Homogeneous microfilled dental composite material and method of preparation

A homogeneous microfilled dental composite material comprised of a mixture of polymerizable monomers and an inorganic filler, wherein said filler is comprised of silane treated fused silica aggregates having a size ranging from submicron to about 160 .mu.m. The aggregates are comprised of agglomerates of fumed silica having an average agglomerate size in the range of approximately 0.5 to 50 .mu.m, and the agglomerates are comprised of primary particles of fumed silica having an average particle size in the range of approximately 1 to 100 nm. The primary particles are interconnected by siloxane bridges formed by burning an organosilane coating on the fumed silica.

BACKGROUND OF THE INVENTION 
This invention relates to a homogeneous microfill composite material for 
use in dental restorations and for layering over microhybrids. 
Microfill composite materials are generally produced by mixing finely 
divided silica with a polymerizable monomer, usually an acrylate or 
methacrylate-based resin, heat polymerizing the mixture in bulk, and 
pulverizing the mixture down to the desired agglomerate size to give a 
filler material comprised of splintered polymerized particles. This filler 
material is then mixed with a polymerizable monomer, again typically an 
acrylate or methacrylate-based resin, and an additional filler material, 
such as a colloidal silica. Thus, there are typically two polymerization 
steps, with the first being referred to as prepolymerization. 
For example, U.S. Pat. No. 4,781,940 describes a process for producing a 
dental composite material using a prepolymerization step. A slurry of 
silica in a polymerizable monomer/solvent solution is prepared, followed 
by evaporating the solvent by heating at atmospheric pressure. The monomer 
coated silica particles are individualized by sieving, then polymerized by 
heating, and again sieved. No pulverization step is required. This filler 
material is then mixed with an additional filler and a resin, which may 
have the same monomers as those used in the monomer/solvent solution. 
The composite material resulting from processes such as that of U.S. Pat. 
No. 4,781,940 is heterogeneous as a result of the prepolymerization step. 
Heterogeneous dental fillers are hydrolytically unstable and suffer from 
catastrophic marginal failures when used in dental restorations, in part, 
as a result of separation along the prepolymerized particle-resin matrix 
interface caused by percolation of aqueous fluids. Thus, it is desirable 
to develop a microfill that does not involve prepolymerization of the 
silica particles. 
U.S. Pat. No. 4,389,497 is directed to a filler material which may or may 
not include a prepolymerization step, and therefore, may be heterogeneous 
or homogeneous. The inorganic filler material used is in the form of 
agglomerates of silicic acid, which can be produced with or without a 
binding agent. For example, the agglomerates could be formed by premixing 
silicic acid with a water glass solution, a boric acid solution, or an 
alcoholic aluminum alcoholate solution. The agglomerated material is then 
adjusted to the desired size by milling and screening. A further manner of 
production involves premixing the silicic acid with organo-silicon 
compounds preferably containing a polymerizable residue and a 
polymerization catalyst if necessary, followed by heat polymerization. The 
organic constituents are then burned and the mixture is brought up to more 
than 600.degree. C. Agglomerate reduction to the desired size is then 
achieved by milling and screening, subsequent to the heating step. 
In addition to the homogeneous composites disclosed in U.S. Pat. No. 
4,389,497, there are other homogeneous composites that have been offered 
commercially. These homogeneous composites have proved unsuccessful in 
that they are difficult to prepare reproducibly and the Theological 
properties are difficult to control. This is due to difficulty in 
controlling the agglomeration of silica particles. Thus, it is desirable 
to develop a process in which the agglomeration can be controlled to 
prepare reproducibly a homogeneous microfill composite material. 
SUMMARY OF THE INVENTION 
The present invention provides a hydrolytically stable, homogeneous 
microfilled dental composite material comprised of a polymerizable monomer 
mixed with bridged silica particles for use in dental restorations, and a 
method for making the same, such that the material is easy to prepare 
reproducibly and the Theological properties of the material are 
controllable. To this end, and in accordance with the principles of the 
present invention, raw fumed silica is coated with silane, dried, milled 
to a fine particle size, and heated to burn the silane coating. Siloxane 
units are thereby formed, which bridge together the fumed silica 
aggregates to produce an interconnected sponge-like mass, allowing for 
interpenetration of resin, without the need for a prepolymerization step. 
The bridged silica particles are then treated with silane, which has the 
advantage of reducing water percolation. The homogeneous resin matrix of 
the present invention leads to higher mechanical properties, such as 
flexural strength and toughness, and reduced crack propagation, thus 
reducing the chance of catastrophic marginal failure in dental restorative 
applications, as well as reducing gouging during finishing. A further 
advantage of the present invention is that the composite material is 
highly polishable and can be brought rapidly to a high luster, which is 
essential for materials used in exterior tooth applications. Furthermore, 
this high luster is retained after tooth brushing and chewing. Thus, there 
is provided a highly polishable, hydrolytically stable, homogeneous 
microfilled dental composite material with excellent mechanical properties 
for use in dental restoration. 
DETAILED DESCRIPTION 
The filler material is comprised mainly of fumed (pyrogenic) silica. Raw 
fumed silica, such as OX-50 commercially available from Degussa Corp., 
Ridgefield Park, N.J., contains primary particles substantially spherical 
in shape, ranging in size from 1-100 nm with an average size of 40 nm. 
OX-50 has a high purity and is of appropriate size, which is important for 
obtaining optimal results according to the teachings of the present 
invention. These primary particles are bound together to form 
agglomerates, ranging in size from about 0.5 to 50 .mu.m. These 
agglomerates become physically entangled, forming aggregates. 
To produce a uniform raw material for subsequent processing, a 
deagglomeration step may be performed in which 30-70% by volume raw fumed 
silica is combined with water and mixed in a colloid mill. For example, 
50% by volume fumed silica is mixed with 50% by volume water and mixed in 
a batch-type Cavimix.TM., available from Arde Barmco Inc., Norwood, N.J. 
The resulting slurry is dried, milled, such as by hammer milling, dry-ball 
milling or impact pulverizer milling, and sieved to produce aggregates 
with a maximum size of 160 .mu.m, such as by sieving through 80-95 mesh. 
The use of a colloid mill greatly increases the cost of the operation. 
Thus, this deagglomeration step is preferably omitted by exercising 
greater control in the subsequent processing steps. 
The uniform raw fumed silica is coated with approximately 20% by volume 
organosilane, such as A-174 (.gamma.-methacryloxypropyltrimethoxysilane) 
available commercially from Union Carbide, Danbury, Conn. This is 
accomplished in a V-blender, for example, using an aqueous solution spray. 
The product is then dried, such as in a forced air oven. The temperature 
and drying time are not critical. Rather, the drying is carried out so as 
to drive off any excess water from the material. For example, drying in a 
forced air oven at approximately 110.degree. C. for about 24 hours is 
sufficient for the purpose. 
The silane-coated silica is milled, such as by passing the material through 
a hammer mill, dry-ball mill or impact pulverizer mill, in order to 
generate particles ranging in size from submicron up to about 80 .mu.m, 
with an average aggregate size of 10 .mu.m. The silane coated fumed silica 
is then heated in a room atmosphere to a temperature in the range of about 
800-1100.degree. C. for 1 to 15 hours. The organic portion of the silane 
is thereby burned generating the following products: 
EQU CH.sub.2 :C(CH.sub.3)COOCH.sub.2 CH.sub.2 CH.sub.2 Si(OCH.sub.3).sub.3 
+O.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O+SiO.sub.2 
The SiO.sub.2 formed has a tendency to associate with itself, thereby 
forming siloxane units (Si--O). These siloxane units can bridge together 
the fumed silica, resulting in an interconnected sponge-like mass, 
referred to as fused fumed silica. The agglomerate reduction step, having 
been carried out prior to heating of the silica, provided greater control 
over the subsequent agglomeration of the bridged silica particles. 
The fused fumed silica is then surface treated with an organosilane to 
produce a hydrophobic material, such as by coating with A-174 using an 
aqueous spray in a V-blender, followed by drying in room atmosphere using 
a two temperature cycle. For example, the silane-coated fused fumed silica 
is dried at approximately 110.degree. C. for 1-4 hours, then at 
approximately 50.degree. C. for 15-20 hours. The product is then sieved to 
produce an average aggregate size of about 10 .mu.m, with a range from 
submicron up to about 160 .mu.m, such as by sieving through 80-95 mesh. 
The filler, silane-treated fused fumed silica, is ready to be compounded 
in paste. 
A homogeneous resin mixture consisting of a polymerizable organic acrylic 
monomer is prepared for mixing with the filler. For example, the following 
components (in parts by weight) may be mixed together to give a preferred 
homogeneous resin mix: 
60-90 parts Diurethane Dimethacrylate (Rohamere 6661-0, Huls America, Inc. 
Piscataway, N.J.) 
10-40 parts Triethyleneglycoldimethacrylate 
0.4-1.0 parts 2-Hydroxy-4-methoxybenzophenone 
0.07-0.4 parts Camphorquinone 
0.1-1.0 parts p-octyl dimethylamino-benzoate. 
This cured resin system is tough and highly cross-linked. The goal in 
selecting a resin system is to provide a resin with a refractive index 
similar to that of silica, such that they are a relatively good match from 
a visual and aesthetic standpoint. 
The filler is then dispersed in resin with additional fumed silica, such as 
silicone treated fumed silica (Aerosil 200, available commercially from 
Degussa Corp.) by mixing together until uniform, approximately 3-10 hours, 
such as by mixing in a planetary mixer. The components are mixed in the 
following amounts: 
45-58 wt. % Filler 
2-7 wt. % Silicone-treated fumed silica (Aerosil 200, Degussa) 
40-48 wt. % Resin. 
A deaeration step is then carried out for at least 10 minutes, by which 
excess air in the composite paste material is eliminated by bubbling the 
air out in a vacuum. Deaerating is advantageously done in an attenuated 
oxygen atmosphere. 
The microfill of the present invention exhibits better physical properties 
than the currently marketed microfills as a result of the combination of 
agglomerate reduction prior to heating the fumed silica particles, the 
silane treatment of the fused silica, and the use of fumed silica bridged 
by siloxane units. For example, substantially higher flexural strength and 
toughness are obtained with the present invention. This assures higher 
fracture resistance than a typical microfill. This high strength and 
fracture resistance help reduce gouging during finishing. Better 
hydrolytic stability from reduced water percolation at the filler-resin 
matrix interface, higher mechanical properties and reduced crack 
propagation even in feathered edges are expected due to the silane 
treatment and the absence of prepolymerized particles. The unique, complex 
morphology of the bridged silica particles presents a higher surface area 
than the original silica, allowing for intimate resin-filler particle 
interactions. A further benefit of the present invention is reduced wear 
due to the complexity of the bridged silica particles-resin matrix 
interface. The fragile nature of this filler material results in the 
exposed filler particles shearing off rather than exfoliating during wear. 
Together, these improvements are expected to lead to better long term 
clinical performance, with the problem of catastrophic marginal failure, 
as observed with heterogeneous microfills, considerably lessened. 
Furthermore, the process of the present invention allows for control of 
the rheological properties due to easier control of the agglomerate of 
fumed silica by agglomerate reduction prior to heat treating the silica. 
Moreover, the homogeneous microfill of the present invention is simpler to 
prepare reproducibly than the heterogeneous and homogeneous microfills of 
the prior art. Esthetic concerns are met in that the present process 
results in a material that is highly translucent, polishable and can be 
brought rapidly to a high luster that is retained after brushing and 
chewing.

EXAMPLE 1 
Raw OX-50 was coated with 20% by weight A-174 organosilane in a V-blender 
using an aqueous solution spray, dried in a forced air oven at 100.degree. 
C. for 24 hours, and hammermilled to a 10 .mu.m average particle size. The 
silane-coated OX-50 was oxidized at 1050.degree. C. for 4 hours, resulting 
in bridged silica particles (fused silica). The fused silica was then 
surface treated with 7% by weight A-174 organosilane in a V-blender using 
an aqueous solution spray. The silane-treated fused silica was dried at 
110.degree. C. for 3 hours and at 55.degree. C. for 16 hours, then sieved 
through 95 mesh. The resulting filler consisted of agglomerates of Si--O 
bridged 0.04 .mu.m fumed silica with aggregates of 10 .mu.m mean size and 
a range of from submicron to 160 .mu.m. The following components (in parts 
by weight) were mixed together and stirred for a few hours to give a 
homogeneous resin mix: 
80.00 parts Diurethane Dimethacrylate (Rohamere 6661-0) 
20.00 parts Triethyleneglycoldimethacrylate 
0.80 parts 2-Hydroxy-4-methoxybenzophenone 
0.12 parts Camphorquinone 
0.60 parts p-octyl dimethylamino-benzoate. 
The following components were mixed in a planetary mixer until uniform 
(approximately 8 hours) followed by deaeration in an attenuated oxygen 
atmosphere: 
50 wt. % filler 
45 wt. % resin 
5 wt. % silicone treated Aerosil 200 fumed silica (Degussa Corp.) 
Table 1 provides the physical properties of the resulting homogeneous 
microfill, as compared to current heterogeneous microfills marketed by 
various companies. 
TABLE 1 
______________________________________ 
Silux 
Heliomolar Plus Durafil Renamel 
Invention (Vivadent) (3M) (Kulzer) (Cosmodent) 
______________________________________ 
Flexural 
110 92 79 83 80 
Strength, FS (11) (13) (10) (12) (9) 
(MPa)* 
Flexural 6,400 6,277 7,000 5,325 5316 
Modulus, (374) (388) (639) (301) (250) 
FM (MPa)* 
Flexural 0.95 0.67 0.45 0.65 0.60 
Toughness 
(FS.sup.2 /2FM) 
Compressive 391 279 248 428 383 
Strength* (23) (81) (96) (47) (31) 
(MPa) 
Rockwell 79.2 77.0 81.7 76.9 78.5 
Hardness 
(15T scale) 
Consistency- 2.6 2.7 4.3 2.7 2.7 
Slump (cm) 
% 25.5 19.1 22.5 22.3 24.6 
Translucency 
(1.0 mm) 
Water 0.66 0.59 -- 0.66 -- 
Sorption* 
(mg/cm.sup.2) 
Water 0 0 -- 0.04 -- 
Solubility* 
(mg/cm.sup.2) 
______________________________________ 
*Testing done in accordance with standard ISO testing protocols. 
() Standard deviation 
While the present invention has been illustrated by the description of an 
embodiment thereof, and while the embodiment has been described in 
considerable detail, it is not intended to restrict or in any way limit 
the scope of the appended claims to such detail. Additional advantages and 
modifications will readily appear to those skilled in the art. For 
example, the raw material may be any commercially available brand of fumed 
silica with a high purity and approximate average particle size of 40 nm. 
Furthermore, variations in time and temperatures may result in a 
homogeneous microfill within the spirit of the present invention. The 
invention in its broader aspects is therefore not limited to the specific 
details, representative method and example described. Accordingly, 
departures may be made from such details without departing from the scope 
or spirit of applicant's general inventive concept.