Characterization of nanofilled compared to universal and microfilled composites.

OBJECTIVES: The purpose of this study was to compare the inorganic fraction and the mechanical properties of three nanofilled composites with four universal hybrid and two microfilled composites. The degrees of conversion of the materials photopolymerized using halogen and LED units were also investigated. METHODS: Three nanofilled (Supreme, Grandio and Grandio Flow), four universal hybrid (Point-4, Tetric Ceram, Venus, Z 100) and two microfilled (A 110, Durafill VS) composites were used in this study. Their filler weight content was measured by thermogravimetric analysis. The morphology of the filler particles was determined using scanning-electron microscopy (SEM). Mechanical properties were measured: dynamic and static elastic moduli, flexural strength and Vickers microhardness. The degree of conversion in relation with the depth of polymerization of every material tested was evaluated using Raman spectrophotometry. RESULTS: Nanofilled resin composites show higher elastic moduli than those of universal and microfilled composites, except for the Z-100. The microfilled composites exhibit by far the lowest mechanical properties. The flexural strength does not appear as a discriminating factor in this study. The degrees of polymerization obtained with the halogen lamp are higher than those obtained with the LED lamp. SIGNIFICANCE: Nanofilled resin composites show mechanical properties at least as good as those of universal hybrids and could thus be used for the same clinical indications as well as for anterior restorations due to their high aesthetic properties.     

The effect of filler loading and morphology on the mechanical properties of contemporary composites

STATEMENT OF PROBLEM: Little information exists regarding the filler morphology and loading of composites with respect to their effects on selected mechanical properties and fracture toughness. PURPOSE: The objectives of this study were to: (1) classify commercial composites according to filler morphology, (2) evaluate the influence of filler morphology on filler loading, and (3) evaluate the effect of filler morphology and loading on the hardness, flexural strength, flexural modulus, and fracture toughness of contemporary composites. MATERIAL AND METHODS: Field emission scanning electron microscopy/energy dispersive spectroscopy was used to classify 3 specimens from each of 14 commercial composites into 4 groups according to filler morphology. The specimens (each 5 x 2.5 x 15 mm) were derived from the fractured remnants after the fracture toughness test. Filler weight content was determined by the standard ash method, and the volume content was calculated using the weight percentage and density of the filler and matrix components. Microhardness was measured with a Vickers hardness tester, and flexural strength and modulus were measured with a universal testing machine. A 3-point bending test (ASTM E-399) was used to determine the fracture toughness of each composite. Data were compared with analysis of variance followed by Duncan's multiple range test, both at the P<.05 level of significance. RESULTS: The composites were classified into 4 categories according to filler morphology: prepolymerized, irregular-shaped, both prepolymerized and irregular-shaped, and round particles. Filler loading was influenced by filler morphology. Composites containing prepolymerized filler particles had the lowest filler content (25% to 51% of filler volume), whereas composites containing round particles had the highest filler content (59% to 60% of filler volume). The mechanical properties of the composites were related to their filler content. Composites with the highest filler by volume exhibited the highest flexural strength (120 to 129 MPa), flexural modulus (12 to 15 GPa), and hardness (101 to 117 VHN). Fracture toughness was also affected by filler volume, but maximum toughness was found at a threshold level of approximately 55% filler volume. CONCLUSION: Within the limitations of this study, the commercial composites tested could be classified by their filler morphology. This property influenced filler loading. Both filler morphology and filler loading influenced flexural strength, flexural modulus, hardness, and fracture toughness.     

Influence of light-polymerization modes on the degree of conversion and mechanical properties of resin composites: a comparative analysis between a hybrid and a nanofilled composite

This study analyzed the influence of the light polymerization mode on the degree of conversion (DC) and mechanical properties of two resin composites: a hybrid (Filtek P60) and a nanofilled composite (Filtek Supreme). The composites were light activated by three light polymerization modes (Standard-S: 650 mW/cm2 for 30 seconds; High intensity-H: 1000 mW/cm2 for 20 seconds and Gradual-G: 100 up to 1000 mW/cm2 for 10 seconds + 1000 mW/cm2 for 10 seconds). The DC (%) was measured by FT-Raman spectroscopy. Flexural strength and flexural modulus were obtained from bar-shaped specimens (1 x 2 x 10 mm) submitted to the three-point bending test. Microhardness was evaluated by Knoop indentation (KHN). Data were analyzed by ANOVA and Student-Newman-Keuls multiple range test and linear regression analysis. The results showed the following DC: H > S > G (p < 0.0001) and hybrid > nanofilled (p < 0.005). Correlation was found between DC and the radiant exposure (R2 = 0.92). With respect to mechanical properties, only KHN was significantly influenced by the light polymerization mode, as follow: H > S = G (p < 0.0001). The hybrid composite presented higher flexural strength and flexural modulus than the nanofilled composite (p < 0.0001). No significant difference was found in KHN between thetwo composites (p = 0.1605). The results suggest that nanofilled composites may present a lower degree of conversion and reduced mechanical properties compared to hybrid composites.     

Sol-gel materials 2. Light-curing dental composites based on ormocers of cross-linking alkoxysilane methacrylates and further nano-components.

OBJECTIVES: The objective of this study was to investigate the use of ormocers, which were synthesized from amine or amide dimethacrylate trialkoxysilanes. Ormocers showed improved biocompatibility in dimethacrylate-diluent-free composite restoratives. Selected mechanical properties, such as flexural strength and flexural modulus of experimental composites containing ormocers were investigated. In addition, the influence of methacrylate-substituted ZrO2 clusters and SiO2 organosols on the mechanical properties of composites was studied. METHODS: The flexural strength and flexural modulus of elasticity were determined according to ISO 4049: 2000. For this purpose, test specimens (2mmx2mmx25mm) of the composites investigated were prepared in stainless steel moulds and light-cured (150mW/cm2, 2x180s). The flexural strength and flexural modulus of elasticity were measured after the samples had been stored in water for 24h at 37 degrees C. RESULTS: While visible light-cured dimethacrylate-diluent-free composite restoratives based on the investigated ormocers showed a similar flexural strength and flexural modulus of elasticity compared to composites that contain only dimethacrylates, their double bond conversion was considerable lower. The simultaneous addition of methacrylate-substituted ZrO2 clusters and SiO2 organosols to the ormocer composite improved the mechanical properties of the composites. SIGNIFICANCE: Ormocers of amine or amide dimethacrylate trialkoxysilanes enabled the preparation of dimethacrylate-diluent-free composite restoratives. Based on the lower cytotoxicity of the ormocers, the prepared restorative composites should show improved biocompatibility. With the addition of nanoparticles, such as methacrylate-substituted ZrO2 clusters or SiO2 organosols, the mechanical properties of composites can be improved.     

The effect of ceramic and porous fillers on the mechanical properties of experimental dental composites.

OBJECTIVES: The purpose of this study was to investigate the effect of ceramic fillers (containing leucite crystals) and their porosity on the mechanical properties of a new experimental dental composite in order to compare with the properties of composites containing conventional glass fillers. METHODS: In this study, experimental composites were prepared by mixing the silane-treated fillers with monomers. Experimental composites were divided into four groups according to their filler type, amount and porosity. The monomers were composed of 70% Bis-GMA and 30% TEGDMA by weight for all groups. Glass and leucite-containing-ceramic were prepared as different filler types. In order to make fillers porous, leucite-containing-ceramic fillers were treated with HF acid. Camphorquinone and DMAEMA were used as photo initiator system. Post-curing was done for all groups before mechanical testing. Degree of Conversion of composites was measured using FTIR spectroscopy. The diametral tensile strength (DTS), flexural strength and flexural modulus were measured and compared among the groups. RESULTS: The results showed that the stronger and more porous filler has a positive effect on flexural strength. Porosity of filler increased flexural strength significantly. No significant difference was found in DTS tests among the groups. Flexural modulus was affected and increased by using ceramic fillers. The type of the filler affected the DC of the composite and DC increased by post-curing. SIGNIFICANCE: Flexural strength is one of the most important properties of restorative dental materials. Higher flexural strength can be achieved by stronger and more porous fillers. Investigation into the effect of filler on dental material properties would be beneficial in the development of restorative dental material.     

Flexural strength and Weibull analysis of a microhybrid and a nanofill composite evaluated by 3- and 4-point bending tests.

OBJECTIVES: The aim of the present study was to evaluate the flexural strength and the Weibull modulus of a microhybrid and a nanofill composite by means of 3- and 4-point bending tests. METHODS: Thirty specimens of Filtek Z250 (3M/ESPE) and Filtek Supreme (3M/ESPE) were prepared for each test according to the ISO 4049/2000 specification. After 24h in distilled water at 37 degrees C the specimens were submitted to 3- and 4-point bending tests using a universal testing machine DL2000 (EMIC) with a crosshead speed of 1 mm/min. Flexural strength data were calculated and submitted to Student's t-test (alpha=0.05) and Weibull statistics. The fractured surfaces were analyzed based on fractographic principles. RESULTS: The two composites had equivalent strength in both test methods. However, the test designs significantly affected the flexural strength of the microhybrid and the nanofill composites. Weibull modulus (m) of Supreme was similar with both tests, while for Z250, a higher m was observed with the 3-point bending test. Critical flaws were most often associated with the specimen's surface (up to 90%) and were characterized as surface scratches/grooves, non-uniform distribution of phases, inclusions and voids. SIGNIFICANCE: Flexural strength as measured by the 3-point bending test is higher than by the 4-point bending test, due to the smaller flaw containing area involved in the former. Despite the large difference in average filler size between the composites, the volume fraction of the filler in both materials is similar, which was probably the reason for similar mean flexural strength values and fracture behavior.     

Ketones in resin composites. Effect of ketone content and monomer composition on selected mechanical properties.

To optimize the improvements by diketones of the mechanical properties of resin composites, diacetyl was added to two different monomer mixtures in different quantities. There was a positive correlation between content of diacetyl and tensile strength, flexural strength, and modulus of elasticity, respectively, of both the BISGMA/TEGDMA- and the UEDMA/HEMA-based materials. Addition of diacetyl did not influence the modulus of resilience significantly. Addition of diacetyl resulted in increases in mechanical properties which were of the same relative size for BISGMA/TEGMA-based materials as for UEDMA/HEMA-based materials. However, because of higher control values, except for modulus of elasticity, the properties of UEDMA/HEMA-based composites were superior to those of the BIGSMA/TEGDMA-based materials. A content of approximately 24 mole% diacetyl seemed to have optimum effect on mechanical properties, giving a mean increase of 25% in tensile strength, flexural strength, and modulus of elasticity.     

Interface effects on mechanical properties of particle-reinforced composites.

OBJECTIVES: Effective bonding between the filler and matrix components typically improves the mechanical properties of polymer composites containing inorganic fillers. The aim of this study was to test the hypothesis that composite flexural modulus, flexure strength, and toughness are directly proportional to filler-matrix interfacial shear strength. METHODS: The resin matrix component of the experimental composite consisted of a 60:40 blend of BisGMA:TEGDMA. Two levels of photoinitiator components were used: 0.15, and 0.5%. Raman spectroscopy was used to determine degree of cure, and thermogravimetry (TGA) was used to quantify the degree of silane, rubber, or polymer attachment to silica and glass particles. Filler-matrix interfacial shear strengths were measured using a microbond test. Composites containing glass particles with various surface treatments were prepared and the modulus, flexure strength, and fracture toughness of these materials obtained using standard methods. Mechanical properties were measured on dry and soaked specimens. RESULTS: The interfacial strength was greatest for the 5% MPS treated silica, and it increased for polymers prepared with 0.5% initiator compared with 0.15% initiator concentrations. For the mechanical properties measured, the authors found that: (1) the flexural modulus was independent of the type of filler surface treatment, though flexural strength and toughness were highest for the silanated glass; (2) rubber at the interface, whether bonded to the filler and matrix or not, did not improve toughness; (3) less grafting of resin to silanated filler particles was observed when the initiator concentration decreased. SIGNIFICANCE: These findings suggest that increasing the strength of the bond between filler and matrix will not result in improvements in the mechanical properties of particulate-reinforced composites in contrast to fiber-reinforced composites. Also, contraction stresses in the 0.5 vs 0.15% initiator concentration composites may be responsible for increases in interfacial shear strengths, moduli, and flexural strengths.     

Development of metal-resin composite restorative material. Part 2. Effects of acid and heat treatments of silver-tin filler particles on flexural properties of metal-resin composite

The effects of acid and heat treatments of silver-tin filler particles on the flexural properties of metal-resin composite restorative materials were investigated. Five metal-resin composite restorative materials containing different silver-tin filler particles treated under different conditions were experimentally prepared. The conditions of the alloy particles were; 1) as atomized (NT), 2) 1.8% HCl acid-treated (AT), 3) heat-treated at 150 degrees C for 5 min after AT (A15), 4) heat-treated at 200 degrees C for 5 min after AT (A20) and 5) heat-treated at 250 degrees C for 5 min after AT (A25). The flexural strength and the flexural modulus of elasticity were measured for the five metal-resin composites to evaluate the effects of the acid and heat treatments. The flexural strength of the prepared composites was significantly influenced by the surface condition of the filler particles (p < 0.01), and increased significantly when the as atomized particles (NT) were acid-treated (AT) or acid- and heat-treated at 150 degrees C (A15), but then significantly decreased as the heat treatment temperature increased (A20 and A25). The strength of the A15 composite was significantly higher than those of the other composites, and exceeded that (about 60 MPa) of the previous composite with no treatment. No significant difference was found in the flexural modulus of the composites.     

Thermo-hydrolytic stability of core foundation and restorative composites

STATEMENT OF PROBLEM: The use of weak and less durable materials in restoring teeth may result in weak restorations unable to withstand intraoral conditions. PURPOSE: The purposes of this study were to evaluate the effect of thermo-hydrolytic stress on the flexural strength and flexural modulus of core foundation composites with direct restorative composites and determine mass percentage of filler content. MATERIAL AND METHODS: A total of 216 specimens, from 9 brands of commercially available composites (Coreflo, DC Core, Photocore, APX, Litefil II A, Surefil, TPH Spectrum, Z100, and Z250) were fabricated following ISO Standard 4049. Flexural strength (MPa) and flexural modulus (GPa) were determined on bar-shaped specimens (25 x 2 x 2 mm) before and after storage in boiling water for 24 hours (n=12). The filler content in composite was determined by incineration using a thermogravimetric analyzer. The data were analyzed using 2-way analysis of variance and the Student t test (alpha=.05). RESULTS: Filler content of the tested composites was 66.6 to 81.8 mass %. Significant differences in both flexural strength and flexural modulus existed among materials, the effect of boiling and interaction (P<.05). Coreflo, DC Core, Z100, and Z250 demonstrated a significant decrease in flexural strength after boiling (P<.05). Z250 showed a significant decrease in flexural modulus after boiling (P=.001), while Surefil showed a significant increase in flexural modulus (P=.007). CONCLUSION: Within the limitations of this study, it can be concluded that composites were affected differently by moist heat stress. Some composites showed a degradation of flexural properties while some retained flexural properties. Stability of the composites varied among brands.     

Influence of veneering composite composition on the efficacy of fiber-reinforced restorations (FRR)

This study investigated the influence of fiber reinforcement on the flexural properties of four commercial (Artglass, Belleglass HP, Herculite XRV and Solidex) veneering composites (Series A) and two experimental composites (Series B&C). This study investigated how the composition of the veneering composites influenced the enhancement of strength and modulus produced by fiber reinforcement. The formulation of the experimental composites were varied by changing the filler load (Series B) or the resin matrix chemistry (Series C) to assess the effect these changes would have on the degree of reinforcement. In Series A, the commercial veneering composites were reinforced by an Ultra-High-Molecular-Weight Polyethylene fiber (UHMW-PE/Connect) to evaluate flexural properties after 24 hours and six months. In Series B, experimental composites with the same organic matrix but with different filler loads (40% to 80% by weight) were also reinforced by Connect fiber to evaluate flexural properties. In Series C, experimental composites (Systems 1-4) with the same filler load (76.5% by weight) but with different organic matrix compositions were reinforced by Connect fiber to evaluate flexural properties. For Series B and C, flexural properties were evaluated after 24 hours water storage. All the samples were prepared in a mold 2 mm x 2 mm x 25 mm and stored in distilled water at 37 degrees C until they were ready for flexural testing in an Instron Universal Testing Machine using a crosshead speed of 1 mm/minute. The results showed no significant differences in the flexural strength (FS) between any of the commercial reinforced composites in Series A. The flexural modulus (FM) of the fiber-reinforced Belleglass HP group was significantly higher than for Artglass and Solidex. Water storage for six months had no significant (p>0.05) effect on the flexural strength of three of the four reinforced veneering composites. The flexural strength for Artglass was significantly reduced (p<0.05) by six-month water storage. In Series B, however, increasing the amount of filler loading improved the flexural modulus of the reinforced experimental composite but had no effect on its flexural strength. In Series C, changing the organic matrix formulation had no affect on flexural strength but affected the flexural modulus of the reinforced experimental composite.     

A partially aromatic urethane dimethacrylate as a new substitute for Bis-GMA in restorative composites.

OBJECTIVES: The objective of this study was to investigate the use of a new, partially aromatic urethane dimethacrylate in visible-light cured resin-based composite restoratives. Selected mechanical properties, such as flexural strength and flexural modulus of elasticity, of model monomer mixtures and composites containing the new urethane dimethacrylate were investigated and compared to the properties of materials that are based on Bis-GMA, at present the most frequently used cross-linker in restorative composites. In addition, the polymerization shrinkage and the water sorption of selected composites were determined. METHODS: The flexural strength, flexural modulus of elasticity, and the water sorption were determined according to ISO 4049:2000. Test specimens (rods: 2 mmx2 mmx25 mm; discs: d=15 mm and h=1 mm) of the investigated composites were prepared in stainless steel molds and light-cured (150 mW/cm2, 2x180 s). The flexural strength and flexural modulus of rods were measured after the samples had been stored under dry conditions or in water for 24 h at 37 degrees C as well as after they had been stored in water for 7 days at 37 degrees C. The water sorption was determined with discs. The polymerization shrinkage was calculated from the densities of the uncured composite pastes and cured composites. RESULTS: Visible light cured mixtures of dimethacrylate diluents with the new urethane dimethacrylate and composites based on these mixtures show a reactivity, flexural strength, flexural modulus of elasticity, polymerization shrinkage and water sorption similar to those of materials that are based on Bis-GMA. The composites did not show any strong deterioration of the mechanical properties after water storage.     

Bis-(acrylamide)s as new cross-linkers for resin-based composite restoratives

OBJECTIVES: The objective of this study was to investigate the use of three new bis-(acrylamide)s as cross-linker in resin-based composite restoratives. Selected mechanical properties such as flexural strength and flexural modulus of model composites containing bis-(acrylamide)s were investigated and compared to the properties of composites that are based on only conventional dimethacrylates. In addition, the hydrolytic stability of composites containing an acidic monomer was examined. METHODS: The flexural strength and flexural modulus of elasticity were determined according to ISO 4049:2000. For this purpose, test specimens (2 mm x 2 mm x 25 mm) of the composites investigated were prepared in stainless steel moulds and light-cured (150 mW/cm2, 2 x 180 s). The flexural strength and flexural modulus were measured after the samples had been stored in dry conditions or in water for 24 h at 37 degrees C as well as after they had been stored in water for 7 days at 37 degrees C, and in certain cases, after they had been boiled for 24 h in water. RESULTS: Visible light cured mixtures of dimethacrylates with bis-(acrylamide)s and composites based on these mixtures show a similar reactivity, flexural strength and flexural modulus of elasticity compared to materials that contain only dimethacrylate. The composites did not show any deterioration of the mechanical properties after water storage. Only when strongly acidic monomers were added to the composites containing dimethacrylates or bis-(acrylamide)s did the flexural strength and flexural modulus of the samples decrease after they were stored in water. SIGNIFICANCE: Bis-(acrylamide)s were similarly reactive than dimethacrylates and therefore can be used as diluents to substitute dimethacrylate diluents in composites. Although the bis-(acrylamide)s are entirely soluble in water, non-ionic materials based on bis-(acrylamide)s did not strongly change their mechanical properties during storage in water.     

Comparison of flexural properties and surface roughness of nanohybrid and microhybrid dental composites

Recently introduced nanohybrid dental composites have promised a smoother surface finish and strength, comparable to that of microhybrid composites. This study compared the mechanical properties and surface finish of nanohybrid and microhybrid composites by measuring the flexural strength and modulus (four-point bend) and surface roughness after polishing (using atomic force microscopy) of six commercial dental composites (three nanohybrid, three microhybrid). Scanning electron microscopy (SEM) was used to qualitatively characterize filler morphology and size. The flexural strength and modulus were significantly higher among the microhybrid composites, while the nanohybrid composites exhibited significantly lower surface roughness. SEM characterization revealed differences in filler particle size and shape that could affect the flexural properties and surface roughness. Composites containing spherical filler particles exhibited higher flexural properties and lower roughness values compared to composites with irregular fillers. These results did not support the premise that nanohybrid composites offer similar mechanical properties to microhybrids in addition to a better surface finish.     

Benzoyl germanium derivatives as novel visible light photoinitiators for dental materials.

OBJECTIVES: The objective of this study was to investigate the use of benzoyl germanium derivatives as a novel visible light photoinitiator of resin-based dental composites. Selected mechanical properties, such as flexural strength and flexural modulus, setting time, storage stability, and UV light stability, of the composites based on the novel photoinitiators benzoyltrimethylgermane (BTMGe) or dibenzoyldiethylgermane (DBDEGe) were investigated and compared to the properties of materials that are cured with a mixture of camphorquinone (CQ) and ethyl 4-(N,N-dimethylamino)benzoate (EMBO). METHODS: The flexural strength and flexural modulus of elasticity were determined according to ISO 4049. For this purpose, test specimens (2 mm x 2 mm x 25 mm) of the composites investigated were prepared in stainless steel moulds and light-cured (150 mW/cm2, 2 s x 180 s). The flexural strength and flexural modulus of elasticity were measured after immersing the cured specimens in water for 24h at 37 degrees C and in certain cases, after they had been boiled for 24h in water. In addition, the setting time, curing depth, storage and UV stability of selected composites were determined. RESULTS: The novel photoinitiators BTMGe or DBDEGe can be used to substitute the binary photoinitiator CQ/EMBO in visible light-cured restorative composites. Especially, DBDEGe showed a significantly higher photocuring activity in composites with a filler load of about 60 wt. % in comparison to that of CQ/EMBO. In addition, composites based on BTMGe or DBDEGe showed an improved UV stability and a storage stability comparable to that of CQ/EMBO-based composites.     

Influence of aldehydes on selected mechanical properties of resin composites.

The present study investigated whether propanol, a monofunctional aldehyde, was able to improve the mechanical properties of dental polymers. The underlying hypothesis was that a cross-linking reaction is possible between various functional groups of different polymers. Propanol was added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to monomer composition and content of aldehyde. Four mechanical properties of the experimental resin composites were determined. Addition of propanol gave rise to significant improvements in mechanical properties, which may be indicative of a cross-linking ability of monofunctional aldehydes. With the exception of modulus of elasticity, the mechanical properties of resin composites based on UEDMA/HEMA were superior to those of BISGMA/TEGDMA-based materials, even though the improvements in flexural strength and modulus of resilience were most pronounced for the BISGMA/TEGDMA-based resin composites.     

Relationship between mechanical properties and bond durability of short fiber‐reinforced resin composite with universal adhesive

The purpose of this study was to determine the relationship between mechanical properties and bond durability of short fiber‐reinforced resin composite with universal adhesive. As controls, micro‐hybrid and nano‐hybrid resin composites were tested. The universal adhesives used were Scotchbond Universal, Adhese Universal, and G‐Premio Bond. The fracture toughness and flexural properties of resin composites, and shear bond strength and shear fatigue strength of universal adhesive with resin composite using both total‐etch and self‐etch modes were determined. In the results, short fiber‐reinforced resin composite showed significantly higher fracture toughness than did micro‐hybrid and nano‐hybrid resin composites. The flexural strength and modulus of short fiber‐reinforced and nano‐hybrid resin composites were significantly lower than were those of micro‐hybrid resin composites. Regardless of etching mode, the shear bond strength of universal adhesives with short fiber‐reinforced resin composite did not show any significant differences from micro‐hybrid and nano‐hybrid resin composites. The shear fatigue strength of universal adhesives with short fiber‐reinforced resin composite and micro‐hybrid resin composites were significantly higher than that of nano‐hybrid resin composites. The results of this study suggest that the mechanical properties of short fiber‐reinforced resin composite improve their bond durability with universal adhesive.     

Shear punch strength and flexural strength of model composites with varying filler volume fraction, particle size and silanation.

OBJECTIVE: The effect of filler volume fraction, particle size and silanation on shear punch strength, flexural strength and flexural modulus of model composites has been evaluated. METHODS: Hybrid type glass filled (0-65.2 vol%), composites some without filler silanation (30.7-51.0 vol%) and microfilled type (0-13.0 vol%) resin matrix (UDMA and EGDMA) composites (Shofu Inc., Japan) were used in the study. For the shear punch test, 10 disc specimens, 0.5mm thick and 9 mm diameter, were prepared for each composite and tested with a 3.2mm dia punch at 1.0mm/min. Flexural strength (n=10) was measured by the method outlined in ISO 4049. Data were analyzed using ANOVA and Fisher's multiple-range test. RESULTS: Shear punch strength and flexural strength increased with increasing filler content up to 52.2% for hybrid composites and between 0 and 9.1% for microfilled composites. Shear punch strength and flexural strength decreased with increasing filler volume fraction for un-silanated composites. Flexural modulus for all materials increased with increasing filler volume fraction. Hybrid composites with silanated fillers have significantly higher values of flexural strength, flexural modulus and shear punch strength than equivalent materials with un-silanated fillers. SIGNIFICANCE: The results showed that filler silanation is an important factor for determining material strength. Additions of small quantities of microfillers appeared to have a greater effect on shear strength than equivalent amounts of hybrid filler. The shear punch test may prove beneficial for routine testing of composites as specimen preparation was simple, specimen quality was easy to maintain and the results are meaningful.     

Mechanical properties of resin cements with different activation modes

Dual-cured cements have been studied in terms of the hardness or degree of conversion achieved with different curing modes. However, little emphasis is given to the influence of the curing method on other mechanical properties. This study investigated the flexural strength, flexural modulus and hardness of four proprietary resin cements. Materials tested were: Enforce and Variolink II (light-, self- and dual-cured), RelyX ARC (self- and dual-cured) and C & B (self-cured). Specimens were fractured using a three-point bending test. Pre-failure loads corresponding to specific displacements of the cross-head were used for flexural modulus calculation. Knoop hardness (KHN) was measured on fragments obtained after the flexural test. Tests were performed after 24 h storage at 37 degrees C. RelyX ARC dual-cured showed higher flexural strength than the other groups. RelyX ARC and Variolink II depended upon photo-activation to achieve higher hardness values. Enforce showed similar hardness for dual- and self-curing modes. No correlation was found between flexural strength and hardness, indicating that other factors besides the degree of cure (e.g. filler content and monomer type) affect the flexural strength of composites. No statistical difference was detected in the flexural modulus among the different groups.     

Dental composite resins containing silica-fused ceramic single-crystalline whiskers with various filler levels.

Currently available direct-filling composite resins are susceptible to fracture and hence are not recommended for use in large stress-bearing posterior restorations involving cusps. The glass fillers in composites provide only limited reinforcement because of the brittleness and low strength of glass. The aim of the present study was to use ceramic single-crystalline whiskers as fillers to reinforce composites, and to investigate the effect of whisker filler level on composite properties. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whiskers, thereby improving retention in the matrix. The composite flexural strength, elastic modulus, hardness, and degree of polymerization conversion were measured as a function of whisker filler mass fraction, which ranged from 0% to 70%. Selected composites were polished simulating clinical procedures, and the surface roughness was measured with profilometry. The whisker composite with a filler mass fraction of 55% had a flexural strength (mean +/- SD; n = 6) of 196+/-10 MPa, significantly higher than 83+/-14 MPa of a microfill and 120+/-16 MPa of a hybrid composite control (family confidence coefficient = 0.95; Tukey's multiple comparison). The composite modulus and hardness increased monotonically with filler level. The flexural strength first increased, then plateaued with increasing filler level. The degree of conversion decreased with increasing filler level. The whisker composite had a polished surface roughness similar to that of a conventional hybrid composite (p>0.1; Student's t). To conclude, ceramic whisker reinforcement can significantly improve the mechanical properties of composite resins; the whisker filler level plays a key role in determining composite properties; and the reinforcement mechanisms appear to be crack pinning by whiskers and friction from whisker pullout resisting crack propagation.