Recovered BHET as a functional additive in starch and keratin biopolymer films

The increasing demand for sustainable packaging materials has driven the development of biodegradable films derived from natural polymers. However, many conventional biopolymer-based films suffer from poor mechanical strength, high water sensitivity, and limited thermal stability, restricting their broader industrial application. In this study, biodegradable films were fabricated using starch and keratin matrices, incorporating varying concentrations of recovered bis(2-hydroxyethyl) terephthalate (BHET), derived from the glycolysis of post-consumer polyethylene terephthalate (PET). The films were characterized for mechanical strength, thermal stability (thermogravimetric analysis/differential thermal analysis, TGA/DTA), and structural properties using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses. Water solubility decreased as BHET content increased, from 2.04 g in SB3 to 1.76 g in SB10 (starch-based) and from 2.12 g in KB1 to 1.51 g in KB10 (keratin-based). Tensile strength reached 1.25 MPa in SB3 and 1.26 MPa in KB10, while KB10 exhibited the highest stiffness (Young's modulus: 40.32 MPa). FTIR confirmed the incorporation of BHET through ester carbonyl peaks (1714 cm −1 ) and shifts in O–H stretching bands, indicating hydrogen-bonding interactions. XRD patterns revealed increased crystallinity, with distinct peaks at 2θ ≈ 23.67° and 29.05°. Enhanced thermal stability was observed in SB10 and KB10, with char yields of 16–18% and 11.4%, respectively. These findings demonstrate that BHET improves the functional performance of starch- and keratin-based films while promoting PET waste valorization. Nevertheless, further studies are needed to evaluate long-term biodegradation behaviour, food-packaging safety, and large-scale processing feasibility to facilitate commercial implementation of these sustainable bioplastic materials.