Direct observation of freeze-thaw instability of latex coatings via high pressure freezing and cryogenic SEM.

Despite its industrial importance, the subject of freeze-thaw (F/T) stability of latex coatings has not been studied extensively. There is also a lack of fundamental understanding about the process and the mechanisms through which a coating becomes destabilized. High pressure (2100 bar) freezing fixes the state of water-suspended particles of polymer binder and inorganic pigments without the growth of ice crystals during freezing that produce artifacts in direct imaging scanning electron microscopy (SEM) of fracture surfaces of frozen coatings. We show that by incorporating copolymerizable functional monomers, it is possible to achieve F/T stability in polymer latexes and in low-VOC paints, as judged by the microstructures revealed by the cryogenic SEM technique. Particle coalescence as well as pigment segregation in F/T unstable systems are visualized. In order to achieve F/T stability in paints, latex particles must not flocculate and should provide protection to inorganic pigment and extender particles. Because of the unique capabilities of the cryogenic SEM, we are able to separate the effects of freezing and thawing, and study the influence of the rate of freezing and thawing on F/T stability. Destabilization can be caused by either freezing or thawing. A slow freezing process is more detrimental to F/T stability than a fast freezing process; the latter actually preserves suspension stability during freezing.

Keywords: SEM, cryogenic SEM, latexes, colloids, emulsions, waterborne, physical properties, freeze-thaw stability, flocculation, acrylics, VOC, architectural

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The development of latex paints based on waterborne emulsion polymer technology has made it possible to substantially reduce the amount of volatile organic compounds (VOCs) released in comparison to organic solvent-based paints. In recent years, environmental concerns and legislative measures have created a new driving force for the coatings industry to develop low- or zero-VOC waterborne coating formulations. The real challenge is that the lower-VOC products must meet or exceed the performance standards expected from conventional higher-VOC (> 250 g/L) compositions. The need to meet or exceed such performance standards places a premium on the characteristics and properties of aqueous polymer dispersions.

Freeze-thaw (F/T) stability of waterborne paints, for example, has long been achieved by the addition of antifreezes such as glycols. (1) The glycol level in a conventional latex paint with 100 to 250 g/L VOC typically ranges from 2% to 6%, based on total paint weight. A lower-VOC requirement (< 50 g/L) means that the glycol level has to be further reduced and new solutions have to be developed. F/T stability is of considerable practical importance because of the need to store and transport paints within cold-climate countries during winter and because of the energy and hardware costs that will otherwise incur if heated transportation and storage are necessary. The F/T stability requirement is such that a formulated paint maintains its original properties, such as rheology and absence of coagulum, after repeated freeze-thaw cycles. In a coating formulation with low or zero VOC, the amount of glycols that can be added is most probably no longer sufficient to impart F/T stability by depressing the freezing point of the aqueous medium and by altering the pattern of ice crystal formation. It is a goal that the next generation of polymer dispersions be improved in such a way that they are capable of achieving F/T stability, which is weakened by glycol level reduction.

Despite its importance, the fundamental aspects of F/T stability have not been studied extensively and are not well understood. Perhaps because of the lack of adequate experimental and analytical techniques, it has not been possible to elucidate microstructural changes that occur during the F/T processes. Blackley (2) gives a comprehensive review of colloidal destabilization of latexes by freezing. The freezing process starts with the reduction of temperature which leads to the appearance of a third phase: pure ice. The formation of ice crystals progressively increases the polymer particle concentration in unfrozen water and the frequency of encounters between particles. Eventually polymer particles are forced into contact with each other under the high pressures from growing ice crystals, resulting in rapid particle aggregation and interparticle coalescence. In the case of F/T instability with polymer latex, it has not been possible to conclude whether coagulation takes place during freezing or during thawing, because one can judge stability only after thawing. For this reason, freeze-thaw stability covers both the freezing and thawing processes without pinpointing the precise stage in which colloidal destabilization actually occurs. Although Blackley discussed several factors expected to affect interparticle coalescence during freezing and thawing, no technique was available to demonstrate that interparticle coalescence was really involved.

In one of the earliest systematic studies of freeze-thaw stability in latex paints, Digioia and Nelson. (3) studied the F/T stability of an entire range of styrene-butadiene copolymer latexes with [T.sub.g] from -85[degrees]C (polybutadiene) to 100[degrees]C (polystyrene). Surprisingly, there did not appear to be a correlation between the F/T stability and the [T.sub.g] of the copolymer. By comparing various test methods, they also found that the freezing conditions had a direct relationship with the ability of the paint to regain its original state. Low freezing temperature, slow freezing rate, and long freezing time all had detrimental effects on freeze-thaw stability. Smaller sample sizes unexpectedly tended to cause more failures in the freeze-thaw test.

Barb and Mikucki (4) have studied the particle size characteristics of coagulate produced by freezing and thawing. Most of their work was based on cationic polystyrene (PS) latexes with fine particle sizes (50-100 nm). They compared the appearance of the electrolyte- and freeze-thaw-coagulated PS latexes. The electrolytically coagulated material is a dull, completely amorphous-looking fine powder whereas the freeze-thaw coagulated one is a very coarse grit of transparent glistening particles of up to several millimeters in diameter that have a strong superficial resemblance to a crystalline material. Electron microscopic examination revealed that the fine powder produced in electrolytic coagulation consists of the primary latex particles, with no significant interparticle coalescence having taken place. During the freezing process, in comparison, the coagulum is formed as a result of compaction or molding of the primary latex particles against the surrounding ice crystals. Neither bulk glass transition temperature nor the molecular weight (60 thousand to 4 million Daltons) appears to correlate with F/T stability.

King and Naidus (5) and Naidus and Hanzes (6) have studied major factors contributing to the stability of polymer emulsions to freezing and thawing: (1) copolymerization with vinyl carboxylic acids, (2) grafting of monomer to surface-active agents, (3) post-additions of glycols, salts, surfactants, etc. to the polymer emulsion, and (4) level and type of alkali used in neutralization. An important contribution by these authors, which forms the foundation of most of the future development in this area, is the clear demonstration that the presence of hydrophilic polar groups on the particle surface greatly improves freeze-thaw stability. Contrary to the findings by Digioia et al. (3) and Barb et al. (4), they have established a quantitative correlation between F/T stability and the [T.sub.g] of the polymer in the particles. The harder polymers require less protective groups to achieve F/T stability than the soft ones.

Blackley and Teoh (7) have confirmed that the [T.sub.g] of the dispersed polymer has a profound effect upon freeze-thaw behavior, although their results do not suggest that hard polymer particles are always more stable. They also found that through post-addition of surfactants it is possible for certain polymer latexes to transition from F/T instability to F/T stability. Investigating the post-additive approach, Cockbain et al. (8) have found that sodium o-hydroxybenzoate is an effective freeze-thaw stabilizer for ammonia-preserved natural rubber latex.

Nakamura and Okada (9) studied the effects of freezing-thawing and particle size on the coagulation of pharmaceutical particles in suspension. They also tested whether large coagulates of "closely packed particles" produced by a freeze-thaw cycle could be redispersed by "vigorous shaking." They concluded that smaller particles tend to form coagulates that are more resistant to redispersion. With optical microscopy up to 80x magnification, Nakamura and Okada (10) observed ice crystal growth and movement of chloramphenicol particles during freezing and thawing of thin layers of suspension. They found that at low rate of freezing, few crystals nucleate and they grow slowly as "needles." The particles are excluded from, and pushed along by the surfaces of, the crystals. Ultimately, their colloidal stability is lost and coagulation takes place. At high rate of freezing, many crystals nucleate almost simultaneously and grow rapidly, particles are left stranded and uncompacted. Nakamura and Okada's studies are quite relevant to the F/T stability of latex suspension. However, the pharmaceutical particles they studied, 0.8 to 70 microns in diameter, were much larger than latex particles. Moreover, the resolution in their microscopy was relatively low.

By incorporating polyethylene glycol moieties, Ottewill et al. (11) have shown that the freeze-thaw stability of polystyrene latexes are improved. Farwaha et al. (12) and Okubo et al. (13) have found that by using polymerizable polyethylene glycol monomer, latex binders can be synthesized for applications in paints which are substantially free of volatile coalescents and freeze-thaw additives. Using polymerizable ethoxylated surfactants, Zhao et al. (14) have also achieved freeze-thaw stability in low-VOC coatings.

Similar to the freezing and thawing conditions, architectural paints may also be exposed to excessive heat during transportation and storage. Exposure to heat may cause a viscosity increase or even gelation of the paint. Achieving heat storage stability is generally a matter of trial and error. Compared with the subject of F/T stability, studies of heat storage stability are even more scarce. Kostansek (15) has studied the interactions at ambient conditions between three key components in latex paints: associative thickener, surfactant, and polymer particles. Two unstable (flocculation) regions have been identified depending on the concentration of each component. Diebold et al. (16) have recently investigated the mechanism of Ti[O.sub.2]/ZnO instability in latex paints. Surface charges of the inorganic pigments were analyzed as a function of pH. It is shown that within a certain pH range, between 6 to 9, for example, Ti[O.sub.2] and ZnO particles are attracted to each other due to their opposite surface charges, resulting in a zone of instability. Like F/T stability, heat storage stability deserves more extensive studies for which Cryo-SEM can be a useful tool. We report on preliminary results which show the microscopic changes occurring during heat storage. These microscopic changes bear some similarities to those taking place in F/T unstable systems.

It would be desirable to compare the efficiency of various stabilizing functionalities based on prior and current work. However, the differences in test methods and in the processes of incorporating the stabilizing functionality make such a comparison unfair and unreliable.

EXPERIMENTAL

Latex Synthesis

A series of acrylic polymer dispersions was synthesized based on methyl methacrylate (MMA) and n-butyl acrylate (nBA) with a minimum film formation temperature target around 0[degrees]C (MMA/nBA = 40/60). They were all stabilized by 1 wt% (based on total monomers) of sodium dodecyl sulfate surfactant (Aldrich) and had particle sizes of ca. 130 nm in diameter, resulting from a seeded emulsion polymerization process. The only variable was the amount of the copolymerizable polyethylene glycol monomer: methoxypolyethylene glycol methacrylate, MPEG 750 MA (Degussa), with approximately 17 units of polyethylene glycol units.

Freeze-Thaw Stability Test

The freeze-thaw stability of polymer dispersions and formulated paints was evaluated based on ASTM standard test method D 2243-95. Typically, a pint of latex or paint (in a one-pint plastic can) is stored at 0[degrees]F (-18[degrees]C) for about 17 hr and then removed and is allowed to thaw for 7 hr undisturbed at ambient temperature. This completes one freeze-thaw cycle. The thawed latex or paint is gently stirred by hand with a tongue depressor before the viscosity is measured (by a Stormer viscometer in the case of paint). The thawed sample was also subjected to cryogenic scanning electron microscope (SEM) examination after being fast-frozen at high pressure, as described below.

High Pressure Freezing, Fracture, Surface Treatment and Cryogenic SEM

The detailed sample preparation procedure has been described elsewhere. (17,18) A droplet of polymer dispersion or latex paint was sandwiched between two brass discs, each with a cylinder-shaped indentation. The assemblage was quickly frozen in a Bal-Tec HPM 010 high pressure freezing machine at 2100 bar. High pressure freezing is critically important for minimizing disturbance by large ice crystals to the original morphology of an aqueous sample. It works as follows: high pressure lowers the temperature range of supercooled water from 233-273 K at atmospheric pressure to 183-251 K at 2100 bar so that the freezing water has a better chance to reach the glass transition temperature and be vitrified before ice crystals grow. The frozen sandwich was then fractured in liquid nitrogen. One side of the fractured sample was mounted on a cryo-transfer stage and was metal-coated. The metal-coated fracture surface sat on the cold stage and was examined in the in-lens Hitachi S900 field emission SEM. Both secondary electron (SE) and backscattered electron (BE) images were recorded. The former provides better topographical contrast while the latter is more preferred for maximum contrast between organic and inorganic phases. Unless otherwise specified, the electron micrographs shown in this article are SE images.

Heat Storage Stability

Formulated paints in one-pint plastic cans were stored for two weeks or longer in an oven with a set temperature of 50[degrees]C. After the storage period, the paint was allowed to reach room temperature and was gently stirred before Stormer viscosity (ASTM D 562-81) was measaured. Selected samples were also examined by cryogenic SEM using the same sample preparation procedure as for the freeze-thaw samples.

RESULTS AND DISCUSSION

Freeze-Thaw Unstable Systems

In order to visualize microscopic changes that have occurred during freezing and thawing in a polymer latex without freeze-thaw stability, we chose an acrylic polymer latex without freeze-thaw stability. The emulsion polymer had a [T.sub.g] about 20[degrees]C, which is typical for a polymer binder used in conventional semi-gloss paint. After one freeze-thaw cycle, the sample was totally coagulated and had the appearance of cottage cheese. Using cryo-SEM coupled with high pressure freezing, we examined the acrylic latex before and after the freeze-thaw cycle. The electron micrographs are shown in Figure 1. It can be clearly seen that polymer particles had coalesced during the freezing and thawing process.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Applying the same methodology, we investigated a semi-gloss paint before and after one freeze-thaw cycle. The paint did not have any freeze-thaw stability. One freeze-thaw cycle had caused total coagulation of the paint. From the three micrographs in Figure 2, we can visualize not only the coagulation of the polymer particles, but also the segregation of Ti[O.sub.2] pigment particles. Thus, in order to achieve freeze-thaw stability in the formulated paint, the polymer particles have to first be stable themselves and additionally prevent aggregation of the inorganic particles. As discussed in the introduction, additives such as antifreezes have often been used in the past for imparting freeze-thaw stability, but such measures are increasingly losing favor in light of VOC concerns.

Freeze-Thaw Stability Improvement

Our efforts have focused on developing new polymer binders that require less than 50 g/L VOC in paint formulations and deliver the performance characteristics normally expected of conventional binders that require higher VOC. One of the key challenges has been freeze-thaw stability. At 50 g/L VOC, one can no longer rely on high levels of volatile additives such as glycols to achieve freeze-thaw stability. The expectation is that the next generation of polymer binders will assume the stabilizing and protective functions.

For feasibility assessment, we re-examined the concept of chemically bound ethylene glycol groups. As described in the experimental section, a series of acrylic polymer dispersions was synthesized based on the same MMA/nBA ratio (target MFFT around 0[degrees]C) and same surfactant. The amount of the copolymerizable polyethylene glycol monomer was varied. A threshold value of about 4 wt% (based on total monomers) of MPEG 750 MA was found. Below this level, the polymer dispersion did not have freeze-thaw stability, as judged by the coagulation of polymer particles revealed by cryogenic SEM. In Figure 3, micrographs (A) and (B) demonstrate the effect of slow freezing (real world situation) on two polymer dispersions containing, respectively, 0.5% and 4% of MPEG 750 MA. Micrograph (C) in Figure 3 shows the case of a latex that has freeze-thaw stability. Here we optimized the incorporation of different copolymerizable functionalities as well as the polymerization process.

Visualization of Freeze-Thaw Stable Low-VOC Paint

Focusing on colloidal chemistry and the emulsion polymerization process, we were able to develop latex binders that were capable of imparting freeze-thaw stability to paints formulated with reduced VOC content. Figure 4 is an example of a semi-gloss paint formulated with less than 50 g/L VOC based on a new polymer dispersion. The semi-gloss paint formulation had a volume solids of 40% and PVC (pigment volume concentration) of 25.5%. Titanium dioxide and aluminum silicates were used in the formulation. In contrast to Figure 2, which shows polymer particle coalescence and pigment segregation in a freeze-thaw unstable semi-gloss paint after the freeze-thaw test, the semi-gloss paint in Figure 4 was stable after five freeze cycles. Polymer, Ti[O.sub.2], as well as extender particles all maintained their individual stabilities during the five freezing and thawing cycles, and the Stormer viscosity increased by only 12 KU.

Freezing and Thawing Rates on Freeze-Thaw Stability

Barb and Mickucki (4) have investigated the rates of freezing and thawing on the freeze-thaw stability of hard polymer latexes. Their results show that coagulum formed at a more rapid rate of freezing has a finer average particle size, and that the rate of thawing has no detectable effect on the particle size of the coagulum. However, the particle size measurement was done on a macroscopic level using 85 to 240 mesh sieves. It was not possible to see what had happened to individual polymer particles. Equipped with cryogenic SEM technique and by dramatically changing the freezing rates, we examined the same latex sample as shown in Figure 3A. The results are shown in Figure 5. In Figure 5A, the polymer particles survived the fast freezing process in liquid ethane (boiling point -88[degrees]C) and maintained individual particle identities. Additionally, we fast-froze the same latex sample under high pressure in liquid nitrogen (-196[degrees]C). As shown in Figure 5A, there was no sign of particle instability. These two experiments show that an extremely fast freezing process does not cause particle coagulation. This may be explained by a "spacer" effect created by small ice crystals formed almost simultaneously between polymer particles when the sample is subjected to fast freezing.

[FIGURE 3 OMITTED]

When the same latex sample was first fast-frozen under high pressure in liquid nitrogen and then thawed slowly overnight, polymer particles were coagulated, as seen in Figure 5B. This experiment indicates that slow thawing was the cause for instability in this sample, as we have already seen in Figure 5A that fast freezing actually preserves polymer particles in a stable state.

Figure 5C shows that slow freezing also causes particle coagulation. As expected, a combination of slow freezing and thawing would be even worse for polymer particle stability. This is indeed the case as shown in Figure 5D.

F/T Process and Mechanisms of Stabilization

Based on published work by other researchers (2-10) and our results presented in this article, we describe the F/T process and mechanisms of stabilization in waterborne latexes and paints as follows. The decreasing temperature eventually reaches the freezing point of water and a new solid phase, pure ice, appears. It should be mentioned that the freezing point of water in a latex or paint can be substantially lower than that of pure water, because of interactions at the interface between water molecules and the surface layer of the latex particles (19) and because of antifreeze if it is present. (1) The formation of ice crystals progressively increases the particle concentration (polymer and inorganic particles, in the case of paint) and frequency of collisions between particles in the unfrozen phase. Eventually, polymer particles are forced into contact with each other under the high pressures from growing ice crystals. At this stage, if the contact is made ineffective, interparticle coalescence is prevented and the system stays stable. Otherwise, coalescence between particles takes place and the system becomes destabilized.

The contact between particles can be made ineffective by a hydration layer resulting from surface-bound polar groups, such as polyethylene glycol functionalities and surface charges. Cansell et al. (19) have revealed that water in the hydration layer may remain unfrozen at temperatures well below -30[degrees]C, which may help explain the effectiveness of the hydration layer in preventing particle coalescence. Antifreeze may function similarly by creating a nonfrozen medium around polymer particles: continued crystallization of water excludes the antifreeze from the expanding ice phase and enriches it in the disperse phase where polymer particles are present.

We have also seen that freezing rates affect the outcome. Polymer particles with poor F/T stability in a slow freezing process can remain stable in an extremely fast freezing process. As mentioned, this may be explained by a "spacer" effect created by small ice crystals formed almost simultaneously between polymer particles when the sample is frozen quickly.

Destabilization can occur during slow thawing after fast freezing. This presumably is due to the lack of a hydration layer effect that would otherwise prevent polymer particle coalescence upon thawing. The mechanism of coagulation upon thawing after slow freezing is postulated as follows. During slow freezing, latex particles are forced into contact with each other at the ice crystal boundaries, where ionic species are concentrated and frozen. As slow thawing follows, the thawing takes place first in the boundary regions between crystals, i.e., in regions between polymer particles. The high concentration of ionic species there further coagulates particles locked close to one another by ice crystals. The coagulation is caused by a reduction in colloidal stability as a result of the compression of the electrical double layer by the high electrolyte concentration.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Visualization of Changes in Paints after Heat Storage

We examined semi-gloss paints before and after heat storage. Figure 6 shows cryogenic scanning electron micrographs of one paint with poor heat storage stability. After two weeks at 50[degrees]C, the Stormer viscosity of the paint had increased by more than 45 KU. Coalescence of polymer particles and limited segregation of Ti[O.sub.2] (white dots) particles are clearly visible in Figure 6B.

SUMMARY

Using model latexes and formulated paints with varying stabilities by design, we applied an advanced analytical technique--cryogenic SEM (scanning electron microscopy)--coupled with a high-pressure freezing apparatus, to study the structural changes occurring during freeze-thaw cycles and heat storage. High pressure freezing avoids artifacts caused by the formation of large ice crystals. We showed that by incorporating copolymerizable functional monomers, it is possible to achieve F/T stability in polymer latexes and in low-VOC paints, as judged by the microstructures revealed by the cryogenic SEM technique. Particle coalescence as well as pigment segregation in F/T unstable systems are visualized. In order to achieve F/T stability in paints, latex particles have to stay stable and to provide protection to inorganic pigment and extender particles. Because of the unique capabilities of the cryogenic SEM, we were able to separate the effects of freezing and thawing and to study the influence of the rate of freezing and thawing on F/T stability. Destabilization can be caused by either freezing or thawing. A slow freezing process is more detrimental to F/T stability than a fast freezing process; the latter actually preserves suspension stability during freezing. We also investigated the microstructural changes in paints with poor heat storage stability. Coalescence of polymer particles and segregation of pigment particles were shown to accompany the extremely high viscosity rise during heat-aging.

ACKNOWLEDGMENTS

This article reports joint research of the Charlotte Technical Center of BASF Corporation and the Coating Process Fundamental Program at the University of Minnesota. The authors express their gratitude to Scott B. Robinson for synthesizing and characterizing the model polymer dispersions, and to Jeanette K. Gilliam for preparing and testing the paint samples.

References

(1) Bosen, S.F., Bowles, W.A., Ford, E.A., and Perlson, B.D., "Antifreezes," Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A 3, VCH Verlag, 1985.

(2) Blackley, D.C., Polymer Latices-Science and Technology, 2nd Ed., Vol. 1, Chapman & Hall, 1997.

(3) Digioia, F.A. and Nelson, R.E., "Freeze-Thaw Studies of Latex Paint Polymers," Ind. Eng. Chem., 745-748 (April 1953).

(4) Barb, W.G. and Mikucki, W., "On the Coagulation of Polymer Latices by Freezing and Thawing," J. Polym. Sci., 37, 499-514 (1956).

(5) King, A.P. and Naidus, H., "The Relationship Between Emulsion Freeze-Thaw Stability and Polymer Glass Transition Temperature," J. Polym. Sci., Part C, 27, 311-319 (1969).

(6) Naidus, H. and Hanzes, R., in "Addition and Condensation Polymerization Processes," Platzer, N.A.J. (Ed.), ACS Advances in Chemistry Series No. 91, American Chemical Society, Washington, D.C., Paper No. 11 (1969).

(7) Blackley, D.C. and Teoh, S.C., Preprints of the International Polymer Latex Conference, Plastics and Rubber Institute, London, Paper No. 9 (1978).

(8) Cockbain, E.G., Gregory, J., Pillai, N.M., and Gorton, A.D.T., J. Rubber Research Institute of Malaya, 22, 408 (1969).

(9) Nakamura, A. and Okada, R., "The Coagulation of Particles in Suspension by Freezing-Thawing I. Effect of Freeze-Thawing Conditions and Other Factors on Coagualtion," Colloid Polym. Sci., 254, p. 718 (1976).

(10) Nakamura, A. and Okada, R., "The Coagulation of Particles in Suspension by Freezing-Thawing II. Mechanism of Coagulation," Colloid Polym. Sci., 254, p. 497 (1976).

(11) Ottewill, R.H. and Satgurunathan, R., "Nonionic Latices in Aqueous Media," Colloid Polym. Sci., 273, 379-386 (1995).

(12) Farwaha, R., Phan, L., and Currie, W., U.S. Patents 5,530,056 (1996) and 5,610,225 (1997), and EP 0 609 756 A3 (1994).

(13) Okubo, M. Sakaguchi, S. Takamatsu, A., Sasaki, H., and Tsuneta, K., U.S. Patent 6,410,655 B2 (2002).

(14) Zhao, C.L., Cunningham, T.A., and Zhuo, L., U.S. Patent 6,933,415 (2005).

(15) Kostansek, E.C., "Using Dispersion/Flocculation Phase Diagrams to Visualize Interactions of Associative Polymers, Latexes, and Surfactants," J. COAT. TECHNOL., 75, No. 940, 27 (2003).

(16) Diebold, M.P., Better, C.R., and Mukoda, D.M., "Mechanism of Ti[O.sub.2]/ZnO Instability in Latex Paints," J. COAT. TECHNOL., 75, No. 942, 29-36 (2003).

(17) Huang, Z., Thiagarajan, V.S., Lyngberg, O.K., Scriven, L.E., and Flickinger, M.C., "Microstructure Evolution in Polymer Latex Coatings for Whole-Cell Biocatalyst Application," J. Colloid Interface Sci., 215, 226 (1999).

(18) Sutanto, E., Ma, Y., Dittrich, U., Zhao, C.L., Zhuo, L., Davis, H.T., and Scriven, L.E., "Cryogenic Scanning Electron Microscopy of Early Stages of Film Formation in Drying Latex Coatings," Proc. 10th International Coatings Science and Technology Symposium, Scottsdale, AZ, pp. 63-66, 2000.

(19) Cansell, F., Grabielle-Madelmont, C., and Ollivon, M., "Characterization of the Aqueous Phase and the Water-Polymer Interface in Latex Suspensions by DSC," J. Colloid Interface Sci., Vol. 144(1), 1-17 (1991).

Cheng-Le Zhao, Shane Porzio, and Alan Smith -- BASF Corporation*

Haiyan Ge, H.T. Davis, and L.E. Scriven -- University of Minnesota ([dagger])

Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27-29, 2004 in Chicago, IL. Tied for first place in The John A. Gordon Best Paper Competition.

* Charlotte Technical Center, Charlotte, NC.

([dagger]) Minneapolis, MN.