# Temperature Changes in Composite Materials during Photopolymerization
## 4 Discussion
measured the temperature rise in class I cavities in third molars in the course of layered filling with a composite material (Filtek Z250, Shade A2, lot N506344, 3M ESPE, St. Paul, MN, USA). The temperature was measured for 110 s using eight thermocouples. The authors demonstrated that the rise in the temperature within the cavity was significantly higher during polymerization of the first layer of material (59.8 \({}^{\circ}\)C) compared to the next layer (58.5 \({}^{\circ}\)C) [19]. The authors' findings do not demonstrate such a temperature increase, even in 20 s mode. This might support the idea of shorter polymerization durations being used for the deepest layers of composite fillings. In a subsequent examination of six composites (Admira, Filtek P60, Premise, Tetric Flow, Tetric Ceram, and Filtek Z250) polymerized using different modes (standard (10 s, full power), pulsed (10 consecutive one-second exposures at full power) or soft start (progressive cycle lasting 20 s)), with a type L thermocouple being used for temperature measurements, higher temperature rises were observed for soft start exposures (in the case of Admira and Tetric Flow materials). The lowest rise in temperature was observed for the Premise material irradiated using the pulsed protocol. None of the exposure protocols tested resulted in temperature being raised to the critical value [20]. In the authors' research the lowest temperature rise was observed for Essentia Medium Dentin and Fast-cure 3 s mode (7.25 \({}^{\circ}\)C). No significant differences in temperature rise were found between modes with similar durations. In another study, three polymerization units were compared, one halogen lamp (QTH LCU XL2500 (3M/ESPE), two LED lamps (Freelight LED LCU (3M/ESPE), and Ultrablue (DMC Equipamentos LTDA) on a one composite material (Filtek Z250, 3M/ESPE). Five types of photoactivation modes were used: 20 s with each of the three light curing units according to the manufacturer's instructions, 50 s with the Freelight LED lamp, and 30 s with the Ultrabule IS lamp. The authors demonstrated that for photoactivation times as per the manufacturer's recommendations, both LED lamps produced a lower temperature rise than the QTH lamp (average temperature rise values in degrees Celsius: Ultrablue LED 1.13 (0.05), Freelight LED 1.05 (0.16) vs. QTH XL 2500 1.57 (0.13)). However, the authors stressed that the choice of the type of the polymerization lamp affected the average temperature rises [21]. However, other studies did not confirm the reports suggesting that LED lamps generated lower temperature rise values. A study using 96 fragments of bovine teeth revealed that higher temperatures were obtained during polymerization of composite materials using a LED lamp as compared to halogen lamps. However, the temperature increase was above 5.5 \({}^{\circ}\)C for both tested lamps, which could be considered as a critical value [22]. The authors' study demonstrated that the temperature rise was lower than the critical temperature value only for the 3 s Fast-cure mode. For other modes and durations, the final temperature was higher than the critical temperature value. [PERSON] et al. used two LED lamps (Bluephase, Elipar Freelight 2) and a halogen lamp (Prismatics). The authors demonstrated that for all light curing units, the exposure of the bonding system resulted in temperature rises being significantly higher than those observed for the exposure of composite materials. However, higher temperatures were achieved during the polymerization of bonding systems and resin-based composite materials using both LED lamps compared to the halogen lamp [23]. [PERSON] et al. demonstrated that for the output values of all units tested (halogen, plasma, LED), the temperature rise exceeded 5.5 \({}^{\circ}\)C; and the temperature rose along with increasing polymerization duration [24]. Type of polymerization unit and irradiation modes also affect the mechanical properties of composite materials. One such parameter is flexural strength. The study by [PERSON] et al.