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Epoxies differ from polyester resins in that they are cured by a ‘hardener’ rather than a catalyst. The hardener, often an amine, is used to cure the epoxy by an ‘addition reac- tion’ where both materials take place in the chemical reaction. The chemistry of this reaction means that there are usually two epoxy sites binding to each amine site. This forms a complex three-dimensional molecular structure which is illustrated in Figure 21: Figure 21 - Schematic Representation of Epoxy Resin (Cured 3-D Structure) Since the amine molecules ‘co-react’ with the epoxy molecules in a fixed ratio, it is essential that the correct mix ratio is obtained between resin and hardener to ensure that a complete reaction takes place. If amine and epoxy are not mixed in the correct ratios, unreacted resin or hardener will remain within the matrix which will affect the final properties after cure. To assist with the accurate mixing of the resin and hardener, manufacturers usually formulate the components to give a simple mix ratio which is easily achieved by measuring out by weight or volume. 5.1.4 Comparison of Resin Properties The choice of a resin system for use in any component depends on a number of its characteristics, with the following probably being the most important for most com- posite structures: 1 Adhesive Properties 2 Mechanical Properties 3 Degradation From Water Ingress 5.1.4.1 Adhesive Properties It has already been discussed how the adhesive properties of the resin system are important in realising the full mechanical properties of a composite.The adhesion of the resin matrix to the fibre reinforcement or to a core material in a sandwich construction are important. Polyester resins generally have the lowest adhesive properties of the three systems described here. Vinylester resin shows improved adhesive properties over polyester but epoxy systems offer the best performance of all, and are therefore 20 - GTC-6-0417PDF Image | GUIDE TO COMPOSITES
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