Effect of boric acid on the color stability and mechanical properties of 3D-printed permanent resins

This study evaluated the effect of incorporating boric acid (BA) at different concentrations on the flexural strength, color stability, and microhardness of 3D-printed permanent crown resins. Disk-shaped specimens (15 × 2 mm) were fabricated for color stability and Vickers microhardness testing, and bar-shaped specimens (25 × 2 × 2 mm) were prepared for flexural strength evaluation using a masked stereolithography (MSLA) printer. BA was incorporated into the resin at 0% (control), 1%, 2%, and 3% (w/w). Flexural strength (n = 7 per group) was measured using a three-point bending test, microhardness (n = 7 per group) was assessed with the Vickers method, and color stability (n = 22 per group) was evaluated using ΔE₀₀ values at 3 time points: immediately after post-curing (T0), 30 min after post-curing (T1), and 60 min after post-curing (T2). Surface morphology was examined using field-emission scanning electron microscopy (FE-SEM). Data were analyzed using one-way and two-way ANOVA followed by Tukey's HSD post hoc tests (α = 0.05). BA incorporation significantly affected flexural strength (p < 0.05), with the 3% BA group showing the highest mean value (177.92 ± 38.58 MPa), significantly higher than the control (p = 0.001), 1% (p = 0.002), and 2% (p = 0.011) groups. Microhardness showed an increasing trend with higher BA concentrations but did not reach statistical significance (p > 0.05). Color stability was adversely affected by BA addition (p < 0.001), as ΔE₀₀ values increased progressively from T0 to T2, with the 3% BA group consistently exceeding both perceptibility (ΔE₀₀ > 0.8) and clinical acceptability thresholds (ΔE₀₀ > 1.8) at all time points. BA incorporation enhanced flexural strength in a concentration-dependent manner, while microhardness improvements were not statistically significant, and color stability was compromised at higher concentrations. BA-modified resins may be suitable for posterior restorations where mechanical performance is prioritized.