Rubber

Rubber has been one of the most important materials derived from plants since the middle of the nineteenth century. In 1839, the American, Charles Goodyear, invented vulcanization, a process in which raw rubber is blended with sulfur to make it both strong and pliable over a range of temperatures. It is only in the last 40 years that synthetic rubbers—products of the petrochemical industry—have seriously challenged the natural material.

Today more than 40 per cent of the world’s natural rubber comes from Malaysia (most of the rest is grown in Indonesia and Thailand), from smallholdings and plantations growing the South American rubber tree Hevea brasiliensis. This plant grows well only within a “rubber belt” that extends about 700 miles (1,100 kilometers) on each side of the equator, and then only at altitudes below 1,000 feet (305 meters). The raw rubber takes the form of a milky latex, which is “tapped” from the trees by making a spiral cut in the bark of the trunk.

The production of natural rubber has increased nearly 95 times since the beginning of the century, largely to meet the demand created for motor vehicle tires; the motor industry uses almost 70 per cent of the 4.75 million short tons (4.3 million metric tons) produced each year. But synthetic rubbers take a larger share of the market—60 per cent of the 10 million short tons (9.1 million metric tons) produced annually goes into making tires—although the soaring oil prices of the 1970’s helped to reduce the threat to natural rubber.

Rubber latex drips into collecting cups from cuts in the bark of rubber trees (Hevea brasiliensis. Almost two-thirds of the world’s production of natural rubber comes from trees grown on smallholdings in Malaysia, Indonesia, and Thailand.

Composition and properties of rubber

Rubber is a natural polymer, consisting of small molecules of the hydrocarbon isoprene linked together to form long coiled chains. When rubber is stretched and then released, the long-chain molecules bounce back to their original length, giving rubber its elastic properties.

Natural rubber is resistant to alkalis and weak acids but swells in many organic liquids, such as gasoline and lubricating oils. It dissolves in various volatile organic solvents, producing a rubber solution that finds application as an adhesive. It is a good electrical insulator, although for this purpose rubber has been almost entirely superseded by synthetic plastics. The individual properties of processed rubbers are largely influenced by chemicals added during the manufacturing process.

The main steps in the processing of natural rubber into an industrially useful material are chemical: the addition of formic acid to coagulate the liquid latex and, at a later stage, heating with sulfur to bring about vulcanization.

Processing rubber

Liquid latex, from rubber trees, has to be solidified before it can be sold or further processed. Some solidifies naturally on the tree, but most is coagulated artificially. Any dirt is filtered out and formic (methanoic) acid is added, which causes the liquid latex to “curdle” and coagulate. The coagulum is granulated, dried, and shaped into bales.

About 10 per cent of natural rubber is used in the latex form, where its elasticity and pliability are important for such products as rubber gloves and catheters. The other 90 per cent must first undergo special processes according to what end-product is required.

Before further processing, rubber is masticated by rollers or rotors that break down the polymer molecules. The heat evolved softens the rubber and it becomes plastic. Strength and elasticity are restored by vulcanization with sulfur at 284°-356° F. (140°-180° C). This causes the material to become thermosetting and thus resistant to wide temperature changes. Vulcanized rubber can be stretched up to eight times its original length and yet remain perfectly elastic—that is, it returns to its original length when released.

During vulcanization, the sulfur atoms form cross-linkages between the chains of rubber molecules to produce a more rigid three-dimensional structure. But too much sulfur and, therefore, too many cross-linkages results in a rigid, nonelastic material. Various other substances are also included. Antioxidants are added to prevent the rubber from perishing; oils to soften it and assist processing; insoluble wax to reduce cracking; pigments to supply color; and fillers that, at a slight cost to strength, increase wear resistance. Rubber that is used for car tires, for example, contains 30 per cent of carbon black as a filler. Other products—erasers and floor tiles, for instance—may have white powder fillers such as chalk, silica or zinc oxide.

An inflatable rubber boat carrying 11 men (above) shoots the rapids on a fastflowing river, testifying to the strength and resilience of rubber. The scientist (below) uses polarized light to study the stress patterns in a motor vehicle tire.
More than two-thirds of the natural rubber produced goes into making tires for the automotive industry.

Shaping rubber

The usual method of making dry rubber goods is by molding. A simple eraser, for example, is heated under pressure in a mold so that it emerges both vulcanized and shaped. For more precision and higher production rates, injection molding is used. In this process a softened rubber mixture is injected at high speed through small holes into a mold or set of molds. Fleat and pressure are again applied to vulcanize it. Surgical rubber goods, such as thin latex gloves, are produced by dipping shaped formers into precompounded liquid latex, and then drying and vulcanizing the products.

Rubber hoses, tubes, and hollow casings are formed by extrusion. The rubber mixture is forced through a die, rather like squeezing toothpaste from a tube, and vulcanized using hot air. Rubber sheeting (used for weather balloons), fabric-reinforced sheeting (used, for ex ample, on hovercraft skirts), and proofed fabric (used for raincoats) are all formed on a calendar. This is a machine with rollers that squeeze the rubber into a sheet. To produce a waterproof material, fabric is fed through the lower rollers so that the rubber sheet is pressed into it.