Composites - GRP & Carbon Fibre

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Glass Reinforced Plastics

Glass-reinforced plastic (GRP) composites is made of a polyester or epoxy resin reinforced by fine fibres of glass in the form of a woven matting. The plastic resins are strong in compressive strength but relatively weak in tensile strength, whereas the glass fibres are very strong in tension but have no compressive strength. By combining the two materials GRP becomes a material that has both compressive and tensile strength. The resin exists in a liquid form and has a catalyst or hardener added to become a solid. 

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The glass fibre strands provide the basic structure while the resin with its additives bonds the fibres together and provides a lightweight rigid structure. The two materials may be used uniformly or the glass may be specifically placed in the portions of the structure that will experience tensile loads. The extemely smooth GRP finish seen on boats and some cars is achieved by a combination of a highly polished surface on the mould used and careful application of the first layer, known as the gel coat. The glass matting which is laid on top of the gel coat to provide the basic structure leaves a very tough finish. Therefore, one surface of GRP is highly polished whilst the other is extremely rough.

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Carbon Fibre

More recently, carbon fibre has been commericially developed in a similar form to that of glass fibre. This carbon composite is made up of carbon fibres, which take tensile loads, set in to a polymer resin matrix that takes the compressive loads. Carbon fibre is a filament material incorporating thousands of filaments that are woven to form a fabric. However, this fabric only has strength in tension. So the woven fabric is placed in different directions to cover tensile loads in all directions, while being supported by a rigid, compression bearing matrix of resin.

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Carbon fibres are much stronger than GRP and are ideal for high performance structural applications in aircraft, sports equipment and F1 racing car manufacture. Carbon composites have unrivalled mechanical properties and, in most load bearing applications where weight is an issue, will easily out-perform any metal alternative. 

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Medium Density Fibreboard

One of the most widespread and commonly used composite materials is medium-density fibreboard (MDF). MDF is primarily made from wood waste (or specifically grown softwoods) in the form of wood chips, which are subjected to heat and pressure in order to soften the fibres and produce a fine, fluffy and lightweight pulp. This pulp is then mixed with a synthetic resin adhesive to bond the fibres and produce a uniform structure and heat pressed to form a fine texture surface. After pressing, the MDF is cooled, trimmed and sanded. In certain applications boards are laminated for extra strength. MDF can be worked like wood but with the added advantage that it has no grain to work with. It finishes well with a variety of surface treatments and is available with a veneered surface for decorative effect.

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As is the case with all composites, there are some potential hazards involved in their use. As well as fumes from glues and resins, as a result of the very fine fibres great care must be taken when undertaking any form of cutting, drilling and especially sanding. Respiratory equipment should be used since the dust can cause irritation of the skin, throat and nasal passage and appropriate dust extraction should also be activated. 

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