Francisco Carballo (Cheko) PLASTICS Francisco Carballo (Cheko)

PLASTICS Contents Introduction Structure of plastics Properties of plastics Types of plastics Thermoplastics Thermosetting plastics (thermosets) Manufacturing with plastics. Processes Extrusion Extrusion blow-moulding Injection moulding Compression moulding Vacuum forming Line bending

PLASTICS Introduction ‘Plastic-like’ natural materials were first used thousands of years ago. Amber, for example, was used by the Egyptians to make jewelry. Today, numerous different plastics are available. Most are made entirely from chemicals obtained from crude oil and, to a much lesser extent, coal. It is the job of plastics manufacturers to convert these chemicals into plastics and then, produce goods. Plastics (polymers and copolymers) are synthetic resinous substances (based on long chain molecules with carbon) that can be moulded with the aid of heat and/or pressure.

PLASTICS Structure All substances are made up of tiny units called molecules. We can learn about the general structure of plastics with an example. Polythene is made by ‘persuading’ thousands of free roaming ethene gas molecules to join together to form long chain polythene molecules. The persuasion is provided by chemicals known as catalysts. The newly formed polythene molecules do attract one another and become tangled and twisted together to form the solid (high density polythene). Small molecules such as ethene, which can link together this way, are called monomers. The process of joining molecules is called polymerization, and the products are called polymers.

PLASTICS Properties The name ‘plastic’ describes a material which, at some point in its manufature, behaves in a plastic way. In other words, it will deform under pressure, and retain the ‘new shape’ when the pressure is removed. The properties of plastics depend on their composition, but in general, their properties are the following: Physical properties: Malleability, ductility, mechanical strength, insulators (acoustic, electric and thermal), light (low density) and waterproof. Ecological properties: Most of the plastics are not biodegradable. Many plastics can be recycled to reuse them. Some can be also burnt to get energy from the combustion, although this is very aggressive to the environment.

PLASTICS Types Thermoplastics Thermosets On first heating, these plastics soften and can be moulded into shape under pressure. However, the heat triggers a chemical reaction in which the molecules become permanently locked together. The reaction is known as cross linking. As a result, the polymer becomes permanently ‘set’ and cannot be softened again by heating. They soften on heating and can be moulded into shape. On cooling they harden again. On heating, the molecules are given the energy to move apart. As a result, the forces between the molecules become weaker. This allows them the freedom to slip over one another to form a new shape when under pressure. This process of softening and hardening can be repeated over and over again because the molecules in a thermoplastic are always free to behave in this way.

PLASTICS Thermoplastics (I) Polythene (high-density) It is made in such a way that the chains are ‘straight’. This allows the molecules to pack close together to produce a high density material. As a result, It is a fairly stiff, strong plastic which is also quite tough. It softens at a fairly high temperature (125 ºC), and is resistant to chemical attack.

PLASTICS Thermoplastics (II) Polythene (low-density) It is made by a process which produces side branches on the chains. This prevent the chains from packing close together. As a result, it is the polymer is weaker, softer and more flexible than high density polythene. It is a good electrical insulator and can be transparent or opaque. It softens at a lower temperature (85 ºC). It is the most consumed polymer.

PLASTICS Thermoplastics (III) Polypropylene It belongs to the same family as polythenes. It is tougher however, and more rigid than high density polythene. It has a great resistance to heat (it softens at 150 ºC). It has the lowest density of the thermoplastics, and yet it has a very high impact strength. Besides, it can be flexed thousands of times without breaking.

PLASTICS Thermoplastics (IV) Polyvinyl chloride PVC can be produced to give a range of properties. The stiff, hard wearing PVC used to make drain pipes and guttering is one example. A more flexible and rubbery material can be produced by adding plasticiser. In its softer forms is used as an insulator for electrical cables. Using a higher proportion of plasticiser ‘leathercloth’ can be produced, to manufacture for instance handbags or car seats.

PLASTICS Thermoplastics (V) Acrylics The acrylic polymer most known is polymethyl methacrylate (perspex). It can have a glass-like transparency or be opaque coloured with pigments. It is fairly hard wearing and will not shatter. However, it can crack when is cut or drilled and is easily scratched. Acrylics can be formed, bent and twisted when heated to temperatures around 170 ºC.

PLASTICS Thermoplastics (VI) Nylon Many different types of nylon are produced, which are identified with a number (for example type 6.6 and type 6.10). It is best known in the form of a fibre, being used in the manufacture of clothing, carpets, brushes, etc. It is a fairly hard material with a good resistance to wear and chemical attacks. ‘Solid nylon is used for engineering purposes, particularly for making fast moving parts such as gears. In this form it is usually creamy white.

PLASTICS Thermoplastics (VII) Polystyrene It is available in two forms mainly. As a crystal clear solid it is very brittle and can be identified by the metallic ring it makes when dropped. If during manufature a gas becomes trapped within its honeycomb structure, then this material will be a good heat insulator, very light and very good absorbing shocks (used for packaging). As a ‘foamed’ plastic, is known as expanded polystyrene (porexpan).

PLASTICS Thermosets (I) Bakelite It was the first plastic to be made artificially from chemicals, in 1909. It is a hard, brittle plastic with a natural dark glossy colour. As it is a ‘thermoset’ plastic, it resists heat without softening but at very high temperatures it will decompose. It is a good thermal and electrical conductor. It is not used extensively these days.

PLASTICS Thermosets (II) Urea formaldehyde It is a colourless polymer, but it can be coloured artificially with pigments to produce articles in a wide range of colours. It is harder than bakelite and has no taste or odour. It is a good thermal and electrical conductor.

PLASTICS Thermosets (III) Melamine formaldehide This polymer has similar properties to urea formaldehide and is used in the manufacture of high quality tableware. Its heat resistant properties make it particularly suitable for the surfaces of laminated kitchen worktops.

PLASTICS Thermosets (IV) Polyester resin This thermosetting plastic polymerizes at room temperature. The resin and a chemical (a hardener), are mixed just before use. When set, the plastic is stiff, hard and brittle. To add strength, it is reinforced with glass fibre to make glass reinforced plastic (GRP).

Manufacturing processes PLASTICS Manufacturing processes Extrusion Extrusion blow-moulding Injection moulding Compression moulding Vacuum forming Line bending

Manufacturing processes PLASTICS Manufacturing processes Extrusion Plastic granules are fed from a hopper on to a rotating screw. The screw forces the plastic through a heated tube where it becomes molten before being forced under pressure through a die with the required shape. This is the process to make ‘long’ products like drain-pipes or curtain rails.

Manufacturing processes PLASTICS Manufacturing processes Extrusion blow-moulding The plastic expands an takes up the shape of the mould. The mould is then opened and removed. Air is blown into a section of extruded plastic tube. This process is used for making articles like bottles and hollow toys. PVC, polythene and polypropylene are common blow-moulding materials.

Manufacturing processes PLASTICS Manufacturing processes Injection moulding The screw is moved backwards, as it rotates, until a measured quantity of plastic is at position ‘A’. The screw is then driven forwards by a hydraulic ram, injecting the molten plastic into the mould. Products manufactured this way have accurate dimensions and a high quality finish.

Manufacturing processes PLASTICS Manufacturing processes Compression moulding Compression moulding is used for thermosetting plastics, because the heat from the mould triggers the chemical reaction known as ‘cross linking’.

Manufacturing processes PLASTICS Manufacturing processes Vacuum forming This process is used to manufature shallow products from thermoplastic sheets (acrylic, polystyrene or PVC).

Manufacturing processes PLASTICS Manufacturing processes Line bending

PLASTICS