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# Plastics

In today's world, we can hardly imagine life without plastic. From cellphones to food packaging, fishing line to plumbing pipes, compact discs to electronic equipment, plastics have become a very important part of our daily lives.

Definition 1: Plastic

The term plastic covers a range of synthetic or semi-synthetic organic polymers. Plastics may contain other substances to improve their performance. Their name comes from the fact that many of them are malleable, in other words they have the property of plasticity.

It was only in the nineteenth century that it was discovered that plastics could be made by chemically changing natural polymers. For centuries before this, only natural organic polymers had been used. Examples of natural organic polymers include waxes from plants, cellulose (a plant polymer used in fibres and ropes) and natural rubber from rubber trees. But in many cases, these natural organic polymers didn't have the characteristics that were needed for them to be used in specific ways. Natural rubber for example, is sensitive to temperature and becomes sticky and smelly in hot weather and brittle in cold weather.

## Interesting Fact:

The first true plastic (i.e. one that was not based on any material found in nature) was Bakelite, a cheap, strong and durable plastic. Some of these plastics are still used for example in electronic circuit boards, where their properties of insulation and heat resistance are very important.

In 1834 two inventors, Friedrich Ludersdorf of Germany and Nathaniel Hayward of the US, independently discovered that adding sulphur to raw rubber helped to stop the material from becoming sticky. After this, Charles Goodyear discovered that heating this modified rubber made it more resistant to abrasion, more elastic and much less sensitive to temperature. What these inventors had done was to improve the properties of a natural polymer so that it could be used in new ways. An important use of rubber now is in vehicle tyres, where these properties of rubber are critically important.

## The uses of plastics

There is such a variety of different plastics available, each having their own specific properties and uses. The following are just a few examples.

• Polystyrene

Polystyrene (Figure 1) is a common plastic that is used in model kits, disposable eating utensils and a variety of other products. In the polystyrene polymer, the monomer is styrene, a liquid hydrocarbon that is manufactured from petroleum.

• Polyvinylchloride (PVC)

Polyvinyl chloride (PVC) (Figure 2) is used in plumbing, gutters, electronic equipment, wires and food packaging. The side chains of PVC contain chlorine atoms, which give it its particular characteristics.

### Interesting Fact:

Many vinyl products have other chemicals added to them to give them particular properties. Some of these chemicals, called additives, can leach out of the vinyl products. In PVC, plasticisers are used to make PVC more flexible. Because many baby toys are made from PVC, there is concern that some of these products may leach into the mouths of the babies that are chewing on them. In the USA, most companies have stopped making PVC toys. There are also concerns that some of the plasticisers added to PVC may cause a number of health conditions including cancer.

• Synthetic rubber

Another plastic that was critical to the World War 2 effort was synthetic rubber, which was produced in a variety of forms. Not only were worldwide natural rubber supplies limited, but most rubber-producing areas were under Japanese control. Rubber was needed for tyres and parts of war machinery. After the war, synthetic rubber also played an important part in the space race and nuclear arms race.

• Polyethene/polyethylene (PE)

Polyethylene ((Reference)) was discovered in 1933. It is a cheap, flexible and durable plastic and is used to make films and packaging materials, containers and car fittings. One of the most well known polyethylene products is 'Tupperware', the sealable food containers designed by Earl Tupper and promoted through a network of housewives!

• Polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (Figure 3) is more commonly known as 'Teflon' and is most well known for its use in non-stick frying pans. Teflon is also used to make the breathable fabric Gore-Tex.

Table 4 summarises the formulae, properties and uses of some of the most common plastics.

Table 1: A summary of the formulae, properties and uses of some common plastics
 Name Formula Monomer Properties Uses Polyethene (low density) $-(CH2-CH2)n-$ $CH2=CH2$ soft, waxy solid film wrap and plastic bags Polyethene (high density) $-(CH2-CH2)n-$ $CH2=CH2$ rigid electrical insulation, bottles and toys Polypropene $-[CH2-CH(CH3)]n-$ $CH2=CHCH3$ different grades: some are soft and others hard carpets and upholstery Polyvinylchloride (PVC) $-(CH2-CHCl)n-$ $CH2=CHCl$ strong, rigid pipes, flooring Polystyrene $-[CH2-CH(C6H5)]n$ $CH2=CHC6H5$ hard, rigid toys, packaging Polytetrafluoroethylene $-(CF2-CF2)n-$ $CF2=CF2$ resistant, smooth, solid non-stick surfaces, electrical insulation

### Exercise 1: Plastics

It is possible for macromolecules to be composed of more than one type of repeating monomer. The resulting polymer is called a copolymer. Varying the monomers that combine to form a polymer, is one way of controlling the properties of the resulting material. Refer to the table below which shows a number of different copolymers of rubber, and answer the questions that follow:

 Monomer A Monomer B Copolymer Uses $H2C=CHCl$ $H2C=CCl2$ Saran films and fibres $H2C=CHC6H5$ $H2C=C-CH=CH2$ SBR (styrene butadiene rubber) tyres $H2C=CHCN$ $H2C=C-CH=CH2$ Nitrile rubber adhesives and hoses $H2C=C(CH3)2$ $H2C=C-CH=CH2$ Butyl rubber inner tubes $F2C=CF(CF3)$ $H2C=CHF$ Viton gaskets
1. Give the structural formula for each of the monomers of nitrile rubber.

2. Give the structural formula of the copolymer viton.

3. In what ways would you expect the properties of SBR to be different from nitrile rubber?

4. Suggest a reason why the properties of these polymers are different.

1.

2.

3. Nitrile rubber is more likely to be flexible and may not be as durable as SBR.

4. SBR has a long chain where nitrile rubber has a triple bonded CN group.

In your home, find as many examples of different types of plastics that you can. Bring them to school and show them to your group. Together, use your examples to complete the following table:

 Object Type of plastic Properties Uses

ObjectType of plasticPropertiesUses
Plastic bottles PET Hard, sterile Cold drinks, milk
Polystyrene trays Polystyrene Flexible, disposable Meat trays, fast foods
Shopping bags PE (HDPE and LDPE) Flexible, durable Packaging
Yoghurt container PP sturdy food packaging

## Thermoplastics and thermosetting plastics

A thermoplastic is a plastic that can be melted to a liquid when it is heated and freezes to a brittle, glassy state when it is cooled enough. These properties of thermoplastics are mostly due to the fact that the forces between chains are weak. This also means that these plastics can be easily stretched or moulded into any shape. Examples of thermoplastics include nylon, polystyrene, polyethylene, polypropylene and PVC. Thermoplastics are more easily recyclable than some other plastics.

Thermosetting plastics differ from thermoplastics because once they have been formed, they cannot be remelted or remoulded. Examples include bakelite, vulcanised rubber, melanine (used to make furniture), and many glues. Thermosetting plastics are generally stronger than thermoplastics and are better suited to being used in situations where there are high temperatures. They are not able to be recycled. Thermosetting plastics have strong covalent bonds between chains and this makes them very strong.

### Case study 1: Biodegradable plastics

Our whole world seems to be wrapped in plastic. Almost every product we buy, most of the food we eat and many of the liquids we drink come encased in plastic. Plastic packaging provides excellent protection for the product, it is cheap to manufacture and seems to last forever. Lasting forever, however, is proving to be a major environmental problem. Another problem is that traditional plastics are manufactured from non-renewable resources - oil, coal and natural gas. In an effort to overcome these problems, researchers and engineers have been trying to develop biodegradable plastics that are made from renewable resources, such as plants.

The term biodegradable means that a substance can be broken down into simpler substances by the activities of living organisms, and therefore is unlikely to remain in the environment. The reason most plastics are not biodegradable is because their long polymer molecules are too large and too tightly bonded together to be broken apart and used by decomposer organisms. However, plastics based on natural plant polymers that come from wheat or corn starch have molecules that can be more easily broken down by microbes.

Starch is a natural polymer. It is a white, granular carbohydrate produced by plants during photosynthesis and it serves as the plant's energy store. Many plants contain large amounts of starch. Starch can be processed directly into a bioplastic but, because it is soluble in water, articles made from starch will swell and deform when exposed to moisture, and this limits its use. This problem can be overcome by changing starch into a different polymer. First, starch is harvested from corn, wheat or potatoes, then microorganisms transform it into lactic acid, a monomer. Finally, the lactic acid is chemically treated to cause the molecules of lactic acid to link up into long chains or polymers, which bond together to form a plastic called polylactide (PLA).

PLA can be used for products such as plant pots and disposable nappies. It has been commercially available in some countries since 1990, and certain blends have proved successful in medical implants, sutures and drug delivery systems because they are able to dissolve away over time. However, because PLA is much more expensive than normal plastics, it has not become as popular as one would have hoped.

Questions

1. In your own words, explain what is meant by a 'biodegradable plastic'.

2. Using your knowledge of chemical bonding, explain why some polymers are biodegradable and others are not.

3. Explain why lactic acid is a more useful monomer than starch, when making a biodegradable plastic.

5. What do you think could be done to make biodegradable plastics more popular with consumers?

## Plastics and the environment

Although plastics have had a huge impact globally, there is also an environmental price that has to be paid for their use. The following are just some of the ways in which plastics can cause damage to the environment.

1. Waste disposal

Plastics are not easily broken down by micro-organisms and therefore most are not easily biodegradeable. This leads to waste dispoal problems.

2. Air pollution

When plastics burn, they can produce toxic gases such as carbon monoxide, hydrogen cyanide and hydrogen chloride (particularly from PVC and other plastics that contain chlorine and nitrogen).

3. Recycling

It is very difficult to recycle plastics because each type of plastic has different properties and so different recycling methods may be needed for each plastic. However, attempts are being made to find ways of recycling plastics more effectively. Some plastics can be remelted and re-used, while others can be ground up and used as a filler. However, one of the problems with recycling plastics is that they have to be sorted according to plastic type. This process is difficult to do with machinery, and therefore needs a lot of labour. Alternatively, plastics should be re-used. In many countries, including South Africa, shoppers must now pay for plastic bags. This encourages people to collect and re-use the bags they already have.

### Case study 2: Plastic pollution in South Africa

Read the following extract, taken from 'Planet Ark' (September 2003), and then answer the questions that follow.

It is difficult sometimes to imagine exactly how much waste is produced in our country every year. Where does all of this go to? You are going to do some simple calculations to try to estimate the volume of plastic packets that is produced in South Africa every year.

1. Take a plastic shopping packet and squash it into a tight ball.

1. Measure the approximate length, breadth and depth of your squashed plastic bag.

2. Calculate the approximate volume that is occupied by the packet.

3. Now calculate the approximate volume of your classroom by measuring its length, breadth and height.

4. Calculate the number of squashed plastic packets that would fit into a classroom of this volume.

5. If South Africa produces an average of 8 billion plastic bags each year, how many classrooms would be filled if all of these bags were thrown away and not re-used?

2. What has South Africa done to try to reduce the number of plastic bags that are produced?

3. Do you think this has helped the situation?

4. What can you do to reduce the amount of plastic that you throw away?