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BDS 1 > Dental Materials > Flashcards

Dental Materials Flashcards

1
Q

What 2 forms can GICs come in?

A

Encapsulated or hand mix (liquid/powder)

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2
Q

Why is encapsulated preferable?

A

Pre-measured
Mixed for you
Can out straight into mouth

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3
Q

How can self mix go wrong?

A

Too much powder or liquid will set slower/faster and alter consistency and binding properties

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4
Q

What are GICs used for?

A

Cement and fillings

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5
Q

What forms can zinc oxide eugenol and calcium hydroxide come in?

A

2 paste (temp-bond, Dycal) or liquid/powder

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6
Q

How are 2 paste ZOE and calcium hydroxide cements mixed?

A

Equal amount of both
15s mix time till creamy, no streaks
5-6 min set time

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7
Q

How is liquid/powder ZOE and calcium hydroxide cement mixed?

A

2 spoons power : 3 drops liquid
Add in powder small amount at time till right consistency
Clean spatula, put powder on spatula and roll towards you until sausage shaped

1-1:30min mix time
4-5 min set time

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8
Q

How is alginate mixed?

A

Powder fluffed - mix ingredients, remove clumps
Scoop in bowl, add room temp water
Mix for 30s, spread against side to remove air bubbles and thoroughly mix
1-2 min set time depending on temp - warm water sets faster

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9
Q

What 3 forms are most dental materials in unset?

A
  1. Power and liquid
  2. 2 paste
  3. 1 paste
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10
Q

What 2 forms do most dental materials cure to?

A
  1. Rigid/stiff

2. Rubbery, elastic material

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11
Q

Describe dis/advantages of powder/liquid materials

A

Depends on what using material for
Alginate is v easy, cements more difficult to incorporate all powder into liquid in short time
Need to avoid incorporating air into material

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12
Q

What are the dis/advantages of 2 paste materials?

A

Difficult to mix to give homogenous resulting mass

Need to avoid incorporating air while hand mixing

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13
Q

What are the dis/advantages of 1 paste materials?

A

Can be difficult to pack into cavity if v viscous

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14
Q

What are the dis/advantages of delivery gun/pentamix materials?

A

Easy to mix
Tips cannot be reused
Could two air while filling impression tray

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15
Q

What are the 3 methods of setting dental materials?

A
  1. Heat
  2. Root temp.
  3. Light cure unit
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16
Q

Compare the physico-mechanical properties of the 3 cure types of materials

A
  1. Heat: optimum physico-mechanical properties (dentures) but not feasible for chairside
  2. RT: compromised compared to heat set - denture repair material
  3. Light: physico-mechanical properties between RT and heat set - composite
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17
Q

What are the 5 ideal properties of dental materials?

A
  1. Biocompatible
  2. Mechanically stable
  3. Chemically resistant
  4. Dimensionally stable
  5. Minimal thermal and electrical conductivity
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18
Q

What are the 11 generic groups of dental materials?

A
  1. Impression
  2. Acrylics
  3. Soft liners and tissue conditioners
  4. Cements
  5. Resin composites
  6. Bonding agent
  7. Casting materials
  8. Investment and die materials
  9. Waxes
  10. Ceramics
  11. Dental alloys/amalgam
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19
Q

What 5 factors influence selection of material?

A
  1. Patient history
  2. Age of patient/dentition
  3. Patient compliance
  4. Location of tooth
  5. Depth of cavity
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20
Q

Compare the important factors for ant. and post. restorations

A

Ant: colour match, natural looking in all lighting conditions, compressive strength not necessity

Post: high compressive strength, aesthetics not necessity

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21
Q

If a carious cavity extends to the pulp what is required when restoring?

A

Cavity liner to act as thermal insulator so tooth not sensitive to heat changes

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22
Q

What is Grassman’s law?

A

Three parameters of colour:

  1. Dominant wavelength (hue)
  2. Excitation purity: saturation of colour; intensity 0-1
  3. Luminous reflectance: brightness (100) or darkness (0)
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23
Q

Why is colour important in DMs?

A

Wavelength material reflects may change w/ age thus colour may change

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24
Q

What is the importance of surface finish of a restoration?

A

When light reflects off a solid some reflects from surface and some reflects from body of solid
Surface reflection dilutes colour: rough surface is much lighter than smooth surface of same material thus colour of restoration could become much lighter w/ wear

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25
Q

What are metameric colours?

A

Colours that appear the same under 1 light source but different under another

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26
Q

What is the significance of metanerism in dentistry?

A

Need to match restoration colour to natural colour in light corresponding to that of use

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27
Q

What is fluorescence?

A

Phenomenon observed when material absorbs colour of one wavelength and emits colour of another wavelength

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28
Q

Why is fluorescence important in DMs?

A

Teeth emit fluorescent light (look whiter under fluorescent light) so materials need to look natural
Some porcelains contain fluorescing agents to mimic natural appearance

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29
Q

Define opacity, translucency, transparency

A

Opacity: selective absorbance of light
Transparency: no interaction w/ light, complete transmission
Translucency: mixture of absorption, refraction, transmission

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30
Q

What is the refractive index?

A

Ratio of velocity of light in a vacuum to velocity in selected medium
A large difference = opaque; identical = transparent

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31
Q

What are 3 areas in why temp. is important in dentistry?

A
  1. Use of drill
  2. Exothermic setting reactions: can’t do in mouth
  3. Effect of hot/cold on restoration
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32
Q

Why is knowing temp. important?

A

Temp change may alter properties of material

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33
Q

What is heat of fusion?

A

Energy required to convert 1g of material from solid to liquid at melting temp.

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34
Q

What is thermal conductivity?

A

Quantity heat/second passing through a 1cm thick substance w/ cross section of 1cm2 w/ temp. inc. of 1 degrees

I.e. ease with which heat is transferred through a material

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35
Q

What is the clinical significance of thermal conductivity?

A

Large amalgam filling close to pulp may be sensitive to hot/cold thus use a non-mental cement between tooth and filling to insulate

Metal denture base material will more closely follow temp. of oral mucosa

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36
Q

What is specific heat?

A

Quantity of heat required to heat 1g material by 1 degrees

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37
Q

What is thermal diffusivity?

A

Measure of transient heat flow: how long does cold end of material take to heat up?
Rate of transfer of heat from hot side to cold side of material

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38
Q

What is the clinical relevance of thermal diffusivity?

A

Good inlay, crown, amalgam have low specific heat but high thermal conductivity thus if close to pulp may get thermal shock

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39
Q

What is thermal expansion coefficient?

A

Change in length of material for a 1 degree change in temp.

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40
Q

What is the clinical significance of thermal expansion?

A

Tooth and material will expand and contract due to hot/cold

Could result in breaking of marginal seal of inlay/filling

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41
Q

What electrical conductivity and resistivity?

A

Ability of material to conduct electrical current

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42
Q

Why is electrical conductivity and resistivity important?

A

Correlation between electrical conductivity and change in physical and mechanical properties of material
E.g. gold alloys: change in resistivity changes internal crystal structure

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43
Q

What is electromotive force?

A

Difference in electrical potential that gives rise to current

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44
Q

What is the electromotive series?

A

List of metals ordered in dec. tendency to oxidise in solution

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45
Q

What is galvanism?

A

Induction of current due to chemical reaction

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46
Q

Why is galvanism important in dentistry?

A

If 2 fillings of different electronegativities touch will cause short circuit which if near the pulp will cause pain

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47
Q

Why does amalgam have a high copper content?

A

Copper-tin complex much harder and less corrodible than tin-mercury

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48
Q

How do GICs bond to enamel?

A

Bind to Ca2+ in enamel

Carboxyl group plays role in setting reaction and bonding mechanisms

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49
Q

What is solubility?

A

Of inorganic salt: number moles of pure solid that will dissolve in 1L solvent at given temp

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50
Q

Compare solubility of hydroxyapatite and fluoroapatite

A

FA less soluble than HA: remains supersaturated for longer than HA even at lower pH

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51
Q

What are the 6 optical factors important in DMs?

A
  1. Colour
  2. Opacity
  3. Fluorescence
  4. Surface finish
  5. Refractive index
  6. Metanerism
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52
Q

What are the 6 thermal factors important in DMs?

A
  1. Heat of fusion
  2. Thermal conductivity
  3. Thermal diffusivity
  4. Specific heat
  5. Temp
  6. Thermal expansion
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53
Q

What are the 3 electrical factors important in DMs?

A
  1. Conductivity and resistivity
  2. Electromotive force
  3. Galvanism
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54
Q

What are the 3 chemical factors important in DMs?

A
  1. Reactions
  2. pH and solubility
  3. Bonding
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55
Q

Define biomaterial

A

Natural or synthetic material that interfaces w/ living or biological tissue

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56
Q

Define biocompatible

A

Ability of material to elicit an appropriate biological response, in given application, in body/mouth

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57
Q

What is biocompatibility dependent on?

A 
Physical function
Biological response required
Location
Composition
Interaction w/ OC
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58
Q

What are the 4 main reasons for restorations?

A
  1. Trauma
  2. Decay/caries
  3. Tooth loss
  4. Revisions: repair failed restoration
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59
Q

Define safe in terms of DMs

A

Must not cause any local or systemic adverse reactions

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60
Q

What are the 2 categories of research?

A
  1. Basic: lab research; pre-market

2. Clinical: post-market surveillance

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61
Q

What are the 2 types of lab research?

A
  1. In vitro

2. In vivo

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62
Q

What 5 basic factors are tested in in-vitro testing?

A
  1. Physico-mechanical properties
  2. Biological properties: cytotoxicity; organ, tissue, cell cultures
  3. Genotoxicity: damage to genetic info causing mutations
  4. Oestrogenic activity
  5. Basic sciences: efficacy and safety of therapeutic, rehabilitative, preventative regimes
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63
Q

What kind of tests are done in-vivo?

A

Implants: material implanted into animal
Allergy tests
Limited usage studies: animal/clinical testing; long, tedious but most clinically relevant

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64
Q

What types of animal tests are there?

A

Past: material ground and fed to animal; implanted into animal
Now: material used in required area

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65
Q

Why is clinical testing preferred?

A

Most accurate and efficacious depending on:
Number of patients
Group of patients
Length of trial

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66
Q

What is the downfall of clinical testing?

A

Clinical symptoms don’t evaluate real world damage

Will have material in OC for years not weeks

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67
Q

What are some of the ways in which post-market surveillance is important?

A

Provide early warning signs of unsuspected adverse effects
Elicit predisposing factors to adverse reactions
Compare adverse reactions between similar products
Permit continued safety monitoring

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68
Q

Who are at risk of adverse reactions from DMs?

A

Dentist
Dental nurse
Dentinal technician
Patient

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69
Q

Who is most at risk of adverse reactions to DMs? Why?

A

Dentist/technician

Inc. risk as inc. exposure to material

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70
Q

How can risks be reduced?

A

Proper packaging
Following manufactures instructions
Non-contact operative techniques

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71
Q

What are the 3 main types of force?

A
  1. Uniaxial
  2. Biaxial
  3. Triaxial
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72
Q

What are the 3 types of uniaxial force?

A
  1. Tensile: away from each other
  2. Compressive: towards each other
  3. Shear: towards, one from top side other from bottom side
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73
Q

What are forces defined by?

A

Where they are applied, in what direction and how big they are

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74
Q

What is stress?

A

Force applied per unit area

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75
Q

Define strain

A

Deformation of object due to stress

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76
Q

What is Hooke’s law?

A

Stress is proportional to strain

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77
Q

What is Young’s modulus?

A

Ratio of stress to strain i.e. stress/strain is a pressure (Pa)

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78
Q

What can be determined from a stress-strain curve?

A

Ductility, strength, elastic modulus, resilience, toughness, flexibility

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79
Q

What is a fracture?

A

Separation of a material into 2+ pieces under action of stress

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80
Q

What are the 2 types of fracture?

A

Brittle: little/no plastic deformation, low toughness
Ductile: significant plastic deformation, high toughness

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81
Q

What are the steps in a fractureb

A
  1. Crack formation

2. Crack propagation

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82
Q

Describe the stress-strain plot for a brittle material

A

Almost linear due to low plastic deformation

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83
Q

Describe a direct tensile measurement

A

Dumbbell shaped test specimens, ensures central fracture

Used for metals, rigid polymers, rubbery polymers

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84
Q

What are compressive tests used for?

A

Ceramics

Hard polymers

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85
Q

What is a diametral/indirect tensile test?

A

Compression across diameter

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86
Q

What are the 6 static strengths?

A
  1. Compressive
  2. Tensile
  3. Shear
  4. Torsion
  5. Flexure
  6. Diametral tensile
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87
Q

What is hardness?

A

Resistance to indentation/permanent deformation when compressive force applied

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88
Q

What is fracture toughness?

A

Resistance of a material to failure from fracture starting at pre-existing crack

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89
Q

Define tear strength and energy

A

Strength: force needed to initiate/continue tearing
Energy: measure of energy per unit area of newly torn surface

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90
Q

Define impact

A

Resistance to fracture from rapid loading measured as energy absorbed at fracture

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91
Q

What is fatigue?

A

Failure of material at force well below static strength due to repeated force/strain cycles

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92
Q

Define fatigue life, strength and limit

A

Life: number of cycles to cause failure at specified stress
Strength: level of stress at which failure will occur after specified number of cycles
Limit: level of stress below which fatigue failure will not occur

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93
Q

What is static fatigue?

A

Failure of material at small load after period of constant loading
Load required to cause failure will dec. w/ inc. time loading

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94
Q

What 4 things is wear a combination of?

A
  1. Abrasive
  2. Adhesive
  3. Fatigue
  4. Corrosive/erosive
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95
Q

Define abrasive wear

A

Materials against each other:
2 body: tooth-tooth
3 body: tooth-tooth w/ food stuff between

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96
Q

What is adhesive wear?

A

Material sticks to tooth and when pulled away removes part of tooth

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97
Q

What is fatigue wear?

A

Propagation and combining of micro-cracks w/ successive loading cycles

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98
Q

Define corrosive/erosive wear

A

Corrosive: acid attack remove weakened enamel
Erosive: grinding, bruxism

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99
Q

What is fluid flow governed by?

A

Strength of intermolecular forces and molecular entanglement

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100
Q

Define viscosity

A

Resistance to flow of a fluid

Measure of internal resistance of material

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101
Q

What is a Newtonian fluid?

A

Fluid in which applied shear stress produces flow w/ constant shear strain rate response

Water, solvents, mineral oils

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102
Q

What are non-Newtonian fluids?

A

Fluids in which there is no define viscosity

Viscosity changes with shear rate or shear rate history

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103
Q

What are the 5 types of non-Newtonian fluids?

A
  1. Pseudoplastic
  2. Dilatant
  3. Viscoplastic
  4. Thixotropy
  5. Rheopexy
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104
Q

Define pseudoplastic fluids

A

Dec. viscosity as shear rate inc.

plaster, stone

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105
Q

Define dilatant fluids

A

Inc. viscosity as shear rate inc.

composites, porcelains in water

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106
Q

Define viscoplastic fluids

A

Will not flow until initial shear stress has been reached then can have Newtonian, pseudoplastic or dilatant properties

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107
Q

Define thrixotropic fluids

A

Viscosity dec. w/ time at constant shear rate

Ketchup, toothpaste, clays, quicksand

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108
Q

Define theopoxic fluids

A

Viscosity inc. w/ time at constant shear rate

Some lubricants, v rare

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109
Q

Define stress relaxation and creep

A

Relaxation: time dependent dec. in stress at constant strain
Creep: time dependent dec. in strain under constant load (stress)

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110
Q

What are the 7 types of adverse reactions?

A
  1. Toxic
  2. Irritant contact dermatitis
  3. Allergic contact dermatitis
  4. Oral lichenoid
  5. Anaphylactoid
  6. Contact urticaria
  7. Intolerance
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111
Q

What are the 2 types of adverse effects?

A
  1. Dermatoses

2. Non-dermatological

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112
Q

Describe irritant dermatose reaction

A

May be of acute toxic nature causing direct and immediate cytotoxic effects to skin cells
Cumulative dermatitis is from repeated contact w/ chemical agent at sub-toxic conc
Localised and restricted to the area of exposure

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113
Q

Describe allergic dermatose reactions

A

Acquired by contact w/ haptens in materials
Tissue develops antigens by contact w/ dermal protein
Next contact produces allergic response

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114
Q

Define haptens

A

In/organic molecule that alone is not antigenic but is when linked to carrier protein

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115
Q

What are some of the materials that can cause dermatose reactions?

A 
Metals
Polymers/monomers
Hydroxyethyl methacrylate
Latex gloves
Formaldehyde 
Eugenol
Disinfectants
Rubber dam: latex or nitrile
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116
Q

What is sodium hypochlorite and its risk?

A

Disinfectant used for irrigation in RCT

Can cause pain if introduced into periapical tissue, periapical bleeding and extensive swelling

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117
Q

What effects can acute systemic toxicity have?

A

Primarily eyes and airway
Long term exposure lead to renal, neural, liver disorders

Associated with repeated dosage

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118
Q

How can mutagenic changes arise from materials?

A

Associated with chemicals in material

Result from breakdown of material

119
Q

How do methacrylates affect the body?

A

Chronic toxicological effects

First respiratory problems then neurological disorders

120
Q

How can mercury and nickel affect the body?

A

Mercury: short term memory loss, irritability
Nickel: carcinogenic

121
Q

How can ceramics, plasters and alginates effect the body?

A

Respiratory problems from inhalation of dust

122
Q

How can trace anaesthetics effect the body?

A

NO, O2 sedation can cause neural and liver disease

123
Q

Define adhesion and cohesion

A

Adhesion: force that binds 2 dissimilar materials together by attraction between atoms and molecules

Cohesion: attraction between atoms/molecules within 1 substance

124
Q

What must there be for adhesion to occur?

A

Attraction between atoms in 2 surfaces

125
Q

What 3 stresses weaken adhesion?

A
  1. Thermal expansion coefficient
  2. Dimensional changes
  3. Moisture hydrating active groups in material
126
Q

What are the 3 criteria for successful adhesion?

A
  1. Surfaces clean and dry
  2. Contaminants removed
  3. In close contact
127
Q

Describe the adhesion between 2 solids

A

Not desirable

Surfaces of both rough (at atomic level) only contact at cusp tips, require adhesive to bind
Surfaces: adherent/substrate
Where they meet: interface

128
Q

Describe solid-liquid adhesion

A

Good

Water clings to glass due to VDWs bonds (secondary), require thermal energy to remove water

129
Q

What are the 5 factors governing adhesion?

A
  1. Surface energy/tension
  2. Wettability
  3. Viscosity
  4. Contact angle
  5. Morphology/Surface roughness
130
Q

Compare the surface energy/tension of bulk and surface molecules

A

Bulk: each molecule attracted in all directions to molecules around it, dynamic equilibrium

Surface: molecules only attracted down and to other surface molecules, thus have higher surface energy
Molecules try to maintain min. surface area so more molecules have lower energy state

131
Q

What 3 things can reduce surface tension?

A
  1. Inc. temp
  2. Impurities
  3. Surface-active agents/species: surfactants
132
Q

What 2 factors make an adhesive effective?

A
  1. Must be in close contact with substrate

2. Spread easily

133
Q

What 3 factors affect spreading?

A
  1. Wettability: resistance to viscosity
  2. If too viscous flow too slow so can’t penetrate crevices before set
  3. Misleading if adhesive is solvent w/ additives
134
Q

What is wettability?

A

Tendency of fluid to spread on a surface

135
Q

What’s an example of good wettability?

A

Metal and water

Water spreads easily over the whole surface

136
Q

What 3 factors can affect wettability?

A
  1. Surface cleanliness
  2. Surface irregularities
  3. Viscosity
137
Q

What is contact angle?

A

Angle between liquid and solid

138
Q

What is contact angle dependent on?

A

Surface tension and surface energy

139
Q

What does a contact angle of 0 mean?

A

Perfect wetting

Surface is completely covered by adhesive, max. bond strength

140
Q

What happens as contact angle is inc.?

A

Air voids are introduced preventing perfect wetting

Rupture of adhesive joints

141
Q

Explain the correlation between surface roughness and bond strength

A

Direct

Roughness inc. bonding area but creates areas difficult for adhesive to penetrate
Need adhesive of high surface tension to penetrate crevices but not too high as must be able to wet substrate

142
Q

What are the 5 types of adhesion?

A
  1. Mechanical
  2. Physical
  3. Chemical
  4. Molecule entanglement
  5. Mixture of all
143
Q

Describe mechanical adhesion

A

Retention by mechanical interlocking of components or penetration of 1 phase into surface of other

Attraction between substrate and adhesive not necessary but is weak, unable to withstand debonding

144
Q

Describe physical adhesion

A

Dipole-dipole attractions between polar molecules
Small: VDWs
Weak adsorption: not suitable for long adhesion

Rapid bonding, reversible
Thermal energy breaks bond

145
Q

Describe chemical adhesion

A

Bonding at molecule/atomic level
Dissociation of molecule after adsorbs to surface, constituents then bond again separately via ionic/covalent forces
Strong adhesive bonds and attraction

146
Q

Describe molecular entanglement

A

Adhesive or component penetrates surface and absorbs into surface of substrate

147
Q

What is molecule entanglement enhanced by?

A
  1. Good wetting
  2. Absorbing component long chain or forms long chain within penetrate layer
  3. Entanglement between adhesive and substrate
  4. V high bond strength
  5. Adhesives must be strongly chemically attracted to surface
148
Q

Why is DM structure important?

A

Understanding structure-property relationship ensures right DM selection for specific clinical situation

149
Q

Compare protons, neutrons and electrons

A

P: +ve charge, 1 amu, relatively large, inside nucleus
N: 0 charge, 1 amu, relatively large, inside nucleus
E: -ve charge, 0 amu, relatively small, outside nucleus

150
Q

What are the 2 classifications of chemical bonds?

A

Primary: strong; 0.5-7 eV
Secondary: weak; <0.5 eV

151
Q

What is a bond?

A

Interaction between atoms

152
Q

What 3 bonds are primary bonds?

A
  1. Covalent
  2. Ionic
  3. Metallic
153
Q

Describe covalent bonds

A

Strongest: 4-6 eV
Atoms share electrons to achieve noble gas configuration
Least reactive bond
Electron orbital overlap forms molecular orbital w/ shared electrons

Directional and rigid

154
Q

Describe ionic bonds

A 
Strong electrostatic (>1.5) interactions: 3-4 eV
Results in formation of ions: cations and anions 
Non-directional as ions interact w/ any ions in vicinity
155
Q

Describe metallic bonds

A

Non-directional, relatively weak 0.5-2 eV

Overlapped orbitals of metallic atoms forms electron gas
Electron has and metallic cations: electrostatic integration

156
Q

What bonds are secondary bonds?

A

VDWs

H bonds

157
Q

Describe secondary bonds

A

No electron sharing

Charge induced dipole interactions

158
Q

Define phase

A

Physically and chemically homogenous part of system/material that has clear boundaries

159
Q

Define phase transition

A

Change from one phase to another

e.g. solid -> gas sublimation

160
Q

What is a phase diagram?

A

Presentation of stability and phase transitions for a system/material of phases in equilibrium at range of temps.

161
Q

Why are phase diagrams useful?

A

Identify phase against the composition at a given temp

162
Q

What is the eutectic point?

A

Lowest melting point of system: lower than constituents and may other mixture of them

163
Q

Describe crystalline solids

A

Consist of crystals

Form crystalline lattice: ordered (symmetric and periodic) and specific arrangements of atoms

164
Q

Describe non-crystalline solids

A

Disordered solids, contain no crystals
Inorganic, rapidly quenched: glass
Organic: polymers
Phase transition specific for non-crystalline solids

165
Q

Describe the phase transitions in non-crystalline solids

A

Glass transition: change in viscosity/rigidity from brittle to rubbery w/ inc. temp

Crystallisation (devitrification): conversion to crystalline solids if heated up to specific temp

166
Q

What is polymerisation?

A

Synthesis of polymers from monomers

167
Q

What are the 2 types of polymerisation?

A
  1. Condensation

2. Addition

168
Q

Describe condensation polymerisation

A

Reaction between 2 molecules producing larger molecule and water/small molecule byproduct
Byproduct has to be removed during polymerisation

169
Q

Describe addition polymerisation

A

Reaction between 2 molecules giving larger molecule with no byproducts

Monomers added 1 by 1 per chain to active site on growing chain

170
Q

What is the most common type of addition polymerisation?

A

Free radical polymerisation

171
Q

What is the Thiokol reaction?

A

Condensation polymerisation that forms polysulphide polymers

Used in base paste of polysulphide impression materials

172
Q

How is 2-polyglycolic acid synthesised and what is its function?

A

Condensation polymerisation

Synthetic, absorbable, suture material
Biodegrades back to acid in 60-90days

173
Q

What is a free radical?

A

Molecules w/ an unpaired electron

174
Q

How is methyl methacylate synthesised and how is it used?

A

Addition polymerisation

Used as denture base

175
Q

How are free radicals formed?

A

When initiation decomposes into 2 highly reactive free radicals in presence of monomers

176
Q

What is the common initiator?

A

Benzoyl peroxide

177
Q

What are the 4 stages of free radical polymerisation?

A
  1. Activation: formation of FRs from initiators
  2. Initiation: FR attacks monomer units
  3. Propagation: chain growth, monomer units add together
  4. Termination: 2 growing chains meet, FRa combine forming stable covalent bond
178
Q

What are the 3 ways in which FRs are formed in dentistry?

A
  1. Heat
  2. Room temp
  3. Light
179
Q

Describe heat formation of FRs

A

Usually powder/liquid, 2-paste or 1-paste

When mixed and heated, BP splits into FRs
FRs attack double bonds in monomers

180
Q

Describe how FRs are formed at room temp

A

Usually powder/liquid or 2-paste

Tertiary amine activator (dimethyl-p-toludine) added to monomer (MMA) reacts w/ BP in powder to form FRs
FRs attack double bonds

181
Q

Describe light formation of FRs

A

Usually 1-paste

Tertiary amine activator (dihydroxymethyl-p-toluidine) and light initiator (camphorquinone) added to monomer and react in presence of visible light source to form FRs

182
Q

How does free radical formation being exothermic lead to porosity?

A

Boiling causes monomer to bubble creating airspaces which then set as material polymerises

183
Q

Why can a molecule weight not be given for a polymer?

A

All polymers are not the same length

184
Q

What 2 methods can be used to give a polymer a MWt?

A
  1. Number average MWt (Mn)

2. Weight average MWt (Mw)

185
Q

Define number average MWt

A

Total weight of sample divided by number of molecules in sample

186
Q

Define weight average MWt

A

Sum of the number of molecules multiplied by their weight average

187
Q

What are the 3 states polymers exist in? Give examples

A
  1. Rubbers: silicone rubber
  2. Hard resins: poly(methyl methacrylate)
  3. Fibres: polyethylene

Rubber and hard resins: long chains exist in randomly coiled configuration and thermal motion
Fibres: long chains stiff and straightened

188
Q

How are polymer chains held?

A

Weak 2ndary bonds: VDWs

Entanglement of chains

189
Q

Describe the effect of stress on rubbers

A

Easy deformation due to low intermolecular forces
Reversible due to 1% crosslinks within polymer
Elastic modulus in MPa

190
Q

Describe the effect of stress on hard resins

A

Difficult to deform due to high intermolecular forces

Elastic modulus in GPa

191
Q

What are crosslinks?

A

Covalent bonds in polymers joining one polymer chain to another
Prevent polymers flowing irreversibly

192
Q

Compare 1% and 30% crosslinks

A

1%: elastic

30%: rigid material

193
Q

Describe the effect of crosslinks on glassy polymers

A

Improves some properties such as impact strength (relevant for dentures)
Too many will make them brittle

194
Q

Define thermosetting resin

A

Initially rubbers w/ low intermolecular forces, due to 30% crosslinks end up as rigid polymers
Vulcanite

195
Q

Compare thermoplastics and thermosetting plastics

A

Thermoplastics: once heated and formed into shape can be reheated and reshaped but red. quality of plastic

Thermosetting: once hearted and shaped can’t be reheated and reshaped, difficult to recycle

196
Q

Define glass transition temp

Explain with an example

A

Temp at which rigid material becomes soft and rubbery

Intermolecular forces are temp dependent
PMMA is glassy, rigid w/ high intermolecular forces
As temp inc., intermolecular forces dec. until material is soft and rubbery

197
Q

Describe the Tg fro natural rubber

A

Elastomer w/ low intermolecular forces

As temp dec, forces inc. until reach Tg temp at which point material becomes glassy rigid

198
Q

What is the significance of Tg to dentistry?

A

Denture bases require Tg for exceeding temps likely to be exposed to
Polishing be technician, cleaning by patient

199
Q

What is a plasticiser?

A

Simple, organic liquid

Dibutyl phthalate

200
Q

What are plasticisers used for?

A

Added to monomer before polymerisation to red. Tg of material i.e. make naturally rigid, glassy material soft and rubbery at RT

201
Q

How do plasticisers function?

A

Depress Tg by lubrication
Flow between chains of polymer red. high intermolecular forces
Rigid material becomes soft and rubbery w/ low intermolecular forces

202
Q

What are the 2 disadvantages to plasticisers?

A
  1. Material becomes hard again w/ time

2. Phthalates are carcinogens and leach out into patients mouth

203
Q

What are crystalline polymers and what are their uses in dentistry?

A

Polymers w/ high degree of order formed by folding and stacking of polymer chains: long, stiff, straightened chains

  1. Polypropylene: Instrument trays; withstand high pressure, temp
  2. Polyethylene: reinforce acrylic dentures
204
Q

What is a homopolymer?

A

Polymer formed when polymerised alone

205
Q

What are the 3 stereoisomers that polymers can obtain?

A
  1. Atactic: random arrangement of head - tail configurations along chain
  2. Isotactic: all substituents on same side of chain
  3. Syndiotactic: alternating head-tail linkages
206
Q

What is a copolymer?

A

Product of polymerisation involving multiple types of monomer

207
Q

What are the 3 types of copolymer?

A
  1. Random: random arrangement of monomers along chain
  2. Block: blocks of monomer groupings of 1 type along chain
  3. Graft: main chain 1 monomer w/ branches of other monomers
208
Q

Why impact can water uptake have on materials?

A
  1. Red. strength
  2. Extract potentially toxic materials
  3. Can induce microorganism formation
209
Q

What are 3 useful water soluble materials used in dentistry?

A
  1. Alginates: impression
  2. Poly(acrylic acid): cements, polymer is solid but dissolves readily and ionises, anion is polymeric
  3. Hydroxyethyl methacrylate: resin modified GIC, can be used as hydrogel
210
Q

Define ceramic

A

Inorganic and non-metallic compound formed between metallic and non-metallic elements
Usually oxides (MgO) can be nitrides, carbides, borides (SiC)
Crystalline or non-crystalline
Combination of compounds

211
Q

Describe the structure of ceramics

A

Have mostly ionic bonds, some covalent
Polymorphic: can exist as 1+ crystalline form OR as both crystalline and non-crystalline
Dependent on how subunits are structured

212
Q

What 3 factors determine shape of ceramics

A
  1. Max. electrostatic attraction between cation and anion (O2)
  2. Min. electrostatic repulsion between anions
  3. Anion to anion size ration (anion usually larger)
213
Q

Describe how ceramic units are connected in crystalline and non-crystalline ceramics

A

Crystalline: regular repeat pattern

Non-crystalline: non-regular, random pattern, short-range molecular order

214
Q

Define ceramic coordination number

A

Number of anions around central cation

215
Q

In crystalline ceramics what 3 areas can unit shapes share?

A

Corners: share 1 ion
Edges: share 2+ ion
Faces: share 3+ ion

216
Q

What are the 7 crystal systems (unit cells) of crystalline ceramics?

A
  1. Cubic
  2. Tetragonal
  3. Orthorhombic
  4. Rhombohedral
  5. Monoclinic
  6. Triclinic
  7. Hexagonal
217
Q

What are the 4 Bravais lattices of crystalline ceramics?

A
  1. Primitive/simple: atoms at corner
  2. Body: simple + atom in centre of shape
  3. Face: simple + atom in centre of all faces
  4. Base: simple + atom in centre of top and bottom face
218
Q

What is a glass?

A

Inorganic product of fusion material that has cooled to rigid condition w/o crystallisation

219
Q

Why can’t glasses be defined by shape?

A

Non-crystalline

Have random, amorphous structure

220
Q

Describe the formation of glasses

A

High viscosity melt cooled rapidly above critical cooling rate
Crystalline structure doesn’t have time to form
As temp. dec., viscosity inc. until form rigid solids w/ random structure of liquids

Are in metastable state
Have no define MP

221
Q

Describe glass transformation

A

Tg depends on cooling rate
Glasses formed when cooling rate > critical rate
At Tg, high viscosity restricts mobility of molecules, can’t move quickly enough to get closer thus shrinkage rate lower

222
Q

Explain correlation between cooling rate and glass density

A

Cooling rate&raquo_space;> critical rate glass will have low density as less time to shrink

Cooling rate just > critical rate glass will have high density as more time to shrink

Higher density = greater physical properties

223
Q

What’re the majority of glasses?

A

Oxide glasses

224
Q

What are the 4 rules of oxide glass formation?

A
  1. O2 atom linked to = 2 glass forming atoms
  2. Coordination number of glass forming atoms small
  3. O2 polyhedral share corners w/ each other; not faces, edges
  4. Polyhedral linked in 3D network
225
Q

What are the 3 different types of oxide in glass oxide composition?

A
  1. Glass former: forms 3D network, form glass alone
  2. Intermediate: can’t form glass alone, takes part in network, cation exchanges for glass forming cation
  3. Modifier: disrupts network; breaks bonds, add O2, red. network connectivity
226
Q

What is the network connectivity of oxide glasses?

A

Av. no. bonds linking each repeat unit in silicate network
Red. viscosity and fusing temp
Inc. coefficient of thermal expansion

227
Q

Describe the processing of ceramics

A

Most formed from powder either dry or in solution
Formed into required shape by: slip casting, throwing, compaction of powders

After shape formed, article sintered (fired)
When particles packed still gaps between them (porosities)
Shrinkage occurs during sintering due to red. in porosity size
Porosity min. by control of particle size and packing density

228
Q

Describe the effect of particle packing on porosity vol.

A

Vol. porosity depends on particle size, shape, distribution, packing

Single size spheres: porosity 40%
Can red. by introducing another size, further red. by introducing more

229
Q

Describe sintering effect on porosity

A

Causes densification as particles merge together
Can occur in solid state (vitrification) or liquid phase
Driving force is red. in surface energy by red. porosity size

230
Q

Explain brittle fracture

A

How all ceramics fail

Propagation and growth of micro-cracks, usually from surface
As cracks grows inc. stress conc., at critical crack length will run through material and cause failure

Fracture below elastic limit
Stress-strain almost linear
Generally fail @ low strain

231
Q

Describe fatigue in ceramics

A

Failure: cyclic loading @ lower load than elastic limit; moist environment red. fatigue life

Static: in presence of water, stress enhanced chemical reaction @ tip of crack causes fracture to occur w/ no inc. load

232
Q

Describe the general properties of ceramics

A

High elastic modulus, brittle, hard
Relatively inert
Some are bioactive and bioresorbable
Crystalline: less reactive, better mechanical properties

233
Q

What are 5 crystalline ceramics used in dentistry?

A
  1. Silica: filler in cements, investment materials
  2. Alumina: high strength core of crown and bridge, filler in cements, reinforcing porcelains
  3. Hydroxyapatite: artificial tooth root, RC filler
  4. Gypsum: stone and plaster as model and die materials
  5. Zinc oxide: power component for cements
234
Q

What are 5 non-crystalline ceramics used in dentistry?

A
  1. Fluoroluminosilicate glasses: GIC
  2. Radiopaque strontium/barium glasses: filled in composite resin
  3. Feldspathic glasses: porcelains
  4. Fumed/colloidal silica: microfine filler in composite resin
  5. Diatomaceous earth: 80-90% silica, filler in alginates impression materials
235
Q

What are the 2 forms of silica and their subtypes?

A
  1. Crystalline
    a: quartz alpha and beta
    b: cristobalite alpha and beta
    c: tridymite alpha and beta1,2
  2. Amorphous
    a: vitreous (fused)
    b: gel
    c: pyrolytic (fumed)
236
Q

What are the 2 types of silica transformation?

A
  1. Reconstructive
    quartz -> tridymite -> cristobalite
    Involve breaking binds; difficult, rarely happen
  2. Displacive
    alpha to/from beta
    Alpha to beta: straighten bonds, causing expansion
    Easy and rapid
237
Q

Describe the properties of silica

A

Crystalline and virtuous relatively inert: only attacked by hydrofluoric acid

238
Q

What are the uses of silica?

A
  1. Filler: composite, investment materials, porcelains, cement, alginate
  2. Component: GICs, porcelains
239
Q

What are the forms of alumina?

A

Most commonly crystalline forms as corundum, alpha-alumina
Also eta, chi, gamma, delta, theta
Can also be produced from bauxite

240
Q

Describe the properties of alumina

A 
Elastic modulus: GPa > zirconia 
Flexural strength: MPa < zirconia 
Fracture toughness: MPa.m1/2 < zirconia 
Hardness 9 on Moh scale
Slightly soluble in strong acids and alkalis
241
Q

What are the 6 uses of alumina?

A
  1. Abrasives
  2. Filler in cements
  3. Reinforcement of restorations
  4. Implants
  5. Maxillofacial reconstruction
  6. Orthopaedics
242
Q

What are the 3 crystalline forms of zirconia?

A
  1. Monoclinic: low temp
  2. Tetragonal: med temp
  3. Cubic: high temp
243
Q

What are the properties of zirconia?

A
  1. Elastic modulus: GPa < alumina
  2. Flexural toughness: MPa > alumina
  3. Fracture toughness: 6-13 > alumina

As chemically stable as alumina

244
Q

What is the transformation toughening of zirconia?

A

Load induced transformation of tetragonal to monoclinic

3-5% expansion

245
Q

What are the uses of zirconia?

A

Similar to alumina

Additives (CaO, MgO, Y2O3) stabilise in either tetragonal or cubic forms
Y2O3 partially stabilised used as high strength core for crown and bridge

246
Q

What are the properties of hydroxyapatite?

A

Biological HA contains: F-, CO3-, Mg2+, Na+; not homogenous

Ca/P ratio 1.67

247
Q

What are the 5 uses of HA?

A
  1. RC filler
  2. Bone filler
  3. Tooth root
  4. Glass ceramic restorative
  5. Bioactive coating
248
Q

What are the 4 uses of porcelains?

A
  1. Artificial teeth
  2. Veneers
  3. Inlays
  4. Crowns and bridges
249
Q

What are the 5 advantages of porcelains?

A
  1. Excellent aesthetics
  2. Relatively inert
  3. Low thermal expansion coefficient: similar to tooth
  4. High MP
  5. High elastic modulus
250
Q

Describe the components and composition of porcelains

A

Clay: kaolin
Feldspar: albite, orthoclase
Crystalline quartz

Modern porcelains mainly feldspar and quartz
Kaolin (4%) only in high temp fusing type

Ratio soda to potash: high K red. fusing temp but less effect on viscosity than Na

251
Q

What are fluxes?

A

Additives to porcelains that red. fusing temp
Include glass formers: added as carbonates
Boric oxide added as borax

252
Q

Describe boric oxide

A

Glass former
Added to porcelains to red. fusing temp
Boron anomaly: red. fusing temp w/o inc. thermal expansion

253
Q

What are some aesthetic additives for porcelains?

A

Metal oxides for colour, opacity, fluorescence

Cobalt for blue
Chrome/tin for pink
Titanium/zirconium for opacity
Terbium/europium/cerium for fluorescence

254
Q

What are the 3 porcelains that may be required for an aesthetic restoration?

A
  1. Core/opaque: mask cement interface or metal alloy core
  2. Body/gingival dentine: bulk colour build up
  3. Enamel: highly translucent
255
Q

What are the 5 uses of metals?

A
  1. Partial dentures and clasps
  2. Inlays and onlays
  3. Direct filling material
  4. Orthodontics
  5. Crowns and bridges
256
Q

Describe a metallic bond

A

Metal atoms lost outer electron (valence) to form cations

Lost electrons able to flow around cations in sea of electrons

257
Q

What are the 3 methods in which metals can be made?

A
  1. Casting: crowns, partial dentures
  2. Cold working: wires, clasps
  3. Amalgamation: amalgam
258
Q

Describe a pure metal cooling curve

A

High temp, no time: liquid state
W/ time and temp dec. ~50% solidification begins, in liquid and solid state
Plateau
Further temp. dec. cause solidification w/ time

259
Q

What causes the plateau in a pure metal cooling curve?

A

Balance between the latent heat of fusion and cooling

260
Q

Describe the 5 stages of solidification of a metal

A
  1. Small nuclei act as centres for crystal growth
  2. Small dendrites grow from nuclei
  3. Dendrites continue to grow
  4. Space between dendrites fill in
  5. Complete: little evidence of dendrite structure remains
261
Q

Describe the grain boundaries of metals

A

Ill defined: almost amorphous, random structure
Attract impurities
More reactive

262
Q

Describe grain structure of metals

A

Fast cool = small size; slow cool = large size
~0.1mm
Key to mechanical performance of metals and alloys
Usually equiaxed
Dependent on conditions at solidification: casting into cold mould

263
Q

@ RT what are the 3 general crystal structures of metals?

A
  1. Body-centred cubic
  2. Face-centred cubic
  3. Close-packed hexagonal
264
Q

Define wrought alloy

A

Cast alloys that have been formed by mechanical processes (cold working) e.g. rolling, hammering, forging, drawing

265
Q

Describe wrought alloys

A

Above yield stress
Grains become elongated resulting in springiness
Under go work (strain) hardening

266
Q

Describe the general properties of metals

A
  1. All polycrystalline
  2. Good strength, high elastic modulus (80-200 GPa)
  3. Good conductors heat and electricity
  4. Lustre: shiny if polished
    - 3 4 result of metallic bonding
  5. Some (Au) resist corrosion
  6. Alloys: better, more controlled mechanical properties
267
Q

Describe the biocompatibility of metals

A

All metals are potentially toxic but some are essential
Toxicity is conc. dependent
Essential: Cu, Mg, Ni, Zn
Non-essential: Ag, Au, Li, Pb, Hg, Sn

268
Q

Describe the reactivity of metals

A

Some v reactive: K, react w/ water
Least Au

Ti, Cr: react w/ O2 but form passive oxide layer; thin, impermeable, prevent further oxidation

269
Q

Define chemical and electrochemical corrosion?

A

Chemical: direct combination of metal and non-metal (including oxidation)

Electrochemical: different metals in an electrolyte (saliva)

270
Q

What does corrosion lead to and what does this cause?

A

Degradation and release of ions:

  • structural breakdown
  • migration of ions around body
  • cytotoxicity or allergic phenomena
  • tissue discolouration
271
Q

What are the 5 types of corrosion?

A
  1. Galvanism: between 2 metals; amalgam and gold
  2. Localised galvanism: between metals within same alloy
  3. Crevice: differences in surface O2 levels; plaque coated and clean surface
  4. Pitting: similar to crevice; damage to passive oxide layer
  5. Stress: sustained force in corrosive environment
272
Q

Why are metals more easily deformed than expected?

A

Due to defects in crystal structure

273
Q

What are the 2 types of crystal defects?

A
  1. Point

2. Line

274
Q

What are the 3 types of point defects?

A
  1. Vacancy: atom removed
  2. Substitutional: atom replaced
  3. Interstitial: atom between metal atoms
275
Q

What are the 2 types of line defects?

A
  1. Edge dislocation: extra plane of atoms

2. Screw dislocation: edge in 3D

276
Q

How do metals deform?

A

By movements of dislocations

277
Q

How do dislocations lead to permanent deformations?

A

Force above yield stress causes atoms in single row to break bonds w/ existing atoms and form new bonds 1 along
Causes dislocation to move along slip plane causing permanent deformation

278
Q

Describe the effect of grain size on deformations of metals

A

Deformations halted by grain boundaries, another dislocation, impurities and point defects

Smaller grain size: less distance to travel, less possible deformation, more rigid

279
Q

Explain how metals can be both ductile and brittle

A

Ductile: dislocations move more easily than cracks grow, deform plastically

Brittle: solid has dislocations but cracks grow at lower stress than that required for dislocation movement, will deform elastically

280
Q

Define ductility and malleability

A

Ductile: withstand permanent deformation under tensile load w/o rupture; draw into wire

Malleable: withstand permanent deformation w/o rupture under compression; hammer into thin sheet w/o cracking

281
Q

Describe work (strain) hardening/cold working

A

Repeated deformation (strain) moves existing dislocations and produces new dislocations
Inc. dislocation density hinders movement
Dislocations stack up @ grain boundary

Inc: yield stress, hardness
Dec: ductility

282
Q

What is annealing?

A

Process by which effect of work hardening can be overcome by heating

283
Q

Describe the 3 stages of annealing

A

Recovery: effects begin to disappear, stress relief
Recrystallisation: old crystal structure disappears, forms new crystal structure, occurs at temp ~50% MP
Grain growth

284
Q

Describe alloys

A

Mix of 2+ metals
Have better properties compared to pure metals
Have no single MP: melt/solidify over range of temps.

285
Q

What are the liquidus and solidus temps. of alloys?

A

Liquidus (Tl): above all liquid, below liquid + solid

Solidus (Ts): above liquid + solid, below all solid

286
Q

What are the 4 types of binary alloys?

A
  1. Solid solution: metals soluble in each other; form single solid containing atoms of both
  2. Completely insoluble: solidify as 2 separate metals; eutectic alloys
  3. Partially soluble: eutectic + solid solution
  4. Metals w/ particular affinity: intermetallic compounds; cementite, amalgam
287
Q

Describe the 3 stages in the construction of a phase diagram for a binary alloy

A
  1. Plot cooling curve of various ratios of 2 metals; measure Tl and Ts for each ratio
  2. Plot Tl and Ts against % composition
  3. Join all Tl and all Ts together to from liquidus and solidus line
288
Q

What is alloy coring?

A

Between Ts and Tl composition of liquid and solid varies w/ temp

Cooling rapidly causes formation of layers of solid of different composition

289
Q

How can coring be rectified?

A

Homogenisation

Reheating to allow diffusion of atoms to give homogenous composition

290
Q

Define solution and order hardening

A

Solution: differing atomic radii hinder movement of planes of atoms (along slip plane) relative to 1 and other

Order: rapid cooling to retain random solid solution (soft) structure then reheat and cool slowly to form superlattice (harder) - ordered solid solution

291
Q

Explain precipitation hardening

A

Supersaturate 1 metal in the other then quench (fast cool), reheat to below Ts/within insoluble region then allow to cool slowly to cause precipitation of fine particles of other metal within metal

292
Q

How does precipitation Harding work?

A

As presence of impurities halts deformation movements

293
Q

Why are constitutional phase diagrams important?

A

Understanding various heat treatments used for dental alloys

Controlling microstructure of alloys and properties

BDS 1 (11 decks)

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