Vulcanisation heats natural rubber with sulfur so that sulfur atoms form cross-links between the poly(isoprene) chains at their C=C double bonds. The rubber becomes harder, stronger, more elastic and more resistant to heat and oxidation.
Natural rubber is a natural polymer, poly(isoprene), whose long chains still contain carbon-carbon double bonds. Because those chains can slide over one another, untreated rubber is soft, becomes sticky when warm and loses its springiness. Vulcanisation is the process of heating rubber with sulfur to lock the chains together with cross-links, and it is a standard part of the Form 5 Polymers chapter. The exam wants the role of sulfur, the change in properties, and the reason behind that change.
The structures involved
The monomer of natural rubber is isoprene (2-methylbuta-1,3-diene), C5H8, whose structure is CH2=C(CH3)−CH=CH2. It polymerises into poly(isoprene), which can be written as a repeating unit:
(-CH2−C(CH3)=CH−CH2−)n
Note that each repeating unit still has a C=C double bond. This leftover double bond is the reactive site that vulcanisation uses.
What happens in vulcanisation
Vulcanisation is not a simple molecular reaction with a single balanced equation; it is a cross-linking process. When rubber is heated with sulfur, sulfur atoms add across the C=C double bonds of neighbouring poly(isoprene) chains and form sulfur bridges (-S−S− cross-links) that tie the chains together. In outline:
rubber chains + sulfur (Sx), heated → chains joined by -S−S− cross-links
Because sulfur atoms bond to two different chains, they act like rungs between the long molecules. The more sulfur used, the more cross-links form and the harder the rubber becomes, a small amount gives flexible tyre rubber, while a large amount gives hard ebonite.
Conditions required
- Sulfur is the cross-linking agent.
- The mixture of rubber and sulfur is heated.
An alternative modern method reacts rubber with disulfur dichloride (S2Cl2) at room temperature (cold vulcanisation), but heating with sulfur is the method the syllabus emphasises.
Observations: the change in properties
Vulcanisation is judged by the change in the rubber’s properties rather than by a colour change:
- Vulcanised rubber is harder and stronger than natural rubber.
- It is more elastic and springs back to its original shape after stretching, instead of staying deformed.
- It is more resistant to heat and does not soften or become sticky as easily when warm.
- It is more resistant to oxidation and wears out more slowly, because many of the reactive double bonds have been used up by the sulfur.
Why the properties change
The explanation is the point most often examined. In natural rubber the chains are held only by weak forces and can slide past each other permanently, so the rubber stretches and stays stretched. The sulfur cross-links form strong bonds between chains; when the vulcanised rubber is stretched the cross-links prevent the chains from sliding away, so the material returns to shape and resists deformation. Using up the double bonds also makes the rubber less easily attacked by oxygen. Always link each improved property back to the cross-links.
Common mistakes to avoid
Say that sulfur forms cross-links between chains, not that it “makes the rubber heavier” or simply “mixes in”. Name the reactive site as the C=C double bond of poly(isoprene). Do not claim vulcanisation gives a colour change; the evidence is the change in hardness and elasticity. Keep the cause (cross-links) and the effect (harder, more elastic, more heat- and oxidation-resistant) clearly paired.
How it appears in the SPM exam
In Paper 2 (4541/2) you may be asked to explain what vulcanisation is, state the substance added (sulfur) and the condition (heat), and explain in terms of cross-links why vulcanised rubber is more elastic and harder. In Paper 1 (4541/1), objective items test the agent used and the property changes. Root every answer in the sulfur cross-links joining the poly(isoprene) chains, and vulcanisation questions become straightforward.
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