Dental restorative composite

The present invention provides a resin-based dental restorative that exhibits low volumetric shrinkage, high filler loading and the high strength required for load bearing restorations, yet maintains a glossy appearance, even after substantial wear. To this end, a dispersant is mixed with a methacrylate resin and a structural filler having a mean particle size between about 0.05 .mu.m and about 0.50 .mu.m. The composite is useful in stress bearing restorations and in cosmetic restorations. The structural filler used is typically ground to a mean particle size of less than 0.5 .mu.m and also includes a microfill having a mean particle size less than 0.05 .mu.m to improve handling and mechanical characteristics. The preferred dental composites maintain their surface finish even after substantial use and also have the strength properties of hybrid composite resins.

FIELD OF THE INVENTION
 This invention relates to resin-based dental restoratives, and more
 specifically to restorative compositions incorporating uniformly dispersed
 submicron sized reinforcing particulate which exhibit high condensability
 and strength, low volumetric shrinkage, improved wear/abrasion resistance
 and improved gloss retention in clinical use.
 BACKGROUND OF THE INVENTION
 In dentistry, practitioners use a variety of restorative materials to
 create crowns, veneers, direct fillings, inlays, onlays and splints.
 Posterior and anterior tooth restoration is typically accomplished by
 excavating a tooth that has decayed or is otherwise in need of repair to
 form a cavity. This cavity is filled with a paste material, which is then
 compacted and shaped to conform to the original contour of the tooth. The
 paste is then hardened, typically by exposure to actinic light. The paste
 material is a tooth colored, packable, light curable, polymerizable
 restorative composition comprising a highly filled material.
 Tooth colored dental restorative composites are usually composed of
 dispersions of glass filler particles below 50 .mu.m in methacrylate-type
 monomer resin. Splintered pre-polymerized particles, which are ground
 suspensions of silica in pre-polymerized dental resins, may also be used.
 Additives such as pigments, initiators and stabilizers have also been used
 in these types of composites. Because the glass particle surface is
 generally hydrophilic, and because it is necessary to make it compatible
 with the resin for mixing, the glass filler is treated with a silane to
 render its surface hydrophobic. The silane-treated filler is then mixed
 with the resin at a proportion (load) to give a paste with a consistency
 considered usable, that is to allow the paste to be shaped without it
 flowing under its own weight during typical use. This paste is then placed
 on the tooth to be restored, shaped and cured to a hardened mass by
 chemical or photochemical initiation of polymerization. After curing, the
 mass has properties close to the structure of a tooth. The restorative
 composites may be dispersion reinforced, particulate reinforced, or hybrid
 composites.
 Dispersion reinforced composites include a reinforcing filler of, for
 example, fumed silica having a mean particle size of about 0.05 .mu.m or
 less, with a filler loading of about 30%-45% by volume. Because of the
 small particle size and high surface area of the filler, the filler
 loading into the resin is limited by the ability of the resin to wet the
 filler. Consequently, the filler loading is limited to about 45% by
 volume. Due to the low loading, the filler particles are not substantially
 in contact with one another. Thus, the primary reinforcing mechanism of
 such dispersion reinforced composites is by dislocation of flaws in the
 matrix around the filler. In dispersion reinforced materials, the strength
 of the resin matrix contributes significantly to the total strength of the
 composite. In dentistry, dispersion reinforced composite resins or
 microfills are typically used for cosmetic restorations due to their
 ability to retain surface luster. Typically, these microfill resins use
 free radical-polymerizable resins such as methacrylate monomers, which,
 after polymerization, are much weaker than the dispersed filler. Despite
 the dispersion reinforcement, microfill resins are structurally weak,
 limiting their use to low stress restorations.
 One example of a dispersion reinforced composite is HELIOMOLAR.RTM., which
 is a dental composite including fumed silica particles on the order of
 0.05 .mu.m mean particle size and rare earth fluoride particle on the
 order of less than 0.2 .mu.m mean particle size. HELIOMOLAR.RTM. is a
 iadiopaque microfill-type composite. The rare earth fluoride particles
 contribute to both flexural strength and radiopacity.
 Particulate reinforced composites typically include a reinforcing filler
 having an average particle size greater than about 0.6 .mu.m and a filler
 loading of about 60% by volume. At these high filler loadings, the filler
 particles begin to contact one another and contribute substantially to the
 reinforcing mechanism due to the interaction of the particles with one
 another and to interruption of flaws by the particles themselves. These
 particulate reinforced composite resins are stronger than microfill
 resins. As with the dispersion reinforced composites, the resin matrix
 typically includes methacrylate monomers. However, the filler in
 particulate reinforced composites has a greater impact on the total
 strength of the composite. Therefore, particulate reinforced composites
 are typically used for stress bearing restorations.
 Another class of dental composites, known as hybrid composites, include the
 features and advantages of dispersion reinforcement and those of
 particulate reinforcement. Hybrid composite resins contain fillers having
 an average particle size of 0.6 .mu.m or greater with a microfiller having
 an average particle size of about 0.05 .mu.m or less. HERCULTLTE.RTM.XRV
 (Kerr Corp.) is one such example. HERCULITE.RTM. is considered by many as
 an industry standard for hybrid composites. It has an average particle
 size of 0.84 .mu.m and a filler loading of 57.5% by volume. The filler is
 produced by a wet milling process that produces fine particles that are
 substantially contaminant free. About 10% of this filler exceeds 1.50
 .mu.m in average particle size. In clinical use, the surface of
 HERCULITE.RTM. turns to a semi-glossy matte finish over time. Because of
 this, the restoration may become distinguishable from normal tooth
 structure when dry, which is not desirable for a cosmetic restoration.
 Another class of composites, flowable composites, have a volume fraction of
 structural filler of about 10% to about 30% by volume. These flowable
 composites are mainly used in low viscosity applications to obtain good
 adaptation and to prevent the formation of gaps during the filling of a
 cavity.
 In U.S. Pat. No. 6,121,344 filed Mar. 17, 1999 and entitled "Optimum
 Particle Sized Hybrid Composite", which is incorporated by reference
 herein in its entirety, it was found that resin-containing dental
 composites that incorporate a main structural filler of ground particles
 of average particle size at or below the wavelength of light (between
 about 0.05 .mu.m to about 0.5 .mu.m) have the high strength required for
 load bearing restorations, yet maintain a glossy appearance in clinical
 use required for cosmetic restorations. Composites containing a main
 structural filler with average particle size of about 1.0 .mu.m or greater
 do not provide a glossy surface.
 Various methods of forming submicron particles, such as precipitation or
 sol gel methods, are available to produce particulate reinforcing fillers
 for hybrid composites. However, these methods do not restrict the particle
 size to at or below the wavelength of light to produce a stable glossy
 surface. U.S. Pat. No. 5,600,67 to Noritake et al., shows an inorganic
 filler composition of 60%-99% by weight of spherical oxide particles
 having a diameter between 0.1-1.0 .mu.m, and 1%-40% by weight of oxide
 particles having a mean particle diameter of less than 0.1 .mu.m. This
 filler is manufactured by a chemical sol gel process. The particle size
 range includes particle sizes up to 1.0 .mu.m and thug a dental composite
 using such filler will not provide a glossy surface in clinical use. The
 particles formed by the sol-gel process are spherical as shown in FIGS. 2A
 and 2B. The formulations described are designed to improve mechanical
 performance, wear and surface roughness of restorations, but do not
 provide for the retention of surface gloss in clinical use. Clinical
 studies of this material have actually shown high wear rates of 22.4 .mu.m
 per year, which cannot establish a stable surface (S. Inokoshi, "Posterior
 Restorations: Ceramics or Composites?" in Transactions Third International
 Congress on Dental Materials Ed. H. Nakajima, Y. Tani JSDMD 1997).
 Comminution by a milling method may also be used for forming the submicron
 particles. 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. 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. 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 0.2 mm and
 about 0.6 mm are also available from Tosoh Ceramics, Bound Brook, N.J.
 Thus, to optimize milling, it is desired to use a milling media
 approximately 1000 times the size of the desired particle. This minimizes
 the time required for milling.
 Previously, the use of a milling process to achieve such fine particle
 sizes was difficult due to contamination of the slurry by the milling
 media. By using yttria stabilized zirconia (YTZ or Y-TZP, where TZP is
 tetragonal zirconia polycrystal), the contamination by spalling from the
 milling media and abrasion from the mill is minimized. Y-TZP has a fine
 grain, high strength and a high fracture toughness. YTZ is the hardest
 ceramic and because of this high hardness, the YTZ will not structurally
 degenerate during milling. 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. The use of Y-TZP provides a suitable milling
 media for providing relatively pure structural fillers having mean
 particle sizes less than 0.5 .mu.m.
 In U.S. Pat. No. 6,010,085 filed Mar. 17, 1999 and entitled "Agitator Mill
 and Method of Use for Low Contamination Grinding", and U.S. Pat. No.
 5,979,805 filed Dec. 4, 1998 and entitled "Vibratory Mill and Method of
 Use for Low Contamination Grinding", both incorporated herein by reference
 in their entirety, there is described an agitator mill and vibratory mill,
 respectively, and method of use designed to grind structural fill to a
 size at or below the wavelength of light with minimal contamination.
 Aside from the need for achieving highly pure structural filler of particle
 size at or below the wavelength of light, an additional factor to be
 considered in developing dental composites is that the coefficient of
 thermal expansion of the glass fillers used in resin-based composites is
 much closer to tooth structure than that of the resins. So it is desirable
 to limit the amount of the resin in a dental composite and maximize the
 amount of filler material. The main factor limiting the volume fraction
 (load) of the inorganic filler in highly filled suspensions is
 particle-particle interactions. Dispersants, through their ability to
 reduce interactions between particles can improve the flow (reduce the
 viscosity) of the suspension, therefore allowing a higher load.
 Dispersants in non-aqueous systems are believed to reduce particle
 interactions by a steric stabilization mechanism. A layer of the
 dispersant is adsorbed on the surface of the particles keeping them apart
 from one another, reducing the viscosity. The dispersant structure must
 contain a chain that allows for steric stabilization in the resin and it
 also must be strongly adsorbed on the particle surface. In U.S. Pat. No.
 6,127,450 filed Jun. 9, 1998, and entitled "Dental Restorative Composite",
 which is incorporated by reference herein in its entirety, the use of
 phosphate-type dispersants is described for increasing the loading in a
 hybrid composite in which the main structural filler has an average
 particle size of about 1.0 .mu.m. There is a need, however, to provide a
 dispersant that will be effective with a non-aqueous, highly filled
 suspension containing a main structural filler having a particle size at
 or below the wavelength of light.
 In summary, the dental profession is in need of a dental restorative that
 has high load capabilities and high strength for load bearing
 restorations, yet maintains a glossy appearance in clinical use required
 for cosmetic restorations.
 SUMMARY OF THE INVENTION
 The present invention provides a resin-containing dental composite
 including a phosphate-based dispersant and structural filler of ground
 particles having an average particle size of between about 0.05 .mu.m and
 about 0.5 .mu.m that has high loading capability and the high strength
 required for load bearing restorations, yet maintains a glossy appearance
 in clinical use required for cosmetic restorations. Further, because the
 structural filler particles are ground, the particles are nonspherical,
 providing increased adhesion of the resin to the structural filler,
 thereby further enhancing the overall strength of the composite. Through
 the use of the phosphate-based dispersant and structural filler particles
 that are ground and that have an average particle size less than the
 wavelength of light, that is less than about 0.50 .mu.m, the dental
 composite of the present invention provides good physical properties and
 the luster and translucency required for cosmetic restorations.
 Specifically, since the structural filler size is less than the wavelength
 of visible light, the surface of a dental restoration will reflect more
 light in some directions than in others even after wear of the composite
 by brushing. The visible light waves do not substantially interact with
 the structural filler particles protruding out of the surface of the
 composite, and therefore, haze is reduced and the luster of the surface is
 maintained even after substantial brushing.
 Known methods of milling, agitator and vibratory milling, have been adapted
 for use in the field of dental composites. As adapted, these methods are
 capable of further reducing the average particle size of the
 HERCULLITE.RTM. filler to an average particle size of between about 0.05
 .mu.m and 0.5 .mu.m. The particle size is at or below the wavelength of
 light, which minimize interaction with light, thus producing a stable
 glossy surface in clinical use. The particles are still large enough to
 reinforce the composite by the particulate reinforcement mechanism, so the
 restorations are also stress bearing. The number of larger particles,
 above 0.5 .mu.m in diameter, are also minimized to help produce the stable
 glossy surface.
 Additionally, because the structural filler particles are ground to an
 average particle size between about 0.05 .mu.m and about 0.50 .mu.m, the
 particles interact with one another to strengthen the composite, in the
 manner of typical hybrid composites, to allow a composite of the present
 invention to be useful in stress bearing restorations.
 In a preferred embodiment, the structural filler is ground, typically by
 agitator or vibratory milling, to the preferred mean particle size. As
 opposed to the particles formed by the known sol-gel process, the grinding
 of the structural filler results in nonspherical particles which due to
 their irregular shape interact with the polymerized resin to a much
 greater extent to increase adhesion of the resin to the structural filler
 and thereby increase the overall strength of the composite.
 Agitator or vibratory milling with selected media and optimized parameters
 produces the required size particles, free of contamination in a narrow
 particle size distribution. This reduces the small percentage of particles
 above 0.5 .mu.m that can contribute to producing a non-glossy surface in
 clinical use.
 In accordance with a further aspect of the invention, microfill particles
 having an average particle size less than about 0.05 .mu.m are added,
 preferably between about 1% by weight and about 15 by weight of the
 composite. The microfill particles contribute to dispersion reinforcement,
 fill the interstices between the larger structural filler particles
 reducing occluded volume, and provide a large surface area to be wetted by
 the resin to increase strength. The microfill particles also contribute to
 the flow properties of the uncured resin.
 Suitable phosphate-based dispersants for use in the present invention
 include phosphoric acid esters according to the formula:
 ##STR1##
 wherein R is a (meth)acrylate group functionalized radical, and wherein n
 represents the number of units of caprolactone.
 DETAILED DESCRIPTION
 The present invention, in a preferred form, is a dental restorative
 composite which includes a curable resin, a dispersant of the phosphoric
 acid ester type, and a ground structural filler having a mean particle
 size between about 0.05 .mu.m and about 0.5 .mu.m. The curable resin is
 preferably a photopolymerizable resin containing methacrylate monomers.
 Such methacrylate monomer resins are cured when exposed to blue visible
 light. The dental composite is applied to teeth by the dental practitioner
 and exposed to a visible light source to cure the resin. The cured resin
 has reduced shrinkage characteristics and a flexural strength higher than
 90 MPa, and preferably greater than 100 MPa, which allows for the use of
 the resin in stress bearing applications.
 To provide ground structural filler having a mean particle size of less
 than 0.5 .mu.m, an extensive comminution step is required. Comminution may
 be performed in an agitator mill or vibratory mill, and more preferably an
 agitator mill or vibratory mill designed to minimize contamination, such
 as that described in U.S. Pat. No. 6,010,085 entitled "Agitator Mill and
 Method of Use for Low Contamination Grinding", C. Angeletakis, filed on
 Mar. 17, 1999 and incorporated herein by reference in its entirety, or
 that described in U.S. Pat. No. 5,979,805, entitled "Vibratory Mill and
 Method of Use for Low Contamination Grinding", C. Angeletakis, filed on
 Dec. 4, 1998 and incorporated herein by reference in its entirety.
 Comminution deagglomerates the structural filler particles by separating
 particles from clusters, decreases the size of the structural filler
 particles, eliminates large particles by breakage and increases the
 specific surface area of the structural filler particles by producing a
 large quantity of very fine particles. Size reduction with an agitator or
 vibratory mill occurs due to a combination of impact with the milling
 media, abrasion with the milling media and attrition of the particles.
 Structural fillers suitable for use in the present invention include barium
 magnesium aluminosilicate glass, barium aluminoborosilicate glass,
 amorphous silica, silica-zirconia, silica-titania, barium oxide, quartz,
 alumina and other inorganic oxide particles.
 Inclusion of a novel dispersant in dental composite formulations of the
 present invention results in increased filler loading and decreased
 viscosity, which after curing provides a dental restorative with reduced
 shrinkage, a lower coefficient of thermal expansion and generally improved
 physical properties. Suitable dispersants useful in the present invention
 are phosphoric acid esters (including mono-, di- and tri-esters).
 Particularly, phosphoric acid esters useful in the present invention
 contains polymerizable groups and are selected from the following: a) a
 phosphoric acid ester containing a carboxylic acid ester group and an
 ether group, and b) a phosphoric acid ester containing a carboxylic acid
 ester group and not containing an ether group. These dispersants are
 effective with nonaqueous, highly-filled suspensions containing
 polymerizable groups (e.g., acrylic and methacrylate esters) used for
 dental purposes and, more particularly, with highly-filled glass
 suspensions containing methacrylate resins. The dispersants useful in the
 present invention preferably comprise 5 weight percent or less of the
 composite paste. To obtain good uniformity of distribution of the
 dispersant in the final composite paste, the dispersant is first mixed
 with the resin, followed by the slow addition of the filler material.
 The dispersant of the present invention is a phosphoric acid ester with the
 following general structure:
 ##STR2##
 where R is a (meth)acrylate group functionalized radical, and wherein n
 represents the number of units of caprolactone.
 The presence of the carboxylic acid ester group of the dispersant results
 in excellent compatibility with (meth)acrylate-baged resin systems. In a
 preferred embodiment, the dispersant of the present invention has the
 structure shown above, wherein R is one of the following:
 Compound 1:R=oxyethyl methacryloyl-
 ##STR3##
 Compound 2:R=oxyethyl acryloyl-
 ##STR4##
 Compound 3:R=polyoxypropyl methacryloyl-
 ##STR5##
 Compound 4:R=glyceryl dimethacryloyl-
 ##STR6##
 Compound 5:R=dipentaerythritol pentaacryloyl-
 ##STR7##
 Compound 6:R=polyoxyethyl methacryloyl-
 ##STR8##
 Each of Compounds 1-6 may be prepared in two steps. In the first step, the
 hydroxy functional methacrylate is condensed with caprolactone under
 ring-opening polymerization conditions in the presence of catalytic
 amounts of SnCl.sub.2 (40-400 ppm) to prepare a polyester. In the second
 step, the polyester is reacted with polyphosphoric acid (117.5%
 concentration) at 65.degree. C. to give the phosphoric acid ester. By way
 of example, the reaction sequence is shown below for the preparation of
 the hydroxyethyl methacrylate (HEMA) derivative Compound 1:
 ##STR9##
 HEMA, MW=130.14 Caprolactone, MW=114.14
 ##STR10##
 Polycaprolactone modified HEMA
 ##STR11##
 Compound 1: Polycaprolactone modified HEMA Phosphate In a further preferred
 embodiment of the present invention, the dispersant is preferably added at
 about 0.5 to about 3.5 weight percent of the composite paste. The
 following examples will further illustrate this aspect of the present
 invention.

EXAMPLE
 In a 4-neck reaction kettle containing an air flow tube, a thermocouple, a
 condenser and a stirrer, 26.0 parts by weight of hydroxyethyl methacrylate
 (HEMA) were combined with 114.1 parts by weight of caprolactone, 0.14
 parts by weight of methyl ether of hydroquinone (MEHQ) and 0.007 parts by
 weight of stannous chlorde under a flow of dry air. The mixture was
 thermostated at 120.degree. C. and stirring was continued for 18 hours.
 The disappearance of the caprolactone was monitored with HPLC (High
 Pressure Liquid Chromatography) using a reverse phase column with 70/30
 acetonitrile/water as eluant. The resultant liquid
 polycaprolactone-modified HEMA was essentially colorless.
 In a three neck flask equipped with a stirrer and a condenser under a
 constant flow of dry air, 70.0 grams of the above product
 (polycaprolactone-modified HEMA) was combined with 8.45 grams of 117.5%
 phosphoric acid. The mixture was heated with stirring for 4 hours at
 70.degree. C. A light yellow oil resulted. Titration with 0.1N NaOH showed
 that the phosphoric acid ester was formed.
 Various methacrylate derivative prepared using the above procedures are
 listed in Table 1.
 TABLE 1
 Polycaprolactone-Modified Methacrylate Monophosphates
 Caprolactone: Molecular
 starting material Weight
 Compound Starting Material (mole ratio) Average
 1a Hydroxyethyl Methacrylate 1:1 324
 (HEMA)
 1b HEMA 2:1 438
 1c HEMA 5:1 780
 1d HEMA 7:1
 2 Hydroxyethyl acrylate 5:1 766
 (HEA)
 3 Polypropylene 5:1 713
 glycomethacrylate
 (PPGMA)
 4a Glycerol Dimethacrylate 2:1 536
 (GDMA)
 4b GDMA 5:1 879
 5a Dipentaerythritol 2:1 713
 pentaacrylate DPEPA)
 5b DPEPA 5:1 1175
 6a Polyethylene glycol 0 459
 monomethacrylate (PEGM)
 6b PEGM 2:1 687
 6c PEGM 5:1 1029
 All of the above compounds may be used as dispersants in highly filled
 glass suspensions containing methacrylate resins. One control sample, two
 test samples and two comparative samples were prepared according the
 following method. A methacrylate resin, as described in Table 2, was
 introduced into a planetary mixer and thermostated to 50.degree. C. It
 should be appreciated that alternative monomers to those listed in Table 2
 may be utilized in the resin composition. For example, diethylene glycol
 dimethacrylate, tetraethylene glycol dimethacrylate, 1,6-hexanediol
 dimethacrylate, 1,12-odecanediol dimethacrylate, diurethane dimethacrylate
 (Rohamere 6661-0, Huls America, Somerset, N.J.), trimethylolpropane
 trimethacrylate, glyceryl dimethacrylate, eopentylglycol dimethacrylate.
 The phosphate ester dispersant with the general structure described above
 was then added to the resin, with the exception of the control sample, so
 as to comprise 1.5 wt. % of the total resin/filler mixture. The test
 samples were prepared with a 74.5 wt. % filler loading; the control sample
 was prepared with a 72 wt. % filler loading; and the comparative samples
 were prepared with an 80 wt. % filler loading. The planetary mixer was
 started for a few minutes to mix the resin phase and then the filler
 containing the physically admixed components listed in Table 3 was slowly
 added over a period of about 3 hours. Mixing was continued for another
 hour and the resultant paste was deaerated under attenuated oxygen
 pressure. Table 3 details the physical properties of the test sample
 pastes prepared along with the properties of control sample 1 and
 comparative samples 1 and 2. All measurements were carried out using
 standard ISO methods except where indicated, and the standard deviations
 are provided in parentheses.
 TABLE 2
 Resin Composition
 BisGMA (Bisphenol A Diglycidyl ether dimethacrylate) 3.0 wt. %
 Triethylene Glycol Dimethacrylate 24.7 wt. %
 Ethoxylated Bisphenol A Dimethacrylate 71.1 wt. %
 2-Ethylhexyl-4-(dimethylamino)benzoate 0.49 wt. %
 Camphorquinone 0.17 wt. %
 2-Hydroxy-4-methoxy Benzophenone 0.49 wt. %
 (BHT) Butylated Hydroxytoluene 0.05 wt. %
 Total 100
 TABLE 3
 Physical Properties of Pastes Prepared
 with Various Dispersants in a Planetary Mixer
 Control Test Test Comparative
 Comparative
 Sample 1 Sample 1 Sample 2 Sample 1 Sample 2
 Dispersant, 1.5 Wt. % None 1c 4b 1c 4b
 Filler Mean Average 0.41 0.41 0.43 1.0 1.0
 Particle Size &gt;50% (.mu.m)
 Filler Mean Average 0.65 0.65 0.75 1.8 1.8
 Particle Size &gt;90% (.mu.m)
 Wt. % 20 nm Hydrophobic 4.0.sup.1 5.0.sup.2 5.0.sup.2 4.0.sup.1
 4.0.sup.1
 fumed silica
 Wt. % 40 nm Fumed Silica, 3.0 4.0 4.0 4.0 4.0
 silanated.sup.3
 Barium Aluminum Silicate, 65.sup.4 65.5.sup.4 65.5.sup.4 72.sup.5
 72.sup.5
 silanated
 Wt. % Filler Load 72 74.5 74.5 80 80
 Depth of Cure at 600 4.0 3.3 3.2 4.6 (0.1) 4.1 (0.3)
 mW/cm.sup.2, 4 mm diameter
 Rockwell Hardness (15 T).sup.6 79.1 81.0 -- 83.4 (0.1)
 83.9 (0.1)
 Compressive Strength 381 (39) 331 (32) 334 (30) 399 (21) 408 (34)
 (MPa)
 Flexural Strength (MPa) 120 (18.6) 100 (16) 139 (12) 129 (12) 125 (26)
 Flexural Modulus (Mpa) 9,521 9,564 11,043 11,189 10,571
 (331) (654) (807) (968) (2,051)
 Penetrometer (mm).sup.7 0 g, 7.5 7.2 5.6 &gt;8.0 6
 (0.2)
 (Needle, 1 mm)
 Penetrometer (mm).sup.8 0 g, 4.3 3.5 2.0 &gt;8.0 4.3
 (0.1)
 (Flathead, 1 mm)
 Slump (cm) 400 g, 30 s 2.6 2.6 2.4 -- --
 .sup.1 TS 530, available from Degussa Corp., Ridgefield Park, N.J.
 .sup.2 US 202, available from Degussa Corp., Ridgefield Park, N.J.
 .sup.3 OX-50, available from Degussa Corp., Ridgefield Park, N.J.
 .sup.4 GM27884 Raw glass (25% barium content) available from Schott
 Glasswerke, Landshut, Germany.
 .sup.5 SP345 Raw glass (30% barium content) available from Specialty Glass,
 Inc., Oldsmar, FL.
 .sup.6 Average of 3 measurements on the surface of a cylindrical sample 10
 mm in diameter and 4 mm in height. The samples were light cured for 40
 seconds, and stored in water for 24 hours at 37.degree. C. prior to
 measurement.
 .sup.7 Precision Penetrometer (GCA Corp., Chicago, IL) with a 1 mm needle
 was used with no additional weight (0 g). The paste was placed in a mold
 10 mm in diameter and 8 mm in height. Penetration was performed for 10
 seconds. An average of 3 measurements is reported.
 .sup.8 Same test as above, but using a flat head rather than a needle, to
 simulate the effect of the impact from dental instruments having a flat
 head on the composite.
 Surprisingly, it has been found that the behavior of dispersants differs
 markedly with the size of the main structural filler. In the comparative
 samples, the physical properties of the composites are essentially
 equivalent, with the main difference being the viscosity, as indicated by
 the penetrometer test. As Table 3 shows, however, dispersant 4b is
 significantly more effective in producing a paste with superior physical
 properties at high loads with a 0.4 .mu.m sized filler than the 1c
 dispersant. Comparison of the penetrometer results show that although the
 1c dispersant is more effective in reducing the viscosity of both size
 fillers, the difference is small in the case of the 0.4 .mu.m fillers. The
 paste containing the 4b dispersant with the 0.4 .mu.m filler, however,
 exhibits a substantially higher flexural strength and modulus. This
 difference is expected to result in better performance when the material
 is placed in vivo.
 To further demonstrate the effects of various dispersants on the viscosity
 of a paste comprising a 0.4 .mu.m filler system and on the final
 properties of the cured composite, seven dispersants prepared as described
 above were added in an amount of 1.5 wt. % to a paste prepared as
 described above comprising the components listed in Table 4, except mixing
 was performed for 60 seconds with a centrifugal type mixer, such as a
 Speed Mix type AM501T, available from Hauschild Engineering, Hamm,
 Germany. The mixing is achieved by applying two centrifugal forces, one in
 the center of the container, and one in the opposite direction a distance
 away from the container.
 TABLE 4
 0.4 .mu.m Filler/Resin Paste Composition at 75% Filler Loading
 0.4 .mu.m Barium Aluminum Silicate Glass,.sup.1 silanated 66 wt. %
 OX-50 Fumed Silica, silanated (40 nm).sup.2 4.0 wt. %
 US202 Hydrophobic Fumed Silica (20 nm).sup.2 5.0 wt. %
 Resin (Table 2) 23.5 wt. %
 Dispersant 1.5 wt. %
 Total 100
 .sup.1 Mixture of 60 wt. % SP345 (Specialty Glass, Inc.) and 40 wt. %
 GM27884 (Schott Glasswerke).
 .sup.2 Mean average particle size.
 Two comparative samples of a dispersant in a 1.0 .mu.m filler system were
 also prepared in the same manner as the test samples, comprising the
 components listed in Table 5.
 TABLE 5
 1.0 .mu.m Filler/Resin Paste Composition at 80% Filler Loading
 1.0 .mu.m Barium Aluminum Silicate Glass (SP345), silanated 72.4 wt. %
 OX-50 Fumed Silica, silanated (40 nm).sup.1 3.6 wt. %
 US202 Hydrophobic Fumed Silica (20 nm).sup.1 4.0 wt. %
 Resin (Table 2) 18.5 wt. %
 Dispersant 1.5 wt. %
 Total 100
 .sup.1 Mean average particle size.
 The properties of the pastes and cured composites for comparative samples
 3-4 and test samples 3-9 are provided in Table 6.
 TABLE 6
 Physical Properties of Pastes Prepared with Various Dispersants in
 a Centrifugal Mixer
 Comparative Comparative Test Test Test Test
 Test Test Test
 Sample Sample Sample Sample Sample
 Sample Sample Sample Sample
 3 4 3 4 5 6
 7 8 9
 Dispersant, 1.5 Wt. % 1c 4b 1c 2 4b
 5b 6a 6b 6c
 Wt. % Filler Load 80 80 75 75 75 75
 75 75 75
 Vickers Hardness 571 591 478 512 484 544
 -- -- 480
 (N/mm.sup.2).sup.1 (9) (3) (5) (7) (20)
 (20) (28)
 Flexural Strength 121 135 127 130 139 93
 110 -- 119
 (MPa) (27) (18) (7) (23) (12) (15)
 (21) (15)
 Flexural Modulus 12,897 13,597 10,837 10,070 10,720
 10,730 11,134 -- 10,034
 (MPa) (408) (549) (640) (560) (389)
 (670) (560) (850)
 Penetrometer (mm).sup.2 -- 4.2 (0.1) -- 7.2 &gt;8.0
 -- 2.7 (0.1) 3.3 (0.4) 7.5 (0.3)
 0 g, (Needle, 1 mm)
 Penetrometer (mm).sup.3 &gt;8 1.0 (0) &gt;8 3.5 4.7 (0.6)
 3.6 (0.6) 1.0 (0.5) 0.7 (0.1) 2.4 (0.2)
 0 g, (Flathead, 1 mm)
 .sup.1 Average of 3 measurements on the surface of a cylindrical sample 10
 mm in diameter and 2 mm in height. The samples were light cured for 60
 seconds, and stored in water for 24 hours at 37.degree. C. prior to
 measurement.
 .sup.2 Precision Penetrometer (GCA Corp., Chicago, IL) with a 1 mm needle
 was used with no additional weight (0 g). The paste was placed in a mold
 10 mm in diameter and 8 mm in height. Penetration was performed for 10
 seconds. An average of 3 measurements is reported.
 .sup.3 Same test as above, but using a flat head rather than a needle, to
 simulate the effect of the impact from dental instruments having a flat
 head on the composite.
 Table 6 demonstrates that dispersant 4b provides an overall best physical
 profile when compared to the other dispersants listed when incorporated
 into a 0.4 .mu.m filler system. The penetrometer data for the Compound 6
 derivatives (Samples 7-9) suggest that increasing chain length of the
 caprolactone units improves the dispersant effect. When compared to the
 use of the dispersants in a 1.0 .mu.m system, the 1c and 4b dispersants
 provided similar results in both filler systems. It should be noted,
 however, that a centrifugal type mixer was used to prepare the samples
 present in Table 6. The centrifugal mixer, by design, applies less shear
 to the components of the mixture than does a planetary mixer. As a result,
 the centrifugal type mixer does not fully mix the components, nor does it
 effectively break large agglomerates of filler particles. This is believed
 to decrease the effectiveness of the dispersants, and the filler
 components are not as effectively dispersed as they are in the planetary
 mixer. Insufficient dispersion of the filler by the mixer is expected to
 lead to decreased effectiveness of the dispersant. Thus, the results of
 Table 6 are believed to be less indicative of the effectiveness of the
 dispersants in a 0.4 .mu.m filler system as compared to the results
 presented in Table 3.
 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 quantity of the dispersant to be added to the resin/filler
 mixture will vary based on the particular compositions used for the resin
 and the filler. The invention in its broader aspects is therefore not
 limited to the specific details, representative method and illustrative
 examples shown and described. Accordingly, departures may be made from
 such details without departing from the scope or spirit of applicant's
 general inventive concept.