Dental Materials Flashcards
Define an alloy
A mixture of two or more metals
The process that occurs in the formation of a crystal from a solution, liquid or vapour
Nucleation
What two types of nucleation are there?
Heterogenous & Homogeneous
Describe heterogenous nucleation
Many sites of nucleation
Describe homogeneous nucleation
Single site and requires specialised equipment
What type of nucleation is more common in dental applications
Heterogenous nucleation
What does grain size refer to?
Crystal size
What promotes a small grain size?
-rapid solidification
-provision of ‘extra’ nucleation sites
What ways can metals and alloys be shaped?
-hammering
-rolling
-pressing
-drawing into a wire
What happens to grains when a metal/alloy is drawn into a wire?
They are elongated
The maximum degree of extension in response to an applied tensile force
Ductility
The maximum degree of compression in response to an applied compressive force
Malleability
What is a solid solution? Give an example.
Where two metals dissolve in each other and then harden to give a solid solution.
E.g. copper and zinc dissolve in each other and harden to give a solid solution called brass.
Ordered, random and interstitial are three types of what?
Solid solution
What are solid solutions in comparison to metals?
-harder
-stronger
-higher elastic limits
-hardening effect
What does Tm indicate on a phase diagram?
Melting point
What does the plateau on a phase diagram indicate?
Temperature constant during crystallisation
Why are phase diagrams useful?
Can construct series of cooling curves for alloys of different composition
When one component in an alloy solidifies before the other, this has less than optimal properties. What is this process called?
Coring
Which line on a binary alloy graph represents the liquidus
The top line
Which line on a binary alloy graph represents the solidus?
Bottom line
What would a large separation between liquidus and solidus lines on a binary alloy graph indicate?
That the alloy is more susceptible to corrosion and there is greater coring
What application do eutectic alloys have?
Solders
Where alloy components are insoluble in each other, there is a specific point where crystallisation occurs at specific temperature. What is this point called?
Eutectic point
In dentistry, what are solders used for?
-attach a metal component to an appliance
-fit golden laced teeth to restore them
When an alloy reacts with mercury what does this form?
Amalgam
What are the two compositions of amalgam?
Conventional and high copper
What effect does copper have on amalgam?
It strengthens it
What is the role of zinc in alloy production?
It acts as a scavenger
How does zinc act as a scavenger?
It oxidises in preference to the other metals, attracts oxygen so that the other metal won’t.
What is another word for mixing?
Trituration
When mercury diffuses into alloy during setting reaction what occurs?
Small shrinkage
What is there risk of if we have a material that expands or contracts too much?
-a gap around the restoration
-forces that could shred off cusps of teeth filled with an alloy
What do the international standards organisation (ISO) limit expansion/contraction to?
+/- 0.1%
What happens to zinc containing alloys if placed in a moist or contaminated environment?
They expand, this is because the zinc ion reacts with water, liberating hydrogen.
How long does it take for final strength of amalgam to be reached?
24 hours
What is the minimum depth of amalgam required to counteract weakness in thin sections?
2mm
What is strength of amalgam dependant upon?
The mercury content
What is the optimum Hg concentration?
44-48%
What does packing of amalgam into a cavity do to the mercury?
Forces it to the surface
Define creep
Plastic deformation under load
How does creep clinically manifest?
As protrusions at restoration margins.
(This can fracture off producing a ditch and predisposing area to caries.)
What is the gamma 2 phase largely responsible for?
Creep
What is the theory of mercuroscopic expansion?
Localised expansion of amalgam due to release of Hg into marginal crevice
What can corrosion products provide?
Marginal seal
What is the most accurate way to dispense mercury?
Capsules containing mercury and alloy
What particle size of an alloy achieves higher strength?
Smaller particle size
What happens if particle size is too small?
It will be too rapid a set and cannot remove Hg by manipulation
What material was used as a restorative material in 1950’s-60’s?
Silicates
What material was used as a restorative material in 1960’s-80’s?
Acrylics
What form could acrylic take?
Powder or liquid
What is the type of reaction in the setting reaction for both powder and liquid acrylics?
Free radical addition polymerisation
What is a consequence of fast setting of acrylics?
Limited working time
What is the bond that splits in a free radical addition polymerisation reaction?
C=C
What is a key organic compound used to demonstrate a free radical addition polymerisation reaction?
Methyl methacrylate (MMA)
What are the four stages of free radical addition polymerisation reaction?
1.activation
2.initiation
3.propagation
4.termination
What happens in the initiation stage?
C=C cleaved
What happens in the propagation stage?
Multiple units join
What happens in the termination stage?
Two free radicals merge/meet which terminates the joining of units
What happens to materials when they undergo polymerisation?
They contract
What type of reaction is the contraction of materials upon polymerisation?
Exothermic
What is very key, important feature of acrylics?
They exhibit low thermal diffusivity (heat transfers slower)
What are some disadvantageous qualities of acrylics?
-can be irritant to pulp
-May interact with Eugenol
-poor abrasion resistance
What is a eutectic alloy?
An alloy that has a very defined, very narrow ranged melting point and properties
What has been used in acrylic to try and reduce shrinkage, however may also weaken the material?
Inert filler
What is the setting reaction for resin composites?
Free radical addition polymerisation
What are the two main components of resin composite?
-resin matrix
-inhibitors
What does the comonomer,TEGMA, control?
Viscosity
What is the role of inhibitors in resin composite?
They prevent polymerisation kicking off too readily
What benefit does the use of siloranes have?
May minimise polymerisation shrinkage
What is the setting reaction for siloranes?
Cationic ring-opening polymerisation
What two important substances do resin composites contain?
Fillers and coupling agents
What are the three types of fillers?
- Quartz
- Silica
- Glasses