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Chapter 28 Plastics and Polymers

Chapter 28 Plastics and Polymers. 28.1 What are plastics?. 28.2 Simple tests on plastics. 28.3 Classifying plastics. 28.4 General uses of plastics. 28.5 Production of plastic articles. 28.6 What are polymers?. 28.7 Alkenes. 28.8 Addition polymerization. 28.9 Common addition polymers.

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Chapter 28 Plastics and Polymers

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  1. Chapter 28 Plastics and Polymers 28.1What are plastics? 28.2Simple tests on plastics 28.3Classifying plastics 28.4General uses of plastics 28.5Production of plastic articles 28.6What are polymers? 28.7Alkenes 28.8Addition polymerization 28.9Common addition polymers 28.10Condensation polymerization 28.11Common condensation polymers 28.12Thermal properties and structures of plastics 28.13Plastics and economy 28.14Problems associated with the use of plastics CONTENTS OF CHAPTER 28

  2. 28.1 WHAT ARE PLASTICS? THE PLASTIC AGE Plastics are now replacing metals, glass, cotton, wool, leather and wood. We can find them everywhere. We are really living in a plastic age. 28.1 WHAT ARE PLASTICS?

  3. Figure 28.2 Many items used to be made of natural materials are now made with plastics. 28.1 WHAT ARE PLASTICS?

  4. A28.1 Soft drink bottles, squeeze bottles, toys, tablecloth, toothbrushes. (Other answers may be given.) DIFFERENT KINDS OF PLASTICS There are about 20 main kinds of plastics. Common plastics include: polythene, polyvinyl chloride (PVC),polystyrene, perspex, nylon, urea-methanal and phenol-methanal. 28.1 WHAT ARE PLASTICS?

  5. WHERE DO PLASTICS COME FROM? Petroleum is the most important raw material used in the production of plastics. About 4% of petroleum is eventually turned into plastics. Plastics come mainly from ethene and other alkenes. Alkenes are obtained by cracking oil fractions (e.g. naphtha and gas oil). DEFINING PLASTICS PLASTICS are man-made polymers which, at some stage during processing, can be softened by heat and then turned into any desired shape. 28.1 WHAT ARE PLASTICS?

  6. Plastics are polymers. Polymers consist of very large molecules, formed by the joining of many small molecules (monomers). For example, 28.1 WHAT ARE PLASTICS?

  7. 28.1 WHAT ARE PLASTICS?

  8. A28.2 Yes. They are made from chemicals derived from petroleum. WHY ARE PLASTICS SO USEFUL? Plastics have properties which make them very useful: 28.1 WHAT ARE PLASTICS?

  9. 28.1 WHAT ARE PLASTICS?

  10. A28.3 (a) A rising general trend. The average mass of plastics in a new car increases steadily over the past 40 years. (b) Bumper. (Other answers may be given.) (c) Yes. (In recent years, most cars from the famous USA manufacturer ‘Saturn’ have the entire car bodies made of a plastic of extra strength.) 28.1 WHAT ARE PLASTICS?

  11. 28.2 SIMPLE TESTS ON PLASTICS It is often difficult to identify a plastic article just from its appearance. This is because plastics can be moulded into any shape and made into various forms. 28.2 SIMPLE TESTS ON PLASTICS

  12. (a) Solid mass (b) Thin films (c) Fibres (d) Expanded foam Figure 28.5 Four common forms of plastics. 28.2 SIMPLE TESTS ON PLASTICS

  13. The following simple tests may help to find out the probable nature of a plastic sample. Strength Bend a thin sample of the plastic. See whether it is flexible or stiff, tough or brittle. Density Put the sample in water. If it floats, it is less dense than water. Melting behaviour Heat the sample gently to find out its melting behaviour. 28.2 SIMPLE TESTS ON PLASTICS

  14. Table 28.1 Properties of some plastics. 28.2 SIMPLE TESTS ON PLASTICS

  15. Figure 28.6 Polythene softens and melts easily, but urea-methanal does not. 28.2 SIMPLE TESTS ON PLASTICS

  16. 28.3 CLASSIFYING PLASTICS We can classify plastics into two broad classes: thermoplastics and thermosetting plastics. A THERMOPLASTIC is a plastic which can be softened by heating and hardened by cooling, the process being repeatable any number of times. A THERMOSETTING PLASTIC (or THERMOSET) is a plastic which, once set hard, cannot be softened again by heating. In Table 28.1, urea-methanal and phenol-methanal are thermosetting plastics. All the rest are thermoplastics. 28.3 CLASSIFYING PLASTICS

  17. 28.4 GENERAL USES OF PLASTICS The uses of a plastic depend on its properties. In general, thermoplastics are flexible, but they melt or catch fire on strong heating. They are mainly used to make plastic bags, bottles, sheets, pipes, textile fibres and so on. Figure 28.7 Polythene (a thermoplastic) burns easily. It is therefore unsuitable for making electrical plugs. 28.4 GENERAL USES OF PLASTICS

  18. Figure 28.8 Objects made of thermoplastics. 28.4 GENERAL USES OF PLASTICS

  19. Thermosetting plastics are usually hard and rigid, and do not melt even at high temperatures. They are used to make objects that have to withstand high temperatures (e.g. casings for electrical appliances and handles of pans). 28.4 GENERAL USES OF PLASTICS

  20. Figure 28.9 Objects made of thermosetting plastics. 28.4 GENERAL USES OF PLASTICS

  21. 28.5 PRODUCTION OF PLASTIC ARTICLES MOULDING PLASTICS Firstly, mix certain additives with the plastic to modify its properties. Secondly, use a mould to turn the plastic into the desired shapes, by applying heat and pressure. A softened (or molten) thermoplastic should be cooled sufficiently in a mould, until it is ‘set’. On the other hand, a softened thermosetting plastic should be heated sufficiently in a mould, until it is set. MOULDING THERMOPLASTICS Injection moulding 28.5 PRODUCTION OF PLASTIC ARTICLES

  22. Figure 28.10 Injection moulding. 28.5 PRODUCTION OF PLASTIC ARTICLES

  23. Figure 28.12 Taking a bucket out of a mould in an injection moulding machine. 28.5 PRODUCTION OF PLASTIC ARTICLES

  24. 28.6 WHAT ARE POLYMERS? POLYMERS AND POLYMERIZATION Polythene is an example of a polymer. Figure 28.15 Polythene consists of very long, chain-like molecules. 28.6 WHAT ARE POLYMERS?

  25. A POLYMER is a compound which consists of very large molecules formed by joining many small molecules repeatedly. POLYMERIZATION is the process of joining together many small molecules repeatedly to form very large molecules. Figure 28.16 In polymerization, many monomer molecules join together to form a polymer molecule. 28.6 WHAT ARE POLYMERS?

  26. A28.4 (a) Yes (b) No 28.6 WHAT ARE POLYMERS?

  27. NATURAL AND MAN-MADE POLYMERS 28.6 WHAT ARE POLYMERS?

  28. We make synthetic polymers from monomers by two basic polymerization processes: Addition polymerization (forming addition polymers) Condensation polymerization (forming condensation polymers) POLYMERS AND PLASTICS All plastics are polymers. On the other hand, not all polymers are plastics. 28.6 WHAT ARE POLYMERS?

  29. A28.5 (a) Nylon is a polymer and also a plastic. (b) Cotton is a polymer but not a plastic. (c) Ethene is neither a polymer nor a plastic. 28.6 WHAT ARE POLYMERS?

  30. 28.7 ALKENES ALKENES ARE STARTING MATERIALS FOR MAKING PLASTICS Many plastics are made from alkenes. Alkenes are usually obtained from the cracking of oil fractions. Alkenes are a homologous series of unsaturated hydrocarbons with the general formula CnH2n (n = 2, 3, 4...). STRUCTURE OF ALKENE MOLECULES The ethene molecule The first member of the alkene series is ethene (molecular formula: C2H4). The structural formula of ethene is: 28.7 ALKENES

  31. Figure 28.18 A ball-and-stick model of ethene molecule. 28.7 ALKENES

  32. Larger alkene molecules Take the example of hex-1-ene: 28.7 ALKENES

  33. Figure 28.19 A ball-and-stick model of hex-1-ene molecule. 28.7 ALKENES

  34. CHEMICAL PROPERTIES OF ALKENES All alkenes have similar chemical properties, as they have the same functional group C = C . Because of the presence of the double bond, alkenes are unsaturated. They are much more reactive than alkanes. Combustion Alkenes burn in excess oxygen to form carbon dioxide and water. For example, 2CH3CH=CH2(g) + 9O2(g)  6CO2(g) + 6H2O(l) 28.7 ALKENES

  35. A28.6 No. Alkenes are important starting materials for making many useful products. It would be a waste to burn alkenes as fuels. Addition reactions Addition reactions are typical reactions of unsaturated hydrocarbons. Most of them take place rapidly at room conditions. 28.7 ALKENES

  36. Reaction with halogens 28.7 ALKENES

  37. hex-1-ene bromine decolorized Br2 in 1,1,1-trichloroethane Figure 28.20 Hex-1-ene (an alkene) decolorizes bromine solution rapidly. 28.7 ALKENES

  38. An ADDITION REACTION is a reaction in which two or more molecules react to give a single molecule. Addition reactions are given only by unsaturated compounds (e.g. alkenes). On the other hand, saturated compounds (e.g. alkanes) can react with halogens only by substitution reactions. Reaction with potassium permanganate solution Alkenes rapidly decolorize an acidified solution of potassium permanganate. For example, 28.7 ALKENES

  39. hex-1-ene KMnO4 decolorized acidified KMnO4 solution Figure 28.21 Hex-1-ene (an alkene) decolorizes acidified potassium permanganate solution rapidly. 28.7 ALKENES

  40. A28.7 Ethene can decolorize purple acidified potassium permanganate solution, but ethane cannot. (Alternative answer: In the dark, ethene can decolorize the red-orange colour of bromine solution immediately, but ethane cannot.) Polymerization Under certain conditions, alkenes can undergo addition polymerization to form plastics. 28.7 ALKENES

  41. To crack medicinal paraffin and test for unsaturation in the gaseous product. 28.7 ALKENES

  42. 28.8 ADDITION POLYMERIZATION WHAT IS ADDITION POLYMERIZATION? ADDITION POLYMERIZATION is a reaction in which monomer molecules join together repeatedly to form polymer molecules, without the elimination of small molecules (such as H2O, NH3 or HCl). 28.8 ADDITION POLYMERIZATION

  43. In most cases, the monomers can be represented by a general formula: Each polymer chain is a macromolecule. Each consists of at least several hundred monomeric units joined together. 28.8 ADDITION POLYMERIZATION

  44. REPEATING UNIT A REPEATING UNIT is the smallest part of a polymer molecule, by repetition of which the whole polymer structure can be obtained. We can thus write the general equation for addition polymerizations as: 28.8 ADDITION POLYMERIZATION

  45. A28.9 28.8 ADDITION POLYMERIZATION

  46. 28.9 COMMON ADDITION POLYMERS MAKING ADDITION POLYMERS POLYTHENE [POLY(ETHENE)] Manufacture The equation for reaction: 28.9 COMMON ADDITION POLYMERS

  47. Properties In general, polythene is light (less dense than water) and low-melting. Uses Its main uses include making plastic bags, wrapping film for food, food boxes, flexible cold water pipes and kitchen wares (e.g. squeeze bottles, wash basins). 28.9 COMMON ADDITION POLYMERS

  48. Figure 28.26 Some polythene products. 28.9 COMMON ADDITION POLYMERS

  49. Figure 28.27 Low-density polythene film used as the roof of a greenhouse. 28.9 COMMON ADDITION POLYMERS

  50. A28.10 28.9 COMMON ADDITION POLYMERS

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