Polymeric Hiding Technologies That Make TiO2 Work Smarter

With the current tight supply and cost run-ups of TiO2, paint companies are looking into options to minimize the effect of the cost increases and reformulate for more efficient utilization of TiO2. This article presents two technologies from Dow Coating Materials that address this issue. ROPAQUE™ opaque polymer is a scattering pigment that partially replaces TiO2, while EVOQUE™ pre-composite polymer directly improves the wet and dry hiding efficiency of TiO2. These technologies can be used individually or in combination to formulate at lower TiO2 levels, and can contribute to overall better paint film properties.

Scattering Efficiency

Titanium dioxide has been used in paints for a century, and in the last 50 years has become the predominant white pigment in architectural coatings. The increase in utility was driven by the desire to reduce the toxicity and environmental impact of white lead. In addition, the higher refractive index and greater optical whiteness allowed formulators to achieve high-hiding white paints. The TiO2 suppliers continued to refine their processes to maximize the scattering and hiding power by reducing impurities and optimizing the particle size and distribution.

Figure 1 

Today’s commercial grades of TiO2 may be nearing the theoretical limit of scattering obtainable from individual particles of this costly raw material. However, when used in most paint formulations, some of this value is lost because the individual particles of TiO2 cannot scatter independently of one another. Particles that are close to one another interfere with their ability to scatter light efficiently. This effect has been quantified as the overlap of the scattering volumes, which are larger than the actual particles,(1) and is known as dependent scattering or more commonly, crowding. There are three factors that contribute to crowding:

• the use level or concentration of TiO2 in the paint film;
• the effect of extender, especially those of larger particle size;
• the quality of the TiO2 dispersion or distribution in the paint film.

At the use level required in white, pastel and medium bases below critical PVC (CPVC) paints, the TiO2 scattering efficiency is compromised by crowding, as depicted by the regular TiO2 line in Figure 1.

When using high levels of TiO2 there is little that can be done to overcome this effect. The distribution of TiO2 in a paint film is, at best, random. As a result, pigment particles are not equally spaced, which results in areas of low and high concentration, as shown in Figure 2a. The high-concentration areas lower the scattering efficiency of TiO2 by exaggerating the effect of crowding. Improving the scattering efficiency of TiO2 would likely reduce the cost of the formulation as well as the environmental footprint by allowing for reduced use levels of this costly and energy-intensive raw material.

Figure 2 

Extenders also play a role in the scattering efficiency of TiO2 by increasing the crowding effect because they reduce the available volume that TiO2 can occupy.(2) Extender crowding is most evident for extenders greater than a few microns in diameter. Small particle size extenders are used to minimize the effect but they do not completely eliminate it. The spacing effect frequently attributed to small extenders does not lead to higher hiding than that attainable with a good dispersion of TiO2 in an unextended paint. It is the non-crowding effect of small particle size extenders, not an active spacing effect, that allows the paint to approach the compromised hiding obtainable with a best random distribution.

The effect of different pigments and extenders can be shown in a simple experiment. A model architectural paint formulation was made with 22 PVC of TiO2. Three different pigments and extenders were added while maintaining the TiO2 PVC and volume solids. The total PVC was allowed to increase with the additional pigment and extender. The pigments and extenders were a 10 µm calcium carbonate, a 1.5 µm calcined clay, and a 0.4 µm opaque polymer.

Figure 3

The scattering of the paint was measured and is plotted in Figure 3. Note that the scattering decreases dramatically with the addition of the large calcium carbonate extender, indicating an increase in the crowding effect. The small-particle-size calcined clay has little effect on scattering, indicating a minimal change in crowding. Opaque polymer actually increases the scattering. Unlike extenders, it is a scattering pigment, so it directly contributes to scattering. Additionally, its small particle size does not further crowd the TiO2. The net effect is an increase in scattering when using opaque polymer, which allows for the removal of TiO2 while maintaining performance properties. In practice, opaque polymer can allow for the removal of half of the TiO2 in a flat formulation and up to 20% in a semigloss or satin formulation. When working with small-particle-size extenders, it is important to keep in mind the relatively high binder demand of these materials. By comparison, opaque polymer is much lower in binder demand due to its uniform spherical shape, which allows for higher use levels without compromising film properties.(3)

ROPAQUE Opaque Polymer

A quality exterior acrylic flat formulation was chosen to study the effect of ROPAQUE Ultra opaque polymer on removing up to 50% of the TiO2 from a paint formulation. The starting formulation and the formulation with the highest opaque polymer level are given in Table 1.

Table 1 

PVC and volume solids were held constant at 49.5 and 34.8, respectively. Note that diatomaceous silica is introduced to maintain the sheen level of the starting formulation. The pigment and extender levels of the test formulation are given in Table 2. Through a 38% reduction in TiO2, performance was similar to the starting formulation. It was found that at higher opaque polymer levels, more diatomaceous silica was required to maintain sheen. However, the higher use level caused a small increase in burnish. The “A” samples have diatomaceous silica added at the rate of 1 PVC for every 5 PVC of opaque polymer. For the “B” samples, the rate is 1 PVC for every 2.5 PVC. At either level of diatomaceous silica, the opaque polymer formulations still have performance similar to the starting formulation, but with a different balance of sheen and burnish properties.

Table 2 

Through the highest levels of ROPAQUE Ultra opaque polymer, cracking and tint retention were maintained, and dirt pick-up resistance was improved after 34 months of exposure testing, as shown in Figure 4.

Opaque polymer can be used to replace up to half of the TiO2 in a flat formulation. In semigloss and satin formulations, up to 20% can be replaced as there is insufficient extender present to balance the gloss and sheen requirements. By replacing TiO2 and lowering its use level, opaque polymer can improve the average efficiency of the remaining TiO2 in the formulation. As a small pigment replacing larger extender, it can reduce the additional crowding effect caused by the large extender that is removed during the reformulation. However, opaque polymer does not directly affect the quality of the dispersion or distribution of TiO2 in the paint film.

EVOQUE Pre-Composite Polymer

Figure 4 

The EVOQUE pre-composite polymer increases TiO2 hiding efficiency while working in combination with opaque polymer to provide the most economic route to developing hiding in white and pastel paints. The pre-composite polymer is designed to interact with the TiO2 surface in such a way that the polymer adheres to the pigment surface. By placing polymer particles on the surface of TiO2, it is more difficult for the TiO2 particles to come in direct contact with each other. This effect can clearly be seen in Figure 2b, which shows a much more uniform distribution of TiO2 as compared to Figure 2a. This more uniform distribution leads to better utilization of the TiO2 and improvements in scattering efficiency. The improvements in scattering are seen in both wet and dry paints.

It also allows for better barrier properties in the paint film by minimizing the pigment-pigment interactions, which ultimately lead to porosity, percolation channels and defects in the film. The improvement in scattering efficiency is shown in Figure 1, which includes the scattering for normally dispersed (regular) TiO2 as well as TiO2 modified with the EVOQUE pre-composite polymer (composite). It should be noted that this approach is similar in concept to highly coated grades of TiO2. Unfortunately, the highly coated grades are lower in TiO2 content, which reduces their fundamental scattering per unit weight. Additionally, they are higher in binder demand than most grades, and it is difficult to use these grades without sacrificing performance properties. Thus the pre-composite polymer technology offers an improved approach to increasing TiO2 scattering efficiency while maintaining performance.

When reformulating to take advantage of EVOQUE pre-composite polymer, TiO2 can be reduced by 10-20%. Sufficient pre-composite polymer is added to the formulation to fully saturate the surface of TiO2 and facilitate good stabilization of the pigment-polymer composite. For typical combinations of TiO2 and pre-composite, that is about one pound of pre-composite polymer (46% TS) for each pound of TiO2 slurry (76.5% TS). The TiO2 slurry is usually added to the pre-composite polymer with good mixing to facilitate the formation of the pigment-polymer composite.

Table 3 

A semigloss paint was modified with pre-composite polymer, resulting in a 20% reduction in TiO2 as shown in Table 4. Since the TiO2 is stabilized by the pre-composite polymer, dispersant demand is reduced and less is required in the formulation. Additionally, less thickener is required, as the hydrodynamic volume of the pigment-polymer composite increases the inherent viscosity of the paint. In this case, no adjustments were made in the other pigments and extenders, and since volume solids was held constant, the total PVC decreased. Other formulation approaches could be used, such as maintaining total PVC with opaque polymer, to further reduce TiO2 level.

Table 4 

It can be seen that the pre-composite polymer technology allows for greater formulation flexibility by reducing TiO2, dispersant and thickener while opening up new options for opaque polymer and extender use while delivering equivalent hiding properties, as shown in Table 4. Pre-composite polymer is useful with all-acrylic binders, as shown in the example, as well as with styrene/acrylic, PVA and EVA latex binders. When replacing non-acrylic binders, the pre-composite polymer may provide films with more “acrylic-like” paint performance.

While the key advantage of EVOQUE pre-composite polymer is hiding efficiency, there are other possible performance benefits with this technology due to its improved pigment distribution. Barrier properties such as household stain removal and humidity resistance are just two examples. A side-by-side drawdown of a conventional paint vs. a composite version of the same paint is shown in Figure 5.

Figure 5 

Tea, coffee and grape juice were applied and allowed to penetrate the dried films for 60 min before they were rinsed with tap water. The central portion of the test panel was then washed with a non-abrasive cleaner for 200 cycles on a Gardner Scrub Machine. Notice how much cleaner the composite paint looks compared to its conventional counterpart. At the bottom of the photo, there is a test strip of lipstick, which also shows the improved stain removal.

The same two test paints were applied over cold rolled steel, allowed to dry for 7 days and then placed in a humidity chamber for 24 hours (Figure 6). Notice the composite paint on the right has fewer rust spots and less tarnishing/yellowing on the surface of the paint film. These performance enhancements observed with the composite paints can be attributed to the tighter films that are formed because of the better distribution of TiO2 particles in the paint film. Exposure testing on early research samples is demonstrating performance similar to the quality acrylic starting formulations that were modified with the pre-composite polymer technology.

Pigment Optimization Using Both Polymeric Hiding Technologies

A natural question when looking at the two polymeric hiding technologies discussed above is – which is best for my formulation? In most cases the answer is – both, in combination. Each delivers hiding to a paint formulation by different hiding mechanisms. For opaque polymer, hiding is directly delivered as it is a scattering pigment and its small particle size helps alleviate crowding of TiO2 caused by large extenders. Pre-composite polymer improves the hiding efficiency of TiO2 by allowing the pigment to be more uniformly distributed in the paint film, minimizing the crowding effect. This improvement in scattering efficiency from pre-composite is seen in both wet and dry paints.

Figure 6 

To illustrate the value of these polymeric hiding technologies, an eggshell architectural paint was reformulated using opaque polymer and pre-composite polymer, individually and in combination. The goal in this study was to match or exceed the hiding and gloss/sheen of the starting formulation while removing TiO2 from the formulation. The main ingredients of the formulations and key appearance properties are given in Table 5. Significant reductions in TiO2 are possible, and note that the reduction is nearly additive when using the polymeric technologies in combination. Once again, the hiding technologies can be used in combination because they contribute to hiding by two different mechanisms. The positive effect of the pre-composite polymer on wet hiding is evident especially when comparing the two paints with similar TiO2 levels [about 265 pounds of slurry TiO2 (76.5% TS)].

Table 5 

Conclusion

TiO2 is the predominant white pigment used in architectural coatings due to its outstanding light-scattering properties. However in white and pastel paints, the full scattering effect is lost due to crowding and less than ideal distribution. ROPAQUE opaque polymer and EVOQUE pre-composite polymer offer two distinct and complimentary approaches to reducing the use levels of TiO2 by as much as 50% while maintaining or increasing hiding. With reduced TiO2 levels other benefits are possible, such as reduced formulation cost, smaller environmental footprint and improved performance of both interior and exterior paints.

How to enhance TiO2 dispersion

TiO2's theoretical optimum particle size is between 0.2 and 0.3 microns, but as received is considerably larger mainly because of the formation of agglomerates in handling during the manufacturing process.

The energy of simply stirring pigment into water or binder is not great enough to overcome the particle attractive forces preventing the breakup of the agglomerates. The presence of these agglomerates adversely affects the hiding power, tinting strength and other end-use properties of the coating.

The graph below illustrates the effect of TiO2's dispersion states vs. particle size distribution on pigment properties:

The process of breaking down agglomerates, otherwise known as dispersion, occurs in 3 steps:

1. Wetting - air and other substances are displaced from the pigment surface by solvent, dispersant / surfactant and binder
2. Grinding - agglomerates of pigment particles are broken and separated into an optimum particle distribution
3. Stabilization - separated particles are maintained by either charge or steric stabilization

Stabilization of Titanium Dioxide in Non-Aqueous and Aqueous Coatings

Titanium dioxide (TiO2) only exhibits its best properties when it is well stabilized against flocculation in the organic binder. This paper deals with the proper selection of additives for the stabilization of TiO2 according to the isoelectric point of the pigment, followed by review of results regarding gloss and haze development and gloss retention improvements in high-quality topcoat applications. Properties of typical additive chemistries that are useful for TiO2 stabilization will be discussed along with a description of their positive and negative side effects, especially the improvement of chalking resistance/gloss retention.

Titanium Dioxide for Coatings

Titanium dioxide, as used for coating applications, is predominantly the rutile-type, high-purity titanium dioxide, manufactured by the chloride or sulphate process. The primary particle size of TiO2 for "hiding"-type applications is usually 0.25-0.3 µm. To reduce the photoactivity of the pigment, the rutile TiO2 crystal is modified with other metal oxides. In addition, very thin films of inorganic oxides (alumina, silica, zirconia) are deposited onto the pigment surface to influence the adsorption of additives. Organic surface treatments (trimethylolpropane, neopentyl glycol, various surfactants, silicones) are often utilized to improve the dispersibility of TiO2 in organic or aqueous media. These organic treatments alone, in most cases, do not provide sufficient stabilization for proper deflocculation of the pigment and, therefore, paint producers must use additional additives in order to achieve the maximum level of pigment stabilization.

Additives for Wetting, Dispersion and Stabilization of TiO2

There are several types of additives used to disperse titanium dioxide. Since titanium dioxide is relatively easy to wet and disperse, the main job of the additive is to improve the stabilization of the dispersed pigment against flocculation. Anionic, cationic, electroneutral or amphoteric materials can be used for solventborne and solvent-free systems. In aqueous applications, anionic, cationic and electroneutral products can be used, sometimes in combination with non-ionic-type surfactants. There is obviously a relationship between the best additive type and the pigment's surface treatment, which can also be influenced by the resin and solvent selection. The target of this study is to provide more precise additive recommendations for specific titanium dioxide pigment types in each system.

Additives to Stabilize TiO2 Dosage Level

Increasing the levels of various additives types (differing in ionic character, molecular weight and chemistry) provides increasing degrees of pigment stabilization, usually characterized by gloss and haze measurements. Figures 1 and 2 characterize these properties of baking-type solventborne coatings. 

This data, and very similar data from a large number of other experiments, indicates that the optimal additive level is typically around 2% active substance, calculated on the TiO2 pigment. The proper amount of additive depends on the following factors:

1. The coating's specific application requirements - i.e., less for architectural or coil coating applications and more for automotive topcoats.

2. The type and chemistry of the binder system - i.e., aqueous systems typically require the highest levels.

3. The additive's chemistry - i.e., lower- to medium-molecular-weight electroneutral additives are usually more effective than their higher-molecular-weight anionic or cationic counterparts.

Transparent TiO2 may require five to eight times more additive for proper stabilization, in comparison to "normal" grades.

Selection of the Optimal Additive Chemistry

There is a relationship between the modification (inorganic surface treatment) of the titanium dioxide pigment and the chemistry of the most suitable additive. For pigments with basic surface treatment, usually anionic or electroneutral additives provide the best stabilization (Figure 3). 

For titanium dioxide grades with neutral surface treatment, electroneutral additives are recommended, and for silica-treated grades, cationic additives are recommended. (Figure 4 and Figure 5).

Titanium dioxides with high levels of dense silica treatment typically show the greatest differentiation in gloss and haze as a function of the type of additive used for stabilization.

Additional studies have shown that results in aqueous and solvent-free systems follow the same rules as those described for solventborne coatings, but the results show even greater differences.

Figure 6 shows the gloss differences of an aqueous high-gloss acrylic emulsion paint, as a function of additive chemistry. Non-ionic polymers - although having good pigment wetting properties - do not provide basic or acidic treated titanium dioxide with a high degree of pigment stabilization. The titanium dioxide dispersions for this study were prepared as aqueous slurries, always with the optimal additive level for each product.

Additive Selection and TiO2 Isoelectric Point

Some titanium dioxide suppliers provide detailed information about the inorganic surface treatment.1,2 However, many commercial suppliers only indicate the nature (alumina, silica, zirconia, etc.) of the treatment, without providing details on the exact amount of the inorganic oxide modification. This lack of information can cause difficulties in the additive pre-selection process. In such cases, it is helpful to determine the isoelectric point of the TiO2 pigment (the pH value where the zeta-potential of the pigment is zero) or obtain the data from the supplier. This information can be used to clearly identify the most suitable additive chemistries for each pigment.

Figure 7 shows the isoelectric points of some common, commercially available titanium dioxides.

Figure 8 identifies the best additive options, as a function of additive chemistry and isoelectric point. In this portion of the study, 13 different polymeric wetting and dispersing additives (four anionic, three electroneutral and six cationic) were evaluated.

There is some overlap between the areas of anionic/electroneutral and electroneutral/cationic additive chemistries, but in most cases the isoelectric point clearly indicates the most suitable additive chemistry for each pigment.

Side Effects of Using Wetting and Dispersing Additives

Negative Side Effects

Negative side effects are rare, since the optimal dosage level of wetting and dispersing additives, as calculated on TiO2, is relatively low (2%). The most common problem is the effect of the additive's chemistry on the storage stability of some sensitive binder systems. Cationic additives can shorten the storage stability of epoxy-functional systems due to their ability to polymerize the epoxy groups. Anionic additives can shorten storage stability due to their acid catalytic activity in some highly reactive urea-formaldehyde or melamine-formaldehyde resin-based formulations.

Another negative side effect is the reduction of the coating's water or humidity resistance, especially with very polar additive structures at higher dose levels. Non-ionic wetting agents, or permanently water-soluble anionic, cationic or electroneutral species can cause these effects.

Positive Side Effects

Colorant acceptance of white base paints can be gradually improved when using certain widely compatible wetting and dispersing additive structures, especially in aqueous systems. Very positive effects have been found with cationic/non-ionic type acrylic wetting and dispersing additives in various types of aqueous latex architectural paints.3,4 Recently, some very positive side effects have been observed in high-gloss automotive topcoat formulations, where phosphate functional anionic and electroneutral wetting and dispersing additives significantly improve the chalk resistance of the coating film (Figure 9). 

These durability tests were carried out in Okinawa Island, Japan, which has similar environmental weather conditions to the established standard Florida test sites.

According to these results, the control formulation lost almost all of its gloss after two years, while phosphate-based wetting and dispersing additives provided much improved gloss retention. Slightly higher levels of additive do not improve initial gloss values, but may increase gloss retention by more than 10 % after two years.

The very great differences between the chalk resistance of the control and additive-containing formulations can be explained not only by better deflocculation of the pigment, resulting in more complete UV absorption by the pigment itself, but also by the fact that some organic phosphates can act as free radical scavengers. This can significantly reduce the degradation of the organic binder by those free radicals (mainly OH_ and HO2_ radicals) formed in UV-induced photochemical reactions at the surface of the TiO2 pigment.5

The greatest improvements were made in polyester-melamine-type binders. In more durable acrylic-melamine resin-based coating formulations, improvements are less pronounced, but still remarkable.

Summary

The inorganic treatment of commercially available titanium dioxide pigments determines to a large extent which type of wetting and dispersing additive will work best for that individual pigment type. For basic surface treatments, anionic and electroneutral additives provide the best deflocculation. For neutral surface treatments, electroneutral additives are best, while cationic additives work best with acidic SiO2-treated pigments. The isoelectric point of each pigment can be used to select the best additive. Organic phosphate-based polymeric wetting and dispersing additives provide not only a high level of pigment stabilization, but may also significantly improve the chalking resistance and durability of high-gloss topcoats.

Acknowledgements

The author gratefully acknowledges some technical data and valuable comments from Ms. Anne Linzmaier, Mr. Bernd Dawid and Mr Akihiro Wakahara from BYK-Chemie, additional acknowledgments to Jotun Paints, Du Pont de Nemours, Inc., Kronos Titan, Inc., Sachtleben Chemie GmbH, Ishihara Sangyo Co., and Kansai Paint.

References

1. Du Pont de Nemours, Inc.: Technical data sheets of Ti-Pure Titanium Dioxides for coatings.
2. Kronos Titan, Inc: Technical data sheets of Kronos Titanium Dioxides for coatings.
3. Proceedings of the ChinaCoat Conference 2003, Shanghai; Janos Hajas, Frank Kother: Improvement of Colorant Acceptance in Architectural Paints.
4. Proceedings Eurocoat Conference 2002 Barcelona: Jaume Figueras, Janos Hajas: How to Improve Colorant Acceptance.
5. Company brochures Du Pont de Nemours, Inc., Kronos Titan Inc., Kerr-McGee, Inc. about theory of chalking.