WO2007101442A2

WO2007101442A2 - Composite material, in particular a dental filling material

Info

Publication number: WO2007101442A2
Application number: PCT/DK2007/000114
Authority: WO
Inventors: Alexander Van Lelieveld
Original assignee: Dentofit AS
Current assignee: Dentofit AS
Priority date: 2006-03-07
Filing date: 2007-03-07
Publication date: 2007-09-13
Anticipated expiration: 2008-09-07
Also published as: WO2007101442A3

Abstract

The present application discloses a composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1: 1.1.

Description

AN IMPROVED COMPOSITE MATERIAL, IN PARTICULAR A DENTAL FILLING MATERIAL

FIELD OF THE INVENTION

The present invention relates to composite materials which have been improved with respect to their optical properties.

BACKGROUND OF THE INVENTION

The applicant's earlier PCT application No. WO 2005/099652 Al discloses a composite material exhibiting a low or even negligible volumetric shrinkage upon curing, or even a small expansion (e.g. up to 0.5%), in particular composite materials in the form of dental filling materials, as well as a method of controlling volumetric shrinkage of a composite material upon curing. According to WO 2005/099652 Al, a volume stable composite material for dental use can, e.g., be obtained by the use of metastable zirconia particles. Since a volume stable composite can minimize crack formation, such a technology is of great commercial importance. However, the use of substantial amounts of, e.g., zirconia particles in a composite material may result in a fairly opaque composite material.

Hence, there is a need for composite materials having improved optical properties, in particular composite materials, e.g. dental filling materials, having improved properties with respect to translucency.

BRIEF DESCRIPTION OF THE INVENTION

Translucency, shading, fluorescence and opalescence are optical properties that give a natural tooth its vital-looking appearance. Translucency and shading have the greatest impact on the total vitality of the tooth because they are the most readily observed. Also for non-dental applications, the optical properties of a hardened composite material may be of importance.

The invention utilizes nanofillers, typically metal oxide nanofillers, to increase the refractive index of the polymerizable resin phase into which the nanofiller is dispersed and stabilized. This, in turn, allows a match in refractive index of the liquid phase of the composite material, with the one or more fillers (e.g. zirconia) leading to a much more optically transparent or translucent material. The liquid phase of a composite material typically comprises of monomers, initiator system, optional additional additives, and a nanofiller that has a refractive index higher than that of the liquid phase without nanofiller. By including a nanofiller in the composite material such that the refractive index of a combined mixture of the monomers and the nanofiller in a way that the component is within about 3-4 percent of the filler (e.g. zirconia), the composite material is provided with improved properties including enhanced aesthetics (e.g., low visual opacity, high optical translucency) as compared to previously known zirconia compositions.

According to the present invention, translucency can be improved by means of the novel composite materials.

Hence, one aspect of the present invention relates to a composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (U) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1:1.1.

Another aspect of the present invention relates to a composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) including metastable zirconia in the tetragonal or cubic crystalline phase, wherein said resin base, upon polymerization and in the absence of any compensating effect from the one or more filler ingredients, causes a volumetric shrinkage (ΔV_resin) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said filler ingredient(s), exhibits a total volumetric shrinkage (ΔV_tota|) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_reSι_n) caused by the resin base, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.95 to 1:1.05, and wherein the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers is in the range of 10:90 to 100:0, preferably 10:90 to 40:60.

The invention further relates to a method of controlling the volumetric shrinkage of a composite material upon curing, comprising the step of: (a) providing a composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1: 1.1;

(b) allowing the resin base to polymerize and cure, and allowing the filler ingredient(s) to undergo a martensitic transformation from said first metastable phase to said second stable phase.

Moreover, the present invention provides the composite materials defined herein for use in medicine, in particular in dentistry.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention, i.a., provides an improved composite material with excellent optical properties useful for applications where volumetric shrinkage upon curing of the material is undesirable or even prohibitive.

More particularly, the present invention provides composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1:1.1.

A particular feature of the present invention is that the one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and that the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1:1.1. Hence, an important feature is that the presence of nanofillers renders the composite material (in particular the composite material when cured) relatively more translucent and thereby more useful for, e.g. dental used.

As used herein the term "nanofiller" is used synonymously with "nanosized particles" and "nanoparticles" and refers to filler particles having a size of at the most 100 nm

(nanometers). As used herein for a spherical particle, "size" refers to the diameter of the particle. As used herein for a non-spherical particle, "size" refers to the longest dimension of the particle.

The nanofillers constitute a fraction of the composite material constituents defined below as fillers (including filler ingredients).

The refractive index of the nanofillers is typically at least 1.8, but more preferably at least 1.9, or at least 2.0, or even at least 2.1.

Suitable nanofillers may be either acid reactive or non-acid reactive and may include, but are not limited to silica; zirconia; oxides of titanium, aluminum, cerium, tin, yttrium, strontium, barium, lanthanum, zinc, ytterbium, bismuth, iron, and antimony; and combinations thereof. More typical nanofillers may include zirconia (ZrO₂ ref. index of 2.19); oxides of titanium (e.g. TiO₂), and ytterium (e.g. Y₂O₃); and other metal oxides with high refractive indices. As used herein, "high refractive index" means a refractive index of typically at least 1.5, and more typically of at least 2.0. Diamond (ref. index of 2.417), Silicon nitride (Si₃N₄ ref. index 2.05), Silicon carbide (SiC ref. index of 2.55), Strontium titanate (SrTiO₃ ref. index 2.47) are particularly useful nanofillers, as they have very high refractive indices, and will require less weight of material than a lower refractive index material to match the refractive indices appropriately.

Taking into account the relatively high refractive index of the nanofillers, the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers can be adjusted so that it lies in the range of 1:0.9 to 1: 1.1, in particular in the range of 1:0.95 to 1: 1.05.

In preferred embodiments, the type and amount of nanofiller is selected so that the refractive index of the combination of the polymerizable resin and the nanofillers is within 4 percent of the refractive index of the fraction of the one or more fillers not being said nanofillers, typically within 3 percent thereof, more typically within 1 percent thereof, and even more typically within 0.5 percent thereof. Refractive index of the nanofillers, the one or more fillers not being said nanofillers, and the combination of the polymerizable resin base and the nanofillers can be determined by conventional means, e.g. The refractive index values of solid materials (e. g. glass fillers) can be measured at room temperature by dispersing a solid test sample into optical liquids with different known specific refractive indexes. Observations of the dispersions are made with a light microscope. The refractive index of the solid material is determined by using the Becke's line as a band of light that appears along the outer edge of the dispersed particles under microscopic investigation. The Becke's line indicates the relative difference or the equality between the refractive indices of the solid material and the optical liquid. The refractive index of the combined mixture of the monomers, the nanofiller, the filler (e.g. zirconia) can be measured in the unhardened state or the hardened state. The refractive index of the mixture in the unhardened state may be the same as it is in the hardened state, but it also may be different. If the refractive index changes after hardening, typically the refractive index after hardening would be matched to the refractive index of the filler (e.g. zirconia). An objective of matching the refractive indices is to obtain the best visual opacity (best clarity) of the hardened composite material.

The nanofillers must have an average particle size of at the most 100 nanometers, but more typically at the most 50 nanometers. On the other hand, nanofillers typically have an average particle size of at least 2 nanometers, more typically at least 5 nanometers, and even more typically at least 10 nanometers. In some embodiments, the nanofiller is in the form of nanoclusters, typically at least 80 percent by weight nanoclusters. In other embodiments, the nanofiller is in the form of a combination of nanoparticles and nanoclusters.

In some embodiments, a portion of the surface of the nanofiller is silane treated or otherwise chemically treated to provide one or more desired physical properties.

The passage of light through the material is affected by the particle size and the difference in the refractive indices of the glass phase and the crystalline phase. If the crystals are smaller than the wavelength of visible light (0.4-0.7 mm) the glass will appear transparent. However a composite material consisting only of nanoparticles may not provide optimal mechanical properties or provide desirable handling properties before hardening, other sizes of particles may be included in the composite. Hence, nanoparticles are preferably combined with larger particles, e.g. glass particles, as will be discussed in the following.

This being said, the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers appears to play a certain role, and is typically in the range of 10:90 to 100:0, preferably 10:90 to 40:60, in particular 10:90 to 30:70. Also the volume relationship between (x) fraction of the nanofillers not at the same time being filler ingredients and (y) the filler ingredients may be of interest. Hence, the volume ratio between (x) the nanofillers not at the same time being filler ingredients and (y) the filler ingredients is preferably at the most 60:40, such as at the most 50:50, e.g. in the range of 5:95 to 50:50.

Another feature of the present invention is that the martensitic transformation of the filler ingredient(s) can be provoked by a trigger mechanism (see further below).

It is well known that many polymeric resin bases (see also below) exhibit volumetric shrinkage upon curing thereof. Thus, a particular feature of the present invention is the presence of a filler ingredient that will reduce or eliminate the volumetric shrinkage caused by the polymerizable resin base, or even counteract this volumetric shrinkage to such an extent that the composite material exhibits a net volumetric expansion upon curing of the polymeric resin base.

Thus, in a preferred embodiment of the composite material, the resin base, upon polymerization and in the absence of any compensating effect from the one or more filler ingredients, causes a volumetric shrinkage (ΔV_reSi_n) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said filler ingredient(s), exhibits a total volumetric shrinkage (ΔVt_otai) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_reSi_n) caused by the resin base. More particularly, the volumetric shrinkage (ΔV_resi_n) is at least

1.00%, such as at least 1.50%, and the total volumetric shrinkage (ΔV_totaι) is at least 0.50%- point less, such as 1.00%-point less than the uncompensated volumetric shrinkage, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.95 to 1: 1.05, and wherein the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers is in the range of 10:90 to 100:0, preferably 10:90 to 40:60.

Alternatively, the present invention provides a composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) including metastable zirconia in the tetragonal or cubic crystalline phase, wherein said resin base, upon polymerization and in the absence of any compensating effect from the one or more filler ingredients, causes a volumetric shrinkage (ΔV_resin) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said filler ingredient(s), exhibits a total volumetric shrinkage (ΔV_totai) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_resiπ) caused by the resin base, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.95 to 1:1.05, and wherein the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers is in the range of 10:90 to 100:0, preferably 10:90 to 40:60.

The composite material typically comprises 5-95%, or 10-90%, by weight of the one or more fillers (including nanofillers and filler ingredient(s)) and 5-95%, or 10-90%, by weight of the polymerizable resin base, in particular 30-95%, or 30-90%, by weight of the one or more fillers and 5-70%, or 10-70%, by weight of the polymerizable resin base.

The amount of nanofiller should be sufficient to provide a composite material having desirable handling properties before hardening and good physical and optical properties after hardening. Typically, the nanofiller represents at least 3% by weight, more typically at least 10% by weight, in particular at least 20% by weight, based on the total weight of the composite material. Typically, the nanofiller represents at most 50% by weight, more typically at most 40% by weight, and most typically at most 30% by weight, based on the total weight of the composite material.

In most interesting embodiments, the nanofillers constitute 10-50% by weight, such as 20- 40% by weight, of the composite material.

Preferably, the composite material is substantially solvent free and water free. By the term "substantially solvent free and water free" is meant that the composite material comprises less than 4.0%, such as less than 1.0% or less than 0.5%, by weight of solvents and/or water.

Filler/Filler ingredient

In view of the above, it is apparent that the one or more fillers, and in particular the one or more filler ingredients and the nanofillers, are important constituents of the composite material.

Fillers are frequently used in connection with polymeric materials in order to provide desirable mechanical properties of such materials, e.g. abrasion resistance, opacity, colour, radiopacity, hardness, compressive strength, compressive modulus, flexural strength, flexural modulus, etc.

Such fillers may be selected from one or more of a wide variety of materials, e.g. those that are suitable for the use in dental and/or orthodontic composite materials.

Fillers can be inorganic materials or cross-linked organic materials that are insoluble in the resin component of the composition. Cross-linked organic materials may as such be filled with an inorganic filler. The filler should - in particular for dental uses - be nontoxic and suitable for use in the mouth. The filler can be radiopaque or radiolucent. The filler typically is substantially insoluble in water.

The term "filler" is to be understood in the normal sense, and fillers conventionally used in composite materials in combination with polymer are also useful in the present context. The polymerizable resin base (see further below) can be said to constitute the "continuous" phase wherein the filler is dispersed.

Some examples of suitable inorganic fillers are naturally occurring or synthetic materials including, but not limited to: quartz; nitrides (e.g. silicon nitride); glasses derived from, for example, Zr, Sr, Ce, Sb, Sn, Ba, Zn, and Al; feldspar; borosilicate glass; kaolin; talc; titania; low Mohs hardness fillers such as those described in U.S. Pat. No. 4,695,251 (Randklev); and silica particles (e.g. submicron pyrogenic silicas such as those available under the trade designations AEROSIL, including "OX 50," "130," "150" and "200" silicas from Degussa AG, Hanau, Germany and CAB-O-SIL M5 silica from Cabot Corp., Tuscola, III.). Examples of suitable organic filler particles include filled or unfilled pulverized polycarbonates, polyepoxides, and the like.

Other illustrative examples of fillers are barium sulfate (BaSO₄), calcium carbonate (CaCO₃), magnesium hydroxide (Mg(OH)₂), quartz (SiO₂), titanium dioxide (TiO₂), zirconia (ZrO₂), alumina (AI₂O₃), lantania (La₂O₃), amorphous silica, silica-zirconia, silica-titania, barium oxide (BaO), barium magnesium aluminosilicate glass, barium aluminoborosilicate glass (BAG), barium-, strontium- or zirconium-containing glass, milled glass, fine YF₃ or YbF₅ particles, glass fibres, metal alloys, etc. Metal oxides, e.g. titanium dioxide (TiO₂) and zirconia (ZrO₂), alumina (AI₂O₃), lantania (La₂O₃), constitute a particularly useful group of fillers for use in the composite materials of the present invention.

In one interesting embodiment, at least 5%, e.g. at least 10%, or even at least 20%, by weight of the one or more fillers are glass particles. It is believed that inclusion of glass particles may further improve the optical (and thereby aesthetic) properties of the composite material by making it more transparent.

The weight content of the one or more filler materials in the composite material is typically in the range of 5-95%, or 10-90%, such as 30-95%, such as 40-95%, e.g. 60-95%. It should be understood that a combination of two or more fillers may be desirable, just as the particle size distribution of the filler(s) may be fairly broad in order to allow a dense packing of the filler and thereby facilitate incorporation of a high amount of fillers in the composite material. Typically, composite materials have a distribution of one or more sizes of fine particles plus microfine and/or nano-size filler (5-15%). This distribution permits more efficient packing, whereby the smaller particles fill the spaces between the large particles. This allows for filler content, e.g., as high as 77-87% by weight. An example of a one size distribution filler would be 0.4 μm structural micro-filler, with the distribution as follows: 10% by weight of the filler particles have a mean particle size of less than 0.28 μm; 50% by weight of the filler particles have a mean particle size of less than 0.44 μm; 90% by weight of the filler particles have a mean particle size of less than 0.66 μm.

Typically, the particle size of the filler(s) is in the range of 0.01-50 μm, such as in the range of 0.02-25 μm, and - as mentioned above - include nanofillers having a particle size of at the most 100 nm.

In some embodiments, the particle size of the filler(s) is/are in the range of 0.2-20 μm with some very fine particles of about 0.04 μm. As an example, fairly large filler particles may be used in combination with amorphous silica in order to allow for a dense packing of the fillers.

The term "particle size" is intended to mean the shortest dimension of the particulate material in question. In the event of spherical particles, the diameter is the "particle size", whereas the width is the "particle size" for a fiber- or needle-shaped particulate material. It should of course be understood that an important feature of such particles is the actual crystal size in that the crystal size (and not the particle size) will be determinative for the preferred crystal phase under given conditions (see also further below).

In the embodiment where the composite material is for dental use, particularly useful fillers are zirconia, amorphous silica, milled barium-, strontium- or zirconium-containing glass, milled acid-etchable glass, fine YtF₃ or YbF₅ particles, glass fibres, etc.

The one or more fillers comprise at least one filler ingredient. The term "filler ingredient" is intended to mean the filler or a fraction of the filler having particular physical properties, namely the inherent ability to compensate (by expansion) for volumetric shrinkage caused by polymerization and curing of the resin base. Thus, a certain filler, e.g. zirconia, may be included in the composite material, and a certain fraction of these filler particles may have particular physical properties, i.e. exist in a metastable crystalline phase (see the following), and thereby constitute the filler ingredient.

The particle size of the filler ingredient(s) is/are typically in the range of 0.01-50 μm.

The filler ingredient(s) typically constitute(s) 20-100% of the total weight of the one or more fillers, e.g. 30-100%, such as 40-100% or 50-100%.

When calculated on the basis of the total weight of the composite material, the filler ingredient(s) typically constitute(s) 15-95% of the total weight of the composite material, e.g. 25-95%, such as 30-95%, more specifically 60-95%.

The one or more filler ingredients are present in a metastable first phase and are able to undergo a martensitic transformation to a stable second phase, where the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) is at least 1.005, such as at least 1.01 or even at least 1.02 or at least 1.03.

In the present context, the term "metastable first phase" means that the filler ingredient existing in such as phase has a free energy that is higher than the free energy of the second phase, and that an activation barrier (F*) must be overcome before transformation from the first phase (high energy state) to the second phase (low energy state) can proceed. Thus, the phase transformation does not proceed spontaneously. It should be understood that the "system" in which the filler ingredient is metastable is the composite material including all its constituents, i.e. the composite material before curing.

The phase transformation is martensitic, which by definition means that the crystal structure of the filler ingredient needs no extra atoms to undergo the transformation. Thus, the transformation can be very fast, almost instantaneous.

The expression "free energy" refers to the sum of free energies from the particle bulk, the particle surface and strain contributions. For most practical purposes, only the free energies from the particle bulk and the particle surface need to be considered.

Thus, when considering various materials as potential filler ingredients, it is relevant to take into consideration the three main requirements: 1. A first requirement for the filler ingredient is that the second crystalline phase thereof, within the selected particle size range, is "stable" under "standard" conditions, i.e. standard pressure (101.3 kPa) and at least one temperature in the range of 10-50⁰C, i.e. corresponding to the conditions under which the product is used.

2. A second requirement for the filler ingredient is that a metastable first crystalline phase of the filler ingredient can exist the under the same "standard" conditions.

3. A third requirement for the filler ingredient is that the specific volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) is at least 1.005.

The expression "stable" refers to a phase which does not transform spontaneously under the conditions required for transforming the filler ingredient from the first metastable phase. Thus, the "stable" phase need not always be the phase with the "globally" lowest free energy, but it often will be.

The filler ingredients relevant in the present context comprise particular crystalline forms of some of the fillers mentioned above, in particular of the metal oxide fillers. A very useful example hereof is ZrO₂ (see in particular the section "Populations of zirconia particles" further below). Zirconia can exist in three major crystalline phases: the tetragonal phase, the cubic phase and the monoclinic phase. The specific volume (density^"1) of the three phases is 0.16, 0.16 and 0.17 cm³/g_/ respectively. Thus, the monoclinic (the second phase) and one of the former two phases (the first phase) have a volume ratio higher than 1.005 (i.e. 1.045 and 1.046, respectively). The tetragonal and the cubic phases have higher bulk energy than the monoclinic phase at the standard conditions.

Illustrative examples of filler ingredients are:

Zirconia in the metastable tetragonal phase (specific volume = 0.16 cm³/g) which can transform into the monoclinic phase (specific volume = 0.17 cm³/g) (volume ratio = 1.045);

Zirconia in the metastable cubic phase (specific volume= 0.16 cm³/g) which can transform into the monoclinic phase (specific volume = 0.17 cm³/g) (volume ratio = 1.046);

Lanthanide sesquioxides (Ln₂O₃), where Ln=Sm to Dy. Transforms from monoclinic to cubic phase at 600-2200⁰C with a volume expansion of 10%. Nickel sulfide (NiS). Transforms from rhombohedral to hexagonal phase at 379°C with a volume expansion of 4%. Density 5.34 g/ml.

Dicalcium silicate (belite) (Ca₂SiO₄). Transforms from monoclinic to orthorhombic phase at 490⁰C with a volume expansion of 12%. Density 3.28 g/ml.

Lutetium borate (LuBO₃). Transforms from hexagonal to rhomhedral phase at 131O⁰C with a volume expansion of 8%.

The surface energy of the tetragonal phase of zirconia is lower than the one of the monoclinic phase at standard temperature and pressure, which results in stable tetragonal (pure) zirconia crystals at room temperature. The crystals must be small (<10 nm) for the difference of surface energy to compete with difference of in bulk energy of the tetragonal and monoclinic phase.

For zirconia in the metastable tetragonal or cubic crystalline phase, the particle size is preferably in the range of 5-80,000 nm, such as 20-2000 nm, though it is believed that a mean particle size in the range of 50-1000 nm, such as 50-500 nm, provides the best balance between optical and structural properties.

In one embodiment, the filler ingredient(s) is/are able to undergo the martensitic transformation under the influence of ultrasound.

In view of the above, the filler ingredient(s) preferably include(s) zirconia (ZrO₂) in metastable tetragonal or cubic crystalline phase (see in particular the section "Populations of zirconia particles" further below).

In another embodiment, the filler ingredient(s) is/are able to undergo the martensitic transformation upon exposure to a chemical trigger.

In some instances, the activation barrier (F*) is not sufficiently large to prevent premature transformation from the first phase to the second phase. This may result in a spontaneous transformation upon storage of the composite material. Thus, in some embodiments, it is advantageous to stabilize the native filler ingredient in order to obtain a metastable phase that will not undergo more or less spontaneous, i.e. premature, transformation upon storage of the composite material. Stabilization of the metastable phase can, e.g., be achieved by doping, by surface modification of the filler particles, etc. as will be explained in the following. In one variant, at least 50% of the nanofillers are zirconia particles.

Many crystal phases can be stabilized using doping materials. Generally, with increasing amounts of dopant, the more the phase is stabilised. In energy-terms, the activation barrier (F*) becomes higher the more dopant used. In order to trigger the phase transformation, the activation barrier must, however, be low enough for the trigger method to overcome the activation barrier, but high enough so that the transformation does not occur spontaneously.

Zirconia is typically stabilized using up to 20 mol-% of one or more dopants. Zirconia can be stabilized with stabilizer such as calcium, cerium, barium, yttrium, magnesium, aluminum, lanthanum, caesium, gadolinium and the like, as well as oxides and combinations thereof. More specifically, the recommended mol-% content for some useful dopants (if it is decided to include a dopant) is: Y₂O₃ (1-8%), MgO (1-10%), CaO (1-18%), CeO₂ (1-12%), and Sc₂O₂ (1-10%). A dopant level of, e.g., Y₂O₃ of 0-1% will typically not sufficiently stabilize the tetragonal phase and the cubic phase of zirconia, and such doped zirconia will, therefore, still undergo a phase transformation spontaneously to the monoclinic phase at room temperature. Adding too high a level Of Y₂O₃, e.g. 8% or more, will stabilise the tetragonal phase and the cubic phase to such an extent that the activation barrier will become too high to overcome with most trigger process. At some point in between the activation barrier, the transformation can be triggered as described below. Adding more dopant will make the triggering more difficult and thus slower. Adding less dopant could make the zirconia unstable and not useful as a filler ingredient. [It should be noted that commercial grade zirconia contains a small fraction of hafnium. Such small amounts of hafnium are neglected in the discussion above, because hafnium is viewed as an integral part of zirconia.]

In a preferred embodiment, the metastable phase of the zirconia is stabilized by doping with an oxide selected from Y₂O₃, MgO, CaO, CeO₂, and Sc₂O₃.

Depending on the activation energy as explained above, the levels of dopants for ZrO₂ could be Y₂O₃ (1-5%), MgO (1-5%), CaO (1-10%), and CeO₂ (1-6%), but for ideal zirconia crystal doping is not necessary so more specifically about 0-2%.

Surface modification

Surface energy can be changed by surface modification. By modification of the surface by adsorption of a chemical constituent, it is possible to lower the surface energy of the first phase so that the sum of the surface energy and the bulk energy becomes lower than the surface energy and the bulk energy for the second phase, and thereby "reverse" the stability order of the first and second phase. In this way, the "metastability" of the first phase arises because the first phase is only "stable" as long as the chemical constituent is adsorbed thereto. Thus, the first phase is stabilised until the surface modification is altered or removed, e.g. by treatment with a chemical trigger.

Generally, the surface of the filler particles (nanofillers, filler ingredients, etc.) can also be treated with a coupling agent in order to enhance the bond between the filler and the resin. Suitable coupling agents include gamma-rnethacryloxypropyltrimethoxysilane, gamma- mercaptopropyltriethoxysilane, gamma-aminopropyltrimethoxysilane, and the like. Examples of useful silane coupling agents are those available from Crompton Corporation, Naugatuck, Conn., as SILQUEST A-174 and SILQUEST A-1230.

For some embodiments of the composite material, the composite materials may include at least 1% by weight, more preferably at least 2% by weight, and most preferably at least 5% by weight other filler, based on the total weight of the composite material. For such embodiments, composite materials of the present invention preferably include at most 40% by weight, more preferably at most 20% by weight, and most preferably at most 15% by weight other filler, based on the total weight of the composite material.

Polymerizable resin base

Another important constituent of the composite material is the polymerizable resin base.

The term "polymerizable resin base" is intended to mean a composition of a constituent or a mixture of constituents such as monomer, dinners, oligomers, prepolymers, etc. that can undergo polymerization so as to form a polymer or polymer network. By polymer is typically meant an organic polymer. The resin base is typically classified according to the major monomer constituents.

The weight content of the polymerizable resin base in the composite material is typically in the range of 5-95%, or 5-90%, e.g. 5-70%, such as 5-60%, e.g. 5-40%.

Virtually any polymerizable resin base can be used within the present context. Polymerizable resin bases of particular interest are, of course, such that upon curing will cause a volumetric shrinkage of the composite material when used without a compensating filler ingredient. The term "curing" is intended to mean the polymerisation and hardening of the resin base.

One class of preferred hardenable resins are materials having free radically active functional groups and include monomers, oligomers, and polymers having one or more ethylenically unsaturated groups. Alternatively, the hardenable resin can be a material from the class of resins that include cationically active functional groups. In another alternative, a mixture of hardenable resins that include both cationically curable and free radically curable resins may be used for the dental materials of the invention.

In the class of hardenable resins having free radically active functional groups, suitable materials for use in the invention contain at least one ethylenically unsaturated bond, and are capable of undergoing addition polymerization. Such free radically polymerizable materials include mono-, di- or poly- acrylates and methacrylates such as methyl acrylate, methyl methacrylate, ethyl acrylate, isopropyl methacrylate, n-hexyl acrylate, stearyl acrylate, allyl acrylate, glycerol diacrylate, glycerol triacrylate, ethyleneglycol diacrylate, diethyleneglycol diacrylate, triethyleneglycol dimethacrylate, 1,3-propanediol diacrylate, 1,3-propanediol dimethacrylate, trimethylolpropane triacrylate, 1,2,4-butanetriol trimethacrylate, 1,4- cyclohexanediol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, sorbitol hexacrylate, the diglycidyl methacrylate of bis- phenol A ("Bis-GMA"), bis[l-(2-acryloxy)]-p-ethoxyphenyldimethylmethane, bis[l-(3- acryloxy~2-hydroxy)]-p-propoxyphenyldimethylmethane, and trishydroxyethyl-isocyanurate trimethacrylate; the bis-acrylates and bis-methacrylates of polyethylene glycols of molecular weight 200-500, copolymerizable mixtures of acrylated monomers such as those in U.S. Pat. No. 4,652,274, and acrylated oligomers such as those of U.S. Pat. No. 4,642,126; and vinyl compounds such as styrene, diallyl phthalate, divinyl succinate, divinyladipate and divinylphthalate. Mixtures of two or more of these free radically polymerizable materials can be used if desired.

An alternative class of hardenable resins useful in the dental materials of the invention may include cationically active functional groups. Materials having cationically active functional groups include cationically polymerizable epoxy resins, vinyl ethers, oxetanes, spiro- orthocarbonates, spiro-orthoesters, and the like.

Preferred materials having cationically active functional groups are epoxy resins. Such materials are organic compounds having an oxirane ring which is polymerizable by ring opening. These materials include monomeric epoxy compounds and epoxides of the polymeric type and can be aliphatic, cycloaliphatic, aromatic or heterocyclic. These materials generally have, on the average, at least 1 polymerizable epoxy group per molecule, preferably at least about 1.5 and more preferably at least about 2 polymerizable epoxy groups per molecule. The polymeric epoxides include linear polymers having terminal epoxy groups (e.g. a diglycidyl ether of a polyoxyalkylene glycol), polymers having skeletal oxirane units (e.g. polybutadiene polyepoxide), and polymers having pendent epoxy groups (e.g. a glycidyl methacrylate polymer or copolymer). The epoxides may be pure compounds or may be mixtures of compounds containing one, two, or more epoxy groups per molecule. The "average" number of epoxy groups per molecule is determined by dividing the total number of epoxy groups in the epoxy-containing material by the total number of epoxy-containing molecules present.

These epoxy-containing materials may vary from low molecular weight monomeric materials to high molecular weight polymers and may vary greatly in the nature of their backbone and substituent groups. Illustrative of permissible substituent groups include halogens, ester groups, ethers, sulfonate groups, siloxane groups, nitro groups, phosphate groups, and the like. The molecular weight of the epoxy-containing materials may vary from about 58 to about 100,000 or more.

Particularly interesting resin bases that are useful for dental applications are those based on compounds selected from the group consisting of methacrylic acid (MA), methylmethacrylate (MMA), 2-hydroxyethyl-methacrylate (HEMA), triethyleneglycol dimethacrylate (TEGDMA), bisphenol-A-glycidyl dimethacrylate (BisGMA), bisphenol-A-propyl dimethacrylate (BisPMA), urethane-dimethacrylate (UEDMA), and HEMA condensed with butanetetracarboxylic acid (TCB), as well as those based on combinations of the above-mentioned compounds. Such resin bases are, e.g., disclosed and discussed in US 6,572,693. A particularly useful combination of compounds is TEGDMA and BisGMA, see, e.g., US 3,066,112.

Other constituents of the composite material

The composite material may comprise other constituents which provide beneficial rheological, cosmetic, etc. properties. Examples of such other constituents are dyes, flavorants polymerisation initiators and co-initiators, stabilizers, fluoride releasing materials, sizing agents, antimicrobial ingredients, fire retardants.

Thus, the resin base may include initiators and co-initiators, and illustrative examples of such compounds, particularly for use in dental applications, are benzoylperoxide (BPO), camphorquinone (CPQ), phenylpropanedione (PPD) and N,N-di(2-hydroxyethyl)-p-toluidine (DEPT), N,N-dimethyl-p-aminobenzoic acid ethyl ester (DAEM). Shading can be achieved by using a number of color pigments. These include metal oxides, which provide the wide variety of colors of the composite; for example, oxides of iron can act as a yellow, red to brown pigment, copper as a green pigment, titanium as a yellowish-brown pigment, and cobalt imparts a blue color.

Fluorescence is a more subtle optical property that further enhance the natural-looking, lifelike appearance or "vitality" of the tooth. Fluorescence is defined as the emission of electromagnetic radiation that is caused by the flow of some form of energy into the emitting body, which ceases abruptly when the excitation ceases. In natural teeth, components of the enamel, including hydroxyapatite, fluoresce under long wavelength ultraviolet light, emitting a white visible light. This phenomenon is subtle in natural daylight but still adds further to the vitality of the tooth. In contrast, under certain lighting conditions, the lack of fluorescence in a restorative material may become alarming. Under "black light" conditions, such as that often used in discotheque-type night clubs, if a restoration does not fluoresce, the contrast between the tooth and restoration may be so great that the tooth may actually appear to be missing. Fluorescence can, e.g., be achieved by adding an anthracene-like molecule.

The weight content of other constituents in the composite material is typically in the range of 0-10%, such as 0-5%, e.g. 0-4% or 1-5%.

Dental filling materials

In view of the above, the present invention also provides a dental filling material in the form of a composite material as defined above. In particular, the filler ingredient(s) of the composite material include(s) zirconia (ZrO₂) in metastable tetragonal or cubic crystalline phase.

In a particularly interesting embodiment, the dental filling material consists of:

40-90% (e.g. 40-85% ) by weight of the one or more fillers, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) include(s) metastable zirconia in the tetragonal or cubic crystalline phase, and wherein said one or more fillers include a fraction of nanofillers, the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers being in the range of 1:0.9 to 1:1.1; 10-60% (e.g. 15-60%) by weight of the a polymerizable resin base, said resin base being based on one or more compound selected from the group consisting of methacrylic acid (MA), methylmethacrylate (MMA), 2-hydroxyethyl-methacrylate (HEMA), triethyleneglycol dimethacrylate (TEGDMA), bisphenol-A-glycidyl dimethacrylate (BisGMA), bisphenol-A-propyl dimethacrylate (BisPMA), urethane-dimethacrylate (UEDMA), and HEMA condensed with butanetetracarboxylic acid (TCB); 0-5% by weight of additives; and 0-4% by weight of solvents and/or water.

In order to avoid premature curing of the polymerizable resin base, it may be advantageous to prepare and store the composite material as a two-component material intended for mixing immediately prior to use.

Use of the composite materials

The composite materials may be used and are cured essentially as conventional composite materials of the same type, except for the fact that the martensitic transformation should be controlled along with the curing of the resin base.

It is believed that the martensitic transformation can be activated either by physical means (e.g. application of mechanical pressure, tension, ultrasound, Roentgen irradiation, microwaves, longitudinal waves, electromagnetic irradiation such as light, near infrared irradiation, heating, etc.) or by chemical means (e.g. modification of the surface free energy by contacting the surface of the filler ingredient particles with a chemical, e.g. a constituent of the composite material or an additive such as water).

It should be understood that the martensitic transformation of the filler ingredient preferably shall take place with the curing (polymerization and hardening) of the resin base. However, since the crystals are small, the expansion due to phase transformation will not cause deterioration of the mechanical properties of the cured compound. Therefore, transformation triggered by slow mechanisms, e.g., diffusion of water into the cured compound or inner tensile stress build up by shrinkage from curing, will happen after the curing. Triggering the transformation before the curing is undesired since the volume compensating effect will be less or lost depending on how much is transformed before curing is initiated. A special note can be made on the ultrasound triggering mechanism, since it uses cavitation to trigger the transformation. In order to have cavitation the molecules should preferably be able to move e.g. at least partly uncured state, thus ultrasound triggering should preferably take place during the curing of the composite.

In one embodiment, the martensitic transformation of the filler ingredient(s) is initiated by application of ultrasound. Ultrasound is defined herein as energy at a frequency in the range of 10 kHz to 10 MHz. More typically, the ultrasound used has a frequency in the range of 10- 1000 kHz, such as 15-100 kHz, and most conventional apparatuses work in the frequency range of 15-50 kHz. Examples of conventional apparatuses are, e.g., ultrasound sealers for removal of tartar within dentistry.

Treatment of the metastable phase in a liquid/fluidable with ultrasound (in the range of 10- 1000 kHz and with a power higher than 1 W/cm²) creates micro-cavitation. The energy in these cavities is higher than the activation barrier and triggers the phase transformation. The energy is, for example, introduced as radicals to make a surface modification or by collision of filler particles.

Example: Treatment of tetragonal zirconia crystals with ethanol in an ultrasound bath (400 kHz) creates a phase transformation. A dispersion of zirconia particles in a resin base can be phase transformed by ultrasound using a sealer, i.e. an apparatus used by dentists to remove tartar.

In another embodiment, the martensitic transformation of the filler ingredient(s) is initiated by exposure of the surface of the filler ingredient(s) to a chemical trigger.

In order to make a phase transformation of a system where the first phase is metastable, but where the activation barrier is high because of a low surface energy of the first phase, the activation barrier can be lowered by surface modification. The activation of the phase transformation can be initiated by surface modification. The activation barrier will be the energy needed to make a surface modification that makes the surface energy of the phase higher (or make it more similar to the surface of the second phase).

Example: It is well-known that treatment of tetragonal zirconia with chemical compounds comprising at least one lone pair can induce phase transformation. The mechanism for this process has not yet been proven, but it involves some surface modification that triggers the phase transformation. Water (H₂O), solution of acids and bases (e.g. 5 M HCIO₄ and 5 M

NaOH) and glycerol proved to trigger the largest conversion of phase transformation at 95°C in 120 h. Other non-aqueous solvents like acetonitrile (CH₃CN), ethanol (C₂H₅OH) and formamide (NH₂CHO) proved to trigger a smaller conversion of phase transformation with same conditions. Non-aqueous solvents like toluene (C₆H₅CH₃) and cyclohexane (C₆Hi₂) without a lone pair cannot trigger a phase transformation under the same conditions.)

Example: Zirconia particles are dispersed in a resin base. The zirconia particles must be of such a size and doping content that water phase transforms the particles. The dispersion is then kept dry in a tube. When applied to a tooth as filling material, water from the tooth and normal air humidity in the mouth will trigger the phase transformation. In this application, the dispersion may only be used in a thin layer in order to provide water to the zirconia particles. Since the crystals are small, the expansion due to phase transformation will not cause deterioration of the mechanical properties of the cured compound. Therefore, transformation triggered by slow mechanisms, e.g., diffusion of water into the cured compound can happen after the curing.

Another example: Zirconia particles are dispersed in a resin base with monomers that release water in curing process. These monomers could contain both an amino group and a carboxylic group that in a condensation process eliminate water e.g. ω-aminocarboxylic acid or by an esterification reaction between monomers contain an acid and an alcohol group. As the curing process begins, the water, released the condensation process, will initiate the phase transformation of the zirconia particles and thereby compensate the shrinkage caused by the polymerisation.

In one variant, the chemical trigger is a constituent of the polymerizable resin base.

In another variant, the chemical trigger is a product arising upon polymerization of the resin base.

In still another embodiment, the martensitic transformation of the filler ingredient(s) is initiated by exposure of the filler ingredient(s) to tensile stress. Tensile stress of 200 MPa for ceramic sintered zirconia has proved to trigger a phase transformation. Upon curing of dental fillings, tensile stress of up to 20 MPa is observed. The making of more unstable metastable zirconia particles will reduce the force needed in the zirconia to induce a phase transformation.

Example: A dispersion of zirconia in a resin base can be light cured. The curing will cause a shrinkage that will result in a tensile stress which will phase transform the zirconia particles.

Method of the invention

In view of the above, the present invention also provides a method of controlling the volumetric shrinkage of a composite material upon curing, comprising the step of:

(a) providing a composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1:1.1;

Preferably, the filler ingredient(s) should be triggered to undergo the martensitic transformation either simultaneous with the curing or subsequent to the curing in order to fully benefit from the volumetric expansion of the filler ingredient(s).

In one embodiment, the martensitic transformation of the filler ingredient(s) is initiated by application of ultrasound (10-1000 kHz). In this instance, the martensitic transformation is preferably triggered simultaneously with or after the curing is initiated, but before the curing is completed.

In another embodiment, the martensitic transformation of the filler ingredient(s) is initiated by exposure of the surface of the filler ingredient(s) to a chemical trigger. In this instance, the martensitic transformation is preferably triggered simultaneously with or after the curing is initiated, but before the curing is completed.

More specifically, the present invention further provides a method of reconstructing a tooth, comprising the step of

(a) preparing a cavity in the tooth;

(b) filing said cavity with a dental filling material as defined above; and

(c) allowing the resin base of the dental filling material to polymerize and cure, and allowing the filler ingredient(s) of the dental filling material to undergo a martensitic transformation from a first metastable phase to a second stable phase.

The above-defined method for the reconstruction of a tooth may generally comprise further steps obvious to the person skilled in the art of dentistry. In one embodiment, the martensitic transformation of the filler ingredient(s) is initiated by application of ultrasound (10-1000 kHz). In another embodiment, the martensitic transformation of the filler ingredient(s) is initiated by exposure of the surface of the filler ingredient(s) to a chemical trigger.

More generally, the present invention also relates to a composite material as defined herein for use in medicine, in particular in dentistry.

The present invention also relates to the use of a filler ingredient for the preparation of a composite material for reconstructing a tooth in a mammal, said filler ingredient having a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1:1.1. The nanofillers, filler ingredient(s) and the composite material are preferably as defined herein.

Combined initiation of martensitic transformation and curing of resin base by means of ultrasound

The present inventors have also found that the initiation of martensitic transformation of the filler ingredient by means of application of ultrasound can advantageously be combined with the curing of the resin base by means of ultrasound.

It is believed that the application of ultrasound will provide the advantage that the curing process which results in a net volume reduction of the bulk will be countered by the volume expansion caused by the martensitic transformation of the filler ingredient.

Thus, in a further embodiment of the above methods, the polymerization of the resin base is initiated by application of ultrasound.

The applied ultrasound is (as above) typically in the range of 10 kHz to 10 MHz, preferably in the range of 15-50 kHz, such as 20-50 kHz. Sonic devices of lower frequency may also be used because cavitation is made from acoustic wave propagation, however, frequencies lower than 15 or 20 kHz can be heard by the normal human ear, and may therefore be inconvenient to use. With respect to the power of the applied ultrasound, this is typically in the range of 0.1-500 W/cm², e.g. 30-100 W/cm². The ultrasound power should, on the one hand, be high enough to create cavitation and, on the other hand, so low that the tooth is not harmed. The ultrasound is typically applied by means of a scaler. The ultrasound can be applied directly on the resin base bulk or indirect via a medium conducting the sound waves to the resin. For dental applications, a suitable medium would be the tooth in which the dental filling material is placed or a metal matrix typically used for making cavities in molar teeth.

Application of ultrasound with the aim of initiating polymerisation typically takes place for a period of in the range of 10-300 seconds, such as 20-120 seconds.

Although it is believed that no polymerisation initiator is strictly needed, it is believed that the polymerizable resin base advantageously comprises a polymerisation initiator, e.g. a polymerisation initiator selected from peroxy-group containing compounds and azo-group containing compounds (e.g. AIBN).

On the other hand, it is believed that polymerisation accelerators/co-initiators (e.g. EDMAB ethyl 4-dimethylaminobenzoate) can be omitted. Co-initiators are often added in order to conduct the initiation at room temperature. Conventional non-photo polymerisable dental materials are based on a two component resin system. The initiator, e.g. benzoylperoxide, and the co-initiator, e.g. EDMAB ethyl 4-dimethylaminobenzoate, are kept separate until use, where the two resins are mixed together. Adding co-initiator to the initiator makes it possible to cure the monomers at room temperature. Thus, contrary to conventional non-photo polymerisable dental filling materials, the dental filling materials of some embodiments of the invention may be prepared, stored and shipped as a one-component system.

The general advantages of this aspect of the present invention are, i.a., that ultrasound has a large penetration depth compared normal light curing (Blue light-curing) used in dentistry, that the packing of the filler particles may be improved, and that curing of the resin base may be conducted while the martensitic transformation of the filler ingredient(s) takes place. Applying ultrasound to a filler-based resin makes the filler particles move and thereby letting the particle find the optimal packing in the cavity. This means that even small cracks in the cavity, will be filled with filler particles (and monomer resin).

The majority of organic polymers are prepared from monomers containing a reactive double bond, which undergo chain growth or addition reactions. The most straightforward preparative method is that initiated by radicals. Ultrasound at high power (at least 1 W/cm²) creates cavitation. As ultrasound passes through a liquid, the expansion cycles exert negative pressure on the liquid, pulling the molecules away from one another. Once made, the cavity will absorb energy and grow. Once the cavity has overgrown, either at high or low sonic intensities, it can no longer absorb energy as efficiently. Without the energy input the cavity can no longer sustain itself. The surrounding liquid rushes in, and the cavity implodes. It is the implosion of the cavity that creates an unusual environment for chemical reactions. Disruption of the cavity bubbles create high temperature and pressure, this is stated in literature as the reason for the radicals being produced with ultrasound. The radicals then initiate the polymerizing reaction leading to a fully cured dental material.

Curing of the resin base of a dental filling material by means of ultrasound

The present inventors have also found that curing of dental filling materials by means of ultrasound in itself provides certain advantages over the use of conventional curing methods, in particular Light-curing, in particular in view of the fact that ultrasound has a large penetration depth compared normal light (e.g UV-light).

Thus, in a further aspect, the present invention provides a method of reconstructing a tooth comprising the step of

(a) preparing a cavity in the tooth;

(b) filling said cavity with a dental filling material comprising a polymerizable resin base; and

(c) applying ultrasound to said dental filling material so as to initiate curing of said resin base of said dental filling material.

The provisions with respect to the frequency (10 kHz to 10 MHz), power (0.1-500 W/cm²) and application time (10-300 seconds) are as defined further above.

The filling material is in particular as defined above, thus in one embodiment, the dental filling material comprises:

30-90% by weight of the one or more fillers (including nanofillers); and 10-70% by weight of the polymerizable resin base.

More particularly, the one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1: 1.1.

Preferably, the filler ingredient(s) should be triggered to undergo the martensitic transformation either simultaneous with the curing or subsequent to the curing in order to fully benefit from the volumetric expansion of the filler ingredient(s). More preferably, the martensitic transformation is preferably triggered simultaneously with or after the curing is initiated, but before the curing is completed.

In one embodiment, however, the polymerizable resin base comprises a polymerisation initiator, e.g. selected from peroxy-group containing compounds and azo-group containing compounds (e.g. AIBN).

A population ofzirconia particles

It has been found that metastable zirconia may be used as a particularly suitable filler in composite materials. In particular, it has been found that zirconia which is capable of allowing a martensitic transformation to a stable second phase is particularly useful in order to counter the shrinkage normally occurring in composite materials.

Thus, a further aspect of the present invention relates to a population of zirconia particles having an average particle size in the range of 50-2000 nm, said particles being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, said transformation being effected to an extent of at least 80% within 300 sec when tested in the "Zirconia Particle Transformation Test" defined herein.

Furthermore, the present invention also relates to method for preparing such populations of zirconia particles.

The zirconia particles of the above-defined populations are present in a metastable first phase and are able to undergo a martensitic transformation to a stable second phase. Preferably, the volume ratio between said stable second phase and said metastable first phase of said zirconia particles is at least 1.005, such as at least 1.01 or even at least 1.02 or at least 1.03. As mentioned above, the particles of the population of the first aspect of the invention are present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, said transformation being effected to an extent of at least 80% within 300 sec when tested in the "Zirconia Particle Transformation Test" defined herein. Preferably, the transformation is effected to an extent of at 80% within 10-100 sec, such as within 20-60 sec.

Thus, when considering various crystal forms and particle sizes of the zirconia particles, it is relevant to take into consideration the two main requirements:

1. A first requirement for the zirconia particles is that the second crystalline phase thereof, within the selected particle size range, is "stable" under "standard" conditions, i.e. standard pressure (101.3 kPa) and at least one temperature in the range of 10-50⁰C, i.e. corresponding to the conditions under which the product (typically a composite material) is used.

2. A second requirement for the zirconia particles is that a metastable first crystalline phase of the zirconia particles can exist the under the same "standard" conditions.

For zirconia in the metastable tetragonal or cubic crystalline phase, the particle size is preferably in the range of 50-2000 nm, though it is believed that a mean particle size in the range of 50-1000 nm provides the best balance between optical and structural properties.

The zirconia particles are able to undergo the martensitic transformation under the influence of ultrasound. The zirconia particles may also undergo the martensitic transformation upon exposure to a chemical trigger.

In view of the above, the filler ingredient(s) preferably include(s) zirconia (ZrO₂) in metastable tetragonal or cubic crystalline phase.

Stabilization of the metastable phase can, e.g., be achieved by doping, by surface modification of the zirconia, etc. as it is explained hereinabove.

Embodiments

In order to obtain zirconia particles that could undergo a fast phase transformation, a large surface area, e.g. 10-250 m²/g or even better 50-200 m²/g, of the particles is preferred and also obtainable by the means described herein. Thus, a further aspect of the present invention relates to a population of zirconia particles having an average particle size in the range of 50-2000 nm and a BET surface area of in the range of 10-250 m²/g, said particles being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase.

Preferably, this population of zirconia particles allows for a martensitic transformation to be effected to an extent of at least 80% within 300 sec when tested in the "Zirconia Particle Transformation Test" defined herein.

As mentioned above, the average particle size is typically in the range of 50-2000 nm, such as in the range of 50-1000 nm, in particular 100-600 nm.

Although the particles size of the zirconia particles generally is in the range of 50-2000 nm, it is believed that the particles may comprise smaller crystal domains with a homogeneous crystal lattice. Accordingly, it is preferred that the particles have crystal domain sizes in the range of 1-100 nm, such as in the range of 4-50 nm, such as 5-9 nm.

Furthermore, it is believed that the zirconia particles advantageously may have a certain porosity in order to allow for a rapid transformation (as described herein). Thus, the average pore size of the particles is preferably in the range of 10-50 nm.

With respect to the porosity, it is believed that zirconia particles having a porosity in the range of 0.1-20%, such as 0.2-10%, are particularly interesting.

Particularly interesting populations are those where the zirconia particles have

a. an average particle size in the range of 50-2000 nm and a BET surface area of in the range of 10-250 m²/g, or

b. an average particle size in the range of 50-1000 nm and a BET surface area of in the range of 10-250

or

c. an average particle size in the range of 100-600 nm and a BET surface area of in the range of 10-250 m²/g/ or

d. an average particle size in the range of 50-2000 nm and a BET surface area of in the range of 50-200 m²/g_/ or e. an average particle size in the range of 50-1000 nm and a BET surface area of in the range of 50-200 m²/g, or

f. an average particle size in the range of 100-600 nm and a BET surface area of in the range of 50-200 m²/g, or

g. an average particle size in the range of 50-2000 nm and a BET surface area of in the range of 50-80 m²/g, or

h. an average particle size in the range of 50-1000 nm and a BET surface area of in the range of 50-80 m²/g, or

i. an average particle size in the range of 100-600 nm and a BET surface area of in the range of 50-80 m²/g, or

j. an average particle size in the range of 50-2000 nm and a BET surface area of in the range of 75-150 m²/g, or

k. an average particle size in the range of 50-1000 nm and a BET surface area of in the range of 75-150 m²/g, or

I. an average particle size in the range of 100-600 nm and a BET surface area of in the range of 75-150 m²/g, or

m. an average particle size in the range of 50-2000 nm and a BET surface area of in the range of 125-200 m²/g, or

n. an average particle size in the range of 50-1000 nm and a BET surface area of in the range of 125-200 m²/g, or

o. an average particle size in the range of 100-600 nm and a BET surface area of in the range of 125-200 m²/g, or

p. an average particle size in the range of 100-350 nm and a BET surface area of in the range of 50-80 m²/g, or

q. an average particle size in the range of 250-500 nm and a BET surface area of in the range of 50-80 m²/g, or r. an average particle size in the range of 400-600 nm and a BET surface area of in the range of 50-80 m²/g, or

s. an average particle size in the range of 100-350 nm and a BET surface area of in the range of 75-150 m²/g_/ or

t. an average particle size in the range of 250-500 nm and a BET surface area of in the range of 75-150 m²/g_/ or

u. an average particle size in the range of 400-600 nm and a BET surface area of in the range of 75-150 m²/g, or

v. an average particle size in the range of 100-350 nm and a BET surface area of in the range of 125-200 m²/g, or

w. an average particle size in the range of 250-500 nm and a BET surface area of in the range of 125-200 m²/g, or

x. an average particle size in the range of 400-600 nm and a BET surface area of in the range of 125-200 m²/g.

Preparation of a population of zirconia particles

The populations of particles defined above may be prepared by one of the methods described in the following.

Method A

One method for the preparation of a population of the above-defined zirconia particles involves heating of amorphous zirconia within a narrow temperature range. Thus, the present invention provides a method for the preparation of a population of zirconia particles as defined hereinabove, said method comprising the step of heating a sample of amorphous zirconia to a temperature within the crystal formation temperature and not higher than the transition temperature of the zirconia from tetragonal to monoclinic both can determined by DSC or XRD. Heating a sample to a temperature that is below the crystal formation temperature will lead to a sample with few or none crystals with no possibility of phase transformation. Heating a sample to a temperature that is much higher (e.g. 200 K higher) than the crystal formation will gradually turn the sample from the tetragonal phase to a monoclinic phase. However this may be preferably to have heated to a temperature somewhat (say 20⁰C) higher than the crystal formation temperature. This ensures that the zirconia is transformed from the amorphous state into the tetragonal phase.

The heating process can be done in normal air standard pressure, but preferably in dry air because humidity (water) promotes the monoclinic phase of zirconia. A dry air flow is therefore preferably, other dry inert atmospheres such as nitrogen, Argon or helium could also be used. Since a controlled heating is necessary in order not to create overshoot depending on the oven a heating ramp of 5⁰C is useful. Once reached the set-point temperature the sample should be kept at that temperature long (say 30-120 min) enough to enable the crystallisation process to occur, but not to long (say 8 hours) since sintering of the crystals could create too much of the monoclinic phase.

Preferably, the amorphous zirconia particles have a BET surface area of in the range of 250- 550 m²/g, or 250-500 m²/g, such as in the range of 350-500 m²/g-

Such amorphous zirconia may be synthesized from a zirconate, e.g. ZrOCI₂ ¹SH₂O₇ by precipitation with a basic solution, e.g. a NH₃ solution. After precipitation and filtration, the zirconia is preferably digested at 100⁰C in deionised water for a suitably period of time, e.g. in the range of ¹/_-48 hours, such as in the range of 6-12 hours. Alternatively, the amorphous zirconia is synthesized from a zirconate, e.g. ZrOCI₂ SH₂O, by precipitation with a basic solution at pH 10, e.g. a cone. NH₃ solution. After precipitation, the zirconia is preferably digested under reflux (at 100⁰C) in the mother liquid for a suitably period of time, e.g. in the range of 6-24 hours, such as in the range of 8-20 hours.

Method B

Another method for the preparation of a population of the above-defined zirconia particles involves the step forming a suspension of a powder of small tetragonal crystals of zirconia in a strong aqueous base e.g. alkali base such as KOH or NaOH under reflux for 24 h. The crystals are then grown in a strong base suspension (1-5 M) to a size, where the bulk energy of the crystals becomes comparable to the surface energy stabilising the tetragonal phase, thus, lowering the activation barrier. The crystals, are grown under hydrothermal conditions e.g. high temperatures in the range of 150-200⁰C using a closed reactor (an autoclave, pressure reactor) only with use of waters vapour pressure (because of the heating) creating pressures up to 20 bars. Under these conditions a resolvation and reprecipitation takes place. To achieve large enough crystals the zirconia particles must remain in the pressure reactor for 24 h.

Preferably, the suspension is heated for a period of not less than 2 hours.

Composite materials

Generally, the populations of particles defined above are believed to be particularly useful as filler ingredients in composite materials. In particular, the zirconia particles of the present invention are useful for applications where volumetric shrinkage upon curing of the composite material would otherwise be undesirable or even prohibitive.

More particularly, the present invention provides a composite material comprising one or more fillers (including the zirconia particles defined herein) and a polymerizable resin base.

A particular feature of the present invention is that the martensitic transformation of the zirconia particles can be provoked by a trigger mechanism.

Thus, in a preferred embodiment of the composite material, the resin base, upon polymerization and in the absence of any compensating effect from the zirconia particles, causes a volumetric shrinkage (ΔV_resin) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said zirconia particles, exhibits a total volumetric shrinkage (ΔV_totai) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_reSi_n) caused by the resin base. More particularly, the volumetric shrinkage (ΔV_resin) is at least 1.00%, such as at least 1.50%, and the total volumetric shrinkage (ΔV_totaι) is at least 0.50%-point less, such as 1.00%-point less than the uncompensated volumetric shrinkage.

The composite material typically comprises 5-95%, or 10-90%, by weight of the one or more fillers (including the zirconia particles) and 5-95%, or 10-90%, by weight of the polymerizable resin base, in particular 30-95%, or 30-90%, by weight of the one or more fillers and 5-70%, or 10-70%, by weight of the polymerizable resin base.

Calculated by volume, the composite material typically comprises 20-80% by volume of the one or more fillers (including zirconia particles) and 20-80% by volume of the polymerizable resin base, such as 25-80%, or 25-75%, by volume of the one or more fillers and 25-75% by volume of the polymerizable resin base. Preferably, the composite material is substantially solvent free and water free. By the term "substantially solvent free and water free" is meant that the composite material comprises less than 4.0%, such as less than 1.0% or less than 0.5%, by weight of solvents and/or water.

Alternatively, the present invention provides a composite material comprising one or more fillers (including zirconia particles) and a polymerizable resin base, wherein said one or more fillers comprises metastable zirconia in the tetragonal or cubic crystalline phase, wherein said resin base, upon polymerization and in the absence of any compensating effect from the zirconia particles, causes a volumetric shrinkage (ΔV_reSi_n) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said filler ingredient(s), exhibits a total volumetric shrinkage (ΔV_tot_ai) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_resIn) caused by the resin base, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.95 to 1:1.05, and wherein the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers is in the range of 10:90 to 100:0, preferably 10:90 to 40:60.

It is apparent that the one or more fillers, and in particular the zirconia particles, are important constituents of the composite material. Fillers are generally described above under "Fillers/Filler ingredients".

The one or more fillers comprise at least one filler ingredient which (for the purpose of this section) at least include the zirconia particles. The term "filler ingredient" is intended to mean the filler or a fraction of the filler having particular physical properties, namely the inherent ability to compensate (by expansion) for volumetric shrinkage caused by polymerization and curing of the resin base.

The zirconia particles typically constitute(s) 20-100% of the total weight of the one or more fillers, e.g. 30-100%, such as 40-100% or 50-100%.

When calculated on the basis of the total weight of the composite material, the zirconia particles typically constitute(s) 15-90% of the total weight of the composite material, e.g. 25-90%, such as 30-90%, more specifically 60-85%. Another important constituent of the composite material is the polymerizable resin base which is described in detail under "Polymerizable resin base".

The composite material may comprise other constituents as disclosed under "Other constituents of the composite material".

The population of zirconia particles is particularly useful in connection with dental filling material, see, e.g., under "Dental filling materials". The general use of the population of zirconia particles in composite materials is described above under "Use of the composite materials".

The initiation of martensitic transformation of the population of zirconia particles by means of application of ultrasound can advantageously be combined with the curing of the resin base by means of ultrasound, see, e.g., under "Combined initiation of martensitic transformation and curing of resin base by means of ultrasound".

EXAMPLES

Zirconia Particle Transformation Test

A test composite material is prepared by mixing 65 vol% of the zirconia particles to be tested and 35 vol% of a polymer resin system (36% (w/w) BisGMA, 43.35% (w/w) UDMA, 20% (w/w) TEGDMA. 0.3% (w/w) camphorquinone (CQ), 0.3% (w/w) N,N~dimethyl-p-amino- benzoic acid ethylester (DABE) and 0.05% (w/w) 2,6-di-te/t-butyl-4-methylphenol (BHT)).

The test composite material is arranged in a cylindrical cavity having a diameter of 4 mm and a depth of 20 mm at 37°C. Ultrasound is applied using an ultrasound sealer EMS PIEZON MAster 400 TM (28.5 kHz; 100 W/cm²) for 300 sec. The tip of ultrasound sealer is placed directly into the mixture.

The phase transformation is measured with the use of powder XRD. The volume fraction of monoclinic zirconia V_m can be determined from the following relationships:

X_m=(I_m(lll) + I_m(ll-l))/( UlU) + I_m(ll-1) + I₁(Hl))

V_m=1.311 X_m/(l+0.311X_m) Where I_m(lll) and I_m(ll-1) are the line intensities of the (111) and (11-1) peaks for monoclinic zirconia and I₁(IIl) is the intensity of the (111) peak for tetragonal zirconia.

Exampel 1 : Composite material comprising silicon carbide nanoparticles

A composite material is prepared by mixing 22 % (w/w) of the SiC nanoparticles into 10 % (w/w) of a monomer resin system (36% (w/w) BisGMA, 43.35% (w/w) UDMA, 20% (w/w) TEGDMA. 0.3% (w/w) camphorquinone (CQ), 0.3% (w/w) N,N-dimethyl-p-aminobenzoic acid ethylester (DABE) and 0.05% (w/w) 2,6-di-tert-butyl-4-methylphenol (BHT)). This transparent suspension is then mixed with 68 % (w/w) of the metastable zirconia particles to achieve a more transparent composite.

Example 2: Composite material comprising SrTiOs nanoparticles

A composite material is prepared by mixing 31 % (w/w) of the SrTiO₃ nanoparticles into 9 % (w/w) of a polymer resin system (36% (w/w) BisGMA, 43.35% (w/w) UDMA, 20% (w/w) TEGDMA. 0.3% (w/w) camphorquinone (CQ), 0.3% (w/w) N,N-dimethyl-p-aminobenzoic acid ethylester (DABE) and 0.05% (w/w) 2,6-di-tert-butyl-4-methylphenol (BHT)). This transparent suspension is then mixed with 60% (w/w) of the metastable zirconia particles to achieve a more transparent composite.

Example 3: Composite material comprising zirconia nanoparticles

A composite material is prepared by mixing 65 % (w/w) of a population of metastable zirconia particles consisting of 30 %(w/w) nanoparticles and 70 % (w/w) of the micro sized zirconia particles into 35 vol% of a polymer resin system (36% (w/w) BisGMA, 43.35% (w/w) UDMA, 20% (w/w) TEGDMA. 0.3% (w/w) camphorquinone (CQ), 0.3% (w/w) N,N-dimethyl- p-aminobenzoic acid ethylester (DABE) and 0.05% (w/w) 2,6-di-te/t-butyl-4-methylphenol (BHT)).

Example 4: Composite material comprising glass particles

A composite material is prepared by mixing 10 % (w/w) glass particles into 11 vol% of a polymer resin system (36% (w/w) BisGMA, 43.35% (w/w) UDMA, 20% (w/w) TEGDMA. 0.3% (w/w) camphorquinone (CQ), 0.3% (w/w) N,N-dimethyl-p-aminobenzoic acid ethylester (DABE) and 0.05% (w/w) 2,6-di-tert-butyl-4-methylphenol (BHT)). This transparent suspension is then mixed with 79 % (w/w) of the metastable zirconia particles to achieve a more transparent composite.

Claims

1. A composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) being present in a metastable first phase and being able to undergo a martensitic transformation to a stable second phase, the volume ratio between said stable second phase and said metastable first phase of said filler ingredient(s) being at least 1.005, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.9 to 1:1.1.

2. The composite material according to claim 1, wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.95 to 1: 1.05.

3. The composite according to any one of the preceding claims, wherein the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers is in the range of 10:90 to 100:0, preferably 10:90 to 40:60.

4. The composite according to any one of the preceding claims, wherein said resin base, upon polymerization and in the absence of any compensating effect from the one or more filler ingredients, causes a volumetric shrinkage (ΔV_reSi_n) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said filler ingredient(s), exhibits a total volumetric shrinkage (ΔV_tota|) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_resjn) caused by the resin base.

5. The composite material according to any one of the preceding claims, comprising:

30-95% by weight of the one or more fillers, including nanofillers; and 5-70% by weight of the polymerizable resin base.

6. The composite material according to claim 5, wherein the nanofillers constitute 10-50% by weight, such as 20-40% by weight, of the composite material.

7. The composite material according to any one of the preceding claims, which comprises less than 4% (w/w) of solvents and/or water.

8. A composite material comprising one or more fillers and a polymerizable resin base, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) including metastable zirconia in the tetragonal or cubic crystalline phase, wherein said resin base, upon polymerization and in the absence of any compensating effect from the one or more filler ingredients, causes a volumetric shrinkage (ΔV_resin) of the composite material of at least 0.50%, and wherein said composite material, upon polymerization of said resin base and upon phase transformation of said filler ingredient(s), exhibits a total volumetric shrinkage (ΔV_tota,) of at least 0.25%-point less than the uncompensated volumetric shrinkage (ΔV_resin) caused by the resin base, and wherein said one or more fillers include a fraction of nanofillers having a refractive index of at least 1.8, and wherein the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers is in the range of 1:0.95 to 1: 1.05, and wherein the weight ratio between (i) the nanofillers and (ii) fraction of the one or more fillers not being said nanofillers is in the range of 10:90 to 100:0, preferably 10:90 to 40:60.

9. The composite material according to claims 8, comprising:

10. The composite material according to claim 9, wherein the nanofillers constitute 10-50% by weight, such as 20-40% by weight, of the composite material.

11. The composite material according to any one of the claims 5-6, which comprises less than 4% (w/w) of solvents and water.

12. The composite material according to any one of the preceding claims, which is a dental filling material.

13. The dental filling material according to claim 12, wherein the filler ingredient(s) of the composite material include(s) zirconia (ZrO₂) in metastable tetragonal or cubic crystalline phase.

14. The dental filling material according to claim 13, consisting of: 40-85% by weight of the one or more fillers, wherein said one or more fillers comprise at least one filler ingredient, said filler ingredient(s) include(s) metastable zirconia in the tetragonal or cubic crystalline phase, and wherein said one or more fillers include a fraction of nanofillers, the ratio between (i) the refractive index of the combination of the polymerizable resin base and the nanofillers and (ii) the fraction of the one or more fillers not being said nanofillers being in the range of 1:0.9 to 1: 1.1;

15-60% by weight of the a polymerizable resin base, said resin base being based on one or more compound selected from the group consisting of methacrylic acid (MA), methylmethacrylate (MMA), 2-hydroxyethyl-methacrylate (HEMA), triethyleneglycol dimethacrylate (TEGDMA), bisphenol-A-glycidyl dimethacrylate (BisGMA), bisphenol-A-propyl dimethacrylate (BisPMA), urethane-dimethacrylate (UEDMA), and HEMA condensed with butanetetracarboxylic acid (TCB); 0-5% by weight of additives; and 0-4% by weight of solvents and/or water.

15. A method of controlling the volumetric shrinkage of a composite material upon curing, comprising the step of:

16. The method according to claim 15, wherein the composite material is as defined in any one of claims 1-14.

17. A composite material as defined in any one of the claims 1-14 for use in medicine, in particular in dentistry.