How do increased irradiance and exposure time affect the repair bond strength of universal adhesives to CAD-CAM composite material?

This study was designed to investigate the effects of increased exposure time and different levels of irradiance on the repair bond strength (µTBS) and degree of conversion (DoC) of two universal adhesives.

Computer-aided design-computeraided manufacturing (CAD-CAM) composite blocks were prepared and subjected to thermal cycling. They were randomly divided into two groups based on the adhesive agent used: Clearfil S³ Bond Universal (CSU) or Gluma Bond Universal (GBU). Within each group, the specimens were further classified into six subgroups (n = 6 blocks) according to irradiance (1,000 or 1,400 mW/cm²) and exposure time (10, 15, or 20 s). The bonded specimens were then repaired using nano-hybrid resin composite. Sticks measuring 0.9 mm² were obtained from the repaired blocks and subsequently tested in a micro-tensile testing machine. Additionally, the DoC was assessed using micro-Raman spectroscopy.

The 1400*10 group of GBU and CSU demonstrated higher µTBS than the 1000*10 and 1400*15 groups, regardless of thermal cycling (P = 0.000). For CSU, the 1400*10 group showed the highest DoC (P < 0.05). However, the DoC of GBU did not differ significantly among the various irradiation/exposure times (P > 0.05).

The polymerization of adhesives at an irradiance level of 1,400 mW/cm² for 10 s (14 J/cm²) increases the repair µTBS and DoC regardless of thermal cycling.