High and low density poly(ethene)
Polythene or poly(ethene)
There are two types of polythene in common use; high density
polythene (HDPE) and low density polythene (LDPE). Both of these
types of poly(ethene) or polythene are made from the
same monomer; ethene. However the differences in the chemical
and physical properties of these two types of polythene are down
to the way in which they are made.
High density poly(ethene) (HDPE)
HDPE is used to make all the items in the collage below:
The long polythene chains which make up HDPE are
very linear and regular in shape with only a few side chains or
branches present; this allows the long polymer chains to pack
closely together which results in a dense polymer. The close
packing of the polymer chains also allows for more intermolecular
bonding between adjacent polymer chains which results in an
increase in the strength of the polymer. HDPE has many uses; this
is mainly due to its many useful and valuable properties which
include:
UV-resistant which means it is suitable for items that will be
used outdoors e.g. wheelie bins, pipes and garden furniture.
It is also a strong and stiff material but is sensitive to
cracking.
HDPE is resistant to most chemicals making it the ideal
material to make bottles to store cleaning solutions, soaps,
detergents and oils in. It is also suitable to store and
package foodstuffs in since it is a non-leaching polymer.
HDPE is also easy to shape and mould into many different
shapes and items. It is lightweight, easy to colour and a very
durable polymer.
However HDPE has numerous drawbacks: it is not biodegradable,
it is flammable and it is made from a monomer obtained from
crude oil; which is of course a finite resource.
Low density polythene (LDPE)
Due to the way in which LDPE is made the polymer chains have
many side branches present; this prevents the polymer chains in
the LDPE from packing together closely which results in a low
density polymer. Since the polymer chains cannot pack together
well there is a reduction in the strength of the intermolecular
bonding present which reduces the strength of the polymer.
The collage below shows a few items which are made from LDPE.
Low density polythene is used to make a wide variety of items
including:
Perhaps the most common use for LDPE is in plastic
shopping bags, rubbish bags, food wrapping bags and cling
film for food wraps. LDPE is resistant to water, oil, acids and
alkalis, it is relatively strong and lightweight so is a suitable
material to use for bags and wraps.
Many soft squeezable bottles, bowls and children's toys are
also made from LDPE.
LDPE is also commonly used to make flexible pipes and
insulation for electrical wires.
The difference between the two types of polythene are down to
the way they are made. The table below summaries the
differences in the way these two variants of the same polymer are
made.
HDPE manufacturers use medium temperature and low
pressure and a catalyst whereas LDPE is manufactured using a
much higher temperature and pressure and an oxygen or
peroxide initiator to start the reaction. These different conditions
produce polymers with different structures and
properties. HDPE has straight polymer chains which pack together
tightly resulting in a polymer with lots of strong intermolecular
bonding present. This produces a polymer with a high melting
point, which is stiffer, stronger and denser than LDPE.
LDPE on the other hand has polymer chains which vary in length
from between 4000 to 40 0000 carbon atoms long with lots of
branches present. The presence of these branches means that
the polymer chains cannot pack together very well and so it has a
low density. It also has a lower melting point, is softer, more
transparent, more flexible than HDPE.
Thermosoftening and thermosetting polymers
If you
could see the polymer chains in a typical plastic or polymer what
do you think they might look like? Well a good picture to have in
your mind is a plate of noodles or spaghetti! Here the long
strands are all tangled and mixed up together. Well in a
typical polymer this is probably how the long polymer molecules
or chains would look. If you pull on any of the strands of spaghetti
it is possible to tease them out and pull them off the plate. This is
because each strand is separate and not actually chemically
joined to any other strands.
Most polymers or plastics when you heat them to fairly
low temperatures will melt easily. This is easily demonstrated if
you take say a plastic bowl or a plastic bottle and place it in an
oven at around 150-2000C. The plastic will soften and melt. This
softened or molten polymer could be processed again in a factory
to make a new plastic object- it could be recycled. Plastics
or polymers which melt when they are heated are
called thermosoftening polymers or plastics. Most plastic
are thermosoftening.
However the image below shows some plastic objects which are
used in high temperature environments yet they do not melt. Car
engines contain many plastic components which are exposed to
very high temperatures; even at home for example pots have
plastic handles, plastic kitchen utensils, electrical plugs and even
the plastic trim on many ovens seem to be able to resist high
temperatures without melting. These polymers are
called thermosetting polymers. They will not melt on heating,
however if they are heated to very high temperatures they will
start to char and eventually they will decompose and they may
catch fire and burn.
The reason for these differences in the properties of these two
types of polymers is in the way the polymers or plastics are made
and the differences in their structures.
Thermosoftening polymers contains of long polymer chains which
are all tangled and mixed together much like the noodles above;
but there are only weak intermolecular forces between
the polymer chains. So when these polymers are heated the
polymer chains begin to move and vibrate more and more; this
increase in movement of the chains will break the weak
intermolecular bonds and the polymer chains become free to
move- the polymer will then start to melt.
Thermosetting polymers- have strong covalent bonds or cross-
links holding the polymer chains in place. These covalent
bonds are much stronger than the weak intermolecular
bonds which hold the chains in place in a thermosoftening
polymer. When a thermosetting polymer is heated the chains are
prevented from moving by these cross-links, so
the polymer cannot melt.
The image below shows how the polymer chains are arranged
in thermosetting and thermosoftening polymer.
Key Points
The properties of polymers can be changed by processing or
manufacturing them using
different catalysts, temperatures or pressures.
Thermosoftening polymers consist of long polymer
chains which are attracted to each other by weak
intermolecular bonds. These weak bonds are easily broken
and this means that when heated these polymers will melt.
Thermosetting polymers have chains which are cross-
linked by strong covalent bonds. This type of polymer will
not melt when heated.
