Reduction of polymerization contraction stress for dental composites by two-step light-activation.

OBJECTIVES: The goal of this study was to assess the reduction of polymerization contraction stress of composites during a two-step light-activation process and to relate this reduction to the process of polymerization shrinkage and specimen thickness.METHODS: Three test procedures were performed to compare two-step light-activation with delay with one-step continuous irradiation of composites: polymerization contraction stress using a closed-loop servohydraulic testing instrument, polymerization shrinkage by a mercury dilatometer, and degree of conversion by FTIR. For the one-step continuous curing method, the samples were light-activated for 60s at 330 mW/cm(2). For the two-step curing method, a 5s light exposure at 60 mW/cm(2) was followed by 2 min without light exposure, and then a second light exposure for 60s at 330 mW/cm(2). The same light parameters were used for measurements of stress, shrinkage, and degree of conversion. Three composites, Heliomolar, Herculite and Z100 were evaluated. The contraction stress experiments were repeated with varying thickness for Herculite using the one-step and two different two-step techniques.RESULTS: Polymerization contraction stress 10 min after light-activation was significantly reduced (P<0.05) by the two-step method: 29.7% for Heliomolar, 26.5% for Herculite, and 19.0% for Z100. Total volumetric shrinkage and degree of conversion were not significantly different for composites cured by the two different techniques. Increasing the thickness of the composite sample reduced the measured contraction stress, especially for one of the two-step curing methods.SIGNIFICANCE: A combination of low initial energy density followed by a lag period before a final high-intensity light irradiation provides a reduction of polymerization contraction stresses in dental composites. The stress reductions cannot be attributed to reductions in degree of conversion or unrestrained volumetric shrinkage.     

Influence of light energy density on effectiveness of composite cure

This study investigated the influence of light energy density (intensity x time) on the effectiveness of composite cure in view of the curing profiles of new light-polymerization units. This investigation used a digital microhardness tester to evaluate the hardness of the top/bottom surfaces and hardness ratio of 2 mm thick composite specimens after exposure to different light energy densities. Parameters included five light intensities (200, 300, 400, 500 and 600 mW/cm2) and nine irradiation times (10, 20, 30, 40, 60, 80, 100, 120 and 180 seconds). Six samples were evaluated for each light energy density. KHN values and the hardness ratio obtained with 40 seconds cure at 400 mW/cm2 was used as control. Results were analyzed with one-way ANOVA and Scheffe's post-hoc test at significance level (0.05). Correlation between curing time and hardness values and ratio was done using Pearson's correlation at significance level 0.01. Results showed that the adequate hardness for surface finishing could be obtained with 20 seconds irradiation at lower intensities of 200 or 300 mW/cm2. Optimal cure of the bottom surfaces could not be achieved with 200 mW/cm2, but was attained with 300 mW/cm2 only after 120 seconds of irradiation. Optimal cure of the bottom surfaces was possible with 30 and 20 seconds irradiation at 500 and 600 mW/cm2, respectively. Effective cure was not achieved with low light intensities (200 to 300 mW/cm2) but could be achieved with high intensities (500 and 600 mW/cm2) after 30 seconds of irradiation.     

Sorption, solubility and residual monomers of a dental adhesive cured by different light-curing units

The aim of this study was to assess polymerization ability of three light-curing units by evaluating the influence of the light source, curing regimen and permeant (water or ethanol) on sorption, solubility and amount of residual monomers of a dental adhesive. Specimens of Adper Single Bond 2 were fabricated using a stainless steel circular matrix (8 mm x 1 mm). One quartz-tungsten-halogen (QTH) lamp and two light-emitting diode (LED) device at three different curing regimes (L1 = 12 J; L2 = 24 J; L3 = 24 J) were used to cure the specimens. Specimens were stored in two types of permeants - deionized water or 75% ethanol - for two storage times (G1 =7 days; G2 = 30 days). The specimens underwent water sorption and solubility tests, according to ISO 4049:2000 standard. After storage, residual monomers were identified and quantified by high performance liquid chromatography (HPLC). For sorption, L1 showed the highest values and QTH, the lowest. For solubility, in ethanol-stored groups, L1 had also the highest values, and QTH, the lowest, and findings were significantly different from the other curing regimens. L1 leached significantly more monomers than the others, and QTH had the lowest results. In conclusion, the type of light source, the curing regimen and the permeant affected sorption, solubility and amount of residual monomers of the adhesive under study.     

Influence of light energy and power density on the microhardness of two nanohybrid composites

The purpose of this study was to investigate the role of light parameters on nanohybrid composite curing. Two nanohybrid resins were cured by two light-emitting diode (LED) devices and by one quartz-tungsten-halogen (QTH) device using different combinations of energy density and power density (8 J cm(-2) and 400 mW cm(-2); 8 J cm(-2) and 1,000 mW cm(-2); 16 J cm(-2) and 400 mW cm(-2); and 16 J cm(-2)-1,000 mW cm(-2)). The effects of these combinations on polymerization were assessed by measuring the Vickers microhardness. Data differed for the two composites and varied according to the light parameters and the nature of the curing device. For both resins, an energy density of 16 J cm(-2) yielded the best microhardness values at both the top and the bottom of the sample, independently of the power density. When using a lower energy density of 8 J cm(-2), a modulated power density was required to achieve proper curing at the bottom of the sample: 8 J cm(-2) and 400 mW cm(-2) induced greater values at the bottom surface. At an energy density of 16 J cm(-2), the power density was not relevant (no significant differences were found between 400 and 1,000 mW cm(-2)), except when the emission spectra of the light-curing units (LCUs) did not match exactly with the absorption spectra of the photoinitators included in the resins (greatest values with 16 J cm(-2) and 1,000 mW cm(-2)). These results suggest that above a certain energy density threshold, the power density may not significantly influence the polymerization kinetics.     

Softening and elution of monomers in ethanol

ObjectivesThe purpose of this study was to investigate the effect of light-curing protocol on softening and elution of monomers in ethanol as measured on a model polymer. It was a further aim to correlate the measured values with previously reported data on degree of conversion and glass transition temperature for the same polymer and curing protocols.MethodsDifferent light-curing protocols were used in order to investigate the influence of energy density, power density, and mode of cure on the properties of a model polymer. The modes of cure were continuous, pulse-delay, and stepped irradiation of the specimens. Wallace hardness was used to determine the softening of the polymer after storage in ethanol for 24 h. Elution of monomers from the polymer was assessed after 7 days in ethanol by means of high-pressure liquid chromatography (HPLC). Data were submitted to two- and three-way analysis of variance (ANOVA), Newman–Keuls’ multiple comparison test, and linear regression analysis.ResultsEnergy density, power density, and mode of cure of the polymer influenced the softening and elution of monomers in ethanol. As energy density increased, softening and elution in ethanol decreased. At same energy density, the influence of power density varied with the mode of cure. When compared to the continuous mode of cure, and at same energy density, pulse-delay irradiation resulted in polymers that in general were more susceptible to softening, but eluted monomers to a lower extent. Less elution was also found with step-cured polymers. Significant, negative correlations were detected between softening and elution in ethanol, respectively, and degree of conversion and between softening and elution in ethanol, respectively, and glass transition temperature.SignificanceA complex relationship exists between curing protocol and the properties selected for investigation. The effect of different combinations of exposure periods and power densities are important to understanding how the curing protocol affects the properties of polymer-based materials.     

The effects of light intensity and method of exposure on the hardness of four light-cured dental restorative materials

AIM: To determine the microhardness of several light-curing dental restorative materials at the top surface as well as at a depth of 2mm. METHODS: Comparisons were made using different light sources. Four groups of five specimens each (3mm diameter, 2mm thick) of Z250 (composite), Filtek Flow (flowable composite), Dyract AP (compomer) and Dyract Flow (flowable compomer) were cured using different methods of exposure. The curing lights used were a Spectrum 800 halogen curing light at settings of 800 mW/cm2 and 400 mW/cm2 and an Optilux 501 ramping light. Vickers microhardness tests were performed at the top surface and at the bottom surface. RESULTS: Significant differences in microhardness between the top and bottom surfaces were demonstrated for all materials and with all light sources (p<0.05). The different exposures also produced significant differences (p<0.05). The light source used had less of an effect on the hardness of the compomers than the composites. CONCLUSIONS: Effective hardness ratios between the top and bottom surfaces were achieved with all three curing protocols. The effect of total energy application must be taken into account before assumptions can be made as to the effect on hardness using different exposure methods.     

Two-step curing: influence on conversion and softening of a dental polymer.

OBJECTIVES: The study investigates the influence of two-step curing of a BisGMA/TEGDMA based polymer on degree of conversion (DC) and on hardness before and after 1 day ethanol storage. METHODS: The specimens were pre-cured at 25, 50, 100, 200, or 400 mW/cm(2) for 10, 20, or 40 s. The final cure as well as the cure of the control group was carried out at 750 mW/cm(2) for 20 s. DC was determined on thin films by FTIR. Hardness was determined on disks by the method of Wallace. RESULTS: At intermediary energy densities of the pre-cure, the DC was reduced as compared to that of the control group. The hardness before ethanol storage was not influenced by the curing mode. Except for the lowest and the two highest energy densities of the pre-cure, the two-step curing mode resulted in polymers that after ethanol storage were softer than the control polymer. SIGNIFICANCE: Two-step curing of resin composites may result in polymers with increased susceptibility to the action of softening substances in food and beverage.     

Polymerization efficiency of different photocuring units through ceramic discs

This study compared the ability of a variety of light sources and exposure modes to polymerize a dual-cured resin composite through ceramic discs of different thicknesses by depth of cure and Vickers microhardness (VHN). Ceramic specimens (360) (Empress 2 [Ivoclar Vivadent], color 300, diameter 4 mm, height 1 or 2 mm) were prepared and inserted into steel molds according to ISO 4049, after which a dual-cured composite resin luting material (Variolink II [Ivoclar Vivadent]) with and without self-curing catalyst was placed. The light curing units used were either a conventional halogen curing unit (Elipar TriLight [3M/ESPE] for 40 seconds), a high-power halogen curing unit (Astralis 10 [Ivoclar Vivadent] for 20 seconds), a plasma arc curing unit (Aurys [Degré K] for 10 seconds or 20 seconds) or different light emitting diode (LED) curing units (Elipar FreeLight I [3M/ESPE] for 40 seconds, Elipar FreeLight II [3M/ESPE] for 20 seconds, LuxOmax [Akeda] for 40 seconds, e-Light [GC] for 12 seconds or 40 seconds). Depth of cure under the ceramic discs was assessed according to ISO 4049, and VHN at 0.5 and 1.0 mm distance from the ceramic disc bottom was determined (ISO 6507-1). Medians and the 25th and 75th percentiles were determined for each group (n=10), and statistical analysis was performed using the Mann-Whitney-U-test (p < or = 0.05). The results showed that increasing ceramic disc thickness had a negative effect on the curing depth and hardness of all light curing units, with hardness decreasing dramatically under the 2-mm thick discs using LuxOmax, e-Light (12 seconds) or Aurys (10 seconds or 20 seconds). The use of a self-curing catalyst is recommended over the light-curable portion only, because it produced an equivalent or greater hardness and depth of cure with all light polymerization modes.     

Curing depths of a universal hybrid and a flowable resin composite cured with quartz tungsten halogen and light‐emitting diode units

This in vitro study evaluated curing depths of a universal hybrid resin composite with two viscosities (Tetric Ceram and Tetric Flow) after curing with 6 different quartz tungsten halogen and light‐emitting diode curing units. Irradiance (light intensity) of the curing units varied between 200 and 700?mW/cm ² . The curing units were used for standard, soft‐start, or pulse curing. Curing times were 20 and 40?s for standard curing, 3?+?10?s and 3?+?30?s for pulse curing, and 40?s for soft‐start. Resin composite specimens, 5 in each group, with a diameter of 4?mm and a height of 6?mm, were made in brass molds and cured from one side at a distance of 6?mm. After 2 weeks, the specimens were ground longitudinally half through the specimen. Curing depth was then determined by measurement of Wallace hardness for each half millimeter starting at 0.5?mm from the top surface. For all curing units and for both resin composites an increased curing time led to statistically significantly higher depth of cure (P?P?r?=?0.89, P?r?=?0.91, P?相似文献   

Halogen and LED light curing of composite: temperature increase and Knoop hardness

This study assessed the Knoop hardness and temperature increase provided by three light curing units when using (1) the manufacturers’ recommended times of photo-activation and (2) standardizing total energy density. One halogen – XL2500 (3M/ESPE) – and two light-emitting diode (LED) curing units – Freelight (3M/ESPE) and Ultrablue IS (DMC) – were used. A type-K thermocouple registered the temperature change produced by the composite photo-activation in a mold. Twenty-four hours after the photo-activation procedures, the composite specimens were submitted to a hardness test. Both temperature increase and hardness data were submitted to ANOVA and Tukey’s test (5% significance). Using the first set of photo-activation conditions, the halogen unit produced a statistically higher temperature increase than did both LED units, and the Freelight LED resulted in a lower hardness than did the other curing units. When applying the second set of photo-activation conditions, the two LED units produced statistically greater temperature increase than did the halogen unit, whereas there were no statistical differences in hardness among the curing units.     

能量恒定时不同光密度对不同颜色纳米树脂硬度的影响

目的：在2种能量水平下采用3种光密度固化2种纳米复合树脂试片,通过检测比较其硬度值评价不同光密度及不同颜色对纳米树脂固化程度的影响。方法：设定12J及16J两个能量水平,用3盏光固化灯,光密度分别为400mW／cm2、800mW／cm2和1000mW／cm2,固化2种纳米树脂,颜色为A1、A2、A3、A3．5。将固化试片浸泡在蒸馏水中,避光37℃保存24h,测量试片表面和底面的维氏硬度值。结果：所有实验组的底面硬度均能达到有效的硬度百分率。16J的试片底面维氏硬度显著高于12J。采用800mW／cm。光密度固化效果较好,底面硬度随着树脂颜色加深而减小。结论：复合树脂固化的总能量恒定的观点不成立。在相同能量下,光密度800mw／cm。的光固化灯及较浅颜色的树脂可获得更高的转化率。能量为16J的固化效果优于12J,但在本实验条件下都能够满足临床需要。     

Diametral tensile strength and Vickers hardness of a composite after storage in different solutions

This study evaluated the Vickers hardness (VHN) and diametral tensile strength (DTS) of the composite Z100 (3M ESPE) cured with: Quartz-Tungsten-Halogen light curing unit (QTH) (700mW/cm2- 40 s) and Argon laser (1,000mW/cm2- 10 s). Specimens of 2 mm depth and 8 mm diameter were immersed for 30 days at 37 degrees C in different storage means: water, alcohol, acetic acid, propionic acid and dry (control). The DTS (n = 8) was determined with a crosshead speed of 0.5 mm/min. The VHN (n = 8) test was carried out using a 50 g load for 60 s. Statistical analysis was performed by two-way ANOVA and Tukey's test (alpha = 0.05). The relationship between VHN and DTS was observed by Pearson correlation. The light source was not significant in both tests (VHN: P < 0.18; DTS: P < 0.92), but the factor storage showed significance (P < .001). Mechanical properties of the control group were statistically superior to those of the other storage groups (VHN = 102.2; DTS = 42.3 MPa). The alcohol group showed the lowest VHN (93.3) and DTS (33.8 MPa) values, which were similar to values for propionic (VHN = 97.5; DTS = 35.9 MPa) and acetic acids (VHN = 97.8; DTS = 36.1 MPa), but different from that of water (VHN = 102.2; DTS = 42.3 MPa). The relationship between VHN and DTS values presented a positive correlation (r2 = 0.90; P < 0.01).     

Influence of curing rate of resin composite on the bond strength to dentin

This study determined whether the strength with which resin composite bonds to dentin is influenced by variations in the curing rate of resin composites. Resin composites were bonded to the dentin of extracted human molars. Adhesive (AdheSE, Ivoclar Vivadent) was applied and cured (10 seconds @ 1000 mW/cm2) for all groups. A split Teflon mold was clamped to the treated dentin surface and filled with resin composite. The rate of cure was varied, using one of four LED-curing units of different power densities. The rate of cure was also varied using the continuous or pulse-delay mode. In continuous curing mode, in order to give an energy density totaling 16 J/cm2, the power densities (1000, 720, 550, 200 mW/cm2) emitted by the various curing units were compensated for by the light curing period (16, 22, 29 or 80 seconds). In the pulse-delay curing mode, two seconds of light curing at one of the four power densities was followed by a one-minute interval, after which light cure was completed (14, 29, 27 or 78 seconds), likewise, giving a total energy density of 16 J/cm2. The specimens produced for each of the eight curing protocols and two resin composites (Tetric EvoCeram, Ivoclar Vivadent; Filtek Supreme XT, 3M ESPE) were stored in water at 37 degrees C for seven days. The specimens were then either immediately subjected to shear bond strength testing or subjected to artificial aging (6,000 cycles between 5 degrees C and 55 degrees C baths) prior to testing. Failure modes were also assessed. The shear bond strengths were submitted to factorial analysis of variance, and the failure modes were submitted to a Chi-square test (alpha = 0.05). All but power density (curing mode, resin composite material and mode of aging) significantly affected shear bond strength. The curing mode and resin composite material also influenced the failure mode. At the selected constant energy density, pulse-delay curing reduced bonding of the resin composite to dentin.     

Influence of curing rate on softening in ethanol, degree of conversion, and wear of resin composite

PURPOSE: To investigate the effect of curing rate on softening in ethanol, degree of conversion, and wear of resin composites. METHOD: With a given energy density and for each of two different light-curing units (QTH or LED), the curing rate was reduced by modulating the curing mode. Thus, the irradiation of resin composite specimens (Filtek Z250, Tetric Ceram, Esthet-X) was performed in a continuous curing mode and in a pulse-delay curing mode. Wallace hardness was used to determine the softening of resin composite after storage in ethanol. Degree of conversion was determined by infrared spectroscopy (FTIR). Wear was assessed by a three-body test. Data were submitted to Levene's test, one and three-way ANOVA, and Tukey HSD test (alpha = 0.05). Results: Immersion in ethanol, curing mode, and material all had significant effects on Wallace hardness. After ethanol storage, resin composites exposed to the pulse-delay curing mode were softer than resin composites exposed to continuous cure (P< 0.0001). Tetric Ceram was the softest material followed by Esthet-X and Filtek Z250 (P< 0.001). Only the restorative material had a significant effect on degree of conversion (P< 0.001): Esthet-X had the lowest degree of conversion followed by Filtek Z250 and Tetric Ceram. Curing mode (P= 0.007) and material (P< 0.001) had significant effect on wear. Higher wear resulted from the pulse-delay curing mode when compared to continuous curing, and Filtek Z250 showed the lowest wear followed by Esthet-X and Tetric Ceram.     

Hardening of a dual-cure resin cement using QTH and LED curing units

Objective

This study evaluated the surface hardness of a resin cement (RelyX ARC) photoactivated through indirect composite resin (Cristobal) disks of different thicknesses using either a light-emitting diode (LED) or quartz tungsten halogen (QTH) light source.

Material and Methods

Eighteen resin cement specimens were prepared and divided into 6 groups according to the type of curing unit and the thickness of resin disks interposed between the cement surface and light source. Three indentations (50 g for 15 s) were performed on the top and bottom surface of each specimen and a mean Vickers hardness number (VHN) was calculated for each specimen. The data were analyzed using two-way ANOVA and Tukey-Kramer test was used for post-hoc pairwise comparisons.

Results

Increased indirect resin disk thickness resulted in decreased mean VHN values. Mean VHN values for the top surfaces of the resin cement specimens ranged from 23.2 to 46.1 (QTH) and 32.3 to 41.7 (LED). The LED curing light source produced higher hardness values compared to the QTH light source for 2- and 3-mm-thick indirect resin disks. The differences were clinically, but not statistically significant. Increased indirect resin disk thickness also resulted in decreased mean VHN values for the bottom surfaces of the resin cement: 5.8 to 19.1 (QTH) and 7.5 to 32.0 (LED). For the bottom surfaces, a statistically significant interaction was also found between the type of curing light source and the indirect resin disk thickness.

Conclusions

Mean surface hardness values of resin cement specimens decreased with the increase of indirect resin disk thickness. The LED curing light source generally produced higher surface hardness values.     

Curing efficiency of various resin-based materials polymerized through different ceramic thicknesses and curing time

PURPOSE

The aim of this in vitro study was to examine the curing efficiency of various resin-based materials polymerized through ceramic restorations with 3 different thicknesses. Curing efficiency was evaluated by determining the surface microhardness (VHN) of the resin specimens.

MATERIALS AND METHODS

Four kinds of resin materials were used. Z350 (3M ESPE Filtek™ Z350: A2 Shade), Z250 (3M ESPE Filtek™ Z250: A2 Shade) and Variolink® II (VL: Ivoclar vivadent, base: transparent) either with or without a self-curing catalyst (VLC: Ivoclar vivadent, catalyst: low viscosity/transparent) were filled into the silicone mold (10 mm diameter, 1 mm thick). They were cured through ceramic discs (IPS e.max Press MO-0 ingot ivoclar vivadent, 10 mm diameter, 0.5, 1 and 2 mm thicknesses) by LED light-curing units for 20 and 40 seconds. Vicker''s microhardness numbers (VHNs) were measured on the bottom surfaces by a microhardness tester. Data were analyzed using a 3- way analysis of variance (ANOVA) at a significance level of 0.05.

RESULTS

The thickness of ceramic disc increased, the VHNs of all four resin types were decreased (P<.05). The mean VHN values of the resins light cured for 40 seconds were significantly higher than that of LED for 20 seconds in all four resin materials (P<.05). VLC showed significantly higher VHN values than VL regardless of other conditions (P<.05). Z350 and Z250 showed higher values than VL or VLC (P<.01).

CONCLUSION

Thinner ceramic disc with increased curing time resulted higher VHN values of all resin materials. The use of a catalyst produced a greater hardness with all polymerization methods. Restorative resin materials (Z350, Z250) showed higher VHN values than resin cement materials (VL, VLC).     

Effect of modulated photo‐activation on polymerization shrinkage behavior of dental restorative resin composites

This study investigated the influence of modulated photo‐activation on axial polymerization shrinkage, shrinkage force, and hardening of light‐ and dual‐curing resin‐based composites. Three light‐curing resin composites (SDR bulk‐fill, Esthet X flow, and Esthet X HD) and one dual‐curing material (Rebilda DC) were subjected to different irradiation protocols with identical energy density (27 J cm^?2): high‐intensity continuous light (HIC), low‐intensity continuous light (LIC), soft‐start (SS), and pulse‐delay curing (PD). Axial shrinkage and shrinkage force of 1.5‐mm‐thick specimens were recorded in real time for 15 min using custom‐made devices. Knoop hardness was determined at the end of the observation period. Statistical analysis revealed no significant differences among the curing protocols for both Knoop hardness and axial shrinkage, irrespective of the composite material. Pulse‐delay curing generated the significantly lowest shrinkage forces within the three light‐curing materials SDR bulk‐fill, Esthet X flow, and Esthet X HD. High‐intensity continuous light created the significantly highest shrinkage forces within Esthet X HD and Rebilda DC, and caused significantly higher forces than LIC within Esthet X flow. In conclusion, both the composite material and the applied curing protocol control shrinkage force formation. Pulse‐delay curing decreases shrinkage forces compared with high‐intensity continuous irradiation without affecting hardening and axial polymerization shrinkage.     

Suboptimal light curing and direct exposure to water of two contemporary composites: degree of conversion,sorption, solubility,and Knoop hardness

The water sorption and solubility of two polymer resin-based dental composite materials were assessed in order to evaluate the effects of immediate post-cure water exposure on the water sensitivity of the composites. Each material was tested with two different light curing setups. The radiant exposure of the two curing setups differed by a factor of 5. After exposure to water and subsequent drying, the Knoop surface hardness was measured. The change in the degree of conversion in both water and air storage medium within the first 24 h after curing was monitored by Raman spectroscopy. No significant differences in the degree of conversion were detected 24 h after curing. Samples exposed to the lower irradiation dose showed higher solubility and a lower surface hardness than the samples exposed to the higher irradiation dose. Early exposure to water did not cause detectable differences in the ongoing polymerization process. Increase in radiant exposure reduced the fraction of unbound constituents and increased the crosslink density, thereby reducing the plasticity of the material.     

Influence of curing modes on crosslink density in polymer structures

OBJECTIVE: This study investigates the influence of curing modes on the crosslinking density of dental composites. METHODS: A light-cure unit (BISCO VIP) that allowed for independent command over time and intensity was selected. Four different light-curing modes with constant light energy density were investigated (control (C), pulse delay (PD), soft-start (SS) and pulse cure (PC)). The degree of crosslinking was assessed directly by measuring the glass transition temperature of 1 mm thick composite (Z100, 3M-ESPE) specimens using differential scanning calorimetry (DSC 2920). Polymer softening in ethanol was used as an indirect method for assessing the degree of crosslinking. After light-curing, specimens were stored in air at 37 degrees C for 24 h and subjected to hardness testing using a digital microhardness tester (n = 6, load=500 g; dwell time=15 s). The specimens were then placed in 75% ethanol-water solution at 37 degrees C for 24 h and post-conditioning hardness was determined. Mean hardness (KHN)/hardness deterioration (DeltaKHN) was computed and data was subjected to analysis using one-way ANOVA/Scheffe's test. RESULTS: Ranking of degree of crosslinking density by DSC was as follows: C>PC>SS>PD. For the indirect method of determining crosslinking density, DeltaKHN ranged from 10.8 to 12.9 and ranking was PC>SS>C>PD. CONCLUSIONS: Specimens polymerized with PD were significantly more susceptible to softening in ethanol than specimens cured with PC. Results of this study suggest that polymerization with PD resulted in a lower crosslink density and gave rise to polymers with an increased susceptibility to softening in ethanol.     

Hardness,Flexural Strength,and Flexural Modulus Comparisons of Three Differently Cured Denture Base Systems

Purpose: This study compared the surface hardness, flexural strength, and flexural modulus of a light‐ and heat‐cured urethane dimethacrylate (UDMA) to two conventional polymethyl methacrylate (PMMA) denture base resins. The effect of less‐than‐optimal processing condition on the hardness of internal and external surfaces of UDMA specimens was also investigated. Materials and Methods: The materials tested were Eclipse (light‐ and heat‐cured UDMA), Meliodent (heat‐cured PMMA), and Probase Cold (auto‐cured PMMA). Eclipse specimens were prepared by adapting the material onto the master cast and light curing in the processing unit for 10 minutes. Meliodent and Probase Cold specimens were prepared according to the manufacturers' instructions. Twenty rectangular specimens measuring 65 × 10 × 2.5 mm³ were prepared for each material. They were stored in water at 37°C for 30 days before testing. The surface hardness was measured using Vickers Hardness (VHN) test, and flexural strength and flexural modulus were measured using a 3‐point bending test. Twenty‐five additional Eclipse specimens were similarly prepared and were processed at various times of less than 20 minutes of curing. Vickers Hardness was determined on both the external and internal surfaces of specimens. Data were analyzed using a one‐way ANOVA for comparisons of hardness, flexural strength, and flexural modulus between the three denture base materials and for hardness values of both the internal and external surface of Eclipse specimens with curing times. Post hoc analyses (Scheffé test) determined the difference between the groups. Student t‐test was used for comparison of hardness between the external and internal surfaces of Eclipse specimens. Results: The hardness (VHN) values were 19.4 ± 0.7, 17.0 ± 0.4, and 16.0 ± 0.4; the flexural strengths (MPa) were 103 ± 4, 78 ± 3, and 63 ± 4; and the flexural moduli (MPa) were 2498 ± 143, 1969 ± 55, and 1832 ± 89 for Eclipse, Meliodent, and Probase Cold materials, respectively. A comparison among the three polymers showed there were significant differences in surface hardness, flexural strength, and flexural modulus (p < 0.05). No significant difference in surface hardness (VHN) between the internal (19.1 ± 0.6 to 19.4 ± 0.7) and external surfaces (18.9 ± 0.4 to 19.2 ± 0.6) of irradiated Eclipse specimens was observed at 10‐, 12‐, and 14‐minute polymerization times. Conclusion: The surface hardness, flexural strength, and flexural modulus of light‐ and heat‐cured UDMA (Eclipse) were significantly higher than the values obtained for heat‐only cured (Meliodent) and auto‐cured (Probase Cold) PMMA denture base systems.