IMPACT OF WETTING/OVEN-DRYING CYCLES ON THE MECHANICAL AND PHYSICAL PROPERTIES OF ORIENTED BOARD

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IMPACT OF WETTING/OVEN-DRYING CYCLES ON THE MECHANICAL AND PHYSICAL PROPERTIES OF ORIENTED BOARD ABSTRACT Siim Kallau, Regino Kask, Harri Lille, Alexander Ryabchikov Estonian University of Life

ee The objective of this study was to explore some physical and mechanical properties and the dimensional stability of oriented strand board OSB/3 after 1 cycles of soaking/oven-drying.

1 IMPACT OF WETTING/OVEN-DRYING CYCLES ON THE MECHANICAL AND PHYSICAL PROPERTIES OF ORIENTED BOARD ABSTRACT Siim Kallau, Regino Kask, Harri Lille, Alexander Ryabchikov Estonian University of Life Sciences, Institute of Forestry and Rural Engineering Address: Fr. R. Kreutzwaldi 5, 5114, Tartu, Estonia Phone: The objective of this study was to explore some physical and mechanical properties and the dimensional stability of oriented strand board OSB/3 after 1 cycles of soaking/oven-drying. The properties to be determined were BS (bending strength), MOE (modulus of elasticity in bending), JH (Janka hardness) and thickness swelling (TS). The moisture content of the samples was altered by soaking for 24 hours and drying in a ventilated drying box. The study consisted of carrying out 3-point bending tests and static hardness tests with the INSTRON Universal Testing System 3369, deflection was measured by an optical gauge (Advanced Video Extensometer ). The sensitivity of the measured data was studied and the expanded uncertainties of the computed mean values are presented. An analytical equation was used for approximation of the change in the physical and mechanical properties of the samples depending on the number of cycles. It was shown that BS and MOE were affected by the number of soaking and drying cycles and they decreased significantly. The values of thickness, swelling and JH of the test pieces decreased significantly faster after the first three cycles and afterwards the values stabilized. The final values of BS, MOE, JH and TS after 1 cycles were 51-56%, 46%, 43% and 133% from the initial values, respectively. Key words: oriented strand board, bending strength, modulus of elasticity, static hardness, thickness swelling INTRODUCTION The oriented strand board (OSB) should be according to EVS EN 3:26 Grade OSB/3 moisture resistant. The mechanical properties of OSB are generally similar to those of particleboard, but it has a greater thickness swelling and poorer surface smoothness. Particleboards are used for cladding wall and ceiling indoors or outdoors as a floor decking material and a wind barrier. This kind of material is also applied in load bearing structures as the rigidity material and is also used as the formwork of concrete casting. The classifications of the OSB and the requirements of its mechanical properties are presented in EVS-EN 3:26. The goal of this study was to explore two OSB/3 panels with the thickness of 12 mm (produced by the manufacturer KRONOSPAN Riga SIA, former Bolderaja Ldt, Latvij. The OSB/3 panel as a wood-based sheet is a hygroscopic material and its dimensional stability and the mechanical and physical properties are dependent on the moisture content, number of soaking/oven-drying cycles and the ambient temperature. Changes in bending strength (BS), modulus of elasticity in bending (MOE), Janka hardness (JH), thickness swelling (TS) at different soaking/oven-drying cycles were investigated (Kallau, 214). The basic method is to soak the samples in water during a fixed period of time (24 h), to dry them in a ventilated drying box a certain number of cycles 59 and then test them using the computer-controlled mechanically actuated universal test machine INSTRON After these procedures, output of data and analysis of the obtained results take place. The sensitivity of the measured data was studied and the expanded uncertainties of the computed mean values are presented. An analytical expression was used to approximate the experimental data for the investigated BS, MOE and JH depending on their soaking/oven-drying cycles. Experimental procedure and methods The BS and the MOE were found by a three-point bending arranged in accordance with EN 31 (1993 using the test machine INSTRON 3369 (Fig. 1). Deflection for calculating the modulus of elasticity was measured by an optical gauge (Advanced Video Extensometer ).

2 Swelling in thickness of the samples was determined according to EN 317:2 before the bending test at the cross section where hardness was determined (see Fig. 1. Bending force, N real curve 2 approximation line Deflection, mm Figure 1. A photo of three-point bending test and of the Advanced Video Extensometer in the upper right corner (; schematic view of threepoint bending test; location of testing area for determining static hardness and thickness swelling ( The experiments were made with 11 series (minimum numbers of samples in a series were twelve). The samples were cut from the board in the following directions: one in the longitudinal (major) axis and the other in the transversal (minor) axis. The MOE was determined by the following formula (see EN 31 (1993) E m ( ) 3 ( ) 3 l1 F2 F1 4bt a a 2 1, (1) where l 1 is the length between the supports, (24 mm); b is the width of the sample, (5±1 mm); t is the thickness of the sample, (12 mm); F 1 and F 2 are 1% and 4% of maximum bending force, respectively; a 1, a 2 are deflections according to the loads F 1, F 2, respectively (see Fig. 2. The BS was calculated by the following formula (see EN 31 (1993) 3Fmax l1 fm, 2 (2) 2bt where F max, is the maximum load, N. The absorption of water of the test samples after a soaking time of 24 h was about 4% and was determined according to EN 322:22. The samples were dried to a moisture content of about 8% in a ventilated drying box at 65 C± 1º and at this moisture content all investigated properties were determined. The JH was determined in the middle of the end area 5 5 mm 2 of the samples before the bending test in accordance with ISO 335:1975 (see Fig Lo ad F Deflection a1 Figure 2. Dependence of deflection on bending force ; load-deflection curve within the range of elastic deformation [EN 31] Calculation of the uncertainty of the measurements was done according to EN 326-1:22. The following linear-fractional function was used to approximate the obtained experimental data for the investigated properties depending on the soaking/oven-drying cycles (Lille et al., 214) ( d( Y Y )/( cx d) ) Y, Y( x) i f f (3) where Y i, Y f are the calculated initial (x = ) and final values of the investigated properties, x is the number of cycles, and c and d are constants. The initial and final values of the properties and constants should be determined so that the measured experimental data are approximated in the best way by minimizing the square of error (least squares regression). This problem was solved by using the programme Mathcad 15. with the regression function genfit(vx,vy,vg,f). RESULTS AND DISCUSSION The investigated OSB/3 panels were tested according to EN 31:22 and ISO (E); the obtained BS, MOE and JH are presented in Figs. 3, 4 and 5, respectively. All experimental data were approximated by Formula 3. In Figs. 3b, 4b and 5b the mean values for one cycle are presented. The

3 experimental data of all properties fluctuated to a great extent. According to EN 3, the minimum allowed value of BS for the major axis was 2 N/mm 2, the experimentally obtained mean after the third cycle was 18.7 N/mm 2. The corresponding values for the minor axis was 1 N/mm 2 and after the second cycle was 8.99 N/mm 2. The BS after the first soaking cycle (24 h) at a moisture content of 4% was 52% and 54% of the values determined for airdry samples (8%) for the major axis and for the minor axis, respectively (Kask et al., 211). This value is below the valid minimum for the standard (EN 3:26). Modulus of elasticity, N/mm B1y 48 B2y 36 B1( t) B2 ( t ) c=.9 cycle -1 d=.472 c=.9 cycle -1 d= Bx, Bx, t, t Figure 4. Dependence of MOE on soaking/ovendrying cycles: bar chart; mean values of the major axis E m,i =5233 N/mm 2 and E m,f =2482 N/mm 2 ; minor axis E m,i =2334 N/mm 2 and E m,f =785 N/mm 2 Bending strength, N/mm 2 3 A1y A2y A1( t) 2 A2( t) c=.59 cycle -1 d=.458 c=.64 cycle -1 d= Ax, Ax, t, t Figure 3. Dependence of BS on soaking/ovendrying cycles: bar chart; mean values of the major axis f m,i =24.6 N/mm 2 and f m,f =2.4 N/mm 2 ; minor axis f m,i =12.2 N/mm 2 and f m,f =2.8 N/mm 2 Modulus of elasticity, N/mm C1y 3 3 C1( t) c=.35 cycle -1 d= Cx, t Figure 5. Dependence of JH on soaking/ovendrying cycles: bar chart; mean values of the major axis H 8c,i =34.2 N/mm 2 and H 8c,f =15.3 N/mm 2 The gradient of the investigated properties was greater after the first soaking/oven-drying cycle: 24% for BS for the major axis and 17% for minor axis; for MOE, 18% for both axes, for JH as high as 4% and for TS, 15%. After ten soaking/ovendrying cycles all properties significantly lost their 61

4 initial values: for MOE less, 44%, for the major axis and for JH more, 57%. The BS and MOE decreased continuously with the increasing number of cycles. In air-dry conditions, also BS and MOE were more than two times greater for the major axis than for the minor axis. We can see that the proposed Formula (3) approximated the experimental data satisfactorily; constants c and d are close for BS and MOE. Figure 6. Dependence of TS on soaking/ovendrying cycles: bar chart, diagram of thickness change The TS of the board did not change significantly after three soaking/oven-drying cycles, i.e % (according to EN 3, the maximum allowed value is 15%). This has been observed in the case of wood. Swelling in wood takes place below the fibre saturation point (about 3%) at which the total amount of the water is present within the cell wall (Hiziroglu) and regardless of the number of soaking/oven-drying cycles, the dimensions do not change. This kind of phenomenon was observed to a certain extent in our experiments in the case of hardness (see Fig. 5). CONCLUSIONS 1. The final mean values determined at a moisture content of 8% after ten soaking/oven-drying cycles were: for MOE, 51% for the minor axis and 56% for the major axis, for BS 46%, for both axis, for JH, 43% and for TS, 133% of the respective initial values. 2. The maximum gradient of the investigated properties was recorded after the first soaking/ovendrying cycle: for BS, 24% for the major axis and 17% for the minor axis; for MOE, 18% for both axes, for JH, as much as 4% and for TS, 15%. 3. The proposed analytical function approximated the experimental data of BS, MOE and JH, depending on the number of soaking/oven-drying cycles, satisfactorily and allowed to predict to a certain extent the mechanical and physical properties of the samples when their values after applying a small number of soaking/oven-drying cycles were known. The presented analysis is limited to the data obtained from the experiments described above. REFERENCES EN 3:26 E. Oriented Strand Boards (OSB) - Definitions, classification and specifications. Brussels. Management Centre. 2 p. EN 31:1993. Wood-based panels - Determination of modulus of elasticity in bending and bending strength. Brussels. Central Secretariat. 7 p. EN 317:1993. Particleboards and fibreboards - Determination of swelling in thickness after immersion in water. Brussels. Central Secretariat. 5 p. EN 322:1993. Wood-based panels Determination of moisture content. Brussels. Central Secretariat. 4 p. EVS-EN 326-1:22. Wood-based panels. Sampling, cutting and inspection. Part 1: Sampling and cutting of test pieces and expression of results. Tallinn. Eesti Standardikeskus. (in Estonian). 14 p. Hiziroglu, S. Dimensional Changes in Wood. Oklahoma Cooperative Extension Service. Avaible oneline at ISO (E). Wood Determination of static hardness. International Standard. 2 p. Kallau, S. (214). Impact of Wetting and Drying Cycles on Mechanical and Physical Properties of Oriented Strand Board Depending on the Cycle Count. M. Eng. Thesis. Tartu: Estonian University of Life Sciences. 96 p. 62

5 Kask, R., Lille, H., Siim, K., Paabo, P., Sillaste, K. (211). Study of Physical and Mechanical Properties of Oriented Board Depending on Moisture Content. In: 3rd International Conference CIVIL ENGINEERING`11 Proceeding, Jelgava, Latvia, pp Lille, H., Kõo, J., Ryabchikov, A., Reitsnik, R., Veinthal, R., Mikli, V., Sergejev, F. (213). Investigation of Residual Stresses and Some Elastic Properties of Brush-Plated Gold and Silver Coatings, Key Engineering Materials, Engineering Materials and Tribology, 527, pp

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