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Rubber

by Chris Woodford. Last updated: July 15, 2014.

Think of rubber and you probably think of elastic bands, car tires, or pencil erasers. But this super-stretchy material actually finds its way into tens of thousands of different products—everything from rubber stamps and waterproof shoes to surfing wetsuits, swimming caps, and dishwasher hoses. Rubber, which has been commonly used for over 1000 years, once came entirely from natural sources; now rubber products are just as likely to be made artificially in chemical plants. That's largely because we can't produce enough natural rubber to meet all our needs. And that, in turn, is because rubber is so fantastically useful. Let's take a closer look at one of the world's most amazing materials!

Photo: Raw rubber waiting to be made into tires at Red River Army Depot in Texarkana, Texas. Photo by Cherie Cullen courtesy of Defense Imagery.

What is rubber?

When people talk about "rubber", they don't usually specify what kind. Although natural rubber and synthetic rubber are similar in some ways, they're made by entirely different processes and chemically quite different.

Natural rubber

A tangle of multicolored latex rubber bands on the palm of a hand.

Guayule: a plant from which rubber can be made

Natural rubber is made from a runny, milky white liquid called latex that oozes from certain plants when you cut into them. (Common dandelions, for example, produce latex; if you snap off their stems, you can see the latex dripping out from them. In theory, there's no reason why we couldn't make rubber by growing dandelions, though we'd need an awful lot of them) Although there are something like 200 plants in the world that produce latex, over 99 percent of the world's natural rubber is made from the latex that comes from a tree species called Hevea brasiliensis, widely known as the rubber tree. This latex is about one third water and one third rubber particles held in a form known as a colloidal suspension. Natural rubber is a polymer of isoprene (also known as 2-methylbuta-1,3-diene) with the chemical formula (C5H8)n. To put it more simply, it's made of many thousands of basic C5H8 units (the monomer of isoprene) loosely joined to make long, tangled chains. These chains of molecules can be pulled apart and untangled fairly easily, but they spring straight back together if you release them—and that's what makes rubber elastic.

Photo: Left: Rubber bands are a very familiar everyday use of latex rubber. Right: Guayule: one of many plants from which rubber can be made. Photo by Peggy Greb courtesy of US Department of Agriculture/Agricultural Research Service (USDA/ARS).

Synthetic rubbers

Synthetic rubbers are made in chemical plants using petrochemicals as their starting point. One of the first (and still one of the best known) is neoprene (the brand name for polychloroprene), made by reacting together acetylene and hydrochloric acid. Emulsion styrene-butadiene rubber (E-SBR), another synthetic rubber, is widely used for making vehicle tires.

For the rest of this article, we'll concentrate mostly on natural rubber.

How is rubber made?

Charles Goodyear

It takes several quite distinct steps to make a product out of rubber. First, you have to gather your latex from the rubber trees using a traditional process called rubber tapping. That involves making a wide, V-shaped cut in the tree's bark. As the latex drips out, it's collected in a cup. The latex from many trees is then filtered, washed, and reacted with acid to make the particles of rubber coagulate (stick together). The rubber made this way is pressed into slabs or sheets and then dried, ready for the next stages of production.

By itself, unprocessed rubber is not all that useful. It tends to be brittle when cold and smelly and sticky when it warms up. Further processes are used to turn it into a much more versatile material. The first one is known as mastication (a word we typically use to describe how animals chew food). Masticating machines "chew up" raw rubber using mechanical rollers and presses to make it softer, easier to work, and more sticky. After the rubber has been masticated, extra chemical ingredients are mixed in to improve its properties (for example, to make it more hardwearing). Next, the rubber is squashed into shape by rollers (a process called calendering) or squeezed through specially shaped holes to make hollow tubes (a process known as extrusion). Finally, the rubber is vulcanized (cooked): sulfur is added and the rubber is heated to about 140°C (280°F).

Photo: In 1939, American inventor Charles Goodyear (1800–1860) developed the vulcanization (heat-treatment) process that makes rubber harder and more durable. He'd spent many years as a struggling inventor, trying desperately to turn rubber into a useful product, when he accidentally dropped some rubber on a hot stove and watched it "cook" itself into a much more useful form: the black, vulcanized material most of us know as rubber to this day. Despite developing one of the most useful materials of all time, Goodyear never made much money from his invention and died deeply in debt. Fortunately, his name lives on in the Goodyear tire company—and his superb contribution to materials technology has never been forgotten. Photo courtesy of US Library of Congress.

Where does rubber come from?

As its name suggests, the rubber tree Hevea brasiliensis originally came from Brazil, from where it was introduced to such countries of the Far East as Malaysia, Indonesia, Burma, Cambodia, China, and Vietnam. During World War II, supplies of natural rubber from these nations were cut off just when there was a huge demand from the military—and that accelerated the development of synthetic rubbers, notably in Germany and the United States. Today, most natural rubber still comes from the Far East, while Russia and its former republics, France, Germany, and the United States are among the world's leading producers of synthetic rubber. The world's largest single source of latex rubber is the Harbel Rubber Plantation near Monrovia in Liberia, established in the 1920s and 1930s by the Firestone tire company.

How does vulcanization make rubber stronger?

Artwork comparing ordinary rubber and sulfur cross links in vulcanized rubber

Rubber—the kind you get from a tree—starts off as white and runny latex. Even when it's set into a product, this latex-based, natural rubber is very squashy, pretty smelly, and not very useful. The kind of rubber you see in the world around you, in things like car and bicycle tires, is vulcanized: cooked with sulfur (and often other additives) to make it harder, stronger, and longer lasting.

So what's the difference between raw, latex rubber and cooked, vulcanized rubber? In its natural state, the molecules in rubber are long chains that are tangled up and only weakly linked together. It's relatively easy to pull them apart—and that's why latex rubber is so stretchy and elastic. When latex is vulcanized, the added sulfur atoms help to form extra bonds between the rubber molecules, which are known as cross-links. These work a bit like the trusses you see on a bridge, tying the molecules together and making them much harder to pull apart.

Artwork: Top: Natural, latex rubber is easy to pull apart because the long polymer molecules it contains (made from carbon and hydrogen atoms) are only weakly linked together. Bottom: When natural rubber is cooked with sulfur, the sulfur atoms form extra cross-links (shown here as yellow bars) "bolting" the molecules together and making them much harder to pull apart. This process is called vulcanization and it makes the strong, durable, black rubber we see on things like car tires.

What do we use rubber for?

A closeup of a car's front wheel showing the brake disk behind

Rubber is elastic and water-resistant, traps air (so it floats), and doesn't conduct electricity. In its unprocessed form, it's used in such things as adhesives, contraceptives, and latex balloons. Vulcanized rubber is harder, less elastic, and more durable and has many more uses, from vehicle tires to hose pipes and from artificial hearts to the waterproof gaskets that seal the doors on washing machines. Tires are still the biggest single use of rubber in the world. About half of all the world's rubber ends up wrapped around the wheels of cars, bicycles, and trucks!

Photo: Half of all rubber is used in vehicles tires. The rubber in this tire has been vulcanized to make it extremely hard-wearing.

A brief history of rubber

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Rubber. Retrieved from http://www.explainthatstuff.com/rubber.html. [Accessed (Insert date here)]

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