Influence of process variables on physical characteristics of spray freeze dried cellulose nanocrystals

This study compares the characteristics of cellulose nanocrystal (CNC) agglomerates prepared using spray drying, freeze drying, and spray freeze drying. Moreover, the effect of the concentration of CNC in the initial aqueous dispersion (~ 0.5–10.0 wt%) on the morphology, particle size distribution, porosity and crystalline structure of the spray freeze dried CNC agglomerates were investigated. Scanning electron microscopy was used to characterize the morphology and particle size distribution, nitrogen adsorption–desorption isotherms were used to analyze the porous structure of the particles, and X-ray diffraction was used to analyze the crystalline structure of the particles. Spray drying of CNC resulted in 0.5–30 μm dense agglomerates of slightly deformed elliptical and mushroom cap shaped particles with no porous structure. Freeze drying resulted in large irregular shape flakes of various sizes ranging mainly between 150 and 350 μm. On the other hand, spray freeze drying of CNC from dilute suspensions (~ 0.5 wt%) resulted in larger (4–240 μm) light spherical particles that were highly porous (~ 110 times larger in BET surface area), with web-like inter-connected structure consisting of 10–30 nm thick nanofibrils. Increasing the concentration to 5 wt% produced slightly denser spherical particles (13–110 μm) with slightly less porous web-like structure.