Plastic See also: Teflon Any synthetic organic material that can be molded under Heat and Pressure into a shape that is retained after the heat and pressures are removed. There are two basic types of plastic: thermosetting, which cannot be resoftened after being subjected to heat and pressures; and thermoplastic, which can be repeatedly softened and reshaped by heat and pressure. Plastics are made up chiefly of a binder consisting of long chainlike Molecule called Polymers. Binders can be natural materials, e.g., Cellulose, or (more commonly) synthetic Resins, e.g., Bakelite. The permanence of thermosetting plastics is due to the heat- and pressure-induced cross-linking reactions the polymers undergo. Thermoplastics can be reshaped because their linear or branched polymers can slide past one another when heat and pressure are applied. Adding plasticizers and fillers to the binder improves a wide range of properties, e.g., hardness, elasticity, and resistance to heat, cold, or acid (see Acids And Bases). Adding Pigments imparts Color. Plastic products are commonly made from plastic powders. In compression molding, heat and pressure are applied directly to the powder in the mold cavity. Alternatively, the powder can be plasticized by outside heating and then poured into molds to harden (transfer molding); be dissolved in a heating chamber and then forced by a plunger into cold molds to set (injection molding); or be extruded through a die in continuous form to be cut into lengths or coiled (extrusion molding). The first important plastic, Celluloid, has been largely replaced by a wide variety of plastics known by such trade names as Plexiglas, Lucite, Polaroid, and Cellophane. New uses continue to be found and include contact lenses, Machine gears, and artificial body parts. The widespread use of plastics has led to environmental problems. Because plastic products do not decay, large amounts accumulate as waste. Disposal is difficult because they melt when burned, clogging incinerators and often emitting harmful fumes, e.g., the Hydrogen chloride Gas (see States Of Matter) given off by Polyvinyl Chloride, and because many products into which they are made, such as plastic bottles, do not compact as readily in landfills. However, recycling is increasingly emerging as an alternative to disposal, particularly in communities with limited landfill capacity.