PBAT composites with cellulosic fillers: from nanocrystals to isometric particles, effect of aspect ratio, crystallinity and dual filler combination

Weak mechanical properties of biodegradable polymers, such as poly(butylene adipate-co-terephthalate), have led to many studies on adding cellulosic fillers to polyesters to develop composites with enhanced properties in order to make them suitable for more applications. This work aims at getting a better understanding of the effect of various cellulosic fillers with different sizes, aspect ratios, and contents on composites of PBAT as well as the impact of combining these fillers in the same blend. Melt mixing at 150 °C was used for the preparation of biocomposites with micro or nano-fibrillated cellulose (MFC), cellulose nanocrystals (CNC), and lignocellulosic fillers (LCF). The resulting biocomposite materials were characterized by tensile tests, dynamic mechanical analysis in solid and melt states, DSC and microscopic observations (SEM or TEM). Results indicated that composites reinforced with 10 wt.% cellulose micro-fibrils, present a percolating network owing to their high aspect ratio, leading to the improvement of the mechanical properties of the polymer by increasing the Young’s modulus by 100% (from 80 MPa to 160 MPa). A synergistic reinforcement effect was identified in PBAT composites containing both CNC and LCF, resulting in a significant increase in stiffness of 35% when 5 wt.% CNC were added simultaneously with 10 wt.% LCF to the blend compared to only 19% improvement when added to the neat polymer. In contrast, the composite obtained with the MFC and LCL fillers kept the exact same improvement of about 35% indicating mainly an additive behavior. This enhancement in CNC/LCF composites occurs without a proportional loss in ductility relatively to MFC/LFC composites, highlighting the potential of combining fillers with distinct aspect ratios, such as LCF and CNC, to optimize mechanical performance.