# Comparative In Vitro Analysis of Composite Resins Used in Clear Aligner Attachments
## 3 Results  
### Thermal Analysis and Combustion Testing  
Figure 3 presents the DSC (differential scanning calorimetry) thermograms for the Aligner Connect (AC) and Connect Flow (CF) composites, detailing their heat flow profiles over the 20-250 \({}^{\circ}\)C temperature range at a constant heating rate of 10 \({}^{\circ}\)C/min. These curves offer key insights into the materials' thermal transitions, phase behavior, and chemical composition.  
The thermal behavior of the AC and CF composites was evaluated through DSC analysis, as shown in Figure 3a,b. Both materials exhibited an initial increase in heat flow around 30-70 \({}^{\circ}\)C during the first heating scan, which was absent in the second scan. This feature is indicative of residual thermal activity, likely related to moisture release, minor reorganization of the polymer network, or low-temperature relaxation phenomena. A more prominent endothermic event was observed in both composites within the 85-120 \({}^{\circ}\)C range, corresponding to the glass transition temperature (Tg) of the polymer matrix. The Tg of CF was slightly lower than that of AC, as reported in the inset table of Figure 3a, with values of 103.8 \({}^{\circ}\)C (first scan) and 81.4 \({}^{\circ}\)C (second scan) for CF compared to 110.8 \({}^{\circ}\)C and 89.6 \({}^{\circ}\)C for AC. The CF thermogram displayed a sharper and more pronounced Tg, suggesting a polymer matrix with reduced crosslink density and greater segmental mobility related to the lower presence of filler. Conversely, the broader and more gradual Tg transition in AC may reflect a more heterogeneous network, potentially due to higher crosslink density or  
Figure 3: DSC curves 1 scan and 2 scan for AC and CF samples, with endo up (**a**); cooling curves for AC and CF samples (**b**).  
-filler-matrix interphase interactions that restrict chain motion. Additionally, CF consistently exhibited higher baseline heat flow than AC, which could be attributed to its lower filler content and greater polymer chain mobility. This trend is further confirmed by the second scan (Figure 3a), where the two materials display different post-Tg thermal responses. At temperatures exceeding \(\sim\)180 \({}^{\circ}\)C a secondary increase in heat flow was noted, particularly for CF. This behavior may point to late-stage, post-curing, residual oligomer reactivity or the onset of thermal softening, consistent with a less thermally stable and more reactive network. In contrast, AC maintained a more subdued heat flow rise, indicating a more thermally inert system. The cooling scans (Figure 3b) further highlight the distinct thermal behaviors. CF retained higher heat flow throughout the cooling phase, consistent with its higher thermal responsiveness and more flexible structure. The flatter cooling curve of AC suggests a more rigid and thermally dampened matrix, likely influenced by its elevated inorganic filler content.  
To support these interpretations, combustion analysis was conducted by incinerating 150 mg of cured composite at 600 \({}^{\circ}\)C for 60 min. The AC composite retained 61% of its initial mass, whereas CF retained only 23%, confirming a significantly higher inorganic content in AC. This aligns with the DSC findings, where AC's reduced heat flow and broader transitions correlate with its higher filler loading. These fillers are likely inert at test temperatures and act as thermal dampeners, reducing matrix flexibility and suppressing thermal events. In contrast, the lower filler content in CF allows for more pronounced thermal activity, increased heat absorption, and sharper transitions, making it potentially better suited for applications requiring thermal responsiveness or elasticity.