Standard Grade Chemistry
Summary Notes
Topic 13. Plastics & Synthetic Fibres
General
Learning Outcomes
Most plastics and synthetic fibres are made from oil.
Synthetic as in ‘synthetic fibre’ means man-made.
You must be able to give examples of how properties of plastics (lightness, durability and insulation) are related to their uses.
You must be able to give examples of advantages and disadvantages of natural and synthetic materials.
Biodegradable means able to be broken down naturally by bacteria.
Most plastics are not biodegradable and their durability can cause environmental problems.
Some plastics burn or smoulder to give off toxic fumes.
A thermoplastic plastic is one which can be melted or reshaped; a thermosetting plastic is one which cannot be shaped in this way.
You must be able to give examples of uses of thermosetting plastics.
You need to be able to state the meaning of : Monomer
Polymer Polymerisation
Plastics are examples of polymers.
Many plastics are made from small, unsaturated molecules produced by cracking.
You must be able to give examples of plastics formed from alkenes.
The making of plastics is an example of polymerisation.
Given the name of a monomer, you must be able to state the name of the addition polymer.
Fibres, both natural and synthetic, are examples of polymers.
You must be able to explain, using full structural formulae, how ethane forms polythene.
Credit
Learning Outcomes
You must be able to relate the toxic gases given off, during burning or smouldering, to the elements present in the plastic, e.g. carbon monoxide, hydrogen chloride (PVC), hydrogen cyanide (polyurethanes).
Small unsaturated molecules join together by the opening of the carbon to carbon double bond.
You should be able to state what is meant by addition polymerisation.
The making of polyalkanes (and substitutes) are an example of addition polymerisation.
Given the structure of an addition monomer, you should be able to construct the polymer.
Plastics General
Plastics surround us in the world today. They are used in our homes, in schools, in hospitals, in fact almost everywhere. Some have very ordinary uses such as the
polythene bag. Others have very specialised uses. Many parts of the space shuttle, for instance, are in fact made of specially designed plastics.
But what is a plastic and where do they come from? A materials which is described as being plastic means that it is a material which is pliable or able to be moulded.
Many plastics used in the world today can be described as synthetic. These materials are not naturally occurring, but are man-made by the chemical industry.
Many of the starting materials of the plastics industry are obtained from crude oil. Crude oil can be processed at an oil refinery to give small molecules which are the basic building blocks of many different plastics. Crude oil first undergoes fractional
distillation and then some of the longer chain fractions undergo catalytic cracking to produce the smaller unsaturated compounds, e.g. ethane, that are commonly used as the starting materials in plastics manufacture. Ethene is sometimes called the building block of the plastics industry.
Plastics are made when small molecules join together to give much larger molecules. The small molecules which are used to make plastics are called monomers which means ‘one bit’. The large molecules which are formed are called polymers which means ‘many bits’, because the many smaller monomers have joined together. The process is called
polymerisation.
Using Plastics General/ Credit
The properties of a plastic affect how it will be made and the uses it will have in our everyday lives.
Common plastic Property Use
PVC flexible, electrical insulator electric cable
polystyrene light, poor conductor of heat hot drinks cups, packaging polythene light, unreactive, hard-wearing kitchen bowls, basins, buckets polypropene strong, high elasticity carpet fibres, ropes
The effect of heat on a plastic is an important consideration. Some plastics will soften on heating. These plastics are said to be thermoplastic. A thermoplastic is a plastic which softens or melts on heating and can be reshaped. Polythene, polystyrene and PVC are all examples of thermoplastics.
One hundred years ago, if we had examined people’s homes and clothes, we would have found that almost everything was made from natural materials. Today plastics have replaced many traditional materials. The replacement of traditional materials with synthetic plastics has advantages and disadvantages.
PVC pipes and gutters have advantages over the traditional metals used for these purposes because PVC is light and does not corrode. Similarly PVC window frames do not rot or require painting, unlike the traditional wooden frames.
Nylon, polyester or poly(propene) fibres used in carpet manufacture have an advantage over the traditional wool in that they are harder wearing, but the disadvantage is that they are not so soft or warm.
Plastics do have other serious disadvantages compared to natural materials.
We live in a throw away society which produces an enormous amount of waste. Materials such as wood, cardboard and paper are broken down naturally by bacteria. These materials are said to be biodegradable. Most plastics are not biodegradable and will remain unchanged for many years. This durability can cause environmental problems such as pollution.
The problem can be tackled in 2 ways. Plastics can be recycled, by collecting them and processing them to form chemicals which can then be used to make them new plastic products. This is particularly important at a time when the world’s oil and coal reserves are beginning to run out and it is becoming more important to recycle materials of all types.
Another way to tackle the problem would be to burn waste plastics. This would provide energy, but would mean that the plastics would be lost as a source of raw material. Another problem with this method is that some plastics burn or smoulder to give off
toxic fumes.
Any plastic containing carbon can burn to give carbon monoxide if burned in a limited air supply; PVC burns to form hydrogen chloride and polyurethane burns to form deadly
Building Polymers General/Credit
One of the commonest plastics that we use in our everyday lives is poly(ethene) usually known as polythene. Poly(ethene) is a polymer made from the monomer ethane, which is the simplest member of the alkene family.
The molecules of ethane join by adding on to each other. As many as 5000 molecules
can join in this way. Since many ethene molecules have joined together, the polymer is called poly(ethene) , (many ethenes). The more common name is polythene.
This type of polymerisation is called addition polymerisation because many small molecules have added onto each other to form a large molecule.
The ethene molecules join to each other by opening the carbon to carbon double bond.
All polymers formed by addition polymerisation are based upon monomers containing
C=C (carbon to carbon double bonds).
Poly(ethene) is formed when all 4 attached groups are H atoms i.e. when the monomer is ethene.
Poly(propene) is formed when 3 attached groups are H atoms and the other is CH3 i.e.
when the monomer is propene.
Addition Polymers General / Credit
The following table shows some information about the most common addition polymers and their monomers.
Name of polymer Polymer
formula
3 unit section of
polymer
Name of monomer
Monomer formula
poly(ethene) ethene
poly(propene) propane
poly (chloroethene) (polyvinylchloride) PVC
chloroethene (vinylchloride)
poly(tetrafluoroethene)
(PTFE) tetrafluoroethene
poly(phenylethene) (polystyrene)
phenylethene (styrene)
The hexagon shape symbol in phenyl(ethene) and its polymer represents the phenyl group which has the formula C6H5. The six carbons of the phenyl group are joined in a
ring.
