Teflon Matrix Research Articles

Color and surface roughness alterations of bulk-fill resin composites submitted to simulated toothbrushing with whitening dentifrices

Aim: To evaluate the surface roughness and color stability of bulk-fill resin composites after simulated toothbrushing with whitening dentifrices. The radioactive/relative dentin abrasion (RDA) and radioactive/relative enamel abrasion (REA) of dentifrices were also assessed. Methods: Specimens (n=10) of Tetric N Ceram Bulk Fill (TNCB), Filtek One Bulk Fill (FOB) resin composites, and Z100(Control) were prepared using a cylindrical Teflon matrix. Surface roughness (Ra, μm) was assessed by a roughness meter and the color evaluations (ΔEab , ΔE00 , WID ) were performed using a digital spectrophotometer based on the CIELAB system. Three measurements were performed per sample, before and after simulated toothbrushing with 3D Oral-B White Perfection (3DW) and Black is White (BW) dentifrices. The abrasivity (REA and RDA values) of the used dentifrices was also determined by the Hefferren abrasivity test. Results: The Ra values increased significantly in all resin composites after 3DW and BW toothbrushing. The acceptable threshold color varied among resin composites, and TNCB and Z100 presented the highest ΔEab and ΔE00 for BW dentifrice. The 3DW dentifrice was significantly more abrasive than BW dentifrice on enamel and dentin. Conclusions: simulated toothbrushing with tested whitening dentifrices increased the surface roughness at acceptable levels. The Tetric N Ceram Bulk-fill and Z100 composite showed the highest color alteration in BW. 3D White Perfection dentifrice was more abrasive on dentin and enamel than Black is White.

Visualizing Water Monomers and Chiral OH-(H2O) Complexes Infiltrated in a Macroscopic Hydrophobic Teflon Matrix.

Insights into the interaction of fluoroalkyl groups with water are crucial to understanding the polar hydrophobicity of fluorinated compounds, such as Teflon. While an ordered hydrophobic-like 2D water layer has been demonstrated to be present on the surface of macroscopically hydrophobic fluorinated polymers, little is known about how the water infiltrates into the Teflon and what is the molecular structure of the water infiltrated into the Teflon. Using highly sensitive femtosecond sum frequency generation vibrational spectroscopy (SFG-VS), we observe for the first time that monomeric H2O and chiral OH-(H2O) complexes are present in macroscopically hydrophobic Teflon. The species are inhomogeneously distributed inside the Teflon matrix and at the Teflon surface. No water clusters or single-file water "wires" are observed in the matrix. SFG free induction decay (SFG-FID) experiments demonstrate that the OH oscillators of physically absorbed molecular water at the surface dephase on the time scale of <230 fs, whereas the water monomers and hydrated hydroxide ions infiltrated in the Teflon matrix dephase much more slowly (680-830 fs), indicating that the embedded monomeric H2O and OH-(H2O) complexes are decoupled from the outer environment. Our findings can well interpret ultrafast water permeation through fluorous nanochannels and the charging mechanism of Teflon, which may tailor the desired applications of organofluorines.

Terahertz time-domain spectroscopy, imaging and density functional theory based studies of 3, 4, 5 Trinitro 1 H Pyrazole

The paper reports the terahertz time-domain spectroscopy (THz-TDS) and imaging on 3, 4, 5 -Trinitro 1-H Pyrazole (explosive) in solid form between 0.1 and 3.5 THz range, including Density Functional Theory was implemented in this study. We have employed a commercially available fiber-coupled THz reflection mode system. We have experimentally ascertained the optical and dielectric properties such as refractive index, absorption coefficients, and dielectric constants in the THz domain. The refractive index values varies between 1.5 and 2.0 range, and absorption coefficients varies between 10 and 400 cm−1 in the stated THz range. In addition, Density Functional Theory was used to assign some important vibrational modes obtained experimentally. In the next step, the Dimer and Trimer configurations of the molecule were also incorporated in the existing theoretical model for developing a clear-cut understanding the role of the intermolecular vibrational modes in the molecule. The simulated results were compared in the IR region (4000–400 cm−1) and the THz region (0.1–3.5 THz). Finally, an improvised version of the imaging system was used to record the THz time and frequency domain images of pure pyrazole and mixed in a teflon matrix with different concentrations. The spatial distributions of pyrazole in the pure and teflon matrix at different absorption frequencies have been analysed.

Avaliação do manchamento de resinas compostas por bebidas de pH ácido

ABSTRACT Introduction: Composite resins are an extensively used esthetic material that has excellent adaptation, reproduces the characteristics of the tooth structure and is suitable for the conditions of the oral environment. However, this material is also very susceptible to staining due to pigmentation coming from the diet and habits of the patient. Objectives: To evaluate the staining of two brands of composite resin, one nanoparticle and a nanohybrid type (Filtek Z350XT® and IPS Empress Direct®), in direct contact with the acid pH of beverages. Methods: In total 40 specimens were made, 20 of each type of resin studied. After being inserted into a Teflon matrix and light activated, the samples were removed, stored in containers with 2 ml of artificial saliva and kept in the refrigerator. Every day 20 of these samples were placed in direct contact with 2 ml Gatorade, and the other 20 samples, in 2 ml of wine, each set for 30 minutes. After 30 minutes, the acid pH substances were removed, and 2 ml of artificial saliva was again placed in the container. This process was repeated every day for 10 days. Results: The results showed that the nanoparticle resin (Filtek Z350XT) was more susceptible to staining than the nanohybrid type (Empress Direct®) when in contact with the wine. When wine and Gatorade were compared, the solution with higher staining capacity was shown to be wine. Conclusion: The two resins tested were susceptible to staining in a 10-day period. Mottling and more staining was visibly detected after samples were immersed in the solution with the higher pH, and nanoparticulate resin was more susceptible to staining.

RESONANCE PROPERTIES OF AN X-BAND RECTANGULAR WAVEGUIDE SECTION WITH AN INHOMOGENEOUS DIELECTRIC INSET

Subject and Purpose. In modern dielectrometry, the problem of detecting foreign inclusions in a radio-transparent material, which are significantly smaller than the operational wavelength, remains very important. The problem becomes even more complicated if it is required to determine complex permittivity of these inclusions. This work analyzes the conditions for the correct use of the original resonance method proposed by the authors earlier for determining permittivity of a local inclusion when its dimensions and dielectric constant change. Methods and Methodology. The measured module consists of a rectangular X-band waveguide, which is partially filled with a dielectric in the form of a rectangular Teflon matrix with a local cubic inclusion inside. The dimensions of the matrix are fixed and are 23 mm × 10 mm × 30 mm. Numerical modeling is performed using the Ansys HFSS software package. The dependences of the resonance frequencies of the module upon changing the dielectric constant of the cube are analyzed. The cube permittivity was changed between 3.8 and 100 in 5-unit steps. Permittivity of the material of the cube is determined by comparing arrays of calculated data with experimental results. Results. Numerical modeling of the module was performed and its electrodynamics properties were determined in the frequency band of 8…10 GHz at different sizes and permittivity of the inclusion. For a cube with a facet size of 2 mm, the resonance frequency decreases with a permittivity increase of the material. For a cube with a facet size of 3 mm and permittivity above 50, additional resonances appear in the structure due to the excitation of resonant modes of the cube itself. Conclusion. It has been shown that by varying the dielectric permittivity of the cubic inset between 3.8 and 100 it proves possible to provide for resonant mode excitation over the frequency range specified. This allows estimating the dielectric permittivity of the cubic inset’s material by way of comparing the calculated versus measured data arrays concerning resonant frequency dependences upon material parameters.

Influence of different power densities during polymerization on the color of composite resins

This study evaluated the influence of different levels of power density on resin polymerization in the staining of composite resins. Thirty-six specimens were made with composite resin (Z350 3M ESPE) with cylindrical Teflon matrix (8 mm X 1 mm). The light curing was performed with different degrees of power density for each group (n=12): 300mW/cm², 700mW/cm² e 1000mW/cm². Afterwards, the specimens had their color checked, with the help of a spectrophotometer and immersed in coffee solution during 7 days. After, a new measuring was done (CIEL*a*b* color system). Results showed, for Δa, only G1000 group obtained a significantly lower value, and the same result was found in the evaluation of Δb. For ΔE and ΔL, no significant differences were found between groups. It is concluded that, low and medium power density entails more reddish and bluish composite resin, while the specimens submitted to high power density and then immersed in coffee did not present color change in a short term.

Investigation of the shock compression behaviors of Al/PTFE composites with experimental and a 3D mesoscale-model

The responses of Al/PTFE reactive materials (RMs) under shock compression were investigated by a single-stage gas gun. A 3D mesoscale-model was established based on micro-computed tomography (micro-CT) slice images and confirmed with experimental results. In the high-pressure stage, the composites reacted partially, whereas there were no deviations between the partially reacted Hugoniot and the inert simulation results. The simulation reveals that the Teflon matrix melting on the high shock pressure. Melts and decomposition of the PTFE accelerated the diffusion of the atoms. Thus, the reactions of the Al/PTFE composites are more like a combustion rather than a detonation.

Comparative analysis of the surface roughness of conventional resins and filling after immersion in mouthwashes

Aim: Bulk Fill composite resins were released on the market in order to reduce the time in clinical sessions by using increments of up to 5.0 mm thickness. The aim of this study was to evaluate the effect of the rinsing solutions on the surface roughness of the conventional composite and Bulk fill composite resins. Methods: 40 specimens were prepared from a 4.0mm x 10.0mm teflon matrix and photoactivated for 20 seconds, with 20 specimens made of Filtek Bulk-Fill composite resin (3M ESPE) and 20 specimens made with Filtek™ Z350 XT composite resin (3M ESPE). Each group was subdivided into 2 subgroups: G1 (Filtek™ Z350 XT-3M ESPE-immersed in Colgate PlaxWhitening®); G2 (Filtek™ Z350 XT-3M ESPE-immersed in PlaxFreshMint®); G3 (Filtek™ Bulk-Fill-3M ESPE-immersed in Colgate PlaxWhitening®) and G4 (Filtek™ Bulk-Fill-3M ESPE-immersed in Colgate PlaxFreshMint®). The surface roughness test was performed initially and after immersion in rinses by the Time Group Inc.-TR200® rugosimeter apparatus and the data were submitted to statistical analysis (two-way repeated measures ANOVA). Results: Surface roughness values of the Filtek™ Bulk-Fill composite resin (3M ESPE) were significantly higher than the Filtek™ Z350 XT composite resin (3M ESPE) (P &lt;0.0001). However, no differences were identified before and after immersion in rinses with or without alcohol. Conclusion: The use of mouthwashes does not interfere with the surface roughness of the tested resins, but the composite resin Filtek Z350 XT (3M ESPE) presents a surface with less roughness.

Effect of nanomaterial incorporation on the mechanical and microbiological properties of dental porcelain

Statement of problemDental porcelain restorations are subject to biological failures related to secondary caries and periodontal disease leading to prosthesis replacement. PurposeThe purpose of this in vitro study was to explore the microbiological and mechanical properties of dental porcelain incorporated with different percentages of silver vanadate (β-AgVO3) through microbiological analysis, roughness tests, and the Vickers microhardness test. Material and methodsIPS InLine porcelain specimens were made by using a cylindrical Teflon matrix in the dimensions of 8×2 mm. For the control group, the porcelain was manipulated according to the manufacturer's instructions. The groups incorporating the nanomaterial were prepared with 2.5%, 5%, and 10% of β-AgVO3, which was added proportionally by mass to the porcelain powder. In vitro microbiologic analysis, roughness tests, and the Vickers microhardness test were performed. ResultsAgainst Streptococcus mutans, the control group showed no inhibition halo (0 mm). All groups with AgVO3 showed a zone of inhibition, the highest for the group with 10% (30 mm) and then the groups with 2.5% (9 mm) and 5% (17 mm). For Vickers microhardness, no statistically significant difference (P<.05) was observed between the evaluated groups. The group with 10% of AgVO3 had the highest mean roughness and was statistically different (P<.001) from the other groups. ConclusionsAdding β-AgVO3 to dental porcelain demonstrated antimicrobial effectiveness at all concentrations (2.5%, 5%, and 10%), with no effect on Vickers microhardness. The 10% group had higher roughness than the other groups.

Repair Bond Strength of a CAD/CAM Nanoceramic Resin and Direct Composite Resin: Effect of Aging and Surface Conditioning Methods.

To evaluate the effect of surface conditioning methods and aging on the repair bond strength between resin composite and nanoceramic CAD/CAM resin. Twenty-four blocks of nanoceramic CAD/CAM resin (NCR) (Lava Ultimate, 3M Oral Care) (10 x 5 x 2 mm3) and resin composite (Filtek Z350, 3M Oral Care) (RC) were made, embedded in acrylic resin, polished (#600, #800, #1200) and randomly divided into 8 groups (n = 12 each) according to surface conditioning methods (air abrasion with 30-μm CoJet [CJ] or air abrasion with 50-μm Al2O3 [AB]) and aging prior to repair (without aging, 24 h in water at 37°C; with aging 6 months in water at 37°C). The blocks were air abraded (20 s, 2.5 bar, 10 mm) using a standardized device. A layer of adhesive resin (Scotchbond Universal) was applied (20 s) and photopolymerized for 20 s. RC cylinders (Ø = 2 mm; h = 2 mm) were then bonded to the NCR substrates using a Teflon matrix and photopolymerized for 40 s. All specimens were thermocycled (10,000 cycles, 5°C-55°C) and submitted to the shear bond test (50 kgf, 0.5 mm/min) to measure repair strength. Data (MPa) were analyzed using 3-way ANOVA and Tukey's test (α = 0.05). Failure analysis was performed using stereomicroscopy (20X). ANOVA revealed a significant effect of only the "material" factor (p = 0.00). The group NCR6mCJ presented bond strengths (29.37 ± 5.41) which were significantly higher than those of the NCR24hCJ (20.88 ± 5.74) and RC groups (p < 0.05). The group RC24hCJ (19.71 ± 4.21) presented the lowest shear bond strength (p < 0.05). Failure analysis revealed predominantly type B mixed failures (adhesive+cohesive in the substrate material) except for the groups NCR24hCJ and NCR6mAB, where mainly type C failure (adhesive+cohesive at the RC) was observed. Air abrasion with Al2O3 particles or silicatization with CoJet followed by adhesive resin application are effective surface conditioning methods for the repair of nanoceramic CAD/CAM resin with resin composite.

Adhesive systems effect over bond strength of resin-infiltrated and de/remineralized enamel

Background: The purpose of this study was to evaluate the effect of different bonding agents on bond-strength to demineralized enamel after remineralizing treatments and resin infiltration. Methods: Buccal enamel of 120 bovine incisors was polished and then were divided into five experimental groups: SE (sound enamel); DE (demineralized enamel); AS (demineralized enamel immersed in artificial saliva for eight weeks); NaF (demineralized enamel treated with 0.05% sodium fluoride solution (one minute) for eight weeks); Ic (demineralized enamel infiltrated with a low-viscosity resin (Icon-DGM). These groups were subdivided according to adhesive system used: self-etching adhesive Adper Easy One (3M/ESPE) and etch-and-rinse adhesive Single Bond (3M/ESPE). The composite resin blocks were fabricated using a Teflon matrix. A thermomechanical cycling machine was used to carry out the artificial aging of the specimens and thus were sectioned into sticks. The microtensile tests were performed using a universal testing machine at a cross-head speed of 1 mm/min. Data (in MPa) were subjected to two-way ANOVA and Tukey’s tests (5%). Results: Significant differences were found for both factors tested and interactions (p<0.05). Tukey’s test results of µTBS (mean ± SD) were: etch-and-rinse SE (28.79±3.93); DE (30.41±7.22); AS (29.03±3.33); NaF (29.81±4.06)a; Ic (29.47±5.5); and self-etching SE (30.37±6.96); DE (14.62±4.47); AS (9.79±2.32); NaF (9.36±2.31); Ic (30.78±8.68). Conclusions: Resin infiltration did not affect the bond strength of demineralized enamel for both adhesive systems tested. For etch-and-rinse adhesive, no differences were observed for the tested groups. For self-etching adhesive, only the resin-infiltrated group showed similar bond strength to sound enamel. Both etch-and-rinse and self-etching adhesive systems can be used in resin-infiltrated enamel, if a composite restoration needs to be further performed. In enamel that has undergone the de/remineralization process, the use of a total-etch adhesive might be preferable for the restorative procedure.

Adhesive systems effect over bond strength of resin-infiltrated and de/remineralized enamel.

Background: The purpose of this study was to evaluate the effect of different bonding agents on bond-strength to demineralized enamel after remineralizing treatments and resin infiltration. Methods: Buccal enamel of 120 bovine incisors was polished and then were divided into five experimental groups: SE (sound enamel); DE (demineralized enamel); AS (demineralized enamel immersed in artificial saliva for eight weeks); NaF (demineralized enamel treated with 0.05% sodium fluoride solution (one minute) for eight weeks); Ic (demineralized enamel infiltrated with a low-viscosity resin (Icon-DGM). These groups were subdivided according to adhesive system used: self-etching adhesive Adper Easy One (3M/ESPE) and etch-and-rinse adhesive Single Bond (3M/ESPE). The composite resin blocks were fabricated using a Teflon matrix. A thermomechanical cycling machine was used to carry out the artificial aging of the specimens and thus were sectioned into sticks. The microtensile tests were performed using a universal testing machine at a cross-head speed of 1 mm/min. Data (in MPa) were subjected to two-way ANOVA and Tukey's tests (5%). Results: Significant differences were found for both factors tested and interactions (p<0.05). Tukey's test results of µTBS (mean ± SD) were: etch-and-rinse SE (28.79±3.93); DE (30.41±7.22); AS (29.03±3.33); NaF (29.81±4.06)a; Ic (29.47±5.5); and self-etching SE (30.37±6.96); DE (14.62±4.47); AS (9.79±2.32); NaF (9.36±2.31); Ic (30.78±8.68). Conclusions: Resin infiltration did not affect the bond strength of demineralized enamel for both adhesive systems tested. For etch-and-rinse adhesive, no differences were observed for the tested groups. For self-etching adhesive, only the resin-infiltrated group showed similar bond strength to sound enamel. Both etch-and-rinse and self-etching adhesive systems can be used in resin-infiltrated enamel, if a composite restoration needs to be further performed. In enamel that has undergone the de/remineralization process, the use of a total-etch adhesive might be preferable for the restorative procedure.

An Evaluation of Er:YAG Laser in an Intact Enamel Treatment on Bonding to the Resin Cement Compared to Four Traditional Techniques

Evaluation of the bond strength of the resin cement after enamel conditioning with erbium-doped yttrium aluminum garnet laser compared to other four different techniques. The study consisted of 50 enamel specimens excluded from the smooth surfaces of newly extracted human 3rd molars. The specimens were randomly divided into five groups in terms of the enamel treatment technique. Each group (n = 10) was conditioned differently by bevel and acid etching group (I), acid etching technique group (II), double acid etching technique group (III), air abrasion followed by acid etching group (IV), and Er:YAG laser followed by acid etching group (V). After enamel treatment, a resin cylinder was installed using split Teflon matrix on each enamel specimen. Then the specimens were exposed to a shear strength device, and shear force was applied. The results were recorded with Newton. The required data were collected and statistically analyzed. A one-way ANOVA test was conducted to compare the five study groups using SPSS version 21 (p < 0.05). All groups showed high shear bond strength values ranging from 19.26 MPa (for group III) to 27.05 MPa (for group V). The enamel treatment with Er:YAG laser followed by acid etching gave the best results with significant differences compared to the other groups. Within the limitations of the present study, it can be concluded that the combination between the Er:YAG laser and the acid etching enhances the bond strength of resin cement with the enamel. The use of Er:YAG laser followed by acid etching can be a successful technique for enamel conditioning and the results showed their superiority over the other groups.

Temperature-dependent terahertz spectroscopy and refractive index measurements of aqua-soluble and plastic explosives.

The paper reports the temperature-dependent time domain terahertz spectroscopy of premium aqua-soluble and plastic explosives such as NH4NO3, TNT, and RDX between 0.1 and 2.2THz. Tunable terahertz radiation was generated using ZnTe crystal as a source, and a photoconductive antenna was used as a detector. The temperature-dependent study was carried out between 30°C and 200°C in a specially designed oven. The signature peaks of RDX and TNT present at 0.82 and 1.60THz, respectively, show a strong redshift, whereas the NH4NO3 molecule shows a comparatively small shift. The high-temperature-based redshift phenomenon is just the opposite of the blueshift recorded at low temperatures. In addition, the temperature-dependent absorption coefficient data of these molecules support the change in the concentration of the NO2 molecule. We have also ascertained the temperature-dependent refractive indices of these molecules between 0.1 and 2.0THz, which confirms the effect of temperature on the refractive indices. Finally, the signature peak of RDX with respect to the reduction in the weight concentration of RDX in the Teflon matrix was studied at 0.82THz.

Surface morphological and physical characterizations of glass ionomer cements after sterilization processes.

The aim of this study was to investigate if sterilization methods would promote changes in the selected adhesion-related surface properties of glass ionomer cements (GICs). Riva self-cure (RSC) and Riva light-cure (RLC) GICs were tested. Thirty samples were prepared according to the type of material (RSC and RLC) and sterilization method: hydrogen peroxide gas plasma (HPGP), steam sterilization (SS), and no sterilization (n = 5 per group). A Teflon matrix (5 × 2 mm) was filled with one of the GICs to produce the samples. For the groups with the RLC material, the samples were light cured using a light curing unit for 20 s. After 24 hr, finishing and polishing were performed in all samples and then they were sterilized. Surface roughness, wettability, and micromorphology were accessed using a profilometer, a goniometer, and a scanning electron microscopy, respectively. Data were statistically analyzed through a two-way ANOVA and Tukey post-hoc test (p < .05). Both sterilization methods promoted similar roughness values to the nonsterilized samples (p > .05). HPGP decreased contact angle for RSC (p < .01), and SS increased contact angle for RLC (p < .01). Samples subjected to HPGP presented similar surface micromorphology to nonsterilized ones, regardless of the material. SS promoted exposition of smaller filler particles in both materials. Although sterilization methods did not alter surface roughness and wettability, the sterilization methods selectively altered the micromorphology of the materials tested. RESEARCH HIGHLIGHTS: This study's main finding suggests that each sterilization method altered the surface of glass ionomers in different ways. Thus, the choice of sterilization methods prior to bacterial adhesion can lead to a bias in antimicrobial studies.

DEVELOPMENT AND APPLICATION OF SOLID FORMS OF CaSO4:Dy THERMOLUMINESCENT DOSEMETERS IN RADIATION PROTECTION DOSIMETRY—A REVIEW

CaSO4:Dy is a reliable high-sensitive themoluminescent phosphor useful for low-level and high-level radiation measurements as it exhibits fading free linear dose response with a single glow peak at ~230°C in these dose regions. For large-scale radiation protection dosimetry service, it is embedded in Teflon matrix with varying thicknesses. Extensive studies have been carried out with such CaSO4:Dy Teflon discs in individual and environmental radiation monitoring applications including its capability to measure International Commission on Radiation Units and Measurements operational quantities. The review highlights their development and application in high-energy photon measurements, thin wafers and graphite-loaded Teflon discs for beta-dosimetry, phosphor-filled aluminium discs for high-dose applications, 6LiF-mixed CaSO4:Dy Teflon discs for thermal and albedo or moderated fast neutrons, sulphur-mixed CaSO4:Dy pellets for fast-neutron exposure even in the presence of gamma-rays and polyethylene-mixed CaSO4:Dy discs for fast neutrons.

Optical response of thin nanocomposite films with transverse inhomogeneity

In-plane homogenization, taking into account inhomogeneity in distribution of nanoinclusions across the film thickness, was performed within the framework of self-consistent approach based on the concept of effective susceptibility and the Green function method. We apply this approach to obtain the optical response of films consisting of metal nanoparticles in a dielectric matrix that exhibits plasmonic absorption. We have calculated the absorption spectra of nanocomposite films with gold spheroidal inclusions in Teflon matrix at a silicon substrate for several different transversal distributions of inclusions, maintaining a constant inclusion volume fraction. The absorption dependence on the light polarization and incidence angle, as well as on the shape of inclusions and their orientation is also under study. From the absorption maps we have extracted dispersion curves for plasmon-polariton resonances in the nanocomposite films.

Evaluation of the shear bond strengths between two alternative metal alloys and porcelain

The success of metal-ceramic restorations is influenced by the compatibilitybetween base metal alloys and porcelains. Although porcelain manufacturersrecommend their own metal systems as the most compatible for fabricating metalceramicprostheses, a number of alloys have been used. The purpose of this study wasto evaluate the bond strength of CoCr alloy (Wironbond C) and NiCr alloy (Wiron 99)using shear forces at the metal-ceramic interface. A stainless steel cylindrical matrixwas used for preparation of the metal dies, application of ceramic and to perform theshear tests. Ten metal dies of each alloy were made for each alloy, and the metallicportion was obtained with the lost wax casting technique with standardized waxing of6.5mm of height and of 6mm of diameter. The ceramic was applied according to themanufacturer’s recommendations with the aid of a Teflon matrix that allowed itsdimension to be standardized in the same size as the metallic portion, and shear testswere performed in a universal testing machine at a cross-head speed of 0.5mm/min.The mean shear bond strength values were 53.06MPa for Wirobond C alloy, withstandard deviation of 10.67, and 45.38MPa for Wiron 99, with standard deviation of9.01. No statistically significant difference was observed between the shear strengthof the two metal-ceramic alloys.

Design of a Polymer-Based Hollow-Core Bandgap Fiber for Low-Loss Terahertz Transmission

We use numerical simulations to design a hollow-core microstructured polymer optical fiber (HC-mPOF) suitable for broadband, terahertz (THz) pulse transmission with relatively low losses and small dispersion. The HC-mPOF consists of a central large air-core surrounded by periodically arranged wavelength-scale circular air holes in a hexagonal pattern, embedded in a uniform Teflon matrix. The THz guidance in this fiber is achieved by exploiting the photonic bandgap (PBG) effect. In our low index contrast Teflon-air (1.44:1) hexagonal periodic lattice, the PBG appears only for a certain range of non-zero values of the longitudinal wavevector. We have achieved PBG over a broad spectral range (bandwidth $\sim 400$ GHz) ranging from 1.65 to 2.05 THz in the proposed HC-mPOF. The achievable loss coefficient in our designed HC-mPOF is $ and the group velocity dispersion parameter is $\cdot $ cm over a 300-GHz bandwidth ( $1.65\sim 1.95$ THz).

Effect of fluoride agents on the color stability of esthetic restorative materials

Objective The aim of the study was to estimate the effect of acidulated phosphate fluoride (APF) gel and fluoride varnish on the color stability of esthetic restorative materials. Methodology The materials included were glass ionomer cement, resin-modified glass ionomer cement, and composite resin. A Teflon matrix (12×2 mm) mold was used to fabricate 108 specimens from all restorative materials. Further, 36 disks of each restorative material were then randomly divided into three groups (n = 12) according to the fluoride application: deionized water (control), APF (1.23%), and fluoride varnish. Color change was measured by means of a spectrophotometer using a CIE L*a*b* (Comission International l´Eclairage) system before and 24 h after fluoride treatment. Statistical comparisons were made using first mixed model repeated measure analysis of variance on the transform ranked data under the assumption of non-normal data and to see which main or interaction effect was significant. Results In control groups, when the baseline and final readings of glagg ionomer cement, resinmodified glass ionomer cement, and composite materials were compared for color changes, similar values were observed and hence results were nonsignificant. When all materials were compared for color changes before and after the application of APF, highly significant mean differences were found among the groups. Hence, application of APF results in a highly significant color change in all materials compared with varnish application. Conclusion Topical fluoride agents – either fluoride varnish or APF gels – cause discoloration in all esthetic restorative materials. However, this discoloration is not visually perceptible.