The Chemistry of Everyday Plastics

Plastics are so common in modern life that it is easy to forget how unusual they are as materials. A single family of substances can be moulded into a rigid water pipe, a flexible carrier bag, a transparent bottle or a stretchy fibre, and all of these share the same basic chemistry. The word plastic refers to a large group of materials known to chemists as polymers. A polymer is a very long molecule built from many small units joined together in a chain. The small repeating units are called monomers, and the process of linking them is called polymerisation. Because a single chain may contain many thousands of these units, the resulting molecules are enormous compared with those of ordinary liquids such as water or alcohol.

The behaviour of a plastic depends heavily on the shape and arrangement of its chains. In some plastics the long molecules lie loosely tangled together, sliding past one another rather like cooked spaghetti in a bowl. Materials of this kind tend to be soft and easy to reshape when heated. In other plastics the chains are packed neatly side by side in ordered regions, which makes the material stiffer, stronger and more resistant to heat. The same basic chain can therefore give very different products depending on how tightly and how regularly the molecules are organised. Manufacturers exploit this by controlling the conditions under which polymerisation takes place.

One of the most useful distinctions in the plastics industry is between thermoplastics and thermosetting plastics. A thermoplastic softens when it is heated and hardens again when it cools, and this cycle can be repeated many times. Because heating merely loosens the tangle of chains without breaking the chains themselves, a thermoplastic can be melted down and moulded into a new shape over and over. This property is what makes many everyday plastics, such as those used in bottles and packaging, relatively straightforward to recycle. A thermosetting plastic behaves very differently. When it is first heated and formed, permanent chemical bonds develop between neighbouring chains, locking them into a rigid network. Once this network has set, further heating will not soften the material; instead it will eventually char or burn. Thermosets are therefore chosen for objects that must keep their shape under heat, such as electrical fittings and the handles of cooking pans.

The raw material for most conventional plastics is crude oil. Refineries separate oil into fractions of different weight, and one of the lighter fractions is broken down further to yield small reactive molecules that serve as monomers. From these building blocks a whole range of familiar plastics can be produced. The plastic used in many drink bottles, for instance, is prized because it is light, clear and resists gases, keeping fizzy drinks from going flat. Other plastics are prized for very different reasons, such as flexibility, electrical insulation or resistance to chemicals. By selecting particular monomers and particular reaction conditions, chemists can design a material to suit a specific job.

The very durability that makes plastics so useful is also the source of a serious environmental problem. The strong chains that resist wear and weather do not break down easily once an object is thrown away, and many plastics can persist in the environment for a very long time. Sunlight and mechanical forces gradually shatter discarded items into ever smaller fragments, but these tiny pieces, often called microplastics, do not truly disappear. They accumulate in soils, rivers and oceans, where they may be swallowed by animals. Dealing with plastic waste has become one of the defining challenges of modern materials science.

Several responses to this problem are being pursued at once. Recycling is the most familiar: thermoplastics can be collected, cleaned, melted and reshaped, although in practice the quality of the material tends to fall slightly each time it is reprocessed. Another approach is the development of plastics designed to break down more readily. Some of these are made from plant materials rather than oil, while others are engineered so that microorganisms can digest them under the right conditions. It is important to note that a plastic made from plants is not automatically one that decomposes quickly, and a plastic that decomposes is not necessarily made from plants; the two properties are separate and must each be built in deliberately.

Despite their drawbacks, plastics are unlikely to disappear from daily life, because few other materials combine such lightness, strength and low cost. The goal of current research is not to abandon them but to use them more wisely: to design plastics that can be recycled again and again without losing quality, to recover far more of the material that is currently discarded, and to ensure that the products which do escape into the environment cause as little lasting harm as possible. Understanding the chemistry of the chains, and how their structure governs their behaviour, lies at the heart of every one of these efforts.