Recent developments have shown that smectite minerals can be used to reduce the cost of TiO₂ by $15 or more per 100 gal of paint.

Smectites consist of a group of layered silicates. These products occur naturally in aggregates of extremely platy crystals. The individual crystals are submicron sized particles with aspect ratios of approximately 500:1. Furthermore, these crystals have an interesting ionic nature. The surface of the crystal has a substantial quantity of negative charges, while the edge of the crystal consists of a network of positive charges (see Figure 1). Counterbalancing the negative ionic charges within the body of the individual platelets are cations, typically sodium or calcium. Monovalent sodium ions will disassociate from the surface of the individual crystals to allow the underlying ionic nature of the crystal itself to be expressed in aqueous environments. Divalent calcium ions do not disassociate from the surface and thus do not allow the ionic nature of the crystal to manifest themselves. As a practical matter, this gives rise to the notion of what is commonly known as “swelling” smectites when the preponderance of the counterions are sodium. Swelling (sodium) smectite will act as a low shear viscosifier in many systems, in addition to having significant titanium dioxide extension characteristics. Calcium smectites, on the other hand, do not possess any significant capacity to viscosify. The widespread commercial availability of both species of smectite (calcium and sodium) allows the formulator a significant degree of flexibility in tailoring the system to customers’ specific needs.

Smectite (particularly the sodium variety) has been used extensively in architectural coatings at relatively low levels (0.2–0.4% by weight of the formula) to control settling and syneresis and to impart sag resistance. Little (if any) work has been done to validate the pigment extension properties of smectite in architectural coatings at higher loadings (up to 1.2% of the formulation by weight).

For purposes of this study, two formulations were chosen that represent fairly straightforward adaptations of typical interior grade architectural coatings. The first is a 30% volume solids, 60 PVC flat vinyl acrylic formula. It contains (per 100 gal of paint) 125 lbs of TiO₂, 245 lbs of 6-micron calcium carbonate, and 75 lbs of delaminated kaolin. The second is a 30% volume solids, 28 PVC semigloss vinyl acrylic with (per 100 gal of paint) 125 pounds of TiO₂ and 106 lbs of 6-micron calcium carbonate. The controls for each formula were thickened with 5 lbs of hydroxyethylcellulose per 100 gal. All formulations used in this study are presented in their totality in the sidebar.

Within the scope of this evaluation, two test techniques were used. The first is the classic Bird bar drawdown, performed at three mils wet film over a Penopac chart. The contrast ratio is simply the reflectance of the black area over the reflectance of the white area. The second procedure is a method developed to more accurately reflect real-world application of architectural coatings. In this test, a piece of Kem-Glo paper is first prepared by bounding a 3-square-foot area. Into this area a 40 ml sample of paint is poured. This sample is then applied with a seasoned roller. The resultant film is dried and the contrast ratio is expressed as the reflectance of the gray area over the white area. While the first method minimizes the impact of rheological differences in the different coatings, the second addresses the fact that homeowners don’t actually paint their homes with Bird bars. In both methods, five readings were taken using a template for each region; these readings were averaged for purposes of calculating contrast ratios.

The first portion of the study was a comparison of four different smectite products to determine the relative ability of each to act as a titanium dioxide extender in a high PVC (PVC=60) paint. The four products were a dry sodium smectite (dry NaSm), dry calcium smectite (dry CaSm), a slurried sodium smectite (slurried NaSm), and slurried calcium smectite (slurried CaSm). The slurrying technique is described in a patent issued to Southern Clay Products Inc. Each of the four smectite products were added to the two different vinyl acrylic paints at four levels — 3.60, 7.18, 10.78, and 14.37 dry pounds per 100 gal of paint. Hydroxyethylcellulose loads were reduced to accommodate any increase in viscosity from the smectite. Figure 2A shows the results of Bird bar drawdowns for these 60 PVC paints.

Figure 2B presents graphically the contrast ratio results for these same paints when they are actually applied with a seasoned roller according to the test method earlier described.

The data clearly shows some substantial trends in this portion of the study. Contrast ratios and reflectance values in general increase measurably with the incorporation of smectite in this coating system. When comparing rollouts to drawdowns, we see an increase in the reflectance values over white substrate and lower contrast ratios with rollouts, despite the fact that the ratio is now being measured over gray substrate vs. black for the drawdowns. This is due to the texture imparted to the film by roller application vs. a Bird bar. This is a much more real-world test and, in fact, closely reflects what has been seen in commercial applications. The contrast ratio differences between the control and the smectite-containing products in rollouts can range up to seven units vs. five units with the drawdowns. It is believed that this difference is due primarily to improved film consistency with smectite containing formulations. This improvement is seen in rollouts due to a more attractive, finer texture in the applied film, resulting in less profile in the film itself. Of further interest is the fact that these contrast ratio improvements were seen in spite of higher reflectance values over white substrate. This forces the improvement over the darker substrate to be even more substantial to record a measurable difference in the contrast ratio. Look, for example, at the readings for dry NaSm applied by drawdown at a 14.37 pound loading. The contrast ratio improves by 3.77 units vs. the control in spite of a 1.91 increase in reflectance over white. This increase in white reflectance forced the black reflectance reading to increase by 5.21 units to register the previously noted 3.77 unit increase in contrast ratio. What does this mean in plain English? Not only does the paint hide better, but it is whiter.

Let us look at the 28 PVC formula next. The data will be presented in the same formats used for the 60 PVC formulations. In these formulations, only the slurried smectite variants were evaluated. Figure 3A shows the results of using the Bird bar drawdown application technique. The quantities of smectite and TiO2 are identical to the amounts added to the 60 PVC formulation. The calcium carbonate is reduced and the kaolin is eliminated.

Figure 3B shows the results of applying these same paints using the roller method. Detailed data is presented in Tables 1A and 1B.

In this evaluation, it is more clearly seen why the roller application test method was developed. Were the sole measurement scheme to be the drawdown bar, no improvements would be seen. When we look at the slurried NaSm in a rollout, a distinct trend toward improved hiding is seen. Slurried CaSm does not show the same improvement in spite of the viscosity being equalized.

Of further note in the evaluation of the 28 PVC coating, however, is an increase in overall reflectance values as noted in the 60 PVC coating. As a consequence, even though the contrast ratio is not measurably improved in many of the above measurements, we are still dealing with a demonstrably whiter paint. In all of the evaluations, there is a distinct trend for NaSm (whether dry or slurried) to more dramatically effect an improvement in contrast ratio than CaSm. There is, however, some indication that CaSm may provide a whiter coating. Preliminary studies indicate that in the 60 PVC coatings more than 20% of the TiO2 can be replaced with calcium carbonate with little loss of contrast ratio. This preliminary data is presented in Table 2.

It is presented graphically in Figures 4A and 4B.

If, for purposes of making a conservative assessment of this technology, we assume that we can replace 20% of the TiO₂ with calcium carbonate at a load of 10.77 lbs of slurried NaSm, we can project a cost savings of $14.81 per 100 gal of paint as seen in Table 3. If, as suspected, further reductions in TiO2 are possible, even greater reductions in cost will be seen.

As is well known in the smectite clay industry, it is strongly advised that formulators either hydrate smectite additives fully in water under high speed agitation or purchase an already slurried product.

Acknowledgements

The author wishes to acknowledge the invaluable assistance of Kevin T. Oakes of Southern Clay Products Inc. in Gonzales, TX. Additional contributions were made by Mr. Carl J. Bauer and Mr. Benjamin W. Knesek, also of Southern Clay Products, Inc. This work could not have been completed without all of these people.

This article was originally presented as a paper at Intertech November 8-10, 2000, in Orlando, FL.

For more information on smectites, contact Southern Clay Products Inc., 1212 Church St., Gonzales, TX 78629; phone 830/672.1984; e-mail tbrennan.scprod@laporteinc.com.