Dental Ceramic Material With a Relatively Low Processing Temperature

The present invention relates to a dental ceramic material for the preparation and repair of metal ceramic and fully ceramic dentures with a processing temperature below 700.degree. C. and a coefficient of thermal expansion .alpha. of 13-14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C.

Using dental ceramic materials, it is possible to prepare fully ceramic dentures, to carry out modifications to change the shape of metal ceramic and fully ceramic dentures, to repair damaged sections or to line metallic dentures with a ceramic layer.

For many years ceramic layers have been used in dentistry for lining metallic frameworks (crowns, bridges), using a type of enamelling, in order to give the denture a natural appearance. In this case, ceramic powders are applied to the metallic framework as an aqueous slurry and fired at elevated temperatures. It is important here that the firing temperature (processing temperature) of the ceramic body is at least 100.degree. C. below the solidus temperature of the material in the metallic framework and that the coefficient of thermal expansion of the ceramic body in the range 25.degree. to 500.degree. C. is only very slightly less than that of the metallic material, so that no cracks are produced in the lining layer during firing and cooling down.

In the same way as these so-called metal ceramic dentures, fully ceramic dental items such as full crowns, part crowns, temporary fillings and lining shells may be prepared in sinter processes, using the same ceramic materials on refractory ceramic molding materials.

Dental ceramics of this type, for lining, generally require high coefficients of thermal expansion, which should be 13 to 14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C., due to the material in the metallic framework. Normally, these ceramics are processed at 950.degree..+-.30.degree. C., but this is too high for some dental alloys. It is very difficult to lower the processing temperature significantly by altering the composition of the dental ceramics without adversely altering the coefficients of thermal expansion and resistance to corrosion. Furthermore, biologically suspect components, such as e.g. lead oxide, must be avoided.

Coefficients of thermal expansion of 13-14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C. are generally only achieved if the dental ceramic materials contain 25 to 30% by weight of alkali metal oxides. However, in an acid environment, such as e.g. in oral cavities under plaque, this means that the limit of resistance to corrosion of these ceramics is reached.

DE-OS 39 11 460 discloses dental ceramic materials for the preparation, correction and repair of metal ceramic and fully ceramic dentures, which contain, apart from SiO.sub.2 as the balance, 5-15% Al.sub.2 O.sub.3, 0.5-2.5% B.sub.2 O.sub.3, 0.5-2.5% Sb.sub.2 O.sub.3, 0.1-0.5% CaO, 0.5-2.5% BaO, 5-10% Na.sub.2 O, 10-15% K.sub.2 O, 0.1-0.5% Li.sub.2 O and 0.1-0.5% F.sub.2. These actually have coefficients of thermal expansion of 13.5 (.+-.1).multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C. but their processing temperatures of 730.degree..+-.30.degree. C. are still too high for some dental alloys during lining.

In German Patent Application P 40 31 168.6, which has not been published previously, ceramic materials for lining metallic dentures, which have a coefficient of thermal expansion of 16-17.5.multidot.10.sup.-6 .multidot.K.sup.-1 and a processing temperature of 770.degree..+-.70.degree. C. are described They consist of 60 to 68% SiO.sub.2, 10 to 15% Al.sub.2 O.sub.3, 0.7 to 1.5% B.sub.2 O.sub.3, 0 to 0.5% Sb.sub.2 O.sub.3, 0 to 0.5% CeO.sub.2, 0 to 0.5% BaO, 0.1 to 0.5% CaO, 9 to 12% K.sub.2 O, 9 to 11% Na.sub. O, 0.8 to 1.4% Li.sub.2 O and 0.2 to 0.4% F.sub.2.

An object of the present invention was to develop a dental ceramic material for the preparation and repair of metal ceramic and fully ceramic dentures, which has a coefficient of thermal expansion of 13 to 14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C. and a processing temperature below 700.degree. C. In addition, it should contain no biologically suspect components and be resistant to corrosion in the mouth.

The present invention discloses a material which consists of 60 to 65% by weight of SiO.sub.2, 8.5 to 11% by weight of Al.sub.2 O.sub.3, 8 to 12% by weight of K.sub.2 O, 10.5 to 12% by weight of Na.sub.2 O, 0.7 to 2% by weight of CaO, 0.5 to 2.5% by weight of B.sub.2 O.sub.3, 0.1 to 0.6% by weight of Sb.sub.2 O.sub.3, 0 to 0.5% by weight of CeO.sub.2, 1.0 to 3.8% by weight of TiO.sub.2, 0.8 to 1.4% by weight of Li.sub.2 O and 0.6 to 2.4% by weight of F.sub.2. The material does not contain BaO.

The present invention also discloses a method of using the dental ceramic material described herein and to a dental prosthesis comprising a metal ceramic denture or ceramic denture and the dental ceramic material described herein.

Preferably the dental ceramic materials contain 60 to 63% by weight of SiO.sub.2, 8.5 to 9.7% by weight of Al.sub.2 O.sub.3, 10 to 12.0% by weight of K.sub.2 O, 10.5 to 11.5% by weight of Na.sub.2 O, 0.7 to 1.5% by weight of CaO, 0.7 to 1.5% by weight of B.sub.2 O.sub.3, 0.2 to 0.4% by weight of Sb.sub.2 O.sub.3, 0.1 to 0.4% by weight of CeO.sub.2, 1.0 to 3% by weight of TiO.sub.2, 0.8 to 1.2% by weight of Li.sub.2 O and 0.6 to 2.0% by weight of F.sub.2.

The dental ceramic materials according to the present invention have processing temperatures of 660.degree..+-.30.degree. C. The glass point is about 450.degree. C. and the softening point is about 510.degree. C. They possess very high material homogeneity and a very high transparency (more than 70% light transmission). The resistance to bending according to DIN 13925 is 110 N.multidot.mm.sup.-2 and is therefore far above their minimum requirement of 50 N.multidot.mm.sup.-2.

The loss in mass during resistance to corrosion testing according to DIN 13925 (16 hours in 4% acetic acid) is about 0.028% by weight. Surprisingly, the resistance to bending is then increased by up to 50%, whereas it decreases by up to 30% with known dental ceramics.

The following Table reproduces the composition of some particularly advantageous materials:

The present invention also concerns a method of making dental prosthesis using the dental material described above and utilizing methods known in the art to form dental prosthesis. The present invention further concerns a dental prothesis, dental bridge, dental crown or other dental items formed from the material described above by methods known in the art. U.S. Pat. No. 4,562,882 is incorporated by reference in its entirety. Phillips, R. W., Skinner's Science of Dental Materials, Eighth Edition, 1982, W. B. Saunders Co., Philadelphia, is referred to in its entirety.

Further variations and modifications of the invention will become apparent to those skilled in the art from the foregoing and are intended to be encompassed by the claims appended hereto.