Characterisation of cellulose treated by the steam explosion method. Part 3: Effect of crystal forms (cellulose I, II and III) of original cellulose on changes in morphology, degree of polymerisaion, solubility and supermolecular structure by steam explosion

Abstract

AbstractAn attempt was made to clarify the effect of the crystal form of untreated cellulose on the morphological and structural changes of cellulose during steam explosion treatment (steam pressure P = 2.9MPa (T = 508K), treatment time t = 15‐300 s). For this purpose, the crystal form of soft wood pulp (cellulose I) was converted by solid‐to‐solid transition, with minimal unavoidable change in other structural characteristics including morphology and average degree of polymerisation, into cellulose II or cellulose III. It was proved by both X‐ray and solid‐state cross‐polarisation/magic‐angle sample‐spinning (CP/MAS) 13C NMR analyses that even a simple addition of water at room temperature brought about a significant structural change in the steam‐untreated cellulose samples. The solubility towards 9.1 wt% aqueous sodium hydroxide, Sa, of the cellulose samples of crystal forms I and III could be improved from 31‐33% up to almost 100% by selecting appropriate steam explosion conditions (for example, P = 2.9MPa, t = 30 s). Such a magnificent increase in Sa by the steam explosion treatment was not observed for the cellulose II sample, even under the rather severe conditions of the steam explosion treatment at which the cellulose III crystal was converted to a large extent to cellulose I, as confirmed by X‐ray diffraction. X‐ray diffraction analysis showed that crystallisation of samples with cellulose I or II crystal occurred to some extent during the steam explosion treatment. Contrary to this, the degree of breakdown of the intramolecular hydrogen bond O3…O'5, as estimated by CP/MAS 13C NMR analysis, significantly increased for cellulose I and I11 during the treatment. The decrease in the viscosity‐average degree of polymerisation, P, observed for all treated samples can be roughly categorised into two or three steps of the first‐order decomposition reaction with different reaction rates.