Utilization of sorghum lignin to improve adhesion strength of soy protein adhesives on wood veneer

Industrial Crops and Products 50 (2013) 501–509

Contents lists available at ScienceDirect

Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Utilization of sorghum lignin to improve adhesion strength of soy protein adhesives on wood veneer Zhigang Xiao a,1 , Yonghui Li b , Xiaorong Wu c , Guangyan Qi b , Ningbo Li c , Ke Zhang c , Donghai Wang c , Xiuzhi Susan Sun b,∗ a

College of Food Science, Northeast Agricultural University, Harbin, Heilongjiang, 150030, China Department of Grain Science and Industry, Kansas State University, Manhattan, KS, 66506, USA c Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS, 66506, USA b

a r t i c l e

i n f o

Article history: Received 17 December 2012 Received in revised form 16 July 2013 Accepted 21 July 2013 Keywords: Lignin Extrusion Softening temperature Thermal weight loss Shear strength Water resistance Soy protein adhesive

a b s t r a c t Features of sorghum lignin (SL) and extruded sorghum lignin (ESL) were examined. Adhesion properties of lignin (SL or ESL) blended soy protein adhesives (SPA) based on soy protein isolates (SPI) or modified soy protein (MSP) were also investigated, respectively. Both SL and ESL exhibited similar softening temperature at around 112 ◦ C; however, the softening enthalpy of ESL was much larger than that of SL. The thermal stability of lignin was significantly improved through the extrusion process. The ratio of relative absorbance for free OH groups to the bands at 1510 cm−1 and 1600 cm−1 went from 0.55 and 0.54 in SL to 0.74 and 0.66 in ESL; carbonyl groups went from 0.81 and 0.80 in SL to 1.08 and 0.96 in ESL. Extrusion changed the microstructure of SL from large masses to small irregular particles. The shear strength and water resistance of lignin (SL or ESL)-blended SPAs (SPI based) on wood veneer joints were obviously improved. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Lignin is one of the most abundant organic polymers in nature, with supplies second only to cellulose and hemicellulose, and is the only non-petroleum resource that can provide renewable aromatic compounds (Toledano et al., 2010). The lignin polymer is derived from phenylpropane units, usually including guaiacyl (G), syringyl (S), and p-hydroxyphenal (H) propane. The main source for lignin polymer is the waste liquid from paper pulp production, but agricultural residues such as crop straws and rice hulls are also promising sources. Sorghum is the third most important cereal crop in the U.S. and the fifth in the world, and the U.S. is the number one producer and explorer of sorghum (Li et al., 2011). In the past few years, sorghum grains and stovers have been extensively investigated for fuel ethanol (Wu et al., 2010; Theerarattananoon et al., 2010; Xu et al., 2011), while sorghum lignin from the stovers still remains as a low value byproduct. Moreover, sorghum lignin

 Contribution no. (12-075-J) from the Kansas Agricultural Experiment Station. ∗ Corresponding author. Tel.: +1 785 532 4077; fax: +1 785 532 7193. E-mail address: [email protected] (X.S. Sun). Former visiting scientist at Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA. 1

0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.07.057

contained substantial amounts of G and S units with fewer H unit (She et al., 2010; Sun et al., 2013). The three most commonly used methods for lignin extraction are high boiling solvent (HBS), acidolysis, and enzymatic hydrolysis (Jääskeläinen et al., 2003; Khan and Ashraf, 2006). Because lignin contains various functional groups and bonds, such as phenolic hydroxyl, alcohol hydroxyl, aromatic units, and ether linkages, it can couple with other organic and inorganic materials by side-chain selection, graft copolymerization, redox, aromatic selection, sulphonation, or phenolic modification (Malutan et al., 2008; Ungureanu et al., 2009), but the properties of lignin vary according to its biological sources and isolation techniques (Ungureanu et al., 2009). As a result, utilization of lignin is always challenging and frequently involves modification or coupling with other macromolecular materials (Alonso et al., 2005). Common techniques for lignin modification use chemical, physical, or biological methods to increase its reactivities (Ungureanu et al., 2009). Recent advances in lignin modification explored microwave, ultrafiltration, and ultrasonic technology to improve the structural and thermal characteristics of lignin. Infantes et al. (2007) pointed out that microwave-assisted oxidation promoted the extention of lateral chains and aromatic rings in lignin. The C O groups increased in the 1700–1720 cm−1 and 1650–1700 cm−1 ranges, and the aromatic signal C C decreased at around 1600 cm−1 . Toledano et al. (2010) used ceramic membranes of different molecular weight

502

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

cutoffs (5, 10, and 15 kDa) to obtain lignins with specific molecular weights from the black liquor of paper pulp. Ren and Fang (2005a,b) found ultrasonic processing very effective in increasing the contents of phenolic hydroxyl and alcohol hydroxyl. 1 H NMR spectra of ultrasonic treated and non-treated lignins revealed that the ratio of G-lignin and S-lignin peak areas to the sum of all peak area was from 3.61% and 0.77% to 0%. Extrusion of biomaterials results in instantaneous changes in physical and chemical properties under high complex energy in the internal environment of a smooth barrel. Extrusion has been successfully applied in the field of biomaterial processing with short reaction time, higher thermal efficiency in internal environment, and improved product properties (Xiao et al., 2008a,b; Li et al., 2011; Li and Sun, 2011). An extruder is a complex machine with operational units for transporting, mixing, heating, and shearing. Biomaterials obtain high internal energy under the action of operational units in the extruder. Squeezing products out of the extruder instantaneously releases energy and water vapor; biomaterial structures can be severely altered by the extrusion process. Soy protein adhesives have long been studied and recognized for their advantages over traditional petroleum-based adhesives, which contain phenol formaldehyde, a highly toxic chemical that causes both environmental and health problems during processing and distribution (Sun and Bian, 1999). Recently, lignin polymers have been widely applied in macromolecule fields, reported as being more efficient than petroleum-based polymers, and touted for avoiding environmental pollution (Alonso et al., 2005). Estimates are that up to 50% phenol can be substituted by bagasse lignin to give lignin phenol formaldehyde (LPF) wood adhesive better bonding strength compared with the control phenol formaldehyde (CPF) wood adhesive (Khan et al., 2004). Moreover, phenol formaldehyde adhesive (PF) can be synthesized with up to 20% enzymatic hydrolysis lignin (EHL) by weight, replacing portions of phenol (Jin et al., 2010). The reactions between lignin and other macromolecules are based on the formation of cross-link bonds such as lignin–lignin, lignin–phenol, phenol–phenol methylene bridges, and urethane bridges (Batubenga et al., 1995). Soy protein adhesives’ low adhesion strength and water resistance greatly limit their practical applications. Fortunately, adhesion strength and water resistance of soy protein adhesives can be improved by modifying polypeptide chains, which improves the unfolded properties of soy protein molecules (Hettiarachchy et al., 1995). The unfolded molecules become entangled during the curing process while retaining bond strength and increasing the adhesive surface area (Lambuth, 1994; Li et al., 2010). Moreover, modified proteins could also expose hydrophobic amino acids previously buried inside the molecule to increase water resistance (Sun and Bian, 1999). Physical, chemical, or enzymatic modification methods can be employed to increase unfolded molecules and expose hydrophobic amino acids (Mo et al., 2004). In their work on thermal properties and adhesion strength of modified soy storage proteins, Mo et al. (2004) found that adhesive strength and water resistance improved significantly for all proteins modified with sodium hydroxide. Furthermore, gluing strength and water resistance also improved for sodium dodecyl sulfate- and urea-modified protein containing large amounts of 7S globulins (Li et al., 2009). Qi and Sun (2010) discussed the features of sodium bisulfite (NaHSO3 )-modified soy protein (MSP) adhesives. Their results indicated that better water resistance of MSP was due to breaking the disulfite bonds in soy proteins during sodium bisulfite modification, which brought the formerly embedded hydrophobic groups to the surfaces of protein molecules. In the aforementioned protocols of soy protein-based adhesives or lignin-based adhesives, the possibility of generating strong bonding effects depends on the coupling reactions of side chains,

link-bonds, and functional groups; thus, we investigated whether shear strength and water resistance of soy protein adhesives can be improved by adding lignin. We added SL and ESL to soy protein adhesives under four reaction mediums: neutral water, alkaline solution, soy protein isolate (SPI), and MSP. Thermal characteristics and structural features of SL and ESL were explored using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). 2. Materials and methods 2.1. Materials Defatted soy flour as the starting material for soy protein adhesives was purchased from Cargill (Cedar Rapids, IA). Cherry wood veneers were provided by Veneer One (Oceanside, NY). Sodium hydroxide, pyridine, acetic acid, ethyl ether anhydrous, and chloroform were purchased from Fisher Scientific (Fair Lawn, NJ), and used as received. 2.2. Sample preparation 2.2.1. Sorghum lignin (SL) The SL was isolated from grain sorghum stalk using a two-stage sulfuric acid hydrolysis process described in the NREL’s Laboratory Analytical Procedure (LAP) (Sluiter et al., 2005). 2.2.2. Extruded sorghum lignin (ESL) SL was rinsed with distilled water until the filtrate was neutral. The washed SL was dried in a vacuum oven (285A, Fisher Scientific, Dubuque, IA) at 50 ◦ C for 24 h. The dried SL was adjusted to moisture content of 15% (w.b.), then fed into a twin-screw extruder (Haake TW100) with a smooth barrel running at 220 rpm to produce ESL. The temperatures of the three barrel zones were maintained at 70, 110, and 110 ◦ C from the feeding port to the die section. 2.2.3. Soy protein adhesives (SPA) Soy protein isolate (SPI) was isolated from defatted soy flour by isoelectric point precipitation at pH 4.2 and then adjusted to neutral pH (Mo et al., 2004). The precipitate was freeze–dried and then milled into power (>90% passed US #100 sieve). Modified soy protein adhesive (MSP) was prepared according to the method described in the literature (Qi and Sun, 2010). Briefly, defatted soy flour was dispersed in water at a 6.25% solid content at pH 9.5, NaHSO3 was added to the slurry at 6 g/L based on water volume, and the slurry was stirred for 2 h at room temperature. The pH of the slurry was adjusted to 5.4 with 2N hydrochloric acid (HCl), and carbohydrates and some glycinin proteins were remove via centrifugation. The pH of the supernatant was then adjusted to 4.8 with the same HCl, and the precipitated modified protein was centrifuged to obtain MSP with a solid content of about 38%. SPI or MSP suspensions were obtained by dispersing SPI or MSP in water (pH of 7) or an alkaline solution of pH 12 to achieve a 10% solid content. SL or ESL were then blended into SPI or MSP suspensions at 0, 10, 20, 30, 40, and 50% (total weight basis) and stirred for 2 h at room temperature. The resulting mixtures as SPA were used to study the shear strength and water resistance of the adhesives. 2.2.4. Wood specimen The cherry wood specimen was prepared according to the methods described by Sun and Bian (1999). Each cherry wood veneer piece was 50 mm × 20 mm × 3 mm (length × width × thickness), and two pieces were glued to form a specimen. The SPA slurries were brushed onto each end (20 mm × 20 mm). The total solid content (SPI or MSP and SL or ESL) was approximately 1.50 mg/cm2 .

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

The two cherry-wood veneer pieces were left at room temperature for 15 min before they were assembled by hand and hot-pressed (Model 3890; Auto “M,” Carver Inc., Wabash, IN) at 170 ◦ C and 2 MPa for 10 min. The pressed wood veneer joints were cooled and stored at 50% relative humidity (RH) and 23 ◦ C for 48 h before measurement. 2.3. Characterization of lignin 2.3.1. Purification SL and ESL were purified by the following the methods described by Lundquist et al. (1977): 6.80 g of SL or ESL was dissolved in 210 mL of pyridine–acetic acid–water (9:1:4) solution. The solution was extracted twice with 270 mL of chloroform (total 540 mL) to avoid contamination due to incomplete separation of the chloroform and aqueous layers. The chloroform layer was a clear solution that contained lignin, and was free from other precipitated materials. The lignin was precipitated out by slowly dropping the chloroform solution (about 250 mL) into stirring ether (1600 mL). The precipitate was washed three times with 325 mL (total 975 mL) ether. The lignin precipitate was purified lignin and was dried in a vacuum oven for future use.

503

2.3.7. Microstructural properties Both SEM and TEM (transmission electron microscope) were used to examine the microstructures of SL and ESL. Ground SL and ESL powder was affixed to an aluminum stub with two-sided adhesive tape and coated with an alloy of 60% gold and 40% palladium by a sputter coater (Desk II Sputter/Etch Unit, Moorestown, NJ). The SEM samples were examined in a Hitachi S-3500N (Hitachi Science System, Ibaraki, Japan), operating at an accelerating voltage of 5.00 kV. A Philips CM 100 (FEI Company, Hillsboro, OR) TEM was used to study the microstructure of SPA with different components (SPI, SPT/SL, SPT/ESL, MSP/SL, or MSP/ESL). All samples were diluted to 1% with deionized water (or a solution with a pH of 12 prepared in deionized water and sodium hydroxide) and sonicated for 5 min using an L&R320 ultrasonic stirrer (L&R Manufacturing Company, Keary, NJ). Samples were absorbed onto Formvar/carbon-coated 200-mesh copper grids (Electron Microscopy Science, Fort Washington, PA) and stained with 2% uranyl acetate (Ladd Research Industries, Inc., Burlington, VT). Morphology properties of each sample were recorded while the TEM was operating at an accelerating voltage of 100 kV. 2.4. Evaluation of adhesives with lignin

2.3.2. Analyses of molecular weight The molecular weights of SL and ESL were determined using Gel Permeation Chromatography (GPC) (PL-GPC 220, Polymer Laboratories Varian, Inc. Amherst, MA) with three Phenogel columns (Phenomenex, Inc., Torrance, CA), a guard column (Phenomenex, Inc.), and a differential refractive index detector. The mobile phase was dimethylsulfoxide (DMSO) flowing at 0.8 mL/min. The molecular weight was calibrated according to dextran standard. The lignin samples were acetylated before the analysis to enhance their solubility in DMSO, as suggested by Toledano et al. (2010). 2.3.3. Elemental analysis Carbon, hydrogen, nitrogen, and sulfur contents in lignin samples were determined according to the modified methods described by Jääskeläinen et al. (2003). The rest of the sample was assumed to be oxygen (100%–C%–H%–N%–S%). 2.3.4. Analyses of SL functional groups The total hydroxyl (OH) groups’ content was determined by employing acetic anhydride in pyridine medium (Faix et al., 1994), phenolic hydroxyl content was found by using a modified ultraviolet–visible spectroscopy (UV–vis) method (Malutan et al., 2008), and alcohol hydroxyl content was the difference between total and phenolic hydroxyl. The carbonyl and carboxyl contents were found by following methods described by Ren and Fang (2005a,b). 2.3.5. Analyses of thermal characteristics The thermal characteristics of SL and ESL were tested using DSC and TGA. The DSC (Q200, TA Instrument, Schaumburg, IL) analysis was carried out by running temperature scans of the sample from 20 to 250 ◦ C with a heating rate of 10 ◦ C/min. The tests were conducted under constant nitrogen flow. The TGA (TGA7, PerkinElmer, Norwalk, CT) test was also carried out in a nitrogen-rich environment, which provided an inert atmosphere during pyrolysis. The samples were heated from 40 to 900 ◦ C at a heating rate of 20 ◦ C/min. 2.3.6. FTIR spectroscopy analysis FTIR spectra of SL and ESL samples were acquired with a PerkinElmer Spectrum 400 FT-IR/FT-NIR Spectrometer (Waltham, MA) over the 4000–400 cm−1 region at a resolution of 4 cm−1 .

2.4.1. Shear strength The shear strength, an indicator of glue strength of wood specimens, was determined according to ASTM method D2339-98 using an Instron testing machine (Model 4465; Canton, MA) with a crosshead speed of 1.6 mm/min. The maximum shear strength to break the glued wood specimens was recorded. All shear strength data reported are means of five replicates. 2.4.2. Water resistance Water resistance for exterior application of the SPA was determined according to the modified methods described by Hettiarachchy et al. (1995). The glued wood specimens were soaked in tap water for 48 h at room temperature, then air-dried in a fume hood at 50% RH and 23 ◦ C for 48 h. Five specimens were used for each treatment. After three repeated soaking and drying cycles, the shear strength of dried wood specimens was tested by using ASTM standard method D2339-98. 3. Results and discussion 3.1. Analysis of molecular weight Compared to SL, ESL showed a wider molecular weight distribution (Fig. 1) with higher percentages of molecules at both high and low molecular weight ranges. Such a pattern of molecular weight distribution indicates that both degradation and cross-linking of lignin molecules probably occurred during the extrusion process. Extrusion-induced cross-linking increased weight average molecular weight from 2699 to 3686 and molecular weight distribution (Mw /Mn , where Mw is the weight average molecular weight, and Mn is the number average molecular weight) from 2.1 to 3.0 (Table 1). The increased molecular weight was beneficial for improvements in shear strength and water resistance of SPA, as will be discussed later. 3.2. Elemental analysis The elemental analysis of carbon, hydrogen, oxygen, nitrogen, and sulfur contents showed little difference between SL and ESL (Table 1), which signifies that the basic chemical composition of SL did not undergo obvious changes during extrusion.

504

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

Fig. 2. Differential scanning calorimetry (DSC) curves of sorghum lignin (SL) and extruded sorghum lignin (ESL).

Fig. 1. Distributions of differential percentage molecular weight for sorghum lignin (SL) and extruded sorghum lignin (ESL) using GPC with a differential refractive index detector.

3.3. Analysis of SL functional groups Functional group analysis on SL revealed the existence of phenolic hydroxyl, alcohol hydroxyl, carbonyl, and carboxyl groups in SL (Table 1). These active functional groups in SL have the potential to interact with functional groups on side chains of SPI. In addition, the hydrophilic and hydrophobic properties of these functional groups can influence the water resistance of SPA.

3.4. Analysis of thermal characteristics 3.4.1. DSC analysis Depending on the sources and history of lignins, some exhibit sharp glass transition during DSC heating scans (Hatakeyama, 2010; Shao et al., 2009), but others do not (Shao et al., 2009; Koullas et al., 2006). We did not observe obvious glass transition for either SL or ESL; however, both showed a broad endothermic peak in the DSC curves (Fig. 2), which was attributed to the softening of lignin during heating (Koullas et al., 2006). The softening point (peak) of SL and ESL was almost the same, both around 112 ◦ C. Nevertheless, the softening enthalpy of ESL was 334.1 J/g, which was much larger than that of SL (94.01 J/g). The higher softening enthalpy of

ESL indicated that lignin probably underwent significant structural changes during extrusion. 3.4.2. TGA analysis Thermal stability of SL and ESL was tested using TGA techniques (Fig. 3a). The first thermal weight loss for both SL and ESL occurred in the 100 ◦ C zone and was caused by the loss of moisture. Significant thermal weight loss of SL and ESL started at about 200 ◦ C (Fig. 3a and b). Thermal weight loss for SL and ESL were 50% and 42% below 600 ◦ C, respectively. Weight loss is mainly due to the thermal decomposition of lignins and the discharge of CO2 , CO, H2 , and CH4 (Hatakeyama et al., 2010). After heating to 800 ◦ C, 45% of SL and 52% of ESL remains unvolatilized due to the formation of highly condensed aromatic structures. The starting point and amount of thermal weight loss for ESL were all lower than that of SL (Fig. 3a and b). TGA measurements showed that extrusion changed lignins’ thermal properties (three obvious mass loss zones for SL vs. two for ESL) and resulted in a more thermal stable ESL. 3.4.3. FTIR spectroscopy FTIR spectra of SL and ESL are shown in Fig. 4. The ratio of relative absorbance for individual bands to the bands at 1600 cm−1 and 1510 cm−1 is listed in Table 2. The two bands at 1600 and 1510 cm−1 were characteristic of aromatic rings, and arose from vibrations of the aromatic skeleton (Nada et al., 1998). The bands were chosen as internal standards because they were strong bands in both lignins (Ungureanu et al., 2009).

Table 1 Properties of SL and ESL. SL Elemental compositions (%)

Carbon Hydrogen Oxygen Nitrogen Sulfur

Functional groups (m mol/g)

OH groups Phenolic hydroxyl Alcohol hydroxyl Carbonyl Carboxyl

Molecular weight

Mn Mw Mz Mp Mw /Mn

ESL 75.07 6.27 13.98 1.69 2.99

76.89 6.82 13.19 1.29 2.38

7.00 1.85 5.15 1.88 0.85 1292 2699 17,072 1150 2.1

1225 3686 63,827 1354 3.0

SL: sorghum lignin; ESL: extruded sorghum lignin; Mn : number-average molecular weight; Mw : weight-average molecular weight; Mz : Z-average molecular weight; Mp : molecular weight at peak point; Mw /Mn : molecular weight distribution.

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

505

Fig. 4. Fourier transform infrared spectroscopy (FTIR) spectra for sorghum lignin (SL, 1) and extruded sorghum lignin (ESL, 2).

Fig. 3. Thermogravimetric analysis (TGA) curves (a) and their differential curves (b) of sorghum lignin (SL) and extruded sorghum lignin (ESL).

Because the samples were dried under vacuum to eliminate the effect of adsorbed water on the spectra, the bands at 3200 cm−1 were characteristic of free OH groups in both SL and ESL; moreover, an increase was observed in the content of free OH groups from 0.55 and 0.54 for SL (Table 2), to 0.74 and 0.66 (Table 2) for ESL. The same increasing trends were observed for the ratio of carbonyl groups and aliphatic/aromatic ratio for SL and ESL (Table 2). The content of carbonyl groups for SL fluctuated between 0.81 and 0.80, whereas the content of the carbonyl groups for ESL reached 1.08 and 0.96. Extrusion also increased the ratio of the aliphatic (A1) and aromatic (Ar) groups. For SL, the ratio varied between 0.68 and 0.67, whereas the values for ESL were 0.80 and 0.72. These

Fig. 5. SEM image of sorghum lignin (SL) and extruded sorghum lignin (ESL) at different magnifications (a) 4000× and (b) 10,000× (scale bar is shown at the bottom of each micrograph).

506

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

Table 2 Characteristics of SL and ESL for different relative absorption values, determined by FTIR spectral analysis. SL

Table 4 Shear strength and water resistance (MPa) of MSP mixed with SL or ESL at different ratios in neutral water.

ESL

SL

OH ratios A3200/A1510 ratio A3200/A1600 ratio

0.55 0.54

0.74 0.66

Carbonyl groups ratio A1700/A1510 A1700/A1600

0.81 0.80

1.08 0.96

Aliphatic/aromatic ratio A2932/A1510 ratio A2932/A1600 ratio

0.68 0.67

0.80 0.72

SL: sorghum lignin; ESL: extruded sorghum lignin.

data supported the fact that the aromatic ring structures of lignin degraded, and phenolic hydroxyl groups were released during the extrusion process. Extrusion resulted in more reactive functional groups in the ESL, and therefore improved lignin functionality. In addition, data in Fig. 4 and Table 2 both indicate that the band at 1200 cm−1 is due to the vibration of the phenolic OH group; bands at 1064 cm−1 and 1148 cm−1 are characteristic of primary and secondary OH groups, respectively. Moreover, the shoulders at 2862 cm−1 and 1430 cm−1 were clearly attributed to vibration of OCH3 groups with the CH bonds (Nada et al., 1998). Bands at 1045 cm−1 are mainly attributed to OH bonds of Guaiacyl propane unit (G type). A band at 810 cm−1 , due to aromatic C H “out of the plane” vibrations in p-hydroxyphenal propane unit (H type), was also detected (Faix, 1991). Syringyl propane unit (S type) was not observed in the FTIR spectra of either SL or ESL. FTIR spectroscopy data identified sorghum lignin as GH-type lignin. 3.4.4. Microstructure of extruded lignin The microstructure of SL and ESL at two magnifications, (a) 4000× and (b) 10,000×, are presented in Fig. 5. SEM showed that the surface of ESL was rougher than the surface of SL. The rougher surfaces and smaller particles of ESL with irregular shapes indicated that the structure of lignin underwent changes during extrusion. This was supported by the findings of Zhang et al. (2008) reported that drastic conditions of steam explosion caused structural changes to lignin. Extrusion and steam explosion have some similarities in that moisture evaporates rapidly while releasing high energy (Zhang et al., 2008). Changes in the microstructure of SL during extrusion improved lignin functional properties and benefited further applications (Khan and Ashraf, 2006).

ESL

Shear strength 0% 10% 20% 30% 40% 50%

6.18 5.70 6.40 6.12 4.86 5.84

± ± ± ± ± ±

0.36cwf 0.92cwf 0.53cwf 0.27cwf 0.56cwf 0.60cwf

6.18 5.68 5.94 4.25 5.36 4.76

± ± ± ± ± ±

0.36cwf 0.51cwf 0.71cwf 0.13cwf 0.64cwf 0.69cwf

Water resistance 0% 10% 20% 30% 40% 50%

2.93 2.34 2.27 2.16 2.77 3.38

± ± ± ± ± ±

0.04 0.27 0.36 0.04 0.83 0.08

2.93 3.05 2.62 2.01 2.45 2.78

± ± ± ± ± ±

0.04 0.42 0.53 0.15 0.08 0.16

SL: sorghum lignin; ESL: extruded sorghum lignin; MSP: NaHSO3 modified soy protein; cwf: 100% cohesive wood failure.

3.5. Evaluation of adhesives with lignin 3.5.1. Shear strength and water resistance of SPA based on SPI Table 3 shows the changes in shear strength and water resistance of SPA based on SPI with different blending percentages of SL or ESL. The shear strength of SPI-based, SL- or ESL-blended, neutral water-suspended SPA was similar. All lignin-blended SPAs exhibited 100% wood cohesive failure (Table 3). Compared with the neutral water-suspended SPAs, the sodium hydroxide solution (pH 12)-suspended SPAs without SL or ESL showed much lower shear strength, but shear strength increased significantly when SPI was blended with SL or ESL. The blended SPAs had 100% wood cohesive failure at lignin-blending levels of more than 20% SL or 10% ESL. Water resistance of all SPAs without lignin was relatively low (Table 2). Furthermore, the water resistance of alkaline solutionbased SPA was even much lower than that of neutral water-based SPA (0.37 MPa vs. 2.01 MPa); however, water resistance increased significantly when blended with increasing percentages of SL or ESL, regardless of suspension medium. For example, water resistance was 2.35 MPa for SL/SPI (10/90) and 2.92 MPa for SL/SPI (40/60) in water, and was 0.83 MPa for SL/SPI (10/90) and 1.73 MPa for SL/SPI (40/60) in alkaline solution. SPI and SL are both viscoelastic polymers with good cohesive properties. When SPI is blended with SL or ESL, their molecules could efficiently entrap each other, thus leading to improved water resistance (Qi and Sun, 2011). The hydrophobic property of SPI blended with SL or ESL was influenced by sodium hydroxide, thus its water resistance in solutions of pH

Table 3 Shear strength and water resistance (MPa) of SPI mixed with SL or ESL at different ratios in neutral water and alkaline solutions. In neutral water SL

In alkaline solution of pH 12 ESL

SL

ESL

Shear strength 0% 10% 20% 30% 40% 50%

4.24 5.58 5.64 5.73 5.29 4.19

± ± ± ± ± ±

0.46cwf 0.30cwf 0.72cwf 0.10cwf 0.48cwf 0.19cwf

4.24 6.00 5.87 5.73 5.16 4.94

± ± ± ± ± ±

0.46cwf 0.58cwf 0.58cwf 0.62cwf 0.82cwf 0.28cwf

3.68 2.58 4.17 5.21 4.95 5.28

± ± ± ± ± ±

0.52 0.22 0.36cwf 0.76cwf 0.48cwf 0.61cwf

3.68 4.14 5.22 3.92 4.42 4.83

± ± ± ± ± ±

0.52 0.07cwf 0.64cwf 0.33cwf 0.06cwf 0.76cwf

Water resistance 0% 10% 20% 30% 40% 50%

2.01 2.35 2.41 2.25 2.92 3.32

± ± ± ± ± ±

0.28 0.10 0.13 0.13 0.47 0.05

2.01 2.50 2.74 2.96 3.18 3.29

± ± ± ± ± ±

0.28 0.16 0.51 0.10 0.29 0.07

0.37 0.83 1.05 1.30 1.73 2.12

± ± ± ± ± ±

0.06 0.07 0.15 0.09 0.15 0.11

0.37 0.42 0.88 1.49 2.06 2.33

± ± ± ± ± ±

0.06 0.14 0.07 0.25 0.32 0.09

SL: sorghum lignin; ESL: extruded sorghum lignin; SPI: soy protein isolate; cwf: 100% cohesive wood failure.

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

507

Fig. 6. TEM images of several SPA samples: SPI in water (A), SPI/SL in water (B), SPI + ESL in water (C), SPI/SL in pH 12 solution (D), SPI/ESL in pH 12 solution (E), MSP in water (F), MSP/SL in water(G) MSP/ESL in water(H). The ratio of SPI or MSP to SL or ESL is 70/30. Scale bar is shown at the bottom of each micrograph, 100 nm. (SPA, soy protein adhesive; SPI, soy protein isolate; MSP, NaHSO3 modified soy protein; SL, sorghum lignin; ESL, extruded sorghum lignin).

12 was lower than solutions of pH 7. The higher water resistance of SPI-blended ESL in water showed that extrusion increased SPI’s ability to combine with sorghum lignin due to the exposed texture (Zhang et al., 2008; Qi and Sun, 2011). The ESL-blended SPAs in alkaline solution were slightly less water resistant than the SLblended SPAs in the 10% to 20% lignin-blending ratio. This could be primarily due to the explosion of hydrophilic groups such as phenolic hydroxy in the ESL during extrusion (Shao et al., 2009). Although the lignin-blending percentages increased from 30% to 50%, the water resistance of ESL-blended SPAs had a more significant improvement than the SL-blended SPAs did (Table 3), which is because the entrapping interaction between SPI and ESL had a more significant effect than the hydrophilic attractions between phenolic hydroxyl and sodium hydroxide.

3.5.2. Shear strength and water resistance of SPA based on MSP The dry shear strength of SPA based on MSP showed 100% wood cohesive failure with blends of SL or ESL (Table 4). The values of water resistance were all lower than the shear strength, and values decreased as the percentage of blended SL and ESL increased from 10% to 30%. When the blended percentage of SL and ESL increased from 30% to 50%, the water resistance again increased gradually. This result indicates that the blended SL and ESL at lower percentages are influenced primarily by the hydrophilic groups in SL or ESL rather than the entrapping interaction between MSP and lignin molecules, but when more SL (or ESL) was blended into the SPAs, the advantage of entrapping interaction dominates. The ESL-blended SPAs from MSP showed better water resistance than the SL-blended ones in the 10% to 30% blending range, but

508

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509

the opposite was observed in the mixing percentage from 30% to 50%. In the lower percentage lignin blending ranges, the ESL in the blended mixture (SPAs) decreased the dynamic viscoelasticity of the mixed system and led to good adhesion strength (Qi and Sun, 2011). Conversely, in higher blending percentages, ESL with more exposed hydrophilic groups such as phenolic hydroxyl led to poor adhesion strength. The extent of adhesion improvement in lignin/MSP systems was less significant comparing with that in lignin/SPI systems. MSP is a sodium bisulfite modified soy protein adhesive developed by Qi and Sun (2011). As a reducing agent, NaHSO3 partially unfolded soy protein by breaking some of the disulfite bonds. Also, as a salt, NaHSO3 altered the environmental ionic strength around the protein molecules, resulting in re-organizing and balancing the protein–protein interactions (electrostatic force, hydrophobic interaction), and forming new protein aggregates as shown in Fig. 6F. The larger aggregates might reduce the efficiency of protein and lignin interaction, leading to the insignificant improvement of the adhesion strength.

3.5.3. Morphology of adhesives Besides the aforementioned synergistic effects of SPA samples on adhesion, the interaction between SPI (or MSP) and SL (or ESL) is further illustrated in TEM images (Fig. 6A–H). Images in Fig. 6B–E are close to Fig. 6A, which shared morphological properties similar to soy protein (Qi and Sun, 2010), and different from the irregular circular shape observed in lignins (Fromm et al., 2003). The observed microstructure indicates that the lignin (SL or ESL)blended SPA systems (SPI- and MSP-based) have good merging properties; SL or ESL was evenly distributed into the SPI and MSP matrix in both neutral and alkaline solutions. Chain-like networks accompanied by many uniformly embedded small soy globular proteins were observed in MSP-based SPA (Fig. 6F), but not in SPI-based SPA (Fig. 6A). Compared with the aggregates in Fig. 6F, the same formations were better defined in SPA blends from SPI and ESL (70/30) in water (Fig. 6C); however, large aggregates were not observed in SPA blends from either SPI/SL (70/30, Fig. 6D) or SPI/ESL (70/30, Fig. 6E) in alkaline solution. Particles in SPAs based on SPI and ESL tend to be more evenly dispersed. In blends of MSP and SL or ESL, aggregates emerged in different enlacing degrees of chain-like networks (Fig. 6G and H). The aggregates in MSP/SL blends were denser than in MSP/ESL. Dense structures limit the penetration of water into the interfacial layer between wood and adhesive, which is beneficial for adhesive performance. Furthermore, large aggregates reduce the water-imbibing property of the adhesive blends system and result in more free water molecules (Yao et al., 1988); therefore, the friction between molecules in the mixture is reduced and a less viscous system is formed. Both high density and large aggregate mass significantly improve water resistance of SPAs, which is consistent with the water resistance shown in Tables 3 and 4.

4. Conclusions Extrusion changed lignin’s thermal properties and resulted in more thermally stable ESL. SL transformed from large aggregates to smaller/irregular particles during extrusion. Dry shear strength of all blends of SPI and MSP, with SL or ESL showed 100% wood cohesive failure except SPI and SPI/SL (90/10) in alkaline solution. Blending of lignin in SPI-based SPAs significantly improved water resistance of SPAs in both neutral and alkaline conditions. The shear strength and water resistance of SPAs could be greatly improved by blending with sorghum lignin, but the improvement with ESL was more significant when the blended SPAs were suspended in water. These results can probably be equally good if other lignins other

than the sorghum one is used, although this has got to be tested further.

References Alonso, M.V., Oliet, M., Rodriguez, F., Garcia, J., Gilarranz, M.A., Rodriguez, J.J., 2005. Modification of ammonium lignosulfonate by phenolation for use in phenolic resins. Bioresour. Technol. 96, 1013–1018. Batubenga, D.B., Pizzi, A., Stephanou, A., Cheesman, P., Krause, R., 1995. Isocyanate phenolics wood adhesives by catalytic acceleration of copolymerization. Holzforschung 49, 84–86. Faix, O., 1991. Classification of lignins from different botanical origins by FTIR spectroscopy. Holzforschung 45 (Suppl. S), 21. Faix, O., Argyropoulos, D.S., Robert, D., Neirinck, V., 1994. Determination of hydroxylgroups in lignins evaluation of H-1 NMR, C-13 NMR, P-31 NMR, FTIR and wet chemical methods. Holzforschung 48, 387–394. Fromm, J., Rockel, B., Lautner, S., Windeisen, E., Wanner, G., 2003. Lignin distribution in wood cell walls determined by TEM and backscattered SEM techniques. J. Struct. Biol. 143, 77–84. Hatakeyama, H., 2010. In: Abe, A., Dusek, K., Kobayashi, S. (Eds.), Biopolymers: Lignin, Proteins, Bioactive Nanocomposites (Advances in Polymer Science). Springer, New York, pp. 1–63. Hatakeyama, H., Tsujimoto, Y., Zarubin, M.J., Krutov, S.M., Hatakeyama, T., 2010. Thermal decomposition and glass transition of industrial hydrolysis lignin. J. Therm. Anal. Calorim. 101, 289–295. Hettiarachchy, N.S., Kalapathy, U., Myers, D.J., 1995. Alkali-modified soy protein with improved adhesive and hydrophobic properties. J. Am. Oil Chem. Soc. 72, 1461–1464. Infantes, M., Ysambertt, F., Hernández, M., Martínez, B., Delgado, N., Bravo, B., Cáceres, A., Chávez, G., Bullón, J., 2007. Microwave assisted oxidative degradation of lignin with hydrogen peroxide and its tensoactive properties. Rev. Tecnica Facultad Ingenieria Univ. Zulia 30 (special issue SI), 108–117. Jääskeläinen, A.S., Sun, Y., Argyropoulos, D.S., Tamminen, T., Hortling, B., 2003. The effect of isolation method on the chemical structure of residual lignin. Wood Sci. Technol. 37, 91–102. Jin, Y., Cheng, X., Zheng, Z., 2010. Preparation and characterization of phenolformaldehyde adhesives modified with enzymatic hydrolysis lignin. Bioresource Technol. 101, 2046–2048. Khan, M.A., Ashraf, S.M., 2006. Development and characterization of groundnut shell lignin modified phenol formaldehyde wood adhesive. Indian J. Chem. Technol. 13, 347–352. Khan, M.A., Ashraf, S.M., Malhotra, V.P., 2004. Development and characterization of a wood adhesive using bagasse lignin. Int. J. Adhes. Adhes. 24, 485–493. Koullas, D.P., Koukios, E.G., Avgerinos, E., Abaecherli, A., Gosselink, R., Vasile, C., Lehnen, R., Saake, B., Suren, J., 2006. Analytical methods for lignin characterization-differential scanning calorimetry. Cellulose Chem. Technol. 40, 719–725. Lambuth, A.L., 1994. In: Pizzi, A., Mittal, K.L. (Eds.), Handbook of Adhesive Technology. Marcel Dekker, New York, pp. 259–281. Li, N., Wang, Y., Tilley, M., Bean, S.R., Wu, X., Sun, X.S., Wang, D., 2011. Adhesive performance of sorghum protein extracted from sorghum DDGS and flour. J. Polym. Environ. 19, 755–765. Li, X., Li, Y., Zhong, Z., Wang, D., Ratto, J.A., Sheng, K., Sun, X.S., 2009. Mechanical and water soaking properties of medium density fiberboard with wood fiber and soybean protein adhesives. Bioresour. Technol. 100, 3556–3562. Li, Y., Sun, X.S., 2011. Mechanical and thermal properties of biocomposites from poly(lactic acid) and DDGS. J. Appl. Polym. Sci. 121, 589–597. Li, Y., Venkateshan, K., Sun, X.S., 2010. Mechanical and thermal properties, morphology and relaxation characteristics of poly(lactic acid) and soy flour/wood flour blends. Polym. Int. 59, 1099–1109. Lundquist, K., Ohlsson, B., Simonson, R., 1977. Isolation of lignin by means of liquid–liquid-extraction. Svensk Papperstidning-Nordisk Cellulosa 80, 143–144. Malutan, T., Nicu, R., Popa, V.I., 2008. Contribution to the study of hydroxymetylation reaction of alkali lignin. BioRes 3, 13–20. Mo, X., Sun, X.S., Wang, D., 2004. Thermal properties and adhesion strength of modified soybean storage proteins. J. Am. Oil Chem. Soc. 81, 395–400. Nada, A.A.M.A., Sakhawy, M.E., Kamel, S.M., 1998. Infra-red spectroscopic study of lignins. Polym. Degrad. Stabil. 60, 247. Qi, G.Y., Sun, X.S., 2010. Peel adhesion properties of modified soy protein adhesive on a glass panel. Ind. Crop. Prod. 32, 208–212. Qi, G.Y., Sun, X.S., 2011. Soy protein adhesive blends with synthetic latex on wood veneer. J. Am. Oil Chem. Soc. 88, 271–281. Ren, S.X., Fang, G.Z., 2005a. Effects of ultrasonic processing on structure of alkali lignin of wheat-straw. Chem. Ind. For. Prod. 25, 82–86. Ren, S.X., Fang, G.Z., 2005b. Effects of ultrasonic processing on structure of alkali lignin of wheat-straw. J. Chem. Ind. For. Prod. 25, 82–86. Shao, S., Jin, Z., Wen, G., Iiyama, K., 2009. Thermo characteristics of steam-exploded bamboo (Phyllostachys pubescens) lignin. Wood Sci. Technol. 43, 643–652. She, D., Xu, F., Geng, Z., Sun, R., Jones, G., Baird, M.S., 2010. Physicochemical characterization of extracted lignin from sweet sorghum stem. Ind. Crop. Prod. 32, 21–28. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., Crocker, D., 2005. Laboratory Analytical Procedure (LAP): Determination of Structural Carbohydrates and Lignin in Biomass. National Renewable Energy Laboratory, Colorado.

Z. Xiao et al. / Industrial Crops and Products 50 (2013) 501–509 Sun, S., Wen, J., Ma, M., Li, M., Sun, R., 2013. Revealing the structure inhomogeneity of lignins from sweet sorghum stem by successive alkali extractions. J. Agric. Food Chem., http://dx.doi.org/10.1021/jf400824p. Sun, X.S., Bian, K., 1999. Shear strength and water resistance of modified soy protein adhesives. J. Am. Oil Chem. Soc. 76, 977–980. Theerarattananoon, K., Wu, X., Staggenborg, S., Propheter, J., Madl, R., Wang, D., 2010. Evaluation and characterization of sorghum biomass as feedstock for sugar production. Trans. ASABE 53, 509–525. Toledano, A., Serrano, L., Garcia, A., Mondragon, I., Labidi, J., 2010. Comparative study of lignin fractionation by ultrafiltration and selective precipitation. Chem. Eng. J. 157, 93–99. Ungureanu, E., Ungureanu, O., Capraru, A.M., Popa, V.I., 2009. Chemical modification and characterization of straw lignin. Cell. Chem. Technol. 43, 263–269. Wu, X., Staggenborg, S., Propheter, J.L., Rooney, W.L., Yu, J., Wang, D., 2010. Features of sweet sorghum juice and their performance in ethanol fermentation. Ind. Crop. Prod. 31, 164–170.

509

Xiao, Z.G., Shen, D.C., Wu, M.C., Shen, X.Y., 2008b. Enzyme hydrolysis of starch syrup by-product with extrusion technology. Trans. Chin. Soc. Agric. Eng. 24, 80–85. Xiao, Z.G., Wu, M.C., Shen, D.C., Xiao, R., 2008a. Analysis on the effect of extrusion on tannin content in rapeseed meal. Trans. Chin. Soc. Agric. Machine. 39, 85–89. Xu, F., Shi, Y.C., Wu, X., Theerarattananoon, K., Staggenborg, S., Wang, D., 2011. Sulfuric acid pretreatment and enzymatic hydrolysis of photoperiod sensitive sorghum for ethanol production. Bioprocess Biosyst. Eng. 34, 485–492. Yao, J.J., Wei, L.S., Steinberg, M.P., 1988. Water-imbibing capacity and rheological properties of isolated soy proteins. J. Food Sci. 53, 464–467. Zhang, L.H., Li, D., Wang, L.J., Wang, T.P., Zhang, L., Chen, X.D., Mao, Z.H., 2008. Effect of steam explosion on biodegradation of lignin in wheat straw. Biores. Technol. 99, 8512–8515.