Origin and Nature of Kraft Colour: 2 The Role of Bleaching in the Formation of the Extraction Stage Effluent Colour

Upon chlorination and extraction, high molecular weight (>1000D) kraft lignin (KL) is degraded to a material which is structurally similar to extraction stage effluent. During bleaching there is a significant decrease in the aromaticity of KL without a concomitant decrease in colour, which would be expected if aromatic residues are the kraft chromophores. Treatment of simple sugars under conditions similar to those of kraft pulping yielded polymeric products which, on treatment with chlorine and subsequential extraction (CE), gave products with similar spectral characteristics to extraction stage effluent. It is proposed that the chromophores responsible for kraft colour are keto-enols probably derived from the degradation of carbohydrates during the kraft cook.     

The Present Status and Potential of Kraft Lignin-Phenol-Formaldehyde Wood Adhesives

Kraft lignin (KL), a phenolic polymer formed during the kraft pulping process, is presently burned as a low value fuel. For decades, researchers have attempted to use KL as an inexpensive substitute for phenol in phenol-formaldehyde (PF) resins, but no one has produced a commercially satisfactory KL-PF resin. This paper reviews the literature on the present status of KL-PF adhesives and makes recommendations on needed research.

Kraft lignin solutions are complex mixtures which have broad molecular weight distributions, high viscosities, relatively low reactivities, and low solubilities. Attempts to overcome these inherent problems include methylolation of lignin to improve reactivity, the use of co-solvents to improve solubility, and ultrafiltration to yield more homogeneous molecular weight fractions. Future research efforts need to focus on understanding the fundamental chemical and physical properties of kraft lignin and its resins. The search for an economic lignin-based wood adhesive should continue.     

Oxidation of Various Kraft Lignins with a Bacterial Laccase Enzyme

Modification of kraft lignin (KL), traditionally uses harsh and energy-demanding physical and chemical processes. In this study, the potential of the bacterial laccase CotA (spore coating protein A) for oxidation of KL under mild conditions was assessed. Thereby, the efficiency of CotA to oxidize both softwood and hardwood KL of varying purity at alkaline conditions was examined. For the respective type of wood, the highest oxidation activity by CotA was determined for the medium ash content softwood KL (MA_S) and the medium ash content hardwood KL (MA_H), respectively. By an up to 95% decrease in fluorescence and up to 65% in phenol content coupling of the structural lignin units was indicated. These results correlated with an increase in viscosity and molecular weight, which increased nearly 2 and 20-fold for MA_H and about 1.3 and 6.0-fold for MA_S, respectively. Thus, this study confirms that the CotA laccase can oxidize a variety of KL at alkaline conditions, while the origin and purity of KL were found to have a major impact on the efficiency of oxidation. Under the herein tested conditions, it was observed that the MA_H KL showed the highest susceptibility to CotA oxidation when compared to the other hardwood KLs and the softwood KLs. Therefore, this could be a viable method to produce sustainable resins and adhesives.     

An NMR Comparison of the Whole Lignin from Milled Wood,MWL, and REL Dissolved by the DMSO/NMI Procedure

Abstract

Lignins isolated from pine milled wood, milled wood lignin (MWL), and residual enzyme lignin (REL) were compared using modified thioacidolysis, modified DFRC, gel permeation chromatography (GPC), two‐dimensional Heteronuclear Multiple Quantum Coherence (HMQC) NMR, and quantitative ¹³C NMR. Dissolution of the lignin for solution‐state NMR was accomplished by utilizing the recently reported DMSO/N‐methylimidazole/acetic anhydride solvent system. Contrary to previous reports, comparison of the lignin preparations by thioacidolysis indicated that REL was more structurally similar to the lignin in the milled wood and Wiley wood meal than MWL. Total monomer yields indicated that the MWL was lower in β‐aryl ether content than the other preparations, and this was verified by quantitative ¹³C NMR. NMR analysis indicated that the inter‐unit linkages present in all the lignin preparations are consistent with the present knowledge about lignin biosynthesis. The contribution of minor end group structures in the MWL are further decreased in the milled wood, indicating that they are preferentially isolated as low molecular weight material, possibly generated during the milling process. All other structural moieties were similar in all preparations. GPC data indicated that the milled wood and REL both contain a portion of lignin with a molecular weight of 55,000 g/mol. Data indicate that the inefficiency of the DFRC method may be related to molecular mobility or accessibility in higher molecular weight portions of the lignin polymer.     

Structural Characterization of Pine Kraft Lignin: BioChoice Lignin vs Indulin AT

BioChoice lignin (BCL) is a newly commercialized pine kraft lignin from Plymouth Mill of Domtar, which is precipitated from black liquor of bleachable-grade pulp. Indulin AT is a pine kraft lignin commercialized by Meadwestvaco for the past 60 years, which is precipitated from black liquor of linerboard-grade pulp. Thus, the two technical lignins are produced under quite different conditions in terms of alkali charge and pulping time/temperature. While the chemical structure of Indulin AT is well documented, that of BCL is totally unknown. In this study, chemical structures of BCL and Indulin AT are characterized using modern analytical techniques and compared with those of pine milled wood lignin (MWL) in order to elucidate the structural changes that occur during kraft pulping and the structural differences, if any, between BCL and Indulin AT. Both BCL and Indulin AT are structurally very different from the native lignin (MWL) in wood, indicating drastic structural modification during the kraft pulping process. Surprisingly, BCL and Indulin AT are structurally very similar, in spite of the fact that they are produced under different process conditions. However, there are subtle structural differences between BCL and Indulin AT. BCL has higher phenolic hydroxyl, catechol, enol ether, and stilbene contents, but lower methoxyl and β-O-4 contents. These differences are explained by the different pulping conditions under which the two technical lignins are produced.     

Effect of Soda-Additive Pulping on Molecular Characteristics of Residual Pulp Lignin

The soda-additive pulps of pine wood were compared to corresponding soda and kraft pulps in respect to the macromolecular properties of the residual pulp lignins. Results of measurements of number and weight—average molecular weights as well as branching parameters show that the presence of additives such as anthraquinone, methanol and O₂—pretreatment significantly causes the breaking of cross-links in the pulp lignin. On the other hand, ethylenediamine and hydroeulfide ion contribute mainly to extensive lignin degradation. The influence of used additives on the condensation reactions of the residual lignin has also been studied.     

Chemical Structure of Residual Lignin from Kraft Pulp

Structural characteristics of wood lignin and residual lignin in conventional and modified kraft pulps were examined employing elemental and methoxyl analysis and ¹³C, ¹H, and ³¹P-NMR. The structural analysis revealed that kraft residual lignin differs significantly from wood lignin while differences between residual lignins in conventional and EMCC® pulps have similar structural characteristics at kappa numbers corresponding to the point where the selectivity of the cook becomes poor. NMR analysis suggested that residual lignin, compared to the wood lignin, is much less reactive towards pulping chemicals due to the low content of aryl ether linkages and the prevalence of condensed type structures.     

紫外光谱定量测定木质纤维预水解液中溶解性木素和糠醛含量

采用紫外光谱探讨了低分子有机酸、糠醛和木质纤维高温预水解液(PHL)的紫外吸收谱图特征。甲酸和乙酸在近205 nm处有显著吸收, 而糠醛在近280 nm处有显著吸收, 以205 nm或280 nm为木素特征吸收位置的传统木素测定方法不适于高温预水解液的溶解性木素测定。研究提出碱性硼氢化钠还原处理预水解液, 消除糠醛的紫外吸收, 进而定量测定溶解性木素含量的方法。结果表明, 硼氢化钠可显著降低近280 nm处吸收值, 预水解条件越高, 还原前后吸收差值降低越多, 说明糠醛类物质含量高。以提取相思木和芦苇木素为标样, 280 nm处吸光度与浓度线性相关系数均达到0.999, 满足定量测定的需要。充分还原前后280 nm吸收差值可用于糠醛的定量, 糠醛标样280 nm处吸光度与浓度线性相关性为0.998, 糠醛测定回收率为98.14%~99.88%, 相对标准偏差为0.17%~0.35%。     

Origin and Nature of Kraft Colour: 1 Role of Aromatics

The chromophores responsible for kraft colour have been generally assumed to be derived from the aromatic portions of the lignin molecule. Analytical studies of the high molecular weight dissolved materials from kraft lignin, kraft cooked sugars, and extraction stage effluent by pyrolysis GC-MS, permanganate/periodate oxidation, and UV-visible spectroscopy showed that these materials had a wide variation in aromatic content but similar spectral properties. It is now proposed that carbohydrate degradation products produced by the kraft cooking of sugars are primarily responsible for the kraft colour.     

Investigation of formaldehyde-free wood adhesives from kraft lignin and a polyaminoamide–epichlorohydrin resin

A formaldehyde-free wood adhesive system consisting of kraft lignin and a polyaminoamide-epichlorohydrin (PAE) resin (a paper wet strength agent) has been investigated in detail. The lignin-PAE adhesives were prepared by mixing an alkaline kraft lignin solution and a PAE solution. Mixing times longer than 20 min had little impact on the shear strength of the wood composites bonded with the lignin-PAE adhesives. The shear strength of the wood composites bonded with the lignin-PAE adhesives increased and then flattened out when the press time and the press temperature increased. The shear strength and water resistance of the wood composites bonded with the lignin-PAE adhesives depended strongly on the lignin/PAE weight ratio. Of the weight ratios studied, the 3:1 lignin/PAE weight ratio resulted in the highest shear strength and the highest water resistance of the resulting wood composites. The wood composites bonded with the lignin-PAE adhesives did not delaminate and retained very high strengths even after they underwent a boiling-water test. The lignin-PAE adhesives could be stored at room temperature for two days without losing their adhesion ability. PAE was the crosslinking agent in this lignin-PAE adhesive. Possible reactions between lignin and PAE are discussed in detail.     

Suitability of Hybrid Willow as a Source of Pulp

Hybrid willow (Salix spp.) is a potential source of pulp as demonstrated by its fibre morphology, chemical composition and pulping kinetics. Fibre length and cell wall thickness measurements on one and two years old clones ranged from 0·49 mm to 0·70 mm and 2·5 μm to 3·6 μm respectively. Fibre length variation by annual growth layer varied from 0·5 mm in the first growth layer to about 1·1 mm in the last growth layer and the maximum growth rate occurred in the first two to three years for fourteen years old clones. The clones were producing more fibre fraction as indicated by volumetric composition. The UV absorptfvities of milled wood lignin was found in the range of 12·17 -14·31 Lg^?1cm^?1 at 278 nm and the presence of syringyl and guaiacyl lignin was observed. ¹³C-NMR results of acetylated milled wood lignin showed clonal structural variations during lignification process and the Klason lignin content of one and two years old clones ranged from 20·8% to 26·1%. Bulk kraft delignification of mature wood resulted in 5–6% higher yield than juvenile wood from the same clone and the pulping activation energy ranged from 98 kJ.mol^?1 to 120 kJ.mor^?1.     

Lignin and Other Aromatic Substances Released from Spruce Wood During Pressurized Hot-Water Extraction,Part 1: Extraction,Fractionation and Physico-Chemical Characterization

Ground spruce wood was extracted with pressurized hot water in an accelerated solvent extractor (ASE) at 170°C during 20, 60, and 100 min. Released aromatic substances (from 2.2 to 2.5% on wood basis) were isolated on XAD-7 resin and fractionated into lignin, oligomeric aromatic substances (OAS), and compounds insoluble in methanol (IMC). The separated aromatic fractions, and the ground wood before and after extraction, were characterized by physico-chemical methods (GC, GC-MS, HPLC-SEC, TG, DSC). The major part of the aromatic substances was oligomeric aromatic substances (OAS). This fraction increased significantly with the extraction time, while the yield of dissolved lignin increased only slightly with extraction time. Isolated lignins had a lower molar mass and differed considerably from spruce milled wood lignin (MWL). The isolated lignins were more stable against oxidative thermodegradation than spruce MWL. The UV-extinction coefficients at 280 nm were lower for the isolated lignins than for MWL.     

Wood bonding by vibrational welding

—Mechanically-induced wood welding, without any adhesive, is shown here to rapidly yield wood joints satisfying the relevent requirements for structural application. The mechanism of mechanically-induced vibrational wood fusion welding is shown to be due mostly to the melting and flowing of some amorphous, cells-interconnecting polymer material in the structure of wood, mainly lignin, but also hemicelluloses. This causes partial detachment, the 'unglueing' of long wood cells, wood fibres, and the formation of a fibre entanglement network in the matrix of molten material which then solidifies. Thus, a wood cells/fibre entanglement network composite having a molten lignin polymer matrix is formed. During the welding period some of the detached wood fibres which are no longer held by the interconnecting material are pushed out of the joint as excess fibres. Crosslinking chemical reactions also have shown to occur. The most likely one of these identified by NMR appears to be a cross-linking reaction of lignin with carbohydrate-derived furfural. The presence of these reactions has been identified by CP-MAS 13C-NMR. These reactions, however, are relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, which explains why long holding times under pressure after the end of welding contribute strongly to obtaining a good bond.     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

Preliminary Results on an Approach for the Quantification of Lignin-Carbohydrate Complexes (LCC) in Hardwood Pulps

Abstract

The literature on biomass research contains many references to lignin-carbohydrate complexes (LCC) decreasing the rate of delignification in chemical pulp production, decreasing the yield of cellulosic ethanol via fermentation, and decreasing forage digestibility. However, it is difficult to find correlations between rates of the processes above and initial LCC concentration. One of the main reasons for the lack of such correlations is the absence of methods for accurate quantification of LCC. In this investigation, repeatable and reproducible determinations of bound sugars at monomeric concentrations as low as 0.3 wt% on enzymatic lignin (EL) have been achieved. The bound sugars are hydrolyzed by H₂SO₄, most likely as low molecular weight oligomers. In the same H₂SO₄ treatment, the oligomers are hydrolyzed to monomers which are subsequently quantified by ¹H NMR analyses. A significant enrichment of bound arabinan was previously reported when a crude milled wood lignin (MWL) was compared to the starting wood meal. A similar arabinan enrichment was observed for ELs from kraft and soda-AQ (SAQ) pulps in the present study. Also, well-resolved cross-peaks have been obtained in 2D HSQC NMR analyses of ELs. It has so far been confirmed that the EL from a 30.6 kappa number SAQ pulp from sugar maple contained ～30% more benzyl ethers linked to primary-OH groups in sugar units than the corresponding EL from a 33.7 kappa number kraft pulp.     

Engineering Plastics from Lignin. IX. Phenolic Resin Synthesis and Characterization

The performance of phenol-formaldehyde (PF) resins, formulated with lignin derivatives previously synthesized as phenolic resin prepolymers, was evaluated by thermal analysis of the curing process, and by a hard maple shear block test. At 54 and 60% phenol replacement levels, respectively, kraft (KL) and steam explosion lignin (SEL)-based resoles exhibited cure behavior very similar to a standard PF resin. Acid hydrolysis lignin gelled prematurely, and was found to be incompatible with the normal synthesis procedure. Differential scanning calorimetry (DSC) was used to compare kinetic parameters for the curing process of neat and lignin derived phenolic resins. Activation energies and cure rates determined by DSC showed no difference between adhesives. High lignin contents had no inhibitory effect on resin cure. Shear strength properties were evaluated in a compression test, and results illustrate that both lignin-based resins have acceptable strength properties, both in a dry and accelerated aging test. Of the lignins tested, kraft lignin consistently demonstrated superior performance as a pre-polymer in phenolic adhesives. This was attributed to differences in the chemical structure of the two lignins, which had been found to vary in terms of their reactivity with formaldehyde and phenol. KL had been noted to be more amenable to derivatization with formaldehyde and phenol, hence its ability to crosslink with a phenol-formaldehyde fraction during resin synthesis was increased. Positive structural features in KL are a high phenolic guaiacyl (3-methoxy, 4-hydroxy phenyl) content, low carbon-to-carbon bonding between aromatic rings, high solubility in alkali, and a higher number average molecular weight than SEL.     

Structural and Concentration Effects on the Diffuse Reflectance Ftir Spectra of Cellulose,Lignin and Pulp

Experiments were conducted to determine the effect of sample type and concentration on the diffuse reflectance infrared fourier transform (DRIFT) spectra of unbleached softwood kraft and thermomechanical pulps, microcrystalline cellulose (Avicel) and kraft lignin (Indulin AT). The absorption of characteristic bands, in Kubelka-Munk units, was followed as a function of sample concentration in potassium bromide, and, in the case of the two pulps, was also determined on handsheets. Anomalous dispersion occurred at concentrations of 1% or below. This phenomenon resulted in a decrease of absorption of specific bands with increasing concentration and was particularly significant for kraft pulp, less important for thermomechanical pulp and microcrystalline cellulose, and practically absent in pure kraft lignin. These effects of specular reflectance could not be completely eliminated by diluting the sample in a non-absorbing matrix, as previously claimed, but could only be minimized at low concentrations.     

Engineering plastics from lignin II. Characterization of hydroxyalkyl lignin derivatives

Several types of hydroxyalkyl lignin derivatives were synthesized from milled wood, organosolv, steam explosion, acid (H₂SO₄) hydrolysis, and kraft lignin with ethylene oxide, propylene oxide, and butylene oxide by either batch reaction in toluene at 180°C using KOH as catalyst, or in aqueous alkali at room temperature. The isolated derivatives were characterized in terms of their chemical structures by H-NMR and FT-IR spectroscopy. Thermal properties were determined by differential scanning calorimetry. Molecular weights were measured by gel permeation chromatography on polystyrene/lignin model compound calibrated high pressure μ-spherogel columns. Solubilities in various organic solvents spanning a solubility parameter (δ) range from 9.3 to 14.5 and a hydrogen bonding index (γ) range from 1.5 to 18.7 were tested using UV₂₈₀ absorption of solutions of up to with degrees of substitution of between 1 and 2.6 (except for ethylene oxide derivatives which were higher) and with lignin contents of around 60%. The drastic reduction of glass transition temperature of between 50° and 100° is explained with increased free volume of the copolymer and with disruption of hydrogen bonds involving especially phenolic hydroxy groups. The greatly enhanced solubility in organic solvents indicates absence of the gel structure typical of network polymers. No molecular breakdown was observed as a consequence of oxyalkylation. The derivatives had molecular weights (M_w) of between 2000 and 50,000 at dispersity factors of between 2.5 and 25. The derivatives seem to constitute useful prepolymers for thermosetting engineering plastics.     

Soy protein isolate/kraft lignin composites compatibilized with methylene diphenyl diisocyanate

Methylene diphenyl diisocyanate (MDI) was used to compatibilize kraft lignin (KL)/soy protein isolate (SPI) blends. The structure and properties of the resultant composite materials were investigated with wide‐angle X‐ray diffraction, differential scanning calorimetry, dynamical mechanical thermal analysis, scanning electron microscopy, and tensile and water absorption tests. The results indicated that graft copolymerization and a moderate degree of crosslinking between KL and SPI occurred in the composites because of the compatibilization of MDI, which favored the strengthening of the materials. Interestingly, the addition of 2 parts of MDI caused a simultaneous enhancement of the modulus, strength, and elongation of KL/SPI blends. The structure with grafting and moderate crosslinks reduced the water absorption of the materials. However, the excess crosslinks hindered the interaction between KL and SPI, resulting in a reduction of the mechanical properties. Scanning electron microscopy showed that the domains of the graft copolymer and crosslinking enrichment existed in the blends. When the MDI content was relatively low, these domains became concentric points of stress, enhancing the mechanical properties. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 624–629, 2004     

新型聚氨酯助剂——酶解木质素的研制

研究了采用无机碱性水溶液和有机溶剂从玉米秸秆发酵制备乙醇的残渣中分离得到酶解木质素的方法.结果表明,用有机溶剂提取后的残渣继续用无机碱水溶液再次提取,酶解木质素的产率为25%～35%,其红外光谱与磨木木质素红外光谱相似.酶解木质素的分子中含有丰富的苯环、酚羟基和醚键等官能团,具有较高的反应活性,能直接用于聚氨酯的改性.