New approaches to the recycling of cross-linked polyethylene

The aim of this study is to explore innovative approaches to improve the recyclability of cross-linked polyethylene. In the context of the circular economy, enhancing the recyclability of materials such as cross-linked polyethylene is critical for reducing waste and conserving resources. Cross-linked polyethylene, widely used for its excellent mechanical and thermal properties, presents significant challenges to recycling due to its cross-linked structure. Developing effective strategies to recycle cross-linked polyethylene aligns with the principles of circularity by promoting material reuse and minimizing environmental impact. This study investigates mechano-chemical methods, such as reactive extrusion and grinding, to break the bonds in cross-linked polyethylene. To evaluate the effectiveness of the de-crosslinking process, various characterization techniques were employed. The Horikx model was applied to determine whether the de-crosslinking mechanism involved random or selective bonds breaking. Findings revealed that milling led to a reduction in the degree of cross-linking, although reactive extrusion emerged as the most promising technique for de-crosslinking XLPE. This conclusion was supported by reductions in cross-link density and observed changes in molecular weight between cross-links (Mc) in the processed materials. Namely, the optimal amount of peroxide (5%) to induce selective de-crosslinking of the material during reactive extrusion was found. Additionally, the synthesis of a new class of polymeric materials known as vitrimers is explored. Research demonstrated the feasibility of developing a potential alternative to conventional XLPE by utilizing a vitrimer composed of dynamic covalent bonds. A key advantage of vitrimers is that they retain excellent mechanical properties at high temperatures, similar to XLPE, while also being fully recyclable due to their reversible cross-linked network, akin to thermoplastic polymers.