Note: Descriptions are shown in the official language in which they were submitted.
CA 0223986~ 1998-06-08
Translucent apatite qlass cer~mic
The invention relates to a 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,
however, have very high CaO and P2O5 contents, 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 ~f ion release in the oral medium.
However, these two types~of apatite glass ceramics are white-
opaque, and have a high level of ion release and/or bioactivity,so they are not suitable for restorative den~istry.
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 light, 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.
CA 0223986~ 1998-06-08
In well known apatite-containing glass ceramics for restorative
dentistry, the main crystal phase is regularly formed not by
apatite but by leucite or mullite. This, however, is undesirable
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 cont~ining 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 results in only a 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 m~rphology.
Moreover, DE-A-34 35 348 describes apatite-cont~; ni ng 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.
CA 0223986~ 1998-06-08
Glass ceramics with good chemical stability are disclosed in EP-A-O 695 726
as alkali-zinc-silicate glass ceramics. The disadvantage of said glass ceramics,however, is that they contain leucite rather than apatite as the crystal phase. As
a result of the high expansion coefficient of leucite, the glass ceramics are
5 therefore usually unsuitable as coatings for substrates with low expansion
coefficients, such as, in particular, lithium disilicate glass ceramics. The glass
ceramic also 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, in terms of their morphology, resemble that of
the carbonate-apatite crystals of natural tooth material but have a greater
15 chemical stability than these and hence confer excellent chemical stability on
the glass ceramic. Moreover, the apatite glass ceramic should have a low
thermal expansion coefficient and should therefore be particularly suitable as adental material and above all as a coating or veneer for dental restorations, such
as crowns or bridges, made of lithium disilicate glass ceramics.
The apatite glass ceramic according to the invention contains the following
components:
CA 0223986~ 1998-06-08
ComPonent Wt. %
SiO2 45.0 to 70.0
Al2O3 5.0 to 22.0
P2O5 0.5 to 6.5
K2O 3.0 to 8.5
Na2O 4.0 to 13.0
CaO 1.5 to 11.0
F 0.1 to 2.5
and the main crystal phase is formed by apatite crystals.
The glass ceramic according to the in~ention may additionally
contain at least one of the following components:
ComPonent Wt.%
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 these additional components are usually 0.05
wt.%.
Preferred quantity ranges exist for the individual components of
the apatite glass ceramic according to the in~ention. Unless
otherwise specified, these may be chosen independently of one
another and are as follows:
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ComPonent Wt. %
SiO2 50.0 to 68.0
Al2O3 7.0 to 21.0
P2O5 0.5 to 4.0
K2O 4.0 to 8.0
Na2O 4.0 to 11.0
CaO 2.0 to 8.0
- F 0.2 to 2.0
B2O3 0.2 to 4.0
LazO3 ~ to 3.0
Li2O 0 to 3.0
BaO 0 to 4.0
MgO 0 to 4.0
ZnO 0 to 4.0
SrO 0 to 5.0
TiO2 + ZrO2 0.2 to 5.0
CeO2 0 to 2.0
Particularly preferred quantity ranges for the individual
components of the apatite glass ceramics 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
CA 0223986~ 1998-06-08
ZnO 0 to 3.0
SrO 0 to 4.0
TiO2 0.5 to 2.0
ZrO2 0.5 to 3.0
CeOz 0.1 tc 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.
It was possible to ascertain 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 particular preference smaller than 5 ~m.
The optical properties of the glass ceramic are controlled by
means of the precipitated apatite crystals which are similar in
appearance to the carbonate-apatite crystals of natural tooth
material. It is thus possible to produce a glass ceramic with an
appearance which corresponds to 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.
In the case of the glass ceramic according to the invention, no
leucite crystals can be detected by radiography, though secondary
crystal phases, such as sodium-calcium-orthophosphate of the
NaCaPO4 type may be present.
CA 0223986~ 1998-06-08
The glass ceramic according to the invention is characterised by
very high translucence. In order to quantify the translucence,
the CR value was determined according to the method as 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~5 Yb = light reflection of the specimen on a black
background, and
Yw = light reflection of the specimen on a white
background.
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
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 composition in
combination with the special precipitated apatite crystals, good
chemical stability is achieved without ZnO necessarily having to
be incorporated.
It is presumed that this stability is attributable to the very
high degree of crystallinity and to the formation of hydroxy and
CA 0223986~ 1998-06-08
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 has become apparent that the apatite glass ceramic according
to the invention is superior to the conventional apatite-
containing glass ceramics in terms of chemical stability.
Surprisingly, particularly good chemical stability may be
achieved if the molar ratio of CaO to P2O5 to F in the glass
ceramic is 1 to (0.020 to 1.5) to (0.03 to 4.2), particularly 1
to (0.1 to 0.5) to (0.1 to 1).
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-1, measured in the
temperature range of 100~C to 400~C.
In order to produce the apatite glass ceramic according to the
invention, ~ ~ .
a) a starting glass cont~ining the above-mentioned 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
CA 0223986~ 1998-06-08
d) the glass granules or glass powder are 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 temperatures.
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 known 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 powder
are subjected to 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.
Volume crystallisation takes place 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) droplet-shaped precipitates which are extremely densely
packed and finely distributed. Even under the scanning electron
CA 0223986~ 1998-06-08
-- 10 --
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 possible to ascertain 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
d~m;xing 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
reproducible manner. The fine apatite crystals in the
microstructure of the glass ceramic according to the invention
bring about a very great similarity to the natural tooth in terms
of optical appearance and structure.
The apatite glass ceramic according to the invention is therefore
used particularly as a dental material and preferably as a
3~ component of dental material.
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When the 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
in which important properties, such as processing temperature,
optical properties, thermal expansion coefficient and chemical
stability 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.
, _
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-alkaline earth-silicate, alkali-aluminosilicate,
alkali-zinc-borosilicate, phosphosilicate or alumino-fluoro-
CA 0223986~ 1998-06-08
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.
5 - 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.%, K20 9.0-22.5 wt.%, Na20 1.0-9.5 wt.%, Li20
0-2.5 wt.%, B2O3 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.%, La203 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.%.
15 - Opalescent glasses having the composition:
SiO2 48.0-66.0 wt.%, B2O3 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)O2 0.5-10.0 wt.%, P2O5 0.5-5.0 wt.%, CeO 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)2O is formed by 3.0-15.0 wt.% of K2O, 3.0-12.0 wt.% of
Na2O 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 ~rO; and the
quantity of Me(IV) ~2 is formed by 0-5.0 wt.% of TiO2 and 0-
5.0 wt.% of ZrO2. ~~
- 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)203 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.S wt.% of BaO and 0-5.0
CA 0223986~ 1998-06-08
- 13 -
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
Al203, 0.2-10.0 wt.% of B203, 4.5-10.0 wt.~ of R20, 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.
Preferrably such glasses are used 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 dèntal crown or bridge.
In particular, the dental material is sintered on to obtain the
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 built-up liquids, 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,
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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 10 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 fully 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 appropriate
starting glasses, fritting and heat treatment at 400~C to 1100~C
have the following composition:
Component Wt.~ --
SiO257.0 to 80.0
Al2O3 ~ 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
P2OS 0 to 11.0
CA 0223986~ l998-06-08
- 15 -
with the proviso that
(a) Al2O3 ~ La2O3 is 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-
6K-l smaller than that of the substrate are particularly
advantageous. The dental material preferably has a thermal
expansion coefficient of 5.5 to 12. 5 X 10 K , measured at
temperatures of 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 with 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
blanks of the desired shape, particularly dental restorations
such as an inlay, an onlay, a bridge, an abutment, a jacket, a
veneer, a facet, a filling, a connector, a crown or a partial
crown.
In contrast to conventional glass ceramics, a leucite crystal
phase could not be detected in the apat te glass ceramic
according to the invention. This is also undesirable since, due
to the high expansion coefficient of leucite, it would also
confer a high thermal expansion coefficient of usually more than
12.5 x 106K on a glass ceramic. If leucite-containing glass
ceramic is used to coat a substrate that has an expansion
coefficient of less than 12.5 x 10-6K-l, such as ZrO2 or lithium
disilicate glass ceramic, very high tensions are therefore also
induced which result in cracks and chipping. The glass ceramic
according to the invention does not exhibit these disadvantages
CA 0223986~ 1998-06-08
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due to its low expansion coefficient, so it is very suitable for
coating substrates with low expansion coefficients.
The invention is illustrated 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 P205 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 -
Image
CA 0223986~ 1998-06-08
- 18 -
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
5 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
10 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 composed
of the respective glass ceramic are given in Table II for some
15 of the glass ceramics. Moreover, details about the heat
treatment actually chosen for the starting glass are given in
Table II--under UHeat treatment~.
The examples illustrate how glass ceramics with different
20 properties may be obtained by altering the chemical composition.
CA 0223986~ 1998-06-08
-- 19 --
Table II:
Ex. Heat Firing Tg ~-value Optical Acid re-
treat- tempe- [~C] x10 K appea- sistan2ce
ment rature (100~C- rance [~g/cm ]
[~C/h] [~C] 400~C)
1 1050/1 860 545 8.4 milky, 21
slightl
y opal,
translu
-cent
8 1000/1 1080 650 7.9 very 23
trans-
lucent
g 1020/1 1050 645 9.7 very 28
trans-
lucent
11 1000/1 890 547 6.6 yellow- 58
ish,
milky,
tran-
slucent
14 1050/1 870 541 9.4 whitish 55
cloudy,
trans-
~ lucent
* Firing temperature = temperature which was used duringproduction of the specimens by sintering onto quartz (1
minute holding time~, vacuum)
15 Determination of the exPansion coefficient--~
In order to measure the thermal expansion coefficient ~, 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
CA 02239865 l998-06-08
- 20 -
obtained.
Determination of acid resistance
5 The acid resistance is a measure of the chemical stability of
glass ceramics used in dentistry in particular, since these are
permanently exposed to the action of acid substances in the oral
cavity.
10 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
15 temperature for 1 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 acid resistance.
In the following examples mixtures of apatite glass ceramics
with additional components were examined. Glasses and/or other
glass ceramics ~that can be used as additional components had the
composition given in Table III. -
i
CA 02239865 1998-06-08
- 21 -
O O N O O
C ~ . . .. . . .
e;~ ~ ~ ~ ~ ~ N O
dP Ocr~ _I o t~~, '
. . . .. . . .
m N ~,~.~ .~ ~
oLnu~ o O O
~-~ ~. . . .. . . .
o o O ~ o o
~a
c oo o o ~ ~ _I O O~
~ h
o
p.,
o .0 ~ Ln ~ ~ ~
E~--I ~ ~ o _I ~ o
_I
o~ ~ ~ _I N~-- O ~ ~~1
O ,~ . . . . . . . . .
~~ m N ~ I O ~) CO O ~ O
~~
~~ ~D OLt) N N ~--
~ ¦ ~ O N--I ~ O O ~1
U~
a o0~ 0 ~ ~D
U~ ~ . . . . . . . .
a) O oCZ~ O O~ ~ o
U r~ ~ . . . . O
UJ
u
r . . . . ~ . . .
~1 O ~~I O O
r,cl O O ~ I r,~ ~ r.~ o~ ~o
~5 ~ ~ ~ ~ .. - . .
U O ~ I N _I~Irr~ _I C~l
a
U , r
O ~ ~D ~
rc ~
e,~ ~ ~ ~ ~ ~ ~ ~ N ~ N
~ O ~O N
~r.~L~~) r.~ ~D N r n ~ U~)
C
O ~0~r o ~r ~
u' OIn ~ ~ r~ ~o~ a~ _I r o
~1 ~ r,~ O r~ ~ D r,~ U7
E~ r~ Dr.~ ~O ~ U~ ~ Ul
o
U
H c c a) ~ a) m a) v a) a a~ ~ a) X ~ H
H ~-- C ~-- ~a ~-- r,~ r ~-- rc~
O _ O Ul _ ~ U) _ ~ ) U~ _ tJ Ul _ O U~ ) U~ l r ,1~ Ul Ei r t~ U! ~ Ul
~~ 1 U t~ -- u~ r~ ~-1 U rC ~- U ~ U~ r~ r l Ul t,) I Id U 1~1 r~ ~,) -_ U ~1 U~
~ ~ -4 ~-1 rC _--rcl -~--I a -~--c ~ c _ ~ ~ h ~ '~ C ~ tC al n5
r ~ O - ~~ - -
c~ rn ~ u~ ~ ~ U ~ u~ ~ ~ Ul ~ ~ U~ ~ O
CA 0223986~ 1998-06-08
- 22 -
Example 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 fully
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 ex~mined by scanning 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 similar way 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 at temperatures from 100~C to 400~C, was
determined for the sample obtained.
This mixture could thus be used for sintering on to a substrate
with a thermal expansion coefficient of 10.6 x 10-6K-l, such as
lithium disilicate glass ceramic, at an advantageous processing
CA 0223986~ 1998-06-08
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 solid ceramic products obtained are characterised by good
chemical stability, an aesthetic appearance and high strength.
~ample 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 thus 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 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 theinvention with glasses and glass ceramics were examined in these
Examples.
The compositions of the individual mixtures and the heat
treatment ca~ried out for the production of the apatite glass
ceramic used in each case are listed in Table IV.
CA 0223986~ 1998-06-08
- 24 -
The properties determined for these mixtures are also given 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.
Table IV: Compositions and properties of mixtures of apatite glass ceramics according
to the invention with glasses and/or glass ceramics
Ex. Composition Heat lreat - ' Mixing Firing temp. Tg [~CI ~-value x 10-ffK-' Optical Acid rç~is~ .
~C/hl ratio [~C] (100~C-400~C) appcal ance ~ILg/cm
[in wt%]
Apatite glass ceramic 9 1000/1 30 880 528 9.5 very translu- 34
Alkali silicate glass (A) cent D
Alkali silicate glass (B) .. 35 O
.. 3
21 Apatite glass ceramic 14 1050/1 50 850 530 9.3 milky cloudy, 38
Alkali silicate glass (A) , lransluccnt
22 Apatite glass ceramic 14 1020/1 50 870 542 8.0 milky translu- <100 Oo~
Alkali silicate glass (F) cent o
.. 50
23 Apatite glass ceramic 8 1000/1 40 910 552 X.8 very translu- 29
Alkali silicate glass (C) cent
.. 60
24 Apatite glass ceramic 1 1050/1 i 70 850 539 8.7 slight1y milky, 26
Alkali silicate glass (D) slightly opal,
.. 30 translucent
Apatite glass ceramic 9 1020/1 20 780 463 1().9 transpa- 24
Alkali zin~ silicate glass rent
(H) .. 80
-- 26 --
26 Apalite glass ceramic 9 1020/l 50 1020 600 10.2 very trans- 27
O~,alesccl~l glass (I) lucent,
slightly
lJlu....;sh ûpal
27 Apatite glass ceramic 14 1050/1 30 910 560 9.7 whitish, 45
Leucite phosphosilicate translu-cent
glass ceramic (G) ..
D
CA 0223986~ 1998-06-08
Example 28
In this Example, the translucence was determined quantitatively
by determining the CR value of selected dental materials
s according to the invention.
The British Standards Institution method of measurement was used
for this purpose, which is described in the test standard 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 = O.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 reflectance
of each of the 5 specimens was measured with an aperture of 10
mm. The samples must not be in optical contact with the
25 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 (Ybl~ck), a black plate with not more than 4%
reflectance was used.
(b) In order to determine the sample emission on a white
background Yw (YWbi~e)~ a white plate with a reflectance of 80%
to 85% was used.
CA 0223986~ 1998-06-08
- 28 -
The contrast value CR was then calculated from the Yb and Yw
values determined according to CR = Yb / Y~, and it was as
follows for the two materials e~mined:
Material l: CR1 = 0.13 - 13% opacity
Material 2: CR2 = 0.50 - 50% opacity
The materials had the following composition:
Material 1: 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, 1 hour)
The above results show that the translucence can be adjusted by
20 a suitable choice of composition of the materials.
