dental amalgam e lec Flashcards
what is amalgam
a combination of different things eg an alloy containing mercury
which industries is amalgam used in
more than just dental industries
how is gold extracted
extraction of gold from an ore
why are companies reducing their use of amalgam
because of the environmental impact the release of mercurious compounds.
what state is mercury in at room temp
liquid
what temp does mercury freeze at
-39C
what is mercury used in
thermometers, manometers, tilt switches, treatment for STDs
what temp does mercury form alloys
room temperature
what is the most damaging form of mercury
organic compounds and mercury vapours
how is mercury considered safe
when rendered with an alloy or inter metallic compound
when does mercury become inert
when it is placed
what is the maximum exposure levels per week
300mg per week
what is the maximum exposure levels in the air regarding the conc of mercury vapour
25mg/m3
how can mercury enter the body
absorbed
ingested
inhaled
how long does it take to half the amount of mercury in th body
60 days
what is mercury presented in to the clinician
into a pre prepared capsule for health and safety reasons
what might a typical modern alloy contain
69 %wt Ag, 19 %wt Sn, 11 %wt Cu and 1 %wt Zn.
what is the amalgam alloy made from
silver and tin with some small additions of copper < 6 %
what is the ratio of silver to tin in amalgam alloys
Ratio of 73% : 17% by weight
what intermediate alloy is formed of silver and tin in the amalgam alloy
Ag3Sn which is known as the gama phase!
what year was the amalgam settled in
1855
why is zinc added to the amalgam alloy
it is not clear
what was amalgams first made from in 1500-1840s
large amounts of silver coins which has silver and cooper
what was the percentage by weight of the silver in these coins
90-95% wt
how does silver set
slow but and was hard
how does tin set
rapidly but does not harden
how does copper set
it set but wasn’t hard
what is the equation of mercury and amalgam alloy reaction
Hg + Ag Sn (γ)( gamma phase)
what is the product of the mercury and amalgam reaction
Ag3Sn (γ) + Ag Hg (γ ) + Sn Hg (γ ) 2 3 1 7 2
what do the gamma 1 and gamma 2 phase do
they precipitate out of the liquid and are not soluble
what is the gamma 1 structure referred to as
the matrix and holds the whole composite together
what is the gamma 2 phase susceptible to
corrosion
what does traditional thinking say about the gamma 2 phase being corroded
that it is not a bad thing as it will prevent leakage of the margins
what do we now know about the gamma 2 phase and corrosion
that it distorts restorations and leads to marginal failure
what are remaining products of corrosion
tin based
what is the dangers of the products of corrosion
can be lost to the body and increase mercury exposure
also causes staining of the surrounding tissues
how was dental amalgam mixed in the start
mixed with a mortar and pestle
how was the process improved from mixing it by hand
the introduction of amalgamators which automatically mixed the alloy and mercury
how are amalgam and mercury now mixed
more recently this has been further refined by the use of encapsulated amalgams
what is the shape of the amalgam alloy
spherical
what is the lathe cut
chips of amalgam alloy which are irregularly cut and do not readily flow
how are spherical amalgam alloys made
spraying liquid alloy into air or water, the latter produced a more varied morphology.
how do we get rid of corrosion
to increase the amount of copper
what happens because we have increased the copper % to 12%
second reaction with the gamma 2 phase forming the products gamma 1 and Cu6Sn5.
Cu6Sn5 + Ag2Hg3 (γ1)
what happens to the additional copper
into the amalgam alloy such that the alloy is of two intermetallic compounds Ag3Sn and Cu3Sn,
what do the crystals of Cu6Sn5
fine rod like
what do we have to consider about dental amalgams
Doesn't adhere to tooth Not aesthetic Base or liner- need therapeutic liner or base material Poor tensile strength Polishing 24 hours later. Long lasting Lathe cut or Spherical Creep
what is the compressive strength of amalgam
350MPA
what is the tensile strength of amalgam
35 MPA
why is the spherical cut of amalgam alloy better
Spherical is thought to lead to the amalgam flowing better during condensation and also to a reduction in the amount of Hg needed to make a workable material
what is creep
the material flows as a response to the various stresses
at what temp does the gamma phase 1 start to melt
80C
