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 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.
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.
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.
AcknowledgementsThe 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 firstname.lastname@example.org.