Science & Technology in Action

14th Edition

The Life of Plastics − from Cradle to Grave

BIM

Bacteria have existed on Earth for a few thousand million years and have evolved the ability to exploit most food sources. Synthetic polymers have been around for less than a hundred years. Nonetheless, in that relatively short time, some strains of bacteria have evolved the ability to use particular plastics as a food source to a limited extent.
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Lesson excerpt

What are plastics?

Originally the word ‘plastic’ referred to materials that were easily moulded into shape. Nowadays the term ‘plastics’ refers to synthetic polymers.

The molecules of synthetic polymers are typically composed of hundreds or even thousands of relatively simple molecules (known as monomers) bonded together to form very large long-chain molecules. For example, polyethylene is made from repeating units of ethene (old name: ethylene) as shown below.

Hydrocarbons in which all the chemical bonds are single bonds are called alkanes: e.g. methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, etc.

Hydrocarbons that have a double carbon-carbon bond in each molecule are called alkenes: e.g. ethene, propene, butene etc.

At a suitable temperature and pressure and in the presence of a suitable catalyst, the double bond in ethene molecules can break.

Why are plastics problematic?

The basic problem with synthetic plastics is that they are not easily broken down by the natural process of decay. In most cases, bacteria and fungi cannot use them as a food source. The main reasons for this are:

    • the plastics are insoluble in water and are often hydrophobic

    • they are generally solid and so only the surface molecules are accessible

    • often their molecules do not have enough variety to provide likely ‘attack sites’ for bacteria.

Which one of the following would be most likely to be attacked by bacteria: polythene, PVC, PVA and polystyrene, assuming that they were formed as thin sheets?

If you draw out the structure of each of these polymers you will see that PVA (poly-vinyl-acetate) has an ‘exposed’ oxygen-containing group on every second carbon atom. This could form hydrogen bonds with water molecules and so would be a likely site for bacteria to attack it. In fact a variety of algae, yeasts and bacteria can slowly break down polyvinyl acetate. (PVA is commonly used as glue for wood and paper but is not used to make bottles etc.)

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True or False?

  1. Polyethylene and polypropylene are produced by addition polymerisation of ethene and propene respectively. true
  2. Hydrocarbons that have a double carbon-carbon bond in each molecule are called alkenes. true
  3. Rigid synthetic plastics are generally not biodegradable. true
  4. It is now possible to make biodegradable plastics based on biomaterials such as cellulose. true
  5. Herbivorous animals produce enzymes to digest cellulose. true
  6. LDPE fl oats on water but HDPE sinks in water, true
  7. Plastics that sink in seawater do not pose an environmental problem. false
  8. ‘Fishing for Litter’ is a practical way of reducing marine litter. true
  9. Most synthetic plastics are hydrophobic and this makes it easy for microorganisms to attack them. false

Glossary of terms

alkanes
alkenes
amino acid