Thermosetting polymers do not contain a fixed arrangement of chains and as such can be classified as amorphous, i.e. they do not contain a distinct crystalline structure [3]. Thermosetting materials are formed by a chemical reaction between a resin and a hardener or catalyst and this reaction is irreversible and produces a hard, infusible material [4]. Cured thermoset materials do not become liquid again when heated, but above a certain temperature their mechanical properties can change substantially. The temperature at which this change can occur is called the glass transition temperature (Tg) and varies depending on the particular resin and hardener/catalyst used, as well as its degree of cure and whether it has been mixed correctly. If the temperature of a thermoset material is increased above Tg, the molecular structure changes from that of a hard crystalline polymer to a more flexible amorphous polymer. At this high temperature the properties of the thermoset material, such as the resin modulus (stiffness), decrease significantly and consequently 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 back to below Tg. Most structural parts made from composites are made from three main types of matrix, namely epoxy, vinylester and polyester.2.3.1. Epoxy ResinEpoxy resin refers to a chemical group consisting of one oxygen atom bonded to two carbon atoms bonded to other elements. The simplest epoxy resin 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 resin has numerous useful properties such as side...... middle of paper... you. The accelerator and catalyst must be added carefully and in controlled quantities to control the polymerization reaction to achieve the best material properties. Adding too much catalyst will cause the resin to cure too quickly, while not enough catalyst will result in insufficient curing and a loss of strength. Fillers are used with polyester resins for a variety of reasons, including:• To reduce the cost of molding• To facilitate the molding process• To impart specific properties to the moldingUp to 50% of the resin weight can be made up by fillers, but the addition of these fillers will affect the strength of the composite to varying degrees. The use of fillers can be beneficial when casting thick components as pure epoxies generate significant amounts of heat during curing, and adding fillers can reduce this exothermic heat production.