Effects of Lignin Content on Mechanical and Thermal Properties of Polypropylene Composites Reinforced with Micro Particles of Spray Dried Cellulose Nanofibrils

In this work, the use of microsized cellulosic particles obtained from spray dried cellulose nanofibrils with high lignin content (>20 wt %) were explored for the first time as reinforcement in polypropylene (PP) composites. Their effect was compared with the results from PP composites reinforced by cellulosic particles of spray dried cellulose nanofibrils with a low lignin content (<5 wt %). Cellulose nanofibrils with diameters less than 100 nm were obtained by mechanically fibrillating unbleached and bleached cellulosic fibers obtained from tree bark after alkaline extraction for removal of extractive. These cellulose nanofibrils were then spray dried to microsized high lignin content cellulose particles (HLCP) and low lignin content cellulose particles (LLCP), respectively. The presence of a large amount of lignin in the nanofibrils alleviated the degree of aggregation during the spray drying process. Both HLCP and LLCP were melt compounded with polypropylene (PP) to make composites films with different cellulosic particle loading levels. Compared to LLCP, HLCP significantly improved water repellency, thermal stability, and tensile properties of the composites films. With an addition of 5 wt % HLCP in PP, the tensile strength and modulus of the composites increased by 25.3% and 41.5% compared to neat PP, respectively. However, composites containing 5 wt % LLCP experienced a decrease in tensile strength by nearly 23.0% instead. Moreover, compatibilizing and stabilizing effects of lignin were also observed during the processing of the composites. This study demonstrated strong potential of HLCP as biobased reinforcement filler in plastic composites.