Chapter 15 (Plastics as Engineering materials) Flashcards
what factors do the results of the stress strain test depend on? (3)
- temperature
- strain rate (speed of pulling)
- Chemical environment
what are the 3 main types of behaviour for plastics?
- Elastic-Fracture since their brittle
- Elastics then Plastic
- Completely elastic
what can be said about the elastic modulus of plastics compared to metals
much less than metals
describe the elastic behaviour of plastics
there is an instantaneous deformation and recovery, it is recoverable (low temps)
describe viscous behaviour of plastics
deformation is time dependent and non recoverable (higher temps)
If a polymer is said to be viscoelastic what will the polymer behave as and what type of behaviour will it exhibit?
polymers can behave as rubbery solids and it will have non-linear elastic behaviour
what does chain slippage depend on? (3)
- the rate of strain(speed of pulling)
- time
- temperature
What happens if the strain is fast on plasitics?
there no time for slippage-» brittle failure occurs
what happens if the strain is slow for plastics?
there is chain slippage and the plastic will react as ductile
what do the graphs of elastic, viscous and visco-elastic behaviour look like respectively?
horizontal line. linear response, positive ln curve
as time increase what happens to stress:
the stress decrease: STRESS RELAXATION
If temperature increases, what happens to the stress
Stress is lower and decreases faster
Describe viscoelastic creeep:
once you apply a constant load:
- intially, the plastics will instantaneous strain
- over time, the plastic will continue to deform over time. This time dependent deformation is known as creep
What else is the Creep modulus also dependent on
temperature
Match these: Ductile response, Brittle response; Above Tg, below Tg
Above: Ductile
Below: Brittle
Describe Crazing
Craze formation prior to cracking:
- plastic deformation of spherulites
- formation of microvoids and fibrillar bridges
Describe elastic deformation of semi crystalline polymers and state whether its recoverable or not
occurs by bond stretching/bending in the chains. It is recoverable
Describe plasic deformation of semi crystalline polymers and state whether its recoverable or not
- the amorphous regions elongate, and the chains slide past each other
- tilting of semi-crystalline regions occurs
- blocks orient towards tensile axis
- Non recoverable unless heated
Where would continued deformation typically occur and why
in the unnecked region because alignment can occur in the necked region which would lead to strengthening
describe predeformation by drawing:
drawing stretches the polymer prior to use, this aligns the chains to the stretching direction
what are the results of drawing: (3)
- increases the elastic modulus in the stretching direction
- increases the tensile strength in the stretching direction
- decreases ductility
What does annealing after drawing do? (2)
- decreases alignment
2. reverses the effects of drawing
Which factors affect the strength of polymers and how?
- Average molecular weight of chains: longer chains(heavier)=more entanglement
- Degree of crystillinity: affects intermolecular secondary bonding. closer packed=stronger.
- Drawing: increases E
- Heat treating
what negative effects does an increase in crystillinty have?
increases brittleness
What does heat treating do for undrawn materials
annealling of semi crystalline polymers leads to increase in tensile strength and yield strength and decrease in ductility
What does heat treating do for drawn materials
it can lead to shrinkage
What is particular about the deformation of elastomers
they can withstand large elastic deformation and spring back
what can be said about the modulus of elasticity for elastomers
low modulus of elasticity
what does the stress strain curve look like for elastomers
non linear
what does a tensile load do regarding the chains for elastomers
causes partial straightening, untwisting and uncoiling
what happens as straightening occurs for elastomers, what about when unloaded
the entropy decreases, when unloaded, the entropy goes back up
what happens as elastomers are stretched
their temperature increases
At melting temp, which regions of polymers will melt
crystalline regions
what will amorphous polymers do at Tg:
they will undergo glass transition like glasses
Below Tg, what is the behaviour of the chains (3)
- fairly immobile
- can’t move easily
- rigid material
Above Tg, wht is the behaviour of the chains (3)
- more mobile
- can flow fast each other under load
- rubbery material
What factors are Tm and Tg affected by (4)
- polymer chain strucutre
- molecular weight
- branching
- polar atoms, bulky side grous, unsaturated bonds
describe the process of addition polymerization
- monomer is initiated-» active site is introduced
- Propogation of reaction (mers are added onto the chain)
- Termination (active sites are deactivated)
Describe Condensation Polymerization
2 different molecules come togeth to form a repeat unit. A by-product is often formed like water
Name the 6 types of polymer additives and what they do
- Fillers: improve strength, abraision resistance, toughness.
- Plasticizers: increase flexibility, toughness, lower Tg
- Stablizers: prevent UV degradation, antioxidants
- lubricants: easier processing
- colourants: dyes or pigments to change colour
- Flame retardants: these additives reduce flammability of polymers
what happens when thermosets are heated?
they form a molecular network and they degrade, they don’t melt
describe compression and transfer moulding processing (4 steps)
- plastic resin and additives are placed into hot mould cavity
- Mold is heated and liquid resin fills the mould
- further heating solidifies the thermoset
- ejected and flash trimmed
which types of polymers is compression moulding used for?
Mostly thermosets but can be used for thermoplastics
describe injection moulding processing
Plastic granules are melted and & mixed together and then injected into mould
what are the advantages of injection moulding processing (5 things)
- can make high quality parts at high rates
- low cost for labour
- good surface finish
- highly automated
- can make detailed shapes
what are the disadvantages of injection moulding processing (2 things)
- high cost of machine and moulds
- requires close control of process
what type of polymer is injection moulding mainly used for
Thermoplastics
steps of injection moulding (4 steps)
- ram retracts, lets in pellets and forces them into the heating chamber
- pellets are melted as they are pushed through the chamber
- molten plastic is forced into the mould cavity and takes the shape of the mould
- cools, solidifies and is ejected
what is extrusion used for
long continuous products like aluminium (to make pipes, rods, wires etc)
describe the extrusion process
- melt pellets
- push melted pellets through shaped die
- cool
what would blow moulding be used for
bottles, plastic bags, containers
blow moulding process
- Heated tube of plastic is pinched in a mould
- air is blown into the tube until it conforms to the sides
- part is ejected from the mould
what is Thermoforming:
heated plastic sheet is formed by presssing onto a mould face using mechanical force, air or vaccum
what is Calendaring
Molten plastic is squeezed between rolls
what is spinning used to form
fibres and filaments
what is casting?
melting of a polymer and pouring into a mould
