A Professor Plastics Feature ArticleSee Other Articles
Quick quiz: How many types of plastics are there? No idea?
I have no exact number. It’s sort of like asking how many types of bread there are. Plastics aren’t simply one material made the same way every time. Although plastics can be broken down into broad types or categories, there actually are thousands of different plastics, each with its own composition and characteristics. One plastic may block oxygen from reaching food. Another may be transparent like glass yet tough. Or stretch and bounce back in shape. Another may trap air inside itself. Or stop a bullet.
That’s why plastics are used in so many ways: they protect our food, cushion our fall, insulate our homes, improve our cars’ gas mileage, keep us dry when it’s raining… and many other things.
Plastics Spotlight: What does HDPE stand for?
Plastics are a result of a mix of chemistry and engineering. As innovation marches on, scientists and engineers can create new plastics to do more and more things.
So even though the number of plastics is unclear, plastics makers tend to group plastics into two general classes: thermoplastics and thermosets.
Can be re-melted and essentially returned to their original state—sort of like the way an ice cube can be melted and then cooled again. Thermoplastics usually are produced first in a separate process to create small pellets; these pellets then are heated and formed to make all sorts of consumer and industrial products. Thermoplastics include plastics you’re likely familiar with: polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylon, polycarbonate, and others.
Are usually produced and formed into products at the same time—and they cannot be returned to their original state. They generally are formed using heat (“thermo”) and become “set,” like a cooked egg. Thermosets include vulcanized synthetic rubber, acrylics, polyurethanes, melamine, silicone, epoxies, and others.
There are other categories of plastics:
Are… well… engineered to have enhanced mechanical properties and often greater durability than other materials. (They often—not always—are thermoplastics.) For example, polycarbonate resists impact. Polyamides like nylon resist abrasion. Some are combinations of plastics, such as incredibly tough ABS (acrylonitrile butadiene styrene). The list of engineering plastics is quite long.
Are precisely that: plastics that have been spun into fibers or filaments that are used to make fabrics, string, ropes, cables—even optical fibers and body armor (such as Kevlar®). Most plastic fibers are strong, stretchable, and stable under heat (so fabrics can be ironed). Some of the most recognizable plastic fibers are polyester, nylon, rayon, acrylic, and spandex, although there are many more. (Side note: polyester sometimes is called polyethylene terephthalate—it’s also used to make plastic beverage bottles that can then be recycled into fibers for clothing, such as fleece jackets and t-shirts).
There are many more categories, such as coatings, adhesives, elastomers and rubbers, covering plastics that are used in everything from the space shuttle exterior to canned vegetables. I could go on (and have been known to), but let’s stop there for now.