Plastics, depending on their physical properties, may be classified as thermoplastic or thermosetting materials. Thermoplastic materials can be formed into desired shapes under heat and pressure and become solids on cooling. If they are subjected to the same conditions of heat and pressure, they can be remolded. Thermosetting materials acquire infallibility under heat and pressure and cannot be remolded.
Plastics may be classified also according to their chemical sources. The twenty or more known basic types fall into four general groups:
- Cellulose Plastics:Include the cellulose nitrates and cellulose acetates. The cellulose nitrate plastics are the oldest in this group, and "Celluloid" is the oldest example. These plastics are made from cotton or wood pulp.
- Synthetic Resin Plastics:Include the phenol formaldehyde, phenolic furfural, urea formaldehyde, vinyl, styrene, and acrylic plastics. These plastics are made from phenol, formaldehyde, urea, acetylene, petroleum, glycerol, and phthalic anhydride.
- Protein Plastics:Casein plastics are the most common type in the protein group. They are made from milk. Other protein plastics are made from soy beans, coffee beans, peanuts, and other agricultural products.
- Natural Resins:Include shellac, asphalt, rosin, amber, and pitch. These materials with fillers are usually cold molded.