Reaction Kinetics and Phase Transformations During Cure of a Thermoplastic‐Modified Epoxy Thermoset

Abstract

A comprehensive thermal and rheological characterisation of a commercially important aerospace resin system modified with a thermoplastic toughener is presented here. The primary focus has been the understanding of how the cure kinetics and mechanism relate to other processes such as phase separation during cure. Differential scanning calorimetry has shown that thermoplastic modification does not affect the cure mechanism significantly and that the process was dominated by an autocatalytic process up to vitrification after which diffusion‐controlled processes dominate regardless of thermoplastic concentration. Thermoplastic addition at 10 wt.‐% PEI was found to increase the final cure conversion compared with the neat resin and inhibit the onset of diffusion control below a cure temperature of 160 °C. At 20 wt.‐% PEI concentration the final conversion and the onset of diffusion control were similar until 150 °C, after which they displayed lower values than the neat resin. Rheological analysis showed that the phase separation process could be followed conveniently through the changes in viscosity and exhibited effects consistent with expected variations in morphology. The 10 wt.‐% PEI system displayed abrupt increases in viscosities which were indicative of a dispersed particulate morphology, while the 20 wt.‐% PEI system displayed more complex behaviour consistent with a phase‐inverted or co‐continuous structure. The gelation process was shown to obey a power law model although the local environment or morphology of the reactive epoxide group at higher PEI concentrations was shown to affect the fit to the model.

Apparent volumetric shrinkage study of RTM6 resin during the curing process and its effect on the residual stresses in a composite

“…Δ H total was found to be 511 J/g. This value is similar to that reported by Varley [16]. The fractional residual epoxide conversion α t was converted to a percentage conversion at time t using the following equation: …”

“…After gelation, the resin behaves like an elastic solid, and its shear modulus had increased by several orders of magnitude at vitrification (i.e., 10 MPa). Kinetic and rheometric data on isothermally cured RTM6 resin between 110°C and 180°C has previously been reported by Varley [16]. Varley reported gelation time varying with temperature from 358 min at 110°C to 18 min at 180°C.…”

“…The high performance HexFlow® RTM6 (Hexcel, Australia) is a one‐part epoxy/amine resin system that has been specifically developed to fulfill the requirements of the aerospace industry in advanced RTM processes. The composition of RTM6 resin is based on a tetra functional epoxy system and a blend of aromatic amine hardeners [16] and its glass transition temperature ( T g ) is 196°C [17].…”

Reaction Kinetics of Polyurethane Formation Using a Commercial Oligomeric Diisocyanate Resin Studied by Calorimetric and Rheological Methods

“…As the network formed however, a 10% strain may have been too large to ensure the newly formed structure was not disturbed, thus not obtaining as high a viscosity build up or storage modulus as one would otherwise have expected. Instrumental limitations disallowed a change in % strain while curing progressed, in order to obtain a full spectrum of the cure profile without structural compromization 48. The gel times established from the crossover of G ′ and G ″ in uni‐frequency mode at 1 Hz are displayed in Table 2.…”

Preparation and characterization of cyanate/epoxy foam

“…On the contrary, low viscosity would cause coalescence and rupture of bubbles during the blowing process. Figure 2 shows the steady shear viscosity of CE60 mixture resin system as a function of time, cured at temperature from 140 C to 180 C. It can be observed that the viscosity of the systems shows a similar tendency and possess an induction period 29,30 (where the increase in viscosity is modest and gradual, followed by an extremely rapid exponential increase in viscosity). The induction period in steady shear viscosity is about 53, 35, 22, and 17 min at 140, 150, 170, and 180 C, respectively, as seen in Figure 2.…”