Properties of Materials: Plastic and Rubber – JSS1 Basic Technology Lesson Note (First Term)

Plastic and rubber are light, waterproof materials used to make many everyday objects, such as buckets, cups, tyres and gloves.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define plastic and name its two classes, with examples;
2. state the properties, advantages and disadvantages of plastic;
3. define rubber and name its two types;
4. state the properties and uses of rubber;
5. list the methods of processing plastics and rubber and describe each one briefly.

What is plastic?

Plastic is a light material made by chemical processes that can be shaped when heated.

• Raw materials: most plastics are made from chemicals obtained from petroleum.
• Making: the chemicals are joined and heated, then allowed to cool and solidify in the shape wanted.
• Everyday examples: plastic plates, cups, buckets, chairs, water bottles and pipes.

What are the classes of plastic?

There are two classes of plastic: thermoplastics and thermosets.

Thermoplastics

• Meaning: plastics that become soft when heated and harden again when cooled.
• Remoulding: they can be softened by heat again and remoulded many times.
• Types: polythene (polyethylene) and polystyrene.
• Examples of objects: plastic plates, buckets and cups.

Thermosets (thermosetting plastics)

• Meaning: plastics that set permanently when heated and moulded once.
• Remoulding: they are not softened by heat again, so they cannot be remoulded.
• Examples of objects: handles of cooking utensils and knives, and parts of telephones.

Difference between thermoplastics and thermosets

What are the properties of plastic?

The properties of plastic are the qualities that make it useful as a material.

• Light: plastic has a low weight for its size.
• Cheap: it costs little to make and buy.
• Insulator: it does not conduct electricity and conducts heat poorly.
• Corrosion resistant: it does not rust or rot easily.
• Waterproof: water does not pass through it.
• Easy to shape: it can be moulded and coloured easily.

What are the advantages and disadvantages of plastics?

Plastics have useful qualities, but they also cause problems for the environment.

Advantages of plastics

• Lightweight: plastic goods are easy to carry.
• Easy to mould: they can be made in many shapes and sizes.
• Low cost: they are relatively inexpensive to produce.
• Long lasting: they do not rust.

Disadvantages of plastics

• Poisonous fumes: some plastics give off poisonous gases when burnt.
• Litter: discarded plastics litter the surroundings and block drains.
• Slow decay: they do not rot easily, so waste stays for a long time.

What is rubber?

Rubber is an elastic, waterproof, non-metal material that stretches easily and returns to its original shape when released.

• Appearance: it looks like plastic but is more elastic.
• Elasticity: it can be stretched and goes back to its original length.
• Waterproof: it does not allow water to pass through it.
• Insulator: it does not conduct electricity.

What are the types of rubber?

There are two types of rubber: natural rubber and synthetic rubber.

Natural rubber

• Source: a milky liquid called latex, taken from the rubber tree.
• Collection: the bark of the tree is cut and the latex is collected in a cup.

Synthetic rubber

• Source: organic material obtained from petroleum.
• Making: it is made in factories by chemical processes.

Difference between natural and synthetic rubber

What are the uses of rubber?

Rubber is used to make goods that must stretch, grip, keep out water or absorb shock.

• Tyres: used on vehicles and bicycles.
• Hand gloves: used for washing, in hospitals and for electrical work.
• Electrical insulation: it covers wires and cables.
• Rain wear: rain coats and rain boots.
• Other articles: buckets and catapults.
• Shock absorber: it is used as an elastic, waterproof cushion.

How are plastics and rubber processed?

Processing is the shaping of plastic or rubber into useful articles.

Methods of processing plastic

1. Vacuum forming: a heated plastic sheet is sucked over a mould by a vacuum.
2. Extrusion: soft plastic is pushed through a shaped opening to make pipes and rods.
3. Calendering: plastic is passed between rollers to make thin sheets.
4. Compression moulding: plastic is pressed in a heated mould.
5. Injection moulding: melted plastic is forced into a mould, as in making cups and buckets.

Methods of processing rubber

1. Extrusion: rubber is pushed through a shaped opening to make tubes and strips.
2. Calendering: rubber is passed between rollers to make sheets.
3. Compression moulding: rubber is pressed in a heated mould.
4. Injection moulding: rubber is forced into a mould under pressure.

Evaluation

1. Name the two classes of plastic and give two examples of each.
2. State three properties of plastic.
3. Define rubber and state three uses of it.
4. State the common methods of processing plastics and rubber.

Assignment

1. State the difference between natural rubber and synthetic rubber.
2. State two advantages and two disadvantages of plastics.