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: Half of the world's rubber is used to make vehicle tires—and at least half of them
are wasted in landfills, burned in incinerators, or otherwise dumped.
When people talk about "rubber", they don't usually specify what
kind. There are many different kinds of rubber, but they all fall into two broad types: natural rubber (latex—grown from plants) and synthetic rubber (made artificially in a chemical plant or laboratory). Commercially, the most important synthetic rubbers are styrene butadiene (SBR), polyacrylics, and polyvinyl acetate (PVA); other kinds include polyvinyl chloride (PVC), polychloroprene (better known as neoprene), and various types of polyurethane. Although natural rubber and synthetic rubbers are similar in some ways, they're made by entirely different processes and chemically quite different.
Natural rubber
Photo: Rubber bands are a very familiar everyday use of latex rubber.
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.
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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?
Photo: Traditional rubber tapping using a machete, photographed in the 1920s.
The latex drips down the cuts into the can on the ground. Photo by Bain News Service courtesy of US Library of Congress Prints & Photographs Division
.
It takes several quite distinct steps to make a product out of
natural 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)
in an autoclave (a kind of industrial pressure cooker).
Photo: Vulcanized rubber is heated in a giant sealed "cooker" like this one, used
for making earthmover tires, pictured at Firestone Tire Company in 1942. At that time, it was
the biggest rubber vulcanizer in the world, standing some 2.5 stories high when opened wide.
I've colored the people in the pictures red to give you an idea of the scale.
Photos by Alfred T. Palmer 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, Africa established in the 1920s and 1930s by the
Firestone tire company.
Charts: Left: Where does rubber come from? Almost three quarters of the world's rubber is produced in Asia, with the rest split mostly between Europe (including Russia) and the Americas. Almost all of the rubber produced in Africa (which, here, includes the Middle East) is natural, whereas most American and all European-produced rubber is synthetic. Asia produces roughly 60% natural and 40% synthetic rubber. Right: Overall, the world now produces more synthetic than natural rubber. Both charts drawn using the latest available data from the International Rubber Study Group, 2020.
How does vulcanization make rubber stronger?
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.
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What do we use rubber for?
Photo: Three everyday uses of rubber. Top: A latex pencil eraser; Middle: the tough vulcanized rubber drive belt from a vacuum cleaner; Bottom: the waterproof rubber gasket that seals a washing machine door tight.
The physical and chemical properties of a material
dictate what we use it for. Even if you know absolutely nothing about
the real-world uses of rubber, you can probably make some very good
guesses. For example, everyone knows rubber is strong, stretchy,
flexible (elastic), durable, and waterproof, so it's no surprise to
find it used in things like waterproof clothes and wellington boots,
sticking plasters, and adhesives.
The most important use of rubber is in vehicle tires;
about half of all the world's rubber ends up wrapped
around the wheels of cars, bicycles,
and trucks! You'll find rubber in the hard, black vulcanized
outsides of tires and (where they have them) in their inner tubes and
liners. The inner parts of tires are usually made from
a slightly different, very flexible butyl rubber, which is
highly impermeable to gases (traps them very effectively), so tires
(generally) stay inflated for long periods of time.
Photo: Swimming caps like this are made from soft and stretchy latex rubber.
The fact that rubber can be made either soft or
hard greatly increases the range of things we can use it for. Soft
and stretchy latex is used in all kinds of everyday things, from pencil erasers,
birthday balloons, and condoms to protective gloves, adhesives (such as sticky white PVA), and
paints. Harder rubbers are needed for tougher applications like
roofing membranes, waterproof butyl liners in garden ponds, and
those rigid inflatable boats (RIBs) used by scuba divers. Because rubber is
strong, flexible, and a very poor conductor of heat and electricity,
it's often used as a strong, thin, jacketing material for electrical
cables, fiber-optic cables, and heat pipes. But the range of
applications is truly vast: you'll find it in everything from
artificial hearts (in the rubber diaphragms that pump blood) to the waterproof gaskets that seal the doors on
washing machines!
Neoprene (polychloroprene) is best known as the
heat-insulating, outer covering of
wetsuits—but it has far more
applications than most people are aware of. Medical supports of
various kind use it because, tightly fitted, it compresses and warms
injured bits of your body, promoting faster healing. Since it's
flexible and waterproof, it's also widely used as a building
material, for example, as a roof and floor sealant, and as a
spongy absorber of sound and vibration in door and window linings.
Although the world has a vast appetite for new
rubber, we also produce a huge quantity of rubber waste, especially
from discarded vehicle tires—and that's becoming an important raw
material in its own right. According to the Rubber Manufacturers
Association, the United States alone produced almost 270 million
waste rubber vehicle tires in 2011, which is about a third of
all the tires used worldwide. While some of these are retreaded and
others are ground up to make a low-grade aggregate that can be used
for the floors in things like children's playgrounds,
over half of them are wasted (either burned as a fuel or buried in landfills). Rubber
manufacturers have recently turned their attention to recycling tires
in all kinds of new ways, making everything from mouse mats and
sports bags to shoe soles and car components.
Photo: Half of all rubber is used in vehicle tires, and hundreds
of millions are wasted each year. I've made a very slight difference to the problem
by buying this recycled rubber mouse mat, made from an old car tire. It's
colored black because it's made from hard vulcanized rubber.
A brief history of rubber
1000CE: Indians living in Central and South America have learned
how to made waterproof clothes and shoes using latex from rubber trees.
They call rubber trees "cahuchu" (crying wood), which is why the French
still call rubber caoutchouc (pronounced
"cow-chew") today.
1731: During an expedition to South America, French explorer
Charles Marie de La Condamine (1701–74) sends back
samples of rubber to Europe, prompting intense scientific interest.
1770: The discoverer of oxygen, English scientist
Joseph Priestley (1733–1804), finds he can use
pieces of
rubber to erase the marks made by pencil on paper. In England, erasers
are still widely called "rubbers" today.
1791: Englishman Samuel Peal develops
a method of waterproofing cloth with a rubber solution.
1818: Scottish medical student James Syme
(1799–1870) uses rubber-coated cloth to make
raincoats.
1823: Scotsman Charles Macintosh
learns of Syme's discovery, refines it, and patents it, earning fame
and fortune as the inventor of the rubberized, waterproof coat.
Waterproof coats have been known as "Mackintoshes" (with a slight
variation of spelling) ever since.
1829: English chemist and physicist Michael Faraday
(1791–1867) analyzes samples of Hevea and works out that
the chemical formula for isoprene-type rubber is C5H8.
1839: American inventor Charles Goodyear
(1800–1860) accidentally discovers how to vulcanize rubber after
dropping a piece of the material (which has been treated with sulfur) onto a hot stove.
Photo: In 1839, 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.
1830s~1840s: Botanist Thomas Lobb discovers a rubbery substance called Gutta-percha
(Palaquium gutta) in Malaysia;
Dr William Montgomerie, a surgeon working in the same region, sends samples back to Britain in 1843.
According to a contemporary account by William Dalton,
it has "remarkable properties, vast utility, and application to scientific and ornamental purposes" in everything from "boots and shoes"
to "prevention of toothache."
1876: Intrepid English explorer Sir Henry
Wickham (1846–1928) smuggles thousands of seeds from the rubber
tree Hevea brasiliensis out of Brazil and
back to England. The
English grow the seeds at Kew Gardens just outside London and export
them to various Asian countries,
establishing the giant plantations that now supply much of the world's
rubber.
1877: US rubber manufacturer Chapman Mitchell
develops the first commercial process for recycling rubber from scratch.
1882: John Boyd Dunlop (1840–1921)
invents
the pneumatic (air-filled) rubber tire. The development of
gasoline-powered cars with rubber tires leads to a huge increase in the
need for rubber.
1883: US chemist George Oenslager (1873–1956) develops a much faster way of vulcanizing rubber using
chemicals called organic (carbon-based) accelerators.
1906–12: Bayer, a German chemical company, develops methyl rubber (a polymer of methylisoprene).
It becomes critically important to Germany during World War I when supplies of natural rubber are cut off,
but falls out of fashion when better alternatives are eventually developed.
1910: English Chemist S.S. Pickles becomes the first person to propose (correctly) that rubber consists of
long chains of isoprene. Technically, Hevea has the chemical name cis-1,4-polyisoprene,
while Gutta-percha is a variation known as trans-1,4-polyisoprene.
1930: German chemical company IG Farben develops a type of general-purpose, synthetic rubber
named Buna-S ("bu" from butadiene, "na" from the chemical symbol for sodium, and "S" for styrene).
Technically, it's a copolymer of butadiene (75 percent) and styrene (25 percent), which is why it's
now more generally known as styrene-butadiene or styrene-butadiene-rubber (SBR); it's also sold under
tradenames such as Goodyear's Neolite®. Today, styrene-butadiene remains by far the world's most important synthetic rubber.
1930: A team of US chemists at the DuPont company, led by
Wallace Carothers (1896–1937), develop a
revolutionary synthetic rubber called polychloroprene and sold as
neoprene. (Shortly afterward, the same group developed an even more
revolutionary material: nylon.)
1940s: Synthetic rubbers are produced in the United States for the first time by companies such as
Firestone, Goodyear, and Goodrich.
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Don't want to read our articles? Try listening instead
The Story of Rubber: Learn about the history and development of rubber with the Polymer Science Learning Center.
International Institute of Synthetic rubber Producers: Website includes some useful PDF summaries of synthetic rubbers, including polybutadiene rubber, butyl rubber, and different kinds of synthetic elastomers. Click on "Synthetic rubber" on the menu.
In Thailand, Rubber Price Plunge Has Political Cost by Thomas Fuller. The New York Times, August 29, 2013. It's easy to forget that commodities like rubber provide a living for many of the world's people. What happens to their lives as prices rise and fall?
Recipe for Green Tires: Plants, Not Petroleum by Steven Ashley. The New York Times, June 18, 2010. From recycling to new kinds of synthetic rubber, this article explores the various ways of reducing the environmental impact of vehicle tires.
Natural Rubber: Biology, Cultivation and Technology by M.R. Sethuraj (Editor) and Ninan T. Mathew. Elsevier, 1992. A detailed, very comprehensive guide to rubber as a crop, including related topics such as guayule.
For younger readers
Rubber versus Glass: I Wonder Why by Laurence Lowry. NSTA, 2014. A 36-page introduction to the contrasting properties of two everyday materials, for ages 5–8.
Rubber by Cassie Mayer. Heineman, 2008. A basic 24-page introduction probably best for ages 6–9.
Rubber by Chris Oxlade. Pearson Education, 2003. A 32-page introduction for ages 9–12.
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