Chapter 15 - Amalgam Flashcards
describe amalgamation
When metal particles are mixed with mercury, the outer portion of the particles dis- solves into mercury. At the same time, mercury di uses into the metal particles. When the solubility of the metal in mercury is exceeded, crystals of mercury-containing com- pounds start to precipitate within the mercury. During this period of reaction the metal particles coexist with the liquid mercury, giving the mix a plastic consistency. is means that the mixture can be adapted to any shape with a light pressure. As the content of liquid mercury in the mixture decreases by the formation of precipitates, the mixture hardens.
why is copper needed for Ag-Sn amalgams?
Antimicrobial, also needed to help blend Ag and Sn uniformly;
why is Zn needed in Ag-Sn amalgams?
deoxidizer, oxygen scavenger; Alloys without zinc are more brittle, and their amalgams tend to be less plastic during condensation and carving.
how are high copper alloys made into smaller particles?
e presence of the higher copper content makes mechanical cutting of ingots into par- ticles difficult. us, they are o en provided in a spherical form that is produced by an atomization process
Spherical alloys require less mercury than typical lathe-cut alloys because spherical alloy powder has a smaller surface area per volume ratio than does the lathe-cut powder
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what is the y1 phase?
Ag2Hg3
what is the y2 phase?
Sn7Hg8
e physical properties of the hardened amalgam depend on the relative percentages of each of the microstructural phases. how does each phase affect strength?
More unconsumed Ag-Sn, the stronger it will be
Sn-Hg is the weakest in corrosive environments
what are the mercury/alloy ratios?
for lathe-cut: 50:50, for spherical: 42% Hg by weight
why is trituration important?
an oxide layer of alloy surface hinders diffusion of Hg into alloy–oxide layer is removed by abrasion
how does undertrituration affect teh Hg-Alloy mix?
too dusty/chalky–>will not be smooth when putting into cavity–>hard to make smooth after burnishing and will tarnish easier
how can burnishing lead to corrosion?
burnish too hard–>heat created (above 60)–>Hg release; significant amounts of Hg in the margins–>accelerated corrosion (and/or fracture)
How can burnishing prevent corrosion?
dull, unburnished surfaces are full of pits, scratches, etc–>areas allowed for concentrated cell corrosion; burnishing gets rids of those pits and scratches
What does the ADA require dimensional change of amalgum to be?
range of 15 to 20 μm/cm mea- sured at 37 °C between 5 min and 24 h
how does mercury content cause expansion?
intitial contraction of mix due to particles dissolving to create gamma-1 crystals; however, if sufficient Hg is present, as gamma-1 crystal grow and are lodged in Hg –> gamma crystals grow and push outwards, pushing on matrix–>expansion
why is moisture bad for amalgam fillings?
causes a large expansion; this type of expansion is known as delayed expansion or secondary expansion. e e ect is caused by the hydrogen produced by electrolytic action involving zinc and water. e hydrogen does not combine with the amalgam but, rather, collects within the lling, increasing the internal pressure to levels high enough to cause the amalgam to creep,
When an amalgam creeps, it is the γ1 phase that deforms plastically. Higher creep rates should be expected for low- copper amalgam with higher γ1 volume fractions and vice versa. e presence of the γ2 phase increases the creep rate. In addition to the absence of the γ2 phase, the very low creep rates in single-composition high-copper amalgams may be associated with η′ phase rods, which act as barriers to defor- mation of the γ1 phase. erefore, the manipulative factors discussed previously that maximize strength also minimize creep for any given type of amalgam.
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what is tarnish?
a layer of silver sulfide on the surface of the amalgam
why can high-Cu amalgam not be used with conventional low Cu amalgam?
High-Cu is cathodic relative to the low-Cu amalgam; will cause corrosion
why can corrosion sometimes be good for amalgam?
can fill in any gaps between amalgam and tooth–bc amalgam does not adhere to tooth structure
how is the ultimate lifetime of an amalgam restoration determined?
1) the material, (2) the dentist and the assistant, and (3) the patient’s environment
