Silylated Softwood and Hardwood Lignin: Impact on Thermomechanical and Interfacial Properties of PLA Biocomposites

Achieving compatibility between biopolymers and natural fillers is a significant challenge in developing sustainable materials. PLA-lignin biocomposites frequently demonstrate poor interfacial adhesion, mostly due to polarity differences. Softwood (LS) and hardwood (LH) lignins vary in composition and reactivity, affecting PLA structure. This study evaluated the surface compatibilization of LS and LH through silylation at 1, 3, and 5 wt % using a GPS coupling agent. Silylation was validated by TGA, DSC (<i>T</i>_g increase of ∼3-7 °C), ash color, and EDX (increased silicon). FTIR assessed structural differences in lignins. Rheological tests and melt flow index indicated that unmodified lignin reduced PLA viscosity, while GPS-modified lignin increased it. DSC showed that LS enhanced crystallization more than LH, and GPS at 1 wt % promoted nucleation. Films containing LH at 10 and 1 wt % GPS exhibited improved mechanical properties. Barrier properties remained unchanged, though all films provided UV-blocking capability.