Natural Rubber and Vulcanisation

This page covers content standard 12.3 of the Form 5 Polymers chapter: natural rubber and vulcanisation. Natural rubber is Malaysia’s most famous polymer, and the exam expects you to know its monomer, how latex is turned into solid rubber, why unvulcanised rubber is weak, and exactly how vulcanisation improves it. The reasoning about cross-links is the heart of this standard.

Natural rubber as a polymer

Natural rubber is a natural polymer made by the addition polymerisation of the monomer isoprene (2-methylbuta-1,3-diene), C₅H₈. The product is poly(isoprene), a long chain in which one carbon–carbon double bond (C=C) remains in each repeating unit. These leftover double bonds matter, because they are the sites where sulfur later forms cross-links during vulcanisation.

The long chains are coiled and can slide over one another, which is why raw rubber can stretch. But because the chains are held together only by weak forces and can slip freely, unvulcanised rubber is not very strong or springy.

Latex and its coagulation

Rubber is tapped from the rubber tree as latex, a colloid in which tiny rubber particles are suspended in water. Each rubber particle is surrounded by a layer that carries a negative charge, so the particles repel one another and stay apart, keeping the latex liquid.

• Coagulation by acid. Adding an acid such as methanoic acid or ethanoic acid supplies hydrogen ions, H⁺. These H⁺ ions neutralise the negative charge on the rubber particles. No longer repelling, the particles collide and combine, so the rubber coagulates into a solid lump.
• Preventing coagulation with ammonia. To keep latex liquid for storage or transport, a little ammonia solution (alkaline) is added. The hydroxide ions, OH⁻, keep the particles negatively charged so they continue to repel and do not coagulate.

Unvulcanised versus vulcanised rubber

Unvulcanised (raw) rubber is soft, has low tensile strength, is not very elastic, becomes sticky when warm, oxidises easily in air and is not very springy. Its chains can slide past one another and do not spring back well.

Vulcanisation is the process of heating rubber with sulfur (sometimes with sulfur monochloride). During vulcanisation, sulfur atoms form cross-links (sulfur bridges) between neighbouring polymer chains at their C=C double bonds. These cross-links lock the chains together, so they can no longer slide far past one another.

Why cross-links help. Because the chains are tied together, a stretched piece of vulcanised rubber springs back to its original shape. The cross-links make the rubber harder, stronger (higher tensile strength), more elastic, more springy, and more resistant to heat and oxidation. The more cross-links formed, the harder the rubber.

Property	Unvulcanised rubber	Vulcanised rubber
Strength	Low	High
Elasticity	Low	High
Effect of heat	Softens, sticky	More resistant
Oxidation	Oxidises easily	More resistant
Hardness	Soft	Harder

Worked example

Question. Natural rubber is poly(isoprene), the addition polymer of isoprene, C₅H₈. A sample of natural rubber has molecules with a relative molecular mass of 204 000. Calculate the degree of polymerisation, n, of this rubber. [Relative atomic masses: H = 1, C = 12]

Step 1, Find the Mr of the repeating unit. In an addition polymer, the repeating unit has the same molecular formula as the monomer isoprene, C₅H₈.

Mr of C₅H₈ = (5 × 12) + (8 × 1) = 60 + 8 = 68

Step 2, Set up the relationship.

Mr of polymer = n × Mr of repeating unit

Step 3, Solve for n.

n = 204 000 ÷ 68
n = 3000

Answer. The degree of polymerisation is n = 3000, about 3000 isoprene units in one rubber molecule.

Practice question

Question. A student adds a few drops of ethanoic acid to natural rubber latex and the latex turns into a solid lump. (a) Name this process. (b) Explain, in terms of charge, why it happens. (c) State one substance that could be added instead to keep the latex liquid.

Answer. (a) The process is coagulation. (b) The rubber particles in latex carry a negative charge and repel one another. The acid supplies hydrogen ions (H⁺) that neutralise this negative charge, so the particles no longer repel, they collide, combine and coagulate into solid rubber. (c) Ammonia solution (an alkali) could be added to keep the latex liquid.

Exam tip

Two ideas score most of the marks here. First, always explain coagulation in terms of charge: acid provides H⁺ that neutralises the negative charge on the rubber particles, so they stop repelling and combine; ammonia keeps them charged and apart. Do not just say “acid makes it solid”, give the charge reason. Second, explain vulcanisation using cross-links: sulfur forms bridges between the chains at the C=C double bonds, stopping the chains from sliding, which makes the rubber harder, stronger and more elastic. A very common slip is to write that vulcanisation “adds sulfur to make rubber soft”, it makes rubber harder and springier, and the more cross-links, the harder it is.

Where this fits

This is content standard 12.3 of the Polymers chapter, and it applies the ideas of the earlier standards to one real polymer. Natural rubber is itself an addition polymer (from types of polymerisation, 12.3 builds on 12.2), and it is the leading example of a natural polymer from 12.1. Practise the poly(isoprene) degree-of-polymerisation question and the coagulation explanation on the chapter worked examples page, and check the charge-and-cross-link wording on the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English from RM50/hr, a one-to-one teacher can make sure your coagulation and vulcanisation answers always include the reason the marking scheme wants.

Quick recap

• Natural rubber = poly(isoprene), an addition polymer of isoprene C₅H₈, with a C=C double bond in each repeating unit.
• Latex is a colloid of negatively charged rubber particles; acid coagulates it (H⁺ neutralises the charge); ammonia prevents coagulation.
• Unvulcanised rubber is soft, weak, sticky when warm and oxidises easily.
• Vulcanisation = heating with sulfur, forming cross-links that make rubber harder, stronger and more elastic.