Sustainable Approach to Thermoplastic Polyurethanes Stabilization: Vanillin as a Natural Additive in Melt-Processing

Thermo-oxidative degradation during high-temperature processing remains a major challenge limiting the performance of thermoplastic polyurethanes (TPUs). This study evaluated vanillin (4-hydroxy-3-methoxybenzaldehyde), a bio-based phenolic compound, as a primary thermo-oxidative stabilizer acting as a free-radical scavenger. Bio-based TPU (bio-TPU) was synthesized via a two-step method using a commercial bio-based polyol, a bio-based chain extender, and a mixture of diisocyanates. During processing in an internal mixer, vanillin was incorporated into both bio-TPU and a commercial TPU at concentrations of 0.03, 0.165, and 0.3 wt%. Changes in chemical structure were assessed by Fourier-transform infrared spectroscopy (FTIR), thermal properties were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), including oxidation onset temperature (OOT) measurements, and dynamic mechanical analysis (DMA). Mechanical properties were assessed by tensile test and hardness. FTIR analysis revealed no significant structural changes after vanillin addition. However, changes in thermal properties were observed, including an increase in the OOT for materials containing the highest vanillin content. TGA results also showed a generally improved thermal stability at higher vanillin concentrations, approaching reference-material values at the highest concentration in the commercial TPU. A slight decrease in mechanical properties was observed. These findings demonstrate a pronounced stabilizing effect of 0.3 wt% vanillin on both bio-TPU and commercial TPU, providing new insights into the potential reuse and further recycling of TPU materials stabilized with bio-based additives.