Thermal degradation of synthetic lubricants under oxidative pyrolytic conditions

Abstract The gaseous phase thermal degradation, under oxidative pyrolysis conditions, of three commercial industrial lubricants (two of different triaryl phosphate composition and one based on fatty acid methyl and ethyl esters) was investigated between 400 and 1000 °C. The main objective of the study was to investigate their behavior in simulated “hot spot” conditions, i.e. compare the thermal stability and identify the products of thermal decomposition as a function of exposure temperature. Experimental results showed that the fluid based on fatty acid methyl and ethyl esters has the lowest thermal stability, being completely degraded at 700 °C with respect to 800 and 900 °C for the two phosphorus-based lubricants. Several by-products were identified during the thermal degradation of all lubricants. The majority of them consisted of aromatic compounds, amongst which benzene was the most abundant, and polycyclic aromatic hydrocarbons (PAHs). The formation of such by-products peaked at 700 and 800 °C respectively for the lubricants without and with phosphorus. Two main mechanisms were proposed for the formation of these products. In first hypothesis, degradation of the fatty acid methyl and ethyl esters lubricant is assumed to start with the breakdown of the aliphatic moieties generating vinyl radicals which, in turn, through displacement and cyclization reactions gave rise to benzene radicals and PAHs. According to the second hypothesis, phosphorus-based lubricants degrade directly forming benzene radicals and PAH structures by consecutive reactions. At temperatures greater than 700 °C, the amounts of benzene and PAHs produced as a result of the degradation of the fatty acid methyl and ethyl ester based lubricant were much lower than those of the two triaryl phosphate lubricants. The experimental results showed that fewer and less hazardous by-products are formed during the thermal degradation of the fatty acid methyl and ethyl ester based lubricant with respect to the lubricants based on triaryl phosphate.