Influence of the variability in pulp quality on the alkalization processes

A comprehensive description and comparison of present market dissolving wood pulps (DWP) is provided. The systematic difference between kraft and sulfite pulps destined for the same application could be shown. Alkalization or alkaline extraction is the most common process operation when using DWP. A method to determine the degree of conversion to alkali cellulose as a result of alkalization based on Raman spectroscopy was developed. Extraction with 18 wt% aq. NaOH gave for all DWP investigated optimal alkalization results regarding yield, purity, and transformation. The choice of steeping temperature was found to be important in order to balance yield and purity of xylan-containing pulps. DWP is usually dried prior to application. Never-dried pulp is supposed to have a better processability due to less hornification. The structural changes induced by drying performed with differing severity as it might occur due to variations in the drying machine were investigated. The determination of the water retention value gave the most consistent results. Evaluations using solid-state NMR and size exclusion chromatography could support the findings. An application study when carboxymethylating alkalized paper and dissolving wood pulp with extreme variation in hemicellulose content, intrinsic viscosity, content of extractives and content of cellulose II found that favorable process conditions using 15 wt% aq. NaOH turn all pulps used into well soluble carboxymethylated cellulose (CMC) with a high degree of substitution (DS). A DS near to the limit for water solubility was obtained when CMC was prepared in the presence of 5 wt% aq. NaOH and gave the possibility to rank different pulps in terms of their solubility. None of the various measured chemical and supramolecular properties could clearly explain the differences between the used DWP which suggests further investigations on the reason for the different applicability of pulps.


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