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Thursday, 20 October 2016

Chapter V Feasibility of Using Larch as Raw Material for Medium Density Fiberboard Panel Manufacturing


Table des matières
Authors: Jun Li Shi, Bernard Riedl, and S. Y. Zhang
La possibilité d'employer le bois de mélèze comme matériau de base pour la fabrication des panneaux de fibres de densité moyenne a été étudiée. Des panneaux de fibres de densité moyenne ont été fabriqués en laboratoire à partir de fibres produites du bois de mélèze (mélange de deux espèces Larix gmelinii et Larix sibirica ), et des panneaux faits à partir des fibres tirées d’épinette, pin, et sapin (É-P-S) ont été étudiés comme contrôle. 10 % de résine d’urée-formaldehyde (UF) et 0,5 % de cire ont été mélangés avec les fibres produites à partir du mélèze et du bois de É-P-S. Le programme de pression et la température utilisés pour la pressage des panneaux ont été maintenus pour les deux types des panneaux. Selon l’american national standard ANSI A 208.2-2002 pour l’évaluation des panneaux de fibres de densité moyenne pour application intérieure, le module de rupture (MOR), le module d'élasticité (MOE), et la cohésion interne (IB) des panneaux faits à du bois de É-P-S satisfont aux conditions de la catégorie 120. Les MOR et MOE des panneaux faits à partir du bois de mélèze étaient inférieurs à ceux des panneaux faits à partir de É-P-S. Cependant, cette différence peut être compensée en optimisant les paramètres de pressage afin d'acquérir un profil de densité optimisé dans les panneaux. De plus, les propriétés des panneaux faits à partir du bois de mélèze ont pu être améliorées par manipulation de paramètres de raffinage pour réduire la proportion de fines dans la pâte. On peut conclure que le mélèze est utilisable comme matériau de base pour la fabrication de panneau de fibres de densité moyenne, bien qu’il soit inférieur au groupe É-P-S.
Feasibility of using two exotic larch species Larix gmelinii and Larix sibirica as raw material for medium density fiberboard (MDF) panel manufacturing was studied. Laboratory MDF panels were fabricated from fibers of two individual larch species Larix gmelinii and Larix sibirica mixed with an approximate proportion of 4:1, and the panels made from fibers of spruce, pine, and fir (S-P-F) were studied as a control. 10 % urea-formaldehyde (UF) resin and 0.5 % wax were blended with the fibers that were generated from larch and S-P-F wood. The schedule and temperature used for panel pressing were kept constant when making the two types of panels. According to American National Standard ANSI A 208.2-2002 for evaluation of MDF panels for interior application, modulus of rupture (MOR) of panels made from larch and S-P-F both met the requirement of Grade 120. MOE of panels made from larch did not meet the ANSI minimum requirement; MOE of panels made from S-P-F met the requirement of Grade 120. Larch panels produced favorable IB strength compared to those panels made from S-P-F. IB of larch panels was 0.78 MPa, and met the requirement of ANSI grade 140. MOR and MOE of panels made from larch wood were slightly lower than those of panels made from S-P-F. This difference can be compensated through optimizing hot-pressing parameters so as to acquire desirable panel density profile. According to ANSI/AHA A 135.4-1995 standard for basic hardboard, thickness swell and water absorption of the two types of panels were below the maximum specified values. It can be concluded that it is feasible to use the two exotic larch species as raw material for MDF panel manufacturing, although they are still inferior to S-P-F.
For a couple of decades, the shortfall of wood and fiber supply that has been experienced by papermaking and composites industries in North America has required extension and diversification in raw material. Recently, composite mills are paying attention to proper utilization of forest residues such as commercial thinnings, tops, branches, etc. as raw materials for composite panel manufacturing (Li et al. 1991). Even tree barks, although it is known that composite panel properties would be very adversely affected (Woodson 1976), have been often studied in recent years in order to develop new technologies for acquisition of qualified panel products.
There are ten species of larch ( Larix ) located mostly in colder climates in the northern hemisphere. Three larch species can be found in North America: the alpine larch ( Larix lyallii ) distributed primarily at high elevations in western Canada; the eastern larch ( Larix laricina ) distributed in boggy areas of the northern forests of eastern North America; and western larch ( Larix occidentalis ) distributed in the American West. The potential role of exotic larch in future sustainable fiber supply cannot be ignored since some introduced larch species such as Japanese larch ( Larix leptolepis ) and European larch ( Larix decidua ) often outperform Canadian native larch, pine, and spruce spieces and can produce two to three times more wood and fiber in the Lake States and southern parts of eastern Canada (Vallee and Stipanicic 1983; Fowler et al. 1988; Palmer 1991).
In this study, MDF panels were made from two individual exotic larch species Larix gmelinii and Larix sibirica . Species Larix sibirica is naturally distributed in west and central Siberia, Russia. Species Larix gmelinii has a larger natural distribution, dispersed from eastern Siberia to northeast China. These two species were introduced to northern Ontario, Canada in 1981. Seeding trials were established in six sites: Fort Frances, Kapuskasing, Ottawa, Espanola, Lindsay, and Huronia. A commercial thinning was carried out when the tree age was around 10-15 years, and the thinnings were collected and transported to Forintek Canada Corp. (Québec, Canada) for wood property characterization and panel manufacture. As a control, properties of MDF panels made from a mix of spruce, pine, and fir (S-P-F) were evaluated as well. S-P-F is a favorable material that is often utilized for MDF panel production in Québec, Canada. The aim of the study was to investigate the feasibility of using the two exotic larch species Larix gmelinii and Larix sibirica for MDF panel manufacturing.
Two larch species Larix gmelinii and Larix sibirica were introduced to Canada and seeded in northern Ontario in 1981. A commercial thinning was operated when the trees were at 10-15 years of age, and the thinnings were collected and transported to Forintek Canada Corp. Hence these larch thinings were mostly juvenile wood. It should be noted that they probably differ from mature larch material. The two individual species Larix gmelinii and Larix sibirica were mixed with an approximate proportion of 4:1. Larch trees were debarked, and a portable chipper was used to produce wood chips. Spruce, pine, and fir (S-P-F) wood chips were collected from a composite panel mill located in Québec, Canada. 
Larch and S-P-F wood chips were fed into a pressurized refiner for fiber conversion. When refining, 30 minutes of transition time was retained to avoid mixing of the two different types of fibers. The refining speed was 2500 rpm, and kept constant for larch and S-P-F wood chips. The specific refining energy spent on larch and S-P-F wood was 125 KWh/t and 182 KWh/t. The clearance between the two plates was 0.038 mm and 0.020 mm for larch and S-P-F, respectively. Normally, the refining energy and clearance between the two plates determine the performance of the generated fibers (Maloney 1993). These two parameters were set differently for larch and S-P-F wood since we attempted to produce fibers with less fiber bundles and broken fibers. The produced fibers were visually checked for the performance at the beginning of refining, and the parameters were adjusted and then fixed accordingly. Larch and S-P-F wood were refined without resin and wax injection.
The two types of fibers were dried in a laboratory-scale dryer until the moisture content was 2-3 %. Before switching to another type of fiber, the walls of the dryer were cleaned up to prevent the two types of fibers from mixing. The dried fibers were passed through a hammer mill immediately to make the fibers fully separated from each other, so the resin and wax can be sprayed more uniformly onto the fibers. Resin and wax were injected to the fibers in a laboratory-scale blade blender. It is known that the properties of composites can be improved by applying more resin to panels (Maloney 1993). Therefore, a relatively low resin level was used in this study in order to avoid any concealment of difference in panel properties. Borden 302 urea formaldehyde (UF) resin with 65 % solid content was first diluted to 10 %. The diluted UF resin and 0.5 % wax were slowly sprayed onto the fibers. The fibers blended with UF resin and wax were passed through the hammer mill once again to disperse fiber balls that were formed during blending. The furnish moisture contents after resin and wax blending for larch and S-P-F were 11.1 % and 14.8 %. Panels were manufactured at a target density of 740 kg/m3, and panel size was 610×610×12 mm. Mats were hand formed in a frame without fiber orientation. Three panels were made for each type of fibers. MDF panels were hot-pressed using the same schedule at the pilot plant of Forintek Canada Corp. The platens were closed within 160 s. Panels were kept pressing for another 160 s and the platens were gradually opened in 40 s. The temperature of the two platens was set at 135 oC. The press settings were initiated based on realization of a flatter density profile, which can reduce the differences in panel properties resulted from different density gradient through panel thickness. 
Before cutting the specimens from each panel, the panels were conditioned in a chamber at 22 oC and 65 % relative humidity (RH) for four weeks until the panels reached equilibrium moisture content. The size of the specimens for bending property testing was 338×75 mm, and three were cut from each panel; ten internal bond (IB) specimens with a size of 50×50 mm were cut from each panel, producing totally thirty for each type of panel. The dimension of the specimens for linear expansion (LE) was 305×76 mm, two were taken from each panel and totally six for each type of panel. The LE specimens were stored in a conditioning chamber at 22 oC and 50 % RH for four weeks until their weight remained constant; the length of the specimens was measured. Then, the specimens were conditioned in another chamber with 80 % RH at the same temperature for another four weeks until they reached equilibrium moisture content; and the length of the specimens was recorded. LE was determined using the change in length divided by the length measured from the specimens equilibrated at 22 oC and 50 % RH. Thickness swell (TS) and water absorption were tested on the specimens with a dimension of 152×152 mm; two specimens were cut from each panel and six were prepared for each type of panel. Before soaking the specimens in water, the thickness and weight were recorded. After 24 h water-soaking, the thickness and weight of the specimens were measured again. TS and water absorption were determined by the variations in thickness and weight divided by the thickness and weight measured from the specimens equilibrated at 22 oC and 65 % RH. The procedures and methods described in the standards ASTM D 1037-99 (2001) and ANSI A 208.2-2002 (2002) were followed for modulus of elasticity (MOE), modulus of rupture (MOR), IB, LE, TS, and water absorption testing. The surfaces of IB specimens were sanded for 1.5 mm before the specimens were glued with the blocks. Density profiles were measured from IB specimens before they were sanded. 
Duncan’s multiple-range test was performed to compare the mean properties of MDF panels made from larch and S-P-F wood. The statistical software Statistical Analysis System (SAS) (1990) was used for the data analysis.
Fiber size distribution was determined using Bauer-McNett classifier while the procedures described in Tappi 233 cm-95 (1995) were followed. Size distribution of the fibers produced from larch and S-P-F wood chips is presented in Table 5-1.The percentages of fibers distributed in the mesh size ranges of 0.828-3.240 mm2 and 0.017-0.281 mm2 were comparable for larch and S-P-F. There were 15.4 % and 16.7 % fibers distributed in 0.828-3.240 mm2 size range, and 8.9 % and 7.3 % in 0.017-0.281 mm2 size range (Table 5-1). However, fibers distributed in >3.240 mm2, 0.281-0.828 mm2, and <0.017 mm2 mesh size ranges apparently differed for larch and S-P-F. Fibers refined from S-P-F wood contained more large particles than those produced from larch wood, which is shown in column ‘>3.240 mm2’ in Table 5-1. Meanwhile, the refining process produced more fines from larch wood than it did from S-P-F wood chips, which can be known from the percentages of fibers distributed in ‘<0.017 mm2’ range (Table 5-1). The more fines in the pulp can have a negative effect on panel properties (Barnes 2002; Woodson 1976). The percentage of larch fibers (26.3 %) distributed in 0.281-0.828 mesh range was higher than that of fibers refined from S-P-F wood (15.9 %) (Table 5-1). More fines weakens fiber to fiber bond, which is due to more resin consumption of the fine fibers (Barnes 2002; Woodson 1976). The higher fine fiber content of larch pulp may affect panel mechanical properties adversely. Nevertheless, higher content of large particles in S-P-F pulp may also have a negative impact on panel properties since it can destroy the overall uniformity of resin distribution on fiber surfaces. 
Panel density profile influences panel properties significantly, especially panel mechanical properties (Wang and Winistorfer 2000; Wang et al. 2001; Wang et al. 2004). High peak density leads to superior bending properties, while high core density results in good IB strength (Woodson 1976). Figure 5-1 shows the average density profiles of larch and S-P-F panels. The peak density of S-P-F panels was slightly higher than that of larch panels and face layer of the former was thinner than the latter, which may slightly improve the bending properties of S-P-F panels. The core density of the two types of panels remained equal. Since the hot-pressing parameters were kept constant, the slight difference in panel density profile between the two types of panels may be mainly due to: 1) the difference in wood density and panel density, 2) different mat moisture content, 3) mat in-plane variation, etc. Low density of face layer can result in low bending properties for both types of panels. 
The density of larch wood is higher than that of S-P-F wood (Table 5-2). Lower density of S-P-F wood can be advantageous when fabricating composite panels. Wood species with low density have the ability to yield higher compaction ratio. Compaction ratio is defined as the ratio of panel density to wood density (Maloney 1993). Normally, good panel mechanical properties were associated with higher compaction ratio (Shupe et al. 1999; Pugel et al. 1989). In Table 5-2, the lower wood density and higher average panel density gave higher compaction ratio to the panels made from S-P-F. The resulting higher compaction ratio of S-P-F panels can bring better mechanical properties to such panels when compared to the panels made from larch wood. 
MOR and MOE of panels made from larch were significantly lower than those of panels made from S-P-F by Duncan’s multiple-range test (Table 5-2). As previously stated, lower compaction ratio and higher fine fiber content in the pulp can negatively influence the bending properties of larch panels. However, the larch panels yielded significantly higher IB than S-P-F panels did (Table 5-2). This could be attributed to other reasons such as fiber chemical properties. Fiber pH and buffering capacity are two important factors influencing the curing rate of UF resin when panels are pressed under heat. If UF resin is used for panel making, higher pH and lower base buffering capacity are always advisable (Maloney 1993; Johns and Niazi 1980). The pH and base buffering capacity of larch and S-P-F fibers were measured using Johns and Niazi’s method (1980). Results show that pH of larch fiber (4.39) was higher than that of S-P-F fiber (4.05), and the base buffering capacity of the two types of fibers were 1.92 ml and 1.83 ml, respectively. The lower pH of S-P-F fiber can result in pre-cure of the UF resin at the initial pressing stage, which weakens IB strength.
LE, TS, and WA are important panel properties reflecting panel stabilities in length with changing relative humidity and either in thickness or in weight when the panel is soaked in water. The LE and TS of larch panels were significantly higher than those of panels made from S-P-F at the 0.05 significance level (Table 5-2). But there was no significant difference in WA between the two types of panels. In the ANSI A 208.2-2002 standard, there are no performance requirements for LE, TS, and WA. According to the maximum TS and WA properties specified by ANSI/AHA A 135.4-1995 (1995) for basic hardboard, which are 25 % and 35 %, respectively, TS and WA of the panels made from larch and S-P-F were below the maximum levels.
According to American National Standard (2002), MOR of panels made from larch and S-P-F wood chips both met the requirement of Grade 120. MOE of panels made from larch did not meet the ANSI minimum requirement, while MOE of panels made from S-P-F wood chips met the requirement of Grade 120. IB of larch panels met the requirement of ANSI Grade 140, but S-P-F panels met the requirement of Grade 120. Although MOR and MOE values of larch panels were lower than those of panels made from S-P-F, this difference can be compensated through optimizing hot-pressing parameters so as to acquire desirable panel density profile. In addition, attention should be paid to the refining process in order to produce larch fibers with less fine content. LE of panels made from larch was higher that that of panels fabricated from S-P-F, which may be a problem when the larch panels are applied to a moist environment.
Using panels made from S-P-F wood chips as a comparison, it is found that it is feasible to use a mixture of two individual exotic larch species Larix gmelinii and Larix sibirica aged between 10 to 15 years as raw material for MDF panel manufacturing. Larch panels had favorable IB strength compared to those panels made from S-P-F. IB of larch panels was 0.78 MPa, and met the requirement of ANSI grade 140. MOR values for panels made from larch and SPF were 18.6 MPa and 23.1 MPa, respectively, and both met the requirement of ANSI Grade 120. MOE of larch panels was 1276 MPa, which did not meet the ANSI minimum requirement. MOE of panels made from S-P-F wood chips met the requirement of ANSI Grade 120. Bending properties of larch panels were lower than those of panels made from S-P-F, but those properties can be improved by adopting an optimal hot-pressing scheme. Compared to the panels made from S-P-F, the large LE of larch panels appears to be a problem. Modifications should be taken into consideration in order to enhance larch panels in linear stability. TS and WA of panels made from larch and S-P-F were below the maximum levels specified in the ANSI/AHA A 135.4-1995 standard for basic hardboard.
American Hardboard Association (AHA). Basic hardboard . ANSI/AHA A 135.4-1995. AHA, Palatine, IL. 1995.
American National Standards Institute (ANSI). Medium Density Fiberboard (MDF) for Interior Application . ANSI A 208.2-2002. Composite Panel Assoc., Gaithersburg. MD. 2002.
American Society of Testing and Materials (ASTM). Evaluating properties of wood-based fiber and particle panel materials . ASTM D 1037-99. Vol. 04.10. ASTM, Philadelphia. PA. Pp. 141-170; 2001.
Barnes, D. A. A model of the effect of fines content on the strength properties of oriented strand wood composites . Forest Prod. J. 52(5): 55-60; 2002.
Fowler, D. P., Simpson, J. D., Park, Y. S., and Schneider, M. H. Yield and wood properties of 25-year-old Japanese larch of difference provenance in eastern Canada . For. Chron. (12): 475-479; 1988.
Johns, W. E. and Niazi, K. A. Effect of pH and buffering capacity of wood on the gelation time of urea-formaldehyde resin . Wood Fiber Sci. 12(4): 255-263; 1980.
Li, M., Gertjejansen, R. O. and Ritter, D. C. Red pine thinnings as a raw material for waferboard . Forest Prod. J. 41(7/8): 41-43; 1991.
Maloney, T. M. Modern Particleboard & Dry-process fiberboard manufacturing . Updated edition. San Francisco. Miller Freeman Inc. 1993.
Palmer, C. L. Short-rotation culture of populus and larix: A literature review . Canada-Ontario For. Res. Dev. Agr. 65p; 1991.
Pugel, A. D., Price, E. W. and Hsu, C. Y. Composites from southern pine juvenile wood. Part 1: Panel fabrication and initial properties . Forest Prod. J. 40(1): 29-33; 1989.
SAS Institute, Inc. SAS/STAT User’s guide . Cary, N.C. 1990.
Shupe, T. F., Hsu, C. Y., Choong, E. T. and Groom, L. H. Effects of silvicultural practice and wood type on loblolly pine particleboard and medium density fiberboard properties . Holzforschung. 53(2): 215-222; 1999.
Tappi Standard. Fiber length of pulp by classification . T233cm-95. 1995.
Vallee, G. and Stipanicic, A. Growth and performance of larch plantations . Proceedings of a symposium sponsored by the Ontario Ministry of Natural Resources and the Faculty of Forestry, University of Toronto. Toronto, Ontario, Nov. 9, 1982. 1983.
Wang, S., Winistorfer, P. and Young, T. Fundamentals of vertical density profile formation in wood composites. Part III. MDF density formation during hot-pressing . Wood Fiber Sci. 36(1): 17-25; 2004.
Wang, S., Winistorfer, P. M., Young, T. M. and Helton, C. Step-closing pressing of medium density fiberboard. Part 1: Influences on the vertical density profile . Holz-als-Roh-und-Werkstoff. 59(1-2): 19-26; 2001.
Wang, S. and Winistorfer, P. M. Fundamentals of vertical density profile formation in wood composites . Part 2: Methodology of vertical density formation under dynamic conditions . Wood Fiber Sci. 32(2): 220-238; 2000.
Woodson, G. E. Effect of bark, density profile, and resin content on medium density fiberboards from Southern hardboards . Forest Prod. J. 26(2): 39-42; 1976.
Table 5-1 Size distribution of the fibers produced from larch and S-P-F wood chips.
Table 5-2 Density of larch and S-P-F wood chips, compaction ratio, average panel density, and properties of MDF panels.
Table 5-1 Size distribution of the fibers produced from larch and S-P-F wood chips.
 
Numbers in columns ‘>3.240’, ‘0.828-3.240’, ‘0.281-0.828’, and ‘0.017-0.281’ were the percentages of fibers retained on the screens with mesh sizes of 3.240 mm2, 0.828 mm2, 0.281 mm2, and 0.017 mm2. Numbers in columns ‘<0.017’ were the percentages of fibers passed through the screen with mesh size of 0.017 mm2.
Methods described in Tappi 233 cm-95 were followed.
Table 5-2 Density of larch and S-P-F wood chips, compaction ratio, average panel density, and properties of MDF panels.
 
SD=standard deviation.
The means with the same letter were not significantly different by Duncan’s multiple range test at 0.05 of significance level.
Methods for panel density measurement were in accordance with ASTM D 1037-99.
Compaction ratios were based on density of panels equilibrated at 22 oC and 65 % RH and density of wood chips. 
Figure 5-1 Average density profiles of MDF panels made from larch and S-P-F wood.
Figure 5-1 Average density profiles of MDF panels made from larch and S-P-F wood.
Figure 5-1 Average density profiles of MDF panels made from larch and S-P-F wood.

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Chapter IV Flexural Properties, Internal Bond Strength, and Dimensional Stability of Medium Density Fiberboard Panels Made from Hybrid Poplar Clones


Table des matières
Chapter IVFlexural Properties, Internal Bond Strength, and Dimensional Stability of Medium Density Fiberboard Panels Made from Hybrid Poplar Clones
Authors: Jun Li Shi, S. Y. Zhang, and Bernard Riedl
Les propriétés en flexion, de cohésion interne, et stabilité dimensionnelle des panneaux de fibres de densité moyenne faits à partir de bois des trois clones du peuplier ( Populus . spp.), dont les codes sont 915303, 915311, et 915313 ont été étudiées. L’analyse de variance (ANOVA) et l’analyse de covariance (ANCOVA) ont été faites dans cette étude pour examiner les différences de module de rupture (MOR) et de module d'élasticité (MOE) des panneaux de fibres de densité moyenne faits à partir des trois hybrides de peuplier. Les résultats indiquent que les MOR des panneaux faits à partir du clone 915311 étaient sensiblement plus élevés que ceux des panneaux faits à partir des clones 915303 ou 915313; cependant, il n’y avait aucune différence significative entre les MOR de panneaux faits à partir des clones 915303 ou 915313. Les MOE des panneaux de fibres de densité moyenne faits à partir du clone 915311 ont été les plus élevés, et sensiblement différents de ceux des panneaux faits à partir de clones 915303 ou 915313 alors que les MOE des panneaux faits à partir de fibres tirées du clone 915303 étaient les plus bas et sensiblement inférieurs à ceux des panneaux faits de fibres tirées du clone 915313. Les panneaux de fibres de densité moyenne faits à partir des deux clones 915303 et 915311 étaient supérieurs aux panneaux faits à partir du clone 915313 en ce qui concerne la cohésion interne; mais il n’y avait aucune différence significative entre la cohésion interne (IB) des panneaux faits à partir des clones 915303 ou 915311. La stabilité dimensionnelle des panneaux de fibres de densité moyenne a été évaluée via l’expansion linéaire (LE), le gonflement en épaisseur (TS), et l’absorption d’eau, et aucune différence significative n'a été trouvée parmi les trois types de panneaux. Cette étude montre un effet significatif de variation clonale de peuplier hybride sur les propriétés en flexion et de cohésion interne des panneaux de fibres de densité moyenne, et elle suggère que des améliorations des panneaux MDF dans les propriétés en flexion et en cohésion interne peuvent être obtenues en choisissant un ou plusieurs hybrides à des fins de sélection reproductive. De plus, la densité des panneaux étant un facteur considérable influençant les MOR et MOE des panneaux de fibres de densité moyenne, des relations linéaires significatives entre les MOR, MOE et la densité des panneaux ont été déterminées.
Flexural properties, internal bond strength, and dimensional stability of medium density fiberboard (MDF) panels made from three hybrid poplar ( Populus spp.) clones with codes 915303, 915311, and 915313 were studied. Analysis of variance (ANOVA) and analysis of covariance (ANCOVA) were both performed in this study to test the differences in modulus of rupture (MOR) and modulus of elasticity (MOE) of MDF panels made from the three poplar hybrids. Results indicate that MOR of MDF panels made from clone 915311 was significantly higher than those of panels made from clones 915303 or 915313; however, there was no significant difference in MOR between panels made from clones 915303 or 915313. MOE of MDF panels made from clone 915311 was the highest value, which was significantly different from those of panels made from either clone 915303 or 915313; MOE of panels made from clone 915303 was the smallest and significantly lower than those of panels from clone 915313.MDF panels made from both clones 915303 and 915311 were superior to those panels made from clone 915313 in internal bond (IB) strength; but there was no significant difference in IB between panels made from clones 915303 or 915311. Dimensional stability of MDF panels was evaluated by linear expansion (LE), thickness swell (TS), and water absorption, and no significant differences were found among the three types of panels. This study shows a significant effect of hybrid poplar clonal variation on flexural properties and internal bond strength. This suggests that improvements in MDF panel flexural properties and internal bond strength may be made through tree breeding. Additionally, panel density was a factor influencing MDF panel MOR and MOE considerably; as significant linear relationships between MOR, MOE and panel density were determined. 
The materials for paper making and composite panel manufacturing have been primarily and conventionally coming from softwood species such as spruce, pine, fir, etc. However, a shortfall in softwood supply is currently being faced by these industries. Recently, forest products companies in Canada and the United States have become interested in fast-growing species as potential replacements for softwood chips to sustain raw material supply.
Hybrid poplar is a hardwood species with a high growth rate and short rotation, which can be expected to produce a promising wood fiber source with high yields (Dix et al. 1999; Cisneroset al. 2000). In Alberta, five major pulping projects have been built recently using aspen resources (Cisneroset al. 2000). Hybrid poplar has also been studied for its potential as a raw material substitute for composite panel products. In fact, the advantages of using hybrid poplar as softwood substitution for composite panel making include not only in its high wood fiber yield, but also good performance of the end products. Geimer (1986) studied the properties of structural flakeboard panels manufactured from poplar, tamarack, and pine. Results indicate that modulus of rupture (MOR), modulus of elasticity (MOE), thickness swell (TS), and linear expansion (LE) of the flakeboards made from tamarack and pine were inferior to those from poplar. Short rotation and intensively cultured hybrid poplar was also investigated as a possible raw material source for hardboard (Myers and Crist 1986). Hardboard panels were tested for strength properties and dimensional stability, and results indicate that hybrid poplar is a suitable raw material for hardboard manufacturing. Moreover, high bending and internal bond (IB) strength and low TS in water of MDF panels made from 19-year-old poplar wood bonded with either a melamine-reinforced urea-formaldehyde (UF) resin, a tannin-formaldehyde resin, or a polymeric diisocyanate (PMDI) resin were also reported (Roffael and Dix 1994). The better performance of composite panels made from poplar wood may be due to its wood and fiber characteristics. As we know, poplar wood is low in density, thus, causing relatively high compaction ratio that has been reported to lead to superior panel strength properties (Maloney 1993; Hsu 1997; Peter et al. 2002; Shi et al. 2005). The thin-walled fiber can be packed better during pressing resulting in more gluelines per unit panel thickness.
The requirements of wood fiber characteristics for various end uses are diverse (Zhang et al. 1997). For some applications such as lumber, construction and plywood, low density wood is not preferred because of the low strength of such wood. On the contrary, some studies showed the advantages of using light wood species for fiberboard making (Nelson 1973; Woodson 1976). In addition, it has been known that wood and fiber properties (e.g. wood density and fiber cell wall thickness) can be affected by genetic control on hybrid poplar trees (Ivkovich, 1996; Law et al. 1997; Xing 2000; Cisneros et al. 2000; Savita 2001). Nevertheless, very little attention has been paid to genetic manipulation and selection of poplar trees for specific end uses. Only a few studies on the utilization of wood and fiber produced from genetically manipulated poplar trees as raw material for composite panels manufacturing were found in the literature. Geimer and Crist (1980) investigated the properties of structural flakeboard panels made from five hybrid poplar clones. It was found that the clonal variation had an effect on structural flakeboard panel properties; some panels performed better than others depending on furnish origins. Peter et al. (2002) studied the flexural properties, IB, density, water absorption, and TS of OSB panels made from eleven hybrid poplar clones. The flexural properties of OSB panels made from some clones were superior to those of panels made from others. It was concluded that these Populus hybrids showed great promise for use in structural panel products because of superior flexural and IB properties.
In this study, laboratory MDF panels were manufactured from three hybrid poplar clones and these panels were evaluated for flexural properties, internal bond strength and dimensional stability. We intend to examine the flexural properties, internal bond strength, and dimensional stability of MDF panels made from these three clones. The information derived from this study is essential to assist in poplar genetic and tree breeding program.
The material for this study came from a hybrid poplar clonal trial established by the Forest Research Branch of the Québec Ministry of Natural Resources in St-Ours, Southern Québec, Canada in 1993. The poplar trees were planted at 1.5×3.5 m spacing on this site, which is a part of the Champlain marine deposit with a rich salty-clay soil (40 % clay). Systematic thinning was carried out in the spring of 1996 and the spacing after the thinning was 2.5×3.0 m (Zhang et al. 2003). Four trees from three clones with codes 915303, 915311 and 915313, coming from a hybrid family of P. maximowiczii and P. balsamifera , were harvested from this site in December 2002 at an age of nine years. Butt logs of the three poplar hybrids were selected, debarked, and subsequently chipped using a portable chipper. Wood chips were collected, pooled by poplar clone, and then refined in a pressurized disc refiner located at the pilot plant of Forintek Canada Corp. Refining was processed without wax and resin injection. Moisture contents of the wood chips in the pre-steaming bin for clones 915303, 915311 and 915313 were 39.0 %, 44.0 % and 40.0 %, respectively. Specific refining energy for clones 915303, 915311, and 915313 were 134 KWh/t, 121 KWh/t, 125 KWh/t, respectively. To prevent fibers converted from different clones from mixing, 30 minutes of transition time was allowed between the three types of clonal wood materials while refining, and the fibers produced in the transition time period were discarded. 
Fibers generated from the three poplar clones were dried in a laboratory-scale dryer until the moisture content reached 2-3 %. Then, fibers were passed through a hammer mill to fully separate the fibers from each other. Since almost all panel properties can be improved by means of increasing resin content (Maloney 1993), in this study resin was blended into the fibers at a relatively low level so as not to conceal any effect of clonal variation on MDF panel properties. Thus, 10 % (by weight of dry fiber) Borden 302 urea formaldehyde (UF) resin (65 % solid content, no catalyst added) was first diluted, and then slowly sprayed onto the fibers using a laboratory-scale blade blender. At the same time, 0.5 % wax was blended together with the fibers. No other additives were added into furnish. Resin- and wax-blended fibers were passed through the hammer mill once again to disperse fiber balls. The average moisture contents of the fibers after wax and resin blending were 10.7 %, 11.7 % and 12.3 % for the clone order listed previously, respectively. Mats were hand-formed immediately in a wood frame of 610 by 610 mm. All panels were manufactured at a target density of 740 kg/m3 at the pilot plant of Forintek Canada Corp. Target panel thickness was 12 mm. Panels were pressed under the same schedule which consisted of a closing period of 160 s, maintained under pressure for another 160 s, and gradual relieved of pressure over 40 s. The temperature of the two platens was set at 135 oC. Due to the fact that panel density profile through thickness is one of the most important factors in determining properties of composite panels (Suchsland and Woodson 1974; Kelly 1977; Harless et al. 1987; Winistorfer et al. 1996; Wang et al. 2001; Wang et al. 2004), initially, we designed the press schedule mentioned previously in order to realize flatter panel density profiles, which may result in reduction of variations in panel properties caused by different density profiles. Three replicate panels were made for each poplar clone. Panels were trimmed immediately after pressing.
All MDF panels were kept for conditioning in a chamber at 22 oC and 65 % relative humidity (RH) for at least four weeks until the panels reached a constant equilibrium moisture content. Testing of modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), liner expansion (LE), thickness swell (TS) and water absorption was carried out in accordance with the standard methods of ASTM D 1037-99 (2001) for evaluating properties of wood-based fiber and particle panel materials and ANSI A 208.2-2002 (2002) for evaluating MDF for interior application. Flexural properties were determined on specimens of 338 by 75 mm. Three specimens were cut from each panel for flexural properties testing, producing nine specimens in total for each clone. Ten IB specimens of 50 by 50 mm were taken from each panel, resulting in thirty specimens for each clone. LE was measured from two specimens of 305 by 76 mm taken from each panel, for a total of six per clone. Specimens of 152 by 152 mm were used for both TS and water absorption assessment; two were cut from each panel, making six specimens in total for each clone. LE specimens were first stored in a conditioning chamber at 22 oC and 50 % RH for four weeks until their weight became constant, and then kept in another chamber at 22 oC and 80 % RH for another four weeks until the moisture content of specimens was checked to be at a constant equilibrium moisture content. LE was obtained by the variation in length caused by the change of relative humidity from 50 to 80 % divided by the length measured at 50 % RH. TS was calculated as the increase in thickness after a 24 h water immersion divided by the thickness measured on the specimens that had reached constant equilibrium moisture content under a condition of 22 oC and 65 % RH. Water absorption was calculated as the percentage of water absorbed in the specimens after 24 h water submersion based on the weight of specimens that had reached constant equilibrium moisture content at 22 oC and 65 % RH. About 1.5 mm was sanded from both surfaces of all IB specimens before they were glued to blocks. The weight and dimension of all specimens for panel flexural properties, internal bond strength and dimensional stability were measured before testing and densities of these specimens were calculated for the purpose of analysis of covariance (ANCOVA). Panel density refers to the density of specimens calculated from the weight and volume obtained from the specimens conditioned at 22 oC and 65 % RH for four weeks. Moisture contents of the three types of panels were measured on the equilibrated moisture content MOR/MOE specimens after four weeks of conditioning while following the methods described in ASTM D 1037-99 (2001). Panel density profiles were determined from the ten IB specimens before they were sanded and glued to blocks, and the collected data were averaged for three types of panels. The surface and bottom of each IB specimen were marked clearly to ensure that the directionality of the surfaces was maintained during density profile testing. 
A Bauer-McNett Classifier was used to determine the fraction distribution of fibers generated from the three poplar clones. The procedures described in TAPPI T 233 cm-95 (1995) were followed for the testing.
Statistical Analysis System (SAS) (1990) software package was used for data analysis in this study. Analysis of variance (ANOVA) was employed to compare the mean property values of MDF panels made from three poplar clones. Since the influence of panel density on panel properties, especially on flexural properties, has been reported in several studies (Maloney 1993; Olson 1996; Peter et al. 2002; Shi et al. 2005), ANCOVA was also performed using panel density as a covariate for the purpose of adjusting the mean MOR and MOE values, which were partly attributed to panel density. The removal of panel density influence on panel properties may improve the precision of the analysis and meanwhile reduce the error. Before performing ANCOVA, assumptions such as homogeneity and linearity of within-group regression, randomization, statistical independence of covariate and treatment, etc, must be tested to be sure that they are met; otherwise, the analysis will lead to misleading results (Huitema 1980). All pairwise multiple comparison procedures were conducted to compare the adjusted means using the PDIFF option in SAS with an α level of 0.05.
Simple linear regression analysis was also performed as a consequent statistical technique to develop equations describing the relationships between MOR, MOE and panel density. The coincidence of the three regression lines was tested by introducing dummy variables into the analysis of variance. 
It is known that fibers with different size distribution can have an impact on MDF panel properties (Groom et al. 1999). The size distributions for the three types of fibers are shown in Table 4-1. Since the percentages of fibers distributed in mesh size ranges of >3.240 mm2, 0.828-3.240 mm2, 0.281-0.828 mm2, 0.017-0.281 mm2, and smaller than 0.017 mm2 for the three clones were similar, we assumed that the refining process produced fibers with similar size for the three clones. Therefore, it can be assumed that the influence of fiber size on properties of the three types of panels was identical. 
Panel average densities and compaction ratios are presented in Table 4-2. Generally, wood density is believed to be the most important wood characteristic in determining final panel properties; the basic requirement of raw materials for making panels with acceptable properties is a relatively low wood density (Maloney 1993; Hsu 1997; Woodson 1976). Basically, low density wood is easier to consolidate into target thickness. As a matter of fact, for the same panel density, compaction ratio of panels made from low density wood is always higher. Compaction ratio is of importance since well-bonded panels are primarily associated with high compaction ratios (Maloney 1993; Hsu 1997). Shi et al. (2005) reported that properties of MDF panels made from black spruce juvenile wood were significantly superior to those of panels made from mature wood. They explained that it was due to the low density of juvenile wood. However, in this study, these relationships were not apparent because only a very narrow range of wood density was involved in the experiment; nevertheless, the adjusted MOR and MOE of the three groups appeared statistically different with the use of ANCOVA since the precision of the analysis was improved.
Slight differences existed among the average density profiles of MDF panels made from the three poplar clones as shown in Figure 4-1 even though we made these panels under the same press schedule. Small difference in mat moisture content (10.7 %, 11.7 % and 12.3 %) seemed not to be a cause of different panel properties. So the slight difference in density profile might result from fiber origin while panels were compressed under heat and pressure. Since the density profiles of the three types of panels were nearly comparable, it is assumed that differences in panel properties were not the cause of density profile. However, the flatter density profile with large face layer thickness may lower the flexural properties of all types of panels. 
The unadjusted mean MOR of MDF panels made from the three poplar clones is shown in Table 4-2. No significant difference was found in MOR among the panels from the three clones by Duncan’s multiple-range test at the 0.05 of significance level if the effect of panel density was not considered in the analysis. However, as mentioned previously, panel density is a factor in determining panel properties (Maloney 1993; Olson 1996; Peter et al. 2002; Shi et al. 2005). Taking panel density into consideration, panel density was then analyzed as a covariate using ANCOVA in order to eliminate the effect on MOR caused by different panel density and and to adjust mean MOR values. First, the assumptions of ANCOVA such as linearity and homogeneity of within-group regressions were tested to ensure the data fits the ANCOVA model. The assumption testing showed that all the assumptions were met. A statistical comparison of ANOVA and ANCOVA for MOR is presented in Table 4-3. F=0.78 (P=0.4702) was obtained from ANOVA, showing that there were no significant differences in panel MOR among the three types of panels. With the use of ANCOVA, F=4.50 (P=0.0236) was acquired, which was to say that here, the clonal variation effect on panel MOR was significant (α=0.05). The results from regression analysis indicate a significant linear relationship between MOR and panel density for all three types of panels. The effect of panel density on MOR was indicated by a significant linear regression test, where F=96.99 (P<.0001) was calculated (Table 4-3, Figure 4-2). The adjusted mean MOR for the three types of poplar panels is listed in Table 4-2. Adjusted mean MOR of MDF panels made from clone 915311 was significantly higher than that of panels from clones 915303 and 915313, there was no significant difference in MOR between panels made from clones 915303 and 915313. The difference in MOR between the panels made from three poplar hybrids indicates a significant effect of raw material on panel performance. 
The same analytical procedures were employed for MOE data. According to ANOVA, no significant differences were found between panels made from the three hybrids by Duncan’s test (Table 4-2). Consequently, ANCOVA was performed in order to eliminate the effect of panel density on MOE. ANCOVA assumptions were tested by verifying linearity and homogeneity of within-group regressions. A linear regression test was significant, and the slopes of within-group regression were all equal, meaning that there were no interactions existing among the three groups (Table 4-3, Figure 4-3). The adjusted mean MOE was calculated by using the same regression slope for the three types of panels, and the results are presented in Table 4-2. MOE of MDF panels made from clone 915311 was significantly higher than that of panels made from clones 915303 and 915313, while MOE of panels made from clone 915303 was the smallest and significantly lower than that of panels from clone 915313 (Table 4-2). The precision of testing was greatly improved and error was reduced by using ANCOVA. Significant difference in MOE between three types of panels was observed meaning that there was an impact of clonal variation on the performance of panels made from hybrid poplar. 
The mean IB of MDF panels made from clones 915303 and 915311 was significantly higher than that of panels from clone 915313; there was no significant difference in IB strength between the panels made from clones 915303 and 915311 by Duncan’s multiple-range test (Table 4-2). The results show that there was no impact of clones 915303 and 915311 on panel IB strength; however, compared to the panels made from those two clones, clone 915313 had a significant effect on panel IB. 
LE, TS, and water absorption of MDF panels made from the three poplar clones were not significantly different at a significance level of 0.05 (Table 4-2). Therefore, the clonal variation effect on MDF panel dimensional stability was not significant. 
Panel density affected considerably panel flexural properties. Significant linear relationships between panel MOR and MOE and panel density for all three types of panels were found as mentioned previously. Equations describing the relationships (Figures 4-2 and 4-3) were developed using simple linear regression from the model Y = b0 + b1x, where Y represents panel MOR or MOE, and x is panel equilibrium density. The relationships between MOR, MOE and panel density indicate the dependence of the two variables on panel density, that is, MOR and MOE of panels made from the three poplar hybrids can be improved by increasing panel density. This is due to more fiber being used in heavier MDF panels resulting in more resistance against mechanical loads (Maloney 1993).
However, a further question is whether there is a difference between the three regression lines, or, they are coincident.
When dummy variables are used in regression analysis, it is important to ensure that all groups can be distinguished in the analysis. Therefore, two dummy variables must be defined in order to index three groups. Thus, variables z1 and z2 need to be defined for the data to distinguish the three different clones, which are shown in the following.
z1 = 1if the panels were made from clone 915303
0otherwise
z2 = 1if the panels were made from clone 915311
0otherwise
Three clones can be obtained by combining z1 and z2.
z1=1 z2=0: panels made from clone 915303
z1=0 z2=1: panels made from clone 915311
z1=0 z2=0: panels made from clone 915313
A model for the relationship between MOR and panel density is given by
Y = b0 + b1x + b2z1 + b3z2 + b4xz1 + b5xz2 + ε
where Y represents MOR of panels;
x is panel density; z1, z2 are clones with codes 915303 and 915311;
ε is the error term.
The model for the panels made from the three poplar clones can be written as:
model for clone 915303: Y = (b0 + b2) + (b1 + b4)x + ε
model for clone 915311: Y = (b0 + b3)+ (b1 + b5)x + ε
model for clone 915313: Y = b0 + b1x + ε
The hypothesis of the coincidence of the three regression lines is the one that the slopes and intercepts are the same for all three types of panels, which can be written as follows:
H0: b2 = b3 = b4 = b5 = 0
Thus, we can test the hypothesis of coincidence by testing significance of the terms z1, z2, xz1, and xz2. The three regression lines shown in Figure 4-2 were tested (Table 4-4), and the terms z1, z2, xz1, and xz2 were not significant indicating that the hypothesis that the three regression lines were coincident was accepted. Thus, we can fit a single overall regression line to the three lines. A model can be given as follows to describe the relationship of MOR to panel density for the three clones.
Y = - 51.0521 + 0.0967 x (R2 = 0.8954)
where Y is MOR of panels made from the three hybrid poplar clones;
x is panel density.
Through the same analysis method, the coincidence of the regression lines for MOE was tested subsequently (Table 4-4). The hypothesis of coincidence was accepted as well. Therefore, only one overall equation can be used to describe the relationship between MOE and panel density for all three clones. The equation is shown below.
Y = - 4214.7052 + 7.9633 x (R2 = 0.8677)
where Y is MOE of panels made from the three hybrid poplar clones;
x is panel density.
The following conclusions can be made from this study:
  1. MOR of MDF panels made from clone 915311 was significantly higher than those of panels from clones 915303 and 915313; however, there was no significant difference in MOR between panels made from clones 915303 and 915313. 
  2. MOE of MDF panels made from clone 915311 was the highest and was significantly different from those of panels from clone 915303 and 915313; MOE of panels from clone 915303 was the smallest and was significantly lower than that of panels from clone 915313.
  3. MDF panels made from either clone 915303 or 915311 were superior to panels from clone 915313 in IB strength; there was no significant difference in IB between panels made from clone 915303 and 915311.
  4. No significant differences were found in LE, TS, and water absorption among panels made from the three poplar hybrids; the effect of clonal variation on the dimensional stability of hybrid poplar MDF panels was not appreciable.
This study shows that clonal variation had a significant effect on the flexural properties and internal bond strength of hybrid poplar MDF panels; however, its effect on panel dimensional stability was not significant. The difference in flexural properties and internal bond of panels may result from different wood fiber characteristics of the three poplar hybrids. MDF panel properties in relation to wood fiber characteristics will be studied and reported in Chapter VI. Knowing the relationships between panel properties and wood fiber characteristics, it is likely that improvement in panel flexural properties and internal bond strength can be achieved through tree breeding. 
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American Society of Testing and Materials (ASTM). Evaluating properties of wood-based fiber and particle panel materials . ASTM D 1037-99. Vol. 04.10. ASTM, Philadelphia. PA. pp. 141-170; 2001.
Cisneros, H. A., Belanger, L., Gee, W. Y., Watson, P. A. and Hatton, J. V. Wood and fiber properties of hybrid poplars from southern British Columbia . TAPPI Journal. 2000.
Dix, B., Thole, V. and Marutzky, R. Poplar and eucalyptus wood as raw material for wood-based panels . Eurowood Technical Workshop Proceedings: Industrial End-uses of Fast-grown Species. CNR/IRL and CNR/ITL. Florence. Pp93-102; 1999.
Geimer, R. L. Properties of structural flakeboard manufactured from 7-year-old intensively cultured poplar, tamarack, and pine . Forest Prod. J. 36(4): 42-46; 1986.
Geimer, R. L. and Crist, J. B. Structural flakeboard from short-ratation, intensively cultured hybrid populus clones . Forest Prod. J. 30(6): 42-48; 1980.
Groom, L. H., Mott, L. and Shaler, S. M. Relationship between fiber furnish properties and the structural performance of MDF . 33rdInternational Particleboard/Composite Materials Symposium Proceedings. April 13-15, Washington State Univ., Pullman, Washington, USA. 1999.
Harless, T. E. G., Wagner, F. G., Short, R. D., Seale, R. D., Mitchell, P. H. and Ladd, D. S. A model to predict the density profile of particleboard . Wood Fiber Sci. 19(1): 81-92; 1987.
Hsu, W. E. Wood quality requirements for panel products . Proceeding of CTIA/IUFRO International Wood Quality Workshop, Timber Management toward Wood Quality and End-product Value. Forintek Canada Corp. Quebec City, Canada. I-7/10; 1997.
Huitema, B. E. The Analysis of Covariance and Alternatives . Jone Wiley & Sons, New York. 1980.
Ivkovich, M. Genetic variation of wood properties in Balsam poplar (Populus balsamifera L.) . Silvae Genet. 45(2/3): 119-124; 1996.
Kelly, M. W. Critical literature review of relationship between processing parameters and physical properties of particleboard . Gen. Tech. Rep. FPL-10. USDA Forest Serv., Forest Prod. Lab., Madison. WI. 1977.
Law, K., Valade, J., Rioux, S. and Lanouette, R. Wood and paper properties of five short-rotation poplar clones grown in Québec . October 19, 1997 pulping conference: proceedings (TAPPI): 847-856; 1997.
Maloney, T. M. Modern Particleboard and Dry-Process Fiberboard Manufacturing . Miller Freeman Publications. San Francisco, CA. 1993.
Myers, G. C. and Crist, J. B. Feasibility of manufacturing hardboard from short-rotation intensively cultured Populus . Forest Prod. J. 36(1): 37-44; 1986.
Nelson, N. D. Effects of wood and pulp properties on medium-density, dry-formed hardboard . Forest Prod. J. 23(9): 72-80; 1973.
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Peter, J. J., Bender, D. A., Wolcott, M. P. and Johnson, J. D. Selected properties of hybrid poplar clear wood and composite panels . Forest Prod. J. 52(5): 45-54; 2002.
Roffael, E. and Dix, B. Mechanical-technological properties of MDF from poplar wood . Holz Roh Werkst. 52(1): 48; 1994.
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Savita, G. Wood and paper properties of poplar clones . IPPTA. 13(4):35-37; 2001.
Shi, J. L., Zhang, S.Y. and Riedl, B. Effect of juvenile wood on strength properties and dimensional stability of black spruce medium density fiberboard panels . Holzforschung. 59(1): 1-9; 2004.
Suchland, O. and Woodson, G. Effect of press cycle variables on density gradient of medium density fiberboard . In proceedings 8thparticleboard symp. Pp 375-396; 1974.
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Wang, S., Winistorfer, T. M., Young, T. M. and Helton, C. Step-closing pressing of medium density fiberboard; Part I: Influences on the vertical density profile . Holz-als-Roh-und-Werkstoff. 59(1-2): 19-26; 2001.
Wang, S., Winistorfer, P. and Young, T. Fundamentals of vertical density profile formation in wood composites. Part III. MDF density formation during hot-pressing . Wood Fiber Sci. 36(1): 17-25; 2004.
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Table 4-1 Size distribution of the fibers produced from the three hybrid poplar clones.
Table 4-2 Mean modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), linear expansion (LE), thickness swell (TS), and water absorption of MDF panels made from the three hybrid poplar clones.
Table 4-3 Comparison of analysis of variance (ANOVA) and analysis of covariance (ANCOVA) for panel modulus of rupture (MOR) and modulus of elasticity (MOE).
Table 4-4 Test for coincidence of the relationships between MOR/MOE and panel density.
Table 4-1 Size distribution of the fibers produced from the three hybrid poplar clones.
 
Note: Numbers in columns ‘>3.240’, ‘0.828-3,240’, ‘0.281-0.828’, and ‘0.017-0.281’ were the percentages of fibers retained on the screens with mesh sizes of 3.240 mm2, 0.828 mm2, 0.281 mm2, and 0.017 mm2. Numbers in columns ‘<0.017’ were the percentages of fibers passed through the screen with mesh size of 0.017 mm2.
Methods described in Tappi 233 cm-95 were followed.
Table 4-2 Mean modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), linear expansion (LE), thickness swell (TS), and water absorption of MDF panels made from the three hybrid poplar clones.
 
Note: Unadjusted means with the same small letter were not significantly different by Duncan’s multiple-range test ( p =0.05). Adjusted means with the same capital letter were not significantly different by multiple comparison procedures for all pairwise comparisons using the PDIFF option in SAS ( P =0.05).
Methods for panel density and moisture content determination were in accordance with ASTM D 1037-99. Compaction ratios were based on panel equilibrium density and density of wood chips.
S represents standard deviation. 
Table 4-3 Comparison of analysis of variance (ANOVA) and analysis of covariance (ANCOVA) for panel modulus of rupture (MOR) and modulus of elasticity (MOE).
 
Note: SS is sum of square. DF is degree of freedom. F values were significant at p =0.05. 
Table 4-4 Test for coincidence of the relationships between MOR/MOE and panel density.
 
Figure 4-1 Average density profiles of MDF panels made from the three hybrid poplar clones.
Figure 4-2 Relationship between modulus of rupture (MOR) and panel density.
Figure 4-3 Relationship between modulus of elasticity (MOE) and panel density.
Figure 4-1 Average density profiles of MDF panels made from the three hybrid poplar clones.
Figure 4-1 Average density profiles of MDF panels made from the three hybrid poplar clones.
Figure 4-2 Relationship between modulus of rupture (MOR) and panel density.
Figure 4-2 Relationship between modulus of rupture (MOR) and panel density.
Note: y1, y2, y3 correspond to the modulus of rupture (MOR) of MDF panels made from hybrid poplar clone 915303, 915311, and 915313, respectively; x refers to panel density.
Figure 4-3 Relationship between modulus of elasticity (MOE) and panel density.
Figure 4-3 Relationship between modulus of elasticity (MOE) and panel density.
Note: y1, y2, y3 correspond to the modulus of elasticity (MOE) of MDF panels made from hybrid poplar clone 915303, 915311, and 915313, respectively; x refers to panel density.

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