How are plastics manufactured?
March 19, 2018 12:57PM
Plastics are made by a chemical process called polymerization. There are several different types of polymerization: Condensation polymerization is a reaction where two molecules join into one larger one, releasing a (generally small) molecule in the process. Examples of condensation polymers are Nylon-6,6 (a polyamide) and PET (a polyester). Addition polymerization is where two molecules join to form one larger one without losing any atoms in the process. Examples of addition polymers are Nylon-6 (also a polyamide, but produced using a ring-opening process rather than a condensation process) and polystyrene (a polyolefin, generally produced using a free-radical propagation mechanism).
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 repeat 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 interest are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone, (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This customization by repeat units molecular structure has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer.
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). 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.
A few biodegradable plastics have been made specifically to solve the issues of disposal. Some examples are: cornstarch polymers, cellulose acetate, celluloid, and polyvinyl alcohol. These will probably see more use in the future, as long as the degradation does not interfere with the intended purpose or create safety hazards.