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.

BEHR BOASTS NANOPARTICLE EXTERIOR COATINGS FAR SUPERIOR TO PRIME AND TOPCOAT SYSTEMS

At 4 to 20 pounds of inorganic oxide nanoparticles per 100 gallons of exterior paint, Behr is sure to brighten the new year prospects of nano pigment manufacturers. Behr has developed a one coat system for the exterior of buildings that can replace the old prime and paint system with long lasting coatings that resists environmental degradation.

Nanophase’s proprietary nanoparticles are now being used in Behr’s Premium Plus Ultra paint. Nanophase’s nanoparticles not only lend the paint improved adhesion and anti-mildew properties, but also allow users to forgo the normal two-step priming and coating process. The new paint reportedly does both in a single step.

In U.S. Patent 7,642,309, Behr Process Corporation (Santa Ana, CA) reveals paints that provide coatings with improved properties such as, tannin blocking, hiding power, stain removal, corrosion resistance and which have high adhesion. Research results show it to be far superior to competing commercial products now on the market .

The formulations basically include two binders, a zinc oxide nanoparticle pigment, and pigmentary titanium dioxide, according to Behr inventors Ming-Ren Tarng, Mark Minamyer, Anh Pham, Stan Brownell, Annie Pham, Anil Alexander, Deven Shah, Kim L. Nguyen, My Linh Pham and Sidney Maxey, who detail a range of coating compositions with inorganic oxide nanoparticles in U.S. Patent 7,642,309.

All nano paint formulations exhibited improvements in adhesion, tannin blocking, stain removal, hiding power, color retention and corrosion resistance when compared to commercial products now on the market.

Nanoparticle-sized metal oxide pigments are used. According to Behr, in a general formulation, the paint is comprised of: an acrylic primer binder in an amount of 340 to 430 lbs per 100 gallons of paint; an acrylic topcoat binder in an amount of 70 to about 100 lbs per 100 gallons of paint; a styrene acrylic topcoat binder in an amount greater than 0 to 180 lbs per 100 gallons of paint; titanium dioxide in an amount greater than 0 to 350 lbs per 100 gallons of paint; zinc oxide in an amount greater than 0 to 7 lbs per 100 gallons of paint; and a nano particle inorganic oxide in an amount 4 to 20 lbs per 100 gallons of paint

FIG. 1 depicts a multi-axis property graph of the test results of Behr’s paint having a satin sheen and white base, as compared to commercially available exterior satin in a white base. 

FIG. 1 depicts a multi-axis property graph of the test results of paint of the present invention having a satin sheen and white base, as compared to commercially-available, exterior satin in a white base. Adhesion, yellowing, stain removal, hiding power, scrub resistance, corrosion, and tannin blocking were measured.

Paint Manufacturing 101

Traditionally consumers have had to choose between the superior durability offered by latex paints and the superior hiding properties of oil-based paint. Conventional latex or emulsion paints require an underlying primer coat prior to their application to provide an adequate level of blocking and to prevent bleed-through, such as of tannins.

In addition, many conventional latex paints require a primer coat to provide adequate adhesion and durability for specific applications. Often, even when a primer is applied, multiple coats of the prior art paints are required to prevent an alteration of color due to the presence of the primer or bleed through of dyes and pigments from the underlying substrate. In fact, multiple applications of prior art paint will not prevent bleed-through on substrates such as cedar since the dyes and pigments, such as tannins contained in the substrates, are often water soluble and diffuse through each layer of the latex paint.

Paint typically contains four essential ingredients, namely pigment, binder, liquid and additives. Any or all of these ingredients may be a single component or may comprise multiple items. Pigment provides color to paint and also makes paint opaque, and pigment is usually of mineral or organic origin although some pigments are artificially produced.

Some pigments possess little or no bulk and must be fixed on a more solid, but at the same time transparent, substance or base. "Prime" pigments provide color and opacity (opaque coverage). The most common prime pigment is titanium dioxide, which is white and is used in latex and oil-based paints.

Traditionally, pigments have also added hiding properties to paint. Specialty or extender pigments may also be used and provide bulk to the paint at a low cost. The extender pigments are often chosen for their impact on properties like scrub resistance, stain resistance and chalk resistance. Alum or clay are frequently used for this purpose.

These pigments are added to the paint to provide certain characteristics such as thickness, a certain level of gloss and durability. They are usually naturally occurring products which originally were in the earth and were mined and purified for use in paint. Such pigments as calcium carbonate, talc and clay are, for example, used extensively in paints.

The binder holds the pigment and also adheres it to a surface, and a binder composition may have more than one component. In latex paint, the latex resin is the binder. Most commonly in latex paint, the binder is 100% acrylic, vinyl acrylic (polyvinyl acetate), or styreneated acrylic.

The pigment particles are insoluble and merely form a suspension in the binder. The binder "binds" the pigment into a tough, continuous film and as noted above helps the paint adhere to the surface. In addition, it has been found previously that the use of 100% acrylic binder provides for maximum adhesion when wet and also provides for resistance to blistering and peeling, resistance to mildew and dirt, and alkali resistance for paint applied over fresh masonry.

Liquids carry the pigment and binders, and this liquid is the part of the paint or coatings product which evaporates. The role of the fluid is to keep the paint in a fluid form for ease of application. Once applied to the surface it evaporates leaving a uniform film which then dries to form a protective coating. The liquid used is primarily determined by the solubility of the binder. In oil-based and alkyd paints, the liquid is typically a paint thinner, and in latex paints, the liquid is typically water. Traditionally, top quality paints have less liquid and more solids (i.e. pigment & binders) as measured by percent solid.

Additives are ingredients used at low levels to provide key properties, such as but not limited to: mildew resistance, better flow and leveling, and splatter resistance. Common additives used in conventional paint formulations include rheology modifiers, surfactants, defoamers, coalescents, and biocides. Other numerous additives are well-known in the art and may be utilized as required to formulate a paint having the desired properties.

Various techniques are known in the art for producing paints having various types of sheens, i.e. "shine" or gloss. For example, by incrementally increasing pigment levels and/or by using larger pigment particles, various gloss levels can be achiev