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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to radiopaque colorless glass ceramics, to
compositions comprising such glass ceramics and a polymerizable synthetic
resin, and in particular to such compositions useful for dental fillings.
To replace the conventional amalgam or gold dental fillings, compositions
have been developed based on rapidly polymerizable synthetic resins, e.g.,
acrylate or methacrylate, and a pulverulent inorganic material, e.g.,
glass, embedded in the synthetic resin.
Heretofore, two groups of inorganic fillers have been added to the
synthetic resins: (a) glasses distinguished by relatively high contents of
BaO (cf. the article by Bowen and Cleek in "Journal of Dental Research,"
1972, pp. 177-182); and (b) Li-Al silicates (DOS [German Unexamined
Laid-Open Application] No. 1,570,971). The reason for the use of
BaO-containing glasses is that barium exhibits a high absorption of short
wavelength X-ray radiation, as utilized in dental diagnostics. As a result
thereof, the composite synthetic resin-Ba-glass element has an absorption
which clearly distinguishes same from the surrounding, natural tooth
material or even from possible cavities.
The second group of fillers, namely the Li-Al silicates, do not possess
this advantageous, high X-ray absorption, but, instead are distinguished
by very low, frequently even negative, coefficients of thermal expansion.
This property is very desirable insofar as low coefficients of thermal
expansion of the filler material can compensate for the very high
coefficients of expansion of the synthetic resins, so that the average
coefficient of thermal expansion of the synthetic resin-Li-Al-silicate
composite can easily be adapted to that of natural tooth substance. This
is a very important advantage, since otherwise there is the danger that
exposure to sudden temperature changes would result in mechanical stress
within the tooth or mechanical stress which might even cause an
unloosening of the filling from the tooth. (To avoid this problem, ground
quartz glass, as well as Li-Al silicates, has also been utilized as the
filling material).
Whereas it would be desirable to provide a glass having both the desired
high X-ray absorption and the described low coefficient of expansion,
heretofore, no materials have been known which would sufficiently satisfy
this combination of requirements. In glasses, a low thermal expansion can
normally be attained only with very high contents of SiO.sub.2 which, in
turn, cannot be coupled with the requirement of a high X-ray absorption.
The number of other solid substances of very low thermal expansion is
relatively limited. Among these substances, insofar as they are
sufficiently resistant to chemicals, there is not a single one with a high
X-ray absorption.
SUMMARY OF THE INVENTION
An object of this invention therefore, is to provide a material exhibiting
a sufficiently high X-ray absorption and a sufficiently low thermal
expansion for dental filling purposes. Whereas the material may have other
uses as well, for dental fillings, it should not contain soluble toxic
components and it must be sufficiently resistant chemically. It must
furthermore be substantially, if not completely, transparent and
colorless, so that pigmentation can render the finished product close to
that of natural teeth. Furthermore, it should be possible to employ the
material in high volumetric proportions in the polymerizable synthetic
resin, and of course, it is necessary that sufficient adhesion be
established between the filling material and the synthetic resin. Also,
the material must be relatively abrasion-resistant and hard.
Other objects relating to such a material are to provide a starting
composition and a method for its manufacture.
Aside from providing the material itself, another object of this invention
is to provide a polymerizable mixture of the material with a synthetic
polymer.
A still further object is to provide articles made from said polymerizable
mixture.
Upon further study of the specification and appended claims, further
objects and advantages of this invention will become apparent to those
skilled in the art.
To attain these objects, it has now been found that it is possible to
employ glass ceramics as fillers for polymerizable dental filling
compositions. These are glass-crystal mixed bodies built up of glass and
one or more types of crystals precipitated from the glass by controlled
crystallization. Such substances can be transparent if the indices of
refraction of the various types of crystals and of the glass do not differ
too much, e.g., by not more than about 0.05, preferably not greater than
0.01, or if the thus-precipitated crystals are sufficiently small, e.g.
having a particle size less than about 1000 A, preferably less than 500 A.
The Li-Al silicates mentioned in the foregoing can be produced in the form
of transparent glass ceramics, but not in the pure form. For, in order to
produce fine-grained glass ceramics, additions of so-called nucleating
agents are required for the formation of a multitude of crystallization
nuclei so that the resultant fine crystals are uniformly distributed in
the volume. In case of Li-Al silicate glasses, the addition of TiO.sub.2,
often in conjunction with ZrO.sub.2, leads to glass ceramics having such a
tiny crystal size that they are transparent. Unfortunately, the TiO.sub.2
content causes a marked brown coloring which is undesirable for the use of
such glass ceramics in dental filling compositions. When abandoning the
simple basic system of Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 and
introducing a number of additional components into the starting glass
materials, it is also possible to obtain an extremely effective nucleus
formation by means of other oxides, among which are also those which do
not impart a coloring. Such glass ceramics have been described in German
Pat. No. 2,132,788. Among these is also a group which can contain up to
12% by weight of oxides of the rare earths. The contents of this German
patent are incorporated by reference herein.
It has now been found surprisingly that the rare earth oxide content of the
glasses in the German patent can even be increased substantially above 12%
by weight without losing the advantageous properties, particularly the
transparency and the low thermal expansion, insofar as such method is
restricted to the production of glass ceramics in a fine-granular form,
either as small spheres according to one of the conventional spraying or
centrifugal processes, or as a granulated material formed by the
introduction of a thin stream of glass into cold water.
It is known that, by the rapid cooling which takes place in the
aforementioned methods, clear glasses can also be obtained even in those
cases wherein the production of larger molded articles is no longer
possible, due to the tendency toward turbidity or crystallization
displayed by the glass. However, it was surprising that the transparency
of the crystallized glass ceramic is not lost upon a further increase of
the content of those oxides which are foreign to the primary crystalline
phases of the glass ceramic, and that, moreover, the thermal expansion
does not rise unduly, since both of these phenomena are customarily
encountered.
The rare earth atoms are distinguished, insofar as they do not belong to
the extremely heavy elements with the highest atomic numbers, by a very
high absorptive power for X-rays of the wavelength of about 0.2 - 0.3 A
(this range being predominantly employed in dentistry). Consequently, even
glass ceramics having proportions of at least 10% by weight,
advantageously more than 12%, especially over 15% by weight, of such
oxides, exhibit a very satisfactory absorbability. The upper limit for the
rare earth content is about 20%, preferably 17 %. Such glass ceramics are
highly suitable for dental filling compositions, especially if lanthanum
is utilized as the rare earth, for lanthanum forms colorless ions and
exhibits an excellent absorption of X-rays having wavelengths of around
0.3 A. Moreover, La.sub.2 O.sub.3 is inexpensive as compared to the
heavier rare-earth oxides.
Accordingly, the glass ceramics of this invention have the following
composition (in % by weight):
SiO.sub.2 40 - 55
Al.sub.2 O.sub.3 15 - 25
Li.sub.2 O 3 - 5
P.sub.2 O.sub.5 3 - 10
MgO 0 - 3
ZnO 0 - 5
ZrO.sub.2 2 - 7
Ta.sub.2 O.sub.5 0 - 7
La.sub.2 O.sub.3 10 - 20
Preferably, the glass ceramics of this invention have the following
compositions (in % by weight):
SiO.sub.2 40 - 50
Al.sub.2 O.sub.3 16 - 20
Li.sub.2 O 3.3 - 4.7
P.sub.2 O.sub.5 4 - 7
MgO 0 - 2
ZnO 0 - 5
ZrO.sub.2 3 - 6
Ta.sub.2 O.sub.5 2 - 6
La.sub.2 O.sub.3 13 - 18
MgO+ZnO 1 - 7
The presence of MgO and ZnO substantially improves the transparency of the
glasses of the invention.
Ta.sub.2 O.sub.5 in combination with ZrO.sub.2 acts as nucleating agent,
which considerably reduces the grain size of the crystallites in the glass
ceramics of the invention.
In addition to the aforementioned oxides, additional oxides can be
contained in the composition, insofar as they are non-toxic and do not
impair the other properties of the glass ceramic. The melting
characteristic can be improved without noticeable disadvantages by the
introduction of Na.sub.2 O and/or K.sub.2 O in amounts of up to 1% by
weight. The total content of possible additional components should
preferably not exceed 5% by weight, for otherwise it would be difficult to
maintain the properties according to this invention. The glasses are
produced in a conventional manner, as described in German Pat. No.
2,132,788.
The conversion of these glass ceramics from the glassy into the partially
crystalline condition is straight-forward; it is sufficient to heat the
glass at a rate of about 6.degree./minute or less from a temperature of
about 0.degree.C to 600.degree.C, to temperatures of at least 800.degree.
C. and at most about 900.degree. C., and to leave the glass at this final
temperature for annealing for about 1 to 5 hours. Annealing can be
conducted at lower temperatures, but longer annealing times are required,
for example about 10 hours at 750.degree. C. The extremely high number of
nuclei necessary to obtain a transparent glass ceramic is attained very
rapidly with these glasses, i.e. during the heating step, and thus does
not require any special measures.
After crystallization, the glass ceramics have thermal expansions (range
0.degree. - 50.degree.) of between -10 and +20 .sup.. 10.sup..sup.-7
.degree..sup..sup.-1 ; they are colorless and transparent. As for the
types of crystals, they contain mixed crystals with a .beta.-quartz
structure in a proportion estimated according to the intensities of X-ray
diffraction diagrams, of about 30-40% by weight, and furthermore mixed
crystals of the structural type of cubic ZrO.sub.2, and a compound
containing La.sub.2 O.sub.3 which heretofore has not been known in detail.
Overall, the percent by weight crystallinity of the glass is about 30 to
70, preferably 40 to 50.
The glass ceramics can be silanized in the usual manner, as has also been
done in the fillers utilized heretofore, to provide good adhesion to the
synthetic resin. The particle size of the glass ceramics for dental
purposes is usually in the range of 5 to 100, preferably 5 to 60 microns.
The X-ray mass absorption coefficient (0.2 - 0.3 A) of the glass ceramics
is about 3 to 7.
A wide variety of synthetic polymerizable resins can be employed in this
invention, both those presently available as well as those available in
the future. Consequently, it is not even possible to catalog a complete
list of specific monomer-polymer systems useful in the context of this
invention. Suffice it to note that all polymers can be reinforced with
glass ceramic of this invention. For dental purposes in particular,
attention is directed to the patent and technical literature for specific
suggestion of synthetic resins, catalysts, monomers, oligomers, polymers,
adducts, initiators, etc. which can be incorporated into a system used in
making dental fillings or in the production of dental prosthetic devices.
With respect to the proportions by volume of the glass ceramic in the
synthetic resin, a wide range can be employed, e.g., about 20 to 70 % of
the total composition being the glass ceramic. For example, with acrylics
(including acrylic and methacrylic acid esters thereof and glycidyl
adducts thereof), considerable amounts of powders or spherical particles
of these glass ceramics are incorporated, normally more than 50% by
volume. The exact upper limit of glass ceramic which can be incorporated
into the composite body depends on the geometrical form and the particle
size distribution of the particulate glass ceramic, but is sufficiently
high in all cases that a satisfactory adaptation of the expansion of the
composite article to that of natural tooth material is possible.
Without further elaboration, it is believed that one skilled in the art
can, using the preceding description, utilize the present invention to its
fullest extent. The following preferred specific embodiments are,
therefore, to be construed as merely illustrative, and not limitative of
the remainder of the disclosure in any way whatsoever. In the following
examples, all temperatures are set forth uncorrected in degrees Celsius;
unless otherwise indicated, all parts and percentages are by weight.
The following table describes examples of glass ceramics of this invention
with their compositions and properties.
TABLE
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Examples for Radiopaque, Colorless Glass Ceramics
No. 1 No. 2 No. 3 No. 4
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% by Weight
SiO.sub.2 45.2 48.3 49.0 52.0
Al.sub.2 O.sub.3
21.7 17.7 18.7 16.0
P.sub.2 O.sub.5
8.2 4.5 5.0 4.0
Li.sub.2 O
4.7 4.0 4.0 4.0
K.sub.2 O 0.6 -- -- --
MgO 1.0 -- -- --
ZnO 1.3 2.0 3.3 1.0
ZrO.sub.2 5.0 5.0 5.0 5.0
Ta.sub.2 O.sub.5
-- 2.5 2.5 5.0
La.sub.2 O.sub.3
12.3 16.0 12.5 13.0
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Heat Treatment:
3 h 900.degree. 3 h 720.degree. 3 h 720.degree. 5 h
800.degree. C.
+2 h 850.degree. +2 h 850.degree.
.alpha. 0.degree. - 50.degree.
(10.sup.-.sup.7 degree .sup.-.sup.1)
+ 3.0 + 6.5 + 4.0 + 5.5
To illustrate the production of the glass ceramics of the invention the
following example is given:
For the production of the glass of example 3 appropriate amounts of the
following raw materials - chemical grade - are thoroughly blended:
SiO.sub.2, Al(OH).sub.3, Al(PO.sub.3).sub.3, Li.sub.2 CO.sub.3, ZnO,
ZrO.sub.2, Ta.sub.2 O.sub.5, and La.sub.2 O.sub.3.
This batch is melted in a quartz crucible for 3 hours at 1550.degree.C, and
is poured into cold water to obtain finely divided glass beads, which are
then remelted for 2 hours at 1550.degree.C, and are again poured into cold
water. Thereafter, the glass beads are heated at a rate of 1.degree.C/min.
to 720.degree.C, are maintained at 720.degree.C for 3 hours, are further
heated with 1.degree.C/min. to 850.degree.C, are again maintained for 2
hours, and are removed from the furnace.
The following recipe is a polymerizing dental filling composition according
to the present invention:
DENTAL FILLING COMPOSITION
(Rapidly Polymerizing Two-Component System)
COMPONENT A
40% Glycidyl methacrylic acid adduct
4% Triphenylborane ammonia complex (cocatalyst)
4% Benzoyl peroxide (catalyst)
COMPONENT B
12% Methacrylic acid
FILLER
(Divided between components A and B):
40% glass ceramic No. 4 of the table.
The preceding examples can be repeated with similar success by substituting
the generically or specifically described reactants and/or operating
conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain
the essential characteristics of this invention, and without departing
from the spirit and scope thereof, can make various changes and
modifications of the invention to adapt it to various usages and
conditions.
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