The aim of this thesis was to address various challenges in the cationic frontal curing of epoxy monomers for composites used in technical carbon fiber reinforced laminates and cured-in-place pipe rehabilitation technology (CIPP). Two types of epoxy resins were investigated throughout the thesis namely bisphenol A diglycidyl ether (BADGE) and 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (CE), that are often used as polymer matrix in numerous industrially relevant composites. To develop a frontally curable epoxy resin suitable for thick-walled carbon fiber reinforced composites, a major challenge was to initiate the curing front with ultraviolet (UV) light since fibers are intrinsic UV blockers and surface irradiation was not sufficient to sustain an autocatalytic curing front.
In the first part of this thesis, varying contents of CE as epoxy diluent were used to increase the reactivity of a BADGE monomer in frontal polymerization by employing a diaryliodonium salt and benzopinacol as classical photoinitiator/thermal initiator pair. The results showed an increase in front velocity and a negligible effect on temperature of the propagating front. In addition, the resins were stable with a pot life of at least eight months. Whilst glass transition temperature and degree of cure of the frontally cured resins decreased with higher CE content, tensile strength and storage modulus increased up to a CE concentration of 25 wt%.
However, it was found that the addition of CE to BADGE was not enough to increase the exothermicity of the curing front when initiated by UV light only. As an alternative approach, triggering the frontal polymerization via heat was studied. For this the neat resins and the composites were subjected to medium temperature (150 °C) in a conventional oven. As the curing yielded composites with a low glass transition temperature, a new strategy namely redox cationic frontal polymerization (RCFP) was introduced. In RCFP, a reducing agent (stannous octoate) is used in combination with the diaryliodonium salt, that allowed the generation of an increased number of radicals and cations at 150 °C. The results showed a glass transition temperature and degree of cure that were equal to those obtained via a classical anhydride cured BADGE monomer.
The RCFP technique was also used to address another significant challenge observed in the frontal curing of cycloaliphatic epoxies. The addition of stannous octoate to a composition of diaryliodonium salt and thermal radical initiator (benzopinacol or even benzoyl peroxide) prevented the decarboxylation of CE-based resin during curing. RCFP was further utilized to develop a frontally curable CE resin suitable for CIPP composites, which are initiated with LEDs emitting at 405 nm. The results showed a successful frontal curing of CE composition on a polyester knitted glass fiber liner with a PVC host pipe without any degradation signs. The thermomechanical properties were compared against acrylate and vinyl ester resins benchmarks cured via classical free radical photopolymerization. It was found that the glass transition temperature and degree of cure of the CE frontal cured resin was higher than in acrylate and vinyl ester resin counterparts. The frontal cured liner exhibited exceptionally high bonding strength to the PVC host pipe in comparison to acrylate and vinyl ester resins, and it was found that host pipes could be safely prevented from heat deformation during on-site operation buried in soil.
In the final chapter of this thesis, a unidirectional (UD) carbon fiber reinforced composite was cured with the RCFP technique, and its thermomechanical properties compared with the classical anhydride cured counterpart. The results showed a lower glass transition temperature of the neat resin but competitive tensile, compression and interlaminar shear properties in comparison to an anhydride cured composite. The results of this study showed that the RCFP cured composite exhibit a slightly brittle behavior in comparison to the anhydride cured composite accounting to the differences in the cure kinetics of the two resins. The newly developed RCFP technique was able to reduce the cure time of a classical anhydride cured BADGE based UD composite from 8 h to only an hour, while being fully intact and free of any microscopic defects. The work presented in this thesis provides an exceptionally significant example for the full utilization of frontal polymerization in various technical applications. Reporting for the very first time, this work demonstrates various solutions to frontal polymerization challenges incurred from industrially relevant composites.