bound diffuse layer shears off. The potential at this shear plane is termed zeta (ζ) potential and is important in assessing the stabilisation of dispersion. The higher the ζ potential, the better a dispersion is protected against flocculation. Traditional, optical methods for determining ζ potential, which are based on electrophoretic mobility, can only be used for very dilute systems. However strong dilution during investigation of coating of the pigment surface by additives leads, for example, to a change in the adsorption equilibrium and thus to measurements which do not correspond to reality.
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Wetting- and dispersing additives
Figure 2.7: CVI principle by using high frequency sound wave
The ζ potential can be measured electroacoustically. This method is also suitable for investigating concentrated dispersions with a pigment concentration of 50 % v/v. There are two different ways of determining the ζ potential electroacoustically depending on the exciting force. If an alternating electrical field is applied to a dispersion of charged particles, the particles are excited to vibrate and emit sound waves. This method gives the value of the electric sonic amplitude. If the exciting force is an ultrasonic wave, an electrical signal can be detected and a colloid vibration current (CVI) measured. Figure 2.7 shows the CVI principle. A high frequency sound wave, generated by a piezo crystal in the measuring sensor passes through the dispersion. The acoustic signal excites the particle to vibrate. The higher the inertia of the particles, the worse their ability to follow the sound wave and hence the larger the phase shift. The diffuse cloud of ions reacts without delay to the sound wave so that each particle with its ionic shell becomes a dipole which constantly changes its direction. At a particular point in time, the dipoles point in one direction so that an electric field arises and the colloid vibration current can be measured with two electrodes.
Figure 2.8: Principle of the use of an ultrasonic wave as exciting forcesors
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Figure 2.8 shows a diagram of a measuring sensor. After applying a radio-frequency pulse, a cylindrical piezo element generates an acoustic pulse which passes through a quartz crystal for internal calibration. The quartz crystal is extended by a buffer section the acoustic impedance of which is more tailored to the dispersion than to the material of the quartz crystal. The end of the buffer rod is coated with gold and forms an
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Test methods
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electrode for measuring the electric signal. The second electrode required is provided by the steel casing. When the measurement sensor is immersed in a sample, a colloid vibration current can be detected between the gold electrode and the stainless steel casing. The measured colloid vibration current is related to the ζ potential as follows:
where ε0 : dielectric constant of the vacuum εm : dielectric constant of medium ζ : ζ potential φ : parts by weight η : dynamic viscosity
Ks : conductivity of dispersion Km : conductivity of medium ρp : density of particle ρs density of system ω : circular or angular frequency
The equation shows which parameters affect the ζ potential. A higher value of dielectric constant causes a lower ζ potential. Water with a dielectric constant of 80 and a very polar character weakens the dipole while, for example, in a non-polar solvent such as heptane, the dipole effects are more pronounced. A greater concentration leads to a lower ζ potential, because the individual particles move closer together and the electrical double layers overlap as the concentration increases. A high dynamic viscosity affects the inertia of the particle which causes the ion cloud to be more easily shifted against the particles in the medium. The difference in density between the particle and medium must be as great as possible so that the medium moves relative to the particle and dipoles are created which contribute to the measurement of the colloidal current.
Figure 2.9: ζ-potential curves based on different wetting and dispersing additives with iron oxide yellow
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Wetting- and dispersing additives
Using additive and pH titrations, one can deduce interactions between pigments and additives and thus characterise the electrostatic stabilisation of pigments. With iron oxide yellow pigments, it has been found that additives with a strongly ionic character promote electrostatic stabilisation while additives with polar anchor groups do not wet the surface [4]. If the pigment surface were wetted by the non-ionic additive, shielding of the partially charged pigment surface by the additive would lead to a measurable change in potential.
2.6
Summary
Wetting and dispersing additives fulfil different functions within a coating formulation. They help pigment wetting and stabilise the dispersed pigment particles. They reduce the grinding time and decrease the viscosity. The dosage of additive has different influences on properties of the grind such as: • • • •
viscosity colour strength compatibility storage stability.
The surface tension has a major influence on pigment wetting. In waterborne formulations two stabilising mechanisms are used: • steric stabilisation • electrostatic stabilisation. The structure of wetting and dispersing additives is determined by the pigment wetting required, the anchoring to the pigment surface and the pigment stabilisation. The structures are mostly surfactant-like. Depending on requirements different chemical structures are available. The simplest are based on fatty acids. Higher demands can be fulfilled by acrylic- and maleic-anhydride-copolymers. 2.7 [1] [2] [3] [4]
Literature Brock, Groteklaes, Mischke, European Coatings Handbook, Vincentz Network 2000 Tego Journal 2006 J. Bieleman, Additives for Coatings, Wiley/VCH 2001 GdCH-Tagung 2008
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Defoaming mechanisms
3
39
Defoaming of coating systems Juergen Kirchner
Foam is a colloidal system of gas surrounded by liquid or solid cell bridges. Foam reduces the quality of paints. Foam formation frequently occurs during the production or application of paints and coatings, resulting in impaired optical or technical performance of a coating. Foam also disrupts production processes, e.g. by reduced energy transfer during dispersion processes, reduced capacities of vessels or problems at filling stations. Effects on the optical performance of coatings include surface irregularities, reduced gloss and transparency. Defects caused by foam also significantly impair the coating’s ability to protect against penetrating media. There are various reasons for the formation of foam including: • mechanical introduction of air by mixing or application processes • displacement of air from surfaces during wetting and dispersing processes • generation of gas by chemical reactions • bubbles caused by too rapid drying The presence of surface active (amphiphilic) substances in paints and coatings often promotes the stabilisation of foam. To prevent its formation during manufacturing and processing of coatings and to destroy foam which has already built up, it is essential to use defoamers or deaerators [1].
3.1
Defoaming mechanisms
To explain the mechanisms of defoaming it is necessary to understand the composition and stabilisation of foam. Defoaming always entails disrupting the mechanisms of foam stabilisation. 3.1.1
Foam
Foam is a dispersion of a gaseous material in a liquid or solid, in which the volumetric content of the gas predominates. The gas bubbles in the foam are separated by liquid or solid walls. Solid foams are often created from existing liquid foams. Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Defoaming of coating systems
Causes of foam
The simultaneous presence of a gas and a liquid does not necessarily result in the formation of foam. Intense mixing of both materials is required for a fine distribution of the gas in the liquid and this results in the creation of new gas/liquid interfaces. In most pure liquids these interfaces are unstable. Consequently, the gas bubbles rise very quickly and collapse on the surface of the liquid. Only the presence of surface active (amphiphilic) substances in the liquid stabilises the gas as foam. (Figure 3.1) [2, 3]. Stabilisation occurs through orientation of the surface active substances at the newly formed liquid/gas interfaces. In practice, the presence of surface active substances cannot be avoided in coating systems, as their use is essential to control properties such as wetting and dispersing, emulsification or substrate wetting [4, 5]. 3.1. 1.2
Types of foam
Classification of foams can be based on their state of aggregation or structure and whether they are liquid or solid. Liquid foams Liquid foams consist of a dispersion of gas in a liquid. Usually freshly formed liquid foams are unstable and, in time, undergo structural changes frequently to a more stable form. If a stable structure is not attained, the foam eventually collapses. Solid foams Solid foams are manufactured for use in specialist applications in the coatings industry such as in assembly. Solid foams also appear undesirably as entrapped air in dried paint films or as craters or pinholes on the surface. Foams can also be differentiated by structure, for example into micro- or macrofoam or dry or wet foam. Microfoam Small gas bubbles trapped either in the liquid phase of a paint or in the solid bulk phase of a coating are termed microfoam. Typically the interface between the gas bubble and the surrounding media is stabilised by a single layer of surfactant. Stabilisation of microfoam is promoted by factors such as a high coating viscosity or low temperatures [1, 5]. For combating micro foam special deaerators are used. These are described in Chapter 7.
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Defoaming mechanisms
Macrofoam Macro- or surface foam is the type visible on the surface. In macrofoam the bubble is separated by a thin foam lamella from the surrounding atmosphere. The surfaces of the lamella orientated to the atmosphere and to the gas bubble are covered with a layer of surfactant. This double surfactant layer is frequently called a duplex film.
Figure 3.1: Formation and stabilisation of foam in a
Macrofoam can be further dif- liquid. Microfoam is stabilised by a single surfactant ferentiated by the liquid con- layer in the bulk phase of the liquid. Foam bubbles tent of the lamella. Freshly penetrating through the surface appear as macro foam and are stabilised by a double layer of surfactants, formed macrofoam created the so called duplex film. by rising bubbles consists of spherical bubbles, which are stabilised by thick lamellas with high water content. This foam is described as ball foam. Due to gravity-induced drainage, the water in the foam lamellas flows down until the bubbles are stabilised by very thin, but very stable lamellas. In this process the original spherical bubbles are transformed into a more stable polyhedral form. The resulting foam consists of a great deal of gas and little liquid and is also known as dry foam or polyhedral foam [1, 4–6]. Ball foam can be differentiated from polyhedral foam according to the volume content of gas as shown in table 3.1 In gas dispersions the volume content of the gas is characteristically higher than the volume content of water. Effect of time on foams/drainage Most foams are thermodynamically unstable and undergo structural changes over
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Figure 3.2: Structure of foam Table 3.1: Volume content of gas in foams [7] ϕ = Vg / (Vg + Vl) gas dispersion
ϕ = 0.52
ball foam
0.52 0.74
Vg = Volume of gas, Vl = Volume of liquid
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Defoaming of coating systems
time. The effect of gravity on foams causes drainage of the liquid from the lamellas. This results in a reduction of the thickness of the lamella. Small gas bubbles disappear by diffusion, which causes a reduction in the number of lamellas and the nodes between them. As drainage continues, ball foam converts into polyhedral foam, causing a deformation of the lamella and an increase in their radii. In some foams drainage creates a sufficiently high tension in the lamella for them and the foam to collapse spontaneously. Steric effects or electrostatic repulsion caused by surfactants can compensate the tension in the lamellas created by drainage. This counteracts further thinning of the lamella. Polyhedral foams stabilised in this manner are highly stable. To prevent the formation of stable polyhedral foams or to destroy them it is necessary to use defoamers [3, 6, 8].
3.2
Defoamers
3.2.1
Composition of defoamers
To understand how defoamers work it is necessary to know that defoamers consist basically of an insoluble or, at least in the application media, partly soluble active ingredient (commonly called oil), a hydrophobic particle or a mixture of both. A more detailed view on the composition of defoamers is given in chapter 3.3. 3.2.2
Defoaming mechanisms
Defoamers are used to destroy macrofoam. An effective defoamer must be able to break the stability of a foam lamella. Various defoaming mechanisms discussed in the technical literature are usually explained in terms of a sequence of individual steps. For clarity each step is first considered and the interaction of the individual mechanisms is explained later in this chapter. 3. 2. 2. 1
Defoaming by drainage/slow defoaming
To cause defoaming a defoamer must be able to penetrate from the inside of a foam lamella to the interface between the liquid and the foam-forming gas. The surfactants stabilizing the lamella act as an entry barrier which impedes penetration of the defoamer droplet into the lamella interface. Some defoamers are unable to pass the entry barrier. These so called slow defoamers only start to act once the foam lamella has been sufficiently thinned by continued drainage. The defoamer droplets (oil droplets) are trapped in the lamellas during this process or migrate to the nodes between the lamellas. If drainage continues, the narrowing lamellas exert a capillary pressure on the defoamer droplets. Once a critical point is reached, the capillary pressure of the upper bubbles in the foam is high enough for the oil
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Defoamers
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droplets to penetrate the surface of the lamella causing its collapse [5, 8] and placing stress on adjacent lamellas. In many cases this stress is sufficient to make the neighbouring lamellas collapse as well. In foams highly thinned by drainage this often leads to a chain reaction until almost complete defoaming occurs. Remaining bubbles are often stable for a long period. The speed and efficiency of defoaming by drainage is inadequate for many processes in the coatings industry. For this reason there is a demand for defoamers providing faster, more efficient defoaming mechanisms [8]. 3. 2. 2. 2
Entry barrier/entry coefficient
The entry barrier describes the kinetic resistance which a defoamer droplet must overcome to break through the lamella surface. The first step of defoaming is always the entry of the defoamer droplet into the surface of the lamella, i.e. the interface to the surrounding air. To do so the defoamer must have a low surface tension [2, 3]. If a defoamer droplet comes into contact with the surface of a lamella as a result of motion in the lamella due, for example, to drainage-induced flow, the droplet needs to overcome the entry barrier to reach the lamella/air interface. The entry barrier of the lamella surfaces is determined by the surfactants stabilising the foam. Systems with low surfactant content, below the critical micelle concentration have a low entry barrier. The entry barrier rises with increasing concentration of surfactant on the lamella interface to a maximum value at total saturation. Another parameter determining the entry barrier is the mobility of the surfactants. Dynamic processes create new interfaces or subject lamellas to stress due to expansion. To create a stable foam, the newly formed surfaces must be quickly covered with surfactant. In systems with less mobile surfactants, saturation of the newly created interfaces with surfactants is delayed, thus facilitating
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Figure 3.3a: Distribution of defoamer droplets in a lamella
Figure 3.3b: Defoamer droplet which has entered the lamella
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Defoaming of coating systems
ingress of the defoamer droplet at these temporarily inadequately stabilised areas in the lamella [8]. Once a defoamer droplet passes the entry barrier and enters the interface of the lamella, the next step is determined by the entry coefficient. The entry coefficient E describes the thermodynamic equilibrium of the oil droplet in the lamella at the liquid/air. interface [4, 5]. E = σAW + σOW – σOA If the entry coefficient is negative, complete wetting of the defoamer by the liquid phase is the most thermodynamically-stable condition. In this case, the droplet would leave the interface and migrate back into the liquid phase. Only if the entry coefficient is positive will the defoamer droplet remain on the surface of the lamella. A positive entry coefficient for the defoamer oil is an important prerequisite for many defoaming mechanisms. However to be effective defoamers also need to fulfil other requirements [8]. 3. 2. 2. 3
Bridging mechanism
The bridging mechanism requires that a defoamer droplet can pass the entry barrier and that it has a positive entry coefficient. Bridging occurs when a defoamer droplet penetrates on both sides of a lamella. The behaviour of a defoamer droplet in the lamella is determined by the bridging coefficient B. Only defoamers with a positive bridging coefficient are able to destabilise the foam lamella by subsequent defoaming mechanisms. If the bridging coefficient is negative, the oil remains as a stable bridge in the lamella. B = σAW2 + σOW2 – σOA2 The value of the bridging coefficient is determined by the contact angles between the three phases, oil, liquid and air, and thus by the geometry of the droplet and the thickness of the lamella.
Figure 3.4: Bridging of a foam lamella
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If changes in the thickness of the lamella occur due to drainage, the absolute value of the bridging coefficient may change. The bridging defoamer drop may also grow by picking up spread-out oil. In both cases this may cause the bridging coefficient to change from negative to positive values, thus converting a stable bridge into an
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Defoamers
Figure 3.5a: A contact angle αW lower than 90° results in a negative bridging coefficient.
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Figure 3.5b: A contact angle αW higher than 90° results in a positive bridging coefficient.
unstable bridge. A positive bridging coefficient is a prerequisite for the subsequent bridging-stretching or bridging-dewetting mechanisms [6, 8]. 3. 2. 2. 4
Spreading mechanism
Defoaming via spreading is only possible with oils of low surface tension. Once a defoamer droplet enters the lamella it spreads on its surface. The ability of defoamer oil to spread is described by the spreading coefficient. S = σAW – σOW – σOA Spreading of oil on the surface of a lamella causes a change in the surfactant distribution on the lamella’s surface [4, 5].The newly created oil/water interface reduces the entry barrier for further defoamer droplets to penetrate into the lamella surface through the spread-out oil. Consequently the area of the spread-out oil increases. Oil lenses caused by spreading may result in surface defects, such as craters or fish eyes, in coatings.
Figure 3. 6a: Defoamer droplet on the surface of a lamella
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Figure 3. 6b: Spreading of a defoamer droplet on the surface of a lamella
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Defoaming of coating systems
A positive spreading coefficient is a prerequisite for defoaming by spread out liquids or by the spreading-wave-mechanism [8]. 3. 2. 2. 5
Bridging stretching mechanism
The prerequisite for the bridging stretching mechanism is a positive bridging coefficient. The mechanism also works in thick foam lamellas. A defoamer droplet penetrating the lamella causes bridging of the latter. In so doing the droplet becomes biconcave with the smallest thickness in its middle. Mechanical stress on the lamella, e.g. caused by uncompensated capillary forces, causes elongation and further thinning of the oil droplet. Once the droplet reaches a state when it can no longer compensate for the mechanical stress, it breaks, causing destabilisation of the lamella and consequent collapse of the foam bubble.
Figure 3.7a: Bridging of a defoamer droplet with a positive bridging coefficient
The prerequisite for the bridging stretching mechanism is that the defoamer droplet is deformable. Due to their inflexible geometry hydrophobic particles cannot defoam via the bridging stretching mechanism [5, 6, 8]. 3. 2. 2. 6 Bridging dewetting mechanism
Figure 3.7b and c: Stretching of a defoamer bridge till collapse
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Analogous to the bridging stretching mechanism the first step of the stretching dewetting mechanism is droplet entry and bridging. This is only possible if the entry barrier of the lamella can be overcome and the defoamer has a positive bridging coefficient. In systems with low surfactant content, spontaneous dewetting of the defoamer droplet would be expected. In surfactant-rich systems, such as coatings, spontaneous dewetting will not occur as the surfactants will wet the defoamer droplets. The ability to wet the defoamer droplet can be characterised by the con-
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Defoamers
tact angle between the defoamer droplet and the surrounding liquid. If the contact angle is less than 90° the defoamer droplet is wetted by the surrounding liquid. However, if the defoamer droplet is able to assume a lens shape, the contact angle at the edges of the lens becomes high enough to cause dewetting of the defoamer droplet followed by collapse of the foam bubble [6, 8].
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Figure 3. 8: Bridging dewetting mechanism
In cases where the lens undergoes further deformation without dewetting, the bridging dewetting mechanism transforms into the bridging stretching mechanism. It is probable that the bridging dewetting mechanism occurs preferentially with higher viscosity defoamer oils, which are able to form more stable oil lenses. For low viscosity defoamers the bridging stretching mechanism is more likely.
Up to now, evidence has been found for the bridging dewetting mechanism for hydrophobic particles only in systems free of strong surfactants. It is unlikely that the bridging dewetting mechanism applies in real surfactant-rich coatings systems. Because of the very fast dewetting process it has not been possible to definitely confirm the mechanism for defoamer oils yet [8]. 3.2.2.7
Spreading fluid mechanism
A possible mechanism for defoaming by spreading is the spreading-fluid mechanism in which the oil creates a flow of the liquid in the lamella in the direction of the spreading (Marangoni flow). This causes a thinning of the lamella, resulting in the lamella breaking and the foam collapsing [3, 5, 6, 8]. Spreading of defoamer oils in lamellas has been already detected experimentally. The destabilisation of the lamella by Marangoni flow seems likely, but has not yet been proven experimentally [8]. 3.2.2.8
Spreading wave mechanism
The spreading wave mechanism has been demonstrated only for slow-acting defoamers. The mechanism only works in very thin lamellas (approx. 1 micrometre). Spreading of a defoamer
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Figure 3.9: Spreading fluid mechanism
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Defoaming of coating systems
causes disruption and thinning of the surfactant film which stabilises the lamella. Resultant stresses in the lamella must be compensated by its deformation. The lamellas assume a wave like structure of varying thickness. This lamella is less resistant to mechanical stress than a defoamer free lamella. The spreading wave mechanism does not act locally at the point of entry of the defoamer droplet; rather the disruption created by the defoamer covers a larger area of the foam lamella. The mechanism has been proven only for slow defoamers in sufficiently large lamellas. Lamellas in the micrometre wavelength region and amplitudes in the nanometre range have been measured experimentally. The disruption caused by the defoamer extended over the whole foam lamella [8]. 3. 2. 2. 9
Effect of fillers on the performance of defoamers
Solid particles can also act as defoamers according to some of the mechanisms described (e.g. bridging dewetting mechanism). The filling of defoamer oil with a solid particle can boost the defoaming characteristics to higher levels than that of the individual components [1, 8]. A solid content of only a few percent is sufficient to achieve this. Excessive solid content often causes excessive viscosity of the oil and impairs flow and deformation, resulting in reduced efficiency of the oil. For optimum performance it is necessary for the solid particles to cover the surface of the defoamer droplet. A strong hydrophobic particle will be too compatible with the oil and will be completely wetted by it. If the solid particle is too hydrophilic, there is a risk that it can be removed from the oil droplet into the aqueous phase of the coating. In both cases the efficiency of the defoamer will suffer. The filler has a major influence on the entry barrier and the bridging mechanism of defoamer oil. Influence on the entry barrier The solid covering of a defoamer droplet creates an irregular surface. The solid disturbs the symmetry between the air, oil and liquid phases and acts like a kind of pin, lowering the entry barrier into the lamella. Many defoamer oils can only overcome the entry barrier and penetrate the lamella surface if they are filled with solids [3, 8]. Influence on bridging Oils spreading at the surface only penetrate into the lamellas to a limited depth. For this reason the lamellas need to be already widely thinned out and the drainage well advanced for the spread out oil to bridge the lamella. A non-deformable filler enables the penetration depth to be significantly increased; bridging then occurs at higher lamella thicknesses [8].
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Chemistry and formulation of defoamers
3.3
Chemistry and formulation of defoamers
3.3.1
Active ingredients in defoamers
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Active ingredients with various chemical compositions are suitable for formulating defoamers. The effectiveness of the defoamer is always dependent on its partial incompatibility in the application medium. This permits formation of defoamer droplets in the system, without causing film or surface defects due to excessive incompatibility. Defoaming is thus always a compromise between efficiency and compatibility. At the same time, a high defoaming power is required. The mostly commonly used defoamer active ingredients are described below. In practice, combinations of active ingredients are used to formulate very powerful defoamers. 3. 3. 1. 1
Silicone oils (polysiloxanes)
The class of silicone oils covers pure silicone oils as well as organic modified polysiloxanes. In many applications, pure silicone oils are too incompatible and are thus mostly used in low dosages as a silicone tip in combination with other defoamer active species. Organic modification of polysiloxanes, for example with polyethers, enables the compatibility of the defoamer to be tailored to specific application systems. The chemistry of silicone defoamers provides a wide range of modifications for polysiloxanes, resulting in an extraordinary variety of possible polysiloxanes tailored to specific applications Thus silicone defoamers are the most widely used defoamers in the coatings industry [4]. 3. 3. 1. 2
Mineral oils
Mineral oils used as defoamers are mostly highly purified paraffin oils or white spirits. They offer the advantage of high stability against environmental influences and do not resinify or become rancid. White spirit is colourless, odourless and free of aromatic compounds. Because of their aliphatic structure, the active ingredients are highly hydrophobic and thus have only limited compatibility with coating systems. This limits the use of mineral oils in many coating systems. Mineral oils are mainly used in architectural coatings with medium to high PVC. In more sensitive applications mineral oils tend to result in flooding or surface defects such as reduced gloss, smearing or fogging which are well known in architectural coatings [9]. 3. 3. 1. 3
Vegetable oils
The requirement for environmentally compliant products and the use of renewable resources are increasingly important in the development of defoamer active ingredients. Vegetable oils consist of triglycerides of saturated or unsaturated fatty
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Defoaming of coating systems
acids. The aliphatic backbone makes them very hydrophobic, which places limitations on the range of uses of vegetable oils. Similarly to mineral oils, vegetable oils are used preferably as defoamer raw materials for medium to high PVC architectural paints. In more sensitive applications vegetable oils tend to produce flooding or impair the surface finish of the coating. 3. 3. 1. 4
Polar oils
The numerous ways of varying synthetic polymers provides the ideal opportunity to tailor-active ingredients for specific coating systems. Commonly used active ingredients are fatty acids, alcohols based on fatty acids, polyethers, alkyl amines, alkyl amides, tributyl phosphate or thio ethers. The compatibility of oils in application systems can be easily controlled by varying their polarity. Polar oils are often used in combination with other defoamer active ingredients to control the compatibility of the formulation. 3. 3. 1. 5
Molecular defoamers (gemini surfactants)
Molecular defoamers or gemini surfactants are a new class of defoamers. Similarly to surfactants, molecular defoamers have pronounced surface active properties and the molecules orientate themselves at the surface of foam lamellas. The foam lamella is destabilised by the molecular defoamer and destroyed. In contrast to all other defoamer active ingredients, molecular defoamers do not act via their incompatibility in the system like an oil droplet. The defoaming mechanisms, described in the previous section, cannot therefore be applied to molecular defoamers. Possible applications of molecular defoamers include UV coatings, furniture coatings, automotive coatings or printing inks [10, 11]. 3. 3. 1. 6
Hydrophobic particles
It is possible to defoam waterborne systems using only hydrophobic particles, but it is more common to use them in combination with other defoamer actives. Commonly used hydrophobic particles are surface-modified silica, aluminium oxide, urea, waxes (e. g. magnesium stearate) or polymer particles (e.g. polyamides, polypropylene). Particles with an irregular amorphous surface are usually more effective than spherical or smooth particles [1, 3, 4]. The hydrophobicity of the particle needs to be such that the particle orientates preferentially on the interface of the defoamer oil to the water phase. If the particle is too compatible with the oil, it will be completely wetted by the oil. If it is too hydrophilic it will migrate into the water phase. In both cases the efficiency of the defoamer will fall. In practice partially hydrophobic particles have proved particularly effective [4].
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Chemistry and formulation of defoamers
3. 3. 1. 7
Emulsifiers
3. 3. 1. 8
Solvents
3.3.2
Defoamer formulations
In the manufacture of waterborne defoamer emulsions emulsifiers are used to prevent the defoamer droplets coalescing or separating. Some defoamers are supplied as solutions of the active substances in solvents. The choice of solvent depends on the type of defoamer active ingredient(s) and the final application of the formulated defoamer. Combinations of the active ingredients described above are frequently used in defoamers. This enables properties such as effectiveness against macrofoam and microfoam or compatibility with the application system to be varied as required. Filling with a hydrophobic particle often boosts efficiency significantly [3]. Defoamers are supplied in different forms: concentrates, emulsions or solutions. The properties of the different supply forms are indicated in Table 3.2. 3.3.3
Suppliers of defoamers
Table 3.3 gives an overview of major global suppliers of defoamers and of defoamer types. Because of the large number of suppliers, no claim is made to completeness. Suppliers only of regional importance or who do not supply the paint industry have not been included. Table 3.2: Comparison of supply forms of defoamers Concentrates
Emulsions
Solutions
active content
100 %
10 to 25 %
R3); b) Rheology profile of two HEUR additives with the same hydrophobic chain ends but with the molecular weight of the PEG1 twice that of PEG2
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Ethoxylated and hydrophobically modified urethanes
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the right hand profile shows that doubling the chain length of the polyethylene glycol in a pseudoplastic HEUR leads to a decline in the rheology profile towards higher values (Figure 4.5b).
The polyurethane used for Figure 4.5b, is the same as that in the first investigation (see Figure 4.3) from which it can be concluded that not only the global viscosity but also the profile is affected by the change of molecular weight. The longer chains usually lead to more viscous behaviour, while shorter chains produce a more elastic one. Therefore by varying those parameters chemists can offer a repertory of formulation aids with different rheological profiles. 4.3.3
From self association to associative behaviour
The self association phenomenon has a strong influence on the manufacture of HEUR additives. In order to supply additives with acceptable viscosities, commercially-available HEUR additives usually comprise a blend of an HEUR, a surfactant and water. The role of the surfactant is to lower the viscosity by breaking the intermicellar linkages and reducing the strength of the associative network by diluting the hydrophobic interactions as depicted in Figure 4.6. With such an approach, the typical viscosities of commercial HEURs range from 2000 to 5000 cts, while the typical HEUR content will vary from 15 to 30 % w/w. This formulation step can also be useful in opening up a wider range of chemical variation for HEURs. For example, when the size of hydrophobic chain ends is increased, the structure of the bulk PU shifts from a continuous polyethylene glycol phase to a continuous hydrophobic phase. When that happens dilution with water is limited by the very slow thickening of the PEG. Adding the surfactant may thus shift the self organisation from equilibrium and allow faster and easier dilution. Thanks to such techniques, HEURs with hydrophobic chains as long as 26 carbon atoms can be formulated in water. The choice of the surfactant is not without side effects as the stability of the blend as well as final application properties may be compromised. The surfactants are non-ionic and, to ensure satisfactory stability, the three-component phase diagram between water, surfactant and HEUR needs to be determined. Stability after storage at various temperatures must then be checked and
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Figure 4.6: Contribution of a surfactant formulated with an HEUR additive. The surfactant dilutes the inter-micelle interaction by increasing the number of micelles and weakens the flower-like micelles by interaction with the hydrophobic groups of the HEUR.
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Rheology modifiers
the application properties of the HEUR in the presence of the surfactant must be reinvestigated. Such checks are very important because, the mode of action of HEUR in coatings is not only to build a network by itself but also to interact with the binder. There are two main types of interaction between a binder and an HEUR and this often explains why changing the binder in a formulation will completely alter the rheology: • interaction with the surfactant layer of the binder • interaction with the polymer of the binder The addition of an external surfactant to formulate the HEUR additive may alter these interactions because it can itself interact with the binder in the same way. Optimization of the chemical structure of an HEUR additive must therefore always take into account the additional factors arising from dilution with a surfactant. 4.3.4
Hydrophobic/hydrophilic equilibrium of waterborne coatings
The sensitivity of HEUR properties to the surfactant is really an illustration of their sensitivity to any extra component which affects the hydrophobic/hydrophilic balance of a formulation. The ability of an HEUR additive to interact with dispersions, in addition to its self associative behaviour, is referred to as its associative properties. They may vary greatly from one dispersion to another.
Figure 4.7: Difference in rheology of two silk formulations with a similar HEUR (balanced profile) depending on the choice of the binder
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Ethoxylated and hydrophobically modified urethanes
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It has been shown that an HEUR additive with Newtonian behaviour can lead to a pseudo plastic paint formulation if the interactions between the hydrophobic chains of the urethane and the surface of the dispersion become significantly stronger than the associative interactions between the chain ends of the polyurethanes. The example in Figure 4.7 shows the rheology profiles of two silk lustre paints, one made with a standard European ethylene-vinyl acetate binder while the other is made with an American styrene-acrylic dispersion. Of course, the associative behaviour is more pronounced in the high gloss formulation with high dispersion content than in silk paints, where the self associative properties will predominate. 4.3.5
Improved colour acceptance with HEUR
Another example of the associative behaviour of HEUR is the phenomenon of colour acceptance which measures the drop in viscosity after the addition of pigments to a white paint formulation. The pigment added to the formulation is usually organic and formulated with surfactant. The presence of such species in the formulation will create interactions with the HEUR additive which modify the structure and strength of the associative network [15]. Depending on the pigment, up to 70 % of the viscosity can be lost when the formulation contains an HEUR. This causes storage and rub-out problems. This drawback is particularly pronounced for pseudo plastic HEUR additives because their properties are based on the very strong associative behaviour of their chain ends. The obvious solution would be to avoid pseudo-plastic polyurethane thickeners when using tint bases and replace them with acrylic or cellulose thickeners. This option is not, however, generally applicable because of the specific advantages of polyurethane thickeners: better application properties (especially in terms of flow and levelling), independence from the pH of the formulation, and improved resistance to wet abrasion or absorption (see Figures 4.2 and 4.3). It was therefore important to supply formulators with HEUR additives which would be less sensitive to variations in formulation. The main parameters studied in tailoring HEURs were the type of hydrophobic segments and the length of the main polyethyleneoxide chain. Linear long hydrophobic chain ends (C12 to C22) were preferred and it was discovered that controlling the degree of branching of the chain ends as well as varying the polydispersion of the polymer produces more robust formulations and improved colour acceptance. Widening the polydispersity index of polyurethane leads to the production of a more complicated network than with a narrow molecular weight distribution. In the intermicellar network, the long chains not only anchor themselves to neighbouring micelles but also further away. The averaging of self organization proper-
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Rheology modifiers
ties of the macromolecules increases the stability of the associative network. Using chain ends that are no longer linear stabilizes viscosity upon tinting. One theory is that the branched hydrophobic groups are less able to entangle when associating. Their main mode of association is through weak Van der Waals or Debye forces. Similar forces can be generated between an organic pigment and the branched hydrophobic species. Hence, when particles of pigment are added to the paint, a new network is. This network can be of a similar energy level to that without pigment. If the associative forces are partly due to entanglement, addition of the pigment will generate weaker associative forces which can lower viscosity. These effects are summarized in Figure 4.8. The performance in a matt paint (Table 4.2) of a high efficiency urethane with a PEG core chain characterized by Mw of 30,000 g/mol and an IP of 2.5 was compared with that of a urethane showing a similar rheology profile. The chain ends consist of a C30 hydrophobic branched group. In addition to this rheology data, the
Figure 4.8: Comparison of the behaviour of narrow molecular weight distribution HEUR with linear end groups (left) and of broad molecular weight distribution HEUR with branched end groups (right) during tinting. On the left, it can be seen that the volume fraction of pigment can increase the distance between particles of latex rendering the linkage by short urethane chains more difficult: the associative network is lost. On the right, the long chains of the broad polydispersity retain a link between particles while some weak additional interactions with the pigment occur.
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Alkali swellable emulsions: ASE and HASE
Table 4 2: Matt paint with styrene acrylic binder, PVC of 76 % and 0.27 % of urethane (a reference and a new generation binder). Rheology measurements were made before and after adding 5 % black pigment to the formulation. Reference before
Variation (%) after tinting
New HEUR before
New HEUR after
Brookfield (100 rpm)
6400
3250
6750
4750
Stormer
125
115.5
126
129
rub out test showed a loss of intensity with the standard PU of 4.2 while the new HEUR showed a variation of only 0.1 %. This rub out test is a strong indication of the high pigment compatibility in the formulation which is indirect proof that the pigment participates in the associative network built by the new generation HEUR. Thanks to this, the formulator is now able to use urethane thickener even when colour acceptance is problematical. However, if the performance profile of an HEUR is not absolutely necessary, the use of acrylic swellable emulsion could well be a very cost effective alternative. This type of product will be considered in the next section.
4.4
Alkali swellable emulsions: ASE and HASE
4.4.1
Synthesis
(Hydrophobic) alkali swellable emulsions (HASE or ASE) designate a type of polymer in emulsion form synthesized mainly from acrylic or methacrylic acid (between 40 and 70 % w/w), an acrylic ester (butyl or ethyl acrylate in a ratio of 60 to 30 %). The H in HASE indicates another type of comonomer consisting of methacrylic macro monomers with the general formula shown in Figure 4.9. The methacrylic structure can be changed into a vinylic or a maleic type but is often chosen because it is ease to copolymerize with acrylic esters and methacrylic acid. The ratio of those three types of monomer governs the solubility of the polymer after neutralization. (H)ASE are typically insoluble at low pH (pH7). The rate of swelling of the chains between these two pH values increases with the number of carboxylic
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Figure 4.9: Methacrylic macromonomer
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end groups. It can also be accelerated by the introduction of highly water-soluble monomers containing sulfonic or phosphonic end groups [16]. Alkali ASE and HASE are reactive products which, in the presence of alkalis, will very rapidly form a gel possibly leading to processing problems. Suitable precautions are therefore required at plant level. 4. 4. 1. 1
ASE
When the polymer becomes soluble, its thickening properties become apparent. Contrary to HEUR additives where, because of the hydrophobic end groups, thickening was mainly based on the associative network, in the case of ASE, binding of water around the anionic groups governs the viscosity. The entanglements of the polymer chains would not be allowed in purely anionic polymers and hence the neutral, flexible hydrophobic monomers of ethyl acrylate weakly link the chains together resulting in a reliable thickening effect. The molecular weight of the ASE is the third parameter affecting the thickening effect of the polymer. The molecular weight is adjusted using a transfer agent such as a water-soluble thiol. Of these three parameters, the molecular weight will decide the rheology profile of the ASE: the lowest leading to almost Newtonian profiles and the highest leading to pseudoplastic behaviour. The typical range of molecular weight is from 50,000 g/mol to 106 g/mol. It is possible to boost the efficiency of pseudoplastic ASE by introducing some level of cross linking in the molecule. Multifunctional monomers, such as divinyl benzene, methylene bis-acrylamide or polyethylene glycol dimethacrylate are used for this purpose. The mechanism by which ASE causes thickening is depicted in Figure 4.10. ASE polymers are self associative. The interactions of the chains in the formulation are forced to entangle by the chain segments which are richer in acrylate esters, and are thus expelled from water. The difficulty in controlling this self associative behaviour stems from the lack of control of the sequencing of monomers during synthesis. Figure 4.10: Thickening mechanism of HASE. Polymer chains immobilize water to solubilise anionic functions all along the chains, while zones rich in ethyl acrylate allow entanglement between the different chains. This network is three dimensional.
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Because ASE polymers mainly interact with water they largely govern the properties of the formulation. They are very good at achieving a given level of rheol-
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ogy cost-effectively but are unsuitable for solving problems resulting from poor compatibility of the different components of the formulation. One of their weaknesses however is their strong dependence on the ionisation of water. The viscosity drops when a certain amount of ions are introduced into the formulation. That might be the case when adjusting the pH of a formulation which can explain why some formulators utilise organic amines rather than sodium carbonate to increase the pH. In order to provide associative behaviour to ASE which is better adapted to the rheology of paints and interacts with dispersions and pigments, hydrophobic monomers must be grafted on to the core structure. Therefore, HASE will be necessary. 4. 4. 1. 2
HASE
HASE must be considered as comb macro molecules characterized by a similar main chain as ASE onto which an organic surfactant has been grafted (Figure 4.11). The surfactant structure is generally based on polyethylene/propylene glycol terminated by a hydrophobic chain end. This structure can be the same as that chosen to make HEUR. Hence the potential associative behaviour in HASE should resemble that of HEUR. However, the fundamental difference between a linear triblock HEUR and a comb polymer HASE leads to different modes of action. This explains why HASE are the best rheology modifiers to ensure colour acceptance and offer the best performance against syneresis. Because of their structure, they offer suitable rheological profiles. Like HEUR, understanding their self-associative properties is a key to understanding their associative properties. It was shown that HEUR behave more or less as difunctional surfactants. Their mobility in dilute solution is high. As HASE, are a sub-group of ASE, they first bind water via the poly-electrolytic main chain. This severely limits the mobility of water around a macromolecule. Consequently, the mobility of the comb surfactant is also limited and is determined by the structure of the main chain. It is well known that, in comb structures, the flexibility of the molecule is first influenced by the density of side chains grafted onto the back bone [17]. Usually, steric hindrance starts to play a
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Figure 4. 11: General structure of a swollen HASE: in black the core backbone of ethyl acrylate and methacrylic acid, in grey, the side polyethylene glycol chains terminated by a hydrophobic end group
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significant role above 60 % w/w of grafted chains. In a typical HASE, the amount of special monomer varies from 0.5 to 15 % which is well under the limits. The HASE macromolecules are therefore quite flexible and can adopt many conformations. To understand the associative behaviour of HASE in a coating formulation, we can start from a solution of HASE to which the latex is slowly added. The initial organization of the solution will vary between two extremes as shown in Figures 4.12 and 4.13. The first case corresponds to HASE with low molecular weight. These molecules are weakly coiled and carry a small amount of special monomer. These molecules exhibit a certain degree of mobility and, depending on the size of the side chains, will lead to different types of association. Varying the number of EO/PO units and the hydrophobicity of the end groups, achieves a balance between intramolecular and intermolecular self association. This behaviour is not without similarities to the flower-like micelles of HEUR. To push the analogy further, the only commercial Figure 4.12: Self association phenomenon of a HASE pre-neutralized HASE with of low Mw. The shape of the micelles depends on the suitable solids content are degree of branching and on the length of the side precisely of this type because chains. they can be handled with similar equipment to HEUR. As in the case of HEUR, the polydispersity index is a key parameter in explaining the stability imparted to formulations. Because of the free radical emulsion polymerization, the polydispersity of a HASE will vary from 2 to 6. Figure 4. 13: Self association phenomenon of a HASE This high dispersity is true of high Mw and illustration of the swelling of the hydrophobic (grey) domains by a hydrophobic domain not only for the chain length (dark grey striped circle). For clarity, only two chains but also for the composition are entangled but the reality is much more complex. of the chain because of the The hydrophobic domains range from 10 nm to different reactivity of all the several hundre nms when swollen by hydrophobic substance. co-monomers. Therefore, the
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self organization of a HASE is more robust than that of a HEUR, because it relies on diverse compositions which can better adapt when an external hydrophobic component is added to the formulation. The second case concerns highly entangled molecules with higher molecular weight (Figure 4.13). In this case, the molecules build a cage of immobilized water in which hydrophobic end groups will assemble, defining small hydrophobic internal domains. The dimensions of the hydrophobic cages formed by self association are in the range of tens of nanometers which explains why HASE gels are transparent. These domains may be swollen when hydrophobic substances are added to a formulation. The swelling of the micelles is a specific phenomenon that explains the very good stability of organic pigments in formulation. The stability of the organic pigment dispersion in the water phase is driven by enthalpy and is less dependent on the initial mixing conditions. In order to achieve all types of rheology profile with HASE, it is essential to vary the different parameters mentioned previously. Figures 4.14 and 4.15 (page 74) show the effects of varying the type of hydrophobic end group, the molecular weight and the size of the attached chains. If there are many types of rheology profiles, it is difficult to achieve a Newtonian behaviour as pronounced as with HEUR. HASE will always retain a certain degree of shear thinning, and the parameter that allows the tuning of this property is clearly the molecular weight. Changing the hydrophobic monomer will mainly impact the medium and high shear viscosity: the larger the hydrophobic group, the lower the viscosity at high shear rate. Its length will also affect the rheology profile; pseudoplastic behaviour will become more pseudoplastic as the length increases. 4. 4. 1. 3
Interaction with binders
The interaction between binders is based exclusively on the type of self association of each HASE and of course, depending on the interaction with the binder and the pigments, the rheology profile can be seriously modified as seen in the case of HEUR. This implies that a formulator will probably need more than one HASE thickener to manufacture all his paint formulations. This is not satisfactory at a time when the emphasis is on rationalization and cost saving. This weakness can be addressed by identifying molecular structures of HASE which have a very predictable rheology profile, whatever the binder used in the formulation. The choice of a specific hydrophobic agent can be useful in obtaining a series of versatile HASE [18]. The following investigation illustrates this particular phenomenon. In this investigation, it is assumed that the formulator would like to manufacture two types of matt paint with 5 and 15 % binder content, respectively. The aim is to find a thickener that will be suitable for use with as many binders as
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Figure 4.14: Variation of the rheology profile of HASE of similar molecular weight as a function of the hydrophobic groups R1 to R3 respectively corresponding to HASE 1 to HASE 3. The hydrophobes are classified according to R1 > R2 > R3.
Figure 4.15: HASE 4 and HASE 5 differ only in their molecular weight: Mw(HASE 4) > Mw(HASE 5). HASE 6 has the same molecular weight as HASE 4 but the length of the attached chains is longer by 20 %.
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possible in order to get some Table 4.3: List of the characteristics of the HASE leverage on his cost structure. used to investigate the versatility of HASE Characteristics Three binders were selected: HASE of hydrophobic group one ethylene vinyl acetate and HASE 7 linear C12 two styrene acrylic binders with HASE 8 octyl phenol different carboxylic content (the EVA was only used for the 15 % HASE 9 linear C22 formulation). Different HASE HASE 10 nonyl phenol as described in Table 4.3 were HASE 11 C16 – guerbet alcohol used in the investigation. Characteristics such as molecular weight, and length of side chains were approximately the same). The Guerbet alcohol is characterized by formulae of the following type: where R1 and R2 are two aliphatic linear chains. The number of carbon atoms in a Guerbet alcohol is the number of carbon atoms in R1 and R2 plus two.
Figure 4.16: Guerbet alcohol with aliphatic linear chains
Figure 4. 17: Brookfield, Stormer and ICI viscosities of 5 formulations containing respectively 5 % of a styrene acrylic binder SA1 and 0.3 % of HASE, 5 % of a styrene acrylic binder SA2 and 0.3 % of HASE, 15 % of binder SA1, 15 % of binder SA2 and 15 % of an EVA binder, the three latter containing 0.14 % of HASE. The dotted lines represent the desired viscosities (middle lines) and the acceptable limits in each case.
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The paints with 5 % binder content were made with 0.3 % of thickener while the ones with 15 % binder content used 0.14 % of thickener. The rheology goals for all the paints were the following: Brookfield viscosity at 10 rpm about 14000 mPa.s (+/– 25 %); Stormer viscosity around 120 KU (+/– 20 %); cone and plate viscosity around 1.1 (+/– 20 %). The results of this investigation are reported in Figure 4.17. Among all the thickeners, only the HASE 11 with a C16 Guerbet hydrophobic end group would be suitable to match the expected rheology with all kinds of binder. HASE 8, 9 and 10 show higher associative behaviour than HASE 11 when combined with styrene acrylic binder but their response depends greatly on the dispersion content and on the carboxylic group content of the binder. They are very fragile with the EVA binder. The behaviour of HASE 7 depends on the acrylic binder content: it is only efficient for binder SA1 at 5 % and binder SA2 at 15 %. In contrast to HASE 9 and 10, HASE 7 could be used in both formulations containing SA1 and SA2 but it would require a higher dosage to fit the viscosities. HASE 8 appears to be very efficient at low shear, but if the amount were decreased, the high shear viscosities would be much worse. This shows that HASE 8 has strong self associating characteristics, while HASE 11 seems to prefer association with the binders in general. HASE 11 carries a specific chain end which is less self associating because it has 2 side chains with 8 carbon atoms. In contrast to the side chains on the other HASE which have a strong tendency to align and entangle (the alignment tendency is reinforced by the phenol groups in the cases of HASE 8 and 10), the iso C16 alcoholic chains result in a certain level of disorder. This implies that the self association behaviour is governed more by the repulsion of the side chains from the aqueous phase than by the associative forces between the hydrophobic groups. Therefore, the C16 chain ends are able to interact mutually with a similar level of intensity and also with external hydrophobic entities such as dispersions. This explains why the rheology profiles of paints formulated with such thickeners are more predictable whatever the binder used. 4.4.2
Thixotropy and HASE
As mentioned in the introduction, pseudoplasticity is often confused with thixotropy. As far as the author knows, only one product currently on the market (a paint containing “Thixol 53L”) exhibits true thixotropic behaviour (see Figure 4.18). The thixotropy aspect arises from the fact that the viscosity at a given shear takes time to reach a plateau. This time dependency adds a kinetic aspect to the treatment of self association which is only based on thermodynamic analysis.
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Outlook
It would appear that several structural parameters influence the kinetics of network disruption. Tam et al. show that the length of the side chain is of prime importance and that 10 units are the optimum for the PEGs for maximum efficiency [19]. The chemical nature of such a side chain is also of prime importance: it needs to build weak interactions between hydrophobes. Therefore an aliphatic moiety is preferred to an aromatic moiety. To avoid crystallization which would increase associative behaviour, the fatty alcohol needs to be a Guerbet alcohol as explained above [20].
77
Figure 4.18: Rheology profile of a paint containing “Thixol 53L”, the only real thixotropic HASE at present. Starting at rest (point A) the viscosity decreases with shear until point B. When the paint is stirred at constant shear the viscosity drops to C. Decreasing the shear returns it to D, and when shear is removed, the viscosity slowly returns to A.
There are many uses for such a thixotropic product. For instance, when used in a wood stain or spray paint, it offers unique levelling characteristics. When the paint is sheared, its viscosity drops very rapidly, for instance, when the paint flows through a nozzle. After this stress, the paint requires time to thicken again, so that the droplets can spread properly thus allowing very good levelling.
4.5
Outlook
The world of waterborne paints is evolving: we see major consolidations in terms of paint manufacturers as well as their suppliers. The number of players is thus shrinking but, at the same time, the demand for a change in paint formulation has never been so great. Many coatings manufacturers are developing their own dispersions for VOC-free waterborne coatings resulting in a massive requirement for new additives suitable for use with these binders. Only recently has the realization dawned that part of the task in improving coatings consists of eliminating components and simplifying formulations. HEUR and HASE are key elements in this simplification process which is necessary not just in the interests of cost-savings but also of sustainability. The introduction of VOCfree solutions must be followed by reduced manufacturing costs. This can only be achieved if suitable rheological additives are used to optimize the performance of the main constituents of coatings.
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4.6 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17] [18] [19] [20]
Literature Mezger T., The Rehology Handbook (Coating Compendia), Ed. Vincentz Network Gmbh & Co., Hannover, 2006 Shear Stress controlled rheometers. Rheometer Haake, Rheostress RS 15, cone and plate 60 mm, 1° angle Suau J.M., in “Les Latex Synthétiques”, Edited by Daniel JC, Pichot C Editions TEC & Doc, Lavoisier, Paris, 2006, 531 Dehm D.C., Hoy K.L., Hoy R.C., Union Carbide Patent US 4496708, 1985 Lundberg D.J., Glass, J.E., Eley R.R., J Rheol., 35 (6), 1255, 1991 Bergh J.S., Lundberg D.J., Glass J.E., Prog. Org. Cat., 17, 155, 1989 Cota I., Chimentano R., Sueiras J., Medina F., Catalysis communications, 9 (11), 2008, 2090 Glass J.E., Journal of Coatings Technology, vol.13, 913, 2001, 79 Lundberg D.J., Glass, J.E., Eley R.R., Proc. ACS Div. Polym. Mater.: Sci. Engin., 61, 533, 1989; Lundberg D.J., Brown R.G., Glass J.E., Eley R.R., Langmuir, 10(9), 3027, 1994; Kaczmarski J.P., Glass J.E., Macromolecules, 26, 51149, 1993 Jenkins R.D., PhD Thesis, Lehigh University, 1990 Emmons W.D., Stevens T.E., Patent US 4079028, 1978, Rhom and Haas Winnik M.A., Yekta A., Curr. Opin. Colloid Interf. Sci. 1997; 2:424 Alami E., Rawiso M., Isel F., Beinert G., Binana-Limbele W., François J., Hydrophilic Polymers, Edited by Glass J.E., Washington DC: Am. Chem. Soc.; 1996, 343 (ACS Adv. Series); Abrahmsen-Alami S., Alami E., François F., J. Colloid Interface Sci 1996, 179, 20 Xu B., Yekta A., Li L., Masounmi Z., Winnick M.A., Colloid Surf. A 1996, 112 : 239 Suau J.M.; Ruhlmann D., patent n° WO 02/02868, 2007, Coatex Kensicher Y.; Suau J.M., patent n° FR 2902103, 2007, Coatex Borget P., Galmiche L., Le Meins J.F., Lafuma F., Colloids and Surfaces A: Phys. Chem. Eng. Aspects 260 (2005) 173; Guerret O.; Dupont D., Mongoin J., patent n° FR 2907347, 2007, Coatex Suau J.M., Ruhlmann D., Kensicher Y., patent n° WO 06/016035, 2007 Coatex ; Suau J.M., Ruhlmann D., patent n° FR 77336, 2007, Coatex Tam K.C., J. Polym. Sci. (1998), 36, 2275 Egraz J.B., Grondin H., patent n° Fr 269303, 1993, Coatex
Acknowledgements The contributions to this chapter by Dr Denis Ruhlmann, Head of R&D for paints and Coatings and by Jean Marc Suau, Head of R&D for new molecules at Coatex are gratefully acknowledged.
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Mechanism of action
5
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Substrate wetting additives Kirstin Schulz
Wetting is a phenomenon which often causes problems in everyday life. For example, when washing up, pure water does not wet an oily pan making it difficult to clean. However a small droplet of surfactant immediately promotes wetting and oily residues can be removed very easily. To use the same metaphor, the “pan” could be an insufficiently cleaned steel or plastic substrate with a low surface energy which is to be coated by a waterborne coating. Such coatings often present difficulties in substrate wetting mainly because of the high surface tension of waterborne coatings compared to that of solventbornes. This weakness of waterborne coatings can be overcome by using substrate wetting additives. The reduction of surface tension enables the waterborne coating to wet the substrate completely. In Chapter 5.1.3 the mode of action and application of substrate wetting additives are discussed.
5.1
Mechanism of action
5.1.1
Water as a solvent
Because of restrictions on VOCs, there is increasing interest in the use of water as a universal solvent. However there are big differences between water and conventional solvents. Water has a dipole moment and is a comparatively small molecule with very strong attractive forces between the molecules. This leads to a very high surface tension. One result is the high boiling point of water. Water is also a very good solvent for many solids, liquids and gases. 5.1.2
Surface tension
Surface tension is a phenomenon at the interface between a liquid and a gas. The surface tension is the interfacial tension of a solid or a liquid against a gas which is often air. The two phase system liquid/air forms a shared interface area in which intermolecular forces are active. Whether the interfacial area is large or small depends mainly on the Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Figure 5. 1: Wetting behaviour of a waterborne paint on PTFE without (left)/ with (right) substrate wetting additive
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surface tension of the liquid. The surface tension is defined as force in the surface per length and has the unit mN/m, (see Equation 5.1). Equation 5.1:
Energy Area
Force Length
Surface work is the work which must be invested to create or increase the surface area under isothermal conditions. Directly after creating the surface area this value, known as the dynamic surface tension, is different from the value when the system is in thermodynamic equilibrium which is known as the static surface tension [1]. 5.1.3
Reason of the surface tension
In a bulk liquid a molecule is homogeneously surrounded by neighbouring molecules and attractive forces inside the liquid are balanced. A molecule in the interface area has neighbouring molecules only on one side and this means that the attracting forces are not balanced with the resulting force directed inwards into the liquid. The directional force pulls molecules into the liquid. The liquid tries to minimize the surface area and the spherical shape of the liquid enables the greatest number of molecules to be within the liquid and to present the smallest interfacial area to the gas. This geometric shape is that of the lowest energy configuration of the system. To enlarge the surface area of the liquid requires molecules from inside the liquid to move to the interfacial area and, for this, energy must be put into the system. The higher the surface tension of the liquid the higher the energy needed [1]. Water has a very high surface tension compared with other solvents and this high surface tension is the reason for the often poor substrate wetting properties of waterborne coatings.
Figure 5.2: Relation of forces inside the liquid and in the interface area to gas
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Usually the enlargement of the surface area occurs while applying a coating, e.g. spraying creates many small droplets. As the surface tension increases, more energy is needed to create a certain surface area. The forces cannot be varied sufficiently to achieve this, because in practice the coating is applied by a brush or spray gun. Some increase in pressure is possible but it is not adequate to create the surface area. As a result the surface tension of a waterborne coating has to be reduced to achieve good application properties. 5.1.4
Effect of the high surface tension of water
The most obvious effect of the high surface tension of water is insufficient substrate wetting on various, mostly low energy, substrates. For optimum adhesion and protection complete wetting of the substrate by the coating is an important prerequisite.
Wetting is the formation of a closed interface area between a liquid and a solid. In terms of coating, substrate wetting is differentiated from pigment wetting and stabilisation. In this chapter only substrate wetting is discussed.
The free specific surface energy is made up of polar and non polar components [1]. Wetting is based on the interaction between the polar and non polar contributions to the free surface energies of the liquid and the solid, which together form a shared interface area. In general, systems try to adopt the configuration of lowest energy. Consequently, in two-phase substrate/liquid systems, a droplet on a substrate only spreads until the interacting forces are in equilibrium. This results in the liquid droplet forming a contact angle on a substrate. The contact angle is characteristic for a particular liquid on a particular substrate. The relationship between the surface energies is given by Young’s equation, see Equation 5.2. Equation 5.2: σsubstrate = σsubstrate/liquid + σliquid · cosα
Substrate wetting depends on the surface tension of the wetting liquid σliquid, the surface tension of the substrate σSubstrate and the interfacial tension σsubstrate /liquid between the liquid and solid. The larger the contact angle, the worse the wetting. In extreme cases with contact angle >120°, water beading occurs (see Chapter 12 “Hydrophobing agents”).
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Figure 5.3: Contact angle of different liquids with different surface tension on a substrate
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Table 5.1: Surface tension of different coatings components Liquid
Surface tension in mN/m
water
73
alkyd resin
33 to 60
butylglycols
30
isopropanols
22
n-octane hexamethylsiloxane
21 16
Table 5. 2: Surface energies of different substrates Substrate
Surface energy in mN/m
PTFE (“Teflon”)
20
PE
31
PS
40
aluminium
40
PMMA
46
steel
50
The smaller the contact angle the better is the wetting behaviour. From the equation, the following rules for wetting can be deduced: • Substrate with high surface energy can be wetted relatively easily.
• Liquid with low surface tension shows generally good wetting behaviour.
In terms of paint this means that the surface tension of the coating must be less than the surface energy of the substrate for the paint to wet successfully and therein lies the difficulty for ordinary coatings when water is used as a solvent. The surface energies in Table 5.1 show that water with a value 73 mN/m has by far the highest surface tension [2].
For water as a universal solvent expected to wet all kinds of substrates, the rules given above are not satisfied and wetting of aqueous coatings on low energy substrates such as plastic is a challenge for the coatings formulator. There are two possible ways of remedying the imbalance and achieving successful wetting on a given substrate. Either the surface tension of the substrate must be increased or the surface tension of the water phase must be decreased. Cleaning metal substrates leads to an increase in surface tension by removing grease and oil present from the production process. The same applies to corona treatment or flaming of plastic surfaces. This produces a high-energy surface by oxidation. These extra steps in coating are very time-consuming and costly. A good alternative for achieving good substrate wetting on low energy substrates is to reduce the surface tension of the waterborne coating. For the most difficult substrates, e.g. polyethylene or propylene, a combination of both procedures is necessary to ensure good wetting and sufficient adhesion of the coatings. 5.1.5
Substrate wetting additives are surfactants
Surfactants are bifunctional compounds with at least one hydrophobic and one hydrophilic group in the molecule. Because of their structure, surfactants orient themselves at every interface. With water as a solvent, the hydrophobic part of the
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substrate wetting additive is pushed out of the aqueous phase, while the hydrophilic part has a high degree of compatibility with the medium. The incompatibility of at least one part of the wetting additive in the Figure 5. 4: Minimised force F , resulting from 2 medium is the force driving sur- adding substrate wetting additives factants to orientate at the interface. Many surfactants are used in detergents, cosmetics and coatings. 5.1.6
Mode of action of substrate wetting additives
Substrate wetting additives accumulate in the aqueous phase at the liquid/air interface, with the non polar part towards the air and the polar part towards the liquid. This enrichment affects the surface tension of the aqueous phase. In pure water, all the water molecules at the water/air interface are surrounded by water molecules in the liquid phase. Water, as a dipole has strong interacting forces, resulting in a high surface tension. The addition of substrate wetting additives to water causes a change in the force balance of molecules interacting in the surface area. The substrate wetting additives displace the water molecules in the interface liquid/air and the interaction forces between the substrate wetting additive and water are much smaller than those between water molecules alone (Figure 5.4). The force diagram no longer represents interaction between water molecules alone, but includes those between surfactant and water molecules. Within this interface area the surfactant molecules reduce the force directed into the aqueous solution resulting in significant reduction of surface tension of the aqueous phase. Consequently, addition of a substrate wetting additive allows a waterborne paint to wet substrates with a low surface tension. This creates a closed film, even at low film thicknesses, and the paint is less liable to cratering (see Chapter 5.6.4.3). 5.1.7
Further general properties of substrate wetting additives/ side effects
The best-known side effect of substrate wetting additives is the relatively strong tendency to promote foam. If a mechanical process such as pumping, pouring or stirring causes air to be incorporated into the system, the substrate wetting additives will move to the newly formed air-bubble/paint interface and stabilise the air bubbles. Small bubbles will remain in the system and can lead to popping and pinholes. Large air bubbles will rise and form stable foam on the surface of the coating (see Chapter 3 “Defoamer” and Chapter 7 “Deaerator”). In general, very hydrophilic, water-compatible substrate wetting additives have the greatest
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tendency to stabilise foam. Another characteristic is that the cured coating remains to a greater or lesser degree hydrophilic and this can affect resistance to moisture.
5.2
Chemical structure of substrate wetting additives
5.2.1
Basic properties of substrate wetting additives
Substrate wetting additives are characterised by the molecule having both hydrophobic and hydrophilic groups. These can be chemically very different. The hydrophobic part can be a hydrocarbon chain varying from 8 to 22 carbon atoms. In powerful substrate wetting additives this hydrophobic part is a siloxane-chain or perfluoridised hydrocarbon chain. The hydrophilic part can be either a negatively or positively charged or neutral head group. This molecular structure with one hydrophobic and one hydrophilic group is termed amphiphilic and leads to the basic properties of substrate wetting additives. They orientate themselves at every interface and form micelles at higher concentrations in the liquid phase. They are classified by their hydrophilic head groups as, e.g., ionic, non ionic and amphoteric surfactants. The classic soaps and also the sulfosuccinates, which are important in paints, are anionic surfactants. Nonylphenylethoxylates are non-ionic surfactants. In coatings, polyether siloxanes, fluoro surfactants, alkoxylates, allylphenylethoxylates, sulfosuccinates and Gemini surfactants are used. 5.2.2
Chemical structure of substrate wetting additives important in coatings
5.2.2.1
Polyether siloxanes
Polyether siloxanes consist of a short siloxane chain and a polyether modification. The linear siloxane chain is hydrophobic and the polyether modification is the polar part of the molecule. In the coatings industry the term “silicone additives” is often used and many formulators associate this with cratering caused by silicone oil so that the use of this product group is sometimes avoided, even though polyether siloxanes are markedly different from silicone oil. Silicones consist of several siloxane units. The silicon atoms are bonded to 1 to 4 oxygen atoms and when the number of oxygens is not sufficient to achieve the electron shell octet of the silicon; this is completed by bonding with organic groups. The siloxane unit is formed by having each oxygen atom as a bridging component between two silicon atoms. One siloxane unit can have one to four more substituents, depending on the number of oxygen atoms with unsatisfied free valencies. Thus siloxane units can be mono-, di-, tri- or tetra-functional. Polydime-
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Chemical structure of substrate wetting additives
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thyl siloxanes (silicone oils) are hydrophobic, non-polar molecules, which are not water miscible. They have a very low surface tension around 22 mN/m and intermolecular forces are so weak that they are classified by viscosity. Over a very large range the viscosity increases linearly with the molecular weight of the silicone oil but Figure 5. 5: Schematic structure of a is almost independent of temperature. These polyether siloxane basic features make silicone oils excellent wetting agents and lubricants. The disadvantage in coatings is the tendency to cratering which rises exponentially with increasing molecular weight. This is good for hammer effects but must be strongly avoided in ordinary coatings. Therefore only modified short siloxane chains are used as substrate wetting additives. Modification is the key factor in excluding the tendency to cratering. The structure of a polyethermodified siloxane, used as substrate wetting additive is shown in Figure 5. 5. This structure retains the positive features of the silicone oil (low surface tension, high interface activity) but additionally improves compatibility in waterborne coatings thus removing the risk of cratering. The products are more or less foam stabilising depending on the polyether. The higher the relative Si content in the molecule, the more surface active is the substrate wetting additive. Substrate wetting additives have up to seven siloxane units; polyether siloxanes with longer siloxane chains are classified as flow and levelling additives (see Chapter 8 “Flow additives”). Substrate wetting additives based on polyether siloxanes are especially suitable for reducing the static surface tension, resulting in improvements in substrate wetting, atomisation in spray application, crater tendency and flow of waterborne coatings. 5.2.2.2
Gemini surfactants
This is the name for ionic and non-ionic surfactants which have at least two hydrophobic and two hydrophilic groups in the molecule. Gemini surfactants are characterized by unusually high interfacial activity. In addition to very good substrate wetting some products show very low foaming tendency. 5.2.2.3
Fluoro surfactants
Perfluoro surfactants are generally called fluorosurfactants. Perfluoro organic compounds have all the hydrogen atoms attached to carbon replaced by fluorine atoms. The hydrophobic part of a fluorosurfactant is such a chain. The hydrophilic part can be an ethoxylate chain or a carboxylate group. Fluorosurfactants are generally
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characterized by high polarity, high thermal and chemical stability and high resistance to UV radiation. They repel dirt, oil, fat and water. Fluorosurfactants (PFT) can be divided into the following groups: • fluoromodified alkysulfonates,
• fluoromodified carboxylic acids and • fluorotelomer alcohols.
Structure of a carboxylate modified fluorosurfactant: Rf-CH2CH2~CH2CH2COO-Li+, Rf = F(CF2CF2)3–8 The fluorocarbon chain (– CF2CF2–) is the reason for the stronger reduction in surface tension compared to that of other chemical classes. The fluorocarbon causes very strong orientation at the interface, an effect which can be traced back to the weak affinity of fluorocarbon to water and the weak interaction between fluorocarbon chains themselves. These relatively expensive additives are powerful wetting agents and so can be used for wetting surfaces with the lowest surface energy. Although fluorosurfactants improve the wetting behaviour, they can be difficult to overcoat because the dried film is itself difficult to wet. The fluorosurfactant orientates in such a way that a fluorocarbon-like surface is formed. The products are used where other substrate wetting additives would be chemically decomposed by, e.g., oxidising agents, acids or alkalis. However fluorosurfactants have limited compatibility and a strong tendency to promote foam [4]. Discussions about toxicity are based on their frequent occurrence worldwide in oceans, lakes and human and animal tissue and blood. Ecological data are available in technical data sheets. 5.2.2.4
Acetylenediols and modifications
The best known substrate wetting additive in the coatings area is probably acetylenediol. It combines wetting and defoaming properties in one molecule and often also promotes flow. Because of its low molecular weight it is very effective in the reduction of dynamic surface tension. It is characterised by good compatibility, easy handling and good moisture resistance. Polyether modifications increase the water solubility and ease of biological degradation [5]. It shows an average reduction of static surface tension. 5.2.2.5
Sulfosuccinate
In practice these are divided into sulfosuccinic acid diesters and sulfosuccinic acid half esters. Sulfosuccinic acid esters with less than 8 carbon atoms per ester group
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are water soluble. Sulfosuccinates are very powerful surfactants which are used as wetting or emulsifying agents. They usually have a strong tendency to foam [6]. 5.2.2.6
Alkoxylated fatty alcohols
Table 5.3: HLB-value HLB-value defoamer
1.5 to 3.0
W/O-emulsions
3.0 to 8.0
wetting additive
7.0 to 9.0
O/W-emulsions
8.0 to 18.0
Alkoxylated fatty alcohols are non-ionic detergents 13.0 to 15.0 surfactants, made by reacting ethylene co-solvents 12.0 to 18.0 oxide, propylene oxide or butylene oxide with long-chain primary fatty alcohols. Alkoxylates based on fatty alcohols show inverse solubility behaviour, the solubility in water decreasing with increasing temperature. They show high cleaning and dispersing ability and the products, which are low foam, biodegrade easily. They do not tend to stabilize foam and show average reduction in dynamic and static surface tension. The HLB-value is a measure of water and oil solubility mainly for non ionic surfactants and the stability of emulsions. HLB is an abbreviation of hydrophilic/ lipophilic balance. The HLB value of a mixture of surfactants or emulsifiers can be calculated in an additive manner from those of its components. This method cannot be used for polypropylenglycol ethers, polyethersiloxanes or anionic surfactants. The values range from 1 to 20. Substances with low HLB values are generally good water in oil emulsifiers, while hydrophobic surfactants with higher HLB values are effective as oil in water emulsifiers [1]. 5.2.2.7
Alkylphenol ethoxylates (APEO)
Chemical constitution of alkylphenol ethoxylates:
These are made by addition of alkylene oxides (usually ethylene oxide) to alkylphenols. The most important are octyl- and nonyl- und tributyl-phenolpolyglycol ether. With an increasing length of ethylene oxide chain, alkylphenolpolyglycol ethers become water soluble but, for a given chain length, the solubility in water decreases with increasing temperature.
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The water soluble alkylphenol polyglycolethers are good wetting and emulsifying additives. The ecological data can be found in the technical data sheets [1].
5.3
Application of substrate wetting additives
5.3.1
Basic properties of various chemical classes
There is frequent criticism about the wetting behaviour of waterborne coatings during development and optimisation of formulations. Usually an additive which is easily available in the lab is added to the formulation and tested. In most cases wetting is not improved sufficiently and additional undesired side effects are created. Further additives are tested often without properly considering the basic properties of the different groups of substrate wetting additives. This procedure is sometimes time consuming and expensive and the final result is still not satisfactory from the point of view of the formulator. Each additive class has typical properties but not all are desirable. It is very import before starting the testing procedure to decide on the most important criteria, so that the most suitable chemical class can be chosen. The investment in time and energy is dramatically reduced if the important factors underlying poor wetting are analysed in advance. The most important chemical classes used in coatings are described above. To clarify this, the following table shows the typical basic properties of the different substrate wetting classes. 5.3.2
Reduction of static surface tension
Generally the fluorosurfactants show the strongest reduction in static surface tension (see Chapter 5.6 “Testing methods”) in waterborne coatings. This property stems from the chemical structure, as the fluoro modification, because of its lipophilic character, shows very little tendency to interact with water. PolyetherTable 5. 4: Properties of different substrate wetting additives Additive
Reduction of static surface tension
Reduction of dynamic surface tension
Foam tendency
Price
sulfo-succinate
medium to good
good
strong
low
alcohol-alkoxylate
few
very good
low
medium
polyether-modified siloxane
good
medium
medium
medium to high
fluorosurfactant
very good
few
very strong
high
acetylenediol and derivatives
medium to good
very good
low
low
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siloxanes constitute the second most efficient chemical group with regard to reduction of static surface tension: the siloxane part of the molecule is lipophilic. The ability of polyethersiloxanes to spread over the surface in aqueous solutions is excellent and this can also be traced back to the structure of the molecule. One small droplet placed on a substrate can easily spread out to an area with a diameter of 15 cm, but this only occurs in pure aqueous solutions. 5.3.3
Possible foam stabilisation
The stronger the reduction of static surface tension in a waterborne coating, the greater is the tendency of the substrate wetting additive to stabilize foam. On one hand they stabilise the incorporated air because the substrate wetting additives move directly to newly formed surface areas and, on the other hand, the lower surface tension causes a smaller pressure difference between the air bubble and the surrounding medium (see Chapter7 “Deaerators”). The foaming tendency of polyethersiloxanes can be reduced by the right polyether modification. A product which is only just water-soluble is made and because of its limited compatibility in water it stabilizes less foam but it is still effective in the reduction of static surface tension. Such freedom to modify does not exist with fluorosurfactants, as they need a very polar modification to be compatible in waterborne coatings without any deleterious effects. To minimize the side effects of substrate wetting additives, the amount used should be as small as possible. 5.3.4
Effective reduction in static surface tension versus flow
The formulator should always bear in mind how much the surface tension has to be reduced, because it is not advisable to immediately use, say, a fluorosurfactant for a small reduction in surface tension. The choice of chemical class should be predicated on the degree of difficulty in achieving the desired wetting. Sulfosuccinates and alkoxylates are not the most efficient reducers of static surface tension but they can be sufficient to overcome minor substrate wetting problems. A moderate reduction in surface tension also improves the final levelling. 5.3.5
Reduction of dynamic surface tension
The sulfosuccinates and alkoxylates are more effective in reducing dynamic surface tension (see Chapter 5.6 “Testing methods”) than polyethersiloxanes or fluorosurfactants. The mobility of the molecule is one key factor for efficient dynamic substrate wetting additives and the mobility correlates directly with the molecular weight. As soon as a new surface is created the small, more mobile molecules orientate much faster to it. This property is very important where fast wetting processes occur, as in the printing industry. Further important factors are tendency to foam and price of the product. A foam promoting substrate wetting additive will
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necessitate choosing a more powerful defoamer with the inherently increased risk of surface defects. No substrate wetting additive is very poor or very good – it should be appropriate for the specific problem. 5.3.6
Which property correlates with which practical application?
A formulator looks for a substrate wetting additive for various reasons. The most common defects which can be cured by substrate wetting additives are: • tendency to cratering, • defoamer craters, • insufficient substrate wetting on surfaces which have not been not perfectly cleaned, • difficulties in substrate wetting on plastics, • contamination by spray droplet • atomisation while spraying and • optimisation of flow. 5. 3. 6.1
Craters
The issue of craters is strongly related to the static surface tension of the coating. If metal substrates are not completely cleaned, oil or grease may remain on the surface from the production process. Craters can also occur if particles and fibres in the air fall into the freshly applied coating. Also carefully chosen defoamer can create craters in sensitive coatings. The basic reason for cratering is the relatively high surface tension of the applied coating against contamination such as fibres, particles or defoamer droplets. If the contaminant comes into contact with the wet paint, the coating material shrinks away from it and the crater is formed. The coating tries to minimise the contact area with the disturbing factor and this can lead to complete dewetting. Thus only substrate wetting additives which are very effective in reducing static surface tension, i.e., fluorosurfactants and polyethersiloxanes, are helpful against
Figure 5. 6: Contamination crater – coating has a higher surface tension than the contaminant
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Figure 5.7: Crater in a clear coat
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craters. The strongest reduction in surface tension is achieved with fluorosurfactants and the second most effective class is the polyethersiloxanes. Both efficiently reduce static surface tension so that wetting and surrounding the fibre or particle by the coating is possible. 5. 3. 6.2
Wetting and atomisation of spray coatings
In spray application the surface area of the coating is enormously enlarged by the formation of many small spray droplets. This process is significantly improved by substrate wetting additives. The smaller the droplets the thinner is the film thickness at which a closed film is formed. One can picture this by imagining the stacking of large or small spheres. Substrate wetting additives enable waterborne coatings to be applied in thin layers without any difficulties. Here additives which reduce the static surface tension are the most helpful. The painting process is often assumed to be very dynamic but in practice the reduction of static surface tension is necessary to achieve good spraying characteristics and allow difficult substrates to be successfully wetted. 5. 3. 6.3
Rewettability, reprintability, recoatability
Up to this point only the coating layer in which the additive is incorporated has been discussed but substrate wetting additives can affect the properties of subsequent coating layers. The use of highly effective polyethersiloxanes or fluorosurfactants can reduce the surface tension of the dried film. If this coating is a primer onto which a top coat is to be applied than recoatability should be tested. The fact that highly surface active substrate wetting additives can impair wetting of the following coating layer must be considered. The higher the molecular weight of the substrate wetting additive or the higher the proportion of low energy mol-
Figure 5.8: Loss of adhesion caused by insufficient dissolving power of the following layer
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ecules, the more difficult it is for additives in the next layer to initiate dissolution. However dissolution of the additives from the dried first coating layer by the following layer is a key requirement for good substrate wetting characteristics of the following layer. This effect becomes increasingly important as the coating becomes more cross linked. In extreme cases not only is wetting poor but adhesion can be completely lost. 5. 3. 6.4
Figure 5.9: Smooth flow versus orange peel
5. 3. 6.5
Flow
Basically substrate wetting is a precondition for good flow. The compatibility of the substrate wetting additives is the main influence on gloss and flow of coatings. A small reduction in static surface tension is also very helpful in improving flow (see Chapter 8 “Flow additives”).
Spray mist uptake
The uptake of spray droplets is mainly influenced by two parameters: the ability of the coating to absorb the droplets and the surface tension of the partially dried coating surface. Both properties are influenced to a greater or lesser degree by substrate wetting additives. On one hand they interact with the evaporating water thus influencing drying characteristics and on the other hand they even out the differences in surface tension between fresh spray droplets and the partially dried coating surface, thus the spray droplets do not create craters in the dried coatings surface. Alkoxylated fatty alcohols and derivatives of acetylenediols are well known for extending open time so that absorption of spray droplets into the dried coating is improved. If the open time is increased, sagging must be tested. If, however, the spray creates craters, only additives such as polyethersiloxanes or fluorosurfactants which reduce static surface tension effectively, can help.
5.4
Use of substrate wetting additives in different market sectors
The use of substrate wetting additives is largely dependent on the requirements of individual market sectors. For example, expensive additives are seldom used in the price sensitive market sector. In printing inks, additives are used which do not stabilise foam and reduce the dynamic surface tension. Low foaming tendency is very important because during the printing process air must not be stabilised even though is incorporated by the
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Tips und tricks
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fast running presses. The substrate wetting additives must also move as quickly as possible to the newly formed interface areas. This is only achieved by small, mobile molecules such as acetylenediols and alkoxylated fatty alcohols. Sometimes sulfosuccinates are used but this also necessitates a very efficient defoamer. Increasingly powerful substrate wetting additives are being used in wood coatings because of the variable quality of wooden substrates. Furthermore they allow the amount of co-solvent to be decreased almost to zero. Hydrophobic constituents in the wood can only be wetted successfully by coatings with appropriate surface tension. Sometimes polyethersiloxanes are used in addition to avoid craters caused by defoamers as coatings are often applied in ever increasing film thickness in a single spray pass. In waterborne dipping coatings it is very important that the substrate wetting additives are highly compatible in the coating system as, depending on the deposition mechanism, the additives can become more concentrated in the dipping bath. Furthermore, the dipping bath is in use for very extended periods and any risk of incompatibility must be excluded. This is why silicon- and fluorine-free additives are often used. The additives also must not stabilise any foam inside the bath. Foam can easily be created when the workpieces are dipped in and lifted out and the wrong choice of additive can result in foam stabilisation. In waterborne automotive and plastics coatings polyethersiloxanes are often used. They result in finely atomised spray coatings and a closed film at very low film thicknesses. They also reduce the tendency of the coatings to crater. Generally the use of additives is determined by their properties and price. All the substrate wetting additives are useful but it is up to the formulator to choose the most appropriate. The more precisely the formulator knows in advance the requirements and properties which need to be improved, the less time will have to be spent in the search for an effective substrate wetting additive.
5.5
Tips und tricks
5.5.1
Successful use of substrate wetting additives in coatings
The correct use of substrate wetting additives is decisive for the success of a waterborne coating. Many formulators are of the opinion that only a coating without any additives is a good coating. This has elements of truth in it: only as much additive should be added as necessary. Over dosage leads to foam stabilisation and undesired side effects with other surface active additives such as wetting and dispersing additives or emulsifiers. Storage stability can also be impaired by over
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dosage. It is not true that more is better! A basic formulation should be structured as follows: • • • • • • • • •
water or resin defoamer wetting and dispersing additive pigment/filler resin thickener if necessary resin/water substrate wetting additive small amount of water to adjust viscosity
The order is very important. It can be seen that substrate wetting additives should be added at the lowest possible viscosity and as late as possible in the formulation. The reason that components should be added in the order shown above is that the tendency to foam is significantly reduced. If substrate wetting additives are added too early they will stabilise foam during the manufacturing process. It is particularly important that no substrate wetting additive should be in the formulation before adding pigments because substrate wetting agents immediately orientate towards every new interface including the pigment surface. However, the pigment surface should be stabilised completely by wetting and dispersing additives. Substrate wetting additives do not have any pigment stabilising effects but promote the wetting process. Although they initially cover the pigment surface, as soon a new interface in the coating occurs the substrate wetting additives will migrate towards the new interface. As a result the pigment surface is not stabilised and flocculation of the pigments can result. Generally it is helpful to select the appropriate defoamer first and then the right substrate wetting additive. The reason for choosing the substrate wetting additive must also be considered: What needs to be improved? Such basic considerations are essential for choosing the right chemical class to obtain this desired improvement. By following the processing rules above, deleterious side effects can be dramatically minimized. 5.5.2
Figure 5.10: Poor and good flop – effect of substrate wetting additives
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Metallic shades
Especially with high gloss metallic coatings, the formulator often finds that, despite many attempts to optimise the formulation, certain L-values cannot be
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reached or the flip-flop cannot be adjusted correctly. Here the choice of substrate wetting additive can have a decisive influence. It is advisable to check the effect of all additives which may have an influence on the orientation of effect pigments. Often silicone-free additives produce more brilliant effects and the colour is significantly easier to adjust.
5.6
Test methods for measuring surface tension
5.6.1
Static surface tension
The best known method for measuring surface tension of liquids is the ring method of Lecompte du Noüy. This method measures the static surface tension. A platinum-iridium ring is placed in the liquid so that the ring is completely covered by the liquid. The ring is slowly withdrawn and a lamella forms below the ring. This lamella is an enlargement of the surface area of the liquid and requires energy for its production. The force is measured while the ring is withdrawn Figure 5.11 Measuring of static surface from the liquid and the maximum force tension by forming a lamella which is needed is a direct measure of the surface tension of the liquid. The measured force corresponds to the energy needed to enlarge the surface of the liquid in the form of a lamella [2]. This method is suitable for low viscosity and non pigmented solutions, dispersions and clear coats. Solid ingredients may lead to incorrect readings because they can cause early rupture of the lamella which will lead to incorrect values of the static surface tension. However measurement of non-pigmented binder dispersions is sufficient to give sufficient information about the surface tension of the final coatings. 5.6.2
Dynamic surface tension
The second procedure for measuring dynamic surface tension is the bubble pressure measure method which is only suitable for low viscosity and non pigmented solutions. The bubble pressure tensiometer is of simple construction. A very thin capillary is placed in the liquid. Using a connected air supply with a pressure gauge, an air bubble is produced in the liquid at the end of the capillary. The force which is needed to form the bubble is measured by monitoring the pressure and this correlates with the dynamic surface tension.
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In this method the newly formed air-bubble/liquid interface area is the enlargement of surface area for which force is needed. The dynamic aspect of this test method is introduced by varying the frequency of bubble formation. This usually starts at one bubble per second and is increased up to ten bubbles per second. At the high bubble rate the air/ liquid interface is formed very quickly. To reduce the dynamic surface tension efficiently, substrate wetting additives have to orientate very quickly to the newly formed interface area and this bubble method allows the mobility of substrate wetting additives to be evaluated [2]. Figure 5.12: Measurement of dynamic surface tension, formation of bubble
5.6.3
Dynamic versus static
The ring method is used to measure static surface tension. The enlargement of surface area occurs very slowly in a thermodynamic sense so that the liquid is in thermodynamic equilibrium. Because of their higher molecular weight, substrate wetting additives need comparatively more time than water molecules to orientate. Thus the new surface area is formed very quickly (in milliseconds) as in a bubble pressure tensiometer and the dynamic surface tension will be a more appropriate measurement of surface tension during the orientation of substrate wetting additives. The system is not in thermodynamic equilibrium when measuring dynamic surface tension. The mobility of the molecules is crucial for achieving thermodynamic equilibrium. The surface tension of a solution with substrate wetting additives is a function of the time. The initially high surface tension decreases very rapidly and has already reached thermodynamic equilibrium after about 100 milliseconds [3]. 5.6.4
Further practical test methods
5. 6. 4.1
Wedge spray application
In spray application, atomisation of the coating material, i.e., the formation of very small droplets, is a decisive factor for producing very thin films. The smaller the droplets, the thinner is the film thickness at which a closed film is achieved. Use of effective substrate wetting additives allows the demands of the coatings industry for ever thinner films to be achieved. One important factor besides viscosity, flow rate and pressure, which influences the atomisation (formation of a very large surface area) is the surface tension. The formation of the large surface area can be
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evaluated very successfully using wedge spray application (applying an increasing film thickness). Before using this method it is essential that critical conditions are chosen. Usually these are low pressure, medium flow rate and slightly increased viscosity. The control should show very poor wetting behaviour so that improvement achieved by a substrate wetting additive can easily be observed. It is important that the same application conditions are rigidly observed and this can be achieved by using a spray robot. The film thickness is measured at which a closed film is formed, that is, the limit of wetting is measured. If panels are always placed in the same position relative to the spray gun/Esta Bell application equipment and coated in the same way the position of the wetting limit on the panel this can also give information about wetting behaviour.
Figure 5.13: Principle of wedge spray application
Figure 5. 14: Poor and optimised wetting limit determined by wedge spray application
If this wetting limit is higher vertically and has the same film thickness then the position of the wetting limit indicates improved substrate wetting. The visual impression of the droplets can also be used for evaluation. If the application conditions are chosen to be as unfavourable as possible for atomisation and the droplets are comparatively small or the test sample is very homogeneously covered it indicates good substrate wetting behaviour. Looking at all the criteria together indicates which is the most efficient substrate wetting additive. 5. 6. 4. 2
One spray path
This method also requires selection of critical application parameters. Furthermore, a substrate must be chosen which will easily show up differences in substrate wetting behaviour. A quick spray pass is performed over the substrate. After drying, the width of the spray dust/spray droplets on the substrate provides information about the substrate wetting behaviour. The more homogeneous the visual impression of the applied paint droplets, the better substrate wetting usually is.
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Substrate wetting additives
Crater test
Before performing this test the component, e.g. a defoamer or an insufficiently cleaned substrate, causing the problem should be identified. This is because the results of crater testing are very specific. If an additive is effective against grease or oil on steel substrates it may not be of any use against craters which result from spray droplets or fibres falling into the freshly applied coating. To obtain reproducible results a substrate which is able to absorb the contaminating solution should be chosen. This can be a primer but not a non-absorbing glass plate, because on the glass plate the contamination droplet moves in a very undefined manner over the substrate until the solvent has completely evaporated. The results obtained using glass are thus unreliable. For testing, appropriate solutions of the contaminant are made (e.g. 0.01 % 10,000 Si-oil in xylene). Known amounts of solutions of various concentrations are placed at defined locations on the appropriate substrate. The solvent must then be allowed to evaporate and the crater-prone coating is applied in the normal way. The control should show large craters where the contaminants were placed. Additives are then added to the crater-prone coating which is then applied in an identical manner on the contaminants. If the diameter of the crater is now reduced this indicates that the additive has made the coating material less crater prone. 5. 6. 4.4
Draw down
A simple but meaningful draw down test is often helpful in identifying components in the coating material which tend to promote cratering. For example, an efficient but incompatible defoamer creates visible craters. Either a more compatible defoamer can be chosen or, if the hydrophobic defoamer is needed because of specific application conditions which strongly incorporate air, the surface defects can be cured by substrate wetting additives. The reduction of static surface tension or the emulsifying effect of some alkoxylates can help cure surface defects. 5. 6. 4.5 Spray drop uptake
Figure 5.15: Poor and optimised substrate wetting indicated by draw down
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The incorporation of spray droplets into the freshly applied coating is tested as follows. A first spray pass is performed. After drying at room temperature for the same time as used in practical application,
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a second thin spray pass is carried out. It can then be seen how well the spray droplets are incorporated into the first coating layer. With all test methods, pigmenting the coating helps to make small differences visible against the background and thus facilitates evaluation. 5.6.5
Analytical test methods
In general, substrate wetting additives can be identified by the reduction of surface tension. Every product shows a typical behaviour in the reduction of static or dynamic surface tension in aqueous solutions. The most important task in the Figure 5.16: Spray mist analysis of surfactants is their determination as raw materials and in formulations and, by trace analysis, in waste water, seas or lakes or as a result of biological degradation. Nowadays HPLC or ion chromatography, mass spectroscopy, thin layer chromatography and capillary electrophoresis are used to identify surfactants. Careful preparation and enrichment of the sample is necessary especially for trace analysis. 5.7 [1] [2] [3] [4] [5] [6]
Literature RÖMPP Online Version 3.2 tego journal, 3rd Edition 2007 www. kruss. de, Theorie zur Blasedruckmethode Technical Information, Zonyl fluortenside, DuPont, Zonyl-FSBR-d-0598 Surfynol 400 Series Surfactants, Air Products and Chemicals 2004 Morell, Samuel P, Coatings Technology Handbook, Surfactants for Waterborne Coatings Applications, Marcel Dekker, Inc.
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6
Improving performance with co-binders
Improving performance with co-binders Dr. Patrick Glöckner
The rest of this book deals with different types of additives for waterborne coatings and printing inks. These products are used in small amounts of between 0.01 and 3 % of the total formulation to improve certain properties of the coating or the printing ink such as substrate wetting or flow and levelling. In contrast to additives, co-binders are used in higher levels of up to 30 % of the total formulation to achieve the required property profile of coatings and printing inks. Because of the large amounts used, co-binders actually influence several properties. By using co-binders it is possible to adjust drying behaviour, hardness, flexibility, blocking resistance, gloss, haptical properties (soft feel) and adhesion to the most diverse substrates. Co-binders are used in formulations for all market segments starting from simple DIY coatings up to high quality industrial and automotive OEM coatings. Often waterborne main binders are combined to obtain the required property profile so that it is not easy to distinguish between main and co-binders. Therefore, the difference between co-binders and main binders is not static and largely a matter of definition. By definition co-binders are not able to form films of high mechanical strength because of their tack or brittleness and thus cannot be used as main binders. Apart from typical co-binders which are used to modify coating and ink properties there are many pigment paste resins with excellent pigment wetting, dispersing and stabilisation properties. Those resins are also often called grinding resins. Co-binders for waterborne formulations range from simple aqueous solutions of polyethers or polyesters, through emulsions of polyurethanes or polyacrylics to dispersions of polyesters or polyurethanes. The manufacture of the products will be discussed in the following section and practical examples of their use for improving coating properties and in pigment concentrates will be given. Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Preparation of co-binders
Typical co-binders are based on dispersions of hydrophilic polyesters, polyurethanes and polyacrylics. Primary and secondary dispersions are differentiated from each other by the type of process by which they are produced. In the case of primary dispersions the monomeric building blocks are emulsified in water and then polymerised in the aqueous phase. A typical example is the emulsion polymerisation of monomeric acrylates to obtain polyacrylate-dispersions [1]. In contrast, secondary dispersions are obtained by first polymerising the monomeric building blocks in a melt or in a solution of organic solvents and then transferring the polymer obtained into the aqueous phase by the addition of water. This type of manufacturing process is used if the monomers cannot be polymerised in the presence of water because they would react with it e.g., in the case of polyurethanes from polyisocyanates, or if a certain property profile can only be achieved in this way, e.g., low molecular weight distribution of polyacrylates. Most of the co-binders available are secondary dispersions which will therefore be the focus of the following discussion. 6.1.1
Secondary dispersions
In order to transfer a more or less hydrophobic polymer to the aqueous phase emulsifiers that impart “water compatibility” are necessary. The emulsifier can be part of the polymer (internal hydrophilisation) or added physically (external hydrophilisation), see Figure 6.1, page 102. The advantage of internal hydrophilisation is that the emulsifiers are covalently bonded to the polymer and cannot then diffuse out of the coating and impair interlayer adhesion to following layers. Moreover, only a very small amount of internal emulsifier is required which may be beneficial for mechanical, chemical and water resistance of the resulting coating film. Emulsifiers can also be classified as ionic or non-ionic. Non-ionic emulsifiers such as polyethers result in final coating films which are permanently moisture-sensitive and subject to polymer degradation. Their advantage lies in the absence of volatile components which would otherwise have to be included in the VOC. Ionic emulsifiers are preferred for high quality applications. As shown in Figure 6.2 ionic emulsifiers are divided into anionic and cationic types. The agents used to neutralise the “potentially hydrophilic” ionic groups of the ionic emulsifiers can 1) In physics, a dispersion is the generic term for two phases, one of which is distributed, but not dissolved, in the other. An emulsion is a mixture of two liquids that are not soluble in each other (e.g., an insoluble oligomer of low Tg in water). The coatings industry uses these terms to describe the film obtained after evaporation of water. Emulsions result in tacky films, whereas dispersions produce tack-free films prior to curing [2].
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Figure 6. 1: Differentiation of emulsifiers for secondary dispersions
be volatile and this results in coatings having a high resistance to water and corrosion. Examples are amines such as dimethylethanol amine or carboxylic acids such as formic acid. The disadvantage is that highly volatile neutralizing agents contribute to the VOC. In contrast, neutralizing agents such as sodium hydroxide do not contribute to the VOC but result in lower water resistance since they remain in the resulting film. Secondary dispersions are obtained by first synthesising the polymer in melt or in solution in an organic solvent. After neutralization the polymer is transferred to the aqueous phase. This transfer can be difficult as the viscosity levels during the process vary greatly (Figure 6.2). In area A of Figure 6.2 the ionomeric polymer is in solution or in melt. The viscosity depends on the ability of the polymer chains to interact with each other by e.g., entanglements, intermolecular hydrogen bonds, or association of the salt moieties. The longer the chain the stronger the entanglements. Thus the viscosity depends strongly on the molecular weight of the polymers.
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103
Figure 6.2: Schematic explanation of the change in viscosity during phase transfer
R1, R2 = aliphatic, cycloaliphatic, aromatic
Figure 6.3: Synthesis of a linear polyester
After adding small amounts of water to the ionomeric polymer the ionic centres are hydrated. The viscosity decreases slightly (B). On adding further amounts of water a water-in-oil emulsion is obtained. The polymer chains are increasingly immobilized, resulting in a very strong increase in viscosity (C). The maximum is achieved immediately before the phase inversion since the mobility of the polymer chains is greatly restricted (D). After this maximum is exceeded only small amounts of water are necessary to lower the viscosity strongly. The polymer chains are unable to form entanglements since they are no longer in contact with each other. The viscosity becomes independent of the molecular weight of the polymers (E). 6.1.1.1
Polyester dispersions
Polyesters are obtained from the esterification and condensation of polyfunctional carboxylic acids and polyols (Figure 6.3). Properties of the polyesters such as reactivity, mechanical properties and chemical resistance can be varied simply by
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Figure 6.4: Transfer of a carboxy functional polyester into the aqueous phase
the choice of the monomers, the molecular weight and the degree of branching of the resulting polyester, as well as the nature of the functional groups. In order to prepare a polyester dispersion, the polyester itself has to contain some carboxylic groups. The carboxylic groups can either be at the end of the polymer chain or, by using tri- or polyfunctional carboxylic acids, close to the polymer backbone. After neutralisation the resin can be transferred to the aqueous phase (Figure 6.4). 6.1.1.2
Polyurethane dispersions
Another important type of polymer dispersion for coatings and printing inks is based on polyurethanes. The basic products are obtained by a polyaddition reaction of polyisocyanates and polyols. Here dimethylol propionic acid (DMPA) is often used for internal hydrophilisation (Figure 6.5). The properties of the final dispersion are influenced strongly by the choice of reactants. By using aliphatic di- and polyisocyanates and aliphatic polyesters, polycarbonate or polyacrylates, products with very high weathering resistance and excellent mechanical properties can be obtained. Figure 6.6 shows the different steps in the synthesis of polyurethane dispersions based on isophorone diisocyanate (IPDI) which is frequently used in high quality polyurethanes. First dimethylol propionic acid (DMPA) is reacted with the isocyanate. This isocyanate functional adduct is then reacted with the polyol components to obtain an
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105
Figure 6.5: Typical monomers for manufacturing polyurethane dispersions
Figure 6. 6: Preparation of polyurethane dispersions based on isophorone diisocyanate (IPDI) and dimethylol propionic acid (DMPA); U = urethane group, HO^^^^OH = saturated polymeric diol [2]
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isocyanate functional prepolymer (Step 1, Figure 6.6). The acid groups are then neutralised and the corresponding polyelectrolyte dispersed in water (Step 2). Chain extension is carried out by the addition of a diamine (Step 3). This last step is an elegant way of avoiding high viscosities associated with phase inversion (cf. Figure 6.2, see page 103) yet obtaining resins with a high molecular weight. Since high molecular weights of the binders are beneficial for mechanical and resistance properties the last step is important especially for high quality binders. Moreover the organic solvent content necessary for processing can be reduced to a minimum.
6.2
Applications of co-binders
6.2.1
Co-binders for better property profiles
Since the number of co-binders for adjusting properties of waterborne coatings and printing inks is considerable and main binders are often combined to achieve the required property profile of the final formulation, only the effects of a few product types will be discussed in this section. Usually co-binders are used if most of the required properties of the formulation are already adequate but some small adjustments are necessary. In general, co-binders have to be as compatible as possible with the different classes of binders and not impair properties such as surface tension or foaming characteristics as this would necessitate the use of further additives. 6.2.1.1
Drying time
During the film forming process, water present between the particles of the dispersions is “pressed” out of the film resulting in a continuous coating film [1]. The higher the minimum film forming temperature (MFT) and thus the glass transition temperature (Tg) of the resins, the faster the water is pressed out of the remaining film. A high Tg results in a high hardness and this usually adversely affects the flexibility. The use of special co-binders with hyperbranched structures and high glass transition temperatures enable hardness and drying speed to be increased significantly without greatly decreasing the flexibility. Figure 6.7 shows the drying speed of a waterborne wood coating (Table 6.1) as a function of the concentration of cobinder added. The co-binder used was an aqueous polyurethane dispersion with a Tg of approx. 120 °C. The higher the amount of co-binder present the higher is the drying speed. At the same time the hardness is increased while the flexibility remains at a similar level up to a concentration of 20 % co-binder.
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Applications of co-binders
Figure 6.7: Reduction of drying time and increase of pendulum hardness of a waterborne wood coating based on a fatty acid modified PUD as a function of the concentration of co-binder (main binder replaced by co-binder solid to solid (w/w)) Table 6.1: Waterborne wood coating based on a fatty acid modified PUD Component
Amount %
mill base fatty acid modified PUD
32.4
defoamer
0.9
substrate wetting additive
0.4
matting agent
2.4
micronised wax
0.6
dryers (25 % Co 6, 25 % Zn 8, 50 % Water)
2.4
let down fatty acid modified PUD dipropylene glycol n-butylether water
33.0 1.2 23.6
PU-thickener (1:1 in Water)
1.6
PU-thickener
1.5
co-binder (replacement of main binder)
0 to 30 % 100.0
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Table 6.2: Parameters influencing the adhesion of waterborne coatings and printing inks Parameter
Suggested action
substrate pretreatment
• cleaning of the substrate (dust, oily substances, etc.) • pretreatment of metals, e.g., phosphating pretreatment of non-polar plastics, e.g., flame or corona treatment
application conditions
• formulation has to be adjusted to the climatic conditions (temperature, humidity)
substrate wetting
• surface tension of the formulation must be lower than that of the substrate • use a substrate wetting agent if required (see Chapter 2).
adsorption between film and substrate
• increase the number and strength of contact points to the substrate: by using binders with hydrogen-bonding groups (urethane, OH, etc.) • in the case of metals: acid groups (phosphates, carboxylic acids) lead to salt formation • better orientation of functional groups of the co-binder due to low Tg • substrate pretreatment often required
diffusion into (absorbent) substrates
increase penetration by using a low-viscosity formulation body, flow and levelling and gloss can however be impaired.
film forming
the minimum film forming temperature of the binder must be chosen so that a closed film is formed on application use coalescent agents if necessary
adjusted flexibility
flexibility of the coating has to be adjusted to the substrate inflexible, hard film on a flexible substrate will not have optimal adhesion.
6.2.1.2
Adhesion
The adhesion of a coating or a printing ink to a particular substrate is influenced by many different parameters. Some of the most important are summarised in Table 6.2 [2]. Many of the commercially available main binders are tailor-made for certain substrates. This is the reason why it is often not possible to coat different kinds of substrates with just one formulation. Today there is a range of commercially available co-binders which optimise the adhesion of coatings or printing inks to different plastic substrates. They are often used in amounts of up to 20 % solid resin based on the main binder to improve poor adhesion of a coating (GT 5) to excellent (GT 0). The substrates can vary significantly, e.g., polycarbonate (PC), polymethyl methacrylate (PMMA), polyvinylchloride (PVC), polystyrene or noryl (PPO/PS).
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Applications of co-binders
To give an example, the primer shown in Table 6.3 has good adhesion to polyamide, PMMA, PVC and noryl but adhesion to ABS and polystyrene is very poor (GT 5). The adhesion can be improved to excellent (GT 0) by using 5 to 10 % of a special polyester resin dispersion or 15 to 20 % of a polyurethane dis persion.
The investigated co-binders had markedly different Tg values (polyester approx. 30 °C and polyurethane polyol approx. 120 °C. This allows the formulator to adjust other coating properties such as hardness, flexibility, scratch and blocking resistance easily.
In the case of metal substrates, excellent adhesion is a prerequisite for good corrosion protection. Pretreated metals such as galvanized steel, but also aluminium, are a particular challenge for formulators. Figure 6.8 (page 110) shows the degree of delamination (corrosion creep) of oxidatively drying acrylic coatings on steel after approx. 850 h exposure to salt spray as a function of the concentration of co-binder. The formulation is shown in Table 6.4. As can be seen from the diagram, 5 % of the co-binder (replacement of solid main binder) is sufficient to obtain a coating with excellent corrosion protection properties.
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Table 6.3: Plastic primer (non pigmented) based on an acrylic dispersion Component
Amount %
acrylic dispersion of low Tg
70.5
defoamer
0.5
substrate wetting additive
0.5
thickener (10 % in water, neutralised with AMP 90)
16.5
water
6.0
butyl glycol
3.0
coalescent agent
3.0
co-binder (replacement of main binder)1,2) total
2.5 to 20.0 100.0
1) waterborne, solvent-free dispersion of a polyurethane polyol, Tg ~ 120 °C 2) waterborne, solvent-free dispersion of a polyester, Tg ~ 30 °C
Table 6. 4: Oxidatively drying acrylic coating for metal Component
amount %
oxidatively drying acrylic dispersion
55.32
dryer
0.27
defoamer
0.18
silica
0.18
corrosion protection pigment
9.71
microtalc
9.71
iron oxide red
9.62
filler
4.67
water
9.16
2-butoxy ethanol
0.82
substrate wetting agent
0.18
thickener
0.18
co-binder (replacement of main binder)1,2) total
2.5 to 20.0 100.0
1) waterborne, solvent-free dispersion of a polyurethane polyol, Tg ~ 120 °C 2) waterborne, solvent-free dispersion of a polyester, Tg ~ 30 °C
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Figure 6. 8: Corrosion creep of an oxidatively drying acrylic coating on steel after approx. 850 h salt spraying as a function of the concentration of co-binder (main binder replaced to co-binder solid to solid (w/w) Table 6. 5: Waterborne metal coating based on an alkyd dispersion Content mill base water
Amount %
thickener
0.55
defoamer
0.05
wetting and dispersing additive
1.25
7.60
substrate wetting additive
0.50
titanium dioxide
24.00
water
5.00
let down alkyd dispersion
53.00
thickener
1.00
thickener
2.90
dryer
0.45
propylene glycol
1.25
water
2.45
co-binder (replacement of main binder)1,2) total
2.5 to 20.0 100.0
1) waterborne, solvent-free dispersion of a polyurethane polyol, Tg ~ 120 °C 2) waterborne, solvent-free dispersion of a polyester, Tg ~ 30 °C
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6.2.1.3
Hardness-flexibility balance
The higher the hardness of a coating film the better are properties such as blocking and scratch resistance. The hardness is significantly influenced by the choice of the main components such as binders, crosslinking agents, and pigments as well as by the crosslink density. The composition of the formulation is usually fixed by the property profile and thus the scope for variation is very limited. Increase of the hardness is limited because flexibility also has to be considered so it is up to the formulator to figure out the best compromise. An efficient tool here is the use of hyperbranched co-binders which
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Applications of co-binders
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Figure 6.9: Pendulum hardness and Erichsen cupping as a function of the concentration of co-binder (main binder replaced by co-binder solid to solid (w/w))
allow the hardness to be increased within certain limits without greatly altering the flexibility as illustrated in Figure 6.9 which shows the performance of a waterborne alkyd-dispersion based metal coating. The formulation is given in Table 6.5. It can be clearly seen that the replacement of alkyd dispersion by the co-binder results in a significant increase in pendulum hardness from 20 to 90 while the flexibility (Erichsen cupping) remains at roughly 9.5 mm. 6.2.1. 4
Gloss
In order to obtain glossy coating films, the components of the formula must be mutually compatible [3]. Co-binders are usually widely compatible and thus act within certain limits as compatibilisers. The gloss of pigmented systems is also significantly influenced by pigment wetting and stabilisation (see Chapter 2). Since many co-binders additionally function as grinding resins they also improve the gloss of pigmented formulations (see Chapter 6.2.2). Gloss also depends on the surface texture, which can be improved by flow and levelling additives (see Chapter 8) and also by co-binders. Figure 6.10 (page 112) shows that the gloss of a 1-pack acrylic coating is increased by adding a cobinder 2. 2) waterborne, solvent-free dispersion of a polyurethane polyol, Tg ~ 120 °C
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Figure 6. 10: Increase in gloss of a 1-pack acrylic coating by adding a co-binder (main binder replaced by co-binder solid to solid (w/w))
6.2.2
Co-binders for pigment pastes
Pigment paste resins (grinding resins) are traditionally used in solventborne coatings. The four main reasons are: • • • •
lower price, higher solids content, greater hardness, little effect on other coatings properties.
It is very probable that the use of solventborne binder-free pigment concentrates will not become established for bulk applications. In contrast, binder-free pigment concentrates nowadays represent the state of the art in waterborne formulations, particularly in view of their universal compatibility (see Chapter 2). This technology is not limited solely to architectural coatings but is also used in printing inks, industrial coatings and wood coatings. Nevertheless, grinding resins are also used in waterborne coatings and printing inks. It is then relatively easy to obtain outstanding mechanical properties, such as hardness-flexibility balance, and excellent chemical or water resistance and also reduced drying times. Apart from many in-house products, a range of grinding resins is commercially available for pigment concentrates as well as direct grinds. These products contain functional groups with affinity for pigments and thus improve the interaction
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Applications of co-binders
113
Figure 6.11: Comparison of gloss, haze, and pendulum hardness of binder-free and resin containing pigment concentrates
between resin and pigment surface. Hence the use of grinding resins in combination with small amounts of pigment wetting and dispersing additives results in excellent pigment stabilisation and colour strength development even at high pigment loadings. (see Chapter 2). Figure 6.11 shows a comparison of binder-free and resin-containing pigment concentrates based on a phthalo blue pigment (PB 15:4). The let down was the same in both cases. Colour strength, gloss and haze-gloss are very similar. The main difference is in the much higher hardness of the resin-containing pigment concentrate. Guide formulations for some waterborne pigment concentrates are shown in Table 6.6 (page 114). After let down with compatible binders, it is possible to obtain high quality coatings. In order to emphasize this, the blue pigment concentrate (pigment PB 15 : 4) was applied after let down as mass tone on a glass plate. After drying the panel was attached to a lamp to demonstrate the excellent transparency (Figure 6.12).
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Figure 6.12: Mass tone (4 % pigment) on glass (150 µm wet film thickness); the image shows the dried paint on a lamp
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Table 6.6: Guide formulations for waterborne pigment concentrate PBk 7
PR 101
PW 6
water
34.2
20.4
11.7
30.7
grinding resin1
33.4
10.0
16.7
33.9
wetting and dispersing additive
8.4
8.0
5.3
4.3
defoamer
1.0
1.0
1.0
1.0
AMP 90
0.3
-
-
-
silica
0.6
0.5
0.2
-
Pigment type
PB 15:4
biocide
0.1
0.1
0.
0.1
pigment
22.0
60.0
65.0
30.0
100.0
100.0
100.0
100.0
total
1) waterborne, solvent-free dispersion of a polyurethane polyol, Tg ~ 120 °C
6.3
Summary
In contrast to additives, co-binders are used in higher amounts of up to 30 % of the total formulation. The formulator can conveniently adjust properties of coatings and printing inks simply by the use of co-binders. In the present chapter the preparation of co-binders as well as their use in different applications has been discussed. Examples of properties that can be influenced are: • • • • • •
drying speed adhesion and intercoat adhesion corrosion protection gloss hardness/flexibility balance blocking and scratch resistance.
Special co-binders are used as grinding resins to wet and stabilise different kinds of pigments. As well as high colour strength, gloss and transparency, very low viscosities can be achieved. At the same time the use of grinding resins results in better drying speeds and better mechanical properties. 6.4 [1] [2] [3]
Literature Thomas Brock, Michael Groteklaes, Peter Mischke, European Coatings Handbook, Vincentz Network, Hannover, Germany, 2000 Patrick Glöckner et al., Radiation Curing, Vincentz Network, Hannover, Germany, 2008 Artur Goldschmidt, Hans Joachim Streitberger, Basics of Coatings Technology, 2nd edition, Vincentz Network, Hannover, Germany, 2007
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Deaerators
7
115
Deaerators Heike Semmler
Deaerators are an essential means of combating foam in coatings. There are two different types of foam, macrofoam and microfoam, both of which often occur together in waterborne formulations. Because it is quite difficult to distinguish between the two types, the problem is generally just termed “foam”. Furthermore magnifying glasses or microscopes are frequently necessary to detect that small bubbles are entrapped in the coating film: the so-called microfoam. In principle it is possible to differentiate between macro- and microfoam: • macrofoam is located on or in the coating surface whereas • microfoam is located in the coating film.
Both types can be distinguished in the liquid and in the dried or cross linked coating film. As a rule macrofoam is made up of air bubbles surrounded by a duplex film with two liquid/air interfaces. In contrast, microfoam bubbles can be considered as air inclusions, characterized by a single liquid/air interface
Many coating problems, which do not at first sight appear directly associated with microfoam, such as turbidity or a reduction in gloss of a coating film can, in fact, be caused by microfoam. This is of course undesirable in, for example, high gloss formulations. Premature corrosion can be promoted by microfoam because the effective film thickness is reduced. Furthermore, during drying of the coating film, microfoam bubbles can be transformed into pinholes which are small channels reaching down to the metallic substrate. The protective function of the coating is impaired resulting in rapid corrosion.
Micro foam bubbles are often not immediately apparent and in most cases they appear during application of the coating so that this poses a particular challenge to formulators. Deaerators and some defoamers provide formulators with an effective means of preventing or eliminating microfoam in coatings or printing inks without restricting the choice of raw material or method of application. Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Figure 7. 1: Microfoam and macrofoamSource: Tego Journal 2007
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Deaerators
1 minute after application
5 minutes after application
10 minutes after application
15 minutes after application
Figure 7.2: Dissolution of microfoam bubbles as a function of time. Microscopic image of an airless-applied waterborne formulation on glass, 300 µm wet, 1, 5, 10, 15 minutes after application.
7.1
Mode of action of deaerators
7.1.1
Dissolution of microfoam
The problem of microfoam is a frequent occurrence, especially in airless/airmix applied coatings. If an airless-applied waterborne coating is observed through a microscope during the drying phase the microfoam bubbles can be seen changing, see Figure 7. 2.
Initially a mixture of microfoam bubbles of various sizes is recognizable. As drying proceeds, relatively large microfoam bubbles slowly increase in size while small microfoam bubbles shrink significantly until they are no longer detectable.
The driving force behind the shrinkage of small micro bubbles is the Laplacepressure. This is given by the Young-Laplace equation which describes the relationship between the internal pressure of the microfoam bubble and the external pressure of the medium surrounding the microfoam bubble. Pin= Paus + 2σ/r where Pin = internal air bubble pres sure Paus = external air bubble pres sure σ = surface tension r = radius of the air bubble The internal pressure is greater than the external pressure because the effect of the surface tension σ has to be overcome.
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Figure 7. 3: Young-Laplace equation
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Mode of action of deaerators
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The ratio of surface tension and radius of the air bubble 2σ/r increases as the size of the air bubble decreases. Consequently the internal pressure Pin will also rise (Figure 7. 3). If the radius of the microfoam bubble decreases further, the internal air bubble pressure will rise significantly compared to the external pressure. This difference in pressure will lead to a diffusion of the air in the microfoam bubble into the surrounding media. The diffusion and with that shrinkage of the microfoam bubble will accelerate as the microfoam bubble becomes smaller. Small microfoam bubbles thus literally dissolve. The phenomena that gases can be solubilised in liquids is described by Henry’s law. Henry’s law states, that the solubility of a gas α in a liquid L is proportional to the partial pressure pα above the solution. For small and medium pressures (p ≤ 5 bar) the mole fraction xα of dissolved gas is given by: xα = pα / Hα,L (T) where Hα, L is Henry’s constant [2, 3] In 1950 P. S. Epstein and M.S. Plesset described mathematically the dissolution of small gas bubbles in liquids [5]. A spray technique patented by Rohm and Haas makes use of the fact that the solubility of CO2-gas in water is up to 52 times higher than that of air. CO2 gas is thus used instead of air for spray application. CO2 filled microfoam bubbles dissolve sufficiently fast, permitting the application of microfoam-free films [6]. However, solubility decreases as the drying/crosslinking of the coating film progresses, because there is less liquid; eventually just a solid paint film remains. It is also conceivable that the air in small microfoam bubbles could diffuse into larger microfoam bubbles which have a significantly lower internal pressure. Larger microfoam bubbles can thus grow further and rise to the liquid surface as long as the coating viscosity does not increase excessively as drying/crosslinking progresses. If these bubbles were in or on the coating film surface they would be described as macrofoam bubbles [4]. Finally the dissolution of microfoam bubbles is basically a physical process. It should be emphasized that this does not only apply to waterborne coatings and airless/airmix applied coatings: small microfoam bubbles also dissolve in solventborne formulations irrespective of the method of application.
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118 7.1.2
Deaerators
Rise of microfoam bubbles in the coating film
As described in Chapter 7.1.1 small microfoam bubbles literally dissolve. Larger microfoam bubbles, however, rise very slowly to the coating surface 1 where they form macrofoam bubbles provided the air bubble is stabilised by a surfactant double layer (duplex film). If no surfactant or amphiphilic substance are present, the air bubble bursts on the coating film surface (see Chapter 3.2.1.1 “Defoamer”). The rise of the microfoam bubble is dependent on its radius r and the coating viscosity η. The relationship of these two parameters is described by Stokes’ law and is immediately apparent from the simplified version 2: v~ r² / η
where v = rising velocity of the foam bubble r = radius of the foam bubble η = viscosity of the coating In its correct form, Stokes’ law describes the resistance Fw of a moving sphere in, for example, a laminar flowing liquid: Fw = 6 π ηvr
where v = velocity of the moving sphere r = radius of the sphere
The velocity v of e.g. particles falling in a liquid can be calculated if the densities ρ1 of the particles and of the liquid are known. The force arising from the weight of a spherical particle is equal to the resistance force Fw so that: Figure 7.4: Velocity of rise as a function of coating viscosity and size of air bubble
v = 2 (ρ1 – ρ 2) gr2/ 9 η
[1]
Stokes’ law can be applied to air bubbles as they are spherical. Furthermore it can be assumed that the same forces operate whether spherical particles are rising or falling and that Stokes’ law can be applied to rising air bubbles/microfoam bubbles, see Figure 7.5. 1) This statement is based on the assumption that the applied coating film is oriented upwards horizontally during drying/ cross linking. However it is still not clear how microfoam bubbles behave or move if the film is oriented vertically or even horizontally facing downwards (e.g. during the application of window frame coatings) [9]
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Mode of action of deaerators
If the coating viscosity η is relatively low, the velocity of rise of air bubbles/microfoam bubbles is, according to Stokes’ law, relatively high. During the rise of larger microfoam bubbles in an applied coating film to the surface, drying/crosslinking of the film proceeds further. The coating viscosity increases and consequently air bubbles/microfoam bubbles persist in the coating film, resulting in the well known problems.
119
Figure 7.5: Velocity of rise as a function of the radius of the air bubble at constant viscosity
Stokes’ law indicates a further important relationship. Since the velocity of rise is dependent on the square of the radius of the bubble, it is affected far more by the size of the bubble than by the viscosity of the coating. Figure 7. 4 shows the velocity of rise of a foam bubble at constant viscosity as a function of its radius, based on a calculation according to Stokes’ law. The velocity of rise of small foam bubbles is very low, even if the bubbles are increasing in size. Only when the bubbles are really large does the velocity increase exponentially. 7.1.3
How to prevent microfoam in coating films
There are several possible methods for preventing microfoam in waterborne coatings. • Increase coating drying time or open time of the coating film Small microfoam bubbles have enough time to dissolve or larger bubbles can rise • Optimise application parameters Particularly relevant for spray or airless/airmix application: increasing the spray distance (substrate to spray gun), use of spray nozzles with small opening and low spray pressure can reduce microfoam.
• Low coating viscosity or Newtonian flow behaviour Microfoam bubbles rise quickly to the surface of the liquid and do not remain in the coating film. • Create large foam bubbles Due to their size they rise quite quickly to the coating film surface • Reduce the amount of air incorporated during coating manufacture and application, especially on porous substrates (wood, stone, etc.). This avoids incorporating large amounts from air right from the start
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Deaerators
• Use low foaming raw materials Avoid surfactant structures able to stabilise microfoam bubbles Some of the possibilities mentioned are not applicable or are not practical. Often the foam problem is just displaced (e.g. macrofoam is created on the coating surface). Many are simply not achievable because coating properties, application methods or substrates are predetermined. The coating formulator has little freedom. In contrast, addition of additives to the coating or printing ink formulation is the simplest way of preventing and effectively eliminating microfoam. In general these coating additives are called defoamers or deaerators. The term “deaerators” re-emphasises the effectiveness of such products against microfoam. Of course some defoamers are also effective against microfoam. For simplification, the term “deaerators” is used to describe both defoamers and deaerators in the following. Ideally, by selecting the right deaerator or combination (e.g. different deaerators or deaerator and defoamer) the coating formulator does not have to accept any compromises on raw materials, production process, application parameters or substrates. Often deaerators or combinations are the only way of achieving a satisfactory finish. 7.1.4
How deaerators combat microfoam
In contrast to the mode of action of defoamers against macrofoam, that of deaerators in eliminating microfoam bubbles is still not fully understood. This may be because macrofoams have been of interest to the industry for decades. The problems of microfoam have, however, been investigated more intensively by the coatings and raw materials industry only over the last 15 years possibly because waterborne and high solid coatings, which are more affected, have become more important. Airless and airmix spray application methods which are used extensively on cost grounds, are particularly affected by microfoam problems. Nowadays there is also increasing emphasis on cutting application steps and production time. Higher coating film thicknesses have to be applied in a single step and faster drying times achieved. Both cause an increase in microfoam. 7. 1. 4. 1
Deaerators promote the dissolution or formation of small microfoam bubbles
As described in Chapter 7.1.1, the dissolution of microfoam bubbles is just a physical phenomenon, which occurs irrespective of the coating system or application method.
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The question thus arises of the role played by deaerators in eliminating microfoam bubbles. Figures 7. 6 and 7.7 are two micrographs of a waterborne coating taken immediately after airless application (substrate glass, 300 μm wet) while the coating film was still wet. The sample in 7.7 contains a deaerator; while the sample in Figure 7. 6 does not. A significant difference between the two images is immediately apparent. While the coating sample without deaerator (Figure 7. 6) contains a mixture of numerous large and small microfoam bubbles, only a few, quite small microfoam bubbles are present in the treated sample (Figure 7.7). The images show that effective deaerators Figures 7. 6 (above) and 7. 7: Airlessprevent the formation of large microfoam applied waterborne, high build coatbubbles immediately or during applica- ing on glass directly after application of the coating and only few very small tion observed through a microscope, Figure 7. 6 without deaerator; Figure microfoam bubbles occur. The latter dis- 7. 7 with deaerator solve rapidly due to the high La Place internal pressure (see Chapter 7.1.1). In the ideal case, after drying or cross linking, the coating film is free from microfoam bubbles. (Figure 7. 9). 7. 1. 4. 2
How deaerators promote the dissolution of microfoam bubbles
Up to now, there is no generally accepted theory of how deaerators act against microfoam. However the following points are discussed in the literature.
Figures 7. 8 (left) and 7. 9: Airless-applied waterborne, high build coating on glass after complete drying observed through a microscope; Figure 7. 8 without deaerator, Figure 7.9 with deaerator. Figure 7.9 shows complete dissolution of microfoam bubbles
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a) The currently known active substances used as deaerators are partially insoluble in the medium to be deaerated. Consequently they orientate themselves strongly at the liquid/air interfaces, i.e., the microfoam bubble. It is assumed, that the active ingredients displace foam-stabilising surfactants and facilitate the diffusion of air into the medium surrounding the microfoam bubble [4, 7]. b) According to C. Bell, microfoam bubbles can occur, if droplets of a liquid splash into the liquid media and thus entrap air. Small droplets of the liquid thus form small air bubbles while larger droplets create large air bubbles [8]. Bell’s explanation was also applied to airless applied coatings and it was shown that effective deaerators are able to reduce the coating droplet size during application. Smaller coating droplets thus splash into the liquid coating film and only a few, very small, microfoam bubbles are generated. They dissolve correspondingly faster [7]. c) Both the surface tension and the rheological profile of a coating formulation can be influenced by deaerators. Although there is, as yet, no proof, it can be assumed with a high degree of certainty that there is a relationship between the two properties and the effectiveness of defoamer/deaerators against microfoam [9].
7. 2
Chemical composition of deaerators
Deaerators based on organically modified polysiloxanes have proved effective against microfoam. In most cases these polysiloxanes are also combined with small amounts ( 0 ≤ 90°), sagging of the film may occur due to the effect of gravity.
The rate of sagging can be calculated using equations 8.2 to 8.4. Equations 8.2 to 8. 4:
where vx = rate of sagging ρ = density h = film thickness dx = change in film thickness η = viscosity
The rate of sagging is therefore proportional to the square of the coating thickness and inversely proportional to the viscosity. The sagging volume can be calculated from Equations 8.5 to 8.8. Equations 8.5 to 8.8:
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Mode of action
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The flow of the coating surface and the sagging of the coating on non-horizontal surfaces are thus mutually conflicting properties when considered from a rheological standpoint. Although a low viscosity improves flow, this leads to sagging on non-horizontal surfaces. Therefore rheological additives are incorporated in the coating material to ensure good flow and consequently low viscosity during and shortly after application while a little later creating a higher viscosity in order to reduce sagging. Products and test methods for rheological additives are discussed in Chapter 4. 8.1.3
Total film flow
Flow and the rate of sagging are mainly influenced by the coating thickness as well as by the viscosity: the coating thickness (h) occurs squared or cubed in Equations 8.4 and 8.8, respectively. The relationship between the applied film thickness (h) and the viscosity (η) (the Total film flow (ψ∞)) is described by Equation 8.9, [3] Equation 8.9:
where h(t) is the average applied film thickness and η(t) the viscosity at the time of measurement (t). It follows that optimal flow is achieved by higher applied film thicknesses and lower viscosity. This formula is valid for a waterborne coating with Newtonian flow characteristics and no solvent content [3]. The “Total film flow” (h3/η) is the film-fluidity as a function of time (t) and corresponds therefore to the area under the fluidity curve. This means that at a certain point after application the “fluidity” of a waterborne coating is very high. During evaporation of the solvent (water) the viscosity increases rapidly (with a simultaneous reduction in
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Figure 8.3: Total film flow as a function of time (schematic)
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applied film thickness) , leading to a smaller and smaller “Total film flow” [3]. Corresponding model-calculations can be found in the literature [4]. 8.1.4
Mode of action in waterborne systems with co-solvents
Film forming aids are frequently used in waterborne coatings, or co-solvents which are more volatile than water are incorporated in the binders during manufacture. This results in viscosity- and surface-tension gradients during film formation which are not taken into account in the equation postulated by Orchard (Equation 8.1). This is why the formula is only valid for solvent-free waterborne coatings and only then if the coating thickness (h) and the amplitude (a) are smaller than the wave length (λ). In cases where there are larger amounts of organic solvents in the coating material, the total film flow becomes a function of the 4th power of the film thickness, because volatilisation of solvent and simultaneous lowering of film thickness result in the viscosity increasing in inverse proportion to the film thickness [3]. The surface tension gradient in particular can either increase or decrease the flow process, even when coatings are applied to horizontal substrates. Overdiep et al. [6] expanded Orchard’s theory and included the surface tension gradients in their mathematical models. These models can describe very accurately the flow (as Total film flow), but assume that the development of surface tension gradients during the drying process is known. Equation 8.10:
δ γ· δ h
‹›
= surface tension = shear rate = interfacial thickness = average coating thickness
8.1.5
Mode of action in an example of a thermosetting waterborne system with co-solvents
Thys and Bosma have experimentally confirmed Overdiep’s mathematical model. A 2-component enamel based on polyacrylate-polyols (with a co-solvent) with isocyanate as a crosslinker was applied to a glass panel. The applied film had an amplitude of a = 4 µm and a wave length of λ = 4 mm which met the require-
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ments postulated by Orchard. After application the film was conditioned at 21 °C and 55 % relative humidity. The glass panel was affixed to a measuring device at a specified angle. Changes in amplitude height were observed by light reflection and photographed and used to determine flow and sagging. The changes were calculated from the photos using specially developed software.
Figure 8.4: Relative amplitude as a function of drying time, source: Thys and Bosma, Resins Nuplex
In Figure 8.4 it is apparent that the relative amplitude initially decreases and then increases for a short period before decreasing again to reach its final level at 1200 s. The sagging characteristic can be calculated from the Total film flow using Equation 8.9). Figure 8.5 shows the continu- Figure 8.5: Decrease in fluidity as a function of drying ous reduction in fluidity as a time, source: Thys and Bosma, Nuplex Resins result of the viscosity increasing as the solvent volatilises. In the subsequent drying process (thermal hardening) the fluidity increases because of the higher temperature and the Total film flow thus also increases until the time at which re-orientation of the binder molecules occurs, and then approaches zero as cross-linking takes place. This means that the flow of waterborne coatings is definitely disrupted at the time when a discontinuous development of the surface tension gradients occurs. This phenomenon can be explained by a phase inversion during film formation. Prior to this phase inversion, the Total film flow corresponds to that of a waterborne system. Subsequently, it behaves like that of a solvent-based system. Figure 8.6 (page 134), shows that the experimentally determined curve of the Total film flow corresponds with the calculated curve thus confirming Overdiep’s math-
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Figure 8.6: Total film flow and amplitude as a function of the surface tension gradients, source: Thys and Bosma, Nuplex Resins
ematical model. These experiments demonstrate that the surface tension and the appearance of surface tension gradients strongly influence Total film flow. This is confirmed by the work of Kojima et. al [8]. 8.1.6
Surface tension gradients
The causes of surface tension gradients are as diverse as their effect on the flow and texture of the applied coating films. One cause is, as already mentioned, the vaporisation of solvents used in waterborne coatings. This leads, on the one hand to a change in the Total film flow and, on the other, to turbulent flow in the coating film. The coating flows from areas of low surface tension to areas of higher surface tension and can give rise to an uneven profile (orange peel effect) .
Figure 8.7: Orange peel effect
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Besides the differences in surface tension occurring in waterborne coatings, defects can also be caused by surface tension differences between the coating and the substrate, or by contaminants. This can often result in localised flow problems such as craters and
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fish-eyes. Various authors agree [5, 9] that good flow also requires a uniform level of surface tension during the drying process. This will be discussed later. 8.1.7
Summary
Good flow requires perfect wetting of the substrate, good flow of the coating (Total film flow) and uniform evaporation of the solvent (water and co-solvent or film formers). Surface active additives, substances which lower the surface tension of a liquid by concentrating at the surface, aid these effects. In waterborne systems with their relatively high surface tensions, many substances are surface-active. This will be the subject of the next section.
8.2
Chemistry of active ingredients
8.2.1
Polyether siloxanes
The most important class of surface-active additives are modified siloxanes. They are derived from low molecular weight polydimethyl siloxanes by replacing individual methyl groups with diverse organic groups such as polyethers. Most of the products have a molecular weight range from 1000 to 15,000 g/mol. In order to be effective in waterborne systems, the right balance between water solubility and surface activity must be found. They must also be sufficiently compatible with the binders to ensure that hazing does not occur in liquid coatings or in dried films. Basically the property profiles of modified siloxanes are dependent on their silicone content, the structure of the siloxane backbone and the organic side chains employed. This allows products to be developed which function preferentially at the substrate/coating interface (see Chapter 5 “Substrate wetting additives”) or are surface active. For waterborne coatings systems the polyethers are the most important modifiers. As a rule the polyethers are derived from ethylene oxide units (EO) and propylene oxide units (PO). Polyethylene oxide is very hydrophilic (polar), polypropylene oxide, in contrast, is hydrophobic (non-polar). Polarity can be adjusted via the EO/PO ratio. Higher EO content improves compatibility in polar coatings systems and water solubility. Very good water solubility can often reduce the surface activity of polyether siloxanes but increase the foam-stabilizing tendency. Higher PO content on the other hand lowers the water solubility and improves defoaming properties. Besides the EO/PO ratio of the polyether and the number of polyether chains in the polyether siloxane, the key determinant is whether the side chains are statistically distributed on the siloxane backbone (comb structure) or the molecule has a block structure or the polysiloxane chain is only end-modified. The way in which
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the polyether side chains are bound to the siloxane backbone is also important. It can be seen in Figure 8.8 that the polyether chain is not bonded directly, but via a (short) alkyl chain to increase resistance to hydrolysis. As a rule, a direct bond results in the polyether chains splitting off easily and to the polysiloxanes condensing Figure 8.8: Structure of a polyether-modified polysito higher molecular weight loxane structures (loss of compatibility and hence a risk of cratering). In general, in waterborne systems, products with Si-C bonds are used. Which active substances are used depends on the binders selected and on the resulting film forming process. Because of the complexity of waterborne coating materials it is difficult to determine the relationship between the structure of the flow control agent and its activity spectrum. Since requirements may be very diverse in practice, only the general advice which follows can be given; universal solutions are, unfortunately, unlikely. However, the effectiveness of such additives is often insufficient in waterborne systems. Substrate wetting additives are related to surface active polyether siloxanes, the considerably lower molecular weight of which leads to a marked surfactant-like structure. Because of this structure, silicone surfactants very strongly lower the surface tension of aqueous systems. The silicone surfactants have the important advantage over fluoro surfactants that they do not stabilize foam. Fundamental investigations into the use of modified siloxanes in waterborne coatings were published in 1996 [10]. • • • • • • •
Polyether-polysiloxane with side-substituted polyether, Polyether-polysiloxane with an end-substituted polyether, Polyether-polysiloxane with both ends-substituted polyether, Polyether-polysiloxane with more than two substituted polyethers, Fluoro-modified polysiloxane, Polysiloxane end-modified with carboxylic acid, and Acrylate-modified polysiloxane
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were tested in three different waterborne coatings systems based on polyesterpolyurethane. In evaluating the flow, effects such as substrate wetting, edge flow and crater prevention were also considered. The results showed that polyether siloxanes with a sideor end-modification with an ether group/polysiloxane unit ratio of 1.6 to 2.5 gave the best results for the waterborne systems based on the binders listed above. 8.2.2
Polyacrylates
In contrast to the polyether siloxanes, polyacrylates hardly affect the surface tension. They exhibit only limited solubility in waterborne coating formulations and therefore migrate to the coating/air interface during the drying process, forming a barrier on the surface. In this way they oppose the material flow which causes the difference in the surface tension [11], see Chapter 4.2.5. Here they hinder solvent evaporation as with a molecular weight of 15,000 to 20,000 mol the molecule is relatively immobile. The acrylate copolymers are made by radical polymerization in a polar solvent. In order to transfer them into the aqueous phase the acid groups are neutralized with amines (e. g. DMEA) to make them water-soluble. In the case of physically drying coatings the films are quite smooth immediately after application. As the surface energy of the dry film material is not reduced, wetting by a subsequent layer is not critical. Neutralized acrylate copolymers are used successfully in forced-drying acrylate, polyester and alkyd resin enamels. The gloss and flow (DOI) are improved (see Chapter 8.8 “Test methods”), without impairing the hardness or solvent resistance. However, in some applications and depending on the molecular weight, stoving temperature and time, there can be problems with intercoat adhesion in multi-coat systems. In addition to the commonly used neutralized acrylate-copolymers, products are therefore also available which contain cross-linkable sites and are modified with fluorine. Such products produce good intercoat adhesion at high stoving temperatures. The polarity of the cross-linkable sites means that surface tension gradients are strongly reduced, leading to a corresponding improvement in the flow [9]. Furthermore the surface active additives (polyether siloxanes or polyacrylates) produce a uniform surface tension across the entire coating surface which remains relatively constant during solvent evaporation. Surface tension gradients on the coating surface are thus avoided resulting in the desired “smooth” surface. The surface activity of the polacrylates is however not as good as that of the polyether siloxanes.
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Depending on the dosage, the total film flow can be negatively influenced by an increase in viscosity (η) or by too great a reduction in surface tension (σ). 8.2.3
Side effects of polyether siloxanes
The term “smooth” can be used in two ways in connection with a coating surface. Firstly, to describe a surface which is free of texture (waves), in other words, geometrically even. “Smooth” can also be used to describe the slip properties of the surface and is then a measure of the frictional resistance or resistance to slip of the surface [12]. 8.2.4
Slip
In order for two surfaces to slide past one another, friction, a force opposing movement, must be overcome. To start the motion, static friction must first be overcome. Sliding friction once motion is underway is less. The main cause of friction is the unevenness of the surfaces. Microscopic observation of even a carefully prepared surface reveals it to be rough and fissured For friction between solids the frictional force (FR) is directly proportional to the load (FN). The proportionality factor between these forces is the dimensionless coefficient of friction (μG). The frictional force is independent of the contact area of the sliding bodies. When a surface having a defined load slides over another surface, the resulting frictional force is dependent only on the coefficient of friction (μG) and hence on the surface characteristics: Equation 8.11:
FR = μG · FN
(at constant speed) High friction is the result of the successive collisions of many microscopically small surface irregularities. This causes abrasion or even penetration of these irregularities into the opposite surfaces and thence to scratching. Susceptibility to scratching can be minimized through the use of flow additives based on the previously-mentioned polyether siloxanes (slip effect). They function as a lubricating film. This means that the surface is well protected especially after drying. These products are all surface active, i.e., they concentrate at the surface of the coating film. While the organic modifications orient themselves into the film, the dimethyl-siloxane units face outward and are therefore responsible for the
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Film formation
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slip effect. As a rule the more dimethyl structures are present, the greater the reduction in the slip resistance by the additive. As with the surface tension of the coating, even the smallest amount of silicone additive (as little as approx. 0.01 % of the formulation) can markedly Figure 8.9: Measurement of slip effect improve the smoothness of the surface. With increasing amounts the effect increases until a plateau is reached. Increasing the silicone concentration above this does not lead to a higher slip because the coating surface is entirely coated with silicone molecules. Determination of the correct dosage to reach the desired slip is of practical importance. Silicone additives with adjusted compatibility characteristics for waterborne coating systems are available commercially. Because of their low dimethyl group content the silicone surfactants, which strongly reduce the surface tension of waterborne systems and are used as substrate wetting additives (see Chapter 5), have practically no effect on the surface smoothness of the coating. If a higher surface slip is desired in waterborne coatings containing silicone surfactants, they must be used in combination with silicone polymers.
8.3
Film formation
As polymer emulsions are used in many market segments, film formation [1] is briefly discussed so as to give a better understanding of flow. After application of the water-based emulsion, film formation is dominated by the evaporation of the water. With water as a solvent the dispersions have a very high surface tension. The conditions fulfil the Orchard requirements for optimal flow. During this phase the polymer particles are able to move freely, but due to the shrinking volume their movement rapidly becomes restricted. Theoretically, after the drying coating reaches solids content of ca. 74 % the evaporation rate of the water slows down exponentially, as the water must then work its way through capillaries which have formed between the film particles. During this phase the polymer particles change shape and the hexagonal close packing of the polymer particles sinters into a rhombic dodecahedral structure. In the next phase the particle surfaces dissolve at places where the surfaces are in contact and the individual particles begin to coalesce. This can only occur as the capillary- and surface tension forces become larger than the deformation resistance
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Figure 8.10: Hypothetical model of film-formation of a polymer emulsion
of the spheres, that is when the ambient temperature is higher than the minimum film forming temperature. Finally the single particles fuse and the film forming process can take place through the interdiffusion of the polymer chain. This process can be improved through the use of film formers. Experimental results have shown that decreasing particle diameter and thus increasing particle surface area continually improve coalescence. In this wetsintering, the decisive role is played by the polymer/water interface and not the polymer/air interface. Finally, it is not the capillary force alone, but rather the accompanying interdiffusion forces which are responsible for complete film-formation [1].
8.4
Main applications by market segment
8.4.1
Industrial metal coating
8. 4.1.1
Electrophoretic coating
In cathodic electro-deposition epoxy adducts are used which are cross-linked by blocked isocyanates. The polymer, which is made water-soluble by the creation
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of salts, is deposited on the substrate as result of the application of an electrical potential on the substrate on which it forms a closed film. The neutralizer evaporates together with the water during the film forming process. As the water/neutralizer ratio has a considerable influence on the rheology, a set drying procedure must be followed. The rheology (Total film flow) has a significant influence on the wetting and resistance properties as well as on the flow. Rheological additives are used to achieve the desired properties. 8. 4.1.2
Waterborne coatings
Resins that are used for metal finishing applications are mostly polymers synthesised in the organic phase by polyaddition, polycondensation or polymerization. They have hydrophilic emulsifying groups built into the polymer structure. The resins are either macro-molecular polyelectrolytes made water-soluble by salt formation, or secondary emulsions (see Chapter 6 “Co-binders”). Nowadays waterborne coatings for the automotive industry use polyester-, polyacrylate-, and polyurethane resins. The OH-functional polymers are frequently thermally cross-linked with melamine resins. Many water-thinnable polymers are not sufficiently hydrophilic to be considered truly water-thinnable. To achieve this, amphiphilic substances in the form of solubilisers (co-solvents) are used. These water-soluble solvents considerably influence the rheology, film forming process and drying properties. As a consequence the Total film flow as well as the “Surface flow” can be negatively affected. To improve the “Surface flow” fluoro-modified polyacrylates are often used as flow modifiers. As these waterborne coatings are often overcoated with solvent-based enamels, polysiloxanes, which are able to migrate, to the coating surface are used. 8.4.2
Industrial coatings
Industrial coatings based on 1- and 2-pack polyurethane dispersions have almost totally replaced solvent-based polyurethanes and alternative polymer emulsions. Polyurethane emulsions are often mixed with other emulsions, such as polyacry lates, to achieve properties which could not be obtained with other waterborne systems. To improve flow, polyurethane emulsions often contain slow-evaporating, highboiling co-solvents. It is worth mentioning that manufacturers of polyurethane emulsions are striving to produce zero-VOC alternatives.
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The film-formation of air-drying 1- and 2-pack polyurethane dispersions follows principles analogous to those of other emulsions (see Chapter 8.3 “Film formation”). Exceptions to this are the 2-pack thermally-drying systems. Phase inversions can occur during the drying process as described by Thys and Bosma [7] (see Chapter 8.1.5 “Mode of action in an example of a thermosetting waterborne system with co-solvents”). In this process surface tension gradients can occur which can negatively influence the flow. The main factor in achieving good flow in a waterborne coating based on a polymer emulsion is the choice of rheological additive which allows the control of Total film flow. In thermally-drying systems surface flow additives can prevent the occurrence of surface tension gradients. Silicone surfactants have proved effective as surface flow additives in practice. 8.4.3
Architectural coatings
8. 4. 3.1
Flat and semi-gloss emulsion paints
Emulsion paints which are nowadays used in both interior and exterior applications are made with dispersions derived from homo- or co-polymers of vinyl acetate, vinyl chloride, vinyl propionate, acrylates, butadiene as well as styrene. As the pigment volume concentration of these coatings lies between 40 and 80 %, the flow (Total film flow) must be controlled by the proper selection of pigment wetting and dispersing agents and rheological additives. 8. 4. 3.2
High gloss emulsion paints
Emulsion paints today are mostly made based on pure acrylics. The emulsions used for this purpose have a minimum film forming temperature of 20 °C and the so-called “core-shell emulsions” from 0 °C. Although the use of film forming agents is not essential with “core-shell emulsions”, they are used to regulate the desired “open time”. Because the “open time” to some extent improves the “surface flow” it is not necessary to use surface-active additives as well. If prevention of the blocking effect (the tendency for coatings surfaces to stick together when in contact) is required, high molecular weight polyether siloxanes with a high concentration of methyl groups on the backbone are recommended.
8.5 Conclusion For optimal flow of a waterborne coating the viscosity must remain low until a smooth surface is created by the action of the surface tension.
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Test methods
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At the same time to avoid sagging from a surface having an angle >0°≤ 90° viscosity must be sufficient to overcome the force of gravity. The requirements of these seemingly contradictory (from a rheological standpoint) tendencies in waterborne coatings are met by using rheological additives which produce shear-thinning properties. The mathematical models of Orchard and Overdiep are suitable for calculations involving total film flow and sagging. Surface defects which arise from surface tension gradients during application and drying of waterborne coatings can be avoided by using polyether siloxanes or polyacrylate-based additives. These “Surface flow control additives” are often used because of their positive side effects. Short-chained polyether siloxanes (silicone surfactants) improve substrate wetting as well as the spraying characteristics but have little influence on surface flow. Long-chained polyether siloxanes create surface smoothness by lessening the slip resistance and reduce the blocking effect in emulsion-based aqueous enamels. Up to 3 times as much additive is required to achieve similar results in waterborne coatings systems when compared with solvent-borne systems.
8.6
Test methods
8.6.1
Measurement of flow
Flow results [13] are best evaluated on dried films which have been affected as little as possible by application conditions (e.g. temperature, relative humidity, coating thickness, thinning, exposure to dust, etc.) which can have a marked effect on the results. As always, visual observation plays a very important role in the evaluation process, although, particularly in the case of automotive coatings, there is increasing emphasis on objective measurements such as DOI (distinctness of image) . In this measurement the unevenness of a mirror reflection of a high-gloss enamel film is used as a basis of evaluation. A glossy surface can be measured with a goniophotometer, which employs lightscattering to obtain the “ALPHA value” (light-scattering at a 45° angle in an arc of 0.6°) which is important in relation to flow. New developments in laser techniques offer further possibilities for evaluating surface characteristics. Instruments which scan the coating surface with laser beams and display the topology as three-dimensional images are used to characterize relatively small surfaces (a few mm2 to a few cm2) and localized surface defects
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(craters, pinholes etc.). The roughness of the surface can be determined very precisely. The “Wave-scan” manufactured by the Byk-Gardner company also uses laserbeam reflection technology. The waviness of a glossy coating surface is analyzed along a 10-cm long line. The measurement of flow is characterised by dimensionless values for the “short wave” and “long wave”. 8.6.2
Measuring flow and sagging by DMA
Dynamic-mechanical analysis [14] involves continuously changing sinusoidal mechanical stressing of the sample. This stressing can be carried out as a function of the temperature. DMA measures the force amplitude, the resulting deformationamplitude and the phase displacement ΔΨ between force- and deformation-amplitudes. Calculation then leads to the complex modulus G* of the sample and the determination of tan δ = G”/G’. The elasticity modulus G* comprises a real part, the storage modulus (G’) , and an imaginary part, the loss modulus, (G”) The storage modulus describes that part of the mechanical energy which can be stored in the system in a shear/elongation experiment. The loss modulus, in contrast, is a measure of the energy dissipated in the material (energy converted from mechanical energy into heat). Flow is calculated by the following equation: Equation 8.12:
ω is the frequency used during the test. Hester and Squire [15] discuss the results obtained by DMA in detail and therefore only a brief overview is given here. 8.6.3
Measuring the surface slip properties
A test method in which the frictional force is measured has proved useful for determining surface slip properties. A 500 g weight with a felt base is pulled at a constant speed over a coating surface. The force required is measured electronically. The friction between two coated/ printed surfaces is measured using small variations in the measuring geometry.
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Literature
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The test is carried out at constant speed which enables reproducible measurement with a high degree of accuracy. The slip resistance is particularly low when the interactions within the slip additive film and between the film and sliding solid are small. 8.7 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Literature Kittel; „Lehrbuch der Lacke und Beschichtungen“, Band 3, S. Hirzel Verlag, StuttgartLeipzig, 2001 S. E. Orchard; Appl. Sci. Res. Section A, Vol. 11 (1962) Martin Bosma et al.; Progress in Organic Coatings, 55 (2006), p. 97–104 T. C. Patton; “Paint Flow and Pigment Dispersion”, 2nd Ed. NY, Wiley, 1979 J. Bielemann; “Additives for Coatings”, Wiley, 2000 W. S. Overdiep; Progress in Organic Coatings, 14 (1986), p. 159–175 F. Thys u. Martin Bosma; Pinture e Vernici (05/2006) S. Kojima; Polymer Engineering and Science, Vol. 33, No. 20 (1993), Vol. 35, No. 13 (1995), Vol. 35, No. 24 (1995) G.. Hobisch et al.; Surface Coatings Intl. Part A (07/2003) K. H. Käsler et al.; Coating (01/1996), p. 25 –27 M. A. Grolitzer; “Surface Defect Control in wb. Coatings”, Lecture in New Orleans, 22nd – 24th of February 1995 W. Heilen et al.; Tego Journal (03/2007) W. Scholz; „Acrylat-Verlaufsadditive und Wachse“, Seminar „Additives in paint industry“ Technische Akademie Esslingen DMA, Wikipedia R. D. Hester u. D. R. Squire; “The Rheology of wb. Coatings”, Lecture in New Orleans, 14th to 16th of February 1996
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9
Wax additives
Wax additives Anne Drewer
When you hear the word wax, you immediately think about terms such as candles or bees. But if you use the word in the context of coatings, then it is significantly more complex as it represents the generic term for the sector of additives based on wax. Wax has been used for thousands of years to protect surfaces. However, surface protection is not always the only motivation; in many cases, other reasons include properties such as texture, structure, rheology or even matting of the surfaces. The coating industry uses these effects in numerous different ways. In most cases, the wax additives specially developed today for the coating industry are products with combined properties. This chapter describes how varied the application of wax additives in aqueous coating systems is, which coating properties can be improved with them and how to find the right product for the required property in the huge range on offer.
9.1
Raw material wax
Wax is a generic term for a specific group of organic compounds. They can be classified by the following properties: • • •
In contrast to oils and fats, the melting point of waxes is at least 40 °C. The melt viscosity of maximum 10 Pas at 10 °C above the melting point differentiates waxes from plastics and resins. With moderate temperature increases, no chemical changes occur in waxes in comparison to natural resins. The solubility of waxes in suitable solvents is improved by increasing temperature.
A specific property of wax is that they can be polished under light pressure. 9.1.1
Natural waxes
Natural waxes can be found in various areas. They have no defined chemical composition, being mixtures of various components which can also vary in their ratios. In addition, natural waxes are more or less coloured due to impurities. Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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Natural waxes have increasingly lost their importance in the coatings industry over the past decades in contrast to synthetic waxes. They are however currently enjoying greater demand due to the trend for renewable resources. 9.1.1.1
Waxes from renewable raw materials
These are wax types that are obtained by purification; a chemical treatment does not occur. They may come from either animal or plant-based sources. With beeswax (animal) the colour of the raw wax varies, depending on the flowers visited by the bees, from pale yellow to dark brown. The wax is processed by purification and bleaching. It consists mainly of myricyl palmitate esters, fatty acids, small amounts of hydrocarbons and cholesterol esters. The wax is soft and can be kneaded by hand. The melting point ranges from 62 to 65 °C. Carnauba wax is the most well-known plant wax. It is obtained from the Brazilian carnauba palm. The tree exudes this wax to protect its leaves and trunk from drying out. The colour can vary between dark brown and yellow, depending on the quality. A main application area for “prime yellow” carnauba wax within the coating industry is for can coatings, which require foodstuff legislation certification. Carnauba wax is also used in floor and automotive care products. The main components in this wax are cerotic myricyl esters and small amounts of acids, alcohols and hydrocarbons. Carnauba wax is hard and brittle, with a melting point between 82 and 86 °C. 9.1.1.2
Waxes from fossil sources
Paraffin is an oil distillate, a part of the so-called wax fraction from a crude oil distillation. Further refining due to different melting points can be implemented. These paraffin’s must therefore also be differentiated by melting point and purity. The melting points of the individual fractions lie between 52 and 64 °C. Paraffin’s consist of saturated hydrocarbons with a linear chain structure (Figure 9.1). They have an opaque appearance and character similar to grease. Microcrystalline waxes, oil deposits, are obtained from the residues of oil distillation. Separation is implemented by refining the wax in different steps and then precipitating it out. This process is significantly more complex than the production
Figure 9.1: Paraffin’s are saturated hydrocarbons with a linear chain structure
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Figure 9.2: Microcrystalline waxes have a branched structure
of paraffin. As microcrystalline wax binds oil more strongly than paraffin, there is always a small percentage of free oil in microcrystalline waxes. The melting points of the waxes lie between 50 and 90 °C. The structure is branched, in contrast to those of paraffin’s (Figure 9.2). Microcrystalline waxes are more flexible and tougher than paraffin and appear to be tackier. Raw Montan wax is obtained from the extraction of brown coal with the aid of suitable solvents. Only certain coal deposits contain enough wax to make extraction with sufficient yields profitable. The main mining areas can be found in Eastern Germany and America. Montan waxes consist mainly of acid esters, free acids and resins. The colour varies between brown and black. The melting point of American wax lies between 85 and 88 °C, German wax between 83 and 89 °C. Montan wax is shiny, hard and brittle. 9.1.2
Semi-synthetic and synthetic waxes
In comparison to natural waxes, the synthetic and semi-synthetic waxes play a significantly greater role in the production of wax additives for coating applications. (Fully) synthetic waxes are chemically well defined and are available in large quantities with uniform and reproducible quality. They are free of impurities. Semi-synthetic waxes are produced from natural raw materials through chemical processes and it is therefore possible to have direct influence on the properties of these waxes. 9.1. 2.1
Semi-synthetic waxes
Amide waxes: A condensation reaction between a fatty acid and an amine produces primary, secondary and tertiary amides, together with secondary bis-amides. The two main groups are the primary amides (R-CO-NH2) and the secondary bisamides (R-CO-NH-X-NH-CO-R) with X= di-amine part, R= e.g. stearic acid, palmitic acid or oleic acid. The melting point varies between 60 and 100 °C for primary amides and between 110 and 145 °C for secondary bis-amides.
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Modified Montan waxes: Raw Montan waxes are refined and oxidised with chromic acid. The ester groups are split by the oxidation into acid and alcohol groups. This also eliminates the biggest disadvantage of Montan wax, the dark colour. The carboxyl groups are then esterified again with alcohols. The alcohol used and the implementation of the reaction determine the properties of this modified Montan wax, i.e. the melting point. 9.1. 2.2
Synthetic waxes
Fully synthetic waxes are subdivided into homo-polymers and copolymers. The homo-polymers include Fischer-Tropsch waxes. These waxes are produced during the hydrogenation of carbon monoxide with cobalt as the catalyser. The reaction takes place at a pressure of 7 Bar and a temperature of 185 to 205 °C. FT waxes consist of hydrocarbons and several oxidised groups. They have long aliphatic chains with relatively short side chains. A wide variety of fractions can be produced, dependent on the melting point, which lies between 90 and 100 °C. The waxes are white and hard. Polyethylene and polypropylene waxes are also homo-polymers, which are produced by polymerisation of the respective monomers ethylene and propylene. Polyethylene wax is one of the best known synthetic waxes and it is available in two variants: LDPE (low density poly ethylene) with low density and HDPE (high density poly ethylene) with a higher density. LDPE waxes are produced by the polymerisation of ethylene at very high pressure with oxygen as the initiator. The difficult to control reaction means that the molecular structure is highly irregular; the wax has very branched chains (Figure 9.3). HDPE waxes form the most important group of polyethylene waxes. They are produced by polymerisation at low pressure. The molecules are not branched and are therefore very tightly packed. The melting point of polyethylene waxes lies between 80 and 140 °C (Figure 9.4, page 150).
Figure 9.3: LDPE waxes are based on ethylene with low density and high branched structure
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Figure 9.4: HDPE waxes are based on ethylene with high density and unbranched structure
Copolymer waxes include products based on ethylene-vinyl-acetate (EVA). The percentage of vinyl acetate is 6 to 20 %. The melting points of the waxes lie between 87 and 92 °C. Waxes that are pure hydrocarbon compounds, like polyethylene waxes for example cannot be emulsified in water due to their low polarity. In order to incorporate them in aqueous systems, a small number of polar groups must be added to the polymer chains. This is possible in various ways: 1. By copolymerisation with a polar monomer, e.g. acrylic acid or vinyl acetate. 2. By radical grafting of unsaturated polar groups on the non-polar hydrocarbon chain. The reaction of polypropylene with 0.5 % maleic anhydride is sufficient to make the polypropylene wax emulsifiable. 3. By oxidising the wax. The hydrocarbon chains are oxidised at high temperatures in combination with a metal catalyst and oxygen in the air, creating ketone, ester and acid groups. This process is often used for HDPE, Fischer-Tropsch and microcrystalline waxes. Overview of raw waxes (Figure 9.5).
Figure 9.5: Overview of raw waxes
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From wax to wax additives
9.2
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From wax to wax additives
The raw waxes are available in various forms and sizes, dependent on their origins, e.g. as granulate, platelets, powder or large wax cakes. In this form, they cannot usually be used in a coating system. The waxes must first be converted into a form that enables easy inclusion in the various coating systems, i.e. wax additives must be produced from the waxes. There are various methods, but they are all based on significant reduction of the wax particle size (< 25 μm). 9.2.1
Wax and water
9.2.1.1
Wax emulsions
A wax emulsion is very finely distributed wax particles in an aqueous medium. In the strictest sense, this is not really an emulsion as the wax particles are still present in solid form within the water. In a transparent emulsion, the main proportion of the wax particles is smaller than 100 nm. One could therefore refer to nanotechnology in this case. In order to emulsify wax in water, emulsifiers are required that act as intermediaries between the polar water and the non-polar wax. Emulsifiers have two different segments in their molecular structure: one part is non-polar and lipophilic (hydrophobic), compatible with wax, and the other segment is polar and hydrophilic, therefore water-compatible. The non-polar segment is based on a longer alkyl chain and the polar segment is either ionic (anionic or cationic) or non-ionic in structure. Each wax particle is surrounded by emulsifier molecules with the non-polar segments oriented towards the wax and the polar segments protruding into the water (Figure 9.6). The wax particles are therefore stabilised in the aqueous phase in this manner. If the amount of emulsifier is insufficient, a part of the wax is not wetted and will cream up on the surface of the emulsion. The majority of the ionic wax emulsions used for aqueous coating applications are anionic emulsions. The emulsifiers consist of a longerchain fatty acid, neutralised with an amine. The selection of fatty acid and amine has a great influence on the quality of the emulsion as well as the properties of the final product, e.g. particle size distribution and transparency. In most cases, oleic
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Figure 9.6: Wax particle in a wax emulsion
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acids are used, but stearic acid or palmitic acid can also be used. One amine that can be used is di-ethyl-amino-ethanol (DEAE). Cationic emulsions are usually used in the leather and textile industry, but are rarely used in the coating industry. The second important group of wax emulsions for coating applications is the non-ionic emulsions. The emulsifiers used here are characterised in that they do not ionise in water; the polar segments contain e.g. hydroxyl and ether groups. The most common non-ionic emulsifiers for waxes are ethoxylised fatty alcohols. It is often useful to add a small amount of KOH in nonionic emulsions to neutralise free acid groups in the wax. 9.2.1.2
Wax dispersions
In comparison to wax emulsions, aqueous wax Figure 9. 7: Particle size measure- dispersions have a different particle size distriment by laser diffraction bution. While wax particles in a wax emulsion are smaller than 1 μ m and have no influence on gloss, the wax particles in a dispersion are usually between 1 and 20 μm in size. They reduce the gloss in a coating system. The reason for this is the different production process. Less emulsifiers are used in a wax dispersion and the product is wet ground. The achievable particle size is dependent on the grinding time (Figure 9.7). 9.2.3
Micronized wax additives
Micronized wax additives are powder-based products with an average particle size of 4 to 15 μm. Because of their extreme fineness and non-polar character, they belong to the category of “non-free-flowing” powders. Micronized wax additives can be characterised by their composition, the properties of the raw waxes and the particle size distribution. These parameters are set by the production process. In the grinding process, the wax particles are accelerated by an expanding air stream to supersonic speeds (500 m/s) and reduced in size by impact. One advantage of this process, which is carried out in an air jet mill, is that no contamination occurs through metal wear in the mill. The milled wax is then classified with a classifier wheel. The fine wax particles are conveyed out of the mill via the classifier wheel,
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while material that is still too coarse falls back into the grinding zone. The product quality is controlled by setting the frequency of the classifier wheel (Figure 9.8). The particle size distribution provides information about the quality of the micronized wax additives. The main characteristics are the average particle size (d50) and the biggest particle size (d100). “d50” means that 50 % of the particles have a particle size that is the same or less than the specified value. The value in d100 gives the maximum particle size, including oversized particles.
Figure 9.8: Air stream mill for the micronizing process of waxes
Due to the great differences in polarity between water and wax, it is a challenge to develop micronized wax additives that can be easily used in aqueous systems. Waxes can be modified chemically; the inclusion of polar structures means they can be relatively easily added to aqueous systems. In addition, polymer chemistry offers the possibility of synthesising substances that are not waxes in the strictest sense, but which act as waxes and have a polarity enabling them to be used in aqueous formulations.
9.3
Wax additives for the coating industry
9.3.1
Mode of action
Two parameters are important when using a wax additive in a coating system: • the influence of the wax base, i.e. the physical/chemical properties of the wax (such as polarity, melting point) and • the influence of the particle size. While the coating is drying, the wax particles distribute themselves in the coating layer. The wax particles orient themselves with the coating surface, dependent on the density of the wax particles and the evaporation rates. In many cases however, they are homogeneously distributed through the coating film like pigments or matting agents. Wax additives are not surface-active in contrast to silicones. The wax additive used influences the coating properties of the dry film depending on the melting point and polarity of the wax (Figure 9.9, page 155). Figure 9.10 (page 155) shows some of these effects and their correlation with the melting point and polarity of the wax base. They clearly show that waxes with a
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Figure 9.9: Impact mechanism of wax additives
Figure 9.10: Effects of wax additives properties and their correlation with the melting point and polarity of the wax base
low melting point (100 °C, such as HDPE waxes on their own or modified with e.g. amide waxes or PTFE, are the right choice for improving mechanical resistance. Surface slip The melting point of the wax is also decisive for increasing surface slip. In contrast to mechanical resistance, additives with a lower melting point are required here, such as paraffin and carnauba waxes. Carnauba wax is frequently used in can and coil coating systems for improving surface slip. But high slip is not required in all cases. A high slip is generally dangerous for sports flooring, parquet and floor coatings, and it can also negatively influence stacking when present in paper coatings. Anti-slip is more in demand for these applications. Polypropylene waxes provide anti-slip effects due to the high melting point. Figure 9.12 shows the influence of melting point on slip and anti-slip properties. The surface slip increase is measured in % in comparison to coating systems without wax additives. The force required to pull a weight over the surface is measured for this test.
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Figure 9.12: Influence of melting point on slip and anti-slip properties in a waterborne parquet laquer
Figure 9.13: Measurement contact angle
Water repellence One property of wax additives that has nothing to do with the melting point, but with the polarity of the base wax, is water repellence. Wax additives based on highly non-polar waxes, such as paraffin, create coating surfaces where water can no longer spread, the so-called duck’s back effect. The water simply pearls off. This effect is used, e.g. for exterior coatings in the painting sector, for garden furniture and also for printing inks. The influence of the base wax can most easily be measured by measuring the contact angle of a water droplet. The higher the contact angle, the more the water droplet pulls together and the more non-polar the surface is (Figure 9.13). Table 9.2 (page 159) shows the effect on a PU exterior wall coating. Non-polar surfaces do however come with the risk of adhesion loss (crawling). Adhesion must therefore always be checked when using non-polar wax additives. The influence on adhesion is relatively low for polar waxes.
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Table 9.2: Water repellency in 100 % acrylic exterior coating Dosage wax emulsion, based on Paraffin
Contact angle water droplet
0%
76°
0.5 %
86°
1%
103°
A second example shows the influence of paraffin on the water absorbency of paper. No moisture should penetrate through the paper used in foodstuff packaging and even washing powder boxes. Paraffin waxes can be used here as a barrier (Figure 9.14).
Anti-blocking Another effect of polarity is the influence on the blocking behaviour of a coating. This refers to the bonding of hardened coating surfaces due to pressure, e.g. when stacked. The effect is also known in window paints. Non-polar coating surfaces with paraffin wax are less prone to bonding than polar surfaces (Figure 9.15). Solid wax on total formulation
In a block test, two coated substrates are placed on top of each other after 24 hours room temperature drying and weighed down with a 1 kg weight. After 1 hour at 40 °C, the surfaces are separated again and the blocking behaviour evaluated.
Figure 9.14: Water up-take measurement in overprint varnish
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Figure 9.15: Anti blocking latex system with different waxes
Figure 9.16: Matting of an aqueous acrylic parquet lacquer
9.3.2.2
Gloss reduction
Coating surfaces appear matt when incidental light is reflected diffusely. This light scattering makes the surface appear matt. Small particles in the coating surface cause this micro-roughness. This effect can also be produced by wax additives. Normally, one would think of silica for matting. The use of wax additives means that mechanical resistance and gloss reduction can be combined. Wax dispersions and micronized wax additives have, due to their particle size distribution, a more or less greater influence on the gloss grade in coating systems (Figure 9.16).
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Table 9. 3: Matting and abrasion resistance test of a UV wood coating Gloss 60°
Steel wool test
17
3 to 4
11
2
without wax additive + 3 % silica 6 % wax additive* + 3 % silica
Depending on the required gloss grade, the added volume of solid wax is between 2 and 10 %. Combination with silica is recommended for dull-matt systems. This combination ensures the most efficient matting with the best surface protection (Table 9.3).
As already mentioned in Chapter 2.3 about micronized wax additives, it is difficult to find wax additives for the aqueous sector that are storage stable in aqueous systems without emulsifiers and which do not cream up at the surface. There are specially modified wax additives for this purpose which, due to their chemistry, are easily incorporated even at low shear forces, demonstrate excellent matting and also provide excellent surface protection because of their high melting points. So-called wax alloys are also new, offering a higher polarity and excellent mechanical resistance due to the alloying of different base waxes. * Wax alloy based on mod. HDPE 1 = excellent 6 = worse
9.3.2.3
Texture and structure
Design and fashion are often the driving forces behind the modification of optical properties in coating surfaces. The surfaces of plastic coatings are particularly subject to this trend due to consumer demand. A trend towards structured surfaces is also apparent for plastic components as used, e.g. in entertainment electronics. Wax additives can also be used here. The wax particles protrude out of the coating film and thus create the structure. Various particle size distributions enable numerous different structures and texture effects. Very fine structures result in very beautiful even matting and a very delicate soft-feel effect. They are often used for the inner cladding of vehicles or airplanes. Very coarse structures can hide uneven backgrounds. Naturally, structured surfaces can also be found in furniture coatings or in the packaging coating sector, such as on mobile phone shells. Table 9. 4: Influence of additive on orientation of effect pigment Flop index BYK-mac Rheology modifying wax emusion based on mod. EVA
19.8
Arcylic thickner
16.8
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9.3.2.4 Rheology control Effect coatings are taking over a greater part of the market, not only in the automotive industry, but also in the industrial coatings sector. This market in Europe is mainly based on environmentally friendly
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Summary
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aqueous coatings. In the solvent-borne systems, ethylene-vinyl-acetate-based (EVA) wax additives have been used for years for the orientation of effect pigments and reduction of sedimentation. Due to the different rheological behaviour of aqueous systems, the use of conventional wax additives in these systems has so far not shown the desired improvements. However, with the development of new modified EVA wax emulsions, it is now possible to positively influence the rheology of aqueous effect coatings and therefore optimise the orientation of effect pigments (see Table 9.4).
9.4
Summary
The use of wax additives offers numerous possibilities of influencing coating properties in aqueous systems. It is important to find the right base wax/wax combination. Decisive factors for the properties aimed for are the melting points of the wax additives for optimal surface protection and polarity for water repellence and anti-blocking. In addition, the particle size of the wax additives is of decisive significance for gloss or matting and surface effects such as structure and texture. It is possible to optimise the orientation of effect pigments through novel rheologymodifying wax additives.
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Light stabilizers for waterborne coatings
10 Light stabilizers for waterborne coatings Dr. Adalbert Braig
10.1
Introduction
Modern paint systems typically have to fulfil along with their decorative function also a protective function for the substrate underneath. Thereby they are themselves affected by a variety of environmental influences, e.g. the harmful effects of UV irradiation. As a result the life time of such coatings is often dramatically reduced. This in turn can be significantly prolonged by the use of suitable light stabilizers. In most cases the UV stabilization of coatings is achieved through the combined use of UV absorbers and radical scavengers. Through absorption of the incoming UV light the UV absorbers are predominantly responsible for colour retention and/or the protection of the substrate underneath. In addition the radical scavengers prevent the photo-oxidative (light induced) degradation/cracking of the binder thereby ensuring gloss retention and the maintenance of the integrity of the coating. Following a short section covering „light and photo-oxidative degradation“, the stabilization possibilities for coatings, the mode of action and the chemical classes of light stabilizers, as well as the application fields and the criteria for proper stabilizer selection will be discussed in greater detail.
10.2
Light and photo-oxidative degradation
The entire electromagnetic radiation spectrum surrounding our planet extends from cosmic radiation, highly energetic and short wavelength to radio waves, low energy and long wavelength. In this context the UV/VIS range as part of the entire spectrum is of particular importance and can be – according to DIN 5031 – subdivided as follows: UV-C: wavelength range 100 to 280 nm UV-B: wavelength range > 280 to 315 nm UV-A: wavelength range > 315 to 380 nm UV-VIS: wavelength range > 380 to 720 nm (“visible light“) Wernfried Heilen: Additives for Waterborne Coatings © Copyright 2009 by Vincentz Network, Hannover, Germany
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The shorter the wavelengths of the incoming light, the higher its energy and the more harmful its effect on polymers (paints/binders). Fortunately the short wavelengths (λ 320 nm). Of these hydroxyphenyl-triazines show the most pronounced absorption towards the short wavelengths. • Due to their second absorption maximum at wavelengths >340 nm hydroxyphenyl-benzotriazoles exhibit the broadest spectral coverage. The exact position of the absorption maxima as well as the extent of extinction ultimately depend on the substitution patterns (see Figure 10.4) of the individual molecules. The extent of the spectral coverage can be even better explained through the transmission spectra (Figure 10.6, page 168) rather than through the absorption spectra of the neat material. The further the absorption is shifted towards the longer wavelength range the more damaging UV light can be filtered out.
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Figure 10. 5: Absorption spectra of different UV absorber classes (c = 10 mg/l in CHCl3; cell 1 cm) 1 = oxalic anilide; 2 = hydroxy-phenyl-benzotriazole; 3 = hydroxy-benzophenone; 4 = hydroxy-phenyl-triazine
Figure 10. 6: Transmission spectra of different UV absorber classes (c = 50 mg/l in CHCl3; cell 1 cm) 1 = oxalic anilide; 2 = hydroxy – phenyl-benzotriazole; 3 = hydroxy-benzophenone; 4 = hydroxy-phenyl-triazine
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Besides the absorption characteristics, the efficacy of UV absorbers is predominantly defined by the Lambert-Beer law: E = Abs = ε . c . d = log I0/ I
where E: extinction Abs: absorbance I0: intensity of the incident light I: intensity of the emergent light ε: extinction coefficient (l/mol cm) c: concentration (mol/l) d: thickness (cm) of the paint film, substrate or cell The extinction depends on the wavelength and can be regarded as a measure of the filter effect of the UV absorber, i.e. the higher the extinction at a certain wavelength the more light is being screened. As shown in the Lambert-Beer law, the extinction is directly proportional to the molecule specific extinction coefficient e, the concentration c of the UV absorber and the film thickness d of the unpigmented polymer (paint film). Cutting the film thickness in half therefore results in the concentration c of the UV absorber having to be doubled in order to achieve the same filter effect. Additionally the Lambert-Beer law clearly indicates that the UV absorber can (by definition) not absorb at the very top surface (d = 0), see also Chapter 10.3.1. Mode of action of UV absorbers As already indicated in Chapter 10.2, the deactivation processes as shown in the Jablonski diagram (Figure 10.1) have to be different in case of UV absorbers, since they have to convert the absorbed energy before undesired side reactions can occur. This means that the energy conversion has to take place in the singlet state S1. Furthermore the crossing from S1 to T1 (Intersystem Crossing) resulting in phosphorescence must be excluded [7].
Concerning the mode of action of UV absorbers one has to distinguish between phenolic UV absorbers (e.g. hydroxy-benzophenones, hydroxy-phenyl-triazines, hydroxy-phenyl-benzotriazoles) and non-phenolic UV absorbers such as oxalic anilides. Figure 10.7 (page 170) shows the energy conversion of phenolic UV absorbers using hydroxy-phenyl-benzotriazoles as an example.
Once the UV absorber has absorbed the light a proton transfer from the oxygen to the nitrogen atom occurs in the excited singlet state thereby forming a photo tautomer (“keto form”). The original molecule subsequently re-forms (“enol form”) through radiation-less deactivation (release of heat) and returns to the ground
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Figure 10.7: Energy conversion of phenolic UV absorbers (e.g.hydroxy-phenyl-benzotriazoles) according to Otterstedt [15]
state. Comparative investigations of oxalic anilides [13, 16] indicate an intramolecular proton transfer occurs during energy conversion. 10.3.2 Radical scavengers As mentioned in Chapter 10.3, excited chromophores Ch* can form radicals subsequently leading to free-radical reactions (see Figure 10.3). In order to prevent such reactions, products are needed which have the ability to scavenge such radicals and thus prevent possible chain reactions from taking place. Such molecules are called radical scavengers. The most important representatives are antioxidants and sterically hindered amines. 10. 3. 2.1 Antioxidants
Depending on the mode of action one can divide antioxidants in two groups, i.e. primary antioxidants (chain terminating effect [17] via a radical mechanism) and secondary antioxidants (decomposition of peroxides through an ionic mechanism). In this context the secondary antioxidants will not be further discussed.
Primary antioxidants are predominantly represented by sterically hindered phenols. The mode of action of this class is schematically shown in Figure 10.8. According to equation (1) in a first step a phenoxy radical is formed. The stability and thus the reactivity of this radical depends on the substituents R1 und R2, i.e.
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Figure 10. 8: Mode of action of sterically hindered phenols (schematic)
the possibility for resonance stabilization (delocalization of the electron), as shown in equation (2). The more stable the phenoxy radical the lower the probability that further chain reactions can be initiated. This is typically achieved through rather bulky R1 and R2 substituents such as tertiary butyl groups. With respect to the stabilization of coatings one important drawback of phenolic antioxidants that exists, is their non-cyclic mode of action, i.e. they are consumed after a certain period of time and are no longer capable of preventing further radical reactions. This in turn means that they are predominantly used as process stabilizers at high processing temperatures (e.g. processing of plastics). 10. 3. 2.2 Sterically hindered amines This class of compound is almost exclusively represented by derivatives of 2,2,6,6-tetramethylpiperidine (Figure 10.9). Typically in the literature they are also referred to as „HALS“, which stands for „Hindered Amine Light Stabilizers“. This term will be used in the remainder of the treatise. Of particular importance is the substituent R, since it directly influences both the basicity of the HALS molecule as well as the rate at which the nitroxyl radical, the actual active substance, is being formed. This will be discussed in greater detail in Chapter 10.3.2.2.
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Basicity of HALS
Generally HALS in which the nitrogen atom of the piperidine group is substituted by hydrogen or methyl display a high basicity and have a pKb value of approximately 5. In some cases this can lead in amine neutralized waterborne coatings to a shift of the pH value resulting in a destabilization of the formulation. Figure 10.9: General structure of sterically hindered amines based on 2,2,6,6-tetramethylpiperidine
HALS in which the nitrogen atom of the piperidine group is substituted by e.g. COCH3 or O-Alkyl are essentially non-basic in nature and have pKb values in the range of 10 to 12.
Mode of action of HALS The mode of action of HALS has been extensively researched. This work was predominantly carried out in liquid or solid model systems. Nevertheless it is unclear if the results obtained truly represent the processes occurring in polymers or paints. In contrast to the mode of action of primary antioxidants the HALS mechanism is cyclic in nature, an important prerequisite for their usefulness as long term stabilizers for coatings. The best known mode of action is based on the research of Denisov and is shown schematically in Figure 10.10 in the so called “Denisov cycle” [18].
As shown in the Figure 10.10 above, in a first step HALS (1) is transformed in the presence of oxygen and UV light into the nitroxyl radical (2), as the actual active species. The formation of the nitroxyl radical starting from a protonated HALS is quite difficult, if not impossible. In the next step the nitroxyl radicals can scavenge the free radicals (e.g. formed form reaction products of excited
Figure 10.10: Mode of action of HALS (schematic), (“Denisov cycle”)
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chromophores Ch*) by formation of the aminoether (3). Reaction of aminoether (3) with peroxide radicals leads to the formation of the intermediate (4), which can readily decompose thereby (re-)forming the nitroxyl radical (2) as well as the harmless bi-products R=O und R’OH. Thereafter the nitroxyl radical is available for a subsequent cycle.
10.4 Light stabilizers for waterborne coatings Generally speaking all light stabilizers initially developed for use in solvent based coatings can also be used in waterborne systems. The “only” difficulty is the ease of incorporation of such products in aqueous systems. Larger paint or resin manufacturers may have the principle option to incorporate such materials in the organic phase, i.e. during resin synthesis. For most paint suppliers, however, this is not considered a valid option. In the last years the industry has made considerable efforts in the development of water compatible product forms based on existing and performance proven light stabilizer technology, which can be easily incorporated or post-added to waterborne coating systems. From a concept point of view the following product forms are to be mentioned: • hydrophilic light stabilizers such as the hydroxy-phenyl-benzotriazole I (BTZ1) shown in Table 10.1. The polarity of the introduced side chain is sufficiently high in order to allow in most cases the incorporation of the compound without additional need for co-solvents [19]. • solid state dispersions: these products are solid UV absorbers and/or sterically hindered amines (Table 10.1), which have been dispersed in water in the presence of dispersants and small amounts of organic solvent (e.g. glycol ether). The solids content (active substance) is in most cases in the range of 50 %. Despite a certain sedimentation tendency these products can be readily re-dispersed whilst stirring [20]. • emulsions: under this category fall preparations in which the light stabilizers were converted into a water compatible form by using non-ionic emulsifiers. Due to the high levels of emulsifiers typically used, there is a certain risk that the water sensitivity of the paint film is increased. • Neat (Novel Encapsulated Additive Technology): nano dispersions, characterized by an average particle size of 1000
5 to 10
5 to 10
Aureobasidium pullulans
0.3
0.1 to 0.5
Chaetomium globosum
4
0.5
0.5
0.5
Aspergillus niger
Cladosporium cladosporoides Gliocladium virens Penicillium glaucum Rhodotorula rubra Sclerophoma pithyophila Sporobolomyces roseus
5
1
2.5
0.5
5
5
2.5
0.5
1
1 to 2
Trichoderma viride
0.5
1 to 2
Ulocladium atrum
2.5
500
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In-can and dry film preservation
The biggest market penetration in the past was made using with a combination of carbendazim and octylisothiazolinone (both fungicides) with either a triazine, e.g. terbutryn, or a phenylurea, e.g. “Diuron”, type algicide. The reason for the combination of fungicides can be seen in Table 11. 2 (OIT/carbendazim). While carbendazim is a very stable fungicide with a low leach rate, it shows a gap in its spectrum of activity against Alternaria and Ulocladium species, this gap can be filled using OIT which is less stable with higher water solubility. In combination, Table 11. 3: Fungicidal active ingredients for film preservation and their main features Fungizid
Main feature
carbendazim
• efficacy gaps against Alternaria, Ulocladium • stable against UV-light and alkalinity • low leaching • classified as CMR-Substance category II (mutagenic und teratogenic) • labeling >1000 ppm with R 46
IPBC
• broad spectrum fungicide • discolouration risk • instable in presence of UV-light, temperature, alkalinity • sensitisation possible • AOX relevance
OIT
• broad spectrum fungicide • stable, also at alkaline conditions • highly water soluble • leachable • sensitization possible
DCOIT
• broad spectrum fungicide • reduced water solubility, low leaching • instable in presence of UV-light, temperature, alkalinity • strong sensitizer • AOX relevance
ZPT
• reduced water soluble and leachable up to pH 90° the substance is said to be hydrophobic. Strongly hydrophobic surfaces have contact angles >140°.
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Mode of action
Facade-protecting coatings with very high contact angles will be discussed in Chapter 12.3.3. It should be noted that the size of a contact angle on a surface is no guarantee that the capillary hydrophobing effect of the coating will be good, rather it provides a measure of the water-beading effect of the coating surface [1]. 12.1.3 How hydrophobing agents work
199
Figure 12.3: Shape of water drops as a function of the contact angle
As described in Chapter 12.1.1, water can rise in a capillary if the capillary can be wetted. For the protection of building materials wetting should not occur. This can be achieved by lining the capillaries with hydrophobing agents such as silicones and paraffin waxes. The low surface tension of the hydrophobing agents increases the Figure 12. 4: Contact angle of a water drop on a hydrosurface energy between the phobic surface capillary wall and the liquid (water). This means that the contact angle in the pore is increased. The relationship between the interfacial energy, surface tension and contact angle given by the Young equation will be discussed in the Chapter 5 “Substrate wetting additives”. The advantage of using silicones as hydrophobing agents is that their low surface tension results in a high degree of spread so that they build a molecular film on the capillary walls. In this way the silicones reduce the free surface energy of the substrate and thus produce their characteristic properties. Siliconising porous materials does not significantly change their permeability to air and water vapour. The Silicones coat the pore walls without clogging them. Liquid water will be unable to enter the pores due to the increase in the interfacial surface tension [2].
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In contrast, organic hydrophobing agents such as paraffin waxes, are unable to spread and also do not provide a good anchor to the substrate. Pores in porous materials will be closed and this will influence the diffusion of water vapour. A further disadvantage of paraffin waxes is that they can lead to adhesion problems when attempting to apply a second coating. As there is no bond with the coating system, the hydrophobing effect is also reduced over time due to weathering effects such as rain. The use of silicone oils and resins results in a permanent hydrophobing effect as they are particularly strongly anchored to the capillary walls. This takes place in different ways, one of which involves the siloxane dipole, inducing the oxygen atoms to orient themselves to the hard surface (see Figure 12.6), and the methyl groups to pack themselves densely together. The packing of the methyl groups leads to an increase in the hydrophobing effect. Alternatively bonds are formed between the reactive groups, such as the alkoxy groups in the silicone resins, and the filler pigments and/or substrate. They react by eliminating alcohol (methanol, ethanol) resulting in a long-lasting hydrophobing effect which cannot be impaired by weathering (see Figure 12.5).
Figure 12. 5: Alkoxy reaction of a silicone resins
Figure 12. 6: Anchoring of polysiloxanes by an oxygen dipoles
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12.2 Chemical structures Silicones can be categorised into oils, fats, resins and rubbers. Silicone oils and resins have become established as hydrophobing agents for waterborne coatings systems. The term silicone refers to organo-silicon-based polymers with Si-OSi bonds. On the remaining free valences of the silicon atom are other organic groups, mostly methyl, connected by Si-C bonds. Due to their structure, silicones occupy a position between inorganic and organic polymers, in particular between silicates and organic polymers [2]. The very stable bonds result in silicones’ highly interesting characteristics including, for example, heat-resistance and weathering resistance. Furthermore, they are hydrophobic and surface active as well as possessing release and slip properties. The simplest linear silicone structure with organo groups is shown in Figure 12.7. Because of the bonding of the silicon atom to both oxygen (sil–oxane) and organic groups, the terms polyorganosiloxanes or organo-polysiloxanes are also used when discussing silicones. Because of the multitude of types of bonding and possibilities introduction of simple silicone building blocks are established. According to this highly complex polymer structures of polysiloxanes can be easily and clear demonstrated. Thus the siloxane units are categorized by their functionality that is by the free valences of the oxygen atoms attached to the silicone atom. These units are termed as mono, di, tri and tetrafunctional. In manufacturing polymers the units are abbreviated as M, D, T und Q units (see Table 12.1). Because of their monofunctionality, M-Siloxane units can be combined with themselves to form a hexaorgano-disiloxane of the type M2. In combination with units of higher functionally the M-siloxanes are used as chain stoppers. R3Si-O-SiR3 (M2) The difunctional D-units can react with each other to produce ring-form siloxanes. The smallest cyclosiloxane is composed of three units (D3), but D4 and D5 siloxanes are also known.
Figure 12.7: The simplest linear silicone structure
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Table 12.1: Mono, di-, tri- and tetra-functional siloxanes Structure
Summary
R3Si-O-
R3SiO
Functionality
Symbol
monofunktionell
M
R2SiO2/2
difunctional
D
RSiO3/2
trifunctional
T
terafunctional
Q
SiO4/2
Combinations with M-units yield chain-form linear siloxanes. Co-reacted T-siloxanes produce spatially cross-linked molecules and in combination with mono- and difunctional units result in resinous macromolecules [2]. 12.2.1
Linear polysiloxanes and organofunctional polysiloxanes
Linear polysiloxanes are generally referred to as silicone oils. Their molecular structure consists only of M- and D-units M-D-D-D-D-D-M Linear organofunctional polysiloxanes are also composed primarily of M- and D-units but they additionally contain at least one functional group involving a silicon-carbon bond. Amino-functional polysiloxanes are important hydrophobing agents and are often referred to as silicone oils. Linear polysiloxanes are supplied either as liquid oil concentrates or dissolved in solvents. They are also available as emulsions. These products exhibit a good hydrophobing effect, reduce the water absorption in coatings and promote a high contact angle. High contact angles with the associated strong water-beading effect are desirable in architectural paints. To achieve strong water beading, the contact
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Chemical structures
angle on the coating surface must be more than 140°. Initially, emulsions give a weaker effect in terms of contact angle, as amphiphilic substances in the emulsion reduce the hydrophobing effect. However when all hydrophilic emulsifiers have been washed away by rain, the contact angle will increase. In contrast, solutions of polysiloxanes immediately impart a higher contact angle to a coating as they do not contain emulsifiers. The assumption that the beading effect from raindrops will automatically lead to a cleaner facade cannot however be proved. Rather the opposite in fact - investigations show an increased affinity to dirt pick-up (see Table 12.3). Linear polysiloxanes are very resistant to UV-radiation, heat and rain and are also very alkali-resistant, which is why they are frequently used in silicate emulsion paints. 12.2.2 Silicone resins/silicone resin emulsions Silicone resins differ from silicone oils in that, in addition to the chain-building dimethyl dichlorosilane, they incorporate the network-building trichlorosilane. A look at the possible combinations of the 4 building blocks M, D, T, and Q will clarify this. There are 15 possible combinations (see Table 12.2). The combinations DM, DD und MM are not branched and are therefore not used in resin production. Combinations with Q-units are of limited use in the hydrophobing of coatings. This leaves the possibilities TD, TM, TDM and TT, all of which are utilised in the production of technical resin products [3]. Table 12.2: Combination possibilities according to Noll: Chemie und Technologie der Silicone [3] Class
Siloxane Units
Class
Siloxane Units
1
QT
8
QTM
2
QD
9
QDM
3
QM
10
TDM
4
TD
11
QTDM
5
TM
12
TT
6
DM
13
DD
7
QTD
14
MM
15
QQ
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For the production of silicone resins for use in silicone-resin architectural paints, resins based on the building blocks TT, TD and TDM are currently used. TTsilicone resins form films at room temperature ranging from clear through hard to brittle depending on the degree of condensation. TD or TDM resins dry to soft or hard films depending on the relative amounts of each building block. Waterborne silicone resin emulsions are the only ones used in architectural paints, especially silicone resin architectural paints. 12.2.3 Other hydrophobing agents In addition to polysiloxanes, paraffin waxes (unsaturated hydrocarbons with the general formula CnH2n+2 where n is in the range from 25 to 50), polyethylene and polypropylene waxes are also used. These polymers have a molecular weight range from 1000 to 4000. They are insoluble in water and organic solvents and are used in the form of a micronized emulsion (waterborne or solvent-based). Polyfluorocarbonates with molecular weights in the range 1000 to 5000 are also used [4]. 12.2.4 Production of linear polysiloxanes The primary raw materials for the production of polysiloxanes are chlorosilanes, which can be hydrolysed into oligomers and/or cyclic dimethylsiloxanes. Dimethylsiloxane is produced from dimethyl dichlorosilane in the usual manner. It comprises a mixture of cyclic and linear OH-containing oligomers. The ratio of cyclic to linear monomers and the chain length depend on the conditions of hydrolysis. Linear OH-functional siloxanes can be produced by the continuous hydrolysis of the chlorosilane. This occurs by breaking the siloxane bond using a catalyst. In this process the cyclosiloxane is continuously distilled off and fed back to the hydrolysis phase of the reaction [3]. The hydrochloric acid produced by both processes is reacted with methanol to produce methyl chloride and this is returned to the silane synthesis. The second step after hydrolysis is either polycondensation of the linear silicone oligomers or polymerization of the cyclosiloxane to linear polysiloxanes. This procedure can be catalysed equally well by acid or alkali. In both cases this is an equilibrium reaction, which can be defined by adjusting the reaction conditions. Depending on the conditions, siloxane chains can be broken or formed. This pro-
Figure 12. 8: Hydrolysis of dimethyl dichlorosilane
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205
Figure 12.9: Addition reaction of allyl amine to an aminofuctional polysiloxane
cedure, known as equilibration [3] can be used to obtain silicone oil with the desired average chain length. Organofunctional polysiloxanes are mainly produced by a hydrosilylation reaction. In this cases siloxanes are used which are Si-H functional. These siloxanes react by a radical addition reaction with the double bonds of vinyl or allyl groups. Platinum is the preferred catalyst. The equation below is a simplified representation of the addition of allyl amine to an aminofunctional polysiloxane. 12.2.5 Production of silicone resin emulsions There are several different processes for the production of silicone resins emulsions. 12. 2.5.1 Secondary emulsification process
This classical method involves initial production of a solid silicone resin. A crosslinked but liquid oligomer is produced from the silane monomers by a catalytic polycondensation reaction, as described in the production of oils. This oligomer is further condensed to a solid resin that is soluble in aromatic as well as aliphatic solvents. In a second step, the solid resin is dissolved in suitable solvents before it is emulsified in water in the third and final step. As a result of this process the silicone resin emulsions can contain up to 15 % solvent. Modern production methods now allow production of secondary emulsions with a lower solvent content. Silicone resin emulsions, which are based on a pre-polymer and then emulsified, are known as secondary emulsions. 12. 2.5.2 Primary emulsification process
Another route for producing silicone resin emulsions, but with very little solvent, is the emulsion polycondensation process. In this primary emulsification process, liquid oligomer resin in the emulsion droplet is further condensed to solid resin. Alternatively, a silane/siloxane/silicone resin mixture is used from the start. Solvents are not required for the production of primary emulsions and the emulsions can therefore be very low in solvent content.
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12.3 Waterborne architectural paints 12.3.1 Synthetic emulsion paints There is a great variety of emulsion paints ranging from non-breathing, high-gloss to open, matt systems. The glossy coatings have higher binder content and are therefore less breathable, i.e. permeable to air/water vapour. Their closed films protect the facade very effectively from driving rain and damp, but they are not water vapour permeable. In contrast the over critical formulated (see Chapter 12.5 “Appendix”) matt emulsion paints which are able to breath require silicone resins or polysiloxanes to coat the capillaries. If silicone resins are used the paints are again in the category of silicone resin emulsion paints (see Chapter 12.3.5). 12.3.2 Silicate emulsion paints The characteristic binder for silicate emulsion paints is waterglass. Potassium silicates as a solution or as a hydrate, also known as potash water glass, are used in silicate emulsion paints. The potassium compounds show advantages over sodium water glass, such as better stability and better resistance to salt efflorescence. For stabilization, organic binders such as styrene acrylic emulsions are used together with waterglass. DIN 18363 specifies that the organic polymer content should not exceed 5 % of the total formulation. When other additives are incorporated, the silicate emulsion paints are more stable over many months than pure silicate paints and are ready-to-use. Pure silicate paints are two-component systems which must be mixed beforehand and allowed to stand before use. Silicate paints dry both physically through water evaporation, and chemically with many reactions taking place simultaneously. In silicification the alkaline silicate reacts with atmospheric carbon dioxide forming carbonates and silica gels [5] (see Figure 12.10).
Figure 12.10: Reaction scheme for silicates
Silicate paints react with mineral substrates to form potassium metasilicates and potassium hydroxide. Coatings based on waterglass are very brittle and have a typical matt, mineral-like appearance. They also have very open pores and therefore exhibit good breathability: To reduce capillary water absorption, linear (functional) polysiloxanes are used. The use of silicone resins is quite limited, due to the high pH value of the waterglass, which requires that only very alkali-stable emulsions, additives and pigments be used. Specific precautions are necessary such as:
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207
• eye protection
• avoidance of skin contact; the skin must be washed with much water immediately upon contact • overspray on glass, aluminium or painted surfaces, must be avoided due to strong corrosion
• only alkaline-stable inorganic pigments, additives and emulsions may be used [6]. 12.3.3 Emulsion paints with silicate character (SIL-paints) Emulsion paints with silicate character have been used for more than 25 years. As their formulations contain quartz powder or other mineral fillers they are particularly open-pored and capillary-active. Because of this, silicone resins or functional polysiloxanes are used to reduce the water absorption in SIL-paints. These kinds of paints are especially useful in maintaining the mineral-like matt optical properties. 12.3.4 Siloxane architectural paints with strong water-beading effect A few years ago paints, which were noteworthy for their strongly hydrophobic effect, became available commercially. The water repellency on the coating surface is so great, that water drops actually form beads, rather like they do on a lotus leaf. For such a water-beading effect contact angles in excess of 140° are needed. Paints of this type have become known as lotus-effect paints. This beading effect is the result of the use of suitable hydrophobing agents and a special surface texture of the coating. The use of certain fillers produces a micro-peaked-structure which, in combination with aminofunctional polysiloxanes, results in a strong water-beading effect. All raw materials must have as low an emulsifier content as possible, otherwise the beading effect will be impaired. Wetting agents and emulsifiers reduce the contact angle and destroy the water-beading effect. For this reason polysiloxanes are used in a solvent solution and not in the form of an aqueous emulsion. These coatings are remarkable for their low water absorption and their high water vapour diffusion. A disadvantage of lotus effect paints is their strong tendency to pick up dirt (see Table 12.3). 12.3.5 Silicone resin emulsion paints Silicone resin emulsions play an important role in silicone resin architectural paints, where they function as co-binders or hydrophobing agents. The amount of silicone resin used in such coatings ranges from 2 to 12 %. Often combinations of silicone resins with modified silicone oils are used. Because of their breathability,
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the coatings are formulated “over-critical” (see PVC), meaning they contain a low level of binder and high amounts of pigments and fillers. The silicone resins react quickly through alkoxy-reaction with themselves and with the fillers to form a network and thereby provide both early- and long-term water-repellency. Because the silicone resin only coats the pores of the coating, there is little or no influence on the coating’s water vapour permeability. The degree of the breathability of the coating film is primarily dependent on the type and amount of organic resin present (for example styrene/acrylic emulsions) and not on the amount of the silicone resin. There is also no difference between the hard silicone resins, which consist mainly of T-units, or the more flexible TD-silicone resins based on a higher amount of D-units. Because of the low surface tension and the resulting spreading potential, the silicone resins distribute themselves along the walls of both large and small pores without changing the pore radius [2]. Because of this it is doubtful whether silicone resins should be considered as binders and used in the calculation of the PVC. Silicone resins have a lower dirt pick-up potential than linear, non-drying polysiloxanes (silicone oils). It can be argued that the oily character of the polysiloxane and its thermoplasticity would favour adhesion of particles to the paint surface and hence a higher affinity for dirt pick-up. The phenomenon of dirt pick-up affinity is still not completely understood. What is known is that the attraction of dirt is influenced by the glass transition temperature (Tg), surface roughness, type of contamination (hydrophobic, hydrophilic) the weather conditions (Sahara weather or industrial) and many other factors. Description of the processes by which the major influencing factors interact with one another is not yet possible. The basic tendency to pick up dirt cannot simply be explained by the thermoplasticity or the hardness of the silicone resins, but is considerably more complex. This is outlined in Table 12.3 which provides a comparison of over-critical formulated coatings containing 8 % of an aminofunctional polysiloxane (emulsion paints 1& 2), a silicone resin with a high T-unit content (hard) and a TD-resin with low T-unit content (soft) (SREP 2 &1). w24 water absorption, breathing characteristics, contact angle and dirt pick-up were tested under natural weathering conditions and simulated conditions in a dirt pickup machine. In the artificial dirt pick-up machine, the softer TD-resin showed the lowest affinity for dirt. The harder silicone resin exhibited the greatest dirt pick-up in the machine, followed by both amino-dimethylpolysiloxanes. However under real weathering conditions both silicone resins showed similar values. The measured sd-values were also similar, whether oil or silicone resins were used. This leads to the conclusion that the degree of crosslinking of a silicone resin neither serves to predict tendency to dirt pick-up nor indicates any other particular advantages.
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Conclusions
Table 12.3: Comparison of different architectural paints Architectural paint
Waterabsorption
Water vapourdiffusion
Dirt pick up degree VG [%]
Contact angle [°]
w24-value
sd-value [m]
outdoor exposure (after 24 months)
simulated dirt pick up
initial
after 3 minutes
SREP paint 1 TD resin (soft)
0.06
0.089
20.1
9.4
104
60
SREP paint 2 TD resin (hard)
0.11
0.099
19.9
17.0
107
88
emulsion paint 1 aminosiloxane emulsion
0.09
0.088
21.4
14.6
120
62
dispersionspaint 2 aminosiloxane solution
0.09
0.083
22.2
16.5
135
121
lotus effect paint
0.08
0.073
25.7
29.7
148
145
silikate paint aminosiloxane emulsion
0.09
0.020
12.1
27.3
94
84
12.4 Conclusions
Because of the large number of silicone combinations, there is a wide variety of products available to the architectural paints sector. They allow the development and formulation of superior coatings for facade protection, which are both breathable and also resistant to driving rain and other weathering effects. Due to their high efficiency, their long-term effectiveness and their recoatability, polysiloxanes and silicone resins have replaced other types of hydrophobing agents com- Figure 12.11: Architectural paints after artificial dirt exposure mercially.
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12.5 Appendix 12.5.1 Facade protection theory according to Künzel In Künzel’s facade protection theory formulated in the 1960s, two properties play an important role: the water absorption capacity of the substrate and the water vapour permeability of the coating. Künzel noted the importance of a protective coating which is both moisture resistant and breathable in the sense of allowing water vapour to pass through. Two parameters are of crucial significance in this theory. The first is the water absorption coefficient (w-value). The second is the water vapour diffusion resistance (sd-value). The w-value is a measure of the timedependent capillary water absorption of a porous building material in kg/m2√h. The water vapour diffusion resistance or sd-value of the coating is expressed in metres. The greater the value, the higher the resistance and the less the diffusion of the paint film. For an optimal coating, both values should approach zero. Künzel proposed the ideal values for coatings at that time to be: Capillary water-uptake
w-value ≤ 0.5 kg/m2√h
Water vapour diffusion sd-value ≤ 2 m
Thus the product w * sd should be ≤ 0.1 kg /m√h (shown as a striped area on the graph, Figure 12.12). Coatings whose values lie inside the yellow area offered the greatest protection at that time (ca. 1960). Künzel’s work and the increasing quality demands on exterior coatings have resulted in the further optimisation of products in the past decade. Silicone resin architectural coatings can now attain w-values under 0.1 kg/m2√h and sd-values under 0.14 m. 12.5.2 Measurement of capillary water absorption (w-value) Capillary water-absorption is measured according to EN 1062-3. The substrate used for this test is calcareous sandstone.
Figure 12.12: Facade protection theory according to Künzel
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DIN EN 1062-3 calls for the coated testing-blocks to be exposed to water three times. This allows the water-soluble components of the
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Appendix
paint to wash out. A fourth exposure to water is then used for the determination of the w24-value. The w-value is defined as the amount of water absorbed in kg per surface area in m2 and the square root of the test time. Example: Water-absorption 0.5 kg/m2 after 24 hours water exposure:
Figure 12.13: Testing for water-pickup to EN 1062-3
The accuracy of the determined w-value becomes less with increasing film thickness. The w-value itself is therefore not dependent on the film thickness, as the ability of the substrate to absorb decreases as long as the barrier-effect of the coating remains intact. 12.5.3 Water vapour diffusion (sd-value)
According to Künzel, a quantitive relationship can be given between the water repellency of a coating and its breathability, i.e. its water vapour diffusion characteristics. The water vapour diffusion rate (V-value) and the diffusion-equivalent air layer thickness (sd-value) for exterior coatings on mineral surfaces and concrete are determined according to DIN EN ISO 7783-2
The sd-value is given in metres [m] and describes an equivalent thickness of an air layer at rest which has the same water vapour diffusion rate as that of the coating. The diffusion capability is classified as shown in Table 12.4. The value depends on the coating thickness and the PVC. The higher the PVC (always greater than the CPVC), the more porous the film and therefore the more breathable the coating. Coatings which have water vapour permeable films similar to those of silicate paints give sd-values < 0.14 m. Films impermeable to water vapour such as solvent-based acrylic enamels whose microporosity is extremely small, have values in the range of 2 m. Table 12. 4: Classification of water-pickup to EN 1062-3 Class
Water absorption
III
low
0.5
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w-value
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212
Hydrophobing agents
Table 12.5: Differentiation by the water vapour transmission rate Class I (high*)) II (medium*)) III (low*))
Water vapour transmission rate V g/(m ·d)
g/(m ·h)
sd m
>150
>6