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

Assessing the degree of conversion is important for evaluating processing quality in additively manufactured dental methacrylates. 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 the 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 estimate surface or near-surface conversion 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 the manufacturer-recommended processing stages, from the green state before postcuring to the final postcured state, the 1320/1352 cm⁻¹ model yielded chemically plausible and statistically stable conversion trajectories. In contrast, the conventional aromatic-normalised method showed greater dependence on formulation and workflow, and lower analytical stability. In the UDMA-dominant resin, conversion increased from 68.44