Resin composite specimens’ preparation
Thirty cylindrical disk-shaped specimens (6 mm diameter and 2 mm thickness) were prepared using a customized Teflon mold, rendering 60 specimens for the two resin composite materials. A Teflon mold and copper ring were fabricated with 2 mm thickness and 20 mm diameter with a centralized hole of (6 mm diameter and 2 mm thickness) in which the resin composite was bulk packed as one increment [According to the manufacturer’s recommendations, the resin composite materials were dispensed, manipulated, and polymerized. A nylon thread was incorporated into the specimen before polymerization to allow their easy handling and their suspension in the immersion solutions during all aging periods. The resin composite was bulk packed inside the central hole of the split Teflon ring. To ensure that specimens would have flat polymerized surfaces free of bubble formation after curing, both top and bottom surfaces were covered with transparent polyester matrix strips (TOR VM, Moscow, Russia) and a microscopic glass slide (1 mm thickness) [Specimens were polymerized for 20 s on both surfaces (top and bottom) using a light-emitting diode unit (LED COXO DB-686 MOCHA) through the glass slide and the polyester matrix strip. LED light curing unit was used in standardized mode, emitting 1200mW/cm2 irradiance as measured with a radiometer (Kerr manufacturing product). Then the two wings of the ring were opened to remove the specimens. The dimensions of the specimens were checked with a caliper (Pirmadent Germany SS). All samples were finished and polished using Sof-Lex disks (three-step system), starting with medium grit, then fine grit, and finally superfine grit, all in a dry media for 15 s. Then rinsing and drying with a water/air syringe for a total of 6 s [