Question
Natural rubber becomes sticky in summer and brittle in winter. How does vulcanization solve this problem? Explain the chemistry of the process and why cross-linked rubber has better mechanical properties.
(NCERT Class 12, Polymers)
Solution — Step by Step
Natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene) with cis-configuration. Its chains are held together only by weak van der Waals forces. When heated, chains slide past each other → rubber becomes soft and sticky. When cooled, chains pack too closely → rubber becomes hard and brittle. It also absorbs organic solvents and swells.
Vulcanization involves heating natural rubber with sulphur (3-5% by weight) at about 150°C for several hours. Charles Goodyear discovered this in 1839.
The sulphur atoms form cross-links (bridges) between adjacent polymer chains through the double bonds present in the rubber backbone:
Each cross-link consists of 1-8 sulphur atoms connecting two chains via C-S bonds at the allylic positions.
Cross-links restrict the movement of polymer chains relative to each other:
- Chains can no longer slide past each other when heated → rubber doesn’t become sticky
- Chains can still stretch (the segments between cross-links are flexible) → rubber retains elasticity
- The network snaps back to its original shape when stress is removed → improved elastic recovery
- Chemical cross-links are permanent, unlike the weak intermolecular forces they replace
| Sulphur % | Product | Properties |
|---|---|---|
| 1-3% | Soft vulcanized rubber | Flexible, elastic — used in tyres, gloves |
| 5% | Standard vulcanized rubber | Good elasticity + strength — tubes, hoses |
| ~30% | Ebonite (hard rubber) | Rigid, non-elastic — used in battery cases, combs |
More sulphur = more cross-links = harder and more rigid material. At 30% sulphur, the rubber becomes so cross-linked it loses elasticity entirely and becomes a hard plastic (ebonite).
Why This Works
Think of natural rubber chains as cooked spaghetti — they slide over each other easily. Vulcanization is like putting clips between adjacent spaghetti strands. Now the strands can’t slide freely (no more stickiness), but individual segments between clips can still flex (elasticity preserved).
The key chemistry involves sulphur reacting with the C=C double bonds in the rubber backbone. Each isoprene unit has one double bond, providing sites for cross-linking. The allylic hydrogens (next to the double bond) are particularly reactive — sulphur radicals abstract these to initiate cross-link formation.
Alternative Method
A quick way to remember the effect: cross-linking converts a thermoplastic (can be reshaped on heating) into a thermosetting polymer (retains shape on heating). Natural rubber = thermoplastic behaviour. Vulcanized rubber = thermoset behaviour.
CBSE board exams ask this as a 2-3 mark question. The expected answer: vulcanization = heating rubber with sulphur, sulphur forms cross-links between chains, improves elasticity and tensile strength, reduces stickiness and solvent absorption. Mention Goodyear’s name for bonus marks in NEET.
Common Mistake
Students write that vulcanization “adds sulphur to the polymer chain.” That’s misleading. Sulphur doesn’t become part of the main chain — it forms bridges between chains. The main chain structure remains polyisoprene. The sulphur cross-links are lateral connections, not insertions into the backbone. Drawing the structure correctly (showing S-S bridges connecting two separate chains) is important for full marks.