Plastic

INTRODUCTION

The word plastic was first used around 1909 and is a commonly used synonym for polymers.
•Plastic from the Greek word plastikos, means ‘ it can be molded and shaped’
•Plastic are groups of synthetic chemical compounds which are high polymers and which are at some stage plastic and could be shape by heat, with or without pressure into useful product.

•Plastics are polymers, large molecules made of repeating units of smaller molecules (monomers) that are chemically bound together. A polymer is like a chain in which each link is a monomer.
•All plastic is made of carbon. Man-made plastic uses carbon derived from oil, while biopolymers or bioplastics use carbon derived from natural materials. 

CHEMISTRY OF PLASTIC 

•All plastics are polymers, but not all polymers are plastics.
•Some familiar nonplastic polymers include starches (polymers of sugars), proteins (polymers of amino acids) and DNA (polymers of nucleotides).
•The simplified diagram below shows the relationship between monomers and polymers. 

The chemical properties of a polymer depend on:

•The type of monomer or monomers that make up the polymer. The chemical properties of homopolymer 1 are different from those of homopolymer 2 or the copolymers.
•The arrangement of monomers within the polymer. The chemical properties of the straight polymers are different from those of the branched polymers.

 

  PROPERTIES OF PLASTIC

•Plastic are light in weight
• resistant to most chemicals ( also corrosion-resistant)
• excellent electrical insulators
• easy to shape into finish parts
•adequately strong 

Compared with metals, polymers/plastic are generally characterized by

•low density
•low strength and stiffness
•Low electrical and thermal conductivity
•Good resistance to chemicals 

•High coefficient of thermal expansion

 

  CLASSIFICATION OF PLASTIC

1. Thermosets
2. Thermoplastics