3Plastics There are two main groups of plastics: thermosetting plastics thermoplastic plastics.The word ‘plastic’ means ‘easily shaped or moulded’.What does the word ‘plastic’ mean?Thermoplastic plastics:Formed by heatSoften when heatedMade of long polymer chains with few cross linksCan be reheated and reshapedMostly recyclable
4Plastics Thermosetting plastics: Initially set by heat Consist of polymer chains with strong bonds between each chainCannot be reshaped once setCommon in powder or resin formsStrong and durableWhat items do you have in your possession that are made of plastic? Are they made from thermoplastic or thermosetting plastics?
5PlasticsThermoplastic plastics are made of long chains of polymers which don’t cross over very often. When heated, the molecules slip easily over one another.Thermosetting plastics also have lots of long chain molecules, but there are links between them. These cross links prevent the molecules from moving over one another.individual monomer moleculeThe one on the left is a thermoplastic plastic and the one on the right is a thermosetting plastic.The teacher might also want to mention elastomers, which only have a few cross links.individual monomer moleculeWhich of these diagrams depicts a thermoplastic and which depicts a thermosetting plastic?
6Thermoplastic plastics The majority of common plastics are thermoplastics.Thermoplastics can be heated and reshaped because of the ways in which the molecules are joined together.This can be repeated many times (as long as no damage is caused by overheating).HeatHardenSoftenCool
8Thermosetting plastics Thermosetting plastics are those which are set with heat and have little elasticity. Once set, they cannot be reheated and reformed.BurnThey are heated and moulded during manufacture.Once cooled, they will not soften again when heated. This breaks the potentially unending cycle that thermoplastic plastics are capable of.If heated too much, they burn.HeatHardenSoftenCool
10Uses of plastics (thermoplastics) Here are some common products made from thermoplastics. Can you say which type of thermoplastic each one is made of?PolystyreneHigh densitypolytheneLow density polythenePolypropyleneABSAcrylicWhy do thermoplastics make good materials for these products?What other uses do these thermoplastics have?
11Uses of plastics (thermosets) Thermosets have different qualities to thermoplastics. Can you say which thermoset each of these products is made from?EpoxyresinPhenol formaldehydeMelamine formaldehydeUreaformaldehydeDiscuss other uses for the thermosets mentioned.Why are thermosets good plastics for the particular products shown?GRPWhy are thermosets good materials for these particular products?What other uses do these thermosets have?
13They can be shaped into very complex shapes. Why do we use plastics?Think of five reasons why we use plastics. How many of them are mentioned below?They are soft to the touch.They can be shaped into very complex shapes.They are tough and durable.They are strong.They are light in weight.They can be shaped easily, quickly and cheaply.They are available in a large range of colours.
14Early and modern plastics Plastic materials were first used at around 2000BC.Plastics can occur naturally in trees or even milk; amber is an example (a resin from trees and insects). Amber was used by early Egyptians to make jewellery and is still used for this purpose today.Today, numerous different plastics are available. Some are still made from natural materials but most modern plastics are manufactured from chemicals obtained from crude oil. Plastics manufacturers convert chemicals into plastics.What implications do you think this has for the environment?
15Oil - the raw material for plastics PumpPipelineWe obtain crude oil from under ground, either on land or at sea.After it has been pumped, it is heated in a fractioning tower and broken down into chemicals.The chemicals consist of nitrogen, carbon, chlorine, sulfur and water.Oil wellLayers of rockGasOil
17Acrylic is available in many different colours. It comes in sheets or as round rods.Acrylic is stronger than glass, but scratches easily.It is also used to make many outer cases for toys.These properties make acrylic ideal for shop signs.Acrylic can resist the weather very well.It can be softened and bent, blown or twisted into many different shapes.
18Polystyrene foamPolystyrene foam can be easily shaped and is very light. This makes it suitable for modelling three-dimensional objects.Which other uses for polystyrene can you think of?
19Bakelite – a very early plastic Bakelite was one of the first artificial plastics and was created in 1909 by Leo Baekeland.It is used for telephones, radios and electrical equipment.Bakelite is a hard, brittle plastic that has a natural dark glossy colour.It resists heat without softening.It is a good thermal insulator and a good electrical insulator.When overheated, it burns and smells.
20Melamine formaldehyde Melamine plastics have similar properties to Bakelite. However, they have the added advantage of being able to be coloured during processing.Melamine formaldehydeUreaformaldehydeMelamine formaldehyde and urea formaldehyde are two types of melamine plastics.
21Polyester resin Polyester resin is a chemical hardener. It will set at room temperature.When set, the plastic is stiff, hard and brittle.When working with it in large projects, it is advised to use gloves and a chemical respirator to protect yourself from the fumes.Polyester resin can be reinforced with glass fibre.Glass-reinforced polyester resin or glass-reinforced plastic (GRP) is used for making motor cycle fairings and boats.
22Vacuum formingWhen plastic is used to make a large number of identical products, industrial processes are used. Specialized machines are used to mass produce products. Press play to see a video of the vacuum forming process.Video courtesy of Formech International Limited (www.formech.com)LinksInformation about vacuum forming and suggested projects can be found atalso details the vacuum forming process, and data sheets about plastic materials, vacuum forming and injection moulding can be downloaded from
23Injection mouldingWhat similarities and differences can you identify between the vacuum forming and injection moulding processes?
24Blow mouldingThere are two main types of blow moulding: the one shown here is the commercial method to blow a bottle. The other uses a simple mould through which compressed air is blown, which forces the plastic into a dome.What similarities and differences can you identify between the blow moulding and vacuum forming processes?
25Line bendingWhat products can you think of that have been manufactured using a strip heater?
26What is the difference between male and female moulds? Compression mouldingWhat is the difference between male and female moulds?
27ExtrusionCan you think of any other products that are manufactured using extrusion?
28Finishing techniquesPlastics have excellent surface qualities. As they are self-finishing, plastics require little or no surface finish. However, when designers want a specific colour or texture, spray paints can be used.Use wet and dry paper (wet) to smooth the surface down and create a ‘key’ for the primer to grip to.Primer is used to create an undercoat to protect the plastic and provide a good surface finish for the top coat. The primer should be rubbed down using wet and dry paper between coats.Top coat is applied once the primer has completely hardened. There are different surface textures; gloss, matt and metallic being the most common. Top coat is applied in several layers to build up a thick, tough surface layer on the top of the primer.
29Plastics combined with other materials Plastics are very useful materials. They are durable, easy to join, self-finishing, easy to mould and not susceptible to rot or sust. However, sometimes their properties are enhanced by combining them with other materials.Carbon fibre can be added to thermosetting plastics to improve their strength. Golf clubs, tennis racquets, cycle frames and fishing rods are made using carbon fibre composite.