Sulfur bridges

Raw rubber turns sticky in the heat and stiff in the cold. Cooking it with sulfur fixes both, and makes the rubber of bands, gloves and tyres.

In 1823 the Scottish chemist Charles Macintosh spread rubber between two layers of cloth to make raincoats. They kept people dry, and they turned soft and tacky in a hot summer and stiff in a cold winter. In raw rubber the chains are held only by weak pulls, so when warm they slide past each other: a piece of raw rubber held stretched slowly creeps longer and stays that way.

The American inventor Charles Goodyear spent years mixing rubber with one substance after another. In 1839 he heated rubber with sulfur, and it came out firm, springy and dry, and stayed that way in heat and cold. Thomas Hancock patented the process in England in 1843, and Goodyear in the United States in 1844. Hancock's friend William Brockedon named it vulcanisation, after Vulcan, the Roman god of fire.

What the sulfur does

When rubber is heated with sulfur, the sulfur's rings of eight atoms break open. Short chains of sulfur atoms bond to carbons next to the double bonds, joining one rubber chain to the next: sulfur bridges. They are the cross-⁠links you met in thermosetting plastics. Nearly all the double bonds stay in the chains, still cis.

A rubber band needs only a few bridges, roughly one for every hundred links of a chain. Between the bridges the chains still coil and uncoil freely. When the band is stretched, the bridges stop the chains from sliding past each other, and when it is let go the chains coil up again, each held to its old place by the bridges.