Effect of Ingredient Loading on Surface Migration of Additives in a Surfactant-Loaded Natural Rubber Vulcanizate

Abstract

Surface migration of additives in a surfactant-loaded natural rubber vulcanizate is investigated as function of ingredient loading. Rubber sheets are compounded according to an L12 orthogonal array using Taguchi design of experiment, where ingredients are treated as factors varied at low and high loadings. Migration experiments are performed by placing the rubber sheets in a natural convection oven at 50°C for 32 days. Weight loss due to removal of migrated additives from surface of rubber sheets is monitored with time. The maximum amount and estimated rate of additive migration are determined from weight loss curves. Attenuated total reflection – Fourier transform infrared (ATR-FTIR) spectroscopy and optical microscopy are used to characterize the chemical structure and surface morphology of rubber sheets during additive migration. Mean effects and analysis of variance (ANOVA) show that high loadings of used oil, paraffin wax, stearic acid, glycerol monostearate (GMS), and cocamide diethanolamide (Coca DEA) increase the amount of maximum migration and migration rate of additives. On the other hand, high loadings of mercaptobenzothiazole (MBT), diphenylguanidine (DPG), mercaptobenzothiazole disulfide (MBTS), sulfur, and zinc oxide (ZnO) decrease the maximum amount of additive migration and migration rate. Used oil has the highest effect on these responses, while sulfur and the accelerators have the least effect. By comparing the of ATR-FTIR spectra of cleaned and migrated rubber surfaces, almost all soluble additives are identified to have migrated to the rubber surface.