Crosslinking in the melt of EVA using tetrafunctional silane: gel time from capillary rheometry

Crosslinking of ethylene vinyl acetate (EVA) copolymers can be performed in the melt by use of a transesterification reaction using tetrapropoxysilane as a crosslinking agent and dibutyl tin oxide as a catalyst [Polymer 43 (2002) 6085]. Whether the kinetics of the reaction can be suitably controlled is an important point that must be answered before any processing method such as extrusion or injection molding is considered. This investigation was carried out in a capillary rheometer. Various stoichiometric ratio and processing temperatures were studied. Viscosity was measured during the reaction and the evolution of the flow curves with reaction time was calculated. Gel point under shear conditions was considered to be achieved at the appearance of a yield stress on the flow curves. It was shown that an excess of propoxy groups slows down the reaction kinetics but until 120 °C, the gelling time is longer than 8 minutes which is long enough for processing with conventional techniques.