- 3.1 Introduction
- 3.2 Material and Methods
- 3.3 Results and Discussion
- 3.4 Conclusions
- 3.5 Literature Cited
- 3.6 List of Tables
- 3.7 List of Figures
Logs coming from different positions in a tree possess different wood and fiber characteristics; butt log wood usually has on average higher density and the fiber cell wall is thicker, whereas, wood density is lower and the fiber cell wall is thinner at the top of a tree since top logs contain a higher proportion of juvenile wood (Panshin and DeZeeuw 1980; Zobel and Sprague 1998). So far, limited studies (Zhang and Chauret 2001; Zhang et al. 2002) have examined lumber quality as well as value recovery in relation to log position in black spruce. In the literature little study has investigated the properties of MDF panels from different log positions in this species. Some studies focused on the properties of composite panels made from different types of wood, such as juvenile and mature wood. For example, Wasniewski (1989) examined the strength properties, dimensional stability, and durability of Douglas fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco) flakeboard panels from five age classes, which were pith-7, 8-14, 15-21, 22-28, 29-bark, and found that panel properties decreased with age class increase. A study by Pugel et al. (1989; 1990) indicates that the properties of flakeboard, particleboard, and fiberboard panels made from four different types of loblolly pine ( Pinus taeda L.) juvenile wood, namely fast-grown trees, the inner core of older trees, branches, and tops, were comparable to or better than those of mature wood composite panels, and that the differences in panel properties between juvenile and mature wood panels were at a minimum in fiberboard. Additionally, MDF panels made from black spruce ( Picea mariana (Mill.) BSP.) juvenile wood performed better in modulus of rupture (MOR), internal bond (IB), and water absorption than the panels fabricated from mature wood; but linear expansion (LE) of the panels from juvenile wood was significantly higher than that of mature wood panels (Shi et al. 2005). Therefore, in most cases, composite panels made from juvenile wood are superior to the panels made from mature wood. This is in accordance with the fact that most panel properties improve with increasing the compaction ratio, and that adding high density wood to the panels made from low density species decreases board quality, for instance, by adding white birch strands to aspen strands in OSB panels.
In this study, MDF panels were manufactured from black spruce top, mid, and butt logs, and the strength properties and dimensional stability of the three types of panels were examined. The objective of this study was to determine the effect of log position in the tree on strength properties and dimensional stability of MDF panels made from black spruce wood.
Compaction ratio refers to the ratio of panel density to wood density (Hsu 1997). The compaction ratio of the panels made from top logs was the largest and the smallest was due to the panels made from butt logs (Table 3-2). Wood density is one of the most important wood characteristics which determines final composite panel properties (Maloney 1993; Hsu 1997). Low-density wood produces higher compaction ratio if panel density is maintained at the same level. It was found that good mechanical properties of composite panels were associated with high compaction ratios (Pugel et al. 1989; Shupe et al. 1999). Compaction ratio was also found to be positively linearly related to panel MOR and MOE in hardboards that were made from fourteen hardwood species (Woodson 1976). MDF panels made of black spruce juvenile wood were superior to the panels made from mature wood, which was believed due to the higher compaction ratio of the former (Shi et al. 2005).
The average vertical density profiles of the panels made from top, mid, and butt logs are shown in Figure 3-1. These density profiles do not represent those obtained from the panel mills because a different pressing program was adopted in this study. The low surface density can result in reduction in bending properties. However, since the density profiles were nearly homogeneous for the three types of panels, this allowed comparison of panel properties between the three groups. The slight difference in vertical density profile between the three groups can be explained by a number of reasons, for example, different density of top, mid, and butt logs, different compaction ratio, in-plane variation in mat density, mat moisture content variations, etc.
X L1 = 519 kg/m3
X L3 = 710 kg/m3
Therefore, Johnson-Neyman suggests that there is no significant difference in MOR between panels made from top and butt logs if panel density is in the region of 519-710 kg/m3. However, if panel density is lower than 519 kg/m3, MOR of panels made from butt logs is significantly higher than that of panels made from top logs. When panel density is higher than 710 kg/m3, panels made from top logs are significantly superior to the panels made of butt log wood in MOR.
Through simple linear regression analysis, the equations describing the relationships between MOR and panel density for the three groups were obtained as shown in Figure 3-2. Two dummy variables were created to identify the three log positions in the tree. Dummy variables z1 and z2 were defined as follows.
z1 = 1if the panels were made from top logs
z2 = 1if the panels were made from mid logs
The three different log positions can be identified by combining z1 and z2.
z1=1 z2=0: panels made from top logs
z1=0 z2=1: panels made from mid logs
z1=0 z2=0: panels made from butt logs
A model for the relationship between MOR and panel density is given by:
Y = b0 + b1x + b2z1 + b3z2 + b4xz1 + b5xz2 + ε
where Y represents panel MOR;
x is panel density, z1, z2 are dummy variables representing top and mid log positions;
ε is the error term.
The models for the relationships between MOR and panel density for the three types of panels can be given by:
for the panels made from top logs: Y = (b0 + b2) + (b1 + b4)x + ε
for the panels made from mid logs: Y = (b0 + b3)+ (b1 + b5)x + ε
for the panels made from butt logs: Y = b0 + b1x + ε
The hypothesis of the coincidence of the three regression lines is that both the slopes and intercepts are equal for the three types of panels, which can be written as follows:
H0: b2 = b3 = b4 = b5 = 0
The hypothesis of the coincidence can be tested by checking the significance of the terms z1, z2, xz1, and xz2. The results indicate that the terms z1, z2, xz1, and xz2 were significant as shown in Table 3-5, which means that the hypothesis of the coincidence was rejected. Tests for parallelism and equality of the intercept of each two of the three regression lines were then followed. Table 3-6 indicates that the relationships between MOR and panel density for the three types of panels must be described using three individual equations, which are presented in Figure 3-2.
The regression functions in Figure 3-3 were developed separately using three datasets measured from top, mid, and butt log panels. Using the same method mentioned previously, the coincidence of the three regression lines for the relationship between MOE and panel density was tested as well (Table 3-5). The hypothesis of the coincidence was accepted meaning that a single regression line can be fit to the relationships for top, mid, and butt logs. Thus, a model to describe the relationship between MOE and panel density is given by:
Y = -5296.8224 + 9.0574 x (R2 = 0.7906)
where: Y is MOE of the panels made from top, mid, and butt logs;
x is panel density;
for the three types of panels.
The better performance of top log panels in static bending properties and internal bond strength indicate a significant effect of fiber origin on MDF panels. Since top logs contain higher proportion of juvenile wood when compared with butt logs, more furnish is needed to make the panels from juvenile wood at the same panel density level. In this case, panels can be bonded better using the thin-walled juvenile wood fibers as raw material, which leads to a greater mechanical load resistance as discussed in Chapter II. The observation here tends to the same conclusion as it is in Chapter II.
MOE and IB of MDF panels made from top and mid logs were significantly higher than those of panels made from butt logs; however, there were no significant differences in MOE and IB between panels made from top and mid logs. Water absorption of MDF panels made from top and mid logs was significantly lower than that of panels from butt logs; again, the difference in water absorption between panels made from top and mid logs was not significant.
Following a 24 h water-soaking period, the panels made from butt logs swelled the most and the panels made from top logs swelled least. There was no significant difference in LE among those panels made from top log, mid log and butt logs.
MDF panels made from top logs performed the best in MOR, while MOR of the panels made from butt logs was the smallest. Johnson-Neyman suggests that there is no significant difference in MOR between panels made from top and butt logs when panel density is between 519 kg/m3and 710 kg/m3. It also suggests that MOR of panels made from top logs is significantly higher than that of panels from butt logs if panel is heavier than 710 kg/m3. If panel density is lower than 519 kg/m3, the reverse is true.
Panel density had a considerable effect on panel MOR and MOE. The relationships between MOR, MOE and panel density were linear. But the linear relationships between IB, LE, TS, or water absorption and panel density were not significant.
It was concluded that most of the properties of MDF panels made from top logs were superior to those of panels made from butt logs. This is due to the high proportion of juvenile wood contained in top logs. Fibers generated from low-density juvenile wood can be easily packed and lead to high performance. This could be helpful for sorting black spruce logs into various uses. Butt logs can be sorted to lumber manufacturers because of the higher density, strength properties and product recovery compared to mid and top logs. Top logs can be allocated to composite mills for fiberboard manufacturing since the performance of the products is better. Thus, values of black spruce forest resource are added and the performance of the products is maximized.
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Table 3-2 Black spruce MDF panel moisture content, average density, compaction ratio, and mean modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), linear expansion (LE), thickness swell (TS), and water absorption.
Table 3-3 Comparison of analysis of variance (ANOVA) and analysis of covariance (ANCOVA) for panel modulus of rupture (MOR) and modulus of elasticity (MOE).
Table 3-4 Homogeneity of regression test and statistics summary of panel modulus of rupture (MOR) where interactions occurred in the analysis of covariance (ANCOVA).
Table 3-5 Test for coincidence of the relationships between MOR/MOE and panel density.
Table 3-6 Tests for parallelism and equality of intercept of the linear 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, respectively. Numbers in column ‘<0.017’ were the percentages of fibers passed through the screen with mesh size of 0.017 mm2.
Methods described in Tappi T 233 cm-95 were followed.
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 all pairwise comparisons ( P =0.05).
Methods for panel density and moisture content determination were in accordance with ASTM D 1037-99.
Compaction ratios were calculated based on panel equilibrated density at 22 oC and 65 % RH and density of wood chips.
The adjusted mean MOR with ‘①’ was calculated using the slope of group 1 and group 2. The adjusted mean MOR with ‘②’ was calculated using the slope of group 2 and group 3.
S is standard deviation.
Figure 3-2 Relationship between modulus of rupture (MOR) and panel density.
Figure 3-3 Relationship between modulus of elasticity (MOE) and panel density.Note: y1, y2, y3 correspond to modulus of rupture (MOR) of MDF panels made from top, mid, and butt logs; x refers to panel density.Note: y1, y2, y3 correspond to modulus of elasticity (MOE) of MDF panels made from top, mid, and butt logs; x refers to panel density.
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