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POLYESTER AND VINYL ESTER RESINS 2 Frank A. Cassis and Robert C. Talbot 2.1 INTRODUCTION AND HISTORY sored production of styrene-butadiene rubber. Organic polymers are divided into two types, reinforced-thermoplastic and thermoset. With thermoset polymers such as unsaturated poly- esters and vinyl esters, a chemical reaction cross links the material so that it cannot be returned to liquid form. Other common ther- mosetting polymers include epoxy and phenolic resins. Thermoset plastics made with polyester and vinyl ester resins represent the major portion of the reinforced plastic com- posites industry today. Early workers on unsaturated polyesters soon learned that despite the possession of reactive double bonds, these resins were slug- gish in reacting with themselves. Even with effective catalysts, they still required high tem- peratures and lengthy cure times to complete the cross linking reaction. The key to modern day application of unsaturated polyesters was the discovery by Carlton Ellis in 1937l that the addition of reactive monomers, such as styrene, gave mixtures that would copolymerize many times faster than homopolymerization. The styrene addition produced the added benefit of an easily handled liquid material that could be pumped, transported and fabricated into a fin- ished plastic by a myriad of molding processes. Developments during the 1940s accelerated the commercial applicability of unsaturated polyesters to the position they hold today. Styrene became readily available and lower in cost as a result of the US Government's spon- Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 At the same time, scientists found that styre- nated polyesters could yield high strength, light weight structures when reinforced with glass fibers. They also learned that fiberglass- reinforced polyesters had excellent electrical properties and that large structures could be molded at low pressures with low cost tooling. As a result, commercial development pro- ceeded rapidly after World War I1 with materials and molding research moving in many directions. In the 20 years that followed, polyester and vinyl ester resins matured rapidly and by the mid-l970s, the composites fabricator and end user had numerous options with these matrix systems to achieve the desired properties in the finished part. 2.2 POLYESTER RESINS The reaction of an organic acid with an alcohol results in the formation of an ester. By using a difunctional acid and a difunctional alcohol (glycol) a linear polyester is produced (Fig. 2.1). 00 II H-(-0 - C - R - C - 0 - R -)" -OH Fig. 2.1 Properties of polyesters can be varied by using different combinations of diacids and glycols. These products are thermoplastic polyesters and they are made with various acids and
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Polyester resins 35 glycols such as the following: In the esterification reaction with maleic anhv- - Acids GZycols Phthalic anhydride Ethylene glycol Isophthalic acid Propylene glycol Terephthalic acid Neopentyl glycol Adipic acid Diethylene glycol
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