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SPECIALTY MONOMERS FOR ENHANCED FUNCTIONALITY IN EMULSION

POLYMERIZATION

Pierre Hennaux, Nemesio Martinez-Castro, Jose P. Ruiz, Zhihua Zhang and Michael D. Rhodes

Solvay Inc. Centre for Research & Technology- Bristol, 350 George Patterson Boulevard, Bristol, PA 19007, U.S.A.

Driven by cost, ease of use, and an improved sustainability profile, the use of waterborne coatings has significantly increased over the last several decades. Emulsion polymer dispersions are utilized as the binder in the majority of these waterborne coatings. The desire to develop improved and differentiated performance in such formulations calls for a tailored design of the makeup of the emulsion polymer. Specialty monomers have been developed to enable specific desired functionality to be built into the polymer dispersion. Although only used at relatively low levels in the emulsion polymer synthesis, these specialty monomers can lead to a significant enhancement of the properties of the resulting paint formulation. The benefits generated by the use of these materials can be divided into four major functional areas: binder stabilization, coating adhesion promotion, coating surface improvement, and coating mechanical properties. In this paper, we will present an overview of these benefits as well as specific examples ranging from dramatic improvements in scrub resistance in high PVC paints to rheology control. POLYMERIZABLE STABILIZERS Surfactants are used to generate micelles which nucleate new polymer particles as well as stabilize the growth of these particles during emulsion polymerization.1 Surfactants also serve to lend stability to the resultant final latex dispersion in use. However, in some cases the use of anionic and/or nonionic surfactants is not enough to stabilize these particles. To improve stability, it is often necessary to incorporate electrostatic charges directly into the polymer backbone. Typically, acrylic or methacrylic acids are incorporated into the backbone to provide this stabilization. However, as a result of the weakly acidic nature of these carboxylic acids, these groups do not always provide sufficient stability to the dispersion. Polymerizable stabilizers are ionic monomers which are used to provide additional stabilization to the polymer particles.1,2 Unlike emulsifiers, these charges are covalently bonded into the polymer chain which limits migration and reduces the potential water sensitivity of the final film. Co-polymerizable stabilizer 1 (COPS-1) and Co-polymerizable stabilizer 3 (COPS-3) are both incorporated during the polymerization and provide additional stabilization to the particles. Furthermore, the incorporation of these monomers improves other properties such as scrub resistance, freeze-thaw stability, etc.

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Monomer COPS-1

This polymerizable stabilizer offers superior emulsion polymer properties by combining the characteristics of monomer and surfactant in one molecule. It is recommended for use in emulsion polymerization with acrylic, vinyl-acrylic and styrene-acrylic copolymers targeted for use in paints, adhesives, paper coatings and textile applications.

Monomer COPS 1 reacts with other co-monomers to incorporate into the polymer chain which restricts the ability to migrate, unlike a conventional surfactant. Table 1 lists the reactivity ratios of Monomer COPS-1 with a number of common bulk monomers. The reactivity ratio data show that the r1 values for Monomer COPS 1 are close to zero, suggesting that homopolymerization is unlikely.

Table 1. Monomer reactivity ratios for Monomer COPS-1 (M1).

Based on the Mayo Plots in Figure 1, this monomer has a preference to react with vinyl monomers such as vinyl acetate and vinyl veova. However, it has been found that this monomer also works well in both acrylic and styrene-acrylic systems.

Figure 1. Mayo plots for Monomer COPS-1

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Conventional surfactants are key components to provide latex stability, but the dynamic nature of their adsorption onto latex particles can lead to issues in both the wet state and the final film. By merely replacing some of the conventional surfactant with Monomer COPS 1, the foaming tendency of the latex can be reduced. Moreover, other dispersion stability properties can also be significantly improved.

In the study below, successful improvement in freeze-thaw stability has been achieved using Monomer COPS-1. The same amount of conventional emulsifier was used to prepare two acrylic co-polymer resins with similar solids content as well as particle size. Table 2 shows that the addition of 0.5 % (BOTM) of Monomer COPS-1 yields five freeze-thaw cycles in the resin. Use of Monomer COPS-1 also leads to improved mechanical stability.

Table 2. Properties of acrylic resins synthesized with and without Monomer COPS-1.

To probe the impact on foaming, conventional surfactant was partially replaced by Monomer COPS-1 (Table 3). Interestingly, it was shown that resins with less susceptibility to foaming and improved freeze-thaw resistance can be synthesized with otherwise comparable properties.

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Table 3. Effect of partial replacement of surfactant by Monomer COPS-1 on freeze-thaw stability and foaming

Monomer COPS-3 Monomer COPS-3 is a newly developed specialty monomer bearing functional groups which provide strong binding power to inorganic pigments and fillers as well as inorganic substrates such as glass, metal, metal oxide, etc. As a result, the use of latexes containing Monomer COPS-3 can promote improved water and scrub resistance, enhanced block and stain resistance, improved color acceptance, and anti-corrosion properties in the downstream paint formulation. During emulsion polymerization, it readily co-polymerizes with common monomers such as styrene, acrylates and vinyl acetate. This section details the benefits of using Monomer COPS-3 in different types of resins and the resulting property improvements when formulated into high PVC architectural paints. Monomer COPS-3 in a Styrene-Acrylic Latex Monomer COPS-3 (0.5% BOTM) was employed in a styrene-acrylic polymerization and Table 4 lists the resulting latex properties. It should be emphasized that the incorporation of Monomer COPS-3 did not affect the conversion and resulted in a resin that exhibited good mechanical stability and excellent freeze-thaw stability.

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Table 4. Properties of a styrene-acrylic resin synthesized with Monomer COPS-3

Additional studies were carried out using resins either without polymerizable stabilizer (control) or with Monomer COPS-3 from a standard styrene-acrylic polymerization procedure. These synthesized resins demonstrated comparable properties in terms of particle size, viscosity, coagulum level and solids content. Subsequently, both systems were cast into homogenous films and were evaluated for their water sensitivity and corrosion inhibition properties. As can be seen in Figure 2, the resin containing Monomer COPS-3 has significantly improved water resistance and anti-corrosion properties.

Figure 2. Water-sensitivity (left) and corrosion resistance (right) tests of resins synthesized without and with Monomer COPS-3. Experimental conditions: Water sensitivity was investigated by casting latex onto a glass substrate to form a 100 micron thick film which was then immersed in water for 24 hours. Corrosion resistance testing was conducted using a phosphate steel panel; the latex was casted onto the substrate to form a 100 micron thick film and was cured at room temperature with 60-70% relative humidity.

A flat paint with a PVC of 60 was formulated with the resins tested above and performance results are summarized in Table 5. The paint formulations are identical with the exception of the change in resin. Both scrub and stain resistance improved with the resin containing Monomer COPS-3.

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Table 5. Scrub and stain resistance of paints formulated with a styrene-acrylic resin without polymerizable stabilizer and one containing Monomer COPS-3

A resin containing Monomer COPS-3 was also evaluated in comparison to a commercially available resin which contains phosphate ester monomer. The commercially available resin is well regarded as a high quality benchmark in the coatings market. High PVC architectural paints (flat paint) were formulated with both resins and Table 6 shows the formulated paint properties.

Table 6. Properties of paints formulated with a commercially available resin and a resin synthesized with Monomer COPS-3

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As can be seen on Table 6, the resin synthesized with Monomer COPS-3 provides excellent scrub resistance at low temperature and improved mud cracking. All other properties are comparable. Monomer COPS-3 in a Vinyl-VeoVa latex

Vinyl based polymer resins such as vinyl-veova, vinyl-acrylic and VAE have been widely used in various applications such as architectural coatings, adhesives, paper coatings, construction, etc. Relative to acrylic and styrene-acrylic resins, vinyl based resins tend to suffer from more issues related to water sensitivity. To determine whether Monomer COPS-3 can be used to improve scrub resistance in high PVC paints, this co-polymerizable stabilizer was incorporated into the synthesis of a vinyl-veova resin. 1% (BOTM) of Monomer COPS-3 was employed in conjunction with both anionic and nonionic surfactants. A vinyl-veova resin control (without Monomer COPS-3) was also synthesized using the same standard polymerization procedure. The resins were formulated into high PVC paints (75) and the results are summarized in Table 7. For comparison purposes, one additional paint was also formulated with a commercially available styrene-acrylic resin which is labeled as commercial benchmark in the table below. By incorporating Monomer COPS-3 into the vinyl-veova polymerization, the scrub resistance of the flat paint with 75 PVC was significantly improved relative to the other resins.

Table 7. Scrub resistance of paints formulated with the resins

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SPECIALTY MONOMERS FOR HASE THICKENERS

Paints and related coatings are complex systems usually comprised of three main components: pigments, a liquid phase (water or organic solvent), and a film forming polymer (latex or solution polymer). In addition to these main components, low levels of other property-enhancing additives are often utilized. The use of waterborne coatings has increased due to factors such as the ease of cleanup, the lower fire hazards due to the lack of solvents as carriers, a reduction in environmental impact, etc. Although waterborne coatings possess some intriguing advantages, there are difficulties that have to be overcome before successful applications can be made. One of the most important issues is the control of rheology, which determines factors such as the quality and appearance of the final film. Thickeners are used in paint to achieve desired rheological properties at various shear rates, which influences properties such as flow and leveling, sag resistance, brush loading, sedimentation resistance, etc. It is also possible to give the paint system a yield point or a time-dependent rheology by choosing the right thickener.3,4

Thickeners used in waterborne coatings are typically classified as either non-associative or associative. Non-associative thickeners adjust viscosity by thickening the continuous aqueous phase through expansion in hydrodynamic volume due to associations with water. Associative thickeners adjust viscosity through interactions with the hydrophobic polymer particles surfaces to form a connected network structure. This associative thickening mechanism can provide a wider range of latitude in controlling the rheology profile over both low and high shear conditions. A type of associative thickener commonly found in latex paints is known as hydrophobically-modified alkali-swellable emulsions (HASE).5,6 HASE thickeners are based on a polyelectrolyte backbone, typically a methacrylic acid and ethyl acrylate copolymer, for water solubility and with pendant hydrophobes (i.e., hydrophobes that are attached to the backbone with polyethylene oxide side chains) for associations. Since the HASE thickeners depend on hydrophobic associations with polymer particle surfaces, the performance of HASE thickeners can be sensitive to variations in the composition of the formulation.7 Changes in the type and level of binder, surfactant, and colorants can have a pronounced effect on paint viscosity. Therefore characterization of HASE thickeners in binder and paint formulations is very important. The work detailed below studied the influence of the composition of HASEs on the rheological profile in various systems by using different specialty monomers. The performance of these HASE thickeners was also compared with commercially available thickeners. The specialty monomers for HASE thickeners are a type of methacrylic monomers with pendant hydrophobes. Typically, the mentioned monomers are comprised of a mixture of active, methacrylic acid and water (Figure 3).

Figure 3. General chemical structure of a specialty monomer for HASE thickeners.

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Several HASE thickeners were synthesized by emulsion polymerization and tested in paints to achieve particular rheological properties. Performance was fine-tuned by changing polymerization parameters such as the monomer type, composition, etc. The potential number of polymerization parameters to synthesize these thickeners is large, but for an initial study a standard formulation was chosen and only the specialty monomer was changed. As can be seen in Figure 4, different rheological characteristics are generated when even just the specialty monomer is changed.

Figure 4. Rheological profiles achieved with HASE thickeners containing various specialty

monomers

To test whether these specialty monomers can be used to synthesize HASE thickeners with viscosity profiles similar to a commercial benchmark, rheological measurements were carried out with an acrylic binder formulation. As shown in Figure 5, only a slight difference in viscosity profile was found between the commercial material and a sample generated with specialty Monomer A. Note that these results will depend on the specifics of the formulation tested.

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Figure 5. Rheological profiles of a HASE thickener containing Monomer A and a commercially available thickener

HASE thickeners synthesized with specialty Monomer D and specialty Monomer E were evaluated in a styrene-acrylic semi-gloss paint formulation. The significant influence of the nature of the hydrophobic component of the monomer is illustrated by the rheological measurements in Figure 6.

Figure 6. Rheological profiles of semi-gloss paints styrene-acrylic binder formulated with HASE thickeners containing Monomer D and Monomer E

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Waterborne paint is typically formulated to target a certain Stormer viscosity (KU) and ICI viscosity. These viscosities at mid and high shear determine several important paint properties. The data in Table 8 illustrate that the change in specialty monomer not only can influence thickener efficiency, but also the relative level of contribution to KU and ICI viscosity to help tune the specific rheology profile desired.

Table 8. Viscosity comparison of semi-gloss paint formulated with HASE thickeners containing Monomer D and Monomer E

In spite of many important advantages, associative thickeners are inherently more sensitive to variations in the composition of the coating formulation since other materials present can disrupt the associations used to form the particle network. For this reason, paints thickened with associative rheology modifiers are also more sensitive to the addition of tinting colorants which contain a significant amount of surfactants and/or dispersants which can result in large viscosity drops. Often, a consequence of this viscosity loss is inferior application properties such as poor film build, sag and severe color float. As tinted and colored paints become more popular, the need for rheology modifiers that can withstand the addition of colorants without significant viscosity loss has gained importance. To demonstrate the influence of the hydrophobe type utilized, HASE thickeners were generated with different specialty monomer types and compared with a commercially available product. As shown in Figure 7, HASE thickeners with improved resistance to surfactant addition can be created by changing the specialty monomer type employed in the synthesis of the thickener.

Figure 7. KU stormer viscosity of HASE thickeners and its drop after surfactant

addition (0.8 % wt SDS)

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References 1) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers, John Wiley & Sons, NY, USA, 1997. 2) Gonzaleza, I.; Mestachb, D.; Leizaa, J. R.; Asua, J. M. Progress in Organic Coatings, 2008, 61, 38. 3) Lau, A. K. M.; Tiu, C.; Kealy, T.; Tam, K. C.Korea-Australia Rheology Journal, 2002, 14, 1. 4) Calbo, L. J. Handbook of Coatings Additives, Marcel Dekker, Inc., New York, 1987. 5) Glass, J. E. Polymers in Aqueous Media, American Chemical Society, USA, 1989. 6) Glass, J. E. Associative Polymers in Aqueous Solutions, American Chemical Society, USA, 2000. 7) Florio, J. J.; Miller, D. J. Handbook of Coatings Additives, Marcel Dekker, Inc., New York, 2004.