An introduction to plastics http://www.calibre.co.nz/plasticg.htm What is plastic? Plastic is a common name for Polymers ( a polymer is a compound of high molecular weight the structure of which is composed of chains of small repeat units): materials made of long strings of carbon and other elements. Each unit in a string is called a monomer, and is a chemical usually derived from oil. The monomer is made into polymer by chain-linking reactions. This is like making a daisy chain. Instead of flowers, carbon atoms are joined together. The appearance of the daisy chain will be different if you use different coloured flowers, and so will polymers. There are many different types of plastic, depending on the starting monomer selected, the length of polymer chains, and the type of modifying compounds added. Each plastic has been developed for a special purpose. There are two main groups of plastics: 1. THERMOPLASTICS soften with heat and harden with cooling. Some typical thermoplastics are: Acrylic (Perspex) Acrylo-nitrile (Nylon) Polyethylene (Polythene) Polypropylene Poly Vinyl Acetate (PVA) Poly Vinyl Chloride (PVC) Polystyrene and ABS PTFE (Teflon) 2. THERMOSETS are cured or hardened by heat. Some typical thermosets are: Bakelite Epoxy Melamine Polyester Polyurethane Properties Plastics are used because they are: Attractive Hard and slippery Soft and rubbery Tough and slippery Flexible Good insulators of heat or electricity Light weight Hygienic Non-rusting Easy to shape and colour Cheap ---------------------------------------- Plastic Manufacturing Methods Plastics are made into shapes in many ways: 1. EXTRUSION Hot molten plastic is squeezed through a nozzle to make long lengths of special shapes like pipes, spouting and wallboard joining strips. It is also used to make large thick sheets of plastic for fabrication. 2. BLOW EXTRUSION (Fig 1) This is used for making plastic films and bags. While it is still hot, an extruded tube is blown up like a balloon, with compressed air. This stretches the plastic and makes it thin. The balloon is made long enough to allow the plastic to cool. The end of the balloon is pinched together by rollers, to hold the air in and make it flat. The flat tube is then wound on to a big roll. You can see continuous rolls of plastic bags in a fruit shop. 3. INJECTION MOULDING Hot molten plastic is squeezed into a mould to make lots of objects all the same. They can be very small like a washer or quite large, like a bowl or a clothes basket. Lots of everyday articles are made this way. 4. BLOW MOULDING (Fig 2) A little bit of hot soft plastic is squeezed into the end of a mould. Compressed air is used to blow a big bubble inside the plastic. The plastic swells out like a balloon until it fills up the whole mould. Many bottles, toys and money boxes are made this way. 5. ROTATIONAL MOULDING Plastic powder is scooped into a mould. The mould is rotated over a big gas burner. As the mould gets hot, the plastic melts and sticks to the mould. This method is used for making big hollow things like water tanks and barrels. 6. COMPRESSION MOULDING This is used for thermoset resins. Dry powder is put in a mould which is squeezed and heated until the plastic is cured. This is used for making ashtrays, cups and plates, and some electrical switches. 7. REACTION INJECTION MOULDING Two chemicals are mixed together and squirted into a mould. The chemicals react together. This is how they make car bumpers, some disposable cups and plates, and the meat trays you get from supermarkets. 8. VACUUM FORMING ( Fig 3) A sheet of plastic is clamped in a frame and heated until it is stretchy. Then it is sucked into a mould. This is how they make the inside of your refrigerator, bath and handbasin. It is also used to make a lot of packaging for cosmetics, chocolates, biscuits, some yoghurt containers and some disposable cups. 9. FABRICATION Some thermoplastics are fabricated like sheetmetal. Sheets of plastic are cut to shape. They can be folded by heating a narrow line through the plastic. When it is soft, the sheet will bend along the heated line. Sheets can be joined together by glueing, or by welding. The join is heated with hot air and a thin filler rod is forced into the gap. These fabrication methods are used to make acrylic signs and displays, and industrial tanks and equipment. Thin flexible plastic sheets are used for making folders, wallets, swimming pool liners, inflatable toys and raincoats. The seams are welded by ultrasonic vibration. 10. A Special Note About Styrene Foam. Styrene foam is made in little pellets. To make blocks of styrene foam, or complicated shapes like a cycle helmet, they scoop lots of pellets into a mould and heat it with steam. The steam makes the pellets swell up and stick together. When you break a moulded block you can see how all the pellets have squashed into each other. ---------------- How Plastics Degenerate Most plastics do not rust or corrode like metals.Plastics are made up of long strings like congealed spaghetti. Solvents seep in between the strings, weakening the structure and making it swell. The material usually softens, and all other properties are also affected. Very strong solvents may even break up and dissolve the strings. Other chemicals, and ultra violet light, cause the chain linking reactions of the plastic to continue and accelerate. The plastic becomes hard and brittle, and small cracks begin to appear at the surface. Once these cracks appear, they continue to grow throughout the material. The cracks usually begin in zones of local stress caused by heating, bending, glueing, or welding. Moulded articles may also have internal stress caused by uneven flow of plastic in the moulding process. Internal stress can be thought of as neighbouring strings within the plastic being stretched by different amounts when the article is made. It is very important for the moulds to be properly designed. ---------------- Some Common Plastic Items I have already mentioned lots of things around your home that are made from plastic, and how they are made. Here are some more: Article How made Plastic Tupperware Injection Polyethylene Bucket Injection Polyethylene Audio Cassette Injection Styrene Ball point pen Injection Styrene Ruler Injection Styrene Shoe soles Reaction Polyurethane Electric cables Extrusion PVC Water pipes Extrusion PVC Water pipes Extrusion PVC Refrigerator liner Vacuum form ABS Milk bottle Blow moulding Polyethylene Plastic bag Blow extrusion Polyethylene I am sure that you can look around and find lots more things and guess how they are made. ---------------- Here are some more points to ponder Plastics are used a lot by doctors and dentists because they are hygienic and will not poison our bodies. Dentists use acrylic to make special white fillings. Can you think of ways that doctors use plastics? Many clothes are made from synthetic fabrics. The fabrics are made from plastic fibres that have been spun out really fine, then made into yarns for knitting or weaving. Plastic fibres are also used to make ropes that are very strong. Fishing lines are made of nylon. Teflon coatings on saucepans and irons are sprayed on as dry powder then baked in a hot oven to make the particles melt together. This is called powder coating and it is also used to put a very tough layer of epoxy or polyester on metal articles like furniture. Many glues are plastics. PVA white glue is a solution of PVA dissolved in water. When it is used to glue two pieces of wood together, the water is soaked up by the wood. The PVA is soaked up with the water. When the water evaporates, the PVA stays behind in the joint, making a strong plastic bridge. Hot melt glue is a thermoplastic. When it is hot it can be squeezed into a joint. When it cools down it goes hard. Other common glues are epoxy, resourcinol, and urea formaldehyde. They are all thermoset plastics made by mixing two chemicals (glue + hardener) to react together. These glues are very important because they are waterproof. They are used to make boats and plywood. Most paints are made from plastic resins. Some, like acrylic house paints and polyurethane varnish are dissolved plastics which dry to form a tough waterproof skin. Other polyurethane, epoxy and polyester paints have to be mixed with a hardener which makes a chemical reaction. Fibreglass resin and automotive body fillers are polyester plastics that you can make yourself with a chemical reaction.
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