Effect of Alkyl Chain Unsaturation on Methyl Ester Thermo-Oxidative Decomposition and Residue Formation
2009
Pillar, Rachel | Ginic-Markovic, Milena | Clarke, Stephen | Matisons, Janis
The formation of persistent residues by esters used in sheet metal-working lubricant formulations interferes with metal surface cleanliness and impairs the integrity of subsequent surface coatings. Understanding the thermal decomposition and residue formation properties of esters is therefore crucial to minimising residue formation during furnace-based cleaning and will aid in the improvement of coated metal products such as galvanised steel. The thermo-oxidative decomposition profiles of four different methyl esters were studied by Thermogravimetric Analysis (TGA) and Pressure Differential Scanning Calorimetry (PDSC). Chemical characterisation of residues remaining at different stages during the thermo-oxidative decomposition process was achieved by infrared spectroscopic analysis of TGA residue samples. Relationships between ester alkyl chain unsaturation levels and the amount/chemical nature of residue present at high temperature (500 °C) were assessed. In the presence of oxygen, polyunsaturated esters decomposed to leave significantly greater amounts of thermally-stable residue, comprising of metal carboxylate and non-volatile oxygenated compounds, than more saturated esters. Given that the formation of analogous residues by more complex esters used in sheet metal-working lubricants may give rise to metal surface coating defects, the results of this study suggest that polyunsaturated esters should be avoided in lubricant formulations where preventing residue formation is of paramount importance.
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