Final Exam Flashcards
What are the three main classes of materials?
Metals, Ceramics, and Polymers
Describe the mechanical properties of metals
Hard, Ductile, Strong
Describe the mechanical properties of ceramics
Hard, Brittle, Strong
Describe the mechanical properties of polymers
Soft, Ductile, Weak
Describe the bonding of metals
1-3 loosely bound electrons that have free mobility.
This contributes to thermal and electrical conductivity as well as the ability to be ductile.
Describe the bonding of ceramics
Form ionic and covalent bonds. The ions are not mobile.
Covalent > Ionic > Metallic
Describe the bonding of polymers
Form covalent bonds and have high molecular weight.
Long molecules primarily of non-metallic elements entangle (spaghetti) which contributes to strength and properties.
Describe the thermal properties of metals
High processing temperatures.
Act as thermal conductors.
Describe the thermal properties of ceramics
High processing temperatures
Describe the thermal properties of polymers
Low processing temperatures.
Used as direct processing materials
Metals:
- Typically used for ___ placement
- ___ processing temperatures
- Exception: ___
- Indirect
- High
- Amalgam
Ceramics:
- Typically used for ___ placement
- ___ processing temperatures
- Exception: ___
- Indirect
- High
- Cements
Polymers:
- Typically used for ___ placement
- ___ processing temperatures
- Exception: ___
- Direct
- Low
- Indirect Composites
Name the 4 steps of polymerization
- Activation
- Initiation
- Propagation
- Termination
Explain the activation step of polymerization
An initiator molecule is activated by either light, heat, or chemical mixing to form free radicals.
Explain the initiation step of polymerization
Free radicals combine with monomer unites. This opens up the double bond on the monomer to create a new free radical.
Explain the propagation step of polymerization
Monomer free radicals bind to grow the polymer chain.
As this happens, the polymer volume decreases and shrinks.
Explain the termination step of polymerization
A floating free radical bonds to the free radical end of the chain.
This creates a double bond and caps off the chain.
What do linear polymers form from?
Monomethacrylates
What is significant about monomethacrylates?
They only have one free radical end
What do linear polymers resemble when intertwined?
Tangled spaghetti
What do branched polymers form from?
Dimethacrylates
What is significant about dimethacrylates?
Short chain with two free radical ends
What do branched polymers resemble when intertwined?
Fishing Net
They have many branched chains that form crosslinks
What can polymerization be initiated by?
Light, heat, and chemical mixing
Define a composite
Physical mixtures of metals, ceramics, and/or polymers
What is the goal of a composite
To achieve some intermediate properties between two material types
What is the rule of mixtures?
By knowing the phases present in the structure of any material and interfacial interactions, it is possible to predict the overall properties fairly well
What happens to the final properties if you increase filler volume?
- Material strength increases
- Modulus of elasticity (stiffness) increases
- Viscosity increases
- Shrinkage during curing decreases
What happens to the final properties if you increase the filler size?
Surface roughness increases
What is the point of using fillers?
They are chemically bonded to resin phase to improve properties
What is the thermal expansion coefficient?
What is the general trend for the materials?
Rate of change of a material’s size upon heating or cooling.
It is measured in ppm/°C
Ceramics < Metals < Polymers
Do metals need a base?
Yes. They require thermal insulator like bases because metals are thermal conductors
Do composites need a base?
No. They have low thermal conductivity, so they do not need a base
Teeth are considered ___ due to high mineral content. Because of this, the pulp requires __ during a restoration
Insulators
Protection
Describe thermal conductivity
Rate of heat conduction
Describe thermal diffusivity
Conductivity/Time
The amount of heat flow over time toward the pulp through the restoration
How is color defined?
In a 3D coordinate system
Define Hue
Wavelength
“Color” (ROYGBIV)
- Left to right
Define Value
Intensity
Brightness
- Top to bottom
Define Chroma
Purity
Density or Concentration
- Inside to Outside
Define metamerism
2 objects appear the same under one light source but different under another.
Due to different spectral characteristics
What does the contact angle measure?
How a liquid interacts with a solid
Good wetting = ___ contact angle
Low (close to 0)
Poor wetting = ___ contact angle
High (close to 180)
Hydrophobicity and Hydrophilicity can help determine:
The wetting properties of a material
The hydrophobic part is the part that polymerizes
Hydrophilic Primers:
- Allows for?
- Mixed with?
- Viscosity?
- Allows for penetration into areas with water AND attachment to composite
- Solvent (alcohol, acetone)
- Very low viscosity
T/F: Molecules can either be hydrophilic or hydrophobic, but not both.
False. They can have both hydrophilic and hydrophobic components.
What are 4 things that surface wetting is important for?
- Anytime 2 different materials come into contact
- Impression materials
- Adhesives
- Bacterial Adhesion
Disposal Issues with Mercury
Ends up in the water systems
Patient issues with Amalgams
Poor esthetics
Operator issues with Amalgams
Brittle, sensitive
Define creep
The tendency of a solid material to deform permanently over time under low constant stresses
- Result from accumulated strain from long-term stresses (chewing, biting, grinding)
Is amalgam ductile or brittle?
Brittle
What can improve corrosion resistance of amalgam?
Zinc - it will also make the material more brittle
What can minimize corrosion?
Polishing
What is the major reaction phase in both low and high-copper amalgams
Gamma 1
What does not form with high copper amalgams?
Gamma 2
What is the strongest microstructural phase in dental amalgams?
Incompletely consumed starting alloy particles (Gamma 1)
What is the weakest microstructural phase in dental amalgams?
Gamma 2 in low-copper amalgams (high corrosion prone)
What causes contraction during amalgam setting?
The absorption of Hg.
The final absorption of mercury by remaining amalgam allow particles also causes contraction.
What causes expansion during amalgam setting?
The formation and growth of the gamma phases
Lathe-cut has more:
Condensation resistance
Spherical has:
Better mechanical properties and is cut with less mercury.
What phases are formed in the first step of the setting reaction?
Both Gamma 1 and 2
What happens in the second step of the setting reactions?
Disappearance of Gamma 2 and formation of n’ (Eta prime)
What does heat treatment of an alloy do?
Relieves stresses in alloy particles and provides control of setting time. Eliminates compositional nonuniformity in ingot before cutting.
3 types of corrosion in metals
Active, Passive, Immune
4 types of active corrosion
- Electrochemical (Galvanic)
- Structure Selective
- Crevice
- Stress
Electrochemical (Galvanic) Corrosion
Electron transfer to allow for oxidation of a metal. (ex: gold next to amalgam)
Structure selective corrosion
Two phases (cathode and anode) that go through redox corrosion (amalgam)
Crevice corrosion
Crack in the structure
What is important about passive corrosion?
Produces a film that prevents further corrosion (implants)
What metals are immune to corrosion?
Noble metals (gold)
Describe corrosion of ceramics
Covalent bonds are broken down by water
Describe corrosion of polymers
Absorption of water leads to hydrolytic degredation
What is a hazard of degredation?
Major concern for amalgams?
Major concern for composites?
Byproducts can be toxic.
Mercury is of concern for amalgams.
Bisphenol A is a concern for composites as it is estrogen like.
Name the 4 regions of the stress-strain curve
- Elastic limit
- Proportional limit
- Ultimate strength
- Elastic modulus
What is the elastic limit?
If a stress is put on an object and released before it exceeds the elastic limit, said material will return to the original shape
What is the proportional limit?
The point where stress and strain are directly proportional. Up to this point the curve is a straight line.
What is ultimate strength?
Where the material breaks; peak of the curve
What is the elastic modulus?
How stiff the material is; between the start of a curve and the proportional limit. Greater slope means greater stiffness.
Curves with a high elastic modulus tend to be…
Resistant to stress but cannot bear a lot of strain. Will break (ex. glass)
Curves with a low elastic modulus will..
Strain greatly under a low amount of stress. (ex. red vine/polymer)
Curves with a medium elastic modulus can..
Withstand a lot of stress and can strain to support more stress (like a tootsie roll or metal)
Define modulus
(Resilience)
The energy absorbed before deformation occurs
Define toughness
The energy absorbed before failure or fracture occurs
How can heat effect mechanical properties?
It can change the setting time of a material
Define fatigue
Multiple cycles of low stress
Define fracture toughness
Ability of a cracked material to resist fracture
Define cyclic stresses
Over time, many cycles of small amounts of deformation occur until failure occurs at low stress
Because teeth are not completely rigid, they can have ___
Abfractions
What is an abfraction?
Mechanical loss of tooth structure near the gum line
Explain Buccal-Lingual Stability
Cusps work as an entity when undergoing B-L forces. Have a high stability.
How can a cavity prep lead to B-L instability and cusp fracture?
When a cavity prep takes out the central groove the cusps become separate entities and now cannot resist B-L stress as well. Subject to fracture.
In general terms, how is each material processed?
Metals - melted together
Ceramics - fused together
Polymers - polymerization
What factors contribute to each materials mechanical properties?
Bonding, Microstructure
When you make an amalgam, will you have a single phase or multiple phases? What does this do to the properties?
Multiple phases. Some of them have the same and others have different properties.
This is why you have corrosion
What is the effect of alloying on amalgam?
Improves properties and corrosion
___ and ___ are useful in alloying
Copper and Zinc
Components of composite resin
Resin system, filler, bonding agent, visible light initiator
What is the role of the resin system
Helps with processing and handling
What is the role of the filler?
Helps modulus and wear resistance
What is the role of the bonding agent?
Bonds filler to the resin system
What are the trade-offs that occur in properties?
Inorganic - allows for wear resistance and strength but is poor for processing and is brittle
Polymer - has poor resistance but is tough and good for processing
Explain crosslinking
Allows for a stronger, one unit matrix to be formed as opposed to multiple linear but independent units to be present - decreasing strength
How does crosslinking control properties?
A greater crosslink density leads to a greater strength; however, shrinkage is a worry
T/F: There is only one size of filler
False. Different filler sizes exist
How does the filler size control properties?
More crosslinking = higher modulus
Smaller filler leads to better wear properties for polishing and finishing. Also better mechanical properties.
What does a coupling agent do?
Coats the surface of the filler and binds it to the resin matrix. This moves groups to the surface to allow for curing (forms covalent bonds and improves properties)
Define polymerization shrinkage
Results from density change of monomer to polymer
What are clinical consequences of shrinkage?
Causes stress on the tooth, leads to microleaks and secondary caries
2 components of glass-ionomer cement
Acid polymer in aqueous solution
Basic glass
How are glass-ionomer cements cured? How is this different from composites?
They are cured by an acid-base reaction. They form salt bridges instead of covalent bonds.
Glass-ionomer cements are a ___ based restorative
Water
What do glass-ionomer cements release?
Fluoride
What is the role of an adhesive?
It contains acid groups to promote bonding of the composite to the hydrophilic tooth surface.
What promotes bonding?
Hydrophilic modifiers
Are both hydrophilic and hydrophobic characteristics needed for bonding?
Yes
How are sealants different from composites?
They’re designed to flow into pits and are attached through mechanical means rather than chemical; usually lack a filler
What is the oxygen inhibition layer?
Oxygen interferes with radical polymerization at the surface leaving a thin layer often uncured; more of a problem for thin layers like sealants
What is hydrocolloid impression material?
Alginate (not as accurate)
What is elastomers impression material?
Polysulfide (good surface detail)
Ideal properties for impression materials
- Low cost
- Long shelf life,
- Biocompatible
- Pleasant to patient
- Dimensionally stable
- Good handling properties
Alginates are ___ based
Water
Alginates are ___ crosslinked
Calcium
Advantages of alginates being water based
- Economical
- Easy to use
- Quick setting
- Fair taste
- Hydrophilic
Disadvantages of alginates being water based
- Dimensional instability
- Low tear resistance
- Limited detail reproduction
- Single pour only
- Quick pouring required
What is dimensional instability
Undergoes shrinkage or expansion upon loss or gain of water
Polysiloxanes are ____
Hydrophobic
Because Polysiloxanes are hydrophobic, there are ___ for taking ____
Consequences for taking impressions
Polysiloxanes are crosslinked by ____ reaction
Hydrosilation
Polysiloxanes have good ___ ____
Dimensional stability
What is the role of surfactants?
Can turn this hydrophobic material into a hydrophilic one
Polyethers are ___
Hydrophobic
Although polyethers are ___, they have better ____ ____
Hydrophobic
Surface characteristics
Polyethers have good ___ ___
Dimensional stability
Gypsum is ___ cured
Water
Explain the dehydration-hydration sequence
Mineral gypsum is heated and dehydrated resulting in a model plaster and water.
When mixed with water, it is rehydrated.
What are gypsum properties dependent on?
Density and crystal structure.
Dental stone is used for ___ not ____
Models not plaster
Where are there differences in composition for denture base and provisionals?
Dentures need to be able to last longer. Provisionals are temporary.
Denture base is normally ___
VLC
___ are normally chemically cured
Provisionals
Mouthguards are ____ not ___
Thermoplastic not Thermoset
_____ controls the properties of a mouthguard
Copolymer
Cements are used to…
Bind restorations of appliances to tooth structure
What do cements require?
Low viscosity with good mechanical properties.
How are viscosity and handling properties modified in cements?
Powder to liquid ratio balances the mechanical properties and viscosity
More filler - more viscosity and modulus
Low powder - increases working and setting time
Two types of cements
Resin and Glass Ionomers
Resin cements have a ___ component system
1
Glass ionomers have a ___ component system
2
Function of bases
Used to provide thermal protection for the pulp and mechanical support for the restoration
Function of liners
Used to protect the dentin from residual reactants that can diffuse out of the restoration or fluids that may leak in from the oral cavity; near pulp exposure.
Explain pulp capping
Near or direct pulp exposure. Use Calcium hydroxide to initiate the formation of reparative dentin (odontoblast formation)
What does ZOE stand for and what is it used for
Zinc oxide and eugenol. It is used as a sedative restoration for pain reduction.
Inhibits polymerization - can’t use with composite.
What are 4 requirements for wax?
- Should melt at lower temperatures
- Able to be carved
- Able to burn off
- Dimensionally stable.
____ molecular weight materials in between polymers and ___ ___
Lower
Organic liquids
