Associative polymer/latex dispersion phase diagrams II: HASE thickeners.

The colloidal interactions of HASE associative polymers and latexes in the presence of surfactant are complicated and subject to a number of variables. Both bridging and depletion flocculation can occur, in addition to good particle dispersion. Dispersion phase diagrams have been developed to help visualize these interactions. The various dispersion states can have a significant effect on coating formulations and film properties. Examples of dispersion phase diagrams are presented for a model HASE anionic associative thickener and various model latexes in the presence of sodium dodecyl-sulfate and nonionic surfactants. The major variables affecting dispersion behavior are associative polymer concentration, latex particle size, latex surface hydrophobicity, electrolyte concentration, and surfactant concentration. The dispersion phase behavior of the HASE systems is compared to that of HEUR thickened systems reported previously. A significant difference is that much less bridging flocculation is observed in the HASE systems. In addition, nonionic surfactants induced depletion flocculation in the HASE systems but not in the HEUR systems.

Keywords: Rheology/flow control, thickeners, colloid stability, acrylics, latexes, colloids, emulsions, application characteristics, physical properties, flocculation, latex

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Over the past two decades, associative polymers have contributed significantly to the improvement of application properties of latex paints. (1) One major reason for this is that associative polymers can produce a uniform dispersion of the latex and pigment particles in the paint. This is in contrast to the more traditional thickeners that tend to flocculate the particles due to the phenomenon of depletion flocculation. (2) In depletion flocculation, the particles do not interact with the thickener and are excluded from the thickener solution space. Consequently, both latex particles and pigment are crowded together, thus degrading optical and rheological properties. Another type of flocculation, called bridging flocculation, must also be considered in solution polymer/latex systems. When polymers such as associative thickeners can adsorb strongly on particle surfaces, there is the possibility of flocculating the particles when single thickener molecules are adsorbed on two different particles. This "bridging" effect can happen when there is more particle surface available than there are adsorbing species (i.e., associative polymers, surfactants, and dispersants). Associative polymers can yield a good dispersion, depletion flocculation, or bridging flocculation, depending on a number of variables. Therefore, a clear understanding of dispersion/flocculation behavior is essential to understanding the optical and rheological properties of coatings.

In the first article of this series, (2) the dispersion/flocculation behavior of latexes thickened with hydrophobically-modified ethoxylated urethanes (HEUR) was explored. These are the most common nonionic associative thickeners used in latex paints. Another type of associative thickener commonly found in latex paints is known as hydrophobically-modified alkali-swellable emulsion (HASE). (3-5) HASE thickeners are based on a polyelectrolyte backbone, usually methacrylic acid (MAA) and ethylacrylate (EA) copolymer, with pendant hydrophobes (i.e., hydrophobes that are attached to the backbone with polyethylene oxide chains). The literature is currently much more extensive on HEUR thickeners than on HASE thickeners, so there is a need to increase the understanding of HASE systems. To this end, the dispersion/flocculation behavior of latexes thickened with HASE thickeners and containing surfactant is the subject of this article. The remaining associative thickener/particle system, interaction with pigments, will be the subject of the third article in this series.

HASE thickeners differ from HEUR thickeners in that they have a much higher molecular weight, are ionic, and have a lower hydrophobe density (i.e., hydrophobe number per molecular volume) than HEUR thickeners. Because of this, HASE thickeners are somewhat of a hybrid thickener, deriving some of their efficiency from hydrophobic interactions and some from molecular volume effects traditionally utilized by nonassociative thickeners such as the nonionic hydroxyethyl cellulose (HEC) and the anionic alkali-swellable emulsion (ASE). HASE thickeners differ from these nonassociative thickeners in that they can produce uniformly dispersed particle dispersions. (1,3,4)

Latex Dispersion States

Whether a latex is well dispersed or flocculated has a profound effect on rheology and the physical and optical properties of films. Associative thickeners have the ability to hold the latex in a well dispersed state versus the flocculated state typical of systems thickened with nonassociative thickeners. This is because of their specific adsorption characteristics onto latex particles. (3,4,6) Two types of flocculation can negatively affect paint and film properties: bridging and depletion flocculation. These two types of flocculation were discussed most recently in the first article of this series (2) and in earlier publications. (3,4,7) The poor particle dispersion in a depletion flocculated system leads to lower gloss, lower hiding, and poorer film integrity and adhesion. HASE systems are often more easily depletion flocculated by surfactant than HEUR systems, due partly to weaker association with the latex. (3,4) Bridging flocculation is generally a stronger interaction, is more difficult to break up with mechanical energy, and reforms faster than depletion flocculation. Depletion flocculation is more commonly encountered in coatings than bridging flocculation, due, in part, to the elevated surfactant concentrations in most commercial coatings. Because the dispersion behavior in systems thickened with associative polymers is complex, it would be useful to have a way to visualize the possibilities.

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Phase Diagrams

One unique way to visualize the regions of latex dispersion and flocculation in thickener-surfactant space is to use dispersion phase diagrams. (2) Figure 1 is a generalized version of such a diagram showing the regions of dispersion and flocculation as previously described. Thickener concentration is increasing on the vertical axis and surfactant is increasing on the horizontal axis. The upper boundary of the bridging region is really a continuum of ever increasing floc sizes until a uniform dispersion is reached. This upper boundary is defined as the points at which individual flocs are no longer noticeable by microscopic inspection of the samples. The lower depletion flocculation boundary is the critical flocculation concentration (CFC) of the thickener, below which depletion flocculation does not occur. These diagrams will be used to illustrate the effect of latex particle size, composition, and electrolyte on dispersion.

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Rheology

When compared with nonassociative thickeners, associative thickeners produce a more favorable (i.e., more Newtonian) rheology profile. Figure 2 shows viscosity versus shear rate curves for two coatings formulations having about the same Krebs-Stormer viscosity, but one is thickened with HASE and the other is thickened with a nonassociative thickener (ASE or HEC). The lower low shear viscosity and higher high shear viscosity of the HASE system leads to improved flow/leveling and higher film build. The rheology of associative polymer/latex systems is complex and is beyond the scope of this article, which concentrates on phase behavior. However, the rheology of HASE systems is their most studied aspect in the literature. In terms of HASE molecule variables, effects of acid level in the backbone (8) and hydrophobe size (9) have been discussed. In terms of additives, both added surfactant (3,4,10,11) and added salt (10,11) have been studied. There has been limited work on the effect of latex parameters on rheology. (12) The most important point to remember relative to the work discussed here is that the desirable properties of associative thickener systems stem from good dispersion and that flocculated associative systems revert to the rheology of nonassociative systems for both HASE and HEUR.

EXPERIMENTAL

Materials

The following materials were used to determine the latex phase behavior:

Model associative polymer -- HASE-type with MAA/EA crosslinked copolymer backbone and pendant [C.sub.12][H.sub.25] hydrophobes (average molecular weight of 400,000).

Nonassociative polymer -- Alkali-swellable emulsion (ASE) or HEC of comparable molecular volume to the HASE.

Latexes -- 50 BA/50 MMA and 50 BA/50 Sty of various particle sizes ranging 50-340 nm in diameter. All latexes contain 1% MAA and 0.05-0.10% sodium dodecylbenzene sulfonate (SDBS) based on total monomer weight, except the smallest particle size, which contains 3% SDBS.

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Added surfactant -- Sodium dodecyl sulfate (SDS), Triton[R] X-100 (octylphenoxypolyethoxyethanol with an average of 10 ethylene oxide units), and Triton[R] X-405 (octylphenoxypolyethoxyethanol with an average of 40 ethylene oxide units).

Electrolyte -- NaCl at 0 and 1%.

Latex Solids -- 25% (by volume).

Ph -- adjusted to 9.0-9.5 with N[H.sub.4]OH.

Note that the concentrations of additives are expressed as wt% of the continuous phase and that the SDBS surfactant levels from the latex synthesis are very low based on the continuous phase (0.02-0.03%) and very low compared to the added SDS in the phase diagrams.

Determination of Particle Dispersion

Mixtures of latex, HASE, and surfactant were prepared in clear glass containers at 25% polymer solids by volume and allowed to equilibrate for at least 60 hr before evaluation. Particle dispersion was assessed by both visual inspection and microscopy. The bridging flocculation region was confirmed by the fact that higher levels of surfactant eliminated the flocculation. Depletion flocculation could be confirmed by the fact that dilution of the sample with water to below the critical flocculation concentration yielded a well dispersed system. An average of 30-40 samples were prepared for each system to define the dispersion and flocculation regions with some precision. The most difficult systems were those containing high levels of HASE and surfactant because the high viscosity led to slow flocculation kinetics and mixing problems. Latexes of about 340 nm in diameter were chosen as a "typical" size for evaluating the variables of latex surface hydrophobicity, composition, surfactant type, and added electrolyte.

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RESULTS AND DISCUSSION

Nonassociative Thickeners

As a control, the phase diagram for a typical nonassociative thickener such as ASE or HEC was generated. A 340 nm BA/MMA latex was chosen for this purpose. Figure 3 is the resulting phase diagram. Note that the latex was depletion flocculated at all but the very lowest thickener concentrations, and that SDS had no effect on the dispersion. In fact, latex composition and particle size also have little effect on the dispersion behavior of nonassociative systems. This is in sharp contrast to what is observed for the latexes with HASE and HEUR thickeners.

HASE Thickeners

PARTICLE SIZE EFFECTS ON BA/MMA LATEXES: One of the latex properties that is commonly varied is particle size. Model BA/MMA latexes of 340, 140, and 50 nm particle diameter were studied in the presence of the HASE and SDS. The resulting phase diagrams are shown in Figures 4, 5, and 6, respectively. The 340 nm diagram represents the baseline behavior to which other systems will be compared. Two important features are that the dispersion region is quite narrow and, unlike the HEUR case, there is no bridging flocculation region. The narrowness of the region attests to the lower hydrophobe density compared to HEUR thickeners and also to the weaker adsorption characteristics. (2-4) The major effect of latex particle size is that the smaller the particle size the larger the good dispersion region. This is due to the increased surface area of the latex available for interaction with the HASE and SDS. Bridging flocculation finally occurs at very small latex particle size. This is presumably because HASE molecules are large and occupy a large amount of surface area on the latex. Bridging flocculation cannot occur unless there is an excess of hydrophobic sites not occupied by surfactant or HASE hydrophobes.

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ELECTROLYTE EFFECTS: Electrolytes have significant effects on HASE/latex systems, as shown in Figure 7, for 0 and 1% NaCl added to the 340 nm BA/MMA latex system. Electrolytes tend to increase the "strength" of the hydrophobic interactions by a salting out effect on both the HASE and the surfactant. In addition, they significantly decrease the hydrodynamic volume of HASE polyelectrolyte while also decreasing the hydrodynamic volume of the latex particles. This has the effect of increasing the area of good dispersion and is used as a formulating tool to enlarge the formulation region. Similar effects are observed in HEUR systems, (2) but less extreme because HASEs are polyelectrolytes. Note that the CFC of the thickener increases when the salt is added due to the decrease in molecular volume. Electrolytes have a profound effect on rheology (decreased viscosity) (10,11) so thickener concentration needs to be adjusted when electrolytes are added.

HYDROPHILIC LATEX SURFACE: When the latex surface is made more hydrophilic, as when carboxylic acid-based monomers are used in the polymerization, the number of hydrophobic sites available for the surfactant and HASE hydrophobes is reduced. This leads to a much decreased region of good dispersion because very little surfactant is needed to displace the HASE from the latex. Figure 8 shows such a phase diagram for a 330 nm BA/MMA latex having a high concentration of surface acid (4% MAA based on total monomer weight). Indeed, there is very little good dispersion region with which to work. The resulting diagram begins to resemble the nonassociative one.

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HYDROPHOBIC LATEX SURFACE: Another common latex variable is hydrophobicity. When the latex surface is made more hydrophobic, interactions with the HASE molecules are increased, as expected, due to the higher number of sites for possible adsorption. BA/Sty latexes having 330, 150, and 50 nm diameters were mixed with the HASE thickener and evaluated. The phase diagrams for these systems are shown in Figures 9, 10, and 11, respectively. As with the BA/MMA system, the smaller the particle size, the larger the region of good dispersion. At the same particle size, the more hydrophobic latex tends to have a larger bridging region (beginning at 150 nm), but also a larger good dispersion region. A similar trend was observed in earlier HEUR work, (2) except that bridging flocculation occurred at all particle sizes including even 600 nm latexes.

NONIONIC SURFACTANTS: Nonionic surfactants are often found in coatings formulations. Triton X-100 is an example of a low HLB (13.5) surfactant of this type, and Triton X-405 is a higher HLB (17.9) example. When Triton X-100 was added to the standard 340 nm BA/MMA latex, a larger region of good dispersion was observed compared to the SDS case. This is shown in Figure 12. Triton X-100 is only about half as efficient, on a molar basis, as SDS at displacing the HASE from the latex surface. Triton X-405, which has a molar efficiency about equal to SDS, yielded a smaller region of good dispersion and was improved by the addition of 1% NaCl, as depicted in Figure 13. In HEUR thickened latexes, only bridging and good dispersion regions were observed with nonionic surfactants. (2) By contrast, these HASE systems exhibit only depletion flocculation and good dispersion regions.

CONCLUSIONS

The following conclusions can be drawn based on the structure of the phase diagrams generated in the work presented here:

(1) HASE polymers, in combination with latex particles and surfactant, exhibit complex dispersion behavior which depends on the concentration of each component plus latex particle size, surface composition, and electrolyte concentration. As with HEUR thickeners, the region of good dispersion is increased by increasing the hydrophobicity of the latex particles, decreasing the size of the latex particles, or increasing the electrolyte concentration.

(2) HASE polymers exhibit far less bridging flocculation behavior than HEUR polymers. This appears to be due to the weaker association and larger molecular size of the HASE polymers. Bridging occurs mainly for small particle sized latexes, especially the more hydrophobic ones.

(3) Good dispersion and both bridging and depletion flocculation of the latex are observed in various HASE systems if one begins at low initial surfactant levels. In contrast, latexes thickened with nonassociative polymers exist only in the depletion flocculated state, except at very low thickener concentrations. HASE systems with very hydrophilic latexes most closely resemble nonassociative systems in terms of phase behavior.

(4) In contrast to HEUR systems, HASE systems of moderate latex particle size containing nonionic surfactant have a depletion flocculation region, but no bridging flocculation region.

(5) Dispersion/flocculation phase diagrams are a unique way to visualize the colloidal dispersion behavior of latex particles with HASE polymers and surfactants. They can help explain the complex behavior observed in coatings formulated with associative polymers.

ACKNOWLEDGMENTS

The author would like to thank the Rohm and Haas Company for its support and permission to publish this work.

References

(1) Glass, J.E., "A Perspective on the History of and Current Research in Surfactant-Modified, Water-Soluble Polymers," JOURNAL OF COATINGS TECHNOLOGY, 73, No. 913, 79 (2001).

(2) Kostansek, E., "Using Dispersion/Flocculation Phase Diagrams to Visualize Interactions of Associative Polymers, Latexes, and Surfactants," JOURNAL OF COATINGS TECHNOLOGY, 75, No. 940, 27 (2003).

(3) Sperry, P.R., Thibeault, J.T., and Kostansek, E.C., "Flocculation and Rheological Characteristics of Mixtures of Latexes and Water-Soluble Polymeric Thickeners," Adv. Org. Coatings Sci. Technol., Series 9, 1 (1987).

(4) Thibeault, J.T., Sperry, P.R., and Schaller, E.J., in Water Soluble Polymers: Beauty with Performance, Advances in Chemistry Series 213, Glass, J.E. (Ed.), American Chemical Society, Washington, D.C., Chapter 20, 1986.

(5) Shay, G., in Polymers in Aqueous Media, Performance Through Association, Advances in Chemistry Series 223, Glass, J.E. (Ed.), American Chemical Society, Washington, D.C., Chapter 25, 1989.

(6) Islam, M., Ou-Yang, H., Jenkins, R., and Bassett, D., "Adsorption of HASE Polymers on Colloidal Surfaces," Abstracts of Papers, 216th ACS National Meeting, August 23-27, 1998, COLL-080, American Chemical Society, Washington, D.C., 1998.

(7) Sperry, P.R., "A Simple Quantitative Model for the Volume Restriction Flocculation of Latex by Water-Soluble Polymers," J. Colloid Interface Sci., 82, 62 (1981); Sperry, P.R., J. Colloid Interface Sci., 87, 375 (1982); Sperry, P.R., J. Colloid Interface Sci., 99, 97 (1984).

(8) Wu, W., Gaynor, C., and Shay, G., "Effect of Acid Content on Rheology of Hydrophobically Modified Alkali-Soluble Emulsion Thickeners," Abstracts of Papers, 224th ACS National Meeting, August 18-22, 2002, PMSE-254, American Chemical Society, Washington, D.C., 2002.

(9) Wu, W., Grieb, R., and Shay, G., "Rheological Characterization of Model HASE Thickeners," Proc. 29th International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans, LA, 343, 2002.

(10) Kaczmarski, J., Tarng, M.-R., Ma, Z., and Glass, J., "Surfactant and Salinity Influences on Associative Thickener Aqueous Solution Rheology," Colloids Surf., A, 147, 39 (1999).

(11) Xing, L.-I., Chen, M., Glass, J., Buchacek, R., and Dickinson, J., "The Influences of Surfactant and Electrolyte on Rheology of HASE Thickener Aqueous Solutions," Polym. Mater. Sci. Eng., 79, 413 (1998).

(12) Quadrat, O., Horsky, J. and Snuparek, J., "Thickening Effect of Commercial Associative Thickeners on the Latices of Copolymers of Acrylic Monomers Carrying Hydrophilic Reactive Groups," J. Dispersion Sci. and Technol., 24, 179 (2003).

Edward Kostansek -- Rohm and Haas Company*

*P.O. Box 904, Spring House, PA 19477-0904, [e-mail].