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Isolation and characterization of lignin from Moroccan sugar cane bagasse: Production of lignin–phenol-formaldehyde wood adhesive
Industrial Crops and Products 45 (2013) 296–302
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Isolation and characterization of lignin from Moroccan sugar cane bagasse: Production of lignin–phenol-formaldehyde wood adhesive Amine Moubarik a,∗ , Nabil Grimi b , Nadia Boussetta b , Antonio Pizzi c a
MAScIR-NANOTECH, ENSET, Avenue de l’Armée Royale, Madinat El Irfane, 10100 Rabat, Morocco Université de Technologie de Compiègne, Unité Transformations Intégrées de la Matière Renouvelable (TIMR, EA4297), Centre de Recherche de Royallieu, B.P. 20529-60205, Compiègne Cedex, France c ENSTIB, Université de Nancy 1, Epinal, France b
a r t i c l e
i n f o
Article history: Received 29 September 2012 Received in revised form 18 December 2012 Accepted 22 December 2012 Keywords: Sugar cane bagasse Lignin Wood adhesive Plywood
a b s t r a c t lignin-based materials were isolated from Moroccan sugar cane bagasse after alkaline delignification. Sugar cane bagasse was subjected to hot water (70 ◦ C) and alkaline aqueous solutions (15% of sodium hydroxide (NaOH), 98 ◦ C) treatments. The dissolved lignin macromolecules were separated and purified. The isolated solid was then characterized by different complementary analysis (FT-IR; 1 H, 13 C NMR; GPC and TGA). In the present work, the possibility of preparing wood adhesives from bagasse lignin has been explored. The results showed that the delignification with 15% NaOH resulted in yields of cellulose and lignin of 42 ± 2.2% and 13 ± 1.5%, respectively. The extracted lignin scaffolds exhibits high reactivity due to the high content of hydroxyl group. Their higher molecular weight (2781 g/mol) and good thermal stability (180 ◦ C) make them excellent candidates for partial substitution of phenol formaldehyde (PF) resin. A resin formulation in which up to 30% of PF can be substituted by bagasse lignin gave good results and was employed for the elaboration of plywood panels which passed relevant international standard specifications for interior-grade panels. © 2013 Elsevier B.V. All rights reserved.
1. Introduction One of the most challenging topics in material science today is to convert biomass-derived waste and feedstock to highly added value-materials. For instance, sugar cane bagasse is a fibrous residue of sugar cane stalks left over after crushing and extraction process of the juice from sugar cane. About 54 million dry tons of bagasse are produced annually throughout the world (Mulinari et al., 2009). Most of the bagasse weight is in the form of the so-called fiber and rind particles, which have high length-width ratios (lengths up to a few cm) and correspond mostly to stalk fibro vascular bundles. For the sugar industries, this waste is mainly converted into energy through combustion (Leibbrandt et al., 2011), applications in pulping (Goncalves et al., 2005), activated carbon production (Devnarain, 2003; Qureshi et al., 2008), cellulosic ethanol production (Carrier et al., 2011) and gasification (Mamphweli and Meyer, 2010). Sugar cane bagasse consists of approximately 50% cellulose, 25% hemicellulose and 25% lignin (Pandey et al., 2000; Ezhumalai and
∗ Corresponding author at: NANOTECH-MAScIR (Moroccan Foundation for Advanced Science, Innovation and Research), ENSET, Avenue de l’Armée Royale, Madinat El Irfane, 10100 Rabat, Morocco. Tel.: +212 6 63 52 81 58. E-mail address: [email protected] (A. Moubarik). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.12.040
Thangavelu, 2010; Ladeira et al., 2010; Sun and Cheng, 2002). Cellulose is a polydisperse linear homopolymer consisting of regio- and enantioselective -1,4-glycosidic linked d-glucose units. The polymer contains three reactive hydroxyl groups at the C-2, C-3 and C-6 atoms (Heinze and Liebert, 2001; Sun et al., 2004). Hemicelluloses are composed of xylan, mannan, b-glucan, and xyloglucan polysaccharides (Vibe Scheller and Ulvskov, 2010). Lignin is a complex amorphous polymer composed of phenylpropanoid units consisting primarily of coniferyl, sinapyl, and p-coumaryl alcohols (Hatfield and Vermerris, 2001; Sugimoto et al., 2002). The use of lignin-based materials in various applications requires a proper investigation of its physico-chemical and thermal characteristics to understand the chemical and physical nature and thermal behaviour of this bio-polymer. To obtain different products from sugar cane bagasse, it is necessary to submit the biomass to separation process of its constituents. However, it is a challenge to isolate original lignin from sugar cane bagasse. A variety of methods like organosolv fractionation (Pye and Lora, 1991; Balogh et al., 1992), steam explosion (Rocha et al., 2012; Ibrahim et al., 2010; Glaser and Wright, 1998) and enzymatic hydrolysis (Shevchenko et al., 1999) have been developed in an attempt to isolate and identify cellulose and lignin. One of the most suitable methods for the extraction and separation of lignin is based on the alkaline delignification (Ibrahim et al., 2010; Sun et al., 2004; Fernandez-Bolanos et al., 1999; Käuper, 2004; Mousavioun and Doherty, 2010).
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Phenol formaldehyde resins are generally synthesized using petro-chemicals such as phenol and formaldehyde under alkaline catalysts. PF resins provide high strength and are extremely resistant to moisture which prevent delaminating and enable excellent thermal stability and low initial viscosity (Pizzi, 1993). Therefore, reduction of cost and substitution of petroleum-based raw materials are the most important direction of development of PF adhesives. Lignin has been incorporated into wood adhesives due to its similar structure to PF resins (Wang et al., 2009; Turunen et al., 2003; Tejado et al., 2007; Kazayawoko et al., 1992). The suitability of lignin-based materials for manufacturing of lignin–PF resins depends on the polysaccharide composition and total hydroxyl content (phenolic hydroxyl and aliphatic hydroxyl). The low content of polysaccharide was researched for the strength and water resistance of the resins, because the presence of polysaccharide decreased the reactivity of the lignin (Pizzi and Mittal, 1994). In the copolymerization of lignin–PF resins, all of the hydroxyl groups were important to activate the lignin. In this contribution, lignin fractions isolated from Moroccan sugar cane bagasse was studied using a combination of several techniques TGA, FT-IR, 1 H, 13 C NMR and GPC. The aim of this study was to get a better understanding on the chemical and thermal properties of the extracted lignin sample and to assess their suitability for partial incorporation into phenol formaldehyde (PF) resin. 2. Experimental methods 2.1. Biological material Sugar cane bagasse was obtained from a local sugar factory (Doukkala, Morocco). The bagasse was stored indoors during the experiments. The bagasse was air-dried for three days at ambient temperature (up to 8–10% equilibrium moisture content) and then cut into small pieces (1–2 cm). The cut bagasse was ground to pass a 1.0 mm size screen. All chemicals used were of analytical or reagent grade (Sigma–Aldrich, France). 2.2. Isolation of lignin The procedure for isolation of lignin by delignification with aqueous solution of sodium hydroxide is illustrated in Fig. 1. Twenty grams (oven-dry matter) of sugarcane bagasse were treated with a hot water at 70 ◦ C for 2 h. The solid-to-liquid ratio used was 1:10. The treatments were carried out in a steel reactor with a Parr 4836 temperature controller (Parr Instrument Company, Moline, IL). At the end of the reaction the pre-treated sugarcane bagasse was cooled (25 ◦ C), washed with water (S:L ratio = 1:10; w/w) and centrifuged (2000 rpm, t = 10 min) to separate the solubilized hemicelluloses. Lignin was then extracted from the pretreated sugar cane bagasse using alkaline treatment. Sugar cane bagasse was placed in a reactor with an aqueous alkaline (15% NaOH (m/v)) solution. The solid-to-liquid ratio was 1:10. The suspension was maintained under agitation (250 rpm) for 90 min at 98 ◦ C. At the end of the reaction the delignified material was filtered to obtain the black liquor without any fibrous materials. Sulfuric acid (5 N H2 SO4 ) was added until reaching pH 2 in order to precipitate the acidified lignin, which was then collected and washed by centrifugation and airdried. The lignin powder, with rather low amount of carbohydrate, was used directly in the preparation of lignin–PF adhesive. No additional purification steps were performed. All the yields of lignins represent the mean of at least triplicate analysis. Lignin samples were subjected to acetylation in order to enhance their solubility in organic solvent in gel permeation chromatography (GPC) and 1 H, 13 C NMR analysis. During the acetylation process, all the hydroxyl functional groups are substituted by new
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acetyl groups. Acetylation was performed using Vázquez et al. (1999) method. 2.3. Lignin characterization FT-IR measurements were performed in an ABB Bomem FTLA 2000-102 instrument by direct transmittance using KBr pellet technique. Each spectrum was recorded over 20 scans, in the range from 4000 to 200 cm−1 with a resolution of 4 cm−1 . Background spectra were collected before every sampling. KBr was previously oven-dried to avoid interferences due to the presence of water. The characteristic bands of lignin were assigned according to the literature. 1 H and 13 C NMR spectra of extracted lignin samples were recorded on a Bruker Avance 500 MHz spectrometer from 80 mg of sample dissolved in DMSO (1.0 mL). Each spectrum was recorded with 32,768 data points, 5.2 s pulse and pulse delay of 1.547 s (relaxation delay 2.5 s; 90◦ pulse). The 13 C NMR spectra were recorded at 25 ◦ C after 30,000 scans. Gel permeation chromatography provides a rapid way to obtain information on the molecular weight of polymers. Samples have been examined through THF-eluted 350A HT-GPC using a MALVERN instrument (TX, USA) equipped with three styrene–divinylbenzene copolymer gel columns of 50, 500, and 104 A´˚ from Polymer Laboratories. The columns were calibrated using polystyrene standards in the 92–66.000 g/mol range. The molecular weight and molecular number were compared to polystyrene standard; the presented results in this work are thus relative values. The flow rate of THF was 1 ml/min and the samples were dissolved in THF at a concentration of 1 mg/ml and stored for 24 h at 5 ◦ C to avoid variations in molecular weight. Thermogravimetric analysis (TGA) was used to determine the thermal stability and degradation of the lignin samples using a TGA Q50 thermogravimetric apparatus. Ten milligrams of each cured sample were placed on a balance located in the furnace and heat was applied over the temperature range from room temperature to 1000 ◦ C at a heating rate of 5 ◦ C/min in air. Mass losses vs. temperature thermograms were obtained showing the different decomposition processes. Three replicates were used for optimal adhesive mix. 2.4. Preparation of resin formulation A phenol-formaldehyde resol with a solids content of 46% and a viscosity of about 450 cp was prepared using a 2.2:1 formaldehyde: phenol ratio and 7.3% (w/w) of NaOH. The resols were prepared in a two liters glass reactor with mechanic stirring and temperature control. The necessary amount of the reactive according to the established formulation was fed into the reactor. When the operating temperature was reached (90 ◦ C), the extension of reaction was monitored, measuring resol viscosity at 25 ◦ C. The lignin–PF adhesives were prepared by copolymerisation of lignin (with variable amounts) at room temperature. 2.5. Plywood manufacture and testing Five ply laboratory plywood panels of dimension 250 mm × 250 mm × 10 mm were prepared from 2 mm thick Maritime pine (Pinus pinaster) veneers of 4% moisture content at a glue mix spread of 225 g/m2 single glue line. Plywood bonded with bagasse lignin–PF resin has been assembled and hot pressed under 12 bar at 160 ◦ C for 6 min. The fixed bonding conditions of 160 ◦ C pressing temperature and 12 bar were selected to reproduce the industrial conditions used to bond plywood panels. The longer pressing time of 6 min was used to assure full reaction.
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Sugar cane bagasse Extracted with hot water at 70°C during 2 hours (solid:liquid ratio; 1:10, w/w).
Separated sample Extracted with 15% NaOH (w/v) aqueous solution at 98 °C for 90 min (solid:liquid ratio; 1:10, w/w)
Residue : cellulose
Filtrate
Neutralized with 5N H2SO4 to pH 2
Acidified lignin
Washing with water and air-drying
Lignin
Fig. 1. Scheme for extractions of original lignins from Moroccan sugar cane bagasse.
Mechanical properties commonly taken into consideration in the general usage areas of plywood panels were investigated. Dry tensile strength and modulus of elasticity values of plywood panels were determined according to EN 314 (1993) and EN 310 (1993), respectively. Fifty specimens were used for each test method, and the results obtained are shown in Table 5.
calculated. The analysis of variance (ANOVA) was applied for the analysis of the results of Table 5. For each analysis, significance level of 5% was assumed. All statistical analyses were carried out using the software Statgraphics Plus 5.1 (Stat-point Technologies, Inc.). 3. Results and discussion
2.6. Formaldehyde emission by desiccator method 3.1. Yield of isolated lignins The formaldehyde emissions from the plywood were determined according to the European Norm (ISO/CD 12460-4) using a glass desiccator. The 24-h desiccator method uses a common glass desiccator with a volume of 10 L. Eight test pieces, with dimensions of 150 mm × 50 mm × 10 mm, which were cut from plywood, are positioned in the desiccator. The formaldehyde released from the test pieces at 23 ± 2 ◦ C and 50 ± 10% relative humidity, during 24 h is absorbed in a Petri dish filled with a 30 ml of distilled water. The released amount was then determined photometrically. Thirty replicates were used for each adhesive. 2.7. Statistical analysis Each experiment was repeated, at least, 10 times. The tests experiments concerning the plywood characterization were repeated 50 times. Means and standard deviations of data were
Alkaline treatment is usually an effective method to extract lignin from agricultural residues. Sodium hydroxide, one of the most common alkaline reagents, has been applied to treat a variety of agricultural residues (Kumar and Wyman, 2009; Ong et al., 2010). In this study, alkaline lignins were isolated with 15% NaOH at 98 ◦ C for 90 min. Yields of lignin and cellulose were calculated on a dry weight basis. The obtained yields of lignin and cellulose are respectively 13% and 42%. These values are in perfect consistency with the literature, which reports cellulose and lignin percentages of 50% and 15% (Pandey et al., 2000; Ezhumalai and Thangavelu, 2010; Ladeira et al., 2010), respectively. The two-stage treatments with hot water and 15% NaOH, totally released 86% of original lignin and 84% of original cellulose from sugar cane bagasse. This sequential treatment enabled the isolation of original lignin from sugar can bagasse.
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Table 1 Band (cm−1 )
Vibration
Assignment
3400–3405 2960–2925 2850–2840 ∼1600 1460 1425 1365 ∼1220 1125 1115 1030 855
st O H st C H st C H st C C C H deformation st C C ıip O H st C O(H) + C O(Ar) ıip Ar C H ıip Ar C H st C O(H) + C O(C) ıop Ar C H
Phenolic OH and aliphatic OH CH3 and CH2 groups OCH3 Aromatic skeleton Asymmetric in CH3 and CH2 Aromatic skeleton Phenolic OH Phenolic OH and ether Guaiacyl (G) Syringyl (S) First order aliphatic OH and ether Guaiacyl (G)
st: stretching vibration. ıip : in-plane deformation vibration. ıop : out-of-plane deformation vibration.
Fig. 2. FTIR spectra of the bagasse lignin.
3.2. Lignin characterization The FT-IR spectra of extracted lignins are illustrated in Fig. 2. FT-IR spectra reflects the chemical structure as well as the purity of lignins. The corresponding assignments and bands for lignins are presented in Table 1. The presence of syringyl (S) and guaiacyl (G) bands for lignins indicate that the lignin extracted from sugar cane bagasse is more similar to wood lignin than annual plant lignin, which is normally HGS lignin. This similarity is due to the absence of the band of hydroxyl phenyl propane (H) at 1166 cm−1 in the spectra (Zhao et al., 2009). The presence of guaiacyl-type (G) confirms that bagasse lignins had potential active site for polymerization. The presence of G-type unit revealed that bagasse lignin could react with formaldehyde and could be cross linked with formaldehyde in the same way as in the phenol formaldehyde condensation reaction. The presence of the signal band at ∼1460 cm−1 , assigned to C H deformation (asymmetric) in methyl, methylene and methoxyl groups, confirms that lignin aromatic structures did not change dramatically during the soda extraction procedure (Nadji et al., 2009).
Fig. 3.
13
Further information on the chemical structure of the isolated lignin samples was obtained by 13 C NMR. The spectra of lignin are shown in Fig. 3. In order to illustrate the distinct attributions, the corresponding assignments identified for the lignin are listed in Table 2 according to the previous literatures (Pan et al., 1994; Peter et al., 1996). The major peaks of the syringyl rings appear at 102 ppm (C2 and C6 ). Furthermore, the characteristic peaks of the side alkyl chains are shown at 76–73 ppm (C␣ OH in b-O-4 linked side chain) and 63–60 ppm (C␥ with C␣ O and C␥ O in p-coumaryl ester). In addition, the signal corresponding to the methoxyl group is observed at around 56–57 ppm. A very weak resonance of carbohydrates between 90 and 100 ppm indicates low concentration of residual sugars in lignin (Yuan et al., 2011). This observation agrees with the results of the FTIR spectra, in which the typical bands of cellulose were not found. The 1 H NMR spectra of bagasse lignin are shown in Fig. 4. Table 3 shows the hydrogen signal integrations subdivided into different structural regions (Goncalves et al., 2000; Guerra et al., 2004; Hiltunen et al., 2006). In the 1 H NMR spectrum, the two signals at 2.50 and 3.46 ppm arise from DMSO-d6 and HDO, respectively. The
C NMR spectrum of the bagasse lignin.
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Table 2
Peak (ppm)
Intensitya
Assignment
102 76–73 63 56
vs vs s w
C2 , C6 (S) C␣ OH in b-O-4 linked; side chain C-␥ in -5, C-␥ in coniferyl alcohol units Methoxyl group
a
Mw (g/mol) Mn (g/mol) Mw /Mn (polydispesity)
Lignin
Lignin acetylated
1315 699 1.88
2781 1365 2.03
vs: very strong; s: strong; w: weak; vw: very weak.
Table 3
Peak (ppm)
Assignment
8.0–6.0 6.9 6.6 3.1–4.2 2.5–2.2 2.2–1.9 1.5–0.8
Aromatic H in S and G units Aromatic H in G Aromatic H in S Methoxyl H H in aromatic acetates H in aliphatic acetates Aliphatic H
broad signal at 5.31 ppm corresponds to H-␣ of -5 structures. The signals at 4.23 ppm and signals between 4.84 and 5.28 ppm were attributed to H-, H-␣ and OH in -O-4 , respectively. The signal of H-␥ in -O-4 was overlapped with that of HDO. The small peak at 1.20 ppm corresponds to methylene groups of aliphatic chain. The molecular weight of lignin samples have been analyzed through THF-eluted GPC. As shown before, lignin samples have been acetylated to enhance their solubility in such solvent. The weight-average (Mw ) and number-average (Mn ) molecular weight, and polydispersity (Mw /Mn ) of the lignin before and after acetylation were determined (Table 4). It was observed that the Mw of lignin samples before being acetylated are comparatively lower than those after acetylation. This observation indicates that the macromolecules of lignin were not completely dissolved in THFeluted GPC. Thus, acetylation should be carried out. Soda bagasse lignins have lower Mw and Mn than organosolv and ethanol process lignins (El Mansouri and Salvado, 2006). This result corroborates the higher phenolic hydroxyl in these lignins in which the cleavage
Fig. 4.
1
of ␣ and -O-4 bonds is the predominant process in alkaline medium. Table 4 shows that the polydispersity for bagasse lignins was relatively low, indicating that bagasse lignins have a high fraction of low-molecular weight molecules. Lignins with high fractions of low molecular weight molecules are very suitable for condensation with PF because they are more reactive than those with high molecular weight molecules (Pizzi and Mittal, 1994). The TGA and DTG curves of lignin obtained by sequential extraction from bagasse are displayed in Fig. 5. As it can be seen, the decomposition of bagasse lignins covered wide temperature ranges from 200 to 1000 ◦ C. The degradation stage can be divided into three stages. The initial weight loss step occurs at 30–120 ◦ C due to the evaporation of absorbed water. The second stage occurred at around 180–350 ◦ C and is ascribed to the degradation of the carbohydrate components of the lignins, which were converted to volatile gases including CO, CO2 , and CH4 . After that, lignin was degraded over a wide range of temperature above 350 ◦ C. The degradation volatile products derived from lignin were phenolics, alcohols, aldehyde acids with the formation of gaseous products (CO, CO2 and CH4 ) (Liu et al., 2008). The TGA curve also shows that thermal degradation began to occur only after the materials have absorbed certain amounts of heat energy. The heat initiates the degradation processes and the breaking down of the structure by causing molecular chain ruptures. At 800 ◦ C, about 20% of non-volatile residue, still remained in solid form and were not completely burned. This result reveals that bagasse lignins are stable at high temperature, which is attributed to the high degree of branching and formation of highly condensed aromatic structure
H NMR spectrum of the bagasse lignin.
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Fig. 5. TGA and DTG curves of the bagasse lignin, ˇ = 5 ◦ C/min in air.
for bagasse lignins. The results obtained with thermogravimetric analysis confirm that in 180 ◦ C these lignins are not damaged.
3.3. Lignin valorization Sugar cane bagasse lignin was selected and used for the elaboration of wood adhesive. The dry tensile strength, wood failure, modulus of elasticity (MOE) and modulus of rupture (MOR) values of plywood glued with resins made with different weight ratio of lignin/PF (10/90, 30/70 and 50/50) was tested and compared to a control PF resin (0:100) at 160 ◦ C and 6 min press time. Table 5 reports the results of the effect of lignin substitution level on the mechanical performance of plywood panels. Up to 30% of lignin, the bond strength and wood failure appear to be relatively unaffected. Furthermore, the test reveals that in most cases, joint failure was cohesive in the wood (wood failure) and it was not due to failure either at the interface or at the adhesive itself. Compared to native PF resins, the addition of lignin up to 30% improves the MOE and MOR by 14% and 79%, respectively. Statically significant difference was present between the control and plywood manufactured with lignin–PF (30/70) resin. However, as it can be seen from Table 5, resin 50/50 exhibited dramatically low mechanical performances (tensile strength, wood failure, MOE and MOR), compared to the other resins. Finally, plywood panels with lignin–PF (30/70) resin shows slight reduction (40%) in formaldehyde emission compared to panels prepared with commercial PF resins. The ANOVA showed a significant difference between panels bonded
Fig. 6. FTIR spectra of PF resin and 30% lignin–PF resins.
with lignin–PF (30/70) and commercial PF resins. The reduction of formaldehyde emission of plywood panels is due to the lignin substitution of PF resins and the PF penetration in the wood. Beyond 30% a sharp increase in formaldehyde emission is observed. This is generally due to poor PF penetration in the wood. Fig. 6 shows a representative FTIR spectrum of PF and optimal resins composition (30% lignin–PF). This measurement was
Table 5
Formulations 0% LPF (control) 10% LPF 30% LPF 50% LPF
Composition lignin/PF (w/w) 0/100 10/90 30/70 50/50
Dry tensile strength (MPa) mean ± SD
Wood failure, mean (%)
MOE (MPa) mean ± SD
MOR (MPa) mean ± SD
Formaldehyde emission (mg/m2 /h) mean ± SD
1.2 ± 0.04a
70a
3114 ± 109a
44 ± 5a
2.52 ± 0.20a
1.5 ± 0.10b 1.8 ± 0.08c 0.8 ± 0.11d
60b 75c 20d
2907 ± 265a 3578 ± 98b 1734 ± 231c
47 ± 4a 79 ± 8b 32 ± 11a
2.03 ± 0.17b 1.51 ± 0.12c 2.11 ± 0.17ab
a, b, c, d values with the same superscript letters were not significantly different; SD: standard deviation. MOR: modulus of rupture. MOE: modulus of elasticity.
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used to monitor the reaction between PF and bagasse lignin. Spectral difference between PF and lignin–PF resins was observed at 980 cm−1 , which was assigned to C H stretching vibration of vinyl in the PF. The peaks at 1020 cm−1 , were ascribed to the presence of C O stretching vibration of aliphatic C OH, aliphatic C O (Ar) and methylol C OH. The peaks at 1020 cm−1 of lignin–PF resins were broader than that of the PF resin, which is attributed to the presence of other species (not only methylol–OH but also alphatic–OH in lignin–PF resins). The bond at 1250 cm−1 of PF resin was bigger than that in the lignin–PF resins, indicating more C O stretching of phenolic–OH groups in PF resin than in lignin–PF resins. 4. Conclusion In this work, the composition and physicochemical properties of lignin isolated from Moroccan sugar cane bagasse by alkali treatments were investigated. FT-IR spectroscopy and 1 H, 13 C NMR spectrometry reveals that lignin is mainly composed of G and S units and less free hydroxyls and highlights the absence of residual sugars in lignin. Molecular weight of lignin samples has been studied through THF-eluted GPC showing that bagasse lignin has much higher MW and Mn . This behavior is related to its higher content in C C linkages. Both structural and thermal properties suggest that bagasse lignin will be a better substitute in the synthesis of lignin–PF resins, as it presents higher amount of activated free ring positions, higher MW and higher thermal decomposition temperature. Lignin–based wood adhesives prepared with PF, were prepared and tested for application to wood panels. As a proof of concept, bagasse lignin could be used to replace 30% of the PF resins used to bond plywood panels, without adversely affecting bond properties. Moreover, the formaldehyde emission levels obtained from panels bonded with lignin–PF (30% lignin–PF) were lower than those obtained from panels made with control PF. References Balogh, D.T., Curvelo, A.A.S., De Groote, R.A., 1992. Solvent effect on organosolv lignin from Pinus Caribaea Hondurensis. Holzforschung 46, 343–348. Carrier, M., Hugo, T., Gorgens, J., Knoetze, J.H., 2011. Comparison of slow and vacuum pyrolysis of sugar cane bagasse. J. Anal. Appl. Pyrol. 90, 18–26. Devnarain P.B., 2003. Production of activated carbon from South African sugar-cane bagasse. M.Sc. (Eng.) Thesis. University of KwaZulu-Natal. El Mansouri, N.E., Salvado, J., 2006. Structural characterization of technical lignins for the production of adhesives: application to lignosulfonate, kraft, sodaanthraquinone, organosolv and ethanol process lignins. Ind. Crop. Prod. 24, 8–16. EN 310, 1993. Wood-based panels. Determination of modulus of elasticity in bending and of bending strength. EN 314-1/-2, 1993. Plywood-Bond Quality, Part 1. Test Methods. European Committee for Standardization, Brussels. Ezhumalai, S., Thangavelu, V., 2010. Kinetic and optimization studies on the bioconversion of lignocellulosic material into ethanol. BioResources 5, 1879–1894. Fernandez-Bolanos, J., Felizon, B., Heredia, A., Guillén, R., Jiménez, A., 1999. Characterization of the lignin obtained by alkaline delignification and of the cellulose residue from stem-exploded olive stones. Bioresour. Technol. 68, 121–132. Glaser, W.G., Wright, R.S., 1998. Steam assisted biomass fractionation. II. Fractionation behavior of various biomass resources. Biomass Bioenergy 14, 219–235. Goncalves, A.R., Ruzene, D.S., Moriya, R.Y., Oliveria, L.R.M., 2005. Pulping of sugarcane bagasse and straw and biobleaching of the pulps: conditions parameters and recycling of enzymes. In: 59th Appita Conference, Auckland, New Zealand, 16–19 May. Goncalves, A.R., Schuchardt, U., Bianchi, M.L., Curvelo, A.A.S., Braz, J., 2000. Piassava fibers (Attalea Funifera): NMR spectroscopy of their lignin. J. Braz. Chem. Soc. 11, 491–494. Guerra, A., Mendonca, R., Ferraz, A., Lu, F., Ralph, J., 2004. Structural characterization of lignin during Pinus Taeda wood treatment with Ceriporiopsis Subvermispora. Appl. Environ. Microbiol. 70, 4073–4078. Hatfield, R., Vermerris, W., 2001. Lignin formation in plants, the dilemma of linkage specificity. Plant Physiol. 126, 1351–1357. Heinze, T., Liebert, T., 2001. Unconventional methods in cellulose functionalization. Prog. Polym. Sci. 26, 1689–1762. Hiltunen, E., Alvila, L., Pakkanen, T.T., 2006. Characterization of Brauns’ lignin from fresh and vacuum-dried birch (Betula Pendula) wood. Wood. Sci. Technol. 40, 575–584.
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Comparing biological and thermochemical processing of sugarcane bagasse: an energy balance perspective. Biomass Bioenergy 35 (5), 2117–2126. Liu, Q., Wang, S.R., Zheng, Y., Luo, Z.Y., Cen, K.F., 2008. Mechanism study of wood lignin pyrolysis by using TG–FTIR analysis. J. Anal. Appl. Pyrol. 82, 170–177. Mamphweli, N.S., Meyer, E.L., 2010. Evaluation of the conversion efficiency of the 180 Nm3 /h Johansson biomass gasifierTM . Int. J. Energy Environ. 1 (1), 113–120. Mousavioun, P., Doherty, W.O.S., 2010. Chemical and thermal properties of fractionated bagasse soda lignin. Ind. Crop. Prod. 31 (1), 52–58. Mulinari, D.R., Voorwald, H.J.C., Cioffi, M.O.H., Da Silva, M.L.C.P., Luz, S.M., 2009. Preparation and properties of HDPE/sugarcane bagasse cellulose composites obtained for thermokinetic mixer. Carbohydr. Polym. 75, 317–321. Nadji, H., Diouf, P.N., Benaboura, A., Bedard, Y., Riedl, B., Stevanovic, T., 2009. Comparative study of lignins isolated from alfa grass (Stipa tenacissima L.). Bioresour. Technol. 100, 3585–3592. Ong, L.G.A., Chuah, C., Chew, A.L., 2010. Comparison of sodium hydroxide and potassium hydroxide followed by heat treatment on rice straw for cellulose production under solid state fermentation. J. Appl. Sci. 10 (21), 2608–2612. Pan, X.Q., Lachenal, D., Neirinck, V., Robert, D., 1994. Structure and reactivity of spruce mechanical pulp lignins: 13 C NMR spectral studies of isolated lignins. J. Wood Chem. Technol. 14, 483–506. Pandey, A., Soccol, C.R., Nigam, P., Soccol, V.T., 2000. Biotechnological potential of agro-industrial residues, I: sugarcane bagasse. Bioresour. Technol. 74, 69–80. Peter, M.F., Arthur, J.R., Jiang, J.E., 1996. Chemical structure of residual lignin from kraft pulp. J. Wood Chem. Technol. 16, 347–365. Pizzi, A., 1993. Wood Adhesives Chemistry and Technology, vol. 1. Marcel Dekker, New York. Pizzi, A., Mittal, K.L. (Eds.), 1994. Handbook of Adhesives Technology. Marcel Dekker, New York. Pye, E.K., Lora, J.H., 1991. The ALCELL process: a proven alternative to kraft pulping. TAPPI 74 (3), 113–118. Qureshi, K., Bhatti, I., Kazi, R., Ansari, A.K., 2008. Physical and chemical analysis of activated carbon prepared from sugarcane bagasse and use for sugar decolorisation. Int. J. Chem. Biol. Eng. 1, 3. Rocha, G.J.M., Gonc¸alves, A.R., Oliveira, B.R., Olivares, E.G., Rossell, C.E.V., 2012. Steam explosion pretreatment reproduction and alkaline delignification reactions performed on a pilot scale with sugarcane bagasse for bioethanol production. Ind. Crop. Prod. 35 (1), 274–279. Shevchenko, S.M., Beatson, R.P., Saddler, J.N., 1999. The nature of lignin from steam explosion/enzymatic hydrolysis of softwood. Appl. Biochem. Biotechnol. 77–79, 867–876. Sugimoto, T., Akiyama, T., Matsumoto, Y., Meshitsuka, G., 2002. The erythro/threo ratio of beta-O-4 structures as an important structural characteristic of lignin – part 2, changes in erythro/threo (E/T) ratio of beta-O-4 structures during delignification reactions. Holzforschung 56, 416–421. Sun, J.X., Sun, X.F., Zhao, H., Sun, R.C., 2004. Isolation and characterization of cellulose from sugarcane bagasse. Polym. Degrad. Stab. 84, 331–339. Sun, Y., Cheng, J., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83 (1), 1–11. Tejado, A., Pena, C., Labidi, J., Echevérria, J.M., Mondragon, I., 2007. Physicchemical characterization of lignins from different sources for use in phenol-formaldehyde resin synthesis. Bioresour. Technol. 98 (8), 1655–1663. Turunen, M., Alvila, L., Pakkanen, T.T., Rainio, J., 2003. Modification of phenolformaldehyde resol resins by lignin, starch and urea. J. Appl. Polym. Sci. 88 (2), 582–588. Vázquez, G., Freire, S., Bona, C.R., González, J., Antorrena, G., 1999. 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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Isolation and characterization of lignin from Moroccan sugar cane bagasse: Production of lignin–phenol-formaldehyde wood adhesive Amine Moubarik a,∗ , Nabil Grimi b , Nadia Boussetta b , Antonio Pizzi c a
MAScIR-NANOTECH, ENSET, Avenue de l’Armée Royale, Madinat El Irfane, 10100 Rabat, Morocco Université de Technologie de Compiègne, Unité Transformations Intégrées de la Matière Renouvelable (TIMR, EA4297), Centre de Recherche de Royallieu, B.P. 20529-60205, Compiègne Cedex, France c ENSTIB, Université de Nancy 1, Epinal, France b
a r t i c l e
i n f o
Article history: Received 29 September 2012 Received in revised form 18 December 2012 Accepted 22 December 2012 Keywords: Sugar cane bagasse Lignin Wood adhesive Plywood
a b s t r a c t lignin-based materials were isolated from Moroccan sugar cane bagasse after alkaline delignification. Sugar cane bagasse was subjected to hot water (70 ◦ C) and alkaline aqueous solutions (15% of sodium hydroxide (NaOH), 98 ◦ C) treatments. The dissolved lignin macromolecules were separated and purified. The isolated solid was then characterized by different complementary analysis (FT-IR; 1 H, 13 C NMR; GPC and TGA). In the present work, the possibility of preparing wood adhesives from bagasse lignin has been explored. The results showed that the delignification with 15% NaOH resulted in yields of cellulose and lignin of 42 ± 2.2% and 13 ± 1.5%, respectively. The extracted lignin scaffolds exhibits high reactivity due to the high content of hydroxyl group. Their higher molecular weight (2781 g/mol) and good thermal stability (180 ◦ C) make them excellent candidates for partial substitution of phenol formaldehyde (PF) resin. A resin formulation in which up to 30% of PF can be substituted by bagasse lignin gave good results and was employed for the elaboration of plywood panels which passed relevant international standard specifications for interior-grade panels. © 2013 Elsevier B.V. All rights reserved.
1. Introduction One of the most challenging topics in material science today is to convert biomass-derived waste and feedstock to highly added value-materials. For instance, sugar cane bagasse is a fibrous residue of sugar cane stalks left over after crushing and extraction process of the juice from sugar cane. About 54 million dry tons of bagasse are produced annually throughout the world (Mulinari et al., 2009). Most of the bagasse weight is in the form of the so-called fiber and rind particles, which have high length-width ratios (lengths up to a few cm) and correspond mostly to stalk fibro vascular bundles. For the sugar industries, this waste is mainly converted into energy through combustion (Leibbrandt et al., 2011), applications in pulping (Goncalves et al., 2005), activated carbon production (Devnarain, 2003; Qureshi et al., 2008), cellulosic ethanol production (Carrier et al., 2011) and gasification (Mamphweli and Meyer, 2010). Sugar cane bagasse consists of approximately 50% cellulose, 25% hemicellulose and 25% lignin (Pandey et al., 2000; Ezhumalai and
∗ Corresponding author at: NANOTECH-MAScIR (Moroccan Foundation for Advanced Science, Innovation and Research), ENSET, Avenue de l’Armée Royale, Madinat El Irfane, 10100 Rabat, Morocco. Tel.: +212 6 63 52 81 58. E-mail address: [email protected] (A. Moubarik). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.12.040
Thangavelu, 2010; Ladeira et al., 2010; Sun and Cheng, 2002). Cellulose is a polydisperse linear homopolymer consisting of regio- and enantioselective -1,4-glycosidic linked d-glucose units. The polymer contains three reactive hydroxyl groups at the C-2, C-3 and C-6 atoms (Heinze and Liebert, 2001; Sun et al., 2004). Hemicelluloses are composed of xylan, mannan, b-glucan, and xyloglucan polysaccharides (Vibe Scheller and Ulvskov, 2010). Lignin is a complex amorphous polymer composed of phenylpropanoid units consisting primarily of coniferyl, sinapyl, and p-coumaryl alcohols (Hatfield and Vermerris, 2001; Sugimoto et al., 2002). The use of lignin-based materials in various applications requires a proper investigation of its physico-chemical and thermal characteristics to understand the chemical and physical nature and thermal behaviour of this bio-polymer. To obtain different products from sugar cane bagasse, it is necessary to submit the biomass to separation process of its constituents. However, it is a challenge to isolate original lignin from sugar cane bagasse. A variety of methods like organosolv fractionation (Pye and Lora, 1991; Balogh et al., 1992), steam explosion (Rocha et al., 2012; Ibrahim et al., 2010; Glaser and Wright, 1998) and enzymatic hydrolysis (Shevchenko et al., 1999) have been developed in an attempt to isolate and identify cellulose and lignin. One of the most suitable methods for the extraction and separation of lignin is based on the alkaline delignification (Ibrahim et al., 2010; Sun et al., 2004; Fernandez-Bolanos et al., 1999; Käuper, 2004; Mousavioun and Doherty, 2010).
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Phenol formaldehyde resins are generally synthesized using petro-chemicals such as phenol and formaldehyde under alkaline catalysts. PF resins provide high strength and are extremely resistant to moisture which prevent delaminating and enable excellent thermal stability and low initial viscosity (Pizzi, 1993). Therefore, reduction of cost and substitution of petroleum-based raw materials are the most important direction of development of PF adhesives. Lignin has been incorporated into wood adhesives due to its similar structure to PF resins (Wang et al., 2009; Turunen et al., 2003; Tejado et al., 2007; Kazayawoko et al., 1992). The suitability of lignin-based materials for manufacturing of lignin–PF resins depends on the polysaccharide composition and total hydroxyl content (phenolic hydroxyl and aliphatic hydroxyl). The low content of polysaccharide was researched for the strength and water resistance of the resins, because the presence of polysaccharide decreased the reactivity of the lignin (Pizzi and Mittal, 1994). In the copolymerization of lignin–PF resins, all of the hydroxyl groups were important to activate the lignin. In this contribution, lignin fractions isolated from Moroccan sugar cane bagasse was studied using a combination of several techniques TGA, FT-IR, 1 H, 13 C NMR and GPC. The aim of this study was to get a better understanding on the chemical and thermal properties of the extracted lignin sample and to assess their suitability for partial incorporation into phenol formaldehyde (PF) resin. 2. Experimental methods 2.1. Biological material Sugar cane bagasse was obtained from a local sugar factory (Doukkala, Morocco). The bagasse was stored indoors during the experiments. The bagasse was air-dried for three days at ambient temperature (up to 8–10% equilibrium moisture content) and then cut into small pieces (1–2 cm). The cut bagasse was ground to pass a 1.0 mm size screen. All chemicals used were of analytical or reagent grade (Sigma–Aldrich, France). 2.2. Isolation of lignin The procedure for isolation of lignin by delignification with aqueous solution of sodium hydroxide is illustrated in Fig. 1. Twenty grams (oven-dry matter) of sugarcane bagasse were treated with a hot water at 70 ◦ C for 2 h. The solid-to-liquid ratio used was 1:10. The treatments were carried out in a steel reactor with a Parr 4836 temperature controller (Parr Instrument Company, Moline, IL). At the end of the reaction the pre-treated sugarcane bagasse was cooled (25 ◦ C), washed with water (S:L ratio = 1:10; w/w) and centrifuged (2000 rpm, t = 10 min) to separate the solubilized hemicelluloses. Lignin was then extracted from the pretreated sugar cane bagasse using alkaline treatment. Sugar cane bagasse was placed in a reactor with an aqueous alkaline (15% NaOH (m/v)) solution. The solid-to-liquid ratio was 1:10. The suspension was maintained under agitation (250 rpm) for 90 min at 98 ◦ C. At the end of the reaction the delignified material was filtered to obtain the black liquor without any fibrous materials. Sulfuric acid (5 N H2 SO4 ) was added until reaching pH 2 in order to precipitate the acidified lignin, which was then collected and washed by centrifugation and airdried. The lignin powder, with rather low amount of carbohydrate, was used directly in the preparation of lignin–PF adhesive. No additional purification steps were performed. All the yields of lignins represent the mean of at least triplicate analysis. Lignin samples were subjected to acetylation in order to enhance their solubility in organic solvent in gel permeation chromatography (GPC) and 1 H, 13 C NMR analysis. During the acetylation process, all the hydroxyl functional groups are substituted by new
297
acetyl groups. Acetylation was performed using Vázquez et al. (1999) method. 2.3. Lignin characterization FT-IR measurements were performed in an ABB Bomem FTLA 2000-102 instrument by direct transmittance using KBr pellet technique. Each spectrum was recorded over 20 scans, in the range from 4000 to 200 cm−1 with a resolution of 4 cm−1 . Background spectra were collected before every sampling. KBr was previously oven-dried to avoid interferences due to the presence of water. The characteristic bands of lignin were assigned according to the literature. 1 H and 13 C NMR spectra of extracted lignin samples were recorded on a Bruker Avance 500 MHz spectrometer from 80 mg of sample dissolved in DMSO (1.0 mL). Each spectrum was recorded with 32,768 data points, 5.2 s pulse and pulse delay of 1.547 s (relaxation delay 2.5 s; 90◦ pulse). The 13 C NMR spectra were recorded at 25 ◦ C after 30,000 scans. Gel permeation chromatography provides a rapid way to obtain information on the molecular weight of polymers. Samples have been examined through THF-eluted 350A HT-GPC using a MALVERN instrument (TX, USA) equipped with three styrene–divinylbenzene copolymer gel columns of 50, 500, and 104 A´˚ from Polymer Laboratories. The columns were calibrated using polystyrene standards in the 92–66.000 g/mol range. The molecular weight and molecular number were compared to polystyrene standard; the presented results in this work are thus relative values. The flow rate of THF was 1 ml/min and the samples were dissolved in THF at a concentration of 1 mg/ml and stored for 24 h at 5 ◦ C to avoid variations in molecular weight. Thermogravimetric analysis (TGA) was used to determine the thermal stability and degradation of the lignin samples using a TGA Q50 thermogravimetric apparatus. Ten milligrams of each cured sample were placed on a balance located in the furnace and heat was applied over the temperature range from room temperature to 1000 ◦ C at a heating rate of 5 ◦ C/min in air. Mass losses vs. temperature thermograms were obtained showing the different decomposition processes. Three replicates were used for optimal adhesive mix. 2.4. Preparation of resin formulation A phenol-formaldehyde resol with a solids content of 46% and a viscosity of about 450 cp was prepared using a 2.2:1 formaldehyde: phenol ratio and 7.3% (w/w) of NaOH. The resols were prepared in a two liters glass reactor with mechanic stirring and temperature control. The necessary amount of the reactive according to the established formulation was fed into the reactor. When the operating temperature was reached (90 ◦ C), the extension of reaction was monitored, measuring resol viscosity at 25 ◦ C. The lignin–PF adhesives were prepared by copolymerisation of lignin (with variable amounts) at room temperature. 2.5. Plywood manufacture and testing Five ply laboratory plywood panels of dimension 250 mm × 250 mm × 10 mm were prepared from 2 mm thick Maritime pine (Pinus pinaster) veneers of 4% moisture content at a glue mix spread of 225 g/m2 single glue line. Plywood bonded with bagasse lignin–PF resin has been assembled and hot pressed under 12 bar at 160 ◦ C for 6 min. The fixed bonding conditions of 160 ◦ C pressing temperature and 12 bar were selected to reproduce the industrial conditions used to bond plywood panels. The longer pressing time of 6 min was used to assure full reaction.
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Sugar cane bagasse Extracted with hot water at 70°C during 2 hours (solid:liquid ratio; 1:10, w/w).
Separated sample Extracted with 15% NaOH (w/v) aqueous solution at 98 °C for 90 min (solid:liquid ratio; 1:10, w/w)
Residue : cellulose
Filtrate
Neutralized with 5N H2SO4 to pH 2
Acidified lignin
Washing with water and air-drying
Lignin
Fig. 1. Scheme for extractions of original lignins from Moroccan sugar cane bagasse.
Mechanical properties commonly taken into consideration in the general usage areas of plywood panels were investigated. Dry tensile strength and modulus of elasticity values of plywood panels were determined according to EN 314 (1993) and EN 310 (1993), respectively. Fifty specimens were used for each test method, and the results obtained are shown in Table 5.
calculated. The analysis of variance (ANOVA) was applied for the analysis of the results of Table 5. For each analysis, significance level of 5% was assumed. All statistical analyses were carried out using the software Statgraphics Plus 5.1 (Stat-point Technologies, Inc.). 3. Results and discussion
2.6. Formaldehyde emission by desiccator method 3.1. Yield of isolated lignins The formaldehyde emissions from the plywood were determined according to the European Norm (ISO/CD 12460-4) using a glass desiccator. The 24-h desiccator method uses a common glass desiccator with a volume of 10 L. Eight test pieces, with dimensions of 150 mm × 50 mm × 10 mm, which were cut from plywood, are positioned in the desiccator. The formaldehyde released from the test pieces at 23 ± 2 ◦ C and 50 ± 10% relative humidity, during 24 h is absorbed in a Petri dish filled with a 30 ml of distilled water. The released amount was then determined photometrically. Thirty replicates were used for each adhesive. 2.7. Statistical analysis Each experiment was repeated, at least, 10 times. The tests experiments concerning the plywood characterization were repeated 50 times. Means and standard deviations of data were
Alkaline treatment is usually an effective method to extract lignin from agricultural residues. Sodium hydroxide, one of the most common alkaline reagents, has been applied to treat a variety of agricultural residues (Kumar and Wyman, 2009; Ong et al., 2010). In this study, alkaline lignins were isolated with 15% NaOH at 98 ◦ C for 90 min. Yields of lignin and cellulose were calculated on a dry weight basis. The obtained yields of lignin and cellulose are respectively 13% and 42%. These values are in perfect consistency with the literature, which reports cellulose and lignin percentages of 50% and 15% (Pandey et al., 2000; Ezhumalai and Thangavelu, 2010; Ladeira et al., 2010), respectively. The two-stage treatments with hot water and 15% NaOH, totally released 86% of original lignin and 84% of original cellulose from sugar cane bagasse. This sequential treatment enabled the isolation of original lignin from sugar can bagasse.
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Table 1 Band (cm−1 )
Vibration
Assignment
3400–3405 2960–2925 2850–2840 ∼1600 1460 1425 1365 ∼1220 1125 1115 1030 855
st O H st C H st C H st C C C H deformation st C C ıip O H st C O(H) + C O(Ar) ıip Ar C H ıip Ar C H st C O(H) + C O(C) ıop Ar C H
Phenolic OH and aliphatic OH CH3 and CH2 groups OCH3 Aromatic skeleton Asymmetric in CH3 and CH2 Aromatic skeleton Phenolic OH Phenolic OH and ether Guaiacyl (G) Syringyl (S) First order aliphatic OH and ether Guaiacyl (G)
st: stretching vibration. ıip : in-plane deformation vibration. ıop : out-of-plane deformation vibration.
Fig. 2. FTIR spectra of the bagasse lignin.
3.2. Lignin characterization The FT-IR spectra of extracted lignins are illustrated in Fig. 2. FT-IR spectra reflects the chemical structure as well as the purity of lignins. The corresponding assignments and bands for lignins are presented in Table 1. The presence of syringyl (S) and guaiacyl (G) bands for lignins indicate that the lignin extracted from sugar cane bagasse is more similar to wood lignin than annual plant lignin, which is normally HGS lignin. This similarity is due to the absence of the band of hydroxyl phenyl propane (H) at 1166 cm−1 in the spectra (Zhao et al., 2009). The presence of guaiacyl-type (G) confirms that bagasse lignins had potential active site for polymerization. The presence of G-type unit revealed that bagasse lignin could react with formaldehyde and could be cross linked with formaldehyde in the same way as in the phenol formaldehyde condensation reaction. The presence of the signal band at ∼1460 cm−1 , assigned to C H deformation (asymmetric) in methyl, methylene and methoxyl groups, confirms that lignin aromatic structures did not change dramatically during the soda extraction procedure (Nadji et al., 2009).
Fig. 3.
13
Further information on the chemical structure of the isolated lignin samples was obtained by 13 C NMR. The spectra of lignin are shown in Fig. 3. In order to illustrate the distinct attributions, the corresponding assignments identified for the lignin are listed in Table 2 according to the previous literatures (Pan et al., 1994; Peter et al., 1996). The major peaks of the syringyl rings appear at 102 ppm (C2 and C6 ). Furthermore, the characteristic peaks of the side alkyl chains are shown at 76–73 ppm (C␣ OH in b-O-4 linked side chain) and 63–60 ppm (C␥ with C␣ O and C␥ O in p-coumaryl ester). In addition, the signal corresponding to the methoxyl group is observed at around 56–57 ppm. A very weak resonance of carbohydrates between 90 and 100 ppm indicates low concentration of residual sugars in lignin (Yuan et al., 2011). This observation agrees with the results of the FTIR spectra, in which the typical bands of cellulose were not found. The 1 H NMR spectra of bagasse lignin are shown in Fig. 4. Table 3 shows the hydrogen signal integrations subdivided into different structural regions (Goncalves et al., 2000; Guerra et al., 2004; Hiltunen et al., 2006). In the 1 H NMR spectrum, the two signals at 2.50 and 3.46 ppm arise from DMSO-d6 and HDO, respectively. The
C NMR spectrum of the bagasse lignin.
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Table 2
Peak (ppm)
Intensitya
Assignment
102 76–73 63 56
vs vs s w
C2 , C6 (S) C␣ OH in b-O-4 linked; side chain C-␥ in -5, C-␥ in coniferyl alcohol units Methoxyl group
a
Mw (g/mol) Mn (g/mol) Mw /Mn (polydispesity)
Lignin
Lignin acetylated
1315 699 1.88
2781 1365 2.03
vs: very strong; s: strong; w: weak; vw: very weak.
Table 3
Peak (ppm)
Assignment
8.0–6.0 6.9 6.6 3.1–4.2 2.5–2.2 2.2–1.9 1.5–0.8
Aromatic H in S and G units Aromatic H in G Aromatic H in S Methoxyl H H in aromatic acetates H in aliphatic acetates Aliphatic H
broad signal at 5.31 ppm corresponds to H-␣ of -5 structures. The signals at 4.23 ppm and signals between 4.84 and 5.28 ppm were attributed to H-, H-␣ and OH in -O-4 , respectively. The signal of H-␥ in -O-4 was overlapped with that of HDO. The small peak at 1.20 ppm corresponds to methylene groups of aliphatic chain. The molecular weight of lignin samples have been analyzed through THF-eluted GPC. As shown before, lignin samples have been acetylated to enhance their solubility in such solvent. The weight-average (Mw ) and number-average (Mn ) molecular weight, and polydispersity (Mw /Mn ) of the lignin before and after acetylation were determined (Table 4). It was observed that the Mw of lignin samples before being acetylated are comparatively lower than those after acetylation. This observation indicates that the macromolecules of lignin were not completely dissolved in THFeluted GPC. Thus, acetylation should be carried out. Soda bagasse lignins have lower Mw and Mn than organosolv and ethanol process lignins (El Mansouri and Salvado, 2006). This result corroborates the higher phenolic hydroxyl in these lignins in which the cleavage
Fig. 4.
1
of ␣ and -O-4 bonds is the predominant process in alkaline medium. Table 4 shows that the polydispersity for bagasse lignins was relatively low, indicating that bagasse lignins have a high fraction of low-molecular weight molecules. Lignins with high fractions of low molecular weight molecules are very suitable for condensation with PF because they are more reactive than those with high molecular weight molecules (Pizzi and Mittal, 1994). The TGA and DTG curves of lignin obtained by sequential extraction from bagasse are displayed in Fig. 5. As it can be seen, the decomposition of bagasse lignins covered wide temperature ranges from 200 to 1000 ◦ C. The degradation stage can be divided into three stages. The initial weight loss step occurs at 30–120 ◦ C due to the evaporation of absorbed water. The second stage occurred at around 180–350 ◦ C and is ascribed to the degradation of the carbohydrate components of the lignins, which were converted to volatile gases including CO, CO2 , and CH4 . After that, lignin was degraded over a wide range of temperature above 350 ◦ C. The degradation volatile products derived from lignin were phenolics, alcohols, aldehyde acids with the formation of gaseous products (CO, CO2 and CH4 ) (Liu et al., 2008). The TGA curve also shows that thermal degradation began to occur only after the materials have absorbed certain amounts of heat energy. The heat initiates the degradation processes and the breaking down of the structure by causing molecular chain ruptures. At 800 ◦ C, about 20% of non-volatile residue, still remained in solid form and were not completely burned. This result reveals that bagasse lignins are stable at high temperature, which is attributed to the high degree of branching and formation of highly condensed aromatic structure
H NMR spectrum of the bagasse lignin.
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301
Fig. 5. TGA and DTG curves of the bagasse lignin, ˇ = 5 ◦ C/min in air.
for bagasse lignins. The results obtained with thermogravimetric analysis confirm that in 180 ◦ C these lignins are not damaged.
3.3. Lignin valorization Sugar cane bagasse lignin was selected and used for the elaboration of wood adhesive. The dry tensile strength, wood failure, modulus of elasticity (MOE) and modulus of rupture (MOR) values of plywood glued with resins made with different weight ratio of lignin/PF (10/90, 30/70 and 50/50) was tested and compared to a control PF resin (0:100) at 160 ◦ C and 6 min press time. Table 5 reports the results of the effect of lignin substitution level on the mechanical performance of plywood panels. Up to 30% of lignin, the bond strength and wood failure appear to be relatively unaffected. Furthermore, the test reveals that in most cases, joint failure was cohesive in the wood (wood failure) and it was not due to failure either at the interface or at the adhesive itself. Compared to native PF resins, the addition of lignin up to 30% improves the MOE and MOR by 14% and 79%, respectively. Statically significant difference was present between the control and plywood manufactured with lignin–PF (30/70) resin. However, as it can be seen from Table 5, resin 50/50 exhibited dramatically low mechanical performances (tensile strength, wood failure, MOE and MOR), compared to the other resins. Finally, plywood panels with lignin–PF (30/70) resin shows slight reduction (40%) in formaldehyde emission compared to panels prepared with commercial PF resins. The ANOVA showed a significant difference between panels bonded
Fig. 6. FTIR spectra of PF resin and 30% lignin–PF resins.
with lignin–PF (30/70) and commercial PF resins. The reduction of formaldehyde emission of plywood panels is due to the lignin substitution of PF resins and the PF penetration in the wood. Beyond 30% a sharp increase in formaldehyde emission is observed. This is generally due to poor PF penetration in the wood. Fig. 6 shows a representative FTIR spectrum of PF and optimal resins composition (30% lignin–PF). This measurement was
Table 5
Formulations 0% LPF (control) 10% LPF 30% LPF 50% LPF
Composition lignin/PF (w/w) 0/100 10/90 30/70 50/50
Dry tensile strength (MPa) mean ± SD
Wood failure, mean (%)
MOE (MPa) mean ± SD
MOR (MPa) mean ± SD
Formaldehyde emission (mg/m2 /h) mean ± SD
1.2 ± 0.04a
70a
3114 ± 109a
44 ± 5a
2.52 ± 0.20a
1.5 ± 0.10b 1.8 ± 0.08c 0.8 ± 0.11d
60b 75c 20d
2907 ± 265a 3578 ± 98b 1734 ± 231c
47 ± 4a 79 ± 8b 32 ± 11a
2.03 ± 0.17b 1.51 ± 0.12c 2.11 ± 0.17ab
a, b, c, d values with the same superscript letters were not significantly different; SD: standard deviation. MOR: modulus of rupture. MOE: modulus of elasticity.
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used to monitor the reaction between PF and bagasse lignin. Spectral difference between PF and lignin–PF resins was observed at 980 cm−1 , which was assigned to C H stretching vibration of vinyl in the PF. The peaks at 1020 cm−1 , were ascribed to the presence of C O stretching vibration of aliphatic C OH, aliphatic C O (Ar) and methylol C OH. The peaks at 1020 cm−1 of lignin–PF resins were broader than that of the PF resin, which is attributed to the presence of other species (not only methylol–OH but also alphatic–OH in lignin–PF resins). The bond at 1250 cm−1 of PF resin was bigger than that in the lignin–PF resins, indicating more C O stretching of phenolic–OH groups in PF resin than in lignin–PF resins. 4. Conclusion In this work, the composition and physicochemical properties of lignin isolated from Moroccan sugar cane bagasse by alkali treatments were investigated. FT-IR spectroscopy and 1 H, 13 C NMR spectrometry reveals that lignin is mainly composed of G and S units and less free hydroxyls and highlights the absence of residual sugars in lignin. Molecular weight of lignin samples has been studied through THF-eluted GPC showing that bagasse lignin has much higher MW and Mn . This behavior is related to its higher content in C C linkages. Both structural and thermal properties suggest that bagasse lignin will be a better substitute in the synthesis of lignin–PF resins, as it presents higher amount of activated free ring positions, higher MW and higher thermal decomposition temperature. Lignin–based wood adhesives prepared with PF, were prepared and tested for application to wood panels. As a proof of concept, bagasse lignin could be used to replace 30% of the PF resins used to bond plywood panels, without adversely affecting bond properties. Moreover, the formaldehyde emission levels obtained from panels bonded with lignin–PF (30% lignin–PF) were lower than those obtained from panels made with control PF. References Balogh, D.T., Curvelo, A.A.S., De Groote, R.A., 1992. Solvent effect on organosolv lignin from Pinus Caribaea Hondurensis. Holzforschung 46, 343–348. Carrier, M., Hugo, T., Gorgens, J., Knoetze, J.H., 2011. Comparison of slow and vacuum pyrolysis of sugar cane bagasse. J. Anal. Appl. Pyrol. 90, 18–26. Devnarain P.B., 2003. Production of activated carbon from South African sugar-cane bagasse. M.Sc. (Eng.) Thesis. University of KwaZulu-Natal. El Mansouri, N.E., Salvado, J., 2006. Structural characterization of technical lignins for the production of adhesives: application to lignosulfonate, kraft, sodaanthraquinone, organosolv and ethanol process lignins. Ind. Crop. Prod. 24, 8–16. EN 310, 1993. Wood-based panels. Determination of modulus of elasticity in bending and of bending strength. EN 314-1/-2, 1993. Plywood-Bond Quality, Part 1. Test Methods. European Committee for Standardization, Brussels. Ezhumalai, S., Thangavelu, V., 2010. Kinetic and optimization studies on the bioconversion of lignocellulosic material into ethanol. BioResources 5, 1879–1894. Fernandez-Bolanos, J., Felizon, B., Heredia, A., Guillén, R., Jiménez, A., 1999. Characterization of the lignin obtained by alkaline delignification and of the cellulose residue from stem-exploded olive stones. Bioresour. Technol. 68, 121–132. Glaser, W.G., Wright, R.S., 1998. Steam assisted biomass fractionation. II. Fractionation behavior of various biomass resources. Biomass Bioenergy 14, 219–235. Goncalves, A.R., Ruzene, D.S., Moriya, R.Y., Oliveria, L.R.M., 2005. Pulping of sugarcane bagasse and straw and biobleaching of the pulps: conditions parameters and recycling of enzymes. In: 59th Appita Conference, Auckland, New Zealand, 16–19 May. Goncalves, A.R., Schuchardt, U., Bianchi, M.L., Curvelo, A.A.S., Braz, J., 2000. Piassava fibers (Attalea Funifera): NMR spectroscopy of their lignin. J. Braz. Chem. Soc. 11, 491–494. Guerra, A., Mendonca, R., Ferraz, A., Lu, F., Ralph, J., 2004. Structural characterization of lignin during Pinus Taeda wood treatment with Ceriporiopsis Subvermispora. Appl. Environ. Microbiol. 70, 4073–4078. Hatfield, R., Vermerris, W., 2001. Lignin formation in plants, the dilemma of linkage specificity. Plant Physiol. 126, 1351–1357. Heinze, T., Liebert, T., 2001. Unconventional methods in cellulose functionalization. Prog. Polym. Sci. 26, 1689–1762. Hiltunen, E., Alvila, L., Pakkanen, T.T., 2006. Characterization of Brauns’ lignin from fresh and vacuum-dried birch (Betula Pendula) wood. Wood. Sci. Technol. 40, 575–584.
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