Chemical structure

Common thermoplastics range from 20,000 to 500,000 in molecular mass, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial interests are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To customize the properties of a plastic, different molecular groups "hang" from the backbone (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This fine tuning of the properties of the polymer by repeating unit's molecular structure has allowed plastics to become such an indispensable part of twenty first-century world.

Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the attractive intermolecular forces are overcome) and one or more glass transitions (temperatures above which the extent of localized molecular flexibility is substantially increased). The so-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets.

Molded plastic food replicas on display outside a restaurant in Japan.

Families

Plastics families

	Amorphous	Semi-crystalline
Ultra polymers	PI, SRP, TPI, PAI, HTS	PFSA, PEEK
High performance polymers	PPSU, PEI, PESU, PSU	Fluoropolymers: LCP, PARA, HPN, PPS, PPA
		Other polyamides
Mid range polymers	PC, PPC, COC, PMMA, ABS, PVC Alloys	PEX, PVDC, PBT, PET, POM, PA 6,6, UHMWPE
Commodity polymers	PS, PVC	PP, HDPE, LDPE

History

This section requires expansion.

The first human-made plastic was invented by Alexander Parkes in 1855 ^[7]; he called this plastic Parkesine (later called celluloid). It was unveiled at the 1862 Great International Exhibition in London. The development of plastics has come from the use of natural plastic materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., rubber, nitrocellulose, collagen, galalite) and finally to completely synthetic molecules (e.g., bakelite, epoxy, polyvinyl chloride, polyethylene).