Plastics are polymers, which is a chain of units. Each unit, or link of the chain, is the “mer” or basic unit that consists of carbon, hydrogen, oxygen and/or silicon. The chain is made of many “mers” which are hooked or polymerized together. Polymers are nothing new. Natural polymers include things like tar, shellac, tortoise shells, animal horns, tree amber and latex. During the 1800s, scientists began to chemically alter natural polymers to produce vulcanized rubber, gun cotton and celluloid. The most common polymers are made of hydrocarbons. They are long chains of carbon atoms with hydrogen atoms bonded to them. Examples of hydrocarbon polymers are polypropylene, polybutylene, polystyrene and polymethylpentene. Other elements can be bonded to the carbon and hydrogen atoms to make up more specialized polymers. For example, nylon contains nitrogen, Teflon contains fluorine, polyvinyl chloride contains chlorine and polyesters and polycarbonates contain oxygen. Some polymers, known as inorganic polymers have silicon or phosphorous atoms in place of carbon atoms. There are two groups of polymers; thermoplastics and thermosets. Thermoplastics are capable of being heated and reheated, thus minimizing wasted material. Thermosets are not capable of being reheated, causing much scrap. Polymers, in general, are excellent in providing the customer with chemically resistant components; light weight, yet strong and durable products; offer thermal resistance; provide electrical insulation; and can be used to produce the most intricate parts.