What are the features of plastics?
- Good thermal/electrical insulation
- Low density
- Easy to manufacture
- Low cost
- Useful as adhesives
- Good transparency
What are polymers?
- Polymers are materials with large macro-molecules, of which plastics are just one group.
- The extent of cross-linking between chains influences the final material.
- Stronger attractive forces between chains leads to a stronger, less flexible polymer (e.g. nylon).
- If chains are less able to slide past each other, the polymer is likely to be flexible.
What are the different types of polymer?
- Thermoplastics
- Thermosets
- Elastomers
- Polysiloxanes
What are thermoplastics?
- Linear/branches chains with minimal cross-linking.
- Weak intermolecular bonds allow melting when heated.
- Greater ability to form crystalline regions.
- Ductile.
- e.g. polyethylene, polypropylene, polystyrene
What are thermosets?
- Heavily cross-linked.
- React and harden causing the material to burn when heated.
- Brittle.
- e.g. vulcanised rubber, epoxies, phenolic resin
What are elastomers?
- Linear with some cross-linking, allowing them to carry much higher strains.
- Shape memory.
- Useful for heat shrink, insulating electrical cables, and structural bearing pads.
What are the different types of chains in polymers?
Linear polymers:
- Single chains
- Flexible
Branched polymers:
- Side branches
- Less efficient packing
- Lower density
- Less crystalline
Cross-linked polymers:
- Adjacent chains are joined at various positions by covalent bonds
- Sometimes referred to as a network
What are copolymers?
Copolymers consist of two ‘met’ units, resulting in an irregular chain structure which can improve or tailor specific properties by modifying the intermolecular bonds.
Random copolymers:
- ABBABA
Alternating copolymers:
- ABABAB
Block copolymers:
- AAABBB
Graft copolymers:
- AAAAAA
BB
(active site)
What is polymerisation?
Polymerisation is the chemical process of reacting monomer molecules to form 3D networks or polymer chains.
Condensation polymerisation:
- Elimination of small molecules
Free radical polymerisation:
- Produces an addition reaction
- Initiation - free bonds produced
- Propagation - chains get longer
- Termination - longer molecules join together
What is the molecular weight?
- The mass of a mol of chains.
- It affects the tensile strength, stiffness and viscosity of the material.
- Longer chains are more likely to entangle and therefore can ‘anchor’ better.
- The critical length before strength increases is 100 ‘mers’ (40 for nylon).
How do you solidify a polymer?
- Annealing causes crystalline regions to grow.
- These crystalline regions are resistant to solvents.
- Secondary bonding such as hydrogen bonds and dipole - dipole reactions are very important.
How are crystalline regions discouraged?
- Random copolymerisation
- Bulky side groups
- Chain branching
What is the difference between semi-crystalline and amorphous structures?
Semi-crystalline:
- Stiff
- Less ductile
- High tensile strength
Amorphous:
- Less stiff
- Ductile
- Low tensile strength
What is the crystallinity and molecular weight of plastics/waxes/grease?
Which factors affect the rate of crystallisation?
- For crystallisation, considerable molecular mobility is necessary.
- The formation rate of these crystals is the product of the nucleation rate and the growth rate.
- Growth rate diffusion is controlled and at a maximum just below the melting point.
- Nucleation rate is at a maximum just above the glass transition temperature.
- Overall formation rate is maximum half way between these two temperatures.
What is the affect of crazing?
- Crazing in linear polymers produces voids which scatter light.
- Molecules are drawn out on a very small scale producing strong strands which bridge microcracks.
Which factors affect the mechanical properties of polymers?
- Molecular structure
- Operating temperature
What is the affect of cold drawing?
- Cold drawing causes molecules to be drawn out and aligned.
- This makes the material much stronger in a particular direction that it once was.
- Therefore, elasticity and tensile strength are increased but the ductility is reduced.
What does the stress-strain graph for different polymers look like?
What effect does temperature have on polymers?
- When heated, polymers deform plastically.
- Therefore ductility increases, but stiffness and strength decrease.
- Chain stiffness and the bulkiness affects the ease of rotation of side groups.
- Bulkier side groups are harder to rotate, and so melting temperature is higher.
- The presence of polar side groups can also lead to significant intermolecular bonding forces.
What is the glass transition temperature equation?
How are polymers formed?
- Injection moulding
- Extrusion
- Thermoforming
- Blow moulding
- Compression moulding
What is injection moulding?
Thermoplastic - two hoppers feed resin and colourant onto a tapered screw root which forces them into a cooled mould.
Thermoset - a hopper feeds resin onto a screw root which forces it into a heated mould.
- Produces complex shapes.
- Rapidly manufactured.
- Moulds are expensive, but production is very cheap once installed.
What is extrusion?
- The polymer is heated causing it to melt into a liquid.
- This is then forced through a die of required cross-section.
- The polymer is then extruded out of the die and cooled.
- This produces complex shapes.
- Rapidly manufactured.
- Moulds are expensive, but production is very cheap.
