Associative polymer/particle dispersion phase diagrams III: pigments.

The colloidal interactions of both HEUR and HASE associative polymers with pigments in the presence of dispersant are complicated and subject to a number of variables. The objective of this work was to clarify the conditions needed to achieve good pigment dispersion in associative thickener systems by characterizing particle dispersion behavior. This had previously been done for latex particles, but not for pigments such as Ti[O.sub.2]. Good dispersion leads to optical properties, such as gloss and hiding, that are superior to nonassociative thickener systems. Pigment dispersion phase diagrams represent a good way to visualize the complex interactions among pigments, dispersant, and thickener. The two most important variables were found to be pigment type (i.e., surface composition) and dispersant composition. Associative thickeners can yield good pigment dispersion or flocculation, depending on the correct matching of dispersants and pigment type. Because of the hydrophobic functional groups governing associative thickener behavior, dispersants having some hydrophobic character yielded the best pigment dispersions and optical properties because they could couple the pigment particles with the associative thickener network. Interior-grade Ti[O.sub.2] tended to yield better dispersions and optical properties than exterior-grade Ti[O.sub.2]. Optimized associative thickener systems generally had improved optical properties over comparable nonassociative systems. Optical properties correlated well with particle dispersion behavior as displayed by the dispersion phase diagrams.

Keywords: Dispersants, thickeners, latexes, colloids, emulsions, pigments, latex, dispersion, flocculation, hiding, titanium dioxide

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Over the past two decades associative polymers have contributed significantly to the improvement of application properties of latex paints. (1) In the first two articles of this series, (2,3) the dispersion/flocculation behavior of latexes thickened with hydrophobically modified ethoxylated urethanes (HEUR) and hydrophobically modified alkali-swellable emulsions (HASE) were explored. These are the most common types of associative thickeners used in latex paints. Only a limited amount of work on the interaction of associative polymers with pigments can be found in the open literature (1) and most of it deals with HEUR-thickened systems, not HASE-thickened systems. Because the dispersion state of the pigments (especially Ti[O.sub.2]) has a profound effect on the physical and optical properties of coatings, the studies described in this article were undertaken. The objective of the work was to clarify the pigment dispersion phase behavior and to explain why the choice of dispersant is critical to obtaining superior coatings properties.

Pigment Dispersion States

Whether a pigment is well dispersed or flocculated has a profound effect on the optical, as well as physical and rheological, properties of films. Associative thickeners have the ability to hold the pigment 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 pigment particles. Two types of flocculation can negatively affect paint and film properties: bridging and depletion flocculation. These two types of flocculation were discussed in the first two articles of this series (2,3) and in earlier publications. (4,5) Basically, bridging flocculation occurs when single associative polymer molecules connect pigment particles causing phase separation, whereas in depletion flocculation the pigment particles are phase separated due to exclusion from the associative polymer solution phase. (6) In practice, depletion flocculation is the more common phenomenon due to the relatively high use rate of dispersants and surfactants in practical coatings systems. Depletion flocculation is the norm for nonassociative systems. The poor particle dispersion in a depletion flocculated system leads to lower gloss, lower hiding, and poorer film integrity and adhesion. Because the dispersion behavior in systems thickened with associative polymers is complex, it would be useful to have a way to visualize the possibilities.

Phase Diagrams

One unique way to visualize the regions of pigment dispersion and flocculation in thickener-dispersant space is to use dispersion phase diagrams. (2-5) 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 dispersant 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 point 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 pigment composition, dispersant type, and thickener type on dispersion.

[FIGURE 1 OMITTED]

Optical Properties

When compared with nonassociative thickeners, associative thickeners often produce more favorable optical properties due to the uniform distribution of the particles in the film. This results in higher gloss and hiding when compared to nonassociative systems. (7,8) 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 optical properties and rheology (9) of nonassociative systems for both HASE and HEUR. In this article, the effects of Ti[O.sub.2], dispersant, and associative thickener type on contrast ratio and gloss will be explored for model latex paints, but first the adsorption characteristics of the dispersants and thickeners and the dispersion phase diagrams need to be discussed.

[FIGURE 2 OMITTED]

EXPERIMENTAL

Materials

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

MODEL ASSOCIATIVE POLYMERS: HEUR-type polyoxyethylene backbone with terminal [C.sub.12] hydrophobes (average molecular weight of 50,000) and HASE-type with MAA/EA copolymer backbone and pendant [C.sub.12][H.sub.25] hydrophobes (average molecular weight of 400,000).

NONASSOCIATIVE POLYMER: HEC of comparable molecular volume to the HASE.

PIGMENTS: Commercial interior grade Ti[O.sub.2] (alumina-rich surface) and commercial grade exterior Ti[O.sub.2] (silica-rich surface).

ADDED DISPERSANT: A hydrophilic dispersant (poly-methacrylic acid) and a hydrophobic dispersant (olefin/maleic acid copolymer).

PIGMENT SOLIDS: 10% (by volume).

pH: Adjusted to 9.0-9.5 with N[H.sub.4]OH.

MODEL PAINTS: 20 PVC model paints containing Ti[O.sub.2], acrylic latex, and thickener (HEUR, HASE, or HEC), drawn down and analyzed for contrast ratio, 20[degrees] gloss and 60[degrees] gloss.

NOTE: Concentrations of thickener are expressed as wt% of the continuous phase and concentrations of dispersant are expressed as wt% of pigment.

Determination of Particle Dispersion

Aqueous mixtures of pigment, thickener, and dispersant were prepared in clear glass containers at 10% pigment solids by volume and were allowed to equilibrate for at least 60 hr before evaluation. Particle dispersion was assessed by both visual inspection and microscopy. In some samples, the bridging flocculation region could not be conclusively distinguished from coagulation resulting from very low dispersant levels. Therefore, the region was treated as a combined bridging flocculation/coagulation region. 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. Typical interior and exterior grade Ti[O.sub.2] pigments were studied in order to assess the effect of surface chemistry on dispersion.

RESULTS AND DISCUSSION

Adsorption of Dispersants on Pigments

Interior and exterior grades of Ti[O.sub.2] have different surface treatments and are known to exhibit differences in dispersant adsorption. (10) In order to establish the relative interactions of the pigments and dispersants used in this study, their adsorption characteristics were determined. The adsorption isotherms at 25[degrees]C for the hydrophilic and hydrophobic dispersants on both interior and exterior grade Ti[O.sub.2] are shown in Figure 2. Both dispersants appear to reach surface saturation on the interior Ti[O.sub.2] at between 0.8 and 1% dispersant on pigment. The same is true for the hydrophilic dispersant on exterior Ti[O.sub.2], but the hydrophobic dispersant had not reached saturation by 1%. The exterior Ti[O.sub.2] has a higher surface area than the interior Ti[O.sub.2], so a higher level of adsorption was expected. Note that at 0.5% dispersant, 36-48% of added dispersant was adsorbed on the interior Ti[O.sub.2] and 60-67% was adsorbed on the exterior Ti[O.sub.2]. At 1% dispersant, only 25-28% was adsorbed on the interior Ti[O.sub.2] and 38-45% was adsorbed on the exterior Ti[O.sub.2]. This means that at typical dispersant levels of 1%, more of the dispersant is in the continuous phase than is on the pigment.

Nonassociative Thickeners

As a control, the phase diagram for a typical nonassociative thickener such as HEC was generated. An interior grade Ti[O.sub.2] with hydrophilic dispersant was chosen for this purpose. Figure 3 is the resulting phase diagram. Note that the pigment is flocculated at all but the very lowest thickener concentrations, and that the dispersant had little effect on the dispersion state. In fact, pigment composition and dispersant type have relatively little effect on the flocculation behavior of nonassociative systems. Some nonassociative thickeners do adsorb minimally onto pigment surfaces, but are easily displaced by small amounts of dispersant. This is in sharp contrast to what is observed for pigments thickened with HASE and HEUR thickeners, as will be described in the following sections.

HEUR Thickener

BACKGROUND: The properties of coatings prepared with HEUR thickeners are known to be sensitive to the choice of dispersant and also the choice of Ti[O.sub.2] grade. In order to achieve maximum benefit, the pigment must become part of the associative polymer network just as the latex does. To achieve good dispersion the dispersant must be bifunctional, having ionic functionality to interact with the pigment and hydrophobic functionality to interact with the associative thickener hydrophobes. Even if a dispersant has hydrophobes, they may not be readily available to the associative thickener if they associate more strongly with the pigment surface. Before analyzing dispersion phase diagrams it is useful to characterize the adsorption of associative polymers as a function of pigment type and dispersant type.

ADSORPTION OF HEUR-THICKENER ON INTERIOR TI[O.sub.2]--EFFECT OF DISPERSANT TYPE: Choice of dispersant has a profound effect on the adsorption of HEUR polymers on interior Ti[O.sub.2]. (11,12) Figure 4 shows the HEUR adsorption isotherms for both hydrophilic and hydrophobic dispersants on interior Ti[O.sub.2]. Since the dispersant serves as a "coupling agent" between the pigment and the associative polymer network, it is clear that the hydrophobic dispersant is the better choice in terms of forming a uniform, unflocculated dispersion. It is interesting that the adsorption increased initially, but decreased as more dispersant was added. This could be related to the increasing amount of dispersant building up in the continuous phase, thus serving as another "sink" for the hydrophobes of the associative polymer. Figure 5 is a series of photomicrographs depicting the dispersion state of the Ti[O.sub.2] as a function of dispersant. Clearly, the hydrophobic dispersant yields the better dispersion by virtue of its ability to link the pigment with the thickener.

[FIGURE 3 OMITTED]

ADSORPTION OF HEUR-THICKENER ON PIGMENT WITH HYDROPHILIC DISPERSANT--EFFECT OF TI[O.sub.2] SURFACE: The differences in surface composition between interior and exterior grade Ti[O.sub.2] are known to affect the adsorption of HEUR polymer. (11-13) Since hydrophobic dispersant is the clear choice to produce good interaction of HEUR and pigment, it was used to study the effect of Ti[O.sub.2] surface on adsorption. Figure 6 shows the resulting isotherms. Surprisingly, the HEUR does not adsorb significantly on the exterior Ti[O.sub.2], even at elevated dispersant concentrations. From this, one may conclude that the dispersant conformation on the exterior Ti[O.sub.2] is very different than on the interior Ti[O.sub.2], with hydrophobic groups available to the HEUR for the interior Ti[O.sub.2], but not for the exterior Ti[O.sub.2]. This behavior should also be reflected in the dispersion phase diagrams.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

DISPERSION PHASE DIAGRAMS OF HEUR-THICKENED INTERIOR TI[O.sub.2] SYSTEMS: Based on the foregoing adsorption results, a hydrophilic dispersant is not going to be a good coupling agent between the Ti[O.sub.2] and the thickener. This should lead to a poor dispersion in the interior Ti[O.sub.2] HEUR-thickened system because the pigment will not be part of the associative thickener network. The corresponding dispersion phase diagram for this system, Figure 7, confirms this. Dispersion is poor at all concentrations of hydrophilic dispersant. By contrast, the same system with a hydrophobic dispersant yields a dispersion phase diagram resembling those of latexes, with a region of good dispersion between two flocculated regions (see Figure 8). If optimum pigment dispersion and optical properties are desired, then a hydrophobic dispersant is the clear choice for the interior Ti[O.sub.2] case. Exterior Ti[O.sub.2], however, should show different behavior due to its significantly different surface composition and adsorption characteristics.

[FIGURE 6 OMITTED]

DISPERSION PHASE DIAGRAMS OF HEUR-THICKENED EXTERIOR TI[O.sub.2] SYSTEMS: Based on the adsorption measurements discussed previously, it was expected that exterior Ti[O.sub.2] systems thickened with HEUR thickener would have only regions of poor dispersion. Surprisingly, good dispersion was achieved at very low dispersant levels for both the hydrophilic dispersant case (Figure 9) and the hydrophobic dispersant case (Figure 10). The region of good dispersion is actually quite narrow, with the hydrophilic dispersant providing a slightly larger region of good dispersion. Apparently the associative polymer adsorbs weakly on the exterior Ti[O.sub.2], thus providing some network structure until enough dispersant is added to displace it. Unfortunately, at typical dispersant levels in coatings formulations, the pigment will be flocculated, although more weakly than in the nonassociative systems. Therefore, optical properties might be expected to be intermediate between the nonassociative and the optimized associative systems.

HASE Thickener

BACKGROUND: HASE polymers differ from HEUR polymers in that the polyelectrolyte nature of the backbone allows them to have some activity as a dispersant for certain pigments. This will of course depend on the surface composition of the pigment. There will be competition between the HASE polymer and the dispersant for the pigment surface. The resulting interactions should eliminate the bridging/coagulation region and limit the phase regions to a good dispersion region and a depletion flocculation region.

[FIGURE 7 OMITTED]

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[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

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[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

DISPERSION PHASE DIAGRAMS OF HASE-THICKENED INTERIOR TI[O.sub.2] SYSTEMS: Figure 11 shows the dispersion phase diagram for interior Ti[O.sub.2] dispersed with a hydrophilic dispersant. A good dispersion is achieved at low dispersant concentration, but depletion flocculation sets in at about 0.25% dispersant. This is presumably because the dispersant displaces the HASE polymer from the pigment surface, and there is no coupling of the pigment into the associative polymer network at this point. A much larger region of good dispersion is achieved when a hydrophobic dispersant is used, as shown in Figure 12. Depletion flocculation eventually sets in when enough dispersant is present in the continuous phase to associate with the HASE molecules, thus converting them to mainly polyelectrolyte characteristics and weakening the associative network. (14) The previous results for the HEUR systems suggest that interior and exterior Ti[O.sub.2] behavior with HASE thickeners will differ.

DISPERSION PHASE DIAGRAMS OF HASE-THICKENED EXTERIOR TI[O.sub.2] SYSTEMS: Dispersion phase diagrams for exterior Ti[O.sub.2] with hydrophilic dispersant (Figure 13) and with hydrophobic dispersant (Figure 14) again demonstrate that it is difficult to achieve a good dispersion with the exterior grade Ti[O.sub.2]. Both diagrams have extremely narrow regions of good dispersion with the hydrophobic dispersant being only slightly better than the hydrophilic dispersion. These results suggest that even though the HASE polymers do adsorb onto the exterior Ti[O.sub.2], they are weakly adsorbed and easily displaced by low levels of dispersant. Again, the conformation of the hydrophobic dispersant appears to favor association of the hydrophobes with the pigment surface rather than the thickener hydrophobes.

[FIGURE 15 OMITTED]

DISPERSION BEHAVIOR OF NON-TI[O.sub.2] PIGMENTS AND USE OF ALTERNATIVE DISPERSANTS: Although the main objective of this work was to characterize Ti[O.sub.2] systems with a hydrophilic and a hydrophobic dispersant, limited work was done with various filler pigments, colorants, and alternative dispersants. Results suggest that they behave as expected based on the Ti[O.sub.2] work presented here. As an example, Figure 15 contains micrographs showing the dispersion state of a hydrophilic clay filler dispersed with hydrophilic and hydrophobic dispersant and thickened with HEUR. Clearly, the hydrophobic dispersant has a similar effect to that observed for the interior Ti[O.sub.2] case. Colorants represent a more complicated situation because of their high levels of proprietary dispersants and surfactants. Nevertheless, colorants such as iron oxide behave as hydrophilic, whereas phthalo blue is hydrophobic. (15)

As expected, other hydrophilic dispersants, such as polyphosphates, produce poor dispersions when used with the associative thickeners. Dispersants with more hydrophobic character than the one used in this study demonstrate a stronger interaction with the associative thickeners. In general, dispersants with some hydrophobic character should be used in associative thickener systems. A word of caution is necessary for HASE systems. It is possible to build up too much structure in HASE-thickened systems containing hydrophobic dispersants. This can lead to "livering," so it may be prudent to use a mixture of hydrophobic and hydrophilic dispersants to obtain the right rheology and structure balance.

Optical Properties of Paints Prepared With Associative Thickeners

It is expected that the pigment dispersion state will have a profound effect on the optical properties of films. (7,8) This should be manifested in both hiding and gloss characteristics. To test this, 20 PVC model paints containing various combinations of thickener, Ti[O.sub.2], and dispersant were formulated and analyzed. Table 1 lists the contrast ratio and gloss results for paints containing interior Ti[O.sub.2]. The HEUR-thickened paint containing the hydrophilic dispersant has contrast ratio and gloss values comparable to the HEC-thickened paint, in accordance with the flocculated state of both paints. The highest contrast ratio and gloss numbers are observed for the HEUR and HASE systems with a hydrophobic dispersant, as expected from the dispersion diagrams. The HASE-thickened paint with the hydrophilic dispersant has intermediate optical properties due to the dual functionality of the HASE molecule to adsorb onto the pigment surface.

The optical properties for paints formulated with exterior Ti[O.sub.2] are listed in Table 2. For these paints, both contrast ratio and gloss are lower than the corresponding interior Ti[O.sub.2] cases. This is to be expected based on the dispersion diagrams. The best results are still obtained using the hydrophobic dispersant with the HEUR and HASE thickeners, but the differences are smaller. Optical properties are still a marked improvement over the nonassociative system. These results confirm the observations in the dispersion phase diagrams and demonstrate how important it is to match the pigment and appropriate dispersant with the thickener system. Although the work here concentrated on optical properties, other film properties dependent on pigment dispersion will also be affected. This includes film permeability and durability.

CONCLUSIONS

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

(1) Dispersion phase diagrams are useful for understanding the complex interactions of associative thickeners, pigments, and dispersants.

(2) The two most important variables for determining pigment dispersion are dispersant type and pigment surface composition.

(3) Optical properties such as hiding and gloss correlate well with pigment dispersion behavior.

(4) HEUR disperses exterior Ti[O.sub.2], but not interior Ti[O.sub.2].

(5) HASE disperses both exterior and interior Ti[O.sub.2].

(6) No bridging or coagulation phase of Ti[O.sub.2] occurs with HASE thickener.

(7) HEUR interior Ti[O.sub.2]-hydrophobic dispersant is the only system showing "typical" bridging/good dispersion/depletion behavior of HEUR phase diagrams of latex.

ACKNOWLEDGMENTS

The author would like to thank the Rohm and Haas Co. for support and for 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," J. COAT. TECHNOL., 73, No. 913, 79 (2001).

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

(3) Kostansek, E., "Associative Polymer/Latex Dispersion Phase Diagrams II: HASE Thickeners," J. COAT. TECHNOL. RES., 2, No. 6, 1 (2005).

(4) 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).

(5) 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.

(6) Sperry, P.R., "A Simple Quantitative Model for the Volume Restric-tion 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).

(7) Lundberg, D.J. and Glass, J.E., "Pigment Stabilization Through Mixed Associative Thickener Interactions," J. COAT. TECHNOL., 64, No. 807, 53 (1992).

(8) Tarng, M-R., et al., in Hydrophilic Polymers: Performance with Environmental Acceptability, Advances in Chemistry Series 248, Glass, J.E. (Ed.), American Chemical Society, Washington, D.C., Chapter 24, 1996.

(9) Bergh, J.S., Lundberg, D.J., and Glass, J.E., "Rheology of Associative Thickener Pigment and Pigmented Commercial Latex Dispersions," Prog. Org. Coat., 17, 155 (1989).

(10) Hulden, M. and Sjoblom, E., "Adsorption of Some Common Surfactants and Polymers on Ti[O.sub.2] Pigments," Prog. Polym. Colloid Sci., 82, 28 (1990).

(11) Svanholm, T., Kronberg, B., and Molenaar, F., "Adsorption Studies of Associative Interactions Between Thickener and Pigment Particles," Prog. Org. Coat., 30, 167 (1997).

(12) Glass, J.E., "Adsorption of HEUR Thickeners on Latex and Titanium Dioxide Disperse Phases," Adv. Colloid Interface Sci., 79, 123 (1999).

(13) Melville, I., et al., "Pigment Thickener Interactions in Emulsion Paints," Polymers Paint Colour Journal, 177 (4187), 174 (1987).

(14) Johnson, E.A., "Interactions Between Rheology-Modifying and Pigment-Dispersing Agents," Farbe und Lack, 100 (9), 759 (1994).

(15) Reiman, H., et al., "Particles in Networks," Farbe und Lack, 108 (9) 91 (2002).

Edward Kostansek--Rohm and Haas Company*

Presented at the Tess Symposium of the 230th American Chemical Society National Meeting, Aug. 28-Sept. 1, 2005, in Washington, D.C.

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

Table 1 -- Optical Properties of 20 PVC Acrylic Latex Paints Containing
Interior Ti[O.sub.2], Hydrophilic or Hydrophobic Dispersant, and HEUR,
HASE, or HEC Thickener

                                 Contrast  20[degrees]  60[degrees]
Thickener Type  Dispersant Type  Ratio     Gloss        Gloss

HEUR            Hydrophilic      90.7      15           61
HEUR            Hydrophobic      94.0      43           83
HASE            Hydrophilic      93.5      34           77
HASE            Hydrophobic      93.8      41           82
HEC             Hydrophilic      91.1       7           51

Table 2 -- Optical Properties of 20 PVC Acrylic Latex Paints Containing
Exterior Ti[O.sub.2], Hydrophilic or Hydrophobic Dispersant, and HEUR,
HASE, or HEC Thickener

                                 Contrast  20[degrees]  60[degrees]
Thickener Type  Dispersant Type  Ratio     Gloss        Gloss

HEUR            Hydrophilic      92.2       5           43
HEUR            Hydrophobic      93.3      14           60
HASE            Hydrophilic      92.3      12           55
HASE            Hydrophobic      92.4      14           60
HEC             Hydrophilic      91.0       4           38