Characterisation of cellulose treated by the steam explosion method. Part 2: Effect of treatment conditions on changes in morphology, degree of polymerisation, solubility in aqueous sodium hydroxide and supermolecular structure of soft wood pulp during steam explosion

Abstract

AbstractAn attempt was made to clarify the effect of steam explosion conditions on the changes in morphology, degree of polymerisation Pv, solubility towards aqueous alkali solution Sa, and supermolecular structure of a soft wood pulp and to elucidate the mechanism by which the steam explosion treatment makes natural cellulose completely soluble in aqueous alkali solution. For this purpose, scanning electron microscopic (SEM) observation and X‐ray diffraction, solid‐state cross‐polarisation/magic‐angle sample‐spinning (CP/MAS) 13C nuclear magnetic resonance (NMR), Sa and Pv measurements were carried out on a series of soft wood pulps treated systematically by the steam explosion method. It was found that (1) the maximum Sa (c. 100%) was obtained when the soft wood pulp was treated under the conditions of steam pressure P = 2.9MPa and treatment time t = 30s, (2) the decrease in Pv of the pulp by the steam explosion resembled conventional acid hydrolysis of cellulose, (3) a higher water content in the sample to be treated gave a lower degree of decrease in Pv, (4) the repeated steam explosion method gave more fibrillated sample with higher Sa than the corresponding batch steam explosion, (5) the amorphous content of the samples as estimated by X‐ray analysis decreased by the steam explosion, in spite of an increase in Sa, and (6) the structural parameters expressing the degree of breakdown in the intramolecular hydrogen bonds at the C3 and C6 positions, Xam(C3) and Xam(C6), of the samples as estimated by CP/MAS 13C NMR changed as functions of P and t, being almost parallel to Sa. This suggests that these parameters may be more closely correlated with Sa than with Xam(X) from X‐ray analysis.