Modified alkyd emulsions for waterborne coatings Authors: Company: Contact: Dr. Carolin Wallenhorst Reinhard Winter ASK Chemicals GmbH Dieselstraße 35 41, 42489 Wülfrath, Germany www.ask-chemicals.com Diógenes de Lima Silva ASK Produtos Químicos do Brasil Ltda Km. 103 da Via Anhanguera, Bairro de Aparecidinha 13068-616 Campinas Sao Paulo, Brazil 0055 (19) 3781 1314 diogenes.silva@ask-chemicals.com Introduction Due to modern ecological- and employment protection guidelines, solvent-based coatings get more and more under pressure and already have strict legal requirements in certain countries. According to those requirements, the legal limit value for VOC contents can only be fulfilled with waterborne or solvent-reduced coatings technologies. Regulations to reduce VOC during production and application of coating materials are only slowly being introduced in Brazil. But even without legislation waterborne coatings have big advantages in comparison to solvent-based types as they provide a lower smell level and are incombustible. Presently, Brazil is a major user of solvent-based coatings especially with regard to industrial applications. The market trends for various coating technologies are listed in table 1 but due to changing economic situation the growth factor might be lower nowadays than published in the market study from 2013. However, the market for waterborne coatings in Brazil is expected to increase within the next years. The improved availability of raw materials for formulating waterborne system over the last five year in Brazil confirms this trend. It is worth mentioning that the amount of raw materials produced by local suppliers is increasing as well. Most commonly known resin technologies for formulation waterborne coatings are acrylic dispersions. For industrial applications, PU dispersions and epoxy systems are used as well. Table 1: Brazilian market for various coating technologies through 2018 ($ Mio.) source: BCC Research, ISBN 0-89336-649-8, 2013 Coating Technology 2011 2012 2013 2018 CAGR% 2013-2018 Solvent-based coatings 1,920 1,910 1,900 1,750-1.6 Waterborne coatings 1,420 1,510 1,640 2,250 5.7 High-solid coatings 660 800 815 1,130 6.8 Powder coatings 480 580 730 1,035 7.2 Radiation cure coatings 320 350 370 515 6.8 Emerging coatings 140 180 230 332 7.6 1
The present paper concentrates on the class of alkyd resins resp. alkyd emulsions. Alkyd resins feature various advantages in paint and coatings formulations. One of the most familiar one is the high content of renewable raw material, e.g. vegetable oils. Well-established benefits are high tolerance against other raw materials and compatibility with other resins systems, good wetting properties of pigments, good adhesion on different substrates and last but not least high gloss and flexibility of coatings. The drying properties as well as the tendency to yellowing during ageing can be unfavorable but depend on the resin modification and coatings formulation. This correlation will be explained within this paper. Alkyd emulsions Alkyd resins are the reaction product of an oil or fatty acid with certain polyols and polyacids. Such polymers having a high viscosity need to be diluted in solvents to become applicable for paint and coatings formulations whereas water is of course the best diluent from an ecological point of view. There are different procedures to emulsify alkyd resins. Depending on the emulsification process the products can be divided into three generations. In the first generation the hydrophilic properties of alkyd resins are adjusted by introduction of carboxyl groups into the resin structure, resulting in high acid values of ~ 45 75 mgkoh/g, followed by neutralization of the resin with amines to increase the water-solubility of the resin. The addition of co-solvents (e.g. butylglycol) is still necessary to reduce the viscosity to an applicable level. The emulsions are alkaline with ph-values of 7.2 8.5. The second generation of alkyd emulsions is produced by addition of an external emulsifier to a mixture of alkyd resin and water followed by emulsification. Compared to emulsions of the first generation a higher molecular weight of the resin is applicable. But in most cases pre-dilution of the resin in a co-solvent is still necessary. Please note that the use of well-known nonylphenolethoxylates (APEO, NPEO) will be liable to registration in the near future. The technology of the third generation is based on alkyd resins, which are modified with hydrophilic monomers followed by emulsification in water. There is no external emulsifier and no co-solvent needed. The amine content is very low, thus the emulsions have neutral ph-values. Alkyd emulsions achieve higher water and corrosion resistance when using an internal emulsification process and avoiding an excess of amine in the binder system. Resin degradation due to saponification is prevented. The paper on hand focuses on this kind of emulsions. The use of alkyd emulsions in coatings applications has certain advantages as mentioned above, like the increased environmental friendliness due to the lower VOC content. From an economic point of view the absence of organic solvent and therewith the production of incombustible paints and coatings has cost benefits as no explosion-protected equipment is needed in manufacturing. In addition, the remaining equipment is easier to clean. Alkyd emulsions feature an unfavorable characteristic, which needs to be mentioned. The evaporation of water is slow and depends on air humidity thus the drying properties of waterborne coatings based on alkyd emulsions appear to be challenging. PU-modified alkyd emulsions Alkyd resins and their emulsions dry according to two different mechanisms. The physical drying mechanism is based on evaporation of the water or the solvent. The evaporation itself cannot be influenced by chemicals treatments but higher molecular weight of the resin systems helps to improve physical drying. The chemical drying mechanism is based on oxidative drying induced by the double bonds of fatty acids and siccatives, e.g. Co, Mn, Zr. Two-component coatings are cured by crosslinking of the alkyd resin with isocyanates or melamines. 2
Due to the reaction of an isocyanate with the alkyd polymer chain, the molecular weight of the alkyds increases (figure 1). This goes without influencing the viscosity of the polymer. The increased molecular weight creates then improved physical drying properties. Figure 1: Illustration of PU-modification of alkyd resins to increase molecular weight with grey = phthalic anhydride, blue = glycerol, yellow = fatty acid, orange = isocyanate PU-modified alkyd emulsions are used to formulate air- and forced drying industrial coatings. For top coats aliphatic PU-modified alkyd emulsions are preferred. They offer fast drying, outstanding gloss, good UV resistance and very low yellowing tendency. Table 2 shows the application properties of three white top coats (T1, T2, T3). All top coats are formulated according to the same coatings recipe (table 3). The alkyd emulsions in the recipe are in each example based on the same raw materials, amongst others drying fatty acids with low yellowing tendency, and the same manufacturing procedure. The oil lengths is 35%, solid content is around 45% and the ph value is neutral. The emulsions only differ in the level of PU-modification. Drying times decrease significantly with increased PU-modification. The hardness of the remaining top coats improve with increasing PU-modification. With a higher PU-level, the minimum film-forming temperature increases as well. With regard to top coat T3, a loss of gloss is possible at drying temperatures below 40 C which can be avoided by forced drying or addition of solvents (1-2% butylglycol). Other application properties, like adhesion and flexibility of the coating remain unaffected by the PU-modification. Table 2: Application properties of white top coats based on alkyd emulsions with aliphatic PUmodifications with T1 = low PU-level, T2 = moderate PU-level, T3 = high PU-level Application properties, top coats T1 T2 T3 PU-modification of alkyd resin: low moderate high Shear resistance: dissolver dispersion pearl mill dispersion Minimum film-forming Temperature: low higher Surface drying (drying recorder): 1 h 30 min < 20 min Through drying (drying recorder): 10 h 4 h 1 h Hardness, König (steel, 30-45 µm): 55 sec 71 sec 102 sec Gloss (Gardner, 60 ): > 90 > 90 > 90 Gloss (Garnder, 20 ): > 85 > 85 > 85 Cross cut test: Gt 0 Gt 0 Gt 0 Cupping test, Erichsen: 8 mm 8 mm 8 mm The PU-modification improves weather and water stability of the coatings. A lack in performance regarding humidity resistance can be produced on purpose by addition of a high amount of hydrophilic additives in the starting formulation based on table 3. The top coat based on alkyd emulsion T2 in figure 2 shows lower humidity resistance i.e. blistering, increased haze, loss of gloss after 72 h in humidity chamber compared to the top coat based on alkyd emulsion T3 with increased PU-level. 3
Figure 2: Comparison of humidity resistance of PU-modified alkyd emulsions resp. their corresponding formulated top coats based on table 3 including additional hydrophilic additives with T2 = moderate PU-level and T3 = high PU-level T2 T3 Table 3: Coating recipe of white top coat based on alkyd emulsions with different levels of aliphatic PU-modification with T1 = low PU-level, T2 = moderate PU-level, T3 = high PU-level Recipe, top coat Wt. (%) Alkyd emulsion T1, T2 or T3 62.80 Pigment paste 35.00 Thickener 0.30 Wetting agent 0.20 Zr-Drier 0.50 Fe-Drier 0.15 Defoamer 0.30 Demin.water 0.75 Total 100.00 For primers on wood and metal cost efficient aromatic PU-modified alkyd emulsions are preferred. They combine fast drying, good corrosion resistance and good compatibility with anti-corrosive pigments. Table 4 summarizes the application properties of an anti-corrosive primer (P). Alkyd emulsion P is based on sunflower oil with oil lengths of 20%, solid content of 45% and neutral ph-value. Table 4: Application properties of anti-corrosive primer with aromatic PU-modified alkyd emulsion P Application properties, anti-corrosive primer P Substrate, film thickness: cold rolled steel, 40-45 µm Gloss: matt Hardness, König: > 90 sec Cross cut test: Gt 0 Cupping test, Erichsen 8 mm Salt spray test, 500 h: m0/g0 no underfilm corrosion 4
Table 5: Coating recipe of anti-corrosive primer based on aromatic PU-modified alkyd emulsion P Recipe, anti-corrosive primer Wt. (%) Alkyd emulsion P 36.5 Demin. water 5.0 Wetting agent 0.5 Defoamer 0.3 Pigment 7.3 Filler 7.3 Filler 10.0 Zinkphophate 7.3 Alkyd emulsion P 18.2 Zr-Drier 0.4 Fe-Drier 0.2 Thickener 0.2 Adhesion promotor 2.5 Demin. water 4.2 Total 100.0 For formulating baking enamels unmodified alkyd emulsions in combination with aminoplasts like melamine or urea/formaldehyde resins are recommended. There is no need to further improve the drying properties by PU-modification of the alkyd emulsions. These enamels exhibit high gloss and low yellowing tendency in combination with good chemical resistance. Unmodified alkyd emulsions can be used in combination with acrylates to improve flow characteristics and gloss as well. Depending on further modifications, alkyd emulsions can be used for various other applications, e.g. stain-blocking paints, 2K PU coatings and trim paints. Modified alkyd emulsions for house and trim paints Waterborne paints already meet the requirements of trim paints and house paints with regard to gloss, hardness, elasticity and weather resistance. However, the usage of solvent-based paints is preferred with regard to flow properties. Compared to solvent-based systems with good brushability, conventional aqueous trim paints have pseudoplastic viscosity behavior, i.e. after the shear stress caused by brushing, the viscosity increases considerably and the paintbrush strokes only spread insufficiently. The flow behavior of aqueous trim paints improve considerably through rheological modification of an alkyd emulsion. The resulting Newtonian viscosity behavior of alkyd emulsion H1 is comparable to those of a good solvent-based system (figure 3). The corresponding coating formulation of the trim paint (H1) is listed in table 6. The flow and levelling properties are excellent and exhibit an open time of more than 10 minutes. The block resistance is more than sufficient as well. Alkyd emulsion H1 is based on drying fatty acids with low yellowing tendency, the oil lengths is 35% and the solid content is 41%. The emulsion is aliphatic PU-modified to optimize drying properties. 5
Figure 3: Viscosity profile of blue = waterborne trim paints with external thickened binder, green = solvent-based trim paint with optimal leveling and red = waterborne trim paints with rheological modified alkyd emulsions H1 As alkyd emulsions based on drying fatty acids tend to exhibit yellowing and odor formation, ASK Chemicals developed a new formulation of alkyd emulsion having improved non-yellowing performance and less odor formation. Figure 4 shows the comparison of yellowing in the dark of the two coatings based on the recipes in table 6 with alkyd emulsion H1 and alkyd emulsion H2. Figure 4: Comparison of yellowing in the dark after four months of a trim / house paint based on rheological and aliphatic PU-modified alkyd emulsion with H1 = original recipe and H2 = improved nonyellowing performance Covered during storage H1 H2 Uncovered during storage Table 6: Coating recipe of waterborne trim / house paint based on a rheological and aliphatic PUmodified alkyd emulsion H1 resp. H2 Recipe, trim / house paint Wt. (%) Alkyd emulsion H1 or H2 63.7 Pigment paste 34.0 Zr-Drier 0.3 Fe-Drier 0.2 Defoamer 0.3 Demin. water 1.5 Total 100.0 6
Coatings based on alkyd emulsion H2 exhibit in other respects the same application properties as coatings based on alkyd emulsion H1, e.g. an open time of more than 10 minutes, excellent block resistance and a high gloss level. Furthermore, the emulsions do not contain both solvents and plasticizers. This gives rise to a binding agent for formulating VOC-free trim paints with excellent flow properties. Conclusion Various options to modify alkyd resins resp. alkyd emulsions for formulating waterborne coatings have been explained in this paper. Alkyd resins can be modified with hydrophilic monomers to become applicable for emulsification without the help of an external emulsifier. There is no need to add cosolvents to the corresponding emulsions. Due to the low amine content and therewith the neutral phvalue of the emulsions, the hydrolysis resistance and storage stability increases. The alkyd emulsion have high shear resistance and provide excellent wetting and adhesion properties and various substrate, also on ABS and PS. Alkyd resins can be modified with different isocyanates to improve drying properties via increased molecular weight of the polymer. Aliphatic isocyanates are used for top coats and cost-efficient aromatic isocyanates are used for primers, if yellowing does not limit the application properties. Additional rheological modification results in alkyd emulsions having a Newtonian viscosity behavior to achieve adequate flow and levelling properties and excellent blocking resistance for formulating waterborne trim paints. By further modification of the resin formulation the yellowing tendency as well as the order formation of the alkyd-based trim paint is decreased. In a nutshell, modified alkyd emulsions exhibit very good application properties in combination with environmental sustainability and prepare the way for converting from solvent-based to waterborne coatings, while simultaneously keeping performance at a high level. Acknowledgements The authors would like to thank their colleagues Dr. Jennifer Bißmeyer and Monika Kokoschka for technical support. 7