Published: Jan 2012
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MIXING A PIGMENT WITH LINSEED OIL AND MAK-ing a paste is a preliminary test applied to pigments that has been in use in a systematic manner for almost a century and perhaps even longer in a less defined manner. This proved to be a practical means to qualitatively characterize the approximate color and texture of pigments quickly with materials and tools readily at hand. The work of mixing and the flow properties of the resulting paste provided an estimate of the rheological impact of a new pigment under consideration or a proposed pigment composition change. Thus, valuable insight of the dispersion performance of a pigment or formulation change in a mill base could be estimated. Further extension of the test was found to be useful in predicting the consistency of finished products. Mills  indicated that if equal volumes of “oil absorption paste” are mixed or thinned with equal volumes of thinners, the paints prepared have equal resistance to flow, viscosity, or consistency. While the application of oil absorption values as a formulation tool originated many years ago in the age of oil- and alkyd binder-based paints, the ability to use this information to calculate the binding power index of latex binders, latex porosity index of latex paints, and free binder continues to demonstrate the value of pigment oil absorption data today to solve coatings formulation problems. This is especially true for the field of interior and exterior architectural paints. If the oil absorption test is conducted in a quantitative manner, the result is a measure of the oil needed to wet the surface area of the pigment and to fill the voids in and among the pigment particles. If this result is achieved, profoundly important information is obtained of high value to pigmented paint and coatings development. A number of relatively simple test procedures have been developed to measure oil absorption of pigments. The difficulty generally to successfully applying these tests has been in two areas. First, great care and effort is required to reach and interpret the oil absorption end point. Secondly, one must obtain particle separation in the test to the same degree as achieved with the formulation in the pigment dispersion manufacturing process to which oil absorption value is to be related. Two ASTM pigment oil absorption test methods are available, which cover two incorporation methods. The different methods result in different end points and therefore different oil absorption values. Stieg  found that if the oil absorption value is determined by the ASTM D281 (Test Method for Oil Absorption of Pigments by Spatula Rub-Out) and expressed on a volume basis, it is proportional to the critical pigment volume concentration (CPVC) of alkyd wall paints. Later he and others demonstrated the extension of oil absorption values to calculate the CPVC of pigment compositions to predict the performance of latex paints and other calculated factors useful in formulating these coatings. Gardner-Coleman oil absorption, ASTM D1483 (Test Method for Oil Absorption of Pigments by Gardner-Coleman Method), values have been related to pigment paste viscosities and found useful in estimating consistencies by comparison to a reference standard pigment behavior in the same dispersion machinery. A number of non-ASTM methods for the determination of oil absorption values have been reported in the literature and a few will be listed below.
Glancy, Charles W.