Novel route to tailoring the performance of liquid thermoplastic acrylic resins for composite applications

Abstract

As strict global government directives continue to incentivise research activities aimed at addressing environmental issues such as the sustainability of fuel resources, reduction of greenhouse gas emissions, and waste management, more and more sectors are adopting ideological shifts towards greener practices. This has positioned continuous fibre-reinforced thermoplastic (CF-RTP) composites as ideal candidate materials in many sectors where their thermosetting (TS) counterparts once dominated. Such a shift is particularly important for the transport sector, which has turned towards lightweighting for improved fuel economy and the minimisation of greenhouse gas emissions. While the attractive characteristics of traditional thermoplastic (TP) resins have long been known, their high-melt viscosities rendered them impractical for many sectors due to the requirement for high temperatures and pressures for the production of CF-RTPs. Consequently, TS matrices, have historically been preferred for composite applications at the expense of matrix toughness, recyclability, thermoformability and weldability, which are intrinsic properties of TPs.

Liquid thermoplastic (LTP) resins are attractive, emerging solutions for the shortfalls of traditional thermoplastics (cost-prohibitive processing methodologies) and thermosets (poor end-of-life resource recovery and brittle failure behaviour). Their use facilitates the low-cost fabrication of CF-RTP composites via liquid composite moulding methodologies. CF-RTP composites possess unique attributes that position them as ideal candidate materials for the production of lightweight vehicles with more favourable end-of-life resource recovery than their TS counterparts.

The recent development of room-temperature-processible, LTP resins for the composites markets has garnered considerable research interest. Elium® resins are LTP acrylics, which have become highly desirable, due to favourably low viscosities under ambient processing conditions without specialised or heated tooling. They have been shown to be thermoformable, recyclable, weldable; and are suitable for the production of large-scale components for the renewable energy sector. Their adoption into other sectors (e.g.: transport and marine) can be beneficial from a post-processibility perspective. Acrylics are, however, commodity TPs resins. They are known to be susceptible to solvent attack and strain-rate-dependent embrittlement and have relatively poor thermal stability. This PhD project aims to explore viable tailoring concepts for liquid reactive acrylic resins, which may be applied without deleterious effects on the glass transition temperature and processibility. The reactivity of the acrylic resin can be exploited to dissolve a telechelic poly(phenylene ether) (PPE) resin for in-situ polymerisation of tailored blends. PPE is a linear, amorphous engineering TP with exceptional thermal stability, and solvent and moisture resistance. It has demonstrable hybridisation efficiency, evidenced by its successful application in blended composite matrices. Thus, a low-molecular, vinyl-functionalised, oligomeric PPE resin was selected based on its ability to reactively participate in the free-radical polymerisation reaction, which can facilitate chemical interactions with the acrylic resin’s chemistry. Complementary analytical techniques have been employed on unreinforced blends with compositional variation as part of a parametric down-selection study to determine the optimum PPE content. In addition to identifying the optimum composition, these analyses revealed the formation of multi-species blends, comprising a cross-linked acrylic/PPE network, with interpenetrating constituents (a grafted acrylic/PPE reacted product and a linear acrylic homopolymer).

The viability of this innovative hybridisation approach for room-temperature resin infusion of CF-RTP composites and their subsequent post-processing (by thermoforming) was assessed. A glass-fibre reinforced PPE-modified acrylic laminate was produced and rigorously characterised against an unmodified acrylic-matrix composite to study the hybridisation efficiency. Benchmarking revealed exceptional solvent resistance and thermal stability in the PPE-modified acrylic-matrix composites, confirming effective contributions of the PPE modifier and this hybridisation methodology. Despite containing a lightly cross-linked network, it was found that the prepared blends retained the desirable thermoformability of the acrylic matrix, confirming that performance tailoring was not performed at the expense of post-processibility.