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Weekly UpdatesThinnings cut from trees are estimated to total 5 million [m.sup.3] annually in Japan. Japanese cedar (sugi) and Japanese cypress (hinoki) account for the most common domestic coniferous thinnings. Hinoki, however, is the second most abundant tree species in Japan, and the material properties of hinoki are similar to sugi. The most effective use of thinnings is as material for particleboard production. Currently, 20 percent of the annual industrial production of particleboard is used for residential construction in Japan. Some studies have examined the properties of wood-based panels constructed with sugi (Mallari et al. 1989, Suzuki et al. 1994); however, few published reports are available on wood-based materials constructed using hinoki.
Formaldehyde emissions from wood-based panels have been a topic of great concern for decades. To reduce formaldehyde emissions from wood-based panels, isocyanate adhesives are frequently being used in wood composite production as a replacement for commonly used amino-based binders. Because isocyanate adhesives are a non-formaldehyde resin, it is believed that using isocyanate adhesives for wood composites can eliminate formaldehyde release associated with the adhesive. Some of the advantages of isoeyanate adhesives include improved board properties, faster pressing rates, lower press temperature, and high moisture content (MC) (Deppe 1977, Johns et al. 1981, Galbraith 1986). Diphenylmethane diisocyanate (MDI) adhesives in particular exhibit high cohesive and adhesive strength, and better resin distribution properties as such tend to produce enhanced panel performance for the manufactured particleboard. Several studies have shown MDI-consolidated panels were clearly better than phenolformaldehyde (PF) resin bonded panels in thickness swelling (TS), internal bond (IB) strength, and bending properties (Hawke et al. 1993; Sun et al. 1994a, 1994b).
MDI-bonded partieleboards were fabricated with laboratory-made hinoki particles to examine the binding properties of MDI resin for wood-based panels. The objectives of this study were to evaluate the effects of manufacturing parameters on mechanical properties and durability performance. Board density, furnish type, and additives were examined.
The following accelerated aging treatments were used to determine the durability performance of MDI-bonded board: the JIS-A and JIS-B tests, the V313 cyclic moisture test, and the ASTM 6-cycle test (ASTM 1993; European Standard EN 321, 1993; JIS 1994).
Materials and methods
Board fabrication
Hinoki (Japanese cypress: Chamaecyparis obtsusa Endl.) particles were used as the main board material. The density of hinoki was 0.39 g/[cm.sup.3]. Hinoki logs were waferized by a disk flaker (wafer size: length 50 mm, width 30 to 40 mm, thickness 0.6 mm), then hammer-milled to reduce the size of the wafers into particles. Panicles were screened to remove dust and dried to a uniform MC of 2 percent. Figure 1 shows the hinoki particles used as furnish. Sugi (Japanese cedar: Co,ptomeria.japonica D. Don) particles were processed in the same manner as the hinoki particles. The density of sugi was 0.35 g/[cm.sup.3]. In addition, a commercially used particle was obtained from a mill line for comparison. An emulsified polymeric MDI adhesive was used as the binder (Cosmonate M-201; Mitsui Takeda Chemicals Co. Ltd.), and wax (E J80/ 147; Exxon Mobil Co. Ltd.) was used as the paraffinic wax sizing additive.
Manufacturing parameters of the varying experimental types of particleboard are shown in Table 1. Conditions for the control board were: MDI resin, resin content 6 percent, target density 0.65 g/[cm.sup.3], homogeneous single-layer panel construction, hinoki panicle furnish, and wax dosage 0.5 percent addition. Experimental parameters used for comparison were: pressed target density from 0.45 to 0.75 g/[cm.sup.3], three-layer panel construction, paraffin wax not added. The resin content was 6 percent for all types of board, and wax doses were blended on the basis of ovendry weight for the screened particle furnish. The board made from commercial particles was consolidated with MDI resin for the core layer and PF resin for the outer two face layers.
[FIGURE 1 OMITTED]
Adhesive was sprayed for blending purposes onto the dried panicle furnish applying a laboratory scale rotary blender. Mat forming was performed over a metal caul plate by hand felting into a 600 by 600-mm forming box with random panicle orientation. The mat MC before pressing was controlled to be approximately 10 percent. The dimensions of the boards were 600 by 600 by 11.7 mm. Metal stops included with the hydraulic pressing system were used to control the final thickness of the pressed board to achieve the desired level of consolidated panel density. Compaction ratio of hinoki board ranged from 1.15 (board density: 0.45 g/[cm.sup.3]) to 1.92 (board density: 0.75 g/ [cm.sup.3]). The compaction ratio ofsugi board (board density: 0.65 g/[cm.sup.3]) was 1.86.
Platen temperature was 180[degrees]C, and the maximum pressure applied in the first step was 3.0 MPa for 1 minute. The pressure was reduced to 1.5 MPa at the second step and to 1.0 MPa at the third step for I minute for each. Maintaining the pressure of 0.5 MPa at the fourth step, the total press time was 5 minutes.
Mechanical tests
After conditioning the boards at 25[degrees]C and 65 percent RH, a bending test was conducted according to JIS A 5908 (JIS 1994) using a specimen with dimensions of 50 by 250 mm. Modulus of elasticity (MOE) and modulus of rupture (MOR) from bending were determined using four replications for each condition. In addition, IB strength was determined using the JIS method (JIS 1994) but with an increased sample to include 10 test replications.
Accelerated aging treatments
Four accelerated aging tests were chosen for the evaluations. The ASTM-6c method is specified in ASTM D1037 for mat-formed panel products (ASTM 1993), which consists of six repetitions of the combined treatments:
1. immersion in water at 40[degrees]C for 1 hour,
2. steaming at 93[degrees]C for 3 hours,
3. freezing at -12[degrees]C for 20 hours,
4. drying at 99[degrees]C for 3 hours,
5. steaming at 93[degrees]C for 3 hours, and
6. drying at 99[degrees]C for 18 hours.
The V313 method is the method specified in the European Standard (EN321) as a test method for the cyclic moisture test (1993). The test specimens are exposed to three cycles of the following exposure treatments:
1. immersion in water at 20[degrees]C for 72 hours,
2. freezing at between -12[degrees]C and -20[degrees]C for 24 hours, and
3. drying at 70[degrees]C for 72 hours.
JIS-A treatment is defined as hot-water immersion for 2 hours followed by 1 hour water immersion. JIS-B includes boiling water immersion for 2 hours and 1 hour water immersion (JIS 1994). Particleboard specimens were exposed to these accelerated aging treatments. TS was measured after each cycle, and IB strength was determined after the treatment was complete.
Results and discussion
Effect of board density
The bending test results conducted on particleboard with various densities of hinoki particle are shown in Figure 2. MOE and MOR increased linearly as board density increased. This is in accordance with previous results reported by numerous researchers (Kawai et al. 1987, Mallari et al. 1989, Suematsu and Okuma 1992, Suzuki et al. 1994). The MOR result for the hinoki particleboard with a density ef 0.70 g/[cm.sup.3] was greater than 40 MPa. Even for the low-density board (0.45 g/[cm.sup.3]), MOR reached approximately 20 MPa, which exceeds the JIS requirement for 18-type particleboard. Hinoki particleboard also exhibited high MOE values of 4.1 GPa and 2.3 GPa for densities of 0.70 and 0.45 g/[cm.sup.3], respectively. The MOE of 4.1 GPa particleboard corresponded to the JAS requirement for Structural Plywood Class-2.
Data from this study about the relationship between MOR and density was compared to data obtained by other researchers. Hinoki particleboard bonded with 10 percent MDI resin (Suematsu and Okuma 1992) showed a linear relationship similar to this study, whereas hinoki particleboard with 10 percent PF resin showed lower MOR values. Sugi particleboard with 8 percent MDI and 8 percent UF resin gave lower values than hinoki particleboard (Mallari et al. 1989). These results indicate that hinoki particleboard bonded with MDI resin has the potential to exhibit high bending performance.
Density, resin content, and furnish type strongly affect the IB strength of particleboard. A clear linear relationship was found between board density and IB strength (Fig. 3). Specifically, IB strengths of 0.8 to 2.5 MPa were obtained at a density range of 0.45 to 0.70 g/[cm.sup.3]. Surprisingly, the IB value of 2.5 MPa from this study was approximately eight times the JIS requirement for particleboard. Compared to reports of sugi MDI, phenol formaldehyde (PF) and urea formaldehyde (UF) boards, and MDI-bonded light red meranti particleboard (Mallari et al. 1989, Suzuki et al. 1994, Kawai et al. 1987), hinoki particleboard bonded with MDI resin exhibited high bending and IB strength qualities.
[FIGURE 2 OMITTED]
TS and IB strength were used as indices of durability performance after the particleboard was subjected to the V313, ASTM-6c, JIS-A, and JIS-B accelerated aging treatments. Figure 4 shows the relationships between board density and TS after the aging treatments. TS was measured under wet conditions for JIS-A and JIS-B treatments immediately following room temperature water immersion. For the V313 and ASTM-6c treatments, TS was based on the greatest thickness in the last treatment cycle.
The results of TS after an aging treatment are dependent upon board density (Kelly 1977). This was demonstrated when TS increased with increasing board density after the JIS-B treatment and the ASTM-6c test. The effect of board density, however, was not clear in the TS following the JIS-A and V313 treatments. This difference could be a reflection of the severity of the treatments. Specifically, the JIS-B test includes a boiling treatment, whereas the ASTM-6c treatment only includes steaming. It is possible that the high TS in the JIS-B treatment was due to high water absorption (about 90%), whereas water absorption was only approximately 30 percent in the JIS-A treatment.
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