Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
1998-11-06
2002-07-16
Koslow, C. Melissa (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C501S005000, C501S064000, C501S068000, C501S069000, C501S072000, C106S035000, C065S033100, C065S033400, C065S033900, C065S102000
Reexamination Certificate
active
06420288
ABSTRACT:
The invention relates to a process for the preparation of shaped translucent lithium disilicate glass ceramic products which can be prepared as blanks, which may be processed to shaped translucent 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 La
2
O
3
, and there is likewise no description of the preparation 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. Moreover, the glass ceramic is intended primarily for the preparation of tableware which naturally has only low translucence.
EP-B-536 572 also describes lithium disilicate glass ceramics which contain no La
2
O
3
. By scattering a finely divided 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 U.S. Pat. No. 4,189,325 which necessarily contain calcium oxide for improving the flow and also platinum and niobium oxide as special nucleating agents in order to produce very fine and uniform crystals. Even though the glass ceramic can be prepared in the form of blanks which have not yet crystallised completely, it is nevertheless free from La
2
O
3
.
WO-A-95/32678 and U.S. Pat. No. 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 ceramic materials are heated to such an extent, however, 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.
Moreover, glass ceramics based on SiO
2
and Li
2
O are known from DE-C-1 421 886 which contain large quantities of arsenic trioxide which is physiologically very harmful.
A lithium disilicate glass ceramic which is suitable for the preparation of dental crowns and bridges but contains no La
2
O
3
at all is disclosed in U.S. Pat. No. 4,515,634.
The glass ceramics described in FR-A-2 655 264 are free from La
2
O
3
. 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 investment material used during pressing occurs when they are processed in the plastic state using elevated temperatures and elevated pressures. Further 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. Moreover, 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 lack 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, therefore, 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 subsequent heat treatment.
Said object is achieved by the process for the preparation of shaped translucent lithium disilicate glass ceramic products.
The invention also relates to the shaped glass ceramic products, the use of such products and the shaped dental products.
The process according to the invention for the preparation of shaped translucent lithium disilicate glass ceramic products is characterised in that
(a) a melt of a starting glass is produced which contains the following components:
Component
Wt. %
SiO
2
57.0 to 80.0
Al
2
O
3
0 to 5.0
La
2
O
3
0.1 to 6.0
MgO
0 to 5.0
ZnO
0 to 8.0
Li
2
O
11.0 to 19.0
where
(i) Al
2
O
3
+ La
2
O
3
accounts for 0.1 to 7.0 wt. % and
(ii) MgO + ZnO accounts for 0.1 to 9.0 wt. %,
(b) the melt of the starting glass is shaped 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:
Component
Wt. %
ZrO
2
0 to 10.0
K
2
O
0 to 13.5
P
2
O
5
0 to 11.0
Colour and
0 to 8.0
fluorescent components
Additional components
0 to 6.0
Surprisingly, it was established that the additional incorporation of ZrO
2
led to an increase in translucence, although the opposite effect was observed in the conventional glass ceramic according to EP-B-536 479.
Ranges that may be chosen independently of one another, unless otherwise specified, exist for the quantities of the individual components, said ranges being as follows:
Component
Wt. %
SiO
2
57.0 to 75.0
Al
2
O
3
0 to 2.5
La
2
O
3
0.1 to 4.0
MgO
0.1 to 5.0
ZnO
0 to 6.0,
particularly 0.1 to 5.0
ZrO
2
0 to 8.0,
particularly 0.1 to 8.0
K
2
O
0 to 9.0,
particularly 0.5 to 7.0
Li
2
O
13.0 to 19.0
P
2
O
5
0 to 8.0,
particularly 0.5 to 8.0
colour and
0.1 to 8.0
fluorescent components
additional components
0 to 3.0.
For example oxides of f-elements may be used as colour components or fluorescent components. In preference, at least one of the following compounds is used.
Component
Wt. %
CeO
2
0.1 to 5.0
V
2
O
5
0.01 to 1.0
Fe
2
O
3
0.01 to 1.0
MnO
2
0.01 to 3.0
TiO
2
0.01 to 5.0
Y
2
O
3
0.01 to 2.0
Er
2
O
3
0.001 to 2.0
Tb
2
O
3
0.001 to 2.0
Eu
2
O
3
0.001 to 2.0
Yb
2
O
3
0.001 to 2.0
Gd
2
O
3
0.001 to 2.0
Nd
2
O
3
0.001 to 2.0
Pr
2
O
3
0.001 to 2.0
Dy
2
O
3
0.001 to 2.0
Ag
2
O
0.01 to 2.0
SnO
2
0.01 to 3.0
Ta
2
O
5
0.001 to 2.0
The special oxides that can be used as colour or f
Cramer von Clausbruch Sascha
Höland Wolfram
Rheinberger Volker
Schweiger Marcel
Ivoclar AG
Koslow C. Melissa
Nixon & Peabody LLP
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