Thursday, 20 October 2016

Chapter II Effect of Juvenile Wood on Strength Properties and Dimensional Stability of Black Spruce Medium Density Fiberboard Panels


Table des matières
Authors: 
Jun Li Shi, S. Y. Zhang, and Bernard Riedl
Les propriétés mécaniques et la stabilité dimensionnelle des panneaux de fibres de densité moyenne (MDF) faits à partir d’épinette noire ( Picea mariana (mill.) BSP.) d’âge de 1-20, 21-40, et plus de 40 ans ont été étudiées. Les matériaux en bois des trois âges ont été rassemblés manuellement en utilisant un scie à ruban. Une analyse de covariance (ANCOVA) a été faite pour examiner les différences dans le module de la rupture (MOR), le module d'élasticité (MOE), et le gonflement en épaisseur (TS) des trois types de panneaux, alors que la densité du panneau était traitée comme une covariante afin d'ajuster les valeurs moyennes qui ont été en partie attribuées à la densité des panneaux. Les résultats indiquent que les MOR et la cohésion interne (IB) de panneaux de fibres de densité moyenne faits à partir de fibres de 1-20 ans, donc contenant 100 % de bois juvénile, étaient sensiblement supérieurs à ceux des panneaux faits à partir de fibres de 21-40 et plus de 40 ans. L’absorption d’eau des panneaux de fibres de densité moyenne faits à partir de fibres de 1-20 ans étaient sensiblement supérieurs à ceux des panneaux faits à partir de fibres de 21-40 et plus de 40 ans; mais l’expansion linéaire (LE) des panneaux de fibres de densité moyenne faits à partir de fibres de 1-20 ans était sensiblement plus grande que celle des panneaux des deux autres classes d’âge. Les différences dans les MOR, IB, absorption eau, et l’expansion linéaire entre les panneaux faits à partir de fibres de 21-40 et plus de 40 ans n'étaient pas significatives. Les comparaisons de MOE et TS des panneaux dépendent relativement de la densité des panneaux et cela est dû à l’existence d’interaction significative entre la densité et les trois catégories d’âge.
Black spruce ( Picea mariana (Mill.) BSP.) is one of the most important commercial species in Eastern Canada. Many studies have been done on black spruce wood physical and mechanical properties, as well as its wood quality in relation to initial tree spacing (Barbour 1987; Barbour et al. 1989; Zhang 1998; Zhang and Chauret 2001). However, little research has been performed to investigate properties of medium density fiberboard (MDF) panels made from black spruce wood.
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.
Fifty mature black spruce trees were harvested from a second growth natural stand in July 1999 in the Réserve Ashuapmushuan, which is 400 kilometres north from Québec City, Canada. The trees were felled into 8-foot long butt, mid and top logs. The butt logs were first debarked by hand and then cut into 1-foot short log sections. On the cross section of each short log section, three age zones were clearly marked using a pen. The zones were from pith to 20th annual ring, from 21st to 40th annual ring, and from 41stannual ring and over. According to a study conducted on black spruce wood that grew in the same stand conditions and collected at the same time, the transition age from juvenile to mature wood was in 20-25 years (Alteyrac 2005). Thus, 1-20 age class contained 100 % juvenile wood; 21-40 age class contained a large proportion of mature wood and a very small proportion of juvenile wood; and over 40 age class yielded 100 % mature wood. A band saw was used to separate the three types of wood along the marked lines on the cross sections from the 1-foot long short logs. Then, the material collected from the three age classes was chipped using a portable chipper. Wood chips were refined by a pressurized disc refiner without resin or wax injection. The moisture content of 1-20, 21-40, and over 40 year-old wood chips in the pre-steaming bin was 46.3 %, 38.9 %, and 47.6 % respectively. The refining speed, cooker pressure, retention time, and temperature for all age groups were 2500 rpm, 7.5 bar, 3 min and 160 oC. The specific refining energy spent on 1-20, 21-40, and over 40 year-old wood, were 98 KWh/t, 170 KWh/t, and 174 KWh/t, respectively. The gap between the two plates for 1-20, 21-40, and over 40 age classes were 0.027 mm, 0.005 mm, and 0.001mm respectively. A transition period of 30 min. was maintained during refining between the two different samples to avoid mixing the fibers that were generated from different age classes, and the fibers produced in the transition period were discarded.
Fibers were dried in a laboratory-scale dryer down to a moisture content of about 2-3 %; then passed through a hammer mill to separate the fibers from each other. A laboratory-scale blade blender was used to inject resin and wax. Since high resin levels are associated with good composite panel properties (Maloney 1993), a relatively low resin level was used in this study in order to avoid overly good panel properties due only to high resin level. Therefore, 10 %, by weight of dry fiber, Borden 302 urea formaldehyde (UF) resin (65 % solid content) and 0.5 % wax were slowly sprayed onto the fibers using the blender mentioned above. After resin and wax blending, the fibers were passed through the hammer mill once again to disperse the fiber balls. 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 forming frame and the fibers were without orientation. Three replicate panels were made for each age class. MDF panels were made under the same pressing program at Forintek Canada Corp. The moisture content of the furnishes after resin and wax blending for 1-20, 21-40, and over 40 age classes were 12 %, 8.4 %, and 10.7 %. Total closing time was 160 s. Panels were kept under pressure for another 160 s and the press platens were gradually opened in 40 s. The temperature of the two platens was set at 135 oC. The pressing program was quite different from those that are usually adopted by composites industry, because we attempted to realize flatter density profile so as to reduce any variation in panel properties caused by the differences in panel vertical density gradient.
All panels were stored in a conditioning chamber at 22 oC and 65 % relative humidity (RH) for four weeks until the panels reached equilibrium moisture content. Three static bending specimens (338×75 mm) were cut from each panel, producing nine specimens in total for each age class; ten IB specimens (50×50 mm) were cut from each panel, equaling thirty specimens for each age class; two linear expansion (LE) specimens (305×76 mm) were cut from each panel, making six in total for each age class; two specimens (152×152 mm) for TS and water absorption were cut from each panel, yielding six specimens for each age class. The procedures and methods described in ASTM D 1037-99 (2001) and ANSI A 208.2-2002 (2002) were followed for MOE, MOR, IB, LE, TS and water absorption testing. LE was determined by the linear variation with change in RH from 50 % to 80 % as the percentage change in length based on the specimens equilibrated at 50 % RH. TS or water absorption were obtained respectively by calculating the variation in thickness or weight with 24 h water immersion divided by the thickness or weight measured on the specimens equilibrated at 22 oC and 65 % RH. All specimens for IB testing were sanded for about 1.5 mm from the two surfaces before being glued with the blocks. The densities of all specimens that were equilibrated at the condition of 22 oC and 65 % RH were measured for the purpose of analysis of covariance (ANCOVA). Panel moisture content was examined from the static bending specimens following the methods described in ASTM D 1037-99 (2001). Panel average density profiles for the three age classes were determined from the IB specimens before they were sanded and glued with the blocks. 
The average panel density and compaction ratio for the three groups are shown in Table 2-2. Attempts have been made to equalize the panel density for the three age groups throughout the experimental design in order to avoid any influence of differences in panel density on measured MDF panel strength properties and dimensional stability. Since in-plane mat density variation normally exists, panel density was treated as a covariate to improve the precision of this analysis and reduce the error.
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. 
ANOVA was carried out to compare MOR of MDF panels made from 1-20, 21-40, and over 40 year-old fibers. The results indicate that there was a significant difference in MOR among the three groups (Table 2-2). MOR of MDF panels made from 1-20 year-old fibers, which contained 100 % juvenile wood, was significantly higher than that of panels from the other two age classes. However, there was no significant difference in MOR between the panels made from 21-40 and over 40 year-old fibers. Nevertheless, these MOR means or their comparisons are not strictly appropriate because the mean values may have been affected by panel density. To be comparable, these means should be adjusted to make them the best estimates of what they would have been. The adjusted means, also called least squares means in statistics, can be computed using the regression equation of MOR to panel density with code ‘LSMEANS MOR/PDIFF;’ in SAS. ‘LSMEANS’ option produces least squares means (adjusted means) for the variable MOR, and ‘PDIFF’ option provides significance probabilities for all pairwise comparisons of these adjusted means.
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. 
ANOVA results show that MOE of MDF panels made from 1-20 year-old fibers was significantly higher than that of panels made from 21-40 and over 40 year-old fibers, but the differences between 21-40 and over 40 age groups were not significant (Table 2-2). A test of regression of panel density on panel MOE was then conducted to verify whether the data fit a linear model for each group. The results show a significant linear relationship between panel density and MOE for the three groups (Table 2-3). The precision of the analysis was highly improved through the analysis of covariance, which can be known by the increased F value produced from ANCOVA (Table 2-3).
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.
L1 = 716 kg/m3
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.
L1 = 642 kg/m3
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.
The regression test of panel density on TS was significant (Table 2-3), which indicates the interactions between density and TS for the three age groups. Additional homogeneity of regression tests on all combinations of pairs were performed. The slope for the pair of group 2 and 3 was examined to be homogeneous, thus, ANCOVA can be employed in these two groups based on the equal slope. However, the slopes of group 1 versus 2 and 3 were heterogeneous. As stated previously, for the pairs that the regression slopes were not equal, the Johnson-Neyman technique (Huitema 1980) was used to compute the limits of nonsignificance region. The limits of nonsignificance region on panel density for group 1 and 2 are shown below.
L1 = 427 kg/m3
L2 = 679 kg/m3
The limits of nonsignificance region for group 1 and 3 were:
L1 = 352 kg/m3
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.
Linear relationships between MOR, MOE, TS and panel density were significant for the three age groups. Thus, panel density could be chosen as an independent variable to predict the bending properties and swell in thickness. The regressions were derived from the linear model, Y = a + bX, where Y is panel MOR, MOE, or TS, and x is panel density. The predictive equations for each group are shown in Figure 2-2, 2-3, and 2-4. The equations can provide estimates for MDF panel bending properties and TS which are likely attained with panel density. The use of these equations is restricted to the species studied here, and to the panels manufactured in a manner similar to the procedures used in this study.
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. 
It was found that some properties of MDF panels made from black spruce juvenile wood, such as MOR, IB, and water absorption, were significantly superior to those of panels made from mature wood. The differences in MOE and TS between the panels made from juvenile and mature wood were dependant on panel density. With some silvicultural practices such as commercial thinning operated in Canada and the United States in recent years, the conclusion can be drawn that a good use of such thinnings is to manufacture fiber-based composite products. So the value of commercial thinnings can be increased, in the meantime, the performance of the products is maximized. However, the linear variation of the fiberboard panels made of juvenile wood must be controlled using effective technologies and modifications.
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. 
Alteyrac, J. Influence de la densité de peuplement et de la hauteur dans l’arbre sur les propriétés physico-mécaniques du bois d’épinette noire (Picea Mariana (Mill.) B. S. P.) . Ph.D. Thesis, Université Laval, Québec, Canada.2005.
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.
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Table 2-1 Size distribution of the fibers generated from black spruce 1-20, 21-40, and over 40 year-old wood.
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).
*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.
*Note: Unadjusted means were computed without consideration of panel density effect on panel properties. Adjusted means were computed using the regression equations of MOR, MOE, and TS to panel density by means of ‘LSMEANS’ option in SAS, and compared pairwisely using the ‘PDIFF’ option in SAS.
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. 
*Note: SS is sum of square. DF is degree of freedom. F values were significant at 0.05 of probability. 


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