Effects of Temperature and Pressure on ZDDP

A recent theoretical study proposed that the anti-wear property of zinc dialkyl dithio phosphate (ZDDP) is due to the formation of chemically connected networks as a result of pressure-induced cross-linkage of phosphate groups of thermally decomposed ZDDP. To investigate the initial decomposition processes and the possibility of linking of phosphate groups in the decomposed product, in-situ high-pressure and high-temperature infrared (IR) spectroscopy using synchrotron radiation were performed on the original ZDDP. At room temperature no substantial structural change was observed up to 21.2 GPa, a pressure far exceeding the predicted onset of a structural transformation for the model zinc phosphate at 7 GPa. The observed Pressure induced broadening of the IR peaks is most likely associated with structural disorder or amorphization of ZDDP which is completely reversible upon decompression. When ZDDP is heated under pressure, an irreversible transformation was observed around 225 C and 18.4 GPa. The experimental results show that ZDDP undergoes substantial decomposition at high pressures and high temperatures but no hint of cross-linkage of phosphate groups was found.