Recycling of Cross‐Linked Polymers by Reincorporation: From Macroscopic Mixing to Molecular Interdiffusion

Mechanical recycling by reincorporation is attractive because it can be implemented directly under melt‐processing conditions. The relevant question is the scale at which virgin and recycled macromolecules become mechanically coupled, from millimetric domains down to interfacial interpenetration and molecular entanglement. In this study, this multi‐scale problem is investigated using a representative system based on cross‐linked regenerated polyethylene (XPE), which is reincorporated into linear low‐density polyethylene (LLDPE). The XPE was regenerated using a high‐temperature extrusion process (320 ≤ T (°C) ≤ 420), which converts a permanent chemical network into a processable material consisting of different populations of chains, including weakly cross‐linked, branched, and soluble chains. This results in a reduction in the gel fraction, with the consequence that these regenerated domains can be processed and blended down to the molecular scale with virgin PE of the same type. The mechanisms of shear mixing between recycled XPE and virgin LLDPE are examined from a conceptual perspective, using distributive mixing via striation, the concept of creep, and characteristic diffusion lengths. The analysis shows that complete molecular homogenization is not a prerequisite for recovering useful mechanical properties. In fact, a sufficiently developed and entangled interphase, equivalent to a thermal sealing mechanism, may be the controlling criterion. However, an excessive temperature of regeneration ( T > 360°C) can generate species with low molar mass and highly branched structures, which may adversely affect the mechanical properties of the blend after reincorporation. These results highlight the need to optimize recycling by striking a balance between domain fragmentation, interfacial repair, molecular diffusion, and the preservation of the chain architecture.