Coupling effect of molecular weight and crosslinking kinetics on the formation of rubber nanoparticles and their agglomerates in EPDM/PP TPVs during dynamic vulcanization†
It is well-known that a fine dispersed rubber phase in thermoplastic vulcanizates (TPVs) is a key to obtain good mechanical properties and high elasticity of TPV products. Previous studies reported that the rubber nanodroplets formed during shearing blending can transform into rubber nanoparticles by in situ rapid crosslinking and these rubber nanoparticles spontaneously form agglomerates dispersed in a plastic matrix during dynamic vulcanization (DV). However, important influencing factors on the formation of rubber nanoparticles and their agglomeration during DV have not been reported yet. In this study, the coupling effect of the molecular weight (MW) of polypropylene (PP) and crosslinking kinetics including the crosslinking rate (CR) and crosslinking degree (CD) on the size of ethylene propylene diene monomer (EPDM) rubber nanoparticles and their agglomerates in EPDM/PP TPVs was systematically studied for the first time. The minimum diameter of EPDM nanodroplets was theoretically calculated by using the critical break-up law of viscoelastic melts for the blend with high MW PP or the critical capillary equation for the blend with low MW PP, and the real size of the EPDM nanoparticles was experimentally verified. Interestingly, the results show that the lower MW of the PP phase, lower CD and higher CR contribute to the formation of smaller rubber nanoparticles, whereas the higher MW of the PP phase and higher CD of the rubber phase contribute to the formation of smaller rubber nanoparticle agglomerates. This study provides guidance to optimize the microstructure of EPDM/PP TPVs for the preparation of high-performance TPV products.