New Universal Bulk-Fill Composites with Translucency Shift: Impact of Rapid Curing and Ageing on Polymerisation and Mechanical Properties

This study investigated real-time polymerisation kinetics and mechanical properties under accelerated ageing of novel universal bulk-fill composites incorporating reversible addition-fragmentation chain transfer (RAFT) agent ß-allyl sulfone, designed for anterior and posterior applications. Five bulk-fill composites were tested: Tetric Plus Fill and Tetric Plus Flow (new universal composites); their predecessors, Tetric PowerFill and Tetric PowerFlow (Ivoclar); and RAFT-free Ecosite Bulk Fill (DMG). Specimens were polymerised for 3 s (~3000 mW/cm²), 10 s (~1200 mW/cm²), or 20 s (~1200 mW/cm²). Degree of conversion (DC) was monitored during and after curing, with mechanical testing after 24 h and after thermal cycling. DC and maximum polymerisation rate at 4 mm depth were significantly lower than at 0.1 mm for all materials and curing times. Three-second curing accelerated the polymerisation rate at both depths. Except for Ecosite cured for 3 or 10 s as RAFT-free material, DC ratios at 4 mm exceeded 80% of surface values. Tetric Plus Fill and Ecosite exhibited the highest flexural strength after 24 h, while PowerFill and Ecosite showed the highest flexural modulus at 24 h and after thermal cycling. Rapid curing did not compromise mechanical properties after 24 h, except for PowerFlow, the composite with the lowest filler vol%, but negatively affected both flowable composites after ageing. Thermal cycling reduced flexural strength in most tested conditions, but all materials and curing conditions more than satisfied the ISO 4049 requirements. The new simplified universal composite Plus Fill has a higher DC and improved polymerisation kinetics compared to its predecessor, PowerFill. Rapid curing is not recommended for the material without RAFT agents.