Consolidation mechanisms and interfacial phenomena in thermoplastic powder impregnated composites
Thermoplastic powder impregnation of continuous reinforcement filaments is studied in this work, focusing on impregnation mechanisms and interfacial phenomena. Various existing techniques to mingle powdered resins to continuous filaments are reviewed; a powder impregnation line designed at the Laboratoire de Technologie des Composites et Polymères (LTC) is presented. Two important types of powder coated towpregs are addressed: FIT bundles (Fibres Imprégnées de Thermoplastique) of powder loaded fibres enclosed in a thin resin sheath, and molten powder towpregs in which the particles are fixed to the fibres in an oven by melting the resin. The impregnation mechanisms of powder coated towpregs are examined. The formation of resin bridges between adjacent fibres is first investigated using a hot stage placed on a microscope. In the absence of externally applied pressure, impregnation is driven by surface energy effects. The driving forces leading to the spreading of the bridge along the fibres are analysed at two levels: at a macroscopic scale, characterising the capillary pressure governing the flow of a liquid into a porous solid, and at a micro-mechanical level analysing the capillary forces in a system defined by a liquid drop in contact with two solid particles. To achieve impregnation at a satisfactory rate, however, it is essential to apply external pressure to most thermoplastic systems during consolidation. An analytical model for the consolidation stage of unidirectional-powder coated towpregs is presented, placing in context effects due to surface energy, viscous flow, externally applied pressure and fibre bed elasticity. The initial conditions for the computation depend on the impregnation technique used. FIT bundles, and molten powder towpregs are examined. The model is compared at each stage to experimental data obtained by compression moulding powder impregnated towpregs in an instrumented hydraulic press using a closed matched-die mould. This model optimises the processing conditions of a given fibre-resin system to achieve a void free laminate with improved mechanical properties. Mechanical properties can further be improved by optimising interfacial adhesion between resin and fibres. Surface energy effects on composite mechanical properties are studied, relating thermodynamic quantities to adhesive strength. A criterion for optimum adhesion is proposed. The influence of the thermodynamic adhesion between fibres and matrix on the mechanical properties of a continuous fibre reinforced composite is studied for two systems: carbon fibre reinforced poly(ether-ether-ketone) and glass fibre reinforced poly(ether-imide). The fibre surface is modified chemically and characterised by optical contact angle measurements of molten resin droplets on the fibres. Unidirectional fibre reinforced laminates are manufactured. Transverse flexural strength is reported as a function of thermodynamic wetting parameters. Adhesion at the fibre-resin interface is found to correlate with both composite strength and void morphology within the laminate after consolidation. Full potential of powder coated towpregs as a precursor for compression moulded composite parts can be reached by the fabrication of drapeable textile preforms. Corrugated sheets are processed usign FIT woven fabrics. Mechanical property measurements show that complex high quality parts can be processed at high rates using powder coated towpregs.