Solid-state healing of resins and composites

Solid-state healing in thermosetting resins, such as epoxies using single and multiphase systems, is described. The first involves dissolution of a linear polymer of similar solubility parameter into the cross-linked matrix bound together in the glassy state by hydrogen bonding. Thermal activation causes this bonding to dissociate, imparting mobility to the linear chains, which diffuse through the free volume in the network and across a microcrack to effect healing. The multiphase approach uses an incompatible thermoplastic in a polymer network. Healing derives from surface-initiated condensation reactions at the solid interface between carboxylic acid groups in the thermoplastic and hydroxyl groups from the epoxy network producing bubbles that become embedded in the thermoplastic. During thermal activation the thermoplastic of low softening temperature is pushed into microcracks by expansion of the bubbles to produce healing. The chapter also demonstrates how healing can be introduced into thermosetting resins and their fiber composites without the need for compartmentalized monomers and catalysts and/or curing agents or a change from traditional fabrication technologies

Thermally activated healing in high performance carbon fibre/epoxy composites

This paper investigated the use of poly(ethylene-co-methacylic acid) (EMAA) as a healing agent in high performance carbon fibre epoxy composites by exploring different curing regimes that contain steps that are above and below the melting point of EMAA (Tm,EMAA). The healing performance of EMAA when the composite was cured via a two-stage process (cure below Tm,EMAA followed by above Tm,EMAA) and a single-stage process (cure above Tm,EMAA only) was compared and assessed. The healing efficiency of EMAA in high temperature cure epoxy resins was also evaluated using diglycidyl ether of bisphenol A (DGEBA) or tetraglycidyl diamino diphenyl methane (TGDDM) as the epoxy resin with two aromatic amine hardeners, namely diethyl toluene diamine (DETDA) and diamino diphenyl sulphone (44 DDS). The healing efficiency was evaluated at three healing temperatures relative to the glass transition temperature of the mendable composites (Tg) comprising below Tg, at Tg or above Tg and followed up to five healing cycles. It was found that EMAA showed excellent healing of delamination cracks when cured according to the two-step cured and with DETDA as hardener but displayed very poor healing when cured in the one step cure or using 44 DDS as a hardener. Healing below Tg, at Tg or above Tg for the two-step cure DGEBA/DETDA resulted in partial, fully recovery and more than fully recovery of the mode I interlaminar fracture toughness of the mendable composite, respectively, and these recoveries were maintained throughout five healing cycles. However, only healing above Tg for the two-step cure TGDDM/DETDA that showed full recovery, while healing at Tg or below Tg showed partial recovery in fracture toughness and these recoveries continuously decrease throughout five healing cycles. Fractographic analysis confirmed the pressure delivery mechanism occurred at the EMAA-epoxy interface and the importance of the initial low-temperature cure stage. The study also reveals the importance of curing at low enough temperature to ensure solid state surface reactions that ultimately maintain the well known pressure delivery mechanism when the cure temperature is raised above the melting point of the EMAA

Improcessability of thermoplastic peek composites via ring opening polymerisation

Thermoplastic composites are becoming increasingly important as the need for reduced weight and recyclability in high performance applications continues to grow in industries such as aerospace, wind energy, automotive and oil and gas. Their inherent recyclability, fire performance, toughness and potentially short cycle times, make them an attractive alternative to crosslinked epoxy composites, particularly for high volume applications. Unfortunately, they tend to have high melt viscosities during composite fabrication, inhibiting fibre wet-out and consolidation, limiting their further application. One strategy to overcome this disadvantage is to start with a low viscosity, low molecular weight macro-cyclic oligomer (MCO) that undergoes entropically driven ring opening polymerisation (ROP) under certain conditions to produce a high molecular weight polymer. In this way, a thermoplastic can be processed similar to a thermoset, retaining all the inherent advantages of being thermoplastic, such as toughness and recyclability, without the complexities associated high pressure and temperature processing. In the case of polyetheretherketone (PEEK), it is particularly difficult to process due to the high melt temperature and melt viscosity as well as a tendency for higher levels of crystallisation. This paper will present research progress towards the goal of achieving a significant improvement in the processability of PEEK. Composites coupons of PEEK synthesised via ROP methods at temperatures below 300 °C will be presented for the first time

Investigation of factors impacting the in-service degradation of aerospace coatings

The impact of in-service environmental stressors on the durability of exterior decorative aerospace coating systems was investigated using accelerated weathering for a high-gloss polyurethane-based monocoat with and without clearcoat. Color, gloss, surface roughness, hardness, and chemical composition changes were studied by varying UV irradiance, temperature, thermal extremes, particulate matter, and acid environment while using constant moisture condensation conditions. The use of a clearcoat was found to enhance the resistance to gloss loss regardless of the stressors applied; however, the clearcoat system also produced a larger increase in hardness under all experimental conditions and a larger color shift for all stressors except for the particulate matter and particulate matter combined with acid. A correlation between color shift and chemical degradation was established by monitoring changes in amide and carbonyl functional groups as a function of UV irradiance, temperature, and thermal extremes. The particulate matter, with or without acid was found not to affect chemical degradation, but produced large color shifts for both coating systems and some loss of gloss at high radiant exposures for the clearcoat system. For the accelerated tests studied here, only the highest UV irradiance and temperature level, with or without additional stressors, produced changes in the clearcoat relative to the monocoat system without clearcoat that correlate with in-service performance observations. © 2011 Elsevier B.V

Self-healing of epoxy resins using self assembling healing agents

We describe the concept of self-assembly through ionomer formation to achieve healing agents of critical molecular weight (MW). Thermal self-healing of epoxy resins could be induced while maintaining a low resin blend viscosity for direct fibre impregnation. DGEBA with a 'relatively low' MW was end-capped with 4-amino salicylic acid sodium salt to introduce ionomer functionality for self-assembly. Healing efficiencies were comparable to those obtained with 'high' MW linear epoxy and Phenoxy polymers of similar molecular structure. The non-reactive end capped DGEBA monomers, which cannot self-assembly had limited ability to heal thermoset epoxy resins used for industrial and aerospace applications

Improving mechanical properties and processability of a very high Tg epoxy amine network via anti-plasticizer fortification

Improving mechanical properties and processability of a very high Tg epoxy amine network via anti-plasticizer fortificatio