Green, Sustainable, and Melt-Compoundable PLA Composites Reinforced with Spray-Dried Lignocellulosic Nanofibrils for Enhanced Barrier and Mechanical Properties

• Development of a fully bio-based and biodegradable spray dried lignocellulose-reinforced PLA-based composite via melt compounding. • Spray dried lignocellulosic nanofibrils are highly dispersible in PLA, capable of nucleating a dense and refined crystal microstructure. • Improvements to mechanical and barrier properties are tied to the induced crystal microstructure of the composite samples compared to neat PLA. • In-situ microscopy combined with micro-tensile testing is applied to observe fracture toughening mechanisms within composite samples. This study presents the first integration of spray dried (SD), non-modified, lignocellulosic nanofibrils (LCNF) into polylactic acid (PLA) by melt blending. Originating from industrial forestry waste, SD LCNFs are an inexpensive, non-toxic, and abundantly accessible drop-in filler whose production is facile, continuous, and highly scalable. Addition of SD LCNF into PLA yields enhanced barrier and mechanical performance due to SD LCNF’s alteration of the crystalline microstructure and fracture dynamics. Incorporating 1–1.5 wt% of SD LCNFs into PLA results in significant enhancements: tensile strength by 32.8%, toughness by 44.6%, water vapor barrier performance by 38.8%, and oxygen barrier properties by 26.4%, compared to neat PLA. Their nucleating capability hastens isothermal crystallization of PLA composites by over 90%, enabling faster processing times. For the first time, in-situ polarized optical microscopy is used to visualize fracture toughening mechanisms in PLA under strain, revealing a direct link between mechanical property improvements and the role of SD LCNFs as craze nucleators in PLA. Additionally, the in-situ observation of crystallization kinetics highlights how SD LCNFs influences PLA microstructure, correlating these structural changes with enhanced barrier and mechanical properties. The composite’s optical clarity and UV shielding capabilities are assessed, confirming its potential for specialty packaging applications.