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Abstract： Bamboo was used as raw material for unbleached pulp production. An acetic acid prehydrolysis process was employed before the pulping process. The effect of acetic acid prehydrolysis on pulp properties was investigated. The results showed that some components， such as hemicellulose and extractives， were degraded or dissolved in the prehydrolysis process prior to kraft pulping. SEM images of the substrate after treatment indicated that the parenchyma cell wall was thinner， and the size of pores between fiber bundle cell walls was larger. The pulping results showed that acetic acid prehydrolysis could improve the pulp quality and make the pulp easier to bleach. The brightness of the pulp reached 59.6%ISO with a single oxygen delignification step. The acetic acid prehydrolysis decreased pulp viscosity and fiber length， but not significantly. The amount of parenchyma cells in the pulp was reduced， which was beneficial for papermaking and improving mechanical strength of paper. This procedure has good potential for unbleached pulp production.
Keywords： acetic acid； prehydrolysis； bamboo； unbleached pulp； kraft pulping
1 Introduction
Bamboo is a fast-growing annual plant and has been demonstrated as a good fiber source for pulping and papermaking. It has long fibers and a suitable chemical composition in many bamboo species[1]. Bamboo fibers also have some interesting natural characteristics， such as antibacterial property and UV resistance. These special properties extend the applications of bamboo fibers. Because of its abundance and good fiber quality， bamboo as a raw material has been attracting increasing attention for conversion into various fiber-based products.
The bleaching process is an important part of papermaking. Oxidizing agents such as hypochlorite and chlorine dioxide are normally used to remove the residual lignin in pulp. Some of the by-products produced from delignification are organic chloride compounds which are toxic， hard to biodegrade， and hostile to the environment. With increasing environmental and health concerns， totally chlorine-free bleaching techniques such as oxygen delignification have been introduced as alternative bleaching processes. In recent years， it has become more popular to use unbleached pulp in paper products. However， consumers are often unwilling to accept too dark a shade of paper in some products such as tissue paper， preferring lightly-bleached pulps that are treated with two stages of oxygen delignification to substitute for darker pulps in these products. Prehydrolysis technology has been successfully used to remove lignin for biomass conversion[2]. This technology has also been introduced for dissolving pulp production to obtain a high cellulose and low hemicellulose content pulp. The treatment can upgrade the cellulose fibers for further reactions and increase the accessibility of lignin during the subsequent pulping process[3]. Hot-water prehydrolysis[4]， steam pretreatment[5]， and ammonia fiber explosion[6] are the most common types of pretreatment procedure. Recently in our group， acetic acid has been applied as a prehydrolysis reagent for dissolving pulp production， which is regarded as an environmentally friendly way to obtain cellulose pulps from lignocellulosic materials. Compared with dilute inorganic acids， it has some desirable advantages， including effective hydrolysis， fewer degradation products， and more production of oligomeric sugars[7].
In this study， the effect of acetic acid prehydrolysis on unbleached bamboo pulp production was evaluated. The chips were hydrolyzed in dilute acetic acid liquor and subsequently cooked by the kraft pulping process. The pulp was bleached by a single step of oxygen delignification. The yield， chemical composition， and properties of the pulp were evaluated to show that this is a viable technology for use in the production of unbleached bamboo pulp.
2 Experimental
2.1 Raw materials and prehydrolysis
Bamboo chips were generously provided by Nanping Forestry Mill （Zhangzhou， Fujian province， China）. All chemicals used were of analytical grade.
The prehydrolysis of bamboo was carried out in an M/K digester equipped with a heat exchanger， circulating pump， and computer-controlled time and temperature. The acetic acid dosage was 2.5% based on dry chip weight. The cooking conditions were as follows： 500 g chips， 4 L/1 kg liquid/solid ratio， maximum temperature 160℃， 60 min to maximum temperature， and 30 min at maximum temperature.
2.2 Kraft pulping and bleaching
The kraft cooking of treated and control chips was performed under the following conditions： 5%～20% effective alkali （EA） and 25% sulfidity on dry chip weight， 4 L/1 kg liquor/bamboo ratio， 60 min to the maximum temperature of 165℃， and 30 min at maximum temperature.
The pulp was bleached by a single step of oxygen delignification where the oxygen pressure was 0.6 MPa at 105℃ for 60 min. NaOH and MgSO4 dosages were 2% and 0.15%， respectively. 2.3 Pulp characterization
The viscosity of the pulp before and after bleaching was measured according to TAPPI T230om-08[8]. The brightness of the handsheets was tested according to TAPPI T525om-06[9].
Fiber characteristics were analyzed by FS300 analyzer and microscope.
2.4 Chemical composition analysis
The chemical compositions of the original and pretreated bamboo chip samples were measured according to the National Renewable Energy Laboratory （NREL） standard. All the substrates were ground using a grinder to pass 80 mesh screen. The milled samples were hydrolyzed using 72% sulfuric acid （w/w） at （30±3）℃ for 1 h and 4% sulfuric acid （w/w） at （121±3）℃ for 1 h. The hydrolysate was then analyzed for carbohydrates using high performance liquid chromatography （HPLC）. The Klason lignin content was measured gravimetrically after washing and drying the solid residues from the acid hydrolysate.
2.5 Scanning electron microscopy （SEM） analysis
The substrates after pretreatment were freeze-dried and cut by an ion beam cutter instrument （Leica EM TIC 3X Triple Ion Beam Cutter system）， then fixed on sample holders and coated with Au for SEM observation. The surface morphology of the as-prepared samples was observed by SEM （Zeiss EVO 18 scanning electron microscope， Germany） with an operating voltage of 10 kV in secondary electron mode.
3 Results and discussion
3.1 Effect of acetic acid prehydrolysis on bamboo chips
As shown in Table 1， the holocellulose content of bamboo is about 63%， and the ash content is less than 1%， which means that bamboo is a good potential biomass for pulp production. After 2.5% acetic acid pretreatment， the content of hemicellulose and extractives decreased significantly. This is because the molecule of hemicellulose is smaller than that of cellulose. Degradation occurs mainly in the hemicellulose area.
Heitz M et al used hot water as a pretreatment agent， and found that water auto-ionizes， releasing acetyl groups on the hemicellulose as acetic acid， and providing hydronium ions for hydrolysis reactions both between hemicellulose and lignin and within the carbohydrates[10]. This pretreatment procedure has been successfully used to remove a part of the hemicellulose from various lignocellulosic materials[11]. Based on this， acetic acid addition could accelerate hemicellulose hydrolysis and improve susceptibility to further reactions in the pulping process[12]. In a SEM image of the raw material （Fig.1（a））， two different areas can be clearly seen： the parenchyma cell area and the fiber bundle cell area. A triangular space exists between the parenchyma cells， while there are pore structures between the fiber bundle cells. After pretreatment by the acetic acid， obvious changes have occurred in the cell wall structure， as shown in Fig. 1（b）. The parenchyma cell wall is thinner， and the pores between fiber bundle cell walls are larger than before acetic acid pretreatment. For the parenchyma cells， fragmentation and collapse mainly occur in the acetic acid pretreatment process. This is associated with the hydrolysis of a part of the substrate and the dissolution of some components.
3.2 Pulp properties
Table 2 shows the pulp yield after various cooking processes. It shows that the total yield of pulp with acetic acid prehydrolysis is not much different from that of pulp without a prehydrolysis step. However， the fine yield of pulp increased after acetic acid prehydrolysis. This indicates that acetic acid prehydrolysis can improve the quality of the pulp. The highest fine pulp yield determined on the basis of the hydrolyzed chip weight is 49.4%， which is a little higher than the results found by Vu T M et al[13]. When the effective alkali dosage is less than 15%， the amount of rejects is high， which is not beneficial for pulping.
Some components， such as hemicellulose and extractives， were degraded or dissolved out into the pretreatment liquor in the prehydrolysis process prior to kraft pulping， and this was beneficial for obtaining a final pulp with less contaminants. Prehydrolysis also results in some lignin removal during subsequent kraft pulping， presumably due to better accessibility and the cleavage of lignin-carbohydrate complex （LCC） bonds[14]. In addition， there is fragmentation and collapse that occur in bamboo chips when acetic acid prehydrolysis is applied. This could provide channels for the cooking liquor.
One single oxygen delignification step was carried out after pulping to meet the requirement of unbleached pulp production. Table 3 shows the viscosity of pulp with or without acetic acid prehydrolysis after one oxygen delignification. Compared to the pulp obtained without prehydrolysis， the viscosity of pulp obtained with prehydrolysis decreased slightly， indicating that the components that are degraded or dissolved in the prehydrolysis process affect the degree of polymerization. However， this effect was not significant. This level of pulp viscosity is still suitable for unbleached pulp production. To investigate the optical properties of the pulp， handsheets were made. The brightness and chromaticity data of the pulp were measured. As shown in Table 4， compared to the pulp obtanined without prehydrolysis， the brightness of the pulp obtained with prehydrolysis prior to cooking increased significantly under the same cooking and bleaching conditions. A pulp brightness of approximately 60%ISO could be reached using one stage of oxygen bleaching when a prehydrolysis process was applied prior to cooking. This indicates that the pulp obtained by cooking combined with the prehydrolysis process is easily bleached. Chromaticity is an important index for the color of paper. Changes in L* value followed a similar trend to that of brightness， where the L* value represents the whiteness of the paper. The values of a* and b* decreased， which indicates that there were less chromophoric groups in the pulp[15]. Therefore， the prehydrolysis process was of benefit to the subsequent cooking and bleaching processes.
In this study， the pulp obtained with prehydrolysis shows good potential for unbleached pulp production. Compared with the conventional bleaching process， one single oxygen delignification employed as a bleaching process is more environmentally friendly. There are no other chemicals needed for bleaching.
3.3 Fiber analysis
The fiber length was analyzed using a FS300 instrument. The data are shown in Table 5. The weighted average fiber length decreased slightly when prehydrolysis was applied. The results correspond to the viscosity data. The fiber width was not affected by the prehydrolysis process. The mechanical properties of paper are mostly affected by fiber length. This indicates that paper made from the pulp with a prehydrolysis process would not significantly lose strength.
A microscope was used to characterize the morphology of the fiber. As shown in Fig.2， all samples exhibited the characteristics of bamboo fibers， which are slender and sharp at both ends. The prehydrolysis process had no effect on fiber morphology. However， the amount of parenchyma cells decreased after cooking combined with the prehydrolysis process. Too many parenchyma cells have an adverse effect on papermaking and paper mechanical properties. SEM analysis of bamboo chips after prehydrolysis showed that the parenchyma cell walls were thinner. It indicated that part of the parenchyma cells were degraded or dissolved in the prehydrolysis process. This is the reason for less parenchyma cells in the pulp with a prehydrolysis process. 4 Conclusions
In summary， acetic acid prehydrolysis combined with the kraft pulping process can improve the properties of bamboo pulp. In the prehydrolysis process， hemicellulose and extractives are degraded or dissolved， which is beneficial for subsequent pulping and bleaching processes. SEM analysis of the substrate after treatment showed that the structure of the parenchyma cell walls and the pores between the fiber bundle cell walls were changed significantly. The brightness of the pulp could reach 59.6%ISO with a single oxygen delignification step. The viscosity and fiber length of pulp obtained by the prehydrolysis process decreased slightly. The amount of parenchyma cells in the pulp was reduced， which is beneficial for papermaking and improving mechanical strength of paper.
Acknowledgments
The authors are grateful for financial support from the National Natural Science Foundation of China （31570569）.
References
[1] Maria S， Raimo A， M?nthihong V. Description of kraft cooking and oxygen-alkali delignification of bamboo by pulp and dissolving material analysis[J]. Industrial Crops & Products， 2008， 28（1）： 47-55.
[2] Kumar P， Barrett D M， Delwiche M J， et al. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production[J]. Industrial & Engineering Chemistry Research， 2009， 48（8）： 3713-3729.
[3] Garrote G， Eugenio M E， D??Az M J， et al. Hydrothermal and pulp processing of Eucalyptus[J]. Bioresource Technology， 2003， 88（1）： 61-68.
[4] Mosier N， Hendrickson R， Ho N， et al. Optimization of pH controlled liquid hot water pretreatment of corn stover[J]. Bioresource Technology， 2005， 96（18）： 1986-1993.
[5] Laser M， Schulman D， Allen S G， et al. A comparison of liquid hot water and steam pretreatments of sugar cane bagasse for bioconversion to ethanol[J]. Fuel & Energy， 2002， 81（1）： 33-44.
[6] Teymouri F， Laureanoperez L， Alizadeh H， et al. Optimization of the ammonia fiber explosion （AFEX） treatment parameters for enzymatic hydrolysis of corn stover[J]. Bioresource Technology， 2005， 96（18）： 2014-2018.
[7] Li G， Fu S， Zhou A， et al. Improved cellulose yield in the production of dissolving pulp from bamboo using acetic acid in prehydrolysis[J]. Bioresources， 2014， 10（1）： 877-886.
[8] TAPPI Standard Test Method. TAPPI T230om-08： 2008 Viscosity of Pulp （Capillary Viscometer Method）[S].
[9] TAPPI Standard Test Method. TAPPI T525om-06： 2006 Diffuse Brightness of Paper， Paperboard and Pulp[S]. [10] Heitz M， Carrasco F， Rubio M， et al. Generalized correlations for the aqueous liquefaction of lignocellulosics[J]. Canadian Journal of Chemical Engineering， 1986， 64（4）： 647-650.
[11] Sabanci K， Buyukkileci A O. Comparison of liquid hot water， very dilute acid and alkali treatments for enhancing enzymatic digestibility of hazelnut tree pruning residues[J]. Bioresource Technology， 2018， 261： 158-165.
[12] Allen S G， Kam L C， Zemann， A J， et al. Fractionation of sugar cane with hot， compressed， liquid water[J]. Industrial and Engineering Chemistry Research， 1996， 35（8）： 2709-2715.
[13] Vu T M， Pakkanen H， Alen R. Delignification of bamboo （Bambusa procera acher）： Part 1. Kraft pulping and the subsequent oxygen delignification to pulp with a low kappa number[J]. Industrial Crops & Products， 2004， 19（1）： 49-57.
[14] Rauhala T， King A W T， Zuckerstaetter G， et al. Effect of auto hydrolysis on the lignin structure and the kinetics of delignification of birch wood[J]. Nordic Pulp & Paper Research Journal， 2011， 26（4）： 386-391.
[15] Kishino M， Nakano T. Artificial weathering of tropical woods. Part 2： Color change[J]. Holzforschung， 2004， 21（6）： 381-565.
Keywords： acetic acid； prehydrolysis； bamboo； unbleached pulp； kraft pulping
1 Introduction
Bamboo is a fast-growing annual plant and has been demonstrated as a good fiber source for pulping and papermaking. It has long fibers and a suitable chemical composition in many bamboo species[1]. Bamboo fibers also have some interesting natural characteristics， such as antibacterial property and UV resistance. These special properties extend the applications of bamboo fibers. Because of its abundance and good fiber quality， bamboo as a raw material has been attracting increasing attention for conversion into various fiber-based products.
The bleaching process is an important part of papermaking. Oxidizing agents such as hypochlorite and chlorine dioxide are normally used to remove the residual lignin in pulp. Some of the by-products produced from delignification are organic chloride compounds which are toxic， hard to biodegrade， and hostile to the environment. With increasing environmental and health concerns， totally chlorine-free bleaching techniques such as oxygen delignification have been introduced as alternative bleaching processes. In recent years， it has become more popular to use unbleached pulp in paper products. However， consumers are often unwilling to accept too dark a shade of paper in some products such as tissue paper， preferring lightly-bleached pulps that are treated with two stages of oxygen delignification to substitute for darker pulps in these products. Prehydrolysis technology has been successfully used to remove lignin for biomass conversion[2]. This technology has also been introduced for dissolving pulp production to obtain a high cellulose and low hemicellulose content pulp. The treatment can upgrade the cellulose fibers for further reactions and increase the accessibility of lignin during the subsequent pulping process[3]. Hot-water prehydrolysis[4]， steam pretreatment[5]， and ammonia fiber explosion[6] are the most common types of pretreatment procedure. Recently in our group， acetic acid has been applied as a prehydrolysis reagent for dissolving pulp production， which is regarded as an environmentally friendly way to obtain cellulose pulps from lignocellulosic materials. Compared with dilute inorganic acids， it has some desirable advantages， including effective hydrolysis， fewer degradation products， and more production of oligomeric sugars[7].
In this study， the effect of acetic acid prehydrolysis on unbleached bamboo pulp production was evaluated. The chips were hydrolyzed in dilute acetic acid liquor and subsequently cooked by the kraft pulping process. The pulp was bleached by a single step of oxygen delignification. The yield， chemical composition， and properties of the pulp were evaluated to show that this is a viable technology for use in the production of unbleached bamboo pulp.
2 Experimental
2.1 Raw materials and prehydrolysis
Bamboo chips were generously provided by Nanping Forestry Mill （Zhangzhou， Fujian province， China）. All chemicals used were of analytical grade.
The prehydrolysis of bamboo was carried out in an M/K digester equipped with a heat exchanger， circulating pump， and computer-controlled time and temperature. The acetic acid dosage was 2.5% based on dry chip weight. The cooking conditions were as follows： 500 g chips， 4 L/1 kg liquid/solid ratio， maximum temperature 160℃， 60 min to maximum temperature， and 30 min at maximum temperature.
2.2 Kraft pulping and bleaching
The kraft cooking of treated and control chips was performed under the following conditions： 5%～20% effective alkali （EA） and 25% sulfidity on dry chip weight， 4 L/1 kg liquor/bamboo ratio， 60 min to the maximum temperature of 165℃， and 30 min at maximum temperature.
The pulp was bleached by a single step of oxygen delignification where the oxygen pressure was 0.6 MPa at 105℃ for 60 min. NaOH and MgSO4 dosages were 2% and 0.15%， respectively. 2.3 Pulp characterization
The viscosity of the pulp before and after bleaching was measured according to TAPPI T230om-08[8]. The brightness of the handsheets was tested according to TAPPI T525om-06[9].
Fiber characteristics were analyzed by FS300 analyzer and microscope.
2.4 Chemical composition analysis
The chemical compositions of the original and pretreated bamboo chip samples were measured according to the National Renewable Energy Laboratory （NREL） standard. All the substrates were ground using a grinder to pass 80 mesh screen. The milled samples were hydrolyzed using 72% sulfuric acid （w/w） at （30±3）℃ for 1 h and 4% sulfuric acid （w/w） at （121±3）℃ for 1 h. The hydrolysate was then analyzed for carbohydrates using high performance liquid chromatography （HPLC）. The Klason lignin content was measured gravimetrically after washing and drying the solid residues from the acid hydrolysate.
2.5 Scanning electron microscopy （SEM） analysis
The substrates after pretreatment were freeze-dried and cut by an ion beam cutter instrument （Leica EM TIC 3X Triple Ion Beam Cutter system）， then fixed on sample holders and coated with Au for SEM observation. The surface morphology of the as-prepared samples was observed by SEM （Zeiss EVO 18 scanning electron microscope， Germany） with an operating voltage of 10 kV in secondary electron mode.
3 Results and discussion
3.1 Effect of acetic acid prehydrolysis on bamboo chips
As shown in Table 1， the holocellulose content of bamboo is about 63%， and the ash content is less than 1%， which means that bamboo is a good potential biomass for pulp production. After 2.5% acetic acid pretreatment， the content of hemicellulose and extractives decreased significantly. This is because the molecule of hemicellulose is smaller than that of cellulose. Degradation occurs mainly in the hemicellulose area.
Heitz M et al used hot water as a pretreatment agent， and found that water auto-ionizes， releasing acetyl groups on the hemicellulose as acetic acid， and providing hydronium ions for hydrolysis reactions both between hemicellulose and lignin and within the carbohydrates[10]. This pretreatment procedure has been successfully used to remove a part of the hemicellulose from various lignocellulosic materials[11]. Based on this， acetic acid addition could accelerate hemicellulose hydrolysis and improve susceptibility to further reactions in the pulping process[12]. In a SEM image of the raw material （Fig.1（a））， two different areas can be clearly seen： the parenchyma cell area and the fiber bundle cell area. A triangular space exists between the parenchyma cells， while there are pore structures between the fiber bundle cells. After pretreatment by the acetic acid， obvious changes have occurred in the cell wall structure， as shown in Fig. 1（b）. The parenchyma cell wall is thinner， and the pores between fiber bundle cell walls are larger than before acetic acid pretreatment. For the parenchyma cells， fragmentation and collapse mainly occur in the acetic acid pretreatment process. This is associated with the hydrolysis of a part of the substrate and the dissolution of some components.
3.2 Pulp properties
Table 2 shows the pulp yield after various cooking processes. It shows that the total yield of pulp with acetic acid prehydrolysis is not much different from that of pulp without a prehydrolysis step. However， the fine yield of pulp increased after acetic acid prehydrolysis. This indicates that acetic acid prehydrolysis can improve the quality of the pulp. The highest fine pulp yield determined on the basis of the hydrolyzed chip weight is 49.4%， which is a little higher than the results found by Vu T M et al[13]. When the effective alkali dosage is less than 15%， the amount of rejects is high， which is not beneficial for pulping.
Some components， such as hemicellulose and extractives， were degraded or dissolved out into the pretreatment liquor in the prehydrolysis process prior to kraft pulping， and this was beneficial for obtaining a final pulp with less contaminants. Prehydrolysis also results in some lignin removal during subsequent kraft pulping， presumably due to better accessibility and the cleavage of lignin-carbohydrate complex （LCC） bonds[14]. In addition， there is fragmentation and collapse that occur in bamboo chips when acetic acid prehydrolysis is applied. This could provide channels for the cooking liquor.
One single oxygen delignification step was carried out after pulping to meet the requirement of unbleached pulp production. Table 3 shows the viscosity of pulp with or without acetic acid prehydrolysis after one oxygen delignification. Compared to the pulp obtained without prehydrolysis， the viscosity of pulp obtained with prehydrolysis decreased slightly， indicating that the components that are degraded or dissolved in the prehydrolysis process affect the degree of polymerization. However， this effect was not significant. This level of pulp viscosity is still suitable for unbleached pulp production. To investigate the optical properties of the pulp， handsheets were made. The brightness and chromaticity data of the pulp were measured. As shown in Table 4， compared to the pulp obtanined without prehydrolysis， the brightness of the pulp obtained with prehydrolysis prior to cooking increased significantly under the same cooking and bleaching conditions. A pulp brightness of approximately 60%ISO could be reached using one stage of oxygen bleaching when a prehydrolysis process was applied prior to cooking. This indicates that the pulp obtained by cooking combined with the prehydrolysis process is easily bleached. Chromaticity is an important index for the color of paper. Changes in L* value followed a similar trend to that of brightness， where the L* value represents the whiteness of the paper. The values of a* and b* decreased， which indicates that there were less chromophoric groups in the pulp[15]. Therefore， the prehydrolysis process was of benefit to the subsequent cooking and bleaching processes.
In this study， the pulp obtained with prehydrolysis shows good potential for unbleached pulp production. Compared with the conventional bleaching process， one single oxygen delignification employed as a bleaching process is more environmentally friendly. There are no other chemicals needed for bleaching.
3.3 Fiber analysis
The fiber length was analyzed using a FS300 instrument. The data are shown in Table 5. The weighted average fiber length decreased slightly when prehydrolysis was applied. The results correspond to the viscosity data. The fiber width was not affected by the prehydrolysis process. The mechanical properties of paper are mostly affected by fiber length. This indicates that paper made from the pulp with a prehydrolysis process would not significantly lose strength.
A microscope was used to characterize the morphology of the fiber. As shown in Fig.2， all samples exhibited the characteristics of bamboo fibers， which are slender and sharp at both ends. The prehydrolysis process had no effect on fiber morphology. However， the amount of parenchyma cells decreased after cooking combined with the prehydrolysis process. Too many parenchyma cells have an adverse effect on papermaking and paper mechanical properties. SEM analysis of bamboo chips after prehydrolysis showed that the parenchyma cell walls were thinner. It indicated that part of the parenchyma cells were degraded or dissolved in the prehydrolysis process. This is the reason for less parenchyma cells in the pulp with a prehydrolysis process. 4 Conclusions
In summary， acetic acid prehydrolysis combined with the kraft pulping process can improve the properties of bamboo pulp. In the prehydrolysis process， hemicellulose and extractives are degraded or dissolved， which is beneficial for subsequent pulping and bleaching processes. SEM analysis of the substrate after treatment showed that the structure of the parenchyma cell walls and the pores between the fiber bundle cell walls were changed significantly. The brightness of the pulp could reach 59.6%ISO with a single oxygen delignification step. The viscosity and fiber length of pulp obtained by the prehydrolysis process decreased slightly. The amount of parenchyma cells in the pulp was reduced， which is beneficial for papermaking and improving mechanical strength of paper.
Acknowledgments
The authors are grateful for financial support from the National Natural Science Foundation of China （31570569）.
References
[1] Maria S， Raimo A， M?nthihong V. Description of kraft cooking and oxygen-alkali delignification of bamboo by pulp and dissolving material analysis[J]. Industrial Crops & Products， 2008， 28（1）： 47-55.
[2] Kumar P， Barrett D M， Delwiche M J， et al. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production[J]. Industrial & Engineering Chemistry Research， 2009， 48（8）： 3713-3729.
[3] Garrote G， Eugenio M E， D??Az M J， et al. Hydrothermal and pulp processing of Eucalyptus[J]. Bioresource Technology， 2003， 88（1）： 61-68.
[4] Mosier N， Hendrickson R， Ho N， et al. Optimization of pH controlled liquid hot water pretreatment of corn stover[J]. Bioresource Technology， 2005， 96（18）： 1986-1993.
[5] Laser M， Schulman D， Allen S G， et al. A comparison of liquid hot water and steam pretreatments of sugar cane bagasse for bioconversion to ethanol[J]. Fuel & Energy， 2002， 81（1）： 33-44.
[6] Teymouri F， Laureanoperez L， Alizadeh H， et al. Optimization of the ammonia fiber explosion （AFEX） treatment parameters for enzymatic hydrolysis of corn stover[J]. Bioresource Technology， 2005， 96（18）： 2014-2018.
[7] Li G， Fu S， Zhou A， et al. Improved cellulose yield in the production of dissolving pulp from bamboo using acetic acid in prehydrolysis[J]. Bioresources， 2014， 10（1）： 877-886.
[8] TAPPI Standard Test Method. TAPPI T230om-08： 2008 Viscosity of Pulp （Capillary Viscometer Method）[S].
[9] TAPPI Standard Test Method. TAPPI T525om-06： 2006 Diffuse Brightness of Paper， Paperboard and Pulp[S]. [10] Heitz M， Carrasco F， Rubio M， et al. Generalized correlations for the aqueous liquefaction of lignocellulosics[J]. Canadian Journal of Chemical Engineering， 1986， 64（4）： 647-650.
[11] Sabanci K， Buyukkileci A O. Comparison of liquid hot water， very dilute acid and alkali treatments for enhancing enzymatic digestibility of hazelnut tree pruning residues[J]. Bioresource Technology， 2018， 261： 158-165.
[12] Allen S G， Kam L C， Zemann， A J， et al. Fractionation of sugar cane with hot， compressed， liquid water[J]. Industrial and Engineering Chemistry Research， 1996， 35（8）： 2709-2715.
[13] Vu T M， Pakkanen H， Alen R. Delignification of bamboo （Bambusa procera acher）： Part 1. Kraft pulping and the subsequent oxygen delignification to pulp with a low kappa number[J]. Industrial Crops & Products， 2004， 19（1）： 49-57.
[14] Rauhala T， King A W T， Zuckerstaetter G， et al. Effect of auto hydrolysis on the lignin structure and the kinetics of delignification of birch wood[J]. Nordic Pulp & Paper Research Journal， 2011， 26（4）： 386-391.
[15] Kishino M， Nakano T. Artificial weathering of tropical woods. Part 2： Color change[J]. Holzforschung， 2004， 21（6）： 381-565.