Mechanical Durability of Several Wood-based Panels Evaluated by Outdoor Exposure Tests and Accelerated Aging Treatments

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1 Mechanical Durability of Several Wood-based Panels Evaluated by Outdoor Exposure Tests and Accelerated Aging Treatments Noboru SEKINO Professor Iwate University, Japan Hideaki Korai Research Scientist FFPRI, Japan Shigehiko SUZUKI Professor Shizuoka University Shizuoka, Japan Summary Towards the establishment of testing method to evaluate mechanical durability of wood-based panels for construction use, the Research Working Group of Wood-based Panels in the Japan Wood Research Society initiated a durability evaluation project in 4. This research project consists of 1-year indoor exposure tests, 1-year outdoor exposure tests, and a variety of laboratory accelerated aging tests. Several panel products from veneer-based to fiber-based for construction uses are being tested for mechanical properties such as bending properties, internal bond strengths and nail-joint performances. To obtain appropriate testing methods for the mechanical durability, correlations between outdoor exposure and laboratory aging treatments are investigated. This paper shows part of the results obtained from 3-year outdoor exposure tests and laboratory accelerated aging tests. 1. Introduction Mechanical durability of wood-based panels is usually evaluated by laboratory-accelerated aging tests such as simple hot water soaking, boiling, steaming, freezing, drying and their combinations. However, relationships between these treatments and actual service environments are still in question. To help answer this, researchers have reported some attempts to correlate degradation caused by outdoor aging with that by laboratory-accelerated aging [1, 2, 3]. Although correlating these results can justify a method of laboratory-accelerated aging tests, it is still uncertain how durable wood-based panels are in actual services, especially in climatic-severer countries with humid and high temperature seasons such as Asia. From the background described above, the Research Working Group of Wood-based Panels in the Japan Wood Research Society initiated a durability evaluation project for commercially made wood-based panels in 4. This research project consists of the following three parts; indoor exposure tests at four different moisture levels plus a construction of an experimental house, outdoor exposure tests conducted at eight places from Hokkaido to Kyusyu that practically cover the typical climatic characteristics over Japan, and lastly, a variety of laboratory-accelerated aging tests. Several panel products from veneer-based to fibre-based for construction uses were tested and estimated for reduction in mechanical properties such as bending properties, internal bond strengths and nail joint performances. Further, this research project is planned strategically so as not only to obtain correlations between outdoor aging and laboratory tests in order to find the most suitable testing methods, but also to propose time-conversion coefficients between results of laboratory tests and service life of wood-based panels under typical moisture/temperature environments by means of conducting a variety of indoor exposure tests [4, 5].

2 The outdoor exposure tests for nail joint durability are being conducted at four places out of eight. Since we have collected test results after three-year outdoor exposure, the present paper focuses on the reduction in bending properties and nail joint performance. Also, part of laboratory aging tests results are shown and what kind of treatment best predicts the results of outdoor exposure test is discussed by means of investigating correlations of nail shear strength in one-plane between the two tests. 2. Materials and methods 2.1 Materials and property testing Eight types of commercial wood-based panels for construction use were selected. As shown in Table 1, there are four groups from veneer-base panel to fibre-base ones and each has two types different in thickness, wood species or binder. The OSB panels were imported in 3 whilst the others were manufactured in Japan in 3. These panels were measured for bending properties (MOR, proportional limit stress and MOE), internal bond strength (IB), thickness swelling caused by a 24-h water soaking according to JIS A598. Also, nail-joint performances were accessed by the following three kinds of tests according to ASTM D 137; lateral nail resistance test, nail-head pull-through test and shear-strength test in one-plane (see Figure 1. a-c). Groups Table 1 Characteristics of the panels used for this research project Panel Types (Symbols) Thickness (mm) Density (g/cm 3 ) Adhesives Note Plywood PW-5 12 (5-ply).64 PW-3 9 (3-ply) OSB a OSB-P PB PB(PF) PF PB(MDI) 12. MDI MDF b MDF(C) 9.72 MDI MDF(NC) MUF PF PF JAS c (Type-special, grade-2, CDgrade veneer, F-four-star JAS (Grade-3, F-four-star), Strand Species; Aspen for and pine for OSB-P Three-layered, % recycles chip, JIS (18-P, F-four-star) JIS (3-M, F-four-star) a Imported from North America and Europe. b Symbol (NC) shows non construction use but high moisture resistant type. c Japanese Agricultural Standards (b) Fig.1 Nail-joint tests. Nails used are made of stainless steel with a diameter of 2.7 mm and a length of 5 mm. (a) Shear strength test in one-plane; nails were driven at a 12 mm form panel edge and load-slip curves were analyzed. (b) Lateral nail resistance test; a nail was driven at a 12 mm from panel edge and a maximum load was recorded. (c) Nail-head pull-through test: a nail was driven at a centre of 5 by 5 mm specimen and a maximum load was recorded. (a) (c) Initial properties of the test panels are shown in Table 2. The bending properties varied with the size of elements (veneer, strand, particle and fibre), degree of the element alignment, and panel

3 density. The OSB with pine strand had less anisotropic bending properties in a plane than that with aspen strand due to relatively week strand orientation, but had a better water resistance in thickness swelling because of plenty of wax added with a binder. The PB bonded with MDI resin showed higher bending properties, a higher IB and a better dimensional stability than that with PF resin. Although the lateral nail resistance depended mainly on the types of element and panel thickness, the maximum load of one-plane nail shear test did not show much difference among panel types because the fracture was caused by pulling-out of nail from lumber at more than half of the test specimens. Symbols MOR (MPa) Table 2 Initial properties of the panels tested (N=3) Nail-joint performances (kn) MOE IB TS(%)* Maximum Nail-head (GPa) (MPa) Lateral resistance shear load pull-through PW PW OSB-P PB(PF) PB(MDI) MDF(C) MDF(NC) In the same row, upper and lower values are parallel and perpendicular to face grain, respectively. *24-h water immersion. 5: (North) Table 3 Climatic conditions of outdoor exposure tests (4, 5, 6) 1: Asahikawa 3: Noshiro 2: No Annual average temperature (C) Annual precipitation (mm) Zone 1 7.2, 6.8, , 99, 153 A 2 1.6, 1.1, , 11, , 11.1, , 1513, , 13.6, , 193, , 13.8, , 17, , 17., , 49, - D B C 6: Shizuoka 7: (South) 8: 4: , 16.4, , 773, 1197 E , 16.8, , 2477, 2285 F 2.2 Outdoor exposure tests Eight places listed in Table 3 were selected for the outdoor exposure tests. They can be classified into six zones by the combination of annual average temperature (T a ) and total precipitation (P t ). The combinations (T a, P t ) for each zone were A (low, little), B (low, middle), C (middle, middle), D (high, middle), E (high, little) and F (high, much). Figure 2 shows a view of the test for each place. Panel specimens with the size of 3 cm by 3 cm were placed on vertical racks facing south. Samples with an exposure angle of 45 degree were added at the place of No.4 to investigate the

4 influence of daylight intensity and remaining rainwater. Four panel edges of the specimens were given a protective coating except the specimens in which the nails were driven (see Fig. 2 c). a) Asahikawa b) c) Nail-driven sample d) Noshiro f) (45 deg.) e) g) (N) h) Shizuoka Fig. 2 View of outdoor exposure tests Nail-driven samples are being tested at four places;,, (South) and i) (S) j) The tests began in April 4, and 2 to 4 specimens for each panel are supposed to be removed from the racks after 1, 2, 3, 5, 7 and 1 years. The removed specimens are dried at for 24 hours and are then reconditioned at and %RH for one week before testing. After calculated for weight and thickness changes to the initial values, the specimens are measured for MOE, proportional limit stress, MOR, IB, and nail-joint performances by the same methods as conducted for initial values. Note that the nail shear strength test was conducted by using non outdoor-exposed lumber (SPF-2x4, g/cm 3 ) to investigate reduction of nail-joint performance caused only by degradation of the panel specimen itself. 2.3 Accelerated aging treatments In general, mechanical durability performances of wood based panels are evaluated by either internal bond strength or bending strength after subjected various aging treatments. Since wet bending tests specified in JIS (Japanese Industrial Standard) are simple and provide results quickly, usefulness of the tests methods were discussed taking correlations with other treatments such as V313 and ASTM-6c into consideration [6], which led to an establishment of the ISO 585 (wet bending strength; Methods A and B). Laboratory aging treatments applied for nail-joint tests in this experiment are based on the wetbending treatments, and the differences are with or without a freezing process and whether nail tests are conducted in a dry or wet condition. Three kinds of nail test described before were conducted for six replicates for each panel after the specimens were subjected to the following eight aging treatments: 1) A-wet; hot water immersion at 7 for 2 h, testing at a wet condition, 2) A-dry; hot water immersion at 7 for 2 h, followed by drying at 7 for 24 h, testing at a dry condition, 3) Afdry; hot water immersion at 7 for 2 h, followed by freezing at -12 for 24 h and drying at 7 for 24 h, testing at a dry condition, 4) B-wet; boiling water immersion for 2 h, testing at a wet condition, 5) B-dry; boiling water immersion for 2 h, followed by drying at 7 for 24 h, testing at a dry condition, 6) Bf-dry; boiling water immersion for 2 h, followed by freezing at -12 for 24 h and drying at 7 for 24 h, testing at a dry condition, 7) ASTM-6c method specified in ASTM D 137, 8) V313 method specified in EN321.

5 3. Results and discussion 3.1 Reduction of bending properties caused by outdoor exposure As shown in Figure 3, the retention of MOR deeply depended on panel types and test places, especially on the degree of precipitation. Among the mat-formed panels tested the MDFs showed the highest retentions and the PB bonded with MDI resin followed, while the OSB with aspen strand lost to percent of the initial strength after exposure for three years. MOR retention (%) Asahikawa Noshiro Shizuoka (S) (N) Miyakonojyo Note: For each plot group, left, center and right plots show 1st, 2nd and 3rd year, respectively. PB(PF) PB(MDI) MDF(NC) MDF(C) OSB-P PW-5 PW-3 Fig. 3 Reduction of MOR caused by 3-year outdoor exposure tests MOR retention (%) PB(PF) PB(MDI) MDF(NC) MDF(C) OSB-P Thickness increase (%) Fig. 4 Relationships between thickness increase and MOR retention Figure 4 shows relationships between panel thickness increase and retention of MOR for matformed panels tested. All data obtained at the eight test locations was used and plotted with a same mark irrespective of exposure period. There was a tendency for MOR retained to decrease linearly to thickness increase. Closer examination of the results, however, suggests that the relationship is

6 affected by panel type since the OSBs with elements larger than the PBs are plotted at upper position. The shape and size of element consisting panel may affect the relationship. 3.2 Reduction of nail-joint performances caused by outdoor exposure Climatic factors, such as rain, wind, sunlight, temperature and humidity changes, may cause panel surface erosion and weight loss can be used as a basic index to evaluate material degradation. Figure 5 shows examples of weight loss of the outdoor-exposed specimen for nail-joint performance. The degree of weight loss depended on panel types and test locations. The panel with the highest resistance was the MDF(C) and the maximum weight loss was less than 3 % after 3-year exposure. Plywood and PB followed this and the weight loss was less than 9 %, whereas the maximum weight loss in reached almost 3 % in due to biological degradation. Weight loss (%) MDF (C) Weight loss (%) Fig. 5 Examples of weight loss caused by outdoor exposure Thickness increase at panel edges (%) MDF(C) Thickness increase at panel edges (%) Fig. 6 Examples of thickness increase at panel edges caused by outdoor exposure Thickness change in mat-formed panels is an important factor in evaluating panel durability, since it relates to the mechanical properties as discussed at the previous section (Fig. 4). This is especially true when nail-joint performance is discussed because the panel specimens for nail-joint performance were exposed outdoors without any protection of the panel edges. Figure 6 shows examples of thickness increase at the panel edges. Among the mat-formed panels tested the most thickness-stable panel was MDF(C) and the thickness increase after 3-year exposure was less than 3 %; this value was equivalent to that of the plywood tested. In contrast to MDF, exhibited the greatest thickness increase ranging 12 to 24 % even after one-year outdoor exposure. 4

7 As shown in Figure 7, retention of lateral nail resistance (LNR) caused by 3-year outdoor exposure ranged from 3 to percent and and PB(PF) tended to show the largest reductions at all test locations. The least reductions were recognized in MDF(C) and it retained more than % of the initial LNR. The reason for this may lie in the small thickness increase shown in Figure 6. Nail-head pull-through resistance (NHR), however, was not as affected as much as LNR; reduction of NHR ranged more than 5 percent. A main reason for this is that test specimens for NHR were prepared from around the centre of the test panels (see Fig. 2 c) at where raindrop penetration is less compared to panel edges. 1 PW-5 // 1 MDF(C) Retainded LNR (%) Retainded LNR (%) Retainded LNR (%) 1 PB(PF) Retained LNR (%) Fig. 7 Examples of reduction of lateral nail resistance (LNR) caused by outdoor exposure 3.3 Laboratory aging method that best predicts load-slip curves of nail joint subjected to outdoor exposure Load (kn) B-dry B-wet ASTM Af-dry Bf-dry A-dry A-wet V313 PB (PF) 2-year outdoor exposure Slip (mm) Figure 8 shows an example of load-slip curves in nail shear strength test in one-plane. The load-slip curve of PB(PF) subjected to 2-year outdoor exposure in seems to be similar to that subjected to B- dry aging treatment. Which laboratory treatment can reproduce a load-slip curve caused by outdoor exposure was evaluated by calculating the value, α, defined by the following equations; pi =(P a i-p e i)/ P e i α=σ( pi) 2 / n where P ai and Pei are load at the same joint slip after laboratory aging and outdoor exposure, respectively. Data acquisition interval was.2 mm for i 3 and.5mm for 3<i. Fig. 8 Load-slip curves of nail joint for PB(PF) subjected to 2-year outdoor exposure in and various laboratory aging treatment

8 Table 4 shows part of the results of this analysis and a laboratory aging method that exhibits the smallest α in a same row is thought to best reproduce a load-slip curve subjected to 2-year outdoor exposure. The results show that B-dry method is likely to fit better than others for the mat-formed panels tested. Table 4 Comparison of α values (indices of reproduction of load-slip curves) Laboratory aging methods A-wet B-wet A-dry B-dry Af-dry Bf-dry V313 ASTM-6c PW-5 PW-3 PB(PF) PB(MDI) MDF(C) Concluding comments In the present paper part of results from three-year outdoor exposure tests were discussed. At the moment this paper is written, however, we have not completed an analysis due to a numerous collection of experimental data from this research project. More detailed analysis data on the effects of climatic condition on reductions in mechanical properties will be showed at the presentation in WCTE 8. Also, correlations of nail-joint performances between 3-year outdoor aging and laboratory tests will be showed. Outdoor exposure tests have a lot of weaknesses such as being time-consuming and difficult to continue, differences caused by the place tested, and last not least being extremely different from actual environments where wood-based panels are used. However, it is true that outdoor exposure tests are ranked as a natural-base accelerated aging treatment and are essential to evaluate the durability performance of wood-based products. As has been mentioned, this research project is planned strategically so as to overcome weaknesses in outdoor exposure tests, and hopefully within 2 years (after we have collected test results of 5-year outdoor exposure and 5-year indoor exposure), we are able to propose test methods that predict service life of wood-based panels under some typical service conditions. Acknowledgement The authors wish to thank all the members of this research project for their valuable contributions and discussions; Mr. Makoto Fukino, Dr. Hidefumi Yamauchi, Mr. Tadashi Higashino, Mr. Kazuo Ohashi, Dr. Kenji Umemura, Dr. Youichi Kojima, Mr. Masayuki Ikeda, Mr. Hidetaka Nogami, Dr. Yoshiyasu Fujimoto, Mr. Shinji Iwasaki, Mr. Hideki Morita and Mr. Akihiro Matsumoto. References [1] Hann R. A., Black J. M. and Blomquist R. F., How durable is particleboard?, Forest Prod. J. 12, 1962, pp [2] River B. H., Outdoor aging of wood-based panels and correlation with laboratory aging, Forest Prod. J 44 (11/12), 1994, pp [3] Okkonen E.A. and River B. H, Outdoor aging of wood-based panels and correlation with laboratory aging, Part 2 Forest Prod. J. 46 (3), 1996, pp [4] Sekino N., Korai H. and Suzuki S., How durable are wood-based panel products -Toward the establishment of durability estimation methods-, Proceedings of the ninth European panel products symposium, 5, pp [5] Sekino N. and Korai H., Secondary research project for evaluation of the durability performance of wood-based panel for construction use, Journal of Timber Engineering, Vol. 18, No. 4, 5, pp [6] Suzuki S. and Sekino N. Wet-bending test for evaluating the durability performance of matformed panel products, Proceedings from The 6 th Pacific Rim Bio-Based Composites Symposium, Vol. 2, 2, pp , Portland, Oregon, USA