Solvent-free pulverization and surface fatty acylation of pulp fiber for property-enhanced cellulose/polypropylene composites

Cellulosic fibers have the potential serving as lightweight and bio-based reinforcement in polymer composites but they suffer from difficulties in obtaining adequate dispersion and a strong interface in the matrixes. The hydrophilic and aggregating nature of the cellulose fiber are the main reasons responsible for the poor processing and mechanical performance. Here we proposed that cellulose fibers be subjected to mechanochemical activation and functionalization to prepare surface modified cellulose powders for properties-improved polypropylene (PP) composites. Specifically, mechanochemical ball milling was used to simultaneously pulverize, activate, and surface modify pulp fibers to produce property-tuned cellulose particles for reinforcing. Acetic-oleic mixed anhydrides were used as esterifying agents for improved processing ability and surface hydrophobicity. The mechanochemical esterification, crystallinity and thermal stability of the modified samples were characterized. Cellulose particles with different degree of substitutions (DS) were obtained and then compounded with PP matrix to prepare composite specimens. The melt processability, matrix crystallization behavior, mechanical performance, composite morphology and water absorption of the composites were investigated. The results reveal that cellulose could be effectively esterified during the solvent-free ball milling reaction, with the successful grafting of both the acetyl group and the fatty acyl group. The modified cellulose particles improved the melt processability, promoted the matrix crystallization temperature and decreased the moisture uptake of the composite. Particle composite produced with a higher level of DS exhibited better processability, higher elongation at break, lower modulus, and better water repellency than those of the lower DS particle reinforced one.