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Many resins are brittle and unstable on theirown, but can be transformed into useful materialsby being combined with a wide range ofadditives such as stabilisers, fire retardants andpigments, and fillers such as wood flour, micaand glass fibres. Fibre addition can greatlyimprove mechanical properties, and this is wellillustrated by plastics boat hulls and car bodypanels moulded from polyester resin reinforcedwith glass fibre. In a similar way, epoxy resinreinforced with carbon fibre produces componentswith outstanding strength which arewidely used in the aeronautics industry.RUBBERIn a category of its own is rubber, natural orsynthetic, flexible or rigid. Rubber is derivedfrom a naturally occurring polymer, latex, andhas many applications ranging from adhesivesand condoms to O-rings and tyres. Syntheticrubbers were pioneered in Germany duringWorld War II and the range of applications forrubber-like materials continues to expandthrough the development of a new family calledelastomers, plastics with rubber-like properties.PRE- AND POST-WAR PLASTICSThe textile industry was to prove a major innovatorin terms of developing new chemicalpolymers, the most notable of which was polyamidedeveloped in 1938 by Dupont in the USAand better known as Nylon. This was used asfilaments in toothbrushes and as yarn in stockingsand parachutes. When nylon mouldingpowder became available it was used for itemssuch as spectacle frames and, reinforced withglass fibre, for engineering applications such asgear wheels. In the 1930s ICI in the UK wasworking on polyethylene (usually abbreviated topolythene), and with the onset of World War IIand the need for radar insulation, research wasspeeded up using a high-pressure process. Thisproduced LDPE (low-density polyethylene),which was flexible and, like many plastics, anexcellent insulator. It quickly found a use asinsulation for electric wires and cables, whichhitherto had been the territory of rubber. In1953 Professor Karl Ziegler discovered thatpolythene could be produced at much lowerpressures by using special catalysts. The resultingmaterial was more rigid and is known asHDPE (high-density polyethylene).Mouldings made from LDPE include washingupbowls as well as films used extensively inthe packaging industry, whereas HDPE is usedto make milk bottles and sewage, water and gaspiping. Both these materials belong to the polyolefingroup of plastics, of which the thirdmember is polypropylene (PP). PP can befound in applications ranging from ropes andfishing nets to garden furniture.PVC AND MISINFORMATIONPolyvinyl chloride (PVC) was patented in Germanyin the 1930s and grew into an importantcommodity plastic owing to its insulation properties,clarity and ability to absorb plasticisers,which makes it extremely flexible. FlexiblePVC can be found in innumerable applicationsfrom leather-cloth in car interiors to most medicaltubing, blood transfusion bags and floortiles. Most electrical cables are insulated withflexible PVC. Unplasticised (rigid) PVC isused for building products such as windowframes and guttering. In recent years PVC hascome under pressure from misguided environmentalgroups due to the theoretical possibilitythat it can cause the production of dioxins whendisposed of by incineration. Extensive testswith modern incinerators have demonstratedthat this is not true. PVC is now probably oneof the best environmentally understood plasticsand its flame-retardant properties have made itfirst choice for many architectural applications.16
A PLASTIC FOR THE TOY INDUSTRYA major group of plastics is based on the simplechemical, styrene. Polystyrene tends to be brittlebut can be modified with rubbers to createimpact-resistant plastics. Examples includeHIPS (high-impact polystyrene), ABS(acrylonitrile–butadiene–styrene) and SAN(styrene–acrylonitrile). Widely used initially inthe toy industry, styrenic plastics are now employedextensively in the automotive, computerhousing and kitchen utensil industries andcontinue to find new applications.THE INDUSTRY’S PET PLASTICAlong with Nylon, another major plastic materialadopted by the textile industry is polyester.Polyester resin for casting was developed in1941 and at the same time an improved polyesteryarn made from extruded filaments was discoveredin the UK and developed by ICI underthe trade name Terylene, being produced commerciallyin the USA under trade names such asDacron and Trevira. In the 1970s an improvedpolyester found widespread application in themanufacture of bottles using the injectionstretch blow process. The name of this materialis polyethylene terephthalate, commonly knownas PET. It has clarity, sparkle, high impactstrength and excellent gas barrier properties,which make it ideal for moulding bottles forcarbonated soft drinks and mineral water. It hassince found wide application in producingcontainers for detergents and mouthwash.Other important plastics include polycarbonate,a material with very high impact resistance andtemperature stability. Typical applications arepanels for bus shelters and products that needsterilisation, such as babies’ feeding bottles, butdevelopments are now taking place in rooflights and car windscreens. The experiencegained from developing polycarbonate-coatedspectacle lenses could help alleviate the problemsof scratches. Polyacetal has been used tomake kettles and polymethyl methacrylate(Perspex), first used for aircraft canopies inWorld War II, has been moulded into car rearlights and can now be found in a wide variety ofuses such as baths and shop signs. Rigid andflexible polyurethane foams are widely usedin acoustical and padding applications, whileexpanded polystyrene conserves heat as an insulatorand protects against damage in packaging.MOULDING PROCESSESPlastics processing usually starts with feedingplastic granules through a hopper onto a rotatingscrew inside a heated cylinder, which melts thegranules as it moves the molten plastic forward.At the end of the cylinder the melt is forced intodifferent types of moulds: a slit-shaped die producesa continuous sheet, a shaped die producesa profile, an annular die extrudes rod, tube orpipe, and can even blow a vertical bubble formanufacturing thin films or a parison (a shortlength of semi-molten material) from whichbottles can be blown. These processes justdescribed are all forms of extrusion moulding.Injection moulding is a fast, mass-productionprocess whereby the screw acts as a plunger andpushes molten plastic into a closed mould.When the plastic in the mould is cool and set,the mould is opened to eject the shaped moulding.Whereas extrusion is a continuous process,injection moulding is an interrupted process.There are many other processes of which thebest known is vacuum forming, whereby a sheetof plastic is heated and softened and suckeddown into a mould. Applications include sandwichpacks and rigid packaging for toys. Rotationalmoulding is a process for moulding largehollow objects such as footballs and storagetanks, in which polymer powders or liquid plastisols,typically polyethylene and PVC, areplaced inside a closed mould and tumbledwhilst inside an oven so that the resin softensand coats the inside of the mould. Powderedplastics can also be sprayed onto wire and heatfusedto make kitchen baskets or furniture in thefluidised-bed coating process, while in anotherprocess, invented in 1956, extrusion dies areoscillated so that the flow of plastic is split intotwo and then joined up, creating Netlon mesh.A MATERIAL FOR DESIGN ENGINEERSPlastics are ideal materials for designers to workwith and can be tailored to suit each application;e.g. they can be temperature- and wear-resistant,or soft to the touch. Most unmodified polymersare clear and can therefore be pigmented tomatch any specified colour, whether transparentor opaque, and surfaces can be matt, glossy ortextured. The ability to create three-dimensionalforms with gentle curves or sharp edges allowsthe designer full imaginative reign.17
Page 2 and 3: Contents:An Interview with Harry Pe
Page 4 and 5: The Bedales Science Magazine
Page 6 and 7: An Interview with Harry PearsonCan
Page 8 and 9: 6-2 Physics Trip to CERN, Genevaver
Page 10 and 11: The Final Century?t the end of Worl
Page 12 and 13: Spring at the North Polehere’s a
Page 14 and 15: PC hazards and the lawHAZARDOUS WAS
Page 16 and 17: Incinerators are some of the larges
Page 18 and 19: IJoseph Rotblatfind myself writing
Page 22 and 23: RECYCLINGThe reprocessing of plasti
Page 24 and 25: and processed to drinking water sta
Page 26 and 27: Interview with Professor Tony Marmo
Page 28 and 29: Finally, what should Bedales be doi
Page 30 and 31: Hydrogen Fuel Cell Car(We built one
Page 32 and 33: • Less land needed for landfill s
Page 34 and 35: The Science of the Sune know very l
Page 36 and 37: The 6-1 Physics Trip to Thorpe Park
Page 38 and 39: Stem Cell Researchlthough medicine
Page 40 and 41: The potential for technological adv
Page 42 and 43: How Aspirin Worksrostaglandins are
Page 44 and 45: effectively a single species model
Page 46 and 47: References and notes:[1] Roy Porter
Page 48: The B-Sci CrosswordAcross1. The pro

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