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 :
- Natural Polymers : e.g. Amber, Keratin, Natural Rubber
- Modified Natural Polymers : e.g. Cellulose, Casein
- 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 :
- Addition Polymerisation
- 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, creating a Covalent Bond, and the Monomer wants to link up with another Mer, in order to be 'happy'.
A Covalent Bond is a type of Primary Bond, which 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.
Copolymerisation
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 :
- Thermoplastic materials
- 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.
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.
Thermosetting materials : Thermosets do not have Secondary Bonds, only Primary Bonds, and Primary Bonds are not affected by heat. As a result heat 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 (ABS) |
TP | Very tough, Resists chemical attack | Telephones, domestic waste pipes |
| Low Density Polyethylene (LDPE) |
TP | Good electrical insulator, translucent, fairly flexible | Food protection film, electric cable covering, plastic bags |
| High Density Polyethylene (HDPE) |
TP | Translucent, fairly stiff and hard | Plastic bottles |
| Polyvinyl Chloride (PVC) |
TP | Transparent if no fillers or colours added | Guttering, car seat upholstery when plasticised |
| Polystyrene (PS) |
TP | Stiff and hard | Yoghurt cartons, disposable cups |
| Polymethalmethacrylate (Perspex) |
TP | Highly transparent, good surface gloss | Illuminated signs, rear lights of motor vehicles |
| Polytetrafluorethylene (PTFE) |
TP | White, smooth texture | Non stick coating on cookware |
| Nylon (Polyamide) |
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 (UF) |
TS | Very hard and stiff, hard wearing | Plug tops, switch covers |
| Phenol Formaldehyde (PF) |
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 |