Forces between molecules determine the properties of plastics. Strong covalent bonds hold the atoms together in long chains. But its the bonds between the different molecule chains tht determine the properties of the plastic.
Weak forces: Individual tangled chains of polymers, held together by weak, intermolecular forces, are free to slide over each other. Thermosoftening polymers don't have cross-linking between chains. The forces between the chains are really easy to overcome, so its really easy to melt the plastic. When it cools, the polymer hardens into a new shape. You can melt these platics and remould them as many times as you want.
Strong forces: Some plastics have stronger intermolecular forces between the polymer chains, called cross links, that hold the chains firmly together. Thermosetting polymers have crosslinks. These hold the chains together in a solid structure. The polymer doesn't soften when its heated. Thermosetting polymers are strong, hard and rigid.
How you make a polymer affects its properties. The starting materials and reaction conditions will both affect the properties of a polymer.
The two types of polythene can be made using different condidtions:
- Low density polythene is made by heating ethen to about 200 degrees under high pressure. Its flexible and is used for bags and bottles
- High density polythene is made at a lower temperature and pressure (With a catalyst). Its more rigid and is used for water tanks and drain pipes.