Morphological characteristics of cellulose nanofibril-filled polypropylene composites

Nano-sized cellulose fillers (cellulose nanofiber [CNF] and microfibrillated cellulose [MFC]) and a micron-sized cellulose filler (microcrystalline cellulose [MCC]) were used as fillers in polypropylene (PP) composites. Cellulose-filled PP composite samples were manufactured and tested, and their morphological properties were examined to describe morphological characteristics of fracture surfaces at different filler loading levels after mechanical testing. Scanning electron microscopy analysis showed polymer stretching as the major component causing plastic deformation in fracture surfaces of CNF- and MCC-filled composites, whereas analysis of MFC-filled composites exhibited brittle deformation. Individual CNF and MFC fibers were separated and dispersed in the matrix polymer, although considerable agglomeration was observed beyond 6% (wt) filler loading, which resulted in sustained tensile and flexural strength. Mechanical property test results showed that in the case of CNF and MFC, composites sustained considerable tensile and flexural strength up to 10% (wt) filler loading, whereas tensile and flexural strength of MCC-filled composites decreased continuously.