Current advances in processing and modification of cellulose nanofibrils for high-performance composite applications

Cellulose nanofibrils (CNFs) have recently emerged as a promising bio-based nanomaterial for high-performance composite applications because of their exceptionally high aspect ratio, large surface area, and outstanding mechanical properties in addition to the sustainability and lifecycle advantages of these materials compared to their synthetic counterparts. However, CNFs are hydrophilic and generally incompatible with most polymer matrices, leading to high levels of aggregation, voids, and poor fiber–matrix interfacial strength, which reduces the final composite properties. Thus, interest in efficient CNF surface modification and hydrophobization is growing in order to improve the nanofibril-polymer interface and dispersion characteristics. We discuss current CNF hydrophobization methods, identifying promising routes and challenges of the different hydrophobization and modification methods available thus far. We demonstrate the effects of various advances in CNF modification on the complex drying and redispersion behavior of nanofibrils, including current limitations and future prospects. We also outline several shortcomings in CNF composite characterization that make it difficult to compare the hydrophobization and composite performance. Moreover, quantifying length scales and energy input of redispersion methods, evaluating the hydrophobicity-CNF crystallinity-composite property relationships, and using X-ray tomography to characterize CNF-matrix distribution are critical quantities with limited investigations to date. This review thus comprises an analysis and recommendations for future work needed to further understand the key aspects of CNF modification to enhance composite properties and processability.