Latex and the rubber polymer
Natural rubber is obtained as latex, a milky liquid tapped from the rubber tree (Hevea brasiliensis). Latex is a colloid in which tiny rubber particles are suspended in water. The rubber itself is a natural polymer whose monomer is isoprene (2-methylbuta-1,3-diene). Long chains of the repeating unit make the material flexible and elastic.
Coagulation of latex
Each rubber particle is surrounded by a thin layer of protein that carries a negative charge. Because like charges repel, the particles stay apart and the latex does not clump. To make solid rubber, the negative charges must be neutralised so the particles can join, a process called coagulation.
Worked example / 例题
Adding a little ethanoic acid (or methanoic acid) to latex supplies H+ ions. These neutralise the negative charge on the protein layer, so the rubber particles collide, combine and coagulate into a solid lump. Adding ammonia solution instead makes the latex more alkaline and prevents coagulation, which is why ammonia is used to preserve latex during transport.
Vulcanisation
Untreated natural rubber is soft, becomes sticky when hot and hard when cold, and is easily oxidised. In vulcanisation, rubber is heated with sulfur. Sulfur atoms form cross-links between the polymer chains, stopping them sliding over one another.
| Property | Unvulcanised | Vulcanised |
|---|---|---|
| Elasticity | lower | higher |
| Strength | weaker | stronger |
| Heat resistance | poor | better |
Remember / 记住
- Acid coagulates latex; ammonia prevents coagulation.
- The monomer of natural rubber is isoprene.
- Vulcanisation with sulfur forms cross-links and improves elasticity, strength and heat resistance.