CA 02239869 1998-06-08
ChemicallY stable translucent apatite qlass cer~mic
The invention relates to a chemically stable, translucent apatite
glass ceramic which is particularly suitable for use in
restorative dentistry and above all for coating or veneering of
dental restorations, such as bridges or crowns.
Apatite glass ceramics are known from the prior art. They are
usually employed as bioactive materials for replacing bone in
human medicine, or as the main component of glass ionomer cements
in dentistry.
In the case of bioactive materials for bone replacement, they
have very high CaO and P2O5 contents, however, in order to
achieve bioactivity, i.e. the direct growing together of glass
ceramic and living bone.
A glass ceramic implantation material is known from DE-A-40 20
893 which has apatite crystals but also contains very large
quantities of CaO in order to achieve bioactivity.
Glass ceramics for glass ionomer cements also have high CaO
contents and mostly also high fluoride ion contents, in order to
obtain the desired high level of ion release in the oral medium.
These two types of apatite glass ceramics are white-opaque,
however, and have a high level of ion release and/or bioactivity,
so they are not suitable for restorative dentistry.
An apatite glass ceramic for restorative dentistry must have
optical properties such as translucence and colour which are
similar to those of the natural tooth. A material which is
impervious to lignt, i.e. opaque, is not suitable for this
purpose. Moreover, bioactivity or a high level of ion release
is undesirable; rather, a high degree of chemical stability is
required which should even exceed that of the natural tooth.
In known apatite-containing glass ceramics for restorative
dentistry, the main crystal phase is regularly formed not by
CA 02239869 1998-06-08
apatite but by leucite or mullite. This is undesirable, however,
since these types of crystals make it difficult, inter alia to
imitate the optical properties of the natural tooth material
composed primarily of needle-shaped apatite.
EP-A-0 690 030 discloses leucite-containing phosphosilicate glass
ceramics which may be used in dental engineering. In view of the
leucite content, however, they have very high thermal expansion
coefficients~ so they are not suitable for the coating of
materials with low expansion coefficients, such as lithium
disilicate glass ceramics.
Moreover, an apatite glass ceramic containing mullite as a
further crystal phase is described by A. Clifford and R. Hill
(Journal of Non-Crystalline Solids 196 (1996) 346-351). The high
mullite content brings about only low translucence.
Apatite-containing glass ceramics are disclosed by S. Hobo et al.
(Quintessence International 2 (1985) 135-141) and Wakasa et al.
(J. Oral Rehabil. 17 (1990) 461-472 and J. Mat. Sci. Lett. 11
(1992) 339-340) for restorative tooth replacement. Said glass
ceramics have high CaO and P2O5 contents, however, so they show
only poor chemical stability. Moreover, the apatite crystals in
these glass ceramics do not have a needle-shaped morphology.
Moreover, DE-A-34 35 348 describes apatite-cont~ining glass
ceramics for the production of dental crowns. The glass ceramics,
however, contain no Al2O3 at all and very large quantities of
CaO, for which reason they have a high tendency to ion exchange
and consequently only poor chemical stability. In addition, the
apatite crystals do not have the needle-shaped morphology which
is characteristic of apatite crystals of natural tooth material.
Glass ceramics with good chemical stability are disclosed in EP-
A-0 695 726 as alkali-zinc-silicate glass ceramics. The
disadvantage of said glass ceramics, however, is that they
CA 02239869 1998-06-08
contain leucite rather than apatite as the crystal phase. As a result of the high
expansion coefficient of leucite, the glass ceramics are therefore usually
unsuitable as coatings for substrates with low expansion coefficients, such as,
in particular, lithium disilicate glass ceramics. The glass ceramic also
5 necessarily contains ZnO in order to achieve good chemical stability.
The apatite glass ceramic described herein resembles natural tooth material in
terms of its optical properties and, in particular, its high translucence, and
contains apatite crystals which have a greater chemical stability than the
10 carbonate-apatite crystals of natural tooth material and hence confer excellent
chemical stability on the glass ceramic. Moreover, the apatite glass ceramic
should preferably contain apatite crystals with a similar morphology to that of
the apatite crystals of natural tooth material and have a low thermal expansion
coefficient and should therefore be particularly suitable as a dental material and
15 above all as a coating or veneer for dental restorations, such as crowns or
bridges, made of lithium disilicate.
The apatite glass ceramic according to the invention contains CaO, P2Os and F
in a molar ratio of:
CaO: P2Os: F 1: 0.020 to 1.5: 0.03 to 4.2
CA 02239869 1998-06-08
and contains apatite crystals as the main crystal phase.
The molar ratio of CaO : P2O5 : F in the glass ceramic is
preferably 1 : 0.1 to 0.5 : 0.1 to 1.0 since this leads to
particularly stable glass ceramics.
Surprisingly, it has become apparent that by adjusting the molar
ratio of CaO to P2O5 to F in the stated manner, apatite glass
ceramics are obtained which show an increased chemical stability
compared with conventional apatite glass ceramics.
In order to quantify the chemical stability, the test method
according to ISO specification 6872:1995 described for glass
ceramics was carried out, in which the resistance to aqueous
acetic acid solution is determined by measuring the loss of mass
in ~g/cm that occurs. The essential stages of this method are
given in the Examples.
The apatite glass ceramic described herein usually
exhibits a loss of mass of less than 200 and preferably less than
100 ~g/cmZ. Particularly preferred glass ceramics have a loss of
mass of less than 60 ~g/cm2.
An advantageous apatite glass ceramic also contains at least
one of the following components:
ComPonent Wt. %
SiO2 45.0 to 70.0
Al2O3 5.0 to 22.0
P2O5 0.5 to 6.5
K~O 3.0 to 8.5
Na7O 4.0 to 13.0
CaO 1.5 to 11.0
F 0.1 to 2.5
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The glass ceramic may additionally contain at least one of
the following components:
Component W _
B2O3 0 to 8.0
La2O3 ~ to 5.0
Li2O 0 to 5.0
BaO --~~0; to 5.0
MgO 0 to 5.0
ZnO 0 to 5.0
SrO 0 to 7.0
TiO2 0 to 4.0
ZrO2 0 to 4.0
CeO2 0 to 3.0
The lower limits for those additional components are usually
0.05 wt%.
Preferred quantity ranges exist for the individual components of
the apatite glass ceramic described above. Unless
otherwise specified, these may be chosen independently of one
another and are as follows:
ComPonent Wt. %
SiO250.0 to 68.0
Al2O37.0 to 21.0
P2O50.5 to 4.0
K2O4.0 to 8.0
Na2O4.0 to 11.0
CaO2.0 to 8.0
F 0.2 to 2.0
B2O30.2 to 4.0
La2O30 to 3.0
Li2O 0 to 3.0
BaO 0 to 4.0
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MgO 0 to 4.0
ZnO 0 to 4.0
SrO 0 to 5.0
TiOz + ZrO2 0.2 to 5.0
CeO2 0 to 2.0
Particularly preferred quantity ranges for the individual
components of the apatite glass ceramic according to the
invention-~are as follows and these may be chosen independently
of one another:
ComPonent Wt. ~
SiO2 54.0 to 65.0
Al2O3 -8.0 to 21.0
P2O5 0.5 to 3.5
K2O 5.0 to 8.0
Na2O 6.0 to 11.0
CaO 2.0 to 6.0
F 0.3 to 1.5
B2O3 0.2 to 3.0
La2O3 0 to 2.0
Li2O 0 to 2.0
BaO 0 to 3.0
MgO 0 to 3.0
ZnO 0 to 3.0
SrO 0 to 4.0
TiO2 0.5 to 2.0
ZrO2 0.5 to 3.0
CeO2 0.1 to 1.5
All the above-mentioned quantities in wt.% relate to the glass
ceramic.
The glass ceramic according to the invention may also contain
e.g. conventional colour components for matching the colour of
a patient's natural tooth material.
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It was ascertained by scanning electron microscope and X-ray
diffraction analyses that apatite, such as hydroxy and/or
fluoroapatite, forms the main crystal phase in the glass ceramic.
The apatite crystals are preferably hexagonal and in particular
needle-shaped. The apatite crystals are preferably smaller than
35 ~m in their greatest extension, particularly smaller than 15
~m, and in partlcular preference smaller than 5 ~m.
The opticaI properties of the glass ceramic are controlled by
means of the precipitated apatite crystals which preferably
resemble the carbonate-apatite crystals of natural tooth material
in terms of their appearance. It is thus possible to produce a
glass ceramic with an appearance which corresponds to that of the
dentine or enamel of the tooth. At the same time, an optical
depth is achieved in the glass ceramic which is not possible by
means of other types of crystals.
The glass ceramic according to the invention is characterised not
only by very good chemical stability but also by translucence.
In order to quantify the translucence, the CR value was
determined according to the method described in the Examples. The
CR value, also known as the contrast ratio, indicates the ratio
of light reflection of a specimen of the glass ceramic on a black
background to the measurement of the light reflection of the
specimen on a white background, and thus serves as a measure of
the translucence of a material. The CR value is defined by the
following formula:
CR = Yb / Yw
where
CR = contrast ratio
Y~ = light reflection of the specimen on a black
background, and~5 Y~ = light reflection of the specimen on a white
background.
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The CR value always lies in the range from 0 to 1, where CR = 0
stands for an opacity of 0% and consequently a completely
translucent material, and CR = 1 stands for an opacity of 100%
and consequently a completely opaque material, i.e. one which is
S impervious to light.
The glass ceramic according to the invention usually has a CR
value of 0 to 0.9 and preferably 0.1 to 0.75.
. .
A further particular advantage of the glass ceramic according to
the invention is that, due to its particular molar ratio of CaO
to P2O5 to F in combination with the precipitated apatite
crystals, good chemical stability is achieved without ZnO
necessarily having to be present.
It is presumed that this stability is also attributable to the
very high degree of crystallinity and to the preferential
formation of hydroxy and fluoroapatite. The stability of the
precipitated fluoro or hydroxyapatite crystals is higher than
that of the rather unstable carbonate-apatite, which is present
in natural tooth material.
It should also be pointed out that the glass ceramic may be
produced in the B2O3-free form. The advantage of adding B2O3,
however, is that the entire sintering behaviour of the glass
ceramic is improved and sintering can take place in the preferred
temperature range of 650~C to 1050~C.
The apatite glass ceramic usually has a very low thermal
expansion coefficient of 6.0 to 12.0 x 10-6K-l, measured in the
temperature range from 100~C to 400~C.
In order to produce the apatite glass ceramic according to the
invention,
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a) a starting glass containing the necessary components is
melted at temperatures of 1200~C to 1650~C,
b) the glass melt obtained is poured into water with the
formation of glass granules,
c) the glass granules are optionally comminuted to a glass
powder with an average particle size of 1 to 450 ~m, based
on the number of;particles, and
d) the glass granules or glass powder is subjected to a heat
treatment at temperatures of more than 900~C and up to
1200~C for a period of 30 minutes to 6 hours.
In stage (a), a starting glass is first melted by intimately
mixing suitable starting materials, such as carbonates, oxides
and fluorides, and heating them to the given temperature.
In stage (b), the glass melt obtained is then quenched by being
poured into water and is thereby converted to glass granules.
This procedure is usually also referred to as fritting.
Optionally, the glass granules are then comminuted in stage (c)
and ground, particularly with conventional mills, to the desired
particle size. The glass powder obtained preferably has an
average particle size of 1 to 450 ~m, based on the number of
particles.
In stage (d), the glass granules or optionally the glass po~der
undergo a heat treatment at temperatures of more than 900~C to
1200~C for a period of 30 minutes to 6 hours, preferably 30
minutes to 3 hours. A temperature of more than 900~C is required
since the development of the apatite crystals in the desired form
and quantity does not take place at lower temperatures.
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Volume crystallisation takes plzce during the heat treatment.
This leads to a homogeneous distribution of the apatite crystals
throughout the glass ceramic, in contrast to leucite-
crystallisation, which can only occur on the internal surfaces
of a glass powder.
The process of glass fritting described in stage (b) is
responsible for freezing a glass structure with very small (~ 100
nm) dropl-et-shaped precipitates which are extremely densely
packed and finely distributed. Even under the scanning electron
microscope with a 30,000 fold magnification, a residual glass
matrix can no longer be detected. It is assumed that the apatite
crystallisation taking place during the subsequent heat treatment
proceeds via these precipitates which can therefore be regarded
as primary nuclei.
It was ascertained by scanning electron microscopy and X-ray
diffraction analyses that apatite, preferably fluoroapatite,
forms the main crystal phase. The size of the crystals obtained
can be controlled by the temperature selected and the duration
of the heat treatment. In addition to the apatite crystals,
further crystal phases may be formed depending on the chemical
composition of the starting glass used. In addition to the
various crystal phases, microheterogeneous demixing regions, i.e.
various glass phases, may also be present. These regions can be
identified under the scanning electron microscope as small
microheterogeneous droplet glass phases about 20 to 400 nm in
size. The droplet glass phases occurring, together with the
crystals, influence the optical properties of the glass ceramics
according to the invention, such as opalescence and translucence.
Surprisingly, the optical properties of the apatite glass ceramic
according to the invention may be adjusted from glassy
transparent to whitish cloudy. This is absolutely vital for use
as dental material or component thereof in order to be able to
produce all the various forms of the natural tooth in a
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reproducible manner. The fine apatite crystals in the
microstructure of the preferred glass ceramic bring about a
very great similarity to the natural tooth in terms of optical
appearance ~nd structure.
The apatite glass ceramic according to the invention is therefore
used particularly as a dental material and preferably as a
component of dental material.
,
When apatite glass ceramic is used as a component of dental
material, it is possible, by a suitable choice of its composition
and of the type of other components, to obtain dental materials
with good chemical stability in which important properties, such
as processing temperature, optical properties and thermal
expansion coefficient, are matched exactly to the respective
requirements. This is often not possible with pure glass ceramic.
A combination of the desired properties may be obtained with the
apatite glass ceramic according to the invention by mi X; ng it
with glasses and/or other glass ceramics. It is preferable in
this case that the dental material contains 10 to 90 wt.~ of the
apatite glass ceramic.
It is in particular possible to use the glass ceramic according
to the invention as a means to modify the optical properties of
glasses and other glass ceramics. In case of a dental ceramic it
is a goal to achieve balance between translucence and lightness,
which closely resembles the natural teeth. A satisfactory dental
restoration must simultaneously have a bright appearance and a
high translucence.
Upon use of conventional opacifiers, such as SnO2, this cannot be
obtained. If the lightness is satisfactory, then the translucence
is too low to match the properties of natural teeth.
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By using the apatite glass-ceramic according to the invention as
opacifier having cristalls of a size of generally up to 15 ~m and
particularly up to 5 ~m a lightness and translucence similar to-
that of natural teeth can surprisingly be obtained.
The dental material according to the invention preferably
contains, in addition to the apatite glass ceramic, at least one
glass and/or glass ceramic of the systems comprising alkali-
silicate,alkali-alkalineearthsilicate,alkali-aluminosilicate,
alkali-zinc-borosilicate, phosphosilicate or alumino-fluoro-
borosilicate. Preferred glass ceramics and glasses of this kind
are given below, the details in wt.% relating to the glass
ceramic in question or the glass in question.
- Leucite-containing phosphosilicate glass ceramic having the
composition:
SiO2 49.0-57.5 wt.%, Al2O3 11.4-21.0 wt.%, P2O5 0.5-5.5 wt.%,
CaO 2.5-11.5 wt.%, K2O 9.0-22.5 wt.%, Na2O 1.0-9.5 wt.%, Li2O
0-2.5 wt.%, B203 0-2.0 wt.%, TiO2 0-3.0 wt.%, ZrO2 0.8-8.5
wt.%, CeO2 0-3.0 wt.%, F 0.25-2.5 wt.%, La2O3 0-3.0 wt.%, ZnO
0-3.0 wt.%, BaO 0-3.0 wt.%, MgO 0-3.0 wt.% and SrO 0-3.0
wt.%.
- Opalescent glasses having the composition:
SiO2 48.0-66.0 wt.%, Bz03 0-1.0 wt.%, Me(III)2O3 5.8-20.0
wt.%, Me(I)20 6.0-22.0 wt.%, Me(II)O 3.5-16.0 wt.%, Me(IV)02
0.5-10.0 wt.%, P2O5 0.5-5.0 wt.%, CeO2 0-3.0 wt.%, wherein
the quantity of Me(III)203 is formed by 5.8-20.0 wt.% of
Al2O3 and 0-6.0 wt.% of La2O3; the quantity of Me(I) 2~ is
formed by 3.0-15.0 wt.% of K20, 3.0-12.0 wt.% of Na20 and 0-
2.5 wt.% of Li2O; the quantity of Me(II)O is formed by 0-
10.0 wt.% of CaO, 0-7.5 wt.% of BaO, 0-9.0 wt.% of MgO, 0-
3.5 wt.% of ZnO and 0-8.5 wt.% of SrO; and the quantity of
Me(IV)02 is formed by 0-5.0 wt.% of TiO2 and 0-5.0 wt.% of
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ZrO~.
- Alkali-zinc-silicate glasses having the composition:
SiO2 52.0-63.5 wt.%, Me(III)2O3 8.5-13.0 wt.%, K2O 0-20.5
wt.%, Na2O 1.5-20.0 wt.~, Li2O 0-5.0 wt.%, ZnO 2.0-8.0 wt.%,
Me(II)O 2.5-6.5 wt.%, TiO2 + ZrO2 0.5-6.0 wt.%, SnO2 0-9.5
wt.%, P2O5 0-4.0 wt.%, F 0-2.0 wt.%, CeO2 0-3.0 wt.%, wherein
the quantity of Me(III)2O3 is formed by 0-13 wt.% of Al2O3
and 0-9.5 wt.% of La2O3; and the quantity of Me(II)O is
formed by 0-3.5 wt.~ of CaO, 0-4.5 wt.% of BaO and 0-5.0
wt.~ of MgO.
In particular preference, however, at least one alkali silicate
glass which can be produced by conventional methods having the
following composition 55.0-71.0 wt.% of SiO2, 5.0-16.0 wt.% of
Al2O3, 0.2-10.0 wt.% of B2O3, 4.5-10.0 wt.% of K2O, 3.0-14.0 wt.%
of Na2O, 0-4.0 wt.% of Li2O, 0-3.0 wt.% of CaO, 0-5.0 wt.% of
BaO, 0-4.0 wt.% of ZnO, 0.2-5.0 wt.% of ZrO2 + TiO2, 0-2.0 wt.%
of CeO2, 0-3.0 wt.% of F and 0-0.6 wt.% of P2O5 is used together
with the apatite glass ceramic. The wt.% details are based on
the glass. Mixtures of the apatite glass ceramic with at least
one glass of this composition produce dental materials which are
particularly suitable as coatings for ceramic frameworks and
hence for the production of fully ceramic dental products with
tooth-like optical properties and good chemical stability.
It is preferred to use glasses which do not crystallise during
further processing of the dental material to dental products and
particularly during sintering or other heating to 600~C to 1000~C
for up to 2 h. Glasses having a sintering temperature from 650~C
to 1050~C are advantageous.
The dental material according to the invention is used preferably
for coating a substrate, particularly a dental crown or bridge.
In particular, the dental material is sintered on to obtain the
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desired coating.
If used as a coating or veneering material, the apatite glass-
ceramic is usually comminuted initially to a powder with an
average particle size of 5 to 80 ~m, based on the number of
particles. Additives such as colour components and in particular
glasses or further glass ceramics, and aqueous solutions for
mixing or modelling, are optionally added to said powder, and the
mixture obtained is applied to the substrate and shaped in the
desired manner. After shaping, sintering finally takes place at
temperatures of 650~C to 1050~C to obtain the coated, shaped
dental product.
It is also possible, however, to bond a dental restoration
produced from the glass ceramic according to the invention to a
substrate.
The apatite glass ceramic according to the invention may be used
as a coating or veneering material for glass ceramic, all-ceramic
or metallic dental frameworks or those based on a composite
material, with a thermal expansion coefficient of 7.0 to 12.0,
particularly 8.0 to 11.0 x lO K . It is used preferably for
coating or veneering of ZrO2 ceramics, Al2O3 ceramics, ZrO2/Al2O3
ceramics, ceramic or glass ceramic composite materials and
titanium.
It is used particularly advantageously, however, for veneering
frameworks based on lithium disilicate glass ceramic in order to
produce in this way aesthetically very attractive solid ceramic
dental products which not only have excellent chemical stability
but are also characterised by very high strength.
Lithium disilicate glass ceramics which have proved to be
particularly suitable and were obtained by melting corresponding
starting glasses, fritting and heat treatment at 400~C to 1100~C
have the following composition:
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ComPonent Wt.%
SiO257.0 to 80.0
Al2O3 0 to 5.0
La2O30.1 to 6.0
MgO 0 to 5.0
ZnO 0 to 8.0
K2O 0 to 13.5
Li2O11.0 to 19.0
P2O5 --0 to 11.0
with the proviso that
(a) Al2O3 + La2O3is 0.1 to 7.0 wt.% and
(b) MgO + ZnO is 0.1 to 9.0 wt.%.
For the production of coatings, dental material according to the
invention having a thermal expansion coefficient that is smaller
than that of the substrate to be coated is advantageous. Dental
materials whose expansion coefficient is not more than 3.0 x
10-6Kl smaller than that of the substrate are particularly
advantageous. The dental material preferably has a linear thermal
expansion coefficient of 5.5 to 12.5 x 10 K , measured at
temperatures from 100~C to 400~C.
The apatite glass ceramic and the dental material according to
the invention may be processed in the usual way together wlth the
additives optionally present to obtain shaped dental products.
Suitable shaped dental products according to the invention
containing the apatite glass ceramic according to the invention
or the dental material according to the invention are, apart from
compacts of the desired shape or ingots, particularly dental
restorations such as an inlay, an onlay, a bridge, an abutment,
a jacket, a veneer, a facet, a filling, a connèctor, a crown or
a partial crown.
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Preferred apatite glass ceramic according to the invention
contains no leucite since, as a result of its high thermal
expansion coefficient, it would also confer a high thermal
expansion coefficient of usually more than 12.5 x 10-6K-1 on the
glass ceramic. If leucite-containing glass ceramic is used to
coat a substrate that has an expansion coefficient of less than
12.5 x 106Kl, such as ZrO2 or lithium disilicate glass ceramic,
very high tensions are therefore also induced which result in
cracks and détachments.
The invention is explained in more detail below on the basis of
examples.
Examples
ExamPle 1 to 17
A total of 17 different glass ceramics according to the invention
were produced. They had the chemical compositions and molar
ratios of CaO to P2Os to F given in Table I and they all had a
chemical stability of less than 100 ~g/cm2 loss of mass according
to ISO 6872:1995.
- 17 -
<IMG>
CA 02239869 1998-06-08
In order to produce said glass ceramics, an appropriate batch of
suitable oxides, carbonates and fluorides in each case was melted
in a platinum/rhodium crucible at a temperature of 1550~C to
1600~C for a homogenisation period of 1 to 1.5 hours. The glass
melt was quenched in water, and the granules of starting glass
formed were dried and ground to an average particle size of less
than 90 ~m.
The powder--of starting glass obtained then underwent a heat
treatment at more than 900~C and up to 1200~C for 30 minutes to
6 hours, whereupon the glass ceramic formed.
Selected properties that were determined on specimens of the
respective glass ceramic are given in Table II for some of the
glass ceramics. Moreover, details about the heat treatment
actually chosen for the starting glass are given in Table II
under 'Heat treatment-.
The examples illustrate how, by altering the chemical
composition, glass ceramics with differing processing properties
and optical properties but always having good chemical stability
can be obtained.
Table II:
Ex. Heat Firing Tg a-value Optical Acid
treat- temper [ C] x 10 K appearance resist-
ment ature (100 C- ance
[ C/h] [ C] 400~C) [~g/cm2]
1 1050/1 860 545 8.4 milky, 21
slightly
opal,
translucent
8 1000/1 1080 650 7.9 very 23
translucent
9 1020/1 1050 645 9.7 very 28
translucent
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11 1000/1 890 547 6.6 yellowish, 58
~ilky,
translucent
14 1050/1 870 541 9.4 whitish, 55
cloudy,
translucent
* Firing temperature = temperature which was used during
production of the specimens by sintering onto quartz (1
minute holding time, vacuum)
Determination of the expansion coefficient a
In order to measure the thermal expansion coefficient a, a rod-
shaped green compact was prepared from powder of the glass
ceramic in question, and said compact was sintered in a vacuum
furnace at a rate of heating of 60~C/min and with a holding time
of 1 minute at the sintering temperature given in each case. A
glaze bake was then carried out without vacuum at a 20~C higher
final temperature and with a holding time of 1 minute. The
thermal expansion coefficient was determined on the specimen
obtained.
Determination of acid resistance
The acid resistance is a measure of the chemical stability of
glass ceramics used in the dentistry, since these are permanently
exposed to the action of acid substances in the oral cavity.
The acid resistance was determined according to the ISO
specification 6872:1995. To this end, small sample plates 12 mm
in diameter and 1 mm thick were prepared initially by sintering
together glass ceramic granules with an average particle size of
90 ~m. The granules were kept at the sintering temperature for
l minute. The sample plates were then treated for 16 hours in a
Soxhlet apparatus with 4 vol.% of aqueous acetic acid and finally
the loss of mass occurring was determined as a measure of the
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acid resistance.
Mixtures of the apatite glass ceramics with additional components-
were e~mined in the following examples. Glasses and/or other
glass ceramics which were used as additional components had the
composition given in Table III.
Table III: Composition of glasses and glass ceramics as additional components
(details in wt.~)
Add. SiO2 Al2O3 P205 CaO F K20 Na20 Li20 B203 TiO2 ZrO2 CeO2 BaO ZnO D
com-
ponent
Alkali 61.5 8.7 .. 1.0 1.7 7.0 8.8 .. 2.4 1.5 1.0 0.5 2.9 3.0
silicale
glass
(A)
Alkali 61.4 8.5 .. 1.1 1.7 7.8 8.7 0.6 1.9 1.5 1.0 0.5 2.1 3.2
si1icale
glass
(B)
Alkali 62.3 8.7 .. 1.3 1.6 7.0 7.0 2.0 1.1 1.4 1.0 0.6 3.0 3.0
silicale
glass
(C)
Alkali 70.8 8.6 .. 2.1 0.9 6.9 8.3 1.5 0.2 0.7 .. .. .. ..
silicate
glass
(D)
Alkali 63.4 6.2 0.4 1.7 .. 6.4 9.6 .. 3.7 1.7 1.1 0.5 2.3 3.0
silicale
glass
(E)
Alkali 61.9 9.9 .. 1.1 1.5 5.8 3.7 0.2 8.0 1.4 1.1 0.5 2.8 2.1
silicate
gl~ss
(F)
Lcucile 56.8 13.G 2.G 3.7 0.3 10.8 7.5 0.2 0.3 0.4 1.1 1.0 0.9 0.8
phos-
phosi-
licate
~~1ass D
ccramic
AlkaG 57.1 9.5 ,. 1.9 0.9 9.6 9.3 1.7 .. .. 1.0 1.0 3.9 4.1 ~ ''
zinc '~
silicate
glass
(H)
Opal- 55.8 15.2 2.6 2.6 .. 11.0 9.6 .. 0.3 .. 1.9 1.0 .. .. ~o
escent
glass (I)
CA 02239869 1998-06-08
E~ample 18
This Example describes the use of the glass ceramic according to
the invention according to Example 9 as a coating material for
ceramic frameworks and thus for the production of all-ceramic
dental products.
Glass powder of the appropriate composition was heat treated for
1 hour at 1020~C for the production of the glass ceramic. The
glass ceramic formed was ~mined by-sc~nni~g electron microscopy
and the crystals formed could be identified by X-ray
diffractometry as needle-shaped apatite crystals.
In order to obtain a suitable expansion coefficient and sintering
temperature, the glass ceramic was mixed with the alkali silicate
glasses (A) and (B) (see Table III).
The production of these alkali silicate glasses took place in a
manner similar to the production of the starting glasses
described above in Examples 1 to 17.
The glass ceramic and the two alkali silicate glasses were mixed
in the form of powders having an average particle size of less
than 90 ~m and in a weight ratio of 40% apatite glass ceramic
according to Example 9 (see Table II), 30% alkali silicate glass
(A) and 30% alkali silicate glass (B).
This mixture was sintered at 870~C to a rod-shaped green compact
in a vacuum furnace at a rate of heating of 60~C/min and with a
holding time of 1 min. A thermal expansion coefficient of 9.5 x
10-6K-l, measured in the temperature range of from 100 to 400~C,
was determined for the sample obtained.
CA 02239869 1998-06-08
- 24 -
This mixture could thus be used for sintering on to a substrate
with a thermal expansion coefficient of 10.6 x 10-6K-I, such as
lithium disilicate glass ceramic, at an advantageous processing
temperature of 830~C. This processing on the tooth substrate can
usually take place at temperatures that are 50~C to 100~C lower
than for sintering onto quartz.
The fully ceramic products obtained are characterised by good
chemical stability, an aesthetic appearance and high strength.
Example 19
In the same way as Example 18, different apatite glass ceramics
according to the invention may also be mixed together or with
other glasses to obtain desired expansion coefficients and
sintering temperatures.
A powder mixture of 25 wt.% of glass ceramic according to Example
4 (heat treatment at 1020~C), 50 wt.% of glass ceramic according
to Example 14 (heat treatment at 1050~C) and 25 wt.~ of alkali
silicate glass (B) (see Table III) was produced. This mixture had
an advantageous sintering temperature of only 830~C and an
expansion coefficient of 9.5 x 10 K .
The mixture had good chemical stability and outstanding optical
properties and was highly suitable as a sintering ceramic for an
all-ceramic dental framework with a low expansion coefficient.
Example 20 to 27
Further mixtures of apatite glass ceramics according to the
invention with glasses and glass ceramics were examined in these
Examples.
CA 02239869 1998-06-08
- 25 -
The compositions of the individual mixtures and the heat
treatment carried out for the production of the apatite glass
ceramic used in each case are listed in Table IV.
The properties determined for these mixtures are also give in
Table IV, and they show that it is possible, by means of a
suitable choice of components, to obtain dental materials with
properties matched to the application in question which in any
event are-characterised by good chemical stability.
CA 02239869 1998-06-08
- 26 -
3 .~ ~ ~ v
~ , . oo C
o
o ~
, ~ o ~
o o o ,~, ~" o oo t o o
X , ~C~ CO ~ ~ ~ ~4
, C ~, o o o o o o ~~ o
~ ~ ~1 ~
." ~
~ ~ 3
-. ~,c ~ 0~0~0~0 ~o~ ~
o
C~
-
t: -- ~ ;~ 2i o ~ ~
.. 3 ~ ~ ~ ~ ~ ~ ~ ~
~ _ . _ . _ . _ . _ . _ . . :
~LL~ ~~ ~ ~
-
E~'"~ ~
CA 02239869 1998-06-08
- 27 -
Example 28
In this Example, the translucence was determined quantitatively-
by determining the CR value of selected dental materials
according to the invention.
The British Standards Institution method of measurement was used
for this purpose, which is described in the test stAn~rd for
dental ceramic "BS 5612:1978".
Five specimens per material with a diameter of 20 mm and a sample
thickness of 1.75 mm were fired at an appropriate sintering
temperature. The specimens were ground with wet SiC powder, grain
size 320, in order to obtain the desired surface quality (surface
roughness Ra = 0.8 ~m - 1.6 ~m). It is important that the plane-
parallelism of the opposite sides does not exceed a tolerance of
+ 0.01 mm since the measuring result depends to a large extent
on the layer thickness. The final sample height / thickness
should be 1.00 ~ 0.025 mm.
The specimens were placed in the designated opening in a Minolta-
CR 300 colour measuring instrument and the reflection of each ofthe 5 specimens was measured with an aperture of 10 mm. The
samples must not be in optical contact with the background during
the measurement, a situation which may be prevented if necessary
by applying a drop of glycerol onto the background.
(a) In order to determine the sample emission on a black
background Yb(Yblack)r a black plate with not more than 4
reflection was used.
(b) In order to determine the sample emission on a white
background Yw (Y~hite), a white plate with a reflection of 80
to 85% was used.
CA 02239869 1998-06-08
- 28 -
The contrast value CR was then calculated from the Yb and Yw
values deterniined according to CR = Yb / Yw, and it was as
follows for the two materials ~x~m;ned:
Material l: CRI = 0.13 - 13% opacity
Material 2: CR2 = 0.50 - 50% opacity
The materials had the following composition:
Material l: Composition like the mixture according to Example
Material 2: 50 wt.% of mixture according to Example 20
- 50 wt.% of apatite glass ceramic according to
Example 14
(heat treatment 1050~C, l hour)
The above results show that the translucence can be adjusted by
a suitable choice of the composition of the materials.
