Note: Descriptions are shown in the official language in which they were submitted.
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Description:
Bath for the electrodeposition of gold and gold alloys,
and its use
The invention relates primarily to a bath for
electrodeposition of gold and gold alloys and to its
use. In this bath, the gold is in the form of a gold
sulfite complex.
It has already been known for a very long time to
electrodeposit gold or gold alloys from preferably
aqueous solutions which contain the gold and/or the
corresponding alloy metals. After predominantly
cyanide-based gold baths were initially used, in recent
times baths based on gold sulfite complexes have been
becoming increasingly important. This trend was
attributable primarily to the fact that the gold
sulfite baths are non-toxic compared to the
cyanide-based gold baths in which hydrogen cyanide is
known to be released. This non-toxicity and the good
quality of the layers deposited has led to the gold
sulfite baths being used to an ever increasing extent,
in particular in the field of dental technology,
despite their higher production costs and despite the
problems with the stability of the baths. Furthermore,
baths based on gold sulfite complexes are relatively
easy to handle, which is an important factor for users
without high levels of specialist chemical engineering
knowledge, such as dental technicians, dentists and
their staff.
Particularly in the field of dental technology,
particular demands are imposed on deposits formed by
electroplating. In addition, these demands also vary
according to the type of dental frame or prosthetic
molding produced. For example, a homogenous layer
build-up, i.e. a homogenous microstructure, a layer
thickness which is as uniform as possible and a
s
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reproducible composition of the deposited layer are
preconditions if it is subsequently to be possible for
a ceramic or plastic veneer to be applied to the
molding. This applies in particular to ceramic veneers,
for which the molding has to be fired at relatively
high temperatures after the ceramic material has been
applied. In these cases, the metallic basic framework
also has to have the required firing stability. Minimum
demands also have to be satisfied with regard to
further properties, such as wear resistance, porosity,
corrosion resistance, inter alia. Moreover, the layers
deposited, in particular in the dental sector, have to
comply with particular aesthetic requirements, for
example relating to the color, the shine or the surface
condition. Finally, certain further demands may be
imposed on the composition of the deposited layers, for
example with regard to biocompatibility.
Biocompatibility of the materials may be particularly
important especially in the dental sector, since gold
or gold alloy layers with the maximum possible purity
are required for example for patients suffering from
allergies.
Irrespective of their field of use and irrespective of
the form in which the gold is present in the bath, gold
and gold alloy baths contain certain additives in order
to at least partially comply with the requirements
imposed on the electroplated deposits. Additives of
this type are also known as fine-grain additives or
shine additives. These may be organic additives, such
as polyamines, polyimines and mixtures thereof or
semimetal compounds, for example of arsenic, antimony
or thallium. All the additives mentioned may be
incorporated to a greater or lesser extent in the
deposited gold layer. In the case of the organic
additives, this causes problems in the dental sector,
since the layer properties (e. g. ductility and firing
stability) may be adversely affected by this
incorporation. The incorporation of the semimetals
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causes problems in the dental sector in particular in
the case of arsenic and thallium, since the required
biocompatibility is then no longer ensured on account
of the use of these toxic substances. The result of
this is that, as far as the Applicant is aware,
currently only antimony has gained any significance as
an additive in the dental sector. However, in
physiological terms it is by no means undesirable for
the antimony compounds used to be replaced. However,
when prosthetic moldings are being lined with dental
ceramic, no metal compounds other than antimony
compounds have proven suitable for reasons of firing
stability.
A further problem involved with the use of previously
known additives for gold and gold alloy baths, in
particular for baths based on gold sulfite complexes,
is that these additives generally have to be metered in
immediately before the baths in question are used. This
is because the compounds which are present in these
additives are not stable in the baths in question, but
rather decompose over the course of time, losing their
effectiveness. This may be caused, for example, by the
pH of the baths in question or by the fact that the
additives react with other constituents contained in
the bath.
In the~case of the addition of antimony compounds to
baths based on gold sulfite complexes, the antimony is
generally used as Sb(III), for example as potassium
antimony tartrate. The latter reacts in the bath to
form jelly-like antimony oxide hydrate gel, which is
likely to destroy the action of this additive. For its
part, the antimony oxide hydrate gel is not stable
under the standard bath conditions and reacts to form
crystalline antimony oxide, which no longer reveals the
desired effect. This is the reason why the additive can
only be added to the bath just before use and why the
additive loses its effect after a certain time.
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Consequently, it is not possible to produce a gold or
gold alloy bath comprising all the required components
which is able to function over a prolonged period of
time.
A further problem is that the additives not only have
to be metered in at a later stage, but also that the
correct metering, i.e. the required quantity of
additive, is dependent on the other bath and process
parameters. Factors of influence in this context are,
for example, the proportions of the other constituents
in the bath, the concentration of the electroactive
ions, the geometry of the deposition vessel
(cell geometry) the temperature and the current
density. In most cases, the user attempts to solve
these problems by, on account of his lack of specialist
chemical engineering knowledge, working according to
what is known as a metering table provided by the
manufacturer of the bath and measuring the quantity of
additive as a function of the number of objects to be
electroplated. Since the size and shape of the objects
which are to be electroplated and the desired layer
thickness of the deposit vary considerably, and
accordingly so does the quantity of metal to be
deposited, metering per object in this way is subject
to a relatively high error level. This can lead to very
varying qualities of the electroplated deposits, and
consequently even objects which have been coated at the
same time in a single operation may differ in terms of
the composition of the deposit. This can make
deposition difficult for the user to manage.
EP-B1 0 126 921 has described an aqueous bath for the
electrodeposition of gold-copper-bismuth alloys that
contains the gold in the form of a gold cyanide
complex. This involves the deposition of ternary alloys
with high bismuth contents. The bath described in that
document is particularly suitable for the deposition of
pink-to violet-colored coatings on decorative objects,
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such as for example jewelry, watches and spectacles.
The technical significance is supposed to lie in the
fact that the bismuth can be incorporated in the alloys
in extremely high levels of up to 30~ by weight and
above. This is intended to open up new application
areas, such as for example the treatment of electronic
components, such as plug connections, since the
corresponding deposits are particularly hard and have a
good electrical conductivity and resistance to
abrasion. The baths described in EP-B1 0 126 921 are
unsuitable for the dental sector, inter alia both on
account of their high toxicity and on account of the
fact that high levels of the bismuth are to be
incorporated in the alloy.
DE-C2 2 723 910 (corresponds to FR-A 2353656) claims a
multiplicity of additive mixtures for baths for the
electrolytic deposition of gold or gold alloys. These'
additional mixtures are intended to improve the
properties of the deposits formed. Compulsory
constituents of these additive mixtures are at least
one organic water-soluble nitro compound of a certain
general formula and at least one water-soluble metal
compound of an element selected from the group
consisting of arsenic, antimony, bismuth, thallium and
selenium. Additive mixtures which, in addition to the
nitro compound, also contain a water-soluble bismuth
compound are in this case too restricted to use in
cyanide-based baths. In the case of baths based on a
gold sulfite complex, this document proposes the use of
an additive comprising nitro acid and
antimony-potassium double tartrate. The use of the
additive mixtures mentioned in DE-C2 2 723 910 and of
the gold baths produced therefrom is restricted to the
technical application of plating electronic components
for semiconductor technology.
Furthermore, US-A-5,277,790 has disclosed an additive
for a bath based on a gold sulfite complex which
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likewise has to contain both an organic polyamine or a
mixture of polyamines and an aromatic organic nitro
compound. DE-A1 3 400 670 describes a bath based on
gold sulfite complex which contains an additive
comprising water-soluble thallium salt and a carboxylic
acid which is free of hydroxyl and amino groups.
The invention is based on the object of providing a
bath for the electrodeposition of gold and gold alloys
which at least partially avoids the drawbacks outlined
above. In particular, it is intended to make the
production of prosthetic dental moldings by
electrodeposition even more reliable and safer and to
further simplify handling of the baths used for this
purpose. Furthermore, it is intended to create the
option of providing the user with a bath which has
already been provided with all the required
constituents and additives and is therefore able to
function. Finally, the intention is for it to be
possible for the baths in question to be operated
substantially with biocompatible, i.e. physiologically
harmless compounds without the quality of the layers
deposited being adversely affected.
This object is achieved by the bath having the features
of Claim 1 and by the uses having the features of
Claims 20 and 21. Preferred embodiments of these
subjects of the invention are explained in the
dependent Claims 2 to 19 and 22 to 27. The wording of
all the claims is hereby incorporated by reference in
the content of the present description.
The bath according to the invention for the
electrodeposition of gold and gold alloys based on a
gold sulfite complex is distinguished by the fact that
in addition to any further metal compounds which may be
present and other standard additives for such gold
sulfite baths, it contains at least one bismuth
compound. This bismuth compound is preferably a
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water-soluble bismuth compound, which results in the
bath itself also preferably being an aqueous bath.
In principle, the bismuth compound used may be any
suitable inorganic or organic bismuth compound. The
bismuth compound is preferably a complex compound,
preferably what is known as a chelate compound.
Compounds of this type are known to be cyclic compounds
in which a ligand (complex-forming agent) occupies a
plurality of coordination sites of a central atom
(metal), and consequently are generally particularly
stable complex compounds. It is also preferable
according to the invention if the bismuth compound
contains an organic complex-forming agent, preferably
an organic chelate-forming agent. Examples of complex-
forming agents or chelate-forming agents in this
context are in particular NTA (nitrilotriacetic acid),
HEDTA (N-(2-hydroxyethyl)ethylenediaminetriacetic
acid), TEPA (tetraethylene pentamine), DTPA
(diethylenetriaminepentaacetic acid), EDNTA
(ethylenedinitrilotetraacetic acid) and the preferred
complex-forming agent/chelate-forming agent EDTA
(ethylenediaminetetraacetic acid).
Other examples of bismuth compounds which can be used
in accordance with the invention are water-soluble
bismuth salts (e. g. sulfates, nitrates, sulfamates,
phosphates, pyrophosphates, acetates, citrates,
phosphonates, carbonates, oxides, hydroxides, inter
alia). In addition to the preferred complex-forming
agents which have already been mentioned above, such as
NTA and the like, other examples of organic
complex-forming agents which can be mentioned include:
organic phosphonic acids, carboxylic acids,
dicarboxylic acids, polyoxycarboxylic acids,
hydroxycarboxylic acids, diketones, diphenols,
salicylaldehydes, polyamines, polyaminocarboxylates,
diols, polyols, dipolyamines, aminoalcohols,
aminocarboxylic acids, aminophenols.
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It is also preferable in the context of the invention
if the bismuth compound (or if appropriate a plurality
of such compounds) is(are) present in the bath in a
concentration of between 0.05 mg/1 and the saturation
concentration of this(these) bismuth compounds) in the
bath. In particular, concentrations of between 0.05
mg/1 and 1 g/1 in the bath are preferred. Low
concentrations are generally preferred, with
concentrations between 0.1 mg/1 and 10 mg/1 being
particularly useful within the latter range.
In a particularly preferred embodiment, the bath
according to the invention is substantially free of
additives which are physiologically harmful (harmful to
health), the bath preferably being free of arsenic,
antimony and thallium compounds. This ensures that
there are no compounds, in particular metals, which are
harmful to health and could restrict the usability of
the layers or of the resulting prosthetic moldings in
dental technology, incorporated in the deposited
layers. Amazingly, it has also been found that the
inventive addition of bismuth compounds is also in a
position to reduce or even prevent physiologically
harmful additives from being incorporated in the
prosthetic molding. As has already been mentioned in
the introduction, conventional gold sulfite baths
contain at least one antimony compound as an additive.
Accordingly, the antimony is incorporated in the
prosthetic molding in a concentration of normally 0.2
part per thousand. If an antimony compound, such as
potassium antimony tartrate, and a bismuth compound,
such as bismuth EDTA, are used simultaneously, however,
it has surprisingly emerged that both antimony and
bismuth are present in quantities of less than 30 ppm
or 40 ppm (these are the detection limits with the
analysis method used for these elements) in the
deposited molding. This shows on the one hand that the
bismuth itself is not incorporated in the molding and
on the other hand that the bismuth is able to
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considerably reduce the extent to which the antimony is
incorporated.
The concentration of gold in the bath according to the
invention is not fundamentally critical. It is
preferable for the gold to be present in the bath in a
concentration of between 5 and 150 g/1. In particular,
gold concentrations of between 10 and 100 g/1,
preferably between 10 and 50 g/1, in the bath are
selected. A particular advantage of the invention is
that it is possible to select gold concentrations in
the bath of between 30 and 48 g/1. These relatively
high concentrations make the bath according to the
invention particularly suitable for the rapid
deposition of thick layers, as is fundamentally
desirable in the field of production of prosthetic
moldings in dental technology. Particularly in the case
of baths with high gold concentrations, it is possible
to obtain prosthetic moldings with layer thicknesses of
approximately 200 ~m in less than 14 hours, preferably
in less than 12 hours. It is even possible, given a
suitable procedure, to deposit moldings with layer
thicknesses of this type in less than 6 hours. The
particular advantages of the invention also manifest
themselves in particular in the case of deposition
operations which are carried out in less than 2 hours,
preferably within one to two hours. In this context,
reference is also made to the examples.
In preferred embodiments of the invention, the bath
contains at least one further metal. This metal can be
incorporated in the deposited layer and is in these
cases referred to as an alloying metal. However, in
other cases it may also be used only for (improved)
deposition of the gold or gold alloy layer. This metal
may in particular be copper andiron and/or at least
one precious metal. If precious metals are added,
precious metals from what is known as the platinum
group are preferred, in particular palladium or
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platinum. Precious metals, in particular those
belonging to the platinum group, are particularly
suitable in the field of prosthetic dental moldings on
account of their high biocompatibility.
By way of example, the concentration of the further
metal in the bath can also be varied within wide limits
as a function of the alloy which it is desired to
deposit. In principle, the metals can be added in the
form of their preferably water-soluble compounds, in
particular salts, or in the form of preferably water-
soluble complex compounds. In this context, it is once
again possible to employ in particular the
complex-forming agents and chelate-forming agents which
have already been mentioned above in connection with
bismuth. The concentrations of the metal compounds may
preferably be selected to be between 0.1 mg/1 and
200 g/1. Within this range, the concentration may be
between 0.1 and 500 mg/1 and in particular between 1
and 20 mg/1. In this case too, low concentrations are
preferred. Concentrations of between 2 mg/1 and 10 mg/1
are further preferred within the latter range.
The gold sulfite complex in the bath according to the
invention may in principle be any complex which is
known from the prior art. It is preferably what is
known as ammonium-gold sulfite complex, in which,
therefore, the gold ion has been complexed by the
sulfite ions and at least one ammonium ion is present
at "counterion".
The baths according to the invention preferably have a
pH of at least 7, i.e. they are either neutral or
alkaline. In particular, the baths are (weakly
alkaline, with pHs of from 7 to 9 being preferred.
The preferred ammonium-gold sulfite complex has a range
of advantages over other gold sulfite complexes. For
example, compared to sodium/potassium-gold sulfite
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complexes, a significantly increased stability of the
complex in the gold bath is responsible for a range of
advantageous properties. These are, for example, a
longer shelf life, reduced sensitivity to impurities
and a lower light sensitivity. Moreover, baths
comprising ammonium-gold sulfite complexes can be
operated at a significantly lower pH of approximately
7-9. This makes baths of this type easier and safer for
users with a lack of specialist chemical knowledge to
handle compared to the Na/Ka-gold sulfite complex baths
with pHs of approximately 10.
Surprisingly, a particularly advantageous relationship
between chemical composition of the gold bath and
chemical composition of the deposit formed by
electrodeposition has resulted in the bath according to
the invention which is based on the preferred
ammonium-gold sulfite complex and contains at least one
bismuth compound. This relationship is improved still
further by the presence of the further metals in the
gold bath, in particular of copper and/or at least one
precious metal and/or iron. In addition, it has been
possible to determine a widened range of applications,
since in addition to plaster it is also possible to use
a wide range of dental modeling and framework materials
in the gold bath.
For example, the use of the bismuth compound has made
it possible in an amazingly simple way to accurately
control and predictably set the composition of the
electroplated gold layer and its functional properties.
This has hitherto been impossible or almost impossible
with the known gold baths which are used in dental
technology, and consequently with those baths the
composition of the deposit is generally determined by
technical factors, such as electrode geometry and the
other apparatus technology factors.
a
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As has already been mentioned a number of times, the
demands imposed on a gold bath and the layers formed by
electrodeposition are of a particular nature in the
field of dental technology, and consequently in this
sector, in addition to the requirements mentioned in
the introduction, the need for biocompatibility and for
the desired gold or gold alloy layers to be as pure as
possible should be emphasized once again. For this
reason, it is particularly important for the
composition of the deposits formed by electrodeposition
to be controlled in a targeted manner and set
reproducibly.
Furthermore, in the case of the preferred further metal
copper, it has emerged that in these preferred baths
according to the invention a specific quantitative
ratio between bismuth and copper is advantageous for
the composition of the gold layer. A surprise in this
context was that the standard parameters of influence
in electrodeposition which are known. to the person
skilled in the art, such as for example electrode
geometry, electrodeposition time, current density,
temperature, form of current, etc., have only a minor
effect on the deposition. Consequently, the copper
content in the gold layer can be set accurately and
reproducibly by setting the bismuth-copper ratio in the
bath. The high level of purity which is advantageous
for dental technology can therefore deliberately be
achieved by targeted control of a low copper content in
the layers without this adversely affecting the
functionality of the layers. The targeted incorporation
of low copper contents in the gold layer, while at the
same time avoiding incorporation of bismuth, in
addition to the high level of purity also enables the
functional properties of the gold layer to be
accurately controlled, such as for example the
hardness, shine, surface properties, color, etc.
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If gold layers which are as pure as possible are to be
deposited with copper and bismuth present in the bath,
the bismuth: copper ratio (based on the metals) is < 1,
in particular between 0.3 and 0.5. If alloys are to be
deposited by the incorporation of copper, this ratio
is > 1.
In the case if iron as a further metal in the gold
bath, surprisingly still further advantages have
resulted in the bath according to the invention. Iron
does not cause any problems in use and is even required
by the body as an essential trace element. In addition,
on the one hand by selection of the quantitative
iron-bismuth ratio in the gold bath and on the other
hand by selection of the type of iron compound/iron
complex compound, it allows the composition and
properties of the gold layer deposited to be controlled
even more accurately.
By way of example, particularly advantageous gold
layers resulted using the iron complexes Fe-DTPA,
Fe-EDTA, Fe-EDNTA with quantitative bismuth-iron ratios
from approximately 1.5 to approximately 2. By contrast,
if iron citrate is used, an advantageous quantitative
bismuth:iron ratio is approximately 0.18 to
approximately 0.3.
In this context, the surprising observation that,
despite the advantageous effect of the iron compounds
during the deposition, no iron (< 10 ppm) is
incorporated in the gold layer, was of particular
significance. This now even makes it possible, by way
of example, to deposit gold layers with a purity of up
to 99.99 and with excellent technical properties, such
as for example absolutely reproducible firing
stability, by electroplating. This was not hitherto the
case with standard gold electrolytes which are able to
deposit such pure layers.
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A further surprise was that despite the different
positions of copper and iron in the electrochemical
series, it is also possible for the two metals to be
used together, in a wide range of quantitative ratios
with respect to bismuth, in the bath according to the
invention. Therefore, the large number of possible
combinations for the bismuth-copper-iron ratios opens
up a wide range of options for new possible ways of
controlling the properties, composition and function of
gold and gold alloy layers formed from baths in
accordance with the invention. In situations in which
bismuth, copper and iron are simultaneously present in
the bath, the quantitative bismuth:copper ratio is
preferably > 0.4 and the quantitative bismuth: iron
ratio is preferably > 0.3.
Furthermore, it has amazingly been found that in the
bath according to the invention (including the bismuth
compound and the interaction between the above-
described additions of compounds of further metals in
the gold bath), the reduction or prevention of the
incorporation of physiologically harmful additives in
the layer formed by electroplating, as described in the
introduction, is nevertheless retained. Therefore, even
when various metals, such as for example copper and/or
iron, are present, the bath according to the invention
is also able to selectively prevent the incorporation
of, for example, antimony.
As mentioned above, the bath according to the invention
may contain further standard additives which are
customarily present in baths of this type based on a
gold sulfite complex. Additives of this type are known
to the person skilled in the art and can be varied in
the standard ranges within the scope of his specialist
knowledge. For example, conductive electrolytes with
their conductive salts, buffer systems/buffer mixtures,
what are known as stabilizers and wetting agents are
present. If appropriate, brighteners and/or fine-grain
CA 02438207 2003-08-12
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additives which are known from the prior art may also
be present in the bath according to the invention.
The invention also comprises the use of the bath
according to the invention as described above for the
production of prosthetic moldings for the dental sector
by means of electrodeposition. A use of this type is
intended in particular for the production of what are
known as dental frames, such as crowns, bridges,
superstructures and the like. The prosthetic moldings
are in this case electrodeposited on a substrate. In
this context, one also refers to the process known as
galvanoforming. The self-supporting, stable molding is
separated from the substrate and processed further. The
substrate may, for example, be a cast molded from a
tooth stump or an implant build-up part (prefabricated
or individually prepared).
In a corresponding way, the invention also encompasses
the use of at least one bismuth compound, preferably of
at least one water-soluble bismuth compound, for the
production of prosthetic moldings for the dental sector
by means of electrodeposition. In particular, the
bismuth compound is used as a constituent of a bath
according to the invention as described above. Bismuth
compounds which can preferably be used have already
been explained extensively above, and consequently
reference can be made to the corresponding parts of the
description.
A particularly important feature of the invention which
should be emphasized is that the bismuth compound which
is used in accordance with the invention and if
appropriate also the compounds of further metals can be
added to the bath directly during its production. This
means that the user is provided with a bath which is
fully functional in terms of having all its
constituents and additives. Unlike with the known baths
from the prior art, the user does not have to meter in
CA 02438207 2003-08-12
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any additive before carrying out the electrodeposition
process, which would entail the drawbacks which have
already been explained above.
However, it is pointed out that the bismuth compound
which is used in accordance with the invention may also
be metered into the bath before or during the
electrodeposition if desired. A variant of this type
may also be provided, for example, when an aqueous bath
to which a completely or partially water-insoluble
bismuth compound, e.g. bismuth oxide, has been added
during production is used. This water-insoluble
compound can then be converted into a water-soluble
bismuth compound immediately before or even during the
electrodeposition by addition of a suitable
complex-forming agent, and this water-soluble bismuth
compound then reveals the desired action in the bath.
The situation in which the bismuth compound is added to
the bath after an electrodeposition operation for
top-up purposes should be mentioned as a further
referred variant of the invention. This relates to the
situations in which the concentration of gold and/or
further metal in the bath is sufficient for a
plurality, in particular a multiplicity, of deposition
operations. In this case, the bismuth compound (and if
appropriate also the compounds of the further metals)
can be topped up appropriately for subsequent
deposition cycles.
As has already been mentioned briefly, the use in
accordance with the invention is intended for the
production of prosthetic moldings which have sufficient
stability in the galvanoforming process. Accordingly,
it is customary to provide molding layer thicknesses of
more than 10 Vim. The layer thicknesses of the molding
are preferably between 100 and 300 ~.m, with in
particular layer thicknesses of approx. 200 ~m being
deposited. The provision of layer thicknesses of this
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type means that the invention is suitable not just for
the production of crowns but also of bridges and other
superstructures.
Finally, the invention also comprises a process for
producing prosthetic moldings for the dental sector
from gold and gold alloys by electrodeposition. This
process is intended in particular for the production of
dental frames, such as crowns, bridges, superstructures
and the Like. In this process, according to the
invention, a gold or gold alloy layer is deposited on a
suitable substrate from a bath in accordance with the
invention, and the layer obtained is separated
(demolded) from the substrate. As mentioned above, the
substrate may, for example, be a cast which has been
molded from a tooth stump or an industrially
prefabricated or individually treated implant build-up
part.
The substrate is preferably composed of an electrically
nonconductive material, in particular plaster or
plastic. This normally relates to situations in which a
cast has been molded from the tooth stump. The surface
of the nonconductive substrate is then made conductive
prior to the electrodeposition, in particular with the
aid of conductive silver.
In other preferred cases, the substrate is composed of
at least one metal which is itself already conductive.
In this case, examples of suitable substrates which may
be mentioned are inner telescopes (usually made from a
cast and milled dental alloy) or implant build-up
parts, such as implant build-up posts. Parts of this
type usually consist of titanium or titanium alloys.
The process according to the invention and also the
uses according to the invention are preferably
characterized in that the deposition takes place at
high current densities, which usually results in short
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electrodeposition times. It is preferable to select
current densities of up to 10 A/dm2, in particular
current densities of up to 8 A/dm2. The bath according
to the invention can still be used very successfully at
such high current densities.
The use according to the invention or the process
according to the invention can preferably be carried
out in such a way that the deposition takes place using
what is known as the pulse-plating process. This type
of electrodeposition of metal likewise uses direct
current. However, this direct current is applied as a
pulsed current, i.e. in the form of current pulses
which are interrupted by pauses. With regard to the
prior art, reference can be made at this point, by way
of example, to the "Pulse-Plating" volume from the
series of papers entitled Galvanotechnik and
Oberflachenbehandlung [Electroplating and Surface
Treatment , Leuze-Verlag, Saulgau, 1990. The use of the
pulse-plating process in dental technology is shown in
DE-A1 198 45 506 in the name of the present Applicant,
the content of which is in this respect incorporated by
reference in the content of the present description.
The use of the pulse-plating process in the present
invention has the advantage that the deposits can be
formed in the desired thickness, for example of approx.
200 ~.m, within relatively short times.
Furthermore, the use according to the invention and the
process according to the invention are preferably
characterized in that the prosthetic molding deposited
is veneered with ceramic and/or plastic during its
further processing. The desired tooth replacement is
produced in this way. A molding which has been veneered
with ceramic is fired in the usual way after the
ceramic has been applied, for example at temperatures
of up to approximately 950°C. A molding which has been
veneered with plastic, after the plastic has been
applied, is irradiated with light, in particular with
CA 02438207 2003-08-12
.,
- 19 -
visible light, in order for it to be cured, after the
surface of the molding has previously been conditioned
using suitable processes which are known to the person
skilled in the art.
As has already been mentioned to some extent and as is
also shown by the examples listed below, a wide range
of advantages are associated with the invention.
For example, the bath according to the invention is
eminently suitable for the production of prosthetic
moldings (dental prostheses). The properties of the
deposits are at least as good as those achieved with
deposits formed from gold sulfite baths which operate,
for example, with antimony compounds being metered in.
The quality of the deposits in the bath according to
the invention tends to match the specific requirements
of dental technology even more successfully.
The pure gold layers obtained with the bath according
to the invention are a golden yellow color and are
extremely shiny, and therefore satisfy particularly
high aesthetic demands. Of course, it is optionally
also possible to produce matt and/or rough surfaces.
The firing stability of these layers, which is
imperative if they are to be veneered with ceramic, is
provided with a reproducible reliability despite the
fact that it is possible to dispense with an antimony
compound in the gold bath. As far as the Applicant is
aware, this has not hitherto been the case in any bath
which is able to operate without an antimony compound.
A further advantage of the bath according to the
invention is that it is clearly insensitive to plastics
which have been introduced into the bath and are
provided, for example, as tooth stump materials or to
cover metallic parts which are not to be coated by
electrodeposition. In the baths of the prior art,
plastics of this type (cast molding plastics) or
CA 02438207 2003-08-12
- 20 -
enamels (covering enamels) during deposition in the
gold bath release constituents which have an adverse
effect on the action of the fine-grain or shine
additives of the gold bath. This adverse effect becomes
more noticeable the higher the current density is
selected to be during deposition. In the invention,
this results in the advantage that, since the bath is
insensitive to such disruptive influences, it is
possible to operate at relative high current densities
(cf. above, up to 8 A/dm2 or 10 A/dm2) .
It must also be mentioned that the efficiency of the
bath according to the invention, for the same profile
of demands imposed on the electrodeposited layers, is
entirely comparable with conventional baths based on
gold sulfite complexes which operate, for example, with
antimony or arsenic additives. It is even possible to
increase the efficiency compared to known baths if the
bismuth additive is selected appropriately.
The possibility of dispensing with compounds which may
be harmful to health, for example of arsenic, thallium
and if appropriate also of antimony, in the bath
according to the invention by using the bismuth
additives has already been explained above.
Surprisingly, a further advantage of the bath according
to the invention is found to be that a bath of this
type with bismuth additive functions without problems,
and in particular with above-average results, in
various devices (including from a number of different
manufacturers) which are in commercial use in the
dental sector for electrodeposition. Hitherto, it has
normally been necessary either for the composition of
the gold or gold alloy bath to be precisely matched to
the device used or for a device of this type to be
precisely matched to a specific bath, in particular
with regard to its process parameters. The result of
this has been that each manufacturer has normally
,' CA 02438207 2003-08-12
- 21 -
offered a specific gold bath for a very specific device
whose process parameters have been matched to this gold
bath.
With the bath according to the invention, it is now
possible, for example, to operate various devices using
this gold bath without these devices having to be
adjusted to this bath in a complicated way. For
example, an AGC micro-appliance produced by the present
Applicant, which achieves a layer thickness of 200 um
usually in 12 hours, can be operated with the bath
according to the invention equally successfully as an
AGC MicroPlus appliance which achieves the same layer
thickness in just 5 hours. The bath according to the
invention is also suitable for use in devices which
operate using the pulse-plating process, for example
the AGC Speed appliance produced by the present
Applicant. In devices of this type, layer thicknesses
of 200 um are achieved, depending on the size of the
part which is to be electroplated, within 1 to 2 hours.
Therefore, the bath according to the invention can
advantageously be matched to existing electroplating
deices owned by the user. The range of applications
from "slow" devices through to the "fastest" devices,
which may also be operated completely automatically,
illustrates how useful the invention is to the user.
Finally, it should be mentioned once again that the
additive comprising a bismuth compound which is present
in the baths according to the invention can be added as
early as during preparation of the bath. The result of
this is that the user is provided with a fully
functional bath without being forced to add further
additives prior to the electrodeposition process.
Furthermore, it has emerged that the baths according to
the invention with the bismuth additive are stable over
prolonged periods of time. This means that the bath
remains able to function even after it has been stored
for prolonged periods and the additive does not lose
CA 02438207 2003-08-12
- 22 -
its effectiveness. All this makes the bath easier to
operate and more reliable when carrying out the
electrodeposition process both for the manufacturer of
the bath and for the user, since all the sources of
faults which can occur with retrospective metering in
excluded of additives are ruled out from the outset.
The features described and further features of the
invention will emerge from the following description of
preferred embodiments in conjunction with the
subclaims. In this context, the individual features may
in each case be realized vn their own or in combination
with one another.
CA 02438207 2003-08-12
- 23 -
Exa~les
Standard electrolysis cells which are known from the
prior art and are commercially available can be used
for the electrodeposition of prosthetic moldings made
from gold or gold alloys which is carried out in
accordance with the present examples. These
electrolysis cells may, for example, be the AGCY
devices produced by the present Applicant under the
names "Micron, "Micro 5h", "Micro Plus" or "Speed",
depending on the desired procedure.
An electrolysis cell which can be used in accordance
with the examples comprises a vessel for holding the
bath. This vessel is usually provided with a cover.
Furthermore, there is an anode, which may comprise a
plurality of parts, and at least one cathode. The gold
or gold alloy is electrodeposited on this cathode,
which is formed, for example, by the substrate, such as
a plaster stump or build-up post. The anode consists,
for example, of platinum-coated titanium. A suitable
current/voltage source is provided for the deposition
itself. Furthermore, there is usually a magnetic
stirrer with heating, which simultaneously ensures a
constant (normally elevated) deposition temperature in
the bath and is responsible for driving a magnetic
stirrer rod which is present in the electrolysis cell.
Accordingly, a temperature sensor is also introduced
into the electrolysis cell.
It is expressly pointed out that the invention does not
require any particular configuration of the
electrolysis cell or of the apparatus which includes
this electrolysis cell. The corresponding apparatus for
deposition from gold sulfite baths are well known to
the person skilled in the art.
CA 02438207 2003-08-12
- 24 -
As has already been explained in the description, in
accordance with the examples (purely as a selection)
- plaster stumps/plaster casts which have been made
conductive using conductive silver,
- cast and milled inner telescopes in which parts
which are not to be electroplated are filled with
a suitable plastic and the surface which is to be
electroplated is covered with conductive silver,
- build-up posts for the production of cap-like
moldings which can be cemented to implant build-up
posts, and
- plaster casts which have a block for connecting
two adjacent teeth and have likewise been coated
with conductive silver,
are electroplated.
Bath composition, deposition parameters, substrate and
deposition result of the examples carried out can be
found in Table 1. In all cases, the particularly
advantageous bath based on an ammonium-gold sulfite
complex was used.
In addition to the constituents listed, the baths used
also contain standard additives for gold sulfite baths.
These additives are known to the person skilled in the
art. For example, they are conductive salts (sulfites,
sulfates and phosphates), wetting agents or
stabilizers, such as for example vitro acids. The bath
according to the invention differs from the known baths
in particular through the addition of the bismuth
compound, and on account of this addition it is if
appropriate possible (although not necessary) to
dispense with additives which are present in
conventional baths, such as for example antimony
compounds or vitro compounds.
Where the deposition result in the following table
refers to a "defect-free" functionality, this is
r . ~ CA 02438207 2003-08-12
- 25 -
intended to mean that the layer obtained during the
deposition does not have any cracks, pores or holes.
Table 1 (cf. next page)
CA 02438207 2003-08-12
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