Thermoset polymers do not contain any defined arrangement of chains and as such they can be classified as amorphous, i.e. they do not contain any crystalline structures distinct [3]. Thermoset materials are formed from a chemical reaction of a resin and a hardener or catalyst and this reaction is irreversible and produces a hard, infusible material [4]. Cured thermosets will not return to liquid form if heated, but above a certain temperature their mechanical properties can change significantly. The temperature at which this change can occur is called the glass transition temperature (Tg) and it varies depending on the particular resin and hardener/catalyst used as well as its degree of cure and whether it has been properly mixed . If the temperature of a thermoset material is raised above the Tg, the molecular structure changes from that of a hard crystalline polymer to a more flexible amorphous polymer. At this elevated temperature, the properties of the thermoset, such as the resin's modulus (stiffness), decrease significantly and, therefore, the compressive and shear strength of the composite will do the same. Other properties such as water resistance and color stability also decrease above the Tg of the resin. This change can be reversed by cooling the material to below the Tg. The majority of composite structural parts are manufactured with three main matrix types, namely epoxy. , vinyl ester and polyester.2.3.1. Epoxy ResinEpoxy refers to a chemical group consisting of one oxygen atom bonded to two carbon atoms bonded to other elements. The simplest epoxy is a three-membered ring structure known as “alpha-epoxy” or “1,2-epoxy” [4]. The idealized chemical structure is shown in Figure 4. Epoxy has many useful properties such as rec...... middle of paper ......te. The accelerator and catalyst must be added carefully and in controlled quantities to control the polymerization reaction so as to obtain the best properties of the material. Adding too much catalyst will cause the resin to cure too quickly, while too little catalyst will cause undercuring and loss of strength. Fillers are used with polyester resins for a variety of reasons, including: • To reduce the cost of casting • To facilitate the casting process • To impart specific properties to the casting Up to 50% of the weight of the resin can be made of fillers but the addition of these fillers will affect the strength of the composite to varying degrees. Using fillers can be useful when casting thick components because neat epoxies generate significant amounts of heat during curing and adding fillers can reduce this exothermic heat production..