GB2051132A

GB2051132A - Alloy Powder for Dental Amalgams

Info

Publication number: GB2051132A
Application number: GB8016365A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-17
Filing date: 1980-05-16
Publication date: 1981-01-14
Also published as: JPS55161003A; FR2456582B1; GB2051132B; DE3018874A1; CA1149107A; IT8021844A0; NZ193738A; FR2456582A1; IT1140843B

Abstract

The powder contains particles comprising a core which contains a minor proportion of silver, and a peripheral layer of a silver-tin alloy surrounding the core. The use of a core containing a minor proportion of silver enables the particles overall to have a low silver content. The powder may be formed by coating the metals on to the core particles and then alloying e.g. by entraining the particles in a stream of inert carrier gas containing the particles through a heating zone so as to melt the surface stratum and then cooling. It has been found that the powders can be used to form dental amalgams with properties comparable to amalgams produced with conventional alloys containing comparatively high amounts of silver.

Description

SPECIFICATION Metallic Powders The present invention relates to metallic powders.

Alloy powders which are used to produce dental amalgams upon admixture with mercury are typically composed of a particulate silver and tin alloy which may contain a proportion of copper or zinc. Each particle contains a relatively high proportion of silver ranging from at least 40% by weight up to as high as 75% by weight. Further, the composition of each particle is essentially the same throughout its entire cross-section although there may be minor variations between the surface and the core of the particle.

Silver is an expensive material and it would be desirable to produce metal particles which could effectively bond with mercury to form an amalgam whilst having a lower silver content than heretofore.

Surprisingly, it has now been found that alloy powders containing particles with a core containing a minor proportion of silver and a peripheral silver alloy layer can form dental amalgams with mercury having properties comparable to amalgams prepared from previously known alloy powders.

In accordance with the present invention there is provided a metallic powder containing particles comprising a core containing a minor proportion of silver and a peripheral layer of a silver-tin alloy surrounding the core. The core may consist of a base metal such as copper or nickel, or a metal alloy containing a major proportion of base metal or consisting of base metals such as a nickelcopper alloy. Any base metal or alloy can be used for the core providing it lends itself to bonding to the peripheral layer. The core preferably contains less than 40% by weight silver, more preferably less than 20%, most preferably less than 10%, yet most preferably less than 5%, and may be entirely free from silver. The cores of the particles of the present invention whether metallic or, as will be described hereinafter, non-metallic, may be solid or hollow.For example, the core could be gasified during production to render it hollow.

The peripheral layer contains silver and tin, preferably in proportions ranging from 35% to 85% (more preferably about 50 to 70%) by weight and 7.5% to 40% (more preferably about 20 to 35%) by weight respectively. The peripheral layer may also contain copper preferably in amounts from 0 to 40% (more preferably about 5 to 20%) by weight. The preferred proportions of silver, tin and copper recited above will result in a peripheral alloy layer which when alloyed with mercury will produce an amalgam suitable for dental purposes. The peripheral layer may also contain small amounts, such as 05% typically 02% by weight, of other amalgamable metals such as zinc, indium, aluminium, gold, gallium and/or cadmium.The inclusion of such other metals may be necessary to achieve physical properties of a satisfactory degree such as compressive strength, static creep, and residual gamma 2 content of an amalgam derived from the alloy.

The particles of the metal powder of the present invention may have a size in the range from 1 to 100 microns preferably in the range from 1 to 45 microns with the majority of particles lying within the size range from 5 to 40 microns.

The base metal core may have any configuration such as spherical or semi-spherical but it may be irregular and may have, for example, chip configuration.

The metallic powders of the present invention may be made by a variety of techniques all of which involve coating the base metal core with layers of the metal components of the peripheral alloy layer.

Where the base metal core is copper the copper can be chemically precipitated from copper sulphate solution using pure zinc anodes placed in the copper sulphate solution which is acidified by means of dilute sulphuric acid. The copper precipitates as a flake which is washed, dried and ball milled to obtain microscopic particles which are then sieved to obtain particles with a maximum size of, for example, up to 40 microns i.e. minus 400 mesh.

The resulting particles are in chip form and may be rendered spherical or semi-spherical by a technique to be described hereinafter. The copper sphere (including semi-sphere where referred to hereinafter) is then coated with the components of the peripheral alloy layer to be produced.

The coating may be achieved by a variety of techniques such as rumbling, electroplating or electroless plating. These techniques will now be described in relation to a copper metal core but it should be understood that they are equally applicable to other cores.

In the rumbling method it has been found that continual rumbling of different types of powder result in the powders physically bonding to each other. In the present case the following procedure may be used. The copper spheres are washed in a solvent in which is dissolved a resin. The copper spheres are then dried leaving a thin layer of resin around each sphere. A predetermined weight of dried copper spheres are then added to a conventional rumbler and predetermined weights of tin and silver flake of minus 400 mesh size are added to the rumbler. The silver and tin can be deposited simultaneously or successively, Rumbling is continued for a time sufficient to ensure that all the silver and tin has adhered to the copper spheres.

After the coated copper spheres are removed from the rumbler they may then be washed in copper sulphate solution to chemically deposit a thin layer of copper around the sphere. The particles are then dried and subjected to a heat treatment to be described hereinafter so that the build up of silver and tin and optionally copper is alloyed together and bonded to the copper core to form the peripheral layer of the present invention.

The particles are then heated and chemically treated by washing in hydrochloric acid to ensure that they are free of oxide. Finally, the particles may be heat treated in conventional manner to subject them to the traditional ageing process used for dental amalgam alloys.

In the electroplating method the coating may be carried out in an electroplating barrel in which each component of the peripheral layer is plated sequentially onto the core. In the present case the following procedure may be used. The electroplating barrel typically contains a number of stainless steel electrodes which are electrically connected in such a manner as to produce the desired polarity. The copper spheres are placed in suspension in an electrolyte containing ions of the metal to be deposited. The barrel is rotated to agitate the materials contained therein and current supplied across the electrodes. This causes deposition of metal from the solution on to the copper spheres. The thickness of the deposited metal can be predetermined by trial and error and measured by microscopic measurement of individual particles and analysis of the electrolyte.In this method, the components of the peripheral alloy layer are plated sequentially on to the core. The coated particles are then heat treated to alloy the coated layers and subsequently treated as described above for the rumbling method.

The electroless plating method may be carried out by the following procedure. Firstly, the copper spheres are acid etched by immersion in sulphuric acid. By using conventional electroless plating techniques the tin component may be deposited on the copper spheres by rumbling the spheres in a container containing a tin supplying electroless plating solution. Once the desired thickness of tin is obtained (determined by electrolyte analysis or particle measurement) the particles are removed from the electrolyte, washed and placed in a similar rumbling container containing the silver supplying electroless plating solution. The procedure used for tin is repeated until the desired thickness of silver has been deposited.

Finally, a copper electroless plating solution may be used to deposit the required amount of copper on the particles. The coated particles are then heat treated to alloy the coated layers and subsequently treated as for the rumbling method.

In each of the above methods the order of deposition of the components which are to produce the peripheral alloyed layer is not critical.

For example, the silver could be deposited first followed by the tin or the copper or the copper could be deposited first followed by the other components. The only important criterion is that the final product has a uniform peripheral alloy layer. Coating the copper last has the advantage that after the alloying procedure the absence of a copper colour could indicate complete alloying.

For dental purposes, the use of pure copper as the core material has the disadvantage that the copper will oxidise on exposure to air. When dental amalgam has been placed in a cavity it is subject to polishing by the dental surgeon.

Particles of the present invention which are subject to the dentists polishing instruments will be cut through and thus the copper core exposed.

This is not a great problem but has the disadvantage that the copper oxidises quickly in the mouth.

Thus, for dental purposes, it is preferred to use other base metal cores which are not so readily oxidised. A nickel-copper alloy or nickel metal could be used for the core. Nickei in particular does not oxidise readily in the mouth and thus the resulting amalgam would always be shiny. As mentioned above any metallic substance can be used for the metal core but for dental purposes it is preferred to use those which do not oxidise readily and which are of high strength.

For dental purposes, the peripheral alloy layer of the particles should be at least 2 micron thick since it has been shown that approximately the outer 1.5 micron of an alloy particle is used in the mercury-alloy reaction when producing amalgam.

Preferably, the layers are at least three microns thick, more preferably at least 4 microns thick. A 4 micron thick layer containing 75% by weight of silver, 25% by weight of tin and 5% by weight of copper would be formed from a 2.8 micron thick layer of silver, a 1.0 micron thick layer of tin and and 0.2 micron thick layer of copper.

The base metal core may either be prepared by chemical double decomposition as described hereinabove for copper or it may be prepared by casting a round ingot of base metal and making filings from the ingot in conventional manner. The filings would be subject to further particle size reduction such as by ball milling or pin milling, and then converted into spherical or semi-spherical or chip configuration.

The present invention is equally as applicable to metal cores in the chip configuration as in the spherical configuration. Providing the peripheral layer is uniform a chip metallic powder of the present invention will perform in the same way as a conventional fully alloyed dental amalgam alloy.

The metallic powder of the present invention with a metallic core preferably contains from 2 to 40% by weight of silver more preferably from 5 to 25% by weight such as about 1 5% by weight.

Correspondingly, the proportion of tin preferably ranges from 1 to 9% by weight. The overall percentage of copper depends on whether it is the core material. If it is then the powder preferably contains 75% to 95% by weight of copper. If copper is not used as the core material but is used in the peripheral layer then it can be used in amounts as low as 0.01%.

Where the core is non-metallic the metallic powder of the present invention preferably contains from about 10 to 90% silver, from about 1 to 30% tin and, optionally, from, 1 to 10% copper.

Apart from the cost saving achieved by reduction in the amount of silver required in the metallic powder of the present invention, it has also been found that the amount of gamma 2 phase produced in the resulting amalgams may be controlled. Gamma 2 phase is a known disadvantageous component of amalgams. Also, the actual strength of the amalgam may be increased because strong base metals can be used as the core material, whilst the uniform peripheral layer of alloy produces the required matrix to ensure strong attachment to the particle.

The present invention also provides in another embodiment a metallic powder in which the core of the particles is non-metallic. In this embodiment, the core may be formed of an inorganic mineral such as glass. An advantage of using a non-metallic core is that it can be so coloured as to produce an amalgam having a toothlike colour, or at least a colour lighter than produced by conventional amalgams, when the amalgam is cut and polished by dental instruments. Further, a core formed of, for example, glass will not oxidise upon exposure to air and thus will not change colour.

A metallic powder in accordance with this embodiment of the present invention may be prepared by first taking a glass powder of suitable strength characteristics and making the particles spherical by a method to be described hereinafter.

Alternatively, the particles may be left in the form of chips.

The glass particles are then subjected to treatment with hydrofluoric acid or other glass etching materials to etch the glass to give it a good surface for attachment of alloy components.

The particles are then subject to treatment with chemicals conventionally used in electroplating prior to plating metals onto glass, such as palladium chloride solution.

Then the particles are treated to coat them with alloy components as is described above for essentially base metal cores and the same parameters apply. The coated particles are treated in the same manner as the base metal core particles to produce the alloy layer and subjected to the same post-treatments.

In this embodiment of the present invention, it is preferred that the particles be fine since in this case less matrix area is exposed at the surface of the amalgam when the amalgam is cut and polished. The handling properties of the amalgams produced using powders of this embodiment of the present invention are more similar to conventional amalgams compared to conventional and anterior cements which are currently in use. These cements contain glass as a filler and are bonded together using resins. They have particularly difficult handling properties being, for example, extremely sticky.In the present invention, the metals coated on the core are preferably alloyed by entraining the particles in a stream of an inert carrier gas under pressure, passing the stream of inert carrier gas containing the particles through a heating zone so as to melt the surface stratum and then cooling the particles.

This process is preferably carried out in a closed container containing an atmosphere of an inert gas such as nitrogen or air. Also, the carrier gas is preferably inert and may be nitrogen or compressed air. The carrier gas may have a pressure of 40 to 100 psi but 45 to 55 psi is preferred. The treated particles are preferably cooled by being immersed in a liquid coolant such as water.

The heating zone can be in the form of a flame and the particles are passed through the reducing section of the flame.

The heating zone may be created by a high frequency induction coil which creates a heat plasma.

Simiiarly, core particles such as the copper chips or glass particles described hereinabove can be made spherical or semi-spherical by being heat treated by the above method. The above described heat treating method is described in detail below.

In accordance with the present invention there is further particle treatment process wherein the particles have at least a surface stratum capable of being melted, which process comprises entraining the particles in a stream of an inert carrier gas under pressure, passing the stream of inert carrier gas containing the particles through a heating zone so as to melt at least the surface stratum and then cooling the particles so as to solidify the melted portions of the particles.

The process of the present invention has a number of applications.

For example, metal alloy particles may be cut from an ingot of the alloy and such particles are of irregular, angular shape. For some uses, such as alloys for producing dental amalgams, it is desirable to have spherical or semi-spherical particles. The alloy particles cut from an ingot may be rendered spherical or semi-spherical by the process of the present invention as is described above.

Also, the process of the present invention can be used to spheridise non-metallic particles such as glass particles, as is also described above.

Further, where a particle contains a peripheral layer or layers of unalloyed metals the metals can be alloyed by the process of the present invention.

Dental amalgam alloy particles containing a peripheral alloy layer are described above.

The process of the present invention is preferably carried out in a closed container containing an atmosphere of inert gas. The inert gas can be any gas which does not interact with the particles so as to alter their chemical composition under the conditions of the process.

Preferably, the inert gas is nitrogen but for most purposes air can be used. Similarly, the carrier gas in which the particles are entrained is also preferably inert and nitrogen is preferred but compressed air is usually satisfactory. The carrier gas in which the particles are entrained may have a pressure in the range from 40 to 100 psi but a pressure in the range from 45 to 55 psi is preferred.

The heat treated particles are preferably cooled by being immersed in a liquid coolant. Preferably, the liquid coolant is water but any liquid which does not interact with the particles can be used.

The heating zone can be in the form of a flame.

The flame can be produced from a combustible gas mixture at high pressure. The particles are preferably passed through the reducing section of the flame so as to avoid the possibility of the particles being exidised although this is clearly not critical where the particles are of a non-oxidisable composition. The flame may be produced by combustion of any gas which produces a temperature sufficient to achieve the desired end result. Examples of suitable gases are oxy acetylene, hydrogen and liquid petroleum gas.

The particles are passed through the flame at a rate which is controlled by the carrier gas pressure. Too fast a speed will result in an insufficiently treated particle whilst too slow a speed will result in the particle being heated excessively. In another embodiment of the process of the present invention, the heating zone is produced by means of a high frequency induction coil which creates a heat plasma. The entrained particles are passed through a tube and whilst in the tube pass through a high frequency field created by the coil which is connected to a conventional high frequency generator.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the process of the present invention in accordance with a first embodiment; and Figure 2 is a schematic illustration of the process of the present invention in accordance with a second embodiment. In Figure 1, there is shown a nozzle 10 in communication with a conduit 12. A combustible gas mixture is passed in the direction of the arrow B through the conduit 12 and nozzle 10 at high pressure and combusted at the exit of the nozzle 10 to produce a flame 14. The flame comprises a reducing section 1 6.

A jet 1 8 is orientated towards the reducing section 1 6 of the flame 14 and terminates a short distance from the flame 14. Particles to be treated are entrained in a carrier gas and passed through the jet 18 in the direction of the Arrow A.

The entrained particles leave the jet 1 8 and pass through the reducing section 1 6 wherein they are heated such that at least a surface stratum is melted.

The particles are then passed into a liquid coolant 20 wherein they are cooled. The cooled particles sink to the bottom of the coolant for subsequent retrieval.

In Figure 2, there is shown a vertically orientated cylindrical tube 22 which may be of 3" diameter and 488 length. A jet 1 8 similar to that shown in Figure 1, is located adjacent the upper end of the tube 22 and is axially aligned therewith. The lower end of the tube 22 is immersed in a liquid coolant 20.

A high frequency induction coil 24 is wrapped around the cylinder 22. The coil 24 is connected to a high frequency generator (not shown).

In use, the entrained particles pass through the tube 22 and the heating zone created by the coil 24. During passage through the heating zone the particles are heated such that at least a surface stratum is melted.

The particles then pass into the liquid coolant 20. Vents 26 are provided in the tube 22 just about the liquid surface to allow excess gas to escape.

In each embodiment described above, the whole treatment is performed in a closed container (not shown) containing an inert gas.

In the embodiment described the treatment process ensures that each particle is separated from other particles during the heating step and during the time the particle is cooling after entering the coolant. This avoids the possibility of the particles adhering to one another whilst the surfaces therof are in melted condition.

The present invention will now be exemplified in the following example.

Example Copper powder was chemically made by depositing from copper sulphate and the resultant powder was dried, ball-milled and atomised during the process described above.

The resultant spherical powder was sieved through a 400 BSS sieve with the oversize being recycled. The portion which was minus 400 consisted of the following particle size analysis: +30 micron diameter 41% -30+20 micron diameter 30% -20 micron diameter 29% This powder blend was acid etched and subjected to electroplating by silver, tin and copper in such a way as to bulid up an overall peripheral thickness of approximately 2 microns per particle. The resultant electroplated alloy was then subjected to the atomising process described above using such conditions as to effectively alloy the peripheral elements and form a layer containing by weight silver 60%, tin 27% copper 13%.

The resultant powder particles were then collected and screened through a 400 BSS mesh sieve and were then heat treated and washed.

This powder was then triturated for 10 seconds with approximately 50% mercury. Tests were performed on the resultant amalgam according to specifications for dental amalgam alloys as dictated byA.D.A. The results obtained were as follows: Tensile strength (24 hours) 8,050 psi Compressive strength (1 hour) 35,353 psi Compressive strength (24 hours) 73,650 psi Static creep (7 days) 0.02% Dimensional change +0.05% Corrosion resistance (Sodium sulphide test) Excellent Modifications and variations such as would be apparent to a skilled addressee are deemed within the scope of the present invention.

Claims

1. A metallic powder characterised in that it contains particles comprising a core which contains a minor proportion of silver, and a peripheral layer of a silver-tin alloy surrounding the core.

2. A metallic powder as claimed in Claim 1, characterised in that the core contains a major proportion of base metal.

3. A metallic powder as claimed in Claim 2, characterised in that the core contains less than 5% by weight silver.

4. A metallic powder as claimed in Claim 3, characterised in that the core is formed of a material selected from the group consisting of copper, nickel or copper-nickel alloy.

5. A metallic powder as claimed in Claim 1, characterised in that the core is non-metallic.

6. A metallic powder as claimed in Claim 5, characterised in that the core is formed of an inorganic material.

7. A metallic powder as claimed in Claim 6, characterised in that the core is formed of glass.

8. A metallic powder as claimed in any one of the preceding claims characterised in that the peripheral layer additionally contains copper.

9. A metallic powder as claimed in any one of claims 1 to 7, characterised in that the peripheral layer contains from about 3596 to about 8596 by weight silver, from about 7.5% to about 40% by weight tin and from about 0 to 40% by weight copper.

10. A metallic powder as claimed in Claim 9, characterised in that the peripheral layer contains from about 50 to 70% by weight silver, from about 20 to 35% by weight tin and from about 5 to 20% by weight copper.

11. A metallic powder as claimed in any one of the preceding claims, in which the particles have a size in the range from about 1 to 45 microns with the majority of particles lying within the size from about 5 to 40 microns.

12. A metallic pcwder as claimed in Claim 1, characterised in that the peripheral layer is applied to the core by a rumbling technique comprising forming a resin coating around the core particles, contacting the coated core particles with particulate silver and tin and rumbling the particles for a time sufficient to bond the silver and tin to the coated core particles, and then heat treating the rumbled particles to alloy the components of the peripheral layer.

13. A metallic powder as claimed in Claim 12, characterised in that the rumbled particles are subsequently contacted with a copper bearing solution to chemically deposit a layer of copper around the rumbled particles.

14. A metallic powder as claimed in Claim 1, characterised in that the peripheral laver is applied to the core by an electroplating technique comprising immersing the core particles sequentially in solutions containing metal ions corresponding to the metals of the peripheral layer and during each immersion applying an electrical potential to the solution to cause deposition of metal on the core particles, and then heat treating the particles to alloy the components of the peripheral layer.

15. A metallic powder as claimed in Claim 1, characterised in that the peripheral layer is applied to the core by an electroless plating technique comprising immersing the core particles sequentially in electroless plating solutions containing metal ions corresponding to the metals of the peripheral layer to cause deposition of metal on the core particles, and then heat treating the particles to alloy the components of the peripheral layer.

1 6. A metallic powder as claimed in any one of the preceding claims, characterised in that the peripheral layer is at least 1 micron thick.

17. A metallic powder as claimed in Claim 16, characterised in that the peripheral layer is at least 2 microns thick.

18. A metallic powder as claimed in Claim 17, characterised in that the peripheral layer is at least 4 microns thick.

1 9. A metallic powder as claimed in any one of claims 2 to 4, characterised in that it contains from about 2 to 40% by weight silver and from about 1 to 9% by weight tin.

20. A metallic powder as claimed in Claim 19, characterised in that it contains from about 5 to 25% by weight silver and from about 1 to 9% tin.

21. A metallic powder as claimed in any one of claims 5 to 7, characterised in that it contains from about 10 to 90% by weight silver, and from about 1 to 30% by weight tin.

22. A metallic powder as claimed in Claim 21, characterised in that it contains from about 10 to 90% by weight silver, from about 1 to 30% by weight tin and from about 1 to 10% copper.

23. A metallic powder as claimed in Claim 1, characterised in that the components of the peripheral layer are alloyed by entraining core particles having the metals of the peripheral layer bonded thereto, in a stream of an inert carrier gas under pressure, passing the stream of inert carrier gas containing the particles through a heating zone so as to melt the metals of the peripheral layer and then cooling the particles.

24. A particle treatment process wherein the particles have at least a surface stratum capable of being melted, which process comprises entraining the particles in a stream of an inert carrier gas under pressure, passing the stream, of inert carrier gas containing the particles through a heating zone so as to melt at least the surface stratum and then cooling the particles so as to solidify the melted portions of the particles.

25. A metallic powder substantially as hereinbefore described in the examples.

26. A particle treatment process substantially as hereinbefore described.