Note: Descriptions are shown in the official language in which they were submitted.
CA 02213390 1997-08-20
1
Sinterable lithium disilicate Qlass ceramic
The invention relates to sinterable lithium disilicate glass
ceramics and in particular those which, by virtue of their
properties, are suitable for the production of shaped dental
products by plastic deformation with the action of pressure and
heat.
Lithium disilicate glass ceramics are known from the prior art.
EP-B-536 479 describes self-glazed lithium disilicate glass
ceramic articles which are not, however, intended for dental
applications . The glass ceramics are free from La203 and are
formed in the usual manner by melting suitable starting
materials, pouring into molds and subsequent heat treatment of
the articles obtained.
Lithium silicate glass ceramics are also disclosed in EP-B-536
572. They are given structure and color by the dispersion of a
finely divided colored glass onto their surface, and they are
used as lining units for building purposes . They are manufactured
in a conventional manner in that suitable starting materials are
melted, the melt is molded to a desired body and the body is
heat-treated together with dispersed colored glass . Laz03 is not,
however, contained in the glass ceramic.
Glass ceramics based on SiOz and Li20 which contain large
quantities of physiologically very harmful arsenic oxide are
known from DE-C-1 421 886.
Moreover, the use of lithium disilicate glass ceramics in dental
technology is also disclosed in the prior art, but said glass
ceramics contain no Laz03 or Mg0 whatsoever and only conventional
methods are used to process them to dental products, wherein a
heat treatment is carried out to precipitate crystals only on
homogeneous bodies, namely monoliths formed from a glass melt,
such as small glass blocks or slabs . Conventional methods of this
kind, however, only allow volume crystallisation to take place,
not surface crystallisation.
CA 02213390 1997-08-20
- 2 -
Examples of such glass ceramics and conventional methods for the
production thereof are described in following documents.
A lithium disilicate glass ceramic with high strength suitable
for the preparation of dental crowns and bridges is described in
US-A-4,515,634.
A high-strength lithium disilicate glass ceramic is also
described in US-A-4,189,325 wherein said glass ceramic necessar-
ily contains Ca0 to improve the flow and also platinum and
niobium oxide to produce very fine and uniform crystals.
Glass ceramics containing lithium oxide and silicon oxide for the
preparation of dental prostheses, which contain very large
quantities of MgO, are described in FR-A-2 655 264.
Finally, US-A-5,507,981 and WO-A-95/32678 describe lithium
disilicate glass ceramics which may be further processed to
formed dental products by special methods, wherein pressing in
the viscous, flowable state at elevated temperatures to the
desired dental product takes place. No further details are given
regarding the production of the slabs or buttons used during this
process. A conventional method is also used to produce the glass
ceramic in that homogeneous glass bodies, such as slabs, for
example, are heat-treated. A disadvantage of these methods is
that they are very elaborate for a dental technician as a result
of the use of a special heat-pressure deformable crucible.
Moreover, the glass ceramic materials are heated to such an
extent that crystals are no longer present in the molten material
since the viscosity would otherwise be too high for pressing to
the desired dental product. Consequently, the product processed
is glass, not a glass ceramic.
The known lithium disilicate glass ceramics have shortcomings,
particularly when they are to be processed further in the plastic
state to shaped dental products. Their viscosity is not ideally
CA 02213390 2001-08-10
- 3 -
adjusted for such processing, so a controlled flow is not
possible and the reaction with the investment material is
undesirably high. Moreover, conventional glass ceramics have
only poor dimensional stability on heating, so that dental
restorations produced from them may be provided with a
sintered-on glass or glass ceramic layer only with
deformation. Finally, conventional lithium disilicate glass
ceramics also frequently lack the necessary chemical stability
for use as dental material, which is permanently being flushed
with fluids of various kinds in the oral cavity.
An object of the invention is, therefore, to provide a lithium
disilicate glass ceramic which exhibits cptimum flow
properties, and, at the same time, little reaction with the
investment material when pressed in the plastic state to
dental products, has high dimensional. stability on heating,
particularly in the range from 700°C to 900°C, and has
excellent chemical stability.
This object is achieved by the sinterable lithium disilicate
glass ceramic of the present invention.
The invention also provides a process for the preparation of
shaped dental products using the glass ceramic, the use of the
glass ceramic, shaped dental products con~~aining the glass
ceram=c, and the starting glass used to produce the glass
ceramic.
The sinterable lithium disilicate glass ceramic according to
the inver.t~_or_ is preferably characterised in that it contains
the following components:
Component Wt.o
SiO~ 57.0 to 80.0
A1~03 0 to 5.0
La~03 0.1 to 6.0
Mg0 0 to 5.0,
CA 02213390 1997-08-20
- 4 -
particularly 0.1 to 5.0
Zn0 0 to 8.0
KZO 0 to 13.5
Li20 11.0 to 19.0
PZOS 0 to 11.0
Color components ~ 0 to 8.0
Additional components 0 to 6.0
wherein
( a ) A1z03 + La203 amount to 0 .1 to 7 . 0 wt . ~ and
(b) Mg0 + Zn0 amount to 0.1 to 9.0 wt.~
and wherein the color components are formed from glass-coloring
oxides (c) and/or coloring bodies (d) in the following quan
tities:
(c) glass-coloring oxides 0 to 5.0 wt.~ and
(d) coloring bodies 0 to 5.0 wt.~.
It is preferred that the glass ceramic essentially consists of
the components mentioned above.
Lithium disilicate was detected by X-ray diffraction analyses as
the main crystalline phase of the glass ceramic according to the
invention.
There are preferred quantity ranges for the individual components
of the lithium disilicate glass ceramic according to the
invention. These may be chosen independently of one another and
are as follows:
Component Wt. ~
Si02 57.0 to 75.0
A1203 0 to 2.5
La203 0.1 to 4.0
CA 02213390 1997-08-20
- 5 -
Mg0 0.1 to 4.0,
Zn0 0 to 6.0 particularly 0.1 to
5.0
KZO 0 to 9.0 particularly 0.5 to
7.0
LizO 13.0 to 19.0
P205 0 to 8.0 particularly 0.5 to
8.0
Color components 0.05 to 6.0
Additional
components 0 to 3Ø
The glass ceramic according to the invention contains preferably
color components, namely glass-coloring oxides (c) and/or
coloring bodies (d) in order to obtain a color match between a
dental product produced from the glass ceramic and the natural
dental material of the patient. The glass-coloring oxides,
particularly TiOZ, Ce02 and/or Fe203 serve only to obtain a
shading, the main coloration being brought about by the coloring
bodies. It is to be noted that TiOZ does not act as a nucleating
agent but, in combination with the other oxides, as a color
component. The coloring bodies are metal oxides conventionally
used in dental glass ceramics and, in particular, commercial
isochromatic coloring bodies, such as doped spinels and/or doped
Zr02. The coloring bodies may be both non-fluorescing and
fluorescing materials.
In addition to the components mentioned above, the lithium
disilicate glass ceramic according to the invention may also
contain additional components, for which B203, F, NazO, Zr02, Ba0
and/or Sr0 are particularly suitable. The viscosity of the
residual glass phase of the glass ceramic may be influenced with
B203 and F, and it is assumed that they shift the ratio of
surface to volume crystallisation in favour of surface
crystallisation.
To produce the glass ceramics according to the invention, the
process described in more detail below for the production of
shaped dental products containing the glass ceramic is used in
CA 02213390 2001-08-10
- 6 -
particular, wherein the forming of special shapes is not
necessary.
The process according to the invention for the production of
shaped dental products containing the sinterable lithium
disilicate glass ceramic according to the invention is
characterized in that
(a) a starting glass which contains the components of the
lithium disilicate glass ceramic as discussed above, with
the exception of coloring bodies, is fused at
temperatures of 1200 to 1650°C to form a glass melt,
(b) the glass melt is poured into water with the formation of
glass granules,
(c) the glass granules are comminuted to a powder with an
average particle size of 1 to 100 um, based on the number
of particles,
(d) the coloring bodies optionally present are added to the
powder,
(e) the powder is compacted to a starting glass blank of the
desired geometry and he=erogeneous structure, and
the starting glass blank is subjected to one or more heat
treatments under vacuum and ;~n the temperature range from
400 to 1100°C in order to achieve a dense sintering and
to give a dental product in the form cf a blank.
In process stage (a), a starting glass is melted, for which
purpose suitable starting materials such as, for example,
carbonates, oxides and fluorides, are intimately mixed with
one another and heated to the specified temperatures, as a
result of which the starting glass forms. If color-imparting
oxides are to be used, these are added to the batch. The
addition of optionally present coloring bodies takes place in
a later stage of the
CA 02213390 1997-08-20
process, since their effect would be lost at the high tempera-
tures prevailing in the glass melt.
The glass melt obtained is then quenched in stage (b) by being
poured into water and is thereby converted into glass granules.
This procedure is usually also referred to as fritting.
The glass granules are then comminuted in stage (c) and in
particular milled to the desired particle size with conventional
mills. An average particle size of the powder obtained of 10 to
50 um, based on the number of particles, is preferred.
The addition of optionally present coloring bodies then takes
place in stage (d).
In stage (e), the powder is then compacted to a glass blank of
the desired geometry and heterogeneous structure. This is carried
out, in particular, at room temperature and pressures of, in
particular, 500 to 2,000 bar are used. This process stage of
pressing to a blank with a heterogeneous structure is important
so that, in contrast to the procedures known from the prior art,
surface crystallisation takes place in addition to volume
crystallisation during the subsequent heat treatment in stage
(f). The heterogeneous structure of the starting glass blank
composed of starting glass powder particles pressed together thus
allows controlled surface crystallisation on the inner surfaces
of the glass powder. This surface crystallisation is identifiable
by the fact that even without conventional volume nucleating
agents, such as metals or P2O5, the heat treatment taking place
in stage ( f ) leads to the formation of a lithium disilicate glass
ceramic containing finely divided crystals. If PZOS is used as a
component of the starting glass, the heat treatment in stage (f)
causes both surface crystallisation and volume crystallisation
to take place. In conventional processes, on the other hand,
blanks with a homogeneous structure are used, i.e. iri which no
CA 02213390 1997-08-20
- g -
particles of starting glass powder are present. The result of
this is that surface crystallisation is not possible.
The purpose of the heat treatment taking place in stage (f) is
to initiate the crystallisation of the starting glass blank and
hence to form the glass ceramic which, after this process stage
has ended, takes the form of a densely sintered glass ceramic
blank. This blank usually has the shape of a small cylinder or
a small slab.
The options for producing the final dental product, such as a
bridge or a crown are, in particular, the two options (g1) or
(g2) given below.
On the one hand, in stage (g1), the dental product taking the
form of a blank is subjected to plastic deformation at a
temperature of 700 to 1200°C and by the application of a pressure
of 2 to 10 bar to form a dental product of the desired geometry.
To this end, in particular the process described in EP-A-231 773
and the pressing furnace disclosed therein are used. In this
process, the blank in the plastic state is pressed into a die
cavity conforming with the dental product of the desired shape.
The pressing furnace used for this purpose is marketed as the
Empress~ furnace by Ivoclar AG, Liechtenstein.
It was ascertained that conventional lithium disilicate glass
ceramics do not satisfy various requirements for further
processing to dental products by plastic deformation. A require-
ment of this further processing is that the blank in the plastic
state should flow in a controlled manner and at the same time
react only to a small extent with the investment material.
Surprisingly, these two properties are obtained with the glass
ceramic according to the invention by the use of La203 and A1203
in the specified quantities. It is very surprising that the
dental product in the form of a blank is free flowing and can be
pressed in the plastic state although it is already a glass
CA 02213390 1997-08-20
- 9 -
ceramic material. In contrast to this, the prior art teaches
always use of a glass as a liquid melt, since otherwise pressing
in the plastic state is not possible because the viscosity is too
high.
It has proved to be particularly advantageous if the dental
product in the form of a blank has a viscosity of 105 to 106 Pa.s
during plastic deformation in stage (g1).
On the other hand, the dental product in the form of a blank may
also be machined in stage { g2 ) to a dental product of the desired
geometry, for which purpose in particular computer-controlled
milling machines are used.
In many cases it is advantageous that the dental product of the
desired geometry obtained after stage (g1) or (g2) is provided
with a coating in stage (h). A suitable coating is, in particu-
lar, a ceramic, a sintered ceramic, a glass ceramic, a glass, a
glaze and/or a composite. Coatings which have a sintering
temperature of 650 to 950°C and a coefficient of linear expansion
that is smaller than that of the dental product to be coated are
advantageous. Coatings whose coefficients of linear expansion
do not differ by more than t 3.Ox10-6K-1 from those of the
substrate are particularly advantageous.
A coating is applied in particular by sintering on, for example,
a glass, a glass ceramic or a composite. During this sintering
process, the dental product containing lithium disilicate glass
ceramic is, however, brought into a temperature range which is
above the transformation point of the residual glass matrix of
the glass ceramic. Conventional lithium disilicate glass ceramics
are often deformed in an undesirable manner during this process
since their dimensional stability on heating is too low. The
dental product according to the invention, however, has an
excellent dimensional stability on heating, for 'which in
CA 02213390 1997-08-20
- 10 -
particular the Laz03 and A1z03 content in the specified quantities
is responsible.
Apart from sintering on, other processes of the kind that are
customary for the manufacture of material composites, e.g.
bonding or soldering, may also be used.
Moreover, the glass ceramic according to the invention also has
very good chemical stability, which is brought about by the use
of A1203, La203, Mg0 and Zn0 in the specified quantities .
Apart from the above-mentioned properties of the lithium
disilicate glass ceramics according to the invention, these also
have the following other important properties, as a result of
which they are particularly suitable for use as dental material
or component thereof:
- High bending fracture strengths of 200 to 400 MPa. The
method of measurement is explained in the Examples.
- High fracture toughness values of 3 to 4.5 MPa~anl~2. The
method of determination is explained in the Examples.
A translucency comparable with that of the natural tooth,
although the production of the glass ceramic according to
the invention takes place at least partially by the mechan-
ism of surface crystallisation. This is surprising because
opacity is often brought about in other glass ceramic
systems due to surface crystallisation effects or initi-
ation of surface nucleation, as in the case of the forma-
tion of surface distortion due to j3-quartz mixed crystal
formation.
- Ability of the color to be matched to that of a natural
tooth by using color components. It is surprising-that in
spite of the color components that can be used, the
CA 02213390 1997-08-20
- 11 -
strength and toughness of the glass ceramic is not adverse-
ly impaired. For example, it is known that the
crystallisation of leucite glass ceramics, which are
likewise produced by the mechanism of surface
crystallisation, is greatly influenced by such additives
and that their strength is often very much reduced thereby.
- Ease of etching of the glass ceramic if this is used as
dental restoration. For example, a retentive pattern is
produced on the inner side of a dental crown according to
the invention by controlled etching. When a retentive
pattern is produced, no layer-like erosion of the glass
ceramic takes place, as is the case, for example, with mica
glass ceramics, but small open-pored structures are pro-
duced in the surface region. As a result of a retentive
pattern of this kind, it becomes possible to fix the glass
ceramic to the natural tooth with the aid of an adhesive
bonding system.
Suitable shaped dental products according to the invention which
contain the glass ceramic according to the invention are, in
particular, dental restorations, such as, for example, an inlay,
an onlay, a bridge, a post construction, a facing, jackets,
veneers, facets, connectors, a crown or a partial crown.
The invention will be explained in more detail below on the basis
of examples.
Examples
Examples 1 to 21
A total of 21 different glass ceramics according to the invention
and shaped dental products with the chemical composition given
in Table I were prepared by carrying out stages ( a ) to '( f') of the
process described.
CA 02213390 1997-08-20
- 12 -
O
N
w , , , . , , , , , ,
' ~ ~
V M r-1 M 00 N M -I
' ~
M O O O O d , O O O
O
C V Os vp ~ In M M 00 O G~
N ~t h M O V1 , ~ .-i M ~Y
M
O
c~ ~ 00 O N O M ~O ~ h M
a o .-i ~ o .-io o .-a ,-ic
w , , , . , , , , ,
N
O Ir
,
~.''~
U , , ,
, o c , , . o
N
O ~n
L !)Q
N , . , c , . , . . ,
H O
H , . . , , , , ,
r-I
E O
N
I 1 n t , n n 1 , 1
0 N CO .~-~h Op M O r1 O~ ~Y
O
N
z , , , ,
, , , , , ,
O ~t h M M O .-~O 00 M
, M M ~-- W ~t et (V N ~t
'ct t
r
C4 M M M (V , M M N N M
n
O
00 ~--~O h h O V'~ h
Q et .-i .-iO , ~ r-iO O r-i
N
O O~ ~ N v-1O M o0 ~ V'7
r
O
W z .-aN M ~t V O
W h 00 Ov
CA 02213390 1997-08-20
- 13 -
N
, , , , . , , O
N N ~ h N d N
O O O . (V , . O O . O
op .-a 'ctC; ~ M I~ N .~-;N
~t ~Y M ~ , v~ et (V N O v7
00 N wt M ~ N O 00 N O M
O O O O O O .~ O M .-iO
~-~ ~ V7 v~ N
O ~ D , , O , , O ,
'~ W C h M M
O O , , , , r.i O O C
i
N I
t 0 i
M , , M , , C ,
i
i
M I
n ~ tn
n O n , n , m -I O n O
O . , , , . , , C ,
~ h tn Cs h N M r-r~O et
~i ~n ~i ~ ~n ~i ~ Vi vG ~ ~i
H ~
Ov M
, , , (V , (V fV , , , ,
N M O h ~t N o0 ~ GI M
~ 'ch et'M ~t , et M ~ M M
h 00 N ~ O Oy 00 00 O Oy
M M M .~ ~ M M M , V M
O G1 00 .-a.-1 O O r1 .-a
r-i .-i O CV .-ie-i .-ir-i m --i
'ct 00 v1 N et .-i M M v0 y 0
h r h
I
N M ~t V1 ~O h 00 Ov O N
H ~--W--i~ .-~.--i~ ~ ~ N
CA 02213390 1997-08-20
- 14 -
Example 22
This Example describes the preparation of a glass ceramic
according to the invention and the potential use thereof as a
framework material for the preparation of a fully ceramic product
which can be formed individually, such as a crown or a multiple-
unit bridge, on which in addition a matching dental sintered
ceramic is fired on.
A starting glass with the chemical composition given in Table I,
Example 21, was prepared initially. To this end, a batch of
oxides, carbonates and phosphates was melted in a plati-
num/rhodium crucible at a temperature of 1500 to 1600°C for a
homogenisation period of one hour. The glass melt was quenched
in water, and the glass frit formed was dried and milled to an
average particle size of 20 to 30 um. Coloring by means of
coloring bodies could be dispensed with due to the use of glass-
coloring oxides, namely Ce02, Ti02 and Fe203.
The colored glass powder was then pressed by means of a uniaxial
dry press at room temperature and at a pressure of 750 bar to
form cylindrical starting glass blanks, hereinafter referred to
as green compacts, with a mass of about 4 g. The green compacts
were sintered in a furnace under vacuum to produce the glass
ceramic according to the invention in the form of a blank. In a
first phase, the green compact was fired for one hour at 500°C.
The blank was then densely sintered in a second sinter treatment
at 850°C for 2 hours, the rate of heating being 30°C/min.
CA 02213390 1997-08-20
- 15 -
Properties of the blanks
Optical properties
The blanks obtained had optical properties, e.g. translucency,
color and opacity comparable with conventional dental ceramic
commercial products, such as IPS Empress OI blanks from IVOCLAR
AG, Liechtenstein.
Biaxial strength
To determine the biaxial strength, sintered blanks were sawn into
discs with a diameter of 12 mm and a thickness of 1.1 mm. The
biaxial strength was determined with three-point bearing test
apparatus (steel balls with a diameter of 3.2 mm) with a force
being introduced at one point by means of a punch with a diameter
of 1.6 mm according to ISO 6872-1995 E "Dental Ceramic" . The rate
at which the load was applied was 0.5 mm/min. The biaxial
strength determined under these conditions was 261 ~ 31 MPa.
The glass ceramic blanks obtained were finally pressed under
vacuum in the viscous state using the pressing method and
pressing furnace according to EP-A-0 231 773 to obtain the sample
geometry required for the test in question. The standby tempera-
ture of the pressing furnace was 700°C, and the rate of heating
to the pressing temperature was 60°C/min; the pressing tempera-
ture was 920°C, the retention time at the pressing temperature
was 10 min and the pressure was 5 bar. After the pressing
process, the die was air-cooled and the specimens were removed
from the die by sand-blasting with A1203 powder and glass beads.
The specimens obtained had the following properties:
CA 02213390 1997-08-20
- 16 -
Properties of glass ceramics sub-~ected to plastic deformation
Optical properties
The glass ceramic having undergone plastic deformation had
translucence properties which enable the dental technician to
prepare fully ceramic dental products from it, e.g. crowns or
multiple-unit bridges which meet the optical requirements of a
natural tooth. Due to the use of glass-coloring oxides in the
basic glass, the hot-pressed glass ceramic was tooth-colored.
The color intensity could be adjusted by controlling the
concentration of the coloring oxides or by the additional use of
coloring bodies.
The combination of translucent framework material and translucent
to transparent dental sintered glass ceramic with a coefficient
of expansion of 9.1 ~m/mK, which was sintered in layers at 800°C
under vacuum onto the crown or bridge structure having undergone
plastic deformation led to translucent, fully ceramic dental
restorations which meet the stringent aesthetic requirements for
such products.
3-point bending strength
Bars with the dimensions 1.5 x 4.8 x 20 mm3 were pressed and
these were ground on all sides with SiC wet-grinding paper (grain
size 1000). The bending strength was determined with a test
specimen span of 15 mm and a load applied at a rate of 0.5
mm/min. according to ISO 6872-1995 E ° Dental ceramic" . The 3
point bending strength determined under these conditions was 341
98 MPa.
Coefficient of linear thermal expansion
Cylindrical specimens with a diameter of 6 mm and a length of 20
mm were pressed. The coefficient of expansion determined for
CA 02213390 1997-08-20
- 17 -
these specimens in the temperature range from 100 to 500°C was
. 6 ,~m/mK .
Fracture touqhness K,
5
Bars with the dimensions 1.5 x 4.8 x 20 mm3 were pressed and
these were ground on all sides with SiC wet-grinding paper ( grain
size 1000). Using a diamond wheel (0.1 mm thick), the specimens
were notched on one side to a depth of 2.8 mm and then tested for
10 their 3-point bending strength. The bending strength was
determined with a test specimen span of 15 mm and a load applied
at rate of 0.5 mm/min. The K1~ value determined was 4.0 ~ 0.2 MPa
dm.
Acid resistance
Disc-shaped specimens with a diameter of 15 mm and a thickness
of 1.5 mm were pressed and then ground on all sides with SiC wet-
grinding paper (grain size 1000). The loss of mass per unit area
of these specimens determined according to ISO 6872-1995 E
"Dental ceramic" was determined after 16 hours' storage in 4
vol . ~ aqueous acetic acid solution, and it was only 73 ug/cmz and
was thus markedly below the required standard value for dental
ceramic materials of 2000 ug/cmZ.
Example 23
This Example describes the preparation of a glass ceramic
according to the invention and the potential use thereof as a
framework material for the preparation of a fully ceramic product
which can be formed individually, such as a crown or a multiple-
unit bridge, onto which in addition a matching dental sintered
ceramic has been fired on.
A starting glass with the chemical composition given i~ Table I,
Example 18, was prepared initially. To this end, a batch of
CA 02213390 1997-08-20
- 18 -
oxides, carbonates and phosphates was melted in a plati-
num/rhodium crucible at a temperature of 1500 to 1600°C for a
homogenisation period of one hour. The glass melt was quenched
in water and the glass frit formed was dried and milled to an
average particle size of 20 to 30 Vim. Commercial coloring bodies
and fluorescing agents were added to the glass powder and
homogenised.
The colored glass powder was then pressed by means of a uniaxial
dry press at room temperature and at a pressure of 750 bar to
form cylindrical green compacts with a mass of about 4 g. The
green compacts were sintered in a furnace under vacuum to obtain
the glass ceramic according to the invention in the form of a
blank. In a first phase, the green compact was fired at 500°C for
20 minutes. The blank was then densely sintered for 30 minutes
at 850°C in a second sinter treatment, the rate of heating being
30°C/min. Unless otherwise specified, the procedure used to
determine the properties of the glass ceramic blank was the one
given in Example 22.
Properties of the blanks
Optical properties
The blanks obtained had optical properties such as translucency,
color and opacity comparable with conventional dental ceramic
commercial products, e.g. IPS Empress Dentin 24 blanks from
IVOCLAR AG, Liechtenstein.
CA 02213390 1997-08-20
- 19 -
Biaxial strength
The biaxial strength was 270 ~ 38 MPa.
The glass ceramic blanks obtained were finally pressed under
vacuum in the viscous state to the desired specimen geometry for
the test in question using the pressing process and pressing
furnace according to EP-A-0 231 773. The standby temperature of
the pressing furnace was 700°C, the rate of heating to the
pressing temperature was 60°C/min, the pressing temperature was
920°C, the retention time at the pressing temperature was 10 min.
and the pressure was 5 bar. After the pressing process, the die
was air-cooled and the specimens were removed from the die by
sand blasting with A1z03 powder and glass beads.
The properties of the specimens obtained were determined
according to the procedure described in each case in Example 22.
Properties of glass ceramic subjected to plastic deformation
Optical properties
The glass ceramic having undergone plastic deformation had
translucence properties which enable the dental technician to
prepare fully ceramic dental products from it, e.g. crowns or
multiple-unit bridges, which comply with the optical requirements
of a natural tooth. The combination of translucent framework
material and translucent to transparent dental sintered glass
ceramic with a coefficient of expansion of 9.1 ~m/mK which was
sintered in layers at 800°C under vacuum onto the crown or bridge
structure which had undergone plastic deformation led to
translucent, fully ceramic dental restorations which meet the
stringent aesthetic requirements of such dental products.
CA 02213390 1997-08-20
- 20 -
3-point bending strength
The 3-point bending strength was 347 ~ 37 MPa.
Coefficient of linear thermal expansion
The coefficient of expansion determined in the temperature range
from 100 to 500°C was 10.7 um/mK.
Fracture toughness K,
The K1~ value determined was 3.8 ~ 0.5 MPa dm.
Acid resistance
The loss of mass per unit area determined according to ISO 6872-
1995 after 16 hours' storage in 4 vol.~ aqueous acetic acid
solution was markedly below the standard value for dental ceramic
materials of 2000 ~g/cm2.
