Beyond High-Speed Curing: Effects of Layer Thickness, Glycerin, and Polishing on the Surface Properties of an AFCT-Based Bulk-Fill Composite

This in vitro study evaluated the effects of layer thickness, photoactivation protocol, glycerin application, and polishing on top-surface roughness and microhardness. Sixty specimens were prepared from bulk-fill (Tetric PowerFill) and conventional (IPS Empress Direct) composites. Bulk-fill specimens were 2 or 4 mm thick and photoactivated with a conventional (1200 mW/cm2, 20 s) or high-speed (3000 mW/cm2, 3 s) protocol, giving nominal radiant exposures of 24 and 9 J/cm2; conventional specimens were 2 mm thick and photoactivated using the conventional protocol. Glycerin was applied or omitted; all specimens were polymerized beneath a polyester strip, with no air-exposed group. Roughness and Vickers microhardness were measured exclusively on the top surface, before and after polishing, and analyzed using General Linear Models for repeated measures (α = 0.05). In the bulk-fill composite, polishing reduced roughness and increased microhardness, with significant polishing × glycerin interactions for both outcomes. In the conventional composite, glycerin increased microhardness and roughness before polishing. Polishing increased microhardness, and a significant polishing × glycerin interaction was observed for roughness. The 20 s protocol produced significantly higher top-surface microhardness than the 3 s protocol, although the difference was small (2.58 VHN); because irradiance and exposure time differed simultaneously, their individual contributions cannot be distinguished. Layer thickness did not significantly affect the top-surface properties evaluated; this cannot be extrapolated to deeper regions or taken as evidence of equivalence between the 2 and 4 mm increments. In the condition in which both composites were treated identically, the effects of glycerin and of polishing differed significantly between the materials for both outcomes; for roughness, the difference in glycerin effects further depended on polishing. With no air-exposed controls and no artificial aging, the findings are restricted to the immediate top-surface response under the conditions evaluated.