Metalwork - Materials - Plastics
Polymer is a Greek word. We can understand it
better when we break it in two : Poly means many,
and mer means parts. Polymers are
made up of long chain like Molecules,
where each molecule is a part or mer. These mers are joined together by Chemical
Bonds.
Look at the picture of the paper clips to the right.
Imagine each paper clip represents a molecule or mer,
and when they are joined together they form long chains. Polymers are made up of similar long chains.
Take a look at the bowl of spaghetti to the left.
This is what a Polymer might look like under a
microscope, magnified many, many times.
Categories of Polymers
Polymers are divided into 3 main categories, depending on how the are obtained :
| Category | Description | Examples |
| Natural Polymers | Natural Polymers occur in nature without any interference from man. These occur much more commonly than you would think, and derive from plants, animals, and even you. Your body includes a Polymer called Keratin which makes up your hair and nails ! |
Shellac, Keratin, Bitumen, Lacquer, Amber, Rubber, etc. |
| Modified Polymers | Modified Polymers are naturally occuring materials which have been altered in some by man. |
Cellulose, Casein, etc. |
| Synthetic Polymers | Synthetic Polymers are totally created by man. They are made from either the by-products of coal or oil manufacture, by chemical methods. |
Acrylic, Polythene, PVC, Nylon, etc. |
Classification of Polymers
Polymers are sub-divided into two classification based on their Chemical Bonds, which determines how they react to heat and / or pressure.
- Thermoplastic Polymers
- Thermosetting Polymers
The reason that Polymers are affected by heat has to do with their Chemical Bonds.
Thermoplastic Polymers
Thermoplastic Polymers make up 80% to 90% of the worlds Polymers. A Thermoplastic Polymer can be reheated and reshaped over and over again. When enough heat is applied to a Thermoplastic Polymer it softens allowing you to reshape it. It hardens again when it cools. You can learn more about reshaping Polymers in Working with Acrylic . Thermoplastic Polymer's mers are joined by Secondary Bonds. Secondary Bonds break when heated as they are weak and so a Thermplastic Polymer can be reshaped. When the Polymer cools down again tghe Secondary Bond reforms between the mers. Generally Thermoplastic Polymers cannot be used in temperatures much above 100°C as the heat resoftens them.
Candle wax is good for a demonstration of Thermoplastic
Properties. When you burn the candle the wax melts
and then solidifies when it cools. If you heat up the
wax again, it will resoften, and can be shaped. You
can keep doing this over and over again.Thermosetting Polymers
Thermosetting Polymers, although not as common as Thermoplastic Polymers, are still an important part of the Polymer industry. Thermosetting Polymers undergo a chemical change called curing when they are moulded and hardened. What happens here is that Primary Bonds form between the mers. Primary Bonds do not break up under heating, so Thermosetting Polymers cannot be resoftened and reshaped. Take the example of boiling an egg. After the egg has been boiled and cooled down, it is solid. If you reheat it the egg will not soften. And that is basically how Thermosetting Polymers operate.
Brief History of Polymers
Natural Polymers have been in use for thousands of years, however it is really only in the last 140 years that the Polymer Industry has grown.
In 1862 the first modified polymer was unveiled by Alexander
Parkes at the 'Great International Exhibition' in
London. The material was called Parkensine and
Alenander Parkes claimed that it could do everything
that Rubber could do but it was cheaper to
produce.
Rubber had been used by Native South Americans, who used it for shoes,coats and capes, long before Christopher Columbus set off in 1492. In 1770 a British chemist called Joeseph Priestley discovered that Rubber could rub out pencil marks on paper, and this is where the name came from.
In 1839 an American called Charles Goodyear, (yes
the tyres), introduced a process called Vulcanisation.
This was a major break through. Vulcanisation made Rubber more elastic, stronger, and mor resistant to abrasion
and weathering.
Nowadays the Rubber Industry is hugh with Bridgestone, Goodyear, Michellen, and Pirelli at the forefront. If you watch a Formulae One motor
car race you will notice how additives to the Rubber produce many different Rubber compounds so that
the cars will have maximum grip in a wide varity of
conditions.
In 1909, when Leo Baekeland from Belgium created Bakelite,
polymers came into their own and started to be
recognised as 'real' materials. However the real big
break for Polymers took place because of World War II.
Because of the War all of the countries involved lost
their easy access to raw materials, such as fabrics
and metals, from other countries. Scientists then set
about really investigating Polymers to see what could
be done to solve the problem. Nylon became a
major source of textile fibres, and Polyester was used in making armor and other materials.
After the War the study and production of Polymers rapidly increased to get us where we are today.
Common Polymers
Most people take Polymers for granted... they all
look the same, they feel the same... etc. Click on Types
of Polymers if you would like to find out more
information about the many common Polymers that
surround you, in terms of discription, properties and uses.
All Polymers are different, and are made in this
way so that they can be used for so many different purposes.
However most most Polymers have the following
charasteristics in common :
1. Polymers can be very resistant to
chemicals
If you think of all of the cleaning fluids you might find in
the kitchen such as, 'Parazone' or caustic soda, you will
notice that the are contained in Polymer containers. Look at
the containers, and you will probably see danger signs
telling you what there chemical liquids can do to your skin,
yet they are perfectly safe when contained within the
polymer container.
2. Polymers can be both thermal and electrical
insulators
If you walk through you house or any other building you will
see that every electric cable is made of some type of
Polymer, because of its electrical insulating properties. In
the kitchen of your house look at the pots and pans. Most
will have Polymer handles. If you have a padded jacket the
inside fibres are probably Polymer, and these points show
the thermal insulating properties of Polymers.
3. Polymers are very light with varying
degrees of strength
If you think about all the things that are made from
Polymers you should get this point. Polymer uses vary from
toys and pantyhose to Kevlar for bullet proof jackets. They
are now building space station frames from Polymers.
4. Polymers can be manufactured in various
different ways to produce thin fibres, intricate shapes or
large objects
Again look around you and you can see the proof of this.
Plastic bottles, parts of toys, plastic bags, car bodies,
oil tanks, Styrofoam, fibreglass, etc., are all manufactured
in different ways, but some are made from the same Polymer.
Recycling of Polymers
Polymers have a lot of advantages as their availability and variety of uses prove. However they have one major problem which is very evident in this age of conservation and being friendly to the enviornment. All Polymers have additives included to increase their range of uses and to improve their physical properties. Some additives are :
| Additive | Change |
| Plasticisers | improves flexability |
| Pigments | colouring of the Polymer |
| Stabilisers & Antioxidants | helps prevent the disintegrating effects of heat, ultra-violet light and oxidation |
| Flame Retardent | prevents the Polymer from bursting into flame |
Some of these additives make the Polymers enviornmentally undegradeable, that is they won't rot
like paper or wood in a landfill. As a result Recycling has emerged as the most practical solution to the
problem.
In 1988 a special code system was introduced by the
Americans to help with recycling. On some Polymer food
or solvent containers you can see the symbol to the
right with a number in the centre. The number tells
the recycler what material the Polymer product is made
from. You can investigate this yourself by using this
handy Chart.
Polymer Experiment
Goto the Polymer Experiment Page to see how you can differentiate between different types of Polymers in the school workshop. You will need a number of different types of Polymers, and you could even bring some used household products with you to experiment with.
Working with Acrylic
The use of Polymers is becoming more and more popular in the Metalwork room. Acrylic or "Perspex" as it is more commonly known is probably the most common. To learn more about workshop techniques with Polymers goto the Working with Acrylic page