Mechanical properties of light-curing composites polymerized with different laboratory photo-curing units

This study aimed to analyze the microhardness (KHN) and diametral tensile strength (DTS) of two hybrid resin composites (TPH Spectrum and Filtek Z250). To this end, the composites were polymerized with six laboratory photo-curing units (LPUs) and the results compared with an alternative polymerization method using conventional halogen light source in conjunction with additional polymerization in an autoclave (15 minutes/100 degrees C). LPUs were used following the manufacturers' instructions. Diametral tensile strength and Knoop hardness tests were conducted for all groups (n=5). Data were statistically compared using ANOVA and Tukey's test (alpha = 0.05). Among the LPUs, the one that provided light curing in conjunction with heat and nitrogen pressure resulted in a significant increase in KHN and DTS of resin composites. Between the resin composites, Filtek Z250 showed higher hardness values than TPH Spectrum. It was concluded that the use of alternative polymerization with conventional light polymerization and autoclave was feasible with a wide implication for the general public in terms of reduced dental treatment cost.     

Diametral tensile strength of composite resins submitted to different activation techniques

The aim of this study was to evaluate the diametral tensile strength (DTS) of composite resins submitted to different curing techniques. Four composite resins were tested in this study: Targis (Ivoclar), Solidex (Shofu), Charisma (Heraeus-Kulzer) and Filtek Z250 (3M Espe). Sixty-four cylindrical specimens were prepared and divided into eight groups according to each polymerization technique (n = 8). The indirect composite resins (Targis and Solidex) were polymerized with their respective curing systems (Targis Power and EDG-lux); Charisma and Filtek Z250 were light-cured with conventional polymerization (halogen light) and additionally, with post-curing systems. Specimens were stored in artificial saliva at 37 degrees C for one week. DTS tests were performed in a Universal Testing Machine (0.5 mm/min). The data were statistically analyzed by ANOVA and Duncan tests. The results were (MPa): Z250/EDG-lux: 69.04 feminine; Z250/Targis Power: 68.57 feminine; Z250/conventional polymerization: 60.75b; Charisma/Targis Power: 52.34c; Charisma/conventional polymerization: 49.17c; Charisma/EDG-lux: 47.98c; Solidex: 36.62d; Targis: 32.86d. The results reveal that the post-cured Z250 composite resin showed the highest DTS means. Charisma composite presented no significant differences when activation techniques were compared. Direct composite resins presented higher DTS values than indirect resins.     

Temperature change and hardness with different resin composites and photo-activation methods

This study verifies whether there is any temperature change during photoactivation of two resin composites (Filtek Z250 and Filtek Flow) with three different light curing methods (conventional halogen light curing unit, light emitting diodes curing unit and xenon plasma arc curing unit) and the relationship of temperature change with resin composite hardness. A type-K thermocouple registered the temperature rise peak in an elastomer mold during photoactivation. After photoactivation, the specimens were submitted to Knoop hardness test performed by an indenter (HMV-2000) under a load of 50g for 15 seconds. Both the temperature change data and results of the Knoop hardness test were submitted to ANOVA and Tukey's test at the 5% significance level. No statistical differences in temperature rise were recorded for the different composites following processing by light curing unit (p>0.05). The conventional halogen source produced statistically higher temperatures (p<0.05) than the other units. The plasma arc source promoted statistically lower (p<0.05) Knoop hardness values and temperature changes than the other light curing units.     

Irradiance effects on the mechanical properties of universal hybrid and flowable hybrid resin composites.

OBJECTIVES: A potential problem with high-intensity lights might be failure of polymer chains to grow and cross-link in a desired fashion, thereby affecting the structure and properties of the polymers formed. The purpose of this study was to evaluate mechanical properties of resin composites polymerized using four different light-curing units. METHODS: A conventional quartz-tungsten-halogen (QTH) light, a soft-start light, an argon-ion laser, and a plasma-arc curing light were used to polymerize disk-shaped (9.0mm diameter x 1.0 mm high) and cylinder-shaped (4mm diameter x 8 mm high) specimens of a universal hybrid and a flowable hybrid composite. Biaxial flexure strength, fracture toughness, hardness, compressive strength, and diametral tensile strength were determined for each composite. RESULTS: The use of the plasma-arc curing light, a high-intensity light, resulted in significantly lower hardness for the universal hybrid composite compared with the hardness obtained using the conventional QTH and the soft-start units. Hardness was the only mechanical property that was adversely affected by the use of a high-intensity light. SIGNIFICANCE: High-intensity lights might affect some resin composite mechanical properties, but this effect cannot be generalized to all resin composites and all properties.     

Establishment of a composite resin inlay technique. Part 1. The effects of various curing modes on mechanical properties of composite resins

The effects of the curing mode on mechanical properties of composite resins were examined. Four resins as inlay, and three chemically-cured and five visible light-cured restorative resins were employed. The resin specimens were prepared by three kinds of curing modes; regular setting (according to the manufacturer's instruction), subsequently added light and heat curing after regular setting, and subsequently added heat and pressure curing after regular setting. Knoop hardness, flexure strength, compressive strength, and diametral tensile strength were determined. All restorative composites were remarkably increased in knoop hardness number due to the subsequently added curing methods. Both subsequently added curing methods provided higher flexure strength to all restorative resins, and particularly in the chemically-cured resins the flexure strength provided by the subsequently added light and heat curing was higher than those by the subsequently added heat and pressure curing. Compressive strength and diametral tensile strength were slightly increased by the subsequently added curing methods with the restorative resins. No correlation was found between the filler distribution and the mechanical properties provided by the subsequently added curing methods. The subsequently added heat curing seems to be preferable for creating higher mechanical properties of resins. The IC-2 resin, experimentally designed for resin inlay, seems to be the most promising resin for inlay restoration, based on the mechanical properties, and further detailed laboratory and clinical researches are required.     

Comparison between a plasma arc light source and conventional halogen curing units regarding flexural strength, modulus, and hardness of photoactivated resin composites

The plasma arc curing light Apollo 95 E (DMDS) is compared to conventional curing lights of different radiation intensities (Vivalux, Vivadent, 250 mW/cm²; Spectrum, DeTrey, 550 mW/cm²; Translux CL, Kulzer, 950 mW/cm²). For this purpose, photoactivated resin composites were irradiated using the respective curing lights and tested for flexural strength, modulus of elasticity (ISO 4049), and hardness (Vickers, Knoop) 24 h after curing. For the hybrid composites containing only camphoroquinone (CQ) as a photoinitiator (Herculite XRV, Kerr; Z100, 3 M), flexural strength, modulus of elasticity, and surface hardness after plasma curing with two cycles of 3 s or with the step-curing mode were not significantly lower than after 40 s of irradiation using the high energy (Translux CL) or medium energy conventional light (Spectrum). However, irradiation by only one cycle of 3 s failed to produce adequate mechanical properties. Similar results were observed for the surface hardness of the CQ containing microfilled composite (Silux Plus, 3 M), whereas flexural strength and modulus of elasticity after plasma curing only reached the level of the weak conventional light (Vivalux). For the hybrid composites containing both CQ and photoinitiators absorbing at shorter wavelengths (370–450 nm) (Solitaire, Kulzer; Definite, Degussa), plasma curing produced inferior properties mechanical than conventional curing; only the flexural strength of Solitaire and the Vickers hardness of Definite reached levels not significantly lower than those observed for the weak conventional light (Vivalux). The suitability of plasma arc curing for different resin composites depends on which photoinitiators they contain. Received: 5 July 1999 / Accepted: 16 March 2000     

PHYSICAL PROPERTIES OF COMPOSITES CURED WITH CONVENTIONAL LIGHT OR ARGON LASER

Objective : This study evaluated various physical properties of two resin composites polymerized by either an argon laser or a conventional visible light. Materials and Methods : A hybrid composite, Herculite XRV (Kerr Corp., Orange, California), and a microfill composite, Durafill VS (Kulzer, Wehrheim, Germany), were used in this study. Three physical properties, diametral tensile strength, compressive strength, and flexural strength, were tested. Five specimens of each composite resin were made for each set of physical properties tested, for a total of 18 groups and 90 specimens. Specimens were fabricated according to the American National Standards Institute and American Dental Association Specification No. 27 for each property tested. The composite was polymerized with either an argon laser (power density of 1000 mW/cm²) for 10 or 20 seconds or a conventional visible light (power density of 354 mW/cm2) for 40 seconds. Specimens were stored in water in light‐proof containers at 37°C for 7 days before testing with a Zwick (Atlanta, Georgia) universal testing machine. Results : The physical properties of Herculite XRV were not affected by the light source or exposure time. For Durafill VS, no significant differences were observed for the diametral tensile strength whether the argon laser or conventional light was used. However, the flexural strength of the microfill was significantly lower when polymerized with the argon laser a t 10 seconds compared with the two other curing methods (20‐second laser cure, 40‐second conventional cure). Also, the compressive strength of Durafill VS polymerized with the argon laser a t 10 seconds was significantly lower than when it was cured with the conventional light for 40 seconds. Conclusions : Hybrid and microfill resin composites cured with an argon laser for 20 seconds had physical properties comparable to composites polymerized with a conventional visible light unit for 40 seconds. Therefore, with adequate exposure time, the argon laser is a potential alternative to conventional visible light‐curing.     

The relationship between composition and properties of posterior resin composites

The effects of filler concentration and resinous components on the properties of highly filled composites were determined for prediction of the durability of the restorative resins. Resinous components of seven proprietary light-cured posterior resin composites were extracted by chloroform solvent and examined by the Fourier Transform Infrared (FTIR) method. Filler concentration was determined by the thermogravimetric method. Diametral tensile strength, Knoop hardness, and Barcol hardness tests for the composite, as well as extracted resinous matrix, were performed by standard experimental procedures. Toothbrush abrasion test of the resin composites was evaluated by a toothbrushing machine giving the equivalent of five years' toothbrushing and examined with a roughness meter. The degree of conversion of resin composites ranged from 43.5 to 73.8%. The volume fraction of filler varied from 58.2% to 74.2%. The ranges of diametral tensile strength and Knoop and Barcol hardness numbers obtained were 39.8 MPa to 60.0 MPa, 41.8 to 81.9, and 76.3 to 89.2, respectively. Significant correlations (p less than 0.01) were obtained between filler fraction and diametral tensile strength (r = 0.89, S.E. = 3.66) and between filler fraction and Knoop hardness number (r = 0.89, S.E. = 8.39). The increase in strength with increased filler concentration might be related to filler/matrix bonding.     

The Knoop hardness of a composite resin polymerized with different curing lights and different modes

AIM: The purpose of this study was to compare the surface hardness of a hybrid composite resin polymerized with different curing lights. METHODS AND MATERIALS: Two 3.0 mm thick composite resin discs were polymerized in a prepared natural tooth mold using: (1) a conventional quartz-tungsten halogen light (QTH- Spectrum 800); (2) a high-intensity halogen light, Elipar Trilight (TL)-standard/exponential mode; (3) a high-intensity halogen light, Elipar Highlight (HL)-standard/soft-start mode; (4) a light-emitting diode, Elipar Freelight (LED); and (5) a plasma-arc curing light, Virtuoso (PAC). Exposure times were 40 seconds for the halogen and LED lights, and three and five seconds for the PAC light. Following polymerization, the Knoop hardness was measured at the bottom and the top surfaces of the discs. RESULTS: Significant differences were found between top and bottom Knoop Hardness number (KHN) values for all lights. The hardness of the top and bottom surfaces of both specimens cured by the PAC light was significantly lower than the other lights. No significant hardness differences were observed between the remaining curing units at the top of the 2.0 mm specimens. Significant differences were found between the LED and two modes of HL on the bottom surfaces. For the 3.0 mm thick samples, while significant differences were noted between LED and TL standard mode and between the two TL curing modes on the top, significant differences were only observed between QTH and the standard modes of TL and HL at the bottom.     

Effect of pre-heating on depth of cure and surface hardness of light-polymerized resin composites

PURPOSE: To evaluate the depth of cure and surface hardness of two resin composites when subjected to three preheating temperatures, three polymerization times and two types of curing lights. METHODS: Two resin composites were used in this study (Esthet-X and TPH), three polymerization times (10, 20, 40 seconds), three preheating temperatures (70, 100, 140 degrees F/21.1, 37.7 and 60 degrees C), and two curing lights (halogen and LED). For depth of cure measurements, 180 specimens (4 mm in diameter and 2 mm in depth) were made for 36 combinations of variables. Four Knoop hardness measurements were obtained from both the top and bottom surfaces. For the surface hardness, another 180 (4 x 6 mm) cylindrical specimens were fabricated. Each specimen was sectioned in half and hardness measurements were made at 0.5 mm intervals. Statistical analyses were performed using the multifactor ANOVA at a level of significance of alpha = 0.05. RESULTS: For depth of cure, there was a statistical difference among all the main effects (time, temperature and curing light) for both composites (P > 0.001) when the % difference from the top was analyzed. Results indicate that there was an increase in hardness as the temperature of the composite was increased from 70 to 140 degrees F for both composites for either the top or the bottom. The percent difference in hardness was greater when the LED curing light was used compared to the halogen curing light. Overall there was a greater change in hardness when the resin composite was polymerized at 140 degrees F. Although the ISO standard was not met in many cases, there was a significant increase in hardness on both the top and bottom as temperature and curing time increased (P < 0.001). Results for the surface hardness showed that there was a significant statistical difference (P < 0.001) in hardness when the surface hardness at 0.5 and 3.5 mm were analyzed separately. There was a general increase in surface hardness for both the hybrid and microhybrid as time and temperature increased. For both hybrid and microhybrid groups, as the temperature increased, there was an increase in hardness and it was statistically different (P < 0.001). When the percent difference between 70 and 100 degrees F or 70 and 140 degrees F was evaluated, the greatest increase occurred between the 70 and 140 degrees F and minimal increase between 100 and 140 degrees F. Overall, the LED curing light provided a greater surface hardness for the hybrid at both depths than the halogen curing light. For the microhybrid, the halogen curing light provided the greatest surface hardness when the resin was polymerized for 40 seconds.     

The physical properties of packable and conventional posterior resin-based composites: a comparison

BACKGROUND: The authors compared the physical properties of three packable hybrid resin-based composites with those of a conventional hybrid and a microfill composite material advocated for use as posterior restorative materials. They evaluated diametral tensile strength, or DTS; compressive strength, or CS; flexural strength, or FS; and depth of cure, or DC. METHODS: The authors studied the following resin-based restorative materials: three packable composites, Alert Condensable Composite (Jeneric Pentron), SureFil High Density Posterior Restorative (Dentsply Caulk) and Solitaire (Heraeus Kulzer); one conventional hybrid composite, TPH Spectrum (Dentsply Caulk); and one microfill, Heliomolar Radiopaque (Ivoclar-Vivadent). The authors evaluated DTS, CS, FS and DC, according to American National Standards Institute criteria. They made scanning electron micrographs of the packable resin-based composites. RESULTS: Results demonstrated that the conventional hybrid, TPH Spectrum, had significantly greater DTS and FS than other resin-based composites. Alert and SureFil had comparable DTS and FS, which were significantly greater than Heliomolar's DTS and FS. Solitaire had significantly lower DTS and FS than all other resin-based composites. SureFil had the highest CS, followed by TPH Spectrum, Solitaire and Alert, which were comparable and had significantly greater CS than Heliomolar. TPH Spectrum and Alert had significantly greater DC than all other resin-based composites, followed in decreasing order by SureFil, Solitaire and Heliomolar. CONCLUSION: While the packable composites tested in this study had physical properties superior to those of the microfill composite, they were no better suited for use as a posterior restorative material than was the conventional hybrid resin-based composite. CLINICAL IMPLICATIONS: Packable composites may be easier for clinicians to handle than conventional resin-based composites; however, their physical properties were not superior to those of the conventional small-particle hybrid resin-based composite. In addition, these materials may have the clinical drawback of increased wear and surface roughness that was seen with early, large-particle composite restorative materials.     

The influence of storage and indenter load on the Knoop hardness of dental composites polymerized with LED and halogen technologies.

OBJECTIVES: The mechanical properties of light cured dental composites are greatly influenced by the light curing unit (LCU) used for the polymerization. Previous studies have shown that for some composites lower mechanical properties were obtained if light emitting diode (LED) LCUs were used for the polymerization instead of halogen LCUs. Previous studies have also shown that light cured composites improve their mechanical properties through a post-curing process after the initial illumination with the LCUs. Therefore, this study investigated the post-curing process, to ascertain if it can compensate for the lower mechanical properties of composites polymerized with LED LCUs. METHODS: The Knoop hardness was measured for four dental composites (Z100, Spectrum, Definite, Solitaire2) polymerized with an LED LCU (LED63 prototype) or a halogen LCU (Trilight), directly after the curing process and after 5 days of storage. In addition, the load on the indenter was varied from 200 to 400 gf to investigate the influence of the load on the measured hardness on the top and bottom of the 2 mm thick samples. RESULTS: In general the Knoop hardness at the bottom of the stored samples, cured with the LED LCU, was the same or statistically significantly greater than for the samples cured with the halogen LCU. A statistically significantly lower (p<0.0001) Knoop hardness was obtained on the top of the samples if the composite Definite was polymerized with the LED LCU instead of the halogen LCU. The load of 200 or 400 gf on the indenter had a statistically significant influence (p<0.0001) on the measured Knoop hardness for the composite Z100. The Knoop hardness measured with an indenter load of 400 gf increased statistically significantly (p<0.0001) for all composites after the 5 days' storage, whether cured with the LED LCU or halogen LCU. SIGNIFICANCE: The post-curing effect cannot compensate for the lower hardness of composites containing co-initiators if polymerized with an LED LCU instead of a halogen LCU. The indenter load had a statistically significant influence on the measured Knoop hardness of composites and has the potential to falsify results if not selected carefully.     

Physical and mechanical properties of the thermosetting resin for crown and bridge cured by micro-wave heating

A heating method using micro-waves was utilized to obtain strong thermosetting resin for crown and bridge. The physical and mechanical properties of the thermosetting resin were examined. The resin was cured in a shorter time by the micro-waves heating method than by the conventional heat curing method and the working time was reduced markedly. The base resins of the thermosetting resin for crown and bridge for the micro-waves heating method were 2 PA and diluent 3 G. A compounding volume of 30 wt% for diluent 3 G was considered good the results of compressive strength, bending strength and diametral tensile strength. Grams of 200-230 of the filler compounded to the base resins of 2 PA-3 G system provided optimal compressive strength, bending strength and diametral tensile strength. A filler gram of 230 provided optimal hardness and curing shrinkage rate, the coefficient of thermal expansion became smaller with the increase of the compounding volume of the filler. The trial thermosetting resin for crown and bridge formed by the micro-waves heating method was not inferior to the conventional resin by the heat curing method or the light curing method.     

Second generation LEDs for the polymerization of oral biomaterials.

OBJECTIVES: New blue, so called second generation light emitting diodes (LEDs) are now available with a high optical power output. These LEDs will potentially find widespread application in commercially available light curing units (LCUs). This study, therefore, investigated the curing performance of a prototype LCU containing one high power LED and a conventional halogen LCU (Polofil). METHODS: The performances of the LCUs were evaluated by measuring the Knoop hardness and depth of cure of the composites. Three dental composites were selected (Z100, Admira and Revolcin Flow) in a light (A2) and a dark shade (A3.5 or A4), respectively, and were polymerized for 40 s each. RESULTS: The LED prototype (irradiance=901 mW/cm2) achieved a statistically significantly greater (p<0.05) depth of cure than the halogen LCU (irradiance=860 mW/cm2) for all composites. Generally, there was no statistically significant difference in Knoop hardness on the top and bottom of a 2 mm thick disk for the composites Z100 and Admira if polymerized with the LED prototype or halogen LCU. The composite Revolcin Flow, however, showed in general a statistically significant lower Knoop hardness if polymerized with the LED LCU. SIGNIFICANCE: The present study shows that second generation LEDs have the potential to replace halogen LCUs if the composites are selected carefully. Furthermore, this study confirmed that the depth of cure test does not discriminate between LCU's performance for composites containing co-initiators, but the Knoop hardness test does.     

Mechanical properties of heat-treated composite resin restorative materials.

Clinical methods for heat treating composite resin restorations have been developed. In this investigation, the effect of heat treatments on the diametral tensile strength of composite resin was determined. The composite resin restorative materials were selected according to the manufacturers' suggested use for anterior or posterior teeth, filler particle composition, and light-cured or chemical polymerization. Samples were prepared according to American Dental Association specification No. 27, and heat treatments were accomplished with a Coltene DI 500 oven for curing at approximately 120 degrees C for 7 minutes. Heat treatment substantially increased the diametral tensile strength tested, with the exception of the anterior hybrid particle (p less than 0.05). Composite resins with fine-particle inorganic fillers were significantly stronger than hybrid and microfilled composite resins.     

The weight change of various light-cured restorative materials stored in water

This study investigated weight changes of seven different light-cured composite restorative materials, one polyacid glass ionomer compomer, and one light-cured glass-ionomer cement following short-term and long-term storage in water. Two packable composites, three universal (hybrid) composites, one microglass composite, one polyacid glass ionomer resin composite (compomer), one microhybrid low-viscosity (flowable) composite, and one light cured glass ionomer composite cement were evaluated in this study. The weight changes of these specimens were measured daily (short-term storage), and they were measured after six weeks (long-term storage) using an electronic analytical balance. A significant difference was found in Ionoliner, Dyract AP, Opticor flow, Charisma, and Solitare 2, but no significant difference was found in the others (Filtek Z 250, Filtek P60, TPH Spectrum, and Valux Plus). Weight change showed a tendency to increase with the time of water storage. The greatest weight change occurred in light-cured glass ionomer composite cement (Ionoliner), which is followed in order by the weight changes in Dyract AP, Opticor Flow, Charisma, Solitare 2, Filtek Z250, Filtek P60, TPH Spectrum; Valux Plus had the least amount of change.     

Depth of cure and hardness of indirect composite materials polymerized with two metal halide laboratory curing units

The purpose of this study was to evaluate the depth of cure and Knoop hardness of indirect composite materials polymerized with different laboratory curing units. Five composite materials designed for fixed restoration veneer (Artglass, Ceramage, Epricord, Prossimo, and Solidex) were filled into a cylindrical mold and then light-exposed by using the respective proprietary laboratory curing unit or two metal halide curing units (Hyper LII and Twinkle X). Depth of cure was determined by a scraping technique, as described in ISO 4049. Composites also underwent Knoop hardness testing after immersion in water. The results (n = 5) were analyzed with the Kruskal-Wallis test and Dunn's multiple comparison test. For three materials (Prossimo, Artglass, and Epricord), depth of cure after polymerization with the Twinkle X unit was greater than that after polymerization with the respective proprietary units. For the Ceramage and Artglass materials, the Twinkle X unit resulted in the highest Knoop hardness number (KHN), whereas, for the Prossimo material, the Hyper LII unit resulted in the highest KHN. The metal halide units were effective in enhancing the post-polymerization properties of specific composite materials while reducing exposure time.     

The effect of composite type on microhardness when using quartz-tungsten-halogen (QTH) or LED lights

This study evaluates the Knoop microhardness of resin composites cured with different light-emitting diode (LED) based light curing units (LCU) or with a conventional quartz-tungsten-halogen light (QTH). Ten experimental groups with 10 specimens each were used. The specimens were prepared by placing two light-cured resin composites with similar VITA shade A2-microhybrid Filtek Z250/3M ESPE and microfill Durafil VS/Heraeus Kulzer--in a 2.0 mm-thick disc shaped mold. The specimens were polymerized for 40 seconds with the use of one QTH LCU (Optilux 501/Kerr-Demetron) and four LED LCUs: Elipar FreeLight 1 Cordless LED (3M ESPE), Ultrablue II LED with cord (DMC), Ultrablue III LED cordless (DMC) and LEC 470 I (MM Optics). Knoop microhardness was determined at the top and bottom surfaces of the specimens 24 hours following curing. Microhardness values in the microhybrid resin composite group showed no statistically significant differences when cured with LED FreeLight 1 LCU and QTH LCU (p<0.05). The other LED devices evaluated in the study presented lower microhardness values in both surfaces (p<0.05) when compared to QTH. In the microfill resin composite group, no statistically significant differences were observed among all LCUs evaluated on the bottom surfaces (p<0.05). However, on the top surfaces, QTH presented the highest KHN values, and the LED devices presented similar results when compared with KHN values relative to each other (p<0.05).     

Physical properties of two new crown and bridge veneering resins

The degree of conversion and physical properties of two contemporary resin veneers based on light-cured microfilled composite formulations and employing proprietary curing systems were evaluated. Visio-gem (V) and Dentacolor (D) were polymerized using the appropriate curing systems. Polymer structure and degree of conversion were determined by Fourier transform infra-red (FTIR) techniques. Physical properties, including depth of cure, compressive and diametral tensile strengths, hardness, thermal expansion and colour stability were determined by standard test modalities. Both materials appear to have the properties of typical microfilled resins including low compressive yield strengths and high thermal expansion coefficients. The curing system for V produced an increased depth of cure compared to conventional light curing techniques and the D system, but no clinically significant increase in the physical properties.     

Effect of light curing modes and light curing time on the microhardness of a hybrid composite resin

AIMS: The aim of this in vitro study was to evaluate the influence of light curing modes and curing time on the microhardness of a hybrid composite resin. METHODS AND MATERIALS: Forty-five Z250 composite resin specimens (3M-ESPE Dental Products, St. Paul, MN, USA) were randomly divided into nine groups (n=5): three polymerization modes (conventional-550 mW/cm2; light-emitting diodes (LED)-360 mW/cm2, and high intensity-1160 mW/cm2) and three light curing times (once, twice, and three times the manufacturer's recommendations). All samples were polymerized with the light tip 8 mm from the specimen. Knoop microhardness measurements were obtained on the top and bottom surfaces of the sample. RESULTS: Conventional and LED polymerization modes resulted in higher hardness means and were statistically different from the high intensity mode in almost all experimental conditions. Tripling manufacturers' recommended light curing times resulted in higher hardness means; this was statistically different from the other times for all polymerization modes in the bottom surface of specimens. This was also true of the top surface of specimens cured using the high intensity mode but not of conventional and LED modes using any of the chosen curing times. Top surfaces showed higher hardness than bottom surfaces. CONCLUSIONS: It is important to increase the light curing time and use appropriate light curing devices to polymerize resin composite in deep cavities to maximize the hardness of hybrid composite resins.