Exploitation of secondary relaxation of recycled polycarbonate for enhanced epoxy toughness and composites performance

Polycarbonate (PC) is an important engineering plastic with excellent mechanical strength, and its recycling and reuse become increasingly attractive along with the growing consumption. In this work, PC was recycled as its oligomeric analogues (denoted as rPC)—and further used to modify epoxy resin, aiming to harness the secondary relaxation behavior. The secondary relaxation of rPC was investigated in four forms: 1) rPC in its free state; 2) rPC in the linear matrix of PC; 3) physical blending of rPC with epoxy resin followed by curing; 4) chemical modification of epoxy resin with rPC followed by curing. It was found that the rPC itself, with an average molecular weight of approximately 2000–4000 g mol⁻¹, exhibited secondary relaxation from −120 °C to −50 °C, similar to that of pristine PC. However, this secondary relaxation behavior were seriously suppressed upon physical blending of epoxy and rPC, which appears to display significant phase separation. By chemically modifying TDE-85 epoxy resin with a moderate amount of rPC (20 %, denoted as EP-rPC4), the cEP-rPC4 resin showed improved elongation at break, flexural strength, and impact strength. The interlaminar shear strength of fiber reinforced/EP-rPC4 composites also increased significantly, both at room temperature and at −50 °C. This study offers an effective strategy for recycling and utilizing waste PC in thermosets and their composites.