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

POLYMERS, cilt.18, sa.18, ss.1-17, 2026 (Hakemli Dergi)

This in vitro study evaluated the effects of layer thickness, photoactivation protocol, glyc-

erin application, and polishing on top-surface roughness and microhardness. Sixty speci-

mens 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 conven-

tional (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 photoac-

tivated 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 distin-

guished. 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 identi-

cally, 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.