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

Abstract An attempt was made to clarify the effect of steam explosion conditions on the changes in morphology, degree of polymerisation P v , solubility towards aqueous alkali solution S a , 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) 13 C nuclear magnetic resonance (NMR), S a and P v 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 S a ( 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 P v 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 P v , (4) the repeated steam explosion method gave more fibrillated sample with higher S a 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 S a , and (6) the structural parameters expressing the degree of breakdown in the intramolecular hydrogen bonds at the C 3 and C 6 positions, X am (C 3 ) and X am (C 6 ), of the samples as estimated by CP/MAS 13 C NMR changed as functions of P and t , being almost parallel to S a . This suggests that these parameters may be more closely correlated with S a than with X am (X) from X‐ray analysis.