Strategies for energy-efficient frontal polymerisation processing and microstructural evaluation of fibre reinforced composites

Liquid composite moulding methods are widely applied for manufacturing of fibre reinforced polymer (FRP) composites and consist of two main stages: impregnation of fibrous reinforcements by a liquid monomer resin and curing of this resin. This thesis work proposes strategies towards the optimisation of both process stages. First an UV-flow freezing method for the microstructural characterisation of dynamic infiltrating flows was optimised by finetuningof the resin formulation and experimental procedure. The method allowed for in-situ photopolymerisation of flow patterns characteristic for viscous-dominated, equilibrated and capillary-dominated regimes. X-ray micro-computed tomography analysis enabled high resolution volumetric imaging and quantification of the saturation levels of the characteristic flow patterns.Frontal polymerisation offers unmatched reductions in energy demand and time for curing of FRPs. Current resin systems are incapable of overcoming the excessive heat uptakes by fibrous reinforcements and are hence limited to fibre volume fractions (Vfs) well-below the desired 55% or more. Optimisation of the resin composition and mould configuration allowed for control of the governing heat balance while a defined processing window related thisbalance to the possibility of forming a self-sustaining polymerisation front but maximum Vfs remained limited by the heat loss to the fibres. Bridging of the Vf gap was achieved via a novel self-catalysed frontal polymerisation approach where a sacrificial resin channel in thermal contact with the FRP ensures enough pre-heating and catalyses the frontal polymerisation process. FRPs were produced with Vfs up to the 60% range, increasing the current maxima by 15-20%, at an energy demand reduced by >99.5% compared to conventional oven-curing. Mechanical properties of the FRPs were comparable to those of traditional carbon FRPs while its glass transition temperature of 177.6±9.3°C was 18.5°C higher. To further evaluate process windows and the role of the mould, reinforcement and resin parameters, finite difference models were developed by derivation of descriptive reaction-kinetics models that were calibrated and validated to experimental temperature recordings. The latter procedure allowed for accurate approximations of front velocities and temperature profiles and slightly overestimated peak temperatures. Application of the model to a series of case studies allowed for complementary insights on the role of the processing conditions and the occurrence of thermal instabilities. The combined strategies proposed in this work are believed to pave the way towards the energy-efficient production of FRPs by means of liquid composite moulding with an improved understanding on ongoing phenomena during resin infiltration.