Chapter 15 (Plastics as Engineering materials)

Question 1

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

Answer 1

1. temperature
2. strain rate (speed of pulling)
3. Chemical environment

Question 2

what are the 3 main types of behaviour for plastics?

Answer 2

1. Elastic-Fracture since their brittle
2. Elastics then Plastic
3. Completely elastic

Question 3

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

Answer 3

much less than metals

Question 4

describe the elastic behaviour of plastics

Answer 4

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

Question 5

describe viscous behaviour of plastics

Answer 5

deformation is time dependent and non recoverable (higher temps)

Question 6

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

Answer 6

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

Question 7

what does chain slippage depend on? (3)

Answer 7

1. the rate of strain(speed of pulling)
2. time
3. temperature

Question 8

What happens if the strain is fast on plasitics?

Answer 8

there no time for slippage-» brittle failure occurs

Question 9

what happens if the strain is slow for plastics?

Answer 9

there is chain slippage and the plastic will react as ductile

Question 10

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

Answer 10

horizontal line. linear response, positive ln curve

Question 11

as time increase what happens to stress:

Answer 11

the stress decrease: STRESS RELAXATION

Question 12

If temperature increases, what happens to the stress

Answer 12

Stress is lower and decreases faster

Question 13

Describe viscoelastic creeep:

Answer 13

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

Question 14

What else is the Creep modulus also dependent on

Answer 14

temperature

Question 15

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

Answer 15

Above: Ductile
Below: Brittle

Question 16

Describe Crazing

Answer 16

Craze formation prior to cracking:

• plastic deformation of spherulites
• formation of microvoids and fibrillar bridges

Question 17

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

Answer 17

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

Question 18

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

Answer 18

• 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

Question 19

Where would continued deformation typically occur and why

Answer 19

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

Question 20

describe predeformation by drawing:

Answer 20

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

Question 21

what are the results of drawing: (3)

Answer 21

1. increases the elastic modulus in the stretching direction
2. increases the tensile strength in the stretching direction
3. decreases ductility

Question 22

What does annealing after drawing do? (2)

Answer 22

1. decreases alignment

2. reverses the effects of drawing

Question 23

Which factors affect the strength of polymers and how?

Answer 23

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

Question 24

what negative effects does an increase in crystillinty have?

Answer 24

increases brittleness

Question 25

What does heat treating do for undrawn materials

Answer 25

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

Question 26

What does heat treating do for drawn materials

Answer 26

it can lead to shrinkage

Question 27

What is particular about the deformation of elastomers

Answer 27

they can withstand large elastic deformation and spring back

Question 28

what can be said about the modulus of elasticity for elastomers

Answer 28

low modulus of elasticity

Question 29

what does the stress strain curve look like for elastomers

Answer 29

non linear

Question 30

what does a tensile load do regarding the chains for elastomers

Answer 30

causes partial straightening, untwisting and uncoiling

Question 31

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

Answer 31

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

Question 32

what happens as elastomers are stretched

Answer 32

their temperature increases

Question 33

At melting temp, which regions of polymers will melt

Answer 33

crystalline regions

Question 34

what will amorphous polymers do at Tg:

Answer 34

they will undergo glass transition like glasses

Question 35

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

Answer 35

1. fairly immobile 2. can't move easily 3. rigid material

Question 36

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

Answer 36

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

Question 37

What factors are Tm and Tg affected by (4)

Answer 37

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

Question 38

describe the process of addition polymerization

Answer 38

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

Question 39

Describe Condensation Polymerization

Answer 39

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

Question 40

Name the 6 types of polymer additives and what they do

Answer 40

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

Question 41

what happens when thermosets are heated?

Answer 41

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

Question 42

describe compression and transfer moulding processing (4 steps)

Answer 42

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

Question 43

which types of polymers is compression moulding used for?

Answer 43

Mostly thermosets but can be used for thermoplastics

Question 44

describe injection moulding processing

Answer 44

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

Question 45

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

Answer 45

- can make high quality parts at high rates - low cost for labour - good surface finish - highly automated - can make detailed shapes

Question 46

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

Answer 46

- high cost of machine and moulds | - requires close control of process

Question 47

what type of polymer is injection moulding mainly used for

Answer 47

Thermoplastics

Question 48

steps of injection moulding (4 steps)

Answer 48

- 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

Question 49

what is extrusion used for

Answer 49

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

Question 50

describe the extrusion process

Answer 50

- melt pellets - push melted pellets through shaped die - cool

Question 51

what would blow moulding be used for

Answer 51

bottles, plastic bags, containers

Question 52

blow moulding process

Answer 52

- 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

Question 53

what is Thermoforming:

Answer 53

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

Question 54

what is Calendaring

Answer 54

Molten plastic is squeezed between rolls

Question 55

what is spinning used to form

Answer 55

fibres and filaments

Question 56

what is casting?

Answer 56

melting of a polymer and pouring into a mould