Plastics are composed of polymers and other substances that may be synthetic or natural organic materials. Polymers are primarily made up of a long chain of hydrocarbons, and may contain other components such as nitrogen, chlorine and sulfur. Plastics can be classified as either thermoplastics or thermosets. Thermoplastics are those plastics that don’t undergo chemical change when subjected to heat - they can be molded repeatedly. Such plastics include polyethylene, polystyrene, polyvinyl chloride, polypropylene, and the polytetrafluoroethylene. Thermosets, on the other hand, are plastics that melt with the application of heat.