Forming and Self-forming of Thermoplastic Polymer Composites
November 2nd, 2004
Paper to Polymer Processing Society Americas Regional Meeting, Florianopolis, Brasil, 7th-10th November 2004
S F Bush
Introduction
It has long been appreciated that the addition of glass or other stiff fibres to a thermoplastic or thermoset in a suitable fashion usually brings increased stiffness and strength to the processed material. In the case of injection moulded thermoplastics, the glass fibres have until the 1980s been very short, usually in the range 0.3-1.0 mm.
In the case of thermoset compositions the fibres have generally either been 25-50 mm discrete fibres as in sheet moulding compounds (SMCs) or continuous woven structures. If 25-50 mm discrete fibres are used, they are usually in tows (bundles) of 30 or more individual filaments, either constructed into a loosely woven mat and then impregnated with thermoset resins or scattered in a random overlapping fashion on to a layer of resin with further resin poured on top. In either case, a form of semicoherent fibre structure is obtained within the polymer liquid, this structure being maintained after the composite sets to solid. This structure is one of the two main reasons why fibre-reinforced thermoset composites commonly show greater strength and stiffness than do the thermoplastic varieties based on short fibres, which do not usually form such structures, the other being the chemical cross-linked character of the thermoset.
Whatever the specific objectives laid down for the composite, two factors in particular will determine how it meets these objectives. These are (i) fibre-polymer contact, and (ii) fibre management. The over-arching requirement is of course that of minimum cost of the product as defined by its required shape and load-bearing characterisation.