Processing

Rubber processing consists of four basic steps: (1) mastication, when the elastomer is sheared and the molecules are broken down to give easier flow, (2) mixing, usually carried out immediately after mastication, when additives are incorporated, (3) shaping of the viscous mass, for example, by extrusion or molding, and (4) curing, when the polymer molecules become interlinked and the shape is fixed.

Mastication

The Banbury mixer, used for mixing polymers and additives in the manufacture of plastic and rubber.

Mastication and softening are usually carried out in batches. The operation is done either in large enclosed mixing machines or on rubber mills. The preeminent example of an enclosed machine is the Banbury (registered trademark) mixer, consisting of heavy steel counterrotating paddles in an hourglass-shaped chamber, holding up to one-half ton of rubber. Rubber mills have two large horizontally opposed, closely spaced steel cylinders, up to 3 metres (10 feet) long, that are rotated slowly in opposite directions and at somewhat different speeds. Rubber is sheared and softened in the gap between the paddles and wall of the Banbury mixer and in the gap between the two cylinders in the roll mill.

Mixing

Mixing is carried out on machines similar to those used in mastication, sometimes immediately after softening. Reactive materials, fillers, oils, and protective chemicals of various kinds, as described above, are incorporated into the base elastomer by a combined shearing and mixing action. An enclosed Banbury-type mixer can produce up to one-half ton of mixed compound in a few minutes. The compound is then sheeted out, coated with a release soap to prevent sticking, and stored until use on steel pallets that can hold up to one ton of rubber.

Shaping

Shaping of the mixture into the desired form takes place in several ways. Extruders are used to produce long continuous products such as tubing, tire treads, and wire coverings. They are also used to produce various profiles that can later be cut to length. Multiroll calenders are used to make wide sheeting. In transfer and injection molds, the rubber mix is forced through channels into a mold chamber of the required shape, where it is cured under pressure. Tires are made of several components: bead wire, sidewall compound, inner liner, cord plies, belt package, and tread; these are brought together and assembled as a complete tire before being transferred to the curing press.

Curing

Curing is carried out in pressurized steel molds, which are heated by steam or electricity to temperatures at which the interlinking reaction takes place. Typical cure conditions are several minutes at a temperature of 160 °C (320 °F). Because heat penetrates rubber slowly, thick articles must be allowed longer curing times, up to several hours, at lower temperatures. Pressures of 1 megapascal (145 pounds per square inch) or more are normally imposed in order to maintain the desired shape and to force trapped air to dissolve in the compound. Other methods of curing the rubber mix after it has been shaped include steam heating in autoclaves, microwave irradiation, and passage through a heated bath of molten metal salts or a fluidized bed. In these cases curing is carried out at near-atmospheric pressure.

Rubber is an incredibly common and versatile material, used for making many items such as elastic bands, footwear, swimming caps and hoses. Indeed, half of all rubber produced goes towards making vehicle tyres. As such a vital material, how is rubber made and where does it come from?

© Panya Studio / Adobe Stock

Origins of rubber

Humans have been exploiting the durable and elastic nature of rubber for making products for over 1,000 years. Although early forms of rubber were made from natural sources, as demand increased for this material, scientists developed artificial or synthetic rubber in laboratories that mimicked the natural material. These days, most rubber produced is of the synthetic kind.

Natural rubber

Natural rubber is made by extracting a liquid sap, called latex, from certain types of tree. There are over 2,500 types of tree that produce this sap (including plants like dandelions), but the overwhelming majority of latex for rubber production stems from the Hevea brasiliensis tree, or the aptly named rubber tree. These trees are native to South America but are today commonly found in Southeast Asia.

Latex is gathered from the trees by making a cut in the bark and collecting the runny sap in cups. This process is called tapping. In order to prevent the sap from solidifying, ammonia is added. Acid is then added to the mix to extract the rubber, in a process called coagulation. This can take about 12 hours.

The mixture is then passed through rollers to remove excess water. Once this is complete, the layers of rubber are hung over racks in smokehouses or left to air dry. Several days later, they will then be folded into bales ready for processing.

Synthetic rubber

It was when natural rubber sources became scarce during World War I that German scientists developed artificial rubber. Although these early forms of rubber were inferior in quality to natural rubber, as research developed, synthetic rubber improved. Today, synthetic rubber is as strong and reliable as natural rubber.

Synthetic rubber differs from natural rubber in that it is made by linking polymer molecules together in a laboratory.

Processing rubber

Both natural and synthetic rubber need to undergo a series of processes to turn it into a usable product. These stages can be adapted slightly according to the intended use of the final product.

Firstly, chemicals are added to the rubber to make it stable. Without this, the rubber would get brittle if it got cold or become sticky during high temperatures. Commonly, a carbon black filler is added to the rubber mix, to improve its strength and durability.

The rubber is then carefully mixed and allowed to cool, before being shaped. It can be shaped by pushing it into rollers, called calendering, or by squeezing it through holes to make hollow tubes, known as extrusion.

Vulcanisation

In order to make rubber strong and durable, it finally goes through a heat-treatment phase known as vulcanisation. This is where the rubber is cooked (often with sulphur) to create extra bonds or cross-links between the molecules of the rubber, so they don’t easily fall apart. Charles Goodyear accidentally discovered this process, when he dropped some rubber onto a hot stove and noticed how the heat made the rubber harder and more durable.

After vulcanisation, any imperfections are removed from the rubber and it is then shaped or moulded into the final product.

As one of the most important inventions ever, rubber continues to enjoy versatile use today.

If you require a wide variety of high-quality rubber products, ranging from matting to seals and extrusions, look no further than Coruba.