Effect of temperature on composite polymerization stress and degree of conversion

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.

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%).

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).

Increasing composite temperature allows for reduced exposure duration and lower polymerization stress (both maximum and final) while maintaining or increasing degree of conversion.