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 atpH 10.5. The oxidation was accomplished within 30min 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 NaCIO 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. (c) 2006 Elsevier B.V. All rights reserved.