Effect of surface-modified incinerated sewage sludge ash as a reinforcing filler on the mechanical and thermal properties of polylactic acid composites

Biodegradable thermoplastic polylactic acid (PLA) has recently been limited in its broader development and application due to its high cost. Therefore, it is crucial to develop low-cost and high-performance PLA-based composites by adding fillers. In this study, surface-modified incinerated sewage sludge ash (M-OH-ISSA) was employed as a reinforcing filler in PLA to prepare PLA/M-OH-ISSA thermoplastic composites with varying M-OH-ISSA ratios (10–30 wt%), and the mold mechanism for the PLA/M-OH-ISSA composites was explored. The surface of incinerated sewage sludge ash (ISSA) was modified to improve interfacial adhesion by NaOH alkali treatment and KH570 silane grafting, which successfully grafted silane groups onto the surface of the ISSA and converted the surface from hydrophilic to hydrophobic, strengthening the compatibility between the ISSA and PLA. The effects of the M-OH-ISSA content on the mechanical and thermal properties of the PLA composites were investigated. The mechanical test results showed that the tensile and flexural properties of the 15 wt% M-OH-ISSA-filled PLA composites were excellent compared to those of pure PLA, with strengths of 33.67 MPa and 73.70 MPa, respectively, which are all 1.6 times greater than those of pure PLA. Morphological analysis via SEM indicated that the composites with 15 wt% M-OH-ISSA exhibited good adhesion between the M-OH-ISSA and the matrix. The thermal performance of the composites was determined by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG/DTG). The results showed that the crystallinity of the composites increased with increasing M-OH-ISSA content and that the thermal stability between 290°C and 400°C decreased slightly with increasing M-OH-ISSA. However, the composites have good thermal stability near 200°C and do not undergo thermal degradation if processed at this temperature. As a filler in composites, M-OH-ISSA can reduce PLA usage and improve polymer properties, applying PLA/M-OH-ISSA composites in packaging and 3D printing applications.