By the late 1930s, the United States was consuming half of the world’s supply of natural rubber. The arrival of World War II led to a natural rubber shortage, which then sparked a shift to create a synthetic substitute. Currently, 70% of rubber used in manufacturing processes is synthetic — a descendant of this original material produced by the U.S. in World War II.
Although most rubber nowadays is synthetic, the U.S. still uses natural rubber; we import over 3 billion pounds of it per year. Rubber is used for a variety of products, such as hoses, gaskets, sealants, and of course, wheels and tires. Below, we’ll take a closer look at how natural resources are collected for rubber, how rubber is produced, and how it is reused and recycled.
Natural rubber begins with latex, which is found in a sap-like form in trees and plants. Rubber trees from South America and Southeast Asia provide much of the latex in natural rubber. A process called rubber tapping is used to harvest latex from rubber trees. A wide cut is made in a tree’s bark, allowing the latex to drip and be collected. After it’s collected, the latex is filtered and washed. Then, an acid is added so that the rubber coagulates, or thickens. Once it’s adequately coagulated, the rubber is dried, squeezed, and pressed into sheets for transport.
When rubber (either natural or synthetic) arrives at a plant, it’s ready to be processed. First, the rubber goes through compounding, which involves adding chemicals and additives based on the intended use for the rubber. For example, a filler made from soot called carbon black is added to improve the rubber’s strength. Carbon black also gives rubber products, like vehicle tires, a black color. Other fillers might include recycled rubber, plasticizers, coloring pigments, and more.
After chemicals and additives are introduced, they must be mixed into the rubber. This mixing phase of processing must balance the mixture of ingredients against premature vulcanization. Because rubber has high viscosity, it’s difficult to mix it with other chemicals without raising the temperature. But if the temperature is raised too high, the rubber can vulcanize too soon.
After mixing is successfully complete, the rubber cools and is ready for shaping. During shaping, rubber might be applied as a coat onto other materials, like conveyor belts or tires. Or, a mold is used to cast rubber into a shape like shoe soles or gaskets. If a mold is used, the rubber is vulcanized then. If not, vulcanization is the final step. In vulcanization, the rubber is heated and cured, which makes it more resistant to damage and temperature changes. Rubber would be much more brittle or sticky without these processes. After the rubber is vulcanized it’s ready to be distributed.
Although rubber is in high demand, the world produces a large quantity of rubber waste, especially from old vehicle tires. Recycling and reusing have helped reduce rubber waste. Rubber in old vehicle tires can be broken down and used for products like playground mulch, car components, mouse pads, and shoe soles.
Reusing old rubber to make rubber mulch, for example, begins by screening recyclable materials with equipment like the GK FINGER-SCREEN™ Primary Vibratory Screen. Once the rubber is separated from the rest of the material, the old vehicle tires are broken apart and cleaned, and smaller pieces are ground up into an even smaller fraction to create mulch. Once in mulch form, color is added, and the mulch is packaged and ready to distribute.
If the rubber is used for other products, it may be further shredded into smaller fibers and then reprocessed. Smaller rubber granules are easy to repurpose into floor mats, shoe soles, buckets, or small wheels. Material handling units like the High-Energy Transfer Conveyor are specifically designed to move this type of material and other parts of the tire like tire wire.
Of the more than 1 billion scrap tires in the United States since the 1990s, about 70 million tires remain. It’s a high number, but scrap tire piles in the U.S. have declined by over 93% in the past 20 years.
At General Kinematics, we’re committed to providing innovative equipment for rubber recycling and recycling of other materials around the world. Contact us today to learn more about our recycling solutions equipment or to discuss your unique equipment needs.