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General Information

1. Definition. The term "plastics" usually refers to a class of synthetic organic materials which, though solid in the finished form, at some stage in their processing are fluid enough to be shaped. There are two basic types of plastics: thermoplastics which, like paraffin, may be softened and resoftened repeatedly without undergoing a change in chemical composition; and thermosetting resins, like phenolformaldehyde, which undergo a chemical change with application of heat and pressure, and cannot be resoftened. Plastics in finished form consist of long chain molecules (polymers), which are built by combining single molecules (monomers), under heat and pressure. Cross-linking is a permanent connection between two polymer molecules binding them together through a system involving primary chemical bonds.