Y. Papadogiannis

Temperature dependence of the dynamic viscoelastic behavior of chemical- and light-cured composites

The relationship of temperature to the viscoelastic properties of six composites, three light-cured and three chemical-cured, was studied, using constant dynamic loading over the narrow range of temperatures (20-60 degrees C) which can be encountered in the mouth. The parameters measured were: storage modulus G(1), loss modulus G(2), tan delta, quality factor Q, coefficient of decay alpha, and dynamic viscosity. It was found that the dynamic viscoelastic properties of the tested materials are temperature-dependent, but probably not to a clinically significant degree. Chemical-and light-cured composites of the same filler loading do not exhibit significantly different viscoelastic dynamic properties.

Viscoelastic properties, creep behavior and degree of conversion of bulk fill composite resins

OBJECTIVES The aim of this study was to investigate the viscoelastic properties and creep behavior of bulk fill composites under different conditions and evaluate their degree of conversion. METHODS Seven bulk fill composites were examined: everX Posterior (EV), SDR (SD), SonicFill (SF), Tetric EvoCeram Bulk Fill (TE), Venus Bulk Fill (VE), x-tra base (XB) and x-tra fil (XF). Each material was tested at 21°C, 37°C and 50°C under dry and wet conditions by applying a constant torque for static and creep testing and dynamic torsional loading for dynamic testing. Degree of conversion (%DC) was measured on the top and bottom surfaces of composites with ATR-FTIR spectroscopy. Statistical analysis was performed with two-way ANOVA, Bonferronis post hoc test and Pearsons correlation coefficient. RESULTS Shear modulus G ranged from 2.17GPa (VE) to 8.03GPa (XF) and flexural modulus E from 6.16GPa (VE) to 23GPa (XF) when the materials were tested dry at 21°C. The increase of temperature and the presence of water lead to a decline of these properties. Flowable materials used as base composites in restorations showed significantly lower values (p<0.05) than non-base composites, while being more prone to creep deformation. %DC ranged from 47.25% (XF) to 66.67% (SD) at the top material surface and 36.06% (XF) to 63.20% (SD) at the bottom. SIGNIFICANCE Bulk fill composites exhibited significant differences between them with base flowable materials showing in most cases inferior mechanical properties and higher degree of conversion than restorative bulk fill materials.

The creep behavior of glass-ionomer restorative materials

The creep of microspecimens of five glass-ionomer filling materials and one glass-ionomer-cermet cement was studied by means of a torsional creep apparatus. The glass-ionomer specimens were aged one week and conditioned in 37 degrees C water. Shear stress of 2.47 x 10(-4) N.m was maintained for three h, and recovery was followed for 50 h. Creep curves were obtained at 21, 37, and 50 degrees C. The effect of temperature increase was studied. All the glass ionomers exhibited linear visco-elastic behavior at low deformations. Their shear moduli and resistance to creep were similar to those of some composites measured by the same method. The increase of temperature influenced the creep behavior and moduli of the materials (i.e., increased creep and residual strains and decreased shear modulus). Although the applied torque was very small, there was permanent deformation, the result of viscous flow in all experiments which was more pronounced at 50 degrees C.

Dynamic viscoelastic behavior of resin cements measured by torsional resonance

OBJECTIVE The purpose of the study was to measure the viscoelastic properties of four dental resin composite cements using a dynamic mechanical analysis technique. METHODS Dynamic torsional loading was conducted in the frequency range from 1 to 80 Hz. Cement specimens were tested after storage in 37 degrees C water for 24 h. One group was thermal cycled prior to testing. Measurements were taken at 21, 37, and 50 degrees C. Storage modulus, loss tangent and other viscoelastic parameters were determined from the amplitude/frequency curves. RESULTS Storage moduli of the cements ranged from 2.9 to 4.1 GPa at 37 degrees C. Loss tangents ranged from 0.054 to 0.084. Storage moduli decreased in a regular way with increasing temperature, whereas, loss tangents increased. Thermal cycling caused small decreases in storage moduli. SIGNIFICANCE Resin cements with higher filler loading were found to have higher storage moduli and lower loss tangents. Since these properties have been associated with better clinical performance in the areas of retention and prevention of fracture of porcelain and resin restorations, the more highly filled cements may be recommended. Temperature variations influenced viscoelastic behavior of the cements. However, within the temperature range studied no sharp drop in modulus was seen, so the materials should function satisfactorily in the oral cavity.

Mechanical viscoelastic behavior of dental adhesives

OBJECTIVES The purpose of the study was to evaluate the mechanical properties of dental adhesive materials at different testing temperatures after dry and wet storage. METHODS Specimens (d=1 mm, l=18 mm) from six materials were tested: Silorane Adhesive System (SL), Heliobond (HE), One-Step Plus (OS), Optibond Solo Plus (OP), cmf Adhesive System (CF) and Protobond (PR). Static and creep testing was performed by applying a constant torque below the proportional limit of the materials, while dynamic testing consisted of dynamic torsional loading. Experiments were performed after 24h of dry and wet storage under temperatures from 21°C to 50°C and various viscoelastic parameters were calculated. RESULTS Shear modulus ranged from 0.19 to 1.99 GPa, while flexural modulus from 0.67 to 5.69 GPa. Most of the materials were affected by the presence of water and increase of temperature. OP showed the highest recovery after creep, while SL exhibited the highest permanent deformation. SIGNIFICANCE Contact with water after polymerization and increase of temperature resulted in a decline of the mechanical properties, especially for the HEMA-containing adhesives.

Creep and dynamic viscoelastic behavior of endodontic fiber-reinforced composite posts

PURPOSE Fiber-reinforced composite (FRC) posts have gained much interest recently and understanding of their viscoelastic properties is important as they can be used in stress-bearing posterior restorations. The aim of this study was to evaluate the creep behavior and the viscoelastic properties of four commercial FRC posts under different temperatures and different storage conditions. METHODS The FRC posts tested were Glassix, C-Post, Carbonite and Snowlight. For the creep measurements a constant load below the proportional limit of the posts was applied and the angular deformation of the specimens was recorded. The viscoelastic parameters were determined by using dynamic torsional loading under four different conditions. RESULTS All materials were susceptible to creep and exhibited linear viscoelastic behavior. Residual strain was observed in all FRC posts. The viscoelastic properties were affected by the increase of temperature and water storage (p<0.001) resulting in their decline. Carbon fiber posts exhibited better performance than glass fiber posts. CONCLUSIONS FRC posts exhibit permanent strains under regular masticatory stresses that can be generated in the oral cavity. Their properties are susceptible to changes in temperature, while direct contact with water also affects them deleteriously.

Dynamic and static mechanical analysis of resin luting cements

Various types of indirect restorations are available for dental treatment and resin cements are commonly used as a luting medium. The aim of this study was to evaluate the mechanical properties of contemporary resin luting agents under different testing conditions and temperatures. The materials tested were Choice 2 (CH), Clearfil Esthetic Cement (EC), Resicem (RC) and RelyX Unicem (RX). Each material was examined after 24 h of storage at 21 °C dry and wet at 21, 37 and 50 °C under dynamic and static testing and parameters such as shear and flexural modulus, loss tangent, dynamic viscosity and Poissons ratio were calculated. The resin cements were also subjected to creep testing under different constant loads for 3 h and a recovery time of 50 h. The material with the highest modulus was CH, while RX had the lowest. All resin cements were affected by the presence of water with RX being the least affected and by the increase of temperature, with RC being the least susceptible. None of the materials exhibited full recovery after creep testing and permanent deformation ranged from 0.43% to 5.53%. The resin cements tested in this study showed no major transitions under the different testing conditions. Their behavior was satisfactory for restorations that do not require increased mechanical properties. However, in the case of stress-bearing restorations the conditions in the oral cavity may affect the performance of these materials.