Evaluation of the kinetics of low intensity ultrasound-assisted sulfuric acid hydrolysis to obtain cellulose nanocrystals (CNCs) from microcrystalline cellulose (MCC)

Cellulose-based materials have been extensively investigated due to their abundance, renewable nature, and low abrasiveness and density. Cellulose nanocrystals (CNCs) stand out as nanometric particles of increasing technological interest, and also due to their potential use as nano-reinforcements to polymer composites. Since the effect of ultrasonic treatment on the structure and properties of CNCs is not yet fully understood, in this study the effect of ultrasonic output and hydrolysis time on yield and morphological, structural, and thermal properties of nanocrystals from microcrystalline cellulose (MCC) was studied. CNCs were prepared by conventional hydrolysis and compared to sonochemical extraction using 64 vol% H2SO4 at 45 °C. Sonication power varied from 50, 70 and 90%, and for each value a kinetic study was performed as a function of hydrolysis time until a colloidal gel suspension was obtained. Results showed that it was possible to obtain CNCs from MCC with reduced extraction time, whose thermal stability, aspect ratio and crystallinity index are comparable to those obtained by conventional acid hydrolysis. The ultrasound power intensity was the most important parameter to increase CNC sonochemical extraction productivity. The optimal CNC yield occurred at predetermined hydrolysis time intervals for each set of variables of low-intensity ultrasound-assisted hydrolysis. The disaggregation of freshly produced CNCs from MCC particles occurred simultaneously to size reduction of previously produced CNCs and conversion of amorphous phase into sugars. Due to the non-selective nature of sonochemical hydrolysis, it was proposed that hydrolysis proceeds regardless of the size and crystallinity degree of cellulose particles dispersed in the acidic medium. Therefore, there was an optimal hydrolysis time as a function of CNC yield for each set of hydrolysis parameters, since prolonged extraction times leaded to deterioration of the morphological and structural properties of CNCs.