What is a composite?

A composite material is a material in a solid, firm condition, consisted of more than one component, where one is called matrix or bound material, usually polymer, ceramics or metal, in which another component has been added, or in other words, that is particle or reinforcement, (fiber, nano-tubes, plates, spherical particles etc...). There are different kinds of divisions of composite materials, the most commonly used is the division of natural ( wood, bone, granite, pearl ) and chemical ( artificial - fabricated ) composites. All kind of materials, polymers, ceramics, metals etc. are used as a matrix in the advanced composites. Subject of our interest is the polymer matrix, which when reinforced with other substances - particles, shows better or more advanced features.

The polymer matrixes for composites are divided into:

• thermosetting and
• Thermoplastics.

The thermoplastic polymer matrix when heated becomes soften or melt, and when cooling becomes solid again. This process can be repeated but no chemical changes happen in the polymer.

The reinforced thermoplastic materials, known under the name - FIBERFILL, compose a special group of thermoplastics and are applied in the areas where high mechanical characteristics of materials are required and where there's a need of dimensional stability, in other words, they are used in the fields where  metals have been mainly used previously.

The thermosetting polymer matrix are composed of molecules that a netted and don't soften when heated. They are formed in the reaction, during which the resin and reinforcement, or the resin and catalyzer mix each other and then endure an irreversible chemical reaction. After the netting, which is mostly performed on higher temperatures, they become rigid,  non-melting and indissoluble   products. At some thermosetting polymers, as phenol resins, volatizing substances are formed like byproduct (condensational reaction ).

90% of all thermosetting matrixes, belong to three bigger groups: epoxy resins, polyester resins and vinyl ester resins.

There are a lot of materials that can be used as reinforcements, but in the technology of advanced composites dominate: glass, carbon, aramide, boron, silicon-carbide and basalt fibers. If there is a necessity,  a hybrid of two or more reinforcements can be used as a component for a certain application.

The reinforcements can be available in various forms: like continuous tow; roving; one-way tape of parallel, continuous fibers (unidirectional); woven with various geometries, like satin and ordinary weaving - plain; unwoven; nonwoven - with occasionally orientated fibers. The constructor of composite materials has the right to determine which type of reinforcement is appropriate for the given function and aim.