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Abstract： In this study， MgO was partially used as an alkali source in the peroxide bleaching process of bleached chemi-thermomechanical pulp （BCTMP）. The effects of substitution percentage of MgO for NaOH on the bulk， optical， and physical properties of bleached pulp， and the main effluent characteristics were analyzed. In addition， the influencing mechanism of Mg-based alkali on the strength properties of the BCTMP was further investigated. Strength properties of the BCTMPs were investigated as a function of charge characteristics， fiber morphology， surface lignin content， relative bonding area， and hydrogen bonds of the BCTMP. The results showed that cationic demand （CD） and chemical oxygen demand （CODCr） of the bleaching effluent decreased as the substitution percentage of MgO for NaOH increased； meanwhile， the bulk and optical properties of the BCTMP increased. Nevertheless， the strength properties （tensile， tear， and burst indices） of the bleached pulp decreased as the substitution percentage of MgO for NaOH increased. The decrease in the fiber charge density and increase in the surface lignin content affected the fiber swelling， resulting in a decline in pulp inter-fibers bonding strength and further loss of the tensile and burst indices.
Keywords： Mg-based alkali； substitution； chemi-thermomechanical pulp； alkaline peroxide bleaching； physical properties
1 Introduction
Alkaline peroxide bleaching， as a major section of the bleaching stage in the high yield pulping process， plays an important role in the quality control of end pulp products. Sodium hydroxide （NaOH） has been widely used as an alkali source in peroxide bleaching. However， as a strong alkali base， NaOH would cause excessive dissolution of carbohydrates， especially hemicelluloses， into bleaching effluents， which will lead to a high chemical oxygen demand （COD） load and bleaching yield loss. Meanwhile， a certain amount of anionic trash might be generated by high alkalinity of NaOH in the process and further carried to the wet-end section of the paper machine， leading to a negative impact on papermaking operations and paper product qualities[1-2].
Mg-based alkalis， e.g.， MgO/Mg（OH）2， partially substituting for NaOH and performing as one alkali source in the alkaline peroxide bleaching of high-yield pulps， provide an effective way to reduce the negative effects caused by NaOH[2-4]. The usage of MgO/Mg（OH）2 in the alkaline peroxide bleaching process can increase the bulk and optical properties of the bleached pulp， decrease the load of the bleaching effluent， improve the bleaching yield， and reduce the oxalate scaling[3-8]. The strength properties of pulp mainly depend on the wood species， pulping method， and fiber characteristics， among other factors. So far， it has been commonly believed that the major reason for the variation of the strength properties of the bleached pulp， when Mg-based alkalis are applied in the alkaline peroxide bleaching process， can be mainly attributed to the weaker alkalinity of MgO/Mg（OH）2[9]. In this study， the effects of some main characteristics （chemical components， charge characteristics， fiber morphology， surface lignin content， the relative bonding area （RBA）， and hydrogen bonding intensity） on the strength properties of the bleached CTMP were investigated. The main reasons for the changes of the strength properties of the peroxide bleached CTMP， when MgO was partially substituted for NaOH， were studied.
2 Materials and methods
2.1 Materials
The poplar CTMP， with the initial Canadian standard freeness （CSF） of 560 mL， was collected from a pulp mill in Shandong province， China， and stored in a cold room at 4℃. The main properties of the CTMP used in this study are listed as follows： bulk of 2.95 cm3/g， tensile index of 10.6 N·m/g， tear index of 1.45 mN·m2/g， burst index of 0.399 kPa·m2/g， ISO brightness of 57.7%， and light scattering coefficient of 14.7 m2/kg.
All chemicals used in this study were of analytical grade.
2.2 Alkaline peroxide bleaching of the poplar CTMP
The bleaching experiments were conducted in polyethylene bags using a water bath under the following conditions： the poplar CTMP of the equivalent to 40 g of oven-dried pulp； 3.0% total alkali charge （on NaOH）， of which 0%， 10%， 25%， 35%， 50%， and 75% of NaOH was replaced with MgO （molar ratio）； 0.2% diethylene triamine pentacetate acid （DTPA）； 2.0% Na2SiO3； 5.0% H2O2； 10% pulp consistency； 80℃ for 90 min.
In the alkaline peroxide bleaching process， NaOH， MgO， Na2SiO3， part of deionized water， and DTPA were mixed well first with the poplar CTMP by kneading the polyethylene bag by hand. Then， H2O2 and the remaining deionized water were added into the suspension. After the pulp and chemicals were mixed well by kneading constantly， the polyethylene bag was sealed and placed into the water bath with a temperature of 80℃. The bleaching duration time began to count as soon as the temperature of the pulp suspension reached 80℃. The polyethylene bag was kneaded once every 10 min in order to make the bleaching reaction uniform. As the duration time reached 90 min， the bag was taken out of the water bath and promptly put into a cold water bath to cool down the pulp to room temperature. The bleached CTMP was then washed thoroughly with deionized water. 2.3 Cationic demand and CODCr analysis
The Müteck PCD-03 charge analyzer （BTG Co.， Ltd.， Germany） was used to measure the cationic demand. The pH value of the filtrate was adjusted to （6.8±0.1） with 0.10 mol/L of H2SO4 before testing[10]. According to the USEPA Reactor Digestion Method[11]， the CODCr was determined using a DRB 200COD instrument （HACH Co.， Ltd.， USA） and a DR 1010 COD instrument （HACH Co.， Ltd.， USA）.
2.4 Handsheets preparation and testing
A well-washed pulp cake was dispersed in deionized water， with 1% of pulp consistency. The pulp suspension was used to prepare handsheets following ISO standards of 5269-1 （2005） and 3688 （1999） after adjusting the pH value to （5.0±0.1） with dilute H2SO4. In addition， bulk， brightness， opacity， light-scattering coefficient， and strength properties of the handsheets were determined according to ISO standards of 534 （2012）， 2470 （2009）， 2471 （2008）， 9416 （2009）， and 5270 （2012）， respectively.
2.5 Fiber quality analysis
The pulp fiber length， fiber width， fine content， and kink index were analyzed by a fiber quality analyzer （FQA） （Model 912， Lorentzen & Wettre Co.， Ltd.， Sweden）.
2.6 Analysis of fiber surface charge
Before the measurement， the bleached pulp samples were converted to their fully protonated form by soaking 1.5 g of the oven-dried pulp in 350 mL of 0.1 mol/L HCl for 60 min. The vacuum filtration of the pulp was then conducted in a Büchner funnel. The carboxyl groups of pulp were converted to their sodium-based form by the treatment with 250 mL of 2 mmol/L NaHCO3 solution and stirred for 30 min. The pulp samples were then filtered and washed with deionized water until the conductivity of the filtrate was lower than 5 mS/cm.
The polyelectrolyte titration method， which was performed using a MUTEK particle charge detector （PCD-03）， was used to determine the fiber surface charge[12-14]. The pulp sample was diluted with 100 mL of 0.1 mmol/L poly-DADMAC and stirred by a magnetic stirrer for 30 min to ensure that the anionic groups in the pulp were neutralized by the cationic polyelectrolyte completely. After the slurry was filtered， a 10 mL of the filtrate was pipetted into the cell of PCD-03 and titrated with 0.1 mmol/L of PES-Na to the endpoint. In addition， a sample of 100 mL of 0.1 mmol/L poly-DADMAC without pulp was treated under the exactly same procedure to determine the blank value. The specific charge density of the test sample was calculated by the equation below[15]： Where， q is the specific charge density of the sample （mmol/kg）， V0 is the titration volume for blank （mL）， V1 is the titration volume for the test pulp （mL）， C is the concentration of the titrate （mol/L）， w is the solid content of the pulp （g）， Vtotal is the total volume of the filtrate （mL）， and Vsample is the volume of the filtrate used for titration （mL）.
2.7 Analysis of pulp fibers’ carboxyl group content
The pulp fibers’ carboxyl group content was measured by using the conductometric titration method[12，14]. About 3.0 g of pulp sample were protonated sufficiently in a 0.1 mol/L HCl solution for 90 min and then washed thoroughly with deionized water in order to remove the CO2 in advance. The protonated pulp was dispersed in 450 mL of 1 mmol/L NaCl. The titration was performed with 0.01 mol/L NaOH using a conductivity meter and a burette under N2 protection and magnetic stirring. The conductivity measurements were carried out every 30 s after each addition of 0.05 mL of alkali solution. The titration curve was drawn when the conductivity reached the starting value. The carboxyl group content was calculated by the following equation[12]：
Where， V2 is volume of NaOH used at the second inflection point in the titration curve （mL）， V1 is volume of NaOH used at the first inflection point in the titration curve （mL）， C is the concentration of NaOH （mol/L）， and w is the solid content of the pulp （g）.
2.8 Analysis of water retention value of pulp fibers
The water retention value （WRV） of the bleached pulp was measured according to the TAPPI Method UM 256.
2.9 Chemical component analysis of pulp fibers
A certain amount of air-dried bleached CTMP was ground into powder using a Wiley mill （Model No.2， Arthur H.， Thomas Co.， Ltd.， USA）. The powders， which passed a 40-mesh screen but were retained on a 60-mesh one， were collected and used for the analysis of chemical components according to the method described in the literatures[15-16].
2.10 Analysis of surface lignin content of pulp fibers
The X-ray photoelectron spectroscopy （XPS） was used to determine the surface lignin content of the bleached CTMP. Before being analyzed by the XPS， the acetone extraction of the bleached pulp was firstly carried out in a Soxhlet extractor for 4 h. The extracted pulp was then washed thoroughly with deionized water. Finally， the well-washed pulp was used to make test handsheets for XPS analysis. The XPS analysis was conducted using an X-ray photoelectron spectrometer （PHI 1600， USA） with a monochromated A1 Ka X-ray source. The photoelectron collection was at 90° in relation to the sample surface. The surface lignin content of the samples was calculated by the following equation[17]： 2.11 Analysis of hydrogen bonds of pulp fibers
The properties of the hydrogen bonds of the bleached CTMP were measured by Fourier Transform Infrared Spectrum （FT-IR） assisted with the Origin software. A 2.0 mg of the grounded pulp powder was mixed well with potassium bromide at a ratio of 1∶100， and pressed into a transparent sheet by a tablet press for FT-IR test. The absorption peak at around 3430 cm-1 was further treated by Origin with Gaussian fitting and peak splitting. The intensities of the intramolecular hydrogen bond and intermolecular hydrogen bond were calculated according to the sub-peak areas[18-19].
2.12 Preparation and physical testing of handsheets
A sample of 30 g of well-washed bleached pulp was diluted with deionized water to 10% consistency， and subsequently refined to a Canadian standard freeness （CSF） of 250 mL in a PFI mill according to the TAPPI Test Method T248 sp-00. The handsheets were then prepared and tested under the procedure described in the TAPPI Test Method T205 sp-02 and T220 sp-01， respectively.
2.13 Analysis of relative bonding area of handsheets
The scattering coefficient measurement was used to determine the relative bonding area （RBA） of the handsheets[21]. The handsheets were prepared with six different pressures， i.e.， 0.1， 0.2， 0.3， 0.4， 0.5， and 0.6 MPa， and their tensile indices and scattering coefficients were measured. The scattering coefficient at the tensile index of zero was determined by using linear extrapolation. The RBA of the handsheets was calculated by the following equation[20]：
3 Results and discussion
3.1 Effect of substitution percentage of MgO for NaOH on the bulk and optical properties of bleached pulp
As the substitution percentage of MgO for NaOH increased， the bulk， light scattering coefficient， and opacity of the handsheets increased， as shown in Table 1. The results showed that the bulk was increased significantly with the increase of the substitution percentage of MgO for NaOH. The weak alkalinity of bleaching， resulting from the addition of MgO， could maintain most of lignin in the pulp fibers and decrease the degree of swelling of cellulose materials. Therefore， the BCTMP bleached with Mg-based alkali is much stiffer， and easy-to-form into a bulky structure that could increase the light scattering coefficient and the opacity of the handsheets. The light scattering coefficient and opacity of the handsheets were increased obviously at higher substitution percentage of MgO for NaOH. However， the brightness was slightly declined with the increase of the substitution percentage of MgO for NaOH. 3.2 Effect of the substitution percentage of MgO for NaOH on the cationic demand and CODCr
The effects of the substitution percentage of MgO for NaOH on the cationic demand and CODCr were analyzed， and the results are shown in Fig.1. The anionic trash in the effluent is mainly derived from the organic components of pulp dissolved in liquid phase during bleaching. Typically， the amount of anionic trash is described as cationic demand. The results showed that， as the substitution percentages of MgO for NaOH in the peroxide bleaching of poplar CTMP increased， the cationic demand and CODCr of the effluent were obviously decreased. When the substitution percentage raised from 0% to 75%， the cationic demand of effluent decreased by 52.3%. The generation of anionic trash is related to the alkalinity of the pulp bleaching process. The MgO could reduce the alkalinity of the bleaching system， resulting in reduced dissolution of the oxide lignin， galacturonic acid， and other organic materials in pulp. The Mg2+ can bind with polygalacturonic acids， oxidized lignin， and resin acids， causing them to be deposited onto pulp ?bers by neutralizing their anionic charges， thus reducing the amount of anionic trash in the bleaching effluent. The results demonstrated that MgO was useful in decreasing the amount of anionic trash in the pulping process. The main reason of generation of CODCr in the effluent is the degradation and dissolution of carbohydrates， lignin， and other organic substances in the pulp during bleaching. The dissolution of these chemical components depends largely on the alkalinity of the bleaching system. Fig.1 indicates that a higher substitution percentage of MgO for NaOH during the bleaching corresponds to lower CODCr of the effluent.
3.3 Strength properties of the handsheets at different substitution percentages of MgO for NaOH
The effects of Mg-based alkali on the strength properties of the peroxide bleached CTMP are presented in Fig.2. It is evident that the tensile， tear， and burst indices were all significantly decreased as the substitution percentage of MgO for NaOH increased. When the substitution percentage raised from 0% to 75%， the tensile， tear， and burst indices declined by 34.0%， 26.2%， and 44.6%， respectively.
To know the influencing mechanism of Mg-based alkali on the strength properties of the peroxide bleached CTMP， some main factors （fiber length， fiber surface charge， carboxyl group content， surface lignin content， and relative bonding area） were chosen and investigated in the following sections. 3.4 Effects of different factors on the strength properties of the bleached CTMP when MgO was partially used as alkali source
Some main factors influencing the strength properties were discussed. The relationships between the strength properties and the main factors， and the corresponding correlation coefficients are summarized in Table 2. The average fiber length of the bleached pulp had a good linear correlation with the tensile and burst indices. The tensile， burst， and tear indices of the test handsheets had well quadratic correlations with fiber surface charge of the bleached CTMP. Meanwhile， these indices also had good linear correlations with the fiber carboxyl group content. The results demonstrated that the fiber carboxyl group content had the highest linear correlation with the tensile index， which further certifies that more carboxyl group content present in the fibers could provide more hydrogen bonds and thus improve the bonding strength between fibers.
Based on the above analysis， it can be concluded that all of these main factors play important and direct roles in the development of the strength properties of the bleached CTMP with partial substitution of MgO for NaOH. However， one question was then raised， i.e.， how did the substitution percentage of MgO for NaOH affect the above main factors and thus influence the strength properties consequently？
3.5 Effect of the substitution percentage of MgO for NaOH on the above main factors
The average fiber length of the bleached pulp is considered as one of the most important factors affecting pulp physical properties. The fiber morphology of the bleached CTMP is listed in Table 3. When the substitution percentage of MgO for NaOH increased， the average fiber length and fiber kink index decreased， but the fine content increased. Meanwhile， there was less change on the average fiber width. The carboxyl group content on the pulp fibers highly affect the fiber swelling and thus result in the change of paper strength properties[21-22]. Therefore， it is also important to determine the fiber charge in order to understand the reason for the reduction of the bleached CTMP’s strength properties. The contents of fiber surface charge and carboxyl groups for the bleached CTMP at different substitution percentages of MgO for NaOH are summarized in Fig.3. It can be seen that a 59.1% decrease in the fiber surface charge （from 29.6 mmol/kg to 12.1 mmol/kg） was observed with the raise of the substitution percentage （from 0% to 75%）， while a 31.8% loss in the carboxyl group content （from 197.5 mmol/kg to 134.7 mmol/kg） occurred. The reason for the above can be attributed to the weakened alkalinity of the bleaching system with the increase of the substitution percentage of MgO for NaOH， leading to less chemical components dissolution （seen in Table 3） and， accordingly， less generation of carboxyl groups （seen in Fig.3）. The effect of fiber carboxyl group content on the WRV of the bleached CTMP was also shown in Fig.3. It is assured that the decrease of the fiber carboxyl group content had a negative effect on the fiber swelling. The O/C value is the ratio of oxygen and carbon atoms on the fiber surface， which can be detected by using XPS to estimate the contents of chemical compositions on the fiber surface[23-24]. The peak areas of C and O atoms， O/C ratio， and surface lignin content of the bleached CTMP are presented in Table 4. The XPS spectra illustrate that the lignin content on the fiber surface of the bleached CTMP increased with an increase in the substitution percentage of MgO for NaOH. When the alkali source was only NaOH in the peroxide bleaching process， the surface lignin content of the bleached CTMP was 61.5%， which was 8.4% lower than that when the substitution percentage of MgO for NaOH was 75%. The weaker alkalinity of the bleaching system slowed the generating rate of HOO-， and thus a smaller amount of HOO- attacked lignin， leading to the increase in the fiber surface lignin content for the bleached CTMP accordingly.
Although the bonding area between the fibers can be measured by many methods， the RBA， however， is still commonly used to reflect the bonding strength of inter-fibers. The RBA values of the bleached CTMP at different substitution percentages of MgO for NaOH are presented in Fig.4. It can be seen that there was a 37.4% decline of the RBA value （from 22.9% to 14.3%） when the substitution percentage of MgO for NaOH increased from 0% to 75%.
In addition to the RBA， the bonding strength was evaluated by the measurement of the intensity of hydrogen bonds since it would further affect the bonding strength between the pulp fibers. Therefore， the effects of the intensity of hydrogen bonds on the strength properties of the bleached CTMP with different substitution percentages of MgO for NaOH were investigated. Four different substitution percentages， i.e.， 0%， 25%， 50%， and 75%， were selected and the FT-IR spectra of the resulted bleached CTMPs were observed. The results showed that there was no obvious difference among these four spectra as the substitution percentage of MgO for NaOH increased. As for the FT-IR analysis， the broad peak at 3430 cm-1 was assigned to the stretching and bending vibration of hydroxyl groups and its red shift could be observed as the increase in the substitution percentage of MgO for NaOH. It is reported that the vibration frequency of hydroxyl groups would shift by 35 cm-1 to lower wave numbers as the intensity of the hydrogen bonds increased by one kilocalorie[20].Thus， it can be concluded that the intensity of hydrogen bonds of the bleached pulp fibers could be decreased as the substitution percentage of MgO for NaOH increased in the peroxide bleaching process. The results of the Gaussian fitting and peak splitting of the characteristic absorption peak of —OH are shown in Fig.5， while the intensities of the sub-peaks are shown in Table 5. The three split out small peaks represent the intramolecular hydrogen bonds O（2）H…O（6）， O（3）H…O（5）， and intermolecular hydrogen bond O（6）H…O（3?）， respectively. The increase in the substitution percentage of MgO for NaOH decreased the intensity of the intermolecular hydrogen bond O（6）H…O（3?）， while increased the intensity of the intramolecular hydrogen bond O（3）H…O（5） of the bleached pulp， as shown in Fig.5 and Table 5. The proportion of the cellulose intermolecular hydrogen bond O（6）H…O（3?） accounted for total hydrogen bonds was decreased from 37.77% to 25.03% when the substitution percentage of MgO for NaOH was increased from 0% to 75%. Correspondingly， the intermolecular hydrogen bond O（3）H…O（5） increased from 52.26% to 65.21%. It seems that the cellulose intramolecular hydrogen bond performed as a dominant factor for the bleached CTMP， after peroxide bleaching of the poplar CTMP.
It has been known that the reduction in strength properties for the bleached CTMP was mainly due to the weakened alkalinity of the bleaching system when MgO was partially used as alkali source. Based on the analyses above， the more important reason is that the weaker alkalinity led to less dissolution of chemical components， thus resulting in the decrease in the fiber charge content and the increase in surface lignin content， which would further negatively affect the fiber swelling. Less fiber fibrillating in the refining process due to the less fiber swelling would weaken the pulp bonding properties and reduce the tensile and burst indices. Meanwhile， less fiber swelling could also result in more fiber cutting in the refining process， and the tear index of the bleached CTMP would be decreased accordingly.
4 Conclusions
In the poplar CTMP peroxide bleaching process， the bulk， light scattering coefficient， and opacity of the resultant handsheets were increased obviously， and the cationic demand and effluent loads， such as CODCr， were reduced greatly at higher substitution percentage of MgO for NaOH， while the physical properties were reduced to certain extent.
The decrease in strength properties for the Mg-based peroxide bleached CTMP was mainly due to the weaker alkalinity of the bleaching system. The decrease in the fiber surface charge and increase in the surface lignin content negatively affected the fiber swelling， thus weakening the pulp bonding properties and decreasing the tensile and burst indices of the bleached CTMP. The tear index of the bleached pulp was decreased correspondingly due to the shortened fiber length. The tensile， tear， and burst indices of the bleached CTMP had a good negative linear correlation with the fiber surface lignin content， while a positive linear correlation with the fiber carboxyl group content and the RBA， respectively. In addition， the fiber surface charge had good quadratic correlations with the tear and burst indices， and the average fiber length had a positive linear correlation with the tensile and burst indices of the bleached pulp， respectively. Acknowledgments
This work was financially supported by the Natural Science Foundation of China （31070528）， Project of China “Twelfth Five-Year” National Science and Technology Supporting Plan （2011BAC11B04）， and the Foundation of State Key Laboratory of Pulp and Paper Engineering.
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Keywords： Mg-based alkali； substitution； chemi-thermomechanical pulp； alkaline peroxide bleaching； physical properties
1 Introduction
Alkaline peroxide bleaching， as a major section of the bleaching stage in the high yield pulping process， plays an important role in the quality control of end pulp products. Sodium hydroxide （NaOH） has been widely used as an alkali source in peroxide bleaching. However， as a strong alkali base， NaOH would cause excessive dissolution of carbohydrates， especially hemicelluloses， into bleaching effluents， which will lead to a high chemical oxygen demand （COD） load and bleaching yield loss. Meanwhile， a certain amount of anionic trash might be generated by high alkalinity of NaOH in the process and further carried to the wet-end section of the paper machine， leading to a negative impact on papermaking operations and paper product qualities[1-2].
Mg-based alkalis， e.g.， MgO/Mg（OH）2， partially substituting for NaOH and performing as one alkali source in the alkaline peroxide bleaching of high-yield pulps， provide an effective way to reduce the negative effects caused by NaOH[2-4]. The usage of MgO/Mg（OH）2 in the alkaline peroxide bleaching process can increase the bulk and optical properties of the bleached pulp， decrease the load of the bleaching effluent， improve the bleaching yield， and reduce the oxalate scaling[3-8]. The strength properties of pulp mainly depend on the wood species， pulping method， and fiber characteristics， among other factors. So far， it has been commonly believed that the major reason for the variation of the strength properties of the bleached pulp， when Mg-based alkalis are applied in the alkaline peroxide bleaching process， can be mainly attributed to the weaker alkalinity of MgO/Mg（OH）2[9]. In this study， the effects of some main characteristics （chemical components， charge characteristics， fiber morphology， surface lignin content， the relative bonding area （RBA）， and hydrogen bonding intensity） on the strength properties of the bleached CTMP were investigated. The main reasons for the changes of the strength properties of the peroxide bleached CTMP， when MgO was partially substituted for NaOH， were studied.
2 Materials and methods
2.1 Materials
The poplar CTMP， with the initial Canadian standard freeness （CSF） of 560 mL， was collected from a pulp mill in Shandong province， China， and stored in a cold room at 4℃. The main properties of the CTMP used in this study are listed as follows： bulk of 2.95 cm3/g， tensile index of 10.6 N·m/g， tear index of 1.45 mN·m2/g， burst index of 0.399 kPa·m2/g， ISO brightness of 57.7%， and light scattering coefficient of 14.7 m2/kg.
All chemicals used in this study were of analytical grade.
2.2 Alkaline peroxide bleaching of the poplar CTMP
The bleaching experiments were conducted in polyethylene bags using a water bath under the following conditions： the poplar CTMP of the equivalent to 40 g of oven-dried pulp； 3.0% total alkali charge （on NaOH）， of which 0%， 10%， 25%， 35%， 50%， and 75% of NaOH was replaced with MgO （molar ratio）； 0.2% diethylene triamine pentacetate acid （DTPA）； 2.0% Na2SiO3； 5.0% H2O2； 10% pulp consistency； 80℃ for 90 min.
In the alkaline peroxide bleaching process， NaOH， MgO， Na2SiO3， part of deionized water， and DTPA were mixed well first with the poplar CTMP by kneading the polyethylene bag by hand. Then， H2O2 and the remaining deionized water were added into the suspension. After the pulp and chemicals were mixed well by kneading constantly， the polyethylene bag was sealed and placed into the water bath with a temperature of 80℃. The bleaching duration time began to count as soon as the temperature of the pulp suspension reached 80℃. The polyethylene bag was kneaded once every 10 min in order to make the bleaching reaction uniform. As the duration time reached 90 min， the bag was taken out of the water bath and promptly put into a cold water bath to cool down the pulp to room temperature. The bleached CTMP was then washed thoroughly with deionized water. 2.3 Cationic demand and CODCr analysis
The Müteck PCD-03 charge analyzer （BTG Co.， Ltd.， Germany） was used to measure the cationic demand. The pH value of the filtrate was adjusted to （6.8±0.1） with 0.10 mol/L of H2SO4 before testing[10]. According to the USEPA Reactor Digestion Method[11]， the CODCr was determined using a DRB 200COD instrument （HACH Co.， Ltd.， USA） and a DR 1010 COD instrument （HACH Co.， Ltd.， USA）.
2.4 Handsheets preparation and testing
A well-washed pulp cake was dispersed in deionized water， with 1% of pulp consistency. The pulp suspension was used to prepare handsheets following ISO standards of 5269-1 （2005） and 3688 （1999） after adjusting the pH value to （5.0±0.1） with dilute H2SO4. In addition， bulk， brightness， opacity， light-scattering coefficient， and strength properties of the handsheets were determined according to ISO standards of 534 （2012）， 2470 （2009）， 2471 （2008）， 9416 （2009）， and 5270 （2012）， respectively.
2.5 Fiber quality analysis
The pulp fiber length， fiber width， fine content， and kink index were analyzed by a fiber quality analyzer （FQA） （Model 912， Lorentzen & Wettre Co.， Ltd.， Sweden）.
2.6 Analysis of fiber surface charge
Before the measurement， the bleached pulp samples were converted to their fully protonated form by soaking 1.5 g of the oven-dried pulp in 350 mL of 0.1 mol/L HCl for 60 min. The vacuum filtration of the pulp was then conducted in a Büchner funnel. The carboxyl groups of pulp were converted to their sodium-based form by the treatment with 250 mL of 2 mmol/L NaHCO3 solution and stirred for 30 min. The pulp samples were then filtered and washed with deionized water until the conductivity of the filtrate was lower than 5 mS/cm.
The polyelectrolyte titration method， which was performed using a MUTEK particle charge detector （PCD-03）， was used to determine the fiber surface charge[12-14]. The pulp sample was diluted with 100 mL of 0.1 mmol/L poly-DADMAC and stirred by a magnetic stirrer for 30 min to ensure that the anionic groups in the pulp were neutralized by the cationic polyelectrolyte completely. After the slurry was filtered， a 10 mL of the filtrate was pipetted into the cell of PCD-03 and titrated with 0.1 mmol/L of PES-Na to the endpoint. In addition， a sample of 100 mL of 0.1 mmol/L poly-DADMAC without pulp was treated under the exactly same procedure to determine the blank value. The specific charge density of the test sample was calculated by the equation below[15]： Where， q is the specific charge density of the sample （mmol/kg）， V0 is the titration volume for blank （mL）， V1 is the titration volume for the test pulp （mL）， C is the concentration of the titrate （mol/L）， w is the solid content of the pulp （g）， Vtotal is the total volume of the filtrate （mL）， and Vsample is the volume of the filtrate used for titration （mL）.
2.7 Analysis of pulp fibers’ carboxyl group content
The pulp fibers’ carboxyl group content was measured by using the conductometric titration method[12，14]. About 3.0 g of pulp sample were protonated sufficiently in a 0.1 mol/L HCl solution for 90 min and then washed thoroughly with deionized water in order to remove the CO2 in advance. The protonated pulp was dispersed in 450 mL of 1 mmol/L NaCl. The titration was performed with 0.01 mol/L NaOH using a conductivity meter and a burette under N2 protection and magnetic stirring. The conductivity measurements were carried out every 30 s after each addition of 0.05 mL of alkali solution. The titration curve was drawn when the conductivity reached the starting value. The carboxyl group content was calculated by the following equation[12]：
Where， V2 is volume of NaOH used at the second inflection point in the titration curve （mL）， V1 is volume of NaOH used at the first inflection point in the titration curve （mL）， C is the concentration of NaOH （mol/L）， and w is the solid content of the pulp （g）.
2.8 Analysis of water retention value of pulp fibers
The water retention value （WRV） of the bleached pulp was measured according to the TAPPI Method UM 256.
2.9 Chemical component analysis of pulp fibers
A certain amount of air-dried bleached CTMP was ground into powder using a Wiley mill （Model No.2， Arthur H.， Thomas Co.， Ltd.， USA）. The powders， which passed a 40-mesh screen but were retained on a 60-mesh one， were collected and used for the analysis of chemical components according to the method described in the literatures[15-16].
2.10 Analysis of surface lignin content of pulp fibers
The X-ray photoelectron spectroscopy （XPS） was used to determine the surface lignin content of the bleached CTMP. Before being analyzed by the XPS， the acetone extraction of the bleached pulp was firstly carried out in a Soxhlet extractor for 4 h. The extracted pulp was then washed thoroughly with deionized water. Finally， the well-washed pulp was used to make test handsheets for XPS analysis. The XPS analysis was conducted using an X-ray photoelectron spectrometer （PHI 1600， USA） with a monochromated A1 Ka X-ray source. The photoelectron collection was at 90° in relation to the sample surface. The surface lignin content of the samples was calculated by the following equation[17]： 2.11 Analysis of hydrogen bonds of pulp fibers
The properties of the hydrogen bonds of the bleached CTMP were measured by Fourier Transform Infrared Spectrum （FT-IR） assisted with the Origin software. A 2.0 mg of the grounded pulp powder was mixed well with potassium bromide at a ratio of 1∶100， and pressed into a transparent sheet by a tablet press for FT-IR test. The absorption peak at around 3430 cm-1 was further treated by Origin with Gaussian fitting and peak splitting. The intensities of the intramolecular hydrogen bond and intermolecular hydrogen bond were calculated according to the sub-peak areas[18-19].
2.12 Preparation and physical testing of handsheets
A sample of 30 g of well-washed bleached pulp was diluted with deionized water to 10% consistency， and subsequently refined to a Canadian standard freeness （CSF） of 250 mL in a PFI mill according to the TAPPI Test Method T248 sp-00. The handsheets were then prepared and tested under the procedure described in the TAPPI Test Method T205 sp-02 and T220 sp-01， respectively.
2.13 Analysis of relative bonding area of handsheets
The scattering coefficient measurement was used to determine the relative bonding area （RBA） of the handsheets[21]. The handsheets were prepared with six different pressures， i.e.， 0.1， 0.2， 0.3， 0.4， 0.5， and 0.6 MPa， and their tensile indices and scattering coefficients were measured. The scattering coefficient at the tensile index of zero was determined by using linear extrapolation. The RBA of the handsheets was calculated by the following equation[20]：
3 Results and discussion
3.1 Effect of substitution percentage of MgO for NaOH on the bulk and optical properties of bleached pulp
As the substitution percentage of MgO for NaOH increased， the bulk， light scattering coefficient， and opacity of the handsheets increased， as shown in Table 1. The results showed that the bulk was increased significantly with the increase of the substitution percentage of MgO for NaOH. The weak alkalinity of bleaching， resulting from the addition of MgO， could maintain most of lignin in the pulp fibers and decrease the degree of swelling of cellulose materials. Therefore， the BCTMP bleached with Mg-based alkali is much stiffer， and easy-to-form into a bulky structure that could increase the light scattering coefficient and the opacity of the handsheets. The light scattering coefficient and opacity of the handsheets were increased obviously at higher substitution percentage of MgO for NaOH. However， the brightness was slightly declined with the increase of the substitution percentage of MgO for NaOH. 3.2 Effect of the substitution percentage of MgO for NaOH on the cationic demand and CODCr
The effects of the substitution percentage of MgO for NaOH on the cationic demand and CODCr were analyzed， and the results are shown in Fig.1. The anionic trash in the effluent is mainly derived from the organic components of pulp dissolved in liquid phase during bleaching. Typically， the amount of anionic trash is described as cationic demand. The results showed that， as the substitution percentages of MgO for NaOH in the peroxide bleaching of poplar CTMP increased， the cationic demand and CODCr of the effluent were obviously decreased. When the substitution percentage raised from 0% to 75%， the cationic demand of effluent decreased by 52.3%. The generation of anionic trash is related to the alkalinity of the pulp bleaching process. The MgO could reduce the alkalinity of the bleaching system， resulting in reduced dissolution of the oxide lignin， galacturonic acid， and other organic materials in pulp. The Mg2+ can bind with polygalacturonic acids， oxidized lignin， and resin acids， causing them to be deposited onto pulp ?bers by neutralizing their anionic charges， thus reducing the amount of anionic trash in the bleaching effluent. The results demonstrated that MgO was useful in decreasing the amount of anionic trash in the pulping process. The main reason of generation of CODCr in the effluent is the degradation and dissolution of carbohydrates， lignin， and other organic substances in the pulp during bleaching. The dissolution of these chemical components depends largely on the alkalinity of the bleaching system. Fig.1 indicates that a higher substitution percentage of MgO for NaOH during the bleaching corresponds to lower CODCr of the effluent.
3.3 Strength properties of the handsheets at different substitution percentages of MgO for NaOH
The effects of Mg-based alkali on the strength properties of the peroxide bleached CTMP are presented in Fig.2. It is evident that the tensile， tear， and burst indices were all significantly decreased as the substitution percentage of MgO for NaOH increased. When the substitution percentage raised from 0% to 75%， the tensile， tear， and burst indices declined by 34.0%， 26.2%， and 44.6%， respectively.
To know the influencing mechanism of Mg-based alkali on the strength properties of the peroxide bleached CTMP， some main factors （fiber length， fiber surface charge， carboxyl group content， surface lignin content， and relative bonding area） were chosen and investigated in the following sections. 3.4 Effects of different factors on the strength properties of the bleached CTMP when MgO was partially used as alkali source
Some main factors influencing the strength properties were discussed. The relationships between the strength properties and the main factors， and the corresponding correlation coefficients are summarized in Table 2. The average fiber length of the bleached pulp had a good linear correlation with the tensile and burst indices. The tensile， burst， and tear indices of the test handsheets had well quadratic correlations with fiber surface charge of the bleached CTMP. Meanwhile， these indices also had good linear correlations with the fiber carboxyl group content. The results demonstrated that the fiber carboxyl group content had the highest linear correlation with the tensile index， which further certifies that more carboxyl group content present in the fibers could provide more hydrogen bonds and thus improve the bonding strength between fibers.
Based on the above analysis， it can be concluded that all of these main factors play important and direct roles in the development of the strength properties of the bleached CTMP with partial substitution of MgO for NaOH. However， one question was then raised， i.e.， how did the substitution percentage of MgO for NaOH affect the above main factors and thus influence the strength properties consequently？
3.5 Effect of the substitution percentage of MgO for NaOH on the above main factors
The average fiber length of the bleached pulp is considered as one of the most important factors affecting pulp physical properties. The fiber morphology of the bleached CTMP is listed in Table 3. When the substitution percentage of MgO for NaOH increased， the average fiber length and fiber kink index decreased， but the fine content increased. Meanwhile， there was less change on the average fiber width. The carboxyl group content on the pulp fibers highly affect the fiber swelling and thus result in the change of paper strength properties[21-22]. Therefore， it is also important to determine the fiber charge in order to understand the reason for the reduction of the bleached CTMP’s strength properties. The contents of fiber surface charge and carboxyl groups for the bleached CTMP at different substitution percentages of MgO for NaOH are summarized in Fig.3. It can be seen that a 59.1% decrease in the fiber surface charge （from 29.6 mmol/kg to 12.1 mmol/kg） was observed with the raise of the substitution percentage （from 0% to 75%）， while a 31.8% loss in the carboxyl group content （from 197.5 mmol/kg to 134.7 mmol/kg） occurred. The reason for the above can be attributed to the weakened alkalinity of the bleaching system with the increase of the substitution percentage of MgO for NaOH， leading to less chemical components dissolution （seen in Table 3） and， accordingly， less generation of carboxyl groups （seen in Fig.3）. The effect of fiber carboxyl group content on the WRV of the bleached CTMP was also shown in Fig.3. It is assured that the decrease of the fiber carboxyl group content had a negative effect on the fiber swelling. The O/C value is the ratio of oxygen and carbon atoms on the fiber surface， which can be detected by using XPS to estimate the contents of chemical compositions on the fiber surface[23-24]. The peak areas of C and O atoms， O/C ratio， and surface lignin content of the bleached CTMP are presented in Table 4. The XPS spectra illustrate that the lignin content on the fiber surface of the bleached CTMP increased with an increase in the substitution percentage of MgO for NaOH. When the alkali source was only NaOH in the peroxide bleaching process， the surface lignin content of the bleached CTMP was 61.5%， which was 8.4% lower than that when the substitution percentage of MgO for NaOH was 75%. The weaker alkalinity of the bleaching system slowed the generating rate of HOO-， and thus a smaller amount of HOO- attacked lignin， leading to the increase in the fiber surface lignin content for the bleached CTMP accordingly.
Although the bonding area between the fibers can be measured by many methods， the RBA， however， is still commonly used to reflect the bonding strength of inter-fibers. The RBA values of the bleached CTMP at different substitution percentages of MgO for NaOH are presented in Fig.4. It can be seen that there was a 37.4% decline of the RBA value （from 22.9% to 14.3%） when the substitution percentage of MgO for NaOH increased from 0% to 75%.
In addition to the RBA， the bonding strength was evaluated by the measurement of the intensity of hydrogen bonds since it would further affect the bonding strength between the pulp fibers. Therefore， the effects of the intensity of hydrogen bonds on the strength properties of the bleached CTMP with different substitution percentages of MgO for NaOH were investigated. Four different substitution percentages， i.e.， 0%， 25%， 50%， and 75%， were selected and the FT-IR spectra of the resulted bleached CTMPs were observed. The results showed that there was no obvious difference among these four spectra as the substitution percentage of MgO for NaOH increased. As for the FT-IR analysis， the broad peak at 3430 cm-1 was assigned to the stretching and bending vibration of hydroxyl groups and its red shift could be observed as the increase in the substitution percentage of MgO for NaOH. It is reported that the vibration frequency of hydroxyl groups would shift by 35 cm-1 to lower wave numbers as the intensity of the hydrogen bonds increased by one kilocalorie[20].Thus， it can be concluded that the intensity of hydrogen bonds of the bleached pulp fibers could be decreased as the substitution percentage of MgO for NaOH increased in the peroxide bleaching process. The results of the Gaussian fitting and peak splitting of the characteristic absorption peak of —OH are shown in Fig.5， while the intensities of the sub-peaks are shown in Table 5. The three split out small peaks represent the intramolecular hydrogen bonds O（2）H…O（6）， O（3）H…O（5）， and intermolecular hydrogen bond O（6）H…O（3?）， respectively. The increase in the substitution percentage of MgO for NaOH decreased the intensity of the intermolecular hydrogen bond O（6）H…O（3?）， while increased the intensity of the intramolecular hydrogen bond O（3）H…O（5） of the bleached pulp， as shown in Fig.5 and Table 5. The proportion of the cellulose intermolecular hydrogen bond O（6）H…O（3?） accounted for total hydrogen bonds was decreased from 37.77% to 25.03% when the substitution percentage of MgO for NaOH was increased from 0% to 75%. Correspondingly， the intermolecular hydrogen bond O（3）H…O（5） increased from 52.26% to 65.21%. It seems that the cellulose intramolecular hydrogen bond performed as a dominant factor for the bleached CTMP， after peroxide bleaching of the poplar CTMP.
It has been known that the reduction in strength properties for the bleached CTMP was mainly due to the weakened alkalinity of the bleaching system when MgO was partially used as alkali source. Based on the analyses above， the more important reason is that the weaker alkalinity led to less dissolution of chemical components， thus resulting in the decrease in the fiber charge content and the increase in surface lignin content， which would further negatively affect the fiber swelling. Less fiber fibrillating in the refining process due to the less fiber swelling would weaken the pulp bonding properties and reduce the tensile and burst indices. Meanwhile， less fiber swelling could also result in more fiber cutting in the refining process， and the tear index of the bleached CTMP would be decreased accordingly.
4 Conclusions
In the poplar CTMP peroxide bleaching process， the bulk， light scattering coefficient， and opacity of the resultant handsheets were increased obviously， and the cationic demand and effluent loads， such as CODCr， were reduced greatly at higher substitution percentage of MgO for NaOH， while the physical properties were reduced to certain extent.
The decrease in strength properties for the Mg-based peroxide bleached CTMP was mainly due to the weaker alkalinity of the bleaching system. The decrease in the fiber surface charge and increase in the surface lignin content negatively affected the fiber swelling， thus weakening the pulp bonding properties and decreasing the tensile and burst indices of the bleached CTMP. The tear index of the bleached pulp was decreased correspondingly due to the shortened fiber length. The tensile， tear， and burst indices of the bleached CTMP had a good negative linear correlation with the fiber surface lignin content， while a positive linear correlation with the fiber carboxyl group content and the RBA， respectively. In addition， the fiber surface charge had good quadratic correlations with the tear and burst indices， and the average fiber length had a positive linear correlation with the tensile and burst indices of the bleached pulp， respectively. Acknowledgments
This work was financially supported by the Natural Science Foundation of China （31070528）， Project of China “Twelfth Five-Year” National Science and Technology Supporting Plan （2011BAC11B04）， and the Foundation of State Key Laboratory of Pulp and Paper Engineering.
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