vulcanization
/ vul-kan-ih-ZAY-shun /
Raw natural rubber is a disappointing material on its own: it is sticky and goes soft and gooey when warm, then hard and brittle when cold, and it takes a permanent set if you stretch it. In 1839 Charles Goodyear found that cooking raw rubber together with sulfur transformed it into the springy, tough, weather-stable rubber we know from tyres and boot soles. That process is vulcanisation, and it is what turned rubber from a curiosity into one of the most important engineering materials in the world.
The mechanism is crosslinking by sulfur. Heating natural rubber with a few percent of sulfur causes short sulfur bridges to form between neighbouring polymer chains, tying the once-separate chains into a light network. The number of crosslinks is a dial for stiffness: a small amount of sulfur (say a few percent) gives a soft, stretchy rubber like a rubber band or a tyre tread, while a large amount of sulfur produces a hard, rigid material — heavily vulcanised rubber is ebonite (vulcanite), once used for bowling balls and pipe stems. In short, vulcanisation is the specific chemistry that crosslinks rubber into a working elastomer.
It matters because vulcanisation is what supplies the crucial crosslinks an elastomer needs: without them, raw rubber would just flow and stay deformed instead of springing back. That is the whole reason a rubber band works. One honest caveat comes with the benefit: because vulcanised rubber is crosslinked, it is effectively a thermoset — it cannot be melted down and remoulded, which makes rubber (like tyres) genuinely hard to recycle. And there is a limit to the good: adding too much sulfur over-crosslinks the rubber and makes it hard and brittle rather than springy.
A rubber band and a hard rubber comb can be made from the same natural rubber — the difference is the amount of sulfur used in vulcanisation. A few percent gives the stretchy band; a lot gives the rigid comb.
Vulcanization crosslinks rubber chains with sulfur bridges; a little sulfur gives a stretchy rubber, a lot gives a hard one.
Vulcanized rubber is crosslinked, so it behaves like a thermoset — it cannot be remelted or reshaped, which is why tyres are hard to recycle. Too much sulfur over-crosslinks it into a hard, brittle solid.