DMT SSA

Question 1

Elastic, plastic and viscoelastic with springs and dashpots - draw them and then also draw the extension and time graphs that go with each one:

Answer 1

see 2019 cheat sheet

Question 2

Effect of plasticisers on glass transition temperature

Answer 2

GTT (Tg) - where a polymer transitions from a hard glassy material to a soft rubbery material. Mobility of the polymer chains increases significantly. Important for dentures; do not want them to fall out due to loss of rigidity at temperature of hot drinks

Gives the material improved flexibility and durability

Embed themselves between the chains of polymers, spacing them apart, and thus significantly lowering the glass transition temperature for the plastic i.e. making it softer

2019cs
Reduces the Tg so makes it more rubbery in the mouth
Acts like a lubricant between the chains

Question 3

3 ways to shape a solid

Answer 3

Metals and alloys:
Working- bend, pull, hammer
Casting- pour into mould (insulator) and put mould in liquid water (quenching)
Amalgamation- mix with mercury

If qs for wrought alloys (cold working involves ) :
Forging
Milling
Drawing and rolling

Question 4

Difference between solid and insoluble alloys and uses of them

Answer 4

Solid alloy- atoms mix well
gold and silver (used in gold alloys for crowns, bridges, etc.)

Insoluble e.g. but lead-tin used in solder is an example

Partially soluble e.g. Ag/Cu – copper and silver (used in gold alloys for crowns, bridges, etc.)

Intermetallic Compounds: (rare) the atoms can form ionic compound Dental example – silver tin (one of the main components of dental amalgam)

Question 5

Equiaxed and fibrous

Answer 5

equiaxed is a lattice like appearance with grains approx the same size separated by grain boundaries.

Cold working produces a fibrous structure which is harder, stronger and with a higher yield stress

Question 6

Explain working time and setting time

Answer 6

WT - time from the start of mixing until beyond which a material cannot be used effectively.
ST - time taken for a material to achieve its minimum required properties

Question 7

What are the causes of gaseous porosity and contraction porosity -

Answer 7

large temperature increases, causing vaporisation of the material. Gaseous porosity - core material is allowed to exceed the boiling point creating air holes in dentures.
• Trapped air bubbles make something weak e.g. filling/denture

Contraction porosity - not enough material in the mould which causes it to come away from the sides during setting as polymerisation causes contraction. Should overfill to avoid this

Question 8

Interstitial alloys

Answer 8

for solid solution to be formed Impurity atoms fill voids or interstices among host atoms
• Difference in atomic radii > 59% - these are termed interstitial solid solution
- Difference in atomic radii is <15%

Question 9

What is order hardening

Answer 9

Solid-solid transitions - where a random solid changes to an ordered solid i.e. super lattice. Requires diffusion of atoms

Question 10

What is precipitation hardening

Answer 10

A heat treatment technique used to increase the yield strength of malleable materials, including steel

Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal’s lattice.

Question 11

force

Answer 11

results from an outside agency acting upon a body to change its momentum

Question 12

Stress

Answer 12

internal force to oppose an external force (equal and opposite)

Question 13

Strain

Answer 13

changes in dimensions resulting from stress. Greater stress therefore results in greater strain

Question 14

Tensile strength

Answer 14

the maximum tensile force that can be given without fracture

Question 15

Ductility

Answer 15

force required to break a solid by stretching it

Question 16

Proportional limit

Answer 16

when irreversible strain occurs; where the graph stops being a straight line

Question 17

young’s modulus/modulus of elasticity

Answer 17

ability of a material to withstand changes in length when under lengthwise tension or compression

modulus- how stiff it is
low modulus is flexible

Question 18

Ideal material for a denture

Answer 18

high glass transition temperature

high impact strength (not notch sensitive), biocompatibility, does not leach, aesthetics

Question 19

Explain viscoelastic

Answer 19

property of materials that exhibit both viscous and elastic characteristics when undergoing deformation

Material slowly return to original dimensions OR material only partially returns to original dimensions

Question 20

Beneficial and detrimental change when when work hardened

Answer 20

increase in yield strength and hardness and grains go from equated to fibrous but decrease in malleability and ductility

hot work- has no ductility issues

in dent- hard work first so sill have eqauixed grains then cold work for good mechanisms properties

Question 21

Work hardening

Answer 21

deformation above the yield stress- strengthening of a metal or a polymer by plastic deformation.

dislocations move
dl form
dl coalesce at grain boundaries
(modulus stays the same)

Occurs in alloys not amenable to heat treatment. Occurs below the recrystallisation temperature and includes bending, drawing and shearing.

Question 22

Eutectic point

Answer 22

when an alloy composition has a single melting point rather than a range

Question 23

relationship of stress and strain

Answer 23

see image 2019 sc

Question 24

A material is made up of A and B which are soluble in each other and have a difference in atomic radii of <15% what would this material be called?

Answer 24

Interstitial alloys

Question 25

After rapid cooling what is the grain structure of the material?

Answer 25

Quenching produces small grains

Cold working grains go from equiaxed to fibrous

Question 26

Rapid cooling below RcT can affect the properties of the material, give an example of one positive impact and one negative impact on the properties of the material and explain why

Answer 26

Increased yield strength- quenching produces fewer grains per unit volume. This means the grains will be smaller and there will be more grain boundaries, and therefore a higher yield stress. Hall petch equation (yield stress is inversely related to the square root of grain size)

Leads to coring- cooling quickly leads to different structures in grains which affects the corrosion resistance which means that these metals are not good for use in the mouth (galvanic cell forms- saliva is the electrolyte)

Question 27

Given Tg data, select which is most appropriate for denture use and give a reason

Answer 27

High glass transition temperature

• If glassy and rigid at room temp then used for denture
• Polymer with a Tg above mouth temperature will Glassy- hard and rigid for denture

don’t want denture converting at a low temp from rigid to rubbery

don’t want them to fall out due to los of rigidity at temperature of hot drinks

Question 28

When would you want to add a plasticiser to a denture?

Answer 28

Aligners for dentures to make more comfy (soft) contain plasticisers
Embed themselves between the chains of polymers, spacing them apart, and thus significantly lowering the glass transition temperature for the plastic i.e. making it softer

Question 29

What is the effect of a plasticiser on the structure of the material?

Answer 29

Reduces the Tg so makes it more rubbery in the mouth

Acts like a lubricant between the chains

Question 30

Drawing a material through dies to form orthodontic wires would cause the grain structure to become what?

Answer 30

Fibrous

Question 31

Draw on the graph dilatant, pseudoplastic and Newtonian curves/lines

Answer 31

easy

Question 32

Material for dental impressions is usually pseudoplastic, using no more than a sentence for each point, give three reasons why this property is beneficial to impression material

Answer 32

Ease of mixing- lower viscosity means the components can mix more easily
Detail- lower viscosity means it can flow more easily so greater detail can be obtained in impression
NEED ONE MORE ANSWER HERE

Question 33

. Cold working produces a

Answer 33

fibrous structure which is harder, stronger and with a higher yield stress

Question 34

work hardening grains go..

Answer 34

from equiaxed to fibrous

Yield strength and hardness increases – work hardening