Increasing the Use of High-Yield Pulp in Coated High-Quality Wood

Feb 21, 2012 - Although high-yield pulps (HYP) are gaining increasing use to replace hardwood kraft pulp in paper grades such as uncoated and coated f...
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Increasing the Use of High-Yield Pulp in Coated High-Quality WoodFree Papers: From Laboratory Demonstration to Mill Trials Hongbin Liu,*,†,‡ Yunzhi Chen,§ Hongjie Zhang,† Zhirun Yuan,∥ Xuejun Zou,∥ Yajun Zhou,⊥ and Yonghao Ni†,# †

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin, China, 300457 College of Packaging & Printing Engineering, Tianjin University of Science and Technology, Tianjin, China, 300222 ∥ FPInnovations, Pointe Claire, Quebec City, Canada H9R 3J9 ⊥ Tembec Inc., Temiscaming, Quebec City, Canada J0Z 3R0 # Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 ‡ Jiangsu Provincial Key Laboratory of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, China 210037 §

ABSTRACT: Although high-yield pulps (HYP) are gaining increasing use to replace hardwood kraft pulp in paper grades such as uncoated and coated fine papers, the amount has been typically limited to less than 20% because there are concerns about its potential impact on papermaking operation and product quality. To address these concerns, laboratory experiments that mimic the actual paper machine operation were carried out and coated paper samples from mill trials were examined to clarify the impact of high-level HYP substitution on the properties of coated wood-free papers. Results showed that the HYP substitution, even at 40%, did not yield negative effects on strength properties such as tensile and tear; in fact the Scott bond increased with the HYP addition. The small increase in the surface roughness from the HYP addition can be eliminated by the filler addition, precalendering, and coating process. The lower brightness and CIE (Commission Internationale d'Eclairage) whiteness of the HYP can be compensated for by the addition of optical brightening agents (OBAs) and dyes, as well as pigments in the coating color. The analysis of samples collected from mill trials indicated that coated paper containing 40% HYP has lower coating penetration than that containing 40% HYP content paper samples. This was attributed to the smaller pore size created by HYP substitution. No significant differences were found between the samples containing 17 and 40% HYP on print gloss, color gamut, and print gloss uniformity. The implication from this study is that the HYP substitution level can be increased up to 40% in the production of coated wood-free paper without significant negative effect on the paper quality.

1. INTRODUCTION The typical furnishes used in wood-free paper grades are softwood bleached kraft fibers and hardwood bleached kraft fibers.1,2 Softwood bleached kraft fibers are long, and they can provide a good wet web strength, thus a good paper machine runability. In addition to that, the long fibers can provide the paper strength, especially the cross-direction tear strength. Hardwood bleached krfat fibers are short, and they can provide the paper with the desired end-use properties, such as good formation, opacity, smoothness, and internal bonding. Hardwood kraft fibers are usually cheaper than the softwood fibers, so they take most of the composition in the wood-free paper grades because of the cost advantage.3 High-yield pulp (HYP) mainly includes the bleached chemithermo-mechanical pulp (BCTMP) and preconditioning refiner chemical treatment alkaline peroxide mechanical pulp (P-RC APMP). In particular, the high-brightness aspen BCTMP pulp has gained more and more interest in the production of high-quality wood-free paper grades.4−6 HYP can provide not only lower furnish cost but also better product quality because using HYP can maintain the caliper and stiffness at a lower grammage. Additionally, HYP contains more fines fraction, so it can improve the paper formation and opacity. HYP is also environmentally friendly because of its © 2012 American Chemical Society

higher yield and lower carbon footprint in the manufacturing process.7 The advances in papermaking and printing technologies as well as the changes in standards for the permanent paper allow the increased use of HYP in the production of wood-free paper grades. A new generation of calender (e.g., hot soft-nip and shoe calendering) preserves more bulk than the conventional supercalender.8 High-temperature calendering helps develop the gloss and smoothness with a lower calendering load.9 The increased dewatering capacity of the paper machine gives more flexibility in the pulp furnish. Smaller ink droplets in inkjet printing and no water involvement in LaserJet printing make the water-induced roughness less of an issue for the use of HYP.10−12 The revised standards of permanent paper grades remove the limit on the lignin content, which promotes the use of HYP in the production wood-free paper grades.12 Market HYP is widely used in nonintegrated paper mills due to its functionality and cost advantages in coated wood-free papers, and this is particularly true in China, which has imported more than 2 million tons of HYP from Canada and Received: Revised: Accepted: Published: 4240

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papermaking conditions were adjusted to meet the final targeted paper properties in terms of smoothness and gloss. The furnish composition and some basic information of the two commercial paper samples are listed in Table 2. These two samples were double-coated (precoating and top-coating), with a coat weight of 25 g/m2 per side.

other countries in 2010 and has more than 3 million tons of HYP production capacity (mainly APMP and P-RC lines).7 HYP has typically been substituting for hardwood kraft pulp at 5−20% rate, and the potential substitution rate can be as high as 50% in some grades, e.g., coated wood-free paper. In the coated paper production, some of the deficiencies, such as surface roughness, caused by using HYP can be mitigated via precoating/top-coating.13 A precoating process would “freeze” or even out the surface of the base paper, therefore paving the way for a high-quality final coating.14 Consequently, the surface properties having strong impacts on the printing quality may only be slightly affected or even not affected at all. A high coat weight also mitigates the slightly lowered brightness and brightness reversion from the HYP substitution.13 Previous studies were performed by using the standard handsheets or dynamic sheet former (DSF) sheets without filler addition and white water circulation.15 Recently, we developed the procedure in using the DSF system with filler addition and white water circulation to study the effects of HYP substitution on the properties of base paper, which makes it possible to more realistically mimic the commercial papermaking process in a laboratory setting.16 This study is a continuation of our effort in determining the effects of increasing the HYP substitution on the base paper, and the coated paper properties, in particular, concentrating on the potential of using a very high HYP substitution (40%). Both laboratory and mill trial results will be presented.

Table 2. Properties of the Commercial Base Papers with Two HYP Contents

softwood bleached kraft pulp (%) hardwood bleached kraft pulp (%) high-yield pulp (%) base paper grammage (g/m2) total coat weight (g/m2)

Table 1. Pulp Properties for given pulp type HWKP

wood species

spruce

eucalyptus

net LC refining energy (kWh/ton) CSF (mL) bulk (cm3/g) tensile index (Nm/g) TEA index (mJ/g) tear index (mN m2/g) brightness (ISO, %) b*

96

70

61.5/38.5 HYP/ HWKP 20

561 1.78 54.8 1470 16.8 86.7 2.49

410 2.15 29.7 373 4.6 87.0 2.96

338 2.45 29.2 298 4.1 84.0 5.00

sample with 40% HYP

5

12

78

48

17 90 51

40 90 52

Experimental Procedure for Laboratory Experiments. In the laboratory program, the percentage of SWKP in the paper furnish was fixed at 35%. The HYP sample (aspen BCTMP) came from the HYP/HWKP mixture, and the 30% HYP percentage was calculated on the basis of the 61.5% HYP in that mixture. A dynamic sheet former (DSF) was used to form the base paper for this study, and the procedure was detailed earlier.16 The wire speed was at 1000 m/min while nozzle speed was set at 800 rpm and the nozzle pump pressure was at 30 psi. A 0.2% pulp consistency was used. A 30% amount of precipitated calcium carbonate (PCC) was added into the mixed paper furnish, followed by the addition of 0.05% cationic poly(acrylamide) (CPAM). The basis weight was controlled at 70 ± 2 g/m2. The DSF sheets were precalendered by a hard-nip calender at 50 °C and 50 m/min. The nip load varied from 0 to 27 kN/m. The strength properties of the paper samples were measured. The surface roughness was measured by a Parker Print Surf (PPS) instrument, and air permeability was determined by the Gurley-Hill method. The optical properties, including ISO brightness, light scattering coefficient, and opacity, were determined with a TechniBrite Micro TB-1C tester. All of the measurements were conducted according to the relevant ISO standard methods. A scanning electron microscope (SEM, Model JSM-840A from JEOL, Tokyo, Japan) was used to obtain the images of coated paper cross-sections. The porous structures of the base paper and coated sheets were characterized on the basis of a mercury intrusion method using a Poresizer 9320 (Micromeritics Instrument Corp., Norcross, GA, USA). The coating penetration was characterized using a SEM cross-section/image analysis method. An X-Rite DTP 22 spectrophotometer was used to collect CIE (Commission Internationale d'Eclairage) L*, a*, and b* values from CMYKRGB colors. The color gamut was determined by measuring the CIE L*, a*, and b* on six color patches, and the gamut area was then calculated. A gloss map was examined by a gloss map tester. The paper samples (50 mm × 50 mm) were scanned with the step size of 50 μm.

2. EXPERIMENTAL SECTION Pulp Furnishes. A softwood bleached kraft pulp (SWKP; Spruce), a hardwood bleached kraft pulp (HWKP; Eucalyptus), and an aspen HYP (85% ISO brightness, and 325 mL of Canadian Standard Freeness (CSF)) were received from a Canadian paper mill. The SWKP and HWKP were refined separately by an Escher Wyss low-consistency (LC) refiner at a 3.5% pulp consistency. To simulate the co-refining practice in paper mills, a pulp mixture consisting of 61.5% HYP and 38.5% HWKP was also refined with the same refiner. Table 1 lists the properties of these refined pulps.

SWKP

sample with 17% HYP

mixed pulp

Paper Samples from Mill Trials. Two coated paper samples, one at 17% HYP substitution and the other at 40% HYP substitution, were obtained from a Chinese coated woodfree paper mill. This mill produces high-quality, double-coated papers, with a coat weight of about 25 g/m2 per side. The coating was applied with two blade coating stations. The

3. RESULTS AND DISCUSSION Laboratory Results. High bulk is an important feature for HYP.4 Previous studies based on handsheets and DSF sheets showed that bulk increased with the increase in the HYP 4241

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substitution level; however, these results were obtained without the filler addition and without calendering.5,15,17 As shown in Figure 1, the bulk of the DSF sheets containing 30% PCC also

The air resistance increases with the increasing HYP percent in the furnish, as shown in Figure 3. The higher fines content in

Figure 3. Effects of HYP substitution on Gurley air resistance of DSF sheets. Figure 1. Effects of HYP substitution on the bulk of DSF sheets.

HYP reduces pore size, thus increasing the tortuosity.19 The fines filled the voids between the fibers and improved the paper formation. The reduced voids and the improved formation also contributed to the increased air resistance.20,21 HYP has lower brightness than bleached kraft pulp because of the presence of a large amount of lignin and other color materials.22 As expected, the brightness of the DSF sheets decreased with the increasing HYP substitution level (Figure 4), similar to the results reported earlier in the handsheet

increased with the increasing HYP substitution level before precalendering, consistent with previous findings from the handsheet studies.5,15 However, Figure 1 further shows that, after precalendering, the bulk increase was marginal from the increasing HYP substitution level. This is different from what Hu et al. found which showed that the bulk advantage with HYP substitution was still largely preserved after coating and calendering.15 The difference can be attributed to the presence of fillers. Fillers may fill the voids between fibers and fines, thus leading to the consistent bulk development with the increase of the HYP substitution level. The implication from these results is that the precalendering conditions need to be adjusted accordingly in order to achieve the desired high bulk from the addition of HYP. Figure 2 shows the tensile strength and Scott bond increased with the increasing HYP substitution levels. At a low HYP

Figure 4. Effects of HYP substitution on the opacity and brightness of DSF sheets.

study.5 This decrease can be recovered by adding a small amount of optical brightening agents (OBAs) and dyes.23−25 The coating process can also improve the brightness of the final product because of the high brightness of pigments and OBA addition.13 The opacity increased with the increasing HYP substitution level (Figure 4). The explanation is as follows: lignin-rich HYP usually have higher opacity than the bleached kraft pulps; the high degree fibrillation of the fiber surface and the presence of a high amount of fines in the HYP are also responsible, because fines usually have higher light scattering coefficient. In addition, the high light absorption coefficient also helps increase the opacity. The higher roughness is a great concern when using mechanical pulps.14 Mechanical pulp has poor bonding ability and they tend to “rise/spring up” when in contact with water (e.g., in coating application). As shown in Figure 5, the roughness increased with the increasing HYP substitution level. This, however, can be resolved by a precalendering process. As

Figure 2. Effects of HYP substitution on the tensile strength and Scott bond of DSF sheets.

substitution level (for example, 0−10%), the tensile strength did not change. The increase was evident when the HYP content was above 10%. This was due to synergistic effect between the HYP and kraft pulp; in particular, the fiber-to-fiber bonding was improved between the HYP and kraft fibers/ fines.4−6 The improved interfiber bonding due to the addition of HYP was further confirmed from the Scott bond data as also shown in Figure 2. The Scott bond increased with the HYP substitution level, which comes from the increased bonded area, as well as increased bond strength.5,6,18 4242

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40% led to a slight decrease in the tensile strength, while the tear index (Machine Direction (MD)) is essentially unchanged; however, the tear index (Cross Direction (CD)) increased somewhat. Although the tear strength is mainly controlled by fiber length, it is also affected by fiber bonding, especially for short fibers. The increased bonding ability will lead to an increase in the tear strength. Another reason for the tear strength increasing may be due to the lignin content of fiber that can provide the stiffness to fiber. In the mill, 10% softwood kraft pulp was added to the mixed furnish; these results are largely in agreement with those obtained in the laboratory study, which were discussed in the previous section. The HYP fibers themselves give lower strength properties than kraft fibers; however, the HYP contains 25−40% fines, which have very high specific surface18,28 and excellent swelling ability,19 resulting in superior bonding areas and bonding capability.5,6 It is noted in Table 3 that the Scott bond is somewhat higher for the sample containing 40% HYP than that containing 17% HYP, while the results on MIT fold endurance are essentially the same. Table 4 lists the optical properties of the mill trial samples. It can be seen that the samples with two levels of HYP

Figure 5. Effects of HYP substitution on the roughness of DSF sheets.

shown in Figure 5, the PPS roughness of the base paper was essentially constant at different HYP substitution levels after precalendering (the precalendering was done under the same pressure). In summary, the laboratory results showed that the addition of up to 40% HYP did not have any significant negative effects on the properties of base papers. A mill trial further confirmed the potential of using a high HYP substitution level (up to 40%). The following section will discuss the properties of paper collected from the mill trial, with two levels of HYP substitution (17% vs 40%). Mill Trial Results: 17% HYP vs 40% HYP Substitution. A 17% amount of HYP is the typical usage for this mill, and the trial was done to see if the HYP content can be pushed up to 40%. The trial was successful as the high level of HYP substitution did not require any significant adjustment on paper machine operation. The softwood kraft content was increased a bit (from 5 to 12%) to ensure good paper machine runnability. Two samples were collected from the mill trial and fully tested and analyzed. It can be found from Table 3 that the

Table 4. Optical Properties of the Mill Trial Samples

brightness D65, top (%) brightness D65, bottom(%) D65 fluor comp, top (%) D65 fluor comp, bottom (%) CIE whiteness, top CIE whiteness, bottom opacity, top (%) opacity, bottom (%) gloss (%)

grammage (g/m2) coat weight (g/m2) bulk (cm3/g) PPS roughness (μm) tensile index (MD) (Nm/g) tensile index (CD) (Nm/g) tear index (MD) (mNm2/g) tear index (CD) (mNm2/g) Scott bond (MD) (J/m2) Scott bond (MD) (J/m2) MIT fold no. (MD) MIT fold no. (CD)

sample with 40% HYP

147 25.0 0.88 0.8 43.6 14.1 3.9 5.9 156 174 48 6

151 25.8 0.88 0.8 38.7 13.6 4.0 6.5 185 181 50 6

sample with 40% HYP

91.0 91.0 8.34 8.37 112.5 112 98.7 98.6 65.0

91.0 91.5 9.04 9.08 112.8 112.8 99.1 99.2 66.0

substitution had the same brightness, CIE whiteness, opacity, and gloss. It is known that HYP has lower brightness and CIE whiteness than the bleached kraft pulp.25 OBA and dyes are usually added in practice to improve the brightness and CIE whiteness requirements.26,27 In the mill trials, more OBA was added when producing the coated paper containing 40% HYP; this is why the 40% HYP containing paper had a higher fluorescence component (D65) than that containing 17% HYP . The amount of coating penetration is shown in Table 5. Coated paper containing 17% HYP showed higher coating

Table 3. Paper Properties of Coated Paper with Different HYP Contents sample with 17% HYP

sample with 17% HYP

Table 5. Properties of the Mill Trial Samples

coating penetration (g/m2) porosity (%) print gloss on black (75°, %)

grammage and coated weights were controlled because the mill was producing the same product. The paper bulk and roughness for these two HYP containing paper products are the same, indicating that, under these commercial conditions, the use of more HYP from 17 to 40% did not materially change the paper bulk and roughness. Table 3 also lists the physical properties of two mill trial samples. It is noted that increasing the HYP content from 17 to

sample with 17% HYP

sample with 40% HYP

2.0 25.3 93

1.5 24.7 94

penetration than that containing 40% HYP, which means coated paper containing 40% HYP content paper samples had better coating holdout than that containing 17% HYP. HYP contains more fines, which lead to higher fiber absorption and swelling. Fiber absorption and swelling reduce absorption rates and decrease coating penetration into the paper.28 The pore size of the basesheets was found to be a key factor in the 4243

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Figure 6. Gloss maps of printed areas for the two mill trial coated paper samples.

Figure 7. Color gamut of the two different HYP content mill trial coated paper samples.

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coating penetration.29 Hui et al. found that HYP fibers have smaller pore size and higher tortuosity,19 and the coating penetration data are consistent with their findings. The earlier results of this study on the air resistance of DSF sheets also support the coating penetration results, that is, the fact that coated paper containing 40% HYP had better coating holdout due to its smaller pore size compared to that containing 17%, as shown in Table 5. The perceived quality of a printed product depends on a number of print quality features. Among them, the print gloss, and in particular variation in print gloss, is the most important. High print gloss is a major attribute of quality for offset printing paper grades.30,31 The print gloss is essentially the same as that shown in Table 5. It means higher HYP content (up to 40%) will not negatively affect the print gloss for the high coat weight coated paper. The map of print gloss further shows there is no difference on printed gloss for the two different HYP content samples,, as shown in Figure 6 which indicates that high HYP content in coated paper sheets will not have any significant negative effects on print mottle. A color gamut is a delimited region in color space, and it contains colors that are physically achieved by a given device or that are present in a given image.32 Color gamut is used for evaluating the color reproducibility of printed samples. As shown in Figure 7, the two coated samples showed similar results. It means that HYP content did not affect the color reproduction on printed coated paper samples. SEM cross-section images show pressure was more applied in the base paper with 40% HYP than that with 17% HYP. As a result, HYP in the base paper with 40% HYP was compressed and became more flat than that with 17% HYP samples, which explains the fact that the HYP fibers in the 40% HYP sample (Figure 9) are more collapsed than those in the 17% HYP samples (Figure 8). It is further illustrated in Figures 8 and 9

samples had the same roughness. These sheets were all doublecoated, and precoating can freeze the sheet structure and reduce the surface roughening during the application of top coating.

4. CONCLUSIONS The results from laboratory experiments and mill trials showed that the HYP substitution (up to 40%) does not yield negative effects on strength properties such as tensile and tear. In fact the Scott bond increases with the HYP addition. The small increase in the surface roughness from the HYP addition can be eliminated by filler addition, precalendering, and a coating process. The lower brightness and CIE whiteness of the HYP can be compensated for by OBA and dye additions, and pigments in the coating color. The mill trial results indicated that coated paper containing 17% HYP had higher coating penetration than that containing 40% HYP because of the larger pore size in the base paper. No significant differences were found between the 17 and 40% HYP coated paper samples on print gloss, color gamut, and print gloss uniformity. The implication from this study is that the HYP substitution level can be increased up to 40% in the production of coated wood-free paper, without any significant negative effect on paper quality. However, some adjustment in paper machine operation may be needed; e.g., the coated sheets with higher HYP substitution level may require more calendering to achieve the same roughness and printability than those with lower HYP substitution level.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the International Scientific and Technological Joint Project of China and Canada (Grant 2008DFA91290) and Grant CCRD07-011 of ISTP Canada, the National Natural Science Foundation of China (Grant 31100435), the Project of Tianjin Scientific Innovation System and Platform Construction (Grant 10SYSYJC28000), the Foundation for the Development of Science and Technology in Tianjin Universities (Grant 20100509), and Tianjin University of Science & Technology Research Funds (Grant 20100222). We also thank Daniel Gilbert, Lyne Cormier, Tony Manfred, Joe Aspler, and Elena Walker of FPInnovations for their technical assistance.

Figure 8. SEM cross-section images of 17% HYP coated paper samples from the mill trial (left, top side; right, bottom side).

that both samples with 17 and 40% HYP substitution levels have very uniform coating layers on the top side and bottom side. The results in Table 3 show these two coated paper

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