that promote the cure reaction but do not themselves serve as direct cross-linking agents. These catalytic curing agents have the advantage of very long shelf life and require significant heat to initiate the reaction. The resultant structure of Lewis acid-cured resin is very tightly cross-linked. Curing of most epoxy resins and composites is done at 120° or 177°C with a typical cycle time of 2 to 3 hours under the molding pressure of up to 1.4 MPa.

The term vinyl ester can be applied to any number of chemical compounds comprising an ester linkage and terminal unsaturation. There are several types of vinyl esters based on epoxy resins as well as nonepoxy resins. However, to the composites industry, the vinyl ester resins usually mean methacrylate esters of epoxy resins (Launkitis, 1978). Unlike polyesters, vinyl esters do not possess internal unsaturation. However, vinyl esters and polyesters are similar in that they both utilize a coreactant or cross-linking agent, such as styrene, and free radical-producing initiators, such as peroxides, to effect cure. As a result, vinyl ester resins can be cured in a very short time like polyesters, but their static strength and modulus properties are similar or comparable to those of epoxy resins.

Typical mechanical properties of polyesters, epoxies, and vinyl esters are shown in Table 3-3. While epoxy resins are at the top of the scale as far as mechanical properties are concerned, they require the longest cure time and are the most expensive. For example, epoxy resins cost about $1.80/lb, whereas unsaturated polyesters cost only $1.00/lb. Vinyl ester resins fall in between these two resins, costing about $1.60/lb. Thus, the final selection of a resin should consider the material cost as well.

The fatigue crack propagation rates of these resins vary with the stress intensity factor range ΔK in the case of other plastics for structural use (Hertzberg and Manson, 1980). In amine-cured DGEBA epoxy resins, the values for exponent m in the equation for the crack growth per fatigue cycle, da/dN = const.(ΔK)^m, range from 7.7 to 20, which are higher than those for other plastics. In general, a lower fatigue crack growth rate and a higher fracture toughness are observed with increasing molecular weight between cross-links.

As discussed previously, all of these resins suffer the problem of brittleness. A brittle resin results in premature matrix cracking in the composite, which in turn facilitates moisture ingress. Although fracture toughness of the resin systems can be raised by increasing Mc or adding diluents, these approaches result in lowering of modulus and temperature resistance (Hertzberg and Manson, 1980; Owen, 1974; Christensen and Rinde, 1979). At present, many commercially available resins utilize toughening agents in the form of discrete particles of elastomers or ductile