A Guide to Common Plastics
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Plastic is everywhere in our day to day lives – but, of course, ‘plastic’ is just a catch-all term for a range of different chemical substances. This graphic takes a look at some of the more common plastics we encounter on a regular basis, and examines their chemical structures. Below, we’ll also talk a little about how these plastics are created.
All plastics we use or encounter are substances called polymers. These polymers are themselves formed from chemical entities called monomers. Monomers can be a range of differing compounds, but specific polymers will generally contain monomers of only one or two types. The polymers are formed by joining together many monomers, like a long chain of paperclips, to form one long molecule.
Let’s look at a simple example to make this clearer. Polyethene is a plastic, or polymer, that is widely used in plastic shopping bags, plastic films, and to make some toys. It’s formed from many monomers of a a small molecule, ethene. At high temperature and pressure, and in the presence of oxygen as a catalyst, one of the two bonds between the two carbons in the molecule can be made to break, and allow them to form linking bonds to other ethene molecules. In the diagram below, n is a large number – each resultant polyethene chain from this process can be formed from as many as 20,000 individual ethene molecules.
Addition polymerisation
Other conditions can also be used to produce versions of the same polymer with varying properties; polyethene comes in a number of different varieties, such as high density polyethene (HDPE) or low density polyethene (LDPE). Different monomers can also be used to give different polymers. For example, if we use propene as the monomer instead of ethene, we obtain polypropene.
Some polymers are better known by their trade names or abbreviations than their full chemical names. For example, most non-chemists probably wouldn’t recognise the name polytetrafluoroethene, but they’d probably know it by its trade name, Teflon. Similarly, the plastic used to make a large proportion of plastic water bottles, polyethylene terephthalate, is better known in that context by its abbreviation, PET. It’s also used in clothing, when it’s often referred to simply as polyester (it’s worth noting that polyesters are really a class of polymers, rather an individual polymer).
So, that’s how these plastics are made – but where do the molecules we use to make them come from in the first place? The raw materials we use to make these molecules come from crude oil, and in fact about 5% of worldwide oil goes into making plastics. There are around 40 different basic chemicals in crude oil, which can be used to make a huge number of further chemicals. Some of these, in turn, can be polymerised to form plastics.
You’ve probably heard of the concept of peak oil, the proposed point in time when the rate of extraction of oil from the Earth reaches its highest point before beginning to decline as reserves dwindle. Of course, if we start to run out of oil, we’ll also be running out of the source of many of the chemicals used to make plastics. The graphic at the top of the page emphasises the range of different uses that these plastics find in our daily lives; should we start to run out of the chemical feedstocks necessary to produce more, we may have to start to find substitutes for this wide range of applications.
Another issue with plastics is their disposal. Many of the different types of plastic are not biodegradable, and can take up to thousands of years to decompose as a consequence of exposure to UV radiation from the sun. Of course, this decomposition time is merely an estimate, as we haven’t been using plastics for long enough to have observed their complete decay. Their disposal therefore poses problems because landfill requires space, and other methods of disposal have their own issues – for example, burning plastics can, in some cases, release toxic gases. These disposal issues are one of the reasons recycling of plastics is encouraged.
Finally, it’s worth noting that polymers aren’t always man-made; DNA is an example of a natural polymer, and both cellulose and lignin, found in plants, are further examples. We’ll be looking further at polymers in some future posts.
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