High strength feldspathic dental porcelains containing crystalline leucite

Translucent feldspathic dental porcelain compositions and dental restoration made therefrom exhibiting a crystalline leucite content of at least about 45% by weight, wherein said leucite crystallites exhibit a size of less than about 35 microns, comprising: ______________________________________ Component Percentage (by weight) ______________________________________ SiO.sub.2 55-70 Al.sub.2 O.sub.3 16-20 CaO 0.5-5.0 MgO 0.5-5.0 Li.sub.2 O 1.0-5.0 Na.sub.2 O 2.0-5.0 K.sub.2 O 12.5-22.5 Ce.sub.2 O.sub.3 0-1.0 ______________________________________ said dental restorations exhibit a compressive strength of at least about 125,000 p.s.i., a flexural strength of at least about 16,000 p.s.i., and a diametral tensile strength of at least about 6,000 p.s.i., thereby obviating the need for metal as ceramic supports.

FIELD OF THE INVENTION 
The present invention relates to a new family of dental porcelain 
compositions exhibiting greater flexural strength, compressive strength, 
diametral tensile strength and crystalline leucite content (K.sub.2 
O.Al.sub.2 O.sub.3.4SiO.sub.2) than present commercial dental porcelains 
and are thus useful in the manufacture of porcelain dental restorations 
such as artificial teeth, crowns, bridges and the like which can be 
employed without metal supporting structures as heretofore required. 
BACKGROUND OF THE INVENTION 
Porcelain is one of the most important materials used in dentistry. It 
lends itself to the manufacture of the most esthetic dental restorations 
since it can be colored to closely resemble the teeth it must replace. 
Porcelain exhibits excellent chemical qualities insofar as dental 
applications are concerned. It is insoluble in the normal fluids of the 
oral cavity and in practically any given food or drink likely to pass 
through the oral cavity. It is also chemically able to resist the acid or 
alkali materials frequently used for washing artifical teeth. Moreover, 
mammalian tissues are very tolerant of its presence and such tolerance 
remains even after years of continuous contact. 
Porcelain does have, however, one great disadvantage. It is relatively 
fragile and repairs are difficult and costly. Because of the hazard of 
fragility, artificial dental crowns and bridges have heretofore been made 
using a metallic framework coated with a fused dental porcelain to provide 
the desired esthetics and strength. 
The type of porcelain that is currently most often employed in dental 
restorations is typified by that described in the Weinstein et al patents, 
U.S. Pat. Nos. 3,052,982 and 3,052,983. The Weinstein et al patents 
address the problem of preparing a porcelain whose coefficient of thermal 
expansion will match that of the metal base so that excessive stress 
formation will not occur during the production of the restoration. 
The solution proposed by Weinstein et al was to make a dental porcelain 
composed of two different frits, one having a high coefficient of 
expansion and the other having a much lower coefficient of expansion, to 
result in a porcelain having a coefficient of expansion intermediate 
between the two materials, and which will match the dental alloy employed 
as the base. 
The major disadvantage of the metal supported porcelain restoration is the 
loss of translucency which is especially noticeable at the gingival margin 
area. The junction of the restoration with the tooth may lack the 
translucent esthetic quality desired even in the case of an all-porcelain 
margin. Also, the added complexity of waxing, casting and metal finishing 
requires an increased amount of labor for the production of the 
restoration. Still, it has been the most versatile restoration heretofore 
employed. 
Mechanisms of strengthening ceramics, other than the use of metal ceramic 
systems, have primarily involved dispersion strengthening (aluminous core 
materials) and controlled crystallization (Dicor.RTM. castable glass 
ceramic from Dentsply International, Inc., York, Pa., and CeraPearl.RTM. 
castable glass ceramic from Kyocera, Inc., San Diego, Calif.). The 
aforementioned systems are not greatly different in their clinical 
strength. Color is applied to the surface of such cast glasses, which 
limits their potential for optimum esthetics. 
An alternative to the cast-glass ceramic systems is the extrusion molded 
system which employs an epoxy die and a shrink-free (expansion/contraction 
controlled) system, for example, Cerestore.RTM. nonshrink alumina ceramic 
of Johnson & Johnson Dental Products, Inc. The extrusion molded and 
cast-glass ceramic systems exhibit disadvantages, including a high initial 
equipment cost and the fact that each system is somewhat labor intensive. 
A further disadvantage of systems employing an aluminous core is the 
reduced translucency produced by the semi-opaque nature of the core 
materials. Both the cast ceramic systems and the dispersion strengthened 
systems also have an inherent disadvantage in the manner in which they 
might be joined to form multiple units. Satisfactory commercially feasible 
systems of joining alumina reinforced ceramic units have not been 
developed. 
Accordingly, it is an object of the present invention to provide a high 
strength dental porcelain for use in making all-ceramic dentures, crowns 
and bridges, thereby obviating the need for a metal or ceramic support. 
A further object of the present invention is to provide a dental porcelain 
composition which is translucent and exhibits the ability to accept colors 
producing a restoration exhibiting desirable dental shades. 
A still further object of this invention is to provide artifical crowns and 
bridges with greater impact strength and hence greater resistance to 
chipping. 
A still further object is to provide a high strength dental porcelain 
composition which can be used employing present laboratory equipment and 
eliminating the need for extensive heat treatment. 
A still further object is to provide permanent dental restorations 
exhibiting high strength, i.e., a minimum compressive strength of at least 
about 125,000 p.s.i., a diametral tensile strength of at least about 6,000 
p.s.i. and a flexural strength of at least about 16,000 p.s.i. 
SUMMARY OF THE INVENTION 
These as well as other objects and advantages can be achieved through the 
present invention which provides a translucent feldspathic dental 
porcelain composition useful for preparing dental restorations having a 
compressive strength of at least about 125,000 p.s.i., a diametral tensile 
strength of at least about 6,000 p.s.i., a flexural strength of at least 
about 16,000 p.s.i. and a crystalline leucite content of at least about 
45% by weight, wherein said leucite crystallite exhibits a size of less 
than about 35 microns, preferably less than about 5 microns, comprising: 
______________________________________ 
Component Percentage (by weight) 
______________________________________ 
SiO.sub.2 55-70 
Al.sub.2 O.sub.3 
16-20 
CaO 0.5-5.0 
MgO 0.5-5.0 
Li.sub.2 O 1.0-5.0 
Na.sub.2 O 2.0-5.0 
K.sub.2 O 12.5-22.5 
Ce.sub.2 O.sub.3 
0-1.0 
______________________________________ 
DETAILED DESCRIPTION OF THE INVENTION 
The translucent feldspathic dental porcelain composition of the present 
invention can be made using a variety of feldspars, including Wyoming, 
Canadian, Norwegian, and Carolinian feldspars. These as well as other 
feldspars which have the following general composition are considered 
suitable for use in conjunction with the present invention: 
______________________________________ 
Component Percentage (by weight) 
______________________________________ 
SiO.sub.2 64-67 
Al.sub.2 O.sub.3 
17-20 
CaO 0.5-1.0 
K.sub.2 O 12.0-14.0 
Na.sub.2 O 1.0-3.0 
______________________________________ 
Preferably, Wyoming feldspar is used in making the dental porcelain of the 
present invention. At least a portion of the K.sub.2 O, Al.sub.2 O.sub.3 
and SiO.sub.2 in such feldspars is currently believed to be present in a 
crystalline leucite configuration. Although not wishing to be bound by any 
theory or mechanism, it is currently believed that such leucite 
crystallites serve as nuclei during the fusing and cooling process in 
order to initiate further crystalline leucite nucleation and growth in the 
magma. As the magma is cooled, the crystalline leucite becomes less 
soluble and precipitates out. 
Na.sub.2 O is an inhibitor of leucite crystal growth during the fusing and 
cooling process. Low Na.sub.2 O in conjunction with high K.sub.2 O in the 
feldspar is believed to be responsible for the resulting high leucite 
content of the translucent feldspathic dental porcelain composition. 
The feldspar is first culled to remove quartz, mica, and biotite. Next the 
feldspar is charged to a ball mill containing a grinding medium to reduce 
it to a fine powder, 95% of which passes through a 180 mesh screen. Then 
the feldspar is passed through a dry magnetic separator to remove any iron 
impurities that may be present. It is next further milled and screened 
through a 200 mesh screen. 
The resultant powdered feldspar is blended with cerium oxide, if included, 
and a flux comprising any or all of the following: potassium nitrate, 
potassium silicate, lithium carbonate, calcium carbonate and magnesium 
oxide in quantities such that the resultant feldspathic dental porcelain, 
after fusing as herein-after described, comprises: 
______________________________________ 
Component Percentage (by weight) 
______________________________________ 
SiO.sub.2 55-70 
Al.sub.2 O.sub.3 
16-20 
CaO 0.5-5.0 
MgO 0.5-5.0 
Li.sub.2 O 1.0-5.0 
Na.sub.2 O 2.0-5.0 
K.sub.2 O 12.5-22.5 
Ce.sub.2 O.sub.3 
0-1.0 
______________________________________ 
Preferably, said resultant feldspathic dental porcelain composition 
comprises: 
______________________________________ 
Component Percentage (by weight) 
______________________________________ 
SiO.sub.2 60-64 
Al.sub.2 O.sub.3 
16-19 
CaO 0.5-2.0 
MgO 0.5-1.5 
Li.sub.2 O 1.0-3.0 
Na.sub.2 O 2.0-4.0 
K.sub.2 O 12.5-14.5 
Ce.sub.2 O.sub.3 
0-0.15 
______________________________________ 
The quantity of flux needed will, of course, depend upon the particular 
composition of feldspar employed. Depending upon the initial fusing point 
of the feldspar, more or less flux will be needed in order that the fusing 
point is adjusted accordingly. For instance, a high fusing point feldspar 
will require more flux and a low fusing point feldspar, less flux. 
The potassium oxide can be introduced by employing a combination of 
potassium nitrate and potassium silicate. It has surprisingly been found 
that this combination produces a much better product than either does 
alone. 
From about 2 to about 7, preferably 3, wt % potassium nitrate can be used 
in the powdered dental porcelain composition of the present invention. The 
potassium nitrate functions to introduce potassium oxide into the silicate 
lattice, from which lattice the leucite crystals precipitate. The 
potassium oxide also lowers the fusing range. 
From about 3 to about 10, preferably 5, wt % potassium silicate can be used 
in the powdered dental porcelain composition of the present invention. The 
potassium silicate functions in the same manner as the potassium nitrate, 
and the silicate tends to increase the silicate phase and to stabilize it 
such that leucite precipitation is more easily controlled resulting in the 
uniform distribution of leucite crystallites having a size of less than 
about 35 microns, preferably less than about 5 microns, throughout the 
glass matrix. 
From about 2.5 to about 12.5, preferably from about 2.5 to about 7, and 
most preferably 3.5, wt % lithium carbonate can be used in the flux 
comprising the translucent feldspathic dental porcelain composition of the 
present invention. The lithium oxide is desired because it controls the 
fusing range without degrading other desirable properties. The 
incorporation of lithium oxide also functions to modify the viscosity 
during fusion so as to favor nucleation and crystalline leucite grain 
growth. The softer feldspars, for example, Carolinian feldspar, require 
less lithium carbonate than the harder feldspars, such as the Wyoming 
feldspar. 
From about 0.75 to about 9, preferably from about 0.75 to about 5.5, and 
most preferably 2, wt% calcium carbonate can be used in the flux of the 
present invention. The calcium carbonate is desired because upon fusing it 
becomes calcium oxide which strengthens the glass phase and reduces its 
solubility in the presence of a high potassium oxide content. 
From about 0.5 to about 5, preferably from about 0.5 to about 1.5, or most 
preferably 0.8, wt % magnesium oxide can be used in the translucent 
feldspathic dental porcelain composition of the present invention. The 
magnesium oxide is desired because it appears to function synergistically 
with the calcium oxide in strengthening the glass in relation to either 
alone. 
From about 0 to about 1 wt %, preferably from about 0 to about 0.15 wt% 
cerium oxide can be used in the translucent feldspathic dental porcelain 
composition of the present invention. After fusion and cooling, without 
the incorporation of the cerium oxide, the resultant fused composition is 
extremely hard but can be milled by high impact comminution processes. 
Milling of such an extremely hard composition by attrition results in 
excessive fines and coarse particles, which are not useful since the 
resulting milled product cannot be wet. The cerium oxide in the 
composition of the present invention is desirable since it releases small 
amounts of oxygen at a point during the fusion where the viscosity is low 
enough such that bubbles are produced. The bubbles so created in the 
matrix allow for ready milling of the fused composition via both impact 
and attrition. The bubbles are also believed to provide extra surface for 
nucleation. 
Nucleating agents, such as niobium oxide, can also be included in the 
translucent feldspathic dental porcelain composition in order to enhance 
leucite crystal formation. The addition of nucleating agents for such 
purposes is well known to those skilled in the art. 
The unfired feldspathic mineral is opaque. Dental porcelain, of course, is 
highly translucent and is largely vitreous. By addition of potassium oxide 
and firing, most of the feldspathic mineral is converted to a vitreous 
phase. 
The translucent feldspathic mixture, after blending, is charged into 
saggers and fused to form a vitreous body containing a uniform dispersion 
of leucite nuclei therein. The fusion can be carried out at about 
2150.degree. to about 2350.degree. F., preferably about 2250.degree. F., 
for from about 2 to about 10, preferably about 5, hours. After the fusion, 
the porcelain composition is furnace-cooled at about 5.degree. F./min to 
about 1900.degree. F., held there for from about one to about four hours, 
and then quenched by immersion into water. The above fusion provides the 
requisite translucency, requisite crystalline leucite content, and desired 
leucite crystallite size of less than about 35 microns, preferably less 
than about 5 microns. The slow cooling to about 1900.degree. F. is 
essential for crystalline leucite nucleation and growth. Quenching at 
about 1900.degree. F. is also essential in order to arrest further growth 
of crystalline leucite such that the requisite translucency is provided. 
The quenched fused porcelain chunks are dried and then crushed and reduced 
to a fine powder by, for example, ball milling. Preferably the powder is 
fine enough to pass through a 180 to 200 mesh screen. 
Since the feldspathic dental porcelain composition of the present invention 
is translucent, it is able to accept pigments and produce a restoration 
after firing exhibiting desirable dental shades. The usual pigments, such 
as chromates, vanadates, and manganates can be added to the feldspathic 
dental porcelain composition in small amounts, such as 0.5-1.5 weight 
percent, as well as opacifiers such as tin oxide, if desired. The thermal 
expansion of the porcelain is controlled to match that of the refractory 
die described below. 
After the porcelain powder has been prepared and blended with the pigments, 
it is then employed in making dental restorations in the conventional 
manner; however, use of a metal or ceramic support is not required. 
The general technique for the construction of a porcelain dental 
restoration (i.e. crown or bridge), is the following: first an impression 
is taken of the area that has been prepared to receive the dental 
restoration. A refractory die is prepared from the impression. The 
porcelain powder is then mixed with water to form a slurry, which is then 
applied to the refractory die by standard procedures. 
Once the dental porcelain material is in its predetermined and desired 
shape, it is fired as is conventional for preparation of the various 
dental porcelain constructions in the art. The composition in its 
predetermined shape is first dried and then fired at a temperature and for 
a time such that the dental porcelain material fuses together as is 
conventional in the art for the preparation of a fired dental porcelain. 
Typically, the composition of the present invention is fired at a 
temperature of from about 1875.degree. F. to about 1975.degree. F., 
preferably 1900.degree. F., for about 30 seconds. The furnace temperature 
is raised from about 1000.degree. F. at the time of insertion to the 
desired temperature at a rate of from about 75.degree. F. to about 
125.degree. F./minute, preferably 100.degree. F./minute. 
Once the composition has been fired, a dental restoration in the 
predetermined shape is provided, i.e., in the shape of a crown or bridge, 
for example, as discussed above. 
By employing the composition of the present invention, the fused 
translucent feldspathic dental porcelain restoration thus obtained 
exhibits a compressive strength of at least about 125,000 p.s.i., 
typically about 140,000 p.s.i.; a diametral tensile strength of at least 
about 6,000 p.s.i., typically about 10,000 p.s.i.; a flexural strength of 
at least about 16,000 p.s.i., typically about 20,000 p.s.i.; and a 
crystalline leucite content of at least about 45% by weight, typically 
about 55 to about 75% by weight; said leucite crystallites exhibiting a 
size of less than about 35 microns, preferably less than about 5 microns. 
These physical characteristics provide a dental composition with 
sufficient strength to obviate the need for a metal or ceramic support. 
Accordingly, the practitioner can make the desired dental porcelain 
structure in one step. 
The coefficient of thermal expansion of the high strength porcelain 
compositions of the present invention are significantly higher than the 
conventional porcelain compositions currently used for 
porcelain-fused-to-metal applications. As a result, the high strength 
porcelains of the present invention cannot be fused to the 
porcelain-fused-to-metal alloys currently available. 
While the porcelain compositions of the present invention cannot be fused 
to any currently available porcelain-fused-to-metal alloys, they can be 
bonded to conventional dental metal substrates, if desired, using 
conventional resin (filled or unfilled) bonding techniques.

The following examples are intended to illustrate, but not to limit, the 
present invention. Unless otherwise stated, all percentages and parts are 
by weight. 
EXAMPLE 1 
Wyoming Feldspar, having the following composition: 
65.3 wt % SiO.sub.2 
19.1 wt % Al.sub.2 O.sub.3 
0.1 wt % CaO 
3.2 wt % Na.sub.2 O 
12.1 wt % K.sub.2 O 
0.2 wt % *L.O.I. 
FNT *L.O.I.=Loss on ignition. 
is culled to remove quartz, mica, and biotite. Next, the feldspar is 
charged into a ball mill containing a grinding medium and reduced to about 
180 mesh. Then, the feldspar is passed through a dry magnetic separator to 
remove any iron impurities. It may then be further milled for 
approximately 2 to 4 hours and screened through a 200 mesh screen. 
The translucent feldspathic dental porcelain composition of the present 
invention was prepared by blending the components listed in Table I: 
TABLE I 
______________________________________ 
Component Grams 
______________________________________ 
Wyoming Feldspar 85.74 
KNO.sub.3 3.00 
K.sub.2 O SiO.sub.2 (1:2.5) 
5.00 
Li.sub.2 CO.sub.3 3.50 
CaCO.sub.3 1.90 
MgO 0.76 
Ce.sub.2 O.sub.3 0.10 
100.00 
______________________________________ 
After weighing the raw materials, the components were blended, ball milled 
for 1 hour, transferred to a high alumina body sagger which was coated 
with a parting agent, for example, Al.sub.2 O.sub.3, and heated to 
2250.degree. F. (at a heat-up rate of 400.degree. F./hour) and maintained 
at that temperature for five hours. After fusion, the porcelain magma was 
cooled at about 5.degree. /min to 1900.degree. F. and held at 1900.degree. 
F. for from about 1 to about 4 hours. The magma was then quenched in water 
and dried. The porcelain composition so produced was crushed and ball 
milled such that the resulting particles pass through a 200 mesh screen. 
The addition of pigments was accomplished by preparing a master batch at a 
pigment concentration of about 10% by weight. The master batch was 
prepared by the addition of the required pigment to a 1 kilogram batch of 
the translucent feldspathic dental porcelain composition and ball milling 
the mixture in order to disperse the pigments uniformly throughout. 
To a 15 kilogram batch of the translucent feldspathic dental porcelain 
composition was added the following pigment master batch to produce a 
composition which, upon firing, resulted in a restoration with a desirable 
dental shade: 
______________________________________ 
Amount (grams) Pigment (10% concentration) 
______________________________________ 
2.25 Iron/Chrome/Zinc 
35.0 Zirconium Praseodymium 
25.0 Zirconium Vanadium Indium 
9.0 Coral Brown 
______________________________________ 
The mixture was then ball milled to evenly disperse the pigments 
throughout. 
EXAMPLE 2 
The translucent feldspathic dental porcelain composition obtained from 
Example 1 and control 1 (Pencraft Porcelain from American Thermocraft 
Corp., 60 Franklin Street, West Orange, N.J. 07017) were formed into 
appropriate shapes for physical testing. 
These structures were then dried at 1000.degree. F. for 6-8 minutes, 
followed by firing in an electric furnace to 1900.degree. F. (at a heat-up 
rate of 100.degree. F./min) and maintained at that temperature for 30 
seconds and then allowed to cool to room temperature in air. 
The following physical properties were measured for the translucent 
feldspathic dental porcelains so produced: 
TABLE II 
______________________________________ 
Example 2 
Control 1** 
______________________________________ 
Compressive Strength (p.s.i.) 
140,000 50,000 
Diametral Tensile Strength (p.s.i.) 
12,000 6,000 
Flexural Strength (p.s.i.) 
20,000 11,000 
*Leucite Content (% by weight) 
Example 2 was about 40% 
greater than control 1. 
Coefficient of Thermal Expansion 
18 .times. 10.sup.-6 
13.2 .times. 10.sup.-6 
(in./in. .degree.C.) 
______________________________________ 
*Estimated by Xray Diffraction. 
**Fired to its fully matured temperature of 1800.degree. F. 
EXAMPLE 3 
Preparation of a Dental Restoration of the Present Invention 
An impression was taken of the area that had been prepared to receive the 
dental restoration. A refractory die was prepared from the impression. The 
die was soaked in water so as not to absorb the water from the porcelain 
slurry when applied thereto. The feldspathic translucent dental porcelain 
composition of Example 1 was mixed with water to form a slurry. The slurry 
was applied to the die using a spatula and forming a rough facsimile of 
the desired dental restoration. Gingival porcelain was built first. Then 
incisal porcelain was blended over the gingival porcelain. The water was 
then removed by a combination of vibration and absorption with a tissue. 
The exact configuration of the desired restoration was then carved by a 
dental laboratory technician. 
The unfired restoration was dried outside a furnace held at 1000.degree. F. 
for 6-8 minutes. It was then placed in the furnace and the temperature 
raised to 1900.degree. F. and held for 30 seconds (at a heat-up rate of 
100.degree. F./min). The restoration was then removed from the furnace and 
allowed to cool to room temperature in air. Appropriate porcelain 
additions were made in order to perfect the configuration of the 
restoration, and the restoration was refired as necessary. Between 
firings, adjustments were made with appropriate grinding instruments. In 
this manner, an all-porcelain dental restoration was obtained exhibiting 
sufficiently high strength such that a conventional metal or ceramic 
support was not necessary for the resulting restoration to meet all 
current dental requirements. 
While the invention has been described in accordance with desirable 
embodiments and details of procedure, it is obvious that many changes and 
modifications may be made in the details thereof and in the 
characteristics of the compositions and articles obtained therefrom 
without departing from the spirit of the invention.