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Bend me, shape me, anyway you want me. Those are the words of an old love song, but it could just as easily be a song about plastics—the most versatile materials in our modern world. Plastics are plastic, which means we can mold them into pretty much anything, from car bodies and washing-up bowls to toilet seats and toothbrushes. That's partly because there are many different kinds of plastic but also because each kind can be used for many things. What exactly is plastic? How do we make it? How do we get rid of it when we no longer need it? Let's take a closer look!

Photo: Fantastic plastic! It's cheap, cheerful, and colorful; tough or gentle; and easy to make into all kinds of shapes. We just have to be careful what we make it from and how we dispose of it when we're done.

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  1. What are plastics?
  2. Types of plastics
  3. Thermoplastics and thermosets
  4. How do we make plastics?
  5. What are plastics like?
  6. What do we use plastics for?
  7. Plastics and the environment
  8. A brief history of plastics
  9. Find out more

What are plastics?

We talk about "plastic" as though it's a single material, but there are in fact many different plastics. What they have in common is that they're plastic, which means they are soft and easy to turn into many different forms during manufacture.

Plastics are (mostly) synthetic (human-made) materials, made from polymers, which are long molecules built around chains of carbon atoms, typically with hydrogen, oxygen, sulfur, and nitrogen filling in the spaces. You can think of a polymer as a big molecule made by repeating a small bit called a monomer over and over again; "poly" means many, so "polymer" is simply short for "many monomers." If you think of how a long coal train is made from many trucks coupled together, that's what polymers are like. The trucks are the monomers and the entire train, made from lots of identical trucks, is the polymer. Where a coal train might have a couple of dozen trucks, a polymer could be built from hundreds or even thousands of monomers. In other words, polymers typically have very large and heavy molecules.

How the polythene polymer molecule is made by endlessly repeating the ethene monomer.

Artwork: Polymers are made from long chains of a basic unit called a monomer. Polyethylene (polythene) is made by repeating the ethene monomer over and over again.

Types of plastics

Sellotape sticky tape dispenser

Photo: Natural plastic: Sticky tape is made from cellulose, a natural polymer found in plants. The first plastic sticky tape was developed in 1930.

There are many different plastics, so we need ways of making sense of them all by grouping similar ones together. Here are a few ways we can do that (and there are others I've not listed):

Thermoplastics and thermosets

The last one on my list is such an important way of grouping plastics that we'd better look at it in a bit more detail. What's the difference between thermoplastics and thermosets—and how can we explain it?


Toes inside brown nylon stockings.

Photo: Thermoplastic: Silky nylon stockings are probably as far away from your idea of plastic as it's possible to get—yet they're just as much plastic as washing-up bowls and toothbrushes. The secret science of condensation polymers, which powers these leggy wonders, was figured out by Wallace Carothers in the 1930s.

You can make something like a plastic bottle by injecting hot, molten plastic into a mold, then letting it cool down. Your bottle stays solid, but if you heat it up again later, it'll soften and melt. We say it's made from a thermoplastic: something that becomes plastic (soft and flexible) when it meets thermal energy (heat). In a thermoplastic, the long polymer molecules are joined to one another by very weak bonds, which easily break apart when we heat them, and quickly reform again when we take the heat away. That's why thermoplastics are easy to melt down and recycle. Some everyday examples you will have come across are polyethylene/polythene (plastic bottles and sheets), polystyrene (crumbly white packaging material), polypropylene (plastic ropes), polyvinylchloride/PVC (toys and credit cards), polycarbonate (hard plastic windows and car headlamps), and polyamide (nylon—used in everything from stockings and swimming shorts to toothbrushes and umbrellas).

Thermosetting plastics (thermosets)

A frying pan coated with nonstick Teflon, a plastic polymer

Photo: Thermosetting plastic: A typical nonstick Teflon (PTFE) cooking pan.

Thermosets are usually made from much much bigger polymer chains than thermoplastics. When they're initially manufactured, they're heated or compressed to form a dense, hard, structure with strong cross-links binding each of these long molecular chains to its neighbors. That's very different from thermoplastics, where the polymer chains are held to one another only by very weak bonds. And that's why we can't simply heat thermosets to remold or reform them. Once they're "set" (cured) during manufacture, they stay that way. You'll be less familiar with thermosets than with thermoplastics; even so, you may have come across examples like polyurethane (insulating material in buildings), polytetrafluoroethylene/PTFE (nonstick coatings on cooking pots and pans), melamine (hard plastic crockery), and epoxy resin (a tough plastic used in strong adhesives and wood fillers).

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How do we make plastics?

A length of anti-static tubular hose pipe.

Photo: Plastic pipes and hoses are made by a process called extrusion, described below.

We've already seen that plastics are made from polymers, but how are polymers made? They're based on hydrocarbons (molecules built from hydrogen and carbon atoms) that we get mostly from things like petroleum, natural gas, or coal. Crude oil drilled from the land or sea is a thick gloopy mixture that contains thousands of different hydrocarbons, which have to be separated out before we can use them. That happens in an oil refinery, through a process called fractional distillation. It's a more involved version of the distillation you might have used to purify water. If we heat water, it eventually turns into steam, which we can then collect, cool, and condense back to water; that's distillation, and it produces highly purified or "distilled" water. We can heat and distill crude oil the same way, but all those many hydrocarbons it contains have molecules that are different sizes and weights, so they boil off and condense at different temperatures. Collecting and distilling the different parts of crude oil at different temperatures gives us a bunch of simpler mixtures of hydrocarbons, called fractions, which we can then use for making different types of plastics.

Hydrocarbons made in this way are the raw materials for polymerization, the name we give to the chemical reactions that make polymers. Some polymers are made simply by fastening hydrocarbon monomers together, like daisy chains, which is a process called addition polymerization. Others are made by joining together two small hydrocarbon chains and removing a water molecule (two hydrogen atoms and one oxygen), making a bigger hydrocarbon chain in a process known as condensation polymerization. The more often you repeat this, the longer the polymer gets.

A selection of plastic household objects made by injection molding.

Photo: Solid plastic things are made by injection molding, described below.

Typically, we need to use other chemicals called catalysts to kick-start polymerization. Catalysts are simply substances we can add that make a chemical reaction more likely to happen and, though they may change temporarily during the reaction, they re-emerge at the end in their original form; in other words, they're not permanently changed as the reaction takes place. Ziegler-Natta catalysts, some of the most important for making polymers, were developed through the work of German chemist Karl Ziegler and Italian Giulio Natta, which won them a joint Nobel Prize in Chemistry in 1963.

Because we need plastics to do all sorts of things, we often have to add other ingredients to the basic hydrocarbons to produce a polymer with exactly the right chemical and physical properties. These extra ingredients include colorants (which, as the name suggests, turn plastics into all kinds of bright and happy colors), plasticizers (which make plastics more flexible, viscous, and easier to shape), stabilizers (to stop our plastics breaking apart in sunlight and heat), and fillers (typically low-cost minerals that mean we need less of the expensive, oil-based hydrocarbons to make our final plastic product—so we can make and sell it more cheaply).

Four plastic-shaping processes: injection molding, blow-molding, extrusion, and calending.

Artwork: Four common processes for making things out of plastic. 1) Injection molding involves squirting hot plastic into a mold. The plastic grains (light blue) are passed over an auger (Archimedes screw) and heated to make molten plastic, which can then be squirted through a needle (injected) into a mold. 2) Blow molding is similar, but air (yellow arrow) is blown into the plastic afterward to make it expand and fill the mold. 3) Extrusion involves squeezing out plastic through a nozzle and shaped die to make something like a pipe. 4) Calendering uses rollers to make flat, thin, smooth sheets of plastic.

The plastic-making process doesn't end there. What we've got at this point is a plastic polymer known as a resin, which can be used for making all kinds of plastic products. Resins are supplied as powders or grains that are loaded into a machine, heated, and then shaped by one or more processes to make our finished plastic product. The shaping processes include injection and blow molding (where we squirt hot plastic through a nozzle into a mold to make things like plastic bottles), calendering (squashing between heavy rollers, for example, to make plastic sheets or films), extruding (squeezing plastic through a nozzle, perhaps to make pipes or straws), and forcing plastics through a kind of microscopically small sieve, called a spinneret, to make thin fibers (which is how fibers are made for things like toothbrushes or nylon stockings). There are many other plastic-making processes as well.

What are plastics like?

The many kinds of plastics all have different properties (if they didn't, we wouldn't need so many of them in the first place). Having said that, they do have things in common. Generally, plastics are flexible and easy to shape in a variety of ways (remember, that's why we call them plastics); easy to make in all different shapes, sizes, and colors; lightweight; electrically insulating; waterproof; and relatively inexpensive. Some of them are meant to be very strong and durable (car bits and prosthetic body parts are examples), while others are designed to fall apart in the environment relatively quickly (biodegradable plastic bags, for example). The properties of a plastic can also be deliberately engineered. Suppose we want plastics to be resistant to static electricity so they don't pick up so much dust; then we can use anti-static additives during the manufacturing process to make them slightly electrically conducting.

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What do we use plastics for?

In the early 20th century, plastics were quite a novelty; there were only a handful of plastics and very few uses. Zoom the clock forward 100 years and it's hard to find things that we don't use plastics for. Materials science means understanding the properties of different materials so we can use them to best advantage in the world around us. Given what we've just learned about the properties of plastics, it comes as no surprise to find them helping us out in building construction, clothing, packaging, transport, and in many other parts of everyday life.

Selection of plastic household articles

Photo: A small selection of the hundreds of plastic things you can find in your home.

In buildings, you'll find plastics in things like secondary glazing, roofs, heat insulation and soundproofing, and even in the paints you slap on your walls. There are plastics insulating your electrical cables and carrying water and waste-water in and out of your home. Look around you now and you'll see plastics everywhere, from picture frames and lamp shades to the clothes on your back and the shoes on your feet. How do all these things get into your life? Up to a third of all the plastic we use finds its way into the packaging we use to protect products (sometimes even plastic products) on the journey from factory to home.

Photo of plastic plane by NASA

Photo: Plastics in action: NASA's plastic Pathfinder aircraft in flight. There's no better way to show that a plastic is strong and lightweight than using it to build a plane! Picture by Nick Galante courtesy of NASA.

Because plastic means flexible, by definition, we tend to think plastics are relatively weak materials. Yet some are incredibly strong and long-lasting. If you have a rotten wooden door or window, for example, you might chisel out the rot and replace it with epoxy resin filler, a very strong thermosetting plastic that will turn rock hard in a matter of minutes and stay that way for years. Car fenders are now mostly made of plastic—and lightweight car and boat bodies are often made from composites such as fiberglass (glass-reinforced plastic), which are plastics mixed with other materials for added strength. Some plastics are soft or hard as the mood suits them. An amazing plastic called D3O® has an astonishing ability to absorb impacts: normally it's soft and squishy, but if you hit it very suddenly, it hardens instantly and cushions the blow. (Find out more about it in our article on energy-absorbing materials.)

Plastics and the environment

Most plastics are synthetic, so they're carefully designed by chemists and laboriously engineered under very artificial conditions. They'd never spontaneously appear in the natural world and they're still a relatively new technology, so animals and other organisms haven't really had chance to evolve so they can feed on them or break them down. Since a lot of the plastic items we use are meant to be low-cost and disposable, we create an awful lot of plastic trash. Put these two things together and you get problems like the Great Pacific Garbage Patch, a giant "lake" of floating plastic in the middle of the North Pacific Ocean made from things like waste plastic bottles. How can we solve horrible problems like this? One solution is better public education. If people are aware of the problem, they might think twice about littering the environment or maybe they'll choose to buy things that use less plastic packaging. Another solution is to recycle more plastic, but that also involves better public education, and it presents practical problems too (the need to sort plastics so they can be recycled effectively without contamination). A third solution is to develop bioplastics and biodegradable plastics that can break down more quickly in the environment.

It's easy to dismiss plastics as cheap and nasty materials that wreck the planet, but if you look around you, the reality is different. If you want cars, toys, replacement body parts, medical adhesives, paints, computers, water pipes, fiber-optic cables, and a million other things, you'll need plastics as well. Maybe you think we struggle to live with plastics? Try imagining for a moment how we'd live without them. Plastic is pretty fantastic—we just need to be smarter and more sensible about how we make it, use it, and recycle it when we're done.

A brief history of plastics

Early history

Early 20th-century plastics

Mid-late 20th-century plastics

21st-century plastics

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Text copyright © Chris Woodford 2017, 2020. All rights reserved. Full copyright notice and terms of use.

Teflon, Nomex, Kevlar, Tyvek, and DuPont are trademarks or registered trademarks of E. I. du Pont de Nemours and Company.
D3O is a registered trademark of D3O.

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