Mobile Edition
Print
Get the Mag
Weekly UpdatesPerformance durability is one of the most important properties of wood-based panels used in housing construction (McNatt and Link 1989, Kajita et al. 1991). Shortfalls in the domestic production of veneer-based panel products in Japan have resulted in an increase in the use of mat-formed panel products for structural purposes. These products include plywood and oriented strandboard (OSB) imported from North America and Europe, as well as domestically produced particleboard and medium density fiberboard (MDF) that has high moisture resistance.
The effects of moisture on wood-based panels determine their properties and possible uses. Thickness swelling (TS) is the most important property that must be assessed when considering the effects of moisture. It can be affected by most process variables, including wood species, element geometry, board density, resin level, blending efficiency, and pressing conditions. In previous studies of the TS of wood-based panels, Halligan (1970) examined the TS of particleboards, Saito et al. (1981) examined TS in relation to internal bond (IB) strength, Sekino (1986) discussed the relationship between modulus of rupture (MOR) and the TS of particleboard and MDF, and Kajita et al. (1991) performed five standard accelerated aging tests on particleboard and measured the TS and IB strength.
In Japan, considerable effort has been focused on correlating degradation resulting from outdoor aging with the results of laboratory-accelerated aging, given that outdoor aging provides baseline criteria for standardizing test procedures (Sekino and Suzuki 2003). Ikeda and Suzuki (1991) performed tests to help determine the relationship between 10-year outdoor exposure of wood-based panels with laboratory-accelerated aging tests, including the wet-bending-A test and the wet-bending-B test (JIS 1994), ASTM D1037 (ASTM 1993), APA D-1 and D-4 (APA 1994), V313 (European Standard EN 321, 1993), and vacuum pressure soaking and drying (VPSD) treatment (Karlsson et al. 1996).
The relationship between accelerated aging and outdoor exposure is usually evaluated in terms of IB strength. Few studies, however, have been conducted in which the evaluation was performed using TS. To understand and explain the performance durability of wood-based panels, it is necessary to determine the TS characteristics of each accelerated aging test. Further, it is crucial to clarify which laboratory-accelerated aging test results correspond to a given outdoor exposure test result.
The objectives of this study were: 1) to clarify changes in TS behavior for five accelerated aging test methods using eight commercial wood-based panels; 2) to investigate the correlations among accelerated aging test methods; and 3) to ascertain the relationships between accelerated aging test methods and outdoor exposure tests conducted in Shizuoka City, Central Honshu, Japan. The main objective of this study was to clarify the durability performance of wood-based panels by using TS.
Materials and methods
Sample panels
The four groups of commercial wood-based panels used in this study are listed in Table 1. They were particleboard, MDF, OSB, and plywood, all of which are widely used for construction purposes in Japan. Each panel group included two panel types of differing specifications, i.e., eight panels in total. The particleboard panels were made from recycled wood with different binders. The MDF panels differed in thickness and binder type and end-use applications. The OSB panels used were imported products with different wood species. The plywood panels also differed in thickness. Because the OSB used in this project was obtained from North America and Europe, these panels were not necessarily representative of the OSB typically used in North America and Europe. Although North America has very little MDI-bonded particleboard or MDF, MDI-bonded particleboard and MDF was selected because fabricators in Japan show a strong preference for particleboard and MDF with high durability performance. The parallel direction on each panel surface was defined by the machine direction for particleboard and MDF, the surface strand alignment for OSB, and the surface veneer grain direction for plywood.
Accelerated aging test methods
Five types of accelerated aging treatment were conducted to evaluate the TS of the eight wood-based panels:
1. cyclic JIS-B treatment,
2. cyclic APA D-1 treatment,
3. V313 procedure,
4. ASTM six-cycle procedure, and
5. VPSD procedure.
With the exception of the VPSD procedure, all of the treatments followed standard methods or modifications of these methods.
1. Cyclic JIS-B treatment consisted of immersion in boiling water for 2 hours, followed by immersion in water at 20[degrees]C for 1 hour, and then drying at 60[degrees]C for 21 hours. The treatment was repeated one, three, and six times and TS was measured after each step. Moreover, TS after reconditioning was measured.
2. Cyclic APA D-1 treatment is specified by APA (APA 1994). It consists of immersion in water at 66[degrees]C for 8 hours, drying at 82[degrees]C for 14.5 hours, and settling under room temperature for 1.5 hours. The treatment was repeated one, three, and six times, and TS was measured after each step. Moreover, TS after reconditioning was measured.
3. V313 is the specified European Standard EN 321 (1993) method for cyclic testing of moisture resistance. The procedure has also been adopted as the JANS (Japanese Australian New Zealand Standard) by the joint committee for Australia, New Zealand, and Japan. The test specimens were exposed to immersion in water at 20[degrees]C for 72 hours, freezing at -12[degrees]C for 24 hours, drying at 70[degrees]C for 72 hours, and settling under room temperature for 4 hours. The treatment was repeated one, three, and six times, and TS was measured after each step. Moreover, TS after reconditioning was measured.
4. ASTM six-cycle method is a common test method and is specified in ASTM D1037 for mat-formed panel products (ASTM 1993). It consists of six repetitions of the combined treatments consisting of immersion in water at 49[degrees]C for 1 hour, steaming at 93[degrees]C for 3 hours, freezing at -12[degrees]C for 20 hours, drying at 99[degrees]C for 3 hours, steaming at 93[degrees]C for 3 hours, and drying at 99[degrees]C for 18 hours. The treatment was repeated one, three, and six times, and TS was measured after each step. Moreover, TS after reconditioning was measured.
5. VPSD consists of a vacuum pressure soaking and drying procedure. It consists of soaking under a vacuum for 0.5 hours, soaking under a pressure of 290 kPa for 1 hour, and drying at 60[degrees]C for 22 hours. The treatment was repeated one, three, five, and ten times, and TS was measured after each step. Moreover, TS after reconditioning was measured.
Reconditioning involved oven-drying for 24 hours at 60[degrees]C followed by 2 weeks of conditioning at 20[degrees]C and 65 percent relative humidity (RH). These five treatments are summarized in Table 2. Eight test pieces measuring 50 by 50 mm were taken from each panel and subjected to the above tests. TS was calculated as follows:
TS = T2 - T1 / T1 x 100% [1]
where:
T1 = initial thickness of each specimen and
T2 = thickness after each treatment.
T1 was measured after drying at 20[degrees]C and 65 percent RH for 1 month.
Outdoor exposure
Twelve sample boards measuring 300 by 300 mm from each panel type were exposed to the outdoor weather at the campus of Shizuoka University (Shizuoka City, Japan; 34[degrees]N, 138[degrees]E). All four edges of each sample were coated with a protective agent to prevent excessive edge swelling from water adsorption during test exposure. The boards were set vertically on a test frame facing South. The outdoor test was started in March 2004 and will run to 2013. In this paper, the results of the first 2 years of exposure are discussed. Two test sample boards of each panel type were removed after 1 and 2 years of exposure, and TS was measured after reconditioning that consisted of drying at 60[degrees]C for 24 hours and conditioning at 20[degrees]C and 65 percent RH for 2 weeks. The thickness of each panel was measured at the center and four edges.
Results and discussion
Characteristics of TS change on each accelerated aging treatment
Figure 1 shows the changes in TS found for the test specimens measuring 50 by 50 mm for each of the five accelerated aging test methods. The TS for the Cyclic JIS-B treatment, Cyclic APA D-1 treatment, V313 method, and ASTM six-cycle method are those of six repeated cycles. The TS of VPSD was determined after 10 repeated cycles. As shown in Figure 1, TS tended to increase following each soaking step, steaming step, or pressure soaking step, and then decrease after each drying step. For the V313 and ASTM six-cycle procedures, TS showed only a slight increase upon freezing (-12[degrees]C). These observations agreed with previous experimental reports (McNatt and Link 1989, West Coast Adhesive Manufacturers' Association 1966). The total TS values were higher in PF-bonded panels (particleboard and OSB), with the TS of PF-bonded particleboard increasing over 40 percent following JIS-B treatment and ASTM six-cycle procedures. On the other hand, MDI-bonded panels, particleboard (MDI), and MDF (MDI) showed lower TS values than PF-bonded panels. The changes in TS of plywood were less than 10 percent for all of the accelerated aging treatments examined.
Particular attention was paid to increases in TS occurring during the wet condition of each cycle when TS was at the maximum. This occurred after soaking in water at 20[degrees]C for JIS-B, soaking in water at 66[degrees]C for APA D- 1, soaking in water at 20[degrees]C for V313, the second steaming step for ASTM six-cycle, and pressure soaking for VPSD. The TS of boards vs. the number of aging cycles (t) is given by:
FEED