Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Materials Science > Chapter 15 (Plastics as Engineering materials) > Flashcards

Chapter 15 (Plastics as Engineering materials) Flashcards

You may prefer our related Brainscape-certified flashcards:
Biology 101 flashcards
1
Q

what factors do the results of the stress strain test depend on? (3)

A
  1. temperature
  2. strain rate (speed of pulling)
  3. Chemical environment
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what are the 3 main types of behaviour for plastics?

A
  1. Elastic-Fracture since their brittle
  2. Elastics then Plastic
  3. Completely elastic
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what can be said about the elastic modulus of plastics compared to metals

A

much less than metals

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

describe the elastic behaviour of plastics

A

there is an instantaneous deformation and recovery, it is recoverable (low temps)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

describe viscous behaviour of plastics

A

deformation is time dependent and non recoverable (higher temps)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

If a polymer is said to be viscoelastic what will the polymer behave as and what type of behaviour will it exhibit?

A

polymers can behave as rubbery solids and it will have non-linear elastic behaviour

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what does chain slippage depend on? (3)

A
  1. the rate of strain(speed of pulling)
  2. time
  3. temperature
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What happens if the strain is fast on plasitics?

A

there no time for slippage-» brittle failure occurs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what happens if the strain is slow for plastics?

A

there is chain slippage and the plastic will react as ductile

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what do the graphs of elastic, viscous and visco-elastic behaviour look like respectively?

A

horizontal line. linear response, positive ln curve

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

as time increase what happens to stress:

A

the stress decrease: STRESS RELAXATION

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

If temperature increases, what happens to the stress

A

Stress is lower and decreases faster

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe viscoelastic creeep:

A

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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What else is the Creep modulus also dependent on

A

temperature

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Match these: Ductile response, Brittle response; Above Tg, below Tg

A

Above: Ductile
Below: Brittle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Describe Crazing

A

Craze formation prior to cracking:

  • plastic deformation of spherulites
  • formation of microvoids and fibrillar bridges
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Describe elastic deformation of semi crystalline polymers and state whether its recoverable or not

A

occurs by bond stretching/bending in the chains. It is recoverable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Describe plasic deformation of semi crystalline polymers and state whether its recoverable or not

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Where would continued deformation typically occur and why

A

in the unnecked region because alignment can occur in the necked region which would lead to strengthening

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

describe predeformation by drawing:

A

drawing stretches the polymer prior to use, this aligns the chains to the stretching direction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

what are the results of drawing: (3)

A
  1. increases the elastic modulus in the stretching direction
  2. increases the tensile strength in the stretching direction
  3. decreases ductility
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What does annealing after drawing do? (2)

A
  1. decreases alignment

2. reverses the effects of drawing

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Which factors affect the strength of polymers and how?

A
  1. Average molecular weight of chains: longer chains(heavier)=more entanglement
  2. Degree of crystillinity: affects intermolecular secondary bonding. closer packed=stronger.
  3. Drawing: increases E
  4. Heat treating
24
Q

what negative effects does an increase in crystillinty have?

A

increases brittleness

25
Q

What does heat treating do for undrawn materials

A

annealling of semi crystalline polymers leads to increase in tensile strength and yield strength and decrease in ductility

26
Q

What does heat treating do for drawn materials

A

it can lead to shrinkage

27
Q

What is particular about the deformation of elastomers

A

they can withstand large elastic deformation and spring back

28
Q

what can be said about the modulus of elasticity for elastomers

A

low modulus of elasticity

29
Q

what does the stress strain curve look like for elastomers

A

non linear

30
Q

what does a tensile load do regarding the chains for elastomers

A

causes partial straightening, untwisting and uncoiling

31
Q

what happens as straightening occurs for elastomers, what about when unloaded

A

the entropy decreases, when unloaded, the entropy goes back up

32
Q

what happens as elastomers are stretched

A

their temperature increases

33
Q

At melting temp, which regions of polymers will melt

A

crystalline regions

34
Q

what will amorphous polymers do at Tg:

A

they will undergo glass transition like glasses

35
Q

Below Tg, what is the behaviour of the chains (3)

A
  1. fairly immobile
  2. can’t move easily
  3. rigid material
36
Q

Above Tg, wht is the behaviour of the chains (3)

A
  1. more mobile
  2. can flow fast each other under load
  3. rubbery material
37
Q

What factors are Tm and Tg affected by (4)

A
  1. polymer chain strucutre
  2. molecular weight
  3. branching
  4. polar atoms, bulky side grous, unsaturated bonds
38
Q

describe the process of addition polymerization

A
  1. monomer is initiated-» active site is introduced
  2. Propogation of reaction (mers are added onto the chain)
  3. Termination (active sites are deactivated)
39
Q

Describe Condensation Polymerization

A

2 different molecules come togeth to form a repeat unit. A by-product is often formed like water

40
Q

Name the 6 types of polymer additives and what they do

A
  1. Fillers: improve strength, abraision resistance, toughness.
  2. Plasticizers: increase flexibility, toughness, lower Tg
  3. Stablizers: prevent UV degradation, antioxidants
  4. lubricants: easier processing
  5. colourants: dyes or pigments to change colour
  6. Flame retardants: these additives reduce flammability of polymers
41
Q

what happens when thermosets are heated?

A

they form a molecular network and they degrade, they don’t melt

42
Q

describe compression and transfer moulding processing (4 steps)

A
  1. plastic resin and additives are placed into hot mould cavity
  2. Mold is heated and liquid resin fills the mould
  3. further heating solidifies the thermoset
  4. ejected and flash trimmed
43
Q

which types of polymers is compression moulding used for?

A

Mostly thermosets but can be used for thermoplastics

44
Q

describe injection moulding processing

A

Plastic granules are melted and & mixed together and then injected into mould

45
Q

what are the advantages of injection moulding processing (5 things)

A
  • can make high quality parts at high rates
  • low cost for labour
  • good surface finish
  • highly automated
  • can make detailed shapes
46
Q

what are the disadvantages of injection moulding processing (2 things)

A
  • high cost of machine and moulds

- requires close control of process

47
Q

what type of polymer is injection moulding mainly used for

A

Thermoplastics

48
Q

steps of injection moulding (4 steps)

A
  • 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
49
Q

what is extrusion used for

A

long continuous products like aluminium (to make pipes, rods, wires etc)

50
Q

describe the extrusion process

A
  • melt pellets
  • push melted pellets through shaped die
  • cool
51
Q

what would blow moulding be used for

A

bottles, plastic bags, containers

52
Q

blow moulding process

A
  • 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
53
Q

what is Thermoforming:

A

heated plastic sheet is formed by presssing onto a mould face using mechanical force, air or vaccum

54
Q

what is Calendaring

A

Molten plastic is squeezed between rolls

55
Q

what is spinning used to form

A

fibres and filaments

56
Q

what is casting?

A

melting of a polymer and pouring into a mould

Materials Science (13 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions