Introduction of aldehyde groups on surfaces of native cellulose fibers by TEMPO-mediated oxidation

Native cellulose fibers were suspended in water and oxidized to various degrees with sodium hypochlorite and catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and sodium bromide at pH 10.5. The oxidation was accomplished within 30 min at room temperature. The TEMPO-oxidized cellulose fibers were then converted to sheets like paper. Tensile strength of the sheets soaked in water, i.e. wet strength, showed a maximum value, when 0.3 mmol NaClO per gram cellulose was used in the TEMPO-mediated oxidation. Aldehyde groups up to 0.225 mmol/g were introduced in native cellulose fibers by the TEMPO-mediated oxidation, and were stably present in there. However, only surface aldehyde groups in the TEMPO-oxidized cellulose fibers contributed to the wet strength development of the sheets. Carboxylate groups were also formed not only surfaces but also insides of cellulose fibers by the TEMPO-mediated oxidation, although they had nearly no contribution to wet strength development of the sheets. These surface aldehyde groups forms hemiacetal linkages with cellulose hydroxyl groups at the inter-fiber bonds, resulting in the clear wet strength development of the sheets prepared thereof. The TEMPO-mediated oxidation is, therefore, applicable to introduction of not only carboxylate groups but also aldehyde groups to native cellulose surfaces as an efficient chemical modification under aqueous conditions.