- 4.1 Introduction
- 4.2 Material and Methods
- 4.3 Results and Discussion
- 4.4 Conclusions
- 4.5 Literature Cited
- 4.6 List of Tables
- 4.7 List of Figures
Authors: Jun Li Shi, S. Y. Zhang, and Bernard Riedl
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.
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.
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.
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
z2 = 1if the panels were made from clone 915311
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.
- 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.
- 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.
- 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.
- 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.
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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.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.
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.
Figure 4-2 Relationship between modulus of rupture (MOR) and panel density.
Figure 4-3 Relationship between modulus of elasticity (MOE) 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.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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