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Eco-friendly fiberboard production without binder using poplar wood shavings bio-pretreated by white rot fungi Coriolus versicolor
Construction and Building Materials 236 (2020) 117620
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Eco-friendly fiberboard production without binder using poplar wood shavings bio-pretreated by white rot fungi Coriolus versicolor Jianguo Wu a,b,⇑, Chunyan Chen a, Haiyang Zhang c, Lin Xia d, Yiming Huang a, Hui Huang a, Yuanyuan Wang a, Dong Qian a, Jing Wang a, Xinfeng Wang a,b, Tong Zhang a,b a Jiangsu Key Laboratory for Eco-Agricultural Biotechnology around Hongze Lake, Jiangsu Key Construction Laboratory for Food Safe and Nutritional Function, School of Life Science, Huaiyin Normal University, Changjiang West Road 111, Huai’an 223300, China b Jiangsu Collaborative Innovation Center of Regional Modern Agriculture and Environmental Protection, Huaiyin Normal University, Changjiang West Road 111, Huai’an 223300, China c College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, China d Key Laboratory of Molecular Biophysics of MOE, Huazhong University of Science and Technology, Wuhan 430074, China
h i g h l i g h t s BS and WSR of the fiberboard reached to 22.7 MPa and 12.4%, respectively. Soluble polysaccharide and reduce sugar had a weak effect on adhesive of fiberboard. Low content of hemicellulose and high content of lignin helped to BS of fiberboard. MnP had a lag enzymolysis effect on adhesive of fiberboard.
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Article history: Received 21 April 2019 Received in revised form 3 November 2019 Accepted 13 November 2019
Keywords: Fiberboard Poplar wood shavings Coriolus versicolor Bio-pretreatment Bending strength
a b s t r a c t Urea-formaldehyde resin (UF) or phenolic resin (PF) is used widely as binder in the production of fiberboard, and formaldehyde emissions from UF or PF is seriously harmful for human health. Eco-friendly fiberboard was produced without binder using poplar wood shavings (PWS) bio-pretreated by white rot fungi Coriolus versicolor in this study, and the correlations between the metabolites and lignocellulose components and the bending strength (BS) of fiberboard were also studied. After PWS were pretreated by C. versicolor for 21 days, the BS and water swelling ratio of the fiberboard were reached to 22.7 MPa and 12.4%, respectively. The soluble polysaccharide and reduce sugar in PWS were detected very low, which had a weak effect on the fiberboard. Lower content of hemicellulose and higher content of lignin were detected and beneficial to the BS of fiberboard. Manganese peroxidase was detected and had a lag enzymolysis effect. Laccase, lignin peroxidase and cellulase were not detected but laccase or cellulase might have a weak influence on fiberboard. The fiberboard production with this bio-pretreatment should be eco-friendly and eliminate the potential formaldehyde emission. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction With continuous development of urbanization and increase of population in China, the consumption and production of fiberboard are increasing day by day. Wood is one of the main materials for the production of fiberboard and widely used in furniture manufacture, packaging, building construction, musical instruments, ⇑ Corresponding author at: Jiangsu Key Laboratory for Eco-Agricultural Biotechnology around Hongze Lake, Jiangsu Key Construction Laboratory for Food Safe and Nutritional Function, School of Life Science, Huaiyin Normal University, Changjiang West Road 111, Huai’an 223300, China. E-mail address: [email protected] (J. Wu). https://doi.org/10.1016/j.conbuildmat.2019.117620 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and other industrial sectors [13]. High-strength fiberboard can be used as load-bearing building materials, and low-strength fiberboard may be used for decoration and ornament. Based on the analysis of recent status of wood consumption in China, 0.85– 0.95 billion m3 wood will be needed to meet the demand of domestic wood consumption and export. However, the forest storage in China is only 1.51 billion m3 [38]. It is very important to utilize effectively the wood processing residue, such as wood shavings and sawdust. Currently, urea-formaldehyde resin (UF) or phenolic resin (PF) is used widely as binder in the production of fiberboard [2,21,26]. Aldehyde-based binder can release some toxic and
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harmful gases like formaldehyde in the consumption and production process of fiberboard [17,31], which is seriously harmful for human health. In order to reduce the toxic gas pollution of fiberboard, a low formaldehyde and VOCs emitting board coated with nanomaterial-added melamine-impregnated papers was developed [18], and the aldehyde-free binders including isocyanate resin, soybean protein binder and binderless fiberboard have becoming the researching hotspot [8,25,39,40]. Based on the issues in production and use of aldehyde-free binder, binderless fiberboard is becoming the focus of research. The key of improving the strength of binderless fiberboard is to increase the adhesive point of raw material. So, physicochemical treatments (such as steam explosion and plasma, catalytic activation and surface treatment), biological or enzyme pretreatments [5,12,19,23,25,27,37] were developed. However, physicochemical and enzyme treatments could cause higher energy consumption and higher cost of enzyme [14,29]. Because some metabolites, such as enzymes and sugars, were produced during biological pretreatment, and the physical and chemical structure of raw materials could be also modified [15,32], which increased the adhesive point of raw materials to product fiberboard by hot pressing method. Moreover, biological pretreatment is environmentally friendly. Therefore, biological pretreatment is greatly concerned by researchers now. In China, there are a large planting area of poplar and many poplar wood furniture manufacture factories, which lead to a large number of poplar wood shavings (PWS). PWS could be used for production of fiberboard, lipids, xylitol and bioenergy [6,7,16], but there had no reports about binderless fiberboard using PWS. The white rot fungi Coriolus versicolor has a strong lignocellulose degrading enzyme systems and has been used in the production of binderless fiberboard [36]. Therefore, the strain C. versicolor was used to pretreat the PWS to produce eco-friendly fiberboard by hot-pressing without binder in this study, and the correlations between the metabolites and lignocellulose components and the strength of fiberboard were also studied.
at 45 °C for 4 h in a reciprocal shaker with 100 r/min. Most of the enzymatic liquid was removed by filtration and squeeze, and the wet enzymatic PWS (about 65% of moisture content) was used for preparation of fiberboard.
2. Materials and methods
WSRð%Þ ¼
2.1. Strain and cultivation
where, F is the maximum force to break the fiberboard specimens, N; L is the span length, L = 60 mm; b is the width of fiberboard specimens, mm; h is the thickness of fiberboard specimens, mm; h2 is the final thickness of fiberboard specimens after immersing in water, mm; h1 is the initial thickness of fiberboard specimens before immersing in water, mm.
The strain Coriolus versicolor was preserved at Jiangsu Key Laboratory for Eco-Agricultural Biotechnology around Hongze Lake in Huai‘an, China. Two or three agar blocks (1 cm 1 cm) from the preserved test tube slant of the strain were inoculated into 100 mL of potato dextrose liquor in a 250 mL round flask. After culturing for 3 days at 29 °C on a reciprocal shaker with 150 r/min, 5 mL of culture was inoculated into 100 mL of potato dextrose liquor for the second culture under the same conditions. 2.2. Bio-pretreatment of PWS by C. versicolor PWS of similar size and weight were selected for biopretreatment. 10 mL culture of C. versicolor was inoculated into 60 g (dry mass) of PWS with 80 mL of distilled water in 250-mL Erlenmeyer flasks for bio-pretreatment. The bio-pretreatment kept still at 29 °C for 28 days. 2.3. Enzymatic pretreatment of PWS by laccase or cellulase About 60 g (dry mass) of PWS was immerged in 300 mL acetic acid buffer (pH = 4.5), and added appropriate enzyme solution (enzyme activity ranged from 30 U to 90 U) of laccase (purchased from Anhui Cool Seoul Bioengineering CO., Ltd., Anhui, China) or cellulase (purchased from Huai‘an Bio-Mass Green Bio Energy Co., Ltd, Jiangsu, China). Enzymatic pretreatment was conducted
2.4. Preparation of fiberboard PWS were manually well-distributed loading into a stainless mold (100 mm 100 mm 3 mm), then pressed flat into a fiberboard by a R-3202 Hot-Press Model (Wuhan Qien Science and Technology Development Co., Ltd., Hubei, China). Hot pressing was conducted under 15 MPa pressure at 170 °C for 10 min. 2.5. Property of fiberboard The density of preparative fiberboard was measured by dividing the fiberboard mass (kg) by its volume (m3). The fiberboard was cut into fiberboard specimens of about 100 mm in length and 20 mm in width, respectively. The span length was set to 60 mm. So, the bending strength (BS) of fiberboard specimens was determined by performing the three point flex test at a crosshead speed of 3 mm/min using a micro-computer control electron universal testing machine (Model ETM104B, Wance Group, Shenzhen, China). The BS could be calculated as Eq. (1). The fiberboard using PWS with 21 days of bio-pretreatment was cut transversely, longitudinally and 45° obliquely, then the BS of the cut fiberboard specimens were determined to investigate the in-plane isotropy of the fiberboard. The water swelling ratio (WSR) was measured from the different thickness (h) before and after immersing in water for 24 h, and the WSR could be calculated as Eq. (2). The length (a), width (b) and thickness (h) of fiberboard specimens were measured accurately by micrometer.
BS ¼
3FL
ð1Þ
2
2bh
h2 h1 100% h1
ð2Þ
2.6. Determination of the polysaccharide and reducing sugar in PWS 5 g of PWS sample was fully immersed in 20 mL of distilled water for 4 h, and the reducing sugar content of the filtrate by filtration with filter paper was determined by 3, 5-dinitrosalicylic acid (DNS) method. The polysaccharide of the filtrate was extracted firstly by 80% ethanol, and the polysaccharide content was determined by Phenol-Sulphate acid method. 2.7. Determination of lignocellulosic components in PWS The contents of cellulose, hemicellulose and lignin of PWS were determined by the National Renewable Energy Laboratory (NREL) method [28]. 2.8. Determination of lignocellulosic enzymes in bio-pretreated PWS 5 g of PWS sample was fully immersed in 20 mL of distilled water for 12 h, then the laccase activity (Lac) of the filtrate by filtration with filter paper was determined by ABTS (2, 20 -Azinobis-
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(3-ethylbenzthiazoline-6-sulphonate)) method, the lignin peroxidase activity (Lip) and manganese peroxidase activity (MnP) of the filtrate were determined by veratryl alcohol method and 2, 6-dimethoxyphenol method, respectively. Cellulase activity of the filtrate was determined by carboxymethyl cellulose (CMC) saccharogenic power method. 2.9. Statistical analysis Microsoft Excel 2016 software was used to calculate the data obtained from determination, and ORIGIN PRO 8.0 software was used to analyze the data and draw figures. The error bars in all figures were corresponded to standard errors of three samples tests, and each sample test was determined for five times. ANOVA was used for analysis of significant. 3. Results and discussion 3.1. Properties of fiberboard by bio-pretreatment The bending strength (BS), water swelling ratio (WSR) and density were the key properties of fiberboard [34]. As shown in Fig. 1, the BS of fiberboard using PWS without pretreatment was 9.8 MPa, and the WSR was higher than 50%. When the fiberboard was immersed in water for 24 h, the fiberboard was decomposed. With increase of bio-pretreatment time, the BS of fiberboard increased and obtained the highest when PWS were pretreated by C. versicolor for 21 days (2.32-fold increase over PWS without pretreatment), and the prepared fiberboard was showed in Fig. 2. The fiberboard was smooth, had beautiful wood grain and delicate fragrance. The BS of the transversely cutting specimen, longitudinally cutting specimen, and 45° obliquely cutting specimen were 22.5 MPa, 23.2 MPa and 21.6 MPa, respectively. So, the fiberboard could be approximated as an in-plane isotropic material. WSR of fiberboard was decreased to 12.4% at 21 days of biopretreatment. However, WSR of fiberboard kept still when further prolonging the bio-pretreatment time. The density of the fiberboard was about 920 ± 58 kg/m3, which meant that the fiberboard in this study belonged to high density fiberboard [30]. In summary, the fiberboard made from bio-pretreated PWS without binder was high density fiberboard and approximated as an in-plane isotropic material, and had higher BS and lower WSR. Because chemical binder was not used in the preparation process of fiberboard, a potential formaldehyde gas emission was eliminated. Nevertheless, comparing with the National Standards of the People’s Republic
Fig. 1. The bending strength (BS) and water swelling ratio (WSR) of fiberboard using poplar wood shavings with different bio-pretreatment time (d) by Coriolus versicolor.
Fig. 2. The fiberboard using poplar wood shavings with 21 days bio-pretreatment by Coriolus versicolor.
of China of High Density Fiberboard (GB/T 31765-2015), the BS of the fiberboard was still below the standard (37 MPa), and the fiberboard in this study could be only used for non-load-bearing decoration. It was important to investigate the adhesive mechanism in order to improve the strength of fiberboard for more applications in building materials. 3.2. Sugar content analysis and the relationship with the strength of fiberboard Soluble polysaccharides in PWS mainly included soluble hemicellulose and fungal polysaccharides. The strain could metabolize the hemicellulose easily and produce some reducing sugar or the fungal polysaccharides. As shown in Fig. 3, the polysaccharide content decreased with increase of bio-pretreatment time. The reducing sugar content increased at first 14 days, and then decreased with the bio-pretreatment time prolonging further. There was a good positive correlation between polysaccharides with the strength of fiberboard [35,36]. Reducing sugar could occur resinification reaction of furfural and condensation reaction between furfural and lignin during the hot pressing process [4,20], and then
Fig. 3. Sugar content in poplar wood shavings with different bio-pretreatment time (d) by Coriolus versicolor.
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produce the stickiness of the fiberboard. In this study, the starch (substitute for polysaccharide) and glucose (substitute for reducing sugar) were used in the preparation of fiberboard, and the BS of the fiberboards increased more than fifteen percent compared to the contrast. However, the total content of the soluble sugar in PWS was very low (lower than 6.5 mg/g), and there was a negative correlation of the content of the sugar and the BS of the fiberboard, which concluded that sugar had a weak effect on the adhesive of PWS for the production of fiberboard in hot-press process in this study. So, it was not easy to improve the strength of fiberboard by increasing the sugar content. 3.3. Lignocellulosic components analysis and the relationship with the strength of fiberboard Lignocellulose was the main component in PWS, and the components changes with bio-pretreatment time were showed in Fig. 4. The hemicellulose content in PWS was steadily decreased from 24.96% to 21% during 21 days of bio-pretreatment, and then kept stable when the bio-pretreatment time was further prolonged. The cellulose content in PWS was kept stable at about 33% in the first 14 days of bio-pretreatment, and then decreased from 33.05% to 30.7% at the end of bio-pretreatment. The lignin content in PWS was increased first and then decreased during the bio-pretreatment time, and the maximum value of lignin content (40.13%) was reached at 21 days of bio-pretreatment. The total content of lignocellulose was kept about 92% in the first 21 days of bio-pretreatment, and then reduced by about 4% after 7 days of bio-pretreatment. From above the change rules of the components of lignocellulose in PWS, it was found that the strain C. versicolor firstly degraded the hemicellulose to grow and produce some lignocellulose hydrolase, and then the cellulose and lignin were degraded successively by C. versicolor. Because of bio-degradation, the mass of PWS was reduced by about 10% during bio-pretreatment process, which led to the increase of lignin relative content. Lignin was composed of polymerized aromatic alcohol, which was hardly to be degrade firstly but could be modified in a way which was consistent with a hydroxyl radical attack [41], which could produce adhesive action. To effectively evaluate the relationship of the lignocellulose component content with the strength of fiberboard, the values of lignocellulose component content were plotted against BS as shown in Fig. 5. There was a significantly (P value = 0.042) positive correlation between the lignin content and the BS, but a very significantly (P value = 0.006) negative correlation between the hemicellulose content and the BS. Cellulose content had nothing to do with the BS of the fiberboard. In a word, lower content of hemicel-
Fig. 4. Lignocellulose components of poplar wood shavings with different biopretreatment time (d) by Coriolus versicolor.
Fig. 5. Relationship between lignocellulose component content and the bending strength (BS) of fiberboard.
lulose and higher content of lignin were beneficial to the BS of the fiberboard. PWS had a higher content of lignin than corn straw and Triarrhena sacchariflora residue, which led to higher BS of fiberboard of PWS bio-pretreated by white rot fungi than corn straw and T. sacchariflora residue [35,36]. Black liquor is a kind of industry wastewater with high COD and high concentration of lignin, the removal of lignin is the bottleneck problem of black liquor treatment [1,11]. So some researchers obtained lignin from paper industry black liquor to prepare phenolic resins (ligninformaldehyde or lignin-glyoxal) or be used as green adhesive to manufacture fiberboard [3,9]. These further indicated that the increase of lignin was beneficial to the strength of fiberboard. 3.4. Lignocellulosic enzymes activity analysis and the relationship with the strength of fiberboard Lignocellulosic enzymes activities in PWS during biopretreatment process were determined. Lac, Lip and cellulase were very low and not determined, but MnP was determined and showed in Fig. 6. MnP increased in the first 14 days of biopretreatment process, and decreased firstly then increased in the next 14 days of the process. Comparing the relationship between MnP and BS of the fiberboard, it was found that there was a
Fig. 6. Manganese peroxidase activity in poplar wood shavings with different biopretreatment time (d) by Coriolus versicolor.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (grant number 31870543); and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (grant number PPZY2015A018).
References
Fig. 7. The bending strength (BS) of fiberboard using poplar wood shavings pretreated by cellulase or laccase.
positive correlation between the 7 days lag MnP and the BS. There was an enzymolysis time in the process of enzyme action, which led to a lag on the enzyme activity to the enzymolysis effect. Lignin degradation enzymes such as MnP could start free radical chain reaction, which led to the bond cleavage, modification and coupling of lignin [22], and increase the specific surface area of the biomass [10]. So, it should be easy to occur the cross-linking and adhesive of lignin and fiber to improve the strength of fiberboard. Consequently, it was important to increase the lignocellulosic enzymes activities, especially MnP in the bio-pretreatment process in order to improve the strength of the fiberboard. Some researchers found that the application of Lac or peroxidase had a high potential for environmentally friendly MDF production [12,33], and there was a positive correlation of Lac with the strength of fiberboard [24,36]. However, there had few reports about cellulase pretreatment for production of fiberboard. In this study, the effects of Lac and cellulase pretreatment on the BS of fiberboard were investigated as shown in Fig. 7. With the increase of enzyme dosage of Lac and cellulase, the BS of fiberboard increased first, and then kept stable. Using only 1 U/g of enzyme dosage of Lac or cellulase, the BS of fiberboard were increased to 1.78-fold and 1.91-fold, respectively. The data indicated that pretreatment of a small amount of Lac or cellulase could significantly improve the strength of fiberboard. Therefore, although the activities of Lac or cellulase were very low in this bio-treatment process, they might also have a weak influence on the strength of fiberboard. The effect of Lac was similar to that of MnP, and the effect of cellulase was mainly to release more reducing sugar by hydrolyzing cellulose, which was beneficial to the adhesive reaction by hot-pressing. 4. Conclusions BS and WSR of the fiberboard without binder reached to 22.7 MPa and 12.4%, respectively, after PWS were exposed to C. versicolor for 21 days. The fiberboard without chemical binder eliminated potential formaldehyde emissions. Soluble polysaccharide and reduce sugar in PWS were detected very low, which had a weak effect on the adhesive of the fiberboard. Lower content of hemicellulose and higher content of lignin were beneficial to the BS of the fiberboard. MnP was detected and had a 7 days lag on the enzymolysis effect, and Lac and cellulase might also have a weak influence on the strength of fiberboard. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Eco-friendly fiberboard production without binder using poplar wood shavings bio-pretreated by white rot fungi Coriolus versicolor Jianguo Wu a,b,⇑, Chunyan Chen a, Haiyang Zhang c, Lin Xia d, Yiming Huang a, Hui Huang a, Yuanyuan Wang a, Dong Qian a, Jing Wang a, Xinfeng Wang a,b, Tong Zhang a,b a Jiangsu Key Laboratory for Eco-Agricultural Biotechnology around Hongze Lake, Jiangsu Key Construction Laboratory for Food Safe and Nutritional Function, School of Life Science, Huaiyin Normal University, Changjiang West Road 111, Huai’an 223300, China b Jiangsu Collaborative Innovation Center of Regional Modern Agriculture and Environmental Protection, Huaiyin Normal University, Changjiang West Road 111, Huai’an 223300, China c College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, China d Key Laboratory of Molecular Biophysics of MOE, Huazhong University of Science and Technology, Wuhan 430074, China
h i g h l i g h t s BS and WSR of the fiberboard reached to 22.7 MPa and 12.4%, respectively. Soluble polysaccharide and reduce sugar had a weak effect on adhesive of fiberboard. Low content of hemicellulose and high content of lignin helped to BS of fiberboard. MnP had a lag enzymolysis effect on adhesive of fiberboard.
a r t i c l e
i n f o
Article history: Received 21 April 2019 Received in revised form 3 November 2019 Accepted 13 November 2019
Keywords: Fiberboard Poplar wood shavings Coriolus versicolor Bio-pretreatment Bending strength
a b s t r a c t Urea-formaldehyde resin (UF) or phenolic resin (PF) is used widely as binder in the production of fiberboard, and formaldehyde emissions from UF or PF is seriously harmful for human health. Eco-friendly fiberboard was produced without binder using poplar wood shavings (PWS) bio-pretreated by white rot fungi Coriolus versicolor in this study, and the correlations between the metabolites and lignocellulose components and the bending strength (BS) of fiberboard were also studied. After PWS were pretreated by C. versicolor for 21 days, the BS and water swelling ratio of the fiberboard were reached to 22.7 MPa and 12.4%, respectively. The soluble polysaccharide and reduce sugar in PWS were detected very low, which had a weak effect on the fiberboard. Lower content of hemicellulose and higher content of lignin were detected and beneficial to the BS of fiberboard. Manganese peroxidase was detected and had a lag enzymolysis effect. Laccase, lignin peroxidase and cellulase were not detected but laccase or cellulase might have a weak influence on fiberboard. The fiberboard production with this bio-pretreatment should be eco-friendly and eliminate the potential formaldehyde emission. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction With continuous development of urbanization and increase of population in China, the consumption and production of fiberboard are increasing day by day. Wood is one of the main materials for the production of fiberboard and widely used in furniture manufacture, packaging, building construction, musical instruments, ⇑ Corresponding author at: Jiangsu Key Laboratory for Eco-Agricultural Biotechnology around Hongze Lake, Jiangsu Key Construction Laboratory for Food Safe and Nutritional Function, School of Life Science, Huaiyin Normal University, Changjiang West Road 111, Huai’an 223300, China. E-mail address: [email protected] (J. Wu). https://doi.org/10.1016/j.conbuildmat.2019.117620 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and other industrial sectors [13]. High-strength fiberboard can be used as load-bearing building materials, and low-strength fiberboard may be used for decoration and ornament. Based on the analysis of recent status of wood consumption in China, 0.85– 0.95 billion m3 wood will be needed to meet the demand of domestic wood consumption and export. However, the forest storage in China is only 1.51 billion m3 [38]. It is very important to utilize effectively the wood processing residue, such as wood shavings and sawdust. Currently, urea-formaldehyde resin (UF) or phenolic resin (PF) is used widely as binder in the production of fiberboard [2,21,26]. Aldehyde-based binder can release some toxic and
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harmful gases like formaldehyde in the consumption and production process of fiberboard [17,31], which is seriously harmful for human health. In order to reduce the toxic gas pollution of fiberboard, a low formaldehyde and VOCs emitting board coated with nanomaterial-added melamine-impregnated papers was developed [18], and the aldehyde-free binders including isocyanate resin, soybean protein binder and binderless fiberboard have becoming the researching hotspot [8,25,39,40]. Based on the issues in production and use of aldehyde-free binder, binderless fiberboard is becoming the focus of research. The key of improving the strength of binderless fiberboard is to increase the adhesive point of raw material. So, physicochemical treatments (such as steam explosion and plasma, catalytic activation and surface treatment), biological or enzyme pretreatments [5,12,19,23,25,27,37] were developed. However, physicochemical and enzyme treatments could cause higher energy consumption and higher cost of enzyme [14,29]. Because some metabolites, such as enzymes and sugars, were produced during biological pretreatment, and the physical and chemical structure of raw materials could be also modified [15,32], which increased the adhesive point of raw materials to product fiberboard by hot pressing method. Moreover, biological pretreatment is environmentally friendly. Therefore, biological pretreatment is greatly concerned by researchers now. In China, there are a large planting area of poplar and many poplar wood furniture manufacture factories, which lead to a large number of poplar wood shavings (PWS). PWS could be used for production of fiberboard, lipids, xylitol and bioenergy [6,7,16], but there had no reports about binderless fiberboard using PWS. The white rot fungi Coriolus versicolor has a strong lignocellulose degrading enzyme systems and has been used in the production of binderless fiberboard [36]. Therefore, the strain C. versicolor was used to pretreat the PWS to produce eco-friendly fiberboard by hot-pressing without binder in this study, and the correlations between the metabolites and lignocellulose components and the strength of fiberboard were also studied.
at 45 °C for 4 h in a reciprocal shaker with 100 r/min. Most of the enzymatic liquid was removed by filtration and squeeze, and the wet enzymatic PWS (about 65% of moisture content) was used for preparation of fiberboard.
2. Materials and methods
WSRð%Þ ¼
2.1. Strain and cultivation
where, F is the maximum force to break the fiberboard specimens, N; L is the span length, L = 60 mm; b is the width of fiberboard specimens, mm; h is the thickness of fiberboard specimens, mm; h2 is the final thickness of fiberboard specimens after immersing in water, mm; h1 is the initial thickness of fiberboard specimens before immersing in water, mm.
The strain Coriolus versicolor was preserved at Jiangsu Key Laboratory for Eco-Agricultural Biotechnology around Hongze Lake in Huai‘an, China. Two or three agar blocks (1 cm 1 cm) from the preserved test tube slant of the strain were inoculated into 100 mL of potato dextrose liquor in a 250 mL round flask. After culturing for 3 days at 29 °C on a reciprocal shaker with 150 r/min, 5 mL of culture was inoculated into 100 mL of potato dextrose liquor for the second culture under the same conditions. 2.2. Bio-pretreatment of PWS by C. versicolor PWS of similar size and weight were selected for biopretreatment. 10 mL culture of C. versicolor was inoculated into 60 g (dry mass) of PWS with 80 mL of distilled water in 250-mL Erlenmeyer flasks for bio-pretreatment. The bio-pretreatment kept still at 29 °C for 28 days. 2.3. Enzymatic pretreatment of PWS by laccase or cellulase About 60 g (dry mass) of PWS was immerged in 300 mL acetic acid buffer (pH = 4.5), and added appropriate enzyme solution (enzyme activity ranged from 30 U to 90 U) of laccase (purchased from Anhui Cool Seoul Bioengineering CO., Ltd., Anhui, China) or cellulase (purchased from Huai‘an Bio-Mass Green Bio Energy Co., Ltd, Jiangsu, China). Enzymatic pretreatment was conducted
2.4. Preparation of fiberboard PWS were manually well-distributed loading into a stainless mold (100 mm 100 mm 3 mm), then pressed flat into a fiberboard by a R-3202 Hot-Press Model (Wuhan Qien Science and Technology Development Co., Ltd., Hubei, China). Hot pressing was conducted under 15 MPa pressure at 170 °C for 10 min. 2.5. Property of fiberboard The density of preparative fiberboard was measured by dividing the fiberboard mass (kg) by its volume (m3). The fiberboard was cut into fiberboard specimens of about 100 mm in length and 20 mm in width, respectively. The span length was set to 60 mm. So, the bending strength (BS) of fiberboard specimens was determined by performing the three point flex test at a crosshead speed of 3 mm/min using a micro-computer control electron universal testing machine (Model ETM104B, Wance Group, Shenzhen, China). The BS could be calculated as Eq. (1). The fiberboard using PWS with 21 days of bio-pretreatment was cut transversely, longitudinally and 45° obliquely, then the BS of the cut fiberboard specimens were determined to investigate the in-plane isotropy of the fiberboard. The water swelling ratio (WSR) was measured from the different thickness (h) before and after immersing in water for 24 h, and the WSR could be calculated as Eq. (2). The length (a), width (b) and thickness (h) of fiberboard specimens were measured accurately by micrometer.
BS ¼
3FL
ð1Þ
2
2bh
h2 h1 100% h1
ð2Þ
2.6. Determination of the polysaccharide and reducing sugar in PWS 5 g of PWS sample was fully immersed in 20 mL of distilled water for 4 h, and the reducing sugar content of the filtrate by filtration with filter paper was determined by 3, 5-dinitrosalicylic acid (DNS) method. The polysaccharide of the filtrate was extracted firstly by 80% ethanol, and the polysaccharide content was determined by Phenol-Sulphate acid method. 2.7. Determination of lignocellulosic components in PWS The contents of cellulose, hemicellulose and lignin of PWS were determined by the National Renewable Energy Laboratory (NREL) method [28]. 2.8. Determination of lignocellulosic enzymes in bio-pretreated PWS 5 g of PWS sample was fully immersed in 20 mL of distilled water for 12 h, then the laccase activity (Lac) of the filtrate by filtration with filter paper was determined by ABTS (2, 20 -Azinobis-
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(3-ethylbenzthiazoline-6-sulphonate)) method, the lignin peroxidase activity (Lip) and manganese peroxidase activity (MnP) of the filtrate were determined by veratryl alcohol method and 2, 6-dimethoxyphenol method, respectively. Cellulase activity of the filtrate was determined by carboxymethyl cellulose (CMC) saccharogenic power method. 2.9. Statistical analysis Microsoft Excel 2016 software was used to calculate the data obtained from determination, and ORIGIN PRO 8.0 software was used to analyze the data and draw figures. The error bars in all figures were corresponded to standard errors of three samples tests, and each sample test was determined for five times. ANOVA was used for analysis of significant. 3. Results and discussion 3.1. Properties of fiberboard by bio-pretreatment The bending strength (BS), water swelling ratio (WSR) and density were the key properties of fiberboard [34]. As shown in Fig. 1, the BS of fiberboard using PWS without pretreatment was 9.8 MPa, and the WSR was higher than 50%. When the fiberboard was immersed in water for 24 h, the fiberboard was decomposed. With increase of bio-pretreatment time, the BS of fiberboard increased and obtained the highest when PWS were pretreated by C. versicolor for 21 days (2.32-fold increase over PWS without pretreatment), and the prepared fiberboard was showed in Fig. 2. The fiberboard was smooth, had beautiful wood grain and delicate fragrance. The BS of the transversely cutting specimen, longitudinally cutting specimen, and 45° obliquely cutting specimen were 22.5 MPa, 23.2 MPa and 21.6 MPa, respectively. So, the fiberboard could be approximated as an in-plane isotropic material. WSR of fiberboard was decreased to 12.4% at 21 days of biopretreatment. However, WSR of fiberboard kept still when further prolonging the bio-pretreatment time. The density of the fiberboard was about 920 ± 58 kg/m3, which meant that the fiberboard in this study belonged to high density fiberboard [30]. In summary, the fiberboard made from bio-pretreated PWS without binder was high density fiberboard and approximated as an in-plane isotropic material, and had higher BS and lower WSR. Because chemical binder was not used in the preparation process of fiberboard, a potential formaldehyde gas emission was eliminated. Nevertheless, comparing with the National Standards of the People’s Republic
Fig. 1. The bending strength (BS) and water swelling ratio (WSR) of fiberboard using poplar wood shavings with different bio-pretreatment time (d) by Coriolus versicolor.
Fig. 2. The fiberboard using poplar wood shavings with 21 days bio-pretreatment by Coriolus versicolor.
of China of High Density Fiberboard (GB/T 31765-2015), the BS of the fiberboard was still below the standard (37 MPa), and the fiberboard in this study could be only used for non-load-bearing decoration. It was important to investigate the adhesive mechanism in order to improve the strength of fiberboard for more applications in building materials. 3.2. Sugar content analysis and the relationship with the strength of fiberboard Soluble polysaccharides in PWS mainly included soluble hemicellulose and fungal polysaccharides. The strain could metabolize the hemicellulose easily and produce some reducing sugar or the fungal polysaccharides. As shown in Fig. 3, the polysaccharide content decreased with increase of bio-pretreatment time. The reducing sugar content increased at first 14 days, and then decreased with the bio-pretreatment time prolonging further. There was a good positive correlation between polysaccharides with the strength of fiberboard [35,36]. Reducing sugar could occur resinification reaction of furfural and condensation reaction between furfural and lignin during the hot pressing process [4,20], and then
Fig. 3. Sugar content in poplar wood shavings with different bio-pretreatment time (d) by Coriolus versicolor.
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produce the stickiness of the fiberboard. In this study, the starch (substitute for polysaccharide) and glucose (substitute for reducing sugar) were used in the preparation of fiberboard, and the BS of the fiberboards increased more than fifteen percent compared to the contrast. However, the total content of the soluble sugar in PWS was very low (lower than 6.5 mg/g), and there was a negative correlation of the content of the sugar and the BS of the fiberboard, which concluded that sugar had a weak effect on the adhesive of PWS for the production of fiberboard in hot-press process in this study. So, it was not easy to improve the strength of fiberboard by increasing the sugar content. 3.3. Lignocellulosic components analysis and the relationship with the strength of fiberboard Lignocellulose was the main component in PWS, and the components changes with bio-pretreatment time were showed in Fig. 4. The hemicellulose content in PWS was steadily decreased from 24.96% to 21% during 21 days of bio-pretreatment, and then kept stable when the bio-pretreatment time was further prolonged. The cellulose content in PWS was kept stable at about 33% in the first 14 days of bio-pretreatment, and then decreased from 33.05% to 30.7% at the end of bio-pretreatment. The lignin content in PWS was increased first and then decreased during the bio-pretreatment time, and the maximum value of lignin content (40.13%) was reached at 21 days of bio-pretreatment. The total content of lignocellulose was kept about 92% in the first 21 days of bio-pretreatment, and then reduced by about 4% after 7 days of bio-pretreatment. From above the change rules of the components of lignocellulose in PWS, it was found that the strain C. versicolor firstly degraded the hemicellulose to grow and produce some lignocellulose hydrolase, and then the cellulose and lignin were degraded successively by C. versicolor. Because of bio-degradation, the mass of PWS was reduced by about 10% during bio-pretreatment process, which led to the increase of lignin relative content. Lignin was composed of polymerized aromatic alcohol, which was hardly to be degrade firstly but could be modified in a way which was consistent with a hydroxyl radical attack [41], which could produce adhesive action. To effectively evaluate the relationship of the lignocellulose component content with the strength of fiberboard, the values of lignocellulose component content were plotted against BS as shown in Fig. 5. There was a significantly (P value = 0.042) positive correlation between the lignin content and the BS, but a very significantly (P value = 0.006) negative correlation between the hemicellulose content and the BS. Cellulose content had nothing to do with the BS of the fiberboard. In a word, lower content of hemicel-
Fig. 4. Lignocellulose components of poplar wood shavings with different biopretreatment time (d) by Coriolus versicolor.
Fig. 5. Relationship between lignocellulose component content and the bending strength (BS) of fiberboard.
lulose and higher content of lignin were beneficial to the BS of the fiberboard. PWS had a higher content of lignin than corn straw and Triarrhena sacchariflora residue, which led to higher BS of fiberboard of PWS bio-pretreated by white rot fungi than corn straw and T. sacchariflora residue [35,36]. Black liquor is a kind of industry wastewater with high COD and high concentration of lignin, the removal of lignin is the bottleneck problem of black liquor treatment [1,11]. So some researchers obtained lignin from paper industry black liquor to prepare phenolic resins (ligninformaldehyde or lignin-glyoxal) or be used as green adhesive to manufacture fiberboard [3,9]. These further indicated that the increase of lignin was beneficial to the strength of fiberboard. 3.4. Lignocellulosic enzymes activity analysis and the relationship with the strength of fiberboard Lignocellulosic enzymes activities in PWS during biopretreatment process were determined. Lac, Lip and cellulase were very low and not determined, but MnP was determined and showed in Fig. 6. MnP increased in the first 14 days of biopretreatment process, and decreased firstly then increased in the next 14 days of the process. Comparing the relationship between MnP and BS of the fiberboard, it was found that there was a
Fig. 6. Manganese peroxidase activity in poplar wood shavings with different biopretreatment time (d) by Coriolus versicolor.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (grant number 31870543); and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (grant number PPZY2015A018).
References
Fig. 7. The bending strength (BS) of fiberboard using poplar wood shavings pretreated by cellulase or laccase.
positive correlation between the 7 days lag MnP and the BS. There was an enzymolysis time in the process of enzyme action, which led to a lag on the enzyme activity to the enzymolysis effect. Lignin degradation enzymes such as MnP could start free radical chain reaction, which led to the bond cleavage, modification and coupling of lignin [22], and increase the specific surface area of the biomass [10]. So, it should be easy to occur the cross-linking and adhesive of lignin and fiber to improve the strength of fiberboard. Consequently, it was important to increase the lignocellulosic enzymes activities, especially MnP in the bio-pretreatment process in order to improve the strength of the fiberboard. Some researchers found that the application of Lac or peroxidase had a high potential for environmentally friendly MDF production [12,33], and there was a positive correlation of Lac with the strength of fiberboard [24,36]. However, there had few reports about cellulase pretreatment for production of fiberboard. In this study, the effects of Lac and cellulase pretreatment on the BS of fiberboard were investigated as shown in Fig. 7. With the increase of enzyme dosage of Lac and cellulase, the BS of fiberboard increased first, and then kept stable. Using only 1 U/g of enzyme dosage of Lac or cellulase, the BS of fiberboard were increased to 1.78-fold and 1.91-fold, respectively. The data indicated that pretreatment of a small amount of Lac or cellulase could significantly improve the strength of fiberboard. Therefore, although the activities of Lac or cellulase were very low in this bio-treatment process, they might also have a weak influence on the strength of fiberboard. The effect of Lac was similar to that of MnP, and the effect of cellulase was mainly to release more reducing sugar by hydrolyzing cellulose, which was beneficial to the adhesive reaction by hot-pressing. 4. Conclusions BS and WSR of the fiberboard without binder reached to 22.7 MPa and 12.4%, respectively, after PWS were exposed to C. versicolor for 21 days. The fiberboard without chemical binder eliminated potential formaldehyde emissions. Soluble polysaccharide and reduce sugar in PWS were detected very low, which had a weak effect on the adhesive of the fiberboard. Lower content of hemicellulose and higher content of lignin were beneficial to the BS of the fiberboard. MnP was detected and had a 7 days lag on the enzymolysis effect, and Lac and cellulase might also have a weak influence on the strength of fiberboard. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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