Some of life's greatest treasures are simple ones. Take erasers, for example. These small pieces of molded rubber are underappreciated but handy tools when it comes to a quick fix of something written in pencil or even in pen.
I had not given a thought to the chemistry behind erasers until a couple of years ago when my family and I stumbled across Claes Oldenburg and Coosje van Bruggen's giant typewriter eraser sculpture. The 14-foot-tall eraser wheel with attached brush, which sits on a patch of lawn in the National Gallery of Art's Sculpture Garden in Washington, D.C., is more than a reminder of the bygone days of typewriters.
||BIG WHEEL Oldenburg and van Bruggen's Typewriter Eraser, Scale X. NATIONAL GALLERY OF ART PHOTO
For Oldenburg, it was a reminder of the simplicity of youth growing up around his father's office desk. For me, there was something hidden to explore in the giant wheel. Although the sculpture is made from stainless steel and fiberglass, the symbolic rubber wheel was begging the question, "What is an eraser?"
Although there are felt-pad chalkboard and white board erasers, the essence of an eraser is a plain piece of rubber--"graphite grabbers," some people in the industry like to call them. Even so, there are many types of these erasers, including handheld flat rectangles, cylindrical plugs attached to a pencil, or caps that fit over the end of a pencil. There are also all sorts of colorful novelty erasers in various geometric shapes with holiday, animal, sports, and other motifs.
THE STORY of the chemistry behind erasers is really a historical tale about rubber. It begins with the development of the pencil. Graphite began to be used as a writing device by the 1560s, and the first crude pencils were fashioned shortly thereafter (C&EN, Oct. 15, 2001, page 35). At first, unwanted pencil marks were rubbed off with a ball of moist bread and probably other similar materials.
In 1752, the proceedings of the French Academy of Sciences noted that caoutchouc (condensed latex) obtained from the Hevea brasiliensis rubber tree could be used to erase pencil marks. The first scientific description of caoutchouc had come during a French geographic expedition to South America in 1735. The name rubber was given to caoutchouc in 1770, and is attributed to none other than British-American chemist Joseph Priestley. He noted that caoutchouc was useful to "rub out" pencil marks; hence the name rubber was born. In most parts of the world, erasers are still called rubbers.
There was a drawback to the early erasers--and all materials made from rubber--since the rubber softened during warm weather, became hard in cold weather, and was stinky as it started to degrade. Enter hardware merchant-turned-chemical engineer Charles Goodyear, who, after several years of work, developed the vulcanization process to cure rubber in 1839. During vulcanization, sulfur is added to rubber and the mixture is heated under pressure to form sulfur cross-links between the rubber's polymer chains. The cross-links increase the strength, stability, and elasticity of the rubber.
After Goodyear's discovery, rubber became broadly used for many common items, including erasers. The first patent on a combined pencil and rubber eraser was granted in the U.S. in 1858. Most pencils made for use outside the U.S. still don't have attached erasers.
Natural rubber was chemically identified in the 1880s as cis-polyisoprene, [CH2C(CH3)=CHCH2]n. It is biosynthesized in the rubber tree from 3-methyl-3-butenyl pyrophosphate, an important building block for many natural compounds. About 30% of the milky white latex obtained from a cut on the rubber tree is cis-polyisoprene. The polymer is recovered from the liquid by using formic acid to coagulate the polymer into curds, which are then pressed into sheets.
Synthetic production of rubber wasn't initially successful, since radical polymerization of isoprene leads to random cis and trans arrangements, giving a sticky and useless product. With the development of Ziegler-Natta catalysts in the 1950s, however, 100% cis-polyisoprene could be manufactured. Trans-polyisoprene, also known as gutta-percha, is a harder material.
Several synthetic rubber compounds have been used to make erasers. These include isoprene-isobutylene (butyl rubber), styrene-butadiene, and ethylene-propylene copolymers. Synthetic rubber began to replace natural rubber in erasers by the 1960s. Since the mid-1990s, erasers have been made nearly exclusively with synthetic rubber, primarily polyvinyl chloride. The driving force to complete the changeover was to help prevent allergic reactions to latex, mainly in schoolchildren.
Erasers and other rubber products are prepared by masticating the natural or synthetic rubber, followed by mixing at low heat to obtain the desired consistency. During mixing, a variety of additives may be introduced: a small amount of petroleum-based oil to aid mixing, sulfur and other reagents for vulcanization (if needed), plasticizers to control firmness, amine or phenol antioxidants, and pigments. For erasers, high-silica pumice or other abrasives may be added, especially if natural rubber is used.
Following mixing, the rubber is shaped by extrusion or by placement in a mold. At this point, the rubber is cured under pressure and elevated temperature. Afterward, the erasers are cut into the final shape or removed from the mold, ready to be used.
For pencil erasers, cylindrical ribbons of rubber are cut into short pieces called plugs. The plugs are placed in a rotating hopper that lines the plugs up on a conveyer belt that carries them to be married up with a pencil. A band of metal called a ferrule is glued onto the end of the pencil where a recess has been cut, while at the same time a plunger presses an eraser plug into the ferrule. When the glue dries, everything is bliss.