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1
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
CONTENTS OF CHAPTER 28
28.8 Addition polymerization
28.9 Common addition polymers
28.10 Condensation polymerization
28.11 Common condensation polymers
28.12 Thermal properties and structures of plastics
28.13 Plastics and economy
28.14 Problems associated with the use of plastics
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 plas
tic 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
28.2 SIMPLE TESTS ON PLASTICS
It is often difficult to identify a plastic article just from its appearanc
e. This is because plastics can be moulded into any shape and m
ade into various forms.
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
Strength Bend a thin sample of the plastic. See whether it i
s 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 mel
ting behaviour.
28.2 SIMPLE TESTS ON PLASTICS
The following simple tests may help to find out the probable
nature of a plastic sample.
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
28.3 CLASSIFYING PLASTICS
We can classify plastics into two broad classes: thermoplastics an
d 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
28.4 GENERAL USES OF PLASTICS
28.4 GENERAL USES OF PLASTICS
The uses of a plastic depend on its properties. In general, thermo
plastics 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
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 propertie
s.
Secondly, use a mould to turn the plastic into the desired shap
es, by applying heat and pressure.
A softened (or molten) thermoplastic should be cooled sufficie
ntly in a mould, until it is ‘set’. On the other hand, a softened ther
mosetting plastic should be heated sufficiently in a mould, until it i
s set.
MOULDING THERMOPLASTICS
Injection moulding28.5 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?
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
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
28.6 WHAT ARE POLYMERS?
NATURAL AND MAN-MADE POLYMERS
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
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
28.7 ALKENES
ALKENES ARE STARTING MATERIALS FOR MAKIN
G PLASTICS
Many plastics are made from alkenes. Alkenes are usually obtain
ed from the cracking of oil fractions.
Alkenes are a homologous series of unsaturated hydrocarbon
s 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 formul
a: C2H4). The structural formula of ethene is:
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
28.7 ALKENES
Reaction with halogens
37
Figure 28.20 Hex-1-ene (an alkene) decolorizes bromine solution rapidly.
hex-1-ene
Br2 in1,1,1-trichloroethane
brominedecolorized
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
Figure 28.21
Hex-1-ene (an alkene) decolorizes acidified potassium permanganate solution rapidly.
KMnO4
decolorized
hex-1-ene
acidified KMnO4
solution
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.)
28.7 ALKENES
Polymerization
Under certain conditions, alkenes can undergo addition
polymerization to form plastics.
41
28.7 ALKENES
To crack medicinal paraffin and test for uns
aturation in the gaseous product.
42
28.8 ADDITION POLYMERIZATION
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).
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
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
28.9 COMMON ADDITION POLYMERS
MAKING ADDITION POLYMERS
POLYTHENE [POLY(ETHENE)]
Manufacture
The equation for reaction:
47
Properties
In general, polythene is light (less dense than water) and low-melti
ng.
Uses
Its main uses include making plastic bags, wrapping film for food,
food boxes, flexible cold water pipes and kitchen wares (e.g. sque
eze 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
51
28.9 COMMON ADDITION POLYMERS
POLYSTYRENE
Laboratory preparation
The equation for reaction:
52
Figure 28.29 Laboratory preparation of polystyrene.
28.9 COMMON ADDITION POLYMERS
53
28.9 COMMON ADDITION POLYMERS
To prepare polystyrene.
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
Figure 28.30 Some polystyrene products.
28.9 COMMON ADDITION POLYMERS
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
Figure 28.34 Foam cups and food boxes made of expanded polystyrene.
28.9 COMMON ADDITION POLYMERS
58
PERSPEX
Manufacture and laboratory preparation
The equation for reaction:
28.9 COMMON ADDITION POLYMERS
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
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
POLYVINYL CHLORIDE
Manufacture
The equation for reaction:
28.9 COMMON ADDITION POLYMERS
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
Figure 28.38
PVC pipes.
28.9 COMMON ADDITION POLYMERS
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
Figure 28.39 Some PVC products.
28.9 COMMON ADDITION POLYMERS
66
A28.11
28.9 COMMON ADDITION POLYMERS
67
28.10 CONDENSATION POLYMERIZATION
28.10 CONDENSATION POLYMERIZATION
CONDENSATION AND CONDENSATION POLYMERIZA
TION
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 H2O, NH3 or HCl).
Consider the following reaction.
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 H2O, HCl or NH3).
An example of condensation polymerization
Consider an example of condensation polymerization. A dioic
acid, , and a diol, HOCH2CH2OH, react as
follows:
28.10 CONDENSATION POLYMERIZATION
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
Figure 28.40
Uses of polyester:
(a) Making textile fibres (b) Making sails
(a) (b)
28.10 CONDENSATION POLYMERIZATION
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
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
28.11 COMMON CONDENSATION POLYMERS
28.11 COMMON CONDENSATION POLYMERS
NYLON
Nylon 6.6 is formed from the two monomers:
74
Figure 28.41 Laboratory preparation of nylon 6.6.
28.11 COMMON CONDENSATION POLYMERS
Laboratory preparation of nylon 6.6
75
28.11 COMMON CONDENSATION POLYMERS
nylon 6.6
76
28.11 COMMON CONDENSATION POLYMERS
Nylon rope trick.
77
A28.13
A28.14
Water molecules, H2O.
28.11 COMMON CONDENSATION POLYMERS
78
Properties
Nylon has a high tensile strength. Its melting point is quite high (a
bout 200oC).
Uses
Nylon has been the most important synthetic fibre. It is used in var
ious kinds of clothing (e.g. stockings, jackets). It resists creasing a
nd ‘drips dry’ quickly. It is not attacked by moth. However, nylon la
cks moisture-absorbing properties of natural fibres.
Nylon fibres are an ideal material for making ropes, carpets, fi
shing lines, fishing nets and strings for tennis rackets.
28.11 COMMON CONDENSATION POLYMERS
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
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
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
28.11 COMMON CONDENSATION POLYMERS
83
28.11 COMMON CONDENSATION POLYMERS
To prepare urea-methanal.
84
A28.15
The laboratory must be well-ventilated.
(Methanal is toxic.)
Wear safety spectacles and handle concentrated sulphuric ac
id 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 d
rops of the acid were added all at once, the reaction would be
come so violent that the mixture spurts out.)
28.11 COMMON CONDENSATION POLYMERS
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
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
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
Figure 28.46 Light-coloured electrical appliances are made of urea-methanal.
28.11 COMMON CONDENSATION POLYMERS
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
Figure 28.47 This radio has a phenol-methanal casing.
28.11 COMMON CONDENSATION POLYMERS
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
28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS
28.12 THERMAL PROPERTIES AND
STRUCTURES OF PLASTICS
Thermoplastics and thermosetting plastics behave differently towa
rds heat.
THERMOPLASTICS
A thermoplastic raw material consists of separate, long flexible pol
ymer chains. These chains are tangled, held in place to one anoth
er by weak intermolecular forces.
93
Figure 28.49
The structure of a thermoplastic in the solid state.
28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS
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
However, as heating continues in the second stage of the mou
lding process, cross-links (covalent bonds) are formed between th
e 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
Figure 28.50
The giant network structure of a thermosetting plastic after being set hard.
28.12 THERMAL PROPERTIES AND STRUCTURES OF PLASTICS
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
28.13 PLASTICS AND ECONOMY
28.13 PLASTICS AND ECONOMY
PLASTICS ARE IMPORTANT TO ECONOMY
Plastics are now very widely used, gradually replacing natural mat
erials such as cotton, silk, wood, leather, wool and metals.
Plastics, however, are not just cheap substitutes. In many cas
es, they have properties which make them superior to natural mat
erials.
99
Figure 28.53 The world production of plastics has increased rapidly in the past 60 years.
28.13 PLASTICS AND ECONOMY
100
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
28.14 PROBLEMS ASSOCIATED WITH THE USE
OF PLASTICS
POISONOUS PLASTIC ARTICLES
Some plastics contain toxic substances. An example is PVC. In fa
ct many large toy shops no longer sell PVC toys.
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
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
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
Figure 28.55 Waste being disposed of at a landfill site.
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
105
Figure 28.56 Burning of plastics produces harmful fumes.
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
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
Re-use plastic scrap
Recycle plastic waste
Figure 28.59
A recyclable plastic
bag.
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
108
Figure 28.61
A biodegradable plastic bag.
Make biodegradable plastics
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
109
Pyrolysis If plastics are heated in the absence of air at
about 700oC, the molecules would break down to form smaller
molecules. The process is called pyrolysis.
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
110
Figure 28.62
A pyrolysis plant.
28.14 PROBLEMS ASSOCIATED WITH THE USE OF PLASTICS
plastic waste
pyrolysis chamber at ~700oC
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
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
SUMMARY
1. Petroleum is the most important raw material used in the pro
duction 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 durin
g processing, can be softened by heat and then turned into a
ny desired shape.
3. Uses of plastics depend very much on their thermal propertie
s. Plastics can be classified into two classes depending on th
eir behaviour towards heat.
SUMMARY
113
SUMMARY
A thermoplastic is a plastic which can be softened by heating
and hardened by cooling, the process being repeatable any n
umber of times.
A thermosetting plastic is a plastic which, once set hard, cann
ot be softened again by heating. This is because of the existe
nce 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
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 CnH2n. Every
alkene molecule contains a C=C d
ouble 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
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 H2O, NH3 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 H2O, NH3 or HCl).
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
SUMMARY
118
SUMMARY
11. The different thermal properties of thermoplastics and thermo
setting plastics can be explained in terms of structure. Therm
oplastics cannot form cross-links between polymer chains. Th
ermosetting plastics can form cross-links to give giant covale
nt 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 la
nd
would be required.
119
SUMMARY
Burning in incinerators
Burning plastics leads to air pollution. Some plastics
even give off poisonous fumes.
14. Thermoplastic waste can be recycled.