Mechanical properties of dental resin/composite containing urchin-like hydroxyapatite

ObjectivesTo investigate the reinforcing effect of urchin-like hydroxyapatite (UHA) in bisphenol A glycidyl methacrylate (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) dental resin (without silica nanoparticles) and dental composites (with silica nanoparticles), and explore the effect of HA filler morphologies and loadings on the mechanical properties.MethodsUHA was synthesized by a facile method of microwave irradiation and studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). Mechanical properties of the dental resin composites containing silanized UHA were tested by a universal mechanical testing machine. Analysis of variance was used for the statistical analysis of the acquired data. The fracture morphologies of tested composites were observed by SEM. Composites with silanized irregular particulate hydroxyapatite (IPHA) and hydroxyapatite whisker (HW) were prepared for comparative studies.ResultsImpregnation of lower loadings (5 wt% and 10 wt%) of silanized UHA into dental resin (without silica nanoparticles) substantially improved the mechanical properties; higher UHA loadings (20 wt% and 30 wt%) of impregnation continuously improved the flexural modulus and microhardness, while the strength would no longer be increased. Compared with silanized IPHA and HW, silanized UHA consisting of rods extending radially from center were embedded into the matrix closely and well dispersed in the composite, increasing filler-matrix interfacial contact area and combination. At higher filler loadings, UHA interlaced together tightly without affecting the mobility of monomer inside, which might bear higher loads during fracture of the composite, leading to higher strengths than those of dental resins with IPHA and HW. Besides, impregnation of silanized UHA into dental composites (with silica nanoparticles) significantly improved the strength and modulus.SignificanceUHA could serve as novel reinforcing HA filler to improve the mechanical properties of dental resin and dental composite.     

Effects of ethanol addition on the water sorption/solubility and percent conversion of comonomers in model dental adhesives

ObjectivesThis study evaluated the kinetics of water uptake and percent conversion in neat versus ethanol-solvated resins that were formulated to be used as dental bonding agents.MethodsFive methacrylate-based resins of known and increasing hydrophilicities (R1, R2, R3, R4 and R5) were used as reference materials. Resins were evaluated as neat bonding agents (100% resin) or they were solvated with absolute ethanol (95% resin/5% ethanol or 85% resin/15% ethanol). Specimens were prepared by dispensing the uncured resin into a circular mold (5.8 mm × 0.8 mm). Photo-activation was performed for 80 s. The water sorption/diffusion/solubility was gravimetrically evaluated, while the degree of conversion (DC) was calculated by Fourier-transform infrared spectroscopy.ResultsWater sorption increased with the hydrophilicity of the resin blends. In general, the solvated resins exhibited significantly higher water sorption, solubility and water diffusion coefficients when compared to their corresponding neat versions (p < 0.05). The only exception was resin R1, the least hydrophilic resin, in which neat and solvated versions exhibited similar water sorption (p > 0.05). Addition of ethanol increased the DC of all resins tested, especially of the least hydrophilic, R1 and R2 (p < 0.05). Despite the increased DC of ethanol–solvated methacrylate-based resins, it occurs at the expense of an increase in their water sorption/diffusion and solubility values.SignificanceNegative effects of residual ethanol on water sorption/solubility appeared to be greater as the hydrophilicity of the resin blends increased. That is, the use of less hydrophilic resins in dental adhesives may create more reliable and durable bonds to dentin.     

Influence of chlorhexidine on the degree of conversion and E-modulus of experimental adhesive blends

ObjectivesThis study examined the effect of chlorhexidine (CHX) content on degree of conversion (DC) and E-modulus of experimental adhesive blends. The hypothesis tested was that increasing concentrations of CHX result in decreased DC and E-modulus in relation to adhesive hydrophilicity.MethodsExperimental adhesive blends with increasing hydrophilicity R2 (70% bisGMA, 28.75% TEGDMA); R3 (70% BisGMA, 28.75% HEMA); R4 (40% BisGMA, 30% TCDM, 28.75% TEGDMA); R5 (40% BisGMA, 30% BisMP, 28.75% HEMA) and different CHX concentrations (1 and 5%) were analyzed. 5% CHX could not be dissolved in R2. A differential scanning calorimeter was used to measure the DC of resin blends. Photopolymerized disks of the experimental comonomer mixtures (n = 10/gp) were used to measure the E-modulus of each specimen using a biaxial flexure test. Data were analyzed with two-way ANOVA (resin type and CHX concentration) and Tukey's post hoc test.ResultsThe addition of 1% CHX did not significantly alter the DC of R2 and R3. Significant decrease in R3 DC values was observed when 5% CHX was added. CHX significantly increased the DC of R4 and R5. 1% CHX reduced the E-modulus of all resins (p < 0.05) except for R2, in which the E-modulus was significantly increased (p < 0.05). 5% CHX significantly reduced the E-modulus of resins R3 to R5 (p < 0.05).SignificanceIn conclusion, increasing concentrations of CHX dissolved in resin blends had little adverse effect on DC but decreased the E-modulus 27–48% compared to controls. Solvation of CHX in ethanol prior to incorporation of CHX into R2 may permit higher CHX concentrations without lower polymer stiffness.     

Fatigue limits of enamel bonds with moist and dry techniques

ObjectiveShear fatigue limit (SFL) testing, coupled with shear bond strength (SBS) measurements can provide valuable information regarding the ability of adhesive systems to bond to mineralized tooth structures. The clinical technique for enamel bonding with adhesive resins has shifted from bonding to a thoroughly dried acid conditioned surface to a moist surface to facilitate dentin bonding. The purpose of this study was to compare the performance of ethanol-containing etch-and-rinse adhesive (ERA) systems on moist and dry enamel by determining the resin composite to enamel SBS and SFL, and examining the relationship of SBS and SFL.MethodsTwelve specimens each were used to determine 24-h resin composite (Z100 – 3M ESPE) to enamel SBS to moist and dry surfaces with two ERA systems, Adper Single Bond Plus (SBP) and OptiBond Solo Plus (OBP). A staircase method of fatigue testing was used in a four-station fatigue cycler to determine the SFL of resin composite to enamel bonds (moist and dry) with the two ERA systems (20 specimens for each test condition) at 0.25 Hz for 40,000 cycles. ANOVA and Tukey's post hoc test were used for the SBS data and a modified t-test with Bonferroni correction was used for comparisons of SFL.ResultsThe two ERA systems each generated statistically similar SBS (p > 0.05) to moist and dry enamel and the SBS of SBP was significantly higher than OBP on dry enamel (p < 0.05). The SFL of SBP was significantly greater to dry enamel when compared to moist enamel and there was not a significant difference in the SFL of OBP on dry and moist enamel. There were no significant differences in SFL values between SBP on either moist or dry enamel and OBP on both moist and dry enamel.SignificanceFatigue testing may provide more useful information than SBS tests regarding the performance of dental adhesive systems. The chemical composition, solvents and filler components of ERA systems may influence their ability to develop long-term durable bonds to both moist and dry enamel surfaces.     

Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites

ObjectivesThe degree of conversion (DC) and polymerization shrinkage of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as an increase in monomer conversion leads to a high polymerization shrinkage. This paper aims to determine the effect of opaque mineral fillers and monomer ratios on the DC and the shrinkage-strain of experimental composites based on (BisGMA/TEGDMA) monomers (traditionally used monomers). A relationship between the shrinkage-strain and the degree of conversion values was also investigated. The radiopacity of these experimental composites has been investigated in a previous paper.MethodsExperimental resin composites were prepared by mixing different monomer ratios of (BisGMA/TEGDMA) with Camphoroquinone and dimethyl aminoethyl methacrylate (DMAEMA) as photo-initiator system. Five different radiopacifying filler agents: La₂O₃, BaO, BaSO₄, SrO and ZrO₂ at various volume fractions ranging from 0 to 80 wt.% were added to the mixture. The degree of conversion of experimental composites containing different opaque fillers contents was measured using FTIR/ATR spectroscopy. The shrinkage-strain of specimens, photopolymerized at circa 500 mW/cm², was measured using the bonded-disk technique at room temperature with respect to time.ResultsThe result revealed that the DC and the shrinkage-strain decrease slightly with the increasing of opaque fillers loadings, but this decrease is not significant. However, these two properties are closely related to the monomer concentration of the organic matrix. The results have also showed a linear correlation between the shrinkage-strain and DC of experimental composites investigated.SignificanceThe nature and the volume effects of the opaque fillers on the DC and shrinkage of the experimental composites investigated were not significant. However, this study has confirmed the importance of viscosity in the system and shrinkage behavior of dimethacrylate monomers studied. Then we confirmed that direct relationships linked the shrinkage and the DC of filled dental resin composites.     

New insight into the "depth of cure" of dimethacrylate-based dental composites

ObjectivesTo demonstrate that determination of the depth of cure of resin-based composites needs to take into account the depth at which the transition between glassy and rubbery states of the resin matrix occurs.MethodsA commercially available nano-hybrid composite (Grandio) in a thick layer was light cured from one side for 10 or 40 s. Samples were analyzed by Vickers indentation, Raman spectroscopy, atomic force microscopy, electron paramagnetic imaging and differential scanning calorimetry to measure the evolution of the following properties with depth: microhardness, degree of conversion, elastic modulus of the resin matrix, trapped free radical concentration and glass transition temperature. These measurements were compared to the composite thickness remaining after scraping off the uncured, soft composite.ResultsThere was a progressive decrease in the degree of conversion and microhardness with depth as both properties still exhibited 80% of their upper surface values at 4 and 3.8 mm, respectively, for 10 s samples, and 5.6 and 4.8 mm, respectively, for 40 s samples. In contrast, there was a rapid decrease in elastic modulus at around 2.4 mm for the 10 s samples and 3.0 mm for the 40 s samples. A similar decrease was observed for concentrations of propagating radicals at 2 mm, but not for concentrations of allylic radicals, which decreased progressively. Whereas the upper composite layers presented a glass transition temperature – for 10 s, 55 °C (±4) at 1 mm, 56.3 °C (±2.3) at 2 mm; for 40 s, 62.3 °C (±0.6) at 1 mm, 62 °C (±1) at 2 mm, 62 °C (±1.7) at 3 mm – the deeper layers did not display any glass transition. The thickness remaining after scraping off the soft composite was 7.01 (±0.07 mm) for 10 s samples and 9.48 (±0.22 mm) for 40 s samples.SignificanceAppropriate methods show that the organic matrix of resin-based composite shifts from a glassy to a gel state at a certain depth. Hence, we propose a new definition for the “depth of cure” as the depth at which the resin matrix switches from a glassy to a rubbery state. Properties currently used to evaluate depth of cure (microhardness, degree of conversion or scraping methods) fail to detect this transition, which results in overestimation of the depth of cure.     

Curing potential of experimental resin composites filled with bioactive glass: A comparison between Bis-EMA and UDMA based resin systems

ObjectivesTo evaluate the degree of conversion, light transmittance, and depth of cure of two experimental light-curable bioactive glass (BG)-containing composite series based on different resin systems.MethodsExperimental composite series based on either Bis-EMA or UDMA resin were prepared. Each series contained 0, 5, 10, 20, and 40 wt% of BG 45S5. Reinforcing fillers were added up to a total filler load of 70 wt%. The degree of conversion was evaluated using Raman spectroscopy, while light transmittance was measured using visible light spectroscopy. The depth of cure was estimated from the degree of conversion data and using the ISO 4049 scraping test.ResultsReplacement of reinforcing fillers with BG can diminish the degree of conversion, light transmittance, and depth of cure. The effect of BG on the aforementioned properties was highly variable between the experimental series. While in the Bis-EMA series, the degree of conversion was significantly impaired by BG, all of the composites in the UDMA series attained clinically acceptable degree of conversion values. The reduction of the degree of conversion in the Bis-EMA series occurred independently of the changes in light transmittance. The UDMA series showed better light transmittance and consequently higher depth of cure than the Bis-EMA series. The depth of cure for all composites in the UDMA series was above 2 mm.SignificanceWhile the Bis-EMA series demonstrated clinically acceptable curing potential only for 0–10 wt% of BG loading, an excellent curing potential in the UDMA series was observed for a wide range (0–40 wt%) of BG loadings.     

牙科纳米氧化铝复合树脂的制备与性能研究

目的 制备一种新型的光固化纳米氧化铝复合树脂,探讨其用于口腔临床的可行性。方法 以双酚A双甲基丙烯酸缩水甘油酯(Bis-GMA)为树脂基质,甲基丙烯酸羟乙酯(HEMA)为活性稀释剂,添加纳米氧化铝填料对树脂基质进行增强增韧改性,制备一种新型牙科纳米氧化铝复合树脂,并表征其固化程度、弯曲强度、硬度、断面形貌、耐磨性、吸水性与水溶解性。结果 添加纳米氧化铝能提高复合树脂材料的刚性和硬度,当添加量达到3wt%时,复合树脂的力学性能、吸水和溶解性能均为最优。结论 复合树脂中加入一定比例的纳米氧化铝可达到增韧和耐磨的效果,该研究为开发新型牙科复合树脂提供了理论和实验基础。     

Release of monomers from different core build-up materials

ObjectiveThe aim of the present study was to evaluate and compare the elution of monomers from three different core build-up composite materials and correlate it with the degree of conversion.MethodsThree different core build-up composite materials (a chemically cured, a photo-cured, and a dual-cured) were tested. Ten samples (diameter: 4.5 mm and thickness: 2 mm) of each material were fabricated to evaluate the release of monomers. The photo-cured samples were polymerized for 40 s and the dual-cured samples for 20 s. The samples remained undisturbed for 10 min and then were stored in 1 ml of 75% ethanol at room temperature, and the storage medium was renewed after 24 h, 7 and 28 days. From the storage medium that was removed, samples were prepared and analyzed by LC–MS/MS. Additionally, four samples of each material were tested for the degree of conversion by using a FT-IR spectrometer.ResultsThe three composite materials differed significantly concerning the elution of monomers (BisGMA: p < 0.0001; TEGDMA: p < 0.0001; and Bisphenol A: p < 0.0001). A significantly higher amount of BisGMA and TEGDMA was released from the chemically cured composite compared to the other two materials. Between the photo-cured and the dual-cured material the latter eluted significantly higher amounts of BisGMA and TEGDMA. During the storage of the samples, the amounts of the eluted substances decreased. The degree of conversion of the chemically cured composite was significantly lower compared to the other two materials.SignificanceUsing the present parameters, the photo-cured material released less monomer and therefore they might be less dangerous with respect to toxicological effects.     

Dynamic covalent chemistry (DCC) in dental restorative materials: Implementation of a DCC-based adaptive interface (AI) at the resin–filler interface for improved performance

ObjectiveDental restorative composites have been extensively studied with a goal to improve material performance. However, stress induced microcracks from polymerization shrinkage, thermal and other stresses along with the low fracture toughness of methacrylate-based composites remain significant problems. Herein, the study focuses on applying a dynamic covalent chemistry (DCC)-based adaptive interface to conventional BisGMA/TEGDMA (70:30) dental resins by coupling moieties capable of thiol–thioester (TTE) DCC to the resin–filler interface as a means to induce interfacial stress relaxation and promote interfacial healing.MethodsSilica nanoparticles (SNP) are functionalized with TTE-functionalized silanes to covalently bond the interface to the network while simultaneously facilitating relaxation of the filler–matrix interface via DCC. The functionalized particles were incorporated into the otherwise static conventional BisGMA/TEGDMA (70:30) dental resins. The role of interfacial bond exchange to enhance dental composite performance in response to shrinkage and other stresses, flexural modulus and toughness was investigated. Shrinkage stress was monitored with a tensometer coupled with FTIR spectroscopy. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine.ResultsA reduction of 30% in shrinkage stress was achieved when interfacial TTE bond exchange was activated while not only maintaining but also enhancing mechanical properties of the composite. These enhancements include a 60% increase in Young’s modulus, 33% increase in flexural strength and 35% increase in the toughness, relative to composites unable to undergo DCC but otherwise identical in composition. Furthermore, by combining interfacial DCC with resin-based DCC, an 80% reduction of shrinkage-induced stress is observed in a thiol–ene system “equipped” with both types of DCC mechanisms relative to the composite without DCC in either the resin or at the resin–filler interface.SignificanceThis behavior highlights the advantages of utilizing the DCC at the resin–filler interface as a stress-relieving mechanism that is compatible with current and future developments in the field of dental restorative materials, nearly independent of the type of resin improvements and types that will be used, as it can dramatically enhance their mechanical performance by reducing both polymerization and mechanically applied stresses throughout the composite lifetime.     

Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites

ObjectivesThe purpose of this study was to evaluate the effect of the amount of 3-methacryloxypropyl-trimethoxysilane (γ-MPS) coupling agent on some physical–mechanical properties of an experimental resin composite for understanding the optimum amount of silanization.MethodsSilica nanoparticles (Aerosil OX 50) used as filler were silanized with 5 different amounts of γ-MPS 1.0, 2.5, 5.0, 7.5 and 10 wt% relative to silica. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (60 wt%) were mixed with a Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of light cured composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by DMA analyzer. Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution. Thermogravimetric analysis was performed in air and in nitrogen atmosphere from 50 to 800 °C.ResultsAt lower silane amounts used (1.0, 2.5 wt%) the silane molecules must have a parallel orientation relative to the silica surface. At higher silane amounts (>2.5 wt%) silane molecules form a layer around the filler particles which now have to occupy a random, parallel and perpendicularly orientation relative to the silica surface. No significant statistic difference was found to exist between the flexural strength and flexural modulus values of composites with different silane contents. Dynamic elastic modulus E′ showed a maximum value for the composite contained 5 wt% silane. The composites with the higher amounts of silane showed the lower values for the tan δ at the T_g revealing that these composites have better interfacial adhesion between filler and matrix.SignificanceThe amount of silane used for the silanization of silica particles affect the orientation of the silane molecules relative to the silica surface. This seems to affect the dynamic mechanical properties of composites.     

Epigallocatechin-3-gallate (EGCG) enhances the therapeutic activity of a dental adhesive

ObjectivesThe purpose of this study was to evaluate the antibacterial potential and physicochemical properties of a dental adhesive incorporated with epigallocatechin-3-gallate (EGCG) in different concentration over time.MethodsEGCG was incorporated at a ratio of 100, 200, and 300 μg/ml into a dental adhesive. The effects of the cured adhesives on the growth of Streptococcus mutans were determined by direct contact test immediately or one month later and by scanning electron microscopy (SEM), respectively. Microtensile bond strength (μTBS) test was used to test the mechanical property of the adhesives immediately or six months later. The degree of conversion (DC) of the adhesives was evaluated by Fourier Transform Infrared Spectroscopy (FTIR).ResultsCompared with negative control, the 200 μg/ml and 300 μg/ml EGCG-incorporated dental adhesive were found to exhibit inhibitory effect on the growth of S. mutans. The μTBS of the EGCG-incorporated dental adhesive was higher than the control. The DC of the adhesive system was not affected by the addition of EGCG.Conclusions200 μg/ml EGCG incorporated dental adhesives could accomplish therapeutic goals that play in antimicrobial function whilst keeping the durability of resin–dentine bond.     

Influence of TiO2 nanoparticles on the optical properties of resin composites

ObjectivesTo determine the influence of titanium dioxide (TiO₂) nanoparticle addition on the opalescence, color, translucency and fluorescence of experimental resin composites.MethodsA light curing resin matrix was made by mixing 60 wt.% Bis-GMA and 40 wt.% TEGDMA. Silane coated glass filler (mean particle size: 1.55 μm) was added in the ratio of 50 wt.% of the resin composites. A fluorescent whitening agent was also added (0.05 wt.%). TiO₂ nanoparticles (<40 nm) were added with the concentrations of 0, 0.1, 0.25 and 0.5 wt.%. Reflected and transmitted colors of 1 and 2 mm thick specimens were measured relative to the illuminant D65 with reflection spectrophotometers. Opalescence parameter (OP), color difference (ΔE*_ab), translucency parameter (TP), fluorescence parameter (FL), and fluorescence and opalescence spectra were calculated.ResultsFor the 1 mm thick specimens measured with 3 mm × 8 mm rectangular aperture, when the concentration of TiO₂ increased from 0% to 0.5%, OP increased from 2.4 to 18.0, TP decreased from 35.4 to 13.1, and fluorescence spectra remained unchanged. Color difference between these specimens was in the range of 3.4–6.6 ΔE*_ab units. OP values were significantly influenced by the thickness of the specimens and the configuration of the spectrophotometers (p < 0.05).SignificanceAddition of TiO₂ nanoparticles significantly increased the opalescence of resin composites while leaving the fluorescence spectra unchanged; however, it significantly decreased the translucency and also changed the color (p < 0.05). Resin composites with 0.1–0.25% TiO₂ nanoparticle would simulate the opalescence of human enamel.     

R-curve behavior and toughening mechanisms of resin-based dental composites: Effects of hydration and post-cure heat treatment

ObjectivesTo test the hypothesis that the fracture resistance of two different particulate resin composites degrade after water hydration and improve after post-cure heat treatment, and to correlate those changes with salient failure micromechanisms.MethodsTwo composites with different filler morphology were selected, denoted microhybrid (Filtek? Z250) and nanofill (Filtek? Supreme plus). Following initial light curing, hydrated samples were aged in water for 60 days at room temperature while post-cured samples were heat treated at 120 °C for 90 min. Fracture resistance was assessed using fracture resistance curves (R-curves) utilizing pre-cracked compact tension, C(T), specimens. The flexural strength of the hydrated composites also was evaluated in four-point bending using unnotched beams. Scanning electron microscopy (SEM) of crack paths and fracture surfaces was performed to determine the micromechanisms of fracture and toughening. The results were compared by two-way ANOVA and Tukey's multiple comparison test (p ≤ 0.05).ResultsSEM observations revealed a predominantly interparticle matrix crack path for all cases except the hydrated nanofill composite, which showed evidence of particle matrix debonding. Hydration lowered the strength for both composites and the peak toughness for the nanofill composite. The strength decrease was attributed to resin matrix plasticization and hydrolytic degradation in both cases, with additional interfacial degradation causing a larger strength decline and concomitant peak toughness decrease in the nanofill composite. The post-cure heat treatment noticeably changed the R-curve shape causing the peak toughness to be reached after shorter amounts of crack extension. Such changes help explain the increases in strength reported in other studies and is attributed to improved resin matrix properties.SignificanceResults from this study provide new insight into the micromechanisms of fracture in resin-based dental composites which should aid the future development and improvement of these materials.     

Multiple correlations of material parameters of light-cured dental composites

ObjectivesThe aim of this study was to explore the correlations between the Knoop hardness, Young's modulus, viscosity, and polymerization shrinkage of an experimental dental composite, in order to determine the temporal variations of the material properties during the polymerization process.MethodsThe digital image correlation method was employed to measure the polymerization shrinkage along the curing depth of bar-shape specimens (cross-section 4 mm × 2 mm and length 10 mm) of an experimental composite RZE045. The shrinkage data were correlated with the Knoop microhardness measured on specimens prepared in consistent conditions. Another series of tests were performed on cuboid composite samples (cross-section 4 mm × 4 mm and height 5 mm) with different degrees of conversions to determine the correlations among microhardness, Young's modulus and viscosity. Further correlations between shrinkage, Young's modulus and viscosity were then derived, from which the temporal variations of the mechanical parameters during curing were estimated.ResultsAlong the curing depth, the Knoop microhardness of the experimental composite RZE045 decreased more rapidly than its volumetric shrinkage. A power function was employed to describe their relation. On the other hand, Knoop microhardness was found to be proportional to Young's modulus and viscosity. These linear correlations also seemed to be applicable to other materials including unfilled resins, silica glass and other dental composites.SignificanceCorrelations between material parameters of dental composites allowed the rapid temporal variations of Young's modulus and viscosity during curing to be estimated based on the measured polymerization shrinkage-strain history.     

Effect of resin-composite filler particle size and shape on shrinkage–strain

ObjectivesThe aim of this study was to investigate the effect of variations in filler particle size and shape on the polymerization shrinkage–strain kinetics of resin-composites.MethodsA model series of 12 VLC resin-composites were studied. The particulate dispersed phase volume fraction was 56.7%: these filler particles were systematically graded in size, and further were either spherical or irregular. The bonded disk method was used to determine shrinkage–strain kinetics. Displacement was recorded following 40 s irradiation (600 mW/cm²) at 23 °C (n = 3). All data were captured for 60 min and the final shrinkage–strain calculated.ResultsFor materials with spherical filler, shrinkage–strain was 2.66% (SD 0.18) for those with irregular filler it was 2.89% (SD 0.11). These differences were statistically significant (p < 0.001): the Scheffé test identified two subsets, those with irregular filler (including materials with a multimodal mix) and those with spherical filler (including materials with a multimodal mix). Additionally, there was a trend for higher shrinkage–strain values with decreasing filler particle size which was apparent for both those composites with spherical filler particles and those with irregular filler particles. For irregular filler particles, linear regression gave a high correlation (r² = 0.99).SignificanceStatistically significant differences were identified in the shrinkage behavior of resin-composites with differing filler size and shape.     

Effect of residual solvent on performance of acrylamide-containing dental materials

ObjectiveMethacrylamide-based monomers are being pursued as novel, hydrolytically stable materials for use in dental adhesives. The impact of residual solvents, due to the chemical synthesis procedures or the need for solvated adhesives systems, on the kinetics of polymerization and mechanical properties was the aim of the present investigation.MethodsTwo base monomers (70 wt% BisGMA or HEMAM-BDI — newly synthesized secondary methacrylamide) were combined with 30 wt% N,N-dimethylacrylamide. Eethyl acetate (EtOAc), or 75 vol% ethanol/25 vol% water (EtOH/H₂O) were added as solvents in concentrations of 2, 5, 15 and 20 wt%. The resins were made polymerizable by the addition of 0.2 wt% 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and 0.4 wt% diphenyliodonium hexafluorophosphate (DPI-PF6). Specimens (n = 3) were photoactivated with a mercury arc lamp (Acticure 4000, 320–500 nm, 250 mW/cm²) for 5 min. Degree of conversion (DC, %) was tracked in near-IR spectroscopy in real time and yield strength and modulus of elasticity were measured in three-point bending after dry and wet storage (n = 6). The data was subject to one-way ANOVA/Tukey’s Test (p ≤ 0.05), or Student’s t-test (p ≤ 0.001).ResultsIn all groups for both BisGMA and HEMAM-BDI-based materials, DC and DC at Rp_max increased and maximum rate of polymerization decreased as solvent concentration increased. Despite the increased DC, BisGMA mixtures showed a decrease in FS starting at 5 wt% EtOAc or 15 wt% EtOH/H₂O. Yield strength for the HEMAM-BDI groups was overall lower than that of the BisGMA groups, but the modulus of elasticity was significantly higher.SignificanceThe presence of residual solvent, from manufacturing or from practitioner’s handling, affects polymerization kinetics and mechanical properties of resins. Methacrylates appear to be more strongly influenced than methacrylamides.     

Evaluation of the filler packing structures in dental resin composites: From theory to practice

Objectives

The aim of this study is to evaluate the packing properties of uniform silica particles and their mixture with secondary particles yielding maximally loaded dental composites. We intend to verify the difference between the idealized models (the close-packed structures and the random-packed structures) and the actual experimental results, in order to provide guidance for the preparation of dental composites. The influence of secondary particle size and the resin composition on the physical–mechanical properties and the rheological properties of the experimental dental composites was also investigated.

Spatial and cure-time distribution of dynamic-mechanical properties of a dimethacrylate nano-composite

ObjectiveThe purpose of this study was to evaluate a nano-filled dental composite, with varying cure irradiation-time, in terms of the spatial distribution of dynamic-mechanical properties determined at nanometre scale and the resultant distinction between filler, matrix and inter-phase regions.Materials and methodsSpecimen groups (n = 5) of the composite Filtek Supreme XT were cured in 2 mm deep molds for 5, 10, 20 and 40 s, and stored for 24 h in distilled water at 37 °C. Properties were measured at 2 mm depth, on the lower specimen surfaces. Nano-dynamic-mechanical parameters (complex, storage and loss modulus, tan δ) were determined at an array of 65,000 locations in a 5 μm × 5 μm area. Micro-mechanical properties (hardness, modulus of elasticity, creep and elastic/plastic deformation) were also measured and additionally the real-time degree of cure, by ATR-FTIR, for 10 min after photo-initiation and after storage.ResultsThe spatial distribution of nano-dynamic-mechanical properties varied significantly enabling four distinguishable matrix, filler-cluster and inter-phase regions to be identified. Proceeding from matrix to filler-cluster locations, complex-moduli increased linearly and loss-factors decreased linearly, consistent with visco-elastic composite theory. Curing time strongly affected all measured properties at 2 mm depth. The organic matrix was shown to be inhomogeneous for all curing times. By increasing cure-time, the proportion of less well polymerized area decreased from 37.7 to 1.1%, resulting in a more homogeneous organic matrix.SignificanceThe experimentally observed graduated transition, in complex modulus and related dynamic-mechanical properties, across the matrix – inter-phases – filler-cluster regions is conducive to low internal stresses, in contrast to the abrupt modulus transitions anticipated or observed in many other particulate composite structures. The identification of these phase-regions provides a realistic basis for accurate nano- and micro-mechanical computational modelling.