Effect of Ingredient Loading on Vulcanization Characteristics of a Natural Rubber Compound

Abstract

Rheometric properties of rubber compounds are usually monitored with time during the course of vulcanization at constant temperature. The measured vulcanization characteristics of rubber compound are used for quality control and evaluation of raw materials and product formulations. With the high number of ingredients used in typical formulations, it is important to identify ingredients which significantly affect the vulcanization characteristics of a rubber compound. This study reports the vulcanization characteristics of a natural rubber compound at 60 °C as function of ingredient loading. Rubber sheets are compounded according to a 212-8 fractional factorial design of experiment, where ingredients are treated as factors varied at low and high loadings. Vulcanization curves, which are time plots of elastic torque S’, viscous torque S”, and tan δ = S”/S’, are measured for each rubber sheet using a moving die rheometer. The following responses are then determined from the vulcanization curves for data analysis: minimum elastic torque ML, maximum elastic torque MH, torque difference ∆S = MH – ML, scorch time ts1, cure time t’90, cure rate index CRI = 100/ (t’90 – ts1), S” and tan δ values at ML and MH. Analysis of variance (ANOVA) shows used oil to be the main ingredient affecting vulcanization of the natural rubber compound (ML, MH, ∆S, ts1, S” at ML and MH), followed by sulfur (MH, ∆S, CRI), calcium carbonate CaCO3 (S” at ML, tan δ at MH) and diphenylguanidine DPG (ts1). High loading of used oil lowers the elastic and viscous response of the rubber compound, while increases the time for scorch. Increased loading of sulfur significantly enhances the elastic torque and cure rate of the compound. High loading of CaCO3 improves the viscous response, while DPG significantly shortens the scorch time of the rubber compound.