Plastic

Plastic is a category of materials made from polymers—long chains composed of identical small units bonded together. The defining characteristic of plastic is that it can be shaped using heat or pressure, and retains that shape once it cools.

Most industrial plastics in use today are derived from crude oil and natural gas. There are also plastics made from plant sources, such as cellulose and starch. Plastic is used in packaging, clothing, household goods, medical equipment, vehicles, and construction.

Types and Classification

Plastics are generally divided into two categories. Thermoplastics soften each time they are heated and can be reshaped—examples include polyethylene, polypropylene, and PET. Thermosets harden only once and do not soften with heat—examples include epoxy and bakelite. This distinction determines how they can be recycled.

Production Process

Monomers are first extracted from gas or oil—for example, ethylene. These then undergo polymerization, a process in which monomers are bonded together to form long chains. The result is small plastic pellets, which are sent to factories. There they are shaped using methods such as injection molding (melted plastic forced into a mold) or extrusion (melted plastic pushed through a die).

Waste and Environment

Most plastic breaks down slowly in the environment, so plastic waste accumulates in soil and water. As it fragments, it produces microplastics—extremely small particles found in water, air, and the food chain. Mitigation measures include recycling, limiting single-use plastics, and developing alternative materials that degrade quickly.