Surface reactions of ethyl stearate and stearic acid with zinc, manganese and their oxides

Abstract

The reactions of stearic acid with zinc and manganese powders have been studied from ambient temperature to 600 °C mainly using differential scanning calorimetry and thermogravimetry. Related, but less extensive, work has been carried out with ethyl stearate replacing the acid and with powders of zinc oxide, MnO, Mn 3O 4, Mn 2O 3 and MnO 2 replacing the metals. The reaction products were analysed by Fourier transform IR spectroscopy, X-ray diffraction, scanning electron microscopy and gas chromatography. For zinc and zinc oxide the first reaction zone began at about 160 °C and extended to 280–290 °C. Water and zinc stearate were formed. From 280 to 375 °C, zinc stearate was present in the reaction products of the acid with both zinc metal and zinc oxide. At 600 °C, no acid soap was detected. The reactions of zinc with the ethyl ester were considerably less exothermic than those with the acid, consistent with the higher amount of soap formed in the acid reaction and with the larger heat evolution arising from the decomposition of any excess acid when compared with that from excess ester. In the acid reactions with manganese powder, the stearate was believed to commence formation at 135 °C. Its presence was detected in residues quenched from 200 °C in reactions of all four manganese oxides with stearic acid as deduced from the IR signals at 1562, 1467 and 1438 cm −1. Only a slight trace of manganese stearate was detected in the quenched residues from reactions between ethyl stearate and manganese up to 200 °C. No evidence of the presence of soap was obtained when the time of exposure at 200 °C was increased to 30 min or when the temperature was raised in stages up to 600 °C. Samples from these experiments gave IR signals at 1630 and 3400 cm −1 consistent with the presence of considerable amounts of molecular water. No clear relationship could be established for the kinetics of any of the reactions over the complete range of temperature because of the difficulties in separating the contributions of a number of simultaneous and overlapping heterogeneous and homogeneous processes. Within the initiation stage of the metal-acid reactions to 300 °C, good correlations were obtained with a Prout-Tompkins type of expression which led to experimental activation energies of 85 kJ mol −1, In A = 18.3, and 78.0 kJ mol −1, In A = 17.0, for zinc and manganese respectively. The oxide film thicknesses on the metals were shown by ion etching to be a maximum of 150 Å on zinc powder and just above 1000 Å on the manganese.