Controlled thermo-catalytic modification of regenerated cellulosic fibres using magnesium chloride Lewis acid

The Lewis-acid catalytic reactions of magnesium chloride with regenerated cellulosic fibres under baking conditions can be interpreted using existing semi-crystalline morphological models. Reaction at 180 °C is associated with chain scission, which takes place randomly within the accessible regions of the fibre structure. This causes a rapid reduction in the cellulose degree of polymerization, which stabilizes at a limiting value, analogous to that observed with wet-state mineral acid catalysed hydrolysis. A slower scission-reaction is also observed, believed to be due to the liberation of single glucan units from crystallite ends, again analogous to wet-state mineral acid hydrolysis. Dry-state catalysis is promoted by thermal molecular motion, allowing migration of catalyst ions and also conformational flexing of the cellulose polymer, which also induces a small amount of recrystallisation at crystallite lateral surfaces. Differences in the dry-state reaction have been observed for lyocell, viscose and modal regenerated fibres, which can be related to differences in crystallinity and resulting accessibility of the magnesium chloride catalyst. For lyocell the accessibility towards magnesium chloride is lower than found with mineral acids, which may be significant in the development of treatments to promote mechanical fibrillation, without sacrificing fibre tensile properties

Controlled accessibility Lewis acid catalysed thermal reactions of regenerated cellulosic fibres

A combination of techniques have been used to characterise lyocell regenerated cellulose fibre subjected to low-moisture thermal-catalytic reactions with zinc chloride Lewis acid. Application from non-swelling ethanol reduces catalyst accessibility, but at high temperatures migration takes place through the internal fibre morphology. The extent of chain scission is reduced at lower temperatures, leading to a higher leveling-off degree of polymerisation (LODP). In contrast, application of zinc chloride from water results in a lower LODP, due to the more even distribution of catalyst. The weights of extractable polymer material increase according to two separate rate constants, following established semicrystalline models. A higher Arrhenius activation energy for chain scission is seen for zinc chloride application from ethanol, which may be due to the physical mobilisation of the cellulose polymer at high temperature, associated with a cellulose Tg. This may also aid recrystallisation. Cellulose dehydration endotherms and pyrolysis exotherms are shifted to lower temperature for application of zinc chloride from ethanol compared to water, which may be the result of a higher local concentration of catalyst and a faster reaction onset