Utilization of Sewage Sludge-Derived Biochar as a Functional UV Stabilizer in Recycled Poly(ethylene terephthalate) Nanocomposite Materials

This work investigates the incorporation of sewage sludge-derived biochar (BC) into a recycled poly(ethylene terephthalate) (rPET) matrix at loadings ranging from 0.5 to 5% wt. in order to develop sustainable nanocomposite materials. The materials were comprehensively characterized using complementary structural, thermal, and morphological techniques to evaluate the effect of BC on the properties of the recycled polymer. XRD, DSC, and PLM analyses demonstrated that BC acts as a heterogeneous nucleating agent, without significantly altering the final crystalline fraction of rPET, while SEM observations confirmed the homogeneous dispersion of particles within the matrix. TGA and Py–GC/MS further indicated that BC moderates thermal degradation, reducing the relative formation of low-molecular-weight degradation products and promoting the retention of larger terephthalate-containing fragments. Accelerated UV irradiation experiments further demonstrated the protective role of BC against photo-induced degradation. Intrinsic viscosity (IV) measurements showed that BC-containing nanocomposites exhibited a smaller molecular-weight reduction after UV exposure compared to neat rPET, indicating reduced chain scission, while complementary DSC analyses confirmed improved preservation of thermal transitions after aging. Overall, sewage sludge-derived biochar is demonstrated to be a promising multifunctional additive for rPET, acting both as a nucleating agent and a UV stabilizer. This approach provides an alternative route for the valorization of two waste streams, contributing to enhanced materials in a circular economy framework.