Biobased Ternary Blends of Lignin, Poly(Lactic Acid), and Poly(Butylene Adipate-co-Terephthalate): The Effect of Lignin Heterogeneity on Blend Morphology and Compatibility

Abstract

Blending of lignin into thermoplastic materials presents a challenge due to the lack of dispersion and compatibility in the thermoplastic matrices. Kraft lignin was fractionated by methanol to homogenize its structure and molecular weight, and blended with poly(butylene adipate-co-terephthalate) (PBAT) and poly(lactic acid) (PLA). It was found through Fourier transform infrared spectroscopy that the lignin–polyester interaction involves aromatic group interactions as well as hydrogen bonding between the polymers. The differences in the intermolecular interactions led to high compatibility of lignin with PBAT and low compatibility with PLA as reflected by glass transition temperature shifts on the differential scanning calorimetry (DSC) curves. The DSC study also indicated that the methanol soluble lignin (MSL) fraction interacts with both PLA and PBAT, but no sign of interaction was evident between PLA and PBAT, which is reflected in the scanning electron microscope images depicting the morphology of the ternary blend. The resulting tensile properties showed retention of toughness at 30 % lignin content, and bridging of stress between PLA and PBAT by MSL.