A modified TP 306 test procedure is used to study the high temperature liqu
id phase oxidation of five base oils and their hydrocarbon type subfraction
s. The base oils are selected so as to cover a wide range of physiochemical
characteristics. Qualitative as well as quantitative analysis of the base
oils and their fractions were done using chromatographic (column, HPLC) and
spectroscopic (MS, NMR, FTIR) techniques to gain a better understanding of
their compositional and structural details. Oxidized materials were also s
tudied and structural changes identified as a result of thermal oxidation.
Relative oxidative degradation of different base oils is discussed in terms
of their NMR derived structural models. The results indicate that polyarom
atic and naphthenoaromatic compounds undergo more oxidative degradation tha
n saturated molecules. The oxygenated polar compounds and deposits formed a
re mainly derived from aromatic structures, which decreases as weight perce
nt of the total base fluid as oxidation progresses. Alkyl substituted aroma
tic structures showed larger decrease in chain length than purely paraffin
(n- and iso-) compounds during oxidation. Spectroscopic data also indicated
that alkylation of aromatics is mainly through alkyl radicals generated fr
om long chain alkyl aromatics. (C) 1999 Elsevier Science Ltd. All rights re
served.