FIELD OF THE INVENTION

The invention relates to porcelains used in dental

prostheses. More particularly the invention relates to

a low-fusing temperature porcelain suitable for use in

combination with metal or porcelain substrates.

BACKGROUND OF THE INVENTION

A dental prosthesis often comprises a crown or

bridge restoration in which a dental porcelain is fused

to a supporting metal or ceramic substrate. Dental

porcelains are generally a mixture of glass frit

components or natural feldspars or the like. There are

a number of key considerations in selecting frit

components to form a porcelain that will be compatible

with the selected substrate.

For example, the porcelain must have a coefficient

of thermal expansion that is as close as possible to

that of the substrate. Otherwise, a porcelain fused to

the substrate will tend to crack and separate from its

supporting structure.

Another key factor in selecting a porcelain

component is its effect upon processing temperature of

the porcelain; that is, the temperature that must be

utilized in fusing the porcelain to the substrate.

Fusing temperatures of porcelain materials of the prior

art have generally been on the order of 900°C or higher.

Such elevated temperatures limit the composition of the

substrate that may be used since some alloys or ceramic

compositions will distort or otherwise suffer

detrimental compositional changes under such processing

conditions.

In selecting glass components and their amounts in

the porcelain composition to achieve low-fusing temperatures and appropriate coefficients of thermal

expansion, care also must be exercised to insure that

the changes in composition do not otherwise adversely

effect the qualities of the resulting fired porcelain with respect to its performance in a dental

environment . Such characteristics of concern are acid

resistance, hardness, flexural strength and the like.

Thus, heretofore, it has remained the goal of

researchers to produce a porcelain glass frit

composition that has a low-fusing or processing

temperature and a coefficient of thermal expansion such

that it can be used in combination with both metal and

porcelain substrates in dental prostheses. Such a

composition would have improved versatility for use in

making, repairing or improving crowns, bridges, linings

for dentures or as finish porcelains and stains.

SUMMARY OF THE INVENTION

A low-fusing temperature porcelain composition for

use in a dental prosthesis is provided. The porcelain composition comprises:

Frit Components Weight Percent

Si0 ₂ 57-61

A1 ₂ 0 ₃ 6.5-11.5

Na ₂ 0 10-12

K ₂ 0 8-16

Li ₂ 0 1-3

CaO 1.5-4

BaO 1.5-2.5

Ce0 ₂ 0.5-2

Tb ₂ 0 ₃ 0.5-2

The porcelain of the invention has a fusing

temperature of about 700°C + 50°C and a coefficient of

thermal expansion compatible with a substrate of metal

or ceramic. Preferably, the coefficient of thermal

expansion is about 12-13 μm.K ^"1 [20 to 500°C] .

DETAILED DESCRIPTION OF THE INVENTION

The invention is a low-fusing temperature

porcelain composition that is a mixture of oxides such

that the resulting frit has a fusing temperature of

about 700 ^o C ± 50°C and a coefficient of thermal

expansion of 12-13 μm.K ^"1 [20 to 500°C] . The low fusing

porcelain may be employed in dental crowns, bridges,

restorations and dentures wherein it is fused to a

substrate or coping of metal or conventional

porcelains, such as Ceramco II ^® , and dental alloy

copings, such as Ceramco II ^® Silver, both manufactured by Dentsply International Inc. of York, Pennsylvania.

The compositional range for the low-fusing

temperature dental porcelains of the invention, having

processing temperatures of less than about 705°C and a

coefficient of thermal expansion (CTE) of 12-13 μm.K ^"1

[20 to 500°C] is a mixture of oxides having the

following composition:

Table 1

Oxide Weight Percent

Si0 ₂ 57-61

A1 ₂ 0 ₃ 6.5-11.5

Na ₂ 0 10-12

K ₂ 0 8-16

Li ₂ 0 1-3

CaO 1.5-4

BaO 1.5-2.5

Ce0 ₂ 0.5-2

Tb ₂ 0 ₃ 0.5-2

An optional ingredient is B ₂ 0 ₃ at about 0-2% by

weight of the composition.

The following examples describe preferred

compositions and characterize the resulting dental

porcelain qualities.

Example 1

A single glass frit, low temperature fusing

porcelain is made by combining and fusing oxides, and

then forming a pulverized mixture, as is well known in

the art, of the composition indicated in Table 1. The

resulting porcelain has a firing temperature of 705°C

and a CTE of 12.5 μm.K ^"1 . Samples of the resulting mixture have the characteristics and properties shown

in Table 2

Table 2

Samples of the low-fusing glass of the invention

are also characterized by their low chemical

solubility. Chemical solubility was determined by

boiling the porcelain samples in acetic acid for

sixteen hours. The maximum allowable weight loss per

ISO standards is 1.0 μg/mm . The measured weight loss

of the glass frit of example 1 of the invention is 0.26

μg/mm . The weight loss of a competitive low-fusing

porcelain in the marketplace (Durceram-LFC,

•*) manufactured by Degussa of Germany is 0.47 μg/mm .

EXAMPLES 2-4

Low-fusing porcelains of the invention are made in

the same manner as Example 1, having the composition

indicated in Table 3 below, and the desired

characteristics within the scope of the invention.

Table 3

The examples of the invention when utilized in

forming dental prostheses are characterized by having

an easier polishability due to the reduced hardness

compared to conventional porcelains. The amounts of

Ce0 ₂ and Tb ₂ 0 ₃ are adjusted to provide desired fluorescence characteristics, ranging from bluish to

white fluorescence.

Variations and modifications of the composition

are within those skilled in the art, for example, to

6/18373 P

adjust the coefficient of thermal expansion to match

the desired value of 12-13 μm.K ^"1 , depending upon the

substrate to which the porcelain will be applied.