Latest Advances in Acid Hydrolysis Technique for Production of Cellulose Nanocrystals (CNCs) as Reinforcing Component in Epoxy-Based Nanocomposites

Cellulose, as one of the most abundant natural polymers with a wide spectrum of applications, has drawn the attention of many researchers. In this regard, cellulose-base structures such as cellulose nanocrystals (CNCs) and nanocomposites are known as materials widely used in many fields. Therefore, development, production and use of cellulose-based structures by gaining the relevant insights are progressively increasing. In the present paper, the latest developments in the field of various extraction techniques of CNCs, their properties and dispersion methods in epoxy matrix to obtain desired mechanical properties in cellulose nano-crystals/epoxy nanocomposite structures are reviewed. First, cellulose nanocrystals are introduced as one of the most promising biodegradable and abundant nanomaterials widely used in various industries. The various production techniques of cellulose nanocrystals are briefly reviewed. Based on the various applications of cellulose nanocrystals in many fields, cellulose acid hydrolysis as one of the most practical and low-cost methods for cellulose nanocrystals preparation is described in details. In this regard, the source of cellulose, time and temperature of the acid hydrolysis, the concentration and type of acid as important factors of the hydrolysis process are investigated. Reinforcing epoxy-based nanocomposites using cellulose nanocrystals is the subject of another section. Dealing with problems associated with cellulose nanocrystals agglomeration phenomenon, the main challenge to achieve a homogeneous dispersion of cellulose nanocrystals within the epoxy matrix, is also thoroughly discussed. In this regard, various methods of chemical surface modification of cellulose nanocrystals are investigated. Finally, considering the investigated cases, the debate on the issue of appropriate methods for dispersing cellulose nanocrystals in the epoxy-based resin is comprehensively covered. It can be said that based on growing demands for high performance cellulose nanocrystals/epoxy nanocomposite structures, research on this field is ongoing. Therefore, in the present review, by providing a comprehensive discussion on the latest researche works conducted in this field, it has been attempted to present the new achievements of researchers and approaches to overcome the involved challenges. The most important achievement of researchers is the emphasis on selection of appropriate dispersion medium considering the cellulose nanocrystals surface energy to produce nanocomposites with suitable mechanical and thermomechanical properties