Reactivity of dissolving pulps modified by TEMPO-mediated oxidation

Abstract

The reactivity of dissolving pulps towards derivatization or dissolution is a crucial quality parameter and is mainly determined by the accessibility of the hydroxyl groups. When dissolving pulps are produced from paper-grade pulps by cold caustic extraction (CCE), their reactivity is often inferior as compared to commercial prehydrolysis kraft dissolving pulps. It was hypothesized that pulp reactivity can be enhanced by the introduction of small amounts of substituents to facilitate interchain accessibility. In this study, CCE-treated Eucalyptus globulus kraft paper pulp was subjected to TEMPO-mediated oxidation to initiate partial oxidation of the C 6 -hydroxyl groups to carboxyl groups. The effect of this pulp modification on the reactivity towards xanthation and the subsequent dissolution in diluted aqueous alkali solution (viscose process) as well as the dissolution in complexing and non-complexing solvents, respectively, was thoroughly examined. The results revealed that the oxidized pulps rich in C 6 -carboxylate groups impeded the xanthation reaction obviously because of the reduced availability of hydroxyl groups. When Nmethylmorpholine-N-oxide monohydrate was used as a direct solvent, a very high content of C 6 -carboxylate groups was found to reduce the solubility of the pulp fibers as less hydrogen bonds can be formed with NMMOÁH 2 O. In the case of dissolution in the complexing solvent cupriethylenediamine, the dissolution mechanism of cellulose was not deteriorated by the high content of C 6 -carboxylate groups. Instead, the oxidation procedure increased the hydrophilic character and the swelling capacity of the outer cell wall layers allowed homogeneous dissolution.

Cellulose oxidation by Laccase-TEMPO treatments

“…In addition, Gehmayr et al (2012) Span test measures the intrinsic strength of a single fiber independently of fiber network 398 (Hagglund, 2004), and indirectly providing information about the effect caused by the treatment 399 to the fiber structure. As observed earlier, cellulose underwent considerable degradation after 400 oxidative treatments but intrinsic fiber strength was not affected.…”

The Influence of Cold Caustic Extraction on the Purity, Accessibility and Reactivity of Dissolving‐Grade Pulp

“…The high molecular weight cellulose may also have an increased susceptibility to fibril aggregation. In contrast, the dissolving grade pulps produced using a PHK process typically retain some of hemicelluloses with low molecular weight since alkaline conditions employed during subsequent Kraft pulping are aimed at solubilizing the lignin component while retaining as much of the carbohydrate components as possible …”

“…Dissolving pulp is a pure, uniform and reactive cellulose polymer. Dissolving pulp can be produced via traditional acid sulfite pulping or alkaline Kraft cooking with additional processing to remove hemicellulose and reduce the degree of cellulose polymerization (DP) ,. Due to issues with chemical recovery and pollutions, the predominant pulping process has globally shifted from sulfite to the Kraft process .…”

“…Compared to prehydrolysis, CCE is performed under alkaline conditions facilitating chemical recovery in the Kraft mill, does not require the extra digester capacity prior to pulping and decrease sacrifices to pulp yield. However, the removal of hemicellulose during CCE compromises downstream cellulose reactivity ,. One possible reason for decreased cellulose reactivity after a CCE treatment is the removal of residual hemicellulose and low molecular weight cellulose result in more cellulose “fibril aggregation” .…”

“…Dissolving pulp can be produced via traditional acid sulfite pulping or alkaline Kraft cooking with additional processing to remove hemicellulose and reduce the degree of cellulose polymerization (DP). [3,4] Due to issues with chemical recovery and pollutions, the predominant pulping process has globally shifted from sulfite to the Kraft process. [5] The increasing demand for dissolving pulp has driven a continuously increasing number of pulp mills using the alkaline Kraft process to convert from producing traditional paper pulp to dissolving-grade pulp.…”