Composite Materials

While two different materials that neither react chemically nor dissolve in each other are combined together, they compose composite materials. One of the two constituents serve as a binder while other might be in micro or macroform. The major purpose for which a composite is built is to attain a property which might not be exhibited by either constituent.

In engineering practice many composites are utilized. The most obvious is reinforced cement concrete (RCC) utilized for building construction. Hard particles in softer metal matrix create composite materials of exceptionally good strength needed for metal cutting tools.

Fibre reinforced plastics (FRP) by putting strong fibres in the matrix of weaker plastic material form most significant materials utilized in engineering practice. The carbon and glass fibres are commonly utilized as reinforcement in a variety of plastics.

The ideal material for structural applications requirement to be tough, strong and light. The metals & their alloys satisfy these needs to a great extent. The metals are not very light, though amply tough and strong. The plastics are extremely light but not tough and strong. Combining materials along complementary properties produced composite materials centuries ago.

Egyptians throughout the days of Pharohs utilized bricks reinforced with chopped straw. Plant fibres were utilized for strengthening potteries in south America and India. Eskimos strengthened ice by utilizing moss as reinforcing constituent.

The glass fibres which serve as reinforcing constituent in softer plastic matrix are conveniently & cheaply drawn from bulk glass. In spite of their higher strength than bulk glass they cannot be utilized in structure for their high brittleness. The softer plastic matrix has good adhesion along brittle and very thin fibres of glass and defends them from atmospheric attacks by surrounding them in a composite. The glass reinforced plastic renders itself with convenience of simply moulding into shapes of varied geometry. The glass fibres laid in a mould along longitudinal direction might be moulded by pouring  molten plastic or even by brushing the plastic on framework composed by fibres of glass. Various products like kitchen sinks, bath tubs, swimming pools, motor car body parts, and even aeroplane bodies may be made in GRP.

Carbon fibres were long known for their electrical resistance and fragility. However, the strong covalent bond between two carbon atoms would make carbon fibres very strong. This fact was also known to scientists. Throughout sixties several companies initiated commercial production of carbon fibres and thus their commercial application in reinforcing of plastic matrix was facilitated. These fibres were discovered to develop high strength adhesive bonding with epoxy resin resulting into FRP as strong as steel but lighter than aluminium.

Boron fibres seem to provide strength higher than carbon fibres but are costlier. Other fibres that have been tried are those of silicon carbide and several ceramics. However, the glass and carbon fibres, by far, are the major reinforcing fibres in plastic matrix.