Selective Decomposition Process and Mechanism of Si–O–Si Cross-Linking Bonds in Silane Cross-Linked Polyethylene by Solid-State Shear Milling

Material recycling of silane cross-linked polyethylene (Si-XLPE) by the solid-state shear milling (S3M) technology could produce thermoplastic polyethylene. To take advantage of thermoplastic polyethylene from Si-XLPE, the de-cross-linked mechanism of Si-XLPE in S3M must be evaluated. The products prepared by the S3M technology were characterized using gel content, molecular weight measurement (GPC), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (29Si-NMR), and re-cross-linking test to obtain insight into the selective de-cross-linking effect of the S3M technology. The results had confirmed that a cross-linking element consisting of Si–O–Si chemical bonds was destroyed during S3M, which was responsible for the destruction of cross-linked structures of Si-XLPE materials, and the molecular structures of the resulting products were close to that of silane-grafted polyethylene. The mechanical properties of Si-XLPE materials prepared by the S3M technology before and after water-cross-linking treatment were compared. It was found that the products obtained by the S3M technology could still undergo the cross-linking reaction and form more silane-grafted polyethylene with the increase of milling cycles. Based on the results of this study, we expected that practical applications of de-cross-linking of Si-XLPE materials could be successfully achieved. In this study, favorable conditions were established for the direct recovery of thermoplastic polyethylene from Si-XLPE by the selective de-cross-linking reaction in S3M and an idea about the recycling of Si-XLPE materials was developed.