- Email: [email protected]
Properties of solid wood and laminated wood lumber manufactured by cold pressing and heat treatment
Accepted Manuscript Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment Jin Heon Kwon, Rang-Ho Shin, Nadir Ayrilmis, Tae Hyung Han PII: DOI: Reference:
S0261-3069(14)00401-4 http://dx.doi.org/10.1016/j.matdes.2014.05.032 JMAD 6510
To appear in:
Materials and Design
Received Date: Accepted Date:
30 March 2014 19 May 2014
Please cite this article as: Kwon, J.H., Shin, R-H., Ayrilmis, N., Han, T.H., Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment, Materials and Design (2014), doi: http:// dx.doi.org/10.1016/j.matdes.2014.05.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment Jin Heon Kwon, Rang-Ho Shin, Nadir Ayrilmis (), Tae Hyung Han Jin Heon Kwon, Professor, Ph.D. Department of Forest Biomaterials Engineering College of Forest and Environmental Sciences Kangwon National University 200-701 Chuncheon city Republic of Korea Email: [email protected]
Rang-Ho Shin, Professor, Ph.D. Samcheok Campus of Design College Kangwon National University Samcheok, Rebuplic of Korea E-mail: [email protected] Nadir Ayrilmis, Associate Prof., Ph.D. () Corresponding Author Istanbul University, Forestry Faculty Department of Wood Mechanics and Technology Bahcekoy, Sariyer, 34473, Istanbul, Turkey Tel.: +90 212 226-1100 / 25083 Fax: +90 212 226-1113 E-mail: [email protected]
Tae Hyung Han, Research Wood Composite Scientist, Ph.D. Department of Forest Biomaterials Engineering College of Forest and Environmental Sciences Kangwon National University 200-701 Chuncheon city Republic of Korea Email: [email protected]
1
Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment Abstract Physical, mechanical, and morphological properties of solid wood lumbers which were cold pressed in a press and then heat treated in a kiln. Two different kinds of domestic thinning small-diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were used in this study. First 50 mm thick lumbers were cold pressed until 35 mm (30% of control lumber) using a stopper for 5 min. Then the cold pressed lumbers were heat treated in an electric kiln at 180 °C for 6, 12, 24, and 48 h. To increase the utilizability of woods, the LVLs were produced from 4 mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48 h-, 24 h-, 12 h-, and 6 h-treated veneers and untreated veneer (from top layer to bottom layer). The shrinkage rates of softwood and hardwood were considerably decreased with increasing temperature. The mechanical properties of heat treated samples were better than those of unpressed control samples. The bending strength and modulus of elasticity of the LVLs manufactured from cold pressed and then heat treated lumbers were slightly lower than those of untreated woods. The colour values obtained from the heat treated wood samples showed a clear effect of the temperature on the colour changes.
Keywords: colour difference, shrinkage, heat treatment, laminated veneer lumber, mechanical properties, wood
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1. Introduction Heat treatment is a process that improves wood performance and leads to improved water repellency, rebuilding duced shrinkage and swelling, higher decay resistance, reduced extractive contents, lower equilibrium moisture content and increased thermal insulating capacity [1-7]. Heat treatment of wood has been developed in Europe during 1990s [5]. Various companies in Holland and France have been producing commercially heat treated wood. Heat treatment adversely influenced most of mechanical characteristics of different wood species [5]. Having higher treatment temperature would also enhance biological durability but some undesirable effects of the treatment such as reduction of strength and hardness of wood are inevitable [6].Therefore, use of heat treated wood in structural application is not suggested due to reduction of mechanical properties of the member ranging from 10% to 30% [5].
More recently the interest in heat treatment processes has been renewed. This renewed interest is due to the declining production of durable timber, to the increasing demand for sustainable building materials, to the deforestation of especially sub-topical forests, and to the increased introduction of governmental restrictive regulations reducing the use of toxic chemicals. It is well known that the heat treatment is often used to improve the dimensional stability of solid wood, but it cases to decrease mechanical properties of solid wood [3]. This restricts the structual applications of heat tretated wood. The densification of solid wood by cold pressing improves the mechanical properties of wood [8]. Based on the extensive literature search, there is no any study regarding properties of solid wood which was cold pressed in a platen press and then heat treated in an oven. Before the heat treatment application to the wood, cold pressing can minimize the decrement in the mechanical properties of heat treated wood. In this study, physical, mechanical, and colour properties of
3
solid wood and laminated veneer lumber (LVL) manufactured by cold pressing and heat treatment were investigated.
2. Experimental details Two different kinds of domestic thinning and small diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were harvested in Taebaek Samcheok, Kangwon in South Korea. The tree ages and breast diameters were 28 and 43 cm for Ginko tree and 25 and 48 cm for Tilia tree, respectively. First 50 mm thick lumbers were cold pressed until 35 mm (30% of control lumber) using a stopper for 5 min. Then the cold pressed lumbers were heat treated in an electric kiln at 180 °C for 6, 12, 24, and 48 h. To increase the utilizability of woods, the LVLs were produced from 4 mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48 h-, 24 h-, 12 h-, and 6 h-treated veneers and untreated veneer (from top layer to bottom layer) (Fig 1-f). The PVA adhesive was spread at the rate of 250 g/m2 on a single bonding surface of the veneers using a laboratory manual cylinder. After applying the adhesive, five veneers were placed with their grain directions parallel to the grain direction of the neighbour veneers. The LVL mats were cold pressed at 1 MPa for 24 h using compression clamps. The physical and mechanical properties of the wood and LVL samples conditioned at 65% relative humidity and 20°C were determined according to Korean Standards.
The effect of heat treatment on the anatomical structure of cold pressed wood samples was also investigated using scanning electron microscope (SEM) images. The samples were previously coated with a thin layer of gold in a vacuum chamber of sputter coater for 2 min. The micrographs were taken from the cross section of the control and heat treated samples.
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3. Results and discussion The physical, mechanical, and colour properties of the untreated and treated wood samples are presented in Table 1. As expected, the density of the samples was increased by cold pressing. The density of the cold pressed Ginko and Tilia woods increased by17% and 15%, respectively, as the 50 mm thick lumbers were cold pressed until the 35 mm (30% of control lumber). The high strain in the platen press drastically reduced the volume of void spaces in the samples and deformed the cell lumens (particularly evident in the vessels). The cell walls of the cold pressed wood samples were deformed without any fracture of the cell walls (Fig. 2). The densities of the cold pressed samples decreased with increasing treatment time at 180 °C. The decreases in the density were mainly related to mass loss, thus increased in the mass loss from the thermal-treated wood.
The shrinkage of cold pressed wood samples was lower than that of the unpressed control samples. The shrinkage of cold pressed wood samples was considerably decreased with increasing heat treatment duration. As the treatment duration increased from 6 to 48 h, the tangential shrinkage ratio (green to oven dry) of Tilia wood samples decreased from 7.41 to 4.92% while it was found to be 8.60% for the unpressed wood samples. A similar trend was found for Ginkgo wood samples.
The compressive strength, modulus of rupture (MOR) and modulus of elasticity (MOE) in bending of the cold pressed samples were higher than those of the unpressed control samples. The mechanical properties of the cold pressed wood samples decreased with increasing treatment temperature. This was confirmed by the SEM images. The SEM images taken from the cross sections of cold pressed samples treated at 180 °C for 6-48 h are presented in Figures 3 and 4. The SEM images revealed that there was some distortion and
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modification of the cell walls due to the heat treatment. The cracks in the cell walls of Ginko and Tilia woods increased as the treatment duration increased from 6h to 48h. The lowest mechanical properties were observed for the samples treated at 180°C for 48 h. Previous studies have clearly shown that strength properties of wood are adversely influenced by heat treatment due to thermal degradation [1,3,6]. This loss becomes more prominent if temperature and exposure time are increased [1]. The cellulose and hemicelluloses in the cell wall are depolymerized resulting in strength reduction of wood [6].
Depolymerization of cellulose and hemicelluloses due to thermal degradation makes wood more brittle and mechanical strength decreases depending on the level of thermal treatment [9-11]. Unsal and Ayrilmis [12] found that the maximum decrease in compression strength parallel to grain in Turkish river red gum (E. camaldulensis Dehn.) wood samples was 19.0% at 180 °C and 10 h. In other study, Esteves et al. [13] determined different strength properties of pine and eucalyptus species reduced ranging from 15% to 40% due to heat treatment. Heat treated spruce and pine also had lower strength properties within the range of 5–49% as a result of heat treatment [14]. In our study, the mechanical properties and dimensional stability of the heat treated samples were better than those of the control samples because the cold pressing was applied to the samples before the heat treatment (Table 1). The MOR and MOE of the wood samples treated at 180°C for 6 and 12 h durations were higher than those of the unpressed control samples (Table 1). Similar properties were observed for Tilia wood samples treated at 180°C for 6 h, 12 h, and 24 h durations. The compressive strength values of the cold pressed Tilia wood samples treated at 180 °C for 648 h were higher than that of the unpressed control samples. It appears that cold pressing can be used as a potential approach to enhance mechanical properties of the heat treated Ginko and Tilia woods used in this experimental study so that they can be used more efficiently
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during their service life without having any mechanical and dimensional movement problems.
The colour values of the wood samples showed a clear effect of the temperature on the colour changes. The UCS (uniform chromaticity scale) colour system-lab diagram of Tilia and Ginkgo woods are presented in Figures 5 and 6. The L*, a* and b* values of the samples decreased with increasing treatment temperature (Table 1). The highest colour difference was found for the samples treated at 180°C for 48 h while the lowest colour difference was found for the samples at 180°C for 6 h. The L, a, and b values of the cold pressed samples decreased with increasing treatment temperature while ∆E*ab increased. According to the results obtained, the samples treated at 180 °C for 48 h were found to be darker than the other samples (Figs. 7 and 8). Similar results were reported in previous studies [15-17]. For example, Gunduz et al. [15] found that that ∆L of wild Pear (Pyrus elaeagnifolia Pall.) wood treated at 180 °C for 6 h was 43.07 while it was 75.85 for the control samples. The cause of colour changes after heat treatment is the mainly the hydrolysis of hemicelluloses [18]. The results revealed that the extent of thermal degradation was directly related to the extent of the darkening of the colour properties.
The composition of LVL samples is presented in Table 2. The MOR and MOE of the LVLs manufactured from the heat treated veneers were slightly lower than those of untreated wood samples. The bottom layer of the LVLs consisted of the untreated veneer while the top layer consisted of the heat treated veneer treated at 180 °C for 48 h. The weakness of a timber beam lies in the brittle tension zone [19-21]. The critical tension zone in the LVL is much more important than the compression zone. For this reason, the heat treated veneers, particularly with high temperature and long duration, were positioned near the top layer. This
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structure helps make the top layer, which is decorative, more visually appealing. The delamination ratio (%) after 24 h water soaking decreased as the thermal treatment duration increased.
4. Conclusions Cold press application to the wood samples before the heat treatment could be a solution to reduce the decrement in the mechanical properties of solid wood. Dimensional stability and mechanical properties of the wood samples which were cold pressed and then heat treated in a oven were considerably higher than the unpressed control samples. The mechanical properties of the cold pressed wood samples treated at 180 °C for 6 h were considerably higher than those of the unpressed control samples. The increment in the temperature decreased the mechanical properties of wood samples while it improved the shrinkage ratio. The MOR and MOE of the LVLs manufactured from the heat treated veneers were slightly lower than those of untreated woods. It appears that dimensional stability and mechanical properties of low quality woods can be improved by using cold pressing and then heat treatment technique. Based on the findings obtained from the present study, it can be said that the wood and LVL manufactured by cold pressing and then thermal treatment at 180 °C for 6 h appear to a practical choice for applications requiring low thickness swelling and high mechanical performance, particularly when exposed to high levels of moisture.
Acknowledgement This study was supported by 2014 Research Grant from Kangwon National University and with the support of the MSIP (Ministry of Science, Ict & Future Planning).
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References [1] Priadi T, Hiziroglu S. Characterization of heat treated wood species. Mater Design 2003;49:575-82.
[2] Nazerian M, Ghalehno MD. Physical and mechanical properties of laminated veneer lumber manufactured by poplar veneer. J Agric Sci Technol 2011;A1:1040-45.
[3] Bekhta P, Niemz P. Effect of high temperature on the change in colour, dimensional stability and mechanical properties of spruce wood. Holzforschung 2005;57: 539-46.
[4] Ringman R, Pilgård A, Brischke C, Richter K. Mode of action of brown rot decay resistance in modified wood: a review. Holzforschung 2013;68: 239-46.
[5] Bakar BFA, Hiziroglu S, Tahir PM. Properties of some thermally modified wood species. Mater Design 2013;43:348-55.
[6] Korkut S, Hiziroglu S. Effect of heat treatment on mechanical properties of hazelnut wood (Corylus colurna L.) Mater Design 2009;30:1853-58.
[7] Ebner M, Petutschnigg AJ. Potentials of thermally modified beech (Fagus sylvatica) wood for application in toy construction and design. Mater Design 2007;28:1753-59.
[8] Kutnar A, Sernek M. Densification of wood. Acta Silvae et Ligni 2007;82:53-62.
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[9] Ayrilmis N, Jarusombuti S, FueangvivatV, Bauchongkol P. Effects of thermal treatment of rubberwood fibres on physical and mechanical properties of medium density fibreboard. J Trop For Sci 2011;23:10-16
[10] Bakar BFA, Hiziroglu S, Tahir PM. Properties of some thermally modified wood species. Mater Design 2013;43:348-55.
[11] Esteves BM, Pereira HM. Wood modification by heat treatment: a review. Bioresources 2009;4(1):370-404.
[12] Unsal O, Ayrilmis N. Variations in compression strength and surface roughness of heattreated Turkish river red gum (Eucalyptus camaldulensis Dehn.) wood. J Wood Sci 2005;51:405-09.
[13] Esteves B, Marques AV, Domingos I, Pereira H. Influence of steam heating on the properties of pine (Pinus pinaster) and eucalypt (Eucalyptus globulus) wood. Wood Sci Technol 2007;41:193-207.
[14] Korkut D, Korkut S, Dilik T. Effect of heat treatment on some mechanical properties of laminated window profiles manufactured using two types of adhesives. Int J Mol Sci 2008;9:454-63.
[15] Gunduz G, Aydemir D, Karakas G. The effects of thermal treatment on the mechanical properties of wild Pear (Pyrus elaeagnifolia Pall.) wood and changes in physical properties. Mater Design 2009:30:4391-95.
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[16] Unsal O, Korkut S, Atik C. The effect of heat treatment on some properties and colour in eucalyptus (Eucalyptus camaldulensis dehn.) wood. Maderas-Cienc Tecnol 2003;5:14552.
[17] Cademartori PHG, Schneid E, Gatto DA, Stangerlin DM, Beltrame R. Thermal modification of Eucalyptus grandis wood: variation of colorimetric parameters. MaderasCienc Tecnol 2013;15(1):57-64.
[18] Yildiz S, Yildiz UC, Tomak ED. The effects of natural weathering on the properties of heat-treated alder wood. Bioresources 2011;6(3):2504-21.
[19] Mohamad WHW, Razlan MA, Ahmad Z. Bending strength properties of glued laminated timber from selected Malaysian hardwood timber. Int J Civil Environ Eng 2011;11(4):7-12.
[20] Saviana J, Sosa Zitto MA, Piter JC. Bending strength and stiffness of structural laminated veneer lumber manufactured from fast-growing Argentinean Eucalyptus grandis. Maderas-Cienc Tecnol 2009;11(3):183-90.
[21] Moody RC, Liu JY. Wood handbook-Wood as an engineering material. Chapter 11: Glued structural members. Gen. Tech. Rep. FPL-GTR–113. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison WI, US, 463 p.
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Table list Table 1. Table 1. The physical, mechanical, and colour properties of the untreated and treated woods. Table 2. The mechanical properties of the LVLs manufactured from veneers of control and heat treated lumbers after cold pressing.
Figure list Figure 1. Treatment process of wood samples and production of experimental LVL samples. Figure 2. SEM images of cold pressed wood samples according to pressure ratio. A: Ginkgo biloba. B: Tilia amurensis. Figure 3. SEM images of heat treated wood samples (Tilia amurensis) after cold pressing. Figure 4. SEM images of heat treated wood samples (Ginkgo biloba) after cold pressing. Figure 5. UCS (uniform chromaticity scale) color system-lab diagram of Tilia amurensis wood samples. Figure 6. UCS color system-lab diagram of Ginkgo biloba wood samples. Figure 7. Photo of untreated and treated Tilia amurensis wood samples after 180 °C heat treatment for 6-48 h. Figure 8. Photo of untreated and treated Ginkgo biloba wood samples after 180 °C heat treatment for 6-48 h.
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Figure 1. Treatment process of wood samples and production of experimental LVL samples.
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A
B Figure 2. SEM images of cold pressed wood samples according to pressure ratio. A: Ginkgo biloba. B: Tilia amurensis
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(A) Heat treatment (180℃, 6hr.)
(B) Heat treatment (180℃, 12hr.)
(C) Heat treatment (180℃, 24hr.)
(D) Heat treatment (180℃, 48hr.)
Figure 3. SEM images of heat treated wood samples (Tilia amurensis) after cold pressing.
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(A) Heat treatment (180℃, 6hr.)
(C) Heat treatment (180℃, 24hr.)
(B) Heat treatment (180℃, 12hr.)
(D) Heat treatment (180℃, 48hr.)
Figure 4. SEM images of heat treated wood samples (Ginkgo biloba) after cold pressing.
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Figure 5. UCS (uniform chromaticity scale) color system-lab diagram of Tilia amurensis wood samples.
Figure 6. UCS color system-lab diagram of Ginkgo biloba wood samples. 17
Temp. hr.
Tilia amurensis / 180℃
0
6
12
18
24
48
Figure 7. Photo of untreated and treated Tilia amurensis wood samples after 180℃ heat treatment for 6-48 h.
Temp. hr.
Ginkgo biloba / 180℃
0
6
19
12
24
48
Figure 8. Photo of untreated and treated Ginkgo biloba wood samples after 180℃ heat treatment for 6-48 h.
Highlights ► Effects of heat treatment on dimensional stability of cold pressed wood. ► Mechanical properties of heat treated samples were better than control samples. ► LVLs were made from cold pressed and then heat treated wood samples. ► Cell wall structure was adversely influenced by the heat treatment.
Table 1. The physical, mechanical, and colour properties of the untreated and treated woods.
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Density Wood species
Tilia amurensis Rupr.
Unpressed wood (control) Cold pressed wood 6h
0.53 (0.02) 0.62 (0.03) 0.47 (0.02) 0.46 (0.03) 0.46 (0.01) 0.44 (0.03) 0.46 (0.02) 0.53 (0.03) 0.44 (0.03) 0.43 (0.03) 0.43 (0.01) 0.41 (0.05)
Cold pressed wood
(g/cm³)
12 h 180°C 24 h 48 h
Unpressed wood (control) Cold pressed wood 6h Cold pressed wood
Ginkgo biloba L.
Treatment
12 h 180 °C 24 h 48 h
Shrinkage
CS (//) MOE
MOR
Colour difference
Green Green (MPa) (MPa) to air to oven (MPa dry (%) dry (%) ) (Tangential)
∆L
∆a
∆b
∆E*ab
5.10 (0.2) 4.60 (0.2) 4.66 (0.03) 4.11 (0.02) 3.86 (0.01) 3.01 (0.01) 6.50 (0.2) 5.90 (0.4) 5.97 (0.03) 5.69 (0.03) 4.23 (0.02) 4.11 (0.01)
-
-
-
-
7.40 (0.2) 6.10 (0.2) 5.16 (0.03) 5.09 (0.02) 4.79 (0.02) 4.77 (0.01) 8.60 (0.01) 8.10 (0.04) 7.41 (0.03) 5.98 (0.03) 5.01 (0.02) 4.92 (0.02)
32.9 (1.7) 35.1 (1.5) 32.7 (1.3) 31.8 (1.1) 30.3 (1.2) 26.9 (1.0) 30.9 (1.6) 42.1 (1.8) 40.9 (1.8) 40.7 (1.7) 40.4 (1.9) 32.7 (1.5)
4603 (164) 5305 (172) 5023 (165) 4852 (143) 4609 (162) 4507 (157) 5370 (166) 6542 (190) 6279 (183) 6034 (178) 5984 (162) 5378 (151)
53.4 (2.4) 58.2 (2.7 58.0 (2.1) 55.4 (1.6) 49.9 (2.0) 49.5 (1.8) 58.1 (1.9) 63.8 (2.3) 62.6 (2.4) 62.5 (2.0) 61.7 (2.2) 57.8 (1.6)
84.5 7.4 24.4 (2.8) (0.5) (1.0) 49.3 16.7 22.6 (1.1) (0.8) (0.5) 45.9 18.0 20.9 (1.5) (0.6) (0.8) 41.2 15.9 16.4 (1.2) (0.8) (0.9) 30.2 20.8 8.6 (1.5) (1.3) (1.5) 75.4 9.8 (2.6) (0.7) 49.1 16.4 (2.2) (1.0) 48.3 15.3 (2.5) (0.9) 41.3 14.9 (0.7) (0.8) 36.5 14.6 (1.7) (0.8)
CS: Compressive strength parallel to grain. MOE: Modulus of elasticity in bending. MOR: Modulus of rupture. L: Lightness. a: green-red. b: blue-yellow: Two chromatic coordinates. ∆E*ab: Colour difference. The values in the parentheses are standard deviations.
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18.4 (0.7) 19.9 (0.9) 21.6 (1.2) 17.9 (0.5) 14.2 (1.4)
30.4 (0.8) 38.9 (1.2) 44.1 (1.0) 56.6 (1.8) 23.3 (0.9) 33.9 (1.3) 41.3 (1.5) 46.7 (1.4)
Table 2. The mechanical properties of the LVLs manufactured from veneers of control and heat treated lumbers after cold pressing. Wood species
Ginkgo biloba L.
Tilia amurensis Rupr.
Arrangement of 5 veneers in the LVL (from top to bottom) 180 °C – 48 h 180 °C – 24 h 180 °C – 12 h 180 °C – 6 h Control (cold pressed veneer) 180 °C – 48 h 180 °C – 24 h 180 °C – 12 h 180 °C – 6 h Control (cold pressed veneer)
Delamination ratio Bending properties after water soaking MOE MOR (24 h) (g/cm³) (Layer ) (%) (MPa) (MPa) 1 0 2 2.1 0.52 4262 49.6 3 4.5 (0.03) (132) (1.5) 4 7.6 All layers 3.6 1 0 2 0 0.49 5012 52.7 3 2.7 (0.02) (144) (1.8) 4 6.3 All layers 2.3
Density
The values in the parentheses are standard deviations.
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S0261-3069(14)00401-4 http://dx.doi.org/10.1016/j.matdes.2014.05.032 JMAD 6510
To appear in:
Materials and Design
Received Date: Accepted Date:
30 March 2014 19 May 2014
Please cite this article as: Kwon, J.H., Shin, R-H., Ayrilmis, N., Han, T.H., Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment, Materials and Design (2014), doi: http:// dx.doi.org/10.1016/j.matdes.2014.05.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment Jin Heon Kwon, Rang-Ho Shin, Nadir Ayrilmis (), Tae Hyung Han Jin Heon Kwon, Professor, Ph.D. Department of Forest Biomaterials Engineering College of Forest and Environmental Sciences Kangwon National University 200-701 Chuncheon city Republic of Korea Email: [email protected]
Rang-Ho Shin, Professor, Ph.D. Samcheok Campus of Design College Kangwon National University Samcheok, Rebuplic of Korea E-mail: [email protected] Nadir Ayrilmis, Associate Prof., Ph.D. () Corresponding Author Istanbul University, Forestry Faculty Department of Wood Mechanics and Technology Bahcekoy, Sariyer, 34473, Istanbul, Turkey Tel.: +90 212 226-1100 / 25083 Fax: +90 212 226-1113 E-mail: [email protected]
Tae Hyung Han, Research Wood Composite Scientist, Ph.D. Department of Forest Biomaterials Engineering College of Forest and Environmental Sciences Kangwon National University 200-701 Chuncheon city Republic of Korea Email: [email protected]
1
Properties of Solid Wood and Laminated Wood Lumber Manufactured by Cold Pressing and Heat Treatment Abstract Physical, mechanical, and morphological properties of solid wood lumbers which were cold pressed in a press and then heat treated in a kiln. Two different kinds of domestic thinning small-diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were used in this study. First 50 mm thick lumbers were cold pressed until 35 mm (30% of control lumber) using a stopper for 5 min. Then the cold pressed lumbers were heat treated in an electric kiln at 180 °C for 6, 12, 24, and 48 h. To increase the utilizability of woods, the LVLs were produced from 4 mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48 h-, 24 h-, 12 h-, and 6 h-treated veneers and untreated veneer (from top layer to bottom layer). The shrinkage rates of softwood and hardwood were considerably decreased with increasing temperature. The mechanical properties of heat treated samples were better than those of unpressed control samples. The bending strength and modulus of elasticity of the LVLs manufactured from cold pressed and then heat treated lumbers were slightly lower than those of untreated woods. The colour values obtained from the heat treated wood samples showed a clear effect of the temperature on the colour changes.
Keywords: colour difference, shrinkage, heat treatment, laminated veneer lumber, mechanical properties, wood
2
1. Introduction Heat treatment is a process that improves wood performance and leads to improved water repellency, rebuilding duced shrinkage and swelling, higher decay resistance, reduced extractive contents, lower equilibrium moisture content and increased thermal insulating capacity [1-7]. Heat treatment of wood has been developed in Europe during 1990s [5]. Various companies in Holland and France have been producing commercially heat treated wood. Heat treatment adversely influenced most of mechanical characteristics of different wood species [5]. Having higher treatment temperature would also enhance biological durability but some undesirable effects of the treatment such as reduction of strength and hardness of wood are inevitable [6].Therefore, use of heat treated wood in structural application is not suggested due to reduction of mechanical properties of the member ranging from 10% to 30% [5].
More recently the interest in heat treatment processes has been renewed. This renewed interest is due to the declining production of durable timber, to the increasing demand for sustainable building materials, to the deforestation of especially sub-topical forests, and to the increased introduction of governmental restrictive regulations reducing the use of toxic chemicals. It is well known that the heat treatment is often used to improve the dimensional stability of solid wood, but it cases to decrease mechanical properties of solid wood [3]. This restricts the structual applications of heat tretated wood. The densification of solid wood by cold pressing improves the mechanical properties of wood [8]. Based on the extensive literature search, there is no any study regarding properties of solid wood which was cold pressed in a platen press and then heat treated in an oven. Before the heat treatment application to the wood, cold pressing can minimize the decrement in the mechanical properties of heat treated wood. In this study, physical, mechanical, and colour properties of
3
solid wood and laminated veneer lumber (LVL) manufactured by cold pressing and heat treatment were investigated.
2. Experimental details Two different kinds of domestic thinning and small diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were harvested in Taebaek Samcheok, Kangwon in South Korea. The tree ages and breast diameters were 28 and 43 cm for Ginko tree and 25 and 48 cm for Tilia tree, respectively. First 50 mm thick lumbers were cold pressed until 35 mm (30% of control lumber) using a stopper for 5 min. Then the cold pressed lumbers were heat treated in an electric kiln at 180 °C for 6, 12, 24, and 48 h. To increase the utilizability of woods, the LVLs were produced from 4 mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48 h-, 24 h-, 12 h-, and 6 h-treated veneers and untreated veneer (from top layer to bottom layer) (Fig 1-f). The PVA adhesive was spread at the rate of 250 g/m2 on a single bonding surface of the veneers using a laboratory manual cylinder. After applying the adhesive, five veneers were placed with their grain directions parallel to the grain direction of the neighbour veneers. The LVL mats were cold pressed at 1 MPa for 24 h using compression clamps. The physical and mechanical properties of the wood and LVL samples conditioned at 65% relative humidity and 20°C were determined according to Korean Standards.
The effect of heat treatment on the anatomical structure of cold pressed wood samples was also investigated using scanning electron microscope (SEM) images. The samples were previously coated with a thin layer of gold in a vacuum chamber of sputter coater for 2 min. The micrographs were taken from the cross section of the control and heat treated samples.
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3. Results and discussion The physical, mechanical, and colour properties of the untreated and treated wood samples are presented in Table 1. As expected, the density of the samples was increased by cold pressing. The density of the cold pressed Ginko and Tilia woods increased by17% and 15%, respectively, as the 50 mm thick lumbers were cold pressed until the 35 mm (30% of control lumber). The high strain in the platen press drastically reduced the volume of void spaces in the samples and deformed the cell lumens (particularly evident in the vessels). The cell walls of the cold pressed wood samples were deformed without any fracture of the cell walls (Fig. 2). The densities of the cold pressed samples decreased with increasing treatment time at 180 °C. The decreases in the density were mainly related to mass loss, thus increased in the mass loss from the thermal-treated wood.
The shrinkage of cold pressed wood samples was lower than that of the unpressed control samples. The shrinkage of cold pressed wood samples was considerably decreased with increasing heat treatment duration. As the treatment duration increased from 6 to 48 h, the tangential shrinkage ratio (green to oven dry) of Tilia wood samples decreased from 7.41 to 4.92% while it was found to be 8.60% for the unpressed wood samples. A similar trend was found for Ginkgo wood samples.
The compressive strength, modulus of rupture (MOR) and modulus of elasticity (MOE) in bending of the cold pressed samples were higher than those of the unpressed control samples. The mechanical properties of the cold pressed wood samples decreased with increasing treatment temperature. This was confirmed by the SEM images. The SEM images taken from the cross sections of cold pressed samples treated at 180 °C for 6-48 h are presented in Figures 3 and 4. The SEM images revealed that there was some distortion and
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modification of the cell walls due to the heat treatment. The cracks in the cell walls of Ginko and Tilia woods increased as the treatment duration increased from 6h to 48h. The lowest mechanical properties were observed for the samples treated at 180°C for 48 h. Previous studies have clearly shown that strength properties of wood are adversely influenced by heat treatment due to thermal degradation [1,3,6]. This loss becomes more prominent if temperature and exposure time are increased [1]. The cellulose and hemicelluloses in the cell wall are depolymerized resulting in strength reduction of wood [6].
Depolymerization of cellulose and hemicelluloses due to thermal degradation makes wood more brittle and mechanical strength decreases depending on the level of thermal treatment [9-11]. Unsal and Ayrilmis [12] found that the maximum decrease in compression strength parallel to grain in Turkish river red gum (E. camaldulensis Dehn.) wood samples was 19.0% at 180 °C and 10 h. In other study, Esteves et al. [13] determined different strength properties of pine and eucalyptus species reduced ranging from 15% to 40% due to heat treatment. Heat treated spruce and pine also had lower strength properties within the range of 5–49% as a result of heat treatment [14]. In our study, the mechanical properties and dimensional stability of the heat treated samples were better than those of the control samples because the cold pressing was applied to the samples before the heat treatment (Table 1). The MOR and MOE of the wood samples treated at 180°C for 6 and 12 h durations were higher than those of the unpressed control samples (Table 1). Similar properties were observed for Tilia wood samples treated at 180°C for 6 h, 12 h, and 24 h durations. The compressive strength values of the cold pressed Tilia wood samples treated at 180 °C for 648 h were higher than that of the unpressed control samples. It appears that cold pressing can be used as a potential approach to enhance mechanical properties of the heat treated Ginko and Tilia woods used in this experimental study so that they can be used more efficiently
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during their service life without having any mechanical and dimensional movement problems.
The colour values of the wood samples showed a clear effect of the temperature on the colour changes. The UCS (uniform chromaticity scale) colour system-lab diagram of Tilia and Ginkgo woods are presented in Figures 5 and 6. The L*, a* and b* values of the samples decreased with increasing treatment temperature (Table 1). The highest colour difference was found for the samples treated at 180°C for 48 h while the lowest colour difference was found for the samples at 180°C for 6 h. The L, a, and b values of the cold pressed samples decreased with increasing treatment temperature while ∆E*ab increased. According to the results obtained, the samples treated at 180 °C for 48 h were found to be darker than the other samples (Figs. 7 and 8). Similar results were reported in previous studies [15-17]. For example, Gunduz et al. [15] found that that ∆L of wild Pear (Pyrus elaeagnifolia Pall.) wood treated at 180 °C for 6 h was 43.07 while it was 75.85 for the control samples. The cause of colour changes after heat treatment is the mainly the hydrolysis of hemicelluloses [18]. The results revealed that the extent of thermal degradation was directly related to the extent of the darkening of the colour properties.
The composition of LVL samples is presented in Table 2. The MOR and MOE of the LVLs manufactured from the heat treated veneers were slightly lower than those of untreated wood samples. The bottom layer of the LVLs consisted of the untreated veneer while the top layer consisted of the heat treated veneer treated at 180 °C for 48 h. The weakness of a timber beam lies in the brittle tension zone [19-21]. The critical tension zone in the LVL is much more important than the compression zone. For this reason, the heat treated veneers, particularly with high temperature and long duration, were positioned near the top layer. This
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structure helps make the top layer, which is decorative, more visually appealing. The delamination ratio (%) after 24 h water soaking decreased as the thermal treatment duration increased.
4. Conclusions Cold press application to the wood samples before the heat treatment could be a solution to reduce the decrement in the mechanical properties of solid wood. Dimensional stability and mechanical properties of the wood samples which were cold pressed and then heat treated in a oven were considerably higher than the unpressed control samples. The mechanical properties of the cold pressed wood samples treated at 180 °C for 6 h were considerably higher than those of the unpressed control samples. The increment in the temperature decreased the mechanical properties of wood samples while it improved the shrinkage ratio. The MOR and MOE of the LVLs manufactured from the heat treated veneers were slightly lower than those of untreated woods. It appears that dimensional stability and mechanical properties of low quality woods can be improved by using cold pressing and then heat treatment technique. Based on the findings obtained from the present study, it can be said that the wood and LVL manufactured by cold pressing and then thermal treatment at 180 °C for 6 h appear to a practical choice for applications requiring low thickness swelling and high mechanical performance, particularly when exposed to high levels of moisture.
Acknowledgement This study was supported by 2014 Research Grant from Kangwon National University and with the support of the MSIP (Ministry of Science, Ict & Future Planning).
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[2] Nazerian M, Ghalehno MD. Physical and mechanical properties of laminated veneer lumber manufactured by poplar veneer. J Agric Sci Technol 2011;A1:1040-45.
[3] Bekhta P, Niemz P. Effect of high temperature on the change in colour, dimensional stability and mechanical properties of spruce wood. Holzforschung 2005;57: 539-46.
[4] Ringman R, Pilgård A, Brischke C, Richter K. Mode of action of brown rot decay resistance in modified wood: a review. Holzforschung 2013;68: 239-46.
[5] Bakar BFA, Hiziroglu S, Tahir PM. Properties of some thermally modified wood species. Mater Design 2013;43:348-55.
[6] Korkut S, Hiziroglu S. Effect of heat treatment on mechanical properties of hazelnut wood (Corylus colurna L.) Mater Design 2009;30:1853-58.
[7] Ebner M, Petutschnigg AJ. Potentials of thermally modified beech (Fagus sylvatica) wood for application in toy construction and design. Mater Design 2007;28:1753-59.
[8] Kutnar A, Sernek M. Densification of wood. Acta Silvae et Ligni 2007;82:53-62.
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[9] Ayrilmis N, Jarusombuti S, FueangvivatV, Bauchongkol P. Effects of thermal treatment of rubberwood fibres on physical and mechanical properties of medium density fibreboard. J Trop For Sci 2011;23:10-16
[10] Bakar BFA, Hiziroglu S, Tahir PM. Properties of some thermally modified wood species. Mater Design 2013;43:348-55.
[11] Esteves BM, Pereira HM. Wood modification by heat treatment: a review. Bioresources 2009;4(1):370-404.
[12] Unsal O, Ayrilmis N. Variations in compression strength and surface roughness of heattreated Turkish river red gum (Eucalyptus camaldulensis Dehn.) wood. J Wood Sci 2005;51:405-09.
[13] Esteves B, Marques AV, Domingos I, Pereira H. Influence of steam heating on the properties of pine (Pinus pinaster) and eucalypt (Eucalyptus globulus) wood. Wood Sci Technol 2007;41:193-207.
[14] Korkut D, Korkut S, Dilik T. Effect of heat treatment on some mechanical properties of laminated window profiles manufactured using two types of adhesives. Int J Mol Sci 2008;9:454-63.
[15] Gunduz G, Aydemir D, Karakas G. The effects of thermal treatment on the mechanical properties of wild Pear (Pyrus elaeagnifolia Pall.) wood and changes in physical properties. Mater Design 2009:30:4391-95.
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[16] Unsal O, Korkut S, Atik C. The effect of heat treatment on some properties and colour in eucalyptus (Eucalyptus camaldulensis dehn.) wood. Maderas-Cienc Tecnol 2003;5:14552.
[17] Cademartori PHG, Schneid E, Gatto DA, Stangerlin DM, Beltrame R. Thermal modification of Eucalyptus grandis wood: variation of colorimetric parameters. MaderasCienc Tecnol 2013;15(1):57-64.
[18] Yildiz S, Yildiz UC, Tomak ED. The effects of natural weathering on the properties of heat-treated alder wood. Bioresources 2011;6(3):2504-21.
[19] Mohamad WHW, Razlan MA, Ahmad Z. Bending strength properties of glued laminated timber from selected Malaysian hardwood timber. Int J Civil Environ Eng 2011;11(4):7-12.
[20] Saviana J, Sosa Zitto MA, Piter JC. Bending strength and stiffness of structural laminated veneer lumber manufactured from fast-growing Argentinean Eucalyptus grandis. Maderas-Cienc Tecnol 2009;11(3):183-90.
[21] Moody RC, Liu JY. Wood handbook-Wood as an engineering material. Chapter 11: Glued structural members. Gen. Tech. Rep. FPL-GTR–113. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison WI, US, 463 p.
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Table list Table 1. Table 1. The physical, mechanical, and colour properties of the untreated and treated woods. Table 2. The mechanical properties of the LVLs manufactured from veneers of control and heat treated lumbers after cold pressing.
Figure list Figure 1. Treatment process of wood samples and production of experimental LVL samples. Figure 2. SEM images of cold pressed wood samples according to pressure ratio. A: Ginkgo biloba. B: Tilia amurensis. Figure 3. SEM images of heat treated wood samples (Tilia amurensis) after cold pressing. Figure 4. SEM images of heat treated wood samples (Ginkgo biloba) after cold pressing. Figure 5. UCS (uniform chromaticity scale) color system-lab diagram of Tilia amurensis wood samples. Figure 6. UCS color system-lab diagram of Ginkgo biloba wood samples. Figure 7. Photo of untreated and treated Tilia amurensis wood samples after 180 °C heat treatment for 6-48 h. Figure 8. Photo of untreated and treated Ginkgo biloba wood samples after 180 °C heat treatment for 6-48 h.
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Figure 1. Treatment process of wood samples and production of experimental LVL samples.
13
A
B Figure 2. SEM images of cold pressed wood samples according to pressure ratio. A: Ginkgo biloba. B: Tilia amurensis
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(A) Heat treatment (180℃, 6hr.)
(B) Heat treatment (180℃, 12hr.)
(C) Heat treatment (180℃, 24hr.)
(D) Heat treatment (180℃, 48hr.)
Figure 3. SEM images of heat treated wood samples (Tilia amurensis) after cold pressing.
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(A) Heat treatment (180℃, 6hr.)
(C) Heat treatment (180℃, 24hr.)
(B) Heat treatment (180℃, 12hr.)
(D) Heat treatment (180℃, 48hr.)
Figure 4. SEM images of heat treated wood samples (Ginkgo biloba) after cold pressing.
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Figure 5. UCS (uniform chromaticity scale) color system-lab diagram of Tilia amurensis wood samples.
Figure 6. UCS color system-lab diagram of Ginkgo biloba wood samples. 17
Temp. hr.
Tilia amurensis / 180℃
0
6
12
18
24
48
Figure 7. Photo of untreated and treated Tilia amurensis wood samples after 180℃ heat treatment for 6-48 h.
Temp. hr.
Ginkgo biloba / 180℃
0
6
19
12
24
48
Figure 8. Photo of untreated and treated Ginkgo biloba wood samples after 180℃ heat treatment for 6-48 h.
Highlights ► Effects of heat treatment on dimensional stability of cold pressed wood. ► Mechanical properties of heat treated samples were better than control samples. ► LVLs were made from cold pressed and then heat treated wood samples. ► Cell wall structure was adversely influenced by the heat treatment.
Table 1. The physical, mechanical, and colour properties of the untreated and treated woods.
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Density Wood species
Tilia amurensis Rupr.
Unpressed wood (control) Cold pressed wood 6h
0.53 (0.02) 0.62 (0.03) 0.47 (0.02) 0.46 (0.03) 0.46 (0.01) 0.44 (0.03) 0.46 (0.02) 0.53 (0.03) 0.44 (0.03) 0.43 (0.03) 0.43 (0.01) 0.41 (0.05)
Cold pressed wood
(g/cm³)
12 h 180°C 24 h 48 h
Unpressed wood (control) Cold pressed wood 6h Cold pressed wood
Ginkgo biloba L.
Treatment
12 h 180 °C 24 h 48 h
Shrinkage
CS (//) MOE
MOR
Colour difference
Green Green (MPa) (MPa) to air to oven (MPa dry (%) dry (%) ) (Tangential)
∆L
∆a
∆b
∆E*ab
5.10 (0.2) 4.60 (0.2) 4.66 (0.03) 4.11 (0.02) 3.86 (0.01) 3.01 (0.01) 6.50 (0.2) 5.90 (0.4) 5.97 (0.03) 5.69 (0.03) 4.23 (0.02) 4.11 (0.01)
-
-
-
-
7.40 (0.2) 6.10 (0.2) 5.16 (0.03) 5.09 (0.02) 4.79 (0.02) 4.77 (0.01) 8.60 (0.01) 8.10 (0.04) 7.41 (0.03) 5.98 (0.03) 5.01 (0.02) 4.92 (0.02)
32.9 (1.7) 35.1 (1.5) 32.7 (1.3) 31.8 (1.1) 30.3 (1.2) 26.9 (1.0) 30.9 (1.6) 42.1 (1.8) 40.9 (1.8) 40.7 (1.7) 40.4 (1.9) 32.7 (1.5)
4603 (164) 5305 (172) 5023 (165) 4852 (143) 4609 (162) 4507 (157) 5370 (166) 6542 (190) 6279 (183) 6034 (178) 5984 (162) 5378 (151)
53.4 (2.4) 58.2 (2.7 58.0 (2.1) 55.4 (1.6) 49.9 (2.0) 49.5 (1.8) 58.1 (1.9) 63.8 (2.3) 62.6 (2.4) 62.5 (2.0) 61.7 (2.2) 57.8 (1.6)
84.5 7.4 24.4 (2.8) (0.5) (1.0) 49.3 16.7 22.6 (1.1) (0.8) (0.5) 45.9 18.0 20.9 (1.5) (0.6) (0.8) 41.2 15.9 16.4 (1.2) (0.8) (0.9) 30.2 20.8 8.6 (1.5) (1.3) (1.5) 75.4 9.8 (2.6) (0.7) 49.1 16.4 (2.2) (1.0) 48.3 15.3 (2.5) (0.9) 41.3 14.9 (0.7) (0.8) 36.5 14.6 (1.7) (0.8)
CS: Compressive strength parallel to grain. MOE: Modulus of elasticity in bending. MOR: Modulus of rupture. L: Lightness. a: green-red. b: blue-yellow: Two chromatic coordinates. ∆E*ab: Colour difference. The values in the parentheses are standard deviations.
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18.4 (0.7) 19.9 (0.9) 21.6 (1.2) 17.9 (0.5) 14.2 (1.4)
30.4 (0.8) 38.9 (1.2) 44.1 (1.0) 56.6 (1.8) 23.3 (0.9) 33.9 (1.3) 41.3 (1.5) 46.7 (1.4)
Table 2. The mechanical properties of the LVLs manufactured from veneers of control and heat treated lumbers after cold pressing. Wood species
Ginkgo biloba L.
Tilia amurensis Rupr.
Arrangement of 5 veneers in the LVL (from top to bottom) 180 °C – 48 h 180 °C – 24 h 180 °C – 12 h 180 °C – 6 h Control (cold pressed veneer) 180 °C – 48 h 180 °C – 24 h 180 °C – 12 h 180 °C – 6 h Control (cold pressed veneer)
Delamination ratio Bending properties after water soaking MOE MOR (24 h) (g/cm³) (Layer ) (%) (MPa) (MPa) 1 0 2 2.1 0.52 4262 49.6 3 4.5 (0.03) (132) (1.5) 4 7.6 All layers 3.6 1 0 2 0 0.49 5012 52.7 3 2.7 (0.02) (144) (1.8) 4 6.3 All layers 2.3
Density
The values in the parentheses are standard deviations.
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