Effect of dwell stage in the cure cycle on toughening of epoxy using thermoplastic multilayers

Epoxies with high cross-linking densities are brittle and hence have a low fracture toughness. However, different methods are known to increase fracture toughness. Numerous approaches are known to incorporate a second phase into the epoxy matrix, such as rubber, inorganic nanoparticles or thermoplastics, referred to as bulk resin modification. These tougheners usually form specific morphologies during the curing phase of epoxy, resulting in improved fracture toughness of the system. Unfortunately, for some tougheners, the addition of second phase into the epoxy system also results in a reduction in overall modulus and limitation in end-use temperature of the system. In the case of thermoplastic tougheners, the second phase is created by diffusion and dissolution, followed by reaction induced phase separation, leading to a morphology in the micrometer range. However, the influence of the curing history beyond phase separation, using two dwell cure cycles with varying dwell time/degree of cure, on the interphase dimension and final morphology for PEI having a contrasting phase behaviour (UCST), is not well understood. The research presented in this work aims to understand the interphase formation, to later attain the desired droplet size and interphase morphology for improved material toughness. This aim is achieved by analyzing the influence of dwell time by considering two main cases for each selected 1st dwell temperature (120-180˚C): (i) wait until the onset of phase separation (OPS) before increasing the temperature to 200°C (second dwell), (ii) wait until 80% degree of cure (80% DOC) before the second dwell. At all processing temperatures, a distinct gradient morphology (Fig. 1a ) was clearly observed for both cases (OPS and 80% DOC). The SEM micrographs revealed the formation of a larger interphase region (71 μm) for the OPS case as compared to the 80% DOC case (56 μm). Figure 1b shows the interphase thickness as a function of 1st dwell temperature for both OPS and 80% DOC cases. It can be seen that the interphase thickness increased with 1st dwell temperature for both cases, until 160˚C after which it slightly decreased for a 1st dwell temperature of 180˚C. This work highlights, i) the importance of the curing process beyond phase separation to control interphase dimension and final morphology and, ii) the influence of both these parameters on the toughness enhancement.