Surface reactivity of tributyl thiophosphate: effects of temperature and mechanical stress

The surface reactivity of tributyl thiophosphate on iron surfaces has been studied in situ by attenuated total reflection Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reaction and desorption spectroscopies. The results show that at temperatures lower than 373 K the molecule forms a physisorbed layer on the iron substrate. At 373 K a reaction takes place with the formation of an organic layer, together with iron polyphosphate and sulfate. At higher temperatures temperature-programmed desorption results suggest that the mechanism involves P-O bond scission to yield butoxy groups. This could be preceded by P=S bond scission to give tributyl phosphite, which then, in turn, undergoes P-O bond scission to produce butoxy groups. The results obtained following tribological testing are in agreement with those of thermal tests: evidence of polyphosphate and sulfate formation is found.