What Are Composites?

Composites are materials that are comprised of strong load-carrying materials (known as reinforcement) imbedded in a weaker material (known as matrix).

Reinforcement provides strength and rigidity, helping to support structural load. The matrix, or binder, maintains the position and orientation of the reinforcement, balance loads between the reinforcements, protect the reinforcement from environmental degradation, and provide shape and form to the structure.

Metal reinforced concrete, plaster casts and wire reinforced tires are all examples of composites. The term composite may also describe newer technology products made from very strong fibrous materials imbedded in thermosetting and thermoplastic resin matrices. Spencer Composites Corporation manufactures these types of composites.

The three most common types of reinforcing fiber are fiberglass, carbon and Aramid (Kevlar). There are many varieties of each of these classes of fibers and each variety may differ from the others in various ways, such as strength, stiffness, fiber diameter and cost. Glass fibers are the heaviest, have the greatest flexibility and the lowest cost. Aramid is a moderate cost and stiffness fiber and is the lightest weight. Carbon is moderate to high in price, slightly heavier than Kevlar but lighter than glass, and features certain varieties that have exceptionally high stiffness. All three types have grades with very high strengths although carbon is the strongest. Hybrids are composites with more than one reinforcing material. Advanced composite is a term used to describe composites reinforced with very high performance fibers such as carbon and Kevlar. Fiberglass is usually included.

Reinforcement Weight Stiffness Cost
Glass heaviest most flexible cheapest
Aramid (Kevlar) lightest moderate moderate
Standard Tow Carbon moderate rigid moderate-high
Large Tow Carbon moderate rigid low-moderate

A filament is an individual fiber of reinforcing material. A primary bundle of filaments is a strand or tow. A collection of strands spooled such that they appear to be on bundle is called roving. Fibers are used by composite manufacturers in the form of roving, cloth, mat and chopped fiber.

The most common types of matrices are thermosetting resins. This is a class of materials that chemically react under certain time and temperature conditions. As soon as this reaction takes place, the material becomes fused in a solid state and will not melt with the application of heat. Epoxy resins, similar to those sold as adhesives, are the most widely used thermosetting resins in the advanced composites field. Others resins include polyester, vinyl ester, phenolic, bismaleimide, epoxy novolac, and polyimide. Each has practical applications and is selected on the basis of required performance and manufacturing technique.