Thermal Decomposition of Tricresylphosphate Isomers on Fe

Triresylphosphate (TCP) [(CH3-C6H4O)3P=O] is the most commonly tested vapor phase lubricant and is readily available as a mixture containing all three isomers (ortho-, meta-, and para-) of the cresyl groups. The surface chemistry of ortho-, meta-, and para-TCP on Fe foil was studied in order to examine the possible differences in decomposition mechanisms among the isomers. All three TCP isomers decomposed by the same reaction mechanisms and with roughly the same kinetics on Fe. Upon heating, they decomposed on the Fe surface to deposit carbon and phosphorous and produce gas phase H2, CO, toluene and cresol. The amounts of carbon and phosphorous deposited onto the Fe surface by TCP (arylphosphate) decomposition were compared to those deposited by decomposition of tributylphosphate (TBP), an alkylphosphate. Thermal decomposition of all three TCP isomers deposits substantial amounts of carbon ono the Fe surface while TBP decomposition does not. We suggest that it is the lack of β-CH bonds in the methylphenoxy intermediates and the high reactivity of the tolyl intermediates generated during TCP decomposition are the primary reasons for the differences in behavior of TCP and TBP and the root cause of the differences in behavior of arylphosphates and alkylphosphates as vapor phase lubricants.