8th Class Science Synthetic Fibers and Plastics NCERT Summary - Polymers

Polymers

Polymers are high molecular mass compounds and their structure is composed of a large number of simple repeating units.

The repeating units are usually obtained from low molecular mass compounds called monomers.

Polymers have very vast spectrum of properties and that is why they are very important for different applications.

Examples of Plastics

Polyethylene terephthalate-PET or PETE

High-density polyethylene-HDPE

Poly vinyl chloride-PVC

Polypropylene-PP

Polystyrene-PS

Low-density polyethylene-LDPE

Fibres

A fibre is a continuous polymer which, when spun into thread, makes fabric.

The polymer strands line up close to one another, forming strong bonds between stands.

If a fibre is pulled, it tightens the bonds, often making the fibre stronger and harder to break.

Natural fibres, such as cotton or wool, come from plants and animals.

Synthetic fibres, such as nylon and polyester, are chemically manufactured from petroleum-based substances by drawing softened plastic through small holes in a machine called a spinneret. These filaments are extremely fine and pliable threads, even when they harden.

Monofilaments are fibres made from molecules that have the same length as the fibre.

Rubber (Natural Rubber)

We get rubber from "Hevea brasiliensis", a tree.

The produce of this tree is "Latex", which is natural rubber.

A natural polymer which possesses elastic properties, also termed as elastomer, is manufactured from rubber latex.

Elastomer is a linear polymer of isoprene (2-methyl-1, 3 butadiene)

Vulcanisation of Rubber

Natural rubber becomes soft at high temperatures (> 335 K) and brittle at low temperatures (< 228 K) shows high water-absorption capacity.

It is soluble in non-polar solvents; is non-resistant to attack by oxidising agents.

Raw rubber is heated with a mixture of sulphur and an appropriate additive, at a temperature range of 373 K to 415 K.

On vulcanisation, sulphur forms cross-links at the reactive sites of the double bonds, and thus, rubber gets stiffened.

For example, in the manufacture of tyre rubber, 5% of sulphur is used as a cross-linking agent.