Optimal dispersion. Seite/Page: 144

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1 Optimal dispersion Latex/pigment composite particles improve waterborne paints. The quality of paint is to a large degree dependent on the quality of the pigment particles dispersion. Latex particles containing a new proprietary acrylic monomer capable of superior binding to pigment surfaces show a significant increase in the dispersion of TiO 2 particles throughout the dry film. This leads to a considerable improvement in key paint performance properties. Marie Bleuzen*, William J. Rosano, Gary R. Larson, Leo J. Procopio. * Corresponding Author. Contact: Dr. Marie Bleuzen, Rohm and Haas, European Laboratories, 371 rue L. van Beethoven, B.P. 249, Sophia Antipolis Cedex, France, Tel , mbleuzen@rohmhaas.com The performance and appearance of pigmented waterborne coatings strongly depend on the degree to which the various pigments and polymer particles are dispersed throughout the dried film [1-4]. The ideal state of dispersion is usually reached when all particles are reduced to and remain as non-flocculated primary particles. Polymer particles, pigments and fillers that are not optimally dispersed can adversely affect the ability of the coating to provide an adequate barrier to electrolytes, water or gases, as well as the mechanical properties such as tensile strength, elongation, scrub resistance, and also optical properties, for example hiding power and gloss. In addition, poorly-dispersed pigments and fillers can affect the ability to add colourant as well as the paint's colour stability under the applied shear forces that occur during application by brushing or spraying. Increasing the latex-pigment interaction When pigments such as TiO 2 are well-dispersed, important properties such as hiding, film gloss, colour and the effectiveness of the coating as a barrier are maximised. A way to provide an optimised pigment distribution is to form polymer-pigment composite latex particles in the aqueous phase, by effectively increasing the interaction between individual TiO 2 and latex particles. During the drying process and film formation, this association can prevent TiO 2 particles from agglomerating, avoiding negative impacts on the performance. A new approach to achieve such composites is offered by using a newly developed, proprietary latex particle containing a specialty monomer ("SM"). The SM contains groups capable of binding to the pigment surface, which significantly improve pigment association, allowing for a more uniform pigment distribution and therefore enhancing coatings performance properties as well as optimising pigment usage. Experimental tests show a range of improvements in property performance such as better binding capacity in paints above cpvc (critical pigment volume concentration). Thus, scrub resistance and hiding power values typical for conventional systems are achieved at up to 20% less binder usage and up to 15% less TiO 2 usage. Alternatively, significant gains in scrub resistance and opacity occur at equal binder and TiO 2 usage. In paints formulated below cpvc, significant improvement occurs in stain-blocking and stain resistance, metal adhesion, corrosion resistance, film gloss and efflorescence resistance. The new technology is designed for high quality, low-voc, low odour interior wall paints as well as those for exterior applications, thanks to its pure acrylic composition. Experimental design The synthesis of emulsion polymers was based on standard emulsion polymerization techniques found in the literature [5,6]. The bulk compositions of the polymers were based on combinations of acrylic or acrylic-styrene monomers and a carboxylic functional monomer ("CFM") (control composition) or a specialty monomer (SM). Latex particle diameters were in the range from 100nm to 300nm, with glass transition temperatures of about 35 C unless noted otherwise. The non-volatile content of the polymers was about 40% by weight or greater. The polymer compositions tested differed in the degree of hydrophobic character where composition A was more hydrophobic (higher styrene content) than composition B. Emulsion polymers were formulated into screening formulations using exterior grades of TiO 2. All paints were prepared from dry-powder pigments and extenders. Film gloss was measured on coatings cast onto phosphate-treated, cold-rolled steel whereas corrosion resistance was determined on blast-cleaned, hot-rolled steel. Paints were applied with a drawdown bar to give about 3 mils (75µm) of dry film. Coatings were allowed to cure at 25 C and ~55% RH for approximately two weeks prior to testing. Homogeneous pigment dispersion helps gloss Table 1 compares the film gloss of paints based on copolymer compositions containing either the CFM or the SM. The data show that the gloss increased significantly in both compositions when CFM was replaced with the SM. The data also show that the level of gloss attained with the SM in both polymer compositions was approximately the same, whereas composition A was slightly higher. The cause for the gloss increase is thought to result from better dry-film morphology. Gloss retention of paints based on composition B was close to 100% for both monomer types for exposure times of about 1000 hours (UV-A/water condensation). The paint based on composition A retained ca 100% of its initial gloss when the polymer composition included the SM while the CFM based paint gloss dropped. Since neither the SM or CFM are expected to affect the response of the polymer to UV light, the observed gloss drop of the CFM based polymer of composition A may be related to differences in film swelling during the condensation cycle of the test. Table 2 shows the improvement in film gloss of binders based on the SM extended to lower TiO 2 loadings. The SM containing binder showed a much less rapid gloss decrease with increasing pigment loading (e.g. only about 10 units in 20 degree gloss at 11 PVC) compared to the CFM containing binder. The data show that, as the PVC approached zero, film gloss for both paints approached the same gloss values. Field emission scanning electron microscopy (FE-SEM) of cross sections of dried films (Figure 1) show that the TiO 2 distribution of the paint based on the SM modified latex was far more uniform compared to the paint based on the CFM modified latex. Atomic force microscopy (AFM) scans (images not provided here) were consistent with the FE-SEM images, showing that paints based on the SM modified latex had a more uniform TiO 2 distribution relative to CFM latex based coatings. In addition, surface roughness (Ra) values, calculated from the AFM image data, were 24 and 29 for the SM and CFM based coatings, respectively. The smoother surface of the SM based coating (smaller Ra)

2 is consistent with its higher film gloss. Improved hiding power A further consequence of a more uniform TiO 2 distribution using SM functional polymers is increased hiding power. Table 3 shows the contrast ratios calculated from Y-reflectance readings for paints formulated at 11 and 18 PVC. As can be seen, the SM containing binder gave higher contrast ratios compared to CFM for both PVCs. In fact, the contrast ratio of SM based paints at 11 PVC was about the same as the CFM based paint at 18 PVC, suggesting a potential reduction of TiO 2. Enhanced corrosion resistance properties The effect of monomer type on the ability of the coating to protect metal surfaces from corrosion during salt-fog exposure significantly improved when the CFM was replaced by the SM in both polymer compositions. Table 4 shows only a few blisters formed after a two-week, salt-fog exposure of paints based on the SM containing polymer in contrast to the dense blister formation on paints based on CFM binder. The reduction of blister formation for the SM containing coating was consistent with improved film-barrier properties as measured by EIS (electrical impedance spectroscopy) [7]. Figure 2 compares the calculated coating resistance of CFM and SM functional compositions as a function of exposure time to the NaCl solution. The results show the resistance (Rc) of the CFM based coating dropped by about two orders of magnitude after about 20 minutes of exposure. On the other hand, the resistance of the SM based coating remained high and unchanged showing better barrier properties compared to the CFM system. Higher abrasion resistance Polymers made with the new specialty monomer showed improved abrasive scrub resistance over the conventional CFM counterparts. Table 5 compares the mass loss after 200 scrub cycles of above critical PVC paints based on all-acrylic polymers. The data show the CFM based paint had a film-thickness loss of about 43µm while the SM based paint lost about 30µm. In addition to the data shown here, improved abrasive scrub resistance was also observed in paints formulated above and below critical PVC. Superior encapsulation The inclusion of the new SM in polymer emulsions results in the pigment particles being better encapsulated by the emulsion particles. Table 6 shows the calculated amount of latex adsorbed onto the TiO 2 for polymer compositions A and B - 25% to ca 40% for the SM containing compositions and 0% to ca 8% for CFM compositions. Assuming a latex particle size of 130 nm, the amount of latex needed to completely cover the TiO 2 (230 nm) surfaces, with ca 210 parts by weight TiO 2 is calculated to be about 30%. This suggests that the association of the latex onto the pigment surfaces was near saturation for the particles considered here, although the actual distribution of latex around the pigment is unknown. For example the latex may not necessarily be arranged in a monolayer, as this simple calculation might imply. Adsorption of latex onto the TiO 2 surfaces in the wet paint is consistent with dry-film SEM scans which show the TiO 2 was more evenly spaced for the SM based polymer compared to the CFM polymer. The samples based on the SM functional polymer show TiO 2 particles covered on the sides and tops with latex particles (Figure 3). That the coverage of latex particles appears to be less than expected is probably due to sample preparation procedures. Latex particles containing a specialty monomer provided waterborne coatings with significant improvements in film gloss, corrosion control, hiding and scrub resistance over the corresponding CFM containing analogues with no increase in the total formulation cost. Alternatively, formulators can, by using lower quantities of binder and pigment, optimise the formulation cost with no loss in performance. References [1] T. C. Patton, "Paint Flow and Pigment Dispersion", 2nd Ed., John Wiley and Sons, [2] S. J. Paul, "Surface Coatings-Science and Technology", John Wiley and Sons, [3] C. H. Hare, "Protective Coatings - Fundamentals of Chemistry and Composition", SSPC: The Society for Protective Coatings, [4] G. Larson, L. Procopio and W. Rosano, 2001 Proceedings of the International Waterborne, High Solids and Powder Coatings Symposium. [5] I. Piirma, "Emulsion Polymerization", Academic Press, [6] R. G. Gilbert, Emulsion Polymerization - A Mechanistic Approach", academic Press Limited [7] L. G.S.Gray and B. Appleman, SSPC 2002 Proceedings. Results at a glance - A new binder technology based on a latex-pigment composite formation results in an optimized dispersion of the pigment particles in water-based paints. - The impact of these acrylic-based, latex-polymer binders is due to a proprietary specialty monomer containing groups that significantly enhance the interaction between the polymer and inorganic particles. - The pigment particles are better encapsulated by the emulsion particles. - Significant improvements in many properties are achieved. - Formulators are able to choose: either get a wide range of performance improvements with no cost penalty, or optimize the formulation cost with no loss in performance - The new binder technology is designed for high quality, low-voc, low odour interior wall paints, as well as exterior applications, thanks for its pure acrylic composition. The authors: > William J. Rosano, received a Ph.D. from the Ohio State University in physical chemistry. He has worked at Rohm and Haas Company at Spring House Technical Center for 20 years. He works in the development of waterborne polymers, cross-linking chemistries and additives for the construction and industrial coatings industries. > Dr. Marie Bleuzen received her Doctorate in Chemistry from the University of Poitiers. In November 1991 she joined Rohm and Haas in their European Laboratories in Sophia Antipolis, Valbonne in Southern France. In January 2000 she moved to the architectural coatings segment and is responsible for the development of new binders and additives for the European market. > Gary R. Larson earned his BSc in chemistry from Western Michigan University in 1972 and a MA in chemistry in Currently he is working at Rohm and Haas Company at their Spring House Pennsylvania research laboratory on waterborne protective coatings for metal and maintenance coatings. > Dr. Leo J. Procopio received his Ph.D. in inorganic chemistry from the University of Pennsylvania. He joined Rohm and Haas Company at their Spring House, PA

3 research laboratories in He is currently a group leader in the architectural and functional coatings technical service department at Rohm and Haas.

4 Figure 1: FE-SEM of dried 18 PVC TiO 2 paints cast one on top of the other, based on composition A using CFM (top half of image) and SM (lower half of image).

5 Figure 2: Coating resistance (Rc) as a function of exposure time to a 5% NaCl solution as derived from EIS spectra of SM (red trace) and CFM (blue trace) based polymers formulated in white gloss paints.

6 Figure 3: FE-SEM (magnification 50,000) of diluted blends of TiO 2 and latex particles based on polymer composition A. The image on the left was based on the SM-containing latex while the image on the right was based on a CFM-containing latex. The image on the left shows latex absorbed around the TiO 2 particles.

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