In: Dental Composites Editor: Frederick C. Calhoun
ISBN: 978-1-61728-933-0 © 2009 Nova Science Publishers, Inc.
Chapter 8
TRENDS IN DENTAL POLYMER NANOCOMPOSITES D. Irini Sideridou* Associate Professor in Chemistry and Technology of Polymers, Lab of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki
BACKGROUND Nanotechnology is offering us the ability to design materials with totally new characteristics. Nanotechnology is also known as molecular nanotechnology or molecular engineering is the production of functional materials and structures in the range of 0.1 to 100 nanometers-the nanoscale-by various physical or chemical methods [1].Today the revolutionary development of nanotechnology has become the most highly energized discipline in science and technology [2]. The intense interest in using nanomaterials stems from the idea that they may be used to manipulate the structure of materials to provide dramatic improvements in electrical, chemical, mechanical and optical properties [3]. A nanometer is 1/1,000,000,000 (one-billionth) of a meter or 1/1,000 of a micron. This is about 10 times the diameter of a hydrogen atom, 1/1,000 the size of a small bacterium or 1/80,000 of a human hair. Frequently, nanotechnology is used to describe research or products where critical component dimensions are in the range of 0.1 to 100 nanomers. In theory nanotechnology can be used to make products lighter, stronger, cheaper and more precise. If this type of material was used to make an airplane instead of metal the airplane could weigh 50 times less but be just as strong. A servey in 1998 revealed that industries where micro-engineered products are the most desirable are in electronics and biomedicine. The current status of this technology is in creating valueadded products. At a nanotechnology fair held in Hanover, Germany (April 2002) some of the products that were displayed, included a lacquer that can be applied over automobile paint to make it a scratch-resistant and transparent coating for glass that shields against UV rays. Other areas currently being commercialized include a nanofiber mesh to prevent body tissues from sticking together preventing the formation of scar tissue, chips-the size of a compact disc-could replace central air conditioning units, a
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Corresponding author: e-mail:
[email protected]
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D. Irini Sideridou ultra thin monitor that looks like paper but can change what it displays like a computer screen or LCD and batteries that last longer. Nanocomposites are a new class of composites that are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range. A large amount of research is being devoted to development of nanocomposites of different types for various applications, including structural materials, high performance coatings, catalysts, electronics, photonics and biomedical systems [4]. Every property has a critical length scale and by using building blocks smaller than the critical length scale –such as nanoparticles one can capitalize on the manifestations of physics at small sizes. An example of this is in light scattering. When a particle shrinks to a fraction of the wavelength of visible light (0.4-0.8 μm) then it would not scatter that particular light resulting in the human eye’s inability to detect the particles. This has tremendous implications for the optical properties of materials [5]. One of the most significant contributions to dentistry has been the development of polymer based composite technology. Adhesively bonded composites have the advantage of conserving sound tooth structure with the potential for tooth reinforcement, while at the same time providing a cosmetically acceptable restoration. .
Dental Polymer Composites Dental polymer composites are interconnected heterogeneous materials that generally have three discernable phases: (1) a polymeric matrix or continuous phase formed by polymerization of one or more monomer/oligomers (2) a higher modulus dispersed phase consisting of fillers of various types (silica, ceramic etc) sizes, shapes and morphologies and (3) an interfacial or interphasial phase that bonds to both the continuous and dispersed phases, thereby enhancing the moduli and mechanical properties of the weaker polymer phase and also facilitating stress transfer between these phases by forming a unitary material. Adhesion of lower moduli polymer matrices to higher moduli inorganic fillers can occur as a result of van der Waals forces, ionic interactions, hydrogen bonding, ionic or covalent bonding, interpenetrating polymer network formation and for certain types of fillers by micromechanical interlocking mechanisms. For most mineral reinforced dental composites the primary interphasial linkage between the polymer matrix and the filler is by chemical bond formation mediated by a silane coupling agent [6]. Usually the organic matrix is based on methacrylate chemistry, especially cross-linking dimethacrylates like : (a) 2,2-bis[4,2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (Bis-GMA) (b) ethoxylated Bis-GMA (Bis-EMA), (c) 1,6-bis[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimetylhexane (UDMA), (d) dodecanediol dimethacrylate (D3MA) or (e) triethylene glycol dimethacrylate (TEGDMA) (Figure 1) [7-10]. The first three monomers are used as base monomers and the last two as diluents which are added in order to control the viscosity of the organic matrix to be suitable to accept a large amount of the inorganic filler.
Evolutionarily Designed Quantum Information Processing of Coherent States… CH3
CH3
CH2=C
C=CH2
C=O
C=O
O
OH
CH2
CH CH2
CH3 O
O
CH2
OH
O
CH
CH2
CH3
(a) Bis-GMA CH3
CH3
CH2=C
C=CH2
C=O
C=O
O
O
CH3
CH2
CH2
O
O
CH2
CH2
CH3
(b) Bis-EMA CH3
CH3
CH2=C
C=CH2
C=O
C=O CH3
O CH2
CH2
O
C
CH3
NH CH2 C CH2 C
O
O CH2
CH2 NH
C
O
CH2
CH2
O
CH3
(c) UDMA CH3
CH3
CH2=C
C=CH2
C=O
C=O
O
O
CH2
CH2
CH2 CH2
O
O
CH2
CH2
(d) TEGDMA CH3
CH3
CH2=C
C=CH2
C=O
C=O
O
O
CH2
CH3
CH2 10
(e) D3MA Figure 1. Dimethacrylates mostly used in dental polymer composites for the preparation of the organic polymeric matrix; (a)-(c): base monomers. (d) and (e): diluents
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Figure 2. Schematic representation of the polymerization of dimethacrylate monomers to form the cross-linked polymer network of dental composites containing many unreacted pendant methacrylate groups (-C=C-) [11]
The free radical polymerization of the matrix monomers leads to a three-dimensional network (Figure 2) [11]. Most of the contemporary dental polymer composites are lightcuring composites which polymerize, harden by irradiation with visible light in the wavelength range 400-500nm. Nearly all composite manufacturers are using camphorquinone as the photoinitiator. The absorption maximum of camphorquinone is at 468 nm. Amines are the most frequently used co-initiators [12]. The fillers used in dental composites directly influence the radiopacity, mechanical properties such as hardness, flexural and compressive strength and thermal coefficient of expansion. The use of heavy metals such as barium and strontium incorporated in the glass provide radiopacity. Dimethacrylate monomers have a high coefficient of thermal expansion. This coefficient is reduced by the addition of fillers and ideally dental composites should have similar coefficients of thermal expansion to enamel and dentine of tooth, which is 17x10-6/oC and about 11x10-6/oC respectively [17]. The fillers provide the ideal means of controlling various aesthetic features such as color, translucency and fluorenscene. Polymerization shrinkage largely correlates with the volumetric amount of the filler in the composite. By incorporating large amount of fillers the shrinkage is mauch reduced because the amount of resin used is reduced and the filler does not take part in the polymerization process. However, shrinkage is not totally eliminated and will depend on the structure of monomers used and the amount of filler incorporated. One of the most important considerations in the selection of filler is the optical characteristics of the composite. The monomers used in dental composites have a refractive index of approximately 1.55. Fillers with refractive indices which differ greatly from this value will cause the composite to appear optically opaque, creating an esthetic and curing problem. Because glasses can have refractive indices ranging from 1.4 to 1.9 the selection of the appropriate filler for dental composites must be guided by a consideration of this important variable. The filler most used until quite recently was fused or crystalline quartz
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and various borosilicate or lithium aluminosilicate glasses. The glass or quartz was ground or milled into particles of various sizes ranging from approximately 0.1μm to 100 μm. The major advantage to using quartz was that it is readily available and has an excellent optical match to the polymer matrix. However quartz has drawbacks in that it is not radiopaque and can be very abrasive to enamel. These characteristics ensured that as the surface of the composite was abraded the polymer would wear away more quickly than the fillers leaving them raised and exposed from the surface. This made the surface of the restoration rough and less enamel-like due to appreciable scattering of incident light. Thus polishability and esthetics were compromised. Most current composites are filled with radiopaque silicate particles based on oxide of barium, strontium, zinc, aluminum or zirconium [11]. The average particle size and particle size distribution of the filler is important as it determines the amount of the filler that can be added to the monomers, without the necessary handling characteristics being lost. Particle size also has a pronounced effect on the final surface finish of the dental composite in that the smaller the filler particle size the smoother the composite will be [13]. The composites have been classified according to the type of filler employed into three main groups, the traditional or macrofilled composites the hybrid or blended composites and the microfilled composites [9,11,13,14-16]. The macrofilled composites contain glass filler particles with a mean particle size of 10-20 μm and a largest particle size of 40 μm. These composites had the disadvantage that the surface finish was very poor with the surface having a dull appearance due to the filler particles produting from the surface as the resin preferentaially removed around them. These composites are significantly less frequently used nowdays because of esthetic reasons. The hybrid composites contain large filler particles of an average size of 15-20 μm and also a small amount of colloidal silica which has a particle size of 0.01-0.05 μm. It should be noted that virtually all composites now contain small amounts of colloidal silica but their behavior is very much dteremined by the size of the larger filler particles [13]. Microfilled composites containing amorphous silica were developed to address the polishing requirements of anterior restorations. Thses silicon dioxide particles are submicroscopic, averaging approximately 0.04 μm in diameter, though the size varies among materials. Because the filler particles in a microfilled composite are so small, they have from 1,000 to 10,000 times as much surface area as filler particles in conventional composites. The increased surface area must be wetted by the monomer matrix and which results in a significant increase in viscosity. This increase in viscosity limits the percentage filler content of the composite to approximately 35 wt%, which in turn limits the strength and stiffness of the composite. During the initial development of dental composites it was shown that the acquisition of good properties in the composite was dependent upon the formation of a strong bond between the inorganic filler particles and the organic polymer matrix. If there is a breakdown of this interface, the stresses developed under load will not be effectively distributed through out the composite. The interface will act as a primary source for fracture leading to the subsequent disintegration of the composite. In most mineral reinforced dental composites the primary interphasial likage between the polymer matrix and the filler phase is by chemical bond formation mediated by a dual functional organosilane, termed a silane coupling agent [6,1719]. In dental composites based on dimethacrylates, adhesion between the polymeric matrix and the reinforcing filler is usually achieved by the used of the silane coupling agent 3methacryloxypropyltrimethoxysilane or γ-MPS. This is a bifunctional molecule capable of
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reacting via its alkoxysilane groups with the filler and itself and with the polymer matrix by virtue of its methacrylate functional group. The overall degrees of reaction of the silane with the glass filler (oxane bond formation) with itself (by siloxane formation) and with the polymer matrix (by graft copolymerization) determine the efficacy of the coupling agent.
Dental Polymer Nanocomposites No one composite material has been able until now to meet both the functional needs of a posterior Class I and II restoration and the superior esthetics required for anterior restorations. Many attempts are made to develop a composite filling material that could be used in all areas of the mouth with high initial polish and superior polish retention (typical of microfills) as well excellent mechanical properties suitable for high stress-bearing restorations (typical of hybrid composites). To this end novel nanofillers were developed and then nanocomposites using advanced methacrylate resins and curing technologies [5]. Nanofillers are very different from traditional fillers and require a shift from a topdown to a bottom-up manufacturing approach. To make filler particles of the mechanically strong composites of today (such as macrofills, hybrids and microfills) one starts from dense, large particles (mined quartz, melt glasses, ceramics) and comminutes them to small particle size. However these milling procedures usually cannot reduce the filler particle size below 100 nm (1nm =1/1000μm). To circumvent this roadblock, synthetic chemical processes were used to produce building blocks on a molecular scale. These materials then were assembled into progressively larger structures and transformed into nanosized fillers suitable for a dental composite. This attempt was made toward the development of a new dental commercial nanocomposite Filtek Supreme Universal Restorative (3M ESPE, Dental Products, St. Paul, Minn.) that has the esthetic properties required for cosmetic restorations and the mechanical properties necessary for posterior restorations [5]. In this study two new types of nanofiller particles were synthesized: nanomeric, or NM, particles and nanoclusters, NCs. The NM particles are monodisperse nonaggregated and nonagglomerated silica nanopartcles. Aqueous colloidal silica sols were used to synthesize dry dowders of nanosized silica particles 20 and 75 nm in diameter. These particles were treated with 3-methacryloxypropyltrimethoxysilane (γ-MPS). γ-MPS makes the filler compatible with the resin before curing to prevent any agglomeration or aggregation. Also two types of NC were synthesized using proprietary processes. The first type consists of zirconia-silica particles synthesized from a colloidal solution of silica and a zirconyl salt. The primary particle size of this NC filler ranges from 2 to 20 nm while the spheroidal agglomerated particles have a broad size distribution with an average particle size of 0.6 μm. The second type of NC filler which were synthesized from 75-nm primary particles of silica has a broad secondary particle size contribution with a 0.6 μm average. The surface of both types of nanocluster filler particles were treated with γ-MPS. The polymer matrix used in this work was consisted of Bis-GMA, Bis-EMA, UDMA and TEGDMA containing photoinitiators for light curing and stabilizers. Using statistically designed experimentation methodology many combinations of NC and NM fillers were studied to determine an optimal formulation for the Filtek Supreme Universal Restorative dental nanocomposite. The
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formulation for the dentine, body and enamel shades of Filtek Supreme Standard pastes contain zirconia-silica NCs and silica NPs. The effective primary particle size is 20 nm. The formulations of Filtek transluscent shades contain a filler predomentally composed of individual NM particles 75 nm in diameter and a minor amount of silica NCs [5] Figure 3 shows three transmission electron micrographs: (A) of a nanocomposite filled with 75-nm diameter NM particles only (B) of a nanocomposite filled with NCs alone (C) of a commercial composite made with large particle-size dense hybrid filler (particle size approximately 1 μm) The compressive and diametral strengths and the fracture resistance of the nnocomposites were equivalent to or higher than those of other commercial composites tested. The three body wear results of the nanocomposites were also statistically better than those of all other composites tested (microfill, hybrid and microhybrid).
Figure 3. Schematics and transmission electron microscopic images of composites. (A): composite with nanometric particles (x 60,000 magnification) (B) composite with nanocluster particles (x 300,000 magnification) (C) composite with hybrid fillers (x 300,000 magnification). nm : nanometers ; APS Average Particle Size; μm micrometer [5]
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Figure 4. Optical effect of nanocomposite material versus that of the other types of composites. (FST: Filtek Supreme Transluscent formulation) [5]
Nano-fillers also offer advantages in optical properties. In general it is desirable to provide low visual opacity in un-pigmented dental composites. This allows the clinician to construct a wide range of shades and opacities and thus provide highly esthetic restorations. In hybrid materials, fillers consist of particles averaging 1 mm in size. When particles and resin are mismatched in the refractive index which measures the ability of the material to transmit light, the particles will scatter light and produce opaque materials. In NM-particle materials the size of the particles is far below the wavelength of light, making them unmeasurable by the refractive index. When light comes in, long-wavelenght light passes directly through and materials show high translucency. As shown in Figure 4 the disks made with hybrid and microfill filler are rather opaque. The nanocomposite made predominantly with the NM particle filler (FST) is very clear as the background can be seen through the composite. In addition when placed on a black background the nanoparticles preferentially scatter blue light, giving the composite an opalescent effect. The ability to create a nanocomposite with a very low opacity provides the ability to formulate a vast range of shade and opacity options from the very transluscent shades needed for the incisal edge and for the final layer in multilayred restorations to the more opaque shades desired in the enamel, body and dentin shades [5]. Palin et al [20] studied the influence of short and medium-term immersion on water uptake and mechanical properties of a nanofilled composite (FiltekTM Supreme in body and translucent shades) compared with a conventional microhybrid composite (FiltekTM Z250). Strength degradation occurred at different rates between material types. Water uptake and mechanical properties of the test materials were influenced by the size and the morphology of the reinforcing particulate phase. The use of nanoparticles and associated agglomerates in modern nanocomposite exhibit distinct mechanical and physical properties compared with the conventional microhybrid composite. In a next work the authors studied composites with microhybrid (FiltekTM Z250) nanohybrid (Grandio) and nanofilled (FiltekTM Supreme) filler particle morphologies. Filler particles were provided by the manufacturer or separated from the unpolymerized resin using a dissolution technique. Filler particles were subjected to compression using a micromanipulation technique between a descending glass probe and a
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glass slide. The number of distinct fractures particles underwent was determined from force/displacement and stress/deformation curves and the force at fracture and pseudomodulus of stress was calculated. [21]. It was found that the “nanoclusters” of FiltekTM Supreme exhibited multiple fractures and a higher force at fracture compared with the spheroidal and irregular filler technologies. This was attributed to the ability of the “nanoclusters” to deform and collapse into pre-existing cluster porosities and through progressive fragmentation of the main cluster structure, which subsequently acted to absorb and dissipate propagating cracks. The authors suggested that incorporation of the “nanocluster” particles into the resin matrix as a complete system would have the potential to produce unique mechanical properties since deformation of the particle may enhance the resistance to crack propagation of the composite and subsequently improve the longevity of the restoration. Consequently as a continuation of this work a hypothesis was proposed where the bi-axial flexure strength (BFS) of a nanocluster-containing composite would differ markedly from microfill, microhybrid and nanohybrid composites following dry and wet cyclic pre-loading regimes. The nanocluster system exhibited distinctive properties in response to the cyclic fatigue pre-loading regimes, such as increased resistance to fracture and improved reliability in strength irrespective of environmental conditions. Consequently the hypothesis was accepted since nanoclusters provided a distinct reinforcement mechanism to the resin matrix. The eagglomerated nanoparticles produced an interconnected network wher the interstices were infiltrated with the silane coupling agent producing an interpenetrating phase composite structure. The combination of unique reinforcement and silane infiltration of structural porosities improved the damage tolerance and may enhance the clinical longevity of nanocluster composite restorations [22]. Characterization of three nanofilled composites (Supreme, Grandio and Grandio Flow) four universal hybrid (Point-4, Tetric Ceram, Venus, Z-100) and two microfilled (A110, Durafill VS) composites showed that nanofilled resin composites exhibit mechanical properties at least as good as those of universal hybrids and could thus be used for the same clinical indications as well as anterior restorations due to their high aesthetic properties [23]. In an effort to improve the mechanical properties of dental resin-based composites TiO2 nanoparticles (with diameter
