• The main source of synthetic plastics is crude oil.
  
  
• Coal and natural gas are also used to produce plastics.
  
  
• Petrol, paraffin, lubricating oils and high petroleum gases are bi-products, produced during the refining of crude oil.
  
  
• These gases are broken down into monomers. Monomers are chemical substances consisting of a single molecule.
  
  
• A process called Polymerisation occurs when thousands of monomers are linked together. The compounds formed as called polymers.
  
  
• Combining the element carbon with one or more other elements such as oxygen, hydrogen, chlorine, fluorine and nitrogen makes most polymers.
  

Sources of plastics

Where does plastic come from?

• Plastics can be either found in natural substances or may be man-made. Most of the plastics used today are man-made.
  
  
• Man-made plastics are known as synthetic plastics.
  
  
• Natural 'plastic products' occur in such things as animals' horns, animals' milk, insects, plants and trees.
  

Long chain molecules

Thermoplastics

• There are a wide range of thermoplastics, some that are rigid and some that are extremely flexible.
  
  
• The molecules of thermoplastics are in lines or long chains with very few entanglements. When heat is applied the molecules move apart, which increases the distance between them, causing them to become untangled. This allows them to become soft when heated so that they can be bent into all sorts of shapes.
  
  
• When they are left to cool the chains of molecules cool, take their former position and the plastic becomes stiff and hard again. The process of heating, shaping, reheating and reforming can be repeated many times.
  

Cross-linked molecules

Thermosetting Plastics

• The molecules of thermosetting plastics are heavily cross-linked. They form a rigid molecular structure.
  
  
• Whereas in thermoplastics the molecules sit end to end, the molecules in thermoplastics sit end-to-end and side-by-side.
  
  
• Although they soften when heated the first time, which allows them to be shaped they become permanently stiff and solid and cannot be reshaped.
  
  
• Thermoplastics remain rigid and non-flexible even at high temperatures. Polyester resin and urea formaldehyde are examples of thermosetting plastics.
  
  
• Bakelite was a thermosetting plastic.
  

Plastic memory

• Each time a thermoplastic is reheated it will try and return to its original shape, unless it has been damaged due to overheating or overstretching. This property is called plastic memory.
  
  
• This is why a shape formed in thermoplastic becomes flat when reheated.
  

Long chain molecules

Cross-linked molecules

Heating and cooling cycle of thermosetting plastic

Substances added to plastics

• Certain plastic products are created in such a way so that they possess specific properties and qualities depending upon where and how the product will be used.
  
  
• Particular products need have mechanical properties. They may need to be hard, strong and have a high resistance to impact.
  
  
• Other products need to possess certain environmental properties. They may need to have a resistance to chemicals such as acids and alkalis.
  
  
• Plastics used outside may need to have a high resistance to ultra violet light and be flame resistant. For some it may be important that they do not absorb water.
  
  
• Before the raw materials can be converted into finished products they need other substances added in order to give the required properties.