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1 Chapter 28Plastics and Polymers 28.1What are plastics? 28.2Simple tests on plastics 28.3Classifying plastics 28.4General uses of plastics 28.5Production.

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Presentation on theme: "1 Chapter 28Plastics and Polymers 28.1What are plastics? 28.2Simple tests on plastics 28.3Classifying plastics 28.4General uses of plastics 28.5Production."— Presentation transcript:

1 1 Chapter 28Plastics 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 CONTENTS OF CHAPTER 28 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

2 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 3 Figure 28.2 Many items used to be made of natural materials are now made with plastics. 28.1 WHAT ARE PLASTICS?

4 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 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 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 7

8 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 9

10 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 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.

12 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 13 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 The following simple tests may help to find out the probable nature of a plastic sample.

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

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

16 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.

17 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.

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

19 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 20 Figure 28.9 Objects made of thermosetting plastics. 28.4 GENERAL USES OF PLASTICS

21 21 28.5PRODUCTION 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 22 Figure 28.10 Injection moulding. 28.5 PRODUCTION OF PLASTIC ARTICLES

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

24 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.

25 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 26 A28.4 (a)Yes(b)No 28.6 WHAT ARE POLYMERS?

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

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

29 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 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 C n H 2n (n = 2, 3, 4...). STRUCTURE OF ALKENE MOLECULES The ethene molecule The first member of the alkene series is ethene (molecular formula: C 2 H 4 ). The structural formula of ethene is:

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

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

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

34 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, 2CH 3 CH=CH 2 (g) + 9O 2 (g) 6CO 2 (g) + 6H 2 O(l) 28.7 ALKENES

35 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 36 28.7 ALKENES Reaction with halogens

37 37 Figure 28.20 Hex-1-ene (an alkene) decolorizes bromine solution rapidly. hex-1-ene Br 2 in 1,1,1-trichloroethane bromine decolorized 28.7 ALKENES

38 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 39 Figure 28.21 Hex-1-ene (an alkene) decolorizes acidified potassium permanganate solution rapidly. KMnO 4 decolorized hex-1-ene acidified KMnO 4 solution 28.7 ALKENES

40 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.) 28.7 ALKENES Polymerization Under certain conditions, alkenes can undergo addition polymerization to form plastics.

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

42 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 H 2 O, NH 3 or HCl).

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

44 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 45 A28.9 28.8 ADDITION POLYMERIZATION

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

47 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 48 Figure 28.26 Some polythene products. 28.9 COMMON ADDITION POLYMERS

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

50 50 A28.10 28.9 COMMON ADDITION POLYMERS

51 51 28.9 COMMON ADDITION POLYMERS POLYSTYRENE Laboratory preparation The equation for reaction:

52 52 Figure 28.29 Laboratory preparation of polystyrene. 28.9 COMMON ADDITION POLYMERS

53 53 28.9 COMMON ADDITION POLYMERS To prepare polystyrene.

54 54 Properties Polystyrene is transparent, hard and brittle. It can be made into expanded polystyrene by heating granular polystyrene with a foaming agent. Expanded polystyrene is a white solid foam. It is very light but still quite rigid. It is an excellent heat insulator and a good shock- absorbent. Uses Being transparent, polystyrene is used to make see-through containers. 28.9 COMMON ADDITION POLYMERS

55 55 Figure 28.30 Some polystyrene products. 28.9 COMMON ADDITION POLYMERS

56 56 Expanded polystyrene is widely used in packaging. It is also used to make disposable foam cups and food boxes. Figure 28.32 Expanded polystyrene is widely used in packaging. 28.9 COMMON ADDITION POLYMERS

57 57 Figure 28.34 Foam cups and food boxes made of expanded polystyrene. 28.9 COMMON ADDITION POLYMERS

58 58 PERSPEX Manufacture and laboratory preparation The equation for reaction: 28.9 COMMON ADDITION POLYMERS

59 59 Properties Perspex is highly transparent. Although it is tough and does not break easily, it can be quite easily scratched. Uses The glass-like transparency of perspex makes it useful as a glass substitute in many ways. Thus it is used in making contact lenses, camera lenses, aircraft windows, street light fittings, optical fibres and illuminated signs. 28.9 COMMON ADDITION POLYMERS

60 60 Figure 28.36 Perspex is used to make the glass of safety spectacles. 28.9 COMMON ADDITION POLYMERS Figure 28.37 Illuminated signs made of perspex.

61 61 POLYVINYL CHLORIDE Manufacture The equation for reaction: 28.9 COMMON ADDITION POLYMERS

62 62 Properties PVC itself is stiff and brittle. It becomes more flexible when mixed with a plasticiser. The properties of PVC can be varied by the addition of different amounts of plasticiser. Uses PVC with little or no plasticiser added is quite rigid, and is used in making bottles for certain chemicals, floor tiles and pipes. 28.9 COMMON ADDITION POLYMERS

63 63 Figure 28.38 PVC pipes. 28.9 COMMON ADDITION POLYMERS

64 64 PVC which has been suitably plasticised has many uses. These include making shower curtains, tablecloths, raincoats, water hoses and artificial leather for making handbags. Being an excellent insulator, it is also used in electrical wire insulation. PVC is not used to make food containers because it is poisonous. 28.9 COMMON ADDITION POLYMERS

65 65 Figure 28.39 Some PVC products. 28.9 COMMON ADDITION POLYMERS

66 66 A28.11 28.9 COMMON ADDITION POLYMERS

67 67 28.10 CONDENSATION POLYMERIZATION CONDENSATION AND CONDENSATION POLYMERIZATION CONDENSATION is a type of reaction in which two or more molecules join together to form a larger molecule, with the elimination of small molecules (such as H 2 O, NH 3 or HCl). Consider the following reaction.

68 68 A special type of condensation reaction is condensation polymerization, in which a polymer is formed. CONDENSATION POLYMERIZATION is a reaction in which monomer molecules join together to form polymer molecules, with the elimination of small molecules (such as H 2 O, HCl or NH 3 ). An example of condensation polymerization Consider an example of condensation polymerization. A dioic acid,, and a diol, HOCH 2 CH 2 OH, react as follows: 28.10 CONDENSATION POLYMERIZATION

69 69 Repeated condensations lead to the formation of long polymer chains, with the structure shown below: The polymer formed here is a polyester, commonly known as Terylene. 28.10 CONDENSATION POLYMERIZATION

70 70 Figure 28.40 Uses of polyester: (a) Making textile fibres (b) Making sails (a)(b) 28.10 CONDENSATION POLYMERIZATION

71 71 The repeating unit of Terylene can be written as, which is derived from one dioic acid molecule and one diol molecule. A28.12 (a)Yes(b)No(c)No(d) No 28.10 CONDENSATION POLYMERIZATION

72 72 Using block diagrams to illustrate condensation polymerization Refer back to the formation of Terylene. The whole process can be represented by the following equation: The repeating unit is: 28.10 CONDENSATION POLYMERIZATION

73 73 28.11 COMMON CONDENSATION POLYMERS NYLON Nylon 6.6 is formed from the two monomers:

74 74 Figure 28.41 Laboratory preparation of nylon 6.6. 28.11 COMMON CONDENSATION POLYMERS Laboratory preparation of nylon 6.6

75 75 28.11 COMMON CONDENSATION POLYMERS nylon 6.6

76 76 28.11 COMMON CONDENSATION POLYMERS Nylon rope trick.

77 77 A28.13 A28.14 Water molecules, H 2 O. 28.11 COMMON CONDENSATION POLYMERS

78 78 Properties Nylon has a high tensile strength. Its melting point is quite high (about 200 o C). Uses Nylon has been the most important synthetic fibre. It is used in various kinds of clothing (e.g. stockings, jackets). It resists creasing and drips dry quickly. It is not attacked by moth. However, nylon lacks moisture-absorbing properties of natural fibres. Nylon fibres are an ideal material for making ropes, carpets, fishing lines, fishing nets and strings for tennis rackets. 28.11 COMMON CONDENSATION POLYMERS

79 79 Figure 28.42 The life of this mountain climber relies very much on the high tensile strength of the nylon rope. 28.11 COMMON CONDENSATION POLYMERS Figure 28.43 Some nylon products.

80 80 UREA-METHANAL Urea-methanal is formed from the two monomers: Laboratory preparation There are two stages in this condensation polymerization. During the first stage, repeated condensations occur with the elimination of water molecules, forming long chains. 28.11 COMMON CONDENSATION POLYMERS

81 81 During the second stage, further condensations occur. Many cross-links (covalent bonds) are formed between the polymer chains. This results in a hard, rigid 3-dimensional giant network. 28.11 COMMON CONDENSATION POLYMERS

82 82 28.11 COMMON CONDENSATION POLYMERS

83 83 28.11 COMMON CONDENSATION POLYMERS To prepare urea-methanal.

84 84 A28.15 The laboratory must be well-ventilated. (Methanal is toxic.) Wear safety spectacles and handle concentrated sulphuric acid with great care. (Concentrated sulphuric acid is corrosive.) Add only one drop of concentrated sulphuric acid. (The polymerization reaction gives out a lot of heat. If a few drops of the acid were added all at once, the reaction would become so violent that the mixture spurts out.) 28.11 COMMON CONDENSATION POLYMERS

85 85 Properties Urea-methanal is white. It is an excellent electrical insulator and is resistant to chemical attack. Being a thermosetting plastic, it cannot be softened by heat after being set hard, and is insoluble in any solvent. It burns only with difficulty; the flame goes out once the heat source is removed. 28.11 COMMON CONDENSATION POLYMERS

86 86 Figure 28.45 Urea-methanal (a thermosetting plastic) only chars when heated strongly. It does not burn. (a)Before heating (b)After strong heating. 28.11 COMMON CONDENSATION POLYMERS (a)(b)

87 87 Uses Urea-methanal is widely used in electrical industry, to make light- coloured electrical switches, plugs, sockets and casings for electrical appliances. 28.11 COMMON CONDENSATION POLYMERS

88 88 Figure 28.46 Light-coloured electrical appliances are made of urea-methanal. 28.11 COMMON CONDENSATION POLYMERS

89 89 PHENOL-METHANAL Phenol-methanal is formed from the two monomers: Properties and uses The properties and uses of phenol-methanal are similar to those of urea-methanal. Phenol-methanal is cheaper, but it has a dark brown colour, which is less attractive. 28.11 COMMON CONDENSATION POLYMERS

90 90 Figure 28.47 This radio has a phenol-methanal casing. 28.11 COMMON CONDENSATION POLYMERS

91 91 A28.16 (a)Addition polymer (b) Condensation polymer (c) Addition polymer (d) Addition polymer (Hint: The repeating units of addition polymers usually take the form, where p, q, r and s stand for any atom or group of atoms.) 28.11 COMMON CONDENSATION POLYMERS

92 92 28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS Thermoplastics and thermosetting plastics behave differently towards heat. THERMOPLASTICS A thermoplastic raw material consists of separate, long flexible polymer chains. These chains are tangled, held in place to one another by weak intermolecular forces.

93 93 Figure 28.49 The structure of a thermoplastic in the solid state. 28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS

94 94 When heated, the chains vibrate more vigorously, becoming further apart. The intermolecular forces are overcome, and the chains can slide over one another easily. The plastic thus softens and melts. We can run the viscous liquid into a mould, where the plastic takes up its shape. THERMOSETTING PLASTICS A thermosetting plastic raw material also consists of separate long polymer chains, with weak intermolecular forces among them. Hence it can be softened by heat and moulded into a particular shape. 28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS

95 95 However, as heating continues in the second stage of the moulding process, cross-links (covalent bonds) are formed between the chains. A hard, rigid 3-dimensional giant network is formed. The chains cannot slide over one another even when heated. Thus the finished article cannot be softened by heat again. 28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS

96 96 Figure 28.50 The giant network structure of a thermosetting plastic after being set hard. 28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS

97 97 A28.17 (a)No(b)Yes(c)No. Plastics consist of molecular chains or have a giant covalent network. There are no delocalized electrons nor mobile ions to conduct electricity. A28.18 28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS

98 98 28.13 PLASTICS AND ECONOMY PLASTICS ARE IMPORTANT TO ECONOMY Plastics are now very widely used, gradually replacing natural materials such as cotton, silk, wood, leather, wool and metals. Plastics, however, are not just cheap substitutes. In many cases, they have properties which make them superior to natural materials.

99 99 Figure 28.53 The world production of plastics has increased rapidly in the past 60 years. 28.13 PLASTICS AND ECONOMY

100 100 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS 28.14PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS POISONOUS PLASTIC ARTICLES Some plastics contain toxic substances. An example is PVC. In fact many large toy shops no longer sell PVC toys.

101 101 Figure 28.54 PVC toys contain poisonous chemicals. Greenpeace has worked hard to stop the sale of such toys in Hong Kong. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

102 102 FIRE RISK Many plastics are flammable. Besides, toxic gases are produced when some plastics are burnt. Thus there is a fire risk associated with plastics, and the fires involved are usually dangerous. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

103 103 DISPOSAL OF PLASTIC WASTE Most plastics, unlike natural materials such as wood or cotton, are non-biodegradable. Plastic waste is often either buried in landfill sites or burnt in incinerators. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

104 104 Figure 28.55 Waste being disposed of at a landfill site. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

105 105 Figure 28.56 Burning of plastics produces harmful fumes. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

106 106 SOLVING PLASTIC WASTE DISPOSAL PROBLEM Reduce the use of plastics 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS Figure 28.58 Reducing use of plastic bags Bring Your Own Bag (BYOB).

107 107 Re-use plastic scrap Recycle plastic waste Figure 28.59 A recyclable plastic bag. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

108 108 Figure 28.61 A biodegradable plastic bag. Make biodegradable plastics 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

109 109 PyrolysisIf plastics are heated in the absence of air at about 700 o C, the molecules would break down to form smaller molecules. The process is called pyrolysis. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

110 110 Figure 28.62 A pyrolysis plant. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS plastic waste pyrolysis chamber at ~700 o C burner gaseous products carbon separation unit fractionating tower residues (wax, tar etc.) pyrolysis gas 50% for heating the plant and 50% as an end product (methane, ethene, propene) carbon

111 111 A28.19 Plastics would burn when heated strongly in air, forming mainly carbon dioxide and water. 28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS

112 112 SUMMARY 1.Petroleum is the most important raw material used in the production of plastics. Plastics are made mainly from ethene and other alkenes, which are obtained by cracking oil fractions (e.g. naphtha, gas oil). 2.Plastics are man-made polymers which, at some stage during processing, can be softened by heat and then turned into any desired shape. 3.Uses of plastics depend very much on their thermal properties. Plastics can be classified into two classes depending on their behaviour towards heat. SUMMARY

113 113 SUMMARY 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 is a plastic which, once set hard, cannot be softened again by heating. This is because of the existence of extensive cross-links in the polymer structure. 4.Plastics can be moulded easily into any shape. (a)A softened thermoplastic must be cooled sufficiently in a mould, until it is set hard. (b)A softened thermosetting plastic must be heated sufficiently in a mould, until it is set hard.

114 114 SUMMARY 5.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. 6.(a)Alkenes are a homologous series of unsaturated hydrocarbons with the general formula C n H 2n. Every alkene molecule contains a C=C double bond. (b)Alkenes are quite reactive. They undergo addition reactions. 7.An addition reaction is a reaction in which two or more molecules react to give a single molecule.

115 115 SUMMARY Addition polymerization is a reaction in which monomer molecules join together repeatedly to form polymer molecules, without the elimination of small molecules (such as H 2 O, NH 3 or HCl). Monomers that can undergo addition polymerization must have a carbon-carbon double bond. 8.A repeating unit is the smallest part of a polymer molecule, by repetition of which the whole polymer structure can be obtained. 9.Condensation is a type of reaction in which two or more molecules join together to form a larger molecule, with the elimination of small molecules (such as H 2 O, NH 3 or HCl).

116 116 SUMMARY Condensation polymerization is a reaction in which monomer molecules join together to form polymer molecules, with elimination of small molecules. 10.Properties and uses of some common plastics:

117 117 SUMMARY

118 118 SUMMARY 11.The different thermal properties of thermoplastics and thermosetting plastics can be explained in terms of structure. Thermoplastics cannot form cross-links between polymer chains. Thermosetting plastics can form cross-links to give giant covalent structures. 12.Plastics are now very widely used, gradually replacing many natural materials. 13.Problems associated with the disposal of plastic waste: Burying in landfill sites Most plastics are non-biodegradable, thus a lot of land would be required.

119 119 SUMMARY Burning in incinerators Burning plastics leads to air pollution. Some plastics even give off poisonous fumes. 14.Thermoplastic waste can be recycled.


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