Softwood kraft lignin: Raw material for the future

Industrial Crops and Products 77 (2015) 845–854

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Softwood kraft lignin: Raw material for the future Göran Gellerstedt Department of Fibre and Polymer Technology, Royal Institute of Technology, KTH, SE-10044 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 23 June 2015 Received in revised form 11 September 2015 Accepted 14 September 2015 Keywords: Condensation Fractionation Kraft lignin Softwood Structure Thermal properties

a b s t r a c t Large quantities of kraft lignin are formed in the pulp industry. Although the vast majority is used for internal energy production at mill sites, modern pulping technology allows for a partial outtake of lignin without disturbance of the energy balance in the mill. At present, lignin from softwood pulping is available in commercial quantities and it can be assumed that this amount will rapidly increase in the future. Therefore, development of material systems based on softwood kraft lignin should be beneficial for the future sustainable society and add value to a renewable resource. In this review, the formation, structure, and properties of softwood kraft lignin is summarized and it is suggested that, depending on final use, an optimization of lignin properties is done through selected fractionation and purification. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7. 8. 9.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 845 Dissolution of lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 846 The isolation procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 846 Lignin reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847 Heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 848 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849 Lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 850 Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 852 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 852 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853

1. Introduction Lignin is second to cellulose in natural abundance among polymers in nature. It is found in all vascular plants and is formed in the cell wall after deposition of the polysaccharides; cellulose and hemicelluloses. Through chemical linkages such as ethers and esters between lignin and the polysaccharide constituents, lignincarbohydrate networks are formed in the cell wall making the direct separation of individual polymers impossible (Lawoko et al., 2006). In annual plants such as cereals, sugar cane or grasses, alkaline delignification is, however, much faster than in hardwood or softwood supporting the view that ester linkage is much more abundant in the former category (Gonzalo Epelde et al., 1998).

E-mail address: [email protected] http://dx.doi.org/10.1016/j.indcrop.2015.09.040 0926-6690/© 2015 Elsevier B.V. All rights reserved.

Traditionally, the dissolution of lignin from wood is done in technical processes such as kraft and sulfite pulping resulting in the formation of cellulosic fibers for further use in commodity products such as packaging, printing papers, and tissue whereas the lignin in most cases is used as internal fuel in the process. In sulfite pulping of wood, the lignin is made water soluble through the introduction of hydrophilic sulfonate groups followed by hydrolytic cleavage of ether groups. In some mills, such lignins have, since a long time, been isolated from the spent pulping liquor and used as dispersing agent in e.g., concrete. In the kraft (and soda) pulping process, on the other hand, the lignin is only water soluble in alkaline solution. Increased water solubility can, however, be achieved through introduction of sulfonate groups via the sulfomethylation reaction (de Groote et al., 1987). At present, such lignins are commercially available, although the production is rather small, with uses such as dispersant for dye

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pigments (Lora, 2008). Despite several decades of research about the possibilities of utilizing kraft and soda lignins in material systems such as carbon fibers, polyurethanes, phenol-formaldehyde plastics, adhesives, and others no commercially viable technology has yet been developed within these areas (Gandini and Belgacem, 2008; Gellerstedt et al., 2013). Rapidly increasing concentration of carbon dioxide in the atmosphere resulting in successive global warming has created a growing awareness that the future society must rely much more on renewable resources both for energy production and in material systems. In this development, biomass such as wood and annual plants will play important roles (Duval and Lawoko, 2014). Consequently, several initiatives have been taken to develop new and simplified separation processes for biomass polymers and many laboratories are also working on the development of new products and material combinations in which cellulose, hemicelluloses, lignin or their degradation/conversion products are involved. Today, this work is still in its infancy and in many cases, it is quite obvious that for biomass and biomass polymers fundamental knowledge about structure, properties, material and polymer behavior, reactivity and other properties is still lacking (Gandini and Belgacem, 2008; Gellerstedt et al., 2013; Ragauskas et al., 2014). Within the forestry industry, on the other hand, much knowledge related to the pulping processes has been developed during several decades of improved and optimized pulp production. Only in a few laboratories, however, has wood polymers like lignin or cellulose been in focus for development work aiming at material systems. While cellulose is readily available in large quantities, development work with either hemicellulose or lignin has been hampered by the lack of suitable sources being able to provide reliable and constant quality over time. For several decades, only one source of commercial softwood kraft lignin has been available and only in limited scale; samples from the MeadWestvaco kraft mill in Charleston, SC, USA. This lignin (Indulin AT), originating from kraft pulping of pine and isolated through a two-step acidification procedure, is comparatively free of ash and has a low content of carbohydrates (Keilen et al., 1952; Ball and Vardell, 1961; Ball et al., 1965). Recently, a similar procedure for the isolation of kraft lignin from softwood (or hardwood) black liquor has been described which is more adapted to modern kraft pulping technology; the LignoBoost process (Öhman et al., 2006; Tomani, 2010). According to this process, rather pure kraft lignin with low contents of residual ash and carbohydrates can be isolated as solid powder. At present, two commercial installations are in operation with primary use of the lignin as lime kiln fuel at the mill. However, a development of more value-added applications for this type of lignin can be expected to result in further industrial installations and rapid capacity increase. Softwood kraft lignin has a higher reactivity in comparison with the hardwood counterpart, e.g., in the production of stabilized lignin fibers (Baker et al., 2012; Norberg et al., 2013) and softwood kraft lignin can be expected to be more uniform between wood species due to structural similarity with the lignin containing only one predominant type of monolignol as building block (guaiacyl-propane units). This makes it important to obtain an understanding of structural features, reactivity and thermal properties for this type of lignin in order to assist in the development of large scale kraft lignin-based products. In this review, the comprehensive but scattered structural information on softwood kraft lignin that is available in the literature has therefore been summarized and critically evaluated. Based on this information, a new concept for softwood kraft lignin utilization is presented.

2. Dissolution of lignin The worldwide production of kraft pulps is in the order of 130 million tons annually with more than 50% being based on softwood pulping. In the kraft process, the lignin structure is modified and depolymerized through the action of strong aqueous alkali with simultaneous presence of hydrosulfide ions. Thereby, up to about 90–95% of the wood lignin can be rendered soluble in the pulping liquor. A large number of wood species is processed by the kraft process to liberate cellulosic fibers. Whereas many softwood species behave rather uniformly with comparatively low rate of delignification, the variability among hardwoods is much greater and pulping conditions can vary considerably. The H-factor is commonly used to describe how much energy in terms of time and temperature is needed to arrive at a certain degree of delignification, usually expressed as the kappa number of the pulp (Grace and Malcolm, 1989). In addition, a high charge of alkali in the pulping liquor will shorten the pulping time and have an influence on the lignin structure (Lindgren and Lindström, 1996; Gellerstedt and Wafa AlDajani, 2003). Once, the targeted H-factor has been achieved, the pulping process is interrupted and pulp and liquor (denoted black liquor) are separated with the pulp being washed and further processed to the final product. The black liquor is evaporated to high dryness and burnt although in modern energy-efficient mills, a partial take-out of black liquor can be tolerated, usually at a medium high consistency (Öhman et al., 2006; Gellerstedt et al., 2010). Subsequently, this black liquor stream can be acidified to recover the kraft lignin as a precipitate. In individual pulp mills using only softwood or only hardwood as raw material, the resulting precipitated kraft lignin will have a rather uniform composition unless the wood supply and pulping conditions are severely changed over time. In the northern hemisphere, it is not uncommon, however, that pulp mills process both softwood and hardwood either in two separate lines or in so called swinging operation in one line. In these mills, the black liquor streams are combined and, consequently, will contain a mixture of softwood and hardwood kraft lignin. A lignin take-out will therefore in fact be a mixture of lignins with differing properties from each other. Lignins obtained from different types of kraft cooks, i.e. largely differing in H-factor (and kappa number) will also show differences in properties as was recently noted by Dodd et al. (2014). 3. The isolation procedure In the kraft pulping process, the lignin that is eliminated from the fiber wall is solubilized in the black liquor as long as the alkalinity is kept high due to the presence of ionized phenolic hydroxyl groups formed through reaction between lignin and the pulping liquor. Minor amounts of carboxylate groups are also formed. Although the pKa -value for phenols can vary widely depending on the substitution pattern in the aromatic ring, the predominant structures in lignin, being of guaiacyl (in softwoods) or guaiacyl and syringyl type (in hardwoods), have pKa -values around pH 10 and can be protonated and precipitated out from the black liquor using a weak acid such as carbon dioxide. This mode of black liquor treatment is made use of in the LignoBoost process as well as in the older industrial methods for lignin isolation with a precipitation pH of about pH 9.5. Around this pH, some 85% of the lignin can be precipitated (Marton, 1971). The lignin precipitate that is formed is filtered off and re-suspended in dilute aqueous sulfuric acid in order to protonate all types of phenolic lignin structures and carboxylate groups thus minimizing the amount of remaining ash in

G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

the lignin. Nevertheless, small amounts of inorganic material (<1%), analyzed as ash, is normally found in the isolated kraft lignin. In many laboratory studies, on the other hand, mineral acid is used to achieve a direct acidification of the black liquor to low pHvalues. Under such conditions, an almost complete precipitation of kraft lignin is obtained but remaining hydrosulfide ions in the black liquor are converted into hydrogen sulfide and must be vented off. Black liquors also contain small amounts of elemental sulfur and polysulfide as well as thiosulfate and sulfate ions (Grace and Malcolm, 1989; Gellerstedt et al., 2004). On acidification, thiosulfate is disproportionated to sulfur and sulfite ions and polysulfide to sulfur and hydrosulfide ions. Consequently, on direct acidification of black liquors, elemental sulfur can be found as impurity in the precipitated lignin. The amount can vary widely depending on mill and processing conditions. For paper grade pulping of softwood, an amount of 1–3% sulfur in the kraft lignin can normally be found with about half being chemically attached to the lignin (Gellerstedt and Lindfors, 1984; Gellerstedt et al., 2010). In addition, kraft lignins from both softwood and hardwood may contain extractives as further impurity. The elemental composition of purified softwood kraft lignin (at kappa number ∼30) has been analyzed as 64.5% carbon, 6% hydrogen, 28% oxygen, 1.5% sulfur and with a methoxyl content of 13.5–14.0%.

4. Lignin reactivity In wood and other biomass, lignin is formed through radicalinduced end-wise polymerization of coniferyl (softwood and hardwood) and sinapyl (hardwood) alcohol moieties with minor proportions of p-hydroxy-cinnamyl alcohol participating in the formation of compression wood in softwoods (Ralph et al., 2004; Gellerstedt and Henriksson, 2008). The latter alcohol has, however, a more prominent role in the formation of lignin in annual plants. All lignin is chemically linked to polysaccharides resulting in network structures (lignin-carbohydrate complexes, LCCs) that cannot be extracted from the tissue unless chemical bonds are broken (Lawoko et al., 2006). The structure of lignin in wood has been discussed in several review articles (Freudenberg, 1968; Adler, 1977; Brunow et al., 1999; Gellerstedt and Henriksson, 2008) and based on this knowledge; numerous experiments with lignin model compounds have been performed to gain an understanding of the chemistry behind lignin dissolution in alkaline media. This chemistry was recently reviewed (Dimmel and Gellerstedt, 2010). In the process, alkylaryl ether (ˇ-O-4) linkages are broken to a large extent, resulting in lignin fragments with a high percentage of free phenolic structures. In other types of lignin structures with side-chain linkages such as phenylcoumaran (ˇ-5), 1,2-diarylpropane (ˇ-1), and pinoresinol (ˇ–ˇ) structures, no lignin fragmentation takes place although the side-chain may undergo various types of modification reactions. Aromatic units linked together in biphenyl (5–5) and biphenyl ether (4-O-5) structures are also stable during kraft pulping conditions. The kinetics for cleavage of ˇ-O-4 structures in both phenolic and non-phenolic units has been investigated with the use of model compounds. By plotting the rate constants for cleavage (expressed as half-lives) against the reaction conditions, it is obvious that whereas phenolic ˇ-O-4 structures are rapidly degraded early in the kraft cook, the non-phenolic counterpart is comparatively unreactive with a half-life in the order of 50 min or more at the maximum pulping temperature as shown in Fig. 1 (Ljunggren, 1980). Detailed analysis of the various lignin-carbohydrate complexes (LCCs) present in softwood shows that the xylan-bound lignin (about 40% of the total) has a strong predominance of ˇ-O-4 linkages between the phenylpropane units (Lawoko et al., 2005).

847

Fig. 1. Rate constants, expressed as half lives, for the cleavage of phenolic and nonphenolic lignin model compounds of the ␤-O-4 type in kraft pulping conditions (data from Ljunggren, 1980).

Thereby, on exposure to kraft pulping liquor and starting from a phenolic end group, this lignin will be easily degraded to low molecular mass fragments and towards the end of the kraft cook, very little xylan-bound lignin will remain in the solid fiber phase (Lawoko et al., 2004). For the glucomannan-linked lignin (about 50% of the total), on the other hand, the opposite is true and a successive relative accumulation takes place as the cook proceeds. Towards the end of a pulping cycle, most of the added sodium hydroxide has been consumed through neutralization reactions with phenols (in lignin) and carboxylic acids (from polysaccharide degradation). For the process, it is, however, vital that sufficient residual alkali is still present in the black liquor to keep dissolved lignin fragments in solution. Most of the hydrosulfide ions added in the pulping liquor are still present towards the end of the pulping cycle since the major function of hydrosulfide is as a delignification catalyst. Due to the harsh conditions prevailing during pulping and to the multitude of redox systems present in wood, some of the added hydrosulfide is, however, converted into polysulfide, sulfur, thiosulfate, and sulfate ions (Gellerstedt et al., 2004). The presence of sulfur in the pulping liquor has been shown to induce radical reactions between dissolved phenols (from lignin) and lignin moieties thereby giving rise to new biphenyl and biphenyl ether structures both in the dissolved and in the residual fiber lignin (Majtnerova and Gellerstedt, 2006). Although many pulping experiments with lignin model compounds give products that are best explained based on heterolytic two-electron reactions, one-electron reactions have been observed as well. The latter type of reaction was first postulated by Kleinert (1965) who suggested that “the final (residual) delignification is related to lignin portions modified during cooking rather than to the solubilization of a distinct type of lignin initially present in the wood”. Later, model compound experiments were used to demonstrate that simple phenols when exposed to kraft pulping liquor with or without the presence of elemental sulfur gave products consistent with radical reaction mechanisms (Smith and Dimmel, 1994; Majtnerova and Gellerstedt, 2006). It is well known that coniferyl alcohol (or possibly a sulfur adduct) is a primary degradation product in the cleavage of phenolic ˇ-O-4 structures during kraft pulping (Brunow and Miksche, 1972; Gierer et al., 1973; Berthold et al., 1998). Being unstable, coniferyl alcohol is rapidly degraded into vinylguaiacol which in turn via the corresponding quinone methide can be converted to apocynol through addition of water (Gierer and Lindeberg, 1978). Both vinylguaiacol and apocynol have been found in alkaline pulping liquors but unlike several other types of low molecular mass

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G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

H3C

OH

CH3

HO-/HS-

OCH3 OH

H3C

OCH3 OH

OCH3 OH

OCH3 O

OCH3 OH

OCH3 OH Fig. 2. Model experiment with apocynol and ethylguaiacol to illustrate the ease of condensation in kraft pulping conditions (Kanungo et al., 2009).

phenols their concentrations are successively reduced as the cook proceeds (Hise et al., 1983). In recent model experiments with apocynol and ethylguaiacol, it could be demonstrated that irrespective of sulfide concentration, a considerable amount of dimer was formed by nucleophilic attack through free ortho position (at C5) to the quinone methide from apocynol (Fig. 2, cf. Fig. 6 below) (Kanungo et al., 2009). From these observations, it seems likely that condensation involving both one- and two-electron reactions can take place during the course of a kraft (or other alkaline) cook. The relative importance of these two mechanistic pathways cannot be estimated based on available data but it seems likely that elemental sulfur, present in the black liquor throughout the cook, to some extent will dissolve in the lignin gel phase thereby acting as initiator for one-electron reactions (Gellerstedt et al., 2004). For lignin end-groups being directly exposed to an aqueous alkaline environment, a preference for two-electron reactions might be operative. In both cases, new phenolic end-groups can be created with detrimental effects on the rate of delignification either by introduction of biphenyl structures or by trapping of available quinone methides. Both types of products will contribute to decreased rate of delignification and can explain the observation that an increased charge of alkali will result in increased rate of delignification and a lower amount of residual lignin in the pulp (Lindgren and Lindström, 1996).

5. Heterogeneity The heterogeneity of the original lignin structure is reflected in the dissolved kraft lignin being highly polydispersive as seen by size exclusion chromatography (SEC) (Fig. 3). There are numerous examples in the literature on SEC of lignin from a variety of biomass sources and several different chromatographic methods have been applied (Gellerstedt 1992; Sjöholm et al., 1999; Baumberger et al., 2007). In a recent survey, different lignins were analyzed by both aqueous alkaline SEC and by SEC in tetrahydrofuran (after acetylation) with the conclusion that the former method was preferable due to less variation in multiple runs (Baumberger et al., 2007). There are, however, several objections to this view, the most important being the fact that (kraft) lignin forms aggregates in alkaline solution (Sarkanen et al., 1984; Norgren et al., 2002) thereby giving rise to erroneously high relative molecular mass values. The alternative, SEC run in tetrahydrofuran (THF), is not ideal either since

Fig. 3. Size exclusion chromatography of silylated softwood kraft lignin in THF (Kringstad et al., 1981). A high molecular mass shoulder at an elution volume of about 25 mL can be ascribed to the presence of lignin-carbohydrate complexes.

derivatization is required but data from many literature sources shows that molecular mass values for e.g. softwood kraft lignin fall in a similar mass range. The method requires, however, that pure lignin samples are used, i.e. impurities present in the crude lignin such as elemental sulfur and inorganic precipitates should be removed prior to analysis. Crude kraft lignins also contain lignincarbohydrate complexes (LCC). If these are left in the sample to be analyzed, a broad shoulder can be seen at low elution volumes (cf. Fig. 3) and all calculated molecular mass values will be affected in an unpredictable way. Therefore, it is common practice to eliminate these prior to analysis (Gellerstedt and Lindfors, 1984). In Fig. 3, SEC of softwood kraft lignin after derivatization with a silylation reagent is shown. With such a reagent, both hydroxyl and carboxyl groups present in the lignin are derivatized thus facilitating the subsequent dissolution in THF. Other reagents employed to derivatize lignin prior to SEC analysis in THF involve methylation with diazomethane or acetylation with either pyridine/acetic anhydride (Gellerstedt, 1992) or acetyl bromide (Asikkala et al., 2012). In the latter case, bromination reactions will, however, take place in addition to acetylation thus facilitating subsequent condensation reactions (Nimz, 1966) which in turn may result in erroneous (higher) molecular mass readings in the SEC analysis. Clear indications of competing condensation reactions in the acetobromination of lignin can be observed when pine wood is treated with acetyl bromide (Peng et al., 1998) and when softwood kraft lignin is acetylated with either acetic anhydride/pyridine or with acetyl bromide

G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

849

Table 1 Determination of absolute molecular mass values of softwood kraft lignins. Comparison with values from SEC analysis of non-acetylated and acetylated samples. Lignin

Mn

Mw

Mw /Mn

Method

Reference

Kraft lignin Kraft lignin Indulin AT Indulin AT Kraft lignin Kraft lignin Kraft lignin, acetylated Kraft lignin, acetylated

1600 n.d. 1200 1340 1710 1820 2320 2350

3500 42900 2990 3400 2570 3100 3770 3600

2.2

a

Marton and Marton (1964) Dong and Fricke (1993) Jacobs and Dahlman (2000) “ “ “ “ “

a

2.5 2.5 1.5 1.7 1.6 1.5

LALLS MALDI-TOF SEC MALDI-TOF SEC MALDI-TOF SEC

Mn : thermoelectrical; Mw : ultracentrifugation.

and subsequently analysed by SEC (Asikkala et al., 2012). Throughout, the latter method gave rise to higher molecular mass readings. The absolute molar mass of softwood kraft lignin has been determined using a variety of analytical techniques. Already in the early 1960s, West Virginia Pulp and Paper Company (now MeadWestvaco) in Charleston SC, used absolute methods for this analysis as shown in Table 1 (Marton and Marton, 1964). Later, the values for Mn and Mw that were obtained have been shown to be in good agreement with values obtained by MALDI-TOF-MS and by SEC in THF (Jacobs and Dahlman, 2000). Determination of Mw by low angle laser light scattering (LALLS) in aqueous alkali, on the other hand, gives much higher values most likely due to lignin aggregation (Dong and Fricke, 1993). The heterogeneous structure of softwood kraft lignin can be visualized by sequential fractionation with organic solvents or by ultrafiltration (Mörck et al., 1986; Brodin et al., 2009; Sevastyanova et al., 2014; Cui et al., 2014; Saito et al., 2014). Various series of solvents have been employed to dissolve lignin fractions of successively larger molecular mass followed by characterization by SEC, 13 C NMR, hydroxyl and carboxyl group determination, and carbohydrate analysis. The results obtained demonstrate that a substantial portion of the kraft lignin (∼30%) has a molecular mass of about 800 Dalton or lower corresponding to lignin structures originating from about one to five phenylpropane units (Helander et al., 2013; Duval et al., 2015). In laboratory pulping experiments, it has been shown that almost 20% of spruce lignin is degraded to monomeric and dimeric phenolic compounds and many such compounds have been identified (Gierer and Lindeberg, 1980; cf Niemelä, 1988). Furthermore, black liquors contain acidic low molecular mass products being the result of condensation reactions between phenols from lignin and reactive aldehydes of carbohydrate origin (Gierer and Wännström, 1984). The presence of a large portion of low molecular mass phenolic products of the guaiacyl type in softwood black liquor will result in ample opportunities for condensation reactions with the polymeric portion of lignin as discussed above. It can also be assumed that the use of unfractionated softwood kraft lignin in material systems such as polyurethanes or formaldehyde-based adhesives will result in preferential reaction between the added chemicals and the low molecular mass portion of lignin. The amount of carbohydrates present in kraft lignin can vary considerably but after kraft pulping of softwood, values in the order of 1–2% by weight have frequently been found. Detailed analysis reveals galactose, xylose and arabinose as predominant sugar moieties (Gellerstedt and Lindfors, 1984; Mörck et al., 1986) indicating chemical linkages between the fiber wall lignin and the hemicelluloses galactoglucomannan and arabinoxylan (cf. Lawoko et al., 2006). The carbohydrates are not evenly distributed in the lignin, however, but preferentially located in the high molecular mass fraction(s). Therefore, any purification of kraft lignin must take these features into account and either solvent fractionation or ultrafiltration can be used to virtually eliminate all remaining LCCs (Mörck et al., 1986; Brodin et al., 2009).

6. Structure In comparison to the native lignin present in wood, kraft lignin has undergone a dramatic change of chemical structure. The network between lignin and polysaccharides have to a large extent been degraded leaving only minor amounts of lignin-carbohydrate linkages intact in the dissolved lignin after the pulping process. In phenolic phenylpropane units, the facile formation of quinone methide intermediates is followed by degradation and modification reactions of the propanoid side chains resulting in elimination of a major portion of the terminal hydroxymethyl groups as formaldehyde. The latter seems to recombine with itself through the Cannizarro reaction and/or with low molecular mass lignin fragments as shown by experiments with radioactive formaldehyde (Araki et al., 1980). Other side chain modifications like elimination and rearrangement reactions can also be envisaged, overall resulting in shortened side chains and a considerable reduction of aliphatic hydroxyl groups. The majority of the predominant inter-lignin linkage, the alkylaryl ether (ˇ-O-4) linkage, is cleaved resulting in fragmentation of the lignin macromolecule (Gellerstedt et al., 1984; Froass et al., 1998). Available data on softwood kraft lignin based on NMR give values of about 7–10% ˇ-O-4 structures of those originally present in wood (Zhang and Gellerstedt, 2001; Berlin and Balakshin, 2014). In addition, some further ˇ-O-4 structures (∼5%) are converted into enol ether structures without cleavage of the ˇ-ether linkage (Gellerstedt and Lindfors, 1987). Other inter-lignin structures such as the phenylcoumaran (ˇ-5) and 1,2-diaryl-propane (ˇ-1) structure are not cleaved but partially reacted to form stilbene structures (Gierer et al., 1972). The inter-lignin units of diaryl (5–5) and diaryl ether (4-O-5) types are unreactive and may even increase in concentration (Gellerstedt and Gustafsson, 1987) due to the presence of sulfur-induced radical coupling reactions towards the end of the kraft cook (Gellerstedt et al., 2004). Structures of the ˇ−ˇ type are present in softwood lignins in structures such as pinoresinol and lariciresinol structures (Zhang et al., 2003). Also these may undergo modification reactions during pulping but are not degraded and can still be found in the kraft lignin (Table 2). By reaction of softwood kraft lignin with formaldehyde in alkaline conditions, it has been shown that about 0.35 mol of formaldehyde per mol of aromatic rings can be introduced as hydroxymethyl groups (Zhao et al., 1994) in fairly good agreement with the number of free C5-positions as given in Table 2. It should be remembered, however, that the vast majority of these, most likely, can be found in that portion of the lignin having the lowest molecular mass. The striking differences in chemical structure between lignin in wood and in kraft pulp and kraft lignin can be illustrated by thioacidolysis followed by SEC of the reaction products. Thus, the quantitative cleavage of ˇ-O-4 structures by the thioacidolysis reaction results in almost exclusive formation of monomeric, dimeric, and trimeric products from wood demonstrating the lack of condensed lignin structures beyond trimers and possibly traces

850

G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

Table 2 Data based on NMR analysis and of permanganate oxidation of native lignin (MWL) and kraft lignin from softwood. Values per aromatic ring. Lignin source/Linkage type

ˇ-O-4

ˇ-5

ˇ–ˇ

5–5

4-O-5

Ar5 -H

Ref.

MWL, Norway spruce Kraft lignin, Norway spruce Kraft lignin, Scots pine Indulin AT, Mead-Westvaco

0.43a 0.08 0.03 0.07

0.12 0.03 0.03 0.04

0.03 0.02 0.05 0.04

0.11b 0.18c

0.04b 0.09c

0.62 0.42

Zhang and Gellerstedt (2001) Zhang and Gellerstedt (2001) Berlin and Balakshin (2014) Berlin and Balakshin (2014)

a b c

The value in wood is much higher but still unknown (cf. Önnerud and Gellerstedt, 2003) Data from Adler (1977). Data from (Gellerstedt and Gustafsson 1987). Table 3 13 C NMR spectra employing the DEPT sequence for quantification of various types of carbon atoms in native softwood lignin and in corresponding kraft lignins. Values denote number of C per aromatic ring (Gellerstedt and Robert, 1987).

Absorbance at 280 nm

1.2 wood

1 0.8

fiber lignin reference, kappa 34

Functional group

Native lignin (MWL)

Kraft lignin, 69% Kraft lignin, delign. 93–95% delign.

kraft lignin reference, kappa 34

Quaternary aromatic C Double bonds Hydroxyl groups, total - Primary - Secondary - Phenolic

3.34 0.06 1.29 0.78 0.31 0.20

3.42 0.18 1.25 0.35 0.25 0.65

3.52 0.22 1.32 0.43 0.21 0.68

80

Aliphatic C, 55–90 ppm 1.34 - CH 0.84 -CH2 - Methoxyl 0.97

0.49 0.35 0.79

0.51 0.35 0.88

Aliphatic C, 0–55 ppm - CH - CH2 - CH3

0.39 0.63 0.18

0.36 0.59 n.d.

0.6 0.4 0.2 0 0

20

40

60

100

Time (min) Fig. 4. Size exclusion chromatography of degradation products from thioacidolysis of spruce wood, kraft pulp and isolated kraft lignin after kraft cook to kappa number 34. The peak at 60–65 min retention time originates from carbohydrate degradation products and the peaks at 50 and 45 min are due to monomeric and dimeric lignin degradation products respectively (Majtnerova and Gellerstedt, 2006).

of tetramers (Suckling et al., 1994; Önnerud and Gellerstedt, 2003; Önnerud, 2003). The same analysis when applied to kraft pulp or kraft lignin reveals, however, a dramatic increase in thioacidolysis products at shorter retention times as shown in Fig. 4. Obviously, the residual lignin in the pulp still contains some ˇ-O-4 structures that can be cleaved to form monomeric and dimeric products but a new peak at about 35 min retention time is predominant and almost exclusively formed on thioacidolysis of the corresponding kraft lignin (Majtnerova and Gellerstedt, 2006). The differences between wood and kraft lignin on thioacidolysis followed by SEC as shown in Fig. 4 are further summarized in Fig. 5. The principles outlined here should be applicable to the lignin linked to glucomannan whereas the xylan-lignin portion, as mentioned before, has a much higher percentage of ˇ-O-4 structures and a higher reactivity in pulping resulting in a more comprehensive degradation to low molecular mass components. The puzzling fact that kraft lignin contains a large amount of aliphatic methylene groups without adjacent oxygen function, as shown in Table 3, has resulted in several suggestions such as reduction reactions during pulping or formation of diarylmethane structures through involvement of formaldehyde in analogy to bakelite type reactions. Indications that the former of these reaction types may play a role has been obtained by isolation and identification of reduced monomeric phenols (Gierer and Lindeberg, 1980) whereas the latter has been ruled out since 2D NMR analysis of kraft lignin did not show the expected cross-correlated peaks (Zhang and Gellerstedt, 2001). More careful analysis by 2D-NMR sequences such as HSQC, HMBC and HSQC-TOCSY has revealed, however, that fatty acids seem to become chemically linked to kraft lignin during pulping (Gellerstedt et al., 2004). Potential candidates could be unsaturated acids such as linoleic acid or pinolenic acid which both is major components among the softwood extractives. Again, radical coupling reactions between phenoxy radicals and fatty acid radicals induced by sulfur biradicals present in the black liquor may explain the behavior. Calculations based on NMR-data gives a frequency of 2–3 fatty acid structures per 100 aromatic rings in kraft

0.25 0.18 0

lignin. Furthermore, dihydroconiferyl alcohol, present as end group in native lignin, will contribute to the total amount of aliphatic methylene groups in kraft lignin (Gellerstedt et al., 2004). The data in Table 3 further supports the view that aliphatic double bonds are introduced (in e.g., stilbene structures), that a predominant portion of the terminal hydroxymethyl groups have been eliminated (as formaldehyde), that new free phenolic groups have been created (through cleavage of ˇ-O-4 and ˇ-5 structures), and that the number of aliphatic methine and methyl groups below ∼40 ppm has increased. Data on the number of phenolic hydroxyl groups in softwood kraft lignin seems to vary between ∼60-70 units per 100 aromatic rings (Robert et al., 1984; Brodin et al., 2009; Berlin and Balakshin, 2014) which can be recalculated as 3.6–4.2 mmol/g of lignin assuming a mean molecular weight of 168 for one phenylpropane unit (Robert et al., 1984). The low molecular mass lignin can, however, be assumed to have a much higher frequency leaving a value of about 45–60 phenolic units per 100 aromatic rings in the major portion of the lignin (2.7–3.6 mmol/g of lignin). 7. Lignin Already in 1964, a tentative structure of softwood kraft lignin based on wet chemistry analysis and spectroscopic data was published (Marton, 1964). The structure was later modified and re-published with the comment that the structure “is not to be considered as a structural formula of softwood kraft lignin. The suggested formulation is simply a scheme compatible with our present stage of knowledge” (Marton, 1971). Much new information about the structure of softwood lignin and its reactions during kraft pulping has been collected from the 1960s and onwards leaving the conclusion that the published formula is rather far from being representative for softwood kraft lignin. Surprisingly enough, in 2013, the formula was, however, reproduced as “a statistical scheme for softwood kraft lignin” (Cui et al., 2013).

G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

851

Fig. 5. Schematic structure of the glucomannan-lignin LCC in softwood and its behaviour on either direct thioacidolysis-SEC or by thioacidolysis-SEC after a preceding kraft pulping operation.

CH3 H

O

H3C

O + others

OH

OH OH HO HO

Lignin-Xyl

OCH3 O

O

H3CO

H3CO

H3CO OH

OH

OH

HO-/HS-

O - CH2O

H3CO

OH OH

H3CO

OH

Lignin-Xylan Complex

+ Lignin-Xylan Residue

Lignin-GM OH

OCH3 R O

H3CO

OH

Lignin-GM OH

HO HO-/HS-

OH

O OCH3

Lignin-GM OH

HO O OCH3

OH

H3CO

O

OH O

OCH3

Lignin-GlucoMannan Complex

OH

H3CO

OH O

OH

OH

O

R O

O OCH3

H3CO

H3CO OH

OCH3

OCH3 R O

OH

+ H2O

H3CO

OH

HO

H3C

CH3 OCH3

OCH3

OCH3 OH OH OCH3

OH

OCH3

CH3 OH OH

H3C OH OCH3

OCH3

Dissolved Lignin + Lignin-GM Residue

Fig. 6. Simplified scheme illustrating some important reactions of the lignin-xylan and the lignin-glucomannan complexes during kraft pulping of softwood.

Based on the discussion in Sections 4–6 and on data from solvent fractionation and ultrafiltration of softwood kraft lignin, it seems possible to distinguish three different types of lignin fractions. 1. A low molecular mass fraction originating from about one to five phenylpropane units and with a molecular mass <∼800 Dalton. The amount of this fraction will vary with exact pulping conditions and with storage time of the black liquor due to the presence of various reactive phenols prone to undergoing condensation reactions but can be in the order of 30%, i.e., approximately corresponding to the ethyl acetate-soluble part from isolated softwood kraft lignin (Duval et al., 2015). 2. The major (middle) fraction of the lignin should constitute 50–70% of the total and be expected to have a fairly narrow

molecular mass distribution. Its structure can be expected to originate to a large extent from the glucomannan-lignin LCC present in the wood. Secondary condensation reactions involving low molecular mass phenols such as vinylguaiacol should, however, result in lignin fragments with low reactivity towards pulping reagents and with low abundance of phenolic phenylpropane units with free aromatic C5-carbons. 3. A high molecular mass fraction in which the molecular mass to a large extent is determined by the presence of carbohydrate chains. The amount of this fraction is estimated to ∼15%. Its structure may resemble that of Fraction 2. In Fig. 6, the three types of fraction is schematically illustrated by a xylan-lignin complex with a predominance of ˇ-O-4 linked

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G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854 Table 4 Glass transition temperature (Tg ) and decomposition temperature (Td ) for softwood kraft lignin samples.

R H3CO

O

Lignin

O H O OCH3

Sample Softwood kraft lignin Indulin ATb Softwood kraft lignin Indulin ATb “d Indulinb “e a b c

pKa1 ~7 pKa2 ~11 Fig. 7. Stabilized mono-anion of a biphenolic biphenyl structure at moderate alkalinity.

phenylpropane units being degraded to low molecular mass products of the type identified by Gierer and Lindeberg (1980) (Fig. 6, upper part). This degradation starts very early in the pulping cycle (cf Fig. 1) and as long as a phenolic ˇ-O-4 structure is available a “peeling off” of coniferyl alcohol takes place. This reacts further to a variety of products, one being elimination of formaldehyde and formation of vinylguaiacol. During the remainder of the cook, the latter, in equilibrium with apocynol, is gradually consumed in oneand two-electron condensation reactions with available (aromatic C-5) sites in the still polymeric glucomannan-lignin complex (Fig. 6, lower part). Simultaneously, this lignin is successively dissolved and degraded through cleavage of non-phenolic and newly liberated phenolic ˇ-O-4 linkages and by further transformation and partial degradation of side chains in phenolic lignin units. Most of the linkages between lignin and glucomannan-derived sugars are cleaved. The formation of phenolic biphenyl structures will, however, result in a change of the pKa -value for such phenols and the formation of quinone methides is prohibited by hydrogen bonding between the rings as shown in Fig. 7. Thus, at a certain point in the pulping cycle, lignin dissolution ceases and can only be regained with new addition of alkali as shown by Lindgren and Lindström (1996). 8. Thermal properties On heating of lignins to temperatures about 150–160 ◦ C, molecular mass increase takes place as reported for organosolv and hardwood kraft lignins (Uraki et al., 1995; Kadla et al., 2002). A similar behavior was recently observed for softwood kraft lignin (Bridson et al., 2013). In a recent paper, these changes have been evaluated in more detail and it could be demonstrated that softwood kraft lignin is susceptible to molecular mass changes already at temperatures above ∼120 ◦ C (Cui et al., 2013). Water elimination with formation of unsaturated side chain carbons together with thermally induced formation of radical centers in aromatic rings and subsequent coupling reactions might well explain these observations (Nakamura et al., 2007). Further support can be found in the thermogravimetric analysis where it has been shown that for both hardwood and softwood kraft lignins, a minor weight loss occurs at temperatures below 200 ◦ C (Brodin et al., 2010). The thermal behavior of various technical lignins makes accurate determination of the glass transition temperature (Tg ) somewhat uncertain since, in order to eliminate any thermal prehistory, the DSC analysis usually involves preheating of the sample prior to actual measurement. Thus, depending on the exact conditions for this preheating cycle, structural changes in the lignin

d e

Tg ,◦ C n.d. 155 148 140 158 153 180

Td ,◦ Ca 306c n.d. 267 280c 305c 235 256

Reference Yoshida et al. (1987a) Kubo and Kadla (2005) Brodin et al. (2009) Bridson et al. (2013) “ Cui et al. (2013)

At 5% weight loss. Manufactured by Mead-Westvaco, Charleston SC. At 10% weight loss. After extrusion at 175 ◦ C. After heating of the lignin for 60 min at 173 ◦ C.

may occur and affect the Tg -determination as illustrated in Table 4. Also the thermogravimetric analysis (TGA) seems to be affected by the thermal history of the lignin sample and heat-induced chemical changes prior to measurement results in increased decomposition temperature (Td ) (Table 4). On further heating of (softwood kraft) lignin beyond the Td -temperature, the yield loss accelerates and between ∼300–600 ◦ C most of the material (∼60%) evaporates leaving a carbonaceous residue. On pre-treatment of the lignin in air at about 250◦ C, oxidative reactions will, however, accompany water elimination and coupling reactions resulting in a kraft lignin with improved thermal stability in a subsequent heating cycle (Brodin et al., 2010). 9. Concluding remarks The heterogenic structure of technical lignins including softwood kraft lignin has been recognized a long time ago. Despite this, only few examples exist in which some kind of pre-fractionation of the lignin has been done in order to improve the properties when applied in various material systems (Yoshida et al., 1987b; Gosselink et al., 2010; Baker et al., 2012; Norberg et al., 2013; Argyropoulos et al., 2014; Gao et al., 2015; Griffini et al., 2015). The development of technically feasible and robust fractionation methods to separate out the very low molecular mass lignin including a variety of rather hydrophilic lignin-based carboxylic acids (Gierer and Lindeberg, 1980; Gierer and Wännström, 1984) accompanied by the selective isolation of the major portion of the lignin in pure form seems, however, desirable for use of the lignin in materials such as polyurethanes (Yoshida et al., 1987b; Cheradame et al., 1989). At the high end of the lignin molecular mass distribution, the fraction containing carbohydrates may also have a negative influence on the reactivity of the lignin since the solubility characteristics are different. Therefore, it seems reasonable to assume that the use of lignin in material systems in which the lignin macromolecule will constitute an integral part of the polymer will require pre-fractionation to improve the homogeneity. In addition, such a fraction should have a proper reactivity for the material to be formed. A suitable molecular mass, low degree of polydispersity and an even distribution of functional groups should constitute desirable prerequisites. In most cases where the lignin is degraded to low molecular mass products such as phenols, aromatic hydrocarbons, bio-oil, or active carbon, pre-extraction seems unnecessary although purity of the lignin may still be important. In the formation of carbon fiber for construction purposes, on the other hand, elimination of the low molecular mass lignin fraction as well as the LCC fraction is again an important issue. It was recently shown that the addition of low molecular mass hardwood kraft lignin to softwood kraft lignin greatly improved the spinnability in melt spinning (Sjöholm et al., 2012; Norberg et al., 2013). Thus, the low molecular mass

G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

(hardwood) lignin may act as a softening agent but the effects of simultaneous presence of low molecular mass softwood lignin is still not known and will require further study. References Adler, E., 1977. Lignin chemistry—past, present and future. Wood Sci. Technol. 11, 169–218. Araki, H., Tomimura, Y., Terashima, N., 1980. Radiotracer experiments on lignin reactions: VI. The polymerization of lignin during pulping involving terminal hydroxymethyl group of phenylpropane side chain. Mokuzai Gakkaishi 26, 102–106. Argyropoulos, D.S., Sadeghifar, H., Cui, C., Sen, S., 2014. Synthesis and characterization of poly(arylene ether sulfone) kraft lignin heat stable copolymers. ACS Sustain. Chem. Eng. 2, 264–271. Asikkala, J., Tamminen, T., Argyropoulos, D.S., 2012. Accurate and reproducible determination of lignin molar mass by acetobromination. J. Agric. Food Chem. 60, 8968–8973. Baker, D.A., Gallego, N.C., Baker, F.S., 2012. On the characterization and spinning of an organic-purified lignin: towards the manufacture of low-cost carbon fiber. J. Appl. Polym. Sci. 124, 227–234. Ball, F.J., Vardell, W.G., 1961. Decantation of Lignin. US Patent 2997466. Ball, F.J., Dimitri, M.S., Schmut, R., 1965. Precipitated Lignin and Products Containing Same and the Production Thereof. US patent 3223697. Baumberger, S., Abaecherli, A., Fasching, M., Gellerstedt, G., Gosselink, R., Hortling, B., Li, J., Saake, B., de Jong, E., 2007. Molar mass determination of lignins by size-exclusion chromatography: towards standardisation of the method. Holzforschung 61, 459–468. Berlin, A., Balakshin, M., 2014. Industrial lignins: analysis, properties and applications. In: Gupta, V.G., Tuohy, M., Kubicek, C.P., Saddler, J., Xu, F. (Eds.), Bioenergy Research: Advances and Applications. Elsevier, 315-336. Berthold, F., Lindfors, E.-L., Gellerstedt, G., 1998. Degradation of guaiacylglycerol-␤-guaiacyl ether in the presence of NaHS or polysulphide at various alkalinities. Part II. Liberation of coniferyl alcohol and sulphur. Holzforschung 52, 481–489. Bridson, J.H., van de Pas, D.J., Fernyhough, A., 2013. Succinylation of three different lignins by reactive extrusion. J. Appl. Polym. Sci. 128, 4355–4360. Brodin, I., Sjöholm, E., Gellerstedt, G., 2009. Kraft lignin as feedstock for chemical products: the effects of membrane filtration. Holzforschung 63, 290–297. Brodin, I., Sjöholm, E., Gellerstedt, G., 2010. The behavior of kraft lignin during thermal treatment. J. Anal. Appl. Pyrolysis 87, 70–77. Brunow, G., Miksche, G.E., 1972. Uber den Abbau des Lignins bei der Sulfatkochung: III. Zur Bildung von Coniferylalkohol aus Guajacylglycerin-␤-arylätherstrukturen. Acta Chem. Scand. 26, 1123–1129. Brunow, G., Lundquist, K., Gellerstedt, G., 1999. In: Sjöström, E., Alén, R. (Eds.), Analytical Methods in Wood Chemistry, Pulping and Papermaking. Springer-Verlag, Germany, pp. 77–124. Cheradame, H., Detoisien, M., Gandini, A., Pla, F., Roux, G., 1989. Polyurethane from kraft lignin. Br. Polym. J. 21, 269–275. Cui, C., Sadeghifar, H., Sen, S., Argyropoulos, D.S., 2013. Towards thermoplastic lignin polymers; Part II: Thermal & polymer characteristics of kraft lignin & derivatives. Bioresources 8, 864–886. Cui, C., Sun, R., Argyropoulos, D.S., 2014. Precipitation of softwood kraft lignin: Isolation of narrow fractions common to a variety of lignins. ACS Sustain. Chem. Eng. 2, 959–968. de Groote, R.A.M.C., Neumann, M.G., Lechat, J.R., Curvelo, A.A.S., Alaburda, J., 1987. The sulfomethylation of lignin. Tappi J. 70 (3), 139–140. Dimmel, D., Gellerstedt, G., 2010. Chemistry of Alkaline Pulping. In: Heitner, C., Dimmel, D., Schmidt, J.A. (Eds.), Lignin and Lignans. Advances in Chemistry. CRC Press, Boca Raton, FL, pp. 349–391. Dodd, A.P., Kadla, J.F., Straus, S.K., 2014. Characterization of fractions obtained from two softwood kraft lignins. ACS Sustain. Chem. Eng. 3, 103–110. Dong, D., Fricke, A.L., 1993. Investigation of optical effect of lignin solution and determination of Mw of kraft lignin by LALLS. J. Appl. Polym. Sci. 50, 1131–1140. Duval, A., Lawoko, M., 2014. A review on lignin-based polymeric, micro- and nano-structured materials. React. Funct. Polym. 85, 78–96. Duval, A., Vilaplana, F., Crestini, C., Lawoko, M., 2015. Solvent screening for the fractionation of industrial kraft lignin. Holzforschung. Freudenberg, K., 1968. The constitution and biosynthesis of lignin. In: Freudenberg, K., Neish, A.C. (Eds.), Constitution and Biosynthesis of Lignin. Springer-Verlag, Berlin, Heidelberg, Germany, pp. 47–122. Froass, P.M., Ragauskas, A.J., Jiang, J.E., 1998. NMR studies Part 3: analysis of lignins from modern kraft pulping technologies. Holzforschung 52, 385–390. Gandini, A., Belgacem, M.N., 2008. Lignin as components in macromolecular materials. In: Belgacem, M.N., Gandini, A. (Eds.), Monomers, Oligomers, Polymers and Composites from Renewable Resources. Elsevier Ltd, Oxford, GB, pp. 243–271. Gao, G., Ko, F., Kadla, J.F., 2015. Synthesis of noble monometal and bimetal-modified lignin nanofibers through surface-grafted poly(2-(dimethylamino) ethyl methacrylate) brushes. Macromol. Mater. Eng., http://dx.doi.org/10.1002/mame.201500006. Gellerstedt, G., 1992. Gel permeation chromatography. In: Lin, S.Y., Dence, C.V. (Eds.), Methods in Lignin Chemistry. Springer-Verlag, Berlin, Heidelberg, Germany, pp. 487–497.

853

Gellerstedt, G., Lindfors, E.-L., 1984. Structural changes in lignin during kraft pulping. Holzforschung 38, 151–158. Gellerstedt, G., Lindfors, E.-L., Lapierre, C., Monties, B., 1984. Structural changes in lignin during kraft pulping. Part 2. Characterization by Acidolysis. Svensk Papperstidn 87, R61–R67. Gellerstedt, G., Gustafsson, K., 1987. Structural changes in lignin during kraft cooking. part 5. Analysis of dissolved lignin by oxidative degradation. J. Wood Chem. Technol. 7, 65–80. Gellerstedt, G., Lindfors, E.-L., 1987. On the formation of enol ether structures in lignin during kraft cooking. Nordic Pulp Paper Res. J. 2, 71–75. Gellerstedt, G., Robert, D., 1987. Quantitative 13 C NMR analysis of kraft lignins. Acta Chem. Scand. B41, 541–546. Gellerstedt, G., Wafa Al-Dajani, W., 2003. Some factors affecting the brightness and TCF-bleachability of kraft pulp. Nordic Pulp Paper Res. J. 18, 56–62. Gellerstedt, G., Majtnerova, A., Zhang, L., 2004. Towards a new concept of lignin condensation in kraft pulping. Initial Results C. R. Biol. 327, 817–826. Gellerstedt, G., Henriksson, G., 2008. Lignins: major Sources, structure and properties. In: Belgacem, M.N., Gandini, A. (Eds.), Monomers, Oligomers, Polymers and Composites from Renewable Resources. Elsevier Ltd, Oxford GB, pp. 201–224. Gellerstedt, G., Sjöholm, E., Brodin, I., 2010. The wood-based biorefinery: a source of carbon fiber? Open Agric. J. 3, 119–124. Gellerstedt, G., Tomani, P., Axegård, P., Backlund, B., 2013. Lignin recovery and lignin-based products. In: Christopher, L.P. (Ed.), Integrated Forest Biorefineries. RSC Publishing, Cambridge UK, pp. 180–210. Gierer, J., Pettersson, I., Smedman, L-Å., 1972. The reactions of lignin during sulphate pulping. Part XII. Reactions of intermediary o,p -dihydroxy-stilbene structures. Acta Chem. Scand. 26, 3366–3376. Gierer, J., Pettersson, I., Smedman, L-Å., Wennberg, I., 1973. The reactions of lignin during sulphate pulping: Part XIII. Reactions of episulphide structures with alkali and with white liquor under pulping conditions. Acta Chem. Scand. 27, 2083–2094. Gierer, J., Lindeberg, O., 1978. Reactions of lignin during sulfate pulping: Part XV. The Behaviour of intermediary coniferyl alcohol structures. Acta Chem. Scand. B32, 577–587. Gierer, J., Lindeberg, O., 1980. Reactions of lignin during sulfate pulping. Part XIX. Isolation and identification of new dimers from spent sulfate liquor. Acta Chem. Scand. 4, 161–170. Gierer, J., Wännström, S., 1984. Formation of lignin and carbohydrate fragments during kraft pulping. Holzforschung 38, 181–184. Gonzalo Epelde, I., Lindgren, C.T., Lindström, M.E., 1998. Kinetics of Wheat Straw delignification in soda and kraft pulping. J. Wood Chem. Technol. 18, 69–82. Gosselink, R.J.A., van Dam, J.E.G., de Jong, E., Scott, E.L., Sanders, J.P.M., Li, J., Gellerstedt, G., 2010. Fractionation, analysis and PCA of properties of four technical lignins for prediction of their application potential in binders. Holzforschung 64, 193–200. Grace, T.M., Malcolm, E.W., 1989. In: Kocurek, M.J. (Ed.), Alkaline Pulping, vol. 5. Pulp and Paper Manufacture, TAPPI, Atlanta; CPPA, Montreal, pp 15–17; pp 45–54. Griffini, G., Passoni, V., Suriano, R., Levi, M., Turri, S., 2015. Polyurethane coatings based on chemically unmodified fractionated lignin. ACS Sustain. Chem. Eng., http://dx.doi.org/10.1021/acssuschemeng.5b00073. Helander, M., Theliander, H., Lawoko, M., Henriksson, G., Zhang, L., Lindström, M.E., 2013. Fractionation of technical lignin: molecular mass and pH effects. BioResources 8, 2270–2282. Hise, R.G., Chen, C.-L., Gratzl, J.S., 1983. Chemistry of Phenolic ␤-Aryl Lignin Structures in Alkaline Pulping. In: 1983 International Symposium on Wood and Pulping Chemistry, Tsukuba Science City, Japan, Proceedings, vol. 2, 166–171. Jacobs, A., Dahlman, O., 2000. Absolute molar mass of lignins by size exclusion chromatography and MALDI-TOF mass spectroscopy. Nordic Pulp Paper Res. J. 15, 120–127. Kadla, J.F., Kubo, S., Venditti, R.A., Gilbert, R.D., Compere, A.L., Griffith, W., 2002. Lignin-based carbon fibers for composite fiber applications. Carbon 40, 2913–2920. Kanungo, D., Francis, R.C., Shin, N.-H., 2009. Mechanistic differences between kraft and soda/AQ pulping. Part 2: results from lignin model compounds. J. Wood Chem. Technol. 29, 227–240. Keilen, J.J., Ball, F.J., Gressang, R.W., 1952 Method of Coagulating Colloidal Lignates in Aqueous Dispersions. US Patent 2623040. Kleinert, T.N., 1965. Lignin grafting onto cellulose during alkaline pulping. Holzforschung 19, 179–183. Kringstad, K.P., Månsson, P., Mörck, R., 1981. Changes in the molecular weight distribution of kraft lignins resulting from various chemical treatments. 1st International Symposium on Wood and Pulping Chemistry, Proceedings, Part V, 91–93. Kubo, S., Kadla, J.F., 2005. Kraft lignin/poly(ethyleneoxide) blends: effect of lignin structure on miscibility and hydrogen bonding. J. Appl. Polym. Sci. 98, 1437–1444. Lawoko, M., Berggren, R., Berthold, F., Henriksson, G., Gellerstedt, G., 2004. Changes in the lignin-carbohydrate complex in softwood kraft pulp during kraft and oxygen delignification. Holzforschung 58, 603–610. Lawoko, M., Henriksson, G., Gellerstedt, G., 2005. Structural differences between the lignin-carbohydrate complexes present in wood and in chemical pulps. Biomacromolecules 6, 3467–3473.

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G. Gellerstedt / Industrial Crops and Products 77 (2015) 845–854

Lawoko, M., Henriksson, G., Gellerstedt, G., 2006. Characterization of lignin-carbohydrate complexes (LCCs) of spruce wood (Picea Abies L.) isolated with two methods. Holzforschung 60, 156–161. Lindgren, C.T., Lindström, M.E., 1996. The kinetics of residual delignification and factors affecting the amount of residual lignin during kraft pulping. J. Pulp Paper Sci. 22, J290–J295. Ljunggren, S., 1980. The significance of aryl ether cleavage in kraft delignification of softwood. Svensk Papperstidn 83, 363–369. Lora, J., 2008. Industrial commercial lignins: sources, properties and applications. In: Belgacem, M.N., Gandini, A. (Eds.), Monomers, Polymers and Composites from Renewable Resources. Elsevier Ltd., Oxford, GB, pp. 225–241. Majtnerova, A., Gellerstedt, G., 2006. Radical coupling—a major obstacle to delignification in kraft pulping. Nordic Pulp Papper Res. J. 21, 129–134. Marton, J., 1964. On the structure of kraft lignin. Tappi 47 (11), 713–719. Marton, J., 1971. Reactions in alkaline pulping. In: Sarkanen, K.V., Ludwig, C.H. (Eds.), Lignins. Occurrence, Formation, Structure and Reactions. Wiley-Interscience, New York, USA, pp. 639–689. Marton, J., Marton, T., 1964. Molecular weight of kraft lignin. Tappi 47, 471–476. Mörck, R., Yoshida, H., Kringstad, K.P., Hatakeyama, H., 1986. Fractionation of kraft lignin by successive extraction with organic solvents. I. Functional groups, 13 C-NMR spectra and molecular weight distributions. Holzforschung 40 (suppl), 51–60. Nakamura, T., Kawamoto, H., Saka, S., 2007. Condensation reactions of some lignin related compounds at relatively low pyrolysis temperature. J. Wood Chem. Technol. 27, 121–133. Niemelä, K., 1988. GLC–MS studies on pine kraft black liquors. Part 1. Identification of monomeric compounds. Holzforschung 42, 169–173. Nimz, H.H., 1966. Ätherspaltungen an Lignin-Modellsubstanzen durch Acetolyse. Justus Liebigs Ann. Chem. 691, 126–133. Norberg, I., Nordström, Y., Drougge, R., Gellerstedt, G., Sjöholm, E., 2013. A new method for stabilizing softwood kraft lignin fibers for carbon fiber production. J. Appl. Polym. Sci. 128, 3824–3830. Norgren, M., Edlund, H., Wågberg, L., 2002. Aggregation of lignin derivatives under alkaline conditions: kinetics and aggregate structure. Langmuir 18, 2859–2865. Öhman, F., Theliander, H., Tomani, P., Axegård, P., (2006) Method for Separating Lignin from Black Liquor. Patent WO 2006/031175. Önnerud, H., 2003. lignin structures in normal and compression wood: evaluation by thioacidolysis using ethanethiol and methanethiol. Holzforschung 57, 377–384. Önnerud, H., Gellerstedt, G., 2003. Inhomogeneities in the chemical structure of spruce lignin. Holzforschung 57, 165–170. Peng, J., Lu, F., Ralph, J., 1998. The DFRC method for lignin analysis: 4. lignin dimers isolated from DFRC-degraded loblolly pine wood. J. Agric. Food Chem. 46, 553–560. Ragauskas, A.J., Beckham, G.T., Biddy, M.J., Chandra, R., Chen, F., Davis, M.F., Davison, B.H., Dixon, R.A., Gilna, P., Keller, M., Langan, P., Naskar, A.K., Saddler, J.N., Tschaplinski, T.J., Tuskan, G.A., Wyman, C.E., 2014. Lignin valorization: improving lignin processing in the biorefinery. Science 344, 1246843.

Ralph, J., Lundquist, K., Brunow, G., Lu, F., Kim, H., Schatz, P.F., Marita, J.M., Hatfield, R.D., Ralph, S.A., Christensen, J.H., Boerjan, W., 2004. Lignins: natural polymers from oxidative coupling of 4-hydroxyphenyl-propanoids. Phytochem. Rev. 3, 29–60. Robert, D.R., Bardet, M., Gellerstedt, G., Lindfors, E.-L., 1984. Structural changes in lignin during kraft coking. Part 3. On the structure of dissolved lignins. J. Wood Chem. Technol. 4, 239–263. Sarkanen, S., Teller, D.C., Stevens, C.R., McCarthy, J.L., 1984. Lignin: 20 associative interactions between kraft lignin components. Macromol 17, 2588–2597. Sevastyanova, O., Helander, M., Chowdhury, S., Lange, H., Wedin, H., Zhang, L., Ek, M., Kadla, J.F., Crestini, C., Lindström, M.E., 2014. Tailoring the molecular and thermo-mechanical properties of kraft lignin by ultrafiltration. J. Appl. Polym. Sci. 131, 9505–9515. Saito, T., Perkins, J.H., Vautard, F., Meyer, H.M., Messman, J.M., Tolnai, B., Naskar, A.K., 2014. Methanol fractionation of softwood kraft lignin: impact on the lignin properties. ChemSusChem 7, 221–228. Sjöholm, E., Gustafsson, K., Colmsjö, A., 1999. Size exclusion chromatography of lignins using lithium chloride/N,N-dimethylacetamide as mobile phase. II. dissolved and residual pine kraft lignin. J. Liq. Chrom. Rel. Technol. 22, 2837–2854. Sjöholm, E., Gellerstedt, G., Drougge, R., Brodin, I., (2012) Method for Producing a Lignin Fiber. Patent appl WO 2012,112,108 A1. Smith, D.A., Dimmel, D.R., 1994. Electron transfer reactions in pulping systems: IX. Reactions of syringyl alcohol with pulping reagents. J. Wood Chem. Technol. 14, 297–313. Suckling, I.D., Pasco, M.F., Hortling, B., Sundquist, J., 1994. Assessment of lignin condensation by GPC analysis of lignin thioacidolysis products. Holzforschung 48, 501–503. Tomani, P., 2010. The LignoBoost process. Cell Chem. Technol. 44, 53–58. Uraki, Y., Kubo, S., Nigo, N., Sano, Y., Sasaya, T., 1995. Preparation of carbon fibers from organosolv lignin obtained by aqueous acetic acid pulping. Holzforschung 49, 343–350. Yoshida, H., Mörck, R., Kringstad, K.P., Hatakeyama, H., 1987a. Fractionation of kraft lignin by successive extraction with organic solvents: II. Thermal properties of kraft lignin fractions. Holzforschung 41, 171–176. Yoshida, H., Mörck, R., Kringstad, K.P., Hatakeyama, H., 1987b. Kraft lignin in polyurethanes I: Mechanical properties of polyurethanes from a kraft lignin–polyether triol–polymeric MDI system. J. Appl. Polym. Sci. 34, 1187–1198. Zhang, L., Gellerstedt, G., 2001. Lignin fractionation and quantitative structural analysis of the lignin fractions with 13 C and HSQC NMR techniques. In: Grenoble Workshop on Advanced Methods for Lignocellulosics and Paper Products Characterization, Grenoble, France, Proceedings, 41-46. Zhang, L., Henriksson, G., Gellerstedt, G., 2003. The Formation of ␤–␤ Structures in lignin biosynthesis—are there two different pathways? Org. Biomol. Chem. 1, 3621–3624. Zhao, L.-W., Griggs, B.F., Chen, C.-L., Gratzl, J.S., 1994. Utilization of softwood kraft lignin as adhesive for the manufacture of reconstituted wood. J. Wood Sci. Technol. 14, 127–145.