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.
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. 
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. 
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.
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