Contribution of Structural Changes in Cellulose Fibers Induced by Alkali Treatment to Their Nanofibrillation

The aim of this study is to explore the effect of strong alkali pretreatment on manufacturing cellulose nanofibrils (CNFs) with mechanical grinding. The structural change (from cellulose I to cellulose II) and a sharp reduction in the yield of the hemicellulose component (xylose) of cellulose fibers (bleached kraft pulp) were observed after strong NaOH (20 wt %) pretreatment. However, the differences depending on treatment time (1 or 2 h) were not significant. The subsequent mechanical grinding did not lead to any significant changes in the chemical composition, specifically the sugar contents, of the cellulose fiber. Morphological analyses showed that the generated micro- and nanofibers in the suspension gradually decreased with an increasing number of mechanical passes . Also, the mean width of the nanofibers produced from alkali-pretreated pulp was relatively thicker than that from untreated pulp. The results can be attributed to changes in the cellulose crystal structure and the loss of internal structural integrity under strong alkali treatment. UV transmittance and turbidity results showed that alkali pretreatment and subsequent mechanical grinding led to an increase in the nanofibril content.