Comparing interphase formation in different thermoplastics combined with RTM6/M18 epoxy systems

The integration of thermoplastics and thermosets through interphases enables novel composite structures with tailored mechanical and processing properties. This study investigates interphase formation between various amorphous thermoplastics (PEI, PES, PSU, PC) and high-temperature epoxy systems (RTM6, M18/1), expanding beyond the well-studied PEI/PES systems to explore candidates for low-temperature processing and welding applications. Hot-stage and autoclave experiments show that PSU and PC form interphases with irregular morphologies and resin displacement effects, differing significantly from the well-defined structures of PEI and PES. Additionally, PC exhibits unexpected spherulitic formations, indicating a distinct phase separation mechanism. In contrast, semi-crystalline thermoplastics fail to form interphases, confirming their limited compatibility with epoxy resins. However, amorphous PEEK shows a tendency for interphase formation, suggesting that amorphous variants of semi-crystalline thermoplastics could be further investigated in this context. These findings highlight the potential of PSU and PC for epoxy bonding at lower curing temperatures (110–160 °C), which could enable cost-efficient composite manufacturing. Further mechanical testing is needed to assess the adhesion strength of these interphases, particularly in fiber-reinforced composites.