The gas phase decomposition of synthetic lubricants under pyrolytic conditions

Abstract

This paper deals with the gas phase thermal degradation, under pyrolytic conditions, of three commercial industrial lubricants, using the system for thermal diagnostic studies (STDS) apparatus between 400 and 1000 °C. Two lubricants of different triaryl phosphates composition and one based on fatty acid methyl-ethyl esters have been studied in order to investigate their behavior in simulated “hot spot” conditions. The main objective of the work was to verify the thermal stability of the three samples and to determine the products generated during the thermal exposure as a function of the experimental conditions. The occurrence of hazardous by-products was investigated as well as their composition and evolution as a function of temperature. Experimental results show that, for each sample tested, the degradation starts at 600–700 °C and is complete within 900 and 1000 °C. A great number of by-products, mainly aromatic and poly-aromatic compounds (PAHs), was always observed at temperatures higher than 700 °C. Two mechanisms are proposed for the formation of these products: the first one is directly related to the production of benzene radicals originated from aryl moieties, that act as building blocks in consecutive reactions; the second one is related to the occurrence of displacement and cyclization reactions which lead to the obtainment of aromatic moieties from vinyl radicals. The amount of PAHs coming from the lubricant based on fatty acid methyl-ethyl esters is always much lower than that of the other two samples. Moreover, the pyrolysis of the two triaryl phosphates lubricants leads to the formation of three phosphorus-containing aromatic by-products (phosphabenzene, isophosphinoline and phosphinoline) and a mechanism for their formation was proposed. On the basis on the experimental results found, it is possible to conclude that the lubricant based of fatty acid methyl-ethyl esters produces less hazardous by-products than triaryl-phosphates ones when exposed to thermally severe pyrolytic conditions.