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Chemical resistance, impact properties

some manner to the fiber. While the method used to manufacture the composite (to be discussed later) can have a large influence on the effectiveness of the loading transfer, reinforcing fibers are usually sold with a sizing, or coating, on them specifically designed to promote chemical bonding between the matrix and the fiber surface.

The matrix also serves as a coating or protector for the fibers and must therefore be chosen not only for its ability to work with the fiber as the load transfer medium but also for its environmental performance. Polymer matrices can be divided into two general classifications: thermoset and thermoplastic. As their names imply, heat is used during processing. A thermoset material is generally processed as a liquid and crosslinked, or cured, through the application of heat to form a nonreversible chemical structure. In contrast, a thermoplastic is melted, formed and then cooled in a reversible process wherein the materials are not crosslinked. There are even materials, such as the polyimides, that exhibit characteristics of both types.

The field of polymer chemistry is very broad. Many excellent reference books exist that detail the molecular structure, processing, and performance of polymers. In this section, only property information on the most common types of polymers used as composite matrices will be presented.

Thermoset matrix materials include epoxies, polyesters, bismaleimides, poly-imides, cyanate esters, and phenolics. Epoxies are by far the most common matrix material for advanced polymer composites. Table 1.5 lists major types of matrix materials available, their physical properties, and service limits.

The curing of a thermoset material usually requires the use of a hardener or catalyst in order to promote the crosslinking process. Three types of materials are common for crosslinking epoxies: amines, anhydrides, and Lewis acids. Each

TABLE 1.5 Matrices for Advanced Polymer Composites

Resin Family

Typical Cure Temperature

Service Temperature

(°F)

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