Oil absorption.

The term oil absorption (OA) refers to the weight of linseed oil that must be taken up by a given weight of dry pigment in order to form a paste. This may seem like an anachronism since pigment no longer is dispersed in linseed oil to make paint, but vehicles other than linseed oil give similar (sometimes only roughly similar) OA values. Linseed oil has remained the vehicle of choice for the measurement because it is readily available and there is no compelling reason to change. Oil absorption values provide valuable information. They are useful in paint formulation because they are measures of vehicle demand for different pigments and give us an idea of how much dispersant will be needed for each one. If a pigment has a high OA number, you know that it will require a lot of vehicle for dispersion. If a film-forming resin is being used to disperse the pigment, then the OA value also indicates the amount or proportion of that resin that will be absorbed by the pigment rather than being available for film formation. Oil absorption values also are related to the critical pigment volume concentration (CPVC) and the pigment packing factor (PPF) for the same pigment in paint and ink films {see T.C. Patton, Paint Flow and Pigment Dispersion, Wiley-Interscience, New York, 1979, pp. 165-168 for details). In addition, OA can be used to compare batches of a pigment or to evaluate the effect of surface treatments on a pigment. However, the precision values of the measurement methods are not good enough to pick out small differences.

There are various methods for measuring the oil absorption. Probably the most common in the U.S. is ASTM D 281, "Oil Absorption by Spatula Rub-out." The rub-out of a weighed amount of pigment is made on a glass plate or marble slab as linseed oil is slowly dripped from a dropping bottle or buret. The endpoint is a stiff, putty-like paste that does not break or separate. The oil absorption value is calculated from the weights of oil and pigment used in the test and is reported as the number of grams (pounds) of oil required to exactly wet 100 grams (pounds) of pigment.

The amount of pigment needed for testing depends on the pigment itself, its particle size and other properties. For example, 10 g of zinc oxide or uncoated Ti[O.sub.2] provides an adequate specimen, but only 1 g of carbon black is needed. The specimen size should be large enough so mat at least 1 g of oil is required. It is a good idea to pretest any new pigment to determine an approximate endpoint. Once this has been established, a second (this one counts) trial should be run with a slower addition of oil and more vigorous rubbing out in the region of the endpoint. The result in this trial is used to calculate the OA for that pigment.

Oil absorption values for pigments vary widely as shown in Table 1. Please note that values for these pigments could be higher or lower depending on particle size, surface treatments, and moisture content (OA tends to go up with pigment moisture content). Oil absorption involves filling pores or interstices as well as adsorbing on the surface of the pigment. Pigments with a high surface area due to porosity or small particle size have high values. Surface treatments (surfactants, silica, or alumina coatings, etc.) may reduce vehicle demand. Pigment density also is important. High density pigments require less weight of oil for a unit weight pigment and, therefore, have lower values.

Good references for additional information on oil absorption and its measurement are Patton's book noted above (pp. 161-178) and J. V. Koleske, "Oil Absorption of Pigments," Paint and Coaling Testing Manual, 14th Edition of the Gardner- Sward Handbook, ASTM, Philadelphia, PA, 1995, Chapter 28 (pp. 252-260).

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Table 1--Oil Absorption Values for Selected Pigments--Rubout Method

Pigment                      Oil Absorption Value

Basic lead carbonate                  9-12
Barium sulfate (Barytes)             10-20
Barium sulfate (Blanc Fixe)             15
Calcium carbonate                    16-18
Zinc oxide                           17-20
Red iron oxide                          20
Uncoated titanium dioxide               11
Amorphous silica                        29
China clay                              30
Phthalocyanine blue                     34
Milori blue                             51
Lampblack                               51
Carbon black (medium)                  124
Diatomaceous earth                     196

Data from a variety of sources.

By Clifford K. Schoff Schoff Associates