The Influence of Various Photoinitiators on the Properties of Commercial Dental Composites

The aim of this article was to compare the biomechanical properties of commercial composites containing different photoinitiators: Filtek Ultimate (3M ESPE) containing camphorquinone (CQ); Estelite Σ Quick (Tokuyama Dental) with CQ in RAP Technology®; Tetric EvoCeram Bleach BLXL (Ivoclar Vivadent AG) with CQ and Lucirin TPO; and Tetric Evoceram Powerfill IVB (Ivoclar Vivadent AG) with CQ and Ivocerin TPO. All samples were cured with a polywave Valo Lamp (Ultradent Products Inc.) with 1450 mW/cm2. The microhardness, hardness by Vicker’s method, diametral tensile strength, flexural strength and contraction stress with photoelastic analysis were tested. The highest hardness and microhardness were observed for Filtek Ultimate (93.82 ± 17.44 HV), but other composites also displayed sufficient values (from 52 ± 3.92 to 58,82 ± 7.33 HV). Filtek Ultimate not only demonstrated the highest DTS (48.03 ± 5.97 MPa) and FS (87.32 ± 19.03 MPa) but also the highest contraction stress (13.7 ± 0.4 MPa) during polymerization. The TetricEvoCeram Powerfill has optimal microhardness (54.27 ± 4.1 HV), DTS (32.5 ± 5.29 MPa) and FS (79.3 ± 14.37 MPa) and the lowest contraction stress (7.4 ± 1 MPa) during photopolymerization. To summarize, Filtek Ultimate demonstrated the highest microhardness, FS and DTS values; however, composites with additional photoinitiators such as Lucirin TPO and Ivocerin have the lowest polymerization shrinkage. These composites also have higher FS and DTS and microhardness than material containing CQ in Rap Technology.

Influence of Cement Thickness on the Polymerization Shrinkage Stress of Adhesively Cemented Composite Inlays: Photoelastic and Finite Element Analysis

The objective of this study was to analyze the effect of cement thickness on the strain and stresses resulting from the polymerization of resin cement using photoelasticity and Finite Element Analysis (FEA). For this study, twenty upper first premolars with inlay cavity preparation were constructed from photoelastic resin and restored with composite resin inlay. The samples were divided into two groups (n = 10) according to the film thickness of resin cement material. For Group 1, the film thickness was 100 μm; for Group 2, the film thickness was 400 μm. After polymerization of the cement, photoelastic analysis and finite element analysis (FEA) were performed. In the photoelastic analysis, Group 2 showed higher strain with the presence of second-order fringe even after 24 h. In Group 1, the formation of first order fringes was not observed, even after 24 h. In the FEA analysis, the greatest cusp deflection and tensile stress occurred in Group 2 (0.00026 mm and 0.305 MPa, respectively) due to the polymerization shrinkage in the lingual cusp compared to Group 1 (0.000107 mm and 0.210 MPa, respectively). It can be concluded that the thickness of the resin cement influences the cusp deflection, with the greater thickness of the cement layer, the greater stresses and deformations in the tooth structure occur.

Stress distribution analysis of novel dental mini-implant designs to support overdenture prosthesis

This study aimed to test and compare two novel dental mini-implant designs to support overdentures with a commercial model, regarding the stress distribution, by photoelastic analysis. Three different mini-implant designs (Ø 2.0 mm × 10 mm) were tested: G1—experimental threaded (design with threads and 3 longitudinal and equidistant self-cutting chamfers), G2—experimental helical (design with 2 long self-cutting chamfers in the helical arrangement), and G3—Intra-Lock® System. After including the mini-implants in a photoelastic resin, they were subjected to a static load of 100 N under two situations: axial and inclined model (30°). The fringe orders (n), that represents the intensity of stresses were analyzed around the mini-implants body and quantified using Tardy's method that calculates the maximum shear stress (τ) value in each point selected. In axial models, less stress was observed in the cervical third mini-implants, mainly in G1 and G2. In inclined models (30°), higher stresses were generated on the opposite side of the load application, mainly in the cervical third of G2 and G3. All mini-implant models presented lower tensions in the cervical third compared with the middle and apical third. The new mini-implants tested (G1 and G2) showed lower stresses than the G3 in the cervical third under axial load, while loading in the inclined model generated greater stresses in the cervical of G2.

Photoelastic Stress Response of Complex 3D-Printed Particle Shapes

Stress visualization within 3-dimensional particles undergoing dynamic processes can greatly advance our understanding of complex particle behaviors. Traditional photoelastic stress visualization methods suffer inherent limitations from lack of available technology for complex particle production. Recently, 3D-printing has created new possibilities for enhancing the scope of stress analysis within physically representative granules. Here, we investigate opportunities offered by 3D-printing a granular material with photoelastic properties. We report the results of X-ray computed tomography and 3D-printing, combined with traditional photoelastic analysis, to visualize strain exhibited within simple discs to reproduced coffee beans. We find that the choice of print layer orientation with respect to the force load affects the optical properties of the discs, without a significant difference in their mechanical properties. Furthermore, we present a first, semi-quantified, measurement of stresses within 3D-printed particles of complex shape. The promising data shows potential for applying this method to complex assemblies of 3-dimensional particles.

The Influence of Cement Layer Thickness on the Stress State of Metal Inlay Restorations—Photoelastic Analysis

The successful restoration of teeth requires a good connection between the inlay and natural tissue. A strong bond may improve retention and reinforce tooth structure. The purpose of this study was to evaluate the influence of cement layer thickness on contraction stress generated during photopolymerization, and to determine the changes in stress state of the cement occurring during aging in water (over 84 days). Two cements were used: resin composite cement (NX3) and self-adhesive resin cement (Maxcem Elite Chroma). A cylindrical sample made of CuZn alloy was used to imitate the inlay. The stress state was measured by photoelastic analysis. The contraction stress of the inlay restoration was calculated for cement layer thicknesses of 25 µm, 100 µm, 200 µm, and 400 µm. For both tested materials, the lowest contraction stress was observed for the thinnest layer (25 µm), and this increased with thickness. Following water immersion, a significant reduction in contraction stress was observed due to hygroscopic expansion. Applying a thin layer (approximately 25 µm) of composite and self-adhesive resin cements resulted in high levels of expansion stresses (over −6 MPa) after water aging.

Exact and linearized refractive index stress-dependence in anisotropic photoelastic crystals

For the permittivity tensor of photoelastic anisotropic crystals, we obtain the exact nonlinear dependence on the Cauchy stress tensor. We obtain the same result for its square root, whose principal components, the crystal principal refractive index, are the starting point for any photoelastic analysis of transparent crystals. From these exact results we then obtain, in a totally general manner, the linearized expressions to within higher-order terms in the stress tensor for both the permittivity tensor and its square root. We finish by showing some relevant examples of both nonlinear and linearized relations for optically isotropic, uniaxial and biaxial crystals.

Effect of Nanographite on Electrical Mechanical and Wear Characteristics of Graphite Epoxy Composites

Effect of weight fraction (WF) of nanographite (NG, 400 nm) on electrical, mechanical and wear and characteristics of graphite epoxy composites (GECs) were investigated. For this purpose, a series of GECs was prepared through dispersion of various WF of NG into epoxy resin, followed by curing with polyamine. Dispersion of NG into epoxy matrix and onward formation of GECs and was revealed through UV, Fourier transformed spectra and atomic force microscopy (AFM). Photoelastic analysis in combination with AFM reveals the presence of uniformly dispersed domain of NG into stress free GECs with fringe order ranging 0.23 - 0.61 under compression of 8 - 20 kgf. GECs have rendered a rising trend in DC conductance ranging 98.32 - 0.54 μS/cm with electrical percolation threshold at 175 WF of NG. GECs have shown enhanced compressive, impact, tensile, strength, Rockwell hardness and wear resistance at 200 WF of NG. In general, GECs has shown a marginal modification in their compressive strength by 5.30 % over cured epoxy. However, their impact and tensile strengths were largely improved to 31.78% and 43.98% over cured epoxy. The present manuscript provides a novel method of modification in electrical, mechanical and wear behaviour epoxy through introducing NG as a novel alternative to traditionally used graphite as filler for development of GECs.