Composite materials are those materials having the following characteristics:
- consist of two or more materials (phases ) of a different nature
- at least two of the phases present have physical properties " sufficiently" different between them, so as to impart to the composite properties different from those of the constituents.
The nature of the phases is a dominant feature in imparting the character of " composite " to mixtures of different materials. However, it is not the only variable . To obtain a composite with high mechanical resistance is essential , besides the use of resistant fibers , to ensure good adhesion between the matrix and the reinforcement. Adhesion is usually promoted by the use of a third component , applied in a thin layer on the surface of the fibers , to make the inorganic surface of the fibers compatible with the organic matrix. Those materials are usely refer to as FRP (Fiber Reinforced Plastics). .
The most widely used reinforcements in polymer matrix composites are glass, carbon and aramid fibers. A careful selection of the types of reinforcement is needed to calibrate the characteristics of strength and endurance of the final product.
Due to its low cost and high strength , glass fibers are the most widely used. The common name of the products is GRP ( Glass Reinforced Plastic). Glass fibers are available as yarn or roving and a large variety of fabrics. The most common fabric is the woven roving suitable for the rapid build up of large size and thickness for structures not requiring optimized performances (eg, boat hulls) . Other textiles are available for specific applications.
The carbon fibers are used for the manufacturing of high performance composites and are characterized by the high modulus and high strength . The main disadvantages in the use of these fibers are the high cost and brittleness . Modern technology of production of carbon fibers is based on the pyrolysis ( thermal decomposition in the absence of oxygen) of various organic precursors (PAN fibers and rayon ) .
The aramid fibers were introduced in 1971 by Dupont under the trade name Kevlar . These fibers are characterized by high toughness and resistance to handling operations . Strength and cost are intermediate between those of the glass and carbon fibers . The main drawback in the use of these fibers is related to the low compressive strength and resistance to ultraviolet radiation. The aramid fibers are commercially available in various forms , of which the most common are yarns and roving as well as fabrics for ballistic uses .
For structural applications, the mostly used matrices are based on thermosetting resins, such as polyesters, phenolics, epoxies and polyimides. Such resins are characterized by a low viscosity at ambient temperature that facilitates the impregnation of the fibers. Once cured , they also offer a good resistance to chemicals. The technological limitations in the use of these materials are due to the need to conduct a chemical reaction during the process of manufacturing of the composite which the chemical reaction may be difficuly tp control in production environments. The inherent limitations of thermosetting resins (low toughness, low operating temperature and tendency to absorb moisture environment) have led in recent years to the development of thermoplastic matrix composites . Thermoplastic resins generally have higher toughness and glass transition temperatures. Maintenance and repairing are also simplified by the possibility of melting the matrix after manufacturing. The main limitation to the use of these matrices is the high viscosity , which makes it difficult to impregnate the fibers and requires relatively expensive processing equipment . The thermoplastic matrix composites with short fibers have also found considerable applications in the field of injection molding .
The production processes of composite materials are various . Some of them are the following :
- Hand lay-up
- Resin transfer molding (RTM)
- Filament windin
- Vacuum assisted infusion
- Centrifugal casting
- SMC/BMC molding
- Autoclave molding