Enhancing Mechanical and Thermal Properties of Recycled Poly(Lactic Acid) with Multi-Branched Polyethyleneimine for Sustainable Recycling Applications

Abstract

The single-use plastic coffee cup made from poly(lactic acid) (PLA) was modified for sustainable recycling to widen its applications by melt blending with multi-branched polyethyleneimine. Different concentrations (0.25 to 1 wt.%) and molecular weights (Mw of 2,000 and 25,000 g/mol) of the multi-branched polyethyleneimine (MPEI) were utilized as a mechanical modifier for the recycled poly(lactic acid) (rPLA). As a result, an attenuated total reflectance accessory equipped with Fourier transform infrared spectroscopy (ATR-FTIR) and carbon nuclear magnetic resonance spectroscopy (13C-NMR) indicated the formation of the amide linkage between the carboxylic acid group of rPLA and the amine group of MPEI under the given melt mixing condition. Incorporating 0.75 wt.% of MPEI, with a molecular weight of 25,000 g/mol, significantly enhances the properties of recycled PLA, increasing tensile strength by about 9 MPa and impact resistance by around 3.6 kJ/m² compared to the unmodified rPLA. The phase morphological property of rPLA/MPEI blend system exhibited a typical miscible blend and ductile microstructure aspect, characterized by uniform homogeneity and visible microfibrils on the impact-fractured surface. A slight decrease in the glass transition temperature (Tg) further indicated enhanced interfacial interactions, contributing to improved mechanical performance. Furthermore, the incorporation of MPEI notably increases the thermal decomposition temperature of the blends, demonstrating its effectiveness in enhancing the mechanical and thermal properties of recycled PLA. This advancement paves a promising strategy for expanding the material’s applicability while fostering sustainable recycling into high-value products.