In recent years, new polymeric wetting and dispersing additives for waterborne formulations have improved pigment color development and in-can shelf stability. Nevertheless, some limitations become apparent when these new additives are incorporated in resin-free pigment concentrates with such pigments as iron oxides, titanium dioxides, and others with small particle sizes and high surface areas. The limitations include the following.

• Efficient wetting of pigment surfaces during incorporation into aqueous formulations,
  
• Efficient development of maximum color strength, and
  
• Long-term shelf stability of the pigment concentrates.

Coating formulations produced with the pigment concentrates require several properties, including the following.

• High-gloss and low-haze levels determined by the compatibility of the wetting and dispersing additives with the resin systems and pigment particles,
  
• Weatherability: UV- and water resistance, and
  
• Compatibility for use with a variety of color shades.

To fulfill all these requirements, the wetting and dispersing additives must possess several performance characteristics, including the following.

• They must migrate to the pigment surface to ensure coating integrity.
  
• They must not be replaced by other polymeric substances, such as resins.
  
• They must not be solubilized with the addition of co-solvent.
  
• They must not break down pigment stability during the coating film drying process as the material changes from water-thinnable to water-resistant.

This article discusses the synthesis of different chemical combinations that result in the development of the next generation of polymeric wetting and dispersing additives for certain transparent pigments used in resin-free pigment concentrates. Application tests and analytical measurements compared pigment stabilization performance of model structures from these different synthetic groups.

Design Synthesis of Wetting
and Dispersing Additives

Several routes for the synthesis of polymeric wetting and dispersing additives exist. Acrylic copolymers can be engineered using different hydrophobic (e.g., styrene/butyl acrylate) and hydrophilic monomers (e.g., carboxylic acid groups after neutralization). After polymerization and neutralization of the acid groups this approach yields aqueous products. The non-polar portion of these molecules adsorbs to the polar parts, while the polar portion dissolves in the aqueous media, creating a stabilizing shell around the pigment particle. If the substances contain acid groups, they stabilize pigment by repulsion of the charged surfaces.

In the presence of chain transfer agents, free radical polymerization of methacrylates produces polymethacrylates with a terminal ester group linked only to the tertiary carbon atom. This ester end group can then be transesterified selectively with various, highly hydrophilic polyoxyalkylenes to yield AB-shaped surfactants like polyethylene glycol (PEG) grafted polymethylmethacrylate (PMMA) (see Figure 1).

Acrylic copolymers (e.g., with styrene and methacrylic acid) can be transesterified with hydrophilic hydroxy functional polyglycols in a random distribution along the polymer chain leading to honeycomb-like structures (see Figure 2).

Modifying copolymers of styrene and maleic acid anhydride (SMA) with polyethers produces honeycomb-like molecules with adsorbing backbones and water-soluble side chains (see Figure 3). They have shown very good performance with different pigments in various resin systems.

The synthesis of wetting and dispersing additives directly in aqueous solutions is very attractive economically but is not possible for modified SMA derivatives due to the water-insoluble nature of the styrene in the backbone. A new synthetic route is now available with the copolymerization of polyethers containing vinyl-terminated groups.1 Addition of acetylene to OH-terminated polyethers produces these polyethers. They can be polymerized with maleic acid, for example, in the presence of suitable initiators (see Figure 4). Non-polar portions in the polyether chain near the terminal group or the backbone contribute to adsorption properties on the pigment surface.

Such products are free of ester linkages, which are unstable against hydrolysis. They can be produced in a few steps from raw material sources at attractive price levels. This synthetic route requires no VOCs.

Changing the vinyl polyether design (e.g., using hydrophobic alkylene oxide monomers) can make the polyethers compatible with different kinds of pigment surfaces.

Model structures from these different synthetic groups were compared in application tests and analytical measurements made for their pigment stabilization performance (see Table 1).

Methods for the Determination of Adsorption Properties

Particle charge detectors from Mutek can measure the streaming potential of highly concentrated formulations (thus, close to real-life conditions). The equipment determines differences in the mobility of counter ions on a charged surface (leading to the formation of dipoles under high shear forces).2-4 In comparison to other methods described in the literature, this method permits adsorption measurements without requiring the strenuous separation of pigment concentrate particles from the surrounding liquid phase.5-8

Titration of aqueous pigment suspensions with additive solutions can determine differences in the adsorption behavior. After adsorption of the wetting and dispersing additives, addition of polyelectrolytes (until electro-neutrality is achieved) can quantify charge intensity on the pigment surface. Addition of substances, such as the surfactants found in resins, provide results about the additive's resistance to replacement from the particle surface.

In addition to titration, thermogravimetric experiments can be helpful to quantify the amount of organic material on a pigment surface. In these experiments, the pigments themselves are quite stable to heat (no significant loss of weight up to 500degC), while the adsorbed organic substances thermally decomposed at lower temperatures.

Microcalorimetric experiments, which detect energetic effects of the additive adsorption on the pigment surface, are performed in mixing cells and provide information about the strength of interaction between additive and particle surface. These methods are common practice in biochemical studies.9-10

Test of Application Properties

The formulation of pigment concentrates using transparent iron oxides, transparent titanium dioxides, and other pigments with small particle sizes and high surface areas remains a challenge. To optimize transparency and brilliancy, the conventional grinding process makes high pigment loadings difficult to achieve. In addition, after storage, these formulations often exhibit dramatic viscosity increases.

As a first step, the additives were tested for performance in a formulation with one standard pigment type and high pigment content (see Table 2).

The additives show significant differences in viscosity and transparency (see Figure 5). Additives D, E and F provide better transparency and viscosity performance than the other products. The very low tendency for additives D,E and F to stabilize foam is also advantageous, allowing the formulation of pigment concentrates without defoamers.

The high transparency achieved with additive E is easily seen in comparison to a commercially available product (see Figure 6).

After storage, viscosity difficulties tend to intensify. Often, there is a strong increase in viscosity, which in turn affects the paste yield. Additive E, however, provides an advantageous rheological profile after long-term storage at elevated temperature (see Figure 7).

Similar results were obtained with other pigments from other suppliers. Among these, the rheological profiles of yellow iron oxides are, in general, more difficult to optimize, leading to slightly lower levels of pigment loading in the formulations (30-40%). The additive level must be optimized for each formulation to reach the maximum level of transparency as shown for Sicotrans L 1916 (see Figure 8).

The visual impression of transparency correlates well with particle-size measurements by way of photon correlation spectroscopy (see Figure 9). All the formulations show a high level of transparency, so distinguishing visual ratings are difficult. A 40% level of additive active matter on pigment leads to optimum results in transparency and particle size.

Transparent titanium dioxides used to achieve a so-called frost effect in metallic basecoats are likewise not easy to stabilize. Formulations with additive E and D show excellent transparency, as well as excellent rheological profiles (see Figure 10). Pigment particle size in the formulations is smaller than in commercially available concentrates (see Figure 11).

The pigment concentrates with transparent iron oxides or transparent titanium oxide did not increase gassing problems in aluminum basecoats when formulated with any of the additives D, E and F.

Pigment Slurries - Adsorption Properties

When additive solutions are added to aqueous pigment slurries, the copolymers E and F - containing vinyl polyethers - show strong shifts in streaming potential measurements. In comparison to additive D, adsorption on the pigment surface is much stronger for additives E and F (see Figure 12).

Additive E also shows a stronger energetic effect than additive D when additive solution and pigment are mixed in the test cell of an Isothermic Titration Microcalorimeter (see Figure 13). This hints at an intense interaction of polymer and pigment surface.

Conclusion

The copolymerization of vinyl polyethers with maleic acid anhydride in an aqueous solution is a new concept for the synthesis of customized wetting and dispersing additives. This concept provides structures that are hydrolytically stable and economically attractive.

They show superior performance compared to other types of dispersing additives, e.g., for the formulation of pigment concentrates with transparent iron oxides. They also provide the desired rheological profiles at high pigment loadings together with excellent transparency. These honeycomb-like, block copolymers do not show significant foam stabilization like many other surfactants. Therefore, no defoamers are required in the formulations. Research opportunities to change the design of the vinyl polyethers offer the possibility to match the requirements of different pigment surfaces.

For more information on pigment additives, contact Tego Coating & Ink Additives, 914 East Randolph Street, Hopewell, VA 23860; phone 800/446.1809; fax 804/541.2783; e-mail frances. eggleston@us.goldschmidt.com; visit www.additiweb.com; or Circle Number 133.

References

1 Internal Report, Goldschmidt AG, EP0736553 A2.
2 M?tek: Messung von Partikelladungen, Keramische Zeitschrift 5/1995.
3 James, M.; Hunter, R.J.; O'Brien, R.W. Languor 8 (1992) P. 42.
4 Osterhold, M.; Schimmelpfennig, K. Farbe und Lack, Volume 11 (1992) P. 841.
5 Osterhold, M. Farbe und Lack, Volume 8 (1995) P. 683.
6 Reck, J.M.; Dulog, L. Farbe und Lack Volume 2 (1993) P. 95.
7 Binford, J.S.; Gessler, A.M. J. Colloid Sci. Volume 63 (1995) P. 1376.
8 van den Haak, H.J.W. J. Coatings Technol., Volume 69 (1997) P. 137.
9 Mohsen, N.M.; Craig, R.G.; Filisko, F.E. J. Biomedical Materials Research, Volume 40 (1998) pp. 224-232.
10 Tewary, Y.B. et al, J. Physical Chemistry, Volume 99, Issue 5 (1995) pp. 1594-1601.
11 Internal Report, Goldschmidt AG, EP0736553 A2
1 M?tek: Messung von Partikelladungen, Keramische Zeitschrift 5/1995
12 M. James, R.J. Hunter, R.W. O'Brien, Languor 8 (1992) P. 42
13 M. Osterhold, K. Schimmelpfennig, Farbe und Lack, Volume 11 (1992) P. 841.
14 M. Osterhold, Farbe und Lack, Volume 8 (1995) P. 683.
15 J.M. Reck, L. Dulog, Farbe und Lack, Volume 2 (1993) P. 95.
16 J.S. Binford, A.M. Gessler, J. Colloid Science Volume 63 (1995) P. 1376.
17 H.J.W. van den Haak, J. Coatings Technol., Volume 69 (1997) P. 137.
18 N.M. Mohsen, R.G. Craig, F.E. Filisko, J. Biomedical Materials Research, Volume 40 (1998) pp. 224-232
19 Y.B. Tewary et al, J. Physical Chemistry, Volume 99, Issue 5 (1995) pp. 1594-1601.

Links

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