section_path: Classifications/Thermoplastics and thermosetting polymers
section_name: Thermoplastics and thermosetting polymers
paragraph_text: Thermoplastics and thermosetting polymers
  
  One important classification of plastics is the degree to which the chemical processes used to make them are reversible or not.
  
  Thermoplastics do not undergo chemical change in their composition when heated and thus can be molded repeatedly. Examples include polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC).
  
  Thermosets, or thermosetting polymers, can melt and take shape only once: after they have solidified, they stay solid. If reheated, thermosets decompose rather than melt. In the thermosetting process, an irreversible chemical reaction occurs. The vulcanization of rubber is an example of this process. Before heating in the presence of sulfur, natural rubber (polyisoprene) is a sticky, slightly runny material; after vulcanization, the product is dry and rigid.