Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components.
Prepregs, comprising a fibre arrangement impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.
Such cured composite materials inevitably contain some degree of entrapped gas, or porosity, which can detract from the mechanical strength of the material. For example, gas bubbles can be present from the resin manufacturing step and the impregnation of fibres may be incomplete, both resulting in porosity being present.
To allow curing to occur, a curing agent is included in the resin which reacts with functional groups on the resin to form cross-links and provide the curing function. Such curing agents are typically blended with the curable resin at low temperatures to prevent thermal hazards and so that premature reaction between the resin and curing agent does not occur. The choice of curing agent strongly governs the mechanical properties of the cured resin.
For aerospace applications composite materials must meet extremely exacting requirements on porosity and mechanical strength. For this reason the curing of aerospace composites is almost exclusively carried out in an autoclave cure. Such curing is carried out at high pressure and temperature and tends to produce very low porosities and hence good mechanical properties. It is believed that the high pressure forces any entrapped gasses into solution, such that the composite material remains essentially void-free during cure.
However, autoclave cure is expensive and time-consuming to operate. Therefore methods of curing aerospace quality composite materials out-of-autoclave have been explored.
The present invention aims to mitigate or at least obviate the above defined problems and/or to provide improvements generally.