The composition, molecular weight (MW), and chemical structure of technical lignins as byproducts of pulping influence their application in terms of physical and chemical properties, reactivity, and performance. It is important to know how the analytical data of technical lignins are influenced by the wood species and the parameters of pulping. The present study focuses on kraft pulping and how the wood species (eucalyptus, pine, and spruce) and variable cooking times influence the characteristics of dissolved lignins. The black liquor (BL) was recovered after three different cooking times and the precipitated lignin was characterized by total acid hydrolysis including the determination of the acid insoluble part (Klason lignin, KL) and the sugars in the hydrolysate, elemental analysis, 31P NMR spectroscopy, analytical pyrolysis (Py-GC/MS), gel permeation chromatography (GPC), thermogravimetry (TG), and differential scanning calorimetry (DSC). The results indicate that the phenolic OH content, MW and glass transition temperature increased with longer cooking times for the softwood (SW) lignins. These lignins had also a higher MW (Mw 5500–8000 g mol-1), than the eucalyptus lignin (Mw 2200–2400 g mol-1). Eucalyptus lignin had higher sulfur content compared to SW.
Lignin is present in nearly all plant materials and thus it is the most abundant natural aromatic polymer. The lignin macromolecules contain a variety of functional groups, such as phenolic and aliphatic hydroxyl, carbonyl, and methoxy groups. It has a high potential for production of various chemicals and materials (Ragauskas et al. 2014). The kraft pulping process gives rise to an easily available technical lignin (kraft lignin), but only 2% of kraft lignin is isolated for utilization as a material; usually it is an important source of energy in the course of recovery of cooking chemicals (Gosselink et al. 2004). The main obstacle to the commercial use of lignins is their heterogeneity concerning the functional groups and molecular weight distribution (MWD), while the technical isolation process additionally influences their natural heterogeneity.
As the main product of the kraft process is the pulp, research is focused on the optimization of the pulp properties and not on those of the byproduct kraft lignin. Studies with kraft lignin in focus are mainly in close relation to the delignification mechanism (Marton 1964; Marton and Marton 1964; Gellerstedt and Lindfors 1984; Sjöholm et al. 1993; Jacobs and Dahlman 2001; Dodd et al. 2015), but there are also data available concerning the recovery and fractionation of lignins (Toledano et al. 2010; Saito et al. 2014; Sevastyanova et al. 2014; Alekhina et al. 2015; Duval et al. 2016). A commercially available kraft lignin recovery process that is suitable for industrial applications is the LignoBoost process, in the course of which a part of the kraft lignin dissolved in the black liquor (BL) is precipitated by acidification, washed, and dried resulting in a solid lignin powder (Tomani 2010). It is important to emphasize that the efforts for improving the lignin quality should not jeopardize the pulp quality as the main product.
The properties of kraft lignins are difficult to compare because the literature data are widely distributed and very different methods were applied for their characterization. Thus the aim of the present study was to investigate the effect of various kraft cooking conditions, with emphasis on the cooking time, and on eucalyptus, pine, and spruce woods concerning the characteristics of dissolved lignins. The degradation method analytical pyrolysis (Py-GC/MS) was included into the extensive characterization protocol as this technique delivers conveniently relevant data from cross-linked polymers with low solubility, which are well suitable for comparative purposes. The expectation is that an improved knowledge with this regard could contribute to the optimization of the quality of the dissolved lignin without compromising pulp properties.
Materials and methods
Fresh pine (Pinus sylvestris) and spruce (Picea abies) chips (softwoods, SW) and dried eucalyptus (Eucalyptus urograndis) chips (with 9% moisture content, MC) were obtained from a Swedish pulp mill. The fresh SW chips were dried in an oven at 65°C for 24 h to a dryness of 95%. The dry chips were sorted manually to obtain fractions without any residual bark or knots and with a thickness between 2 and 8 mm.
Kraft cooking stock solutions were prepared by dissolving NaOH pastilles of puriss grade (VWR International AB) and technical grade flakes of Na2S hydrate with purity >60% (VWR International AB) in deionized water. The NaCl was of puriss grade (VWR International AB). H2SO4 (72%, LabService AB) was used for acid hydrolysis and lignin precipitation.
Kraft pulping was performed in steel autoclaves (2.5 dm3) filled with 250 g o.d. chips. The filled autoclaves were evacuated for 30 min and filled with the liquor via suction. The liquor:wood ratio was 4 dm3 kg-1 wood. For the SW cooking, the initial [HO-] in the cooking liquor was 1.2 M and the initial [HS-] was 0.26 M. For the eucalyptus wood cooking, the initial [HO-] was 1.0 M and the initial [HS-] was 0.26 M. The autoclaves were heated in a glycol bath (157°C) under rotation.
The beginning of the cooking time was defined after an initial heating time of 10 min. After specific cooking times (100, 200, and 260 min for SWs and 30, 60 and 100 min for eucalyptus), the cooking was terminated by cooling the autoclaves in a water bath. The BL was separated from the chips by filtration through a sieve and subsequent pressing of the delignified chips. For each wood species and cooking time, BL from two autoclaves was collected and pooled. Before precipitation, the BL was stored at 4°C for 12–15 h. The delignified chips were washed in de-ionized water for 12–15 h.
For lignin precipitation, the BLs were heated to 70°C (SW) or to 60°C (eucalyptus). The precipitation procedure was performed according to Lin and Dence (1992). Twenty percent H2SO4 was added during rapid magnetic stirring until pH 9. The acidified BL was then cooled in an ice bath and stored in a fridge overnight for lignin precipitation. Lignin was filtered through a Büchner funnel (20 μm). The filter cake was re-dispersed in de-ionized water, and the pH was lowered to pH 2 by addition of H2SO4 under rapid magnetic stirring. The lignin was filtered again and dried in a ventilated oven at 65°C for 12 h.
The delignified chips after kraft cooking were washed in deionized water over night. After the longest cooking time, the delignified chips were defibrated into pulp in a water jet defibrator. The pulp and the delignified chips were dried at 40°C until reaching ca. 92% dry content.
The kappa number (KN) of pulps obtained after the longest cooking time was determined according to ISO 302:2004. The Klason lignin (KL) content and carbohydrate composition were analyzed according to SCAN-CM 71:09 on 200 mg of wood and pulp samples and on ca. 80 mg of BL precipitates. The carbohydrate content was determined in the hydrolysate by high-performance anion exchange chromatograph equipped with pulsed amperiometric detection (HPAEC-PAD) and with a CarboPac PA1 column (Dionex, Sunnyvale, CA, USA). The reported data are averages of two determinations (with two samples). The coefficient of variation (mean value divided by StD) was <10%. The total dry solid content in the BL was determined by drying 10 ml of the BL at 105°C. The ash content was determined by placing 0.3 g of the dry solids into an ash oven at 500°C for 6 h. The presented results are averages of two samples.
For elemental analysis, homogenized lignin samples (30 mg) were packed in a tin foil, weighed, placed into the carousel of an automatic sample feeder and analyzed with an Elementar Analysensysteme GmbH (Germany) Vario MACRO CHNS. The oxygen content was calculated by difference to 100. The results are from triplicate experiments.
Analytical pyrolysis, (pyrolysis followed by gas chromatography coupled with mass spectrometry, Py-GC/MS), was performed by means of a Frontier Lab (Japan) Micro Double-shot Pyrolyser Py-2020iD, at 500°C and a heating rate of 600°C s-1. The instrument was coupled directly to a Shimadzu (Japan) GC/MS-QP 2010 apparatus (capillary column RTX-1701, Restek, Bellafonte, PA, USA with the dimension of 60 m×0.25 mm×0.25 μm film thickness). Injection temperature 250°C, ionization energy 70 eV (EI mode), and the MS scan range was m/z 15–350. Flow rate of He: 1 ml min-1; split ratio: 1:30. The loaded sample size was between 1.0 and 2.0 mg. Temperature program: 1 min isothermal at 60°C, →270°C with 6°C min-1 →10 min at 270°C. Substance identification: Library MS NIST 147.LI13. The relative areas of the peaks was calculated by the Shimadzu software and, if necessary, corrected or integrated manually on the basis of the GC/MS data. The relevant peaks were averaged from the two samples. The summed molar areas of the relevant peaks were normalized to 100%, while the peak areas were also summed separately as originating from lignin, carbohydrates, and lipophilic extractives.
The MWDs of the lignin samples were determined by the gel permeation chromatography (GPC) according to Guerra et al. (2006). The samples were acetobrominated prior to analysis by stirring a 5 mg sample for 2 h at room temperature (r.t.) in 1 ml of a 9/1 (v/v) mixture of glacial acetic acid and neat acetyl bromide before surplus reagents were removed rapidly in vacuo. The residue was dissolved in 1 ml of high-performance liquid chromatography (HPLC) grade tetrahydrofuran (THF), and the resulting solution was filtered through a 0.45 μm syringe filter. GPC analysis was performed in a Waters instrument system (Waters Sverige AB, Sollentuna, Sweden) consisting of a 515 HPLC-pump, 2707 autosampler, and 2998 photodiode array detector (operated at 254 and 280 nm). HPLC-grade THF, filtered through a 0.2 μm polytetrafluoroethylene membrane filter and degassed, served as the mobile phase with a flow of 0.3 ml min-1. Separation was achieved on Waters Ultrastyragel HR4, HR2 and HR0.5 4.6×300 mm solvent-efficient columns connected in series and operated at 35°C. For analysis, a sample volume of 20 μl was injected via the technique “partial loop needle overfill injection”. Data was collected at both 254 and 280 nm to ensure minimal peak drift. Calibration was performed at 254 nm with polystyrene standards with nominal MWs in the range 480–176 000 Da. Quantification was performed with the Waters Empower® 3 build 3471 software suite.
Phosphorus-31 NMR (31P NMR) analysis, with a 90° pulse angle, an inverse gated proton decoupling and a delay time of 10 s, was used for the identification and quantification of the OH and COOH groups. Prior to analysis, a 20 mg sample of each lignin fraction was functionalized for 2 h at r.t. with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane in a 1/1.6 mixture (V/V) of CDCl3 and pyridine (Granata and Argyropoulos 1995).
The metal analysis was performed by Eurofins Environment Sweden AB (Lidköping), Sweden.
Thermogravimetric analysis (TGA) on lignin samples was carried out with TGA/SDTA 851 Mettler Toledo (Mettler Toledo, Columbus, OH) instrument equipped with STAR software for data analysis. To determine the thermal degradation behavior of lignins, 5–10 mg was tested under N2 atmosphere (50 ml min-1) at a heating rate of 10°C min-1 from 25 to 800°C. Differential scanning calorimetry (DSC) was performed on a Mettler Toledo DSC 820 instrument. Each sample was heated under N2 atmosphere (50 ml min-1) at a heating rate of 10°C min-1 from 25 to 150°C, cooled down again to 25°C (to delete a thermal history of samples) and heated again over a temperature range from 25 to 300°C. All measurements were made with 2–3 mg samples placed in standard aluminum pans under N2 atmosphere. Glass transition temperature, Tg, was recorded at the midpoint temperature of the heat capacity transition of the second heating run by the Mettler software. The presented results are an average of two runs.
Results and discussion
The BL and the delignified residues from kraft cooks of eucalyptus, pine, and spruce were collected after different cooking times and analyzed (Table 1). Consistent with existing knowledge, the softwoods (SW) have higher lignin and galactoglucomannan (GGM) contents, whereas the xylan content is higher in the eucalyptus. Expectedly, increased cooking time leads to decreased lignin and increased cellulose contents in the pulps as visible on the higher glucose fraction in the delignified chips.
|Species||Cooking time (min)||Kappa number and results of acid hydrolysis|
|KN||KL (%)||Ara (%)||Gal (%)||Glc (%)||Xyl (%)||Man (%)|
KL, Acid insoluble “Klason lignin”; Ara, arabinose; Gal, galactose; Glc, glucose; Xyl, xylose; Man, mannose. Kappa number (KN) determination and total acid hydrolysis followed by AE-HPLC analysis of the hydrolysates. Zero minutes cooking time refers to the extractive-free wood. Percent data are based on dry wood or pulp.
The lignin in the BL was precipitated by acidification to pH 9. It is known that several parameters leads to higher lignin yields, such as lower pH (Alén et al. 1979; Zhu et al. 2014), higher dry solids content in the BL (Zhu et al. 2015), lower temperature (Alén et al. 1979; Zhu et al. 2014), and higher ionic strength (Alén et al. 1979). In the current study, the precipitation conditions were not optimized; the only parameter changed was the lower precipitation temperature for eucalyptus BL (60°C) compared to 70°C in case of SWs. As shown in Table 2, it was not possible to obtain any precipitates after the shortest cooking time of eucalyptus. Either the size of the lignin particles precipitated was too small or the lignin did not agglomerate. It was observed that a longer cooking time resulted in a more easily filterable lignin precipitate. The acid insoluble lignin content, designated as KL, was higher in case of the eucalyptus BL precipitates than in case of SW precipitates. Approximately 72% of the eucalyptus precipitates consisted of lignin, whereas only half of the SW precipitates can be considered as KL.
|Species||Cooking time (min)||Lignin precip. (g kg-1)||KL (%)||C (%)||Ash (%)||Results of sugar analysis. Relative carbohydrate compostion (%)|
The composition of the carbohydrates and the relative amount of different monosugars is also presented in Table 2. The carbohydrates co-precipitated with the lignin reflect the main hemicelluloses present in the wood species. Lignins from SW pulping have a higher mannose content, presumably from co-precipitated GGMs and a larger amount of arabinose (from the arabinoxylan). Lignin precipitates from eucalyptus pulping have significantly higher xylan content and less arabinose substituents.
All lignins have very high galactose content. Even in the case of SWs with a high GGM content, the galactose exceeded the amount expected based on the mannose:galactose ratio. For eucalyptus wood, the mannose:galactose ratio was approximately 1:1, while for the SWs the ratio was 7:1 (spruce) and 9:1 (pine). In the precipitates, however, the galactose content was 5–10 times higher than that of the mannose. The relative amount of galactose increased with cooking time, whereas the mannose content decreased. High galactose contents in kraft lignin have been reported (Gellerstedt and Lindfors 1984; Robert et al. 1984; Zhu et al. 2014; Alekhina et al. 2015). In the present study, no extensive purification of the precipitates was performed with the exception of an acid wash at pH 2. According to Gellerstedt and Lindfors (1984), the galactose content remains high even after several purification steps. It is likely that lignin-carbohydrate complexes (LCCs) are linked via galactose units and, as the lignin is dissolved from the wood matrix, the galactose units remain connected to the lignin (Lawoko et al. 2006). LCCs remaining in pulp have also been shown to be highly enriched in galactose (Laine et al. 2004).
Py-GC/MS is in the meanwhile an established rapid and highly sensitive technique for studying the chemical composition and structure of complex and insoluble natural materials, including lignocelluloses and technical lignins (Faix et al. 1992; Meier and Faix 1992; Gutierrez et al. 1996; Rodriges et al. 2001; Ohra-aho et al. 2005; Ponomarenko et al. 2015). The chemical composition of the precipitates was determined by Py-GC/MS, and the list of the compounds detected is presented in Table 3. Accordingly, the content of lignin derivatives was in the range of 55–65% and a higher lignin portion was detected in the SW precipitates. This is mainly due to the higher content of acid soluble lignin in hardwoods (Musha and Goring 1975).
|60 min||100 min||100 min||200 min||260 min||100 min||200 min||260 min|
|Lignin derivatives (%)a||55.5||53.7||62.7||61.0||61.1||63.3||62.8||63.0|
|Compounds with S (%)a||9.2||7.4||2.6||2.8||3.2||3.9||3.8||4.0|
|Not identified (%)a||2.9||3.4||4.3||2.6||4.0||4.4||3.7||4.4|
aRelative %; baliphatic, aromatic and cyclic monomers.
Table 4 shows essential thermal degradation products (obtained via Py-GC/MS) originating mainly from carbohydrate residues in the acid BL precipitates. The main difference between the SW and eucalyptus lignins was the amount of furans and aldehydes/ketones. Two-thirds of the carbohydrates in the eucalyptus precipitate were decomposed to furans (mainly to furfural) during pyrolysis, compared to 20–25% in the case of the SW precipitates, while the carbohydrates in the latter decomposed to a higher extent to aldehydes and ketones (30–35%). This type of degradation products amounted only to 6% in the case of eucalyptus. Similar results were reported by Dizhbite et al. (2011). These results reflect the differences in the type of hemicelluloses impurities present in the lignin samples of different origin.
|60 min||100 min||100 min||200 min||260 min||100 min||200 min||260 min|
|Propanoic acid, 2-methyl-||0.0||0.0||0.0||0.0||0.0||0.0|
|Propanoic acid, 2-oxo-, Me-ester||0.1||0.1||0.1||0.1||0.1||0.1|
|Aldehydes, Ketones (%)||0.3||0.3||0.8||1.2||0.9||1.1||1.5||1.1|
|Cyclopentane derivates (%)||0.2||0.2||0.5||0.7||0.6||0.6||0.7||0.6|
|2-Cyclopenten-1-one, 2-hydroxy- 3,4-dimethyl-||0.0||0.1||0.2||0.3||0.2||0.2||0.3||0.3|
Bold font indicates total values.
Concerning the lignin-type pyrolysis products, the pyrogram peaks were also normalized 100%; thus the individual peaks are presented as relative percentages (Table 5). The individual phenols are classified according to the structure of the side-chain and ortho-substitution in aromatic rings (Ponomarenko et al. 2015). Expectedly, the eucalyptus lignin is composed of both syringyl (S) and guiacyl (G) units with an S:G ratio of around 2. The SW lignins consists mainly of G units, (86–87%), while the remaining 13–14% are phenyl- and benzyl-type derivatives. The proportion of the different lignin units is not affected by the cooking time.
|Sample||Cooking time (min)||G units (%)||S units (%)||Units with phenolic OH and||(ArC1+ArC2)/ArC3c|
|Saturateda side chain (%)||Cα=Cβ bondsb (%)||Oxygen in side chains (%)|
aNon-substituted; bdouble bond between α and β carbon; cratio between phenols with 1 and 2 Cs and 3 Cs in the side chains.
The data are relative percentages calculated from the pyrograms.
The amount of phenols with non-substituted saturated side chains (such as 4-ethylguaiacol, 4-ethyl phenol) and with double bond in the α-position in the side chain (such as eugenol, iso-eugenol, sinapyl aldehyde, etc.) are lower in case of eucalyptus lignin. As cooking time increased, fewer units are present with oxygen containing groups (such as vanillin, acetovanillin, syringyl aldehyde, coniferyl aldehyde, etc.) in the side chain of phenolic units.
The Py-GC/MS observations are helpful for estimating the potential use of lignin. The high content of methoxylated phenyl units in lignin may have a positive effect on its antioxidant activity and CH2 groups in the α-position of the phenols aromatic ring Ar-CH3 (ArC1), Ar-CH2-CH3 (ArC2), Ar-CH2-CH2-CH3 (ArC3) can also increase the antioxidant activity of lignins due to their electron-donating properties (Ponomarenko et al. 2014 2015). Furthermore, the high ratio between the summed portion of phenols with shortened side chains (ArC1+ArC2) and phenols with propanoid side chain (ArC3) in the lignin pyrolysis products has a strong positive impact on lignin antioxidant activity. This value (ArC1+ArC2)/ArC3) was slightly higher for the eucalyptus lignin.
Table 6 presents the sulfur-containing decomposition products, as recorded by Py-GC/MS. The higher sulfur content of the eucalyptus lignin led to hydrogen sulfide and sulfur dioxide. As expected, methane-containing sulfur compounds are also significantly higher in the eucalyptus precipitates, because eucalyptus lignin contains mostly syringyl units which are rich in methoxy groups.
|Σ S-containing compounds (%)||9.2||7.4||2.6||2.8||3.2||3.9||3.8||4.0|
|H2S and SO2 (%)||6.3||4.9||1.9||2.1||2.6||3.3||3.1||3.3|
|Dimethyl sulfide and CS2 (%)||0.4||0.4||0.3||0.4||0.3||0.3||0.5||0.4|
|Disulfide, dimethyl- (%)||0.3||0.4||0.0||0.0||0.1||0.1||0.1||0.1|
The data are based on normalized pyrograms. Bold font indicates total values.
The elemental analysis (Table 7) also confirms the higher sulfur content in the eucalyptus precipitates compared to those of SWs. In case of SW lignins the elemental analysis and Py-GC/MS gave similar results, whereas in the case of eucalyptus a much higher sulfur content was recorded by Py-GC/MS, which is indicative for a preferred pyrolytic cleavage of sulfur containing lignin subunits.
|Species||Cooking time (min)||N (%)||C (%)||H (%)||S (%)|
According to the 31P NMR results (Table 8), the S:G ratio of the eucalyptus lignin is 2.4–2.45, which is higher than that derived from the Py-GC/MS results. This is a little surprising as analytical pyrolysis leads usually to elevated contents of syringyl-type degradation products, which should be corrected by multiplication of a factor <1 (Choi et al. 2001). For the SW lignins, it is apparent that the number of both the condensed structures and the OHphen groups increased and the number of OHaliph groups decreased with cooking time, which was also demonstrated previously (Alekhina et al. 2015). There is also a slight tendency for increasing COOH group contents as a function of cooking time.
|Species||Cooking time (min)||OHaliph (%)||Phenolic OH (%) in||-COOH (%)|
|Cond.||S units||G units|
The eucalyptus lignin precipitates are dark and greenish in color, whereas the SW lignin precipitates are brown. The possible influence of metals to the color can be seen in Table 9. Accordingly, the eucalyptus lignin precipitates contain more calcium, which is common for eucalyptus species (Ziesig et al. 2015). Moreover, this lignin also contains more aluminum, iron, magnesium, and manganese compared to the SW precipitates. Conversely, the spruce lignin has a very high barium content. The green color of the eucalyptus lignin might be due to ferrous hydroxide, Fe(OH)2, which is also green.
|Al (mg kg-1)||50||26||8.6|
|Ba (mg kg-1)||4.4||0.4||20|
|Pb (mg kg-1)||0.3||0.1||0.2|
|B (mg kg-1)||7.8||<5.0||13|
|P (mg kg-1)||52||<52||<53|
|Fe (mg kg-1)||58||10||12|
|Ca (mg kg-1)||1800||30||47|
|K (mg kg-1)||270||<110||180|
|Co (mg kg-1)||0.1||0.0||0.1|
|Cu (mg kg-1)||1.2||1.8||1.7|
|Cr (mg kg-1)||4.4||2.0||2.4|
|Mg (mg kg-1)||190||<52||<53|
|Mn (mg kg-1)||57||22||32|
|Mo (mg kg-1)||0.6||0.5||0.4|
|Na (mg kg-1)||24 000||7100||11 000|
|Ni (mg kg-1)||10||4.9||8.2|
|Se (mg kg-1)||0.1||<0.1||<0.1|
|Sn (mg kg-1)||0.1||0.1||0.1|
|V (mg kg-1)||0.1||0.1||<0.1|
|Zn (mg kg-1)||3.4||3.9||1.2|
|Si (mg kg-1)||<520||<520||<530|
The MWs were determined by GPC, showing that SW has higher MW than eucalyptus (Table 10). In the case of pine lignin, the longer cooking time results in a significant increase in the Mw data after 260 min compared to that of 200 min cooking time. However, this tendency was not observed for spruce lignin. Generally, the Mw:Mn ratio (polydispersity) is higher in case of SW lignins than that of eucalyptus. As seen in the MWD curves (Figure 1), there are two maxima for the SW lignins (bimodal distribution), quite similar in size, whereas the eucalyptus lignin has one broad distribution profile flanked by two smaller peaks.
|Sample||Cooking time (min)||Mn||Mw||PDIa||Tg (°C)||Decomp. temp., (°C)b|
Thermal analysis measurement uncertainty is ±2°C. aPolydispersity index=Mw/Mn; bby TGA, 5% of weight loss.
The results of the TGA and DSC analysis are presented in Table 10. The thermal stability of lignins is dependent of many factors, such as number and kind of functional groups and intermonomeric linkages and MW data. The Tg is also affected by the MW and MWD (Saito et al. 2014; Sevastyanova et al. 2014; Dodd et al. 2015), thermal history, the presence of contaminants with low MW (including water and solvents), and the cross-linking density. In conventional DSC studies, the Tg is deduced from the second heating scan, while the initial scan (heating to a temperature above its Tg) is usually used to eliminate the thermal history stored within the polymer’s glassy state (Rials and Glasser 1984; Hofge 1994). For a better comparability of the data, an annealing temperature of 150°C was selected for all samples. The SW lignins, which have higher MWs and more condensed structures, show both higher thermal stability (their decomposition starts later) and a higher Tg than the eucalyptus kraft lignin. Similar observations were reported by Gordobil et al. (2014). With the SWs, the amount of condensed structures in the dissolved lignins (as shown by 31P NMR analysis), as well as their MWs, increased with longer cooking times. This is reflected in a slight increase in the Tgs for the corresponding samples. For the eucalyptus kraft lignin, a MW decrement was observed for lignins dissolved after a longer cooking time, which may explain the lower Tg data for such samples.
The reported results are very preliminary in terms of lignin utilization, as it is obvious that for value-added lignin applications fractionation and upgrading is necessary (Brodin et al. 2009, 2012; Gosselink et al. 2010; Ropponen et al. 2011; Duval et al. 2015, 2016; Fang et al. 2015). Selective precipitation and ultrafiltration are potentially useful techniques, depending on the intended lignin application (Brodin et al. 2009; Toledano et al. 2010; Ponomarenko et al. 2014; Sevastyanova et al. 2014; Duval et al. 2015). It has been demonstrated that an increased content of OHaliph and OHphen groups improves the acetone solubility of lignin, which would be beneficial for fractionation based on acetone and hexane (Cui et al. 2014). As hardwood lignins have a higher OHphen content, they would be more suitable for such fractionation. The better thermal stability of the SW lignins suggests that they would be preferable for thermosets, while the lower Tg of the eucalyptus lignin might be more favorable for thermoplastic applications. Eucalyptus lignin seems to contain larger quantity of structural descriptors which are favorable for a more effective antioxidant activity.
Eucalyptus, pine and spruce chips were subjected to kraft cooking, and the dissolved lignins in the resultant BL were precipitated at pH 9 after different cooking times and then extensively characterized. The sulfur content was much higher in the eucalyptus precipitates than in the SW precipitates. The carbohydrate portion of the lignin precipitates contained higher amounts of galactose than was expected from the galactose moiety in the GGM. The relative amount of galactose increased with cooking time. The MW of SW lignins was higher with a higher polydispersity. Increasing the cooking time generally resulted in an MW increment. The amount of condensed structures and OHphen groups of SW lignins increased with cooking time. Due to their higher MWs, SW lignins showed a better thermal stability. Both the onset temperatures for decomposition and the Tg data were higher for spruce and pine than for eucalyptus. The results of the Py-GC/MS analysis indicated a higher sulfur content for the eucalyptus lignin than those of the elemental analysis, whereas the sulfur contents observed by means of the two methods were similar in case of SW lignins.
Dr. Rosana Moriana Torró is gratefully acknowledged for her assistance with the thermal analysis and the evaluation of the results. Dr. Liming Zhang is gratefully acknowledged for the assistance with NMR analysis. COST Action FP1105 WoodCellNet is acknowledged for its financial support for the visit of Antonia Svärd to the Latvian State Institute of Wood Chemistry, Riga, Latvia (Grant No. ECOST-STSM-FP1105-190114-039623).
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