A new simple numerical model based on experimental scorch curve data fitting for the interpretation of sulphur vulcanization

Sulphur was the first agent used to vulcanize commercial elastomers (e.g. natural rubber) and allows meaningful cost reductions during the industrial process (production cost ratio between peroxides and accelerated sulphur is around 5). Therefore, accelerated sulphur vulcanization is the most popular technique for the production of polydiene and EPDM elastomers items. At present, crosslinking mechanisms are not analytically known in detail, therefore reticulation kinetic has to be deduced from mechanical properties obtained during standardized tests, as for instance the oscillating disc rheometer. In the present paper, we propose a numerical model to fit experimental rheometer data based on a simple composite three functions curve, able to describe the increase of the viscosity at successive curing times at different controlled temperature to use during the production of thick items vulcanized with sulphur. It is believed that rheometer curve is able to give an indirect information on the rubber reticulation kinetic at different temperatures, to use in a successive step to establish simplified analytical kinetic formulas to adopt in the accelerated sulphur vulcanization of polydiene and EPDM elastomers. In the model, it is necessary to collect rheometer curves at different specimen temperatures, because vulcanization in industrial practice occurs at variable temperatures during curing, with considerable differences from the core to boundary of the item. Once that rheometer curves are suitably collected in a database, they are used to predict the optimal vulcanization of real items industrially produced. Finally, a so called alternating tangent approach (AT) is implemented to determine optimal input parameters (curing external temperature Tn and rubber exposition time t) to use in the production process. Output mechanical property (objective function) to optimize is represented by the average tensile strength of the item. A meaningful example of engineering interest, consisting of a thick 2D EPDM cylinder is illustrated to validate the model proposed.