Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background of the Invention
,''
The invention relates generally to the dental alloys
~7hich are used for filling teeth from which decayed portions ` -~
have been removed. More particularly, the invention relates to
an improved dental alloy having both corrosion resistance and
improved handling characteristics compared to alloys of the prior
art.
The prior art emphasized the development of alloys ' ~`
which are corrosion resistant. While typical dental alloys are
10 principally composed of silver and tin, they usually contain small
-
amounts of copper and zinc. A typical alloy of the prior art
ould contain at least 65 wgt. % silver, about 1-2 wgt. % zinc,
ancl about 2-4 wgt. % copper, with the remainder being tin. Such
alloys are not completely resistant to corrosion. It has been
15 found that increasing the copper content of such alloys provldes ~ '
increased strength and also avoids the formation of what is `
known in the art as the gamma-two phase, a tin and mercury phase ~;
which has low resistance to corrosion and thus may lead to early ;~
deterioration of fillings. Typical of such high coppex alloys
20 are those disclosed in United States Patent 3,871,876 and United
States Patent 3,997,32~. Such dental alloy compositions increase
the cop~er content from the typical 2-4 wgt. ~ to the rang~ of ~-
- B-27 wgt. ~ in the first-mentioned patent, and in the latter ~ ~-
patènt from 20-40 wgt. %. -
While such alloys have improved corrosion resistance~
another important characteristic of dental alloys is its handling
characteristics. The success of a dentist in filling a dental
cavity is related to the handling characteristics o the alloy
after it is amalgamated with mercury. For example, the high
L546~.6 `~ ~
copper alloy disclosed in U.S. 3,371,~76 is typically produced by
air atomization from the molten state which results in a
; spherical or spheriodal form for the finished alloy. It is
characteristic of alloys having a spherical shape that they feel
5 relatively soft to the dentist and appear to re~uire delicate
handling. They are sometimes difficult to pack into a dental
cavity since they have a tendency to be forced up the wall of the
cavity if too much pressure is exerted or an instrument is used
which has too small a bearing area. Consequently, many dentists
10 find that such spherical material is not well adapted to their
individual technique. As a result, they may be unable to take
advantage of the corrosion resistance inherent with spherical
alloys having a high copper content~
One method of improving handling characteristics of
15 conventional dental alloys is disclosed and claimed in United
States Patent 3,997,327. In that invention a major portion of
spherical par-ticles is combined with a minor portion of microcut
irregular particles, or flakes. Typical dental alloys in the
prior art ~enerally have been of the flake type, which inherently
20 re~uires a hi~her pressure in order to be packed into a dental
cavity than is characteristic of the spherical particle type
alloys. By combining spherical particles with flake particles
having the same composition, it is possible to improve the handling
characteristics of the resulting mixture. Such a combination,
25 having a conventionally low copper content, has less resistance `~
to corrosion than the higher copper content alloys previously
discussed.
One object of the present invention is to provide
improved handling characteristics to corrosion-resistant dental
30 alloys.
,
Summar~j of the Inverltion ~5~
The present inveLItion has three basic embodiments.
In the first embodiment, ~here is provided a corrosion-
resistant dental alloy in particulate form for use as a filling
for dental cavities after amalgamation with mercury and
comprising a mixture of silver, tin and copper. The alloy
consists essentially of particles having a mean particle size
of between about 20 and 26.5 microns, and having a particle ;
size distribution such that substantially all of the particles
fall within a particle size range of from about 1 to 75 microns.
The particles further have a surface area ranging from about
0.22 m2/gm to 0.31 m2/gm, so that the alloy~ after amalgamation,
retains a corrosion resistance comparable to that of spherical
particles having substantially the same composition and having
a surface area of about 0.21 m2/gm, while retaining handling
characteristics comparable to those of flake-like particles
also having substantially the same composition and having a
surface area of at least about 0.33 m2~gm.
In the second embodiment, there is provided a corrosion-
resistant dental alloy mixture for use as a filling for dental `~;
cavities after amalgamation with mercury comprising a sub~
stantially uniform mixture of particles of a first dental alloy
and a second dental alloy, both of the dental alloys comprising ;
a mixture of silver, tin and copper. The first alloy comprises
spherical particles having a mean particle size of from about
20 to 26.5 microns, and further having a particle size
distribution such that substantially all of these particles
fall within a particle size range of from about 1 to 75 microns.
These particles have a surface area of about 0.21 m2/gm. The
second alloy comprises particles of the type described in the
preceding paragraph. By combining suitable proportions of
spherical particles and randomly-shaped micro-crystalline
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particles, the handling cl~aracteristics of amalgams prepared
from such mixtures can be adjusted to suit the requirements
of the individual user, while retaining the corrosion resistance
of the alloy. Since both types of particles are corrosion
resistant, any desired proportions may be used in a mixture
according to the invention.
In the third embodiment, there is provided a corrosion-
resistant dental alloy mixture for use as a filling for dental
cavities after amalgamation with mercury comprising a
substantially uniform mixture of particles of a f irst dental
alloy, a second dental alloy and a third dental alloy, each
of the dental alloys comprising a mixture of silver, tin and
copper. The f irst and second dental alloys are of the tyes
described in the preceding paragraph. The third alloy
comprises flake-like particles having a mean particle size
of between about 25 and 30 microns, and further having a
particle size distribution such that substantially all of
these particles fall within a particle size range of from
about 1 to 75 microns. These particles have a surface area
of about 0.33 m2/gm, and comprise not more than about 25%
by weight of the dental alloy mixture. By combining suitable
proportions of spherical particles, randomly-shaped micro-
crystalline particles, and f]ake-like particles, the handling
; characteristics of amalgams prepared from such mixtures can be
adjusted to suit the requirements of the individual user.
~lthough any suitable proportions of the first and second types
. of particles may be employed, the third type of particle is
limited to a maximum of 25% by weight of the alloy mixture in
order to retai~ the corrosion resistance provided by the f irst ~-
and second types of particles.
In one particular aspect the present invention provides
à corrosion-resistant dental alloy in particulate form for ;
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6 - ~ ~
use as a filling for dental cav;.tles after amalgamation with
mercury, comprising a mixture of silver, tin and copper,
said alloy consisting essentially of random].y shaped particles
having a mean particle size of between about 20 and 26.5
microns, and further having a particle size distribution
such that substantially all of said particles fall within a
particle size range of from about 1 to 75 microns, said
particles having a surface area which, exclusive of both
spherical particles and flake~ e particles as defined
below, ranges from 0.22 m2/gm to.about 0.31 m2/gm, so that
said alloy, after amalgamation, retains a corrosion resistance ~ -
comparable to that of spherical particles having substantially
the same composition and having a surface area of about 0.21
m2lgm, but in any event, less than 0.22 m2/gm, while retaining
handling characteristics comparable to those of flake-like .~
particles also having substantially the same composition and ~ .
having a surface area of at least about 0.33 m2/gm and
approximately the same particle size and distribution as
said randomly shaped particles. ;; .
In a related aspect the present invention provides
a corrosion-resistant dental alloy in particulate form for
use as a filling for dental cavities after amalgamation with -~
mercury comprising a mixture of silver, tin and copper, said :.
alloy consisting essentially of randomly shaped particles
which, exclusive of both spherical particles and flake-like
particles as defined below, have a surface area at least
about 20 percent greater than the surface area of spherical
particles but less than about 0.33 m2/gm, whereby said
alloy, after amalgamation, retains a corrosion resistance
comparable to that of spherical particles having substantially
the same composition and having a surface area of about 0.21
m2/gm, but in any event, less than 0.22 m2/gm, while retaining
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llandling characteristics comparable to those of flake-like
particles also havillg substantially the same composition and
having a surface area of at least about 0.33 m2/gm and
approxin~ately the same particle size and distribution as
said randomly shaped particles.
In another particular aspect the present invention
provides a method of preparing a corrosion-resistant dental
alloy adapted for amalgamation with mercury comprising:
(a) preparing a composition of silver, tin and copper;
(b) melting the composition of (a); and
(c) microcasting randomly-shaped microcyrstalline alloy
particles from said melted composition of (b), said particles
having a mean particle size in the range of from about 20 to
26.5 microns, and further having a particle size distribution
including particles ranging between about 10 and 52 microns,
said particles having a surface area ranging from about 0.22
m2/gm to about 0.31 m2/gm, such that said alloy, after
amalgamation, retains a corrosion resistance comparable to
that of spherical particles having substantially the same
composition, and retains handling characteristics comparable
to those of flake-like particles.
In ye~ another particular aspect the present invention
provides a corrosion-resistant dental alloy mixture for use as
a filling for dental cavities after amalgamation with
mercury conæisting essentially of a substantially uniform
mixture of particles of a first dental alloy and a second
dental alloy, both of said dental alloys comprising a mixture
of silver, tin and copper, said first alloy comprising
spherical particles having a mean particle size of from
about 20 to 26.5 microns, and further having a particle size
distribution such that substantially all of said particles
fall within a particle size range of from about 1 to 75
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microns, said particles having a surface area of about 0.21
m2/gm, and said second alloy comprising particles having a
mean particle size of between about 20 and 26.5 microns, and
furtller having a particle size distribution such that
substantially all said particles fall within a particle size
range of from about 1 to 75 microns, said particles having a
surface area of from about 0.22 m2/gm to 0.31 m2/gm.
In a further particular aspect ~he present invention
provides a corrosiorl-resistant dental alloy mixture for use as a
. 10 filling for dental cavities after amalgamation with mercury
: . consisting essentially of a substantially uniform mixture of :
pareicles of a first dental alloy and a second dental alloy,
both of said alloys comprising a mixture of silver, tin and
copper, sald first dental alloy comprising spherical particles
having a mean particle size of between about 20 and 26.5 :
'. microns, and further having a particle size distribution
such that substantially all of said particles fall within
the particle size range of from about 1 to 75 microns~ said
spherical particles having a surface area of about 0.21
m2/gm, and said second alloy comprising randomly shaped :: ~
, particles having a surface area at least about 20 percent :
, greater than the surface area of said spherical particles
but less than 0.33 m2/gm, and having approximately the same
particle size and distribution as said spherical particles.
In yet a further particular aspect the present invention
provides a corrosion-resistant dental alloy mixture for use as a
filling for dental cavities after amalgamation with mercury,
consisting essentially of. a substantially uniform mixture of
a first corrosion-resistant alloy comprising a mixture of
silver, tin and copper, said first alloy being in the form
of spherical particles having a mean particle size of from
about 20 to 26.5 microns, and further having a particle size
jl/ -8-
distr:ibutioll such ~hat substantially all of said particles
fall within a particle siæe range of from about l to 75
microns, said spherical particles having a surface area of
about 0.21 m2/gm, a second corrosion-resistant alloy comprising
a mixture of silver, tin and copper, said second alloy being
in the form of randomly shaped particles having a mean
particle size of from about 20 to 26.5 microns, and further
having a particle size distribution such that substantially
all of said particles fall within a particle size range of
from about 1 to 75 microns, said randomly shaped particles
having a surface area which, exclusive of both spherical
particles and flake-like particles as herein defined, ranges
from about 0.22 to 0.31 m2/gm, and a third alloy comprising
a mixture of silver, tin and copper, said third alloy comprising
flake-like particles having a mean particle size of between
about 25 and 30 m:icrons, and further having a particle size ~-~
distribution such that substantially all of said particles `~
fall within a particle size range of from about ]. to 75
microns, said particles having a surface area of about 0.33 ~
m2/gm, said third alloy comprising not more than about 25% ;
by weight of the dental alloy mixture.
Brief Description of the Drawings
Figure la shows the spherical purticles of the prior
art corresponding to United States 3,871,876.
Figure lb shows particles corresponding $~ a dental
~r~
alloy of the present invention.
Figure lc shows particles corresponding to microcut or
flake-like particles of the prior art.
Figure ld shows a mixture of the particles of la and lb.
Figure le shows mixtures of the particles of la, lb and lc.
Figure 2 plots the results of tests described herein and
applied to several dental amalgams.
j 11 _9_
Description of the Preferred Embodiments
A dentist in packing an amalgam prepared from a
dental alloy and mercury into a dental cavity considers two
factors to be of articular importance. First, what may
; be termed "condensation" relates to the resistance of the
alloy to being packed into the cavity by the dentist using
typical instruments. It will be clear that an amalgam
must have sufficient plasticity when under pressure to
enable it to flow into and completely fill all portions of
the cavity, thereby preventing the formation of open spaces
in the finished filling which could weaken it or permit
further decay tG the tooth structure~ At the same time,
the amalgam must not be so fluid as to flow out from
beneath the dental instruments during condensation of the
amalgam and move up the wall of the cavityv In such
situations, a non-uniform degree of packing necessarily
results, with poor adaptation to the cavity and increased
porosity which weakens the filling and may result in further
`;~ decay. Thus, one important handling characteristic of an
amalgam is its ability to be pressed into a dental cavity
to fill all the small openings under the desired condensation
~ressure, while not being so soft that the dentist cannot
adequately compact the amalgam. This condensation pressure
may be approximated by an empirical test which ~ill be
hereinafter described and which is useful in connection
with the present invention~
The second handling characteristic of importance -~
to the dentist is the ability of an amalgam to be carved
or shaped in order to finish the exterior surface of the
com~acted filling. An amalgam also must be of a desired
plasticity in order to be satisfactorily carved or shaped.
An amalgam may be satisfactorily packed into a dental
-- 10 --
mS/r / ~
cavity but be difficult to smooth and shape when the packing
process is completed On the other hand~ an amalgam which
is easy to carve and shape may be difficult to pack properly
into a dental cavity. Another empirical test to be
described hereinafter may be related to the carving
characteristic of the amalgams derived from various dental
alloys.
As described in UOS. Patent 3,253,783 and elsewhere,
the gas atomization technique may be used to produce spherical
or spheroidal particles from molten dental alloys. Particles
are screened after cooling to provide a powdered alloy
having particles in the size range of about 1 micron to
about 65 microns. Larger and smaller particles are
separated and recycled to be remelted and recast. Spherical
~articles such as are illustrated in Figure la have an average
surface to volume ratio of about 0,21 m /gm as measured
by the usual BET apparatus~ The randomly-shaped micro- ;
crystalline particles typically having a BEI' surface area
of 0.22-0.31 m2/gm~ By way of contrast, the flake-like
part~cles commonly used heretofore have a BET surface area
of about 0.33 m /gm. It should be noted that the surface
area is related in part to the particle size, thus the values
given herein relate to a particle size distribution suitable
for dental alloys and as specified hereinbelow for the alloy
of the invention.
It should be further noted that the surface area
measured by the BET apparatus is much larger than the
- geometric exterior surface of the particles. For example,
a perfect sphere would have a surface area only about 10%
of that measured for the generally spherical particles of
Figure la. The additional 90% of the measured surface
is evidently due to surface roughness and porosity. Since
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:
this additional surface seems less likely to have a large
effect on the handling properties of amalgams than the
geometric surface, the geometric surface of the particles
should be compared rather than the BET surface. E~owever,
the geometric surface has not been measured although it may
be approximated by subtracting about 90% of the BET value
for comparison purposes.
Amalgams are produced by mixing mercury with dental
alloys of the invention. Generally, the dental alloys of
this invention are mixed with sufficient mercury to form
a workable plastic amalgam, and generally about 1 part
by weight of these dental alloy mixtures are mixed with
from about 0.8 to 1 parts by weight of mercury. At the
completi~n of the amalgamation process, the amalgam is
condensed into a tooth cavity by a dentist and then the
~. .. : ,
filling is carved or shaped until the amalgam has become
so hard that it cannot be worked. This period is typically
about six minutes. The dentist packs or condenses the
amalgam into the tooth cavity while the amalgam is still
soft enough to do so. The pressure required is quite
important to the dentist as has been previously discussed
and to characterize dental alloys of the invention we have
chosen to designate the resistance of the amalgam one minute
after amalgamation is complete as the condensation factor.
A lower value indicates that an amalgam is stiffer and
requires more pressure to pack or condense it into a tooth
cavity than an amalgam having a higher numerical value.
The test used to obtain values reported herein for
condensation factors may be described as follows. ~ pellet
of dental alloy is mixed with the recommended amount of
mercury in an amalgamator for the manufacturer's recommended
timeO A commercially available Wig-L-Bug* Model SAR
.,,1 .
* trade mark - 12 -
ms/~
manufactured by Crescent Corporation was used in the tests
repor-ted herein, although other amalgamators would be
acceptable. After the amalgamation is complete, the
amalgam is immediately placed on a flat glass plate and
covered by another such glass plate and pressed to a one
millimeter thickness, as determined by one millimeter
spaces placed between the plates. The top plate is removed
and measurements are made of the resistance of the flattened
amalgam disc during the hardening period. For thP measure-
ments reported herein an Instron* testing unit model llOl
produced by Instron Corporation was employed. A constant
' ~ load of ~ive pounds was placed on a two millimeter steel
ball in contact with the amalgam. The depth of the -
indentation made by the ball when the load was applied ~or
fifteen seconds is used as a measure of the resistance of
the amalgam. Tests were made at one minute intervals for
a period of five minutes, or until no further change in the
resistance was measured~ The period of time during which
measurements were made approximates the time which a dentist
uses to fill a tooth cavity and to carve the filling. Test
results obtained with prior art dental alloys in spherical
and flake form are compared with the dental alloy of the
invention in the examples below.
The car~ability factor relates to the ability of a
hardening dental amalgam to be carved and shaped by dental
instruments after it has been compacted. It will be apparent
that after the compaction or condensation period (about 2
minutes) the dentist will have a limited time in which to
shape or carve the hardening amalgam. A variant of the
test preYiously described is used to obtain a carving factor.
The two millimeter ball loaded by a five pound weight is
replaced with a one pound Gilmore needle having a one milli-
* trade mark - 13 -
ms/l ,l
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meter point. The Gilmore needle is normally used for
measuring setting rates of cements and plastic materials
and has been described in an article by Peyton and Craig
in Restorati~e Dental Materials, 4th _., 1971. It has
been found that the lighter loaded Gilmore needle will fail
to penetrate an amalgam after it is sufficiently hardened.
The time between the end of the amalgamation process and
the failure of the Gilmore needle to penetrate the hardening
amalgam may be used as an index of the carvability of the
amalgam.
, .
EXAMPLE 1
A dental alloy is prepared by mixing individual
metal powders and resulting in an overall composition 58 wt.
% Ag, 29 wt. % Sn, 13 wt. ~ Cu. The powdered mixture is
melted and processed in an air atomization apparatus modified
to minimize the formationof spherical particles by contacting
the molten droplets during the cooling process, thereby
producing the randomly-shaped microcrystalline particles
of the invention. The particles formed have a surface
area o~ 0.24 m /gm~ They are sieved to produce a powdered
alloy according to the invention as shown in Figure lb and
having particles sized within the range of 1 micron to
45 microns. The powdered alloy is then pelleted and mixed
with sufficient mercury to form an amalgam having an alloy
to mercury ratio of 1:1. The amalgam is measured for its
resistance to condensation pressure according to the test
hereinbefore described and the results plotted on Figure 2.
EXAMPLE 2
` A dental alloy is prepared by mixing individual
metal powders and resulting in an overall composition 58 wt.
% Ag, 29 wt. % Sn, 13 wt. % Cu. The powdered mixture is
melted and processed in an air atomization apparatus
- 14 -
ms/"
, .,,.,
according to U.S~ 3,871~876 to produce spherical particles
as shown in Figure la. The particles have a surface area
of 0.2I m2/gm. After sieving, the particles are within the
range of l micron to 40 microns. The powdered
alloy is then peIleted and mixed with sufficient mercury
to form an amalgam having an alloy to mercury ratio of l:l.
The amalgam is subjected to the condensation factor test
described hereinbefore and the results plotted in Figure 2. --
.. , :
EXAMPLE 3
: ' ' ,
A dental alloy is prepared by mixing individual
metal particles and resulting in an overall composition
68 wt. % Ag, 27 wt. % Sn, 4.4 wt~ % Cu, 0~5 wt ~ Zn. The
powdered mixture is melted and cast into a bar, from which
it is cut on a lathe into flake-like particles as shown in
Figure lc according to the usual technique of the prior art.
The partic'es have a surface area of 0.33 m /gm. After
sieving, the particles are within the range of 2 microns to
50 microns. The powdered alloy is then peLleted and mixed
with sufficien~ mercury to form an amalgam having an alloy
to mercury ratio of l.2:l. The amalgam is subjected to
the condensation factor test described hereinbefore and the
results plotted in Figure 2
As shown in Figure 2 flake-like alloys of the prior
art ~Figure lc and Example 31 are firmer when freshly
amalgamated than amalgams made with spherical particles.
Amalgams made with flake particles require heavier pressure
when being condensed or packed into a tooth cavity. The
condensation factors expressed as millimeters of indentation
after one minute from completion of the amalgamation process
are 10.75, 18, and 10.3 for the alloys of Example 1-3
respectively. The spherical particles of Exmaple 2 and
Figure la produce an amalgam which is soft when freshly
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- 15 -
m.s/~'rll
:
mixed with mercury. As previously indicated dentists often
find amalgams made with spherical particles to be delicate
to handle and difficult to condense properly. The alloy
of the invention (Figure lb and Example 1) has a unique
morphology and is neither spherical nor flake-like. The
handling characteristics are similar to those of the
flake-like particles of the prior art during the condensation
of the amalgam into a tooth cavity.
EXAMPLE 4
A dental alloy is prepared by mixing 40~ ~y~
weight of the particles of Example 1 with 60~ by weight of
the particles of Example 2. The mixed particles have a
surface area of about 0.23 m2/gm and are within the size
range of 1 micron to 45 microns. The powdered alloy is
then pelleted and mixed with sufficient mercury to form
an amalgam having an alloy to mercury ratio of 1 to 1. The
amalgam is subjected to the condensation factor test
described hereinbefore.
The mixed particles of Example 4 provide amalgams
; 20 having handling properties intermediate amalgams made with
the spherical particles of Example 2 and the randomly-
shaped particles of Example 1. The condensation factors
- expressed as millimeters of indentation after one minute
from completion of the amalgamation process, are 10.75,
18 and 14.S for the alloys of Examples 1, 2 and 4 respectively.
The spherical particles of Example 2 and Figure la produce
an amalgam which is soft when freshly mixed with mercury.
As previously indicated, dentists often find amalgams made
with spherical particles to be delicate to handle and
; 30 difficult to condense properly. The randomly-shaped particles
(Figure lb and Example 1) have a uni~ue morphology which
provides handling characteristics similar to those of the
''-~7
- 16 -
ms/~
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5~
flake-like particles of the prior art during the condensation
of the amalgam into a tooth cavity. The mixture of spherical
particles and randomly-shaped particles (Figure ld and
Example 4) provides intermediate handling characteristics
which will be experienced by the dentist as a moderateIy
soft amalgam which requires less pressure for proper
condensation into a cavity. The combination of Example 4
is only one possible mixture. Clearlyl mixtures of any
proportions could be made to suit the individual requirements
of the user. Another satisfactory mi~ture combines 85%
by weight of the particles of Example l with 15% by
weight of the partlcles of Example 2. The surface area
of such a mixture is about 0.22 m /gm and the size
distribution is within the range of about l micron to about
45 microns. ~n amalgam made of such a mixture will be
generally firmer than the mixture of Example 4 and its
conde~sation factor after one minute would be about 12
millimeters.
EXAMPLE 5
A dental alloy is prepared by mixing individual
metal particles to provide an overall composition of 68 wt.
% Ag, 27 wt. % Sn, 5 wt. % Cu. The mixture is melted and
cast in a bar, from which it is cut into flake-like
particles as shown in Figure lc according to the usual
technique of the prior art. The particles have a surface
area of 0.33 m2/gm and are within the size range of about
l to 50 microns. The powdered alloy is then pelleted and
mixed with sufficient mercury to form an amalgam having an
alloy to mercury ratio of 1.2 to l. The amalgam is
subjected to the condensation factor test described herein-
before.
- 17 -
mS/fy~ Jk~
E ~IPLE 6
A dental alloy is prepared by mixing 25~ by
weight of the particles of Example 1 with 60% by weight
of the particles of Example 2 and with 15% by weight of
the particles of Example 5. The mixed particles have a
surface area of about 0.24 m2/gm and are within the size
range of 1 micron to 45 microns. The powdered alloy is
then pelleted and mixed wlth sufficient mercury to form ``
an amalgam having an alloy to mercury ratio of 1 to 1.
The amalgam is subjected to the condensation factor test
described hereinbefore.
The mixed particles of Example 6 and Figure le
provide amalgams having handling properties intermediate
amalgams made with the spherical particles of Example 2
and the flake-like particles of Example 5. The condensation
factors expressed as millimeters indentation after one
minute from completion of the amalgamation process are
10.75, 18,14.0, and 13.0 for the alloys of ~xamples
1, 2, 5 and 6,respectively. The spherical particles of
~20 Example: 2 and Figure la produce an amalgam which is
soft when freshly mixed with mercury. As previously
~ .:
indicated, dentists often find amalgams made with spherical
particles to be delicate to handle and difficult to
condense properly. The randmoly-shaped particles (Figure
lb and Example 1~ have a uni~ue morphology which provides
handling characteristics similar to those of the flake-like
particles of the prior art during the condensation of the
amalgam into a tooth cavity. The mixture of spherical
particles with randomly-shaped particles and flake-like
particles (Figure le and Example 6) provide intermediate
handling characteristics which will be experienced by the
dentist as a moderately soft amalgam which requires less
18
ms 1'i
pressure for proper condensation into a cavity. The
combination of Exam~le 6 is only one possible mixture.
Clearly, other mixtures could be made to sui-t the
individual requirements of the user. Another satisfactory
mixture combines 60% by weight of the particles of Example
1 with 25% by weight of the particles of Ex~mple 2 and
15% by weight of the particles of Example 5. The surface
area of such a mixture is about 0.25 m2/gm and the size
distribution is within the range of about l micron to
about 45 microns. An amalgam made of such a mixture
- will be generally firmer than the mixture of Example 6
and its condensation factor after one minute would be
about 12.5 millimeters.
The particles of Examples l and 2, having a
relatively high copper content overall and a higher than
average silver and copper content at the surface than in
~he interior of the particles are corrosion resitant and
may be combined in any proportions found desirable to
adjust the handling characteristics of amalgams made
from alloys of the invention. The relatively high silver
and low copper content of the flake-like particles of
Exampl~ 5 are not so resistant to corrosion. Consequently,
the flake-like particles may be included in an alloy
according to the invention as desired to adjust handling
characteristics OL amalgams made therewith, but limited
to a maximum of ~5% by weight of the alloy in order to
retain the corrosion resistance of the other two particles.
The flake~like particles typically will have a composition
of about 55% to 75% bv weight silver, 20% to 40% by
weight tin, 0% to 10% by weight copper and 0% to 2% by
weight zinc.
The alloy cQmpositions of the various particles
~ j -- 1 9
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will generally be within the range of about 47% to 70%
by weight silver, 20% to 32% by weight tin, and 7% to
27~ by weight copper which corresponds to that of the
spherical particles of U.S. 3,871,876.
The carving period (typically 2 to 5 minutes
after amalgamation) represents the time period when the
dentist shapes the compacted filling tosuit the patient's
bite. After a certain period the amalgam becomes unduly
hard and can no longer be worked with the usual dental ~-
instruments. After about one hour a typical amalgam
has reached substantial strength and can withstand the
pressure of normal use. Another test may be used to
discriminate between amalgams made from alloy particles
of various shapes. Measurements of the three particles
in the preceding examples made by substituting a Gilmore
needle for the two millimeter ball as previously described,
give the followin~ results.
TABLE I
Carving Factor
Particle TypeTime, minutes - Penetration Ceased
Spherical (Ex. 2) 4.15
Microcrystalline (Ex. 1) 3.15
Flake-like (Ex. 3) 2.15
Mixed particles (Ex. 4) 3.50
Mixed particles (Fx. 6) 3.50
The above results indicate that spherical particles
can be carved with less force and for a longer time than
the randomly-shaped microcrystalline particles, which
have the same advantage over flake-like particles. The
mixture of particles, as would be expected, can be carved
with less force for a longer period than the amalgams
made with the random-shaped microcrystalline particles
ms/~
&,~6
of Example 1 but the mixture is firmer and hardens
quicker than amalgams made with spherical particles.
Mixing particles according to the invention
provides a means by which the handling characteristics of
dental amalgams may be adjusted to suit the requirements
of the individual user. At the same time, when the flake-
like particles are limited to a maximum of 25 weight
percent, the resulting mixtures are found to have
satisfactory corrosion resistance.
Alloy particles are sieved to provide a typical
particle size distribution as follows:
Microns Wt.
52-7S 0.2 to 1.4
44-52 1.4 to 12.2
38-44 1.6 ~o 8.9
30-38 20.9 to 2~.6
20-30 26.1 to 35.7
10-Z0 24.0 to 35.~i
1-10 3.6 to 7.2 ;~
The mea~ particle size is typically 20 to 26.5 microns.
~lthough some variation about the above typical size
distribution may be made to adjust the handling
characteristics, an amalgam prepared with particles ;~ -
having a significantly different size distribution from
that given above will have handling characteristics differing
from those reported herein. In general, the smaller
the average particle size, the firmer the amalgam will
be and the shorter the working time.
As previously discussed, the surface area of
the alloy particles of the invention having the size
distribution as given above will be found to have a
BET surface area between those of the component particles,
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ms/ I'~J'
p~
namely from about 0.24 m2/gm to about 0.27 m2/gm. With
other size distributions, the surface to volume ratio
may be wider.
Particles may be used directly to form amalgams,
especially if employed in pre-mi~ed dental capsules.
Often the particles are pelletized for use in dispensers
designed to provide the desired amount of mercury needed
to amalgamate with the pelleted alloy. The pelletizing
process has been found to alter the handling properties
of the re~htingamalgam, generally providing a dry and
less plastic amalgam than if the powdered alloy were used
~ directly. I~ has been found that by heat treating the
pellets in a vacuum or under an inert atmosphere (e.g.,
argon, nitrogen) for a suitable time, the mechanical
properties and useful working time of the alloy can be
returned to their original and more desirable values.
Typically a vacuum of about ten microns (0.01 mm Hg
absolute pressure) has been found to be acceptable, the
determining factor being the need to avoid oxidation of
the metals with the consequent degradation of physical
properties and corrosion resistance. The heat treatment
is carried out typically between 100 and 700F (37.8
to 370C) as requir~d until the handling characteristics
of an amalgam made from the pellets matches those of
the unpelleted powder, as measured by the condensation
and carving factors.
Generally, at least about 90% of the particles
of the alloy of the invention will fall within the size
range of from about 10 to 52 microns. Particles of a size
greater than 52 microns should comprise not more than
about 1.4% by weight of the alloy particles. With
particles larger than about 52 microns, such oversized
22 -
mS/t'` '
particles coul`d pose difficulties in filling small
apertures in a tooth. The lower limit of particIe size
is determined by the fact that with very small particle
sizes the desired effect provided by the defined specific
shape of the particles of the invention is lost. Further,
very fine particle sizes of the alloy use up a proportion-
ately greater amount of mercury in the amalgam and tend
to increase the proportion of mercury beyond the desired
limit.
Obviously, particle xange sizes expressed herein
are maximum ranges; the actual particle size range of
specific embodiments of the invention may fall within a
narrower range encompassed by the broadly stated ranges.
The foregoing discussion of the preferred
embodiments of the invention is not intended to limit ~`
the scope of the invention, which is defined by the claims
which follow.
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