DMT

Question 1

Elastic, plastic and viscoelastic, explain using models and graphs what these mean

Answer 1

Elastic
Model = spring
When load applied - deforms quickly
If load held it stays deformed and when removed return to original dimensions

Plastic
Model = dashpot
When load applied deforms quickly
If load is held it stays deformed and when removed it stays deformed

Viscoelasticity
Model = dashpot and spring (in series = Maxwell model) (in parallel = Voigt model)
Maxwell = only some deformation returns
Voigt = all deformation returns but slowly

Question 2

What effect does adding plasticiser to a polymer have on Tg?

Answer 2

Reduces Tg
Acts as a lubricant between chains so they cannot get as close together
Residual monger can act as a plasticiser

Question 3

Force?

Equation?

Answer 3

Results from an outside agency acting upon a body to change its momentum
load x acceleration

Question 4

Define stress

Answer 4

Internal forces set up inside a body to oppose and externally applied force

Question 5

Define strain

Answer 5

The amount of deformation that occurs due to an applied stress

Question 6

Define tensile strength

Answer 6

Maximum stress a material can withstand before breaking

Question 7

Define stiffness

Answer 7

The mount of resistance against deformation

Question 8

Define ductility

Answer 8

How much a material can be pulled

Question 9

Define proportional limit

Answer 9

The point where stress is not proportional to strain (switch from elastic to plastic)

Question 10

Define Young’s modulus

Answer 10

Measures stiffness - defining relationship between stress and strain

Question 11

Define resilience

Answer 11

How much energy something can take before it deforms

Question 12

Define toughness

Answer 12

How much energy something can take before it breaks

Question 13

Define malleability

Answer 13

How much something can be compressed

Question 14

Define yield strength

Answer 14

Point on Young’s modulus graph (stress and strain) where plastic deformation occurs

Question 15

3 methods to shape metals and alloys

Answer 15

Casting
Working
Amalgamation

Question 16

Soluble vs insoluble alloys

Answer 16

Soluble - atoms form into planes

Insoluble - atoms don’t form into planes

Question 17

Working time

Answer 17

Time from start of mix until the material can no longer be effectively used (should be in the mouth by this point)
At RT

Question 18

Setting time

Answer 18

Time from start of mix until the material has minimum properties for function
At suitable temp e.g. mouth

Question 19

What is quenching?

Answer 19

Rapid cooling of a solid in a cooler liquid

Question 20

Effect of quenching?

Answer 20

Increases speed of cooling - atoms closer together - more grains per volume, more grain boundary per unit of volume
More grain boundaries = higher yield strength (atoms cannot move as much)

Question 21

What is glass transition temperature?

Answer 21

The temperature at which the material switches from being rubbery to glassy

Question 22

What type of material does glass transition temperature affect?

Answer 22

Thermoplastic polymers

Question 23

Thermoplastic

Answer 23

Type of polymer that hardens on cooling and softens on heating

Question 24

Thermosetting

Answer 24

Type of polymer that hardens on setting and does not soften on heating
All cross-linked polymers are thermosets

Question 25

Gaseous porosity

Answer 25

Increased temperature leads to vaporised monomer causing weakness and failure

Question 26

Where does porosity occur?

Answer 26

Thickest part of the denture (most volume)

Question 27

Contraction porosity

Answer 27

Polymers are denser than monomers so polymerisation causes shrinkage = contraction porosity = weakness and failure

Question 28

Interstitial alloy

Answer 28

When the atoms forming the alloy are very different in size the smaller element sits int he interstitial gaps between the larger ones = solid solution

Question 29

The type of molecule is the molecule forming free radicals?

Answer 29

An initiator molecule - weak bond e.g. BPO

Question 30

Through what step does an initiator molecule become a free radical?

Answer 30

Activation (heat or light)

Question 31

What increases working time?

Answer 31

Retarder by delaying setting

Question 32

How to reduce polymerisation contraction?

Answer 32

Use larger monomers or add a filler that will not polymerise

Question 33

Composition of orthodontic wire

Answer 33

18/8 austentitic stainless steel | 18% Cr 8% Ni added to steel

Question 34

What is steel?

Answer 34

An alloy of carbon and iron (carbon much smaller = so in interstitial sites)

Question 35

Why is steel so useful?

Answer 35

Number of phases with different properties Austentite = stable at useful temperatures Ferrite Cementite = ionic solution (hard but brittle) Pearlite = ferrite and cementite (depends on concentration of carbon)

Question 36

How does C concentration affect mechanical properties of perlite?

Answer 36

Increase C = increased hardness, yield strength, ductility

Question 37

Purpose of light cures

Answer 37

Protect the eyes of the dentist because high intensity visible light can cause retinal damage

Question 38

What increase Tg?

Answer 38

``` Increased polymerisation (lower residual monomer) Change monomer = C=C bond is more rigid than Si-o ```

Question 39

What decreases Tg?

Answer 39

Plasticiser | Large pendant groups

Question 40

Order hardening

Answer 40

Heat treatment on a solid to increase hardens and yield strength by making the atoms form ordered solid solution

Question 41

Precipitation hardening

Answer 41

Heat treatment on a solid to increase yield strength and hardens in partially soluble alloys Must be below RcT

Question 42

Eutectic point

Answer 42

Point at which homogenous mixture of substances has a single melting point

Question 43

Why does steel have no more than 2% carbon?

Answer 43

2% carbon is its solubility limit | unstable above this

Question 44

Rheology?

Answer 44

Study of deformation and flow of material, measurement of how viscosity changes

Question 45

Newtonian Pseudoplastic Dilatant

Answer 45

Viscosity does not change as shear rate increases/decreases Viscosity decreases as shear rate increases (ketchup) Viscosity increases as shear rate increases (bullet proof vest)

Question 46

Thixotropy

Answer 46

No flow until sufficient pressure is applied