The Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has declared war on flat tires by inventing a new form of rubber that heals after being punctured. The invention emerged as an extension of SEAS research with self-healing hydrogels, which use water to create reversible bonds that can heal after being broken. While SEAS was able to use the properties of water to create self-healing bonds, applying self-healing to dry substances such as rubber proved more challenging. Rubber is formed by permanent covalent bonds, which are much stronger than the reversible hydrogen bonds used in hydrogels and not prone to rebond once broken. However, after experimentation, the SEAS team was able to develop a molecular rope that knit reversible and covalent bonds together, enabling self-healing rubber. Researchers envision the new material being used for tires, electronic equipment and medical devices.
Harvard’s achievement is the latest chapter in a long story of elastic innovation. Here’s a look at three other types of rubber that scientists have discovered and developed, along with their uses.
Latex is a milky substance that occurs in 10 percent of flowering plants, where it serves as a defense against insects. Pre-Columbian Central American civilizations such as the Mayans, Olmecs and Aztecs harvested latex, mixing it with juice from morning glory vines to make it less brittle when it solidified, according to National Geographic. They were able to apply the resulting latex rubber to make balls and sandals, using it for an ancient ancestor of basketball. When Columbus visited Haiti on his second voyage of 1493 to 1496, he saw natives playing with rubber balls, according to a historical account of his trip written a century later, which forms the first written record of rubber, says the International Rubber Study Group (IRSG). French scientist Charles de la Condamine brought samples of latex rubber from Ecuador back to Europe in 1735, setting the stage for the modern development of rubber.
Trade in rubber shoes between the Americans and Europe soon began. Meanwhile in 1770, English chemist Joseph Priestley discovered that latex could be used for erasers, one of the first new applications of rubber. Scientists soon also discovered that hardened latex dissolved in turpentine formed a waterproofing liquid for cloth, leading to the development of raincoats by Scottish chemist Charles Macintosh in 1823. Elastic bands and hoses were other early applications of rubber, says IRSG.
The most important application of latex rubber followed American inventor Charles Goodyear’s accidental discovery that he could make rubber more heat-resistant by heat-treating it with sulfur, a process that became known as “vulcanization,” named after the Roman god of fire. Vulcanized rubber was heat-resistant, elastic, airtight and watertight, making it suitable for applications such as tires and seals. Today latex-derived natural rubber continues to be used for purposes such as tires, rubber gloves and mattresses.
Demand for rubber tires in the late 19th century inspired experimentation with synthetic rubber manufacturing, which accelerated during the World Wars as natural rubber supplies became scarce. During the 1930s and 1940s, German and American scientists used copolymers of acrylonitrile and butadiene to create a new form of rubber called nitrile, also known as nitrile butadiene rubber (NBR) and Buna-N.
Nitrile is resistant to oil as well as temperature changes, with an operating range of -40 degrees F to 257 degrees F. Special treatment can increase its upper temperature range past 300 degrees F. These properties make it the most widely-used form of rubber. It is used for applications such as non-latex gloves, automotive belts and hoses, and o-rings for purposes such as engine gaskets, medical seals and SCUBA tanks.
Another important synthetic rubber is Viton, a Chemours Company brand name for a type of rubber referred to generically as an FKM. FKMs are a type of fluorocarbon-based rubber known as a fluoroelastomer. Fluoroelastomers have strong carbon-fluorine bonds, giving them high resistance to chemicals as well as temperature extremes, with an operating range from -13 F degrees to 446 F degrees. FKMs are also resistant to deformation and aging.
DuPont first introduced Viton in 1957 for applications in the aerospace industry, where it was initially used for high-performance seals. Today Viton continues to be used for this purpose as well as automotive parts, chemical processing, oil and gas exploration, and petroleum refining.