Influence of irradiant energy on degree of conversion, polymerization rate and shrinkage stress in an experimental resin composite system.

OBJECTIVE: This study evaluated the degree of conversion (DC), maximum rate of cure (Rpmax), and polymerization stress (PS) developed by an experimental dental composite subjected to different irradiant energies (3, 6, 12, 24, or 48 J/cm2) under constant irradiance (500 mW/cm2). METHODS: DC and Rpmax were monitored for 10 min on the bottom surface of 2-mm thick disks and on 150-microm thick films (representing the top of the specimen) using ATR-FTIR. PS was monitored for 10 min in 2-mm thick disks bonded to two glass rods (Ø=5mm) attached to a universal testing machine. One-way ANOVA/Tukey tests were used and differences in DC and R(p)(max) between top and bottom surfaces were examined using Student's t-test. Statistical testing was performed at a pre-set alpha of 0.05. RESULTS: For a given surface, DC showed differences among all groups, except at the top between 24 and 48 J/cm2. Rpmax was similar among all groups at the same surface and statistically higher at the top surface. PS also showed significant differences among all groups. Data for 48 J/cm2 were not obtained due to specimen failure at the glass/composite interface. SIGNIFICANCE: Increases in irradiant exposure led to significant increases in DC and PS, but had no effect on Rpmax.

Influence of radiant exposure on contraction stress, degree of conversion and mechanical properties of resin composites.

OBJECTIVE: To verify the influence of radiant exposure (H) on contraction stress (CS), degree of conversion (DC) and mechanical properties of two restorative composites. METHODS: Filtek Z250 (3M ESPE) and Heliomolar (Ivoclar) were photoactivated with 6, 12, 24, or 36 J/cm2 at continuous irradiance of 600 mW/cm2. CS at 10 min was determined in a low compliance testing system. DC, flexural strength (FS), flexural modulus (FM) and Knoop microhardness (KHN) were measured after 24 h storage at 37 degrees C. KHN and DC measurements were conducted on the irradiated surface of 1mm thick disk-shaped specimens. Bar-shaped specimens were submitted to three-point bending to determine FS and FM. Data were analyzed by one-way ANOVA/Tukey's test (alpha = 0.05) separately for each composite. RESULTS: For Filtek Z250, no significant increase in CS was observed above 12 J/cm2. DC and FM were similar at all H values, while FS increased significantly between 6 and 24 J/cm2. KHN was significantly different among all H levels, except between 12 and 24 J/cm2. For Heliomolar, CS and KHN increased significantly with H, except between 24 and 36 J/cm2. DC, FM and FS did not vary, regardless of the radiant exposure. SIGNIFICANCE: Variables tested behaved differently. CS and KHN were more sensitive to increasing radiant exposures than the other properties evaluated. FS varied only for Filtek Z250, while, for both composites, DC and FM were not affected by different H values.

Influence of photoactivation method on conversion, mechanical properties, degradation in ethanol and contraction stress of resin-based materials.

OBJECTIVES: To investigate the influence of photoactivation method on degree of conversion (DC), flexural strength (FS), flexural modulus (FM) and Knoop hardness (KHN) of a composite and an unfilled resin (Filtek Z250 and Scotchbond multi-purpose plus, 3M ESPE) after storage in water or ethanol, and on composite contraction stress (CS). METHODS: Specimens 1x2x10 mm were prepared for FS test, photoactivated by 600 mW/cm2 x 40 s (A), 200 mW/cm2 x 120 s (B), or 600 mW/cm2 x1 s +3 min delay +600 mW/cm2 x 39 s (C), and tested after 24 h in water or ethanol. Load and displacement values were used to calculate FM. Specimen fragments were used to measure KHN. DC was determined by FT-Raman spectroscopy. CS was determined by mechanical testing. Data were submitted to ANOVA/Tukey's test (alpha=0.05). RESULTS: Composite DC was not affected by photoactivation (A: 65+/-1.8%; B: 66+/-3.4%; C: 65+/-2.9%). Unfilled resin DC was statistically higher using method A (79+/-0.3%) than B (74+/-1.0%) and C (73+/-0.9%). Photoactivation did not influence composite properties, regardless of the storage medium (p>0.05). After ethanol storage, FS of the unfilled resin was lower for specimens irradiated by method B (p<0.001). Pulse-delay curing (C) significantly reduced CS (7.7+/-1.3 MPa), compared to A (10.7+/-1.2 MPa) and B (10.1+/-1.3 MPa). SIGNIFICANCE: Photoactivation method did not affect composite properties or susceptibility to ethanol degradation. For the unfilled resin, DC was lower with the use of low intensity and pulse-curing, while FS after ethanol storage was reduced by low intensity curing. Pulse-delay curing significantly reduced CS.

Relationship between contraction stress and degree of conversion in restorative composites.

OBJECTIVES: To verify the relationship between contraction stress and degree of conversion (DC) in different composites (Filtek Z250, Filtek A110, Tetric Ceram and Heliomolar). METHODS: For the contraction stress test, composite (2 mm thick) was applied between two 5-mm diameter glass rods, mounted in a tensilometer. DC was determined by Infrared Photoacoustic spectroscopy in specimens with similar dimensions and geometry, submitted to identical curing conditions. Specimens were exposed to different energy densities (4.5, 13.5, 27.0, 54.0 and 108.0 J/cm2) by varying exposure time. Contraction stress and DC were recorded 10 min after the beginning of photoactivation. Results were analyzed by ANOVA/Tukey's test and regression analysis. RESULTS: For contraction stress, the interaction between composite and energy density was significant. Stress values ranged between 0.6+/-0.2 and 2.0+/-0.3 MPa at 4.5 J/cm2, 2.3+/-0.5 and 4.3+/-0.4 MPa at 13.5 J/cm2, 3.8+/-0.5 and 5.8+/-0.9 MPa at 27.0 J/cm2, 4.2+/-0.8 and 7.9+/-0.9 MPa at 54.0 J/cm2 and 6.6+/-0.8 and 8.1+/-0.9 MPa at 108.0 J/cm2. Tetric Ceram (39+/-5.8%) showed a higher average DC than the other materials. Heliomolar (28+/-5.2%) showed an average DC similar to Filtek Z250 (32+/-6.6%) and to Filtek A110 (24+/-7.5%) regardless of the energy density level. No significant increase in DC was observed above 27 J/cm2. CONCLUSIONS: At high energy levels, DC had a tendency to level off earlier than contraction stress values. SIGNIFICANCE: Using high energy densities may cause a significant increase in stress values, without producing a significant increase in conversion.

Effect of temperature on composite polymerization stress and degree of conversion.

OBJECTIVE: To test the following hypotheses: (1) degree of conversion (DC) and polymerization stress (PS) increase with composite temperature (2) reduced light-exposure applied to pre-heated composites produces similar conversion as room temperature with decreased PS. METHODS: Composite specimens (diameter: 5mm, height: 2mm) were tested isothermally at 22°C (control), 40°C, and 60°C using light-exposures of 5 or 20s (control). DC was accessed 5min after light initiation by FTIR at the specimen bottom surface. Maximum and final PS were determined, also isothermally, for 5min on a universal testing machine. Non-isothermal stress was also measured with composite maintained at 22°C or 60°C, and irradiated for 20s at 30°C. Data were analyzed using two-way ANOVA/Tukey and Student's t-test (α=5%). RESULTS: Both DC and isothermal maximum stress increased with temperature (p<0.001) and exposure duration (p<0.001). Isothermal maximum/final stress (MPa) were 3.4±2.0b/3.4±2.0A (22°C), 3.7±1.5b/3.6±1.4A (40°C) and 5.1±2.0a/4.0±1.6A (60°C). Conversion values (%) were 39.2±7.1c (22°C), 50.0±5.4b (40°C) and 58.5±5.7a (60°C). The reduction of light exposure duration (from 20s to 5s) with pre-heated composite yielded the same or significantly higher conversion (%) than control (22°C, 20s/control: 45.4±1.8b, 40°C, 5s s: 45.1±0.5b, 60°C, 5s s: 53.7±2.7a, p<0.01). Non-Isothermal conditions showed significantly higher stress for 60°C than 22°C (in MPa, maximum: 4.7±0.5 and 3.7±0.4, final: 4.6±0.6 and 3.6±0.4, respectively). CLINICAL SIGNIFICANCE: Increasing composite temperature allows for reduced exposure duration and lower polymerization stress (both maximum and final) while maintaining or increasing degree of conversion.

Degree of conversion and mechanical properties of a BisGMA:TEGDMA composite as a function of the applied radiant exposure.

OBJECTIVE: Verify the influence of radiant exposure (H) on composite degree of conversion (DC) and mechanical properties. METHODS: Composite was photoactivated with 3, 6, 12, 24, or 48 J/cm(2). Properties were measured after 48-h dry storage at room temperature. DC was determined on the flat surfaces of 6 mm x 2 mm disk-shaped specimens using FTIR. Flexural strength (FS) and modulus (FM) were accessed by three-point bending. Knoop microhardness number (KHN) was measured on fragments of FS specimens. Data were analyzed by one-way ANOVA/Tukey test, Student's t-test, and regression analysis. RESULTS: DC/top between 6 and 12 J/cm(2) and between 24 and 48 J/cm(2) were not statistically different. No differences between DC/top and bottom were detected. DC/bottom, FM, and KHN/top showed significant differences among all H levels. FS did not vary between 12 and 24 J/cm(2) and between 24 and 48 J/cm(2). KHN/bottom at 3 and 6 J/cm(2) was similar. KHN between top and bottom was different up to 12 J/cm(2). Regression analyses having H as independent variable showed a plateau region above 24 J/cm(2). KHN increased exponentially (top) or linearly (bottom) with DC. FS and FM increased almost linearly with DC/bottom up to 55% conversion. CONCLUSIONS: DC and mechanical properties increased with radiant exposure. Variables leveled off at high H levels.