Dental Amalgam
United States Patent 3871876
An amalgamable silver-base metal powder containing about 8% to about 27% copper, 20% to about 32% tin, and the balance consisting essentially of silver, which upon admixture with about 42% to 54% mercury forms a dental amalgam of improved strength and corrosion resistance.
US Patent References:
Dental alloy and process of making the same
Vogt et al. - December 1926 - 1612782
Comminuted alloy
Gray - June 1934 - 1963085
Dental amalgam alloy
Poetschke - May 1942 - 2281991
Dental amalgam
Youdelis - February 1967 - 3305356
/3554738.html
Beldham et al. - January 1971 - 3554738
Dental alloy and process of making the same
Vogt et al. - December 1926 - 1612782
Comminuted alloy
Gray - June 1934 - 1963085
Dental amalgam alloy
Poetschke - May 1942 - 2281991
Dental amalgam
Youdelis - February 1967 - 3305356
/3554738.html
Beldham et al. - January 1971 - 3554738
Inventors:
Asgar, Kamal (Ann Arbor, MI)
Reichman, Steven H. (Ann Arbor, MI)
Reichman, Steven H. (Ann Arbor, MI)
Application Number:
05/451840
Publication Date:
03/18/1975
Filing Date:
03/18/1974
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
75/955, 148/400, 75/338, 148/442
International Classes:
A61K6/05; C22C1/04; C22C7/00; A61K6/02; C22C7/00; C22C5/00
Field of Search:
75/173C,169,134B,134C,.5R
US Patent References:
| 3676112 | July 1972 | Muhler |
Primary Examiner:
Rutledge, Dewayne L.
Assistant Examiner:
Weise E. L.
Attorney, Agent or Firm:
Harness, Dickey & Pierce
Claims:
What is claimed is
1. An amalgamable silver alloy powder in which each of the particles are of substantially the same composition and consist essentially of silver, tin and copper; said silver and tin being present at a ratio of from about 1.8:1 to about 3.0:1 and said tin and copper being present at a ratio of about 1:1 up to 3:1, said powder further characterized as being of an average particle size less than about 100 microns and wherein each particle is of a gradient composition on moving from the surface toward the core thereof.
2. The amalgamable silver alloy powder as defined in claim 1 in which said ratio of silver-to-tin ranges from about 2.2 to about 2.6.
3. The amalgamable silver alloy powder as defined in claim 1 in which said ratio of tin-to-copper ranges from about 1.5 to about 2.5.
4. The amalgamable silver alloy powder as defined in claim 1 in which the average particle size thereof ranges from about 45 microns to about 5 microns.
5. A dental amalgam consisting essentially of a continuous matrix composed of an amalgam having interspersed therethrough a plurality of discrete phases chemically bonded by said continuous phase into an integral matrix, said discrete phases comprising particles of an alloy consisting essentially of silver, tin and copper and wherein the particles are of a gradient composition on moving from the surfaces thereof toward their center, said continuous phase comprising intermetallic compounds of mercury with silver, tin and copper; said dental amalgam containing from about 42% to about 54% mercury, and said silver, tin and copper being present in amounts so as to provide a ratio of silver-to-tin of from about 1.8 up to about 3.0 and tin-to-copper of from about 1:1 up to 3:1.
6. The dental amalgam as defined in claim 5, wherein the concentration of mercury ranges from about 46% to about 50%.
7. The dental amalgam as defined in claim 5 in which said ratio of silver-to-tin ranges from about 2.2 to about 2.6 while the ratio of tin-to-copper ranges from about 1.5 to about 2.5.
8. The amalgamable silver alloy powder as defined in claim 1, further including up to 2% zinc.
9. The dental amalgam as defined in claim 5, further including up to about 2% zinc.
1. An amalgamable silver alloy powder in which each of the particles are of substantially the same composition and consist essentially of silver, tin and copper; said silver and tin being present at a ratio of from about 1.8:1 to about 3.0:1 and said tin and copper being present at a ratio of about 1:1 up to 3:1, said powder further characterized as being of an average particle size less than about 100 microns and wherein each particle is of a gradient composition on moving from the surface toward the core thereof.
2. The amalgamable silver alloy powder as defined in claim 1 in which said ratio of silver-to-tin ranges from about 2.2 to about 2.6.
3. The amalgamable silver alloy powder as defined in claim 1 in which said ratio of tin-to-copper ranges from about 1.5 to about 2.5.
4. The amalgamable silver alloy powder as defined in claim 1 in which the average particle size thereof ranges from about 45 microns to about 5 microns.
5. A dental amalgam consisting essentially of a continuous matrix composed of an amalgam having interspersed therethrough a plurality of discrete phases chemically bonded by said continuous phase into an integral matrix, said discrete phases comprising particles of an alloy consisting essentially of silver, tin and copper and wherein the particles are of a gradient composition on moving from the surfaces thereof toward their center, said continuous phase comprising intermetallic compounds of mercury with silver, tin and copper; said dental amalgam containing from about 42% to about 54% mercury, and said silver, tin and copper being present in amounts so as to provide a ratio of silver-to-tin of from about 1.8 up to about 3.0 and tin-to-copper of from about 1:1 up to 3:1.
6. The dental amalgam as defined in claim 5, wherein the concentration of mercury ranges from about 46% to about 50%.
7. The dental amalgam as defined in claim 5 in which said ratio of silver-to-tin ranges from about 2.2 to about 2.6 while the ratio of tin-to-copper ranges from about 1.5 to about 2.5.
8. The amalgamable silver alloy powder as defined in claim 1, further including up to 2% zinc.
9. The dental amalgam as defined in claim 5, further including up to about 2% zinc.
Description:
BACKGROUND OF THE INVENTION
Conventionally, silver-base alloys adapted for use as dental amalgams contain a minimum of about 65% silver, a maximum of about 6% copper, a maximum of about 2% zinc and a minimum of about 25% tin. The admixture of such silver-base alloys with from about 45% to about 55% mercury by the dentist as needed are adapted to harden within a matter of a few minutes, enabling shaping by carving for a period of up to about 15 minutes and the amalgamation reaction is usually complete after about 24 hours. Typically, silver alloys suitable for forming amalgams in accordance with American Dental Association specifications contain from about 26% to about 28% tin, about 1% to about 2% zinc, about 2% to about 4% copper, with the balance essentially all silver. Of the foregoing alloying constituents present, it has been generally accepted that the quantity of copper should be restricted to a maximum of about 6% due to the tendency of dental amalgams incorporating greater quantities of copper to corrode and/or discolor in actual use. On the other hand, quantities of copper in excess of about 6% are known to improve the compressive strength of the resultant dental amalgam.
The general requirements of a dental amalgam include a retention of shape, color and appearance; a biological compatibility, a restoration of the tooth to its original function, and a long durable operating life. Extensive investigation and test work of dental restorative materials including dental amalgams has revealed that creep resistance is perhaps the single most important mechanical property followed by compressive strength as it relates to failure or fracture of dental restorations. These tests have convincingly established a relationship between a high incidence of marginal fracture of dental restorations with high dynamic creep values. Attempts to improve the marginal fracture characteristics and other mechanical properties of dental amalgams by increasing the quantity of copper in the alloy have been unsuccessful due to the increased corrosivity and tendency of discoloration of the resultant amalgams.
The present invention provides for an amalgamable silver alloy containing amounts of copper substantially greater than those heretofore considered useable which improves not only the resistance of the dental amalgam toward corrosion and/or discoloration, but also provides for substantially superior mechanical properties, including increased resistance to dynamic creep.
SUMMARY OF THE INVENTION
The benefits and advantages of the present invention are achieved by an amalgamable silver alloy powder derived from a microcasting of a molten mass of the alloy, such as by fluid atomization techniques or the like, and wherein each particle is of substantially the same composition and is further characterized by having a gradient composition in which the concentration of the individual alloying constituents changes on moving in a direction from the surface of the particle toward the center thereof. The amalgamable powder contains tin and copper in combination with silver as the essential alloying constituent and wherein the silver-to-tin ratio is controlled from about 1.8:1 to about 3.0:1, while the tin-to-copper ratio is controlled from about 1:1 to about 3:1. On a weight percent basis of the individual alloying constituents, the amalgamable silver alloy powder contains from about 8% to about 27% copper, from about 20% to about 32% tin, with the balance consisting essentially of silver but, in any event, a minimum of 47% silver. The alloy powder can also contain up to about 2% zinc without any adverse effects. The powder particles are of an average size less than about 100 microns and preferably range from about 5 microns to about 44 microns.
The dental amalgam is prepared employing the amalgamable silver alloy powder by admixture with mercury in an amount of from about 42% up to about 54% and preferably in an amount such that mercury in the final product is present from about 46% to about 50% by weight.
Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments, the specific examples provided, taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
The drawing graphically portrays the range of composition gradients of the amalgamable silver alloy particles between the surface and interior of the particle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The proportions of the individual alloying constituents comprising the amalgamable silver alloy powder and the resultant dental amalgam prepared therefrom are herein defined in terms of percentages by weight and on a weight ratio basis unless clearly indicated to the contrary.
The permissible as well as the preferred ratios and corresponding percentages of the three essential alloying constituents are set forth in Table 1.
TABLE 1 ______________________________________ Metal Powder Composition Permissible Range Preferred Range ______________________________________ Ag/Sn Ratio 1.8 - 3.0 2.2 - 2.6 Sn/Cu Ration 1.0 - 3.0 1.5 - 2.5 Cu 8% - 27% 10% - 18% Sn 20% - 32% 23% - 28% Ag 47% - 70% 57% - 65% ______________________________________
As shown in Table 1, the ratio of silver-to-tin is broadly controlled within the range of about 1.8 up to about 3.0, while the permissible ratios of tin-to-copper are from about 1 to about 3. Expressed in terms of percentages of the three alloying constituents, based on the foregoing weight ratios, the minimum percentages are specified with the recognition that when any one of the three is at the minimum level, one or both of the remaining alloys are present in amounts greater than their respective minimums in order that the total percent is 100. The foregoing is also applicable to the preferred percentages of these three alloying constituents as set forth in Table 1. For the purposes of convenience, the relative proportions of the three metals are set forth in terms of their weight ratios in which the silver-to-tin ratio preferably is controlled within a range of about 2.2 to about 2.6, while the tin-to-copper ratio is preferably controlled within a range of about 1.5 to about 2.5. It is also contemplated that zinc can be included in amounts up to about 2% by weight of the total alloy.
It is an important feature of the present invention that the amalgamable silver alloy powder is derived by a microcasting technique in which molten fine-sized droplets of the desired composition are cooled in a manner to effect a progressive solidification during the cooling commencing on the surface and moving inwardly thereof. The differential cooling and solidification rate of each droplet contributes toward a stratification or composition gradient in the final solidifed particle which is at least in part responsible for the unexpected improved mechanical and chemical properties of dental amalgams prepared therefrom.
The microcasting of a molten mass of the silver alloy can be achieved by a variety of techniques including gas atomization, airless spraying and centrifugal fragmentation for effecting a subdivision of the molten mass into a plurality of fine-sized liquid droplets which are all of substantially the same composition. Of the foregoing techniques, the gas atomization technique has been found particularly satisfactory and a specific embodiment thereof is described in U.S. Pat. No. 3,253,783, the teachings of which are incorporated herein by reference. In accordance with the apparatus disclosed in the aforementioned United States patent, a molten stream of the metal alloy passes downwardly and is atomized in response to the impingement of a conically-shaped vortex of gas and wherein the resultant droplets progressively solidify as they fall downwardly through the collection chamber. Metal powder produced in accordance with the arrangement described in the aforementioned patent under proper operating conditions generally is of a spherical configuration and where the predominant portion thereof is of a particle size within the desired range. The amalgamable powder can be of any particle configuration, although particles of a substantially spherical configuration are preferred. The powder can be of an average particle size of less than about 100 microns, although powders having average particle sizes ranging from about 45 microns to about 5 microns are usually preferred.
While air can be satisfactorily employed as the atomizing and collection medium, the contact of the molten silver alloy with the oxygen in the atomizing gas occasions the formation of undesirable oxides on the particle surfaces. While such oxides can subsequently be reduced by heating the powder in a reducing atmosphere, such as hydrogen, for example, it is preferred to employ a substantially inert gas, such as a substantially dry argon, nitrogen or helium gas, for effecting the atomization of the powder and also as the cooling medium within the powder collection chamber.
The amalgamable powder of the desired composition and particle size can be employed for forming a dental amalgam at the time required by admixture with from about 42% up to about 54% mercury and, more usually, from about 46% to about 50% mercury. The wetting of the particle surfaces with mercury initiates the amalgamation reaction, whereby the intermetallic compounds formed by a consumption of the surface strata of the particles serves to bond the residual unreacted particle cores to each other, forming a matrix characterized as a continuous phase of amalgam, having interspersed therethrough discrete discontinuous phases of the unreacted particle cores. Of the several intermetallic compounds formed, the tin-mercury intermetallic compound comprises the weakest alloy with respect to compressive strength and also is the most susceptible to corrosion and/or discoloration.
A possible explanation of the different behavior of the spherical alloy containing high copper to that of the comminutive alloys having the same composition is the difference in their manufacturing and solidification pattern of the alloy. In the spherical alloy, the outer surface of each particle contains a higher concentration of silver and copper, whereas the tin-rich phase is concentrated in the inner part of the particle. Using a hydrochloric acid solution as a washing agent, some tin ions are dissolved from the outer surface, while the silver and copper ions on the surface are left almost unaffected. This process would actually increase the relative concentration of silver-to-tin on the surface, which should cause amalgam alloys to harden in a very short period of time. However, after acid washing of particles, the surface of particles becomes spongy in appearance, producing some microscopic channels to the central part of the sphere where the higher concentration of tin exists. During heat-treating of spherical particles, a considerable amount of tin ions, through the microscopic channels, migrate toward the surface. Increased amount of tin on the surface would increase the working time of the alloy. The affinity of mercury to tin is greater than to silver. The affinity of the copper ion to mercury at room temperature is very low and thus the copper will remain unreacted, during trituration, increasing the ratio of Cu/Ag to a higher value than before amalgamation.
While the particular mechanism by which the unexpected results of the present invention are obtained is not entirely understood at the present time, a possible explanation is that the outer surfaces of each of the particles contains a higher concentration of silver and copper with the inner core of the particle comprising a tin-rich phase. This minimizes the formation of detrimental tin intermetallic compounds, whereby the properties of the resultant dental amalgam are substantially improved. The foregoing is substantiated by microprobe analyses of the spherical alloy particles employing x-ray beams which show a variation in the composition between the inner portions of each particle relative to the peripheral portions thereof. The range of composition values obtained by microprobe analyses of large amalgam alloy particles are set forth in Table 2.
TABLE 2 ______________________________________ Microprobe Analysis of Large Spherical Particles Location of Composition, Percent by Weight Analysis Silver Tin Copper ______________________________________ Center of particle 51-58 27-29 9-19 Edge of particle 64-84 10-20 6-16 ______________________________________
The results of incremental microprobe analyses of an individual particle to determine the range of variation in the composition of the three alloying constituents on moving from the particle surface toward the central core of the particle is graphically illustrated in the drawing. As shown, the concentration of the silver constituent progressively decreases on moving from the surface toward the particle core. In contrast, the concentration of tin progressively increases on moving from the particle surface toward the particle core, while the quantity of copper remains substantially constant but with a reduction in the breath of the range of values on moving toward the core.
In order to further illustrate the amalgamable silver alloy powder of the present invention and the improved properties of amalgams prepared therefrom, the following specific example is provided. It will be understood that the example is included solely for illustrative purposes and is not intended to be limiting of the scope of the invention as herein described and as set forth in the subjoined claims.
EXAMPLE
An amalgamable silver alloy powder is prepared by the argon atomization of a molten mass of material containing a silver-to-tin ratio of 2.23:1 and a tin-to-copper ratio of 2.10:1, resulting in an alloy consisting essentially of 13% copper, 27% tin and 60% silver. The resultant powder particles are generally of a spherical configuration and the collected powder is screened to separate and recover particles of an average particle size ranging from 44 microns to about 5 microns. The balance of the powder particles are recycled.
The resultant heat-treated and washed amalgamable powder is triturated for ten seconds with approximately 46% mercury, whereafter the mixture is shaped into a test specimen and allowd to set for a period of 7 days. Test evaluations of the resultant dental amalgam test specimen reveal a compressive strength of greater than about 75,000 psi, a creep resistance of less than 0.20% in 3 hours, and an observable resistance to tarnish and corrosion.
While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages set forth above, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.
Conventionally, silver-base alloys adapted for use as dental amalgams contain a minimum of about 65% silver, a maximum of about 6% copper, a maximum of about 2% zinc and a minimum of about 25% tin. The admixture of such silver-base alloys with from about 45% to about 55% mercury by the dentist as needed are adapted to harden within a matter of a few minutes, enabling shaping by carving for a period of up to about 15 minutes and the amalgamation reaction is usually complete after about 24 hours. Typically, silver alloys suitable for forming amalgams in accordance with American Dental Association specifications contain from about 26% to about 28% tin, about 1% to about 2% zinc, about 2% to about 4% copper, with the balance essentially all silver. Of the foregoing alloying constituents present, it has been generally accepted that the quantity of copper should be restricted to a maximum of about 6% due to the tendency of dental amalgams incorporating greater quantities of copper to corrode and/or discolor in actual use. On the other hand, quantities of copper in excess of about 6% are known to improve the compressive strength of the resultant dental amalgam.
The general requirements of a dental amalgam include a retention of shape, color and appearance; a biological compatibility, a restoration of the tooth to its original function, and a long durable operating life. Extensive investigation and test work of dental restorative materials including dental amalgams has revealed that creep resistance is perhaps the single most important mechanical property followed by compressive strength as it relates to failure or fracture of dental restorations. These tests have convincingly established a relationship between a high incidence of marginal fracture of dental restorations with high dynamic creep values. Attempts to improve the marginal fracture characteristics and other mechanical properties of dental amalgams by increasing the quantity of copper in the alloy have been unsuccessful due to the increased corrosivity and tendency of discoloration of the resultant amalgams.
The present invention provides for an amalgamable silver alloy containing amounts of copper substantially greater than those heretofore considered useable which improves not only the resistance of the dental amalgam toward corrosion and/or discoloration, but also provides for substantially superior mechanical properties, including increased resistance to dynamic creep.
SUMMARY OF THE INVENTION
The benefits and advantages of the present invention are achieved by an amalgamable silver alloy powder derived from a microcasting of a molten mass of the alloy, such as by fluid atomization techniques or the like, and wherein each particle is of substantially the same composition and is further characterized by having a gradient composition in which the concentration of the individual alloying constituents changes on moving in a direction from the surface of the particle toward the center thereof. The amalgamable powder contains tin and copper in combination with silver as the essential alloying constituent and wherein the silver-to-tin ratio is controlled from about 1.8:1 to about 3.0:1, while the tin-to-copper ratio is controlled from about 1:1 to about 3:1. On a weight percent basis of the individual alloying constituents, the amalgamable silver alloy powder contains from about 8% to about 27% copper, from about 20% to about 32% tin, with the balance consisting essentially of silver but, in any event, a minimum of 47% silver. The alloy powder can also contain up to about 2% zinc without any adverse effects. The powder particles are of an average size less than about 100 microns and preferably range from about 5 microns to about 44 microns.
The dental amalgam is prepared employing the amalgamable silver alloy powder by admixture with mercury in an amount of from about 42% up to about 54% and preferably in an amount such that mercury in the final product is present from about 46% to about 50% by weight.
Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments, the specific examples provided, taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
The drawing graphically portrays the range of composition gradients of the amalgamable silver alloy particles between the surface and interior of the particle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The proportions of the individual alloying constituents comprising the amalgamable silver alloy powder and the resultant dental amalgam prepared therefrom are herein defined in terms of percentages by weight and on a weight ratio basis unless clearly indicated to the contrary.
The permissible as well as the preferred ratios and corresponding percentages of the three essential alloying constituents are set forth in Table 1.
TABLE 1 ______________________________________ Metal Powder Composition Permissible Range Preferred Range ______________________________________ Ag/Sn Ratio 1.8 - 3.0 2.2 - 2.6 Sn/Cu Ration 1.0 - 3.0 1.5 - 2.5 Cu 8% - 27% 10% - 18% Sn 20% - 32% 23% - 28% Ag 47% - 70% 57% - 65% ______________________________________
As shown in Table 1, the ratio of silver-to-tin is broadly controlled within the range of about 1.8 up to about 3.0, while the permissible ratios of tin-to-copper are from about 1 to about 3. Expressed in terms of percentages of the three alloying constituents, based on the foregoing weight ratios, the minimum percentages are specified with the recognition that when any one of the three is at the minimum level, one or both of the remaining alloys are present in amounts greater than their respective minimums in order that the total percent is 100. The foregoing is also applicable to the preferred percentages of these three alloying constituents as set forth in Table 1. For the purposes of convenience, the relative proportions of the three metals are set forth in terms of their weight ratios in which the silver-to-tin ratio preferably is controlled within a range of about 2.2 to about 2.6, while the tin-to-copper ratio is preferably controlled within a range of about 1.5 to about 2.5. It is also contemplated that zinc can be included in amounts up to about 2% by weight of the total alloy.
It is an important feature of the present invention that the amalgamable silver alloy powder is derived by a microcasting technique in which molten fine-sized droplets of the desired composition are cooled in a manner to effect a progressive solidification during the cooling commencing on the surface and moving inwardly thereof. The differential cooling and solidification rate of each droplet contributes toward a stratification or composition gradient in the final solidifed particle which is at least in part responsible for the unexpected improved mechanical and chemical properties of dental amalgams prepared therefrom.
The microcasting of a molten mass of the silver alloy can be achieved by a variety of techniques including gas atomization, airless spraying and centrifugal fragmentation for effecting a subdivision of the molten mass into a plurality of fine-sized liquid droplets which are all of substantially the same composition. Of the foregoing techniques, the gas atomization technique has been found particularly satisfactory and a specific embodiment thereof is described in U.S. Pat. No. 3,253,783, the teachings of which are incorporated herein by reference. In accordance with the apparatus disclosed in the aforementioned United States patent, a molten stream of the metal alloy passes downwardly and is atomized in response to the impingement of a conically-shaped vortex of gas and wherein the resultant droplets progressively solidify as they fall downwardly through the collection chamber. Metal powder produced in accordance with the arrangement described in the aforementioned patent under proper operating conditions generally is of a spherical configuration and where the predominant portion thereof is of a particle size within the desired range. The amalgamable powder can be of any particle configuration, although particles of a substantially spherical configuration are preferred. The powder can be of an average particle size of less than about 100 microns, although powders having average particle sizes ranging from about 45 microns to about 5 microns are usually preferred.
While air can be satisfactorily employed as the atomizing and collection medium, the contact of the molten silver alloy with the oxygen in the atomizing gas occasions the formation of undesirable oxides on the particle surfaces. While such oxides can subsequently be reduced by heating the powder in a reducing atmosphere, such as hydrogen, for example, it is preferred to employ a substantially inert gas, such as a substantially dry argon, nitrogen or helium gas, for effecting the atomization of the powder and also as the cooling medium within the powder collection chamber.
The amalgamable powder of the desired composition and particle size can be employed for forming a dental amalgam at the time required by admixture with from about 42% up to about 54% mercury and, more usually, from about 46% to about 50% mercury. The wetting of the particle surfaces with mercury initiates the amalgamation reaction, whereby the intermetallic compounds formed by a consumption of the surface strata of the particles serves to bond the residual unreacted particle cores to each other, forming a matrix characterized as a continuous phase of amalgam, having interspersed therethrough discrete discontinuous phases of the unreacted particle cores. Of the several intermetallic compounds formed, the tin-mercury intermetallic compound comprises the weakest alloy with respect to compressive strength and also is the most susceptible to corrosion and/or discoloration.
A possible explanation of the different behavior of the spherical alloy containing high copper to that of the comminutive alloys having the same composition is the difference in their manufacturing and solidification pattern of the alloy. In the spherical alloy, the outer surface of each particle contains a higher concentration of silver and copper, whereas the tin-rich phase is concentrated in the inner part of the particle. Using a hydrochloric acid solution as a washing agent, some tin ions are dissolved from the outer surface, while the silver and copper ions on the surface are left almost unaffected. This process would actually increase the relative concentration of silver-to-tin on the surface, which should cause amalgam alloys to harden in a very short period of time. However, after acid washing of particles, the surface of particles becomes spongy in appearance, producing some microscopic channels to the central part of the sphere where the higher concentration of tin exists. During heat-treating of spherical particles, a considerable amount of tin ions, through the microscopic channels, migrate toward the surface. Increased amount of tin on the surface would increase the working time of the alloy. The affinity of mercury to tin is greater than to silver. The affinity of the copper ion to mercury at room temperature is very low and thus the copper will remain unreacted, during trituration, increasing the ratio of Cu/Ag to a higher value than before amalgamation.
While the particular mechanism by which the unexpected results of the present invention are obtained is not entirely understood at the present time, a possible explanation is that the outer surfaces of each of the particles contains a higher concentration of silver and copper with the inner core of the particle comprising a tin-rich phase. This minimizes the formation of detrimental tin intermetallic compounds, whereby the properties of the resultant dental amalgam are substantially improved. The foregoing is substantiated by microprobe analyses of the spherical alloy particles employing x-ray beams which show a variation in the composition between the inner portions of each particle relative to the peripheral portions thereof. The range of composition values obtained by microprobe analyses of large amalgam alloy particles are set forth in Table 2.
TABLE 2 ______________________________________ Microprobe Analysis of Large Spherical Particles Location of Composition, Percent by Weight Analysis Silver Tin Copper ______________________________________ Center of particle 51-58 27-29 9-19 Edge of particle 64-84 10-20 6-16 ______________________________________
The results of incremental microprobe analyses of an individual particle to determine the range of variation in the composition of the three alloying constituents on moving from the particle surface toward the central core of the particle is graphically illustrated in the drawing. As shown, the concentration of the silver constituent progressively decreases on moving from the surface toward the particle core. In contrast, the concentration of tin progressively increases on moving from the particle surface toward the particle core, while the quantity of copper remains substantially constant but with a reduction in the breath of the range of values on moving toward the core.
In order to further illustrate the amalgamable silver alloy powder of the present invention and the improved properties of amalgams prepared therefrom, the following specific example is provided. It will be understood that the example is included solely for illustrative purposes and is not intended to be limiting of the scope of the invention as herein described and as set forth in the subjoined claims.
EXAMPLE
An amalgamable silver alloy powder is prepared by the argon atomization of a molten mass of material containing a silver-to-tin ratio of 2.23:1 and a tin-to-copper ratio of 2.10:1, resulting in an alloy consisting essentially of 13% copper, 27% tin and 60% silver. The resultant powder particles are generally of a spherical configuration and the collected powder is screened to separate and recover particles of an average particle size ranging from 44 microns to about 5 microns. The balance of the powder particles are recycled.
The resultant heat-treated and washed amalgamable powder is triturated for ten seconds with approximately 46% mercury, whereafter the mixture is shaped into a test specimen and allowd to set for a period of 7 days. Test evaluations of the resultant dental amalgam test specimen reveal a compressive strength of greater than about 75,000 psi, a creep resistance of less than 0.20% in 3 hours, and an observable resistance to tarnish and corrosion.
While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages set forth above, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.