- Plastics are flexible synthetic long-chain molecules (polymers). They are made from joining many smaller molecules (monomers) together. The raw materials for making plastics come from crude oil and the petrochemical industry.
- Examples of Plastics include bakelite, nylon, polyester and PVC (polyvinyl chloride).
- Polymerisation is the chemical reaction that joins repeating units called monomers together to form polymers. There are two types of polymerisation reactions.
- Two Polymerisation Reactions:
- Addition Polymerisation occurs when the double bonds of hydrocarbons called alkenes are broken, and then joined into a long-chain molecule (e.g. Many ethene molecules join to form polyethene in plastic bags.
- Condensation Polymerisation occurs when two types of monomers join to form a long-chain polymer and water is given off (e.g. Many molecules of urea and formaldehyde are joined to form glue.)
- Natural Polymers are also formed from polymerisation of natural monomers. Examples include Proteins made from amino acids, Silk and Wool.
Types of Plastics
- Thermoplastics consist of long chains with no cross-links that allow the molecules to slide over each other when heated. This allows them to be moulded into new shapes. Common examples are polyethene in plastic bags, nylon, polyvinyl chloride (PVC), perspex and polystyrene.
- Thermosetting Plastics have cross-linking between many molecules and they cannot slide over each other. Once they are first set into a certain shape, they cannot be remoulded. If they are heated, they will not melt but will eventually burn. Common examples are bakelite in light switches and saucepan handles, polyurethanes and epoxy resins.
Types of Shaping Plastics
Plastics can be moulded into objects in 3 main ways:
- Injection moulding
- Blow moulding
- Injection moulding consists of heating a thermoplastic and injecting it into a mould where it cools and hardens into the new object.
- This process is fast and economical for producing large numbers of objects.
- Injection moulding leaves a small line on the object where the mould splits to allow the object to be taken out.
- Examples: cups, bottle caps, dessert bowls
- Softened thermoplastics are forced through an opening in a piece of metal called a die. Once through the die, the plastic cools and hardens.
- The shape of the die determines the cross-section of the object.
- Examples: pipes, tubes, straws and plastic rods
- A small amount of thermoplastic is softened by heating, and then expanded by compressed air to fill the walls of a mould.
- Blow moulding is fast and economical for making hollow items.
- Examples: soft drink bottles