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% to 91.61% after light postcuring. For the Bis-EMA-dominant resin, the same model revealed a significant processing-stage effect, with a practical plateau near 91% after combined light and thermal treatment. The coefficients of variation for the 1320/1352 cm⁻¹ protocol were low (0.54-5.58%) compared with those of the conventional method (29.13-673.09%), which also yielded nonphysical negative estimates in some groups. Under the conditions tested, these findings support the 1320/1352 cm⁻¹ ATR-FTIR workflow as a practical approach for surface or near-surface characterisation of the two material-workflow systems studied. However, broader validation against orthogonal analytical methods and performance-related outcomes is needed before wider adoption.