Note: Descriptions are shown in the official language in which they were submitted.
CA 022~2660 1998-11-03
A Process for the PreParation of shaPed
translucent lithium disilicate ~lass ceramic Products
The invention relates to a process for the préparation of shaped
translucent lithium disilicate glass ceramic products which can
be prepared as blanks, which may be processed to shaped translu-
cent dental products with high strength, particularly by plastic
shaping with the action of pressure and heat or by machining.
Lithium disilicate glass ceramics are known from the prior art.
Thus, self-glazed lithium disilicate glass ceramic articles are
described in EP-B-536 479 but are not intended for dental
purposes. The glass ceramics also contain no La2O3, and there is
likewise no description of the prepar2tion of blanks from the
glass ceramic which, after processing, undergo a further heat
treatment in order to complete crystallization. It is also
necessary to carry out the heat treatment at a very low rate of
heating of 5K/min in order to prevent stresses in the structure
of the glass ceramic. ~oreover, the glass ceramic is intended
primarily for the preparation of table-~are which nzturally has
only low translucence.
EP-B-536 572 also describes lithium disilicate glass ceramics
which contain no La2O3. By scattering a finely di~ided coloured
glass onto their surface, they receive structure and colour, and
are used as lining elements for building purposes.
Lithium disilicate glass ceramics are disclosed in US-A-4,189,325
which necessarily contain calcium oxide for improving the flow
and also platinum and niobium oxide as special nucle2ting agents
in order to produce ve~y fine and uniform c yst21s. Even though
the glass ceramic can be prepared in the form of blanks which
have not yet crystallised completely, it is nevertheless free
from La2O3.
WO-A-95/32678 and US-A-5,507,981 describe lithium disilicate
glass ceramics which may be processed to shaped dental products
by hot pressing using a special pressable crucible. The glass
CA 022~2660 1998-11-03
ceramic materials are heated to such an extent, howe~er, that
crystals are no longer present in the molten material, otherwise
- the viscosity is too high for pressing to the dental product.
Tests have shown that when the materials described are pressed
by means of the process described in EP-A-231 773 and using the
pressing furnace disclosed therein, an undesirably strong
reaction occurs with the investment material used. Moreover, the
glasses used show a very high rate of crystal growth, so that
large crystals are produced during the heat treatment which
impair the structure of the glass ceramic produced and
consequently lead to products with poor strength.
~oreover, glass ceramics based on SiO~ and Li2O are known from
DE-C-1 421 886 which contain large ~uantities of arsenic trioxide
which is physiologically very harmful.
A lithium disilicate glass ceramic which is suitable for the
preparation of dental crowns and bridses but contains no La2O3 at
all is disclosed in US-A-4,515 63~.
The glass ceramics described in FR-A-2 6;5 264 are free from
La.O3. They contain lithium oxide and silicon oxide and very
large quantities of MgO and are suitable for the preparation of
dental prostheses.
--
Blanks of sintered ceramic based on leucite, feldspar or mica
which are processed to dental products by computer-aided milling
processes are also known from the prior art. These products have
low strength, however, which is why said materials have not
become established for highly stressed dental restorations.
The known lithium disilicate glass ceramics exhibit shortcomings
when they are further processed to shaped products since an
undesirably strong reaction with the inves-~ment material used
during pressing occurs when they are processed in the plastic
state using elevated temperatures and ele~ated pressures. Purther
. .
CA 022~2660 1998-11-03
processing of the glass ceramics by machining, such as milling,
cannot usually be carried out satisfactorily due to the strength
and toughness of the glass ceramics. Morec~er, the conventional
lithium disilicate glass ceramics frequently do not exhibit the
high strengths and optical properties such as high translucence
required for dental products and in many cases they also lac~ the
chemical stability required for use as dental material which is
permanently flushed with fluids of various kinds in the oral
cavity.
The object of the invention is, there-fore, to provide a process
for the preparation of shaped translucent lithium disilicate
glass ceramic products which have good chemical stability, a low
density of defects, and high translucence with simultaneously
good mechanical properties and exhibit only little reaction with
the investment material used when further processed by pressing
in the plastic state, and the glass ceramic products may also be
prepared in the form of blanks with a low degree of
crystallisation which may be shaped easily in the desired manner
by mechanical means such as machining and may be converted to a
high-strength glass ceramic product by a subse~uent heat
treatment.
Said object is achieved by the process for the preparation of
shaped translucent lithium disilicate glass ceramic products
according to claims 1 to 12.
The invention also relates to the shaped glass ceramic products
according to claims 13 to 15, the use according to claim 16 and
the shaped dental products according to claims 17 to 19.
The process according to the invention for the preparation of
shaped translucent lithium disilicate glass ceramic products is
characterised in that
.,
CA 022~2660 1998-11-03
_ 4
(a) a melt of a starting glass is produced which contains the
following components:
Com~onent Wt.%
SiO2 57.0 to 80.0
Al2O3 ~ to 5.0
LazO3 0.1 to 6.0
MgO 0 to 5.0
ZnO 0 to 8.0
Li2O 11.0 to 19.0
where
(i) Al2O3 + La~O3 accounts for 0.1 to 7.0 wt.% and
(ii) ~gO + ZnO accounts for 0.1 to 9.0 wt.%,
(b) the melt of the starting glass is shzped in the desired
manner and cooled, and
(c) the shaped glass product is subjected to at least one heat
treatment in the temperature range from 400 to 1100~C in
order to obtain a shaped glass ceramic product in the form
of a blank.
In process stage (a), a melt of a starting glass is produced, to
which end suitable starting materials, such as carbonates,
oxides, phosphates and fluorides, are intimately mixed and heated
to temperatures of, in particular, 1200 to 1600~C. In order to
obtain a particularly high degree of homogeneity, the glass melt
obtained may be poured into water to form glass granules and the
glass granules obtained are melted again at temperatures of, in
particular, 1200 to 1600~C for 1 to 4 hours.
The melt of the starting glass preferably contains at least one
of the following further components:
CA 022~2660 1998-11-03
Com~onent Wt. %
ZrOz 0 to 10.0
R2O 0 to 13.5
P2O5 0 to 11.0
5 Colour and
fluorescent components 0 to 8.0
Additional components 0 to 6.0
Surprisingly, it was established that the additional incorpor-
ation of ZrO2 led to 2n increase in translucence, although the
opposite effect was observed in the con~entional glass ceramic
according to EP-B-536 47g.
Ranges that may be chosen independently of one another, unless
otherwise specified, exist for the quantities of the individual
components, said ranges being 2s follows:
Com~onent Wt.%
SiO2 57.0 to 75.0
Al2O3 ~ to 2.5
La2O3 0.1 to 4.0
MgO 0.1 to 5.0
ZnO 0 to 6.0, p2 ticularly 0.1 to 5.0
ZrOz 0 to 8.0, particularly 0.1 to 8.0
.
R2O 0 to 5.0, p2rticularly 0.5 to 7.0
Li2O 13.0 to 1~.0
PzO5 0 to 8.0, p2rticularly 0.5 to 8.0
colour and
fluorescent components 0.1 to 8.0
additional components 0 to 3Ø
For example oxides of f-elements m2y be used as colour components
or fLuorescent components. In preference, at least one of the
following compounds is used.
, .. . . .. ..
CA 022~2660 l998-ll-03
-- 6 --
ComPonent Wt . %
CeOz 0.1 to 5 . 0
V205 0.01 to 1.0
Fe203 0.01 to 1 . 0
MnO2 0.01 to 3.0
TiO2 0.01 to 5 .0
Y2O3 0 . 0 1 to 2 . 0
Er203 0.001 to 2 . 0
Tb203 0.001 to 2 . 0
Eu203 0.001 to 2.0
Ybz03 - 0.001 to 2.0
Gdz03 0.001 to 2.0
Nd203 0.001 to 2.0
pr2o3 0.001 to 2.0
Dy2O3 0 . 001 to 2.0
Ag20 0.01 to 2.0
SnO2 0.01 to 3.0
Ta205 0.001 to 2 . 0
20 The special oxides that can be used 25 colour or fluorescent
components in the process according to the invention ensure that
the colour of the glass ceramic product can be matched easily to
the application in question. This is particularly important if
the glass ceramic products are to be used as dental products, the
colour of which must be matched specially to that of the natural
tooth material of the patient in ~uestion. The colour spectrum
that can be obtained with these special oxides ranges from very
pale shades to deep grey-brown shades e.g. in the case of non-
vital tooth stumps. The fluorescence of the natural tooth
material is imitated by any fluorescent components present. A
particular advantage of the colour and fluorescent components
used according to the invention is that they do not interfere
with the structure of the glass ceramic products produced in such
a manner that non-homogeneous materials with a high density of
defects and high porosity are produced. This problem frequently
occurs with sintered ceramics, the colour of which is altered by
.
CA 022~2660 1998-11-03
the addition of pigments. In order to prevent any deterioration
in their colouring effect, said pigments are not usually added
until prior to the sintering process carried out at relati~ely
low temperatures so that they are always present as crystals or
crystallites which lead to non-homogeneities.
Apart from the components mentioned above, the starting glass may
also contain additional components for which in particular B~03,
Na20, BaO, F and/or SrO are suitable.
Preferably, the melt of the starting glass is composed of the
components mentioned in the stated quantities.
Moreover, the melt of the starting glass is shaped in the desired
way in stage (b) and cooled. Shaping taXes place in particular
by pouring the melt into a desired mould. It is also possible for
compaction of the melt by pressure to take place after pouring
in order to achieve a particularly good homogeneity and accurate
reproduction. It is possible to proceed in such a manner that a
glass droplet is introduced into the desired mould and then
compacted by pressing.
The melt is cooled particularly in a controlled manner so as to
prevent stresses in the structure associated with rapid tempera-
ture changes and to prevent cracks and fissures that may possiblyresult from said stresses. As a rule, the melt is therefore
poured into preheated moulds or cooled slowly in a furnace.
Finally, the shaped glass product formed undergoes at least one
heat treatment in stage (c) in order to bring about the
crystallisation thereof. When this process stage has ended, 2
shaped glass ceramic product in the form of a blank is obtained.
This blank usually takes the form of a small cylinder or a
rectangular block. The heat treatment takes place preferably at
a temperature of less than 1000 and particularly less than 900~C.
The shaped glass product is preferably introduced into a furnace
CA 022~2660 1998-11-03
already heated to the temperature mentioned. In contrast to
cor.~entional materials, it is not necessary to select a slow rate
of heating in order to prevent stresses. The special composition
and method of preparation of the material according to the
invention is apparently responsible for this advantageous behav-
iour.
The degree of crystallisation and the crystal size in this glass
ceramic blank may be varied very widely by the type of heat
treatment selected. On the one hand it is possible to produce a
glass with only nuclei or very small crystals in the sub-micron
region, which thus represents the slmplest form of a glass
ceramic, or on the other hand to form a fully crystallised glass
ceramic. In each case, the ceramic production process takes place
by way of the mechanism of volume crystallisation, and volume
nucleating agents such as e.g. P2O5 present in the starting glass
used play an important part in the formation of finely divided
crystals in the structure.
In particular the following two possibilities (dl) and (d2) are
available for producing the final glass ceramic product, such as
a dental bridge or a dental cro~n.
On the one hand, the glass ceramic product in the form of a blank
may undergo plastic shaping in stage (dl) to a glass ceramic
product of the desired geometry at a temperature of 700 to 1200aC
and by the application of pressure, particularly of 8 to 40 bar.
It is preferable for this forming stage to use the process
described in EP-A-231 773 for the production of dental restora-
tions and to use the pressing furnace likewise disclosed therein.In said process, the blank is pressed in the plastic state into
a mould cavity corresponding to the desired shaped dental
product, such as crowns, using heat and pressure. The pressing
furnace used in particular for this purpose is sold as the
Empress~ furnace by Ivoclar AG, Liechtenstein.
. . .
CA 022~2660 1998-11-03
It has become apparent that conventional lithium disilicate glass
ceramics exhibit an unacceptabiy strong reaction with the invest-
ment material used during further processing to glass ceramic
products, have insufficient flow properties or exhibit uncon-
trolled crystal growth. These disadvantages are avoided in theprocess according to the invention by the use of La2O3 and
optionally Al2O3 in the stated quantities in the starting glass.
As a result, the glass ceramic product in the form of a blank may
be processed in an advantageous manner by pressing in the plastic
state to a glass ceramic product of the desired geometry,
particularly a dental product such as a dental restoration.
It is also possible to process the glass ceramic product in the
form of a blank by machining in stage (d2), particularly by
CAD/C~-based milling devices, to obtain a glass ceramic product
of the desired geometry. A so-called chair-side treatment is thus
possible for the dentist. When this variant of further processing
is carried out, the glass ceramic blank used in particular is one
which is not yet fully crystallised but is present e.g. only as
a nucleus-containing glass blank or glass ceramic blank with very
small crystals. Such glass ceramic blanks that have not yet fully
crystallised have the particular 2dvzntage that they may be
machined to the finished glass ceramic product of the desired
geometry in a mar~edly easier manner than conventional glass
ceramics. In order to produce a glass ceramic blank in which the
glass matrix contains only nuclei o_ very small crystallites, it
has proved to be particularly advantageous to carry out the heat
treatment performed in stage (c) at a temperature of 400 to
900~C. In each case, the degree o~ crystallinity of the glass
ceramic blank used may be adapted to the type of machining
desired so that said machining may be carried out as easily as
possible.
After the subsequent machining in stage (d2), the shaped glass
ceramic product obtained then undergoes at least one further heat
treatment, particularly at 700 to 900~C, in order to achieve
CA 022~2660 1998-11-03
-- 10 --
further crystallisation and hence solidification of the glass
ceramic product. The fracture strength, colour and translucence
are improved by this further heat treatment.
The finished glass ceramic product of the desired geometry
present after further processing, particularly in stages (dl) and
(d2), may ultimately be provided with a coating, which is
advantageous if it is used in the dental field. Suitable coatings
are in particular a ceramic, a sintered ceramic, a glass ceramic,
preferably an apatite glass ceramic, a glass, a glaze and/or a
composite. Those coatings that have a sintering temperature of
650 to 950~C and a linear thermal expansion coefficient that is
smaller than that of the glass ceramic product to be coated are
advantageous. Coatings whose thermal expansion coefficient
deviates by not more than + 3.0 x 10-6 K~l from that of the
substrate are particularly suitable.
A coating is applied in particular by sintering on. ~uring this
sintering process, the glass ceramic product containing the
lithium disilicate glass ceramic is, howe~er, brought to a
temperature range which lies above the transformation point of
the residual glass matrix of the glass ceramic. In so doing,
con~entional lithium disilicate glass ceramics are frequently
deformed in an unwanted manner because their dimensional
stability on heating is too low. The glass ceramic product
prepared according to the invention, however, shows excellent
dimensional stability on heating, for which in particular the
La2O3 content and possibly the ~12O3 content in the stated
quantities is responsible.
The glass ceramic products prepared according to the invention
are particularly suitable for use as dental products or constitu-
ents thereof due to their properties. Preferred glass ceramic
products have a 3-point bending strength of more than 400 MPa if
they are prepared according to process variant (dl) and of more
than 250 MPa if they are prepared according to process variant
,
CA 022~2660 1998-11-03
(d2). The process used to determine the 3-point bending strength
is explained in the Examples.
Moreover, the glass ceramic products according to the invention
have a transLucence comparable with that of the natural tooth.
In order to quantify the translucence, the CR value was deter-
mined according to the method described in the Examples. The CR
value, also known as the contrast ratio, indicates the ratio of
light reflection of à specimen of the glass ceramic on a black
background to the measurement of light reflection of a 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 = YD/Y~
where
CR = contrast ratio
Yb = light reflection of the specimen on a black background,
and
Yw = light reflection of the specimen on a white background.
The CR value is always between 0 and 1, whith CR = 0 standing for
an opacity of 0~ and consequently a completely translucent
material, and CR = 1 standing for an opacity of 100~ and
consequently a completely opa~ue material, i.e. one which is
impervious to light.
_ .
The glass ceramic product accordins to the invention usually has
a CR value of 0.05 to 0.9 and preferably 0.1 to 0.75, in each
case measured with a sample thickness of 1.2 mm.
Analyses of the glass ceramic product according to the invention
have also shown that this has a very homogeneous structure with
uniformly distributed fine crystals. It is assumed that this
special structure is brought about by the particular composition
of the starting glass used and by the shaping, particularly
pouring solid glass blanks in stage (b), and is responsible for
CA 022~2660 1998-11-03
-- 12 --
the particularly high strength of the glass ceramic product
eventually obtained.
It is also surprising that the colour, translucence and fluor-
escence of the glass ceramic product according to the invention
may be matched to that of a natural tooth without the colour and
fluorescent components used adversely affecting the strength and
toughness of the glass ceramic. In contrast, it is known that
with glass ceramics based on leucite, crystallisation is
considerably affected by such additives and the strength is often
very much reduced. It is known that~the pigments used in many
cases in sintered ceramics lead to a very high density of defects
and to pore formation in the glzss ceramic, which in turn impairs
the properties thereof.
Finally, the glass ceramic product according to the invention is
characterised by excellent acid resistance, which is preferably
less than 100 ~g/cm2 loss of mass. Said loss of mass was
determined by the method explained in the Examples in which the
glass ceramic is treated with aqueous acetic acid over a certain
period and the loss of mass ascertained after the treatment
serves as a measure of acid resistance.
Preferred shaped dental products which contain the glass ceramic
product according to the invention are dental restorations such
as an inlay, an onlay, a bridge, an abutment, a facing, a veneer,
a facet, a crown or a partial crown.
Moreover, preferred shaped dental products are those in the form
of blanks or ingots, i.e. which undergo further processing to the
final dental product, e.g. according to stages (dl) and (d2).
Such blanks may be present in vzrious forms adapted to the
further processing method in question, such as small cylinders
or rectangular bloc~s.
CA 02252660 1998-11-03
-- 13 --
The invention is explained in more detail below on the basis of
Examples.
E~amples
Exam~les 1 to 20
A total of 20 different glass ceramic products according to the
invention with the chemical compositions given in Table I were
prepared by carrying out stages (a) to (c) of the process
described.
T;ll)le I (amollnts in Wt.%)
PY:~mpl- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 lS 16 17 18 19 20
SiO2 67.9 66.6 69.42 68.86 68.5 68.1 64.9 68.1 67.6 74.95 61 71.7 67.6 66.5 67.9 61.3 63.7 65.6 66.7 65.8
K2O 4.2 4.1 4.3 4.3 4.2 4.2 5.2 4.2 4.1 2 7.8 4.4 4.1 3.5 4.2 13.5 4 4,1 4.1 41 o
Li2O 15 14.7 15.4 15.3 15.1 15.1 16.1 15 lS 17 11 15.9 14.9 15.6 lS.l 13.8 14 14.5 14.8 14.6
Al2O3 1.1 1 1.1 1.1 1.1 1.1 I.l 1.1 1.1 ().S 2 1.1 l.S 1.1 1 1 1.1 1.1 1.1 _ ~
P2(), 3.X 3.7 3.X 3.X 3.8 3.R 3.7 3.8 3.X 1.8 7 3.8 2.5 3.7 3.4 3.6 3.7 3.7 3.7 1-
M~O ().l 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.2 0.1 0.1 0.1 0.1 0.1 0.1 n.l 0.1 ().l
TiO2 1.6 1.6 0.3 0.2 0.2
Zr~2 2 6.1 4 3
ZnO 4.8 4.7 4.9 4.9 4.8 4.8 5.8 4.8 4.8 2.3 8 5.1 4.8 5 4.8 4.1 4.6 4.7 4.7 4.7
CeO2 0.5 (1.5 0.6 0.6 2 2 2 2 2 1 2 2 2 2 2 2 2 0.5 2
MnO2 0.53 n.53 0.32 1.8
~e2~3 0.17 ().17 0.12
La2O3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 ().3 0.3 ().l5 0.5 0.3 0.30.3 0.3 0.3 0.3 1 0.3 2
Ag2O 0.08
V2~5 0.1 0.1 0.2 ().f. ().f~ n.1 ().2 0.2 ().10.2 ().2 ().2 ().2
Er2~3 n.l 0.3 0.1 ~ D
Tb~07 l.l
E~l2O3 0.3 , ! ~
pr2o3 0.15 0.3 n.3 0.3 0.6 0.3 0.6 0.3 ~ 1-
Y2Oa 0.2 o~5 0.6
DY2O3 05 ' ~
SnO2 2
Ta2~s 0-3
CA 022~2660 1998-11-03
- 16 -
Example 21
Dental Product PrePared by hot pressinq
accordinq to EP-A-231 773
This Example describes the preparation of a glass ceramic blank
according to the invention which may be used for the preparation
of an individually shapable all-ceramic dental product, such as
a crown or a multi-span bridge. In addition, a matched dental
sintered ceramic may then be sintered onto the dental product.
Initially, a starting glass with the chemical composition given
in Table I for Example 14 was prepared. To this end, a batch of
appropriate oxides, carbonates and phosphates was mixed in a ball
mill and melted in a platinum-rhodium crucible at 2 temperature
of 1500~C and for a homogenisation time of 2 hours. The glass
melt obtained was granulated by pouring into water and the glass
frit formed was dried. The glass frit was then milled again in
a ball mill and melted again at 1500~C for a homogenisation time
of 2 hours. The homogeneous, transparent and slightly yellow
coloured melt obtained was then poured into a steel mould
preheated to 500~C to form cylindrical rods which were cooled
slowly in a controlled manner in a furnace from 500~C to room
temperature. The glass rods obtained were sawn into specimens of
2 g, 3 g and 4 g and then heat-treated for 30 minutes at 870~C
in order to form the corresponding glass ceramic blanks. The
--_,
cooled glass rods were placed directly in the furnace preheated
to 870~C. A slow rate of heating was not required.
Pro~erties of the blanks
The glass ceramic blanks obtained had optical properties such as
e.g. translucence, colour and cloudiness comparable with those
of commercial dental ceramic products, e.g. I~S Empress blanks
from Ivoclar AG, Liechtenstein.
CA 022~2660 1998-11-03
A. 3-Point bendinq strenqth
In order to determine the 3-point bending strength, rods were
prepared as specimens from the glass ceramic blanks according to
ISO dental standard 6872/1995. The 3-point bending strength W2S
then determined likewise according to ISO 6872-1995 E UDental
Ceramic~ with a rate of advance of load application of 0.5 mm/min
and a distance of 15 mm between the supports of the specimen. The
bending strength determined under these conditions W25 408 i 63
MPa.
Properties of alass ceramic havinq underaone ~lastic sha~inq
The glass ceramic blanks obtained were pressed using the hot
pressing process according to EP-A-231 773 and the pressing
furnace likewise described therein in vacuo in the viscous state,
to obtain the desired specimen geometry for the test in question.
The standby temperature of the pressing furnace was 800~C, the
rate of heating to the pressing temperature was 60~C/min, the
pressing temperature was ~10~C, the holding t-me at the pressing
temperature was 15 minutes and the pressing pressure reading was
5 bar. After the pressing process, the pressing mould was cooled
in the air and the shaped glass ceramic products obtalned were
removed from the mould by sand blasting with Al2O3 powder and
glass beads. The products had the following properties.
A. Optical pro~erties
In order to quantify the translucence of the glzss cerzmic
products, the CR value was determined according to the method of
measurement of the British Standards Institution which is
described in the dental ceramic test standard UBS 5612: 1978~.
To this end, 5 specimens with a diameter of 16 mm and a sample
thickness of 1.4 mm were prepared. The specimens were ground with
wet SiC powder, grain 320, in order to obtain the desired surface
,
CA 022~2660 1998-11-03
- 18 -
wet SiC powder, grain 320, in order to obtain the desired surface
quality (surface roughness Ra = O.8 ~m to 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
S depends to a large extent on the film thickness. The final sample
height/thickness was 1.2 10.025 mm.
The specimens were placed in the provided opening of a Minolta-
CR300 colour measuring instrument and the reflection of each of
the 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 necess-
ary, by applying a drop of glycerine onto the backsround.
(a) In order to determine the sample emission on a black back-
ground Yb(Yblack), a black plate with not mo-e than 4%
reflection was used.
(b) In order to determlne the sample emission on z white back-
ground Y~(Y.~ ~), a white plate with a reflection of 80% to
85% was used.
The contrast value CR = Y~/Y.~ was then determined frcm the values
Yb and Y~ determlned, and this was 0.63.
- 25
As a result of the translucent properties, this gl~ss ceramic
product was suitable as an all-ceramic dental prcduct which
conforms optically with the specifications of a natural tooth.
Due to the use of glass-colouring oxides in the stz-_lng glass,
the hot-pressed glass ceramic product was tooth-coloured, and it
was possible to adjust the intensity and shade of the colour by
means of the concentration of the colourant oxides.
A translucent to transparent dental sintered glass ceramic with
an expansion coefficient of 9.5 x 10 R (100 to 400~C) could be
applied as a coating to the translucent glass cerzmic product
.
CA 022~2660 1998-11-03
-- lg --
employable as a framework material. The dental sintered glass
ceramic was sintered in layers onto the glass ceramic product in
vacuo at 760~C, which led to translucent all-ceramic dental
restorations which meet the stringent aesthetic requirements of
such products.
B. 3-~oint bendinq strenqth
The 3-point bending strength was determined on hot-pressed glass
ceramic rods in accordance with the method used above for the
blanks. A 3-point bending strength of-450 +85 M~a was determined.
C. Thermal ex~ansion coefficient
To this end, cylindrical glass ceramic samples with a diameter
of 6 mm and a length of 20 mm were hot-pressed. The expansion
coefficient determined for these samples in the temperature range
of 100 to 500~C was 10.8 x 106 K1.
D. Fracture touahness RT~
To this end, glass ceramic rods with the dimensions 20 x 4.4 x
1.4 mm3 were hot-pressed and then reground on 211 sides with SiC
wet-grinding paper (1000 grain). Using a diamond cutting wheel
(0.2 mm thick), the samples were notched on one side as far as
the centre to a depth of 2.2 mm and then tested according to DI~
51 109 with an outer distance between the su~ports of 15 mm and
a rate of advance of load application of 0.5 mm/min using the 4-
point bending test arrangement. The ~IC value determined was 3.0
+0.3 MPa ~ m.
E. Acid resistance
To this end, disc-shaped glass ceramic samples with a diameter
of 15 mm and a thickness of 1.5 mm were hot-pressed and then
reground on all sides with SiC wet-grinding paper (1000 grain).
CA 022~2660 1998-11-03
- 20 -
according to ISO 6872-1995 E ~Dental Cera~ic" wzs determined
after 16 hours' storage in 4 vol.% aqueous acetic acid solution.
The value was 36 ~g/cm and was markedly below the standard value
for dental ceramics of 2000 ~g/cm2.
Exam~le 22
Dental product pre~ared by comPuter-aided millina technolo~v
This Example describes the preparation of a glass ceramic blank
according to the invention which is processed by machining and
subsequently by z further heat treatment to an individually
shaped all-ceramic dental product, such as a crowr. or a multi-
span bridge, onto which a matched translucent to transparent
dental sintered ceramic may be sintered on in addition.
The dental product was produced by means of a CAD/CAM method,
such as CEREC 2 , from Siemens AG.
Only the heat treatment carried out after mzchinins resulted in
the dental produc~ with the good mechanical characteristics such
as 3-point bending strength and the good optical properties
required for a dental ceramic product.
A starting glass with the composition gi~en in Table I for
Example 7 was prepared initially. To this end, a batch of oxides,
carbonates and phosphates was mixed in a ball mill and melted in
a platinum/rhodium crucible at a tempe-ature of 1500~C and for
a homogenisation time of 2 hours. The glass melt wzs fritted by
pouring into water and the frit was milled zfter drying in a ball
mill and melted again at 1500~C for a homogenisation time of 2
hours. The homogeneous, transparent and slightly yellow coloured
melt obtained was then poured into a steel mould preheated to
500~C to obtain rectangular blocks with the dimensions 65 x 20
x 16 mm3 and cooled slowly in a controlled manner from 500~C to
room temperature in a furnace. The rectangular slass blocks
obtained were sawn into samples with the dimensions 18 x 14 x 20
CA 022~2660 1998-11-03
- 21 -
obtained were sawn into samples with the dimensions 18 x 14 x 20
mm3. These samples were then heat-treated for 60 minutes at
650~C. The cooled glass blocks were introduced directly into the
furnace heated to 650~C. The glass ceramic bl2nks obtained after
this first heat treatment stage had the following properties.
Pro~erties of the blanks after a sinale
heat treatment (650~C~lh)
A. O~tical pro~erties
The glass ceramic blanks had a violet-whitish colour. They had
a CR value of 0.36 determined according to the method described
in Example 21.
B. 3-w int bendina strenath
The 3-point bending strength determined according to Example 21
for the glass ceramic blanks was 171 +20 ~Pa.
Pre~aration and Pro~erties of the finished alass ceramic
The glass ceramic blanks which had undergone a single heat
treatment were processed to dental ceramic restorations such as,
e.g., crowns using a computer-aided milling machine. In viet~ of
the relatively low strength and toughness of the glass ceramic
blanks, processing proved to be easy to carry out. Compared with
known milling ceramics, they brought about less tool wear and
fewer breakages were formed, this being attributable to the finer
structure and absence of defects.
The milled dental restoration then underwent a further heat
treatment at 760aC for 1 h. This temperature was selected because
there is no risk of deformation of the framework at said
temperature. This additional heat treatment led to a more
thorough crystallisation and hence a change in the properties of
-
CA 022~2660 1998-11-03
- 22 -
the restoration. The glass ceramic obtained which had undergone
two treatments had the following properties.
A O~tical Pro~erties
The glass ceramic was translucent and tooth-coloured due to the
use of glass-colouring oxides in the starting glass.
The CR value dete mined for this glass ceramic according to
Example 21 using cylindrical samples with a diameter of 16 mm and
thickness of 1.2 mm was 0.23.
B. 3-point bendinc strenqth
The 3-point bending strength of the glzss ceramic determined
according to Example 21 was 272 +24 ML~a.
C. Linear thermal ex~ansion coefficient
To this end, rectangular specimens with the dimensions 30 x 4 x
3 mm3 were sawn out of solid blocks that had unde_gone two heat
treatments. The expansion coefficient dete~ined for these
samples in the temperature range from 100 to 500~C was 10.~ x 10-
6K-I
_ Fracture touchness KTC
The fracture toughness determined according to Example 21 on rods
from solid blocks that had undergone two heat treatments was 2.1
+0.1 ~a ~ m.
E. Acid resistance
The acid resistance determined according to Example 21 on samples
of glass ceramic which had undergone two heat treatments was 16
~g/cm2 and was thus mar~edly below the standard value for dental
, . . . .
CA 022~2660 1998-11-03
- 23 -
ceramic materials of 2000 ~g/cm2 and lower than that of conven-
tional dental framework materials.
Finally a translucent to transparent sintered glass cer~mic with
an expansion coefficient of 9.5.10-5x~l was sintered in layers in
vacuo at 760~C and with a holding time of 2 minutes in each case,
onto the milled, twice heat-treated glass ceramic. A finished
dental restoration was thereby o~tained.
E~amples 23 to 26
In these Examples, hot-pressed glass ceramic products were
prepared according to Ex~mple 21 and their prope~ties were
tested. The starting glasses used, however, were glasses with the
composition given in Table I for Ex~mples 1, 4, 18 2nd 20.
The properties determined for these glass cer~mics 2nd the glass
ceramics according to Ex~mple 21 and 22 are listed in Table II.
T;ll-le 11
I;YO r'~S I'rocess 13ending F~ncture Tllermal Acid resi~:h~ Fh nresc Tr3r ~IJr ~ Colour
(sl~lrting glnss) strenglll [Mr~3] tou~ no;or [~g/cm2] (at Jl .. ~ r.bll- CRvulue
[Ml~l$~/m~ r ~l~ t of 366 nm) (saml)le Illlck-
(1nO~C - 5l)~~C) ~ nes~
[,um/mK] 1.2 mm) D
F~ '~ 21 dl 450 l 85 3.0 + 0.3 10.8 36white-yellow 0.63 white-beige I ' ~
(glass no. 14)
F~ r'~ 22 d2 272 + 24 2.1 + 0.1 10.9 16 orange 0.23 grey-beige I ~
(~lass no. 7) 1-
F--- r~ ' 23 dl 386 + 71 10.7 22 white-beige
(~lass no. 1)
F--- rlf 24 dl 453 + 93 1().5 34 white-yellow ~
(~lass no. 4)
p~- r~- 25 dl 336 + fi3 white-yellow 0.41 yellowish-
(glass no. 18) tra~.~pdl ci t
Examl~le 26 d1 343 ~ 15 dark violet 0.37 whitish-
(~lass no. 2û) t.a.. ~,.a~ t
