Modelling the Vulcanization Reaction of Devulcanized Rubber

Abstract

The generation of scrap tires/rubber is increasing annually and thus there is a need for a safer and more efficient method of disposal or reuse of this rubber. As the direct disposal (burning/ land filling) of scrap rubber has harmful effects on the environment, recycling and reusing is given great consideration. For reusing, scarp rubber can be devulcanized and used in many applications. Some methods for devulcanization of rubber have been developed and devulcanized rubber can be blended with virgin rubber and re-cured. However, the curing behavior of devulcanized rubber will vary compared to that of virgin rubber so it would be helpful if a proper model could be obtained in order to characterize and optimize the curing properties of devulcanized rubber with respect to the curatives added. The main objective of this study is to characterize the vulcanization reaction of devulcanized rubber by using a kinetic model which will help in predicting the kinetics of vulcanization with change in temperature and the amount of curatives added. Also, it will help to obtain a better understanding of the relationship between curing behavior and properties of devulcanized rubber. It may further help in minimizing the time required for choosing the appropriate cure system for compounds involving devulcanized rubber. This study involves fitting the cure kinetic reaction model on differential scanning calorimetry (DSC) data. Vulcanization is an exothermic process in which the energy released during the reaction is assumed to be proportional to the bonds formed and thus can be related to the degree of vulcanization. DSC records this heat released which is later used to calculate the reaction rate and degree of cure. As vulcanization is a chemical process, its simulation involves characterization of kinetic parameters. Experiments have been done using 7 different samples of devulcanized rubber which vary in curative composition. Those samples have been tested using DSC at different temperature scanning rates and a kinetic model was used to fit the DSC data and to determine the kinetic model parameters. The Kissinger model along with the Arrhenius equation was used to determine the vulcanization activation energy and the Kamal-Sourour model was used to fit the data obtained from DSC through non-linear least square techniques and estimate the rest of the parameters. To have a comparison between virgin rubber and devulcanized rubber, natural rubber samples with similar formulations have been mixed and tested in the same way. Also, to further investigate the cure kinetics of devulcanized rubber, blends of virgin rubber (tire tread compound) with different amounts of devulcanized rubber were analyzed in similar ways.