How Elastic Works

Elastic is so ubiquitous today that we barely give it a second thought. Like paper clips and zippers, we simply expect it to work without ever wondering what it is, how it's made or what people did before it existed. Take the elastic waistband. In fact, fetch a pair of underwear (preferably clean) from your bedroom and give them a good once-over. You'll notice the familiar stretch of the band followed by the satisfying springing action as it returns to its original shape. It's like a rubber band, but not. When you put your hands on a rubber band, you touch, well, raw rubber. When you do the same with an elastic waistband, you touch fabric.

Believe it or not, the briefs and boxers so common today, equipped with elastic waistbands, weren't invented until the 1930s and 1940s. Before then, people had to find others mechanisms to hold their undergarments in place.

What took so long? Some of it was a sort of fashion inertia -- if it ain't broke, don't fix it -- but some of it was industrial necessity. Textile manufacturers either had to adapt their operations to produce elastic or find partners that could supply it economically. Either way, making elastic looked no different than making other woven fabrics. It required a loom, which was a machine that allowed lengthwise threads known as the warp to be interlaced with widthwise threads known as the weft. In normal woven fabric, those threads would consist of yarn derived from natural fibers, such as cotton or wool. But in elastic, strands of yarn were laced together with strands of natural or synthetic rubber.

Today, automated looms handle the weaving process, though the results are the same: a stretchy fabric that can be incorporated into an array of garments. So far, we've focused on the elastic waistbands found in boxers and briefs because they make a convenient example. But elastic finds its way into everything from bras and belts to suspenders and flex-waist trousers. Even the ever-handy shock cord, or bungee, begins its life in a textile manufacturing plant.

Cut into any of these stretchable items, and you'll find one common element: fine rubber threads or thick rubber bands just like the ones you use in your office or kitchen. Interestingly, rubber bands are not ancient inventions. Like the waistbands that contain them, rubber bands are a snappy, modern success story.

In the late 1700s and early 1800s, the world had grown quite small. Ships of all sizes carried sailors, pirates and explorers to exotic ports on every continent. Along the way, they saw strange foods, minerals and other natural materials known to the indigenous people of the lands they visited. One of the strangest materials they came across, in Central and South America, was caoutchouc, the French spelling of an Indian term for "weeping wood." The wood in question referred to trees and shrubs in the Hevea genus. When incisions were made in the bark of Hevea trees, they oozed a milky substance known as latex. South American Indians discovered that latex, left to evaporate, produced a pliable, flexible material that could be shaped into balls, applied to cloaks and fashioned into shoes and bottles.

Sailors returning to Europe from Central and South America brought some of the raw material, as well as items made from it. The famed British chemist Joseph Priestley experimented with the stuff in the late 1700s and remarked how it rubbed away pencil markings with great effectiveness. By the time Priestley died in 1804, his so-called "rubber" was in high demand all over the world. Venture capitalists, inventors and get-rich-quick schemers scrambled to transform the stretchy, waterproof material into useful goods and garments. Unfortunately, rubber was stable across a narrow range of temperatures.

Two men, working on both sides of the Atlantic, struggled to make a stable form of rubber. One was Charles Goodyear, a sickly merchant who carried out his experiments in Philadelphia, New York City and Woburn, Mass. The other was Thomas Hancock, an English inventor who had partnered with Charles Macintosh to manufacture waterproof cloaks. Hancock had also devised a way to create simple elastic threads by cutting slices from rubber bottles and raw lumps of rubber. Dismayed by how much waste this process generated, he went on to design and develop, in 1820, the masticator -- a machine that could chew and churn rubber scraps until they became melded together into a single sheet of useful material. And yet elucidating the chemical steps necessary to stabilize rubber at extreme temperatures eluded even the resourceful Hancock.

Goodyear hit pay dirt first, in 1839. By slow-cooking latex with sulfur, he finally discovered a process to transform rubber into a durable material with nearly limitless applications. He sent some of the new rubber to his brother-in-law, a textile manufacturer, who immediately saw the potential, incorporating rubber into men's shirts to create the shirred, or ruffled, effect that was popular at the time. Goodyear also sent samples to British rubber companies. Eventually, a few pieces ended up in the hands of Thomas Hancock, who reverse-engineered Goodyear's sulfur-based manufacturing technique. In 1843, Hancock filed for a patent for the process, which he now called vulcanization, after Vulcan, the Roman god of fire.

Now, finally, the rubber boom could begin in earnest. New companies emerged and rubber products -- from shoes to sheets -- flooded the market. In 1845, Stephen Perry and Thomas Barnabas Daft of London invented the modern rubber band by slicing narrow rings from a vulcanized rubber tube. Today, manufacturing rubber bands happens in much the same way. First, workers create rubber by mixing latex with a range of chemicals depending on the desired elastic qualities. Then they extrude the raw rubber compound so it forms a long, hollow tube. They slip this sleeve of rubber over a round pipe known as a mandrel and expose the material to high heat and pressure. This is the vulcanization process, which cures the rubber and stabilizes it indefinitely. Finally, they cut narrow bands of rubber from the end of the tube, wash and dry them, then package them for shipment.

Rubber threads are made in the same way, except they're cut from sheets of rubber instead of tubes. These slivers of natural rubber make their way to textile manufacturers, who weave the stretchy threads, with natural-fiber yarns, into elastic products. Some manufacturers also use threads from synthetic elastic materials.

The transformation of latex into usable rubber may be one of the greatest accomplishments of science. But on the heels of that breakthrough came another -- the development of synthetic, or man-made, rubber from petroleum and other minerals. Much of this work occurred during and immediately after World War II, when wartime shortages of natural rubber prompted governments to invest in the appropriate research and development.

All of this R&D led to three important synthetics: butadiene rubber, styrene-butadiene rubber and ethylene-propylene monomer. All of these materials were soft, highly flexible and elastic and could be used, at least theoretically, as direct replacements in products normally made with natural rubber. And yet they all lacked the same flex resistance -- the ability to withstand numerous flexing cycles without damage or deterioration -- as their naturally derived counterpart. While these synthetics were perfectly suited for automobile tires and other industrial applications, they were less popular with textile manufacturers, who still preferred fibers cut from natural rubber.

All of that changed in 1959, when two DuPont chemists introduced a synthetic fiber made from polyurethane, a synthetic resin in which the polymer units are linked by urethane groups. Most people know polyurethane as a major ingredient of paints, varnishes, adhesives and foams. But when the polymer is diluted and then forced through a metal plate containing tiny pores, it forms strands of liquid polymer, which can then be heated and dried to form solid fibers. The DuPont chemists called these fibers spandex, and the company marketed its new product as Lycra, targeting the makers of "ladies foundation garments."

Spandex has some useful qualities for the textiles industry. The fibers, normally dull white, readily accept dyes. They absorb little moisture and remain stable when machine washed and dried at moderate temperatures. But their most attractive feature is extreme elasticity. Spandex fibers can be stretched hundreds of times beyond their original length without breaking and quickly return to their original length. That makes them ideal for an array of garments, in which they are normally accompanied by other synthetic fibers, such as nylon, or natural fibers, such as cotton. For example, a typical Spanx body shaper might contain 80 percent nylon, 18 percent spandex and 2 percent cotton.

When it was first introduced, spandex found its way into the usual garment suspects -- bras, jockstraps and workout gear. Today, spandex might be considered the modern elastic, a key structural material found in 80 percent of all clothing bought by Americans [source: Penaloza]. Manufacturers weave it into everything, from underwear to outerwear, including innovative designs like PajamaJeans, which combine spandex with cotton denim to create form-fitting, curve-hugging pants.

The properties of spandex we love so much are the direct result of some interesting science. Up next, we'll go from the catwalk to the chemistry lab.

We've learned that elastic stretches and returns to its original shape because it contains fibers of rubber or rubber-like material. But why does rubber behave that way in the first place? Scientists now know that rubber belongs to a group of compounds known as elastomers, a mash-up of "elastic" and "polymers." Polymer molecules look like long chains of repeating units. These units, or monomers, are identical along the entire length of the molecule and stay connected by strong covalent bonds. In most polymers, carbon atoms form the backbone of the chain.

Here's where it gets interesting. Many polymers contain big, bulky monomers, which means the individual building blocks of the molecule crowd together. This makes the polymer stiff and inflexible at room temperature. Other polymers have subunits that fit together so well that they have a crystalline arrangement at normal temperatures. These kinds of polymers give us plastics and resins. Then there are elastomers, which have an altogether different arrangement. Under normal conditions, the long molecules of an elastomer are irregularly coiled like a slithering mass of snakes. When a force is applied to these molecules, they straighten out in the direction in which they're being pulled. Imagine the mass of snakes suddenly aligning and stretching out to their full length, from end to end. As soon as the force is released, the molecules spontaneously return to their normal, coiled-up arrangement.

The dividing line between plastic polymers and elastic polymers is a temperature known as the glass transition temperature, or T_g. Extremely flexible molecules have glass transition temperatures in the range of negative 195 degrees Fahrenheit (negative 125 degrees Celsius). Rigid or crystalline molecules can have glass transition temperatures as high as 419 degrees Fahrenheit (215 degrees Celsius). For example, T_g for polystyrene (used in Styrofoam) and polymethyl methacrylate (used in Lucite) is 212 degrees Fahrenheit (100 degrees Celsius).

Natural rubber is a polymer known as polyisoprene. It's built from repeating units of isoprene, a hydrocarbon consisting of five carbon atoms and eight hydrogen atoms. In its long-chain form, polyisoprene can take one of four three-dimensional shapes, which chemists refer to as isomers. Natural rubber consists almost entirely of one of these isomers, cis-1, 4 polyisoprene.

As we've already noted, natural rubber is stable across a fairly narrow range of temperatures. As the isoprene molecule begins to cool, it crystallizes. As it heats up, it starts to lose its elasticity. And if it reacts with oxygen and ozone in the atmosphere, the molecule starts to fall apart at the carbon-to-carbon double bonds. During vulcanization, polyisoprene is heated with sulfur, which causes adjacent polymer chains to link together via sulfur atoms, forming a loose molecular network that overcomes all of the disadvantages of untreated rubber.

Not that you need to know any of this to enjoy your elastic-infused clothing and accessories. Then again, it's kind of cool to think that the elastic in your waistband might have hailed from the jungles of South America or that your Spanx body shaper owes its existence to two pocket-protector-toting, polymer-slinging chemists from DuPont.