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Copolymerisation And Rubber

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Copolymerisation And Rubber - Lesson Summary

Copolymerisation is a type of polymerisation reaction in which more than one kind of monomers polymerises to form a copolymer. Copolymers can be obtained by either chain growth polymerisation (or) step growth polymerisation.

Butadiene-styrene copolymer (Buna-S) formed from 1, 3-butadiene and styrene and is elastomeric and tough in nature.

Homo-polymers are obtained by the polymerisation of a single mono-meric species when compared with copolymers.

Polystyrene is a homo-polymer formed from styrene monomers and is brittle in nature.

As a mixture of two or more monomers undergoes copolymerisation, the copolymers produced have better physical and mechanical properties than homo-polymers.

The butadiene-styrene copolymer useful in the manufacture of automobile tyres, floor tiles, footwear components and cable insulation.

Rubber:

Natural rubber is a polymer with elastomeric properties. It is obtained from the bark of the rubber tree in the form of latex. Rubber latex is a colloidal dispersion of rubber in water.

Isoprene 2 -methyl 1, 3 -butadiene polymerises to form cis -1, 4 polyisoprene which is natural rubber. Thus, natural rubber is a linear polymer of 2-methyl 1, 3-butadiene.

Natural rubber can be stretched. When stretched, the chains get aligned in the direction of the external force. When the external force is released, the chains back to their original positions.

Untreated natural rubber is quite sticky. It becomes soft at high temperatures (>335 K) and brittle at low temperatures (<283K). Natural rubber absorbs water. It is soluble in non-polar solvents and is susceptible to attack by oxidising agents. These properties of natural rubber limit to its usage. To improve the physical properties of natural rubber a process called vulcanisation is employed.

Most rubber used today is vulcanised rubber. To vulcanise rubber, raw rubber is mixed with sulphur and heated between 373K and 415K. This forms sulphur cross-links at the reactive sites of double bonds such as allylic positions and thus, the rubber gets stiffer.

The vulcanisation process is generally irreversible. The cross-links help vulcanised rubber to maintain its shape when stretched.

Vulcanised rubber is not very sticky, unlike natural rubber. Vulcanised rubber has high tensile strength and is usable over a wide temperature range. It is also resistant to abrasion. Vulcanised rubber has many uses: as car tyres and shoe soles.

Synthetic rubbers are more flexible, tougher and durable than natural rubber. Synthetic rubber can be defined as any rubber-like polymer that can be vulcanised. Synthetic rubber must be elastomeric can be stretched and then must return to its original shape.

Neoprene is an example of a synthetic rubber. Chloroprene undergoes free-radical addition polymerisation to form neoprene.

Neoprene is resistant to vegetable and mineral oils. It is used to manufacture conveyer belts, gaskets and hoses.

Buna-N, known as nitrile rubber, is a copolymer of 1, 3-butadiene and unsaturated monomer, acrylonitrile. A peroxide catalyst is required for the reaction.

Buna-N or nitrile rubber is resistant to petrol, lubricating oil and organic solvents. It can be used over a wide temperature range and so it has many applications in automobiles. Buna-N is used to make oil seals and tank linings, Non-latex disposable gloves are typically made from nitrile rubber.

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