Enhancement of nanofibrillation of softwood cellulosic fibers by oxidation and sulfonation

In this study, sulfonic acid groups were introduced to milled softwood (SW) bleached kraft fibers by oxidation and sulfonation with sodium periodate followed by sodium bisulfite under relatively mild conditions. The effect of variable amount of sulfonated groups on nanofibrillation of sulfonated cellulose samples was investigated. The cellulose samples, with contents of sulfonated groups of 371-501 μmol/g, were readily nanofibrillated by homogenization at relatively low pressure. These samples converted to viscous and transparent gels without clogging the homogenizer. By passing through the homogenizer one to four times, the transmittances of homogenized suspensions were up to 98%. SEM characterization of the homogenized and lyophilized fibril suspension indicates that the nanofibrillated fibrils are a network structure with lateral sizes of ∼15-45 nm and lengths ˃1 μm. The consecutive periodate oxidation and sulfonation with bisulfite was shown to be an effective pretreatment method to facilitate the nanofibrillation of softwood pulp cellulose and can be expectedly used with other cellulosic resources.