Note: Descriptions are shown in the official language in which they were submitted.
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Title: Gold alloy and method for manufacturing a dental restoration
The invention relates to a gold alloy and more in particular to a gold
alloy having a high gold content. The invention further relates to a method
for manufacturing a metal-ceramic dental restoration.
For a number of decennia, gold alloys with a high gold content have
been used in dental restorations, particularly because of their biological and
chemical inertia and their attractive deep yellow color.
For aesthetical reasons, these alloys are fired on with porcelain. This
porcelain was manually built up in layers. In recent years, ceramic is also
pressed on. This pressing procedure comprises the following steps: a
supporting structure from a metal alloy is, either provided with a thin
coating layer or not, pressed over with a press ceramic by use of the "lost
wax method". During the burning out or drying out of the wax, while
heating for a long time at a relatively high temperature, with many alloys, a
thick oxide layer is formed on the alloy. In many cases, this oxide layer
results in a dark edge or contour.
From an aesthetic point of view, steps need to be taken to form as
little oxide as possible on the surface of the structure from the metal alloy.
However, for a good bond between press ceramic and metal alloy
structure, an oxide layer is highly desired.
2 o Therefore, there is a need for a high gold alloy with a solidus
temperature which is sufficiently high in relation to the application
temperature of the ceramic - and is, as a rule, at least 50°C higher
than the
firing temperature or pressing temperature of the ceramic or porcelain,
which alloy forms an oxide layer upon heating in air, which layer is
2 5 necessary for a good bond with the porcelain to be fired on or pressed on,
and which oxide layer is hardly or not visible.
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According to the invention, an alloy with a high gold content, a
so-called "high gold alloy" has now been found which remains yellow upon
oxidation, which alloy is suitable for use in a metal-ceramic system, in
which a gold alloy with an aesthetic yellow color is fired on or pressed on
with a dental ceramic or porcelain tailored thereto. In addition, the alloy
according to the invention has a high degree of biocompatibility.
This alloy according to the invention comprises 0.01-0.05 wt.% zinc;
0.01-0.05 wt.% indium; 0.01-0.05 wt.% silver and 0.01-0.05 wt.% manganese
in a gold base.
1o The gold base substantially consists of gold, but may contain small
amounts of pollutants, as long as they produce no adverse color effects and
do not affect the biocompatibility.
In a preferred embodiment, the alloy according to the invention
comprises at least 99 wt.°/~ gold. A very suitable alloy substantially
consists
of 99.0 wt.% gold; 0.05 wt.% zinc; 0.05 wt.% indium; 0.05 wt.% silver and
0.05 wt.% manganese.
With the high gold alloy according to the invention, it has been found
possible to fire or press it with ceramic, while the intense gold color is
preserved.
2 o The alloy according to the invention provides a stable, very thin,
possibly monomolecular oxidation layer which is so light in color than no
adverse color effects occur. However, the oxidation layer is sufficiently
strongly bound to the underlying alloy and appears to be capable of a very
good metal-press glass or metal-porcelain binding.
Particularly the presence of manganese provides a good bond. Tin and
indium ensure a reinforcement of the oxide. Incidentally, the gold alloy
according to the invention does not have a very great strength, but this has
not been found necessary for the applications for which this alloy is
intended.
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The alloys according to the invention have a solidus temperature of
between 1030 and 1100°C; for the preferred alloys, the solidus
temperature
is between 1045 and 1065°C. For the alloys according to the invention,
the
coefficient of thermal expansion (measured from 25 to 500°C) is between
14.5 and 15.5 ~.mlm.°C; and for the preferred alloys between 14.3 and
15.3 ~,m/m.°C.
High gold alloys were already known in the state of the art. For
instance, I)E-OS 44 19 408 describes a dental alloy with 95-9~ wt.% gold;
1-4 wt.% titanium; and 0.05-1.5 wt.°/ of one or more elements from the
1 o group of Re, Rh, Ru, Tr and Ta.
Further, US-A-5,922,276 relates to a dental alloy with an excellent
oxide color, which alloy contains at least 99.5 wt.% gold, 0.1-0.25 wt.% zinc,
0.1-0.25 wt.% indium and up to 0.3 wt.% Rt, Pd, Rh, Ir, Re or combinations
thereof It is explicitly stated that elements like copper, manganese and iron
should be avoided because they produce dark or colored oxides.
Further, the invention relates to a method for manufacturing a
metal-ceramic dental restoration, comprising pressing, with heating, a
tooth-colored press glass onto a wholly or partly supporting structure from
the alloy according to the invention, with the press glass having a
coefficient
2 0 of thermal expansion (CTE) of between 12.5 and 14.5, and preferably
between 13.0 and 14.5 km/m.K, measured in the range from 25°C to
500°C
or to the glass transformation temperature, depending on which of the two
is the lowest, and with the press glass having a pressing temperature which
is at least 50°C lower than the solidus temperature of the alloy.
In this method, the press glass is pressed with heating in a mold
manufactured by use of the "lost wax" method. Such a method is much more
effective and economical than the conventional method in which porcelain
was applied layer by layer. In addition, fewer bubbles and cracks are formed
during pressing compared to applying the porcelain layer by layer.
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Incidentally, the alloy according to the invention can also be coated with
this conventional method.
In more detail, in the pressing method according to the invention, a
wax model of one or more teeth and/or molars is made, which model is
embedded in a fire-resistant material, for instance Currur a ~ Zlrziue~~s~l
Z)ustless Ircuestment (ex Elephant Dental B.V., Hoorn, The Netherlands).
Then, after curing of the die from fire-resistant material, the wax is burnt
out. After this, a closed pellet of press glass is brought, on the connecting
channels, to the mold, by pressing the glass therein with a fire-resistant
s o cylinder with thermal plasticization. In the die, the structure from the
high
gold alloy according to the invention is present, as stated. This structure
may, for instance, be formed by CAD/CAM methodologies.
Preferably, the press glass is available in a tooth color. The coloring of
porcelain is known to a skilled person. A suitable method is described in
DE-OS-1999 04 522, which document is understood to be inserted in this
specification for the description of the coloring method.
In a preferred embodiment, before the pressing, first, a so-called liner
may be applied onto the alloy. This liner will, as a rule, have a melting
point
which is less than 50°C lower than the pressing temperature of the
press
glass. A suitable liner consists of 58.5 wt.% SiO~, 12.6 wt.% Al~Os, 11.0 wt.%
K2O, 7.1 wt.% Na2O, 10.4 wt.% Ce02, 0.4 wt.% Li02. This liner can be
applied as a single coating in a thickness of 20-40 ~.m and be burnt up at
about 900°C.
A suitable press glass may have the following (preferred) composition:
7-15 wt.% AhOs; 13-23 wt.% (Kz0 + NazO), 1-3 wt.% (Ba0 + CaO), 1-3 wt.%
(Sb~Os + Li~O) and 0.2-1.2 wt.% fluorine, rest Si02 including coloring
compositions. '7-15 wt.% AhOs; 6-14 wt.% K~O, 5-11 wt.% Na~O,
0.2-2.5 wt.% BaO, 0.1-1.5 wt.% CaO, 1.2-2.5 wt.% Sb~Os, 0.05-0.5 wt.% Li~O
and 0.5-1.0 wt.% fluorine, rest SiO~ including coloring compositions.
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The powder formed from these glass compositions preferably has a
particle size smaller than 106 p~m. This powder is granulated with a binder
and uniaxially dry-pressed at room temperature and sintered at a
temperature of, for instance, 800-1000°C, preferably 900-960°C,
for
5 1 minute to 1 hour, preferably 1-30 minutes.
In addition, the invention relates to a method for manufacturing a
metal-ceramic dental restoration, comprising firing a dental porcelain onto a
supporting structure from the alloy according to any one of claims 1-3, with
the porcelain having a coefficient of thermal expansion of between 12.5 and
14.5 ~.m/m.K, measured in the range from 25°C to 500°C or to the
glass
transformation temperature, depending on which of the two is the lowest,
and with the porcelain having a bring temperature which is at least 50
°C
lower than the solidus temperature of the alloy. A suitable firing ceramic
has the following (preferred) composition: 64.1-67.0% Si02, 11.0-12.5%
A120s, 10.1-11.6% K2~, 6.6-8.6% Na20, 0.7-1.1% CaO, 0.4-l.3% Ba~, 0-2.1%
Sb203, 0-0.2% Li2~, and 0-0.6% fluorine with pigments. A more preferred
firing ceramic has the following composition: 64.1% Si02, 14.2% A120a,
11.1% K~O, 6.6% Na~O, 1.1% CaO, 0.4% BaO, 1.4% Sb~Os, 0.2% LizO and
0.6% F~ with pigments.
2 o Every CTE described in this specification or the claims is measured in
the range from 25°C to 500°C or to the glass transformation
temperature,
depending on which of the two is the lowest. Also, every percentage is a
weight percentage related to the weight of the total composition, unless
indicated otherwise.
The pressing or firing temperature needs to be at least 50°C lower
than the solidus temperature of the alloy in order to avoid deformation of
the metal structure during pressing. The CTE of the press glass or porcelain
needs to be such that the CTE of the alloy is 0.5-2.0 ~,m/m.K higher than
that of the press glass or porcelain. When the difference is greater than
3 0 2.0 ~,m/m.K, cracking in the porcelain may occur; when the difference is
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smaller than 0.5 ~,mlm.K, possibly, a structure is obtained in which the
bond between press glass or the porcelain and alloy is insuf~.cient. Tn the
range mentioned, the porcelain is subjected to such. pressure after cooling
that a strong restoration is obtained.
The invention will now be illustrated in more detail in and by the
following non-limiting examples.
Example 1 (comparative
Into a crucible of pure alumina, in a vacuum induction furnace, the
so following metals were weighed and melted under a partial pressure of
400 Torr of argon gas and then cast to a bar in a die which had already been
present in the vacuum chamber: 97.625 wt.% gold, 1.5 wt.% platinum,
0.5 wt.% zinc, 0.375 wt.% rhodium. After casting, the die was removed from
the vacuum induction furnace and the die was opened.
The bar was rolled out to plate with, optionally, glowing between
whiles to bring the plate back into a rollable condition. After this, the
plate
was cut into strips and the alloy was cut into cubes.
The alloys were then cast in an electric casting device at 1200°C
into
a graphite-containing, phosphate-bound embedding mass die, which had
2 o been preheated to 750°C. After oxidation, the alloy has a grey-
yellow color.
The binding with porcelain is given in Table 3.
Example 2 ~comparative~
In the same manner as in Example l, an alloy was produced with the
following composition: 98.2 wt.% gold, 1.2 wt.% platinum, 0.1 wt.% zinc,
0.3% xhodium and 0.2% indium. After oxidation, the alloy has a grey-yellow
color.
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Example 3
In the same manner as in Example l, an alloy was produced with the
following composition: 99.~ wt.°/~ gold, 0.05% zinc, 0.05°/~
indium, 0.05%
silver, 0.05°/~ manganese. After oxidation, the alloy has an intensely
yellow
color.
Example 4 (comparative)
In the same manner as in Example 1, an alloy was produced with the
following composition: 99.7 wt.% gold, 0.1% zinc, 0.2% indium (see
so US-A-5,922,276). After oxidation, the alloy has an intensely yellow color,
but did not have the good bond of Example 3.
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Results of Examples 1-4 are shown in the following Table.
Example
number
(wt./~
components)
Metal component 1 2 3 LTS 5922276
Gold 97.625 98.2 99.8 99.7
Platinum 1.5 1.2 - -
Iridium - - - -
Zinc 0.5 0.1 0.05 0.1
Indium - 0.5 0.05 0.2
Silver - - 0.05 -
Manganese - - 0.05 -
Rhodium 0.375 0.3 - -
Tensile strength, 180 160 133 142
MPa
Yield point, MPa 79 63 51 54
Elongation at break, 23.4 31.9 53 34
%
Vic7zers hardness, 75 43 37 38
HV
Liquidus, C 1080 1070 1060 1060
Solidus, C 1060 1050 1050 1050
Coefficient of thermal
exp ansion
(20-500C) ~.m/m.C 15.3 15.1 15.0 15.1
Oxidation color yellow/ yellow/ yellow yellow
grey grey
Binding porcelain, 75 71 70 61
%
Binding press ceramic,71 73 72 63
%
A round disk of the alloys was cast with a diameter of 25 mm and a
thickness of 1.0 mm. After casting, the casting pieces were ground with
coarse and fine aluminum oxide. The metal-ceramic disk was then deformed
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from the top, with the porcelain downwards, by a stamp with a spherical
end. The disk was bent 0.4 mm in the centre to achieve a consistent
deformation of the disk and r emoval of the ceramic with minimal cracks in
the metal. After the breaking off of the porcelain, loose particles of
porcelain
were removed from the surface of fracture with a nylon brush, after which
the surface of fracture was placed in an ultrasonic bath for 10 minutes.
After breaking, the samples were tested for the amount of
remaining porcelain surface by means of a scanning electron microscope.
The percentage of oxidized metal surface which was still coated with
s o ceramic was measured by measuring the amount of silicon on the surface of
fracture by means of E.D.A.X. and comparing this to the uncovered part of
metal surface and a surface 100% covered with porcelain. The average
surface fractions or remaining ceramic were given in the above Table. The
values for remaining surface still covered with porcelain show that the
majority is still attached to the alloy after the breaking off of the mass of
the
porcelain. Tests of other alloy systems have shown that a percentage higher
than 50% does not cause problems in practice.
