Patent Publication Number: US-2005124762-A1

Title: Dental compositions containing core-shell polymers with low modulus cores

Description:
FIELD OF THE INVENTION  
      This invention relates to composite materials for restorative dentistry. More particularly, it relates to a dental composite material that combines reduced shrinkage with sufficiently low viscosity, high polymerization rate, and good mechanical properties.  
     BACKGROUND OF THE INVENTION  
      In recent years, composite materials comprising highly filled polymer have become commonly used for dental restorations. A thorough summary of current dental composite materials is provided in N. Moszner and U. Salz,  Prog. Polym. Sci.  26:535-576 (2001). Currently used dental filling composites contain crosslinking acrylates or methacrylates, inorganic fillers such as glass or quartz, and a photoinitiator system, enabling them to be cured by radiation with visible light. Typical methacrylate materials include 2,2′-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (“Bis-GMA”); ethoxylated Bis-GMA (“EBPDMA”); 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (“UDMA”); dodecanediol dimethacrylate (“D 3 MA”); and triethyleneglycol dimethacrylate (“TEGDMA”).  
      Dental composite materials offer a distinct cosmetic advantage over traditional metal amalgam. However, they do not offer the longevity of amalgam in dental fillings. The primary reasons for failure are believed to be excessive shrinkage during photopolymerization in the tooth cavity, which causes leakage and bacterial reentry, and inadequate strength and toughness.  
      The incumbent low-shrink monomer, Bis-GMA, the condensation product of bisphenol A and glycidyl methacrylate, is an especially important monomer in dental composites. However, it is highly viscous at room temperature and consequently insufficiently converted to polymer. It is therefore typically diluted with a less viscous acrylate or methacrylate monomer, such as trimethylol propyl trimethacrylate, 1,6-hexanediol dimethacrylate, 1,3-butanediol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, TEGDMA, or tetraethylene glycol dimethacrylate. However, while providing fluidity, low molecular weight monomers contribute to increased shrinkage. Increasingly, Bis-GMA and TEGDMA have been combined with UDMA and ethoxylated-methacrylated versions of bisphenol A, but shrinkage remains too high.  
      Increasing the amount of inorganic filler in the composite has moderated shrinkage. However, the amount of filler that can be added is severely limited by the resulting increase in viscosity. Also, it has been reported that the increase in modulus more than offsets this benefit and can lead to an increased build-up of stress during shrinkage. This “contraction stress” is of great importance in that it can lead to mechanical failure and debonding of the composite from the tooth, creating a gap that can permit microleakage of oral fluid and bacteria, causing a reinfection.  
      Another approach has been to prepolymerize the monomer, reducing the ultimate degree of polymerization and attendant shrinkage. However, this reduces the amount of inorganic filler that can be added below current levels, thus decreasing the modulus and other mechanical properties.  
      Spiro-type, “expanding” monomers, introduced in the 1970s, eliminate shrinkage, but they have never been commercialized because they polymerize too slowly and they, or their polymerization products, are too unstable. Diepoxide monomers are similarly limited in that they polymerize slowly for practical application, and the monomers and photosensitizers may be too toxic. They do not entirely eliminate shrinkage.  
      Slow cure and the so-called “soft start” photocure are also reported to reduce contraction stress.  
      Other systems have been reported in the literature but are not commercial. Liquid crystalline di(meth)acrylates shrink far less, but there is a tradeoff in mechanical properties. Branched polymethacrylates and so-called “macromonomers” offer lower viscosity at reduced shrinkage, but cost of manufacture may be excessive.  
      U.S. Pat. No. 5,182,332 issued to Yamamoto et al. on Jan. 26, 1993, discloses a dental composition comprising grafted rubber, wherein the grafted rubber comprises a core consisting of polybutadiene, polystyrene, or a copolymer of styrene or methyl methacrylate with butyl acrylate, and an outer shell consisting of acrylate rubber.  
      U.S. Pat. No. 5,210,109 issued to Tateosian et al. on May 11, 1993, discloses hardenable compositions comprising rubber-modified polymer, wherein the rubber-modified polymer comprises elastomeric compounds overpolymerized with monomeric species.  
      Published German Application DE1 9617876 discloses the use of polysiloxane elastomers as an impact strength modifier in dental composites. The polysiloxane elastomers can be used as the core of a grafted rubber.  
      Japanese Patent JP4275204 discloses core-shell particles comprising rubber cores and glass polymer shells. The core-shell toughening technology is useful in compositions for denture bases.  
      There remains a need for a dental composite material that combines reduced shrinkage with sufficiently low viscosity, high polymerization rate, and acceptable mechanical properties.  
     SUMMARY OF THE INVENTION  
      The present invention provides a dental composite material comprising at least one (meth)acrylic ester compound, at least one polymerization initiator, at least one inorganic filler, and at least one core-shell polymer compound. The invention also provides a method of producing a dental restoration article using at least one (meth)acrylic ester compound, at least one polymerization initiator, at least one inorganic filler, and at least one core-shell polymer compound. The invention further provides a method of treating dental tissue with a direct composite, comprising the steps of: 
          (a) placing a composite material, as described herein, on a dental tissue;     (b) curing the composite material; and     (c) shaping the composite material.       

    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Applicants specifically incorporate the entire content of all cited references in this disclosure. Applicants also incorporate by reference the co-owned and concurrently filed applications entitled “Dental Compositions Containing Liquid and Other Elastomers” (Attorney Docket # CL 2368), “Branched Highly-Functional Monomers Exhibiting Low Polymerization Shrinkage” (Attorney Docket # CL 2452), and “Bulky Monomers Leading to Resins Exhibiting Low Polymerization Shrinkage” (Attorney Docket # CL 2428).  
      In the context of this disclosure, a number of terms shall be utilized.  
      The terms “(meth)acrylic” and “(meth)acrylate” as used herein denote “methacrylic or acrylic” and “methacrylate or acrylate” respectively.  
      The term “dental composite material” as used herein denotes a composition that can be used to remedy natural or induced imperfections of, and relating to, teeth. Examples include filling materials, reconstructive materials, restorative materials, crown and bridge materials, inlays, onlays, laminate veneers, dental adhesives, teeth, facings, pit and fissure sealants, cements, denture base and denture reline materials, orthodontic splint materials, and adhesives for orthodontic appliances.  
      The term “core-shell” as used herein denotes a compound comprising a soft core comprising a rubber or elastomeric polymer, terms used interchangeably herein, surrounded by a shell comprising a rigid polymer. Core-shell polymers of the type used in the present invention are generally described in “Core-Shell Impact Modifiers” [Carlos A. Cruz-Ramos,  Polymer Blends, Vol.  2:  Performance,  137-75 (D. R. Paul &amp; C. B. Bucknall eds., 2000)]. The core comprises, for example, a cross-linked rubber derived from polybutyl acrylate, polybutadiene, and/or polystyrene-co-butadiene. The shell comprises a rigid polymer, preferably with a glass transition temperature (“T g ”) that is much higher than that of the core, that is chemically grafted onto the core. The shell comprises, for example, polymethyl methacrylate or its copolymer with styrene, optionally containing functional groups such as epoxy, carboxylic acid, or amine.  
      A core-shell polymer may also be made up of multiple layers, prepared by a mult-stage, sequential polymerization technique of the type described in U.S. Pat. No. 4,180,529 issued to Hofmann on Dec. 25, 1979. Each successive stage is polymerized in the presence of the previously-polymerized stages. Thus, each layer is polymerized as a layer on top of the immediately preceding stage. In one embodiment, the first stage of the polymerization produces a nonelastomeric polymer, the second stage produces an elastomeric polymer, and the third stage produces a nonelastomeric polymer. In another embodiment, the first stage of the polymerization produces an elastomeric polymer, the second stage produces a nonelastomeric polymer, the third stage produces an elastomeric polymer, and the fourth stage produces a nonelastomeric polymer. In other embodiments, this process may be extended to five or more stages.  
      Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.  
      The (meth)acrylic ester compound used in the present invention can comprise either a monofunctional compound or a polyfunctional compound which means a compound having one (meth)acrylic group and a compound having more than one (meth)acrylic group respectively. Specific examples of monofunctional (meth)acrylic ester compounds include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, hydroxyethyl (meth)acrylate, benzyl (meth)acrylate, methoxyethyl (meth)acrylate, glycidyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, and methacryloyloxyethyltrimellitic mono ester and its anhydride.  
      Specific examples of polyfunctional (meth)acrylic ester compounds include di(meth)acrylates of ethylene glycol derivatives as represented by the general formula  
                 
 
 wherein R is hydrogen or methyl and n is an integer in a range of from 1 to 20, such as ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and polyethylene glycol di(meth )acrylate; 1,3-butanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, dodecanediol dimethacrylate, glycerol di(meth)acrylate, bisphenol A di(meth)acrylate, bisphenol A diglycidyl di(meth)acrylate and ethoxylated bisphenol A diglycidyl di(meth)acrylate; urethane di(meth)acrylates; trimethylolpropane tri(meth)acrylate; tetrafunctional urethane tetra(meth)acrylates; pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, and hexa(meth)acrylates of urethanes having an isocyanuric acid skeleton. 
 
      These (meth)acrylic ester compounds may be used alone or in admixture of two or more. The mixtures can be mixtures of monofunctionals, polyfunctionals, or both.  
      The (meth)acrylic ester compound used in the dental compositions preferably comprises at least one polyfunctional (meth)acrylic ester compound, and more preferably comprises at least two polyfunctional (meth)acrylic ester compounds.  
      Surprisingly, it has been found that a core-shell polymer compound dispersed in the organic phase, comprising the (meth)acrylic ester compound, of a dental composite dissipates shrinkage stress through cavitation of the core-shell polymer&#39;s low-modulus core. By “low-modulus” is meant a modulus of elasticity at 100% elongation, M 100 , below about 2,000 psi, preferably below about 1,000 psi, and more preferably below about 500 psi.  
      This core-shell polymer compound comprising a low-modulus core also offers the potential to enhance the fracture toughness of the composites in a manner similar to that of the soft, rubbery phases that toughen thermoplastics.  
      The core-shell polymer compound of the present invention comprises a low-modulus interior, which can cavitate, and a plastic shell that aids in uniform dispersion and preserves small particle size by preventing coalescence of the low modulus interior. When the core-shell polymer compound is added to the aforementioned (meth)acrylate monomers, polymerization shrinkage is significantly reduced. Additionally, the core-shell polymer compounds can improve toughness and other mechanical properties.  
      Core-shell polymer compounds can be prepared by any of the methods known to one of ordinary skill in the art, for example, the methods described in U.S. Pat. No. 3,808,180 issued to Owen on Apr. 30, 1974, and U.S. Pat. No. 4,180,529 issued to Hofmann on Dec. 25, 1979. For example, a core-shell polymer compound can be prepared by emulsion polymerization in water or other suitable fluid medium with suitable initiators, first feeding in and polymerizing a monomer or set of monomers from the core. Feeding in a second monomer or set of monomers then forms the shell. Though the predominant monomers contain a single polymerizable vinyl group, smaller quantities of monomers with multiple vinyl groups may optionally be used for preparation of core and/or shell. Core-shell polymer compounds can be obtained commercially, for example, Paraloid® EXL-2330 (with a core prepared from butyl acrylate monomer), Paraloid® EXL-2314 (epoxy functional polymer, with a core prepared from butyl acrylate monomer), Paraloid® EXL-2691 (methacrylate-butadiene-styrene), and Paraloid® KM-365 (with a core prepared from butyl acrylate monomer) from Rohm and Haas (Philadelphia, Pa.).  
      Emulsion polymerization produces core-shell polymer compounds of a well-defined size with a narrow size distribution, wherein each core-shell polymer compound is individually polymerized and cross-linked during the synthesis process. The core-shell polymer compounds retain their well-defined size and narrow size distribution even after dispersion in the organic phase, although they may be somewhat swollen by absorption of a portion of the (meth)acrylic ester compound(s).  
      The low-modulus interior of the core-shell polymer compound comprises a rubber or elastomer added to the aforementioned (meth)acrylate monomers in order reduce polymerization shrinkage. By “drubber or elastomer” is meant a compound with T g  of less than about 20° C. Preferably the T g  of the rubber or elastomer is less than about 0° C., and more preferably the T g  is less than about −20° C. Furthermore, the low-modulus interior is substantially noncrystalline. By “substantially noncrystalline” is meant less than about 10% of the low-modulus interior is crystalline.  
      The low-modulus interior of the core-shell polymer compound can comprise any elastomeric polymer except polysiloxane, for example, polybutyl acrylate, polybutadiene, and polystyrene-co-butadiene. Elastomeric polymers are optionally cross-linked.  
      The shell of the core-shell polymer compound has two primary functions. First, the shell prevents the low-modulus interior of each core-shell polymer compound from adhering to the low-modulus interior of other core-shell polymer compounds. Second, when the core-shell polymer compound is dispersed in the organic phase, the shell physically binds the (meth)acrylic esters of the organic phase to the low-modulus interior of the core-shell polymer compound.  
      The shells of the core-shell polymer compound can comprise any non-elastomeric polymer, for example, polymethyl methacrylate or its copolymer with styrene, optionally containing functional groups such as epoxy, carboxylic acid, or amine. The non-elastomeric polymers are generally prepared from olefinic monomers by free radical polymerization.  
      In a preferred embodiment, the core of the core-shell polymer comprises about 50 to about 90 weight percent of the core-shell polymer, the percentages being based on the total weight of the core-shell polymer.  
      The core-shell polymer compound can be used in the range of at least 10 weight percent to about 30 weight percent and preferably in the range of about 12 weight percent to about 20 weight percent, the percentages being based on the total weight exclusive of filler.  
      In another embodiment, the core-shell polymer compound can be combined with any of the compounds disclosed in the aforementioned co-owned and concurrently filed applications. In fact, any combination of the disclosed compounds in the aforementioned co-owned and concurrently filed applications, including combinations where core-shell polymer compound is not present, can be used successfully together to produce a composite material ideal for dental composites that exhibit reduced polymerization shrinkage yet maintain good mechanical properties.  
      The elastomeric compound of co-owned and concurrently filed “Dental Compositions Containing Liquid and Other Elastomers” comprises a liquid rubber or other elastomer, terms used interchangeably herein, added to the aforementioned (meth)acrylate monomers in order reduce polymerization shrinkage. Additionally, the elastomeric compounds can improve toughness and other mechanical properties. By “elastomeric compound” is meant a compound with a glass transition temperature (T g ) of less than about 20° C. and a melt index of at least about 100 g/10 min. at 190° C. Preferably the T g  of the elastomeric compound is less than about 0° C., and more preferably the T g  is less than about −20-° C. Furthermore, the elastomeric compound is substantially noncrystalline. By “substantially noncrystalline” is meant less than about 10% of the elastomeric compound is crystalline. It is essential that the elastomeric compounds of the invention are polysiloxane-free.  
      The term “liquid rubber” as used herein denotes a substantially noncrystalline polymer with a T g  less than about 20° C. and a molecular weight low enough so that the compound flows at room temperature, that is the compound is pourable. Preferably, the liquid rubber has a viscosity of less than about 2,000 Pa.s.  
      Preferred elastomeric compounds have a molecular weight less than about 10,000 and more preferably less than about 5,000.  
      Preferable elastomeric compounds include liquid poly(butadiene-co-acrylonitrile), liquid polybutadiene, liquid hydrogenated polybutadiene diol, ethylene-(meth)acrylic ester copolymers, poly(meth)acrylate ester elastomers, polychloroprene copolymers, hydrogenated poly(butadiene-co-acrylonitrile), polyepichlorohydrin, polysulfides, chlorinated polyethylene, chlorosulfonated polyethylene, fluoroelastomers, polyethylene plastomers, ethylene/propylene copolymers, and polystyrene-co-butadiene.  
      The elastomeric compound can be used in the range of about 2 weight percent to about 30 weight percent, preferably in the range of about 5 weight percent to about 25 weight percent, and more preferably in the range of about 10 to about 20 weight percent, the percentages being based on the total weight exclusive of filler.  
      The branched, low-viscosity, high-equivalent weight polymerizable monomers of co-owned and concurrently filed “Branched Highly-Functional Monomers Exhibiting Low Polymerization Shrinkage” are based on polyalkoxylates of polyfunctional nucleus molecules, for example, trimethylolpropane or pentaerythritol. Preferably, branched, low-viscosity, high-equivalent weight polymerizable monomers are derived from pentaerythritol. The branched, low-viscosity, high-equivalent weight polymerizable monomers can be compounded into composites that, upon polymerization, have low viscosity and exhibit low volumetric shrinkage and good mechanical properties.  
      Preferred derivatives of pentaerythritol are polyalkoxylated wherein the average number of alkoxylate units per pentaerythritol hydroxy group is in a range of from 1 to 10. Polyalkoxylate derivatives of pentaerythritol are esterified to form (meth)acrylate monomers, for example, by reaction with (meth)acrylic anhydride.  
      In one embodiment, the pentaerythritol derivatives are completely (meth)acrylated. In another embodiment, pentaerythritol derivatives are partially (meth)acryated. Alternatively, branched, low-viscosity, high-equivalent weight polymerizable monomers can be partially (meth)acrylated and partially modified with terminal alkyl carbonyl; alkyl carbonyl, wherein the alkyl carbonyl is substituted with at least one alkyl; cycloalkyl carbonyl; or aralkyl carbonyl chains.  
      In dental composite materials, branched, low-viscosity, high-equivalent weight polymerizable monomers can be used in the range of about 1 weight percent to about 100 weight percent, preferably in the range of about 20 weight percent to about 80 weight percent, and more preferably in the range of about 40 weight percent to about 60 weight percent, the percentages being based on the total weight exclusive of filler.  
      The space-filling compound of co-owned and concurrently filed “Bulky Monomers Leading to Resins Exhibiting Low Polymerization Shrinkage” is a monomer comprising a rigid, angular, bulky moiety that can be compounded into composites, which upon polymerization exhibit low volumetric shrinkage. By “space-filling compound” is meant a monomer comprising a moiety with an inability of a significant fraction of its constituent atoms to be place in a common plane. By “significant fraction” is meant greater than about 15%. Additionally, the constituent atoms have a relative lack of mobility with respect to one another; that is, the moiety&#39;s structure is highly rigid and preferably has less than two freely rotating internal bonds.  
      The space-filling compounds comprise, for example, derivatives of at least one of the moieties spirobisindanediol (“SBID”), phenylindane dicarboxylic acid (“PIDA”), t-butylisophthalic acid (“BIPA”), cyclohexyldiphenol, fluorenyl bisphenol A, tetrahydrodicyclopentadiol, phenol-alkyl levulinate, and isosorbide.  
      Preferably, the space-filling compound is functionally terminated with 2-(carbonylamino)ethyl acrylate; 2-(carbonylamino)ethyl methacrylate; 2-(2-ethoxycarbonylamino)ethyl acrylate; 2-(2-ethoxycarbonylamino)ethyl methacrylate; 2-[1-(2-propoxy)carbonylamino]ethyl acrylate; 2-[1-(2-propoxy)carbonylamino]ethyl methacrylate; 2-[2-(2-ethoxy)ethoxycarbonylamino]ethyl acrylate; 2-[2-(2-ethoxy)ethoxycarbonylamino]ethyl methacrylate; 2-(omega-polyoxyethylenecarbonylamino)ethyl acrylate; 2-(omega-polyoxyethylenecarbonylamino)ethyl methacrylate; 2-(omega-polyoxypropylenecarbonylamino)ethyl acrylate; 2-(omega-polyoxypropylenecarbonylamino)ethyl methacrylate; 2-(2-ethoxycarbonylamino)ethyl acrylate; 2-(2-ethoxycarbonylamino)ethyl methacrylate; 2-[1-(2-propoxy)carbonylamino]ethyl acrylate; 2-[1-(2-propoxy)carbonylamino]ethyl methacrylate; 2-[3-(2,2-dimethylpropoxy)carbonylamino]ethyl acrylate; or 2-[3-(2,2-dimethylpropoxy)carbonylamino]ethyl methacrylate.  
      In dental composite materials, space-filling compounds can be used in the range of about 1 weight percent to about 100 weight percent, preferably in the range of about 20 weight percent to about 80 weight percent, and more preferably in the range of about 40 weight percent to about 60 weight percent, the percentages being based on the total weight exclusive of filler.  
      When combined with any of the compounds disclosed in the aforementioned co-owned and concurrently filed applications, the core-shell polymer compound can be used in the range of about 1 weight percent to about 30 weight percent and preferably in the range of about 12 weight percent to about 20 weight percent, the percentages being based on the total weight exclusive of filler.  
      The production of the crosslinked polymers useful in the practice of this invention from monomers and crosslinking agents may be performed by any of the many processes known to those skilled in the art. Thus, the polymers may be formed by heating a mixture of the components to a temperature sufficient to cause polymerization. For this purpose, peroxy-type initiators such as benzoyl peroxide, dicumyl peroxide, lauryl peroxide, tributyl hydroperoxide, and other materials familiar to those skilled in the art may be employed, and the use of activators may be advantageous in some formulations. Suitable activators include, for example, N,N-bis-(hydroxyalkyl)-3,5-xylidines, N,N-bis-(hydroxyalkyl)-3,5-di-t-butylanilines, barbituric acids and their derivatives, and malonyl sulfamides, including specific examples of these activators found in published U.S. patent application Ser. No. 2003/0008967. Azo-type initiators such as 2,2′-azobis(isobutyronitrile), 2,2′-azobis(2,4-dimethyl valeronitrile), 2,2′-azobis(2-methyl butane nitrile), and 4,4′-azobis(4-cyanovaleric acid) may also be used. Alternatively, the crosslinked polymers of the invention may be formed from the constituents by photochemical or radiant initiation utilizing light or high energy radiation. For photochemical initiation, photochemical sensitizers, or energy transfer compounds may be employed to enhance the overall polymerization efficiency in manners well known to those skilled in the art.  
      Suitable photoinitiators include, for example, camphor quinone, benzoin ethers, α-hydroxyalkylphenones, acylphosphine oxides, α,α-dialoxyacetophenones, α-aminoalkylphenones, acyl phosphine sulfides, bis acyl phosphine oxides, phenylglyoxylates, benzophenones, thioxanthones, metallocenes, bisimidazoles, and α-diketones.  
      Photoinitiating accelerators may also be present. Such photoinitiating accelerators include, for example, ethyl dimethylaminobenzoate, dimethylaminoethyl methacrylate, dimethyl-p-toluidine, and dihydroxyethyl-p-toluidine.  
      According to another aspect, an inorganic filler is included in the composite. Included in the inorganic fillers are the preferred silicious fillers. More preferred are the inorganic glasses. Among these preferred inorganic fillers are barium aluminum silicate, lithium aluminum silicate, strontium fluoride, lanthanum oxide, zirconium oxide, bismuth phosphate, calcium tungstate, barium tungstate, bismuth oxide, tantalum aluminosilicate glasses, and related materials. Glass beads, silica, especially in submicron sizes, quartz, borosilicates, alumina, alumina silicates, and other fillers may also be employed. For example, Aerosil® OX-50 fumed silica from Degussa can be used. Mixtures of fillers may also be employed. The average diameter of the inorganic fillers is preferably less than 15 μm, even more preferably less than 10 μm.  
      Such fillers may be silanated prior to use in this invention. Silanation is well known to those skilled in the art and any silanating compound known to them may be used for this purpose. By “silanation” is meant that some of the silanol groups have been substituted or reacted with, for example, dimethyidichlorosilane to form a hydrophobic filler. The particles are typically from about 50 to about 95 percent silanated. Silanating agents for inorganic fillers include, for example, γ-mercaptoproyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-aminopropyltriethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and γ-methacryloyloxypropyltriethoxysilane.  
      The at least one (meth)acrylic ester compound or its combinations with branched, low-viscosity, high-equivalent weight polymerizable monomers and/or space-filling compounds can be used in the range of about 70 weight percent to about 90 weight percent, preferably in the range of about 80 weight percent to about 88 weight percent, the percentages being based on the total weight exclusive of filler.  
      The polymerization initiator with, optionally, a photoinitiating accelerator can be used in the range of about 0.1 weight percent to about 5 weight percent, preferably in the range of about 0.2 weight percent to about 3 weight percent, and more preferably in the range of about 0.2 weight percent to about 2 weight percent, the percentages being based on the total weight exclusive of filler.  
      The inorganic filler can be used in the range of about 20 weight percent to about 90 weight percent, preferably in the range of about 40 weight percent to about 90 weight percent, and more preferably in the range of about 50 weight percent to about 85 weight percent, the percentages being based on the total weight of the (meth)acrylic ester compound, the polymerization initiator, the inorganic filler, and the core-shell polymer compound.  
      In addition to the components described above, the blend may contain additional, optional ingredients. These may comprise activators, pigments, radiopaquing agents, stabilizers, antioxidants, and other materials as will occur to those skilled in the art.  
      Suitable pigments include, for example, inorganic oxides such as titanium dioxide, micronized titanium dioxide, and iron oxides; carbon black; azo pigments; phthalocyanine pigments; quinacridone pigments; and pyrrolopyrrol pigments.  
      Preferred radiopaquing agents include, for example, ytterbium trifluoride, yttrium trifluoride, barium sulfate, bismuth subcarbonate, bismuth trioxide, bismuth oxichloride, and tungsten.  
      Preferred stabilizers include, for example, hydroquinone, hydroquinone monomethyl ether, 4-tert-butylcatechol, and 2,6-di-tert-butyl-4-methylphenol.  
      Primary antioxidants, secondary antioxidants, and thioester-type antioxidants are all suitable for use in the dental compositions of the invention. Preferred primary antioxidants comprise hindered phenol and amine derivatives such as butylated hydroxytoluene, butylated hydroxyanisole, t-butyl hydroquinone, and α-tocopherol. Preferred secondary antioxidants include phosphites and phosphonites such as tris(nonylphenol) phosphite, tris(2,4-di-t-butylphenyl) phosphite, distearyl pentaerythritol diphosphite, bis(2,4-dicumylphenyl) pentaerythritol diphosphite, and Irgafos® P-EPQ (Ciba Specialty Chemicals, Tarrytown, N.Y.). Preferred thioester-type antioxidants, used synergistically or additively with primary antioxidants, include dilauryl 3,3′-thiodipropionate, dimyristyl 3,3′-thiodipropionate, distearyl 3,3′-thiodipropionate, and ditridecyl 3,3′-thiodipropionate.  
      Organic fillers, comprising prepolymerized material, optionally comprising at least one of the (meth)acrylic ester compounds and core-shell polymer compounds, and optionally comprising inorganic filler, may also be included in the composite material. Prepolymerization filler can be produced by any method known in the art, for example, by the method described in published U.S. patent application Ser. No. 2003/0032693. Optionally, uniformly-sized bead methacrylate polymers, such as Plexidon® or Plex® available from Röhm America LLC (Piscataway, N.J.), may be utilized as organic fillers.  
      The core-shell polymers can be added directly to the monomers of the invention, followed by the addition of the fillers, or they can be added together with or after the fillers.  
      The dental composite materials of the present invention can be used in any treatment method known to one of ordinary skill in the art. Treatment in this context includes preventative, restorative, or cosmetic procedures using the dental composites of the present invention. Typically, without limiting the method to a specific order of steps, the dental composite materials are placed on a dental tissue, either natural or synthetic, the dental composite materials are cured by any method known to one of ordinary skill in the art, and the dental composite materials are shaped as necessary to conform with the target dental tissue. Dental tissue includes, but is not limited to, enamel, dentin, cementum, pulp, bone, and gingiva.  
      The dental composite materials of the present invention are suitable for a very wide range of dental uses, including fillings, teeth, bridges, crowns, inlays, onlays, laminate veneers, facings, pit and fissure sealants, cements, denture base and denture reline materials, orthodontic splint materials, and adhesives for orthodontic appliances. The materials of the invention may also be utilized for prosthetic replacement or repair of various hard body structures such as bone and may be utilized for reconstructive purposes during surgery, especially oral surgery. They are also useful for various non-dental uses as, for example, in plastic construction materials.  
     EXAMPLES  
      The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can 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 uses and conditions.  
      The meaning of abbreviations is as follows: “hr.” means hour(s), “min.” means minute(s), “sec.” means second(s), “ml” means milliliter(s), “cm” means centimeter(s), “mm” means millimeter(s), “μm” means micron or micrometer, “g” means gram(s), “mmol” means millimole(s), “in.” means inch(es), “wt %” means weight percent(age), “mW” means milliwatt(s), “atm.” means atmosphere(s), “M n ” means number average molecular weight, “T g ” means glass transition temperature, “d50” means 50% of particles have a diameter below a given size, “d” means density, “MW” means weight average molecular weight, “cps” means centipoise, “mp” means melting point, “HQ” means hydroquinone, “MPa” means megapascal(s), “CQ” means camphor quinone, “EDB” means ethyl 4-dimethylaminobenzoate, “THF” means tetrahydrofuran, “PTFE” means polytetrafluoroethylene.  
     Materials and Supplies  
      Bis-GMA adduct was obtained from EssTech (Essington, Pa.)—product code X 950-0000. TEGDMA was obtained from EssTech—product code X 943-7424, inhibited with HQ (50-70 ppm). Photosensitizers were obtained from Sigma-Aldrich (St. Louis, Mo.): CQ (97%, catalog #12,489-3) and EDB (99+%, catalog #E2,490-5). Aerosil® OX-50 fumed silica was obtained from Degussa (Parsippany, N.J.). Schott 8235 UF1.5 glass powder was obtained from the Schott Corp. (Yonkers, N.Y.); it had a mean diameter, d50, of 1.5 μm and was treated with C 10 H 20 O 5 Si to a level of 2.3 wt % silane. Core-shell polymers were obtained from Rohm and Haas (Philadelphia, Pa.): Paraloid® EXL-2330 (with a core prepared from butyl acrylate monomer), Paraloid® EXL-2314 (epoxy functional polymer, with a core prepared from butyl acrylate monomer), Paraloid® EXL-2691 (methacrylate-butadiene-styrene), and Paraloid® KM-365 (with a core prepared from butyl acrylate monomer).  
     Examples 1-4  
      A monomer-photosensitizer masterbatch was prepared under yellow light to avoid premature polymerization, with the ingredients indicated in Table 1.  
                           TABLE 1                                      Bis-GMA (EssTech), g   15.0           product code × 950 0000           TEGDMA (EssTech), g   15.0           inhibited with HQ (50-70 ppm), product           code × 943 7424           Photosensitizers:           CQ (97%, Aldrich), g*   0.40           EDB (99+%, Aldrich), g*   0.40                         *Sigma-Aldrich                Photo(co)sensitizers from Sigma-Aldrich:                1. Ethyl 4-dimethylaminobenzoate, 99+%, mp = 64-6° C., #E2,490-5, MW = 193.2                2. Camphorquinone, 97%, mp = 198-200° C., #12,489-3, MW = 166.2             
 
      Using the first recipe shown in Table 2, core-shell polymer was mixed with a portion of the masterbatch in a beaker under yellow light. The remainder of the ingredients was added in the amounts shown in Table 2. The fumed silica was added to the contents of the beaker and mixed briefly in the beaker. Then, the mixture was turned out onto a 7 in×12 in glass plate. The mixture of masterbatch, core-shell polymer, and fumed silica was mixed on the plate with a larger spatula until uniform. The glass powder was then added in several portions to the beaker, stirred to combine it with the remainder of the previous mixture, and then added to the mixture on the plate. Mixing was continued for a total of 10 min. The mixture was kneaded between PTFE sheets (flattened, folded over, and flattened again) for 65 cycles. The procedure was repeated for each recipe shown in Table 2.  
                       TABLE 2                                      Example:                                     1   2   3   4                                             Monomer/photosensitizer masterbatch, g   4.5   4.5   4.5   4.5       Core-shell polymer:       Paraloid ® EXL-2330, g* (butyl   0.5   —   —   —       acrylate core)       Paraloid ® EXL-2314, g* (butyl acrylate   —   0.5   —   —       core, epoxy functional)       Paraloid ® EXL-2691, g*   —   —   0.5   —       (methacrylate-butadiene-styrene)       Paraloid ® KM-365, g* (butyl acrylate   —   —   —   0.5       core)       Added to pre-mixed       monomers/photosensitizer/       rubber:       1 st : Degussa OX-50 fumed silica (0.04 μm),   1.0   1.0   1.0   1.0       g**       2 nd : Schott 8235 (Ba silicate) UF1.5   14.0   14.0   14.0   14.0       glass powder (d50 = 1.5 μm, 2.3 wt %       silane), g**       Hand-mix time, min.   10   10   10   10       Kneading, fold cycles [˜10 min.]   65   65   65   65                 *Core-shell polymers from Rohm and Haas.            **Degussa OX-50 fumed silica: 5 wt % of total composition - added to masterbatch first. Schott 8235 UF1.5* (d50 = 1.5 μm, d99 &lt; 5 μm) 2.3 wt % silane [*B 2 O 3  (10%), Al 2 O 3  (10%), SiO 2  (50%), BaO (30%), plus silane, C 10 H 20 O 5 Si (˜2%)].             
 
      The mixtures were degassed in a desiccator with a vacuum pump, cycling between atmospheric pressure and full vacuum every 10 min. for 1 hr, then holding at 50 mm Hg overnight (˜16 hr). The mixtures were further degassed overnight at 45° C. in a vacuum oven with just enough vacuum to keep the oven door closed, then isolating the oven by closing off all gas inlet/outlet valves. The mixtures were wrapped in foil to exclude light and stored in a refrigerator until used. For measurements or curing under a Dentsply Specturm 800 dental light, they were removed from the refrigerator, and the mixtures were allowed to warm to room temperature prior to use.  
      Shrinkage was determined by measuring the densities (with a Micromeritics Corp AccuPyc 1330 Helium Pycnometer) of uncured mixtures and of the bars cured under the dental lights under the following conditions. In a mold cut from PTFE, were cured three bars of dimension ˜2(depth)×4×25 mm. The uncured mixture was packed into the mold and sandwiched between two polyester plastic sheets and two glass plates. Three dental curing lamps (model Spectrum 800 from Dentsply, set at a visible light intensity of 550 mW/cm 2 ), each bearing an 8-mm light tip, were lined up and tied together to cure one side of one bar all at once. The light tips were brought up to the glass plate that covered the polyester sheet, which covered the dental composite and mold. Each bar was cured for 2 min. on the top and then 2 min. on the bottom. The volumetric shrinkage was calculated from the formula: Shrinkage=(cured density−uncured density)/(cured density).  
      The degree of monomer polymerization (“conversion”) was measured by Fourier Transform Infrared spectroscopy, using the total attenuated reflectance (ATR) method. A new, small metal file was cleaned with soap/water (scrubbing), then deionized water, then acetone, gently dabbed with a towel to absorb moisture, and air-dried. A bar of each composition was cured for the times specified in Table 3, at 550 mW/cm 2  under the following conditions. The uncured composition was packed into a stainless steel mold with a 2×2×25 mm cavity and sandwiched with two polyester sheets and two glass plates. Cured bars were obtained in the same manner as described for the bars used for shrinkage determination, except that the cure times (top and bottom of the bar) were varied as shown in Table 3.  
      Each bar was broken near center, just before filing it down to obtain powder for analysis. The powders were stored in vials wrapped in aluminum foil. The degree of conversion was obtained by comparing the relative peak heights ratios before and after cure. The peak ratio was calculated by dividing the height of the methacrylate C═C peak at 1,640 cm −1  by the height of the aromatic peak at 1,610 cm −1 .  
                           TABLE 3                                      Example                                         1   2   3   4                                                     No cure   1A   2A   3A   4A           Light cure time (top and           bottom of bar)            60 sec.   1B   2B   3B   4B           120 sec.   1C   —   3C   —                      
 
      Fracture toughness (K IC ) and flexural strength (ISO 4049) were obtained by standard methods on bars cured under the following conditions. Each uncured composition was packed into a stainless steel mold with a 2×2×25 mm cavity and sandwiched with two polyester sheets and two glass plates. Cured bars were obtained in the same manner as described for the bars used for shrinkage determination, except that the cure time was limited to 60 sec. on the top and bottom of each bar. Five bars were used for each of the two mechanical tests. The bars were stored in glass vials until use and conditioned in water for 24 hr at 37° C., just prior to the tests.  
      The fracture toughness test was based on both the ASTM polymers standard (ASTM D5045) and the ASTM ceramics standard (ASTM C1421, precracked beam method). Testing was conducted at a test speed of 0.5 mm/min. at room temperature and ambient humidity using a three-point bend fixture (span to depth ratio of 10). The specimens were molded using the flex bar mold specified in ISO 4049. The specimens were precracked halfway through the depth. Two modifications to the test procedures were made. The first was the use of smaller test specimens than those recommended in the ASTM C1421 standard (2 mm×2 mm×25 mm instead of the recommended minimum dimensions of 3 mm×4 mm×20 mm). The second was the use of a slitting circular knife to machine the precracks. The knife was 0.31 mm in thickness with a 9 degree single bevel. Tests have shown that this technique produced precracks that were equivalent to precracks produced using techniques recommended in ASTM D5045.  
      The properties of the compositions are summarized in Table 4.  
                                   TABLE 4                               Con-       Flexural   Shrinkage               version   K IC     Strength   (He       Sample #   Cure time   (FTIR)   (MPa-m 1/2 )   (MPa)   pycnometry)                                                        Example 1                           1B    60-sec.   88.5%   1.37   90       1C   120-sec.   88.3%           3.34%       Example 2       2B    60-sec.   87.2%   1.63   92       2C   120-sec.               4.04%       Example 3       3B    60-sec.   83.9%   1.78   107       3C   120-sec.   86.8%           3.85%       Example 4       4B    60-sec.   88.5%   1.67   93       4C   120-sec.               4.17%                  
 
     Comparative Examples A and B  
      A monomer-photosensitizer masterbatch with the same ingredients shown in Table 1 was prepared under yellow light. The ingredients shown in Table 5 were mixed according to the procedure shown in Examples 1-4 and the composition tested as described in Examples 1-4, except that flexural strength and conversion were not determined.  
                       TABLE 5                                   Example:           A                                                    Monomer/photosensitizer masterbatch, g   5.0           Added to pre-mixed           monomers/photosensitizer:           1 st : Degussa OX-50 fumed silica (0.04 μm), g   1.0           2 nd : Schott 8235 (Ba silicate) UF1.5 glass powder   14.0           (d50 = 1.5 μm, 2.3 wt % silane), g           Hand-mix time, min.   10           Kneading, fold cycles [˜10 min.]   65                      
 
      Duplicate sets of properties were determined and are summarized in Table 6 as Examples A and B. The shrinkage value is greater than for the compositions of Examples 1-4.  
                                   TABLE 6                               Con-       Flexural   Shrinkage               version   K IC     Strength   (He       Sample #   Cure time   (FTIR)   (MPa-m 1/2 )   (MPa)   pycnometry)                  Example A    60-sec       1.85                   120-sec               4.50%       Example B    60-sec       1.84           120-sec               4.51%       Example C    60-sec   85.2%   1.74   120           120-sec               4.30%       Example D    60-sec   87.5%   1.77   119           120-sec               4.73%                  
 
     Comparative Examples C and D  
      The ingredients in Table 7 were mixed according to the procedure described for Examples 1-4, except that no monomer masterbatch/photosensitizer was prepared, and tested in the same manner. The properties are summarized in Table 6. The shrinkage values are greater than for the compositions of Examples 1-4.  
                       TABLE 7                                      Example:                             C   D                                     Bis-GMA (EssTech), g   3.0   2.0       product code × 950 0000       TEGDMA (EssTech), g   2.0   3.0       inhibited with HQ (50-70 ppm), product code × 943 7424       Photosensitizers:       CQ (97%, Aldrich), g   0.05   0.05       EDB (99+%, Aldrich), g   0.05   0.05       Added to pre-mixed monomers/photosensitizer:       1 st : Degussa OX-50 fumed silica (0.04 μm), g   1.0   1.0       2 nd : Schott 8235 (Ba silicate) UF1.5 glass powder   14.0   14.0       (d50 = 1.5 μm, 2.3 wt % silane), g       Hand-mix time, min.   10   10       Knead time, min. [10 min. = 65 fold cycles]   10   10                  
 
     Example 5  
      A monomer-photosensitizer masterbatch with the same ratio of ingredients as shown in Table 1 was prepared under yellow light. The ingredients shown in Table 8 were mixed according to the procedure shown in Examples 1-4 and the shrinkage measured as described in Examples 1-4. The composition contained 5 wt % core-shell polymer or 20 wt % of the combined weights of the monomer and polymer portion. The composition showed 2.58% shrinkage during light cure, a lower value than the 3.85% found for the composition of Example 3, which contained the same core-shell polymer, Paraloid® EXL-2691. The core-shell polymer was only 2.5 wt % of the composition of Example 3, or 10 wt % of the combined weights of the monomer and polymer portion.  
                       TABLE 8                                   Example:           5                                            Monomer/photosensitizer masterbatch, g   4.0       Core-shell polymer:       Paraloid ® EXL-2691, g* (methacrylate-butadiene-   1.0       styrene),g       Added to pre-mixed monomers/photosensitizer/rubber:       1 st : Degussa OX-50 fumed silica (0.04 μm), g**   1.0       2 nd : Schott 8235 (Ba silicate) UF1.5 glass powder   14.0       (d50 = 1.5 μm, 2.3 wt % silane), g**       Hand-mix time, min.   10       Kneading, fold cycles [˜10 min.]   65