Polyethene
Polyethylene also known as polyethylene or
polythene, was the first of the
polymers to be discovered. Polyethene is a
polymer produced by reacting oxygen
and ethene, in this reaction the small
ethene molecules attach together to form
long chain polymer molecules. This
process is known as addition polymerisation.
Polyethene in our world
today has many uses, some of these are: · mouldings-plastic
bottles, lids and
caps, different types of containers. · films-glad wrap and
various plastic
bags. · cable coverings-various pipes and insulating wire and
cables As you
can see polyethene has a huge variety of both domestic and
industrial uses,
this is fairly impressive when you see that polyethene has only
been around
sense 1933. Polyethene is a thermoplastic material which is often
described
as wax-like it is extremely tough and is has an excellent
chemical
resistance. It is also less dense than water and is the simplest
polymer, these
attributes lead to polythene being an extremely useful
substance. Ethene (C2H4),
is a simple hydrocarbon molecule which consists of
2 carbon atoms and 4 hydrogen
atoms. Ethene's main use is in the production
of polythene yet it is one of the
most widely used petrochemicals in the
world. Ethene is an unsaturated
colourless gas which can be ignited in the
presence of oxygen. Below is a
diagram of ethene: Polyethene is produced by
allowing the free roaming ethene
gas molecules to bond together to form long
chain polyethene molecules. In order
for this to work a catalyst must be
used, a catalyst is a substance that can
alter the rate of a chemical
reaction without undergoing any chemical change
itself. During this process
thousands of ethene molecules bond to from each
molecule of polyethene.
Polyethene is simply a set of ethene molecules bonded
together to form a
chain, these chains can often stretch up to many many times
longer than the
original ethene molecule. Below is a diagram of polyethene:
Although
normally ethene monomers have little attraction for one another, yet
the
polyethene molecules have a strong attraction for one another.
When
polyethene molecules are attracted and bond high-density polyethene is
formed,
thus polyethene is either formed by low-density or high-density
polymerisation.
Ethene can undergo the process called polymerisation due
to the fact that it is
unsaturated and because it has a double bond between
its two carbon atoms. Both
High-density and low-density polyethene have
different uses. Low-density
polyethene is used in the production of products
such as various bags, plastic
bottles, cling wraps, and insulating cables.
Low pressure polymerisation with
the use of certain catalysts; has meant that
the process of polymerization can
be achieved at fairly low pressure (20
atmospheres or 2000 kPa) and at
temperatures of approximately 100°C. The
reactor itself contains a polyethene
bed placed on a perforated plate. It
works by allowing the recycled gas to enter
near its base, the gas then
passes through the plate and pushes up through the
bed causing it to bubble.
Finally the catalyst converts the ethene to polyethene,
once the polyethene
is cooled it forms a fine powder called 'fluff' which is
then collected and
transported to a storage bin. Any ethene gas which has not
reacted passes
through a compressor and then a cooler, and is processed
again.
High-density polyethene is used to produce items such as lids,
caps, baskets,
bowls and large containers such as garbage bins. These
polyethene products are
created in what is know as high-pressure
polymerisation, this is a process that
was originally used before
low-pressure polymerisation was discovered and thus
it is a fairly simple
process. Firstly Ethene gas is compressed and liquefied,
from here it is
pumped into a large reactor at a pressure of up to 2660
atmospheres(266 Mpa).
Amongst this oxygen and peroxides are pumped in to
initiate the
polymerisation reaction. This process generates a huge amount of
heat so the
most complex part of the system is the cooling facilities. The many
products
of polyethene which are most commonly used are generally manufactured
using
any of these five different techniques Extrusion - film, this technique
is
used for the production of items such as plastic bags such as garbage bags
and
glad wrap. These are the most common applications, while there are many
others
these are the best examples of this technique. This final product is
achieved by
blowing air into a tube of molten plastic this allows for
extremely fine layers
of the plastic to form. Blow moulding, this is used to
make plastic bottles and
some motor oils. This technique is much like the
previous one but rather that
using such fine walls to create fine layers of
the plastic here the blow mould
allows for the molten plastic to be forced
downwards into a mould. Air is them
used to force the plastic against the
walls this way this final product is
thicker. Injection moulding, this is
used to make anything from lids and caps to
toys, baskets and even garbage
bins. Injection moulding, is the process where
the polyethene pellets are
melted and shot into a mould where the plastic
re-cools and hardens leaving a
rigid and firm final product. Rotational moulding,
this technique is used in
the production of things such as boats, playground
equipment and even canoes.
In this process a specific amount of solid polyethene
is placed inside a
mould, this mould is then spun at a high speed while being
heated. Once the
mould is removed a hard object is left much like that of
injection moulding.
Extrusion, this is used to insulate wires and cables as well
as making pipes.
To make pipe, the molten polyethene is passed through through a
ring where
the shape is decided, it then enters a cooling chaimber thus creating
a hard
product. For wire it is shot through a mould thus gaining a coating, it
then
enters a cooling agent which hardens it and thus the wire is coated with
a
hard insulation cover. After investigating the uses and properties of
polyethene
I have found that it is useful due to the following factors:
1. Insulation
properties 2. Chemical resistance 3. Strength 4. Flexability 5.
Non-toxic 6.
Water proof 7. Unreactive 8. Ability to be used in an
extremely thin film 9.
Fairly low production cost 10. Ease of production
11. Ability to be formed in
two seperate ways thus allowing for differnent
products with different
properties from the same polymer. Polyethene is used
in millions of applications
all around the world, due to the many different
uses of this substance it is
hardy unexpected. Polyethene it is so widely
used that for some of us life would
be almost impossible, for everyone of you
reading this has probabily come into
contact with at least one polyethene
product today. BIBLIOGRAPHY (1993)
Production of SBR, Kemcor Australia
Resources Kit - Section 6, Kemcor Australia,
Melbourne. Jones, Elvins.
Miskin, Lukins. Sanders, Ross (1995) Chemistry One,
Reed International
Books, Melbourne.
Bibliography
(1993) Production of SBR, Kemcor
Australia Resources Kit - Section 6, Kemcor
Australia, Melbourne. Jones,
Elvins. Miskin, Lukins. Sanders, Ross (1995)
Chemistry One, Reed
International Books, Melbourne.