News from CRIS: Real-time Science - What's Plastic?

January 30, 2024

What is plastic?

There is no singular agreed-upon definition for plastic. Plastics, as a term, is used to describe a vast array of materials composed of polymer-containing compounds. 

Generally, folks consider plastic to be synthetic or semi-synthetic polymers that are lightweight, strong, durable, and, when desired, flexible. Polymers identified as plastics can be made by extrusion, injection molding, vacuum forming, and compression to make fibers, thin sheets, and other specific objects.

What is a polymer?

Before discussing polymers, we need to discuss their building blocks, monomers.

Let’s look at monomers.

Monomers are compounds that can be interconnected through chemical reactions. When you string together four or more monomers, you create a polymer. 

Common monomer compounds include

  • Ethylene: Ethylene is the monomer for polyethylene. We use polyethylene to create packaging, containers, pipes, and more. 

  • Propylene: Propylene is the monomer for polypropylene. We use polypropylene to make automotive parts, textiles, packaging, and more.

  • Vinyl Chloride: Vinyl Chloride is the monomer for polyvinyl chloride (PVC). We use PVC in construction materials like pipes, cable insulation, window frames, and more. 

  • Styrene: Styrene is the monomer for polystyrene. We use polystyrene to create products like foam insulation, disposable cups, containers, and more.

  • Acrylonitrile: Acrylonitrile is the monomer for polyacrylonitrile. Polyacrylonitrile is used to make synthetic fibers like acrylics and carbon fibers, as well as to make heat-resistant components for aerospace applications.

Let’s look at polymers.

Polymers are compounds containing four or more monomers. Thousands of polymers exist.

Polymers differ in their composition, degradability, biodegradability, structure, function, solubility, and more. 

There are two general categories of polymers: naturally occurring polymers and synthetically created polymers.

  • Natural polymers include proteins, nucleic acids (like DNA and RNA), polysaccharides (like cellulose and starch), silk, wool, and more. 

  • Synthetic polymers include nylon, polyethylene, resins, rubbers, and more.

Some polymers, like polyvinyl alcohol, will dissolve in water over the course of days, while other polymers, like HDPE pipes, may maintain their structure and function for thousands of years.

Is there a legal definition of plastic?

Some states, like Illinois, defined a microbead as a specific type of plastic, but federally in the United States, there isn't one readily available, legally agreed upon definition of plastic. 

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