INTRODUCTION TO PLASTICS

 INTRODUCTION TO PLASTICS

 

            The class of materials known as polymers is characterized by a structure made up of molecular chains with a large aspect ratio, that is, they are very long relative to their transverse dimensions, corrosive resistance, good electrical and thermal insulation properties, low co-efficient of friction and so on. Natural polymers such as shellac horn, bitumen, lacquer gutta percha etc. are available for many centuries and have provided much of the basic knowledge of mould making.

 

          When considering the evolution of plastics, it is apparent that they have had on unfortunate history as substitute materials. Special new materials could be fabricated with sufficiently consistent properties to facilitate the manufacture of products with relatively intricate geometries and a unique combinations of properties coupled with ease of fabrication has revolutionized the modern industry. To the enlightened designer, plastics are a major complement to the range of more traditional materials.

 

          The plastic industry is composed of three divisions:       

1.     The manufacture of plastic resins, synthesizing polymeric materials from chemical compounds.

2.     The manufacture of compound plastic mixtures known as moulding compounds.

3.     The process, which produces plastic articles by moulding and fabricating methods.

 

The primary function of the plastics resin producer is to make available various types and grades of polymeric substances which the plastic compound can formulate with other ingredients into powders, pellets, flakes, granules, liquid resins or solutions for processing by moulder in to the finished plastics products.

 

There are many ways of transforming plastic materials into finished plastic articles. The basic principle consists of partially melting the plastic mass by the application of heat and compression. By this process the plastics softened and formed into desired shapes by using of rolls, dies, moulds etc. with the aid of pressure.

 

The intent of using the plastic material as useful articles usable in different areas like is intended.

 

Finally, the shaped hot plastic is solidified through further polymerization in curing stage. Solidification of plastic in case of thermo set materials is by applying pressure at curing stage and by chilling in the case of thermoplastic materials.

 

          Plastics may be classified in various ways depending upon their basic raw materials, their distinctive properties and their applications.

  However the two important divisions namely:

 

THERMO-PLASTICS

            In thermoplastic material the individual molecules are often described as being chain like but not actually bonded together. When they are heated the intermolecular forces are weakened so that it becomes soft and flexible and after cooling they will solidify again. They are obtained from the substituted derivatives of ethylene polystyrene, acetyl, nylons, vinyl’s acrylics and fluorocarbons. Generally they are processed by injection or blow moulding.

 

THERMOSET-PLASTICS

          They are formed by chemical reactions in which first long chain line molecules form further a close network structure is formed usually under the application of heat and pressure.

 

            Since molecules are cross linked by covalent bonds, they are characterically quite rigid. So if once it is heated then it cannot be reheated. They are generally processed by compression or transfer moulding. However some properties can be achieved by reinforcement of chemicals or additives.

 

E.g.: Formaldehydes, Epoxies, Glyptal, Formica etc.