Effects Of Thermoset Polymers

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Thermoset polymers contain no set arrangement of chains and as such they can be classified as amorphous i.e. they contain no distinct crystalline structure [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 and infusible material [4]. Cured thermosets will not become liquid again if 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 it varies depending on the particular resin and hardener/catalyst used as well as its degree of cure and whether it was mixed properly. 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 resin modulus (stiffness) drop significantly and as a result the compressive and shear strength of the composite will do the same. Other properties such as water resistance and colour stability also reduce above the resin’s Tg This change can be reversed by cooling the material back down to below the Tg. The majority of structural parts made of composites are made with three main types of matrix, namely epoxy, vinylester and polyester. 2.3.1. Epoxy Resin Epoxy refers to a chemical group consisting of an oxygen atom bonded to two carbon atoms that are bonded to other elements. The simplest epoxy is a three-member ring structure known by the term ‘alpha-epoxy’ or ‘1,2-epoxy’ [4]. The idealised chemical structure is shown in Figure 4. Epoxy has numerous useful properties such as the fac... ... middle of paper ... ...te. The accelerator and catalyst must be added carefully and in controlled amounts in order to control the polymerisation reaction such that the best material properties will be achieved. Adding too much catalyst will cause the resin to harden too quickly while too little catalyst will result in under curing and a loss of strength. Filler materials are used with polyester resins for a variety of reasons including: • To reduce the cost of the moulding • To facilitate the moulding process • To impart specific properties to the moulding Up to 50% of the resin weight can be fillers but the addition of these fillers will affect the strength of the composite to varying degrees. The use of fillers can be helpful when casting thick components as pure epoxies generate substantial amounts of heat when curing and the addition of fillers can reduce this exothermic heat output.

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