Cellulose phosphorylation for cellulose nanofibrils production: impact of curing on the color change and chemical surface characterization

Cellulose phosphorylation as a pretreatment for CNF production is a robust process allowing to modify the surface charge of the fiber. However, cellulose phosphorylation is associated with a yellowing of the substrate and variations in the grafting of phosphate groups leading to crosslinking during intense curing. In this study, a design of experiment was conducted on a process of paper impregnation to limit the color change and maximize the degree of grafting of cellulose. The impregnation of the substrate was found to occur very rapidly, namely at 10s maximum. Whereas the color change was found to be dependent only on the curing parameters (time and temperature). The charge content model was reduced to 2 variables, i.e., the curing parameters, since the impregnation time was previously established to be very short and complete. Consequently, both models were combined allowing to determine optimal curing parameters: a temperature of 150 degrees C for 20 min., yielding a minimum color change (6.0) and maximum charge content (1.0 mmol/g). Moreover, cellulose phosphorylation was characterized in bulk and surface by XPS and NMR, respectively. It did not reveal crystallinity changes in the pretreated cellulose. 31P NMR allowed the estimation of the amount of crosslinking within cellulose matrix.