ATR-FTIR Assessment of Degree of Conversion Across Curing-State Transitions in Bis-EMA- and UDMA-Based Microfilled Dental Resins for Additive Manufacturing

Reliable assessment of the degree of conversion (DC) is essential for evaluating the processing quality and anticipated clinical performance of additively manufactured dental methacrylate resins. However, the widely used attenuated total reflectance–Fourier transform infrared (ATR‑FTIR) method based on the 1637/1608 cm⁻¹ ratio may be unstable in contemporary methacrylate systems because of spectral congestion and formulation‑dependent behaviour of the aromatic reference band. This study evaluated a low‑variance ATR‑FTIR protocol based on a polymerisation‑sensitive, ester‑associated band pair at 1320/1352 cm⁻¹ to assess DC in two commercially available microfilled dental resins for additive manufacturing with matrices dominated by ethoxylated bisphenol A dimethacrylate (Bis‑EMA) and urethane dimethacrylate (UDMA). The conventional 1637/1608 cm⁻¹ ratio was analysed in parallel as a comparative method. Across clinically relevant processing stages, including the green state and post‑curing, the ester‑associated model produced chemically plausible and statistically stable conversion trajectories, whereas the conventional aromatic‑normalised method showed greater formulation dependence and lower stability. In the UDMA‑dominant resin, mean surface DC increased (68.44–91.61%) after post‑curing. In the Bis‑EMA‑dominant resin, the same model detected a significant processing‑stage effect, with a practical conversion plateau near 91% following post‑curing and thermal treatment. Coefficients of variation for the ester‑associated protocol were low (0.54–5.58%) compared with the conventional method (29.13-673.09%), which also produced non‑physical negative DC estimates in some groups. These findings support the proposed workflow as a practical surface‑state characterisation tool for printed dental resin systems, while highlighting the need for broader validation across multiple materials and orthogonal methods before wider adoption.