Properties and uses
The special properties of these polymers decide what they are used for (table 14.6).
| Polymer | Special properties | Uses |
|---|---|---|
| Polythene | Electrical insulator, waterproof, airtight, light, withstands knocks, lasts long | Plastic bottles, toys, polythene sheet, polythene bags, bins for collecting rubbish, hard plastic crates |
| Polyvinyl chloride (PVC) | Resists fire, electrical insulator, waterproof, light | Rainwear, water pipes, conduit, flexible pipes |
| Teflon | Withstands heat, electrical insulator | Non-stick pans for cooking, ski boots |
Why each property fits its use
All three are made of long chains held by strong covalent bonds, with no ions or free electrons, so all three are electrical insulators — the reason for PVC conduit round house wiring. Polythene is light and does not let water or air through, which suits bags and bottles. PVC's chlorine makes it harder to set alight, which matters for pipes and conduit inside buildings. Teflon's chain is wrapped in fluorine atoms, so it stands up to heat and food does not stick to it. Compare iron, the material PVC pipes often replace: iron is heavy, rusts in water (chapter 12), and conducts electricity.
Insulator does not mean heat-proof
All three insulate electricity, but only Teflon stands up to the heat of a stove. A polythene bag or a PVC pipe left on a hot pan softens and melts.
When you choose a material for a job, work backwards from the job to the property: a raincoat must keep water out, a pipe must not rust or crack, a pan coating must survive heat. Table 14.6 is that reasoning done for three polymers.
Properties decide uses
Polythene: light, waterproof — bags, bottles. PVC: fire-resistant, insulating — pipes, conduit. Teflon: heat-resistant — non-stick pans.