Spray-dried Cellulose Nanocrystal Reinforced Polypropylene Composites

Abstract

Cellulose nanocrystal (CNC), a nano-scale structure with unique properties such as high crystallinity, Young’s modulus, and strength, is dominantly extracted from lignocellulosic materials and primarily dispersed in aqueous suspension, which cannot meet the requirement of large-scale industrial production of CNC-based composites manufactured by melt compounding process. This research aims to use CNC after spray-drying (SDCNC) to develop high-performance homopolymer polypropylene (HPP) composites with a thermal compounding process. Two specific objectives are to: 1) develop composites with different loading levels of SDCNC particles (5, 10, 15, 20, 30, 40, and 50 wt.%) and characterize the effect of various loadings of SDCNC on composite properties to identify the percolation threshold of SDCNC particles in the matrix and 2) understand the effect of the compatibilizer of maleic anhydride polypropylene (MAPP) at 3, 5 and 7 wt.% on composite mechanical and thermal properties near the SDCNC percolation threshold. The SDCNC particle reinforced HPP composites (CNC/HPP) with different SDCNC loading levels were prepared through a masterbatch concept. The mechanical behaviors, including tensile, flexural, and impact properties, as well as morphological and thermal properties of the composites, were investigated. Compared to the neat HPP, the tensile and flexural moduli of elasticity (MOE) of the composites significantly increased with increasing SDCNC loading levels. The impact strength of CNC/HPP composites was significantly improved due to the establishment of mechanical interlocking networks between the SDCNC particles and the HPP matrix. The SDCNC percolation threshold in the HPP matrix was identified between 30 – 40 wt.%. The crystallization behaviors of HPP in the CNC/HPP composites were also changed with the presence of the SDCNC particles due to their nucleation function. The crystallization peak temperatures increased with the increasing loadings of SDCNC particles at 20 and 30 wt.%. They remained constant when the SDCNC particle loading levels (40 and 50 wt.%) were higher than the percolation threshold because the nucleation function of the SDCNC particles saturated at the percolation threshold. The effect of MAPP on the mechanical and thermal behaviors of the SDCNC particle reinforced HPP composites near the SDCNC percolation threshold was assessed. The introduction of MAPP significantly enhanced the mechanical properties of the composites due to the improved interfacial adhesion between the particles and the matrix. The crystallization behaviors of HPP were also altered in the CNC/MAPP/HPP composites compared to the CNC/HPP composites due to the coverage of the SDCNC particle surface with MAPP through a chemical reaction. In addition, adding MAPP in the composites with an SDCNC loading level higher than the percolation threshold may change the dispersion and distribution of the particles in the matrix.