Composition–Property Balance in PETG / SiO 2 / PBT Composites for Transparent Orthodontic Thermoforming Films

ABSTRACT Glycol‐modified polyethylene terephthalate (PETG) composites were examined through two formulation steps to balance mechanical properties, optical performance, and water uptake. Fumed and precipitated silica were compared at 0.5–2.0 wt%, followed by poly(butylene terephthalate) (PBT) blending with the selected silica‐containing precursor. The materials were prepared by melt blending and injection molding. Mechanical properties, light transmittance, and haze were evaluated, while water absorption was measured separately during immersion in distilled water at 37°C. In the silica series, 1.5 wt% fumed silica gave a tensile strength of 54.8 MPa and a flexural modulus of 2708 MPa. Transmittance and haze were 80.36% and 19.27%, respectively. Water absorption at 288 h remained 0.615%, compared with 0.583% for neat PETG. PBT blending lowered water uptake but also reduced strength and stiffness. The selected formulation contained 73.875 wt% PETG, 1.125 wt% SiO 2 ‐G, and 25 wt% PBT. It showed a tensile strength of 50.6 MPa, a flexural modulus of 2189 MPa, water absorption of 0.395%, and transmittance of 80.15%. Selection was based on the measured balance within the tested series, with optical performance constraining further PBT addition. The decrease in water uptake occurred alongside changes in polymer composition and dilution of the silica‐containing precursor. These results identify a candidate composition for transparent orthodontic thermoforming materials.