Molecular evolution during dynamic vulcanization of polyolefin mixtures for lead-free thermoplastic vulcanized

The process of vulcanization has been exploited for decades for rubber production. The original process involves heating natural or synthetic diene polymers with Sulphur which accelerates and activates the formation of crosslinks between the long molecules (macromolecules) resulting in improvement of rubber elastic properties. Also, vulcanization of polyolefins has been widely exploited industrially. Specifically, medium voltage (MV) insulation compounds based on EPDM and EPDM/LDPE blends have been produced. In recent literature, a lead free MV insulation compound was subjected to dynamic crosslinking with peroxide in a melted polypropylene matrix, in the presence of various ethylene polymers, propylene and additives to optimize the ultimate properties. In fact, an analysis of the MV TPV compounds properties revealed that it passed thermomechanical testing for specific operating temperature and short circuit emergency. Nonetheless, it was also shown that a deeper analysis of the molecular evolution and structural morphology during the vulcanization and the role of the various components of the starting mixture was still required.

To address this, Italian scientists from the University of Pisa: Professor Francesco Ciardelli and Stefano Riolo, in collaboration with Dr. Stefano Dossi, Dr. Andrea Galanti and Dr. Andrea Magri at the Mixer SpA company further characterized the thermoplastic vulcanized (TPVs) prepared by MIXER from the molecular viewpoint to the best level allowed by the complexity of the system. Their aim was to reach a control of the final properties based on the feed composition and the structure evolution during the dynamic vulcanization. Their work is currently published in the research journal, Polymers Advanced Technologies.

In their approach, the TPV MV insulation compounds were prepared starting from a peroxide curable lead free MV insulation, which was the actual market benchmark. For this reason, they were extensively investigated in comparison to the standard lead free MV insulation. The researchers then undertook a detailed analysis of the molecular changes induced by the peroxide vulcanization reaction with reference to the different polymers and low molecular weight additives present in the starting mixture.

The results of the dynamic vulcanization process were evaluated and related to the formulation. In addition, the authors found that the presence of the unsaturated polymer (EPDM in their case) highly favored the crosslinking but as shown by their data, the inter chain coupling of saturated ethylene polymers (POE and LDPE) was very modest living a consistent non crosslinked material fraction. This last, with the polypropylene giving degradation rather than crosslinking, granted a good thermoplastic behavior to the TPV.

In summary, the study by University of Pisa and Mixer SpA company scientists presented for the first time at the molecular level, the preparation process and the final properties of new thermoplastic lead-free medium voltage insulation compounds based on the TPV technology. Remarkably, the team achieved their objective, i.e. the design and production of TPVs with better processing behavior and predesigned performances in the final application with particular reference to the possibility of processing them as thermoplastic elastomers.