Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OI; TilE INVENTION
Conventionally, silver-bas~ alloys adapted for use
as dental amalgams contain a minimum of about 65% silver, a
maximum of about 6~ copper, a maximum of about 2% zinc and a
minimum of about 25~ tin. The aclmixture of such silver-base
alloys with from about 45~ to about 55% mercury by the dentist
as needed are adapted to harden within a matter of a few minutes,
enabling shaping by carving for a period of up to about 15 min-
utes and the amalgamation reaction is usually complete after
about 24 hours. Typically, silver alloys suitable for forming
amalgams in accordance with American Dental Association specif-
ications contain from about 26% to about 28~ tin, about 1~ to
about 2% zinct about 2~ to about 4% copper, with the balance
essentially all silver. Of the foregoing alloying constituents
present, it has been generally accepted that the quantity of
copper should be restricted to a maximum of about ~% due to
the tendency of dental amalgams incorporating greater quantities
of copper to corrode and/or discolor in actual use. On the
other hand, quantities of copper in excess of about 6% are
known to improve the compressive strength of the resultant
dental amalgam.
The general requirements of a dental amalgam include
a retention of shape, color and appearance; a bioli~ical com-
patibility~ a restoration of the tooth to its original function,
and a long durable operating life. Extensive investigation and
test work of dental restorative materials including dental
amalgams has revealed that creep resistance is perhaps the
single most important mechanical property followed by compres-
sive strength as it relates to failure or racture of dental
restorations.`--These tests have convincingly established a
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relationship between a hig incidence of marginal fracture
of dental restorations with high dynamic creep values. Attempts
to improve the marginal fracture characteristics and other
mechanical properties of dental amalgams by increasing the
quantity of copper in the alloy have been unsuccessful due to
the increased corrosivity and tendency of discoloration of the
- resultant amalgams.
- The present invention provides for an amalgamable
silver alloy containing amounts of copper substantially greater
10than those heretofore considered useable which improves not
only the resistance of the dental amalgam toward corrosion
and/or discoloration/ but also provides for substantially
superior mechanical properties, including increased resistance
to dynamic creep.
SUMMARY OF THE INVENTION
-
The benefits and advantages of the present invention
are achieved by an amalgamable silver alloy powder derived from
a microcasting of a molten mass of alloy, such as by fluid
atomization techni~ues or the like, and wherein each particle
is of substantiàlly the same composition and is further character-
ized by having~a gradient composition in which the concentrati~n
of the individual alloying constituents changes on moving in a,
direction from the surface of the particle toward the center
thereof. The amalgamable powder contains tin and copper in com-
bination with silver as the essential alloying constituent and
wherein the silver-to-ti~ ratio is controlled from about 1.8:1
to about 3.0:1, while the tin-to-copper ratio is controlled
from about 1:1 to about 3 1. On a weight percent basis of the
individual alloying cons~ituents, the amalgamable silver alloy
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po~Jder contains from about 8~ to about ~7~ copper, from about
20% to about 32% tin, with the balance consisting essentially
of silver but, in any event, a minimum of 47% silver. The alloy
powder can also contain up to about 2% zinc without any adverse
effects. The powder particles are of an average size less than
about 100 microns and pref~rably range from about S microns
- to about 44 microns.
The dental amalgam is prepared employin~ the amal-
gamable silver alloy powder by admixture with mercury in an
amount of from about 42~ up to about 54% and preferably in an
amount such that mercury in the final product is present from
about 46~ to about 50~ by weight.
Additional benefits and advantages of the present
invention will ~ecome apparent upon a reading of the description
of the preferred embodiments, the specific examples provided,
taken in conjunction with the accompanying drawing.
BRIEF DESGRIPTION OF THE DRAWING
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The drawing graphically portrays the range o com-
position gradients of the amalgamable silver alloy particles
between the surface and interior of the particle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS .
- The proportions of the individual alloying constit-
uents comprising the amalgamable silver alloy powder and the
resultant dental amalgam prepared therefrom are herei~ defined
in terms of percentages by weight and on a weight ratio basis
unless clearly indicated to the contrary.
The permissible as well as the preferred ratios and
corresponding percentages of the three essential alloying con-
stituents are set forth in Tabl~ 1.
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TABLE 1
METZ~L POWDI~:R COMPOSITION
Perr.lissible Range Preferred Range
Asg Ratio 1.8 - 3.0 2.2 - 2.6
Cu Ratio l.0 - 3.0 1.5 - 2.5
Cu 8~ - 27~ 10% - 18~
Sn 20~ - 32% 23% - 28%
Ag 47% - 70% 57% - 65%
As shown in Table l, the ratio of silver-to tin is
broadly controlled within the range of about 1.8 up to about
3.0, while the permissible ratios of tin-to-copper are from
about l to about 3. Expressed in terms of percentages of the
three alloying constituents, based on the foregoing weight ~ -
ratios, the minimum percentages are specified with the
-~ recognition that when any one of the three is at the minimum
level, one or both of the remaining alloys-are present in
amounts greater than their respective minimums in order that
the total percent is lO0. The foregoing is also applicable
to the preferred percentages of these three alloying con-
stituents as set forth in Table 1. For the purposes of conven~
ience, the relative proportions o~ the three metals are set
forth in terms of their weight ratios in which tha silver-to-tin
ratio preferably is controlled within a range of about ~.2 to
- about 2.6,while the tin-to-copper ratio is preferably controlled
within a range o~ about 1.5 to about 2.5. It is also contem-
plated that zinc can be included in amounts up to about 2~ by
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weight o~ th~ total alloy. ~3~53~
It is an important feature of the present invention
that the amalgamable silver alloy powder is derived by a micro-
casting technique in which molten fine sized droplets of the
desired composition are cooled in a manner to effect a progres-
sive solidification during the cooling commencing on the sur~
face and moving inwardly thereof. The differential cooling and
solidification rate of each droplet contributes toward a
stratification or composition gradient in the final solidified
particle which is at least in part responsible for the unex-
pected improved mechanical and chemical properties of dental
amalgams prepared therefrom.
The microcasting of a molten mass of the silver alloy
can be achieved by a variety of techniques including gas
-- atomization, airless spraying and centrifugal fragmentation for
effecting a subdivision of the molten mass into a plurality of
fine-size liquid droplets wh ch are all of substantially the
same composition. Of the foregoing techniques, the gas atomiz-
ation technique has been found particularly satisfactory and a
specific embodiment thereof is described in United States Patent
No. 3,253,783. In accordance with the apparatus disclosed in
; the aforementioned United States patent, a molten stream of
the metal alloy passes downwardly and i5 atomized in response
to the impingement of a conically-shaped vortex of gas and
wherein the resultant droplets progressively solidify as they
fall downwardly through the collection chamber. Metal powder
produced in accordance with the arrangement described in -the
aforementioned patent under proper operating conditions gen-
- erally is of a spherical configuration and where th~-predom-
inant portion thereof is of a particle size wi hin the desired
range.
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The amalg~mable powder can be of any particle configuration,
although particles of a substantially spherical configuration
are preferred. The powder can be of an average particle size
of less than about 100 microns, although powders having average
particle sizes ranging from about 45 microns to about 5 microns
are usually preferred.
While air can be satisfactoril~ empLoyed as the atom-
izing and collection medium, the contact of the molten silver
alloy with the oxygen in the atomizing gas occasions the form-
ation of undesirab].e oxides on the particle surfaces. While
such oxides can subsequently be reduced by heating the powder
in a reducing atmosphere, such as hydrogen, for example, it is
preferred to employ a substantially inert gas, such as a sub-
stantially dry argon, nitrogen or helium gas, for effecting the
atomization of the powder and also as the cooling medium
within the powder collection chamber.
The amalgamable powder of the desired composition and
particle size can be employed for forming a dental ama`lgam
at the time required by admixture with from about 42% up to
about 54% mercury and, more usually, from about 46% to about 50%
mercury. The wetting of the particle surfaces with mercury
initiates the amalgamation reaction, whereby the intermetallic
compounds formed by a consumption of the surface strata of the
particles serves to bond the residual unreacted particle cores to
aach other, forming a matrix characterized as a continuous phase
of amalgam, having interspersed therethrough discrete discontin-
uous phases of the un~eacted particle cores. Of the several inter-
metallic compounds formed, the tin-mercury intermetallic com-
pound comprises the weakest alloy with respect to compressive
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strength and also is the most susceptible to corrosion and/or
discoloration.
A possible explanation of the different behavior of the
spherical alloy containing high copper to that of the commin-
utive alloys having the same composition is the difference in
their manufacturing and solidification pattern of the alloy.
In the spherical alloy, the outer surface of each particle con-
tains a higher concentration of silver and copper, whereas the
tin-rich phase is concentrated in the inner part of the part-
icle. Using a hydrochloric acid solution as a washing agent,
some tin ions are dissolved from the outer surface, while the
silver and copper ions on the surface are left almost unaffected.
This process would actually increase the relative concentration
of silver-to-tin on the surface, which should cause amalgam
alloys to harden in a very short period of time. ~ However,
after acid washing of particles, the surface of particles be-
comes spongy in appearance, producing some microscopic channels
: to the central part of the sphere where the higher concentration
of tin exists. During heat-treating of spherical particles, a
considerable amount of tin ions, through the mi¢roscopic
channels, migrate toward the surface. Increased amount of tin
on the surface would increase the working time of the alloy.
The affinity of mercury to tin is greater than to silver. The
affinity-of the cbpper ion to mercury at room temperature is
very low and thus the copper will remain unreacted, during tri-
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turation, increasing the ratio of Cu/Ag to a higher value than
before amalgamation.
While the particular mechanism by which the un-
expe~ted res-~lts of the present invention are obtained i~ not
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entirely und~rstood ~t the pr~sent time, a possible explan-
ation is that the outer surface of each of the particles con-
tains a higher concentration of silv~r and copper with the inner
core of the particle comprising a tin-rich phase. This minimiz-
es the formation of detrimental tin intermetallic compounds,
whereby the properties of the resultant dental amalgam are
substantially improved. The forlegoing is substantiated by
microprobe analyses of the spherical alloy particles employ-
ing x-ray beams which show a variation in the composition be-
tween the inner portions of each particle relative to the per-
ipheral portions thereof. The range of composition values
obtained by microprobe analyses of large amalgam alloy particles
are set forth in Table 2.
TABLE 2
MICROPROBE ANALYSIS OF LARGE SPHERICAL PARTICLES
Location of Composition, Percent by Weight
Analysis Silver Tin Copper
-` Center of particle 51-58 27-29 9-19
~- Edge of particle 64-84 10-20 6 16
The results of incremental microprobe analyses of
an individual particle to determine the range of variation in
the composition of the three alloying constituents on moving
from the particle surface toward the central core of the part-
icle is graphically illustrated in the drawing. As shown, the
concentration of the silver constituent progressively decreases
on moving from the surface toward the particle core. In con-
trast, the concentration of tin progressively increases on mov
ing from the particle surface toward the particle core, while
- the quantity of copper remains substantially constant but with
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a reduction in the breath of the range of values on moving
toward the core.
In order to further illustrate the amalgamable silver
alloy po~der of the present invention and the improved
properties of amalgams prepared therefrom, the following specific
- ~xample is provided. It will be understood that the example is
included solely for illustrative purposes and is not intended
to be limiting of the scope of the invention as herein described
and as set forth in the subjoined claims.
EXAMPLE
An amalgamable silver alloy powder is prepared by
the argon atomization of a molten mass of material containing
a silver-to-tin ratio of 2.23:1 and a tin-to-copper ratio of
2.10:1, resulting in an alloy consisting essentially of 13%
copper, 27% tin and 60% silver. The resultant powder particles
are generally of a sphexical configuration and the collected
powder is screened to separate and recover particles of an aver-
age particle size ranging from 44 microns to about 5 microns.
The balance of the powder particles are recycled.
T~ resultant heat~treated and washed amalgamable
powder is triturated for ten seconds with approximately 46% mer~
cury, whereafter the mixture is shaped into a test specimen
allowed to set for a period of seven days. Test evaluations of
the resultan~ dental amalgam tast specimen reveal a compres
sive strength of greater than about 75,000 psi, a creep resist~
ance of less than 0.20~ ~in three hours, and an observable re-
sistance to tarnish and corrosion.
- While it will be apparent that the invention herein
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disclosed is well calculated to achieve the benefits and
advantages set forth above, it will be appreciated that the
invention is susceptible to modification, variation and change
~ without departing from the spirit thereof.
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