CA1154616A - Corrosion-resistant dental alloy having improved handling characteristics - Google Patents
Corrosion-resistant dental alloy having improved handling characteristicsInfo
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- CA1154616A CA1154616A CA000343195A CA343195A CA1154616A CA 1154616 A CA1154616 A CA 1154616A CA 000343195 A CA000343195 A CA 000343195A CA 343195 A CA343195 A CA 343195A CA 1154616 A CA1154616 A CA 1154616A
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Abstract
Abstract of the Invention A corrosion-resistant dental alloy is disclosed which has improved handling characteristics during the filling of a dental cavity. The alloy is substantially uniform blend of three types of particles having the same chemical components, but differing in morphology and, optionally, in proportions of components. One type of particle is spherical or spheroidal in form. The second type of particle is a randomly-shaped microcrystalline form. The third type of particle is a flake-like particle. handling character-istics of an amalgam prepared from such three particles can be adjusted to suit the requirements of the user by varying the relative proportions of the three types of particles while still retaining the corrosion resistance of the particles.
Description
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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 jl/ -4-~5~ 6 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 ;
jl/ -5-.; , . : .
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 jl/ -6-. .
,, L~L6:~,.6 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 jl/ -7-$.~
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 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 ~ .
ms/l~ j'J j .
:
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 ) ~,~ !."
L~
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.
, .
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.
` 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 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. --.. , :
: ' ' , 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 .~
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.
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 ms/~
.
.
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.
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.
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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 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 .~ i ms/~ ;J?
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, 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 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.
ms/~ -
.
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 jl/ -4-~5~ 6 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 ;
jl/ -5-.; , . : .
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 jl/ -6-. .
,, L~L6:~,.6 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 jl/ -7-$.~
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 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 ~ .
ms/l~ j'J j .
:
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 ) ~,~ !."
L~
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.
, .
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.
` 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 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. --.. , :
: ' ' , 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 .~
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.
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 ms/~
.
.
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.
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.
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 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 .~ i ms/~ ;J?
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, 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 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.
ms/~ -
Claims (38)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. 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 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-like 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 m2/gm, 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.
2. The corrosion-resistant dental alloy of Claim 1 wherein said particles have a surface area ranging between about 0.23 m2/gm and 0.26 m2/gm.
3. The corrosion-resistant dental alloy of Claim l in combination with mercury to provide a dental amalgam.
4. The corrosion-resistant dental alloy of Claim l in combination with about 1 part by weight of mercury for each part by weight of said corrosion-resistant dental alloy.
5. The corrosion-resistant dental alloy of Claim l wherein said mixture of silver, tin and copper comprises from about 47 to 70 percent by weight of silver, from about 20 to 32 percent by weight of tin, and from about 7 to 27 percent by weight of copper.
6. The corrosion-resistant dental alloy of Claim 1 wherein at least about 90% by weight of said particles fall within a particle size range of from about 10 to 52 microns.
7. 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 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.
8. The corrosion-resistant dental alloy of Claim 7 wherein said mixture of silver, tin and copper comprises from about 47 to 70 percent by weight of silver, from about 20 to 32 percent by weight of tin and from about 7 to 27 percent by weight of copper.
9. The corrosion-resistant dental alloy of Claim 7 wherein said alloy particles have a surface area of less than about 0.31 m2/gm.
10. The corrosion-resistant dental alloy of Claim 7 wherein said alloy particles have a mean particle size of between about 20 and 26.5 microns, and further wherein said particles have a particle size distribution such that substantially all of said particles fall within a particle size range of from about 1 to 72 microns.
11. The corrosion-resistant dental alloy of Claim 10 wherein at least about 90% by weight of said particles fall within a particle size range of from about 10 to 52 microns.
12. 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.
(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.
13. The method of Claim 12 including pelletizing said particles, and heat treating said pellets for a period of time such that the handling characteristics of amalgams produced with said pellets are substantially identical to the handling characteristics of amalgams produced from said particles.
14. The method of Claim 13 wherein said heat treating is conducted at a temperature of between about 100°F (37.8°C) and 700°F (371°C).
15. The method of Claim 14 wherein said heat treating is carried out under vacuum.
16. The method of Claim 13 wherein at least about 90%
by weight of said particles fall within a particle size range of about 10 to 52 microns.
by weight of said particles fall within a particle size range of about 10 to 52 microns.
17. The corrosion-resistant dental alloy of Claim 7 wherein said alloy particles have a surface area which is about 20-30 percent greater than the surface area of spherical particles but less than about 0.33 m2/gm.
18. 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 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 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 further 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.
19. The corrosion-resistant dental alloy mixture of Claim 18 comprising from about 15 to 60 percent by weight of said first alloy and from about 40 to 85 percent by weight of said second alloy.
20. A dental amalgam comprising a combination of the corrosion-resistant dental alloy mixture of Claim 18 with mercury.
21. The dental amalgam of Claim 20 comprising from about 0.8 to 1 parts by weight of mercury for each part by weight of said corrosion-resistant dental alloy mixture.
22. The corrosion-ressitant dental alloy of Claim 18 wherein each of said first and second dental alloys comprises a mixture of silver, tin and copper including from about 47 to 70 percent by weight of silver, from about 20 to 32 percent by weight of tin, and from about 7 to 27 percent by weight of copper.
23. The corrosion-resistant dental alloy of Claim 18 wherein at least about 90% by weight of said particles fall within a particle size range of from about 10 to 52 microns.
24. 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 particles of a first dental alloy and a second dental alloy, both of said alloys comprising a mixture of silver, tin and copper, said 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.
25. The corrosion-resistant dental alloy mixture of Claim 24 wherein both of said first and second alloys include from about 47 to 70 percent by weight of silver, from about 20 to 32 percent by weight of tin, and from about 7 to 27 percent by weight of copper.
26. The corrosion-resistant dental alloy mixture of Claim 24 including from about 15 to 60 percent by weight of said first alloy and from about 40 to 85 percent by weight of said second alloy.
27. The dental alloy mixture of Claim 24 wherein said second alloy has a surface area of less than about 0.26 m2/gm.
28. The corrosion-resistant dental alloy of Claim 24 wherein at least about 90% by weight of said particles fall within a particle size range of about 10 to 52 microns.
29. The corrosion-resistant dental alloy mixture of Claim 24 wherein said alloy particles of said second alloy have a surface area at least about 30 percent greater than the surface area of spherical particles.
30. The corrosion-resistant dental alloy of Claim 24 wherein said alloy particles of said second alloy have a surface area which is about 20-30 percent greater than the surface area of spherical particles but less than about 0.33 m2/gm.
31. 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 distribution such that substantially all of said particles fall within a particle size range of from about 1 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 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 of about 0.33 m2/gm, said third alloy comprising not more than about 25% by weight of the dental alloy mixture.
32. The corrosion-resistant dental alloy mixture of Claim 31 wherein said first and said second alloys comprise from about 47 to 70 percent by weight of silver, from about 20 to 32 percent by weight of tin, and from about 7 to 27 percent by weight of copper.
33. The corrosion-resistant dental alloy mixture of Claim 31 wherein said third alloy comprises from about 55 to 75 percent by weight of silver, from about 20 to 40 percent by weight of tin, and up to about 10 percent by weight of copper.
34. The corrosion-resistant dental alloy mixture of Claim 31 comprising from about 25 to 60 percent by weight of said first alloy, from about 25 to 60 percent by weight of said second alloy, and from about 15 to 25 percent by weight of said third alloy.
35. A dental amalgam comprising a combination of the corrosion-resistant dental alloy mixture of Claim 31 with mercury.
36. The dental amalgam of Claim 35 comprising about 1 part by weight of mercury from each part by weight of said corrosion-resistant dental alloy mixture.
37. The corrosion-resistant dental alloy of Claim 31 wherein at least about 90% by weight of said particles fall within a particle size range of from about 10 to 52 microns.
38. The corrosion-resistant alloy of Claim 31 including about 25 to 60 percent by weight of said second alloy.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000343195A CA1154616A (en) | 1980-01-07 | 1980-01-07 | Corrosion-resistant dental alloy having improved handling characteristics |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000343195A CA1154616A (en) | 1980-01-07 | 1980-01-07 | Corrosion-resistant dental alloy having improved handling characteristics |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1154616A true CA1154616A (en) | 1983-10-04 |
Family
ID=4115993
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000343195A Expired CA1154616A (en) | 1980-01-07 | 1980-01-07 | Corrosion-resistant dental alloy having improved handling characteristics |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1154616A (en) |
-
1980
- 1980-01-07 CA CA000343195A patent/CA1154616A/en not_active Expired
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