Characteristics of Poplar Preconditioning Followed by Refining

Mar 2, 2013 - High-yield pulp (HYP) has various fractions, and they not only play an important role in the papermaking process but also affect paper p...
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Characteristics of Poplar Preconditioning Followed by Refining Chemical Treatment Alkaline Peroxide Mechanical Pulp Fiber Fractions and Their Effects on Formation and Properties of HighYield Pulp Containing Paper Ming Lei, Hongjie Zhang,* Jianguo Li, and Junlei Duan Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China S Supporting Information *

ABSTRACT: High-yield pulp (HYP) has various fractions, and they not only play an important role in the papermaking process but also affect paper properties. Hence, it is necessary to clarify the impact of different HYP fractions on paper formation and properties. In this study, the characteristics of poplar preconditioning followed by refining chemical treatment alkaline peroxide mechanical pulp fractions were determined, and their effects on paper formation and paper properties were investigated. The results show that the HYP fiber is not as flexible as the bleached wheat straw pulp fiber. Compared with the HYP long-fiber fraction, the short-fiber fraction can improve the formation index more favorably. It was also found that the long-fiber fraction can maintain the strength properties, while having a negligible effect on the light-scattering coefficient; on the contrary, the shortfiber fraction can improve the light-scattering coefficient effectively but reduces the strength properties.

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INTRODUCTION In recent years, high-yield pulps (HYPs) have been found in increasing applications in the production of value-added paper grades, due to their own unique characteristics.1−7 Much work has been conducted in addressing various practical aspects of using HYP for the production of fine papers, such as the optical properties by using optical brightening agents (OBA)8−10 and the physical properties by adjusting the characteristics of pulp fines.11,12 The preconditioning followed by refining chemical treatment alkaline peroxide mechanical pulp (P-RC APMP) is another high-quality HYP.13 Many studies regarding its processes of various hardwoods have been reported,14−17 and this technology has undergone some important developments for different raw materials, different pulp grades, and different economic situations.18 However, the physical strength and fiber bonding of these pulps are usually lower than those of chemical pulps, which can hinder their application in many paper grades. During the defibration and fiber development of the mechanical pulping process, various fiber fractions and large amounts of pulp fines, consisting of a wide range of different fibrous particles, are produced.19,20 These fiber fractions not only play an important role during the papermaking process, such as filler retention, drainage, and white water recycling, but also determine the paper properties, including the physical and optical properties and printability. The impact of different fractions on formation and properties of HYP-containing paper sheet is different. In addition to the fiber fraction, the fines fraction is also an important part of HYP. The effect of pulp fines on the paper properties has been recognized and studied extensively in the literature.20−23 Fines are critical during the paper consolidation process19 and can affect paper properties, such as tensile strength, elongation, and light-scattering ability. It has been shown earlier that, without fines, paper sheets consisting only of © 2013 American Chemical Society

the long-fiber fraction have poor bonding strength and that paper strength could be improved by the addition of HYP fines.27 From the mentioned above, various HYP fiber fractions play their irreplaceable role in the paper manufacturing process and affect the properties of the resulting paper sheets. How to use these fractions rationally and effectively is the key point of increasing their application in many paper grades. How to utilize the fines is also critical. Therefore, it is necessary to determine the functions of different HYP fractions. The aim of this work was to examine the effect of different poplar P-RC APMP fractions, including the fines, on paper formation and properties of HYP-containing paper sheets. Also, the fiber characteristics of such a typical HYP were determined comprehensively.

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EXPERIMENTAL SECTION Materials. Three pulp samples were obtained from a paper mill in China, including hardwood bleached kraft pulp (HWBKP) with a beating degree of 28°SR, P-RC APMP with the Canadian Standard Freeness (CSF) of 400 mL, and bleached wheat straw pulp (BWSP) with a beating degree of 43°SR. The P-RC APMP was classified by the Bauer−McNett fiber classifier into different fiber fractions, including R30, P30/ R50, P50/R100, and P100/R200. Four fiber fractions were collected: R30 and P30/R50 as the long-fiber fraction and P50/ R100 and P100/R200 as the short-fiber fraction. Furthermore, P-RC APMP fines were obtained from the dynamic drainage jar Received: Revised: Accepted: Published: 4083

September 10, 2012 February 22, 2013 March 2, 2013 March 2, 2013 dx.doi.org/10.1021/ie3024356 | Ind. Eng. Chem. Res. 2013, 52, 4083−4088

Industrial & Engineering Chemistry Research

Article

through a subtraction method), and the long-fiber fraction (R30 and P30/R50) accounted for 30.8% (5.4% + 25.4%). The fiber flexibility coefficient was calculated according to the literature.25 Theoretical Bauer−McNett fiber length of different mesh size was shown in the Supporting Information (Table S1), and the mean fiber length (weight av and arithmetic av) of poplar P-RC APMP fractions and BWSP fractions was obtained in Table 2. The dynamic flexibility

(DDJ) with a 200 mesh screen. All the procedures were according to the TAPPI standard methods. Evaluation of Fiber and Fines Quality. The characteristics of each fiber fraction, including average fiber length, average fiber width, and other properties, were measured by using the L&W Fiber Tester 912. A method24,25 for evaluating the fiber flexibility was followed for both chemical pulp (BWSP) and HYP (P-RC APMP). First, the weight average and arithmetic average fiber length of different pulp fiber fractions were measured using the L&W Fiber Tester. Then, the dynamic flexibility coefficient and static flexibility coefficient were calculated. The method consists in calculating the slopes of the linear regression straight line by plotting the fiber length (weight av and arithmetic av) against three kinds of given theoretical Bauer−McNett length.25 The lower the coefficient value, the more flexible the pulp fiber. Paper Sheet Preparation. Two sets of handsheets with basis weight of 60 g/m2 were prepared in a laboratory sheet former (made by Labtech in Canada) following the TAPPI standard methods: (1) handsheets were fully made of each fractionated P-RC APMP fiber fraction; (2) the specified P-RC APMP fiber fractions and the fines fraction were added into the HWBKP with different addition ratios of 10%, 20%, 30%, and 40%, and then, the mixtures were used for handsheet preparation. The reference sheets were made from 100% HWBKP. Measurement of Paper Properties and Evaluation of Sheet Formation. The laboratory handsheet properties, including optical and physical properties, were measured according to the TAPPI standard methods. The equipment used for paper formation measurement was a Paper Formation Analyzer (made by OpTest Equipment Inc.), and a formation index can be obtained to evaluate the sheet formation. The higher the formation index, the better the sheet formation.

Table 2. Mean Fiber Length (Weighted Av and Arithmetic Av) of Poplar P-RC APMP and BWSP

RESULTS AND DISCUSSION Characterization of Fiber and Fines. The fiber properties of each fraction measured using a L&W Fiber Tester were listed in Table 1. As shown, the fiber length, width, and coarseness decreased with the increase in the mesh size of 30−200. The short fractions, P50/R100 and P100/R200, were 47.0% (33.6% + 13.4%), the fines (P200) content was 22.2% (obtained Table 1. Fiber Analysis of Different Fiber Fractions of Poplar P-RC APMP

mean fiber length, mm weight av arithmetic av weight weighted av mean fiber width, μm weight av arithmetic av weight weighted av coarseness, μg/m mean kinks index content, % on pulp (22.2% fines content)

R30

P30/ R50

P50/ R100

P100/ R200

1.28 1.051

0.881 0.797

0.665 0.603

0.395 0.351

1.525

0.99

0.737

0.459

33.4 31.2

27.9 27.4

26.5 26.3

26.2 26.1

35.8

28.7

27.1

26.3

246 0.785 5.4

165 1.004 25.4

140 1.012 33.6

114 1.082 13.4

mean fiber length of BWSP, mm

mesh size

weight av

arithmetic av

weight av

arithmetic av

30 50 100 200

1.28 0.881 0.665 0.395

1.051 0.797 0.603 0.351

1.021 0.812 0.568 0.336

0.815 0.675 0.481 0.308

coefficient and static flexibility coefficient were obtained respectively based on the corresponding slopes of the linear regression straight line by plotting the fiber length against the theoretical Bauer−McNett fiber length.25 As shown in Table 3, the obtained dynamic fiber flexibility coefficients of the poplar P-RC APMP and BWSP were 0.49 and 0.38, respectively. The obtained static flexibility coefficient of the poplar P-RC APMP and BWSP were 0.38 and 0.28. These results indicate that the HYP fibers are not as flexible as the BWSP. The physical properties of handsheets made from the fractionated HYP fractions were shown in Table 4. It can be seen that the strength properties were higher for the higher fiber length fraction. The bulk and light-scattering coefficients were also presented in Table 4. Effect of Poplar P-RC APMP Fractions on Paper Formation of HYP-Containing Sheets. Paper properties depend strongly on the sheet formation. It was found that critical paper properties, such as printability and strength properties, are strongly correlated to the uniformity of paper sheet.26,27 It can be found from Figure 1 that the paper sheet formation improved with the decrease in the fiber length. Figure 2 showed the mean fiber length, coarseness, and formation index of paper sheets made of different fractions. Evidently, the formation index increased with the fiber length or as the coarseness decreased. The paper strength increased if the paper formation became more uniform, which was confirmed in this study, as shown in Figure 2. In commercial practice, a paper sheet consists of fibers, fiber fragments, mineral fillers, and chemical additives. In this study, fillers and chemicals were not added. During the paper formation process, fibers and fiber fragments settled stochastically to form a fiber matrix. The effect of HYP fractions on paper formation was shown in Figure 3. The addition levels of 0−40% were used to replace the HWBKP in the pulp furnish. Figure 3 also showed that the paper formation index decreased with the increase in substitution ratio of HYP fractions. Furthermore, compared to the reference paper (100% hardwood kraft paper) with the initial formation index of 284.5, the formation index of HYP-containing paper sheets decreased to 192.7−259.3, suggesting that the formation here was 10−30% worse than the control (100% hardwood kraft pulps). The initial freeness of the HYP fraction makes a significant influence on the formation of HYP-containing paper sheets.

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characteristic

mean fiber length of P-RC APMP, mm

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Table 3. Flexibility Coefficient of Poplar P-RC APMP and BWSPa Lmax

Lmed

flexibility coefficient

Lmin

pulp type

SW

Sa

SW

Sa

SW

Sa

dynamic

static

P-RC APMP BWSP

0.359 0.276

0.278 0.205

0.463 0.360

0.361 0.267

0.656 0.506

0.509 0.375

0.493 0.381

0.383 0.282

Sw: corresponding slope of the linear regression straight line by plotting the weighted average fiber length against the theoretical Bauer−McNett fiber length. Sa: corresponding slope of the linear regression straight line by plotting the weighted average fiber length against the theoretical Bauer− McNett fiber length. a

Table 4. Properties of Handsheets Made of Different Fractions of P-RC APMP property bulk, cm3/g tear index, (mN·m2)/g tensile index, (N·m)/g elongation, % burst index, (kPa·m2)/g light-scattering coefficient, m2/kg ISO brightness, % opacity, %

R30

P30/ R50

P50/ R100

P100/ R200

2.57 1.57 11.9 0.892 0.38 35.8

2.7 1.54 10.9 0.74 0.32 37.5

2.78 1.31 9.7 0.574 0.32 41.9

2.81 1.08 7.9 0.468 0.22 42.5

54.4 92.6

55.5 92.3

56.9 94.1

57.6 94.6

Figure 3. Effect of HYP fractions on the formation index of HYPcontaining paper sheets.

Figure 3 showed that the formation index for most of the components was lower with the substitution of high-freeness HYP fractions for HWBKP, implying that paper formation is negatively affected from the increase in the high-freeness HYP substitution; however, this is only true for the cases with the addition of a single high-freeness HYP fraction. As shown in Figure 3, compared to the long-fiber fractions, the short-fiber fraction can improve the formation index more favorably. A possible explanation could be that the flocculation of long-fiber fraction is stronger than that of the short-fiber fraction. Paper formation is a nonuniform distribution of particles. For practical purposes, a useful definition of formation is the small-scale basis weight variation in the plane of the paper sheet. Variability of the basis weight of paper partially depends on the random deposition of pulp fibers and the interactions of the fibers. The flocculation of long-fiber fraction increases the variability of the basis weight, compared to the short-fiber fraction, thus leading to a lower formation index. The other possibility is the hydrodynamic smoothing, which refers to the fact that the suspension flow during drainage can improve the formation. The flow rate through the settled fiber mat is highest where the flow resistance is lowest. The fines or short-fiber fractions can change this situation through delaying the draining time so that the distribution of fibers is more uniform. Effect of Poplar P-RC APMP Fractions on Properties of HYP-Containing Paper Sheets. In this study, the effect of different fiber fractions on properties of paper sheet was studied. The R30 fraction (a long-fiber fraction) and the P50/ R100 fraction (a short-fiber fraction) are listed in Tables 5 and 6, respectively. As shown, strength properties of handsheets with long-fiber fraction addition can be maintained, while the strength properties decreased for the short-fiber fractions. A probable explanation is that the long-fiber fractions contained

Figure 1. Formation index of the handsheets made of fractionated poplar P-RC APMP fractions.

Figure 2. Correlation between mean fiber length, coarseness, and formation index of paper sheets made of different fractions.

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Usually, HYP has lower initial brightness than the bleached chemical pulp. As shown in Figure 5, the ISO brightness was

Table 5. Effect of R30 Fraction on Properties of HYPContaining Paper Sheets addition level, %

0

10

20

30

40

bulk, cm3/g burst index, (kPa·m2)/g tear index, (mN·m2)/g tensile index, kN/m TEA, J/m2 elongation, % light-scattering coefficient, m2/kg ISO brightness, % opacity, %

2.10 1.38 4.70 24.9 20.6 1.86 40.8 82.9 80.7

2.12 1.38 4.11 22.7 17.8 1.69 40.6 79.3 82.7

2.14 1.18 3.87 21.4 15.4 1.54 40.5 75.1 85.2

2.21 1.06 3.62 19.9 11.9 1.43 39.8 73.5 86.9

2.32 0.92 3.50 19.1 10.4 1.43 39.0 71.8 88.4

Table 6. Effect of P50/R100 Fraction on Properties of HYPContaining Paper Sheets addition level, % 3

bulk, cm /g burst index, (kPa·m2)/g tear index, (mN·m2)/g tensile index, kN/m TEA, J/m2 elongation, % light-scattering coefficient, m2/kg ISO brightness, % opacity, %

0

10

20

30

40

2.10 1.38 4.70 24.9 20.6 1.86 40.8 82.9 80.7

2.16 1.11 3.83 22.3 17.1 1.69 41.4 81.0 81.3

2.14 1.08 3.41 20.5 13.2 1.44 41.8 78.1 83.0

2.21 0.97 2.96 19.7 10.4 1.26 42.2 76.2 83.6

2.37 0.86 2.70 18.9 10.0 1.24 42.7 74.3 84.3

Figure 5. Effect of HYP fractions on ISO brightness of HYPcontaining paper sheets.

reduced significantly with the increase of the substitution ratio of HYP fractions. Figure 6 shows the effect of different HYP

more fibrillar-like fiber, which gave excellent specific surface area due to the fiber development during the refining process.12,19−21 Moreover, the fibrillar-like fiber contributes more hydrogen bonding between fibers. For these reasons, the strength of the paper sheet with long-fiber fraction addition is better than that containing the short-fiber fractions. It can be seen from Figure 4 that the tear strength decreased with the increase of the substitution ratio, and it is true for all of

Figure 6. Effect of HYP fractions on light-scattering coefficient of HYP-containing paper sheets.

fractions on the light-scattering coefficient. It can be observed that the short-fiber fractions (P50/R100 and P100/R200) had a positive effect on the light-scattering coefficient of paper sheets, while the long-fiber fraction (R30 and P30/R50) had almost no effect. For the short-fiber fraction, the light-scattering coefficient increased with the increase of HYP fraction substitution ratio, up to 40%. This may be due to the higher surface area of the shorter fiber fractions, resulting in more free surface area for light scattering. Effect of Poplar P-RC APMP Fines on Properties of HYP-Containing Paper Sheet. HYP has higher fines content than bleached kraft pulps. There are two kinds of fines: flakelike fines (also known as primary fines) and fibrillar-like fines, (also known as secondary fines).29 Flakelike fines, originating from the outermost parts of the fiber wall within the middle lamellae, are created upon shattering of the middle lamellae during the refining process. These primary fines can effectively increase

Figure 4. Effect of HYP fractions on the tear index of HYP-containing paper sheets.

the fractions. Because of the lignin presence in HYP, the pulp fibers are stiffer,28 especially for the long-fiber fractions. The negative effect on the tear index from the short-fiber fractions (P50/R100) was more pronounced than the long-fiber fractions (R30). 4086

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Table 7. Effect of P-RC APMP Fines Addition on Properties of HYP-Containing Paper Sheets fines addition level, %

thickness, μm

bulk, cm3/g

burst index, (kPa·m2)/g

tear index, (mN·m2)/g

tensile index, (N·m)/g

Z-tensile strength, J/m2

0 5 10 20 40

127 130 123 134 130

2.08 2.08 2.02 2.00 1.94

1.14 1.20 1.25 1.14 1.28

4.65 4.38 4.04 3.73 3.34

20.8 21.0 22.2 22.8 23.7

11.2 13.4 15.4 16.3 19.9

These particles can substantially affect the development of light scattering. HYP fibers are not as flexible as chemical pulp fibers; thus, the bonding ability of HYP is inferior to the chemical pulp, and as a result, there would be more free surface for HYP after the paper making drying process, thereby increasing the light-scattering coefficient.34

light scattering but have a minor influence on the strength properties.34 Fibrillar-like fines, originating from the S2 fiber wall, contribute significantly to the strength development,23,30,31 which was due to the fact that the fibrillar-like fines have strong bonding ability.32 The effect of HYP fines on the properties of paper sheet was shown in Table 7. It can be found that the strength properties, except the tear strength, increased as the addition ratio of pulp fines increased. It is known that the paper strength is significantly influenced by the fiber length and bonding ability. Properties that are mainly determined by interfiber bonding, such as tensile index, burst index, and Z-tensile strength, increased as the addition level of pulp fines increased. P-RC APMP fines had a negative influence on the tear strength, which was mainly determined by the mean fiber length (Table 7). The pulp fines can improve fiber bonding and increase the number of bonds between long fibers, resulting in the enhancement of paper sheet strength, while the bulk of paper sheets can be reduced significantly even with a small amount of pulp fines, as shown in Table 7. A small amount of pulp fines can also decrease the thickness of the fiber network.33 The origin and morphology of pulp fines are very different, and their effects on the optical properties of paper sheets were also different. Figure 7 showed that the opacity increased with

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CONCLUSIONS The poplar P-RC APMP was fractionated, and the properties of the fractions were determined. The flexibility coefficient results indicate that the HYP fibers are not as flexible as the BWSP fibers. For the handsheets made from each fraction, the formation index increases with the fiber length. The formation of HYP-containing paper sheets is negatively affected with the increase of the high-freeness HYP fraction. Compared to the long-fiber fractions, the short-fiber fraction can improve the formation index more favorably. The different HYP fractions and fines, when used in HYPcontaining paper sheets, can lead to differences in the paper properties: (1) the R30 fraction can maintain the strength properties, while having negligible effect on the light-scattering coefficient; (2) the short-fiber fractions, for example, P100/ R200, can improve the light-scattering coefficient effectively, while reducing the strength properties; (3) the long-fiber fraction makes more contribution to the strength properties than the short-fiber fractions, and the short-fiber fraction makes more contribution to the optical properties such as lightscattering and opacity; (4) the P-RC APMP fines are critical in determining the light-scattering coefficient and opacity, while also improving the strength properties of the resulting paper.

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical Bauer−McNett fiber length of different mesh size (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 7. Effect of P-RC APMP fines on optical properties of HYPcontaining paper sheets.

*Tel.: +86-22-6060-1988. Fax: +86-22-6060-2510. E-mail: [email protected].

increasing the substitution ratio of pulp fines. At a low addition ratio of pulp fines, the P-RC APMP fines exhibited a great effect on the opacity of paper sheets. It can also be seen from Figure 7 that the light-scattering coefficient increased significantly with the addition ratio of pulp fines from 5% to 40%. For example, in the case of adding 40% P-RC APMP pulp fines, the lightscattering coefficient of handsheet increased by 35% over the control. During the HYP manufacturing process, lots of small particles, such as lignin granules and fiber shives, are generated.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the Foundation for the Development of Science and Technology in Tianjin Universities (grant no. 20110518) and Tianjin University of Science and Technology Research Funds (grant no. 20110107). 4087

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