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Characterization of Jack Pine Early- and Latewood Fibers in Thermomechanical Pulping Fang Huang,*,† Robert Lanouette, and Kwei-Nam Law Centre de recherche sur les materiaux lignocellulosiques, Universite du Quebec a Trois-Rivieres, Quebec QC G9A 5H7, Canada ABSTRACT: The principal objective of this research was to study the morphological changes of Jack pine (Pinus banksiana) earlywood (EW) and latewood (LW) in thermomechanical pulping (TMP). The results indicate that EW fibers tend to separate at the P/S1 interface, whereas LW fibers commonly fail in the regions between the primary wall and the transition layer or outer layer of the secondary wall (P/S1) and between the outer and central layers of the secondary wall (S1/S2). LW fibers exhibit mostly intrawall failure and lower curl and kink indices, whereas EW fibers tend to fail in transwall mode (splitting) and suffer fiber cuttings. In addition, the thin-walled EW fibers show higher collapsibility and conformability than their thick-walled LW counterparts. Moreover, EW fines have higher surface lignin coverage, whereas LW fines have a higher specific volume (SV).
’ INTRODUCTION In the temperate region, when the atmospheric temperature rises in the spring, tree growth activities begin with the division of cells in the cambium. This activity is believed to be regulated by the growth hormone auxin. The early growth is fast and gives rises to fibers or tracheids in conifers (softwood) with large diameters and relatively thin cell walls. This wood tissue is called springwood or earlywood (EW). As the late summer approaches and the temperature falls, tree growth gradually slows, producing fibers with smaller cell lumens and thicker cell walls. This zone of thick-walled fibers is called summerwood or latewood (LW). The tree growth season ends in the fall as trees begin to shed leaves or needles. The most visible differences between EW and LW fibers are that the former tend to have larger diameters, thinner cell walls, and larger radial widths than the latter.1,2 These differences affect not only fiber properties, such as density and mechanical strength,3,4 but also chemical5,6 and mechanical pulping7,8 and paper properties.8 Because of their differences in morphology, EW and LW fibers behave differently in thermomechanical pulping. Studies1,911 have indicated that splitting of the cell walls occurs principally in EW fibers, particularly in the first stage of refining, and that reduction of the wall thickness takes place more often in the thick-walled LW fibers. Moreover, EW fibers tend to change form easily because of their greater compressibility and flexibility. Conversely, LW fibers are more resistant to the refining action including flexion and kneading, which makes changing their cross-sectional shape difficult. It was reported that EW absorbs energy easily and requires more energy to reach the same freeness than LW.8 EW tends to break into fragments of irregular forms and sizes (shives), whereas LW disintegrates into slender bundles or individual fibers.2 Despite these findings, it remains unclear how these two types of wood tissue are transformed from solid wood into individual fibrous elements in refining, especially thermomechanical refining. This study systematically characterized the changes in EW and LW during the thermomechanical pulping (TMP) pulping process, including those in cell-wall thickness, fiber length, r 2011 American Chemical Society
coarseness, curl and kink indices, specific volume (SV) of fines, and water retention value (WRV). Light microscopy and scanning electronic microscopy were used to qualify and quantify fiber fibrillation and cell-wall damage in refining. Electron spectroscopy for chemical analysis (ESCA) was used to analyze the fiber surface coverage of lignin. The results obtained from this research provide a better understanding of the breakdown mechanism of these two types of wood tissue and are expected to help improve refining efficiency.
’ EXPERIMENTAL SECTION Materials and Preparation. Logs of freshly cut Jack pine (Pinas banksiana Lamb.) were used in this study. The Jack pine trees were taken from a 30-year old plantation in the St. Maurice region of Quebec, Canada. EW and LW chips were prepared by means of a chisel, as discussed in previous studies.10 In this study, the basic density (oven-dry weight/volume) of EW/LW chips was measured following TAPPI method T-258 om-02. The cross-sectional features of EW and LW, such as cellwall thickness and lumen area were measured based on TAPPI method T-263 sp-06 and the ImageJ algorithm.12 Some major chemical components, namely, Klason lignin, holocellulose, αcellulose, dichloromethane (DCM) extractives, and ash were determined by TAPPI methods, as listed in Table 1. Chemical Maceration of EW and LW Fibers. To study the fiber characteristics of the starting EW and LW, such as fiber length and cell-wall thickness, it was necessary to liberate the fibers from the wood matrix by means of chemical maceration.13,14 Thermomechanical Pulping. A Sunds Defibrator 300 CD pilot plant (Metso Paper) was used for refining of the wood chips. Its refining capacity is 2 t/day. The models of the refiner rotor and stator plate employed are R3809BG and R3803, respectively. EW Received: September 2, 2011 Accepted: October 10, 2011 Revised: October 6, 2011 Published: October 10, 2011 13396
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Table 1. Test Methods for Chemical Analysis component
test method
fraction
R14
R28
R48
R100
TAPPI T-9 wd-75
EW
0.7
34.0
19.2
10.0
7.5
28.6
TAPPI T-203 om-99
LW
7.6
36.8
16.4
6.9
4.5
27.8
Klason lignin
TAPPI T-222 om-98
Holocellulose α-cellulose
Table 4. Weight Distribution of BauerMcNett Fractions of EW and LW from Pulp with a CSF of 150 mL
dichloromethane (DCM) extractives
TAPPI T-204 cm-97
ash
TAPPI T-211 om-02
R200
P200 (fines)
Table 2. Basic Property of Jack Pine EW and LW Fibers basic
fiber
cell-wall
outer
density
length
thickness
perimeter
area
cell-wall area
(μm)
(μm)
(μm2)
(μm2)
(g/cm3) (mm)
lumen
EW
0.30
3.34
2.12
130
400
240
LW
0.49
3.55
4.75
105
260
350
Table 3. Major Chemical Components of EW and LW Fibers
EW LW
Klason lignin
holocellulose
α-cellulose
DCM extractives
ash
(%)
(%)
(%)
(%)
(%)
28.30 27.09
68.68 71.01
42.34 44.55
1.95 1.62
0.17 0.17
Figure 2. Fiber length reduction of the R28 fraction as a function of specific refining energy.
Figure 3. Cell-wall reduction of the R28 fraction as a function of specific refining energy. Figure 1. Freeness as a function of specific refining energy.
and LW chips were pulped separately. In the process, the chips were presteamed at atmospheric pressure for 10 min and then screw-fed into a digester using a 2:1 compression ratio. The refining was carried out in two stages. The first stage was under pressure at 160 °C, and the pulps were produced with a freeness of about 500 mL. The Canadian standard freeness (CSF) was measured following TAPPI method T-227 om-04. The primary pulps were refined at atmospheric pressure to a freeness range of 50200 mL. The specific energies were recorded for the pulps of each freeness level. The first-stage refining consistency was about 2024%, whereas that at the second stage was around 1014%. After being refined, all the pulp samples were disintegrated with 90 °C hot water to remove latency prior to further analysis.15,16 Fractionation of Pulps. The second-stage pulps were fractionated in a BauerMcNett classifier to obtain six fractions denoted as R14, R28, R48, R100, R200, and P200 (fines). The BauerMcNett fiber classification is a commonly used method to characterize the fiber-length distribution of mechanical pulps. The fibers in different BauerMcNett fraction are
Figure 4. Coarseness of the R28 fraction as a function of specific refining energy.
morphologically different and have different effects on paper properties. For example, the R14 fraction contains long and stiff fibers that have poor bonding characteristics. The fines (P200) fraction comprises flakelike particles and fibrils that strengthen the fiber network. For these reasons, it was necessary to fractionate 13397
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Figure 5. Curl and kink indices of the R28 fraction as a function of specific refining energy.
Figure 6. Specific volume of fines as a function of specific refining energy.
the pulps before characterization. Because of space considerations, only the R28 and P200 (fines) fractions are discussed in this article. FQA Analysis. The Fiber Quality Analyzer (FQA, OpTest Equipment Inc.) employed in this study was an optical device used to measure the length-weighted mean fiber length (llw), fiber coarseness, and fiber curl and kink indices.1719 Cell-Wall Thickness Measurements. Cell-wall thickness is an important morphological characteristic that is related to a fiber’s stiffness or its ability to form interfiber bonds. In this study, the cell-wall thickness was measured by means of a MorFi cell-wall thickness analyzer (Techpap, St.-Martin-d'Heres, France).20 Rejects Analysis. The reject contents were determined by means of a Pulmac shive analyzer equipped with a 0.1-mm screen plate as described in TAPPI test method T-274 sp-97. The rejects were collected for microscopic analysis of fiber types and rupture modes. Specific Volume (SV) Analysis of Fines by Sedimentation. The quality of the fines is an important characteristic of pulp, and it can be qualified in terms of the specific volume (SV) of these particles. The specific volume of the fines was measured as follows: First, 10 g of pulp was diluted to 0.20.3% consistency with demineralized water. The pulp suspension was then passed through a 200-mesh wire into a 2-L dynamic draining jar (DDJ). The filtrate containing fines was condensed to a consistency of 0.40.5% by centrifugation. In this work, 50 mL of fines suspension was first mixed with a solution containing 120 mg/L Na2SO4 and 30 mg/L CaCl2 (mixed in a volume ratio of 1:2) to equilibrate the ion content, and then the pH was adjusted to 66.5 with 1 g/L NaOH or 1 g/L HCl.11 Next, the suspension was transferred into a 100 mL graduated glass cylinder, and the surplus was discharged. Before
Figure 7. Water retention value of the R28 fraction as a function of specific refining energy.
the settling test, the air in the suspension was removed with a vacuum pump for 10 min. After air removal, the cylinder was sealed with parafilm and manually shaken well to disperse the suspension. Then, the suspension was kept still for 24 h, after which the volume of the settled fines was read (accuracy of (1 mL), and the suspension was filtered to recover the fines. The mass of fines was obtained after they had been dried at 105 °C; the weight of the recovered fines was used to determine the SV value. The ratio of the volume of the fines suspension to the oven-dry weight of the recovered fines is defined as the specific volume (SV) of the fines. Water Retention Value (WRV) Determination. The WRV reflects the fiber cell-wall damage occurring during refining, such as splitting and internal fibrillation (delamination). The WRV was determined using TAPPI method UM 256. Microscopy Analysis. This study employed both light microscopy and scanning electronic microscopy (SEM). Light microscopy and image analysis were used to quantitatively evaluate fiber cross-sectional characteristics in refining, including cell-wall damage21 and collapsibility.22 In this study, the proportion of fibers damaged in the cell wall was assessed by counting the fibers present in the features of interest, namely, transwall and intrawall failures. The form factor (FF) was used to evaluate the fiber collapsibility.23,24 It is defined as FF ¼
4πAt P2
where At is the area of filled fibers (fiber wall area + lumen area) and P is the outer perimeter of the cell wall without fibrils. 13398
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Table 5. Failure Percentages in Refining intrawall failure (%)
transwall failure (%)
EW
0.3
26.3
LW
15.3
0.6
’ RESULTS AND DISCUSSION
Figure 8. Cell-wall failure of EW and LW fibers in refining: (A) intrawall failure in an LW fiber, (B) transwall failure in an EW fiber.
The samples for light microscopy analysis were prepared as follows: First, the fibers were aligned,25 and then the aligned fibers were progressively dehydrated using waterethanol mixtures and impregnated with a resin medium.26 The resin-cured samples were sectioned into 3-μm-thick sections using a sledge microtome. The thin sections were stained with Toluidine Blue O (T161, Fisher Scientific Co.) and then mounted on microscope slides for observation using a Zeiss photomicroscope. SEM (JEOL, JSM-500) was employed to qualitatively examine fiber surface failures, particularly exposures of the outer layer of the secondary wall (S1), the central layer of the secondary wall (S2), and the interface between the outer and central layers of the secondary wall (S1/S2). Fibers were air-dried and coated with coal and gold prior to SEM analysis. Electron Spectroscopy for Chemical Analysis (ESCA). During refining, fiber cell walls are peeled off in different ways (e.g., transwall and intrawall), which influences their surface chemical composition, especially in terms of the lignin. ESCA was used to study the fiber surface coverage of lignin. ESCA measurements were performed on an AXIS ULTRA electron spectrometer equipped with a monoenergetic Al X-ray source. The analyzed area was 2 2 mm, and the angle from the X-ray detector to the sample was 45°. Peak intensities were determined by peak area integration. The sensitivity factors used were 0.278 for C 1s and 0.780 for O 1s. The matching of the C 1s peaks was carried out with a Gaussian curve-fitting program. Sheets with a layer grammage of 60 g/m2 were made for the ESCA analysis. In the ESCA technique, the chemical composition is normally evaluated using the C 1s and O 1s peaks by calculating the total O/C ratio. The surface coverage of lignin, ϕlignin, was calculated from the O/C atomic ratio as27 "
ϕlignin
O=Cðpulp sampleÞ O=CðcarbohydrateÞ ¼ 100 O=CðligninÞ O=CðcarbohydrateÞ
#
where O/C(carbohydrate) = 0.833 and O/C(lignin) = 0.333. Statistical Analysis. For statistics reasons, at least 300 fibers per sample were measured. The standard error for each analysis was (5%.
Physical Properties. Table 2 indicates that EW fibers have thinner cell walls and larger lumens than their LW counterparts. The cell-wall thickness of LW (4.75 μm) is more than twice that of EW (2.12 μm). EW fibers have greater outer perimeters and lumen areas than LW fibers. LW fibers have larger cell-wall areas because of their thicker cell walls and smaller lumens. As a result, the density of LW (0.49 g/cm3) is greater than that of EW (0.30 g/cm3). In addition, LW fibers (3.55 mm) are generally longer than EW fibers (3.34 mm). These findings are well in line with those reported earlier.28 Chemical Composition. The experimental data on the chemical components of EW and LW of Jack pine are presented in Table 3. Because of its thicker cell walls, LW has about 5.2% more α-cellulose than EW. However, EW has a 4.5% higher lignin content than LW, which is probably due to its relatively thicker lignin-rich compound middle lamella (CML), as explained by Fengel.29 Interestingly, EW shows 20.4% higher dichloromethane (DCM) extractives than LW. This difference in DCM extractives might be due to the fact that EW contain more resinrich canals than LW.30 In contrast, there are no significant differences in the ash content between EW and LW. Refining Energy. Figure 1 clearly shows that, at a given freeness value, refining EW was found to require more energy than refining LW. As discussed later, EW was defibrated into pulp fibers with relatively little fibrillation compared to LW. As a result, the EW pulp had higher freeness for a given energy consumption. This finding is in agreement with that reported by other researchers.8 Weight Distribution of BauerMcNett Fractions. The weight distributions of BauerMcNett fractions of EW and LW fibers in pulp with a CSF of 150 mL are listed in Table 4. It can be seen that the LW fibers had a much higher content of long fibers (R14 fraction) than the EW fibers. This might be because the original LW fibers were longer than the EW fibers and/or the thin-walled EW fibers suffered more fiber cutting during refining than the thick-walled LW fibers. In contrast, there were no significant differences between EW and LW fines. Fiber Quality Analysis. Fiber Length. As can be seen in Figure 2, as the specific energy increased, the fiber length decreased for all EW and LW fibers. The reduction in fiber length was determined from a comparison with the initial fiber length in the wood. Because the thin-walled EW fibers are more flexible and collapsible than the thick-walled LW fibers, they experienced a greater reduction in mean fiber length than the LW fibers. Cell-Wall Thickness. Cell-wall thickness is an important morphological characteristic that affects fiber stiffness and, as a result, influences interfiber bonding. Data on cell-wall thickness provide information on how the fibers respond to the refining actions. As indicated in Figure 3, the cell-wall thicknesses of fibers in the R28 fractions of EW and LW fibers decreased with increasing refining energy. The reductions in cell-wall thickness shown in Figure 3 were based on the initial cell-wall thickness in the wood samples. They indicate that the thick-walled LW fibers suffered a greater reduction 13399
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Figure 9. Surface nature of EW fibers in refining.
than the thin-walled EW fibers, which means that the LW fibers exhibited more cell-wall peeling and external fibrillation than the EW fibers under the same refining conditions. Fiber Coarseness. Changes in fiber coarseness constitute another feature of fiber development. As seen in Figure 4, the coarseness of the EW and LW fibers decreased with increasing refining energy, with the fiber wall being peeled off progressively as the refining action proceeded. Evidently, the LW fibers were coarser than the EW fibers because of their initial thicker cell walls. Greater coarseness means fewer fibers per gram of pulp and, consequently, fewer surfaces for fiber bonding.8 Curl and Kink Indices. Changes in the curl and kink indices of the EW and LW fibers in refining are shown in Figure 5. It can be observed that the curl and kink indices increased with increasing specific refining energy. Note also that the thin-walled EW fibers had higher curl and kink indices than the thick-walled LW fibers, indicating that the former are more curlable or more flexible than the latter. This finding implies that LW fibers are more resistant to the mechanical actions of refiner bars. Specific Volume (SV) of Fines. Measurements of the specific volume (SV) of fines, which indicates the physical nature of the fines fraction, provide useful information on the mechanism of the breakdown of EW and LW during refining. Figure 6 shows that the SV of fines increased with increasing refining energy because more fibrils were generated at higher refining energies. The increase in fibril component in the fines led to an increase in SV. In addition, refining of the thick-walled LW fibers produced more fibril elements than that of the thin-walled EW fibers. As a consequence, the LW fines had a higher SV than the EW fines. Water Retention Value (WRV). The water retention value (WRV) is a useful parameter for evaluating the water holding capacity or wetness of fibers.31 The WRV of pulp is related to both the external fibrillation and the internal fibrillation or delamination of cell wall. Splitting and delamination of the cell wall under refining action facilitate water absorption, increasing the water holding capacity of fibers. Therefore, the WRV reflects the refining response of the fibers: external fibrillation, cell-wall splitting, and internal fibrillation (delamination). All of these effects increase the swellability of the fibers and interfiber bonding.19 Figure 7 shows that the EW fibers had a higher WRV than the LW fibers. According to Law,32 internal fibrillation is mostly influenced by compression forces in refining. Under such forces, the thin-walled and flexible EW fibers collapsed, their cell walls fractured, and cell corners became separated. These changes
Table 6. Form Factors (FFs) of EW and LW Fibers in Refining form factor EW
0.48
LW
0.69
would improve the fibers’ capability of absorbing of water. On the other hand, the thick-walled LW fibers, being more rigid and resistant to compression forces, tended to retain their form by separating in interfiber mode and exhibiting little internal fibrillation.33 Consequently, it is understandable that the EW fibers had higher WRVs than the LW fibers and those in mixed samples. Microscopy Analysis. Cell-Wall Damage. As shown in Figure 8, fiber failure could be of two types: transwall and intrawall. In this study, the percentage of each type of failure was assessed using a light microscope. As seen in Table 5, most of the transwall failures occurred in EW fibers, whereas the intrawall failures took place uniquely in LW fibers. Under the shear and compression actions, the thinwalled and flexible EW fibers tended to collapse (as shown in Figure 9) and split, resulting in transwall failure. On the contrary, the thick-walled and rigid LW fibers, which are more resistant to mechanical forces, retained their form and exhibited intrawall failures. Cross-Section Deformation. Great differences between EW and LW fibers were observed in terms of cross-section characteristics. Knowledge on the cross-section deformations of fibers is useful for assessing the overall quality of the fibers. Such information would helpful in determining whether special treatment should be given to some fibers to improve papermaking properties and in predicting the effects of processing variables on the fibers. Cross-sectional parameters, such as the form factor (FF), reflect the fiber flexibility and collapsibility, which are essential for paper consolidation.3436 Table 6 shows that the EW fibers had a lower FF than the LW fibers, which means that the former had a higher collapsibility. Under the compression forces of refining, the thin-walled EW fibers were readily deformed and collapsed. Because the thickwalled LW fibers were rigid and resistant to mechanical forces, they tended to retain their forms after refining. Fiber Surface Failure. After refining, the EW fibers were completely collapsed, diminishing their lumen volume, as shown 13400
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Figure 10. Surface nature of LW fibers in refining.
Figure 11. Surface lignin coverages of different samples.
in Figure 9. In fact, the compression and shear forces left the EW fibers split and twisted. The majority of the failures took place at the interface between the primary wall and the transition layer or outer layer of the secondary wall (P/S1), especially around the pits. Failures at the interface between the outer and central layers of the secondary wall (S1/S2) were also occasionally noticed. Fiber external fibrillations associated with shear forces were not evident in EW fibers; a few fibrils were occasionally observed. For LW fibers, the compression forces had limited effects on the change in lumen dimensions because collapsed fibers were rarely observed (Figure 10). Most of the LW fibers showed exposed surfaces with various natures, such as the primary wall (P), the outer layer of the secondary wall (S1), and the central layer of the secondary wall (S2). However, S1/S2 separation was commonly seen in LW fibers. Fibrils were frequently seen along the cell walls of LW fibers. Some long and ribbonlike layers were detached from the cell walls. Surface Lignin Coverage. ESCA is a useful and efficient means for characterizing the surface chemical nature of fibers. The principle of this analysis is based on the fact that the lignin concentration decreases gradually across the fiber wall, with the highest concentration being in the middle lamella. At the same time, the concentration of cellulose, which is practically absent in the middle lamella, increases from the primary wall toward the inner secondary layer.29 In this study, ESCA was conducted on two types of pulp samples including the whole pulp without fines (fines-free pulp) and the fines. Because the fines include flakelike particles and fibrils37,38 generated from the fiber surface and the secondary
layer, respectively, analysis of both the fines and fines-free pulp permit an understanding of the mode of fines formation. As shown in Figure 11, the surface lignin coverage of the fines was always higher than that of the fines-free pulp for both EW and LW. This is not surprising because the flakelike fines are the materials detached from the outer layer of cell wall. Because the lignin-rich middle lamella is the outermost layer of the cell wall, it is always the first to be peeled from the cell wall. As a result, the surface lignin coverage is higher in the fines than in the fines-free pulp. The surface lignin coverage of EW pulp was found to be higher than that of LW pulp for both the fine-free pulp and the fines. This finding seems to support the conclusion that the thin-walled EW fibers break down more readily with less surface fibrillation. The reduced fibrillation means that the surface of the fibers maintained more lignin than the LW fibers that suffered more fibrillation. On the other hand, EW fibers had greater surface perimeters and hence produced more lignin-rich flakes in terms of the surface. This might be another reason that the EW fines had higher lignin contents than the LW fines.
’ CONCLUSIONS LW fibers of Jack pine have a cell-wall thickness of 4.75 μm, which is approximately twice that of EW fibers (2.12 μm), and a cell diameter (radial width) one-half that of EW. Because of their morphological differences, EW and LW fibers behave differently in thermomechanical pulping (TMP). Studies based on the fiber fractions R28 and fine P200 revealed the following: The thin-walled EW fibers tend to separate in the P/S1 interface and show little external fibrillation. In contrast, thick-walled LW fibers commonly fail in the P/S1 and S1/S2 regions, generating considerable amounts of external fibrillation. LW fibers exhibit mostly intrawall failure and lower curl and kink indices, whereas EW fibers tend to fail in transwall mode (splitting) and show higher curl and kink indices. As a result, LW yields higher fibers length, whereas EW suffers more fiber cutting. In addition, the thin-walled EW fibers show higher collapsibility and conformability than their thick-walled LW counterparts. Moreover, EW fines have higher surface lignin coverage, whereas LW fines have a higher specific volume (SV). ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 13401
dx.doi.org/10.1021/ie2019992 |Ind. Eng. Chem. Res. 2011, 50, 13396–13402
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School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
’ ACKNOWLEDGMENT The authors gratefully appreciate the financial support from the Natural Science and Engineering Research Council of Canada (NSERC). ’ REFERENCES (1) Reme, P. A.; Johnson, P. O. Changes Included in Early- and Latewood Fibers by Mechanical Pulp Refining. Nordic Pulp Paper Res. J. 1999, 14 (3), 256–262. (2) Law, K. N. Mechanical Behaviour of Early- and Latewood under Compression Load. Proc. Intl. Mech. Pulp Conf. 2001, 159–166. (3) Gamov, V. V. Tensile Strength of Larch Spring- and Summerwood in the Direction Perpendicular to the Fibers. Izv. Vuz. Lesnoi. Zh. 1975, 18 (4), 161–163. (4) Hartler, N.; Nyren, J. Influence of Pulp Type and Post-treatments on the Compressive Force Required for Collapse. In The Physics and Chemistry of Wood Pulp Fibers; Page, D. H., Ed.; STAP No. 8; TAPPI Press: Atlanta, GA, 1969; Vol. 265270; pp 271277. (5) Jones, T. G.; Richardson, J. D. Relationships between Wood and Chemimechanical Pulping Properties of New Zealand Grown Eucalyptus Nitens Trees. Appita J. 1999, 52 (1), 51–56. (6) Law, K. N.; Valade, J. L.; Daneault, C. Chemimechanical Pulping of Tamarack, (2). Effects of pH and Sodium Sulfite. Cellular Chem. Technol. 1989, 23 (6), 733–741. (7) Reme, P. A.; Helle, T. Quantitative Assessment of Mechanical Fiber Dimensions during Defibration and Fiber Development. J. Pulp Paper Sci. 2001, 27 (1), 1–7. (8) Murton, K. D.; Richardson, J. D.; Corson, S. R.; Duffy, G. G. TMP Refining of Radiata Pine Earlywood and Laterwood Fibers. Proc. Intl. Mech. Pulp Conf. 2001, 361–371. (9) Reme, P. A.; Helle, T. Fiber Characteristics of Shives Initiating Web Rupture. Nordic Pulp Paper Res. J. 2000, 15 (4), 287–291. (10) Huang, F.; Lanouette, R.; Law, K.; Li, K. Microscopic Analysis of Early- and Latewood in Thermomechanical Pulp Refining. Appita J. 2008, 61 (6), 445–449. (11) Lanouette, R.; Law, K.; Huang, F. Impact of Early- and Latewood on Thermomechanical Pulping. Appita J. 2010, 63 (2), 120–125. (12) Rasband, W. S. ImageJ: Image Processing and Analysis in Java; U. S. National Institutes of Health: Bethesda, MD, 19972011; available at http://www.ninds.nih.gov/ (accessed Nov 2005). (13) Franklin, C. L. Preparing Thin Sections of Synthetic Resin and Wood Composites and a New Maceration Method for Wood. Nature 1945, 155, 51–54. (14) Law, K. N.; Kokta, B. V.; Mao, C. B. Fibre Morphology and Soda-Sulphite Pulping of Switchgrass. Bioresour. Technol. 2001, 77, 1–7. (15) Beath, L. R.; Neill, M. T.; Masse, F. A. Latency in Mechanical Wood Pulps. Pulp Paper Mag. Can. 1966, 67 (10), T423–T430. (16) Karnis, A. Latency in Mechanical Pulp Fibers. Paperi Ja Puu 1993, 75 (7), 505–511. (17) Bently, R. G.; Scudamore, P.; Jack, J. S. A Comparison between Fiber Length Measurement Methods. Pulp Paper Can. 1994, 95 (4), 41–45. (18) Page, D. H.; Seth, R. S.; Jordan, B. D.; Barbe, M. C. Curl, Crimps, Kinks and Microcompressions in Pulp Fibers—Their Origin, Measurement and Significance. In Transactions of the 8th Fundamental Research Symposium; Punton, V., Ed.; Mechanical Engineering Publications Ltd.: London, 1985; pp 183227. (19) Law, K. N.; Yang, K. C.; Valade, J. L. Fiber Development in Thermomechanical Pulping: Comparison between Black Spruce and Jack Pine. In Preprints of the CPPA Technical Section Annual Meeting; PAPTAC: Montreal, Canada, 1997; pp B113B127.
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