Compositions: coating or plastic – Coating or plastic compositions – Dental
Reexamination Certificate
2002-07-03
2004-09-28
Turner, Archene (Department: 1775)
Compositions: coating or plastic
Coating or plastic compositions
Dental
C433S215000, C501S002000, C501S003000, C501S004000, C501S014000, C501S016000, C501S021000, C501S024000, C521S025000
Reexamination Certificate
active
06797048
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for preparing a glass-ceramic. The pulverized glass-ceramics (glass-ceramic powders) obtained by the method of the invention is useful particularly as a porcelain to be built up and fused for coating the surface of a metal frame and thereby fabricating a dental prosthesis with excellent aesthetic qualities.
BACKGROUND ART
A technique is known in which a porcelain powder (hereinafter referred to simply as “porcelain”) comprising a glass-ceramic is built up on and fusion-bonded to the surface of a metal frame to fabricate a dental prosthesis which has an appearance similar to natural teeth and high mechanical and chemical durability. The metal frame is mainly made of a precious metal alloy which has an approximately constant coefficient of thermal expansion (about (14.2±0.5)×10
−6
/° C.).
In such dental prosthesis fabrication, a coating process comprising building up, fusion-bonding and cooling a porcelain on the metal frame surface is repeated several times to form the external shape of the dental prosthesis. Stated more specifically, the fabrication includes the basic steps of forming three layers by sequentially building up and fusion-bonding an undercoat opaque for concealment of the metal color and fusion bonding, a dentine porcelain that determines the basic color of the prosthetic teeth, and an enamel porcelain that reproduces the characteristics of tooth enamel. In addition, the fabrication involves the step of forming a margin that matches with natural teeth, and adjusting steps such as coloring, color tone modification and the like. Thus, in the known technique, the coating process comprising building up, fusion-bonding and cooling a porcelain on a metal frame is repeated at least 3 times, up to about 10 times.
Therefore, the porcelains are required to have a coefficient of thermal expansion approximating that of the material of the metal frame and have thermal stability such that their coefficients of thermal expansion scarcely change during the repeated coating process.
Leucite crystals are represented by the chemical formula 4SiO
2
.Al
2
O
3
.K
2
O(═KAlSi
2
O
6
). Since leucite crystals have a large coefficient of thermal expansion, a glass-ceramic containing a specific amount of leucite crystals has a coefficient of thermal expansion approximately equal to that of the material of the metal frame. Further, when the leucite crystal phase coexists with a glass matrix phase, the two phases as a whole have uniform light transmittance (transparency) because the refractive indices of the two phases are close to each other. Therefore, addition of a coloring ingredient to a porcelain comprising the coexisting mixture enables desired coloration of a restored dental prosthesis to impart a highly aesthetic appearance similar to the appearance of natural teeth. Because of these excellent characteristics (coefficient of thermal expansion and transparency) of leucite crystal-containing glass-ceramics, the use of leucite crystal-containing glass-ceramics as materials (porcelains) for coating metal frames has been proposed.
For example, U.S. Pat. No. 4,604,366 discloses a method for preparing a ceramic porcelain, comprising blending a matrix glass with several types of leucite crystal-containing glass-ceramic frits having different leucite crystal contents and different coefficients of thermal expansion. However, this method necessitates troublesome procedures for determining the ratio of the at least three types of ingredients and blending these ingredients, in order to suitably control the leucite crystal content and coefficient of thermal expansion of the final product porcelain. Moreover, the dental prosthesis obtained by firing the ceramic porcelain has the serious problem of non-uniform leucite crystal distribution.
U.S. Pat. No. 4,798,536 discloses a method for preparing a ceramic porcelain, comprising mixing a natural feldspar, such as Wyoming feldspar, as a leucite crystal origin point with a glass matrix-forming ingredient, and melting, slowly cooling and then suddenly cooling the mixture. However, this method includes troublesome procedures for purifying the natural feldspar, and is complicated as a whole. In addition, trace impurities derived from the natural feldspar are liable to remain in the porcelain and decrease the transparency, thereby deteriorating the color of the resulting dental prosthesis.
When a porcelain is fusion-bonded to a metal frame, a lower fusion-bonding temperature is desirable from a workability point of view. However, the leucite crystal phase is instable in “low-fusing porcelain” which is softened and fluidized at 750 to 950° C. and fusion-bonded to a metal surface. Thus, if the porcelain is maintained at 950° C. or lower, the leucite crystal phase undergoes transformation to a different type of crystal phase, or shifts to a state of coexistence with a different type of crystal phase. More specifically, when a powder of a known low-fusing, leucite crystal-containing glass-ceramic (a composite of leucite crystals and matrix phase) is fired at 750 to 950° C. to coat a metal frame, Na—K feldspathic crystals (high temperature-type Na—K sanidine) start to precipitate after a given period of time, and then the leucite crystals begin to decrease and finally disappear, since the leucite crystals contained are a metastable crystal phase. The precipitation of the Na—K feldspathic crystals lowers the coefficient of thermal expansion and causes opacification in the glass-ceramic. Thus, the coefficient of thermal expansion of the glass-ceramic gradually decreases during the repeated coating process comprising building up, fusion-bonding and cooling of the glass-ceramic on a metal frame. As a result, the glass-ceramic has defects such as cracks owing to the stress of strain between the frame material and the ceramic coating layer, leading to low adhesion between the frame material and the ceramic coating layer. Further, the opacification of the glass-ceramic impairs the transparency of the ceramic coating layer.
In the above situation, the development of a novel porcelain which can be easily prepared and is free of deterioration in characteristics (i.e., decrease in the coefficient of thermal expansion, or opacification) during a process of coating a metal frame is desired.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a leucite crystal-containing glass-ceramic which can be easily prepared.
Another object of the invention is to provide a leucite crystal-containing glass-ceramic whose leucite crystal content does not substantially change when heated, and which exhibits a stable coefficient of thermal expansion and excellent transparency, and a porcelain comprising a powder of the glass ceramic.
A further object of the invention is to provide a leucite crystal-containing glass-ceramic porcelain in which the leucite crystals, once precipitated, do not substantially decrease in amount during the process of coating a metal frame.
A further object of the invention is to provide a porcelain in which crystals of types other than leucite crystals (e.g., Na—K feldspathic crystals) do not substantially precipitate during the process of coating a metal frame, in other words, to provide a leucite crystal-containing glass-ceramic porcelain in which crystals of types other than leucite crystals (e.g., Na—K feldspathic crystals) begin to precipitate sufficiently long after the precipitation of leucite crystals reaches the saturation point.
A further object of the invention is to provide a leucite crystal-containing glass-ceramic porcelain which is free of opacification and decrease in the coefficient of thermal expansion during the process of coating a metal frame with the porcelain.
A further object of the invention is to provide a dental prosthesis obtainable by building up and fusion-bonding the leucite crystal-containing glass-ceramic porcelain onto the surface of a metal frame.
The above objects of the invention can be achieved by mixing a glassy material a
Anraku Teruo
Hoshikawa Takeshi
Yamamoto Hirohisa
Yamazoe Masatoshi
Armstrong Kratz Quintos Hanson & Brooks, LLP
Turner Archene
Yamamoto Precious Metal Co., Ltd.
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