"I'm rubber, and you're glue. Whatever you say bounces off me and sticks to you." Although you probably remember this saying from when you were a smart-alecky kid, it's an apt description for the substance we know as rubber.
The peoples of Mesoamerica, an ancient region of Central America and Mexico, are thought to be the first to have used this elastic chemical compound. They used rubber to make balls for a game that Columbus, and later the Spanish conquistadors, watched them play. To these peoples, rubber was called "caoutchouc." The English chemist Joseph Priestley was the one who later came up with the term "rubber" in 1770.
Rubber is a specific type of polymer called an elastomer: a large molecule that can be stretched to at least twice its original length and returned to its original shape. Early forms of rubber had many gluelike properties, especially in hot weather. In cold temperatures, rubber became hard and brittle. It was only after an accidental discovery by Charles Goodyear in 1839 that modern rubber became possible.
Since that time, rubber has become an important natural polymer in society. We make rubber from rubber trees (natural latex) and from oil (synthetic rubber). We use both types of rubber in many products. Like the Mesoamericans (Aztecs and Mayans) before them, athletes and children today play with rubber balls. Of course, the most common use for rubber is in automotive tires. But pencil erasers, shoes, gloves, dental dams and condoms contain the ubiquitous substance, too. In many products, rubber is added as a protective coating for either weatherproofing or shockproofing.
In this article, we'll look at the chemistry of this stretchy substance, where and how it's produced, and what Charles Goodyear's remarkable discovery was. We'll also look at the different types of rubber, some of your favorite products made from it and the industry responsible for producing it.
Up next: a journey to Central America.
Tapping Trees for Natural Rubber
Explorers and colonists brought samples of these trees when they headed back to Europe. Eventually, seeds from these trees were transported to rubber plantations in other tropical climates during the era of European colonialism.
Currently, most natural rubber comes from Latin American-derived trees transplanted to Southeast Asia (Thailand, Indonesia, Malaysia), as well as India, Sri Lanka and Africa. In these areas, you can find other rubber-producing trees including:
- Ficus elastica: found in Java and Malaysia. This species is also a common tropical houseplant.
- Funtumia elastica: grows in West Africa
- Landolphia owariensis located in the Congo basin
Of all of these trees, the best rubber-producing tree is H. braziliensis.
It takes about six years for a rubber tree to grow to a point where it's economical to harvest the sap, which is called latex. Here's how you tap one: The collector makes a thin, diagonal cut to remove a sliver of bark. The milky-white latex fluid runs out of the bark, much as blood would run out of a small superficial wound on your skin. The fluid runs down the cut and is collected in a bucket. After about six hours, the fluid stops flowing. In that six-hour period, a tree can usually fill a gallon bucket. The tree can be tapped again with another fresh cut, usually the next day.
The Mesoamericans would dry the collected rubber latex and make balls and other things, like shoes. They would dip their feet in the latex and allow it to dry. After several dips and dryings, they could peel a shoe from their feet. Next, they smoked their new rubber shoes to harden them. The Mesoamericans also waterproofed fabrics by coating them with latex and allowing it to dry. This process was used to make rubber items until around the 1800s.
Columbus brought back rubber balls with him upon returning from his second voyage to the New World, and in the early 1700s, rubber samples and trees were brought back to Europe. At that time, rubber was still a novelty. Rubber made in the Mesoamerican way resembled a pencil eraser. It was soft and pliable. In 1770, the chemist Joseph Priestley was the first to use rubber to erase lead marks. He coined the word "rubber" because he could remove the lead marks by rubbing the material on them.
While it was useful for waterproofing fabrics and making homemade shoes, rubber had its problems. You can see these problems for yourself with a simple rubber pencil eraser. Take that eraser and place it under intense heat for several minutes. What do you see? The eraser should get very soft and sticky. Next, do the opposite -- place the eraser on ice or in a freezer for several minutes. What do you see? The eraser should get hard and brittle. The same thing happened to early rubber. Imagine what it would be like to walk around in your rubber shoes on a hot or cold day back then. The shoes wouldn't wear well. Likewise, your rubberized clothing might stick to your chair while you were sitting, especially on a warm day.
Keep reading to learn what makes rubber so intrinsically stretchy.
What makes rubber so elastic? Like plastic, rubber is a polymer, which is a chain of repeating units called monomers. In rubber, the monomer is a carbon compound called isoprene that has two carbon-carbon double bonds. The latex fluid that seeps from rubber trees has many isoprene molecules. As the latex dries, the isoprene molecules crowd together and one isoprene molecule attacks a carbon-carbon double bond of a neighboring molecule. One of the double bonds breaks, and the electrons rearrange to form a bond between the two isoprene molecules.
The process continues until you have a long strands of many isoprene molecules linked like a chain. These long strands are called polyisoprene polymer. Each polyisoprene molecule contains thousands of isoprene monomers. As the drying continues, the polyisoprene strands stick together by forming electrostatic bonds, much like the attraction between opposite poles of two bar magnets. The attraction between these strands holds the rubber fibers together and allows them to stretch and to recover.
However, temperature changes can affect the electrostatic interactions between the polyisoprene strands in latex rubber. Hot temperatures reduce the interactions and make the rubber more fluid (sticky). Colder temperatures increase the interactions and make the rubber more solid (hard, brittle).
In the early 1800s, several scientists and inventors set out to make rubber more durable. One famous inventor, Charles Goodyear, reasoned that you could reduce rubber's stickiness by mixing it with various dry powders. He experimented by combining talcum and other powders with rubber. In 1838, Goodyear met Nathaniel Hayward, who had made progress in treating rubber sheets with a solution of sulfur and turpentine and then drying them in the sun. Hayward's sun-dried rubber was harder and more durable, so he patented the process, which he called solarization.
Goodyear purchased the patent rights to solarization and began experimenting with sulfur compounds. By trial and error, the inventor mixed latex rubber with sulfur and lead oxide. Legend has it that some of the mixture fell onto a hot stove, and the resulting rubber was hard, flexible and durable. Goodyear's accidental process eventually became known as vulcanization. He also found that changing the amount of sulfur changed the rubber's characteristics. The more sulfur used, the harder the rubber became. So what happens when rubber is vulcanized?
When polyisoprene strands are heated with sulfur and lead oxide, the sulfur atoms attack the double bonds in the polyisoprene strands and bind to the carbon atoms. Sulfur atoms also can form bonds among themselves (disulfide bonds) and cross-link adjacent polyisoprene strands to form a netlike structure in the rubber.
This cross-linking strengthens the polyisoprene to make it harder, flexible and more durable. As Goodyear found, the more sulfur used, the more cross-links can form, and the harder the rubber gets. Goodyear's vulcanization process involved combining latex rubber, sulfur and lead oxide in high-pressure steam for up to 6 hours to achieve the best results.
One-upping Mother Nature: Synthetic Rubber
The rubber industry really started booming with the invention of the automobile. All those tires created and still create a huge market for vulcanized rubber. In the early 20th century, most tires were made from vulcanized latex rubber, which was derived from trees. The United States was a leading producer of automobiles and a big consumer of world rubber, which was controlled by British plantations throughout Asia.
It was only a matter of time before scientists asked if rubber could be made artificially. As early as 1860, they had already worked out the chemistry of rubber and its vulcanization process. Chemists had heated rubber to break it apart and found that it produced isoprene, oil and tar. They could make isoprene from oil and then combine isoprene artificially to make rubber. It seemed the stuff of pencil erasers and automotive tires could be made from petroleum products.
You've probably heard of a few of these synthetic rubbers. For example, neoprene is a soft, foamy, insulating rubber that's often used in wet suits. Scuba divers and surfers have DuPont chemist Wallace Carothers, who turned a polymer of chloroprene into polychloroprene in 1930, to thank for keeping them warm during their ocean adventures. Incidentally, Carothers later invented another popular polymer called nylon.
Here's another synthetic rubber we bet you've heard of: silicone. In 1945, chemists at Dow Corning developed a synthetic rubber that relied on silicone-based polymers instead of carbon-based ones. They created a light, flexible and chemically inert material that's often used in the medical device industry and that may even sit in the body in the form of breast implants.
Latex is the last synthetic rubber we'll talk about. You probably remember that latex refers to the rubber tree sap. It's also the name of a thinner, stronger synthetic rubber that retains the natural milky color of that sap, at least initially. In 1921, chemist Peter Schidrowitz discovered that you could vulcanize latex by adding polysulfides to it and heating it at lower temperatures than traditional vulcanization over a long time. Furthermore, because this type of vulcanization occurred at lower temperatures, you could add dyes to the latex to produce colors. This is the type of latex that you find in latex gloves, dental dams and condoms.
Learn how much money there is to be made in latex gloves and the like, next.
Processing and Profiting from Rubber
How much rubber is bouncing around out there? According to the International Rubber Study Group, 9.7 million tons of natural rubber was produced in 2007, most of which came from Asia. At approximately $2,321 per ton, the natural rubber market involved about $22.5 billion in 2007 [source: International Rubber Study Group]. But before that natural rubber is transformed into stuff like hoses, rubber bands and little yellow ducks, it has to be processed.
Processing natural rubber consists of the following steps:
- Companies begin by obtaining the latex fluid, which means tapping the sap from the rubber trees, filtering the latex and then packaging it in drums for export or processing.
- They make smoked sheets of latex rubber. They clump the latex by adding acid, roll the clumped fluid into sheets in a mill to remove water, and then they dry, smoke and export the sheets.
- The latex is chemically treated and heated at low temperatures to prevulcanize it. Prevulcanized latex is easier to transport and can be converted to regular rubber later by gentle heating.
The profit and process picture is a little different for synthetic rubber. In 2007, about 13.6 million tons of synthetic rubber was produced, mostly in the United States, Europe and Asia [source: International Rubber Study Group]. Priced at about $2,012 per ton, there was $26.2 billion to be made in the synthetic rubber market in 2007.
Synthetic rubber polymers are made from petroleum-based chemicals, clumped and dried for transport. Once at a manufacturing plant, the synthetic rubber polymers are mixed, ingredients may be added and the rubber is rolled into sheets. The sheets can be cut into strips for subsequent molding and processing. There are three basic processing techniques:
- Extrusion: Rubber polymers are heated and mechanically mixed in a long chamber, forced through a small opening and vulcanized or cured. This method is used to make large strands for compression molding.
- Injection molding: The rubber strips are heated and mechanically mixed in a chamber, forced under high pressure into a mold. The rubber is steam vulcanized in the mold and then cooled. Once cooled, the rubber product is released from the mold.
- Compression molding: The rubber strips are compressed around a mold under pressure and vulcanized to form to the mold. The cooled product is then removed from the mold.
Rubber may not be the first thing you think of when you think reduce, reuse, recycle, but some products can be recycled. For example, tires can be shredded and heated in an anaerobic environment (pyrolysis) to break down the rubber and to reclaim the oil as oil by-products such as benzene. In other recycling methods, shredded rubber can be compression molded into different products.
Don't leave the rubber room just yet. We have more links than you can shoot a rubber band at next.
HowStuffWorks looks at brazing to see how it works and why someone might choose the technique over welding. Read more about brazing at HowStuffWorks.
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