Degree of Conversion and Microshear Bond Strength of New Composite to Postcured Composite Resin Surface Treated With Ethyl Acetate

<b>Objectives:</b> This study aimed to investigate the impact of ethyl acetate (E) surface treatment on the degree of conversion (DC) of postcured composite, measured by the Fourier transform infrared spectroscopy (FTIR) method, and its microshear bond strength to the new composite. <b>Materials and Methods:</b> A total of 42 composite discs were prepared in a plexiglass mold with a thickness of 2 mm. Both sides of the discs were postcured in the Labolight LV-ӀӀӀ machine for 5 min. The composite discs were then divided into three groups, each containing 14 discs (<i>n</i> = 14), based on the surface treatment method used. It is worth noting that the mentioned division was employed randomly to ensure an unbiased study. The control group received sandblast and silane treatment. The second group was treated with E, while the third group underwent treatment with both E and silane (ESi). Cylindrical composite blocks were bonded to the prepared discs in a tygon tube. All groups underwent 5000 thermal cycles. The microshear bond strength was measured using a universal testing machine, and DC before and after treatment with E was determined through FTIR testing. Additionally, samples were examined under a stereomicroscope with a 40x magnification to investigate the failure mode. <b>Results:</b> The microshear bond strength of the control group was significantly higher than that of the other two groups; however, no significant difference was observed between the E and ESi groups (<i>p</i>-value = 0.18). In the control group, cohesive failure whereas in the other two groups, adhesive failure was the most frequent. The FTIR results showed that the application of E resulted in an 11% reduction in DC. <b>Conclusions:</b> The surface treatment of postcured composite, utilizing both chemical and mechanical methods, still results in better bond strength. Although the use of E led to a higher bonding potential of the postcured composite, it did not improve bond strength compared to the control group.