Dental Materials Flashcards
(293 cards)
1
Q
What 2 forms can GICs come in?
A
Encapsulated or hand mix (liquid/powder)
2
Q
Why is encapsulated preferable?
A
Pre-measured
Mixed for you
Can out straight into mouth
3
Q
How can self mix go wrong?
A
Too much powder or liquid will set slower/faster and alter consistency and binding properties
4
Q
What are GICs used for?
A
Cement and fillings
5
Q
What forms can zinc oxide eugenol and calcium hydroxide come in?
A
2 paste (temp-bond, Dycal) or liquid/powder
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
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
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
9
Q
What 3 forms are most dental materials in unset?
A
- Power and liquid
- 2 paste
- 1 paste
10
Q
What 2 forms do most dental materials cure to?
A
- Rigid/stiff
2. Rubbery, elastic material
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
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
13
Q
What are the dis/advantages of 1 paste materials?
A
Can be difficult to pack into cavity if v viscous
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
15
Q
What are the 3 methods of setting dental materials?
A
- Heat
- Root temp.
- Light cure unit
16
Q
Compare the physico-mechanical properties of the 3 cure types of materials
A
- Heat: optimum physico-mechanical properties (dentures) but not feasible for chairside
- RT: compromised compared to heat set - denture repair material
- Light: physico-mechanical properties between RT and heat set - composite
17
Q
What are the 5 ideal properties of dental materials?
A
- Biocompatible
- Mechanically stable
- Chemically resistant
- Dimensionally stable
- Minimal thermal and electrical conductivity
18
Q
What are the 11 generic groups of dental materials?
A
- Impression
- Acrylics
- Soft liners and tissue conditioners
- Cements
- Resin composites
- Bonding agent
- Casting materials
- Investment and die materials
- Waxes
- Ceramics
- Dental alloys/amalgam
19
Q
What 5 factors influence selection of material?
A
- Patient history
- Age of patient/dentition
- Patient compliance
- Location of tooth
- Depth of cavity
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
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
22
Q
What is Grassman’s law?
A
Three parameters of colour:
- Dominant wavelength (hue)
- Excitation purity: saturation of colour; intensity 0-1
- Luminous reflectance: brightness (100) or darkness (0)
23
Q
Why is colour important in DMs?
A
Wavelength material reflects may change w/ age thus colour may change
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
25
What are metameric colours?
Colours that appear the same under 1 light source but different under another
26
What is the significance of metanerism in dentistry?
Need to match restoration colour to natural colour in light corresponding to that of use
27
What is fluorescence?
Phenomenon observed when material absorbs colour of one wavelength and emits colour of another wavelength
28
Why is fluorescence important in DMs?
Teeth emit fluorescent light (look whiter under fluorescent light) so materials need to look natural
Some porcelains contain fluorescing agents to mimic natural appearance
29
Define opacity, translucency, transparency
Opacity: selective absorbance of light
Transparency: no interaction w/ light, complete transmission
Translucency: mixture of absorption, refraction, transmission
30
What is the refractive index?
Ratio of velocity of light in a vacuum to velocity in selected medium
A large difference = opaque; identical = transparent
31
What are 3 areas in why temp. is important in dentistry?
1. Use of drill
2. Exothermic setting reactions: can't do in mouth
3. Effect of hot/cold on restoration
32
Why is knowing temp. important?
Temp change may alter properties of material
33
What is heat of fusion?
Energy required to convert 1g of material from solid to liquid at melting temp.
34
What is thermal conductivity?
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
35
What is the clinical significance of thermal conductivity?
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
36
What is specific heat?
Quantity of heat required to heat 1g material by 1 degrees
37
What is thermal diffusivity?
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
38
What is the clinical relevance of thermal diffusivity?
Good inlay, crown, amalgam have low specific heat but high thermal conductivity thus if close to pulp may get thermal shock
39
What is thermal expansion coefficient?
Change in length of material for a 1 degree change in temp.
40
What is the clinical significance of thermal expansion?
Tooth and material will expand and contract due to hot/cold
| Could result in breaking of marginal seal of inlay/filling
41
What electrical conductivity and resistivity?
Ability of material to conduct electrical current
42
Why is electrical conductivity and resistivity important?
Correlation between electrical conductivity and change in physical and mechanical properties of material
E.g. gold alloys: change in resistivity changes internal crystal structure
43
What is electromotive force?
Difference in electrical potential that gives rise to current
44
What is the electromotive series?
List of metals ordered in dec. tendency to oxidise in solution
45
What is galvanism?
Induction of current due to chemical reaction
46
Why is galvanism important in dentistry?
If 2 fillings of different electronegativities touch will cause short circuit which if near the pulp will cause pain
47
Why does amalgam have a high copper content?
Copper-tin complex much harder and less corrodible than tin-mercury
48
How do GICs bond to enamel?
Bind to Ca2+ in enamel
| Carboxyl group plays role in setting reaction and bonding mechanisms
49
What is solubility?
Of inorganic salt: number moles of pure solid that will dissolve in 1L solvent at given temp
50
Compare solubility of hydroxyapatite and fluoroapatite
FA less soluble than HA: remains supersaturated for longer than HA even at lower pH
51
What are the 6 optical factors important in DMs?
1. Colour
2. Opacity
3. Fluorescence
4. Surface finish
5. Refractive index
6. Metanerism
52
What are the 6 thermal factors important in DMs?
1. Heat of fusion
2. Thermal conductivity
3. Thermal diffusivity
4. Specific heat
5. Temp
6. Thermal expansion
53
What are the 3 electrical factors important in DMs?
1. Conductivity and resistivity
2. Electromotive force
3. Galvanism
54
What are the 3 chemical factors important in DMs?
1. Reactions
2. pH and solubility
3. Bonding
55
Define biomaterial
Natural or synthetic material that interfaces w/ living or biological tissue
56
Define biocompatible
Ability of material to elicit an appropriate biological response, in given application, in body/mouth
57
What is biocompatibility dependent on?
```
Physical function
Biological response required
Location
Composition
Interaction w/ OC
```
58
What are the 4 main reasons for restorations?
1. Trauma
2. Decay/caries
3. Tooth loss
4. Revisions: repair failed restoration
59
Define safe in terms of DMs
Must not cause any local or systemic adverse reactions
60
What are the 2 categories of research?
1. Basic: lab research; pre-market
| 2. Clinical: post-market surveillance
61
What are the 2 types of lab research?
1. In vitro
| 2. In vivo
62
What 5 basic factors are tested in in-vitro testing?
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
63
What kind of tests are done in-vivo?
Implants: material implanted into animal
Allergy tests
Limited usage studies: animal/clinical testing; long, tedious but most clinically relevant
64
What types of animal tests are there?
Past: material ground and fed to animal; implanted into animal
Now: material used in required area
65
Why is clinical testing preferred?
Most accurate and efficacious depending on:
Number of patients
Group of patients
Length of trial
66
What is the downfall of clinical testing?
Clinical symptoms don't evaluate real world damage
| Will have material in OC for years not weeks
67
What are some of the ways in which post-market surveillance is important?
Provide early warning signs of unsuspected adverse effects
Elicit predisposing factors to adverse reactions
Compare adverse reactions between similar products
Permit continued safety monitoring
68
Who are at risk of adverse reactions from DMs?
Dentist
Dental nurse
Dentinal technician
Patient
69
Who is most at risk of adverse reactions to DMs? Why?
Dentist/technician
| Inc. risk as inc. exposure to material
70
How can risks be reduced?
Proper packaging
Following manufactures instructions
Non-contact operative techniques
71
What are the 3 main types of force?
1. Uniaxial
2. Biaxial
3. Triaxial
72
What are the 3 types of uniaxial force?
1. Tensile: away from each other
2. Compressive: towards each other
3. Shear: towards, one from top side other from bottom side
73
What are forces defined by?
Where they are applied, in what direction and how big they are
74
What is stress?
Force applied per unit area
75
Define strain
Deformation of object due to stress
76
What is Hooke's law?
Stress is proportional to strain
77
What is Young's modulus?
Ratio of stress to strain i.e. stress/strain is a pressure (Pa)
78
What can be determined from a stress-strain curve?
Ductility, strength, elastic modulus, resilience, toughness, flexibility
79
What is a fracture?
Separation of a material into 2+ pieces under action of stress
80
What are the 2 types of fracture?
Brittle: little/no plastic deformation, low toughness
Ductile: significant plastic deformation, high toughness
81
What are the steps in a fractureb
1. Crack formation
| 2. Crack propagation
82
Describe the stress-strain plot for a brittle material
Almost linear due to low plastic deformation
83
Describe a direct tensile measurement
Dumbbell shaped test specimens, ensures central fracture
Used for metals, rigid polymers, rubbery polymers
84
What are compressive tests used for?
Ceramics
| Hard polymers
85
What is a diametral/indirect tensile test?
Compression across diameter
86
What are the 6 static strengths?
1. Compressive
2. Tensile
3. Shear
4. Torsion
5. Flexure
6. Diametral tensile
87
What is hardness?
Resistance to indentation/permanent deformation when compressive force applied
88
What is fracture toughness?
Resistance of a material to failure from fracture starting at pre-existing crack
89
Define tear strength and energy
Strength: force needed to initiate/continue tearing
Energy: measure of energy per unit area of newly torn surface
90
Define impact
Resistance to fracture from rapid loading measured as energy absorbed at fracture
91
What is fatigue?
Failure of material at force well below static strength due to repeated force/strain cycles
92
Define fatigue life, strength and limit
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
93
What is static fatigue?
Failure of material at small load after period of constant loading
Load required to cause failure will dec. w/ inc. time loading
94
What 4 things is wear a combination of?
1. Abrasive
2. Adhesive
3. Fatigue
4. Corrosive/erosive
95
Define abrasive wear
Materials against each other:
2 body: tooth-tooth
3 body: tooth-tooth w/ food stuff between
96
What is adhesive wear?
Material sticks to tooth and when pulled away removes part of tooth
97
What is fatigue wear?
Propagation and combining of micro-cracks w/ successive loading cycles
98
Define corrosive/erosive wear
Corrosive: acid attack remove weakened enamel
Erosive: grinding, bruxism
99
What is fluid flow governed by?
Strength of intermolecular forces and molecular entanglement
100
Define viscosity
Resistance to flow of a fluid
| Measure of internal resistance of material
101
What is a Newtonian fluid?
Fluid in which applied shear stress produces flow w/ constant shear strain rate response
Water, solvents, mineral oils
102
What are non-Newtonian fluids?
Fluids in which there is no define viscosity
| Viscosity changes with shear rate or shear rate history
103
What are the 5 types of non-Newtonian fluids?
1. Pseudoplastic
2. Dilatant
3. Viscoplastic
4. Thixotropy
5. Rheopexy
104
Define pseudoplastic fluids
Dec. viscosity as shear rate inc.
| plaster, stone
105
Define dilatant fluids
Inc. viscosity as shear rate inc.
| composites, porcelains in water
106
Define viscoplastic fluids
Will not flow until initial shear stress has been reached then can have Newtonian, pseudoplastic or dilatant properties
107
Define thrixotropic fluids
Viscosity dec. w/ time at constant shear rate
| Ketchup, toothpaste, clays, quicksand
108
Define theopoxic fluids
Viscosity inc. w/ time at constant shear rate
| Some lubricants, v rare
109
Define stress relaxation and creep
Relaxation: time dependent dec. in stress at constant strain
Creep: time dependent dec. in strain under constant load (stress)
110
What are the 7 types of adverse reactions?
1. Toxic
2. Irritant contact dermatitis
3. Allergic contact dermatitis
4. Oral lichenoid
5. Anaphylactoid
6. Contact urticaria
7. Intolerance
111
What are the 2 types of adverse effects?
1. Dermatoses
| 2. Non-dermatological
112
Describe irritant dermatose reaction
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
113
Describe allergic dermatose reactions
Acquired by contact w/ haptens in materials
Tissue develops antigens by contact w/ dermal protein
Next contact produces allergic response
114
Define haptens
In/organic molecule that alone is not antigenic but is when linked to carrier protein
115
What are some of the materials that can cause dermatose reactions?
```
Metals
Polymers/monomers
Hydroxyethyl methacrylate
Latex gloves
Formaldehyde
Eugenol
Disinfectants
Rubber dam: latex or nitrile
```
116
What is sodium hypochlorite and its risk?
Disinfectant used for irrigation in RCT
| Can cause pain if introduced into periapical tissue, periapical bleeding and extensive swelling
117
What effects can acute systemic toxicity have?
Primarily eyes and airway
Long term exposure lead to renal, neural, liver disorders
Associated with repeated dosage
118
How can mutagenic changes arise from materials?
Associated with chemicals in material
| Result from breakdown of material
119
How do methacrylates affect the body?
Chronic toxicological effects
| First respiratory problems then neurological disorders
120
How can mercury and nickel affect the body?
Mercury: short term memory loss, irritability
Nickel: carcinogenic
121
How can ceramics, plasters and alginates effect the body?
Respiratory problems from inhalation of dust
122
How can trace anaesthetics effect the body?
NO, O2 sedation can cause neural and liver disease
123
Define adhesion and cohesion
Adhesion: force that binds 2 dissimilar materials together by attraction between atoms and molecules
Cohesion: attraction between atoms/molecules within 1 substance
124
What must there be for adhesion to occur?
Attraction between atoms in 2 surfaces
125
What 3 stresses weaken adhesion?
1. Thermal expansion coefficient
2. Dimensional changes
3. Moisture hydrating active groups in material
126
What are the 3 criteria for successful adhesion?
1. Surfaces clean and dry
2. Contaminants removed
3. In close contact
127
Describe the adhesion between 2 solids
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
Describe solid-liquid adhesion
Good
Water clings to glass due to VDWs bonds (secondary), require thermal energy to remove water
129
What are the 5 factors governing adhesion?
1. Surface energy/tension
2. Wettability
3. Viscosity
4. Contact angle
5. Morphology/Surface roughness
130
Compare the surface energy/tension of bulk and surface molecules
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
What 3 things can reduce surface tension?
1. Inc. temp
2. Impurities
3. Surface-active agents/species: surfactants
132
What 2 factors make an adhesive effective?
1. Must be in close contact with substrate
| 2. Spread easily
133
What 3 factors affect spreading?
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
What is wettability?
Tendency of fluid to spread on a surface
135
What's an example of good wettability?
Metal and water
| Water spreads easily over the whole surface
136
What 3 factors can affect wettability?
1. Surface cleanliness
2. Surface irregularities
3. Viscosity
137
What is contact angle?
Angle between liquid and solid
138
What is contact angle dependent on?
Surface tension and surface energy
139
What does a contact angle of 0 mean?
Perfect wetting
| Surface is completely covered by adhesive, max. bond strength
140
What happens as contact angle is inc.?
Air voids are introduced preventing perfect wetting
| Rupture of adhesive joints
141
Explain the correlation between surface roughness and bond strength
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
What are the 5 types of adhesion?
1. Mechanical
2. Physical
3. Chemical
4. Molecule entanglement
5. Mixture of all
143
Describe mechanical adhesion
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
Describe physical adhesion
Dipole-dipole attractions between polar molecules
Small: VDWs
Weak adsorption: not suitable for long adhesion
Rapid bonding, reversible
Thermal energy breaks bond
145
Describe chemical adhesion
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
Describe molecular entanglement
Adhesive or component penetrates surface and absorbs into surface of substrate
147
What is molecule entanglement enhanced by?
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
Why is DM structure important?
Understanding structure-property relationship ensures right DM selection for specific clinical situation
149
Compare protons, neutrons and electrons
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
What are the 2 classifications of chemical bonds?
Primary: strong; 0.5-7 eV
Secondary: weak; <0.5 eV
151
What is a bond?
Interaction between atoms
152
What 3 bonds are primary bonds?
1. Covalent
2. Ionic
3. Metallic
153
Describe covalent bonds
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
Describe ionic bonds
```
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
Describe metallic bonds
Non-directional, relatively weak 0.5-2 eV
Overlapped orbitals of metallic atoms forms electron gas
Electron has and metallic cations: electrostatic integration
156
What bonds are secondary bonds?
VDWs
| H bonds
157
Describe secondary bonds
No electron sharing
| Charge induced dipole interactions
158
Define phase
Physically and chemically homogenous part of system/material that has clear boundaries
159
Define phase transition
Change from one phase to another
| e.g. solid -> gas sublimation
160
What is a phase diagram?
Presentation of stability and phase transitions for a system/material of phases in equilibrium at range of temps.
161
Why are phase diagrams useful?
Identify phase against the composition at a given temp
162
What is the eutectic point?
Lowest melting point of system: lower than constituents and may other mixture of them
163
Describe crystalline solids
Consist of crystals
| Form crystalline lattice: ordered (symmetric and periodic) and specific arrangements of atoms
164
Describe non-crystalline solids
Disordered solids, contain no crystals
Inorganic, rapidly quenched: glass
Organic: polymers
Phase transition specific for non-crystalline solids
165
Describe the phase transitions in non-crystalline solids
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
What is polymerisation?
Synthesis of polymers from monomers
167
What are the 2 types of polymerisation?
1. Condensation
| 2. Addition
168
Describe condensation polymerisation
Reaction between 2 molecules producing larger molecule and water/small molecule byproduct
Byproduct has to be removed during polymerisation
169
Describe addition polymerisation
Reaction between 2 molecules giving larger molecule with no byproducts
Monomers added 1 by 1 per chain to active site on growing chain
170
What is the most common type of addition polymerisation?
Free radical polymerisation
171
What is the Thiokol reaction?
Condensation polymerisation that forms polysulphide polymers
| Used in base paste of polysulphide impression materials
172
How is 2-polyglycolic acid synthesised and what is its function?
Condensation polymerisation
Synthetic, absorbable, suture material
Biodegrades back to acid in 60-90days
173
What is a free radical?
Molecules w/ an unpaired electron
174
How is methyl methacylate synthesised and how is it used?
Addition polymerisation
Used as denture base
175
How are free radicals formed?
When initiation decomposes into 2 highly reactive free radicals in presence of monomers
176
What is the common initiator?
Benzoyl peroxide
177
What are the 4 stages of free radical polymerisation?
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
What are the 3 ways in which FRs are formed in dentistry?
1. Heat
2. Room temp
3. Light
179
Describe heat formation of FRs
Usually powder/liquid, 2-paste or 1-paste
When mixed and heated, BP splits into FRs
FRs attack double bonds in monomers
180
Describe how FRs are formed at room temp
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
Describe light formation of FRs
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
How does free radical formation being exothermic lead to porosity?
Boiling causes monomer to bubble creating airspaces which then set as material polymerises
183
Why can a molecule weight not be given for a polymer?
All polymers are not the same length
184
What 2 methods can be used to give a polymer a MWt?
1. Number average MWt (Mn)
| 2. Weight average MWt (Mw)
185
Define number average MWt
Total weight of sample divided by number of molecules in sample
186
Define weight average MWt
Sum of the number of molecules multiplied by their weight average
187
What are the 3 states polymers exist in? Give examples
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
How are polymer chains held?
Weak 2ndary bonds: VDWs
| Entanglement of chains
189
Describe the effect of stress on rubbers
Easy deformation due to low intermolecular forces
Reversible due to 1% crosslinks within polymer
Elastic modulus in MPa
190
Describe the effect of stress on hard resins
Difficult to deform due to high intermolecular forces
| Elastic modulus in GPa
191
What are crosslinks?
Covalent bonds in polymers joining one polymer chain to another
Prevent polymers flowing irreversibly
192
Compare 1% and 30% crosslinks
1%: elastic
| 30%: rigid material
193
Describe the effect of crosslinks on glassy polymers
Improves some properties such as impact strength (relevant for dentures)
Too many will make them brittle
194
Define thermosetting resin
Initially rubbers w/ low intermolecular forces, due to 30% crosslinks end up as rigid polymers
Vulcanite
195
Compare thermoplastics and thermosetting plastics
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
Define glass transition temp
| Explain with an example
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
Describe the Tg fro natural rubber
Elastomer w/ low intermolecular forces
| As temp dec, forces inc. until reach Tg temp at which point material becomes glassy rigid
198
What is the significance of Tg to dentistry?
Denture bases require Tg for exceeding temps likely to be exposed to
Polishing be technician, cleaning by patient
199
What is a plasticiser?
Simple, organic liquid
| Dibutyl phthalate
200
What are plasticisers used for?
Added to monomer before polymerisation to red. Tg of material i.e. make naturally rigid, glassy material soft and rubbery at RT
201
How do plasticisers function?
Depress Tg by lubrication
Flow between chains of polymer red. high intermolecular forces
Rigid material becomes soft and rubbery w/ low intermolecular forces
202
What are the 2 disadvantages to plasticisers?
1. Material becomes hard again w/ time
| 2. Phthalates are carcinogens and leach out into patients mouth
203
What are crystalline polymers and what are their uses in dentistry?
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
What is a homopolymer?
Polymer formed when polymerised alone
205
What are the 3 stereoisomers that polymers can obtain?
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
What is a copolymer?
Product of polymerisation involving multiple types of monomer
207
What are the 3 types of copolymer?
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
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Why impact can water uptake have on materials?
1. Red. strength
2. Extract potentially toxic materials
3. Can induce microorganism formation
209
What are 3 useful water soluble materials used in dentistry?
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
Define ceramic
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
Describe the structure of ceramics
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
What 3 factors determine shape of ceramics
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
Describe how ceramic units are connected in crystalline and non-crystalline ceramics
Crystalline: regular repeat pattern
| Non-crystalline: non-regular, random pattern, short-range molecular order
214
Define ceramic coordination number
Number of anions around central cation
215
In crystalline ceramics what 3 areas can unit shapes share?
Corners: share 1 ion
Edges: share 2+ ion
Faces: share 3+ ion
216
What are the 7 crystal systems (unit cells) of crystalline ceramics?
1. Cubic
2. Tetragonal
3. Orthorhombic
4. Rhombohedral
5. Monoclinic
6. Triclinic
7. Hexagonal
217
What are the 4 Bravais lattices of crystalline ceramics?
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
What is a glass?
Inorganic product of fusion material that has cooled to rigid condition w/o crystallisation
219
Why can't glasses be defined by shape?
Non-crystalline
| Have random, amorphous structure
220
Describe the formation of glasses
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
Describe glass transformation
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
Explain correlation between cooling rate and glass density
Cooling rate >>> 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
What're the majority of glasses?
Oxide glasses
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What are the 4 rules of oxide glass formation?
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
What are the 3 different types of oxide in glass oxide composition?
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
What is the network connectivity of oxide glasses?
Av. no. bonds linking each repeat unit in silicate network
Red. viscosity and fusing temp
Inc. coefficient of thermal expansion
227
Describe the processing of ceramics
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
Describe the effect of particle packing on porosity vol.
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
Describe sintering effect on porosity
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
Explain brittle fracture
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
Describe fatigue in ceramics
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
Describe the general properties of ceramics
High elastic modulus, brittle, hard
Relatively inert
Some are bioactive and bioresorbable
Crystalline: less reactive, better mechanical properties
233
What are 5 crystalline ceramics used in dentistry?
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
What are 5 non-crystalline ceramics used in dentistry?
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
What are the 2 forms of silica and their subtypes?
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
What are the 2 types of silica transformation?
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
Describe the properties of silica
Crystalline and virtuous relatively inert: only attacked by hydrofluoric acid
238
What are the uses of silica?
1. Filler: composite, investment materials, porcelains, cement, alginate
2. Component: GICs, porcelains
239
What are the forms of alumina?
Most commonly crystalline forms as corundum, alpha-alumina
Also eta, chi, gamma, delta, theta
Can also be produced from bauxite
240
Describe the properties of alumina
```
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
What are the 6 uses of alumina?
1. Abrasives
2. Filler in cements
3. Reinforcement of restorations
4. Implants
5. Maxillofacial reconstruction
6. Orthopaedics
242
What are the 3 crystalline forms of zirconia?
1. Monoclinic: low temp
2. Tetragonal: med temp
3. Cubic: high temp
243
What are the properties of zirconia?
1. Elastic modulus: GPa < alumina
2. Flexural toughness: MPa > alumina
3. Fracture toughness: 6-13 > alumina
As chemically stable as alumina
244
What is the transformation toughening of zirconia?
Load induced transformation of tetragonal to monoclinic
| 3-5% expansion
245
What are the uses of zirconia?
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
What are the properties of hydroxyapatite?
Biological HA contains: F-, CO3-, Mg2+, Na+; not homogenous
| Ca/P ratio 1.67
247
What are the 5 uses of HA?
1. RC filler
2. Bone filler
3. Tooth root
4. Glass ceramic restorative
5. Bioactive coating
248
What are the 4 uses of porcelains?
1. Artificial teeth
2. Veneers
3. Inlays
4. Crowns and bridges
249
What are the 5 advantages of porcelains?
1. Excellent aesthetics
2. Relatively inert
3. Low thermal expansion coefficient: similar to tooth
4. High MP
5. High elastic modulus
250
Describe the components and composition of porcelains
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
What are fluxes?
Additives to porcelains that red. fusing temp
Include glass formers: added as carbonates
Boric oxide added as borax
252
Describe boric oxide
Glass former
Added to porcelains to red. fusing temp
Boron anomaly: red. fusing temp w/o inc. thermal expansion
253
What are some aesthetic additives for porcelains?
Metal oxides for colour, opacity, fluorescence
Cobalt for blue
Chrome/tin for pink
Titanium/zirconium for opacity
Terbium/europium/cerium for fluorescence
254
What are the 3 porcelains that may be required for an aesthetic restoration?
1. Core/opaque: mask cement interface or metal alloy core
2. Body/gingival dentine: bulk colour build up
3. Enamel: highly translucent
255
What are the 5 uses of metals?
1. Partial dentures and clasps
2. Inlays and onlays
3. Direct filling material
4. Orthodontics
5. Crowns and bridges
256
Describe a metallic bond
Metal atoms lost outer electron (valence) to form cations
| Lost electrons able to flow around cations in sea of electrons
257
What are the 3 methods in which metals can be made?
1. Casting: crowns, partial dentures
2. Cold working: wires, clasps
3. Amalgamation: amalgam
258
Describe a pure metal cooling curve
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
What causes the plateau in a pure metal cooling curve?
Balance between the latent heat of fusion and cooling
260
Describe the 5 stages of solidification of a metal
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
Describe the grain boundaries of metals
Ill defined: almost amorphous, random structure
Attract impurities
More reactive
262
Describe grain structure of metals
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
@ RT what are the 3 general crystal structures of metals?
1. Body-centred cubic
2. Face-centred cubic
3. Close-packed hexagonal
264
Define wrought alloy
Cast alloys that have been formed by mechanical processes (cold working) e.g. rolling, hammering, forging, drawing
265
Describe wrought alloys
Above yield stress
Grains become elongated resulting in springiness
Under go work (strain) hardening
266
Describe the general properties of metals
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
Describe the biocompatibility of metals
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
Describe the reactivity of metals
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
Define chemical and electrochemical corrosion?
Chemical: direct combination of metal and non-metal (including oxidation)
Electrochemical: different metals in an electrolyte (saliva)
270
What does corrosion lead to and what does this cause?
Degradation and release of ions:
- structural breakdown
- migration of ions around body
- cytotoxicity or allergic phenomena
- tissue discolouration
271
What are the 5 types of corrosion?
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
Why are metals more easily deformed than expected?
Due to defects in crystal structure
273
What are the 2 types of crystal defects?
1. Point
| 2. Line
274
What are the 3 types of point defects?
1. Vacancy: atom removed
2. Substitutional: atom replaced
3. Interstitial: atom between metal atoms
275
What are the 2 types of line defects?
1. Edge dislocation: extra plane of atoms
| 2. Screw dislocation: edge in 3D
276
How do metals deform?
By movements of dislocations
277
How do dislocations lead to permanent deformations?
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
Describe the effect of grain size on deformations of metals
Deformations halted by grain boundaries, another dislocation, impurities and point defects
Smaller grain size: less distance to travel, less possible deformation, more rigid
279
Explain how metals can be both ductile and brittle
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
Define ductility and malleability
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
Describe work (strain) hardening/cold working
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
What is annealing?
Process by which effect of work hardening can be overcome by heating
283
Describe the 3 stages of annealing
Recovery: effects begin to disappear, stress relief
Recrystallisation: old crystal structure disappears, forms new crystal structure, occurs at temp ~50% MP
Grain growth
284
Describe alloys
Mix of 2+ metals
Have better properties compared to pure metals
Have no single MP: melt/solidify over range of temps.
285
What are the liquidus and solidus temps. of alloys?
Liquidus (Tl): above all liquid, below liquid + solid
| Solidus (Ts): above liquid + solid, below all solid
286
What are the 4 types of binary alloys?
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
Describe the 3 stages in the construction of a phase diagram for a binary alloy
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
What is alloy coring?
Between Ts and Tl composition of liquid and solid varies w/ temp
Cooling rapidly causes formation of layers of solid of different composition
289
How can coring be rectified?
Homogenisation
| Reheating to allow diffusion of atoms to give homogenous composition
290
Define solution and order hardening
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
Explain precipitation hardening
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
How does precipitation Harding work?
As presence of impurities halts deformation movements
293
Why are constitutional phase diagrams important?
Understanding various heat treatments used for dental alloys
| Controlling microstructure of alloys and properties
