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