Microstructure-properties interrelationships in poly(lactic acid)/Plant fiber biocomposites: Impacts of low-toxic boric acid pulping reagent and epoxidized soybean oil compatibilizer

The depletion of fossil fuel resources and increasing global concerns about synthetic plastic wastes have motivated researchers to seek sustainable alternatives from biopolymers and natural fibers. In this study, rice straw (RS) fibers were utilized as a reinforcement agent in poly(lactic acid) (PLA). To tailor the filler/matrix interface and improve the mechanical performance, the lignocellulosic fibers were chemically treated with a low-toxicity boric acid (BA) solution in distilled water for 24 h at reflux conditions before melt-compounding with PLA. The untreated and treated RS were incorporated into PLA in the presence of epoxidized soybean oil (ESO), a plant-based reactive compatibilizer, and soybean oil (SO), a green plasticizer. Then, the mechanical, morphological, chemical, rheological, thermo-mechanical, physical, and flammability properties of the green biocomposites were characterized. The findings clarified that BA was successful in pulping, fibrillating, and increasing the aspect ratio of the RS fibers, and ESO noticeably improved the PLA/treated RS interfacial adhesion, fiber dispersion state, and embedment in the PLA matrix. Pulping of RS with BA increased the notched impact resistance, ultimate strain, and dissipated energy of PLA/untreated RS specimens. ESO, with its compatibilization effect in the PLA/treated RS fiber composite and chain extension influence on PLA, increased the melt viscoelastic properties. The ESO addition improved the yield stress, stress at break, and notched impact strength of the PLA/treated BR fiber systems. The considerable enhancements in the water resistance (51%) and linear burning rate (38%) of the PLA/untreated RS composites were attained by the RS pulping and ESO-induced compatibilization effect.