Plastics can be made from many materials found in our earth, such as coal, natural gas, salt, cellulose and crude oil (Plastics Europe, 2020). Amongst these, crude oil is the most important for making plastics. Crude oil is the unrefined oil obtained from drilling and pumping oil to the Earth’s surface. It is a mixture of compounds known as hydrocarbons (compounds consisting of the elements of hydrogen and carbon). The process of making plastics is quite complex and is outlined below.
The key takeaways from an environmental perspective are that:
Any plastic made from crude oil is not naturally occurring. Instead, plastic is a man-made material which is highly durable. Therefore, unlike traditional materials such as paper, metals, wood and glass; plastic is not easily incorporated into natural biological recycling processes in the Earth (Andrady and Neal, 2009). This partly explains why plastic is such a pollution problem.
Also, the process of producing plastic produces greenhouse gases, see: How does plastic affect climate change?
The process of turning crude oil to plastic:
Crude oil is separated by a process known as fractional distillation where the liquid mixture of hydrocarbons is heated to form a gas. In this process, crude oil is separated into different parts called fractions such as refinery gas, petrol, naphtha, kerosene, diesel and residue. This can be done because different hydrocarbons have different boiling points. Longer chains of hydrocarbons have higher boiling points because more energy is required to break the larger number of bonds.
A fraction from this process - naphtha - is frequently used to make plastics (Plastics Europe, 2020). While naphtha has been separated by fractional distillation, it is still a mixture of many hydrocarbons. Therefore, the naphtha fraction is separated again by a long vertical tube into different components.
These components then undergo a process known as cracking where longer hydrocarbon chains are broken down into smaller, more useful hydrocarbons. This can be done using heat but is more commonly done using a catalyst –a substance that causes or accelerates a chemical reaction. The result of this process is to form individual hydrocarbons called monomers.
Monomers undergo polymerization: the process of joining a large number of useful individual monomers to produce a longer chain polymer. These polymers are plastic resins. For example, the monomer ethene is polymerized to form the plastic resin: polyethene.
These resins are cooled and chopped up to form nurdles which are little plastic pellets. Nurdles are sent to manufacturers who will heat and mold them to form the wide variety of plastics materials in use (National Geographic, 2019).
A video to visualise this process can be found here (1:28 – 2:51). (National Geographic, 2018).
A wide variety of monomers which are polymerized into plastics. Furthermore, chemicals called additives are added to alter properties and appearance of plastics (Napper & Thompson, 2020). Some examples of different types of plastics include:
Polyester – textiles and clothing
Polyvinylchloride (PVC) - building and construction (e.g. pipes)
Polystyrene – Packaging, disposable cutlery and smoke detector housings
Acrylic – As an alternative to glass
Nylon – For fabrics, rope and clothing
Andrady, A.L and Neal, M.A., (2009). Applications and societal benefits of plastics. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1526), pp. 1977-1984.
Plastics Europe. (2020). How plastics are made. Plastics Europe. Available at: <https://www.plasticseurope.org/en/about-plastics/what-are-plastics/how-plastics-are-made> (Accessed: 19 June 2020).
National Geographic. (2018). Plastics 101 | National Geographic. Available at: <https://www.youtube.com/watch?v=ggh0Ptk3VGE> (Accessed: 19 June 2020).
Napper, I.E. and Thompson, R.C., (2020). Plastic Debris in the Marine Environment: History and Future Challenges. Global Challenges, p.1900081.