Article pubs.acs.org/IECR
Effect of Chip Mixing Ratio of Pinus pinaster and Populus tremula on Kraft Pulp and Paper Properties Sezgin Koray Gulsoy*,† and Saduman Tufek† †
Faculty of Forestry, Forest Products Engineering, Bartin University, Bartin, Turkey ABSTRACT: In this study, Pinus pinaster and Populus tremula chip mixtures were kraft cooked and resulting pulp and paper properties were investigated. The chemical properties and fiber morphology of both species were examined. Cooks were made for five different pine/aspen chip mixtures (0, 25, 50, 75, and 100%). Results of this study showed that pulps with higher kappa number, viscosity, and reject ratio were observed for chips mixtures with higher pine ratio. Higher pine in the mixture resulted in lower total and screened yield. Increase in poplar ratio in mixture gave pulps that were easier to beat. Pine chips improved the strength properties and lowered the brightness and smoothness.
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INTRODUCTION Maritime pine (Pinus pinaster Ait.) grows naturally in the western Mediterranean region and south Atlantic coasts of Europe.1 The annual consumption of maritime pine for pulp production is approximately 3 million raw tons.2 On the other hand, the European aspen (Populus tremula L.) is the most widely distributed tree species in the world and it grows throughout Europe, Scandinavia, and across north Asia to China and Japan.3 Both maritime pine and European aspen are important species for the pulp and paper industry. The most significant factor affecting pulp quality is wood species. The chemical composition and fiber properties of woods vary among species and even with trees. Therefore, properties of pulps produced under certain pulping conditions depend on wood species.4 Much stronger paper can be produced from softwood pulps. These pulps are generally used as reinforcement pulp in paper production.5 On the other hand, hardwood pulps are generally preferred for producing smooth and high-quality writing papers. Because of differences between species in terms of chemical composition and fiber morphology and wood-supply problems, a pulp mill is seldom operated using a single species. Wood raw material is generally composed of two or more species.4 On the other hand, using single species is provided for more specific paper properties and more consistent fiber properties compared to using two or more species.5 The kraft process is less demanding than other pulping processes in terms of wood species. Most lignocellulosic materials can be pulped by kraft process under correct cooking conditions. The strong alkaline conditions lead to a partial removal of polysaccharides, especially hemicelluloses. On the other hand, residual lignin content in the pulp (kappa number) varies with rate of delignification and lignin content of wood. Therefore, chemical composition of wood used in the pulping is one of the important factors in terms of pulp yield and kappa number.4 The fiber morphology of wood species has a significant effect on the paper properties.6,7 Softwood fibers have generally higher fiber length than hardwood fibers. Higher strength papers are generally obtained from long-fiber species. But, these papers have a rough surface and are unsuitable for high-quality © 2013 American Chemical Society
writing. Short-fiber species give a smooth surface, good formation, and weaker papers than those of long-fiber species. Fiber coarseness is defined as the mass per unit fiber length.8 Coarseness is affected by fiber diameter, cell wall thickness, cell wall density, and fiber cross section.9 Softwood fibers have higher fiber coarseness value than hardwood fibers, because fiber coarseness increases with the increase of fiber length and fiber wall thickness. Coarse-fiber species give papers having higher tear strength and bulk, but lower smoothness compared to thin-fiber species. In an earlier study,10 it was noted that kraft pulping of mixed softwood/hardwood chips provides some advantages, on the condition that the minor wood component does not exceed 20% of the total wood raw material. In addition, kraft pulping of mixed softwood/hardwood leads to higher pulp yield, shorter cooking time (or lower kappa number), and improved beatability compared to using of single species in pulping. Some earlier studies related to mixed alkaline pulping of several hardwood and softwood species were reported by various authors.11−14 Recently, effects of mixed hardwood and softwood kraft pulping on paper properties have been evaluated.15 The limited results in the literature have been reported on the properties of kraft pulps and papers from cooking of mixed hardwood and softwood chips. Populus tremula is a cheaper and more abundant species compared to Pinus pinaster. The aim of this study is to investigate the effect of chip mixing ratio on properties of kraft pulps produced with chip mixtures of Pinus pinaster and Populus tremula, which are important species for pulp and paper industry. Thus, the possible benefits and drawbacks of Populus tremula addition to pulping were demonstrated.
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MATERIALS AND METHODS The logs of Pinus pinaster Ait. and Populus tremula L. were taken from the Bartin province of Turkey. Ten-cm thick wood Received: Revised: Accepted: Published: 2304
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discs were taken from breast height of each log. These discs were debarked and subdivided into four discs in terms of disc height (25 mm disc height). They were manually chipped to 25 × 15 × 5 mm size for pulping using a chisel as homogeneously as possible. The wood chips were air-dried to a final humidity of 10% and stored in dry conditions. Wood specimens for the chemical analysis were also sampled and prepared according to the TAPPI T 257 standard. The holocellulose,16 α-cellulose (TAPPI T 203), klason lignin (TAPPI T 222), and ash (TAPPI T 211) contents of samples were determined according to the relevant methods. The solubility properties were also determined based on alcohol (TAPPI T 204), cold-hot water (TAPPI T 207), and 1% NaOH (TAPPI T 212) methods. Densities of woods were determined according to ISO 3131. On the other hand, chlorite method17 was used in the maceration of wood samples of each species. After maceration, the samples were agitated gently to disintegrate individual fibers18 and dehydrated with ethyl alcohol and stored in glycerin after staining with safranin. The fiber length, fiber width, lumen width, and cell wall thickness of 50 randomly selected fibers were measured. The slenderness ratio (fiber length/fiber width), flexibility ratio [(lumen width/fiber width) × 100], and Runkel ratio [(2 × cell wall thickness)/lumen width] were calculated using the measured fiber dimensions. The length of 50 randomly selected vessel elements was also measured. Five kraft cooks were carried out in this study for varying pine/aspen chip mixtures (100:0, 75:25, 50:50, 25:75, and 0:100). Each kraft cook was performed under the following conditions: 18% active alkali as Na2O, 25% sulfidity, 4:1 liquor/ wood ratio, 170 °C cooking temperature, 90 min to cooking temperature, and 60 min at cooking temperature. Air-dried chips corresponding to 650 g of oven-dried were cooked in a 15-L electrically heated laboratory cylindrical-type rotary digester. At the end of cooking, the pressure was reduced to atmospheric pressure. After digestion, pulps were washed to remove black liquor and were disintegrated in a laboratory-type pulp mixer with 2-L capacity. Disintegrated pulps were screened using a Somerville-type pulp screen with 0.15-mm slotted plate (TAPPI T 275). Pulps were then beaten to 35 and 50°SR in a Valley Beater according to TAPPI T 200. Kappa number (TAPPI T 236), screened yield (TAPPI T 210), viscosity (SCAN-CM 15-62), and freeness of pulps (ISO 52671) were determined according to relevant methods. Handsheets (75 g/m2) made by a Rapid-Kothen Sheet Former (ISO 52692) at three different beating levels were conditioned according to TAPPI T 402. Tensile (TAPPI T 494), burst (TAPPI T 403), tear (TAPPI T 414), opacity (TAPPI T 519), brightness (TAPPI T 525), yellowness index (ASTM E313), roughness (ISO 8791-2), air permeability (ISO 5636-3), and bulk (TAPPI T 220) of the handsheets were measured using relevant standard methods.
Table 1. Chemical Composition and Density of Maritime Pine and European Aspen experiment
maritime pine
European aspen
holocellulose (%) α-cellulose (%) Klason lignin (%) 1% NaOH solubility (%) hot water solubility (%) cold water solubility (%) alcohol solubility (%) ash content (%) wood density (g/cm3)
70.21 47.11 28.23 9.71 3.67 1.64 4.15 0.27 0.43
82.68 49.03 16.69 15.34 3.04 1.73 3.22 0.31 0.39
Table 2. Fiber Properties of Maritime Pine and European Aspen experiment
maritime pine
European aspen
fiber length (mm) fiber width (μm) lumen width (μm) cell wall thickness (μm) vessel element length (mm) slenderness ratio flexibility ratio Runkel ratio
2.4 43.7 29.5 7.1
1.1 23.9 11.4 6.3 0.6 46.0 47.7 1.1
54.9 67.5 0.5
pine. A similar kappa number increase was observed in kraft pulps of eucalyptus/pine chip mixtures.19,20 This result could be ascribed to higher lignin content of maritime pine wood (Table 1).21 On the other hand, softwoods consist mainly of guaiacyl type (G) lignin, hardwoods also contain up to 50% syringyl type (S) lignin.22 In alkaline pulping, S-lignin has higher reactivity in comparison with G-lignin.23,24 Therefore, higher S/G ratio of raw material means faster delignification.25−27 Higher and lower kappa numbers were determined in the 100% pine and 100% aspen pulping, respectively. The total and screened yields decreased with increasing maritime pine chip ratio in the mixture (Table 3). This is in agreement with kraft pulps of eucalyptus/pine chip mixtures.19 Another study20 noted that total yield of pine/eucalyptus (50:50) pulps was found as 47.31%, while total yield of 100% pine pulps was 43.04%. This finding could be attributed to more lignin content of maritime pine wood compared to European aspen (Table 1).21 Reject ratio in pulping depends on homogeneity of chip thickness, wood species, and other factors. Increase in European aspen chip ratio in mixture lowered the reject ratio (Table 3). This result could be explained with easier penetration of cooking liquor to hardwood chips compared to softwood.28,29 Also, it could be attributed to slightly higher density of pine compared to aspen (Table 2) owing to more difficult penetration of denser wood.30 Pulp viscosity is an indirect measure of fiber strength31 and decrease in fiber strength has a linear correlation with cellulose degradation.32 Increase in pulp viscosity with maritime pine chip ratio used in mixed pulping could be explained by higher fiber strength of maritime pine than European aspen. The beatability of pulps is given in Table 4. Beatability of pulps is very important for the energy consumption of mills and generally depends on the chemical composition of pulps. Pulps with higher hemicellulose content22,33,34 and lower lignin content35 are easier to beat. The pulps having higher
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RESULTS AND DISCUSSION The pulp and paper properties of wood species depend greatly upon their chemical composition and fiber properties. Therefore, the chemical composition (Table 1) and fiber properties (Table 2) of maritime pine and European aspen were determined to better understand the changes occurring in the pulp and paper properties with the changing chip mixing ratio. Pulp properties of Maritime pine and European aspen chip mixtures are given in Table 3. Kappa number of pulps was significantly higher for chips mixture including more maritime 2305
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Table 3. Pulp Properties experiment
Kappa number
screened yield (% o.d. wood)
reject ratio (% o.d. wood)
total yield (%)
viscosity (cm3/g)
100% pine 75% pine 50% pine 25% pine 100% aspen
60.3 48.8 36.9 23.2 12.8
45.2 47.6 50.3 51.2 53.8
1.7 1.4 1.2 0.5 0.1
46.9 49.0 51.5 51.7 53.9
1008.4 971.1 955.7 919.9 894.1
Table 4. Beatability of Kraft Pulps Obtained from Different Mixing Ratios of Maritime Pine and European Aspen Chips
Table 5. Strength Properties of Pulps
mixing ratio (%)
unbeaten freeness (°SR)
beating time up to 35°SR (min)
beating time up to 50°SR (min)
mixing ratio (%)
100% pine 75% pine 50% pine 25% pine 100% aspen
15 15 14 15 18
44.5 39 32 22.5 20.5
54.5 49 41 30 28
100% pine
75% pine
50% pine
hemicellulose content provide an improvement in pulp beating efficiency with shorter beating time. A linear correlation was observed between beating time and maritime pine chip ratio in mixture (Figure 1). Increase in European aspen in the mixture
25% pine
100% aspen
freeness level (°SR)
tear index (mN·m2/g)
burst index (kPa·m2/g)
tensile index (N·m/g)
stretch (%)
15 35 50 15 35 50 14 35 50 15 35 50 18 35 50
19.59 8.82 8.23 15.00 8.66 7.99 12.06 8.01 7.60 8.21 7.29 6.38 5.20 6.38 5.51
2.86 7.05 7.12 2.18 6.79 6.99 1.97 6.39 6.53 1.72 4.90 5.32 1.71 4.71 4.92
32.67 81.27 81.92 28.79 77.87 77.36 27.58 75.71 75.75 27.47 63.87 69.03 27.42 61.13 65.91
2.80 2.99 2.69 2.39 2.91 2.68 2.13 2.89 2.69 1.65 2.83 2.72 1.46 2.75 2.63
bonding ability of fibers. Less flexible fibers do not produce large contact areas for fiber-to-fiber bonding. Besides, lower stretch of aspen paper compared to pine paper could be due to the shorter fibers of aspen.36 In this study, aspen and pine chips were cooked together and aspen chips may be overcooked due to the use of more severe cooking conditions. This is a disadvantage for pulping mixture of softwood and hardwood chips together. Nonuniform cooking of chips may decrease the strength properties of paper.15 An earlier study10 noted that 10% hardwood addition to pulping did not cause strength loss in paper compared to 100% softwood pulping. But, 20% hardwood addition resulted in small loss in paper strength. Another study49 indicated that 20% and 30% softwood addition to hardwood pulps led to increasing in tear index. On the other hand, tear and tensile indices of mixed pulp decreased when hardwood in mixture increased.50 Similar strength losses were reported for spruce/ aspen and spruce/birch mixtures.15 Another study20 noted that tear and burst indices of pine/eucalyptus (50:50) pulps were determined as 11.5 mN·m2/g and 2.3 kPa.m2/g; they were 13.9 mN·m2/g and 3.6 kPa.m2/g for 100% pine pulps, respectively. Air permeability, roughness, and optical properties of handsheets at three different beating levels are listed in Table 6. The air permeability of beaten mixed pulp increased with increasing aspen chip ratio except for 100% aspen pulp. This was probably due to longer fibers of maritime pine (Table 2).41 A similar result was reported in radiate pine and acacia mixed pulp.51 The roughness of unbeaten and beaten mixed pulps decreased with increasing aspen chip ratio. This finding could be explained by shorter and finer aspen fibers (Table 2).41 The bulk of unbeaten and beaten mixed pulps slightly increased with increasing pine chip ratio. Increasing in bulk could be attributed to longer and thicker walled fibers of maritime pine (Table
Figure 1. Relationship between beating time and kappa number of mixed pulps.
resulted in easier beating. A similar response to beating was reported for kraft pulps of eucalyptus/pine chip mixtures.19 This result could be due to higher residual lignin content of 100% maritime pine pulp than 100% European aspen pulp. The relationship between beating time and the kappa number was positively linear, as seen in Figure 1. Some strength properties of pulps at three different beating levels are shown in Table 5. Based on the results, it is clearly visible that tear, burst, and tensile indices of unbeaten and beaten pulps decreased with an increase in aspen in chip mixture. Results also indicated that a linear correlation was found between stretch ratio of paper and maritime pine chip ratio in pulping. The higher and lower strength values were determined in 100% maritime pine pulps and 100% European aspen pulps, respectively. Increase in tear index with the increase in maritime pine chips in the mixture could be attributed to longer,31,35−43 thicker,41,44 and more flexible35 fibers of maritime pine, and also its higher slenderness ratio30,45 (Table 2). On the other hand, lower burst and tensile indices of aspen pulp could be explained by lower fiber flexibility41,46 (lower bonding ability), lower slenderness ratio,45 and higher Runkel ratio47 of aspen fibers (Table 2). Tensile43,48 and burst indices36 depend on 2306
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Table 6. Air Permeability, Roughness, and Optical Properties of Handsheets mixing ratio (%)
freeness level (°SR)
100% pine
15 35 50 15 35 50 14 35 50 15 35 50 18 35 50
75% pine
50% pine
25% pine
100% aspen
air permeability (mL/min)
roughness (mL/min)
bulk (cm3/g)
ISO brightness (%)
opacity (%)
yellowness index
675.72 387.63 389.28 613.61 365.39 366.01 582.02 320.04 330.58 563.03 298.31 288.57 486.01 271.94 264.02
2.03 1.35 1.29 1.97 1.32 1.26 1.88 1.31 1.21 1.83 1.29 1.20 1.78 1.26 1.17
20.22 15.60 14.31 22.30 17.35 16.36 25.21 20.27 18.98 29.01 24.64 22.54 32.35 27.78 24.61
99.91 98.73 97.66 99.83 98.53 97.90 99.67 98.49 97.75 99.56 98.48 97.74 99.28 97.38 95.47
49.80 51.67 53.16 46.63 49.96 50.78 43.98 47.02 49.19 38.82 42.23 44.09 36.85 39.58 42.25
53.30 11.70 71.84 12.30 96.21 18.61 132.03 23.19 90.88 9.51
2).41 In an earlier study, it was revealed that increasing pine ratio in a pine/birch mixed pulp resulted in increasing in the bulk of paper.49 The brightness of unbeaten and beaten mixed pulps tended to increase along with an increase in aspen chip ratio in pulping. Increasing in brightness could be ascribed to a linear correlation between kappa number and pine chip ratio in mixed pulping (Figure 2). Moreover, brightness of all pulps decreased with
on desired pulp and paper properties. Results of this study have demonstrated that higher yield and better beatability of pulps could be possible when European aspen was more in the mixture whereas kappa number of pulp was negatively correlated. Higher amount of European aspen in the mixture resulted in better brightness and smoothness. On the other hand, higher maritime pine in the mixture increased the pulp strength. Consequently, using cheaper and more abundant hardwood species in mixture may result in economic benefits.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +90 378 2235073. Fax: +90 378 2235066. E-mail:
[email protected]). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Ilvessalo-Pfäffli, M.-S. Fiber Atlas: Identification of Papermaking Fibers; Springer-Verlag: Berlin, 1995. (2) Chantre, G.; Perez, D. D. S. ; Najar, M. ; Nougier, P. Wood supply strategies to enhance the maritime pine kraft pulp quality. In Engineering, Pulping and PCE&I Conference, Atlanta, GA, 2004. (3) Worrell, R. European aspen (Populus tremula L.): A review with particular reference to Scotland I. Distribution, ecology and genetic variation. Forestry 1995, 68 (2), 93−105. (4) Perez, D. D. S. ; Fauchon, T. Wood quality for pulp and paper. In Wood Quality and its Biological Basis; Barnett, J. R., Jeronimidis, G., Eds.; Blackwell Publishing CRC Press, 2003, Ch 7. (5) Shackford, L. D. A comparison of pulping and bleaching of kraft softwood and eucalyptus pulps. In 36th International Pulp and Paper Congress and Exhibition; São Paulo, Brazil, 2003. (6) Paavilainen, L. Fiber structure. In Handbook of Physical Testing of Paper; Mark, R. E., Habeger, C. C. Jr., Borch, J., Lyne, M. B., Eds.; Marcel Dekker Inc., New York, 2002. (7) Wathén, R. Studies on Fiber Strength and Its Effect on Paper Properties; KCL Communications 11; KCL: Espoo, Finland, 2006. (8) Robertson, G.; Olson, J.; Allen, P.; Chan, B.; Seth, R. Measurement of fiber length, coarseness, and shape with the fiber quality analyzer. Tappi J. 1999, 82 (10), 93−98. (9) Wang, B.; Li, R.; He, B.; Li, J. The impacts of lignin coverage, relative bonded area, and fiber properties on sheet strength. BioResources 2011, 6 (4), 4356−4369. (10) Hunt, K.; Hatton, J. V. Increased pulp production by use of hardwood in softwood kraft mills. Pulp Pap. Can. 1976, 77 (12), 119− 123.
Figure 2. Relationship between kappa number and brightness of mixed pulps.
increasing beating degree. The brightness losses could be attributed to more even distribution of residual lignin containing highly colored chromophoric groups on fiber surfaces after beating.52 The opacity of unbeaten and beaten mixed pulp slightly increased with increasing pine chip ratio (Table 6). In contrast, a recent study noted that opacity increased with increasing hardwood ratio in a softwood/ hardwood mixed pulp.51 On the other hand, yellowness index of unbeaten and beaten mixed pulp increased with increasing pine chip ratio (Table 6).
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CONCLUSIONS The most important factors affecting pulp and paper properties are chemical composition and fiber properties of wood. These factors show great variability for different wood species. The differences among species sometimes provide advantages or disadvantages in terms of pulp and paper properties depending 2307
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