Investigating the structure-property relationships of a high-temperature epoxy matrix for structural and thermal applications

This project explores the network formation and structure-property relations of some high temperature epoxy resins cured with aromatic diamines. A highly aromatic dimeric naphthalene epoxy resin monomer, bis(2,7 glycidyl ether naphthalenediol) methane (NNE), is cured with 4,4'-diaminodiphenyl sulfone (DDS) to produce a network that exhibits rapid gelation, vitrification, and an ultimate glass transition temperature approaching 350°C. An array of thermal, spectral, rheological, and thermo-mechanical analysis techniques are used to comparatively assess the formation of the NNE-DDS network to a commercial high-Tg epoxy resin comprised of 4,4'-tetraglycidyldiaminodiphenyl methane (TGDDM) and DDS. The difference between the glycidyl ether (NNE) resin and glycidyl amine (TGDDM) resin through cure was explored. It was determined that the NNE resin showed primarily epoxy-amine addition and no signs of etherification or side reactions which was, in contrast to TGDDM, attributed to the absence of tertiary amine within the epoxy backbone. The time-temperaturetransformation (TTT) diagrams of the resins are constructed to explain the processing and formation of the glass transition temperature and show competition between devitrification (Tg) and the ultimate glass transition temperature of NNE. The structure-property relations of the NNE and TGDDM networks are explored via thermal analysis and mechanical testing of the cured resins and their carbon fibre composites. It is shown that the structure of the NNE monomer and limited mobility produces a network with high free volume and poor equilibrium packing density. The rapid vitrification at low degree of conversion prevents a high degree of cure and the network becomes topologically constrained. That in turn results in higher moisture ingress, lower strength and modulus, but achieves better thermal stability than TGDDM at temperatures above 200°C. Further ageing of the resins and composites at 250°C for 504hr indicate some benefit for high temperature, short duration thermal performance of NNE compared to TGDDM. That is attributed to the limited mobility of the matrix after cure and the effects of the tertiary amine contained in TGDDM providing more pathways to degradation. The NNE resin, whilst possessing superior thermal performance, is difficult to process and exhibits poor mechanical performance compared to TGDDM. An approach to modify the network for improved strength and stiffness combined with reduced viscosity was taken at the molecular level. A monofunctional reactive epoxy diluent, partially reacted substructures (PRS), a molecular fortifier (MFN), and pure naphthalene were synthesised and cured with NNE in percentages ranging from 5-20 mol% or wt%. This survey showed an antiplasticising effect where strength reduced, and modulus increased significantly. Further, the monofunctional epoxy and naphthalene, when added at 10 mol % and 10 wt%, respectively, reduced the melt viscosity and increased the processing window to values in the range of conventional resin transfer moulding parameters. Carbon fibre composites of the modified resins at 10% loading showed the neat resin properties transferred to improvements in the composite flexural modulus with minimal impact on strength and interlaminar shear. Finally, the toughening of a multi-functional epoxy resin was achieved by use of core-shell rubber (CSR) particle toughening. The improvement to room temperature fracture toughness of nearly 40% of the neat resin at 20 wt% particle addition was then used to create a carbon fibre composite. The composite mode I fracture toughness was evaluated by means of in situ acoustic emission (AE) spectroscopy and scanning electron microscopy (SEM) imaging of the fracture surface. A nearly two-fold increase in G1C was shown. AE spectroscopy indicated that toughening by CSR contributed to high frequencies of interply-, fibre bridging-, and fibredominated failure events than the unmodified composite where the frequencies of events showed lower energy matrix-dominated failure. This project investigates the capabilities of a high temperature naphthalene-based epoxy-amine resin formulations and routes to achieve simultaneous improvements to thermal resistance and stability, mechanical performance, and fracture toughness for structural composite applications.