Effect of drying conditions on cellulose nanocrystal (CNC) agglomerate porosity and dispersibility in polymer nanocomposites

The microstructure of CNC particles depends strongly on the drying method employed during manufacturing. The effect of different drying techniques, including spray drying, freeze drying, and spray freeze drying (SFD), on microstructure of cellulose nanocrystal (CNC) particles was studied. Conventional drying methods (i.e. spray and freeze drying) yielded packed and dense agglomerates of CNC. Capillary forces and ice crystal growth play key roles in CNC aggregation during spray and freeze drying, respectively. It was found that, in the absence of these forces, in the SFD technique, the dispersed state of CNC in water could be frozen in. Thus, SFD of CNC suspension resulted in a powder with porous structure. A filament-like agglomerate structure consisting of nanofibers was formed at low CNC concentrations (ca. < 2 wt.%), while a spherical foam structure was obtained at higher CNC concentrations. Polypropylene (PP) nanocomposites containing spray dried CNC, freeze dried CNC, and spray freeze dried CNC (CNCSFD) were prepared via melt mixing in an internal batch mixer. Optical light and scanning electron microscopy coupled with rheological properties showed better dispersion of CNCSFD agglomerates. Moreover, PP samples containing CNCSFD showed significant increases in mechanical properties.