Coagulation

Coagulation is the joining together of the dispersed rubber particles in latex into a solid mass. It happens when an acid is added, because the acid neutralises the negative charge that keeps the particles apart. Malay: penggumpalan · Chinese: 凝固.

The particles in latex float freely because their protein coats carry the same negative charge and so repel one another. When acid such as methanoic acid or ethanoic acid is added, the hydrogen ions neutralise this charge. Without the repulsion the particles can now collide, stick together and grow into larger and larger lumps until the rubber separates from the water as a soft solid. This is a physical change in the colloid rather than a change to the rubber molecules themselves, the poly(isoprene) chains are unchanged; only their arrangement changes.

Example. In a rubber factory, methanoic acid is stirred into latex in shallow tanks. The latex thickens and sets into a spongy mass of coagulated rubber, which is then rolled into sheets and dried before being sold or sent for further processing.

Confusion to avoid. Do not confuse coagulation with vulcanisation. Coagulation only makes the rubber particles clump into solid rubber, using acid, and changes no bonds within the molecules. Vulcanisation is a later, separate process that heats rubber with sulfur to form cross-links and improve its properties. Coagulation comes first and makes solid rubber; vulcanisation comes after and makes it better.

In the exam you may be asked why latex is preserved with ammonia during transport: ammonia is an alkali that keeps the latex slightly alkaline, so any acid produced by bacteria is neutralised and premature coagulation is prevented. This links coagulation neatly to the acid–base ideas from an earlier chapter.

Coagulation is the step that turns liquid latex into usable solid rubber, sitting between latex and vulcanisation in the natural-rubber sequence tested in SPM Chemistry.