Sulfur-free dissolving pulps and their application for viscose and lyocell

Abstract

In this study, the concept of multifunctional alkaline pulping has been approved to produce high-purity and high-yield dissolving pulps. The selective removal of hemicelluloses was achieved by either water autohydrolysis (PH) or alkaline extraction (E) both applied as pre-treatments prior to cooking. Alternatively, hemicelluloses were isolated after oxygen delignification in a process step denoted as cold caustic extraction (CCE). Eucalyptus globulus wood chips were used as the raw material for kraft and soda-AQ pulping. In all process modifications sulfur was successfully replaced by anthraquinone. By these modifications purified dissolving pulps were subjected to TCF bleaching and comprehensive viscose and lyocell application tests. All pulps met the specifications for dissolving pulps. Further more, CCE-pulps showed a significantly higher yield after final bleaching. Morphological changes such as ultrastructure of the preserved outer cell wall layers, specific surface area and lateral fibril aggregate dimension correlated with the reduced reactivity towards regular viscose processing. The residual xylan after alkali purification depicted a lower content of functional groups and a higher molecular weight and was obviously entrapped in the cellulose fibril aggregates which render the hemicelluloses more resistant to steeping in the standard viscose process. Simultaneously, the supramolecular structure of the cellulose is partly converted from cellulose I to cellulose II by the alkaline purification step which did not influence the pulps reactivity significantly. Nevertheless, these differences in pulp parameters did not affect the lyocell process due to the outstanding solubility of the pulps in NMMO. Laboratory spinning revealed good fiber strength for both, regular viscose and lyocell fibers. The high molecular weight xylan of the CCE-treated pulps even took part in fiber forming.

Hydrotropic Fractionation of Birch Wood into Cellulose and Lignin: A New Step Towards Green Biorefinery

“…However, this difference was partially compensated by a lower content of non-cellulosic polysaccharides and a higher content of cellulose in HF pulp (Table 5). Based on the viscosity values and the content of hemicelluloses, the pulps were comparable with other dissolving pulps found in the literature (Sixta and Borgards 1999;Sixta et al 2004;Sixta 2006;Schild and Sixta 2011).…”

“…With the applied bleaching sequence, the pulps did not reach a high brightness (Table 5), which is usually close to or exceeds 90% ISO for dissolving pulps (Sixta and Borgards 1999;Sixta et al 2004;Schild and Sixta 2011). However, bleaching optimization was not a purpose of this work.…”

Purification of cellulosic pulp by hot water extraction

“…In addition to a good quality rayon pulp, the SEW process allows high flexibility in terms of raw materials, short cooking times, and a significant recovery of sugars from the pulping liquor (Iakovlev and Heiningen 2012;Sixta et al 2013). The production of viscose-grade pulps has also been investigated by applying a CCE stage to paper-grade pulps, which allows the removal of substantial amounts of shortchain hemicellulosic sugars while preserving the yield of cellulose (Gehmayr et al 2011;Schild and Sixta 2011). Recently, the novel IONCELL process has been shown to be successful in the conversion of paper-grade pulps to dissolving pulps.…”

“…In the last years, several research efforts have been directed towards producing dissolving pulp by applying a hot water extraction (HWE) to the lignocellulosic material before alkaline pulping (Borrega et al 2013a;Li et al 2010;Reguant et al 1997;Ribas Batalha et al 2011;Schild and Sixta 2011). This approach is similar to the existing industrial PHK process, but the vapor phase is replaced by a liquid water prehydrolysis, and Kraft pulping is often replaced by a non-sulphur sodaanthraquinone (SAQ) pulping.…”

Reactivity of dissolving pulps modified by TEMPO-mediated oxidation

“…Because of their mobility, these short-chain hemicelluloses are supposed to act as spacer in-between the cellulose microfibrils preventing them from aggregation and allowing better surface accessibility. CCE-treated kraft pulps instead, contain long-chained hemicelluloses with less functional groups (Schild and Sixta 2011). Thus, the cellulose microfibrils tend to aggregate upon intense dewatering or drying, resulting in additional hydrogen bonds and ester formation upon crosslinking (Fernandes Diniz et al 2004).…”