GB1594643A

GB1594643A - Corrosion-resistant dental alloy having improved handling characteristics

Info

Publication number: GB1594643A
Application number: GB430/78A
Authority: GB
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: Engelhard Minerals and Chemicals Corp
Priority date: 1977-01-17
Filing date: 1978-01-06
Publication date: 1981-08-05
Also published as: JPS5390122A; DE2801721A1; FR2377191A1; FR2377191B1; CA1069347A

Description

(54) CORROSION-RESISTANT DENTAL ALLOY HAVING IMPROVED HANDLING CHARACTERISTICS (71) We, ENGELHARD MINERALS & CHEMICALS CORPORATION, a corporation organized under the laws of the State of Delaware, United States of America, of 70 Wood Avenue South, Metro Park Plaza, Iselin, New Jersey, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the -method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates generally to the dental alloys which 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 principally composed of silver and tin, they usually contain small amounts of copper and zinc. A typical alloy of the prior art would contain at least 65 wt.% silver, 1--2 wt.% zinc, and 24 wt.% copper, with the remainder being tin. Such alloys are not completely resistant to corrosion. It has been found that increasing the copper content of such alloys provides increased strength and also avoids the formation of what is known in the art as the gamma-two phase, a tin and mercury phase which has low resistance to corrosion and thus may lead to early deterioration of fillings.

Typical of such high copper alloys are those disclosed in United States Patent No.

3,871,876 and United States Patent No.

3,997,328. Such dental alloy compositions increase the copper content from the typical 24 wt.% to the range of 8-27 wt.% in the first-mentioned patent and in the rattler patent, from 20-40 wt.%.

While such alloys have improved corrosion resistance, another important characteristic of dental alloys has been neglected heretofore. The success of a dentist in filling a dental cavity is related to the handling characteristics of the alloy after it is amalgamated with mercury. For example, the high copper alloy disclosed in U.S. Patent No. 3,871,876, is typically produced by air atomization from the molten state which results in a spherical or spheroidal form for the finished alloy. It is characteristic of alloys having a spherical shape that they feel relatively soft to the dentist and appear to require delicate handling. They are sometimes difficult to pack into a dental cavity since they have a tendency to be forced up the wall of the cavity if too much pressure is exerted or an instrument is used which has a small bearing area. Consequently, many dentists find that such spherical material is not well-adapted to their individual technique. As a result, they may be unable to take advantage of the corrosion resistance inherent with spherical alloys having a high copper content.

One method of improving handling characteristics of conventional dental alloys is disclosed and claimed in United States Patent No. 3,997,327. In that invention a major portion of spherical particles is combined with a minor portion of microcut irregular particles, or flakes. Typical dental alloys in the prior art generally have been of the flake type, which inherently requires a higher pressure in order to pack it into a dental cavity than is characteristic of the spherical particles. By combining spherical particles with flake particles having the same composition, it is possible to improve the handling characteristics of the resulting mixture. Such a combination, having a conventionally low copper content, has less resistance to corrosion than the higher copper content alloys previously discussed.

The present invention has as its objective providing improved handling characteristics to corrosion-resistant dental alloys.

The present invention provides a dental alloy which comprises particles consisting of 47 to 70% by weight of silver, 20 to 32% by weight of tin, and 7 to 27% by weight of copper, any balance being unavoidable impurities, the particles having a higher than average silver and copper content at the surface thereof, being randomly-shaped and microcrystalline, and having a surface area 20 to 30% greater than that of spherical particles and 20 to 30% less than that of flakelike particles of the same mean particle size.

Preferably, the particles of alloy have a BET surface area of from 0.22 to 0.31 m2/g.

and a mean particle size of I to 45 microns, most preferably, a BET surface area of from 0.23 to 0.26 m2/gm. and a mean particle size of 20 to 26.5 microns.

One embodiment of the invention provides an alloy composition which comprises a first alloy which is an alloy as defined above, and spherical particles of a second alloy having substantially the same composition and mean particle size as the first alloy and having a spherical shape.

Preferably such compositions comprise from 15 to 40% by weight of said first alloy and from 85 to 60% by weight of said second alloy.

Another embodiment of the invention provides an alloy composition which, in addition to the first and second alloys comprise particles of a third alloy consisting of 55 to 75% by weight of silver, 20 to 40% by weight of tin, 0 to 10% by weight of copper, and 0 to 2% by weight of zinc any balance being un-avoidable impurities, and being in the form of flake-like particles having a mean particle size of from 25 to 30 microns.

Preferably such compositions comprise from 25 to 60% by weight of said first alloy, from 60 to 25% by weight of said second alloy, and from 15 to 25 /" by weight of said third alloy.

The invention also provides a dental amalgam which comprises substantially equal amounts of mercury mixed with any of the alloys or alloy compositions defined above.

The invention also provides a method of preparing the first alloy as defined above, which comprises microcasting a molten metal composition consisting of 47 to 70% by weight of silver, 20 to 32% by weight of tin, and 7 to 27% by weight of copper, any balance being unavoidable impurities.

The invention also provides a method of preparing alloy compositions which comprises mixing microcast particles of the first alloy with spherical particles of the second alloy, having substantially the same composition and mean particle size.

The invention further provides a method of preparing alloy compositions which comprises mixing microcast particles. of the first alloy with spherical particles of the second alloy and with flake-like particles of the third alloy.

A preferred embodiment of these methods comprises pelletizing the particles of alloy or alloy composition and heat treating the resulting pellets at a temperature between 100 and 700"F for a period of time sufficient to match the handling characteristics of amalgams made with said pellets to amalgams made with the particles of the alloy or alloy composition.

The dental alloys of this invention combines corrosion resistance and good handling qualities. It is corrosion resistant in that it has a relatively high copper content.

Its composition corresponds generally to that of the spherical material disclosed in United States Patent No. 3,871,876 being within the range of 47% to 70% by weight silver, 20% to 32% by weight tin, and 7% to 27% by weight copper. As is true of the particles of U.S. Patent No. 3,871,876, the particles have a higher than average silver and copper content at the surface of the particles.

In the present invention the alloy is produced in an irregular shape rather than the spherical form typical of U.S. Patent No.

3,871,876, but differing from the irregular flake-like particle typical of the prior art.

The alloy particles according to the present invention preferably have a surface area of 0.23-0.26 m2/gm, which is 2030% greater than the typical spherical particle and about 2030% less than typical flake-like particles. They may be produced by a variant of the air atomization process used to form spherical particles, although other techniques may be used.

When amalgamated with mercury, the alloy particles of the invention have handling qualities similar to those of flake particles as shown by empirical tests as described hereinafter, which have been found to relate to subjective experience with the condensation and carving characteristics of amalgams.

In another aspect of the invention, the corrosion-resistant alloy particles having the unique shape of the invention may be combined with particles of conventional shapes, either spherical and/or flake-like forms. One type of particle may be the spherical material disclosed in United States Patent No. 3,871,876, having a composition within the range of 47% to 70% by weight silver, 20% to 32% by weight tin, and 7% to 27% by weight copper (i.e. the second alloy). Another useful type of particle has the relatively low copper and high silver content typical of the prior art and has a flake-like shape. In a preferred embodiment, such particles will have a composition of 55% to 75% by weight silver, 20% to 40% by weight tin, 0% to 10% by weight copper and 0% to 2% by weight zinc, (i.e. the third alloy). By combining suitable proportions of spherical particles, randomly-shaped microcrystalline particles of this invention and flake-like particles, the handling characteristics of amalgams prepared from such mixtures can be adjusted to suit the requirements of the individual user. Although any proportions may be used of the particles which have a substantial copper content, the particles having a relatively low copper content are limited to a maximum of 25% by weight of the alloy mixture in order to retain the corrosion resistance provided by the particles having higher copper content.

In another aspect of the invention the dental alloy is prepared by the steps of formulating a metal composition as given above, melting said composition, microcasting particles according to the invention, pelletizing said particles, and heat treating said pellets to adjust the handling characteristics of amalgams made with said pellets to correspond with characteristics of amalgams made directly with said particles.

When particles of conventional spherical or flake-like shapes are added, they will be mixed with the irregular particles of the invention prior to the pelletizing step.

Figure la shows the spherical particles of the prior art corresponding to United States No. 3,871,876.

Figure lb shows particles corresponding to a dental alloy of the present invention.

Figure lc shows particles corresponding to microcut or flake-like particles of the prior art.

Figure Id shows a mixture of the particles of la and Ib.

Figure le shows mixtures of the particles of la, lb, and Ic.

Figure 2 plots the results of tests described herein and applied to several dental amalgams.

A dentist in packing an amalgam prepared from a dental alloy and mercury into a dental cavity considers two factors to be of particular importance. First, what may be termed "condensation" relates to the resistance of the alloy to being packed into the cavity by the dentist using typical instruments. It will be clear that an amalgam must have sufficient plasticity when under pressure to enable it to flow into and completely fill all portions of the cavity, thereby preventing the formation of open spaces in the finished filling which could weaken it or permit further decay to the tooth structure. At the same time, the amalgam must not be so fluid as to flow out from beneath the dental instruments during condensation of the amalgam and move up the wall of the cavity. In such situations, a nonuniform degree of packing necessarily results, with poor adaptation to the cavity and increased porosity which weakens the filling and may result in further decay. Thus, one important handling characteristic of an amalgam is its ability to be pressed into a dental cavity to fill all the small openings under the desired condensation pressure, while not being so soft that the dentist cannot adequately compact the amalgam.

This condensation pressure may be approximated by an empirical test which will 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 compacted 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 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 U.S. Patent No. 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 I micron to 65 microns.

Larger and smaller particles are separated and recycled to be remelted and recast.

Spherical particles such as are illustrated in Figure 1 a have an average surface to volume ratio of about 0.21 m2/gm as measured by the apparatus conventionally used to determine surface area by the BET method, "the BET apparatus". Flake alloys of approximately the same size as illustrated in Figure Ic are substantially different, having a surface to volume ratio of about 0.33 m2/gm. A mixture of spherical particles with flake particles as disclosed in U.S.

Patent No. 3,997,327 will have a ratio between the two extremes. Rather than mixing spherical and flake particles, the alloy of the present invention is preferably produced in a single step process to provide a new particle shape. The air atomization technique or other microcasting method may be altered to cause distortion of the particles, which otherwise freeze in a spherical or spheroidal shape. A suitable morphology is illustrated in Figure Ib. The spherical form of Figure la is no longer predominant. Neither do the particles have the distinctive shape of microcut, flake-like particles, as seen in Figure Ic, nor do they have the striations characteristic of such particles.

The alloy particles according to the invention need not be exactly the same as those of Figure Ib. Rather, the particles of the invention may be characterized by their surface area and the handling characteristics measured as hereinbefore described. Typically, particles of the invention will have a surface area within the range of 0.22 to 0.31 m2/gm and preferably in the range of 0.23 to 0.26 m2/gm.

Specifically, the particles of Figure lb have a median surface area of about 0.24 m2/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 specifically reported hereinafter for the alloy of the invention.

It should be further noted that the surface area measured by the BET apparatus is much larger than the geometric exterior surface of the particles. For example, a perfect sphere would have a surface area only about 10% of that measured for the generally spherical particles of Figure la.

The additional 90% of the measured surface is evidently due to surface roughness and porosity. Since this additional surface seems less likely to have a large effect on the handling properties of amalgams than the geometric surface, the geometric surface of the particles should be compared rather than the BET surface. However, 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.

At the completion of the amalgamation process, the amalgam is condensed into a tooth cavity by a dentist and then the filling is carved or shaped until the amalgam has become so hard that it cannot be worked.

This period is typically about six minutes.

The dentist packs or condenses the amalgam into the tooth cavity while the amalgam is still soft enough to do so. The pressure required 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. A pellet of dental alloy is mixed with the recommended amount of mercury in an amalgamator for the manufacturer's recommended time. A commercially available Wig-L-Bug Model 5A R manufactured by Crescent Corporation was used in the tests reported herein, although other amalgamators would be acceptable. After the amalgamation is complete, the amalgam is immediately placed on a flat glass plate and covered by another such glass plate and pressed to a one millimeter thickness, as determined by one millimeter spacers 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 the measurements reported herein an Instron testing unit model 1101 produced by Instron Corporation was employed. A constant load of five pounds was placed on a two millimeter steel ball in contact with the amalgam. The depth of the indentation made by the ball when the load was applied for fifteen seconds is used as a measure of the resistance of the amalgam. Tests were made at one minute intervals for a perod 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 carvability factor relates to the ability of a hardening dental amalgam to be carved and shaped by dental instruments after it has been compacted. It will be apparent that after the compaction or condensation period (about 2 minutes) the dentist will have a limited time in which to shape or carve the hardening amalgam. A variant of the test previously described is used to obtain a carving factor. The two millimeter ball loaded by a five pound weight is replaced with a one pound Gilmore needle having a one millimeter 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 Restorative Dental Materials, 4th ed., 1971. It has been found that the lighter loaded Gilmore needle will fail to penetrate an amalgam after it is sufficiently hardened. The time between the end of the amalgamation process and the failure of the Gilmore needle to penetrate the hardening amalgam may be used as an index of the carvability of the amalgam.

EXAMPLE 1 A dental alloy is prepared by mixing individual metal powders, resulting in an overall composition 58 wt.% Ag, 29 wt.% Sn, 13 wt.% Cu. The powdered mixture is melted and processed in an air atomization apparatus modified to minimize the formation of spherical particles by contacting the molten droplets during the cooling process, thereby producing the randomly-shaped microcrystalline particles of the invention. The particles formed have a surface area of 0.24 m2/gm. They are sieved to produce a powdered alloy according to the invention as shown in Figure lb and having particles sized within the range of I micron to 45 microns. The powdered alloy is then pelleted and mixed with sufficient mercury to form an amalgam having an alloy to mercury ratio of 1:1. The amalgam is measured for its resistance to condensation pressure according to the test hereinbefore described and the results plotted on Figure 2.

EXAMPLE 2 A dental alloy is prepared by mixing individual metal powders, resulting in an overall composition 58 wt.% Ag, 29 wt.% Sn, 13 wt.% Cu. The powdered mixture is melted and processed in an air atomization apparatus according to U.S. 3,871,876 to produce spherical particles as shown in Figure la. The particles have a surface area of 0.21 m2/gm. After sieving the particles are within the range of 1 micron to 40 micron. The powdered alloy is then pelleted and mixed with sufficient mercury to form an amalgam having an alloy to mercury ratio of 1:1. The amalgam is subjected to the condensation factor test described hereinbefore and the results plotted on Figure 2.

EXAMPLE 3 A dental alloy is prepared by mixing individual metal particles, resulting in an overall composition 68 wt.% Ag, 27 wt.% Sn, 4.4 wt.% Cu, 0.6 wt.% Zn. The powdered mixture is melted and cast into a bar, from which it is cut on a lathe into flake-like particles as shown in Figure Ic according to the usual technique of the prior art. The particles have a surface area of 0.33 m2/gm.

After sieving the particles are within the range of 2 microns to 50 microns. The powdered alloy is then pelleted and mixed with sufficient mercury to form an amalgam having an alloy to mercury ratio of 1.2:1.

The amalgam is subjected to the condensation factor test described hereinbefore and the results plotted on Figure 2.

As shown in Figure 2, flake-like alloys of the prior art (Figure Ic and Example 3) 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 indentation after one minute from completion of the amalgamation process are 10.75, 18, and 10.3 for the alloys of Examples 1--3 respectively. The spherical particles of Example 2 and Figure la produce an amalgam which is soft when freshly mixed with mercury. As previously indicated, dentists often --find amalgams made with spherical particles to be delicate to handle and. difficult to condense properly. The alloy of the invention (Figure lb and Example 1) has a unique morphology and 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.

The carving period (typically 2 to 5 minutes after amalgamation) represents the time period when the dentist shapes the compacted filling to suit the patient's bite.

After a certain period the amalgam becomes unduly hard and can no longer be worked with the usual dental instruments.

After about one hour a typical amalgam has reached substantial strength and can withstand the pressure of normal use. As is indicated by Figure 2, the effect of particle shape on the handling characteristics of amalgams is more significant during the condensation period than during the carving period. In fact, one might conclude from Figure 2 that amalgams made according to the invention would be more difficult to carve than those made with either spherical or flake-like articles. However, measurements of the three particles in the preceding examples were made by substituting a Gilmore needle for the two millimeter ball as previously described, with the following results.

TABLE 1 Carving Factor Time, minutes Penetration Particle Type Ceased Spherical (Ex. 2) 4.15 Microcrystalline (Ex. 1) 3.15 Flake (Ex. 3) 2.15 The above results indicate that spherical particles can be carved with less force and for a longer time than the randomly-shaped microcrystalline particles of the invention which in turn can be carved with less force than the flake-like particles.

As previously discussed, the alloy of the invention may be produced by modification of the air atomization process so that molten metal is distorted instead of frozen into spherical form. Such particles may be produced by other processes, for example, by splat-cooling of a stream of molten alloy and by modifying the conventional metallizing process. However produced, the particles will have a surface area intermediate that of spheres and that of flakes in the preferred form characterized by having a surface area of 0.230.26 m2/gm, which is 2030% greater than the typical spherical particle and 2030% less than the typical flake-like particles.

The composition will be within the range of 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. Patent No.

3,871,876. It has been found that the alloy in the unique form of the invention still has corrosion resistance as measured by the anodic polarization test described in U.S.

Patent No. 3,997,329, even though the particles are no longer spherical in form.

The anodic polarization test indicates by the absence of the gamma-two phase that the amalgam is resistant to corrosive attack. It is believed that the higher than average silver and copper content found at the surface of the irregular particles of the invention as well as in the spherical particles of U.S.

Patent No. 3,871,876 is related to the relatively high copper content of the alloy and the speed at which it is cooled from the molten state. It is expected that many methods of forming particles from molten metal which involve rapid cooling can be employed.

Although no explanation is presently available, it has been found that if the alloy of the invention is prepared as a mixture of about 60% by weight spheres and about 40% by weight flakes having the same composition, the handling properties are similar to that of particles of the invention but the amalgam is no longer corrosion resistant by the anodic polarization test.

However, with their unique morphology the particles of the invention unexpectedly combine both corrosion resistance and improved handling characteristics.

EXAMPLE 4 A dental alloy is prepared by mixing 40% by weight of the randomly-shaped microcrystalline particles of Example 1 with 60% by weight of the spherical particles of Example 2. The mixed particles have a surface area of about 0.23 m2/gm and are within the size range of I micron to 45 microns. The powdered alloy is then pelleted and mixed with sufficient mercury to form an amalgam having an alloy to mercury ratio of 1:1. The amalgam is subjected to the condensation factor test described hereinbefore.

The mixed particles of Example 4 provide amalgams having handling properties intermediate amalgams made with the spherical particles of Example 2 and the randomly-shaped particles of Example 1.

The condensation factors expressed as millimeters indentation after one minute from completion of the amalgamation process are 10.75, 18, and 14.5 for the alloys of Examples 1, 2, and 4 respectively. The spherical particles of Example 2 and Figure la produce an amalgam which is soft when freshly mixed with mercury. As previously indicated, dentists often find amalgams made with spherical particles to be delicate to handle and difficult to condense properly. The randomly-shaped particles (Figure Ib and Example 1) have a unique morphology which provides handling characteristics similar to those of the flakelike 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 Id) provides intermediate handling characteristics which will be experienced by the dentist as a moderately soft amalgam which requires less pressure for proper condensation into a cavity. The combination of Example 4 is only one possible mixture. Clearly, mixtures of any proportions could be made to suit the individual requirements of the user.

Another satisfactory mixture combines 85% by weight of the particles of Example 1 with 15% of the particles of Example 2. The surface area of such a mixture is about 0.22 m2/gm and the size distribution is within the range of I micron to 45 microns. An amalgam made of such a mixture will be generally firmer than the mixture of Example 4 and its condensation factor after one minute would be about 12 millimeters.

Measurements of the individual types of particles with the mixture of Example 4 were made by substituting a Gilmore needle for the two millimeter ball as previously described, with the following results.

TABLE 11 Carving Factor Time, minutes Penetration Particle Type Ceased Spherical (Ex. 2) 4.15 Microcrystalline (Ex. 1) 3.15 Mixed spherical and micro crystalline (Ex. 4) 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. The mixture, as would be expected, can be carved with less force for a longer period than the amalgams made with the randomly-shaped particles of Example 1 but the mixture is firmer and hardens quicker than amalgams made with spherical particles.

Mixing particles provides a means by which the handling characteristics of dental amalgams may be adjusted to suit the requirement of the individual user. At the same time, when only spherical and irregularly-shaped particles are used, both the component particles are corrosion resistant and the resulting mixtures preserve the corrosion resistance of the components.

For this reason no composition limits need be set on mixtures of these particles, which may be varied to meet the handling characteristics of the intended user, and thus could approach the softness characteristic of amalgams made solely of spherical particles or the firmness characteristic of randomly-shaped microcrystalline particles.

EXAMPLE 5 A dental alloy is prepared by mixing 25% by weight of the irregularly-shaped micro crystalline particles of Example 1 with 60% by weight of the spherical particles of Example 2 and with 15% by weight of the flake-like particles of Example 3. The mixed particles have a surface area of about 0.24 m2/gm and are within the size range of 1 micron to 45 microns. The powdered alloy is then pelleted and mixed with sufficient mercury to form an amalgam having an alloy to mercury ratio of 1:1. The amalgam is subjected to the condensation factor test described hereinbefore.

The mixed particles of Example 5 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 3. The condensation factors expressed as millimeters indentation after one minute from completion of the amalgamation process are 10.75, 18, 14.0, and 13.0 for the alloys of Examples 1, 2, 3, and 5 respectively. The mixture of spherical particles with randomly-shaped particles and flake-like particles (Figure le) provides intermediate handling characteristics which will be experienced by the dentist as a moderately soft amalgam which requires less pressure for proper condensation into a cavity. The combination of Example 5 is only one possible mixture. Clearly, other mixtures could be made to suit the individual requirements of the user. Another satisfactory mixture combines 60% by weight of the particles of Example 1 with 25% by weight of the particles of Example 2 and 15% by weight of the particles of Example 3. The surface area of such a mixture is about 0.25 m2/gm and the size distribution is within the range of 1 micron to 45 microns. An amalgam made of such a mixture will be generally firmer than the mixture of Example 4 and its condensation factor after one minute would be about 12.5 millimeters.

The particles of Examples I and 2, having a relatively high copper content overall and a higher than average silver and copper content at the surface are corrosion resistant and may be combined in any proportions found desirable to adjust the handling characteristics of amalgams made from alloys of the invention. The relatively high silver and low copper content of the flake-like particles of Example 3 are not so resistant to corrosion.

Consequently, the flake-like particles may be included in an alloy according to the invention as desired to adjust handling characteristics of amalgams made therewith, but limited to a maximum of 25% by weight of the alloy in order to retain the corrosion resistance of the other two particles. The flake-like particles typically will have a composition of 55% to 75% by weight silver, 20% to 40% by weight tin, 0% to 10% by weight copper, and 0% to 2% by weight zinc.

Measurements of the three particles in the preceding examples made by substituting a Gilmore needle for the two millimeter ball as previously described, give the following results.

TABLE III Carving Factor Time, minutes Penetration Particle Type Ceased Spherical (Ex. 2) 4.15 Microcrystalline (Ex. 1) 3.15 Flake-like (Ex. 3) 2.15 Mixed particles (Ex. 5) 3.50 The above results indicate that spherical particles can be carved with less force and for a longer time than the randomly-shaped microcrystalline particles, which have the same advantage over flake-like particles.

The mixture of particles, as would be expected, can be carved with less force for a longer period than the amalgams made with the flake-like particles of the alloy of the invention (Example I) 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.

After the particles have been produced, they are sieved to provide a typical particle size distribution as follows: Microns Wt.% 52 0.3 to 1.4 4452 1.4 to 12.2 3844 1.6 to 8.9 3--38 20.9 to 24.6 20--30 26.lto35.7 120 24.0to35.4 10 3.6 to 7.2 The mean particle size is typically 20 to 26.5 microns. Although some variation about the above typical size distribution may be made to adjust the handling characteristics, an amalgam prepared with particles having a significantly different size distribution from that given above will have handling characteristics differing from those reported herein. In general, the smaller the average articles size, the firmer the amalgam will be and the shorter the working time.

As previously discussed, the surface area of the alloy particles of the invention having the size distribution as given above will be found to have a surface to volume ratio of 0.23-0.26 m2/gm. With other size distributions, the surface to volume ratio may be as wide as 0.22 to 0.32 m2/gm.

Particles may be used directly to form amalgams, especially if employed in premixed dental capsules. Often the particles are pelletized for use in dispensers designed to provide the desired amount of mercury needed to amalgamate with the pelleted alloy. The pelletizing process has been found to alter the handling properties of the resulting amalgam, generally providing a dry and less plastic amalgam than if the powdered alloy were used directly. It has been found that by heat treating the pellets in a vacuum for a suitable time, the mechanical properties and useful working time of the alloy can be returned to their original and more desirable values.

Typically a vacuum of about ten microns (0.01 mm Hg absolute pressure) has been found to be acceptable, the determing 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 700"F (37.8 to 370"C) as required until the handling characteristics of an amalgam made from the pellets matches those of the unpelleted powder, as measured by the condensation and carving factors.

The foregoing discussion of the preferred embodiments of the invention is not intended to limit the scope of the invention which is defined by the claims which follow.

WHAT WE CLAIM IS: 1. A dental alloy which comprises particles consisting of 47 to 70% by weight of silver, 20 to 32% by weight of tin, and 7 to 27% by weight of copper, any balance being unavoidable impurities, the particles having a higher than average silver and copper content at the surface thereof, being randomly-shaped and microcrystalline, and having a surface area 20 to 30% greater than that of spherical particles and 20 to 30% less than that of flake-like particles of the same mean particle size.

2. An alloy as claimed in Claim 1 having a BET surface area of from 0.22 to 0.31 m2/g.

and a mean particle size of 1 to 45 microns.

3. An alloy as claimed in Claim 1, having a BET surface area of from 0.23 to 0.26 m2/gm. and a mean particle size of 20 to 26.5 microns.

4. An alloy composition which comprises a first alloy which is an alloy as claimed in any preceding Claim, and spherical particles of a second alloy having substantially the same composition and mean particle size as said alloy of any preceding Claim, and having a spherical shape.

5. An alloy composition as claimed in Claim 4, which comprises from 15 to 40% by weight of said first alloy and from 85 to 60% by weight of said second alloy.

6. An alloy composition as claimed in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

TABLE III Carving Factor Time, minutes Penetration Particle Type Ceased Spherical (Ex. 2) 4.15 Microcrystalline (Ex. 1) 3.15 Flake-like (Ex. 3) 2.15 Mixed particles (Ex. 5) 3.50 The above results indicate that spherical particles can be carved with less force and for a longer time than the randomly-shaped microcrystalline particles, which have the same advantage over flake-like particles.

The mixture of particles, as would be expected, can be carved with less force for a longer period than the amalgams made with the flake-like particles of the alloy of the invention (Example I) 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.

After the particles have been produced, they are sieved to provide a typical particle size distribution as follows: Microns Wt.%

52 0.3 to 1.4 4452 1.4 to 12.2 3844 1.6 to 8.9 3--38 20.9 to 24.6 20--30 26.lto35.7

120 24.0to35.4

10 3.6 to 7.2 The mean particle size is typically 20 to 26.5 microns. Although some variation about the above typical size distribution may be made to adjust the handling characteristics, an amalgam prepared with particles having a significantly different size distribution from that given above will have handling characteristics differing from those reported herein. In general, the smaller the average articles size, the firmer the amalgam will be and the shorter the working time.

As previously discussed, the surface area of the alloy particles of the invention having the size distribution as given above will be found to have a surface to volume ratio of 0.23-0.26 m2/gm. With other size distributions, the surface to volume ratio may be as wide as 0.22 to 0.32 m2/gm.

Particles may be used directly to form amalgams, especially if employed in premixed dental capsules. Often the particles are pelletized for use in dispensers designed to provide the desired amount of mercury needed to amalgamate with the pelleted alloy. The pelletizing process has been found to alter the handling properties of the resulting amalgam, generally providing a dry and less plastic amalgam than if the powdered alloy were used directly. It has been found that by heat treating the pellets in a vacuum for a suitable time, the mechanical properties and useful working time of the alloy can be returned to their original and more desirable values.

Typically a vacuum of about ten microns (0.01 mm Hg absolute pressure) has been found to be acceptable, the determing 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 700"F (37.8 to 370"C) as required until the handling characteristics of an amalgam made from the pellets matches those of the unpelleted powder, as measured by the condensation and carving factors.

The foregoing discussion of the preferred embodiments of the invention is not intended to limit the scope of the invention which is defined by the claims which follow.

WHAT WE CLAIM IS: 1. A dental alloy which comprises particles consisting of 47 to 70% by weight of silver, 20 to 32% by weight of tin, and 7 to 27% by weight of copper, any balance being unavoidable impurities, the particles having a higher than average silver and copper content at the surface thereof, being randomly-shaped and microcrystalline, and having a surface area 20 to 30% greater than that of spherical particles and 20 to 30% less than that of flake-like particles of the same mean particle size.
2. An alloy as claimed in Claim 1 having a BET surface area of from 0.22 to 0.31 m2/g.

and a mean particle size of 1 to 45 microns.
3. An alloy as claimed in Claim 1, having a BET surface area of from 0.23 to 0.26 m2/gm. and a mean particle size of 20 to 26.5 microns.
4. An alloy composition which comprises a first alloy which is an alloy as claimed in any preceding Claim, and spherical particles of a second alloy having substantially the same composition and mean particle size as said alloy of any preceding Claim, and having a spherical shape.
5. An alloy composition as claimed in Claim 4, which comprises from 15 to 40% by weight of said first alloy and from 85 to 60% by weight of said second alloy.
6. An alloy composition as claimed in

Claim 4 or 5, which comprises a third alloy consisting of 55 to 75% by weight of silver, 20 to 40% by weight of tin, 0 to 10% by weight of copper, and 0 to 2% by weight of zinc any balance being unavoidable impurities and being in the form of flakelike particles having a mean particle size of from 25 to 30 microns.
7. An alloy composition as claimed in Claim 6, which comprises from 25 to 60% by weight of said first alloy, from 60 to 25% by weight of said second alloy, and from 15 to 25% by weight of said third alloy.
8. An alloy as claimed in Claim 1 and substantially as hereinbefore described with reference to Example 1.
9. An alloy composition as claimed in Claim 4 and substantially as hereinbefore described with reference to Example 4.
10. An alloy composition as claimed in Claim 6, and substantially as herein-before described with reference to Example 5.
II. A dental amalgam which comprises substantially equal amounts of mercury mixed with an alloy or alloy composition as claimed in any of the preceding Claims.
12. A method of preparing an alloy according to any of Claims 1 to 3 which comprises microcasting a molten metal composition consisting of 47 to 70% by weight of silver, 20 to 32% by weight of tin, and 7 to 27% by weight of copper, any balance being unavoidable impurities.
13. A method of producing an alloy composition according to Claim 4 or 5, which comprises mixing microcast particles obtained according to Claim 12 with spherical particles of an alloy having substantially the same composition and mean particle size.
14. A method of producing an alloy composition according to Claim 6 or 7, which comprises mixing microcast particles obtained according to Claim 12 with spherical particles of an alloy having substantially the same composition and mean particle size, and with flake-like particles having a mean particle size of 25 to 30 microns consisting of 55 to 75% by weight of silver, 20 to 40% by weight of tin, 0 to 10% by weight of copper, and 0 to 2% by weight of zinc, any balance being unavoidable impurities.
15. A method as claimed in any of Claims 12 to 14, which comprises pelletizing the particles of alloy or alloy composition and heat treating the resulting pellets at a temperature between 100 and 700"F for a period of time sufficient to match the handling characteristics of amalgams made with said pellets to amalgams made with the particles of the alloy or alloy composition.