(72b) Morphological Characteristics and Mechanical Properties of Thermoplastic Composites Using Surface Modified Cellulose Nanofibril (CNF) Fillers

Cellulose based nanofillers have potential as lightweight, sustainable, and biodegradable components of polymer composites. However, in order to be commercialized, there is an increasing need in the production of surface modified nanomaterials that can be easily dispersed in hydrophobic polymers via normal processing methods (e.g. melt blending and extrusion.) This study reports on the surface modification of cellulose nanofibrils (CNFs) via spray-drying and their application in the reinforcement of polymer composites. Reactive spray drying was used with the aim of both reducing the size of CNF aggregates and to add alkyl surface functionality to facilitate the dispersion of the nanoscale additives into the polymer matrix. Synthesized nanofibers were characterized using light microscopy, scanning electron microscopy (SEM), contact angle measurements, and Fourier-Transform Infrared spectroscopy (FTIR). Modified nanofiber characterization showed that CNFs are reduced in size from large agglomerates into particles after spray-drying, and that agglomeration is reduced as a consequence of spray-drying; moreover, sample hydrophobicity was increased as a result of the alkyl modifications. Freeze-dried CNF, unmodified spray-dried CNF and alkyl-modified spray-dried CNFs were used as fillers for the formation of composites by compounding with a thermoplastic polymer. Polypropylene(PP)/CNF composites were prepared using a melt mixing technique. Processing conditions, alkyl-modification, and nanofiber concentration was varied to determine the ideal processing conditions and to understand the effects of CNF concentration and alkyl-modification on the morphological and mechanical properties of the final polypropylene composites. Surface morphology of freeze-fractured composite samples was studied using SEM and atomic force microscopy (AFM) to analyze the presence of fiber agglomerates in the polymer matrix, and determine the effects of CNF concentration and alkyl-modifications on the dispersion of the nanofibers. Morphological studies were correlated to tensile testing results to better understand the effect of filler dispersion. Morphology studies of freeze-fractured samples suggested an improvement in the CNF dispersion inside the polymer matrix as functional alkyl-groups were added to the CNFs, the number of fiber agglomerates are reduced when modified samples are compared to samples prepared using freeze-dried and unmodified spray-dried CNFs. This is further confirmed by the tensile tests, which demonstrated an increase in sample modulus as fibers were added to the polymer matrix; this increase was more pronounced and linear when modified CNFs are used as composite fillers.