Plastics are made up of long chain molecules. These are things made up of a series of atoms. A large number of long chain molecules are called hydrocarbons. This is because the long chain molecules are made up of hydrogen and carbon atoms in different combinations.
A polymer is the scientific term for a plastic
There are many sources of raw materials which are used in production:
- When making semi synthetic polymers animal and vegetable by products are used.
- The source of purely synthetic polymers is coal, oil and gas. Synthetic polymers are carbon based as they are made through the process of thermal cracking.
There are 3 main types of plastic:
- Thermoplastics - can be repeatedly reheated and remoulded e.g. acrylic, nylon, polyethylene and polystyrene.
- Thermosetting - undergo a chemical change, resulting in them being permanently rigid e.g. epoxies and polyester.
- Elastomers - polymers which have good elasticity. They can be distorted under pressure but will return to their original shape when the pressure is removed.
Some general properties are:
- good electrical and thermal insulators.
- good strength to weight ratio - which means they have a good strength compared to their weight.
- good atmospheric and chemical corrosion resistance.
- low melting temperatures (thermoplastics).
Thermoplastics can be repeatedly reheated and remoulded.
Properties of thermoplastics include:
- Deformable upon heating.
- High melting points(100 - 1000 degrees Celsius).
Thermoplastics have linear branches with no cross links as this is what allows them to be repeatedly reheated and remoulded. Another reason for this is that they have weak intermolecular forces between the atoms.
Some example of thermoplastics are:
- Acrylic - this plastic is durable and tough. It can be easily machined as well. Acrylic is commonly used for light units and illuminating signs.
- Polystyrene - this plastic is lightweight and colourless. It also have low impact strength. It is commonly used for packaging and disposable cups and plates.
- Nylon - this plastic is tough, hard and resistant to wear. Therefore, it is commonly used for clothing and curtain rail fittings.
Thermosetting plastics undergo a chemical change, resulting in them becoming permanently rigid.
Properties of themosets include:
- Permanently rigid.
Thermosets have strong intermolecular forcesbetween individual hydocarbons. The bonds are prevent which means that they can only have relative movement and they have extensive cross linking, stopping them from being able to be remoulded.
Some examples of thermosets are:
- Epoxy resin - This is a high strength plastic with good chemical resistance. This means that it is commonly used for surface coating of other materials.
- Urea Formaldehyde - This plastic is brittle, rigid and hard which means that it is commonly used for electrical fittings.
Elastomers are plastics which good elasticity and distort under pressure, but once the strain is removed, the material returns to it's original shape.
Properties of elastomers include:
- Good insulators.
- Resistant to corrosion so they are durable and long lasting.
- Waterproof and resistant to humidity.
- Don't conduct electricity.
In elastomers, the long chains of molecules are tangled together. However when they are stretched, they are distorted and the chains are forced to line up in the direction in which the material is being stretched.
Some example of elastomers are:
- Liquid Silicon Rubber (thermosetting elastomer) -This elastomer is often used for medical uses such as tubing, plasters and bandages. This is because it is non toxic.
Improving the properties of plastics
Fillers - these are used to reduce the bulk of the plastic. They are cheaper than plastics and so often used to maximise profit. They can also increase strength and hardness, whilst reducing brittleness e.g. sawdust, crushed quartz and limestone.
Flame Retardants - these are used to reduce the risk of combustion. They work by creating a chemical reaction once the combustion begins, and then they release subtances which stop the combustion continuing e.g. foams used to fill seating cushions.
Anti Static Agents - these reduce the effects of static charges that could build up on a product e.g walking on a carpet made from synthetic materials.
Plasticiser - these are added to plastics to improve the flow of properties of plastics when being moulded. This is achieved by reducing the osftening temperature and also by making the material less brittle.
Stabiliser - these are used to reduce the effects of ultra violet light. They are used in products which are exposed to a lot of light e.g. conservatory components.
Biodegradable is a term given to materials that will break down with the aid of natural processes such as sunlight and rain.
Some applications for biodegradable polymers are:
- shopping bags, food trays, soft drink bottles and catering
- disposable pots, bowls, plastic cutlery and medical uses such as gloves
Most biodegradable plastics come from renewable raw materials. When materials are renewable they are extracted from managed sources such as forests where trees are replanted.
Some biodegradable polymers are:
- Polyactides (PLA) - this polymer is transparent and can be processed in similar ways to conventional thermoplastics. Applications include, bottles, films for carrier bags, plant pots and disposable nappies.
- PHA/Biopol (made from natural cellulose) - these are biodegradable plastics made by using bacteria to aid in fermenting plant sugars. The most popular of this type of polymer is PHB. Variations of this polymer are used in packaging as it has similar properties to polypropylene. Applications include dispersable fixatives (films, screws and bone plates), applying slow release medication. These polymers are stable in their environment, but are fully compostible when in contact with soil microrganisms.
- Oxo - degradable polymers - These have additives which promote short degradation time. They help the polymer break down into a powder, making it more reeadily diagestable by microorganisms e.g. Photo-degrable polymers break down when exposed to UV light. You can also add these additives (bio-batch) to normal polymers, and it has the same effect.
- Water Soluble Polymers - Liquid detergent packages make use of this technology, and they dispense the detergent only when in contact with warm water. Another example is laundry bags which hold the dirty washing until placed in the washing machine which protects people such as hospital staff.
Thermoplastics can be processed in a number of ways, depending on what it is that is going to be manufactured. However, thermosets are limited by the manufacturing methods used.
Processing Plastics - Injection Moudling
Injection Moulding is a method of processing thermoplastics and is often used to produce complex 3D shapes.
Processing Plastics - Injection Moudling
Stages of the Process
Step 1: Plastic granules and any additives are placed into the hopper. The granule mixtures falls through the hopper and onto the Archimedean Screw.
Step 2: The Achimedean Screw is rotated using the motor and gearbox. This forces the granules towards the heater, where it becomes softened, ready to be injected into the mould
Step 3: The hydraulic ram forces the softened polymer through the feedhole into the mould. Pressure from the ram ensures that the mould is full.
Step 4: After a few seconds, the polymer has cooled and solidified. The mold halves are opened. The ejector pins are activated to push the product from the mould.
Step 5: The mould closes and another cycle can begin.
Advantages and Disadvantages of Injection Moulding
- The mould can be very complex, which allows complex shapes to be produced.
- High volumes of products can be produced, all at the same quality
- Metal inserts can be included in the item when produced.
- The initial set up costs of the machine itself are expensive
- The process of making the moulds, and therefore the mould themselves are very expensive.
Processing Plastics - Blow Moudling
Blow Moulding is a processes of manufacturing thermoplastic materials into re-entrant shapes with just one opening. For example, drinks bottles and detergent bottles.
Stages of the Process
Step 1 - A tube (a Parison) of heated and softened polymer is extruded vertically downwards.
Step 2 - The mould halves then close which traps the top end of the Parison, this is now sealed.
Step 3 - Hot air is blown into the Parison which causes it out to follow the shape of the mould.
Step 4 - The mould cools down the polymer which allows it to be released from the mould.
Step 5 - The mould halves are opened which then means that the product can be taken out.
Advantages and Disadvantages of Blow Moulding
- Blow moudling is a quick way of creating hollow shapes once the mould has been initially set up
- Shapes which are not circular can be made
- The moulds for the process can be very expensive
- Some shapes cannot be made using this process i.e. shapes which don't allow easy extraction such as triangular shapes bottles.
Processing Plastics - Rotational Moulding
Rotational Moulding is a method of moulding thermoplastic polymers that involves plastics in powder form taking the shape of a fully enclosed mould as the mould passes through a heating section. The material become rigid once cooled.
Stages of Rotational Moulding Process
Stage 1 - The moulds are loaded with the correct amount of thermoplastic powder e.g. polyethylene. The moulds are clamped together so that no powder can escape
Stage 2 - The moulds are rotated around an arm spindle and the whole arm rotated towards a heated chamber. This causes the thermoplastic to be heated to it's melting point. The rotation ensures that all of the mould is coated with thermoplastic.
Stage 3 - The material is then cooled so that it can be extracted from the mould.
Stage 4 - The mould is returned to it's starting point, the halves are separated and the product removed.
The arms on the machines are attached to a fixed central point. The moulds themselves are attached to the end of each arm and are rotated all the time. They only stop rotating at the start of the processes and at the end.
Advantages and Disadvantages of Rotational Moulding
- Products made out of one piece of plastic can be produced
- It can be used to make rigid, tough and flexible shapes.
- A wide size range of products can be made
- Textures can be applied in the mould to create surface textures on the product
- High pressures are not required and therefore the mould is cheaper than in both blow and injection moulding. This means that there can be lower production runs.
- Only hollow shapes can be made using this process
Processing Plastics - Thermoforming
Thermoforming is very similar to vacuum forming. Vacuum Forming only has a vacuum to pull the softened material towards the mould. Whereas Thermoforming uses air pressure and an additional female mould to assist the forming processes which means that the products being formed can have more detail.
Stages of the Thermoforming Process
Stage 1 - The sheet of thermoplastic is heated to it's softening point and held in the frame between the male and female mould halves.
Stage 2 - The mould halves are closed and a vacuum is applied through the lower mould.
Stage 3 - The upper mould is lowered onto the sheet of plastic which ensures the required amount of detail is achieved.
Note: This is the same as Vacuum Forming, but vacuum forming doesn't have the upper mould.
Advantages and Disadvantage of Thermoforming
- The process is relatively low cost
- Thermoforming is good for smooth shapes which have a lot of detail
- Moulds which are deep cause the walls to be thinner where it has been stretched.
- Thermoforming can only produce simple designs
- Once it has been removed from the mould, you often need to trim off a lot of excess material.