Rethinking sustainable composite resin systems

 based in Loughborough, UK, Hive Composites delivers world-class composite solutions, supporting companies with design, prototyping, testing and certification of new products. Gerry Boyce and colleagues at Hive have been working with thermoplastic composites for more than 30 years, both infusible and melt processed. They could see the benefits of thermoplastics, but the high viscosity of molten thermoplastics makes it very difficult to impregnate dense fibre layups without the use of high pressure. This limits their use to relatively small parts with high-cost tooling.

In 2022, Hive was granted funding through the Innovate UK funding agency and embarked on a two-year research project focused on wind turbine blade applications (Figure 1). Their aim was to develop a new thermoplastic polymer, aiming for the following characteristics:

• low viscosity for ease of fibre impregnation;
• higher mechanical properties than epoxy or polyester resins;
• able to be processed using conventional techniques for thermosetting resins;
• recyclable at the end of the structure’s life into added value applications;
• similar price to an epoxy resin.

An innovative chemical approach

Polybutylene terephthalate (PBT) is an engineering thermoplastic with excellent mechanical and electrical properties, good thermal stability and superior chemical resistance. PBT has a high Molecular Weight (MW) and typically melts at 235°C with a high melt viscosity requiring high pressure processes, such as injection moulding, to manufacture parts. It is used in applications which require good durability under thermal stress, such as electrical connectors and switches, automotive under-bonnet applications including engine covers, air intake manifolds, radiator tanks and valve housings where chemical resistance is also required.

An alternative way to use PBT in composites involves first cracking the high MW PBT into a low MW polymeric oligomer form. This melts at 140°C into a very low viscosity liquid which can rapidly infuse into fabrics. Then, with the addition of a catalyst and an increase in temperature, the polymer is converted in-mould back into high MW PBT, with all the associated benefits in properties. Hive’s innovative research resulted in developing a much more efficient way of producing a stable, low MW PBT directly from precursor chemicals. This is supplied as a powder, trademarked MET-OL™ combining the terms meta‑OLigomer (pronounced with the emphasis on ‘OL’) and supplied through spin-off company Metol Ltd. They were also able to compare properties of composites with epoxy and polyester, and Loughborough University completed a preliminary Life Cycle Assessment (LCA).