Note: Descriptions are shown in the official language in which they were submitted.
~9~
The present invention relates to metallic p~wders~
Alloy powders which are used to produce dental
amalgams upon admixture with m~rcury are typically composed of
a particulate silver and tin alloy which may contain a
proportion of copper or zinc. .~ach particle contains a
relatively high proportion of silver ranging from at least 4~/0
by weight up to as high as 75% by weight. Further, the
composition of each particle is essentially the same through-
out its entire cross-section although thexe may be minor
variations between the surface and the core of the particle.
Sil.ver 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 o~ silver and a peripheral
layer of a silver-tin alloy surrounding the coreO The core
may consist of a base metal such as copper or nic~el, or a
metal alloy containing a major proportion of base metal or
consisting of base metals such as a nickel-copper alloy.
Any base metal or alloy can be used for the core providing it
lends itself to bonding to the peripheral layerO The core
preferably contains less than 40/0 by weight silver~, more
preferably less than 2~/o~ mos~ preferably less than 10%~ yet
most preferably less than 5%, and may be entirely ~ree from
silver. The cores of the particles of the present invention
whe~er metallic or, as will ba described hereinafter, non-
metallic, may be solid or hollow. For example, the core
could be gasified during produ~i-on to render it hollow.
Tha peripheral layer contains silver and tin, pre-
ferably in proportions ranging from 35% to 85%(more preferably ,
about 50 to 70%)by weight and 7.5% to 40/O (more preferably
about 20 to 35%) by weight respectively. The peripheral layer
may also contain copper preferably in amounts from to 40/0
(more preferably about 5 to 2~/o) 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 0-5%
typically 0 - 2% 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 resid!.lal galnma 2 content of an
amalgam derived from the alloy.
The paxticles of the meta~ powder of the present
invention may have a size in the range rom 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 con~iguration.
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.
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Where the base metal core is copper th~ copp~r can be
chemically precipita~ed from copper sulphate solution using pure
zinc clnodes 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 particl~s ~hich are then seived to obtain
particles with a maxim~m size of, ~or 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 b~r a technique to be des-
cribed hereinafter. The copper sphere ~including semi-sphere
where referred to hereinafter) is then coated with the c~mponents of
~heperipheral 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 contin-
ual rumbling of different types of powder result in the p3wders
physically bonding to each other~ In the presen~ case the following
procedure may be used. m e copper spheres are washed in a solvent
in which is dissolved a resin. The copper spheres are then dri0d
leaving a thin layer of resin an3und each sphere. A predetermined
weight of dried c~pper spheres are then added to a conventio i
rumbler and predetermined weights of tin and silver flake of DLLnus
400 mesh size are added to the r~nbler. The silver and tin can be
desposited sinultaneously or successively. R~bling is continued
for a time sufficient of ensure that all the silver and tin has
adhered to the copper spheres.
After the coated copper spheres are rem~v~d from the
ru¢~31er they may then be washed in copper sulphate solution
to ch~ically deposit a thin layer of copper around the
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f
sp~ere. The p~rticles are then dried and subjected to a heat
txeatment to be described hexei~after 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
treaked 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 coa~ing may be
carried out in an electroplating barrel in which each compon-
ent 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 o~ 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 thick-
ness o~ the deposited metal can be predetermined by trial and
error and measured by miscroscopic measurement of individual
particles and analysis of ths 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 ~nd subse~uently
treated as described above for the rumbling method.
The e~ectroless plating method may be carried out
;
by the following procedure. Firstly, the copper spheres
are acid etched by immersion in ~ulphuric acid. By using
conventional electroless plating techni~les 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 remov~d from the electrolyte, washed and
placed in a similar rumbling container containing the silver
supplying electrole~s plating solution. The prccedure used
~or tin is repeated until the desired thickness of silver has
been deposited.
Finally, a copper electroless pla~ing solution may
be used to deposit the required amount of copper on the
particles. The coated parti~les are then heat treated to
alloy the coated layers and subsequently treated as for the
rumbling method.
In each of the above ~ethods 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 compon-
ents. 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 com-
plete alloying.
For dental purposes, the use of purP copper as the
core material has the disaavantage that the copper will
oxidise on exposure to air. When dental amalgam has been
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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 he 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~
1~ ~ickel in particular does ~ot 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 w~ich 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 7~% by weight of silver, 25% by weight
of tin and 5% by weight of copper would be formed from a 2.8
micron ~hick layex of silver, a 1.0 micron thick layer of tin
and an O.Z ~icron thick layer of copperO
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
9~
netal an~ naXing fil mgs frcm the ingot in conventional ~nner.
The filings wDuld ke subject to further particle size reduction
such as by kall 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 perfonm in the same way as a con-
ventional fully alloyed dental ~malgam alloy.
The metallic powder of the present invention with a
metallic core preferably contains from 2 to 40% by weight of si~ver
more preferably from 5 to 25% by ~ight such as about 15% by weight.
Cbrrespondingly, tlle proportion of tin preferably ranges from
1 to 9% by weight. The overall percentage of copper depends on
whe~her it is the core material. If it is then the powder pre-
ferably contain~ 75% to 95% by weight of copper. If copper is
not us~d as the core material but is used in the peripheral
lay~r 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, frcm about 1 to 30% tin a~d, optionally, from 1 to 10% copper.
Apart from the cost saving achieved by redw tion in the
am~und of silver required in the metallic powder of the present
invention, it has also been found that the am~unt of
gamma 2 phase produced in the resulting amalgams may be con-
trolled. 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
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the particle.
The present invention also provides in another
embodiment a ~etallic powder in which the core of the particles
is non-metallicO 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 that 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 embodi-
ment of the present invention may be prepared by ~irst taking
a glass powder o~ 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 ~hips.
The glass particles are then subjected to treatment
with hydrofluoric acid or o~her 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 ~or essentially base
metal cores and the same parameters apply. The coated paxt-
icles 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
9~37
.
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
pr~sent invention are more similar to conventional amalgams
compared to conventional and anterior cements which are
currently in use. rrhese cements contain glass as a filler
and are bonded together using resins. They hav~ particularly
difficult handling properties being, for example, extremely
stic~y. 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 carxied 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. r~he 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.
rrhe heating zone can be in the form of a flame and
the particles are passed through the reaucing section of the
flame.
The heating zone may be created by a high frequency
induction coil which ~reates a heat plasma.
Similarly, core particles such as the copper chips
or g}ass particles described hereinabove can be made spherical
or semi-spherical by being heat treated by the above method.
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t ',
~he above described heat treating method is described in
detail below,
In accordance with the present invention there is
fuxther provided a particle treatmen~ 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 throuyh
a heating zone so as to melt at least the surface stratum and
lo 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 xendered 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 conkaining a peripheral alloy layer axe
described above.
The process of the present invention is preferably
carried out in a closed container containing an atmosphere of
. ~4 . . .
~9~L~7
inert gas~ The inert gas can be any gas which does not inter-
act with the particles 50 as to alter their chemical comp~sit-
ion under the conditions o 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 com-
pressed air is usually satisfactory. ~he carrier gas in w~ich
the particles are Pntrained 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 liguid coolant. Preferably, the liquid
coolant is water but any 1 quid 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 comhustible gas mixture at
high pressure. The particles are preferably passed through
the reducing section of the flame so as to avoid the psssibil-
ity of the particles being oxidised 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 temp~rature 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 prassure. 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 em~odiment of the process of
the present invention, the heating zone is produced by means
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37
>
of a high frequency induction coil which creates a heat
plasma. The entrained particles are passed through a tube
ana whilst in the tube pass through a high frequency ~ield
created by the coil which is connected to a conventional high
frsquency 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 yas mixture is passed in the
direction of the arrsw 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
16.
A jet 18 is orientated towards the reducing section
16 of the ~lame 14 and terminates a short distance from the
flame 14. Particles to be treated are entrained in a carrier
gas ~nd passed through the jet 18 in the direction of the
Arr~w A. The entrained particles leave the jet 1~ and pas~
through the reducing section 16 wherein they are heated such
that at least a surface stratum i5 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 i5 shown a vertically orientated
cylindrical tube 22 which may be of 3" diameter nd 48"
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:
length. A jet 18 similar to that shown in Figure 1, i5
located adjac~nt 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 fre~uency induction coil 24 is wrapped around
the cylinder 22. The coil 24 is connect~d 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 possib-
ility of the particles adhering to one another whilst the
- surfaces thereof 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 sp~erical powder was sieved through a
400 BSS sieve with the oversize being recycled. The portion
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which wa~ minu~ 400 c~nsi ~ed of the following particle size
analysis:
~ 30 micron diameter 41%
- 30 ~ 20 micron diamet~r 3~/~
- 20 mi~ron diameter 2~/o
This powder blend was acid etched and subjected to
electroplating by silver, tin and copper in such a way as to
build up an overall periphexal thicknass 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 60D/o, 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 trituxated for 10 saconds
with approximately 5~/O mercury. Tests were performed on the
resultant amalgam according to specifications for dental amalgam
alloysas dictated by A.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) 73,65~ psi
Static creep (7 days) O~Q2%
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.
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