Effect of Pulp Fines on the Dye−Fiber Interactions during the Color

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Effect of Pulp Fines on the Dye-Fiber Interactions during the Color-Shading Process Hongbin Liu,*,†,‡ Shuhui Yang,† and Yonghao Ni‡ Tianjin Key Laboratory of Pulp and Paper, Tianjin UniVersity of Science and Technology, Tianjin 300457, China, and Limerick Pulp and Paper Centre, UniVersity of New Brunswick, Fredericton, NB E3B 5A3, Canada

Fines play a very important role in the papermaking process and paper properties. High-yield pulp (HYP) contains a large amount of higher specific surface area fines, which may lead to the absorption of more dye at the wet end section. Better understanding of the dye-fines interaction will help improve the dye efficiency in HYP-containing furnish. This study was focused on the fines from high-yield pulp and hardwood bleached kraft pulp (HBKP) on optical properties, particularly on the CIE (Commission Internationale d’Eclairage) whiteness and b* (negative values indicate blue, and positive values indicate yellow). The characteristics of both HYP fines and HBKP fines were presented, and their effects on dyeing (color-shading) process were investigated. Fines have a higher specific surface area and more dissolved and colloidal substances (DCS) than do the fibers. It was found that for a system made of HYP fibers, HYP fines had a negative effect, while HBKP fines had a positive effect on the color shading process. For a system made of HBKP fibers, a low content (5%) of HYP fines can increase the dye effectiveness, although when the HYP fine content increased further, the dye performance showed a decrease; HBKP fines retarded the dye effectiveness for the HBKP fiber system. For the mixture of HYP fines and HBKP fines, the turbidity measurement was used to explain the interactions between the HYP fines and HBKP fines. The dyes and HYP fines can form complexes, which then retain in the fiber network, thus improving the dye effectiveness and resulting in a higher CIE whiteness and lower b* of the paper sheets. 1. Introduction Fines have been traditionally defined as the fiber fraction that passes a 200 mesh (75 µm diameter) screen, and they are not a homogeneous fraction; various types of particles have been identified in the fine fraction.1 The quantity and quality of fine fractions in mechanical pulps have a critical impact on the optical and mechanical properties, thus affecting the quality parameters of the final paper.2,3 Generally, mechanical pulps have more fines than chemical pulps and may contain as high as 25-40% fines by mass.4 The characteristics of fibers and fines, which are important to wet end chemistry, include the sorption capacity, swelling behavior, and ion exchange properties.5 The high specific surface area of mechanical fines enhances the light scattering ability and opacity. Also, the fines can have high light absorption because of their high lignin content. The success of mechanical pulps as part of the pulp furnish for the production of writing and printing grades is largely due to the unique properties of their fine fraction. Chemical pulp fines are also an important component in papermaking furnish in terms of the mechanical and optical properties of the paper.6 Different from fines from mechanical pulps, chemical pulp fines contribute little to the light scattering coefficient of the paper.7 They are flexible and readily bond to fibers during sheet drying and lose their free surface that scatters light. Chemical pulp fines have chemical compositions similar to the fibers; however, the fine fractions are somewhat richer in hemicellulose and lignin than is the fiber fraction.8 Chemical pulp fines retard dewatering of the pulp suspension due to their high water holding capacity. Fines are extremely important to the papermaking chemistry and play a decisive role in retention, drainage, and paper * To whom correspondence should be addressed. E-mail: [email protected]. † Tianjin University of Science and Technology. ‡ University of New Brunswick.

properties.9 Many studies have been completed regarding the effect of fines on the optical properties,10,11 paper properties,12 drainage, and wet end chemistry.13 In the production of many high-quality paper grades, bleached mechanical pulps, also known as bleached highyield pulp (HYP), including bleached chemical mechanical pulp (BCTMP), alkaline peroxide mechanical pulp (APMP), and preconditioning refiner chemical-alkaline peroxide mechanical pulp (PRC-APMP), can substitute for part of the hardwood bleached kraft pulp (HBKP) in the fine paper furnish, largely due to some functional advantages associated with the HYP.14-20 In particular, the aspen HYP has gained much attention due to its unique properties of brightness, bulk, and strength.14,15,19,20 When the HYP is used in such an application, it is still desirable to enhance the optical properties, for example, the CIE whiteness and b*. Commercial dyes, including cationic basic dyes and cationic direct dyes, can serve for this purpose effectively.21-24 There is a concern that increasing HYP level in fine paper causes poorer dye efficiency because of the higher fines content of HYP and yellowish color of the HYP, therefore having an overall negative impact on the optical properties of the HYPcontaining paper products. For both fundamental and practical consideration, it would be of interest to determine the interaction of dyes with fines in the process. The objectives of this study include: (1) compare the HYP fines and BKP fines and their effects on dyeing (color shading) process; and (2) investigate the effect of mixed HYP fines and HBKP fines, which is the practical situation in the paper industry, on the dye effectiveness. 2. Experimental Section Materials. An aspen HYP sample, with a freeness of 325 mL of CSF and 85% ISO brightness, was obtained from a mill

10.1021/ie101169s  2010 American Chemical Society Published on Web 08/17/2010

Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010 Table 1. Characteristics of HYP Fines and HBKP Fines (0.1% Consistency) fines type

HYP fines

HBKP fines

turbidity (NTU) cationic demand of well-washed fines (µequiv/g pulp) water retention value (g water/g fines)

383 9.2

330 2.8

1.34

2.07

in Quebec, Canada. A commercial eucalyptus bleached kraft pulp was refined in a PFI mill to 420 mL of CSF. A cationic basic violet dye (Violet 97 NA) was obtained from BASF. Fiber and Fines Separation. A dynamic drainage jar was used to separate fibers and fines for both HYP pulp and HBKP pulp according to the method from Rundlof et al.3 The wire of the jar was 200 meshes. The fines passing through the screen were collected; the pulp retained on the wire was diluted with a small amount of distilled water and collected as the fiber fraction. The filtrate was allowed to settle overnight, and the concentrated materials were referred to as fines. It should be noted that the fines thus obtained contain a significantly less amount of dissolved and colloidal substances (DCS). The deionized water was used in all of the experimental procedures. Determination of Turbidity. The turbidity measurements were carried out by using a HACH 2100AN turbidimeter, and the changes in turbidity can be correlated to the changes in specific surface area of the fines based on the method from Wood and Karnis.2 Fines Characterization. Light microscopic images of the fine fractions were captured on a DM LAM microscope (Leica, Germany). The cationic demand of the fines was measured on a Mu¨tek PCD 03 equipped with PCD-titrator. Handsheet Preparation. A 0.1% pulp slurry was made to prepare handsheets with or without dyes added. The desired amount of fibers and fines was mixed in a plastic beaker. Next, the pH of the pulp slurry was adjusted to 7.0 with a diluted NaOH solution. The mixing was provided by a magnetic stirrer. Subsequently, the target dosage of dyes (4 ppm, unless specified otherwise) was added into the pulp suspension in the beaker, followed by 10 min contact time. The handsheets were subsequently prepared by following the Tappi standard method, T272 sp-02. We measured the residual dye in the filtrate, which was negligible, indicating that the dyes added were retained well onto the pulp fibers under the conditions studied. Optical Property Measurements. The optical properties were determined after reconditioning the handsheets at 23 ( 1 °C and 50% ( 2% relative humidity overnight, and they were measured with a Technidyne Micro TB-1C reflectometer. 3. Results and Discussion Characterization of HYP Fines and HBKP Fines. In Table 1 are shown the characteristics of HYP fines and HBKP fines. It can be found that the HYP fines had a higher turbidity (383 NTU) than the HBKP fines (330 NTU), and the turbidity increase could be due to the higher surface area of the particulates. According to the large particle scattering theory, the turbidity is directly related to the specific surface of the scattering particles.2 The higher turbidity of the HYP fines than the HBKP fines indicates a higher specific surface of the HYP fines than the HBKP fines (it may be that the dissolved and colloidal substances (DCS) present may also be partially responsible for the higher turbidity of HYP fines). This is confirmed in the literature in that the specific surface areas of

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the fines from a HYP and a HBKP are 14.3 and 5.0 m2/g, respectively.26 Table 1 further shows that for the well-washed fines (see Experimental Section for the preparation procedure), the HYP fines have a higher cationic demand than HBKP fines (9.2 µequiv/g pulp versus 2.8 µequiv/g pulp); it should be noted that the cationic demand data here would be close to the surface characteristics of the fines, rather than the representation of the amount of anionic trash, and, as reported earlier, the amount of anionic trash of the HYP fines was much higher.26 The fines are more swollen than the fibers, and earlier studies reported that the fines carry approximately double the amount of the water than the fibers per unit dry mass.25 The HBKP fines have a higher water holding capacity than the HYP fines, as shown in Table 1, largely because of the fact that HBKP fines are lignin-free while HYP fines contain a significant amount of lignin, which overweights the higher surface area of the HYP fines. Li et al. reported that the zeta potential of HYP fines is more negative than that of HBKP fines because the HYP fines contain unique sulfonic groups and more carboxylic groups than the BKP fines.26 For this reason, the sizing behaviors of the HYPcontaining systems are different from those of HYP-free systems.27,28 In Figure 1 are presented the microscopic images of HYP fines and HBKP fines; the HYP fines are more flourlike, while the HBKP fines are more fibril-like. The flour-like fines consist of granular fiber pieces, fiber fragments, and ray cells, and the fibril-like fines are good bond-forming materials. All of these different characteristics may give different behavior for both HYP fines and HBKP fines in the color-shading process in this study. Effect of HYP Fines and HBKP Fines on the HYP Fiber System. In Figure 2 are shown the results of adding HYP fines and HBKP fines on the CIE whiteness and b* of the HYP fibers system. The CIE whiteness decreased and b* increased with the increasing of both fines contents; the decreasing of CIE whiteness and b* was significant when the fines content was over 20%. The negative effect of HYP fines is slightly more than that of the HBKP fines. This is expected because the HYP fines contain more lignin and extractives than the HBKP fines, and these colored materials adversely affected the CIE whiteness and b* of the paper sheets. The addition of mechanical pulp fines to a fiber fraction substantially increases the light absorption capacity,29 which was also responsible for the lower CIE whiteness and higher b* in Figure 2. The effect of two types of fines on color-shading of the HYP fibers is shown in Figure 3. The dye performance on HYP fibers decreased, that is, the CIE whiteness decreased and b* increased, with the increase in the HYP fine content. For example, for the fine-free HYP, the highest CIE whiteness of 76.1 and the lowest b* of 2.1 were obtained at 4 ppm dye dosage, while the CIE whiteness was 70.3 and b* was 3.0 when the HYP fine content was 30%. However, the HBKP fines increased the CIE whiteness and decreased b* when added into the HYP fibers system. The CIE whiteness increased by about 3 units for the sample containing 30% HBKP fines as compared to the fine-free sample. The explanation for the differences in HYP and HBKP fines on the dye performance of the HYP fiber system can be given as follows: (1) The turbidity results in Table 1 indicated that the HYP fines had higher specific surface area than the HBKP fines. Once dyes were added into the HYP fines-HYP fibers system, HYP fines absorbed more dyes because of their higher specific surface area, and consequently less amount of dyes would be available for the HYP fibers so that the

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Figure 1. Microscopic image of HYP fines (left) and HBKP fines (right).

Figure 2. Effect of fines content on the CIE whiteness and the b* of the HYP fiber system in the absence of dyes (0 ppm dyes, pH 7.0, room temperature).

Figure 3. Effect of fines content on the CIE whiteness and the b* of the HYP fiber system in the presence of dyes (4 ppm dyes, pH 7.0, room temperature).

overall dye effectiveness decreased. The HBKP fines had relatively lower specific surface area, and thus less dyes would be attached to the HBKP fines, and more dyes would be available for the HYP fibers. (2) The HBKP fines contain more fibril-like fines, and they had higher CIE whiteness and lower b*; their retention in the network of HYP fibers can contribute to the increased CIE whiteness and decreased b* in Figure 3. Effect of HYP Fines and HBKP Fines on the HBKP Fiber System. The effect of HYP fines and HBKP fines on the CIE whiteness and b* of HBKP fibers is illustrated in Figure 4. Both the HYP fines and the HBKP fines had negative effects on the CIE whiteness and the b* of the HBKP fibers system. Some differences of the fine effects between the HYP fiber system and HBKP fiber system can be found when comparing the results in Figures 2 and 4. It is evident that the negative effect of fines on the CIE whiteness was more pronounced for the HBKP fiber system than for the HYP fiber system. For example, for the HBKP fiber system, the CIE whiteness lost

8.2 units at a 30% HYP fine addition and 5.5 units at a 30% HBKP fine addition (Figure 4). However, for the HYP fiber system, the CIE whiteness lost 2.6 units at a 30% HYP fine addition and 2.0 units at a 30% HBKP fine addition (Figure 2). It is noted that the HBKP fibers had higher base (at 0% fines content) CIE whiteness than the HYP fibers (75.3 versus 63.8). The HYP fines and HBKP fines, which had their CIE whiteness similar to that the HYP fibers, but lower than that of the HBKP fibers, will have less effect on the HYP fiber system than on the HBKP fiber system. As noted in Figure 3, the HYP fines had a negative effect while the HBKP fines had a positive effect on the dyeing process for the HYP fiber system. However, the behavior of HYP fines and HBKP fines was very different for the HBKP fiber system from the HYP fiber system. In Figure 5, the CIE whiteness of the HYP fines-HBKP fiber system had a maximum at a 5% HYP fines content, and decreased when the HYP fine content further increased. On the other hand, the negative effect of

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Figure 4. Effect of fines content on the CIE whiteness and the b* of the HBKP fiber system in the absence of dyes (0 ppm dyes, pH 7.0, room temperature).

Figure 5. Effect of fines content on the CIE whiteness and the b* of the HBKP fiber system in the presence of dyes (4 ppm dyes, pH 7.0, room temperature).

HBKP fines on the color-shading process of the HBKP fiber system was linear with the HBKP fine content. It can be further seen in Figure 5 that the HYP fines-HBKP fiber system always had a higher CIE whiteness and a lower b* than did the HBKP fines-HBKP fiber system. HYP fines have a higher specific surface, higher dye absorption, and also higher scattering coefficient, all of which can result in a higher CIE whiteness. On the other hand, the HYP fines have a lower

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Figure 6. Effect of mixed fines on the CIE whiteness and b* of the fiber system in the presence of dyes (fines content: fixed at 20%, pH 7.0, room temperature, 4 ppm dyes).

whiteness than HBKP fibers. For these two reasons, one may expect a maximum CIE whiteness at a low HYP fines substitution. Also, the higher scattering coefficient of the HYP fines prevails in comparison with the HBKP fines, which is responsible for the fact that the HYP fines-HBKP fiber system always had a higher CIE whiteness and a lower b* than the HBKP fines-HBKP fiber system. Effect of Mixed HYP Fines and HBKP Fines on the HYP Fiber System. The above results presented so far indicated that fine behaviors were different in the dyeing process for different fiber systems. In the commercial paper manufacturing process, pulp furnish contains both types of fines, that is, HYP fines and HBKP fines, when the paper mills use some HYP as a substitution for HBKP pulp. The interactions between the HYP fines and HBKP fines are of practical importance. We fixed the total fine content at 20%, but different ratios of HYP fines and HBKP fines were made for the HYP fiber system and HBKP fiber system. The mixed fines were added into the fiber system, followed by the dye addition. The results shown in Figure 6 indicated that, at 20% HYP fines and 80% HBKP fines, the mixed fines led to the maximum CIE whiteness (86.5) and lowest b* (-0.40) for the HBKP fiber system. For the HYP fiber system, the presence of HYP fines always resulted in a lower dye effectiveness. The fact that the mixed fines had a better response for the HBKP fibers than for the HYP fibers (the differences in the whiteness and b* of 9.3 and 1.91, respectively) may suggest that the HBKP fiber system can retain more dye-absorbed fines, and thus a higher dye efficiency, than the HYP fiber system. To further study the interactions among HYP fines, HBKP fines, and dyes, the turbidity of the mixed fines was determined by using 0.1% fines consistency; the ratio of HYP fines and HBKP fines varied from 0/100 to 100/0. The two types of fines were mixed thoroughly before the measurement. The turbidity of HYP fines, HBKP fines, and their mixtures is shown in Figure 7. The turbidity decreased with increasing the amount of HYP fines. The turbidity of a suspension is determined principally by the light scattering, therefore, by the specific surface of the particles. The turbidity of the mixed fines after the addition of

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because the dye-fines complexes are formed and retained in the fiber networks, consequently improving the CIE whiteness and b*. In the mixture of HYP fines and HBKP fines in the colorshading process, the dyes and HYP fines can form complexes, which then retain in the fiber network, thus improving the dye effectiveness and resulting in a higher CIE whiteness and lower b* of the paper sheet; the HYP fines had a stronger absorption for dyes than did the HBKP fines. Acknowledgment Figure 7. Turbidity of different mixtures of HYP fines and HBKP fines with or without 4 ppm of dyes.

the dyes decreased, as shown in Figure 7. This suggests that some of the fines combined with the dyes and some complexes are formed (especially for the HYP fines), and consequently the specific surface of the fines decreased. It can also be found from Figure 7 that the HYP fines have more turbidity loss than do the HBKP fines after the addition of dyes. HYP fines are more negative than the HBKP fines on the zeta-potential measurement;26 this is because the HYP fines contain unique sulfonic groups and more carboxylic groups than the HBKP fines.26 The dyes used in this study are cationic basic dyes. Once added into the fines, the dyes were absorbed on the negatively charged fines, which can decrease the repulsion force between the fines, and can induce fine aggregation and flocculation, consequently resulting in decreased specific surface of fines and their turbidity. Because of their higher specific surface area and higher lignin content, the HYP fines can absorb more dyes, and the dyes absorbed on the HYP fines improved the overall dye effectiveness. The hypothesis is supported by the literature results that toluidine blue basic dye had more adsorption on high lignin containing TMP fibers than kraft fibers.30 These dye-fines complexes can then be trapped in the fiber networks like fillers. It is known from the particle science31 that when a suitable amount of very small particles is added to coarser particles, the maximum possible packing density is achieved at a particular ratio, with the small particles occupying the interstices between the large ones. Evidently, the HYP fines, rather than HBKP fines, can preferably form complexes with dyes, which function as fillers for the fiber network made of HBKP fibers (Figure 6). 4. Conclusions The HYP fines have a higher specific surface area and more negative charge than the HBKP fines. Both HYP fines and HBKP fines had a detrimental effect on the CIE whiteness and b*, although the negative effect of the HYP fines is more pronounced because of the higher lignin content and some other color materials; however, the use of dyes can improve these optical properties. For the HYP fiber system, the HYP fines decreased the dye effectiveness in the color shading process; this is because HYP fines had a higher specific surface area and absorbed more dyes, resulting in less amounts of dyes available to HYP fibers, and thus a lower overall dye effectiveness. For the HBKP fiber system, a small amount of HYP fines can improve the dye retention, leading to an increased dye effectiveness; this is

The financial support from an international scientific and technological joint project between China and Canada (Grant no. 2008DFA91290) is greatly acknowledged. Literature Cited (1) Sundberg, A.; Sundberg, K.; Holmbom, B. Chemical characterization of different types of fines in mechanical pulp. Proceedings, 11th Symposium Wood Pulping Chemistry; Nice, France, 2001; Vol. III, pp 605-608. (2) Wood, J. R.; Karnis, A. Determination of specific surface area of mechanical pulp fines from turbidity measurements. Paperi Ja Puu 1996, 78, 181–186. (3) Rundlof, M.; Htun, M.; Hoglund, H.; Wagberg, L. The importance of the experimental method when evaluating the quality of fines of mechanical pulps. J. Pulp Pap. Sci. 2000, 26, 301–307. (4) Luukko, K.; Kemppainen-Kajola, P.; Paulapuro, H. Characterization of mechanical pulp fines by image analysis. Appita J. 1997, 50, 387–399. (5) Kerr, A. J.; Goring, D. A. I. The ultra structure arrangement of the wood cell wall. Cellul. Chem. Technol. 1975, 9, 563–573. (6) Kang, T.; Paulapuro, H. Characterization of chemical pulp fines. Tappi J. 2006, 5, 25–28. (7) Seth, R. S. The measurement and significance of fines. Pulp Pap. Can. 2003, 104, 41–44. (8) Htun, M.; De Ruvo, A. The implication of the fines fraction for the properties of bleached kraft sheet. SVen. Papperstidn. 1978, 81, 507. (9) Liu, X. A.; Whiting, P.; Pande, H.; Roy, D. N. The contribution of different fractions of fines to pulp drainage in mechanical pulps. J. Pulp Pap. Sci. 2001, 27, 139–143. (10) Luukko, K.; Paulapuro, H. Mechanical pulp fines: effect of particle size and shape. Tappi J. 1999, 82, 95–101. (11) Mohlin, U. B. Mechanical pulp properties-the importance of fines retention. SVen. Papperstidn. 1977, 80, 84–88. (12) Mohlin, U. B. Properties of TMP fractions and their importance for the quality of printing papers. SVen. Papperstidn. 1980, 83, 513–519. (13) Marton, J. The role of surface chemistry in fines-cationic starch interactions. Tappi Papermakers Conf. 1980, 205–213. (14) Zhou, Y. Overview of high yield pulps (HYP) in paper and board. Preprints, 90th Annual Meeting, PAPTAC, 2004; pp B143-B148. (15) Reis, R. The increased use of hardwood high yield pulps for functional advantages in papermaking. Proc. 2001 Papermakers Conf. 2001, 87–108. (16) Yuan, Z.; Schmidt, J.; Heitner, C.; Zou, X. Coating improves the brightness stability of wood-free coated papers containing high-yield pulp. Tappi J. 2005, 5, 9–13. (17) Hu, K.; Ni, Y.; Zou, X.; Zhou, Y. Substitution of hardwood kraft with aspen high-yield pulp in lightweight coated wood-free papers. I: Synergy on basestock properties. Tappi J. 2006, 5, 21–26. (18) Hu, K.; Ni, Y.; Zou, X. Substitution of hardwood bleached kraft pulp with aspen high-yield pulp in LWC wood-free papers. Part II. Impact on coated paper quality. Tappi J. 2007, 6, 26–32. (19) Hu, K.; Ni, Y.; Zou, X. Substitution of aspen high-yield pulp for hardwood kraft pulp in fine papers and its effect on AKD sizing. Tappi J. 2004, 3, 13–16. (20) Reis, R. J.; Nielsen, G. Aspen BCTMP: Proven performance. Solutions! For People, Process and Paper 2001, 84, 28–30. (21) Liu, H.; Yang, S.; Ni, Y. Using dyes for improving optical properties of high yield pulps. Pulp Pap. Can. 2007, 108, 25–29. (22) Liu, H.; Yang, S.; Ni, Y. Applying dyes to HYP-containing paper grades. Appita J. 2008, 61, 128–140. (23) Liu, H.; Yang, S.; Ni, Y. Dye stability in the presence of hydrogen peroxide and its implication for using dye in the HYP manufacturing process. J. Wood Chem. Technol. 2009, 29, 1–10.

Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010 (24) Liu, H.; Yang, S.; Ni, Y. Comparison of dye behavior from aspen HYP: Dyes added in the HYP manufacturing process versus dyes added at the papermaking wet end. J. Wood Chem. Technol. 2010, 30, 118–128. (25) Laivins, G. V.; Scallan, A. M. The influence of drying and beating on the swelling of fines. J. Pulp Pap. Sci. 1996, 22, 178–183. (26) Li, H.; Ni, Y.; Sain, M. Characterization of BCTMP fines and their effect on sizing. Tappi J. 2002, 1, 3–7. (27) Li, H.; Ni, Y. Sizing behaviour of BCTMP at neutral to alkaline pH. Pulp Pap. Can. 2001, 102, 45–48. (28) Li, H.; Ni, Y.; Sain, M. M. The presence of dissolved and colloidal substances in BCTMP and their effect on sizing. J. Pulp Pap. Sci. 2002, 28, 45–49. (29) Lindholm, C. A. Comparison of some papermaking properties of groundwood, pressure groundwood and thermomechanical pulp by means

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of artificial blends of pulp fractions, Part 2. The fines fractions. Paperi Ja Puu 1980, 62, 593–606. (30) Van de Ven, T. G. M.; Saint-Cyr, K.; Allix, M. Adsorption of toluidine blue on pulp fibers. Colloids Surf., A 2007, 294, 1–7. (31) Fedors, R. F.; Landel, R. F. An empirical method of estimating the void fraction in mixtures of uniform particles of different size. Powder Technol. 1979, 23, 225–231.

ReceiVed for reView May 27, 2010 ReVised manuscript receiVed August 2, 2010 Accepted August 4, 2010 IE101169S