For many applications, initially dominated by the use of metals, composite materials have been proposed consisting of a polymer matrix and an inorganic reinforcement, particularly glass or carbon fibres. For conditions in which their performance has necessarily to be high from the mechanical strength viewpoint, composites comprising long or in particular continuous fibre are proposed.
The first type of long or continuous fibre composite developed as an alternative to metals was characterised by a matrix consisting of thermosetting polymers, of which epoxy resins, unsaturated polyester resins and bismaleimides are characteristic examples. The methods for transforming this type of material were and still are preimpregnation and pultrusion.
With both these methods the fibres are passed through suitable solutions of the thermosetting material, leading to environmental problems and problems of process cost in view of the need to make provision for reusing and/or storing the solvents used. To obviate such drawbacks, composite materials have recently been introduced which use thermoplastic polymers as the matrix. In this case the relative transformation requires the formation of a preliminary product if the composite is of the long or continuous fibre type.
An example of the preparation of such a preliminary product is reported in U.S. Pat. No. 3,742,106, which describes the formation of a composite by impregnating a continuous reinforcement filament in a molten thermoplastic material, or in U.S. Pat. No. 4,614,678 which describes the formation of a composite by impregnating a fibre roving with thermoplastic powder and covering the roving with a thermoplastic sheath.
A further example of a preliminary product for composites of thermoplastic matrix type is reported in the Journal of Applied Polymer Science - Applied Polymer Symposium, vol. 47, page 501, 1991, which describes the preparation of a continuous filament by extruding a mix consisting of reinforcement and thermoplastic fibres.
After preparing the preliminary product, the final piece is prepared by consolidation methods which can comprise heating and applying pressure, as reported for example at various points in the Encyclopedia of Composites, edited by S. M. Lee, published by VCH Publishers, New York, 1991, or pultrusion of the preliminary product itself.
The use of thermoplastic composites has lead to undoubted improvements from the environmental viewpoint as solvents are not used in the production cycle. Certain thermal and mechanical problems are however observed, which are not present with thermosetting resins, in particular when thermoplastic polyester-based matrices are used. In the case for example of polyethyleneterephthalate or polybutyleneterephthalate, limited performance is observed under shear, which makes it difficult to design components from which a certain level of performance is required, or viscoplastic behaviour is observed, i.e. a considerable variation in mechanical properties with time. This effect is particularly evident at high temperature, in that the thermoplastic material tends to suffer increased creep in response to increased temperature.
Attempts to combine the characteristics of thermoplastic matrix composites with the characteristics of thermosetting matrix composites are illustrated in the literature. Published European patent application 391,581 describes composites obtained by laminating thermoplastic matrix layers with thermosetting matrix layers. However this method as a considerable point of weakness linked to the consolidation and interface of layers of different mechanical characteristics.