Q. Differentiate the thermoplastic and thermo sets.

                                  OR

Write note on thermoplastic and thermosetting plastic resins.

Ans. 

Thermoplastic resins: Thermoplastic materials do not undergo any chemical change while moulding and do not become permanently hard with the application of pressure and heat. These plastic remains soft at elevated temperatures and becomes hard/rigid only on cooling. They can be again melted and given any desired shape by moulding. Plasticity of thermoplastics increases with temperature. They are generally linear polymers, the chemical structure or molecular weight of which does not change on heating or moulding. Molecular weight of thermoplastic compounds is smaller than that of thermosetting plastics. The intermolecular usually inorganic solvent, some swell up in solvent.

Example-Polyethylene, Polypropylene, Polystyrene.

Thermosetting plastic: These are materials which during the moulding, get hardened and once articles solidified, they cannot be softened again. It is a irreversible change. A permanently hard product, in the first instance. Heat makes the material soft but on additional heat or by adding special chemicals the plastics are hardened by by polymerization and cannot be softened by heat, change is irreversible. Whole structure is one large mass because of primary covalent bonds are present throughout and does not change with temperature. They are stronger than thermoplastic resins.

Examples- poly form aldehyde or Bakelite, urea, formaldehyde and melamine formaldehyde, alkyl or glyptic resins, polyesters.