The structure of plastic

Plastic can be produced cheaper, faster and with a lower energy consumption than other types of packaging such as aluminium and steel. Plastics are synthetic polymers, which means that they are created artificially by humans. Polymers are large molecules (macromolecules) that are made up of many of the same smaller molecule unit, called the monomer. Poly means many. The various plastic polymers are made from crude oil or gas and thereby from fossil fuels.

Crude oil is the oil that is extracted directly from the subsurface, which is still untreated. Crude oil is mainly made up of hydrocarbons, which are molecules that consist only of hydrogen and carbon. The crude oil is first treated so that the different hydrocarbon molecules are separated from each other. In this way, you isolate the monomers that are needed to make the polymer. For example, ethene can be isolated (_C2H₄, see Figure 9) and possibly new functional groups can be added for different reactions. After you have the desired monomer, the polymers are formed by polymerization, which is an exothermic process where the monomers are linked.

Figure 9: The structure of ethene.

Figure 10 provides an overview of the monomer of 5 most common types of plastic. The bracket and the n show that they form a polymer. The R groups are the next monomers in the chain.

Figure 10: Overview of the monomer of the 5 most common types of plastic. Please note that these are sketches where binding angles have not been taken into account.

Plastic consists mainly of carbon and hydrogen (hydrocarbons). It is seen that the types of plastic have in common that they have carbon-carbon bonds that form the backbone of the molecule. Oxygen, nitrogen and chlorine can also be included in plastics, but since they mainly consist of C and H, they are organic compounds and also have many chemical similarities with some molecules found in nature such as wood and resin. Plastics are hydrophobic materials, making them ideal for packaging. Their hydrophobicity comes from the fact that their structure consists primarily of hydrocarbons, which are either aliphatic or cyclic.