Engineering - Plastics / Polymers

Plastic is a state or condition of a material. What we call "Plastics" are referred to correctly in Engineering as Polymers.

Polymers can be divide into 3 main categories :

  1. Natural Polymers : e.g. Amber, Keratin, Natural Rubber
  2. Modified Natural Polymers : e.g. Cellulose, Casein
  3. Synthetic Polymers

Synthetic Polymers are either made from :

  • the by-products of the production of gas from coal or
  • the by-products of the distillation of crude oil

Polymers are manufactured in 2 main ways :

  1. Addition Polymerisation
  2. Condensation Polymerisation

If you would like to learn more about the basics of Plastics or Polymers, as are studied in the Junior Cert. Metalwork course, goto the Polymer page.

Addition Polymerisation

Addition Polymerisation is the adding of single Mers together by the use of a free radical. The vast majority of Polymers are manufactured in this way.

How Mers are Addition Polymerised

Here we will see how an Ethylene Monomer is Polymerised into Polyethylene.(Polythene).

To the right is a single Mer of Ethylene,(Ethyene) called a Monomer.

It is made up of 4 Hydrogen atoms and 2 Carbon atoms. There is a Double Bond between the two Carbon atoms.

Imagine a pot full of millions of Ethylene Monomers.

A Catalyst, called an Initiate Free Radical is added to the pot. The Free Radical is a reactive atom containing an unpaired electron, represented by the dot. Therefore it is 'unhappy', so it joins onto one of the Monomers

When this happens the Double Bond is broken and the Monomer wants to link up with another Mer, in order to be 'happy'.

The bonding between the atoms is a Covalent Bond which is a type of Primary Bond. It is very strong and has a high resistance to heat. In this type of Polymer the Primary Bonds only exist along the chain.

The Mers now join up in a fraction of a second to form long chain molecules. This chain is called a Polymer. It should be easy to see why this method of forming Polymers is called Addition Polymerisation.

In order to stop the reaction a second Free Radical is added, called a Terminate.

The result is that the pot is now full of a large number of interwoven chain molecules, twisted around eachother like spaghetti. Where these chains touch off eachother Secondary Bonds are formed. Therefore the Secondary Bonds provide the three dimensional structure. These bonds are much weaker than Primary Bonds, and are susceptible to heat.

Condensation Polymerisation

Each of the Phenol molecules give up a Hydrogen atom, and the Formaldehyde molecule gives up an Oxygen atom, and these join to form water. The resultant polymer has a Cross Linked atomic structure, meaning that all bonds formed between the molecules are Primary Bonds.


This type of Polymerisation involves different types of Mers. This allows many different Polymers to be manufactured. Below is shown the Copolymer Polyvinylethylchloride. This is a combination of PVC and Polythene.

Classification of Polymers

Polymers can be classified under two main headings :

  1. Thermoplastic materials
  2. Thermosetting materials

Polymers fit into either class according to how they react to heat, and how the Polymer reacts to heat depends on the type of bonds that hold the Polymer together.

  1. Thermoplastic materials : Thermoplastics soften when heated because the heat breaks down the weak Secondary Bonds. This allows the long chains of Mers to 'slip' over eachother, thus allowing the Polymer to be reshaped. When the Polymer cools down the Secondary Bonds reform and the Polymer solidifies.Thermoplastic polymers can be resoftened and reshaped many times.

  2. Thermosetting materials : Thermosets do not have Secondary Bonds, only Primary Bonds, and Primary Bonds are not affected by heat. As a result additional heat, after polymerisation, does not have the effect of softening the Polymer. The Polymer will retain its mechanical properties until it starts to burn. Heat, along with pressure, is used in the production of Thermosets, in order to set or cure them.

Additives Used in Polymers

Additives are used either to improve the properties, or change the properties of a Polymer.

Some common additives are listed below :

Fillers : used to change the mechanical properties of the Polymer, e.g., glass fibre adds strength.

Plasticisers : improve the flexibility of Polymers.

Colour Pigments : give the Polymer the desired colour.

Stabilisers & Antioxidants : prevent the degrading effects of heat and the elements on the Polymer.

Properties and Uses of some common Polymers

( TP = Thermoplastic TS = Thermosetting )

Polymer Type Main Properties Main Uses
Acrylonitrile Butadiene
TP Very tough, Resists chemical attack Telephones, domestic waste pipes
Low Density Polyethylene
TP Good electrical insulator, translucent, fairly flexible Food protection film, electric cable covering, plastic bags
High Density Polyethylene
TP Translucent, fairly stiff and hard Plastic bottles
Polyvinyl Chloride
TP Transparent if no fillers or colours added Guttering, car seat upholstery when plasticised
TP Stiff and hard Yoghurt cartons, disposable cups
TP Highly transparent, good surface gloss Illuminated signs, rear lights of motor vehicles
TP White, smooth texture Non stick coating on cookware
TP Opaque, stiff and hard Curtain rails, bearings and gears, ropes
Polyester film TP Transparent, stiffer than other films Audio tape, cooking bags
Urea Formaldehyde
TS Very hard and stiff, hard wearing Plug tops, switch covers
Phenol Formaldehyde
TS Good electrical insulator, hard and stiff, flame resistant Handles for domestic cooking utensils
Polyester resin TS Very stiff and hard, excellent binder for fibre reinforcement Boat building, car and aircraft parts