Although the reinforcement-to-matrix ratio may be varied between 30 - 70% and respectively 70 - 30% by weight, it is not unusual to also see several other materials incorporated with composites, as a means of further modifying the mechanical or physical properties of the moulded component.
These ‘additives’ - which may account for some 15 - 20% of the total weight - may form either an inherent part of the composite itself, or be employed in a somewhat ‘external’ or ‘added-on’ manner. Both techniques are of importance, and the reasons therefore demand a brief explanation.
Disregarding the pigmentation present to provide decoration, certain minerals - such as marble dust, silicon dioxide and alumino-silicate ‘sands’ - added as filler to the matrix, can for example enhance the compressive strength or reduce the moulding shrinkage.
Fire Retardant Additives
Others such as antimony trioxide and alumina trihydrate, improve the fire retardancy – or fire hardness - of the finished composite, if chemical modification of the matrix with for example halogenated compounds, is neither practical nor desirable.
In the ‘external’ case, the additional materials are of two types and typically employed to improve the structural rigidity or stiffness. The simplest are timber or metal sections – but possibly also profiles made from composites by hand or machine - perhaps bolted, or adhesive bonded to, or otherwise encapsulated within, the initial composites structure or moulding. Alternatively, paper, metal, thermoplastic or other honeycombs, or other ‘core’ materials such as foam slabstock, bonded fibre fabrics, heavily mineral-filled resin compounds - or perhaps even timber - can all be very successfully employed between opposing composite skins (i.e. laminates), to provide a sandwich construction.
Structurally these are highly effective, to also frequently offer, enhanced thermal insulation.
Additives provide a third route by which the composite components can be critically tailored to very cost-effectively answer the performance specification demanded.