Isolation and characterization of cellulose nanofibers from sweet sorghum with different ultrafine grinding modes

Sorghum bicolor (L.) Moench baggase is abundant in Indonesia which can be a potential source of cellulose. Cellulose nanofibers (CNF) is one of the cellulose-based nanomaterial that have a good psychochemical properties for composite. This study aims to characterize the isolated cellulose nanofibers from sweet sorghum bagasse using a combination of chemical and mechanical treatment. Cellulose fibers were isolated from sorghum bagasse powder by alkalinization using sodium hydroxide (NaOH), then the bleaching process using hydrogen peroxide (H2O2) followed by alkaline hydrogen peroxide (AHP). Furthermore, cellulose fibers were fibrillated using ultrafine grinder with six different distances of gap in grinder (from zero position): -10; -30; -50; -70; -90; and -110 μm with 5 passes in each gap. Chemical content analysis of the Sorghum bicolor (L.) Moench fibers showed that the content of cellulose, hemicellulose, and lignin are 35.06%, 11.25%, and 20.07%. The the combination of soda pulping and bleaching process successfully removed almost all the lignin content up to 94.5% and significantly purified the sorghum fiber with achieved 86.92% of cellulose content. The FT-IR spectra showed a peak around 1427 cm-1, indicating the presence of crystallinity band or the vibration of symmetrical CH2 in the FT-IR spectrum, which only appears in bleached pulp and cellulose pulp. FE-SEM images showed that the diameter of fibrillated fiber could reach 22.4 nm. The XRD analysis indicated the highest crystallinity using an ultrafine grinder through-110 μm, which was 57.25%. These results showed that the different distances of gap affected the size and crystallinity index of sorghum cellulose nanofibers.