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Effect of Boric Acid Treatment on Mechanical Properties of Laminated Beech Veneer Lumber

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Effect of Boric Acid Treatment on Mechanical Properties of Laminated Beech Veneer Lumber Silva Fennica 37(4) research articles Effect of Boric Acid Treatment on Mechanical Properties of Laminated Beech Veneer Lumber Gürsel Colakoglu, Semra Colak, Ismail Aydin, Umit C. Yildiz and Sibel Yildiz Colakoglu, G., Colak, S., Aydin, I., Yildiz, U.C. & Yildiz, S. 2003. Effect of boric acid treat- ment on mechanical properties of laminated beech veneer lumber. Silva Fennica 37(4): 505–510. Laminated veneer lumber (LVL) made from beech wood veneers treated with boric acid by using dipping method was tested for some mechanical properties following differ- ent standards. The values for treated LVL varied around the mean values of untreated LVL panels for static bending strength in the grain direction and modulus of elasticity; decreased for compression and splitting strengths, perpendicular to the grain; increased for compression strength parallel to the grain, and for Brinell hardness and pull-out strength of screw, perpendicular to surface. Keywords treated LVL, boric acid, beech veneer, mechanical properties Authors’ address Karadeniz Technical University, Faculty of Forestry, Forest Industry Engineering Section, 61080 Trabzon, Turkey E-mail [email protected] Received 24 April 2003 Accepted 4 July 2003 1 Introduction the structural properties of wood. Veneers from medium or small diameter logs are converted In spite of many alternative materials like iron, into glued parallel laminates or LVL, which is a plastic, and concrete, which can be used instead competitive alternative for structural purposes, of solid wood in different areas, wood remains as its properties are superior to those of wooden superior as a natural engineering material. Due planks (Kamala et al. 1999). to depletion of forest resources, forest products As known, boron compounds have several great industries are faced with a crisis in obtaining advantages for application as wood preservatives suitable raw materials in suffi cient amount. and fi re-retardants, including a broad spectrum Therefore, layered composite materials like ver- of activity against insects and fungi, low mam- tically and horizontally glued, laminated wood malian toxicity, low volatility, and absence of and Laminated Veneer Lumber (LVL) may be color and odour (Yalınkılıç et al. 1999). Due to used as a substitute for solid wood as they retain their hygroscopic character, they may affect the 505 Silva Fennica 37(4) research articles dimensional stability of wood. Yalınkılıç et al. Dipping method was applied to the veneers for (1995, 1999) reported that the addition of water impregnation. For that reason, a tank with 50 cm repellent polymers or hydrophobic reagents to × 30 cm × 55 cm dimensions was prepared and boron solutions could reduce this effect. the interior part of the tank was divided at equal The changes of physical properties caused by the intervals to provide equal amounts of impregna- preservative treatment of wood and the chemical tion solution to each veneer. Before impregna- and physical mechanisms of these changes have tion, veneers were conditioned at 20 °C until the been studied by numerous researchers for a long moisture content of the samples reached 7%, and time. Two major reviews have been published weighed. Veneers were dipped into 5% aqueous (Winandy 1988; LeVan and Winandy 1990). solution of boric acid for 20 minutes. After The effects of boron treatments on mechanical, impregnation, the veneers were subjected to a biological, and dimensional properties of wood second drying process in industrial conditions, and wood based materials have been widely inves- then conditioned to 7% moisture content again tigated (Laks et al. 1988, Dimri and Shukla 1991, and re-weighed. Net uptake of boric acid was Hashim et al. 1992, Dimri et al. 1992, Laks and calculated from the difference between the last Manning 1995, Yalınkılıç et al. 1999). However, weight and the initial weight. The mean reten- little is known about the strength properties of LVL tion of veneers was found as 11.5 kg/m3 (min. treated with wood preservative in general. Kamala 8.6, max. 13.2). Three replicate panels with 0.72 et al. (1999) indicated that LVL of rubber wood g/cm3 density were produced for both test and can be compared with teak in many properties. On control groups. the other hand, Militz (1991) determined that the 47% phenol formaldehyde glue resin was used diffusion of the borate in laminated beams made for producing LVL. Before the gluing process, the from spruce, pine and larch was poor when the veneers were stored in a acclimatization chamber borate preservative was applied as rod. Gomben until they had reached about 7% moisture content. and Gorman (1994) reported that the penetration Production of LVL was conducted in laboratory was inhibited in the face ply direction, apparently conditions. The adhesive mixture was applied on due to glue-lines and, incising the face plies will single bonding surfaces of veneers at approxi- likely be required for lodgepole pine LVL treated mately 160 g/m2 by using a cylinder gluing with pentachlorophenol. These studies suggest that machine. After the gluing process, nine 50 cm × the impregnation of each veneer individually 50 cm veneers were pressed with the grain direc- before the manufacturing of LVL panels may be tions of all veneers being the same. The applied an appropriate way to treat LVL material. pressure, temperature and duration were 12 N/ The aim of the present research was to exam- mm2, 140 °C and 15 minutes, respectively. ine changes in the mechanical properties of LVL obtained from beech veneers treated with boric acid. 2.2 Method Mechanical properties summarized in Table 1 were determined on LVL produced from beech 2 Material and Method veneer impregnated with boric acid. Thirty repli- cate specimens were used for each test method. 2.1 Material To evaluate the changes in mechanical proper- ties of the untreated and impregnated beech LVL Beech (Fagus orientalis Lipsky) veneers (2.1 mm panels statistically, F-test was fi rst used for each thick) obtained by rotary cutting at a plywood mill variable to test if variances were different at 0.05 in industrial conditions were used in production of signifi cance level. Depending on the test result, LVL. Test and control specimens were obtained the means were then compared with equal-vari- from the outer part of the same log. Veneers were ance t-test or unequal-variance t-test using SPSS dried up to 6–8% moisture content by using a statistical software package (Özdamar 1999). veneer dryer. 506 Colakoglu et al. Effect of Boric Acid Treatment on Mechanical Properties of Laminated Beech Veneer Lumber Table 1. Mechanical tests applied to LVL. Test Standard Dimensions of Shape of sample sample (in cm) Static bending strength DIN 52186 2 × 30 × thickness of LVL in grain direction (MOR) Modulus of elasticity DIN 52186 2 × 30 × thickness of LVL in static bending (MOE) Compression strength DIN 52185 2 × 3 × thickness of LVL in longitudinal direction Compression strength in DIN 52192 2 × 3 × thickness of LVL tangential direction of veneers Splitting strength According to 2.5 × 4.5 × thickness of LVL in transverse direction of board Kollmann (1968, p. 331) Pull-out of screw strength DIN EN 320 7 × 7 × thickness of LVL - perpendicular to surface Hardness perpendicular to According to 5 × 5 × thickness of LVL - surface Brinell 3 Results The mean values of compression strength parallel to grain of the untreated and the treated The mean values (x) of mechanical properties panels were calculated as 50.7 N/mm2 and 51.4 investigated and the related statistics are sum- N/mm2, respectively. Boric acid treatment had marized in Table 2. no remarkable effect on compression strength in As can be seen in Table 2, the mean strength longitudinal direction of LVL. However, com- values obtained from static bending test (MOR) pression strength in tangential direction of veneer were 111.5 N/mm2 for untreated panels and was signifi cantly affected by the treatment. The 105.8 N/mm2 for treated panels with boric acid. mean values obtained for the untreated and the According to the t-test, the effect of boric acid on treated panels were 18.6 N/mm2 and 12.0 N/mm2, static bending strength was statistically signifi - respectively. The differences between the mean cant but less signifi cant in practical use of LVL. values of the untreated and the treated panels were The modulus of elasticity (MOE) values of the statistically signifi cant. untreated panels were also higher than those of The mean values of splitting strength of the treated panels. However, no statistically signifi - untreated and the treated LVL panels perpendicu- cant difference was found between the untreated lar to grain and glue line were 0.343 N/mm2 and and the treated LVL panels. 0.144 N/mm2, respectively. The effect of boric 507 Silva Fennica 37(4) research articles Table 2. Mechanical test results of LVL panels (n a) = 30). Mechanical properties Untreated panels Panels treated with % changes F-test for t-test for boric acid variances means x sd b) x sd p-value p-value Static bending strength 111.5 10.62 105.8 6.42 –5.12 0.0123 0.0298 in grain direction (N/mm2) (MOR) Modulus of elasticity in 17230 2405 16584 1845 –3.75 0.1837 0.27 static bending (N/mm2) Compression strength 50.7 1.59 51.4 2.3 +1.38 0.0489 0.20 in longitudinal direction (N/mm2) Compression strength 18.6 1.88 12.0 0.56 –35.48 0.0005 1.16 E-20 in tangential direction of veneers (N/mm2) Splitting strength 0.343 0.036 0.144 0.026 –58.02 0.1029 2.7 E-32 in transverse direction of board (N/mm2) Pull-out of screw strength 1676 137.9 2100 177.1 +25.30 0.1837 8.37 E-15 perpendicular to surface (N) Brinell hardness 21.4 4.49 23.5 3.76 +9.81 0.3727 0.0557 perpendicular to surface (N/mm2) a) Number of samples b) Standard deviation acid treatment on splitting strength was statisti- 4 Discussion cally signifi cant according to the t-test.
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