1 chapter 28plastics and polymers 28.1what are plastics? 28.2simple tests on plastics...

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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

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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.


3

Figure 28.2

Many items used to be made of natural materials are now made with 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.


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.


6

Plastics are polymers. Polymers consist of very large

molecules, formed by the joining of many small molecules

(monomers). For example,


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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:


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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.)


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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.

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(a) Solid mass (b) Thin films

(c) Fibres (d) Expanded foam

Figure 28.5 Four common forms of 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.


The following simple tests may help to find out the probable

nature of a plastic sample.

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Table 28.1 Properties of some plastics.


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Figure 28.6

Polythene softens and melts

easily, but urea-methanal does

not.


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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.

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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.

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Figure 28.8 Objects made of thermoplastics.


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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).


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Figure 28.9 Objects made of thermosetting plastics.


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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

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Figure 28.10

Injection moulding.


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Figure 28.12 Taking a bucket out of a mould in an injection moulding machine.


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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.

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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.


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A28.4

(a) Yes (b) No


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NATURAL AND MAN-MADE POLYMERS

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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.

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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.


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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:

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Figure 28.18

A ball-and-stick model of

ethene molecule.

28.7 ALKENES

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Larger alkene molecules

Take the example of hex-1-ene:

28.7 ALKENES

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

28.7 ALKENES

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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

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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

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28.7 ALKENES

Reaction with halogens

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Figure 28.20 Hex-1-ene (an alkene) decolorizes bromine solution rapidly.

hex-1-ene

Br2 in1,1,1-trichloroethane

brominedecolorized

28.7 ALKENES

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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

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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

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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.

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28.7 ALKENES

To crack medicinal paraffin and test for uns

aturation in the gaseous product.

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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).

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Each polymer chain is a macromolecule. Each consists of at

least several hundred monomeric units joined together.


In most cases, the monomers can be represented by a

general formula:

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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:


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A28.9


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28.9 COMMON ADDITION POLYMERS


MAKING ADDITION POLYMERS

POLYTHENE [POLY(ETHENE)]

Manufacture

The equation for reaction:

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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).


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Figure 28.26 Some polythene products.


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Figure 28.27

Low-density polythene

film used as the roof of a

greenhouse.


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A28.10


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POLYSTYRENE

Laboratory preparation


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Figure 28.29 Laboratory preparation of polystyrene.


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To prepare polystyrene.

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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.


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Figure 28.30 Some polystyrene products.


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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.


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Figure 28.34 Foam cups and food boxes made of expanded polystyrene.


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PERSPEX

Manufacture and laboratory preparation



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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.


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Figure 28.36

Perspex is used to make the ‘glass’ of

safety spectacles.


Figure 28.37

Illuminated signs made of perspex.

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POLYVINYL CHLORIDE

Manufacture



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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.


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Figure 28.38

PVC pipes.


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.


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Figure 28.39 Some PVC products.


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A28.11


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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.

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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:


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.


70

Figure 28.40

Uses of polyester:

(a) Making textile fibres (b) Making sails

(a) (b)


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


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:


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28.11 COMMON CONDENSATION POLYMERS


NYLON

Nylon 6.6 is formed from the two monomers:

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Figure 28.41 Laboratory preparation of nylon 6.6.


Laboratory preparation of nylon 6.6

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nylon 6.6

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Nylon rope trick.

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A28.13

A28.14

Water molecules, H2O.


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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.


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Figure 28.42

The life of this mountain

climber relies very much on

the high tensile strength of

the nylon rope.


Figure 28.43

Some nylon products.

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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.


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.


82


83


To prepare urea-methanal.

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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.)


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.


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Figure 28.45

Urea-methanal (a thermosetting plastic) only chars when heated strongly. It does not burn.

(a) Before heating (b) After strong heating.


(a) (b)

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Uses

Urea-methanal is widely used in electrical industry, to make light-

coloured electrical switches, plugs, sockets and casings for

electrical appliances.


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Figure 28.46 Light-coloured electrical appliances are made of urea-methanal.


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.


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Figure 28.47 This radio has a phenol-methanal casing.


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.)


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.

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Figure 28.49

The structure of a thermoplastic in the solid state.


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.


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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.


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Figure 28.50

The giant network structure of a thermosetting plastic after being set hard.


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


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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.

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Figure 28.53 The world production of plastics has increased rapidly in the past 60 years.


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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.

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Figure 28.54

PVC toys contain poisonous chemicals. Greenpeace has worked hard to stop the

sale of such toys in Hong Kong.


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.


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.


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Figure 28.55 Waste being disposed of at a landfill site.


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Figure 28.56 Burning of plastics produces harmful fumes.


106

SOLVING PLASTIC WASTE DISPOSAL PROBLEM

Reduce 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.


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Figure 28.61

A biodegradable plastic bag.

Make biodegradable 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.


110

Figure 28.62

A pyrolysis plant.


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.


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.

1 chapter 28plastics and polymers 28.1what are plastics? 28.2simple tests on plastics...

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plastics plastics

thermosetting plastics

common plastics

production of plastics

classifying plastics

structures of plastics

use of plastics slide

plastic age plastics