Thermoplastic resin transfer moulding of tough recyclable composites for high volume manufacturing

Abstract

The aim of this project was to develop tough thermoplastic composite materials and moulding processes aimed at high rate production applications. The use of anionically polymerised polyamide 6 (APA-6) as a matrix in continuous fibre reinforced composites, manufactured using thermoplastic resin transfer moulding (TP-RTM) was investigated. The requirements for high speed manufacturing of composite parts and the importance of the polymer matrix choice in terms of temperature and pressure were outlined. In order to manufacture laminates, a TP-RTM setup had to be designed and built. The system designed consisted of a mixing/injection unit with a set of tanks and pumps heated above the melt temperature of the raw materials. During infusion of a composite part, the materials would be melted in the tanks, pumped through a mixing head for homogenising and delivered to the mould which contained a textile. Together, the system is capable of producing 340 mm x 390 mm panels with varying thicknesses, depending on that desired. Decisions for the choice of raw materials, their mixing ratios and processing temperatures were outlined. These decisions were based on literature and experiments carried out in order to produce a tough matrix material suitable for its purpose. 4 mm thick polymer plates were manufactured using the TP-RTM system and were characterised in terms of their chemical, thermo-morphological and mechanical properties. The distribution of thickness and density across plates was studied to better understand the effects of the mould geometry, molten flow and heat transfer on the degree of shrinkage in the material. It was found from this work that the APA-6 had a melt temperature of ~219 °C, glass transition temperature of ~70-85 °C and degree of crystallinity of ~42%. The tensile strength and modulus of the APA-6 were found to be ~83 MPa and 2.8 GPa respectively. Following analysis and testing of pure APA-6, unidirectional composite laminates were manufactured using the TP-RTM system. A non-crimp stitched glass fabric was used such that fibre volume fractions of approximately 51-52% would be achieved. As for the pure polymer, the distribution of thickness and density was determined across laminates. Likewise, the thermo-morphological properties were determined and compared with those of the pure polymer to determine the role of the fibres and the degree to which they effect the matrix properties. Finally, extensive mechanical testing was carried out to determine the transverse and longitudinal properties of the composite. In order to observe the effects of flow during manufacture on the final internal geometry of the composite, X-ray CT and micro section studies were carried out. The studies confirmed the presence of asmall percentage of voids in the materials as a result of the stitching and determined the degree to which this effected the properties. The mechanical properties of the composite compared extremely well with similar products which are commercially available. The strength and modulus of the composite were ~1109 MPa and ~41 GPa in tension respectively and ~691 MPa and ~42 GPa in compression respectively. The effects of different fibre sizings on fibre-matrix interface was investigated. Two different sizing types were looked at, one of which consisted of a reactive coating which would theoretically result in greater bonding. Firstly, scanning electron microscopy and atomic force microscopy of the fibres was carried out to observe distribution of the sizings on fibres at the microscale and nanoscale respectively. Thermogravimetric analysis was carried out to determine the temperature at which oxidation occurred in each sizing. Pyrolysis was carried out on larger quantities of fibres to determine the amount of sizing by mass on each fibre type. Unidirectional composite laminates using both fibre types were manufactured and tested to determine the effects on transverse, interlaminar shear and fracture toughness properties. The results indicated that the composite with the reactive sizing performed around 16% better in terms of strength on average but showed little difference in modulus. Results from testing indicated that it might also perform better in terms of Mode I fracture toughness; however, high scatter in results meant that this was inconclusive. Finally, quasi-isotropic laminates were manufactured using both of the aforementioned fibre sizing types and the effects of out-of-plane impact were investigated. Tests were firstly carried out on the pure APA-6 material and were compared to those for a standard epoxy in terms of energy absorption and force induced due to impact. The epoxy only absorbed around 44% the amount of energy before fracture compared to the APA-6. Laminates made using both fibre types were compared by testing at different energy levels and carrying out post-impact mechanical tests. The fibre type wasn’t shown to significantly influence the energy absorbed by the materials before break; although, differences in the nature of failure were observed. Carbon fibre laminates made from both APA-6 and epoxy matrices were manufactured and tested to measure energy absorption and force induced in each. The epoxy composite absorbed around 84% the amount of energy before fracture compared to the APA-6 composite. Overall, it is shown that thermoplastic composites with excellent strength, toughness and impact performance were manufactured using a production process with potential for high production rates. Unidirectional laminates were produced using injection pressures of around 10% of those required to achieve the same fibre volume fraction and degree of wet-out using a typical thermoset RTM resin, negating the need for expensive equipment.