Plastics

Plastics

Plastics are a group of very important materials that have a wide variety of uses. Plastics can be engineered for specific uses by matching up the properties of the plastic to the job it will ultimately be used for. Plastics are examples of polymers - very large molecules formed by the joining of many small molecules called monomers. The uses of plastics are related to their chemical propertie. Polymers are made by joining alkenes together. Their carbon to carbon double bond breaks open so that they can join to another alkene.


Naming Polymers

The name of the polymer comes from the name of the monomer used to make it. For example, the monomer ethene can be used to make the polymer poly(ethene). Notice how the prefix poly- is put in front of the name of the monomer. Brackets are put around the name of the monomer.


Polymerisation

The alkenes ethene and propene are two important raw material (“feedstock”) in the petrochemical industry which can also be used to make polymers through this process. Polymerisation is the process in which many small monomer units combine to form one large polymer molecule.

Small ethene monomers join to each other by the opening of the double bond allowing them to join up to form one long carbon chain of poly(ethene).


For example :

propene → poly(propene)

Where propene is a monomer and poly(propene) a polymer.


The video below shows examples of Polymerisation:-

Thermosoftening and Thermosetting

There are two main categories of plastic:

  • Thermosoftening (also called thermoplastics) are plastics which will soften when heated and can be reshaped.

  • Thermosetting plastics are plastics do not soften on heating. They are used when resistance to heat is important (eg kettles, plugs, laptop chargers etc).

The properties and uses of some common thermosoftening plastics are shown below.


Thermosoftening plastic examples:-

Thermosetting plastic examples:-

Plastics and Pollution

Nearly all plastics are non-biodegradable. This means that they will not rot away naturally either through the weather or by bacteria in soil, and can therefore cause a long term litter problem.

There is another pollution problem produced when all plastics burn. As all plastics contain carbon, they can produce the toxic gas carbon monoxide (CO) when they are burned. However some plastics can give off other toxic gases when they burn, depending on which elements are present in the formula of the plastic.

Plastics and the Ocean

The problem with plastic is that most of it isn't biodegradable. It doesn't easily degrade, like paper or food, so instead it can be present in the environment for hundreds of years.

All animals, whether they live on land or in the sea, can be hurt by plastic. They can get trapped in bigger items such as carrier bags or food packaging.

Birds, fish and shellfish can mistake plastic for food when it has broken down into smaller pieces. They can't digest plastic so their stomachs can become full, meaning they don't have room for their food.

The video below shows some of the impacts of marine plastic pollution:-