Upgrading of paper grade pulps to dissolving pulps by nitren extraction: yields,molecular and supramolecular structures of nitren extracted pulps

Different paper grade pulps were extracted with nitren in order to produce dissolving pulps and polymeric xylan. The yield and molecular structure of the extracted pulps were investigated by carbohydrate analysis and HPSEC combined with fluorescence labelling in order to additionally monitor the carbonyl and carboxyl group profiles of the pulps. The supramolecular structure of selected pulps were further studied by solid state ¹³C-CP/MAS-NMR and wide-angle X-ray scattering (WAXS). These supramolecular data of nitren extracted pulps were compared to samples extracted with NaOH and a conventional dissolving pulp in order to classify the properties of nitren extracted pulps. Nitren extraction results in selective xylan removal without noticeable degradation or oxidation of the cellulose fraction. The resulting dissolving pulps have high molar masses, a narrow molar mass distribution and the typical contents of carbonyl and carboxyl groups. The supramolecular structure of cellulose is less affected by nitren compared to strong NaOH, and the resultant dissolving pulps still have the cellulose I structure. All laboratories are members of the European polysaccharide network of excellence EPNOE.     

Influence of xylan on the degradability of laboratory kraft pulps from hardwood and reed canary grass in acid hydrolysis

The depolymerisation of laboratory-prepared kraft pulps from birch, eucalyptus and reed canary grass during acid hydrolysis was studied. The intention was to study especially the influence of xylan content on the levelling-off degree of polymerisation (LODP) and on the dissolution of carbohydrates during the acid hydrolysis. The xylan content in the pulps was varied by prehydrolysis prior to the kraft pulping or by alkali-extraction of the bleached pulps, and the levelling-off degree of polymerisation was compared with the amount of xylan left in the pulps at LODP. The dimensions of the original fibers in the pulps and of the fiber fragments after hydrolysis were also measured. It was found that the fiber fragments after hydrolysis were longer in the pulps containing a higher amount of xylan. Xylan thus appears to prevent degradation during acid hydrolysis, both on the fiber level and on the individual cellulose fibril level.     

Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods

Solid-state ¹³C NMR spectroscopy was used to determine the degree of cellulose crystallinity (CrI) in kraft, flow-through kraft and polysulphide–anthraquinone (PS–AQ) pulps of pine and birch containing various amounts of hemicelluloses. The applicability of acid hydrolysis and the purely spectroscopic proton spin-relaxation based spectral edition (PSRE) method to remove the interfering hemicellulose signals prior to the determination of CrI were also compared. For softwood pulps, the spectroscopic removal of hemicelluloses by PSRE was found to be more efficient than the removal of hemicelluloses by acid hydrolysis. In addition to that, the PSRE method also provides information on the associations between cellulose and hemicelluloses. On the basis of the incomplete removal of xylan from the cellulose subspectra by PSRE, the deposition of xylan on cellulose fibrils and therefore an ordered ultrastructure of xylan in birch pulps was suggested. The ordered structure of xylan in birch pulps was also supported by the observed change of xylan conformation after regeneration. Similarly, glucomannan in pine pulps may have an ordered structure. According to the ¹³C CPMAS measurements conducted after acid hydrolysis, the degree of cellulose crystallinity was found to be slightly lower in birch pulps than in the pine pulps. Any significant differences in cellulose crystallinity were not found between the pulps obtained by the various pulping methods. Only in pine PS–AQ pulp, the degree of cellulose crystallinity may be slightly lower than in the kraft pulps containing less hemicelluloses.     

The effect of eucalypt pulp xylan content on its bleachability,refinability and drainability

Currently, bleached eucalypt pulps are largely used for printing and writing (P&W) and sanitary (tissue) paper grades. Among the many pulp quality requirements for P&W and tissue paper production the xylan content is one of the most significant. For P&W papers, increasing xylans improve pulp refinability and strength properties but negatively affect bulk and drainability. For tissue paper, xylans are purportedly advantageous during paper drying in the Yankee cylinder but negatively affect paper bulk and may increase dusting during paper manufacture. On the other hand, bleachability is a very important parameter for both P&W and tissue grade pulps since bleaching cost is the second most significant in eucalypt bleached kraft pulp production. The aim of this study was evaluating the influence of eucalyptus pulp xylan content on its bleachability, refinability and drainability. A sample of industrial unbleached eucalyptus kraft pulp containing 15.6?% xylans was treated with various alkali charges at room temperature in order to obtain materials with different xylan contents. The pulps were bleached to 90 % ISO brightness with the O–D_HT–(EP)–D sequence and evaluated for their refinability and drainability. By increasing the alkali concentration in the range of 10–70 g/L pulps of 14.5–5.9 % xylans were produced with no significant impact on cellulose crystallinity. The decrease of xylan content significantly decreased pulp bleaching chemical demand, water retention value and refinability and increased pulp drainability.     

Effects of nitren extraction on a dissolving pulp and influence on cellulose dissolution in NaOH–water

A commercial dissolving pulp was treated with aqueous solutions containing 3, 5 and 7 % of an organometalic complex (nitren) with the aim to selectively extract xylan and study its impact on the conventional physical–chemical properties of the pulp. The influence of these treatments on the pulp dissolution in a moderate solvent (8 % NaOH aqueous solution) was assessed by measuring the dissolution yields and the dissolution mechanisms. The results of this study show that nitren treatment has the effect of removing a large part of the xylan present in a dissolving pulp. It is also removing mannans and most important, it is influencing cellulose in two ways, (1) extracting it with more intensity when the nitren concentration increases, and (2) decreasing its mean molecular mass, also more evident with nitren concentration increase. The nitren extractions are favourable for the dissolution in cold NaOH–water, being more effective with higher concentrations. This chemical modification of the fiber surface leads to the disassembly of the primary wall. This allows an easier access of the NaOH reagent to regions not accessible on the initial fibres, which with the decrease of the cellulose molecular weight allows an easier dissolution and gives different dissolution mechanisms.     

On the accessibility and structure of xylan in birch kraft pulp

The structure of -(14)-xylan, both in isolated form and as a component of bleached birch kraft pulp, was studied employing CP/MAS ¹³C NMR spectroscopy. Bleached birch kraft pulp was treated with xylanases or alkali in order to distinguish between accessible and inaccessible xylan. In xylan which was alkali-extracted from bleached birch kraft pulp, the relative contents of xylose and 4-O-methylglucuronic acid were 99.4 and 0.6 weight %, respectively, and the degree of polymerization was 70. The supermolecular structure of xylan is very sensitive to the surrounding environment. All extracted xylan chains were accessible to water and methanol and the solvent molecules easily exchanged. In bleached birch kraft pulp, cellulose fibrils interact with xylan chains, causing these to adopt a conformation similar to one of the configurations observed for dry xylan. In birch pulp, about 1/3 of the xylan was found to be accessible to digestion by xylanases or extraction with 5% w/w potassium hydroxide (aq). A signal at 81.7ppm in the C-4 region of the CP/MAS ¹³C NMR spectrum of bleached birch kraft pulp originated from xylan at the accessible fibril surfaces. A portion of a broad signal at 83.5ppm reflected inaccessible xylan, which is probably present as co-aggregates with cellulose fibril aggregates.     

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

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.     

Purification of cellulosic pulp by hot water extraction

Hot water extraction (HWE) of pulp in a flow-through reactor was evaluated as a method to purify paper-grade pulps. About 50–80 % of the xylan and up to 50 % of the lignin in unbleached birch Kraft pulp was extracted by the HWE without losses in cellulose yield. The residual xylan content in the extracted pulps was predominantly too high for dissolving-grade applications, but some of the pulps with a xylan content of 5–7 % might still be suitable as rayon-grade pulps. Increasing extraction temperature lowered the xylan content at which cellulose yield started to decrease. Furthermore, at any given xylan content, increasing extraction temperature resulted in cellulosic pulp with higher degree of polymerization. The extracted xylan was recovered almost quantitatively as xylo-oligosaccharides. The results suggest that HWEs at elevated temperatures may be applied to purify cellulosic pulps, preferably containing a low xylan content, and to recover the extracted sugars.     

Bleachability and characterization by fourier transform infrared principal component analysis of acetosolv pulps obtained from sugarcane bagasse

Sugarcane bagasse Acetosolv pulps were bleached by xylanase and the pulps classified by using Fourier transform infrared (FTIR) spectroscopy and principal component analysis (PCA). Pulp was treated with xylanase for 4–8 h with stirring at 30°C. Some samples were further extracted with NaOH for 1 h at 65°C. FTIR spectra were recorded directly from the dried pulp samples by using the diffuse reflectance technique. Reduction in kappa number of 69% was obtained after sequence xylanase (4 h)-alkaline extraction. During bleaching the viscosity decreased only 12%. FTIR-PCA showed that the first three principal components (PCs) explained more than 90% of the total variance of the pulp spectra. PC2×PC1 plot showed that the points related to pulps from sequence xylanase (4 h)-alkaline extraction are different from the other. This group isenlarged by plotting PC3×PC1 or PC3×PC2 containing all pulps submitted to alkaline extraction. PC2 and PC3 are the principal factor for differentiation of the pulps. These PCs suffer influence of the ester bands (1740 and 1244 cm⁻¹). On the other hand, the pulps bleached only with xylanase could not be differentiated from the nonbleached pulps.     

Adsorption of cationized barley husk xylan on kraft pulp fibres: influence of degree of cationization on adsorption characteristics

A water-soluble (glucurono)arabinoxylan (GAX) was isolated from barley husk using chlorite delignification followed by alkaline extraction and enzymatic purification of the extract. The isolated xylan was shown to adsorb on bleached softwood kraft fibres, but the degree of adsorption was rather low under the applied conditions. This can be explained by the inhibited adsorption of GAX molecules with a relatively high degree of arabinofuranosyl substitution, as indicated by iodine complexation and neutral carbohydrate analysis of the non-adsorbing xylans. In order to increase the driving force for adsorption of the more highly substituted GAX, the xylan was cationized through a reaction in an aqueous alkaline medium with 2,3-epoxypropyltrimethylammonium chloride (EPTMAC). The chemical modification of xylan was confirmed by using ¹H-¹³C HSQC (Heteronuclear Single Quantum Coherence) NMR, and was quantified by using elemental analysis. The GAX cationization, which introduced cationic charge densities ranging from 110 to 740 μeq/g, was shown to increase the rate and magnitude of adsorption extensively, due to the induced electrostatic interaction between the anionic fibres and the cationic xylan. Similar to non-modified xylan, cationic xylan possessed a non-electrostatic cellulose surface affinity, as shown by adsorption at high ionic-strength and on esterified (carboxyl-free) pulp fibres.     

Possibilities for Optimization of Industrial Alkaline Steeping of Wood-Based Cellulose Fibers

Steeping of cellulosic materials in aqueous solution of NaOH is a common pre-treatment in several industrial processes for production of cellulose-based products, including viscose fibers. This study investigated whether the span of commonly applied process settings has the potential for process optimization regarding purity, yield, and degree of transformation to alkali cellulose. A hardwood kraft dissolving pulp was extracted with 17–20 wt% aq. NaOH at 40−50 °C. The regenerated residue of the pulp was characterized regarding its chemical composition, molecular structure, and cellulose conformation. Yield was shown to be favored primarily by low temperature and secondly by high alkali concentration. Purity of xylan developed inversely. Both purity of xylan and yield varied over the applied span of settings to an extent which makes case-adapted process optimization meaningful. Decreasing the steeping temperature by 2 °C increased xylan content in the residue with 0.13%-units over the whole span of applied alkali concentrations, while yield increased by 0.15%-units when extracting with 17 wt% aq. NaOH, and by 0.20%-units when extracting with 20 wt%. Moreover, the yield-favoring conditions resulted in a narrower molecular weight distribution. The degree of transformation via alkali cellulose to cellulose II, as determined with Raman spectroscopy, was found to be high at all extraction settings applied.     

The Degree of Disorder in Hardwood Kraft Pulps Studied by Means of LODP

The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed kraft pulp. A model explaining the LODP-results is proposed.     

Molecular properties of hemicelluloses located in the surface and inner layers of hardwood and softwood pulps

The molecular properties of hemicelluloses located in the surface and inner layers of fibers present in hardwood and softwood pulps, together with the effects of different bleaching processes on these properties, have been investigated in this study. In order to separate the hemicelluloses located in these two layers, fibers were subjected to mechanical peeling and then separated by filtration into surface (filtrate) and inner layer materials. The materials thus obtained were characterized with respect to their polysaccharide compositions and uronic acid contents. The molar mass parameters of the hemicelluloses (extracted by alkali) were determined by employing size-exclusion chromatography in combination with off-line MALDI mass spectrometry. For all of the pulps examined, the relative content of xylan was found to be greater in the surface layer of the fiber than in the corresponding inner layer. The xylan polymers of the surface layer exhibited higher molar masses and lower frequencies of uronic acid side groups than did the xylans in the inner fiber layer. In connection with ozone treatment, hexenuronic acid residues in the surface layer xylan were removed to a greater extent than in the case of the inner layers, indicating a gradient for the reaction with ozone across the fiber wall. The xylan polymer remaining on the surface of the softwood pulps after completion of the chlorine dioxide bleaching process was predominantly uncharged.     

Properties of spruce sulfite pulp and birch kraft pulp after sorption of cationic birch xylan

In this study, correlations between the charge density of adsorbed cationic xylans and the mechanical properties of selected pulps are discussed. Hand-sheet experiments were carried out using birch sulfate pulp and spruce sulfite pulp after the adsorption of 2-hydroxypropyl-trimethylammonium-4-O-methylglucuronoxylans (HPMAGXs) with different molar degrees of substitution (MS) in the range of 0.06–0.19. The charge density of the HPMAGX in water was determined by polyelectrolyte titration. Properties such as the tensile and burst index increased after HPMAGX addition and showed an optimum depending on the MS, which was 0.1 for both pulps. Other properties like the tear-index or the specific volume changed depending on the pulp. Beating experiments were also performed and showed an increasing tensile index of birch kraft pulp in the range of 53.7 to 85.7 N m g⁻¹ for a beating time of 10 min. The adsorption of HPMAGXs with different MS onto thiol-based self-assembled monolayers (SAMs) on gold and regenerated cellulose surfaces were studied using surface plasmon resonance (SPR). Electrostatic interactions were found to be the most important factors affecting HPMAGX adsorption, and a strong correlation between HPMAGX adsorption onto carboxyl-terminated SAMs (SAM-COOH) and paper strength when HPMAGX was used as a papermaking additive was observed.     

Efficient total halogen-free photochemical bleaching of kraft pulps using alkaline hydrogen peroxide

Total halogen-free bleaching of kraft pulps was conducted by an oxidative photochemical process at room temperature using alkaline hydrogen peroxide. Selection of an appropriate wavelength of light was crucial for effective bleaching and avoiding degradation of cellulose. The wavelength of the light has to be selected so that the light is absorbed only by the colored compounds in the pulps and not by the bleaching reagents or the pulp itself. When a long-wavelength black-light fluorescent lamp was used in combination with aqueous hydrogen peroxide solution at pH 11, the bleaching efficiency for hardwood and softwood kraft pulps reached the same level as that obtained by conventional two-stage elemental chlorine-free processes.     

Mutagenicity of Tectona grandis Wood Extracts and Their Ability to Improve Carbohydrate Yield for Kraft Cooking Eucalyptus Wood

Considering the toxicity of the impurities of synthesized anthraquinone, this study clarified new catalytic compounds for kraft cooking with improved carbohydrate yield and delignification and less mutagenicity, which are important for ensuring the safety of paper products in contact with food. The 2-methylanthraquinone contents of teak (Tectona grandis) woods were 0.18–0.21%. Acetone extracts containing 2-methylanthraquinone from Myanmar and Indonesia teak woods as additives improved lignin removal during kraft cooking of eucalyptus wood, which resulted in kappa numbers that were 2.2–6.0 points lower than the absence of additive. Myanmar extracts and 2-methylanthraquinone improved carbohydrate yield in pulps with 1.7–2.2% yield gains. Indonesia extracts contained more deoxylapachol and its isomer than 2-methylanthraquinone. The residual content of 2-methylanthraquinone in the kraft pulp was trace. Although Ames tests showed that the Indonesia and Myanmar extracts were mutagenic to Salmonella typhimurium, 2-methylanthraquinone was not. The kraft pulp obtained with the additives should be safe for food-packaging applications, and the addition of 0.03% 2-methylanthraquinone to kraft cooking saves forest resources and fossil energy in industries requiring increased pulp yield.     

The effect of barley husk arabinoxylan adsorption on the properties of cellulose fibres

This study investigates the adsorption of (glucurono)arabinoxylan (GAX) on cellulose fibres and the properties thereof. A water-soluble GAX, from barley husks (Hordeum vulgare), was isolated using chlorite delignification and alkaline extraction followed by enzymatic purification. The isolated GAX fraction showed an arabinose to xylose ratio of 0.22 and a weight average molar mass of 20,200 g/mol, as determined by size exclusion chromatography (SEC) in DMSO:H₂O. The GAX was adsorbed on cellulose fibres under well controlled conditions, where temperature and initial concentration of GAX proved to be important parameters in controlling the level of adsorption. The adsorption process was also dependent on xylan molecular structure. Carbohydrate analysis on the modified fibres showed a preferential adsorption of low substituted xylans (arabinose to xylose ratio of ∼0.10). During the adsorption process the GAX solution was analyzed using SEC-RI-MALLS in aqueous solvent, which demonstrated a molecular xylan adsorption on cellulose fibres. Additionally, a decrease in light scattering responses, which corresponds to an adsorption of aggregated xylan and/or xylan with a great tendency towards self-association, could be observed during the adsorption process. This was demonstrated by adsorption of GAX on regenerated cellulose fibres (Lyocell), which compared to native fibres possesses a relatively smooth fibre surface. Atomic force microscopy analysis visualised a heterogeneous decoration of the Lyocell fibres with xylan agglomerates. The effect of GAX adsorption on paper strength was also investigated. A GAX modified kraft pulp showed an evident increase in tensile strength, which might be due to a retained fibre–fibre bonding ability for xylan coated fibrils after drying and rewetting.     

Properties and treatments of pulps from recycled paper. Part II. Surface properties and crystallinity of fibers and fines

Surface properties of bleached kraft pulps were evaluated before and after recycling, and after a series of chemical treatments designed to improve and/or modify the pulp characteristics. The surface free energy characteristics of the pulps were determined using the Wilhelmy technique, and ESCA and ATR-FTIR methods were used to evaluate the chemical composition of the surfaces of the pulp fibers. In general rather small changes were noted at the fiber surfaces with recycling and chemical treatment. Recycling tended to increase the acid component and decrease the base component of the surface free energy of the pulps. This could result from exposure of carboxyl groups from hemicelluloses and/or from oxidized layers from the bleaching process. ESCA analyses also indicated increased carboxyl concentration at the surfaces of the recycled fibers. Although treatment with aqueous bases and organic solvents tended to increase the hydroxyl content on the surface of recycled pulps, the chemical treatments were not beneficial to pulp quality. AFM and SEM of fiber and fine surfaces of kraft pulps revealed that the fines fraction was altered to a much greater extent with recycling. Although recycled fibers appeared to have improved wettability, these small changes in the surface characteristics do not appear to play the dominant role in the characteristics of recycled pulps. Recycling did not change the crystallinity of whole pulps, but it increased the crystallinity of the fines fraction. The increase in the crystallinity of the fines fraction and the reduction in the water retention value (WRV) and the bulk carboxyl content (xylan) of the recycled pulps, as noted in Part I of this paper, appear to play the predominant role in determining the characteristics of recycled pulps. It appears that the loss of the hemicelluloses in the bulk of the fiber with recycling is much more important for internal fibrillation than the apparent small increase of hemicelluloses at the surface of recycled fibers.     

Ionic liquid extraction method for upgrading eucalyptus kraft pulp to high purity dissolving pulp

High purity cellulose from wood is an important raw material for many applications such as cellulosic fibers, films or the manufacture of various cellulose acetate products. Hitherto, multi-step refining processes are needed for an efficient hemicellulose removal, most of them suffering from severe cellulose losses. Recently, a novel method for producing high purity cellulose from bleached paper grade birch kraft pulp was presented. In this so called IONCELL process, hemicelluloses are extracted by an ionic liquid–water mixture and both fractions can be recovered without yield losses or polymer degradation. Herein, it is demonstrated that bleached Eucalyptus urograndis kraft pulp can be refined to high purity acetate grade pulp via the IONCELL process. The hemicellulose content could be reduced from initial 16.6 to 2.4 wt% while persevering the cellulose I crystal form by using an optimized 1-ethyl-3-methylimidazolium dimethylphosphate-water mixture as the extraction medium. The degree of polymerization was then reduced by a sulfuric acid treatment for subsequent acetylation of the pulp, resulting in a final hemicellulose content of 2.2 wt%. When pre-treating the pulp enzymatically with endoxylanase, the final hemicellulose content could be reduced even to 1.7 wt%. For comparison, the eucalyptus kraft pulp was also subjected to cold caustic extraction and the same subsequent acid treatment which led to 3.9 wt% of residual hemicelluloses. The performance in acetylation of all produced pulps was tested and compared to commercial acetate grade pulp. The endoxylanase-IONCELL-treated pulp showed superior properties. Thus, an ecologically and economically efficient alternative for the production of highest value cellulose pulp is presented.     

Comparison of the properties of chemical cellulose pulps

The literature related to differences between chemical cellulose pulps produced by different pulping processes has been reviewed. Kraft pulps tend to be stronger, particularly in tear strength, while sulfite pulps hydrate and beat more readily. Organosolv pulps tend to mirror the properties of sulfite more than those of kraft pulps. A number of theories have been offered to explain the different properties of the chemical pulps; however, none has been universally accepted. It may be that acidic processes develop weak points in the fibers which are magnified in tear strength losses since, at a constant tensile strength, a 10% loss in fiber strength can lead to a 25–30% loss in tear strength. The effects of acidic pulping may also be magnified in greater fiber breakage and damage in the subsequent refining stages. However, strength improvements for inferior pulps can be realized through post-chemical treatments. Caustic treatments appear to give the greatest improvements, presumably due to increases in acidic group content which results in enhanced swelling properties, and possible subtle reorientation of cell wall polymers. The strength of hornified, recycled fibers can also be enhanced with such treatments, although simple beating will restore considerable strength, but at the expense of drainage rates. It is clear that the processes are complex and involve both the chemistry and physics of the fibers and how these attributes combine to affect the subsequent beating of the fibers for bonding and strength development.