Experience with evaluating biodegradability of lubricating base oils

Biodegradability analysis of lubricants by standardised tests provides valuable information for both legislation purposes and assessment of how chemical structure influences biodegradability. The choice of an appropriate test for evaluating the ultimate biodegradability of oils raises serious problems as the majority of lubricating base oils display a poor water solubility. The paper summarises the experience gained and the results achieved from the evaluation of primary and ultimate biodegradability of lubricating base oils differing in chemical structure, such as rapeseed oil, synthetic polyolester oils, poly(α-olefin) oils (PAO 4 and PAO 6) and the conventional mineral oil. Primary biodegradability was evaluated using the CEC L-33-A-93 test. To evaluate the ultimate biodegradability of oils in an aerobic aquatic environment, use was made of the testing methods OECD 301 B and OECD 310 for ready biodegradability. Oils that fail to fulfil the “ready biodegradability” criterion, e.g. pentaerythrite tetra(sec-capronate) oil (polyolester with steric hindrance around the ester bonds), PAO 4, PAO 6, and mineral oil, were evaluated for inherent biodegradability using the OECD 302 D (draft) test and the OECD 302 B method where the test vessels were prepared via a modified procedure. The oils belonging to this group differred in inherent biodegradability. Thus, PE tetra(sec-capronate) oil reached an extent of biodegradation amounting to 65%, that of PAO 4 and mineral oil being equal to 48% and 38%, respectively. Experiments have shown that ISO 14593 offers a convenient method for evaluating the ready biodegradability of base oils according to OECD 310 and makes it possible to evaluate (with the same apparatus and reagents) the inherent biodegradability of oils when the conditions and criteria recommended by the OECD 302D (draft) CONCAWE test are satisfied.