1. 7.4 Polymers
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2. Polymers
The correct term for what we normally call plastics is polymers. The first ‘synthetic’ polymer, called Bakelite, was invented in Synthetic means that it was made from chemicals. It is amazing to think that in only a hundred years, hundreds of different polymers have been invented.
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3. Polymers
Most polymers are made from crude oil. The oil is processed, and a complex series of chemical reactions takes place to make the polymers.
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4. Polymers
Polymers are good insulators of electricity and heat. Some of the stronger polymers can be as strong as metals. Polymers are not normally painted, but their colour can be changed by adding colours to the plastic before it is made into a product.
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5. Types of polymers There are two main types of polymer:
Thermoplastic polymers
Thermosetting polymers
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6. Thermoplastic polymers
The term ‘plastic’ that we often use for all polymers is an abbreviation of thermoplastic polymer.
When thermoplastic polymers are heated they soften and can be easily shaped. The polymer will harden when cooled and retain the new shape. It can be reshaped when reheated.
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7. Thermoplastic polymers
Properties
Examples of use
Low-density polyethylene (LDPE)
Soft and flexible. Not very strong
Carrier bags, detergent bottles, packaging
High-density polyethylene (HDPE)
Strong
Bowls, buckets, milk crates
Polypropylene (PP)
Can be bent or flexed many times without breaking. High impact strength for a polymer
Lunch boxes, plastic chairs, children’s toys
High-impact polystyrene (HIPS)
Light but strong
Vacuum-formed packaging and casings
Acrylic (polymethyl methacrylate – PMMA)
It can be transparent, like glass, or coloured with pigments. Hard wearing and will not shatter on impact
Plastic windows, bath tubs, display sign
Polyvinyl chloride (PVC)
Stiff and hard wearing
Pipes, coverings for electric cables, floor and wall coverings, packaging
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8. Thermosetting polymers
Thermosetting polymers are also known as thermosets. They are normally formed into a shape by a chemical reaction between liquids or powdered ingredients. Once they have set their shape cannot be changed again. They are harder and more rigid than thermoplastics. They cannot be recycled.
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9. Thermosetting polymers
Properties
Examples of uses
Epoxy resins
Good resistance to chemicals and wear. Strong when reinforced
Adhesives, printed circuit boards
Melamine formald- hyde
Good strength. Resistant to scratches
Laminates for work surfaces, plastic plates
Urea formald- hyde
Good strength, but brittle – it will break rather than change shape
Electrical fittings, light sockets, switches, plugs
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10. Summary There are two types of polymer: thermosets and thermoplastics.
Once a thermoset has been made into a product, its shape cannot be changed again.
The shape of a thermoplastic can be changed by heating it up again.
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