Most of the rubber made today ends up rolling along roads. About half of it comes from trees, and the rest is made from oil.
In 1876 Henry Wickham shipped about 70,000 rubber tree seeds from Brazil to Kew Gardens in London, and the seedlings were sent on to Asia. Today most natural rubber grows on plantations in Southeast Asia, with Thailand the largest grower, and the world produces about 15 million tonnes of it a year.
Synthetic rubber
The most widely made synthetic rubber is SBR, styrene-butadiene rubber. It is mostly buta-1,3-diene, with phenylethene, the styrene of polystyrene, making up about a quarter of its weight. Buta-1,3-diene has the same C=C–C=C run as isoprene, so it joins the chain in the same way, and most of its links keep a double bond between their two middle carbons.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Chemists in Germany developed SBR around 1930. When Japan took the rubber plantations of Southeast Asia in 1942, the United States built a whole industry to make it, and within three years its factories made most of the country's rubber.
Other synthetic rubbers suit other jobs. Neoprene, first made in 1930 by Wallace Carothers's team at DuPont, is poly(chloroprene): its monomer is isoprene with the methyl group swapped for a chlorine atom. It stands up well to oil, sunlight and weather, and wetsuits are made of neoprene foam, whose tiny gas bubbles keep a diver warm. In the 1950s chemists found catalysts that make poly(isoprene) itself with nearly every double bond cis, very close to natural rubber.
What goes into a tyre
Natural rubber's chains are so regular that, stretched hard, some of them line up into crystals, which makes it very strong and hard to tear, and it warms up less as it flexes. Truck tyres carry heavy loads, so they use more of it. Car tyres use more SBR, which grips wet roads well.
| Tyre | Natural | Synthetic |
|---|---|---|
| car | 30 to 40% | 60 to 70% |
| truck | 60 to 80% | 20 to 40% |
About a quarter of a tyre's weight is carbon black, a fine soot made by burning oil with too little air. It makes the tyre black, strengthens the rubber and shields it from sunlight. Along with the rubber, a tyre holds steel wire, fabric, and the sulfur and other chemicals of vulcanisation.
Quick check
That's the end of lesson 31
In this lesson you found out:
- Why rubber's long cis chains coil, stretch and snap back
- How sulfur bridges between chains make rubber tough
- Why a stretched rubber band warms up
- What goes into tyres and synthetic rubber