Characterization of Kraft Lignin Fractions Obtained by Sequential

Research Article pubs.acs.org/journal/ascecg

Characterization of Kraft Lignin Fractions Obtained by Sequential Ultrafiltration and Their Potential Application as a Biobased Component in Blends with Polyethylene Caoxing Huang,†,‡ Juan He,† Robert Narron,‡ Yuhan Wang,‡ and Qiang Yong*,† †

Co-Innovation Center for Efficient Processing and Utilization of Forest Products, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, China ‡ Department of Forestry Biomaterials, North Carolina State University, 2820 Faucette Drive, Raleigh, North Carolina 27695-8005, United States ABSTRACT: Fractionation of Kraft lignin from black liquor is necessary to reduce its heterogeneity, which exerts negative effects upon lignin-commercial polymer composite materials. In this work, Kraft lignin was dissolved in acetic acid to get low viscosity lignin solution and fractionated into specific molecular weight fractions by sequential ultrafiltration with different molecular weight cut-offs. The fractionated lignins were characterized, and their suitability in polyethylene−lignin composites was evaluated. After ultrafiltration with 5k and 3k Da cutoff, fractionation resulted in three fractions of differing molecular weights and lower polydispersity than the original Kraft lignin (F1 (7010 g/mol), F2 (3540 g/mol), and F3 (1890 g/mol)). The results from spectroscopic characterization of lignin (31P, 13C, and 2D-HSQC NMR) and thermal stability analysis (TGA and DSC) indicated that the contents of various linkages and functional groups and the thermal properties of each fraction varied as a function of its molecular weight. Compared to F1 and F2, F3 (lowest molecular weight and high quantity of phenolic hydroxyl groups) most positively contributed to the mechanical properties of a polyethylene−lignin composite. KEYWORDS: Lignin, Sequential ultrafiltration, Molecular weight, Physicochemical structures, Polyethylene−lignin composite

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INTRODUCTION For the pulp and paper industry, Kraft pulping is the dominant delignification process for producing wood fibers. The lignin that is solubilized can be found in a solution colloquially referred to as black liquor due to its darkened appearance. Pulp mills typically burn black liquor to recover the original cooking chemicals, simultaneously consuming the dissolved lignin and hemicelluloses as a source of energy.1,2 Although commonly viewed as a waste byproduct of the paper mill, solubilized Kraft lignin is being considered by many material manufacturers for incorporation into formulations. For example, the most accepted approaches for lignin valorization is to make lignosulfonate, which has been commercialized for many years.3 In addition, lignin is an excellent reinforcement filler for thermoplastics, hydrogels, thermosets, rubbers, and highvalue bioadditive for multifunctional nanocomposites. A small amount of lignin is currently commercialized as plasticizers, antioxidants, coatings, lubricants, and surfactants.4−8 Kraft lignin is inherently heterogeneous, is highly polydisperse, and possesses both nonuniform functionality and chemical linkages. Each of these chaotic properties hamper lignin commercialization. Hence, how to get the lignin with high homogeneous property is of interest for its valorization. Technically, various methods have been applied to reduce the © 2017 American Chemical Society

previously bemoaned heterogeneity of lignin, primarily focusing upon increasing the abundance of phenolic hydroxyl group (OH) contents in lignin.9−11 Increasing quantities of phenolic OH groups in Kraft lignin have been shown to confer beneficial effects to the materials it is blended with, such as greater oxidative, thermal, and light stability.7,12 Studies on various lignin extracts have shown that the actual chemical structure and functional groups present in differing lignin extracts depends significantly upon the method of extraction/ fractionation, resulting in lignin “cuts” which possess specific chemical and molecular properties.13 For Kraft lignin, it has been confirmed that its chemical structures and functional group profile varies by lignin molecular weight distribution. For example, Jiang et al.9 fractionated softwood Kraft lignin into four different fractionations via sequential precipitation and found that the contents of various structural linkages and thermal properties of each fraction varied as a function of molecular weight. The most common method for procuring Kraft lignin from black liquor is inducing lignin precipitation through lowering Received: September 24, 2017 Revised: October 23, 2017 Published: October 31, 2017 11770

DOI: 10.1021/acssuschemeng.7b03415 ACS Sustainable Chem. Eng. 2017, 5, 11770−11779

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ACS Sustainable Chemistry & Engineering

obtained a HDPE/lignin composite. This material exhibited a decrease in tensile strength when compared to the lignin-less material (neat HDPE). In attempt to explain the poor mechanical performance exhibited by lignin-containing PE composites, Alekhina et al.26 concluded that large molecular weight lignins with few phenolic hydroxyl functionalities disproportionately lowered composite material’s elongation and hardness. Hence, it is speculated that fractionating low molecular weight and high phenolic hydroxyl groups portion from Kraft lignin to produce lignin−commercial polymer composite materials can achieve acceptable mechanical performances. Also, increasing phenolic hydroxyl groups in lignin can improve its dispersion and compatibility with PE.26 In the present work, a practical technology was carried out to reduce black liquor viscosity by precipitating and redissolving Kraft lignin in acetic acid (AcOH) solution. Afterward, the low viscosity lignin solution was processed by ultrafiltration with 5k and 3k Da cutoff to get lignin fractionations with low PDI ( 13) and found the fractionationed lignin was in accordance with molecular weight cut-offs of membranes, while, the resulting lignin fractionations were still with high polydispersity index (PDI > 2.0). In addition, the high viscosity of black liquor might problematically hinder mass transfer of liquor through membrane mediums when authors just used raw black liquor. Hence, it is necessary to employ membrane technology with different molecular weight cut-offs to obtain specific lignins from lignin solution low viscosity. Polyethylene (PE) is the largest produced and consumed polymer in different grades, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE). Both of these grades are convertible to a wide variety of commodities including flexible and semiflexible plastic items for packaging, personal care products, agricultural mulch films, and disposable items,23 while PE is a synthetic biostable polymer with slow biodegradation rate in the various environmental compartments, which has caused the “white pollution” for environment. Lignin is one of the renewable biopolymers and has been considered as economic and environmental components for synthetic polymer blends.7,23 Hence, lignin and its derivatives have been blended with polyethylene to produce biodegradable thermoplastic polymer/ lignin composites. For example, Rusu et al.24 produced a low density polyethylene (LDPE)/lignin (30 wt %) composite and found the composite showed a 42% decrease in tensile strength and a 96% decrease in elongation at break with respect to neat LDPE. Hu et al.25 incorporated 20−40 wt % lignin in high density polyethylene (HDPE) through melt mixing and

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MATERIALS AND METHODS

Materials. Moso bamboo (Phyllostachys pubescens) residues were provided by Shaowu Bamboo Processing Factory in Fujian, China. The residues were subjected to Kraft pulping, and the black liquor produced was utilized as starting material for this work. In accordance with our previous works,27,28 Kraft cooking conditions were 12% effective alkali (EA) charge (as Na2O on dry material), 20% sulfidity (on Na2O basis), 1:6 solid/liquor ratio, 150 °C cooking temperature, and 60 min holding time. The low density polyethylene (428043) and polyethylene graft maleic anhydride (456624) were purchased from Sigma-Aldrich, U.S.A. Sequential Ultrafiltration of Kraft Lignin. Kraft lignin was precipitated from black liquor by adjustment of solution pH to ∼2−3 using 1 M H2SO4. After precipitation was induced, suspended solids were collected through filter paper. The obtain Kraft lignin was next washed with 0.1 M H2SO4 three times, followed by distilled water washing until the resultant water filtrate was pH to ∼6−7. After sequential acid and water washing, the Kraft lignin was dried in a laboratory oven at 40 °C until constant mass. The sequential ultrafiltration performed upon the Kraft lignin to obtain low molecular weight oligolignin centered around a centrifugal device with different molecular weight cut-offs (5 kDa and 3 kDa), described in Figure 1. Specifically, 3 g of Kraft lignin was dissolved in 24 mL of 90% (v/v) acetic acid (AcOH) solution to produce lignin solution with dissolved solids content of 12.5% (w/v), a replicated value similar to that of real black liquor. Next, the lignin solution was subjected to ultrafiltration with 5 kDa cutoff, producing two fractions: trapped fluid (L1) and permeate fluid (L2). The permeate fluid (L2) was next centrifuged with 3 kDa cutoff to produce another trapped fluid (L3) and permeate fluid (L4). To obtain the lignin dissolved in trapped and permeate fluid, deionized water (10 mL/mL 90% AcOH) was added to the fluid in dropwise manner to induce lignin precipitation. Precipitated Kraft lignin from L1, L3, and L4 are designated as F1, F2, and F3, respectively. The precipitates (F1, F2, and F3) were sequentially washed with diethyl ether and petroleum ether and finally dried to constant mass prior to analysis. Gel Permeation Chromatographic (GPC) Analysis. The molecular weight of each Kraft lignin sample was analyzed by GPC, 11771


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measurement was first performed at a scan rate of 10 °C/min from 40 to 105 °C to remove residual moisture from the lignin sample. The second scan was from 40 to 180 °C to measure the lignin Tg, which was defined as the midpoint of the temperature at which heat capacity occurred. Preparation of LDPE/Lignin Composites and Their Mechanical Testing. The fractionated lignins were blended with low density polyethylene (LDPE) to investigate lignin processing characteristics and the mechanical properties of the ensuing blends. The LDPE/lignin composites were produced using a laboratory-scale twin-screw extruder (DSM Xplore, Netherlands) at 170 °C and 100 rpm.26,30 The LDPE/lignin blend (15 g) contained 20% (w/w) of fractionated lignin and 77% (w/w) of LDPE. Meanwhile, 3% (w/w) of polyethylene grafted maleic anhydride was added in the blend as coupling agent, according to the work of Alekhina et al.26 The physical mixture of LDPE and lignin was manually introduced into the feeder of the extruder. To improve dispersion, the mixture was circulated inside the extruder’s inner chamber for 10 min prior to extrusion. The extruded filament was cooled by atmosphere and cut into pellets after sufficient cooling. Pellets were injected into a mold (DAKA 50000, U.S.A.) to create dog bone shaped samples for tensile strength testing. Each dog bone sample weighed about 1.1 g and had a test span of 25 mm. Sample width was 4 mm, and thickness was 1.5 mm. The dog bone samples were placed in a piston for 5 min at 150 °C before injection. Five specimens were prepared from each extruded LDPE/lignin blend sample. The tensile test was carried out using an Instron Universal Testing Machine (Instron 4443, U.S.A.) at 25 °C with a cross head speed of 2.5 mm/min. This translates to an effective strain rate of 10% strain per minute. The values reported are an average of five separate measurements.

Figure 1. Scheme of sequential ultrafiltration of Kraft lignin. a mode of size-exclusion chromatography. Before analysis, all lignin samples were acetylated to enable mobile phase solubility, in accordance with our previous work.29 The GPC instrument was equipped with a PL-gel 10 mm mixed-B 7.5 mm i.d. column and an ultraviolet eluent detector. The column was operated at ambient temperature and eluted with tetrahydrofuran (THF) at a flow rate of 1.0 mL/min. Monodisperse polystyrene was used as calibration standards. NMR Analysis. NMR spectra (13C and 2D-HSQC) were acquired on a Bruker AVANCE 600 MHz spectrometer equipped with 5 mm BBO and BBI probe, respectively. For the quantitative 13C NMR experiments, 160 mg of lignin was dissolved in 0.5 mL of DMSO-d6 and 40 μL of chromium(III) acetylacetonate (0.01 M) was added to provide complete relaxation of all carbon nuclei. The solution was transferred to a 5 mm Shigemi microtube and subjected to analysis by the spectrometer. The acquisition parameters were 25 °C, 90° pulse width, 1.7 s relaxation, and 1.2 s of acquisition time. A total of 20 000 scans were collected. For the 2D-HSQC NMR experiments, 40 mg of lignin was dissolved in 0.5 mL of DMSO-d6. The acquisition parameters used were 160 transients (scans per block) acquired using 1024 data points in the F2 (1H) dimension with an acquisition time of 53 ms and 256 data points in the F1 (13C) dimension with an acquisition time of 5.14 ms. Total running time was 18 h. The coupling constant (1J C−H) of 147 Hz was applied. Functional Groups Analysis. Quantities of aliphatic hydroxyl, phenolic hydroxyl, and carboxylic acid functional groups were determined by quantitative 31P NMR using a Bruker AVANCE 600 MHz spectrometer. Specifically, an accurate weight (about 40 mg) of a dried lignin sample was introduced into NMR tubes. Next, 500 μL of an anhydrous pyridine/CDCl3 mixture (1.6:1, v/v) was added, and the lignin was allowed to dissolve. Then, 200 μL of an endo-N-hydroxy-5norbornene-2, 3-dicarboximide (e-NHI) solution (9.23 mg/mL, serving as internal standard) and 50 μL of chromium(III) acetylacetonate solution (5.6 mg/mL, serving as a relaxation reagent) were added. Finally, 100 μL of phosphitylating reagent (2-chloro4,4,5,5-tetramethyl-1,2,3-dioxaphospholane) was added, and the tubes were vigorously shaken and immediately subjected to 31P NMR observation. Thermal Analysis. To describe the thermal properties of each lignin fraction, thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) were used to analyze the decomposition temperature (Td) and glass transition temperature (Tg), respectively. Analysis was performed using TA Instruments Q500 TGA and Q100 DSC. For TGA, the sample was heated from 30 to 600 °C at a heating rate of 10 °C/min under dry nitrogen atmosphere. For DSC, the

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RESULTS AND DISCUSSION Fractionation of Specific Molecular Weight Kraft Lignin by Sequential Ultrafiltration. Ultrafiltration’s retentate depends upon the size cutoff of the membrane and the viscosity of the solution.16 Hence, reducing black liquor viscosity is of particular interest when using ultrafiltration on black liquor to fractionate Kraft lignin by molecular weight. In this work, the viscosities of crude black liquor (containing lignin) and Kraft lignin AcOH-solution were measured for comparison. As shown in Figure 1, the dissolved solids content of black liquor was 12.5% (w/v) and the viscosity was 12.4 cps. With the same solid content (12.5%), the viscosity of Kraft lignin dissolved in AcOH-solution was 2.1 cps, much lower than what was measured for rough black liquor. For ultrafiltration, fluxing energy is required to thrust low molecular weight solutes beyond the membrane barrier.31 Technically, the flux of permeate liquid depends on both membrane cutoff and solution viscosity. Based on this, it is speculated that a lesser degree of energy consumption would be needed to fractionate Kraft lignin AcOH-solution by membrane ultrafiltration compared to isolation of the same solutes in crude black liquor.16,17 Due to lowered solution viscosity, Kraft lignin dissolved in AcOH-solution served as the feedstock for centrifugal fractionation (5k and 3k Da cut-offs). The centrifugal fractionation protocol is described in Figure 1. After sequential ultrafiltration, the yield of each fraction varied depending upon each stage’s cutoff. As shown in Table 1, yields of F1, F2, and F3 were 50.4%, 38.7%, and 10.5%, respectively. The molecular weight distributions of each fractionated lignin as well as the original Kraft lignin are shown in Table 1 and Figure 2. Results indicate that ultrafiltration with different molecular weight cut-offs successfully fractionated original Kraft lignin along the lines of molecular weight. The average Mw of F1, F2, and F3 were 7010, 11772


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In Figure 3, the signals for S, G, and H units were observed in the aromatic region (160−103 ppm). Signals observed in the region of 86−50 ppm are attributed to the oxygenated and nonoxygenated interunit linkages in lignin. For example, the linkages of β-O-4, β−β, and β-5 could be identified by their unique Cβ signals at 86.1, 53.8, and 52.2 ppm, respective. In the 2D-HSQC spectra of all lignin samples (Figure 4), the differences between Kraft lignin, F1, F2, and F3 are most clearly observable in the side-chain region (δC/δH 50−90/ 2.5−6.0 ppm). The intensity of signals for β-O-4 (A), β−β (B), and β-5 (C) substructures decreased with the decreasing molecular weight of each lignin fraction, i.e., F1 to F3. In the aromatic region (δC/δH 100−135/5.5−8.5 ppm), the differences of S, G, and H units for the fractionated lignins also can be observed through comparison of signal intensities. The S units in lignin showed a prominent signal for the C2,6-H2,6 correlations at δC/δH 104.1/6.74 ppm. Signals for the C2−H2, C5−H5, and C6−H6 in G units were clearly observed at δC/δH 111.0/7.01, 114.4/6.73, and 119.0/6.82 ppm, respectively. The C2,6−H2,6 correlations of H units appeared at δC/δH 127.8/7.22 ppm. Based on the signal integrals of S and G units in 2D HSQC spectra, it can be known that the S/G ratios of F1, F2, and F3 were 1.5, 1.6, and 1.8, respectively. Generally, lignin distributions with high percentages of G units are associated with higher molecular weight constituents due to higher probability of C5 bonding (aryl−aryl or aryl−aliphatic). In the case of S units, it is impossible to form C5 bonds due to native C5 methoxyl groups.35 Because of this, it was observed that the membrane’s low molecular weight cutoff tended to yield lignin fractions with higher amounts of S units. This observation is in good agreement with another study by Alriols et al.36 In order to quantitatively compare the chemical structure differences between original Kraft lignin and each fraction obtained through sequential ultrafiltration, substructural linkages were quantified by the method coupling quantitative 13C NMR and 2D-HSQC NMR described by Balakshin et al.38 The amount of each individual lignin substructure was calculated per 100 Ar (C600) as follows:

Table 1. Fractionation Yields and Molecular Weight of Fractionated Kraft Lignin by Sequential Ultrafiltration Kraft lignin F1 F2 F3

yield (%)a

Mw (g/mol)

Mn (g/mol)

PDI (Mw/Mn)

50.4 ± 1.2 38.7 ± 0.5 10.5 ± 0.8

5410 7010 3540 1890

1830 3520 1820 1230

3.0 2.0 1.9 1.5

a

Yields of fractionation, based on the reallocation of original lignin dissolved in AcOH solution.

Figure 2. GPC chromatograms of Kraft lignin and resultant fractionated lignins.

3540, and 1890 g/mol, respectively. It is of value to note that chromatograms obtained from original Kraft lignin and F1 showed bimodal distribution, while the distributions of F2 and F3 GPC curves were unimodal. For gel permeation chromatography, bimodal distributions are attributed to a high degree of polymeric polydispersity. Alternatively, unimodal distributions indicate that the separated polymers are of low polydispersity. As shown in Table 1, the polydispersity index (PDI) of Kraft lignin, F1, F2, and F3 were 3.0, 2.0, 1.9, and 1.5, respectively, indicating that ultrafiltration results in more homogeneous lignin fractions along the lines of molecular weight distribution. It is of significant interest to find that applying the ultrafiltration on a Kraft lignin solution resulted in procurement of highly monodisperse fractions. These findings indicate that sequential ultrafiltration could prove useful as a mode of lignin isolation from black liquor, potentially resolving the current heterogeneity issues associated with Kraft lignin.9,10 Such findings benefit industrial efforts to valorize Kraft lignin beyond combustible potential energy. 13 C and 2D HSQC NMR Analysis. In order to understand if the applied sequential ultrafiltration preferentially distributes lignin with specific substructures, the original Kraft lignin and its fractionated samples (F1, F2, and F3) were characterized by 13 C NMR and 2D-HSQC NMR techniques. The 13C spectra and 2D-HSQC spectra from each lignin sample are shown in Figures 3 and 4, respectively. All of the observed signals in NMR spectra have been assigned according to previous studies.29,32−34 The cross signals assigned in the 2D-HSQC spectra are listed in Table 2.

Ix = 2DIx /2DI78 − 90/2.5 − 6.0 × 13C78 − 90 /13C103 − 163 × 612 (1)

where Ix is the integration of Aβ (both Aβ (G/H) and Aβ (S)), Ba, Ca, and Da in the 2D spectra region of 78−90/2.5−6.0 ppm and 2DI78−90/2.5−6.0 is the total integration of this region. The quantities calculated from each structurally specific signal and their integrals are shown in Table 3. Results show that the amount of β-O-4 structures in Kraft lignin, F1, F2, and F3 were 11.8/100Ar, 13.2/100Ar, 10.9/100Ar, and 6.9/100Ar, respectively. The abundances of the β-5 structure in the fractionated lignins also decreased with the decreasing molecular weight. An explanation for this observation may be due to the presence of stilbene structures, which is derived mainly from the β-5 structure.9 In addition, G-type lignin units (with free aromatic C5 position) are able to form C−C bonds at the C5 position, which is blocked on S-type units by a methoxyl group. Thus, lignins that contain low β-5 structural abundance tend to also possess lower molecule weights.36−38 Overall, it is clear that the contents of various substructures in the fractionated lignins are influenced by the applied molecular weight membrane gradient. This confirms that the fractionation process exerts influence upon the membrane-fractionated lignin’s substructure profile. Functional Group Analyses. The major chemical functional groups in Kraft lignin include hydroxyl, methoxyl, and 11773


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Figure 3. Spectra of 13C NMR for lignin samples.

lignin), were not significantly affected by the fractionation, i.e., not dependent on the molecular weight of isolated lignin fractions. Overall, the findings suggest that the profile and abundance of the functional groups present in bamboo Kraft lignin fractions is directly related to the molecular weight of the sequential ultrafiltration. Since the presence of a phenolic OH group in lignin is of vital importance for cross-linking reactions, the F3 fractionation is speculated to demonstrate good performance if it is blended to produce synthetic polymers.41,42 Thermal Stability. Information on the thermal properties of the lignin samples is particularly pertinent when lignin is considered for incorporation into moldable plastics. In this work, the thermal properties of fractionated lignins by sequential ultrafiltration were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC). The results are shown in Table 5 and Figure 5. At temperatures below 130 °C, the detected (but minor) mass decrease corresponded to the vaporization of residual moisture in the lignin samples. In general, 10% weight loss of lignin is defined as decomposition temperature (Td).9 In this work, it was found that the decomposition temperature decreased along lines of decreasing molecular weight. The Td of F1, F2, and F3 were observed at 280, 252, and 231 °C, respectively. This observation was in good agreement with the results obtained by Jiang et al.,9 who studied the thermal

carboxylic acid structures in various amounts and proportions, depending upon pulping process variables.39 To understand how the sequential ultrafiltration affected the distribution of hydroxyl functional groups, the contents of aliphatic hydroxyl, phenolic hydroxyl, and carboxylic groups contents were determined by quantitative 31P NMR analysis. The results of functional group analysis are shown in Table 4. 31 P NMR determination of hydroxyl functional groups showed that the low and high molecular weight fractions (F1 and F3) are significantly different in terms of functional group abundance. Quantitation found that the sequentially ultrafiltered lignin was of lower aliphatic hydroxyl group content and of higher phenolic hydroxyl content as the cutoff molecular weight decreased. Specifically, F3 has a higher phenolic hydroxyl content (4.21 mmol/g) and a low aliphatic hydroxyl content (1.51 mmol/g) than F1 (2.97 and 4.11 mmol/g, respectively). Similar phenomena have been previously reported by Jiang et al.9 and Cui et al.,10 who indicated that aliphatic hydroxyl group content of organic solvent-fractionated Kraft lignin decreased with the decreasing molecular weight, whereas phenolic hydroxyl content exhibited the reverse trend. Possible explanation for this trend may be that new phenolic hydroxyl groups are formed during Kraft pulping through degradative reactions upon native aliphatic hydroxyl groups.40 Carboxylic groups, measured to be minor (∼0.1 mmol/g 11774


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Figure 4. Spectra of 2D-NMR for lignin samples.

Tg of lignin is positively related to its molecular weight.25 In addition, it is reported the amount and type of functional groups present in the lignin structure may have an impact on the Tg of lignin.26 Kubo and Kadla45 discovered that the polar functional groups (hydroxyl groups and carboxylic groups) participate in intermolecular interactions, further reducing thermal mobility of the lignin molecules. In this work, the phenolic hydroxyl groups in F2 and F3 were 3.87 and 4.21 mmol/g, respectively, with the corresponding Tg of 149 and 117 °C. Therefore, it is speculated that high phenolic hydroxyl groups in lignin may explain its accompanying low Tg. LDPE/Lignin Composites Preparation and Testing. To investigate if the fractionated lignin can be used in thermoplastic formulations to make lignin-containing thermoplastics, the F1, F2, and F3 lignin fractions were blended with low density polyethylene (LDPE) by way of twin-screw extruder. Results from mechanical testing (tensile strength and tensile modulus) of the LDPE/lignin composites are shown in Table 6. It can be seen from the presented results that the addition of fractionated lignins to neat LDPE resulted in PE-lignin composites with dampened mechanical properties. For example, the maximum tensile strength of injection molded composites decreased from 11.2 MPa for LDPE to 7.4 MPa when F1 was incorporated into LDPE. A significant reduction of the tensile properties was also detected from the PE blended with biorefinery lignins, such as prehydrolysis beech wood lignin,46 straw steam explosion lignin,47 and industrial pine black liquor lignin.43 The addition of F3 and F2 resulted in a

properties of Kraft lignin fractions obtain from sequential precipitation with various organic solvents. From these results, it can be speculated that F3 would serve as a viable candidate for biofuel and chemical production due to its lower degradation temperature. At 600 °C, some lignin remains incompletely burned, existing in solid phase as residual char. The extent of residual char mass can be interpreted as a given lignin’s thermal stability. In this work, the amounts of residual chars (at 600 °C) for F1, F2, and F3 were determined to be 48.9, 46.0, and 40.9 wt %, respectively. These results indicate that the nonvolatile residue at 600 °C increased with increasing molecular weight of fractionated lignins. It can be speculated that F1 is a good carbon precursor to produce activated carbon or carbon fibers due to the highest residual char yield.43 Lignin is an amorphous solid that can undergo transition from glassy state to rubbery state at a glass transition temperature (Tg).44 The Tg of lignin is affected by several factors, such as molecular weight, thermal history, and degree of cross-linking. Hence, it is important to evaluate the Tg properties of fractionated lignins if they are to be blended with synthetic polymers to produce thermoplastic polymer/ lignin composites. As shown in Table 5, there was no obvious glass transition temperature for F1 (7010 g/mol), indicating that this fraction did not sufficiently soften in the DSC system running to 180 °C. The Tg of F1 maybe over 200 °C. For the high Tg property of F1, it may be explained by the fraction’s high molecular weight. The Tg of Kraft lignin, F2, and F3 were 162, 149, and 117 °C, respectively. These results are in good agreement with previously published works suggesting that the 11775


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ACS Sustainable Chemistry & Engineering Table 2. Assignment of Main Lignin Signals in the 2D HSQC Spectra of Lignin Samples δC/δH

labels lignin structure Cβ

assignment

53.7/3.05 55.9/3.73

Iγ

61.4/4.09

A′γ

63.6/4.36

phenolic hydroxyl

Cβ−Hβ in phenylcoumaran substructures (C) Cβ−Hβ in resinol substructures (B) C−H in methoxyls

53.1/3.49

Bβ −OCH3 (OMe) Aγ Dβ

Table 4. Contents of Aliphatic Hydroxyl, Phenolic Hydroxyl, and Carboxylic Groups in Fractionated Lignins (mmol/g)

59.6−60.8/3.37−3.72 59.8/2.78

Bγ A′a

71.3/4.18,3.82 71.8/4.86

Da

81.2/5.09

A″β

82.8/5.23

Aβ (G/H)

83.9/4.30

Ba Aβ (S)

84.9/4.69 86.0/4.11

Ca

86.8/5.49

S2,6

104.1/6.74

S′2,6

106.6/7.32

G2 FA2 PCAβ G5 PCA3,5

111.0/7.01 111.1/7.34 113.8/6.29 114.4/6.73 116.2/6.77

G′6

118.7/7.33

G6 FA6 H2,6

119.0/6.82 123.1/7.19 127.8/7.22

PCA2,6

130.1/7.48

PCAa, FAa

144.7/7.46

Kraft lignin F1 F2 F3

Cγ−Hγ in β-O-4 substructures (A) Cβ−Hβ in spirodienone substructures (D) Cγ−Hγ in cinnamyl alcohol endgroups (I) Cγ−Hγ in γ-acylated β-O-4 substructures (A′) Cγ−Hγ in resinol substructures (B) Ca−Ha in β-O-4 substructures (A′) linked to a S-unit Ca−Ha in spirodienone substructures (D) Cβ−Hβ in β-O-4 substructures with CaO groups (A″) Cβ−Hβ in β-O-4 substructures linked to a G unit (A) Ca−Ha in resinol substructures (B) Cβ−Hβ in β-O-4 substructures linked to a S unit (A) Ca−Ha in phenylcoumaran substructures (C) C2,6−H2,6 in etherified syringyl units (S) C2,6−H2,6 in syringyl units with CaO groups (S′) C2−H2 in guaiacyl units (G) C2−H2 in ferulate (FA) C8−H8 in p-coumarate (PCA) C5−H5 in guaiacyl units (G) C3−H3 and C5−H5 in p-coumarate (PCA) C6−H6 in guaiacyl units with CaO groups (G′) C6−H6 in guaiacyl units (G) C6−H6 in ferulate (FA) C2,6−H2,6 in p-hydroxyphenyl units (H) C2,6−H2,6 in p-hydroxyphenyl units (H) Ca−Ha in p-coumarate (PCA) and ferulate (FA)

aliphatic hydroxyl

condensed phenolic OH

noncondensed phenolic OH

total phenolic hydroxyl

COOH

3.13

1.76

1.33

3.09

0.14

4.11 2.43 1.51

1.46 1.95 1.92

1.51 1.89 2.29

2.97 3.87 4.21

0.12 0.15 0.16

Table 5. Td, Tg, and Residue Char of Kraft Lignin and Fractionated Lignins Td (°C)a residue char (%)b Tg (°C)c

Kraft lignin

F1

F2

F3

263 47.4 162

280 48.9

252 46.0 149

231 40.9 117

a Decomposition temperature at 10% weight loss of lignin. bThe nonvolatile residue of lignin at 600 °C. cGlass transition temperature.

Table 3. Quantitative Informations of the Substructures on Fractionated Lignin Samples by 13C NMR and 2D-HSQC NMR amount of lignin linkagea

Kraft lignin F1 F2 F3

percentage of lignin unitb

β-O-4 (A)

β−β (B)

β-5 (C)

S

G

H

S/G ratio

11.8

2.4

0.7

53.9

23.4

22.7

2.3

13.2 10.9 6.9

2.4 2.1 1.9

0.8 0.6 0.6

46.8 47.8 49.8

30.9 29.9 27.1

22.3 22.3 23.1

1.5 1.6 1.8

Figure 5. TG and DSC curves of Kraft lignin and fractionated lignins.

slightly lower decrease in tensile strength, from 11.2 to 9.6 MPa and 8.3 MPa, respectively. These results indicate that PE-lignin composites with lower molecule weight lignins incorporated exhibit higher tensile strength compared to high molecular weight lignin−PE composites. An explanation for this observation could be that low molecule weight lignin (F1 >

a

Expressed as per 100 Ar (C600). bMolar percentages S + G + H = 100%.

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induce a better compatibility between polyolefin and lignin structure.12,49 In this work, it has to be pointed out that the addition of lignin decreased the property of LDPE. This is the common issue when using lignin or fractionated lignin as a biobased component in blends with polyethylene, which has also been reported by Sadeghifar and Argyropoulos,7 Toriz et al.,19 Kubo and Kadla,45 and Alexy et al.46 Also, the chemical modification, such as as esterification,5 methylation,30 and atom transfer radical polymerization,50 has been applied on lignin to improve the number of possible reaction sites for a better compatibility with PE. Hence, it is speculated that an improved mechanical property of LDPE/lignin composition can be achieved when adding the chemically modified F3 lignin. Kraft lignin fractions obtained along lines of molecular weight by sequential ultrafiltration exerted different effects upon the tensile properties of ensuing PE−lignin composites. Compared to high molecular weight lignin (F1 and F2), low molecular weight lignin (F3) exhibits better contribution to composite material mechanical properties. This indicated that F3 or chemically modified F3 are of potential interest for use as a biobased component in blends with polyethylene. This finding is in agreement with the work reported by Alekhina et al.,26 who indicated that lignin with lower molecular weight exhibits greater affinity toward polyethylene. It can be summarized that the fractionated lignin obtained through sequential ultrafiltration is more homogeneous than original Kraft lignin in term of molecular weight and polydispersity, which could be selected for composite incorporation based upon their positive contribution to mechanical properties.

Table 6. Mechanical Properties of LDPE/Lignin Composites with 20 wt % Fractionated Lignins LDPE/lignin composites tensile strength (MPa) tensile modulus (MPa) elongation at break (%)

neat LDPE

F1

F2

F3

11.2 ± 0.8

7.4 ± 0.5

8.3 ± 0.3

9.6 ± 0.2

171 ± 4

262 ± 8

233 ± 1

197 ± 7

90.2 ± 0.5

32.1 ± 0.2

38.5 ± 0.3

46.8 ± 0.1

F2 > F3) bears higher contents of phenolic OH groups (F1 < F2 < F3), which assist with dispersing lignin into the LDPE matrix.7 Also shown in Table 6, the tensile modulus of all PE-lignin composites increased significantly compared to that of neat LDPE. For example, the tensile modulus of LDPE increased from 171 to 197 MPa with addition of F3 (1890 g/mol), accounting for 15.2% relative increase in tensile modulus. In addition, the tensile modulus further increased with the addition of lignins with greater molecule weights. The addition of F2 (3540 g/mol) and F1 (7010 g/mol) resulted in the tensile modulus of LDPE increased from 171 to 233 MPa and 262 MPa, respectively. This indicates that the addition of high molecule weight lignin may be decreasing the rigidity and dimensional stability of PE−lignin composites. Relative to neat polymer, Iyer et al.48 also reported that melt-mixed polymer and lignin composites generally show enhanced tensile modulus but at the cost of significant tensile strength deterioration. From the stress−strain curves of LDPE/lignin composites in Figure 6, it can be seen that the elongation at

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CONCLUSIONS Kraft lignin was fractionated into fractions of specific molecular distributions by sequential ultrafiltration with different cutoff points. The fractionated lignins were more homogeneous than the original Kraft lignin in terms of molecular weight and polydispersity. Quantitation of interlignin substructures and hydroxyl functional groups revealed both quantities to be a function of fractionated lignin molecular weight. Blending lignin in LDPE to make composite materials significantly reduced tensile properties compared to neat LDPE. However, incorporation of lignin with lower molecular weight into LDPE resulted in an increase in tensile properties of LDPE−lignin composites. Overall, the results demonstrate that fractionated Kraft lignins can be more easily utilized for various applications due to decreased heterogeneity.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 25 85427797. E-mail: [email protected].

Figure 6. Stress−strain curves of LDPE/lignin composites with 20 wt % fractionated lignins.

ORCID

Qiang Yong: 0000-0001-5266-7278 Notes

break behavior was significantly reduced for all of the composites with 20 wt % fractionated lignins when compared to neat LDPE. When F1, F2, and F3 fractionations were added with LDPE, the elongation at break were reduced from 90.2% (neat LDPE) to 32.1%, 38.5%, and 46.8%, respectively. However, it is important to note that the addition of F3 lignin showed somewhat more improved elongation at break of LDPE/lignin composition, compared to the behaviors of addition of F1 and F2. For this phenomenon, it maybe explained that F3 with high phenolic hydroxyl groups can

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (31570561) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD). Meanwhile, the authors thank Dr. Richard Venditti (Department of Forestry Biomaterials, North Carolina State University) for providing twin-screw extruder equipment. 11777


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