JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010 - 51

Liquid thermoplastics provide the benefits of thermoplastics (toughness, durability, impact resistance, recyclability) whilst using low-cost polymers and the ability to include more reinforcement due to the low viscosity of the resin. The issues with liquid thermoplastics in the past have been related to their limited moulding size and to their physical processing as these materials tend to be susceptible to processing conditions which can influence polymerization. Therefore, a material and process investigation programme was the cornerstone of this project. Cyclics Corporation’s range of thermoplastic polyesters, CBT® (the cyclic form of polybutylene terephthalate, PBT), were used because of their very low viscosity during polymerization and negligible volatile organic compounds (VOCs) during processing. CBT® resin is produced from linear PBT through catalytic depolymerization which takes place at elevated temperatures, converting PBT into a lower molecular weight cyclic form or CBT® resin. A reversible reaction allows CBT to re-polymerize to PBT, an engineering thermoplastic providing excellent stiffness and superior chemical resistance. Process development was carried out to limit intolerance of the base resin to moisture and other chemicals and to control the polymerization rate. A form of CBT was thus developed whereby the polymerization and closely-linked crystallization of the resulting PBT was engineered to a state that could be satisfactorily moulded in processes such as RTM, vacuum infusion and lowpressure bag moulding. Closely matched to the development of the base resin was an investigation into compatibility with fibre reinforcements. Ahlstrom Glass Fibre Oy developed a range of sizings that did not affect the polymerization of PBT while providing a good bond between the glass fibre and PBT.

Fig. 1: Thermoplastic composite prepreg

Whilst the raw glass fibre was useful for processes such as RTM, the aim of the project was to develop materials and processes for very large transportation components (larger than RTM is efficiently capable of making). Therefore, a prepreg material that incorporated the resin on a non-crimp glass fibre fabric was developed by Ahlstrom (see Figure 1).

devised a means of achieving high levels of polymerization and thus high-volume-fraction components through careful control of processing variables. EPL also used equipment and technology that were readily available to many composite manufacturers. Hightemperature tooling is needed as the resin starts to polymerize at about 160°C. Statistical methods were applied to a range of variables in order to determine what affected the quality of components. This resulted in an optimized processing methodology capable of creating composites with flexural strengths in excess of 800 MPa.

Design and manufacture
With materials manufactured and processes developed, the task of translating the concept of a thermoplastic composite material into the reality of a 44-tonne, triaxle semi-trailer was undertaken by EPL

Processing
Research in processing was critical to the successful production of components using the liquid thermoplastic composites. This research was carried out by IKV in Aachen, Germany and by EPL Composite Solutions in the UK. For smaller components, IKV developed processing routines for closedmould RTM and single-sided-tool vacuum infusion processes. Teams concentrated on eliminating moisture from the process, as well as developing optimal injection and thermal profiles. Closed-mould RTM resulted in glass composites with volume fractions up to 63% and mechanical properties similar to industry-standard thermoset composites, i.e. 40 GPa flexural modulus and 850 MPa flexural strength. Moreover, these components have the added benefits of increased toughness, thermoformability and full recyclability. In order to commercialise the process for the transport industry, very large components need to be manufactured economically. This requires using lowpressure moulding techniques which were developed by EPL Composite Solutions. Using the prepreg from Ahlstrom, EPL

Figure 2 – CAD design for the trailer

No59 August - September 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010