Brown's GCSE/IGCSE/O Level KS4 science-CHEMISTRY Revision Notes
Oil, useful products, environmental problems, introduction to
7. Making Polymers, plastics, uses, problems and recycling
Polymers are very long molecules
and the main component in many common plastic materials. On this page the
molecular structure, production-manufacture, uses and recycling of the polymers
poly(ethene), poly(propene), poly(chloroethene)/PVC and polystyrene are
described. Problems with waste plastics and methods of recycling of
plastics are discussed.
Index of KS4 Science GCSE/IGCSE
Chemistry Oil & Organic Chemistry Pages: 1.
Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3.
ALKANES - saturated hydrocarbons and combustion : 4.
Pollution, carbon monoxide, nitrogen oxides, what
makes a good fuel?, climate change-global warming :
5. Alkenes - unsaturated hydrocarbons :
6. Cracking - a problem of supply and demand, other products :
7. Polymers, plastics, uses and problems :
8. Introduction to Organic Chemistry - Why so many series of
organic compounds? : 9. Alcohols - Ethanol
- properties, reactions, biofuels :
10. Carboxylic acids and esters : 11. Condensation polymers, Nylon & Terylene,
comparing thermoplastics, fibres and thermosets
12. Natural Molecules - carbohydrates - sugars
- starch : 13. Amino acids, proteins,
enzymes & chromatography : 14. Oils, fats,
margarine and soaps :
15. Vitamins, drugs-analgesic medicines & food
additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free
radicals : 17. Extra notes, ideas and links on
Global Warming and Climate Change : Multiple Choice and Gap-Fill Quizzes:
m/c QUIZ on Oil Products (GCSE/IGCSE easier-foundation-level)
m/c QUIZ on Oil Products (GCSE/IGCSE harder-higher-level) :
IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry
3 Easy linked GCSE/IGCSE Oil Products word-fill worksheets
Introducing, polymerisation, polymers and plastics
7a. The formation
of POLYMERS and the USES of PLASTICS - Macromolecules
Reactions of alkenes (4)
(See also section 11.
More on other types of polymers)
joining lots of small molecules (called monomers) into very large
long molecules (called polymers)
This page describes the formation of big
polymer molecules called polyalkenes from small molecules called alkenes
Alkenes are made by
cracking some of the
fractions obtained by the fractional distillation of crude oil
The diagram below shows the general equation
for the formation of a plastic polymer from alkenes.
There are many different alkenes, so there
are many different polymers with a range of physical properties eg strength.
This is a general equation for the addition
polymerisation of ANY alkene
The name of the polymer is quite simply derived from
the monomer name in () preceded by poly
i.e. poly(alkene monomer name)
When catalysed and heated under pressure,
link together when 'half' of the double bond opens. The spare
bond on each carbon atom of the double bond are
used to join up the molecules.
The general equation for polyalkene
formation is shown in the diagram above.
In terms of the 'backbone'
of carbon atoms, the essential molecular
linking is equivalent to ...
... C=C + C=C + C=C
... etc. ===> ... C-C-C-C-C-C ... etc.
.... to give a long
polymer chain molecules, hundreds-thousands of carbon atoms long!
... and the whole chain of
atoms is held together by strong carbon-carbon covalent bonds.
This process is called
addition polymerisation, the monomer molecules all add together,
with no other product formed other than the long polymer molecule.
Full equations showing all
the atoms involved are given in the next section 7b.
The attractive forces
between the molecules, the so-called 'intermolecular forces' or
'intermolecular bonding' are much weaker than the chemical bonds
between the atoms, but they are most important in determining the
properties of the polymer AND as you will read on later, they can be
changed to change the physical properties of a polymer.
Think of the process as
springing open half of the C=C double covalent bond of an alkene and using the
'spare bonds' to link across from one alkene molecule to another.
This can only happen if the
hydrocarbon molecule is unsaturated, like alkenes (with a C=C
You cannot polymerise
saturated hydrocarbon molecules like alkanes (all C-C single bonds).
Most polymers (plastics) are made
from alkene compounds containing the -C=C- bond by addition polymerisation.
The general reaction is
small monomer molecule ==> long polymer molecule as the
small molecules link together to form a long chain.
The original small
molecule is called the monomer
and the long molecule is called the polymer,
which is the sort of molecule most plastics consist of.
So lots of small molecules join up to form a
big long molecule in a process called addition polymerisation
and the polymers are named as poly(name of original alkene), i.e.
Several examples are shown
and their formation and structure described below, namely for
poly(ethene), poly(propene), poly(chloroethene) PVC, polystyrene.
Their properties and uses of
these thermoplastic materials are also described and explained.
7b. Examples of
poly(alkene) polymer molecules - formation, structure
polymers described on this page are typical addition polymers
(formed by simple addition of monomer molecules), just like polythene
and polystyrene etc.
AND they are all
examples of thermoplastic polymers (thermoplastics),
that is, the polymer can be heated and softened, reshaped and cooled
to keep their new moulded shape.
The equation for the
formation of poly(ethene) is shown below.
displayed formula style
equation for poly(ethene) formation, or simpler molecular formula style
The molecular structure
A 'picture' of a section of a very long chain
molecule, essentially consisting of hundreds or thousands of -CH2-CH2-
ethene units all joined together.
Above is a 'space-filling'
model 'ball and below a 'stick' type of model.
Although the poly(ethene)
molecules look straight, in reality, the long molecule will be all
twisted, jumbled and tangled up as in the thermoplastic diagram on the
Using these models' to
explain the general physical properties of a thermoplastic polymer.
The intermolecular forces
(electrical attractive forces) between the polymer molecules are quite
weak compared to the strong covalent bonds (C-C) holding the chain of
Because this 'intermolecular
bonding' is weak, when heated, these 'plastic' materials will
soften/melt quite easily (hence easily remoulded) at relatively low
temperatures (e.g. for poly(ethene) 110-150oC), which is why
they are called 'thermoplastic' and have relatively low
Even when 'cold' and
apparently solid, the plastic is easily distorted (easily bent when
stressed) because the polymer chains can slide over each other i.e. the
external physical force applied on bending overcomes the intermolecular
forces between the polymer molecules.
Despite their relative
weakness, on controlled heating, until they are quite soft (but NOT
molten), they are readily extrusion moulded or drawn out into useful
shapes (even fibres) which retain their new formation on cooling.
Also, the flexibility and
sometimes stretchability is actually very useful - think of plastic
'polythene' bags and PVC coated electrical cables.
How is poly(ethene)
manufactured? e.g. for commercially available plastics like
are used in the manufacture of polymers?
Does changing the
reaction conditions have any effect on the physical properties of the
Can we make different
forms of the same polymer for different uses?
polymers have their own characteristic physical properties depending on
their specific molecular structure.
The strength of the
intermolecular forces (intermolecular bonding) between the polymer
molecules is crucial to how strong, rigid, flexible etc. the manufacture
plastic material is.
If the molecules can be made
more 'lined up way' and more 'densely' packed, the intermolecular forces
(intermolecular bonding) increases, the physical properties change e.g.
the polymer density and rigidity increases, so it is stronger and less
It is possible to achieve
variations in the physical properties of plastics like 'polythene' by
varying the manufacturing conditions to give subtle differences in the
molecular structure and composition of the polymer-plastic produced..
Two different types of
poly(ethene) and their molecular structure and physical properties are
described with reference to their different uses.
like poly(ethene) are usually produced under conditions of relatively high pressure,
and a catalyst.
However, by changing and
reaction conditions you can make subtle changes to the physical
properties of the polymer obtained from the polymerisation process e.g.
changing temperature, pressure or catalyst. This allows the manufacture
of a weaker more flexible lower
referred to as
LDPE, and a stronger less flexible
form of poly(ethene),
In the original high pressure process developed in the 1930s, poly(ethene)
was made at a temperature of 80oC to 300oC and a
pressure of 1000 atm to 3000 atm. pressure (1 atm = normal atmospheric
pressure at sea level), the product has a lower density and the
poly(ethene) very flexible. Small controlled traces of oxygen (< 10 ppm)
are used to initiate the polymerisation process.
The polymer chains are quite
jumbled up and the polymer chains also have many branches (NOT cross-links),
and the intermolecular forces between molecules are weaker, so the
plastic is less strong and less rigid, more flexible than HDPE
This lower density weaker
form of poly(ethene) melts at around 115oC, but softens well
before that temperature, so don't have very hot drinks from an LDPE
This 'low density
poly(ethene)' is known by the acronym LDPE and is used for plastic bags,
cling film wrap, laminating paper, car covers, squeezy bottles, liners
for tanks and ponds, moisture barriers in construction.
Low density poly(ethene) is
not very heat resistant and softens easily if heated, so it can't be
used for any application where temperatures well above room temperature
will be encountered!
in the 1950s,
it became possible to make poly(ethene) in a low pressure process at around a temperature of
50oC to 300oC and using a
much lower pressure of a 2 atm to 80 atm., AND a catalyst, the polymer
product is more dense and less flexible.
Here the polymer chains are
much more aligned with each other, maximising the intermolecular forces/bonding
between the molecules, so the plastic is stronger and more rigid and
less flexible than LDPE poly(ethene).
The polymer molecules are
longer with far fewer branches in the chains and this allows the
poly(ethene) molecules to get closer together (just compare the two
This higher density stronger
form of poly(ethene) melts at around 135oC and can withstand
contact with hot water without distortion!
This 'high density
poly(ethene)' is known by the acronym HDPE and is used for freezer bags,
water pipes (a bit flexible but quite strong), or more rigid uses like
drainpipes and water tanks, plastic milk bottles, strong plastic crates, wire and cable insulation, extrusion coating.
In general, in terms of how
the polymer molecules are packed ...
the closer the polymer
chains are packed together, the stronger the intermolecular
forces/bonding, the density increases and the strength of the
polymer increases (this tends to increase the hardness and
crystallinity of the plastic),
the more jumbled or
'spread out', the polymer chains are, the weaker the intermolecular
forces/bonding, the polymer material is less dense and weaker e.g.
Summary of the properties and uses
(old or commercial names:
First and foremost poly(ethene)
from ethene is a cheap, if not very strong, useful plastic.
Poly(ethene) on heating is
readily moulded into any shape e.g. 'plastic'
is because poly(ethene) is an example of a thermoplastic - it can be heated to soften
it, reshape it e.g. in an injection mould system, and on cooling retains
its new shape - bottle, bowl, toy etc.
Lower density poly(ethene) is lighter and 'stretchy'
and used for plastic bags, plastic piping,
laminating paper etc.
Higher density poly(ethene)
is used where a less flexible/stretchy more rigid and stronger form is
needed e.g. drainpipes, milk crates
A general important point
to make here is that the use of a polymer doesn't just depend on its
chemical composition and molecular structure, but mainly on its physical
properties (which of course are derived from the polymers structure).
These differences become
accentuated when comparing
thermosoftening plastics and thermosetting plastics which also
includes a discussion on modifying polymers to change their properties,
hence varying their uses.
(old or commercial names:
polypropylene, polyprene and polypropene).
The polymerisation of
propene and the molecular structure of poly(propene) is shown below.
displayed formula style
equation for poly(propene) formation, or simpler molecular formula style
properties and uses of Poly(propene), incorrectly 'polyproprene'
Poly(propene) is made from propene
and is stronger, more rigid and hard wearing than
polythene but still flexible.
Poly(propene) is also a
thermoplastic and can be moulded into many useful objects.
Poly(propene) can be drawn
out into strong fibres (just like nylon
Poly(propene) is used for making
strong containers like milk crates (a thermoplastic moulding), carpet fibres,
thermal underwear and ropes (last three are examples of using
(old name polyvinyl
made from chloroethene, CH2=CHCl, (old name vinyl chloride).
PVC formation and
molecular structure is shown in the diagrams below
The equation for the
polymerisation of ethene is shown in different ways.
displayed formula style
equation for poly(chloroethene) formation, or simpler molecular
formula style equation below.
The molecular structure
of PVC poly(chloroethene)
Above is a 'picture' of a section of a very long chain
molecule (PVC), essentially consisting of hundreds or thousands of
-CH2-CHCl- chloroethene units all joined together.
diagram is that of a 'ball and stick' type of model and although it looks
straight, in reality, the long molecule will be all
twisted-jumbled up like in the diagram on the right.
chlorine atoms are shown as a regular arrangement, but they will
be more randomly distributed down the ...-C-C-C-... chain
depending on which way round the chloroethene molecule added.
The properties and uses of PVC
another thermoplastic, is
much tougher than poly(ethene), very hard wearing with good heat
stability, so is used for covering insulation electrical wiring and plugs.
also replacing metals for use as gas pipes and water drain pipes because
it is strong and durable.
PVC can be manufactured
to be quite rigid or quite flexible depending on its intended use,
and, you can vary the composition so that rigidity or flexibility to
suit a particular use.
More flexible poly(chloroethene) has found
a use in the clothing industry e.g. as artificial leather and is readily dyed
with bright colours!
Rigid PVC is replacing wood
for e.g. window frames, metals for guttering and piping because it is tough
and hard wearing (durable) with excellent weather proofing
PVC can be coloured to suite aesthetic taste and unlike
wood, it doesn't rot and it doesn't have to be painted, so saving
time as well as money!, but PVC windows are not as aesthetically
pleasing as nicely painted wooden windows (I have to say this,
because living with persistently rotting windows in an old grade II
listed Victorian schoolhouse, I'm not allowed to use PVC windows!).
More general points -
ways of modifying the properties of a particular polymer-plastic
If you increase the
chain length of the polymer molecules you increase the
intermolecular forces between the chains so it is stronger and less
flexible and has a higher softening/melting point.
If you shorten the
average chain length the polymer has lower softening/melting and is
easier to shape, and the plastic is more flexible.
If you can reduce the
branches in a polymer chain, the molecules can pack closer together
(rather like in a fibre), this increases the intermolecular forces
giving a more dense, stronger and more crystalline polymer with a
higher softening/melting point.
Another way of changing
the physical properties of a polymer is to add a plasticiser.
additive is a relatively large non-volatile molecule (but
much smaller than a polymer molecule) that 'pushes' the polymer
chain apart a little.
This reduces the
intermolecular forces, making the plastic more softer, flexible and
easier to shape.
By this means you can
make a rather more 'stretchy' PVC for use as synthetic leather.
PCBs) are used in PVC to make it more flexible e.g. when it is
used for electrical insulation cable.
Plasticisers known as PCBs are toxic and can leach out into the
environment, and, just like pesticides contaminating water, they can
build up in food chains to potentially poison fish and humans, so
there use is strictly controlled.
Polystyrene is made from
'styrene* (another alkene monomer)
New modern systematic name:
polymer made from polymerising phenylethene monomer.
old name 'styrene', hence the common name 'polystyrene'
You can tell that this
molecule can theoretically act as a monomer, that is it can be
polymerised to a polymer, because it is an alkene with the
characteristic C=C carbon-carbon double bond which can partially
open up to form linking -C-C-C-C- bonds.
Phenylethene (styrene) is like ethene, but one
of the hydrogens is replaced with a phenyl group derived from a
benzene ring (a -C6H5 group actually,
but don't worry about this at GCSE level).
Properties and uses
Polystyrene is used in a
gas expanded form for packaging and insulation - 'polystyrene foam'
or 'expanded polystyrene'. This is light and useful for packing
breakable objects and valuable items like computers!
(EPS), with its trapped gas (from the 'blowing agent' e.g. CO2)
gives the material excellent heat insulation properties (thermal
insulation material, due to the trapped gas - poor conductor of heat).
or PTFE for a short acronym! PTFE's commercial name is
made by polymerising tetrafluoroethene (its like ethene except all the H
atoms are F atoms!)
PTFE is one of the most hard
wearing of commonly used thermoplastics.
PTFE is chemically a very unreactive
plastic and, unlike poly(ethene) and poly(propene), it doesn't burn
easily, so has good flame resistant properties.
PTFE with its excellent heat
resistance it can with stand quite
'non-stick' properties, combined with its thermal stability and flame
resistance has made an excellent coating for 'non-stick' cooking pans.
PTFE has very low
coefficient of friction and can be moulded into moving parts in machines
(sliding/rotating surfaces etc.).
The use of PTFE in
'breathable' fabrics like GORE-TEX are described and discussed on the
smart materials page 6. Gore-Tex and
Polymers have many useful applications and new
uses and new polymers are being developed all the time
There are lots of examples!
- some described in section 7b. above and more listed below (and described above) are on many GCSE syllabuses.
A wide range of
polymers are available for use in an even wider range of
applications, it seems endless these days from when I was a
chemistry student in the 1960s when some of the properties of these
new polymers were unthinkable! e.g. electrical conducting, light
Its important to realise
that polymer plastics are alternatives to traditional materials like
wood and metals and in many cases, if not completely, replacing them
on a large scale e.g. water pipes, window frames.
properties can range from being very rigid or
flexible-stretchy-elastic, very strong, easily moulded if gently
heated and pressed (thermoplastic) or set hard after moulding
and very heat resistant (thermoset).
considerably in strength e.g. polyamides or nylons are much stronger
than poly(ethene) and can even be manufactured to provide a variety
of strengths and flexibilities (see example of
LDPE and HDPE commercial products of
Many metals have become
quite expensive so things made from plastics derived from oil and
quite cheap at the moment, but prices will rise as cheap oil gets
packaging materials which are light and stretchable.
Waterproof coatings for fabrics
Dental polymers for
Wound dressings made
from hydrogels - polymers that hold water and keep wounds moist.
materials (including shape memory polymers)
eg 'memory foam' is a polymer that gets softer when it gets warmer
so you can make a mattress which adopts your body shape for
Elastic fibres that are
very stretchable to make tight fitting clothing eg Lycra fibres for
tights and sports clothing.
Heat resistant polymers
are usually thermosets e.g. like melamine resin (plastic plates),
but even thermoplastics like poly(propene) can be used in hot
situations e.g. plastic electric kettles.
Many polymers are not biodegradable
- they don't rot easily!, so they are
not broken down by microbes and this can lead to
problems with waste disposal - see also section 7c on
pollution-environmental problems when disposing of plastic materials.
Because of lack of
biodegradability by microorganisms, you get environmental problems with landfill sites and
street litter and it may take years before the plastic materials
break down - ie decompose!
Most plastic don't rot
away because synthetic polymers are chemically very molecules and
most microorganisms don't have the enzymes to break up the bonds of
the polymer chain and break the plastic down.
This is fortunate enough
when we want to use plastics, but not so good when we want to
dispose of waste plastics!
There is also a growing
problem with vast quantities of plastic debris circulating around
our seas and oceans.
You can bury waste
plastics in landfill sites BUT even after many years most plastics
will not have rotted.
Burning waste plastics
produces all sorts of polluting gases like hydrogen chloride,
hydrogen cyanide and sulfur dioxide, incineration seems a good idea,
but would you like to live down-wind of the combustion plant!
However, all is not lost and
chemistry and technology can come to the 'rescue' and lots of new
improved polymers for existing applications as well as new polymers
for new applications are constantly being developed.
eg plastic bags are being made from polymers
derived from cornstarch making biodegradable plastics that break down more easily
via microorganisms in the environment.
AND, wherever possible,
recycle and reuse! (see section below).
Extra note on natural
Starch and cellulose are
natural polymers but are not used as 'material polymers' commercially.
DNA is actually a polymer,
but of rather a delicate constitution!
Rubber has been used for
centuries as a natural elastic polymer, but it has been replaced by
synthetic polymers like neoprene.
Silk has been used as a
clothing and decorative fabric for thousands of years, and, like sheeps
wool, is essentially a protein polymer material.
polymers like nylon and terylene where developed and designed to try and
mimic silk's useful clothing properties, and nylon's excellent
properties eg strength have meant it has found many and diverse
applications. You need to read the nylon and terylene
page in conjunction with this page.
11. for a comparison of thermoplastics, fibres and thermosetting
More on Pollution
with using and disposing of Polymers or Plastic
Polymers or plastics cannot be easily broken down by
micro-organisms i.e. most, at the moment, most are NOT biodegradable
which leads to
waste disposal and other environmental problems eg
'Non-rotting' litter around the environment.
Land-fill sites are getting full
and recycling isn't as easy as it may seem. (see point 3.below)
using waste plastic as fuel must be very efficient to avoid any other
pollution problem. (see point 2.)
Even our lakes, seas and
oceans are carrying waste plastic materials causing harmful effects on
Can we burn waste plastic?
When plastic materials burn they can
produce highly toxic gases such as carbon monoxide, hydrogen cyanide and
hydrogen chloride (particularly from PVC and other plastics containing
chlorine and nitrogen).
The toxic fumes cause deaths in house fires and controversial problems with alleged inefficient waste incinerators as they will
definitely cause environmental problems if burned on waste tips!
can we recycle plastics? What ways are used to recycle plastics?
It is highly desirable to
recycle plastics for several reasons
Less environmental impact
It conserves valuable oil
based resources, especially as oil is becoming increasingly expensive.
to recycle plastics because of separation into the different types of plastics and their different physical properties.
There are problems in trying
to sort out the different plastics into useful categories, they are not
easy separate, you can't just use any old mixture of polymers.
people are coming up with ideas. A company in Scarborough, England, is
collecting waste plastic. This is shredded and compressed into porous
pads and used for good 'underground' drainage layers for footpaths, golf
greens and sand bunkers etc. and has a good working life because the
material isn't biodegradable!
Clear soft drink bottles are
made from PET (polyethene/polyethylene terephthalate) which can be
recycled as fibre-fill for pillows and carpets.
This save 90% of the energy
costs compared to the original manufacturing process.
Energy costs are a big
economic recycling factor, its not just about making naturally occurring
resources like oil last longer.
However, it takes about
20,000 drinks bottles to make a tonne of recycled PET.
are being developed which are more biodegradable
or can be recycled, so will the
paper bag and cardboard package make a comeback? (in Ireland you have to
bring your own bag or buy one, and not necessarily a plastic one!), this
isn't a recycling process BUT it does reduce environmental pollution.
You can mix starch granules
(very biodegradable) with plastics like poly(ethene) which enables
microorganisms to grow and feed on/in the plastic and eventually help
biodegrade the plastic itself. By breaking down the starch the
microorganisms also break down the poly(ethene).
You can also manufacture
plastics whose break down is initiated by exposure to sunlight. This is
particularly useful in agriculture where plastic bags of fertiliser are
in common use.
In 1988 Australia issued
bank notes made from recycled poly(propene). These plastic notes
apparently have the advantage of being more difficult to forge and they
Other ideas include making
more durable plastic bags that can be used many times for shopping.
Ideally some recycled
thermoplastics and scrap material from a plastic product manufacturing
process, can be heated and remoulded in the same process or another
It is possible in
some cases to break the plastic material down with heat (a sort of
'cracking') into smaller organic hydrocarbon molecules that can act as
chemical feedstock like those from oil itself, from which you can make other valuable
products including plastics.
Less green, but useful
purpose, is to use scrap plastic as a fuel, but complete incineration is
not always easy to be efficient and clean burning.
All of these will slow down
the rate at which valuable oil-petroleum deposits are depleted - the
latter are finite, so we should make the best use of them.
section 4. Pollution
Carbon monoxide etc.
Multiple Choice Quizzes and Worksheets
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry
3 linked easy Oil Products gap-fill quiz worksheets
ALSO gap-fill ('word-fill') exercises
originally written for ...
... AQA GCSE Science
Useful products from
crude oil AND
... OCR 21st C GCSE Science
Worksheet gap-fill C1.1c Air
pollutants etc ...
... Edexcel 360 GCSE Science
Crude Oil and its Fractional distillation
... each set are interlinked,
so clicking on one of the above leads to a sequence of several quizzes
Level Organic Chemistry revision notes
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