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Sunday, 6 October 2019

Classification of Thermosetting Plastics

Thermo-setting Resins:


It is an intermediate product which undergoes chemical changes of condensation and polymerisation to form a rigid final shape under the influence of heat and pressure etc. On the application of heat, they first become soft and plastic and on further heating they undergo change and set hard. This process is called thermo-setting or thermo-hardening. When material is thermoset, it is permanently set and does not soften to any appreciable extent when being heated. However, intense heating will bring about the breakdown of the material by burning. When moulded, they undergo chemical change, and cannot be resoftened by heating to shape them. Compared to thermo-plastics thermo-setting materials are harder, mechanically strong, and are therefore suitable for high temperatures. These materials are less flexible and have heat resisting properties. To improve various properties, fillers and reinforcing materials are added to thermo-setting resins. The classification of thermosetting plastics are explained below.
Bakelite :

It is also known as Phenolics and is made by the reaction of Phenol and formaldehyde. Therefore, it is sometimes referred as Phenol Formaldehyde resins. They are strong, rigid and dimensionally stable. They are heat and solvent resistant. It is a non conductor of electricity and are cheap and easily available. They find a wide application in the electrical engineering. They are moulded into appliance handles, sockets and switches, cabinets etc.


Silicon Resins :

These are organic compounds of Silicon and are produced by complicated process of hydrolysis and condensation of chlorisilances. The catalyst used normally is zinc-octoate. Materials like cotton, paper, asbestos, glass, fibre, mica are used as fillers to improve the quality of the resins. They have good insulating properties and are resistant to heat. It can work at temperatures as high as 180oC. Silicon resins are attacked by aromatic solvents and mineral insulating oils. They are good water repellent, chemically inert and resistant to weathering effect. They are used as cooling and impregnating liquids for capacitors and transformers, as varnishes, as moulding for high temperature applications. They are also used as additives with rubber to make silicon rubber, which has improved abrasion resistance and high operating temperatures.

Epoxy Resins :

These resins are produced by alkaline condensation of epichlorohydrin and polyhydric compounds. They are hardened by the use of hardeners like organic acids. Epoxy resins have good mechanical strength, less shrinkage, excellent dimensional stability after casting good adhesion to metals. They are used in the manufacture of laminated insulating boards and as insulating varnishes. Moulds made with epoxy resins are employed for the production of metal components for aircrafts and automobile.


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