Eco-Friendly Flame-Retardant Construction Composites Based on Bio-Based TPU, Recycled Rice Husk, and Ammonium Polyphosphate

This study explores the use of agricultural waste rice husk powder (RH) as a sustainable alternative to the petrochemical-derived carbon source, pentaerythritol (PER), in expandable flame retardants. RH is combined with halogen-free ammonium polyphosphate (APP), which serves as both an acid and a gas source. The resulting APP/RH system is incorporated into bio-based thermoplastic polyurethane (Biobased TPU) to prepare a halogen-free, flame-retardant composite material consistent with circular economy principles and environmental sustainability. The optimal APP-to-RH ratio in bio-based TPU was determined to be 2:1, with the best flame-retardant performance observed in the composite containing 20 wt% APP/RH. This formulation achieved a limiting oxygen index (LOI) of 27% and a UL-94 V-0 rating, indicating excellent flame resistance. Thermogravimetric analysis (TGA) showed a significant increase in char residue—from 0.51 wt% in pure TPU to 26.1 wt%—demonstrating improved thermal stability. Further characterization using cone calorimetry, thermogravimetric analysis–Fourier transform infrared spectroscopy (TGA-FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy confirmed that the addition of APP/RH significantly enhances the flame-retardant properties of the TPU composite. Consequently, the application of TPU in construction materials can be advanced through improved fire safety performance and alignment with sustainability goals.