Elastomers: synthetic rubbers (Buna-S, Buna-N, Neoprene)

Elastomers: Buna-S, Buna-N, Neoprene

1. What an elastomer is

Elastomers = lightly cross-linked polymers with long flexible chains ($T_g$ below room temperature) that stretch several hundred percent and snap back.

• Requirement: coiled chains + few cross-links (vulcanization with sulphur) so chains uncoil under load but can't slide past each other permanently.
• Natural rubber (polyisoprene) is the reference; synthetics below improve oil/heat/weather resistance.

2. Buna-S (SBR — styrene-butadiene rubber)

Copolymer of butadiene ($CH_2=CH-CH=CH_2$, ~75%) + styrene (~25%), emulsion polymerization with free-radical initiator.

• Cheapest synthetic rubber; good abrasion resistance, poorer oil resistance and heat build-up than natural rubber.
• Uses: car tyres (major), footwear soles, conveyor belts, hoses.

3. Buna-N (NBR — nitrile rubber)

Copolymer of butadiene + acrylonitrile ($CH_2=CH-CN$, typically 20–40%).

• Polar –CN groups → excellent oil/fuel/solvent resistance (more acrylonitrile = better oil resistance, less flexibility at low T).
• Uses: oil seals, gaskets, fuel hoses, gloves (nitrile disposable gloves), printing rollers.

4. Neoprene (polychloroprene, CR)

Monomer chloroprene ($CH_2=CCl-CH=CH_2$) → emulsion polymerization.

• Cl atom gives flame resistance, weather/ozone resistance, good chemical stability, moderate oil resistance; bonds well to metals/fabrics.
• Uses: wetsuits, cable jackets, gaskets, bearing pads, adhesives (Fevicol-type contact adhesive).

Rubber	Monomers	Star property	Main uses
Buna-S (SBR)	butadiene + styrene	cheap, abrasion-resistant	tyres, belts, soles
Buna-N (NBR)	butadiene + acrylonitrile	oil/fuel resistant	seals, fuel hose, nitrile gloves
Neoprene (CR)	chloroprene	flame + weather resistant	wetsuit, cable jacket, adhesive

Vulcanization (all three): heating with 2–5% sulphur forms –S– bridges between chains → elasticity, strength, temperature stability. Too much sulphur → hard ebonite.

Exam Points

• Monomer pairs for Buna-S, Buna-N, neoprene (3 marks — most asked as direct recall).
• One distinguishing property + two uses each (3–5 marks); vulcanization purpose and sulphur role (2 marks).