ABSTRACT
Although a variety of thermosetting resins are utilized in the polymer-composites industry. the most common ones are unsaturated polyesters. vinyl esters, epoxies. phenolics. and polyurethanes. Unsaturated polyesters are less expensive compared to vinyl esters and epoxies and account for almost 75% of total thennoset usage in FRPs [3]. Typically one reacts an unsaturated dicarboxylic acid (such as maleic anhydride) with a dihydric alcohol (such as ethylene glycol) to form a (linear) prepolymer containing double bonds in the prepolymer backbone. The prepolymer is then dissolved in an almost equal amount by weight of a reactive monomer such as styrene to give a low-viscosity liquid. It is this liquid that would be contained in the resin bath shown in Figure 15 .I. Crosslinking is carried out with the help of an organic peroxide such as benzoyl peroxide that decomposes on heating to give free radicals. These free radicals attack the unsaturation sites (double bonds) in the prepolymer and also react with the styrene, resulting in the growth of polystyrene branches which ultimately unite all the prepolymer molecules into a gel.
