Vibratory Mill and Method of Use for Low Contamination Grinding

The present invention is related to a vibratory mill and method of use for grinding submicron-sized reinforcing particulate having high purity suitable for use in a dental composite. Uniformly dispersed in the dental composite, the high purity submicron-sized reinforcing particulate provides transparency, high strength, improved wear resistance and gloss retention in clinical use.

In dentistry, practitioners use a variety of restorative materials in order to create crowns, veneers, direct fillings, inlays, onlays and splints. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a polymerizable resin matrix. These materials may be dispersion reinforced or particulate reinforced, depending on the type of filler, or may be hybrid composites or flowable composites, depending on the filler loading. A full discussion of these materials is included in U.S. patent application Ser. No. 09/270,999, entitled "Optimum Particle Sized Hybrid Composite," C. Angeletakis et al., filed on Mar. 17, 1999, now pending (incorporated herein by reference in its entirety). Highly pure submicron particles are useful in these composite resin materials because they impart the desirable optical properties of high gloss and high transparency.

In a composite material, such as a tooth colored dental restorative, the resin matrix and the filler have to be matched in their refractive index to achieve a transparency similar to tooth structure. In addition, this transparency is necessary for the material to be cured using visible light initiation of polymerization. The formulator has relatively wide choices for adjusting the refractive index of the resin because resins in the range of about 1.48 to about 1.55 are easily available commercially. However, glass fillers are much more complex to formulate. The purity of the glass filler must be very high because small amounts of impurities, down to the low ppm level, show very prominently when the filler is dispersed in the resin. Moreover, the refractive index of the polymerized resin must closely match the refractive index of the filler. This is particularly critical with small particle sizes since it is known that light scattering, which is responsible for the opacity in a dental composite, is proportional to the absolute difference between the refractive index of the filler and the refractive index of the resin. If the ground filler dispersed in a resin matrix does not have a transparency value of at least 45, measured as described hereinafter in the Detailed Description of the Invention, then it is difficult to formulate a final restorative material, including pigments and other additives, having the desired shading to match the patient's tooth color.

Milling methods previously used to produce submicron particles have been found unacceptable for filler in dental composites because of the impurities that result from abrasion and spalling. The inclusion of impurities in dental composites can decrease transparency and negatively affect color, making the composites unacceptable for use in dental cavities. Examples of prior art agitator mills are set forth in U.S. Pat. Nos. 5,335,867; 4,129,261; and 4,117,981, all assigned to Draiswerke GmbH and each incorporated herein by reference in its entirety; and 5,065,946, assigned to Matsushita Electric Industrial Co. and incorporated herein by reference in its entirety. These prior art mills typically include ceramic or metallic agitators and grinding chambers. During milling, the ceramic or metallic material of the agitator and grinding chamber spalls and abrades, and the abraded particles become intimately mixed with the material being ground. In the case of fillers for dental restoratives, these abraded particles are unacceptable due to their negative impact on the optical properties of the restorative. The abraded particles may cause decreased transparency due to light scattering and may impart an unnatural color. Draiswerke, Inc., Mahwah, N.J., has applied a polyurethane coating on the agitator and grinding chamber for their PML-H/V machine. The pigment from this coating, however, also contaminates the composites, making them unacceptable for dental use.

The predominant types of milling methods are dry milling and wet milling. In dry milling, air or an inert gas is used to keep particles in suspension. However, fine particles tend to agglomerate in response to van der Waals forces which limits the capabilities of dry milling for submicron particle sizes. Wet milling uses a liquid such as water or alcohol to control reagglomeration of fine particles. Therefore, wet milling is typically used for comminution of submicron-sized particles.

A wet mill typically includes spherical media that apply sufficient force to break particles that are suspended in a liquid medium. Milling devices are categorized by the method used to impart motion to the media. The motion imparted to wet ball mills includes tumbling, vibratory, planetary and agitation. While it is possible to form submicron particles with each of these types of mills, the agitation or agitator ball mill is typically most efficient.

The agitator ball mill, also known as an attrition or stirred mill, has several advantages including high energy efficiency, high solids handling, narrow size distribution of the product output, and the ability to produce homogeneous slurries. The major variables in using an agitator ball mill are agitator speed, suspension flow rate, residence time, slurry viscosity, solid size of the in-feed, milling media size and desired product size. As a general rule, agitator mills typically grind particles to a mean particle size approximately 1/1000 of the size of the milling media in the most efficient operation. In order to obtain mean particle sizes on the order of 0.05 .mu.m to 0.5 .mu.m, milling media having a size of less than 0.45 mm can be used. Milling media having diameters of about 0.2 mm and about 0.6 mm are available from Tosoh Ceramics, Bound Brook, N.J. Thus, to optimize milling, it is desirable to use a milling media approximately 1000 times the size of the desired particle. This minimizes the time required for milling.

As discussed briefly above, the use of known milling processes to achieve such fine particle sizes was difficult due to contamination of the slurry by the internal components of the mill. Further contamination has also been introduced into the slurry by abrasion and spalling of the milling media. By using hard materials for the milling media, such as yttria stabilized zirconia (YTZ or Y-TZP, where TZP is tetragonal zirconia polycrystal), which has a Vickers hardness of equal or greater than about 11 GPa, the contamination by spalling from the milling media and abrasion from the mill is minimized, thereby minimizing opacification of the filler. Y-TZP has a fine grain, high strength and a high fracture toughness. High strength Y-TZP is formed by sintering at temperatures of about 1550.degree. C. to form tetragonal grains having 1-2 .mu.m tetragonal grains mixed with 4-8 .mu.m cubic grains and high strength (1000 MPa), high fracture toughness (8.5 MPa m.sup.1/2) and excellent wear resistance. Due to its resistance to abrasion and spalling, the use of Y-TZP provides a suitable milling media for providing relatively pure, highly translucent structural fillers having mean particle sizes less than 0.5 .mu.m. The YTZ milling media, however, is very expensive. So although low contamination agitator milling with YTZ milling media is time efficient, it is costly due to the expense of the milling media as well as the cost of the machine.

Furthermore, despite some reduction in contamination of the ground filler particulate by the use of the abrasion-resistant YTZ milling media, agitator ball mills still introduce an unacceptably high level of contamination into dental composites containing the ground filler. The high intensity of the grinding action produced by the agitator, and the high momentum of the media, result in abrasion and spalling of the grinding chamber wall, as discussed above. One proposed solution is described in Applicants' copending U.S. patent application Ser. No. 09/271,639 filed Mar. 17, 1999, and entitled "Agitator Mill and Method of Use for Low Contamination Grinding," which is incorporated by reference herein in its entirety. It is proposed that various internal components of the mill be fabricated from the abrasion-resistant YTZ material and that the interior of the grinding chamber be coated with a non-pigmented polymer that would not negatively affect the optical properties of the ground material.

Another possible solution to the problem of contamination in ground dental fillers is to use the less harsh method of vibratory milling. Vibratory ball mills are often used for submicron particle grinding because they provide a high production rate at low capital cost, fine and uniform product size distribution, low power consumption, and low contamination. The rate of milling is a function of the shape and size of the media. Cylindrical media are generally preferred, according to Engineered Materials Handbook.RTM., Desk Edition, ASM International, p 742 (1995), because they spin on an axis and therefore produce small shear forces. The major variables in using a vibratory mill are the amplitude of vibration, energy developed in the mill, slurry viscosity, solid size of the in-feed, milling media size and desired product size. Because vibratory milling involves low intensity grinding, abrasion and spalling of the grinding chamber wall and the milling media are less of a concern, as compared to agitator mills. Even this low intensity grinding, however, may still contribute to an unacceptable level of contamination in the ground filler.

Furthermore, the vibratory mill has not previously been found useful for low contamination grinding of particles having an average size less than the wavelength of visible light. U.S. Pat. No. 4,544,359 describes a dental restorative material with a borosilicate glass/barium silicate filler having an average particle size diameter of from about 0.5 .mu.m to 5.0 .mu.m. The filler is ground by a conventional wet milling process, such as vibratory milling, using a grinding or milling media such as low alumina, porcelain balls, stainless steel balls, borosilicate glass rods, or any other low alumina, non-contaminating grinding medium. Thus, there is a need for a low contamination, vibratory grinding mill and method of use to produce particles having a mean particle size of less than 0.5 .mu.m, where the filler does not opacify upon exposure to visible light.

The present invention is directed to a vibratory mill and method of use in which the vibratory mill has a grinding chamber charged with glass beads having a size and refractive index suitable for providing substantially pure ground particles in the size range of 0.05 .mu.m-0.50 .mu.m having a transparency value of at least about 45 when mixed with a resin matrix and measured as described herein. To this end, and in accordance with the principles of the present invention, filler having a predetermined refractive index is ground with a glass milling media having a refractive index of substantially similar magnitude such that the disparity between the refractive indices of the media and filler is insufficient to cause opacification of the ground filler containing media particles when dispersed in the resin matrix and subjected to visible light. Fillers suitable for use in dental restorative composites typically have a refractive index in the range of about 1.48 to about 1.55. According to the principles of the present invention, glass milling media of hardness less than that of yttria stabilized zirconia such that abrasion of the milling media may occur during grinding should have a refractive index substantially similar to that of the filler, more specifically in the range of about 1.46 to about 1.56. A radiopaque filler suitable for use in dental restorative composites has a refractive index in the range of about 1.51 to about 1.55. According to the principles of the present invention, glass milling media for grinding the radiopaque filler have a refractive index of about 1.48 to about 1.56. A non-radiopaque filler suitable for use in dental restorative composites has a refractive index in the range of about 1.48 to about 1.51. According to the principles of the present invention, glass milling media for grinding the non-radiopaque filler has a refractive index of about 1.46 to about 1.52.

In a further embodiment of the present invention, the ground filler is mixed with a methacrylate resin having a refractive index of about 1.48 to about 1.55 to produce a curable composite paste for use in cosmetic dental restorations.

The filler particles produced using the vibratory mill and media of the present invention may vary greatly in mean particle size; however, it has been discovered that particles having a mean particle size of between about 0.05 .mu.m and about 0.5 .mu.m provide the high strength required for load-bearing dental restorations, yet maintain a glossy appearance in clinical use required for cosmetic restorations. Further, the mill produces nonspherical particles which provide increased adhesion of the resin when used as filler in a dental composite, thereby further enhancing the overall strength of the composite. The filler particles ground with the mill and media of the present invention are highly pure, preferably having an average particle size less than the wavelength of visible light, that is less than about 0.50 .mu.m, and when incorporated into a methacrylate resin and cured produce a composite with a transparency value of at least about 45. Highly pure, ground particles of various sizes and size distributions for uses other than dental composites may also be formed using the present invention.

The present invention, with glass bead media and optimized parameters, produces the particles of desired size, which are free of harmful contamination and exhibit a narrow particle size distribution. The narrow particle size distribution minimizes the small percentage of particles above 0.5 .mu.m which, when present, contribute to producing a non-glossy surface in clinical applications.

FIG. 1 is a plan view of a vibratory mill of the present invention;

FIG. 2A is a scanning electron micrograph, at 20,000.times. magnification, of the particulate ground in the vibratory mill of the present invention;

FIG. 2B is a scanning electron micrograph, at 5,000.times. magnification, of the particulate ground in the vibratory mill of the present invention;

FIG. 3A is a scanning electron micrograph, at 20,000.times. magnification, of the prior art filler particles formed by sol-gel processes; and

FIG. 3B is a scanning electron micrograph, at 100,000.times. magnification, of the prior art filler particles formed by sol-gel processes.

The present invention, in a preferred form, is a vibratory mill, such as types M45-L (521 liter capacity) and M18-L (44 liter capacity) available from Sweco, Florence, Ky., which have an abrasion resistant liner of polyurethane on the grinding chamber.

As seen in FIG. 1, the vibratory mill 2 of the present invention includes a generally cylindrically shaped outer housing 10 supporting an inner housing 12, which is also generally cylindrically shaped but forms an annular ring about a center column 14. The inner housing 12 carries a polymer lining 16 of a type described hereinafter to prevent abrasion of inner housing 12. The inner housing 12 with the polymer lining 16 generally define the grinding chamber 18. The grinding chamber 18 is charged with milling media 20 of a type described hereinafter. Grinding chamber 18 is supplied with material to be ground by slurry inlet 22a and slurry is removed from the grinding chamber by slurry outlet 22b. The grinding chamber 18 vibrates in an up and down motion with an amplitude of approximately 1/4 inch by means of a motor 24 equipped with appropriate centric weights, thereby imparting motion to the milling media 20 to grind the particle slurry. The slurry outlet 22b may include a media retainer 26 in fluid communication with slurry outlet 22b. The media retainer 26 allows the slurry to pass out of the vibratory mill through slurry outlet 22b, while retaining the media 20 within the grinding chamber 18. For larger capacity vibratory mills, such as a 521 liter capacity M45-L mill, circulation of the slurry from the top to the bottom of the chamber with a pump may be necessary for ensuring homogeneity of the slurry during grinding.

The milling media 20 of the present invention are glass beads, preferably soda lime glass beads, such as Dragonite Type LFK glass beads (Jaygo Inc., Union, N.J.). These beads are available in a diameter range of 0.1 mm to 10.0 mm, but 2 mm to 10 mm sized beads are preferred. Soda lime glass beads have an approximate hardness of about 5 GPa. In the case of agitator milling, smaller sized beads are generally preferred, but with vibratory milling, better results appear to be obtained with medium to larger sized beads, albeit with longer grinding times. The beads are preferably spherically shaped; however, beads of other shapes with corresponding dimensions of those of spherically shaped beads can also be utilized, such as oblate spheroids or cylinders.

The milling media 20 of the present invention further possess a refractive index of magnitude substantially similar to that of the filler material to be ground. Because there exists the possibility that the milling media will abrade during the grinding process, thus contaminating the filler material, the present invention negates the harmful optical effect of this contamination by substantially matching the refractive indices of the contaminating media particles and the filler particles. It is the disparity between the refractive indices of the milling media and the filler that causes opacification of the composite material upon dispersion of the filler in a resin matrix and exposure to visible light. Thus, the transparency value (translucency) of the filler/resin mixture may be maintained at a value of at least about 45 by grinding the structural filler according to the principles of the present invention.

The present invention recognizes that a correlation exists between the hardness and refractive index of the milling media and the hardness and refractive index of the filler. The hardness of the milling media must be equal or higher than that of the filler it is grinding, but need not be as hard as that used in the more harsh grinding technique of agitator milling (e.g. YTZ, which has an approximate Vickers hardness of 11 GPa or above). The harder the glass bead milling media, i.e., as the hardness approaches that of zirconia, the less the refractive index of the milling media becomes a significant factor because the milling media becomes more and more resistant to abrasion and spalling. These harder materials, however, tend to be prohibitively expensive. The softer the glass bead milling media, the more likely abrasion and spalling will occur, thus making the refractive index more and more significant for preventing contamination. Furthermore, the softer the media material, the closer the refractive index value must match. By way of further general explanation, and not intended to represent actual values, for a glass material on the softer end of the spectrum, it may be necessary to match the refractive index of the milling media to within .+-.0.02 of the refractive index of the filler. For a glass material in the middle of the hardness spectrum, the refractive index may need only be within .+-.0.04 of the refractive index of the filler for there to be a substantial match for purposes of preventing contamination. Toward the upper end of the hardness spectrum, the refractive indices become irrelevant, because little to no abrasion is occurring. Thus, according to the principles of the present invention, relatively soft and inexpensive glass media may be used in vibratory milling, but as a result, the refractive indices of the glass media and filler must substantially match.

The focus of the present invention, therefore, is on a method of grinding high purity dental fillers in which milling media is used that has a hardness such that abrasion and/or spalling of the milling media may occur during the grinding process. Recognizing this general correlation between hardness and refractive index, one skilled in the art will be capable of determining when the refractive index becomes a significant factor and how closely the refractive indices of the glass milling media and filler must match so as to obtain a translucency value of the filler/resin mixture of at least 45.

To measure the translucency of the filler when dispersed in resin, the following method is used. The dry ground filler is sieved through an 100 mesh plastic screen and then silane treated by spraying in a V-blender a 2.5% by weight hydrolyzed solution of gamma-trimethoxy-methacryloxy-propylsilane to make it hydrophobic. A paste is then prepared by mixing the ground filler with methacrylate resin at a ratio of 70% by weight filler to 30% by weight resin either in a planetary mixer (Charles Ross and Son Company, Hauppage, N.Y.) or a centrifugal force type mixer such as the Speed Mix (Hausschild Engineering, Hamm, Germany). The resultant paste is then deaerated by stirring in a bowl under vacuum for 5 minutes. Cured samples are then prepared by placing in a mold of 20 mm in diameter and 1 mm thickness and curing each side for 60 seconds with a tungsten halogen lamp. The translucency is then measured in the transmission mode using a TCS Colorimeter (BYK-Gardner, Columbia, Md.), by recording the Y value after the sample is placed on the transmission accessory. Using the above procedure, the value obtained for the SP345 barium glass (Specialty Glass, Oldsmar, Fla.) ground to 0.4 microns using the vibratory mill is 54.3 and the corresponding value for the GM27884 barium glass (Schott Glasswerke, Landshut, Germany) ground to 0.4 microns in the vibratory mill is also 54.3. However, when the GM27884 is ground to 0.4 .mu.m in an agitator mill using YTZ milling media, the value is 48.6, which although acceptable is lower than that obtained with the vibratory mill. Thus, the present invention provides a method for efficiently grinding dental filler by an inexpensive technique and with excellent optical results.

Using the same procedure for the translucency measurement of the final fully formulated composite, which typically includes silanated fumned silica, pigments and other additives, a correlation is established that indicates that these values as reported above lead to acceptable translucency for the fully formulated shaded composite. For example, for the A3 shade using barium glasses ground in the vibratory mill, a corresponding value for the formulated restorative of 37 is achieved. Because the purity of the filler is controlled by the grinding method of the present invention, the formulator is able to match the refractive indices of the ground filler and the resin matrix to produce a final restorative composite acceptable for use in cosmetic restorations.

Filler materials useful in dental restorative composites generally have refractive indices in the range of about 1.48 to about 1.55. Thus, the relatively soft glass milling media used to grind the particles according to the principles of the present invention should have a refractive index in the range of about 1.46 to about 1.56, keeping in mind the general correlation described above between hardness and refractive index. In general, the higher the refractive index of the glass milling media, the more likely that abraded particles will produce harmful optical effects.

By way of example and not limitation, for filler materials that are radiopaque, the refractive index is typically about 1.51 to about 1.55. The relatively soft glass milling media used to grind radiopaque fillers should thus have a refractive index of about 1.48 to about 1.56 to eliminate any harmful optical effects produced by introduction of abraded media material into the filler.

For filler materials that are non-radiopaque, the refractive index is typically about 1.48 to about 1.51. The relatively soft glass milling media used to grind non-radiopaque fillers should thus have a refractive index of about 1.46 to about 1.52 to eliminate any harmful optical effects produced by introduction of abraded media material into the filler.

By the milling method of the present invention, abrasion of the milling media may introduce foreign particles into the ground filler particulate, but these foreign particles are non-contaminating because they do not opacify the filler. Thus, the necessary optical qualities of the final restorative material incorporating the ground structural filler are maintained despite foreign particulate from the grinding media.

Although abrasion and spalling of the interior housing or wall 12 of the grinding chamber 18 is less likely in vibratory milling due to the low intensity nature of the grinding, as discussed above, it is still preferable to coat the interior 12 with a polymer lining 16 to prevent any abrasion that might occur. In the case of the agitator mill, as described in Applicant's copending application Ser. No. 09/271,639, entitled "Agitator Mill and Method of Use for Low Contamination Grinding", C. Angeletakis, filed Mar. 17, 1999, now U.S. Pat. No. 6,010,085 and incorporated herein by reference in its entirety, the polymer material for the coating of the grinding chamber is a non-pigmented polymer. Pigment from a pigmented polymer lining has been found to contaminate the structural filler ground in the agitator mill, and thus, the resulting dental composite. Thus, a non-pigmented or clear polymer is applied to the inner surface of the grinding chamber, preferably at a thickness of at least 0.005 inches, to prevent contamination of the structural filler. Although the likelihood of abrasion of the polymer lining is less in the vibratory mill, it is nonetheless preferable to use a non-pigmented or clear polymer lining on the interior of the grinding chamber of the vibratory mill of the present invention.

The vibratory mill of the present invention is especially useful in forming particles with a desired mean particle size between about 0.05 .mu.m and about 0.50 .mu.m to be used as the structural filler in dental restorations. Structural fillers suitable for use in the present invention include barium magnesium aluminosilicate glass, barium aluminoborosilicate glass, amorphous silica; silica-zirconia; silica-titania; silica titania barium oxide, quartz, alumina and other inorganic oxide particles.

To provide ground particles having a mean particle size of less than 0.5 .mu.m, the mill of this invention extensively comminutes the particles. Comminution provided by the mill of this invention deagglomerates the ground particles by separating particles from clusters, decreases the size of the particles, eliminates large particles by breakage and increases the specific surface area of the particles by producing a large quantity of very fine particles. Size reduction with the vibratory mill of this invention occurs due to a combination of: impact with the milling media, abrasion with the milling media and attrition of the particles. The vibratory mill of this invention uses a charge of glass beads as the milling media, which produces highly pure particulate of about 0.05 .mu.m to about 0.5 .mu.m mean particle size in about 40 to about 150 hours at a very low cost. Abrasion or spalling of the relatively soft glass milling media may occur during vibratory milling, which introduces foreign media particulate into the ground filler material. However, because the refractive index of the glass media of the present invention substantially matches the filler material, the transparency of the filler and of the final restorative composite is not affected, and thus the foreign particles are not considered to be harmful contaminants. For dental restorative composites, the optical properties are of paramount importance. Thus, composites containing foreign particles that do not negatively affect the optical properties of the material are, for all practical purposes, contaminant-free.

To prepare a structural filler for inclusion into a dental composite, 3 kg. of the filler material to be milled, such as barium aluminoborosilicate glass (for example, type SP345, Specialty Glass, Oldsmar Fla. having a refractive index of 1.541 or type GM27884, Schott Glasswerke, Landshut, Germany having a refractive index of 1.528), is charged into a 44 liter total capacity vibratory mill (available from Sweco, Florence, Ky., type M18-L) having a clear polyurethane clad grinding chamber and containing 110 lbs. of Dragonite glass beads from Jaygo, Inc. as the milling media. These glass beads have a refractive index of 1.52. Four separate tests were run using different sized glass beads. Test A used 2.0 mm diameter glass beads, Test B used 4.0 mm diameter glass beads, Test C used 6.0 mm diameter glass beads, and Test D used 8.0 mm diameter glass beads. After charging the mill with the glass filler particles, the mill is filled with water up to 10 mm above the surface of the beads. The mill was then vibrated to comminute the particles. During the milling process, rough edges and facets were created on the structural filler particles by the impact with the milling media, abrasion with the milling media and attrition of the particles. Each of these edges provide an adhesion site for the resin which increases the overall strength of the cured composite. These edges are visible in FIGS. 2A and 2B.

When the filler slurry is removed from the mill, the mean particle size is measured, typically by laser scattering. Laser scattering is a method of measuring mean particle size by sensing the average relative angular intensity of scattered light. A beam of monochromatic light with a uniform wave front is directed at the sample, the light is diffracted or scattered by the particles and a detector is used to measure the relative average intensity of the scattered light at various angles. The mean particle size and size distribution may then be calculated from the relative average intensity. One such laser scattering device is disclosed in U.S. Pat. No. 5,610,712 to Schmitz et al., incorporated herein by reference in its entirety. For the present example, a Horiba Model 2A-910 Laser Scattering Mean Particle Size Analyzer was used. The particle size range of the structural fillers prepared by the above-described method is set forth in TABLES 1-4. TABLE 2 shows, for example, that for type SP345 barium aluminum silicate glass ground for 144 hours with 4.0 mm diameter glass beads, 10 percent by volume of the filler particles have a mean particle size of less than 0.26 .mu.m; 50 percent by volume of the filler particles have a mean particle size less than 0.45 .mu.m; and 90 percent by volume of the filler particles have a mean particle size less than 0.73 .mu.m.

As TABLES 1-4 above indicate, average particle sizes of below 0.5 .mu.m can be achieved with a vibratory mill using glass beads as milling media. The difference in grinding times between the SP345 structural filler and the GM27884 structural filler is believed to be attributable to the difference in barium content. SP345 contains 30% barium, while GM27884 contains 25% barium. The lower content of barium makes the GM27884 filler particles harder and more fragile, and therefore easier to grind. With respect to the size of the glass beads, the grinding efficiency increases with increasing bead size going from 2 mm to 6 mm in diameter. This can be attributed to higher momentum during impact playing a dominant role in the grinding mechanism. Essentially, no improvement in grinding efficiency is observed going from 6 mm to 8 mm in bead diameter. The same is true using a 10 mm bead diameter. This may indicate that increased distance between contact points becomes an increasingly important factor compared to the weight of the bead.

As shown in FIGS. 2A and 2B, the SP345 particulate ground in the vibratory mill of the present invention using 4.0 mm glass beads has the desired average particle size of less than about the average wavelength of visible light, and a narrow particle size distribution. As compared to the filler particles formed by the sol-gel processes, a shown in FIGS. 3A and 3B, the vibratory mill produces superior results. Furthermore, the results obtained with the vibratory mill are similar to the results obtained with the agitator mill claimed and described in Applicant's copending application Ser. No. 09/271,639, entitled "Agitator Mill and Method of Use for Low Contamination Grinding", C. Angeletakis, filed on Mar. 17, 1999, now U.S. Pat. No. 6,010,085 and incorporated herein by reference in its entirety, except that the vibratory mill required significantly longer grinding times due to the low energy consumption nature of the vibratory mill. The cost of the glass bead milling media results, however, in low contamination grinding but at a significantly lower cost than the agitator mill using YTZ milling media. The dental composite produced using the particles ground by the vibratory mill of the present invention as a structural filler provide restorations having the high strength useful for load bearing restorations, and also provide good transparency and surface gloss, useful in cosmetic restorations. Various properties of dental composites using the structural filler prepared by the agitator mill of Ser. No. 09/271,639 now U.S. Pat. No. 6,010,085 were measured and reported in U.S. patent application Ser. No. 09/270,999, entitled "Optimum Particle Sized Hybrid Composite", C. Angeletakis, et al., filed on Mar. 17, 1999 now pending and incorporated herein by reference in its entirety. The gloss is apparent even after substantial wear as can be observed in a recall appointment 6 months or longer after the placement of the restoration. The ground particles produced by the vibratory mill and media of the present invention are expected to produce similar results. Through the use of structural filler particles ground using the vibratory mill and media of the present invention and having a average mean particle size less than about the average wavelength of visible light, a dental composite having high surface gloss and transparency with high strength may be formed.

While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant 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, various sized glass beads can be used to grind the particulate to the desired size with variable grinding times but equal results with respect to size and low contamination. The invention, in its broader aspects, is therefore not limited to the specific details, representative composition as shown and described. This has been a description of the present invention, along with the preferred coating composition as currently known. However, the invention itself should only be defined by the appended claims.