Note: Descriptions are shown in the official language in which they were submitted.
lQ69347
Back~round of the Invention
.
The invention relates generally to the dental ~lloys which
are used for filling teeth from which decayed portions have been
removed. More particuiarly, the invention relates to an improved
dental alloy having both corrosion resistance and improved
handling characteristics compared to alloys of the prior art.
. ` ` ~ ... ..
`;` 10~93~7
The prior art emphasized the development of alloys which are
corrosion resistant. While typical dental alloys are principally
composed of silver and tin, they usually contain small amounts of
copper and zinc. A typical alloy of the prior art would contain
S at least 65 wt.% silver, about 1-2 wt.% zinc, and about 2-4 wt.%
copper, with the remainder being tin. Such alloys are not
completely resistant to corrosion. It has been found that
increasing the copper content of such alloys provides increased
strength and also avoids the formation of what is known in the
art as the gamma-two phase, a tin and mer~ury phase which has low
resistance to corrosion and thus may lead to early deterioration
of fillings. Typical of such high copper alloys are those
disclosed in United States Patent 3,871,876 and United States
Patent 3,997,328. Such dental alloy compositions increase the
copper content from the typical 2-4 wt.% to the range of 8-27 wt.
in the first-mentioned patent and in the latter patent, from
20-40 wt.%.
While such alloys have improved corrosion resistance,
another important characteristic of dental alloys has been
neglected heretofore. The success of a dentist in filling a
dental cavity is related to the handling characteristics of the
alloy after it is amalgamated with mercury. For example, the
high copper alloy disclosed in U.S. 3,871,876, is typically
produced by air atomization from the molten state which results
in a spherical or spheroidal form for the finished alloy. It is
characteristic of alloys having a spherical shape that they feel
relatively soft to the dentist and appear to require delicate
handling. They are sometimes difficult to pack into a dental
,, ; , ~ :
347
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 a small bearing area. Consequently, many dentists 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 conven-
tional dental alloys is disclosed and claimed in United States
Patent 3,997,327. In that invention a major portion of spherical
particles is combined with a minor portion of microcut irregular
particles, or flakes. Typical dental alloys in the prior art
generally have been of the flake type, which inherently requires
a higher pressure in order to pack it into a dental cavity than
is characteristic of the spherical particles. By combining
spherical particles with flake particles having the same
composition, it is possible to improve the handling characteris-
tics of the resulting mixture. Such a combination, having a
conventionally low copper content, has less resistance to
corrosion than the higher copper content alloys previously dis-
cussed.
The present invention has as its objective providing improve
handling characteristics to corrosion-resistant dental alloys.
Summary_of the Invention
The dental alloy of the invention combines corrosion resis-
tance and good handling qualities. It is corrosion resistant in
that it has a relatively high copper content. Its composition
``
I ` 106934~7
corresponds generally to that of the spherical material disclosed
in United Sta~es Patent 3,871,876 being within the range of about
47~ to 70~ by weight silver, 20~ to 32~ by weight tin, and 7% to
27% by weight copper. As is true of the '876 patent, the
particles have a higher than average silver and copper content at
the surface of the particles.
In the present invention the alloy is produced in an
irregular shape rather than the spherical form typical of the
'871 patent, but differing from the irregular flake-like particle
typical of the prior art. The alloy particles according to the
present invention characteristically have a surface area of about
0.23-0.26 m2/gm, which is 20-30% greater than the typical
spherical particle and about 20-30% less than typical flake-like
particles. They may be produced by a variant of the air atomi-
zation process used to form spherical particles, although othertechniques may be used.
When amalgamated with mercury, the alloy particles of the
invention have handling qualities similar to those of flake
particles as shown by empirical tests as described hereinafter,
which have been found to relate to subjective experience with the
condensation and carving characteristics of amalgams.
In another aspect of the invention, the corrosion-resistant
alloy particles having the unique shape of the invention may be
combined with particles of conventional shapes, either spherical
and/or flake-like forms. One type of particle may be the
spherical material disclosed in United States Patent 3,871,876,
having a composition within the range of about 47% to 70% by
weight silver, 20~ to 32% by weight tin, and 7~ to 27% by weight
copper. Another useful type of particle has the relatively low
10~934'~
copper and high silver content typical of the prior art and has a
flake-like shape. In a preferred embodiment, such particies will
have a composition of about 55% to 75% by weight silver, 20% to
40~ by weight tin, 0% to 10% by weight copper and 0% to 2~ by
weight 2inc. By combining suitable proportions of spherical
particles, randomly-shaped microcrystalline particles of this
invention and flake-like particles, the handling characteristics
of amalgams prepared from such mixtures can be adjusted to suit
the requirements of the individual user. Although any proportions
may be used of the particles which have a substantial copper
content, the particles having a relatively low copper content are
limited to a maximum of 25% by weight of the alloy mixture in
order to retain the corrosion resistance provided by the particles
having higher copper content.
In another aspect of the invention the dental alloy is
prepared by the steps of formulating a metal composition as given
above, melting said composition, microcasting particles according
to the invention, pelletizing said particles, and heat treating
said pellets to adjust the handling characteristics of amalgams
made with said pellets to correspond with characteristics of
amalgams made directly with said particles. When particles of
conventional spherical or flake-like shapes are added, they will
be mixed with the irregular particles of the invention prior to
the pelletizing step.
Brief Descri~tion of the Drawinys
Figure la shows the spherical particles of the prior art
corresponding to United States 3,871,876.
Figure lb shows particles corresponding to a dental alloy of
the present invention.
~ 10~i9347
Figure lc shows particles corresponding to microcut or
flake-like particles of the prior art.
Figuxe 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.
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
particular 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 to 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 cavity. In such situations, a nonuniform 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 characteric _
tic of an amalgam is its ability to be pressed into a dental
cavity to fill all the smàll openings under the desired conden-
sation pressure, while not being so soft that the dentist cannot
adequately compact the amalgam. This condens~ation pressure may be
approximated by an empirical test which will be hereinafter des-
cribed and which is useful in connection wit~ the present inven-
tion.
- 6 -
i - , - .
10~ '7
The second h~ndllng chLIrllcterL:~lc oE lmport.lnce ~o ~he
dentist is the ab:Llity of an amal~am to bc c~rvcd or shaped in
order to finish the exterior surface of the compacted filling.
An amalgam also must be of a desired plasticity in order to be
satisfactorily carved or shaped. An amalgam may be satisfact-
orily packed into a dental 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 f~om various
dental alloys.
As described in U.S. Patent 3,253 9 783 and elsewhere, the gas
atomization technique may be used to product 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 particles such as are
illustrated in Figure la have an average surface to volume
ratio of about 0.21 m2/gm as measured by the usual BET apparatus,
which is a well-known conventional apparatus for the measurement
of surface by the Brunover, Emmett and Teller method. Flake
alloys of approximately the same size as illustrated in Figure
lc are substantially different, having a surface to volume ratio
of about 0.33 m /gm. A mixture of spherical particles with
flake particles as disclosed in U.S. Patent 3,997,~27 will
have a ratio between the two extremes. Rather than mixing
spherical and flake particles9 the alloy of the present
invention is preferably produced in a single step process to
provide a new particle shape.
jl/ -7-
10693~7
The air atomization technique or other microcasting method may be
altered to cause distortion of the particles, which otherwise
freeze in a spherical or spheroidal shape. A suitable morphology
is illustrated in Figure lb. The spherical form of Figure la is
no longer predominant. Neither do the particles have the
distinctive shape of microcut, flake-liXe particles, as seen in
Figure lc, nor do they have the striations characteristic of such
particles.
The alloy particles according to the invention need not be
exactly the same as those of Figure lb. Rather, the particles of
the invention may be characterized by their surface area and the
handling characteristics measured as herein~efore described.
Typically, particles of the invention will have a surface area
within the range of 0.22 to 0.31 m2/gm and preferably in the range
of 0.23 to 0.26 m /gm. Specifically, the particles of Figure 2
have a median surface area of about 0.24 m /gm. It should be
noted that the surface area is related in part to the particle
size, thus the values give~ herein relate to a particle size
distribution suitable for dental alloys and as specifically
reported hereinafter 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 this additional surface seems less likely to
have a large effect on the handling properties of amalgams than
the geometric surface, ~he geometric surface of the particles
10~i93~7
, should be compared rather than the BET surface. However, the
l geometric surface has not been measured although it may be
¦ approximated by subtracting about 90~ of the BET value for
l comparison purposes.
5 ¦ Amalgams are produced by mixing mercury with dental alloys
of the invention. At the completion 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 i5 quite important to the dentist
as has been previously discussed and to characterize dental
alloys of the invention we have chosen to designate the resistanc~
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 conden-
sation factors may be described as follows. A pellet of dental
alloy is mixed with the recommended amount of mercury in an
amalgamator for the manufacturer's recommended time. A com-
A mercially a~ailable Wig-L-Bug Model 5AR manufactured by Crescent
Corporation was used in the tests reported herein, although other
amalgamators would be acceptable. After the amalgamation is
complete, the amalgam is immediately placed on a flat glass
plate and covered b~ another such glass plate and pressed to a
one millimeter thickness, as determined by one millimeter spacers
101~9~7
placed between the plates. The top plate is removed and measure-
ments are made of the resistance of the flattened amalgam disc
during the hardening period. For the measurements reported hereir
an Instron testing unit model 1101 produced by Instron Corporatio
was employed. A constant load of five 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 for
fifteen seconds is used as a measure of the resistance of the
amalgam. Tes~s 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 car~e 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 example~ below.
The carvability 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 previously 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 millimeter poi~t. 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
Restorative Dental Materials, 4th ed., 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.
- 10-
:: . . . .
lOti9347
EXAMPLE 1
A dental alloy is prepared by mixing individual metal powderc
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 formation of
spherical particles by contacting the molten droplets during the
cooling process, thereby producing the randomly-shaped micro-
crystalline particles of the invention. The particles formed hav~
a surface area of 0.24 m2/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 suf~
ficient mercury to form an amalgam having an alloy to mercury
ratio of 1:1. The amalgann is measured for its resistance to
condensation pressure according to the test hereinbefore describe
and the results plotted on Figure 2.
EXAMPLE 2
A dental alloy is prepared by mixiny individual metal powder
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 according to U.S. 3,871,~76 to produce
spherical particles and shown in Figure la. The particles have a
surface area of 0.21 m2/gm. After sieving the particles are
within the range of 1 micron to 40 micron. The powdered alloy is
then pelleted and mixed with sufficient mercury to form an amalgan
having an alloy to mercury ratio of 1:1. The amalgam is subjectec
to the condensation factor test described hereinbefore and the
results plotted on Figure 2.
~ 10ti9347
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, 0.6 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 particles have a surface
area of 0.33 m2/gm. After sieving the particles are within the
range of 2 microns 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 1.2:1. The amalgam is sub-
jected to the condensation factor test described hereinbefore
and the results plotted on Figure 2.
As shown in Figure 2 flake-like alloys of the prior art
(Figure lc and Example 3) are firmer when freshly amalgamated thar
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 milli-
meters indentation after one minute from completion of the
amalgamation process are 10.75, 18, and 10.3 for the alloys of
Examples 1-3 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 difficult to condense properly. The alloy of the
invention (Figure lb and Example 1) has a unique morphology and ic
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.
` 101~9347
The carving period (typically 2 to 5 minutes after amalga-
mation) represents the time period when the dentist shapes the
compacted filling to suit 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 with-
stand the pressure of normal use. As is indicated by Figure 2,
the effect of particle shape on the handling characteristics of
amalgams is more significant during the condensation period than
during the carving period. In fact, one might conclude from
Figure 2 that amalgams made according to the invention would be
more difficult to carve than those made with either spherical or
flake-like particles. However, measurements of the three
particles in the preceding examples were made by substituting a
Gilmore needle for the two millimeter ball as previously
described, with the following results.
TABLE I
Carvino Factor
Particle Type Time, minutes - Penetration Ceasec
.
Spherical (Ex. 2) 4.15
Microcrystalline (Ex. 1) 3.15
Flake (Ex. 3) 2.15
The above results indicate that spherical particles can be carved
with less force and for a longer time than the randomly-shaped
microcrystalline particles of the invention, which in turn can be
carved with less force than the flake-like particles.
10693~7
As previously discussed, the alloy of the invention may ~e
produced by modification of the air atomization process so that
molten metal is distorted instead of frozen into spherical form.
Such particles may be produced by other processes, for example,
by splat~cooling of a stream of molten alloy and by modifying the
convPntional metallizing process. However produced, the particle~
will have a surface area intermediate that of spheres and that of
flakes in the preferred form characterized by having a surface
area of 0.23-0.26 m2/gm, which is 20-30~ greater than the typical
spherical particle and 20-30% less than the typical flake-like
particles.
The composition will 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 particle
of U.S. 3,871,876. It has been found that the alloy in the
unique form of the invention still has corrosion resistance as
measured by the anodic polarization test described in U.S. Patent
3,997,329, even though the particles are no longer spherical in
form. The anodic polarization test indicates by the absence of
the gamma-two phase that the amalgam is resistant to corrosive
attack. It is believed that the higher than average silver and
copper content found at the surface of the irregular particles of
the invention as well as in the spherical particles of U.S.
3,871,876 is related to the relatively high copper content of
the alloy and the speed at which it is cooled from the molten
state. It is expected that many methods of forming particles
from molten metal which involve rapid cooling can be employed.
- 14 -
10~à93~ ~
Although no explanation is presently available, it has been
found that if the alloy of the invention is prepared as a mixture
of about 60~ by weight spheres and about 40% by weight flakes
having the same composition, the handling properties are similar
to that of particles of the invention, but the amalgam is no
longer corrosion resistant by the anodic polarization test.
However, with their unique morphology the particles of the
invention unexpectedly combine both corrosion resistance and
improved handling characteristics.
EXAMPLE 4
A dental alloy is prepared by mixing 40% by weight of the
randomly-shaped microcrystalline particles of Example 1 with 60%
by weight of the spherical particles of Example 2. The mixed
particles ha~e 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 amalgan
having an alloy to mercury ratio oE 1:1. The amalgam is sub-
jected to the condensation factor test described hereinbefore.
The mixed particles of Example 4 provide amalgams having
handling properties intermediate amalgams made with the spherical
particles of Example 2 and the randomly-shaped particles of Exam-
ple 1. The condensation factors expressed as millimeters indenta-
tion after one minute from completion of the amalgamation process
are 10.75, 18, and 14.5 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.
10~9347
As previously indicated, dentists often find amalgams made with
spherical particles to be delicate to handle and difficult to
condense properly. The randomly-shaped particles (Figure lb and
Example 1) have a unique 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 and randomly-shaped
particles (Figure ld) provides intermediate handling characteris-
tics which will be experienced by the dentist as a moderately
soft amalgam which requires less pressure for proper condensation
into a cavity. The combination of Example 4 is only one possible
mixture. Clearly, mixtures of any prGportions could be made to
suit the individual requirements of the user. Another satis-
factory mixture combines 85% by weight of the particles of
Example 1 with 15~ of the particles of Example 2. The surface
~ area of such a mixture is about 0.22 m2/gm and the size distri-
c bution is within the range of about 1 micron to about 45 microns.
' An amalgam made of such a mixture will be generally firmer than
the mixture of Example 4 and its condensation factor after one
minute would be about 12 millimeters.
Measurements of the individual types of particles with the
mixture of Example 4 were made by substituting a Gilmore needle
for the two millimeter ball as previously described, with the
~ following results.
;~ 25 TABLE II
Carvinq Factor
Particle Type Time, minutes - Penetration Ceased
Spherical (Ex. 2) 4.15
Microcrystalline (Ex. 1) 3.15
Mixed spherical and 3.50
microcrystalline (Ex. 4)
10~i93~7
The above results indicate that spherical particles can be carved
with less force and for a longer time than the randomly-shaped
microcrystalline particles. The mixture, as would be expected,
can be carved with less force for a longer period than the
amalgams made with the randomly-shaped particles of Example 1
but the mixture is firmer and hardens quicker than amalgams made
with spherical particles.
Mixing particles 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
only spherical and irregularly-shaped particles are used, both
the component particles are corrosion resistant and the resulting
mixtures preserve the corrosion resistance of the components.
For this reason no composition limits need be set on mixtures of
these particles, which may be varied to meet the handling
characteristics of the intended user, and thus could approach the
softness characteristic of amalgams made solely of spherical
particles or the firmness characteristic of randomly-shaped
micrycrystalline particles.
- 20 ~XAMPLE 5
A dental alloy is prepared by mixing 25% by weight of the
irregularly-shaped microcrystalline particles of Example 1 with
60% by weight of the spherical particles of Example 2 and with
15~ by weight of the flake-like particles of Example 3. 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 with sufficient mercury to form
an amalgam having an alloy to mercury ratio of 1:1. The amalgam
is subjected to the condensation factor test described herein-
before.
llD69347
The mixed particles of Example 5 and Fig~re le provideamalgams having handling properties intermediate amalgams made
with the spherical particles of Example 2 and the flake-like
particles of Example 3. 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 Examples 1, 2, 3, and 5 respectively. The mixture of spherica
particles with randomly-shaped particles and flake-like particles
(Figure le) provides intermediate handling characteristics which
will be experienced by the dentist as a moderately soft amalgam
which requires less pressure for proper condensation into a
cavity~ The combination of Example 5 is only one possible
mixture. Clearly, other mixtures could be made to suit 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 Example 2 and 15% by weight of
the particles of Example 3. The surface area of such a mixture
is about 0.25 m2/gm and the size distribution is within the range
of about 1 micron to about 45 microns. An amalgam made of such a
mixture will be generally firmer than the mixture of Exampl~ 4
and its condensation factor after one minute would be about 12.5
millimeters.
The particles of Examples 1 and 2, having a relatively high
copper content overall and a higher than average silver and
copper content at the surface are corrosion resistant 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 Example 3 are not so resistant to
corrosion.
106~3~7
Consequently, the flake-like particles may be included in an
alloy according to the invention as desired to adjust handling
characteristics of amalgams made therewith, but limited to a
maximum of 25% 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%
by weight silver, 20% to 40% by weight tin, 0% to 10% by weight .-
copper, and 0% to 2% by weight zinc.
Measurements of the three particles in the preceding example
made by substituting a Gilmore needle for the two millimeter ball
as previously described, give the following results.
,
TABLE III
Carving Factor
Particle TypeTime, minutes - Penetration Ceasec
Spherical (Ex. 2) 4.15
Microcrystalline tEx. 1) 3.15
Flake-like (Ex. 3) 2.15
Mixed particles (Ex. 5) 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 flake-like particles of the alloy of
the invention (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 timer when the flake~ e particles are limited to a maximum
of 25 weight percent, the resulting mixtures are found to have
satisfactory corrosion resistance.
~1 -19- ~
lC~3~7
After the particles have been produced, they are sieved to
provide a typical particle size distribution as follows:
Microns Wt.%
52 0.3 to 1.4
44-52 1.4 to 12.2
` 38-44 1.6 to 8.9
30-38 20.9 to 24.6
20-30 26.1 to 35.7
10-20 24.0 ~o 35.4
~ 10 10 3.6 to 7.2f
The mean particle size is typically 20 to 26.5 microns. Although
some variation about the above typical size distribution may be
; made to adjust the handling characteristics, an amalgam prepared
with particles having a siqnificantly different size distribution
from that given above will have handling characteristics differin
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 surface to volume ratio of about
0.23-0.26 m2/gm. With other size distributions, the surface to
volume ratio may be as wide as 0.22 to 0.32 m2/gm.
Particles may be us~d directly to form amalgams, especially
if employed in pre-mixed 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 resulting amalgam, generally providing a dry
and less plastic amalgam than if the powdered alloy were used
.~: -
- ~
~0~;3~7
directly. It has been found that by heat treating the pellets in
a vacuum 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 required 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.
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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