Oil absorption number (OAN) is a measure of the ability of a carbon black to absorb liquids. This property is a function of the structure of the carbon black. Either dibutyl phthalate (DBP) or paraffin oil is acceptable for use with standard pelleted grades, including N‑series carbon blacks found in ASTM D1765, although OAN testing using paraffin oil on some speciality blacks and powder blacks might result in unacceptable differences as compared to OAN testing using DBP oil. While studies have shown the two oils to give comparable precision, paraffin oil offers the advantage of being non-hazardous.
ISO 4656:2012 specifies a method using an absorptometer for the determination of the oil absorption number of carbon black for use in the rubber industry.
The same method is used for the determination of the oil absorption number of compressed samples of carbon black. The procedure for the preparation of the compressed samples is described in an annex.