- 2.1 Introduction
- 2.2 Material and Methods
- 2.3 Data Analysis and Statistical Methods
- 2.4 Results and Discussion
- 2.5 Conclusions
- 2.6 Literature Cited
- 2.7 List of Tables
- 2.8 List of Figures
As well known, juvenility varies in terms of tree age, and juvenile wood is undesirable for some forest products such as lumber and plywood because of resulting low strength and dimensional instability (Bendtsen and Senft 1986; Zobel and Sprague 1998). However, plenty of juvenile wood such as commercial thinnings is produced each year. As the case stands, proper utilization of such undesired wood resource becomes of paramount importance. In fact, the impact of juvenile wood on composite panels is not well known. Species may be the first concern. In other words, panels made from juvenile wood of some species may perform better than those made from mature wood, while properties of panels made with juvenile wood of other species may be worse than those of panels made from mature wood (Maloney 1986). Several studies in the literature are cited here, which were concerned with using juvenile wood as raw material to make composite panels. Groom et al. (1998) indicated that stiffness and strength properties of un-oriented MDF panels made from 51 year-old loblolly pine ( Pinus taeda L.) wood could be improved by increasing the proportion of juvenile wood. Peter et al. (2002) also found that oriented strand board (OSB) panels made from young hybrid poplar ( Populus spp.) clones showed superior flexural and internal bond (IB) properties. Dix et al. (1999) reported that juvenile poplar wood is a good raw material for particleboard and fiberboard. Shupe et al. (1999) also found higher modulus of rupture (MOR), modulus of elasticity (MOE) and IB values in juvenile wood particleboard and fiberboard panels made from loblolly pine ( Pinus taeda L.). Pugel et al. (1989; 1990) studied durability and dimensional stability of flakeboard, particleboard, and fiberboard composites made from four different types of loblolly pine ( Pinus taeda L.) juvenile wood: fast-grown trees (8 year-old), the inner core of older trees (the first 10 year of growth of 40 to 50 year-old trees), branches, and tops. Mature wood was obtained from the outer growth increments of 40 to 50 year-old trees and used for flakeboard, particleboard, and fiberboard making as well. Fibers were manufactured from chips through a single disk atmospheric refiner with plate clearance set at 0.51 mm. The press schedule was 30 s to stops, reduction of initial pressure after 2 minutes, and gradual relief of pressure during the last minute of the 6-minute press cycle. Durability of these panels was evaluated by subjecting specimens to an ovendry-vacuum-soak (ODVPS) treatment, and then tested for MOE, MOR, and IB. Dimensional stability was assessed by measuring thickness swell (TS) and linear expansion (LE) of specimens subjected to an ODVPS treatment and specimens exposed to a single cycle of 30 to 90 % relative humidity (RH). Durability of those composites made from juvenile wood was comparable to that of composites from mature wood. It was found that LE of composites made from juvenile wood was significantly greater than that of panels made from mature wood. TS was also higher for those composites made from juvenile wood after ODVPS treatment. Among the three types of composites studied, fiberboard showed the fewest differences in properties between juvenile and mature wood composites.
Wasniewski (1989) made structural composite panels with strands from Douglas fir ( Pseudotsuga menziesii var. menziesii [Mirb.] Franco) of five age classes, which were 0-7, 8-14, 15-21, 22-28, and 29-bark, and evaluated the strength properties and dimensional stability of those panels. It was found that MOR, MOE, IB, TS, and LE reduced with increasing the age of the furnish. Moreover, a study by Li et al. (1991) shows that red pine ( Pinus resinosa ) thinnings was comparable to aspen as raw material for waferboard, but IB and TS increased with red pine thinnings content.
Our emphasis on the effect of juvenile wood on black spruce MDF panel properties is not simply an attempt to compare the performance of MDF panels made from juvenile and mature wood. More importantly, this study would address the potential for using furnish produced from black spruce young trees, while also providing essential data for forest management on black spruce tree rotation period which will aid in the economical and effective utilization of forest resources.
The objective of this study was to determine the effect of juvenile wood on strength properties and dimensional stability of black spruce MDF panels.
The average density of wood chips collected from 1-20 year age class was the smallest. The compaction ratio was the highest for the MDF panels from 1-20 year age class and decreased with increasing the group age. It is well recognized that wood density is one of the most important factors in determining final panel products properties (Maloney 1993; Hsu 1997). Compaction ratio is defined as the ratio of panel density to wood density (Maloney 1993; Hsu 1997). At a given board density, panels made with low density wood yield higher compaction ratio. Generally, a compaction ratio of higher than 1.3:1.0 is considered to be acceptable for making well bonded composite panels (Maloney 1993). Pugel et al. (1989) and Shupe et al. (1999) both found that higher compaction ratios were associated with superior mechanical properties of composite panels.
The average density profiles of MDF panels made from 1-20, 21-40, and over 40 year-old fibers are shown in Figure 2-1. Although the manufacturing variables, such as press schedule, temperature, resin and wax content, etc. were kept consistent for the three age groups, there still existed slight differences in panel density profile. For example, the density profile of MDF panels made from 1-20 year-old fibers possessed slightly higher peak density and more marked variation tendency from low to peak near the two surfaces. Many causes are possible to explain the differences in panel density profile for the three groups, e.g. wood density differences, panel density variation, in-plane variation in mat density, mat moisture content variation, etc.
Analysis of panel density as a covariate was then performed. It is very important for the data to meet specific assumptions before evaluating the fit of data to ANCOVA model, such as randomization, homogeneity of within-group regression, independence of covariate and treatment, etc. The test of regression was significant, indicating that panel density accounted for a significant portion of the variation in panel MOR (Table 2-3). While comparing the F values produced from ANOVA and ANCOVA, the precision of the analysis was improved when the influence of panel density on MOR was taken into account (Table 2-3). The assumption of homogeneity of regression lines was also met, meaning that the regression slopes associated with the three age groups were equal (Table 2-3). This is also to say that the changing magnitude of MOR was kept constant for the three groups when changes in panel density occurred. Thus, MOR mean values can be computed based on the same slope. The results indicate that MOR of MDF panels made from 1-20 year-old fibers was significantly higher than those of panels from the other two age classes, but the difference in panel MOR between 21-40 and over 40 age classes was not significant (Table 2-2).
Generally, compared to mature wood fibers, the cell wall of juvenile wood fiber is thinner and lumen diameter is larger (Panshin and DeZeeuw 1980). Thus, the ratio of cell wall thickness to lumen diameter of juvenile wood fibers is larger than that of mature wood fibers. This could be particularly of advantage in fiberboard manufacture. The thin-walled cellulose fibers can be easily consolidated to the target thickness and lead to high bending strength.
However, the results presented in Table 2-3 also indicate significant interactions between density and the three age groups, meaning that the regression slopes of the three groups were not equal over the density range. The assumption of homogeneity of regression slopes was rejected, but this does not lead to the conclusion that the three slopes differ from each other. Thus, the overall test with additional homogeneity of regression tests on all combinations of pairs was followed up. This test shows that the slopes of group 2 and 3 were homogeneous (Table 2-4), so ANCOVA can be carried out using the same regression slope for this pair. Using ‘PDIFF’ option in SAS, the adjusted mean values were computed and the results indicate that there was no significant difference in MOE between group 2 and 3 (Table 2-2). The slope of group 1 was different from that of group 2 and 3: here, Johnson-Neyman technique (Huitema 1980) was applied to identify the values of the covariate (panel density) that are associated with the significant group differences in the dependent variable (MOE). It can make a statement about the regions of nonsignificance and significance. The equations used for computing the limits of nonsignificance region on the covariate can be found in Huitema (1980). Therefore, the limits of nonsignificance region for the pair of group 1 and 2 were calculated, and they are shown here.
X L1 = 716 kg/m3
X L2 = 743 kg/m3
Thus, Johnson-Neyman technique suggests that group 1 and 2 do not differ on MOE if panel density falls in this region. Group 2 is significantly superior to group 1 regarding MOE if panel density is lower than 716 kg/m3. Alternatively, if panel density is higher than 743 kg/m3, group 1 is significantly superior to group 2 concerning MOE.
The same equations were used to compute the limits of nonsignificance region for group 1 and 3. The limits are shown below.
X L1 = 642 kg/m3
X L3 = 748 kg/m3
Johnson-Neyman’s suggestion is that group 1 and 3 do not differ in MOE if panel density is in the region of 642-748 kg/m3. But group 3 is significantly superior to group 1 concerning MOE if panel density is lower than 642 kg/m3; and group 1 is significantly superior to group 3 if panel density is higher than 748 kg/m3. According to the results shown above, for those heavier MDF panels where the panel density is higher than 748 kg/m3, the panels made from juvenile wood are always superior to the panels fabricated from mature wood in terms of MOE.
X L1 = 427 kg/m3
X L2 = 679 kg/m3
The limits of nonsignificance region for group 1 and 3 were:
X L1 = 352 kg/m3
X L3 = 662 kg/m3
Johnson-Neyman suggests that there is no significant difference in TS between group 1 and 2 when panel density drops in the region of 427-679 kg/m3. However, if panel density is lower than 427 kg/m3, TS of panels made from 1-20 year-old fibers is significantly higher than that of panels made from 21-40 year-old fibers; if panel density is larger than 679 kg/m3, TS of panels made from 21-40 year-old fibers is significantly higher than that of panels made from 1-20 year-old fibers (Figure 2-4). For the pair of group 1 and group 3, if panel density is between 352-662 kg/m3, the difference between the two group means is not significant. If panel density is lower than 352 kg/m3, group 3 is significantly better than group 1 on TS since group 3 yields smaller TS; whereas, when panel density is larger than 662 kg/m3, group 1 is significantly better than group 3 (Figure 2-4). Based on Johnson-Neyman’s suggestion, the panels made from black spruce juvenile wood may swell less when compared with those panels from mature wood if the panels are heavier than 679 kg/m3.
It was found that the panels made from black spruce juvenile wood produced larger LE, however, caused smaller TS and water absorption. As we discussed previously, the thin-walled juvenile wood fibers are much easier to consolidate to tighter and good bonded panels. And the highly compressed thin-walled panels may be hardly permeated by moisture or water, eventually, can result in lower TS and water absorption. In many cases, heavier composite panels are associated with higher TS because there are more materials in such panel to absorb the water (Maloney 1993). But this may not be applicable to the panels that are manufactured from juvenile wood.
MDF panels improved in MOR and MOE with increasing panel density for the three age groups. More material was used to make heavier panels resulting in more intimate fiber to fiber contact, thus, leading to a high resistance to mechanical loads. However, the increasing magnitude of MOR and MOE for the panels made from 1-20 year-old fibers was larger than that for the other two types of panels. To make panels with the same density, more furnish is needed for juvenile wood than it does using mature wood material. TS of those panels made from 21-40 and over 40 year-old fibers increased when increasing panel density. This is due to more furnish used in heavier panels and subsequent larger variation in thickness after soaking in water (Maloney 1993). However, panels made from 1-20 year-old fibers swelled less in thickness with panel density increasing.
MOR, IB, and water absorption of MDF panels made from 100 % juvenile wood were significantly superior to those of panels made from mature wood. LE of MDF panels made from 100 % juvenile wood was significantly larger than that of panels made from mature wood.
Johnson-Neyman suggests that MOE of MDF panels made from 100 % juvenile wood is not significantly different from that of panels made from a mixed material that contains a high proportion of mature wood and a small proportion of juvenile wood if panel density is in the region of 716-743 kg/m3. If panel density falls below 716 kg/m3 or above 743 kg/m3, the difference in MOE between the two groups is significant. There is no significant difference in MOE between those panels made from juvenile and mature wood within a panel density range of 642-748 kg/m3; however, if panel density is lower than 642 kg/m3or higher than 748 kg/m3, the difference between these two age groups become significant. Also, the difference in TS of MDF panels between 1-20 and 21-40 age groups is not significant when panel density drops in the region of 427-679 kg/m3; however, if panel density is lower than 427 kg/m3, TS of panels from 1-20 age class is significantly larger than that of 21-40 age class; if panel density is higher than 679 kg/m3, TS of panels from 21-40 age class is significantly larger than that of panels from 1-20 age class. There is no difference in TS between the panels made from 1-20 and over 40 year-old fiber while panel density is in the region of 352-662 kg/m3. But if panel density is lower than 352 kg/m3, TS of the panels from 1-20 age class is larger than that of panels from over 40 age class; if panel density is higher than 662 kg/m3, panel TS of over 40 age class is larger than that of 1-20 age class.
Significant linear relationships between MOR, MOE, TS and panel density were found for the three age groups.
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Table 2-2 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 2-3 Comparison of analysis of variance (ANOVA) and analysis of covariance (ANCOVA) for panel modulus of rupture (MOR), modulus of elasticity (MOE), and thickness swell (TS).
Table 2-4 Homogeneity of regression test and statistics summaries for panel modulus of elasticity (MOE) and thickness swell (TS) where interactions occurred in the analysis of covariance (ANCOVA).
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 density at equilibrium moisture content and density of wood chips.
S is standard deviation.
Figure 2-2 Relationship between modulus of rupture (MOR) and panel density.
Figure 2-3 Relationship between modulus of elasticity (MOE) and panel density.
Figure 2-4 Relationship between 24 h thickness swell (TS) and panel density.
Note: y1, y2, y3 correspond to modulus of rupture (MOR) of MDF panels made from 1-20, 21-40, and over 40 year-old fiber, respectively; x refers to panel density.
Note: y1, y2, y3 correspond to modulus of elasticity (MOE) of MDF panels made from 1-20, 21-40, and over 40 year-old fiber, respectively; x refers to panel density.Note: y1, y2, y3 correspond to 24h thickness swell (TS) of MDF panels made from 1-20, 21-40, and over 40 year-old fiber, respectively; x refers to panel density.
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