Effect of a Dwell Stage in the Cure Cycle on the Interphase Formation in a Poly(ether imide)/High T_g Epoxy System

Abstract

Epoxies are inherently brittle materials and to overcome this brittleness, a second microphase (i.e., thermoplastic) is typically added. This modification of epoxy resin using thermoplastics results in reaction-induced phase separating morphologies in the micrometer range. In this study, the influence of the curing history, beyond phase separation, on the interphase formation and final morphology of PEI and the high T g epoxy system is investigated. Several cure cycles were examined, each with a first dwell temperature ranging from 120 to 180 °C for a given time up to the onset of phase separation (OPS) or up to the 80% degree of cure (80% DOC) and then with a second dwell at 200 °C for 20 min to complete the cure cycle. Hot-stage microscopy experiments were carried out at several first dwell temperatures before final conversion at the second dwell. The morphologies and resulting droplet size distribution at the interphase, after the final cure, were analyzed through scanning electron microscopy. Results showed that the diffusion distance was significantly higher in the case of OPS as compared to the 80% DOC case, particularly at lower first dwell temperatures. This behavior was attributed to the fact that, in the case of OPS, polymeric chains were still in a mobile state and diffused further during the second dwell curing stage, while at 80% DOC, polymeric chains were completely bound but still diffuse due to non-stoichiometric curing. This restricted mobility of polymeric chains after phase separation (80% DOC) resulted in a larger number of smaller droplets as compared to the OPS case.

The Mechanical Characterization of Welded Hybrid Joints Based on a Fast-Curing Epoxy Composite with an Integrated Phenoxy Coupling Layer

“…The actual time for interphase formation is much less than 60 s, as the highest diffusion rate prevails in the beginning of the curing. As the chain mobility reduces with a higher degree of curing, the diffusion rate drops rapidly, eventually leading to the maximum interphase thickness within the first 10 s of curing [ 28 , 30 , 31 , 50 ]. Voleppe et al [ 51 ] showed that the penetration at the front of the thermoset continued beyond phase separation, whereby both mechanisms overlapped.…”

“…The affinity between thermoset and thermoplastic is the crucial element for creating joints with high strength [ 13 , 26 , 27 , 28 ]. A gradient interphase forms between the reactive epoxy resin and the thermoplastic material, whereby the two components partially dissolve, diffuse, and, finally, decompose due to a reaction-induced phase separation [ 29 , 30 , 31 ]. The overall composition or cross-link density of the material varies from location to location at the macroscopic level [ 32 ].…”

In Situ Characterization of the Reaction-Diffusion Behavior during the Gradient Interphase Formation of Polyetherimide with a High-Temperature Epoxy System

“…To measure the interphase formation and characterize its microstructural properties as a function of temperature, an optical hot-stage microscopy setup was used, as used by Teuwen and Farooq et al [ 9 , 12 , 24 ], where a controlled heating device (Linkam THMS600, Tadworth, Great Britain) was coupled with optical microscopy (Keyence VHX 600, Osaka, Japan). Figure 1 shows the adapted measurement stack.…”

“…The general concept proposes incorporating a second polymer in the epoxy matrix that is used as a toughening agent to prevent the crack propagation, thus leading to the improvement of the fracture toughness [ 5 ]. Several thermoplastics have been studied to toughen high-performance aerospace epoxy systems, such as poly(hydroxy ether) (phenoxy), poly(ether sulfone) (PES), poly(ether imide) (PEI) and poly(etherether ketone) (PEEK) [ 4 , 6 , 7 , 8 , 9 ]. The formation of interpenetrating polymer networks (IPN) and phase separating morphologies is an essential mechanism to improve fracture toughness [ 4 , 9 , 10 ].…”

“…Several thermoplastics have been studied to toughen high-performance aerospace epoxy systems, such as poly(hydroxy ether) (phenoxy), poly(ether sulfone) (PES), poly(ether imide) (PEI) and poly(etherether ketone) (PEEK) [ 4 , 6 , 7 , 8 , 9 ]. The formation of interpenetrating polymer networks (IPN) and phase separating morphologies is an essential mechanism to improve fracture toughness [ 4 , 9 , 10 ].…”