2Düzce University, Faculty of Technical Education, Department of Furniture and Design,. Duzce, Turkey. Heat treatment is a wood modification method that has ...
Drying Technology, 27: 1240–1247, 2009 Copyright # 2009 Taylor & Francis Group, LLC ISSN: 0737-3937 print=1532-2300 online DOI: 10.1080/07373930903267161
Effect of High-Temperature Treatment on the Mechanical Properties of Rowan (Sorbus aucuparia L.) Wood Su¨leyman Korkut1 and Mehmet Budakc¸ı2 1
Du¨zce University, Faculty of Forestry, Duzce, Turkey Du¨zce University, Faculty of Technical Education, Department of Furniture and Design, Duzce, Turkey 2
Heat treatment is a wood modification method that has been used to some extent in improving timber quality. The high temperature thermal treatment of wood is an environmentally friendly method for wood preservation. This technique has attracted considerable attention both in Europe and in North America in recent years. This article presents the results of experimental studies on influence of heat treatment on the mechanical properties of Rowan (Sorbus aucuparia L.) wood performed in order to understand its role in wood processing. Samples were exposed to temperature levels of 120, 150, and 180 C for time spans ranging from 2 to 10 h. Mechanical properties including compression strength, modulus of elasticity, modulus of rupture, Janka hardness, impact bending strength, tension strength perpendicular to grain, tension strength parallel to grain, shear strength, and cleavage strength of heat-treated samples were determined. Maximum reduction values of 34.12, 28.40, and 26.37% were found for impact bending strength, tension strength parallel to grain, and cleavage strength for the samples exposed to 180 C for 10 h, respectively. Overall, the results showed that treated samples had lower mechanical properties than those of the control samples. Statistically significant difference was determined (P ¼ 0.05) between mechanical properties of the control samples and those treated at 180 C for 10 h. Keywords Heat treatment; Mechanical properties; Rowan
INTRODUCTION The genus Sorbus encompasses of about 100–200 species of trees and shrubs in the subfamily Maloideae of the Rose family Rosaceae. Rowan (Sorbus aucuparia), the bestknown species of the genus Sorbus, is native to most of Europe, except for the far south, and northern Asia. In the south of its range in the Mediterranean region it is confined to high altitudes in mountains.[1] In Turkey, Rowan is found primarily in the north and northwest Anatolia as small groups in angiosperm mixed forests. It is a smallto medium-sized deciduous tree, which typically grows up to 8–20 m (rarely 20 m and exceptionally 28 m) and can live Correspondence: Su¨leyman Korkut, Du¨zce University, Faculty of Forestry, 81620 Duzce, Turkey; E-mail: suleymankorkut@ hotmail.com
over 100 years. It is very tolerant of a wide range of soil conditions.[2] Rowan wood is one of the underutilized species although it has limited use in the manufacture of veneer, furniture, decorative inlays, and novelty items. The improved characteristics of heat-treated rowan wood would make this species more attractive to manufacture value-added products with possible potential opportunities. Heat treatment is a way of drastically changing the properties of wood and in some sense of producing a ‘‘new’’ material. The treatment is performed in air or inert surroundings such as superheated steam, nitrogen gas, or vegetable oil at 150–250 C.[3,4] The process is generally conducted under the influence of heat and pressure. Temperature during thermal treatment usually ranges from 150 to 250 C, and treatment time spans between 15 min and 24 h, depending on the type of the process, wood species, stock dimensions, initial moisture content, the desired level of alteration of mechanical properties, resistance against biological deterioration, and dimensional stability of the product. The presence of air or other oxidative medium can accelerate the degradation process of wood components during heat treatment and that is why the process is usually carried out in a protective gaseous medium (nitrogen, steam, CO2) or various oils.[5–7] The thermal modification of wood has long been carried out to change its properties. Since ancient times, the method of heat treating wooden poles over open fire has been used to enhance its durability in ground contact, and in African countries the tips of wooden spears were hardened by alternately pounding and heating the wood.[8] In the period from 1930 to 1950, mainly in the United States, research and development on heat-treated wood focused on hygroscopicity. A lot of work was done, mainly in Germany, in the period from 1950 to 1970 to deepen knowledge about heat treatment of wood and the effects on its properties. Since the late 1980s an interest in finding environmentally friendly wood materials has grown, and heat treatment of wood has been suggested as a possible
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HEAT TREATMENT OF ROWAN WOOD
mothod to enhance biological resistance without adding harmful additives to the wood.[3,9,10] During the last decade, heat-treated wood has been commercialized and produced on a larger scale. Presently, the following processes have been commercialized: PlatoProcess (The Netherlands), Retification Process (France), Bois Perdure (France), Oil Heat Treatment Process (Germany), and ThermoWood Process (Finland).[11] The common ground of these five processes lies in modifying the chemical structure of lumber at temperatures ranging from 160 to 260 C. The main differences between these methods are based on the materials used (e.g., wood species, fresh or dried wood, moisture content, dimensions), process conditions applied (e.g., one or two process stages, wet or dry process, heating medium, oxygen or nitrogen as sheltering gas, heating and cooling rate), and the equipment necessary for treatment (e.g., process vessel, kiln).[12,13] The high-temperature treatment of wood offers an alternative to chemical treatment of wood, which uses tar oil– based chemicals (creosote and pentachloro-phenol) and chromated copper arsenate (CCA). These chemicals are toxic to humans and other living species when released to the environment. Traditional industrial wood treatment methods, such as chemical impregnation, have significant environmental impacts. Growing environmental awareness, the risks that contaminated ground pose to the environment, and stricter environmental regulations have encouraged researchers to look for new, more environmentally friendly methods of wood processing. Correspondingly, hardwood species that are attractive, in terms of durability and appearance, are being cut down in endangered rainforests, which in turn poses a threat to ecosystem diversity and the climate of the entire world. Alternative treatment methods that can provide other wood species with durability, structure, and appearance similar to that of hardwood also reduce the demand for tropical wood species. Chemical treatment of wood and the consumption of endangered wood species can be avoided by using heat-treated wood. Heat treatment is an environmentally friendly method that makes it possible to modify the properties and color of the wood to meet market demand.[14–16] The most important properties of thermally treated wood are dimensional stability (due to changes in the molecular structure of cell walls where water molecules can no longer bond with the cellulose molecular structure) and the beautiful shades of colors that can be obtained, varying from light brown to almost black. However, losses in the mechanical strength of wood may also occur, and this drawback is a limitation for the use of heat-treated wood in a broad range of products.[3] It is well known that the effect of heat treatment on the strength properties of wood is complex and some effects might be less or more severe as a result of additional factors such as exposure
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period, temperature, heating medium, wood moisture content, and pressure.[17,18] Giebeler[19] treated wood at 180 to 200 C in the presence of moisture and found that these treatments resulted in a large reduction in the resistance to shock, modulus of elasticity (MOE), modulus of rupture (MOR), and compression strength. Bekhta and Niemz[20] found that MOR decreases when the heat treatment temperature decreases, whereas MOE does not seem to be affected significantly by the temperature. Some studies showed the reduction in bending strength of the treated wood.[21,22] The degree of such decrease is dependent on the wood species to be treated, the maximum temperature reached in the process, the holding time at that temperature, etc.[23] To our knowledge, there is no information about the influence of heat treatment on mechanical properties, such as compression strength, bending strength, modulus of elasticity in bending, Janka hardness, impact bending strength, tension strength perpendicular to grain, tension strength parallel to grain, shear strength, and cleavage strength of Rowan wood grown in Turkey. Therefore, the main objective of this study is to evaluate the effect of heat treatment on hazelnut wood characteristics to provide preliminary data so that treated wood can be used more widely and effectively by the timber product industry. The data can provide firsthand information to industry and academia with regard to the mechanical behavior of Rowan wood species treated by heat-treatment processes. MATERIALS AND METHODS Lumber from the logs was sawn and planed. Initial moisture content of the sawn and planed green (fresh) wood samples was 75% on a dry basis. Small clear specimens were cut from the lumbers for compression strength parallel to grain (20 20 30 mm), MOR (20 20 360 mm), MOE in bending (20 20 360 mm), Janka hardness (50 50 50 mm), impact bending strength (20 20 300 mm), tension strength paralel to grain (7 20 450 mm),shear strength(30 60 80 mm),cleavagestrength (50 50 94 mm), and tension strength perpendicular to grain (20 30 70 mm).[24,25] To reduce possible deformations, the samples were sawn with the annual rings at a 45 angle to the surface. The specimens were randomly divided into nine treatment groups, each having 20 specimens, and a group of nontreated 20 specimens was used as control samples. The wood samples were dried during thermal treatment. Heat treatments were carried out at three temperature levels (120, 150, and 180 C) and three time spans (2, 6, and 10 h) in a laboratory heating unit securing controlled temperature of air with an accuracy of 1 C. These conditions represented typical opearating conditions because temperature during thermal treatment usually ranges from 150 to 250 C and treatment time spans from 15 min to 24 h.
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KORKUT AND BUDAKC ¸I
Because of moisture evaporation during thermal treatment, both treated and control samples were then conditioned in a climatic chamber at 20 2 C and 65 5% relative humidity to reach equilibrium moisture content (EMC).[26] The equilibrium moisture content of the samples was 12 2% after 2 weeks of conditioning. Prior to each test, the dimensions of the samples were measured with a micrometer to the nearest 0.001 mm and their weight was recorded at accuracy of 0.01 g. Compression strength parallel to grain (rck),[27] MOR,[28] MOE in bending,[29] Janka hardness (Hj),[30] impact bending strength (ri),[31] tension strength parallel to grain (rzk), shear strength (saB), cleavage strength (rs), and tension strength perpendicular to grain (rz?) measurements were carried out for treated and untreated samples, based on Turkish Standards (TS).[32–35] After strength tests were completed, the moisture content of samples was also measured.[36] For all parameters, multiple comparisons were first subjected to an analysis of variance (ANOVA), and significant differences between average values of control and treated samples were determined using Duncan’s multiple range test. P-values of 0.05 were considered to determine significance level. The control (nontreated) samples were compared with heat-treated samples of the same moisture content. RESULTS AND DISCUSSION Table 1 displays results of tests for the control and heattreated samples for three exposures and time combinations. Based on the findings in this study all of the mechanical properties tested decreased with increasing temperature and time. The effect of the heat treatments was significant for all the variables analyzed. The maximum decrease of all properties were observed when samples were treated with a temperature of 180 C for 10 h. The lowest compression strength value was 46.170 N=mm2, which is 24.33% lower than that of control samples at this condition. Similarly, the lowest average bending strength value was also found as 106.559 N=mm2, which is 13.42% lower than that of control samples under the condition above. The loss of tensile strength perpendicular to grain, hardness reduction cross section, and radial and tangential grain orientations were 21.67, 12.83, 15.89, and 11.39%, respectively. Table 2 shows the percentage decrease of values in relation to the control for each treatment and each measured parameter. The parameters measured varied in their rate of decrease; some had a gradual loss, others exhibiting more dramatic changes as illustrated in Fig. 1. In general, the results of this study on the effect of heat treatment on Rowan wood are compatible with the findings of previous studies related to the effect of heat treatment on various properties of different tree species.
Viitaniemi and Ja¨msa¨[37] and Viitaniemi[38] have reported that the bending strength of thermally modified wood is reduced by up to 30% depending on the treatment conditions. An average decrease in bending strength as high as 44–50% has also been reported.[20] Mburu et al.[39] found that MOE decreased insignificantly for G. robusta samples having weight loss of less than 16% after thermal treatment. There was significant reduction in MOE for heat-treated G. robusta with weight loss of 16% and above. Reduction in MOE progressed with increase in weight loss due to thermal treatment. Shi et al.[40] found that the reduction in MOE was smaller than that in MOR; in hardwood such as aspen and birch, MOE increased by 15 and 30% compared with untreated controls. Esteves et al.[41] found that MOE of pine wood decreased with mass loss during the heat treatment. The decrease was less than 5% until about 4% mass loss but increased subsequently and reached 16% for about 6% mass loss. Yildiz[42] reported a decrease in MOE of about 45% for beech wood treated at 130–200 C for 2–10 h but mass loss was not referred. Kamdem et al.[43] studied the mechanical properties of spruce and beech after heat treatment between 200 and 260 C. Spruce showed a reduction in MOR of 8% and in MOE of 11%, whereas beech showed a reduction in MOR of 40% and in MOE of 20%. Borrega and Ka¨renlampi[44] found that as a function of mass loss, the elastic toughness was the highest for wood heated at intermediate relative humidity. Kocaefe et al.[45] obtained similar changes in mechanical properties of jack pine (Pinus banksiana Lamb.) and aspen (Populus tremuloides Michx.) for the same treatment duration and temperature. A material property that is clearly altered during heat treatment is the weight of the boards and thus the density of wood. The main reasons for the decrease of the density of wood after heat treatment are degradation of wood components (mainly hemicelluloses) into volatile products, which are released during treatment; evaporation of extractives; and a lower equilibrium moisture content of the boards because heat-treated wood is less hygroscopic. Although a lower density after heat treatment implicates a decrease of the strength properties, this conclusion can be premature. Degradation of the main wood components with its corresponding loss in woody material and weight decreases the strength properties because the internal stresses must be distributed over the less-molecular material. On the other hand, a lower moisture content does have a positive effect on the strength properties, reducing the effect of mass loss.[13] Because of the amorphous structure, hydroxyl groups in hemicellulose are much more accessible to water than those of cellulose. Removal of hemicelluloses increases the
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150 C
120 C
None
Heat treatment
2h
10 h
s s2 V N
s s2 V N Avg.
s s2 V N Avg.
6h
Avg. s s2 V N Avg.
Unitb
s s2 V N Avg.
2h
Time
61.017 A 7.225 52.213 11.842 20 54.417 BJK 5.1981 27.02 9.5523 20 53.796 CJK 5.9979 35.975 11.149 20 52.89 DK 4.0352 16.283 7.6294 20 51.83 EK 6.2183 38.667 11.997 20
Compression strengthc (N=mm2)
7.744 59.976 6.728 20 113.785 A 10.338 106.881 9.085 20 112.434 B 17.2513 297.607 15.3435 20
14.580 212.584 12.507 20 115.095 A
123.079 A 8.085 65.371 6.569 20 116.572 A
Bending strengthc (N=mm2) 10,710.51 A 1204.175 1,450,038 11.242 20 10,052.95 AD 1907.606 3,638,961 18.975 20 9901.861 AD 1471.875 2,166,416 14.864 20 9818.578 AD 1378.283 1,899,664 14.037 20 9768.536 AD 1624.592 2,639,299 16.630 20
Modulus of elasticity in bendingc (N=mm2) 271.341 A 39.977 1598.17 14.733 20 265.968 AE 31.695 1004.6 11.917 20 263.845 AE 30.665 940.375 11.622 20 262.436 AE 40.500 1640.27 15.432 20 258.209 A 31.076 965.731 12.035 20
Cross section (N=mm2) 127.059 A 10.360 107.34 8.154 20 121.9 ADEFGH 16.061 257.979 13.176 20 119.605 AGH 9.605 92.271 8.031 20 117.362 BGH 16.215 262.932 13.816 20 116.815 CGH 15.152 229.589 12.971 20
Radial (N=mm2)
Janka hardnessc
147.384 A 15.870 251.858 10.767 20 146.218 AB 13.100 171.615 8.959 20 146.200 AB 17.695 313.119 12.103 20 145.644 AB 12.784 163.453 8.778 20 140.315 A 16.743 280.335 11.932 20
Tangential (N=mm2) 16.376 A 3.9152 15.328 23.908 20 15.984 ACDEF 4.1769 17.447 26.132 20 14.639 AEF 4.193 17.581 28.642 20 14.098 AEF 3.4196 11.693 24.257 20 13.947 AEF 4.0377 16.303 28.95 20
Impact bendingc (J=cm2) 6.976 A 1.235 1.526 17.713 20 6.513 AH 0.826 0.682 12.68 20 6.367 AH 0.915 0.836 14.36 20 6.099 B 1.116 1.247 18.31 20 6.057 C 1.328 1.765 21.93 20
Tension strength perpendicular to grainc (N=mm2) 133.222 A 25.389 644.6 19.057 20 128.031 ACDE 23.2176 539.057 18.134 20 124.837 ADE 33.9123 1150.04 27.165 20 119.051 AE 30.93687 957.0902 25.986 20 115.958 AE 19.4269 377.404 16.753 20
Tensile strength parallel to grainc (N=mm2) 12.991 A 0.582 0.339 4.486 20 12.868 AEF 0.996 0.993 7.746 20 12.766 A 0.718 0.516 5.629 20 12.742 A 0.820 0.673 6.438 20 12.592 A 0.757 0.574 6.017 20
Shear strengthc (N=mm2)
TABLE 1 The effect of heat treatment for different durations on mechanical properties in Rowan (Sorbus aucuparia L.) wooda
(Continued )
1.133 A 0.138 0.019 12.14 20 1.091 ABCDEFGHK 0.133 0.018 12.23 20 0.947 BHK 0.149 0.022 15.7 20 0.911 C 0.105 0.011 11.56 20 0.891 D 0.138 0.019 15.5 20
Cleavage strengthc (N=mm2)
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10 h
6h
2h
10 h
Avg.
6h
s s2 V N Avg. s s2 V N Avg. s s2 V N Avg. s s2 V N Avg. s s2 V N
Unitb
Time
51.249 F 13.773 189.69 26.874 20 50.05 G 4.454 19.844 8.9 20 49.421 H 5.142 26.443 10.405 20 48.132 J 5.674 32.201 11.789 20 46.17 K 12.306 151.43 26.653 20
Compression strengthc (N=mm2) 111.987 C 19.275 371.554 17.212 20 109.296 9.921 98.444 9.077 20 108.076 12.747 162.491 11.7947 20 106.816 13.786 190.073 12.906 20 106.559 17.753 315.193 16.660 20 G
F
E
D
Bending strengthc (N=mm2) 9704.798 AD 1471.239 2,164,543 15.159 20 9663.377 AD 1763.968 3,111,583 18.254 20 9490.58 B 1673.351 2,800,105 17.631 20 9325.858 C 1548.294 2,397,213 16.60216 20 8578.179 D 1180.49 1,393,556 13.761 20
250.475 A 35.656 1271.36 14.235 20 245.795 38.212 1460.23 15.546 20 243.465 27.897 778.292 11.458 20 241.805 28.172 793.712 11.651 20 236.525 24.724 611.299 10.453 20 E
D
C
B
Cross section (N=mm2) 113.049 D 9.111 83.015 8.059 20 112.473 9.812 96.289 8.724 20 111.015 13.894 193.043 12.515 20 107.345 9.290 86.315 8.654 20 106.875 9.087 82.591 8.503 20 H
G
F
E
Radial (N=mm2) 140.17 A 16.411 269.350 11.708 20 139.02 A 12.186 148.515 8.766 20 137.439 A 13.391 179.328 9.743 20 137.095 A 11.834 140.044 8.632 20 130.595 B 12.447 154.940 9.531 20
Tangential (N=mm2) 13.701 BEF 4.1002 16.812 29.926 20 13.367 CF 3.6534 13.348 27.332 20 12.784 D 3.5397 12.529 27.688 20 11.095 E 2.5056 6.278 22.583 20 10.788 F 2.6009 6.7645 24.108 20
Impact bendingc (J=cm2) 6.024 D 1.19 1.416 19.75 20 6.018 0.998 0.995 16.58 20 5.943 1.09 1.187 18.33 20 5.807 1.239 1.534 21.33 20 5.464 0.946 0.895 17.31 20 H
G
F
E
Tension strength perpendicular to grainc (N=mm2) 115.272 AE 31.1854 972.53 27.053 20 110.727 B 30.1949 911.735 27.269 20 105.216 C 28.3351 802.88 26.93 20 102.68 D 26.7592 716.053 26.006 20 95.392 E 25.658 658.34 26.898 20
Tensile strength parallel to grainc (N=mm2) 12.39 B 0.717 0.514 5.789 20 12.385 C 0.749 0.562 6.055 20 12.36 D 0.935 0.875 7.571 20 12.258 E 0.772 0.596 6.298 20 12.222 F 0.459 0.210 3.756 20
Shear strengthc (N=mm2) 0.875 E 0.101 0.01 11.55 20 0.86 F 0.136 0.018 15.79 20 0.853 G 0.178 0.032 20.84 20 0.839 H 0.177 0.031 21.05 20 0.835 K 0.131 0.017 15.68 20
Cleavage strengthc (N=mm2)
b
Number of samples used in each test is 20. Avg ¼ average; s ¼ standard deviation; s2 ¼ variance; V ¼ coefficient of variation; N ¼ number of samples used in each test. c Homogeneity groups: the same letters (A, B, C, D, E, F, G, H) in each column indicate that there is no statistical difference between the samples according to the Duncan’s multiply range test at P < 0.05. Comparisons were made between each control and its test.
a
180 C
Heat treatment
Modulus of elasticity in bendingc (N=mm2)
Janka hardnessc
TABLE 1 Continued
0.94 1.73 1.91 3.07 4.62 4.66 4.85 5.64 5.92
3.79 16.41 19.63 21.36 22.79 24.11 24.75 25.96 26.37
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3.90 6.29 10.64 12.96 13.47 16.89 21.02 22.93 28.40 6.63 8.73 12.58 13.17 13.65 13.73 14.80 16.76 21.67 2.39 10.60 13.91 14.83 16.33 18.38 21.93 32.25 34.12 0.79 0.80 1.18 4.80 4.90 5.68 6.75 6.98 11.39 4.06 5.87 7.63 8.06 11.03 11.48 12.63 15.52 15.89 1.98 2.76 3.28 4.84 7.69 9.42 10.27 10.89 12.83 6.14 7.55 8.33 8.80 9.39 9.78 11.39 12.93 19.91 5.29 6.49 7.55 8.65 9.01 11.20 12.19 13.21 13.42 10.82 11.84 13.32 15.06 16.01 17.97 19.00 21.12 24.33 180 C
150 C
2h 6h 10 h 2h 6h 10 h 2h 6h 10 h 120 C
Tensile Impact Tension strength strength Shearing Cleavage Compression Bending Modulus of Cross bending Heat strength strength elasticity in section Radial Tangential strength perpendicular parallel to strength strength (%) (%) (%) to grain (%) grain (%) treatment Time (%) (%) bending (%) (%) (%) (%)
Janka hardness
TABLE 2 Percentage decrease of mechanical properties in Rowan (Sorbus aucuparia L.) wood following heat treatment for different durations
HEAT TREATMENT OF ROWAN WOOD
FIG. 1. Percentage decrease of technological properties in Rowan (Sorbus aucuparia L.) wood following heat treatment for different durations, in relation to control (nontreated samples). () compression strength; (&) bending strength; (D) modulus of elasticity in bending; ( ) Janka hardness cross section; () Janka hardness radial; () Janka hardness tangential; () impact bending strength; (.) tension strength perpendicular to grain; (þ) tension strength parallel to grain; (~) shear strength; (&) cleavage strength.
crystalline part in wood material due to relative increase of cellulose component. When the relatively flexible hemicellulose-cellulose-hemicellulose bond is replaced by a more rigid cellulose-cellulose bond, the flexibility of the material decreases. The changes taking place in wood’s molecular structure due to decomposition of the long polymers can cause a decrease in the elasticity, and the wood becomes more fragile.[46–48] According to Bengtsson et al.,[49] the heat treatment decreases the strength and stiffness of the wood. However, the extent of loss may vary either by the heat treatment schedule or the wood species. Softwoods have shown larger reductions in strength than hardwoods. Usually, the bending and tensile strength of heat-treated material are reported to fall between 10 and 30%. CONCLUSIONS In this research, strength values of the samples decreased with increasing time and temperature of the treatments. The smallest decrease was determined at the heat treatment at 120 C for 2 h. The largest decrease was for impact bending strength, followed by tension strength parallel to grain, cleavage strength, compression strength parallel to grain, and tension strength perpendicular to grain when heat treated at 180 C for 10 h under the conditions stated. According to the results, it was found that 120 C and 2 h are the most reasonable heat treatment parameters. If heat treatment of this species is a solution to moisture vulnerability, and consequently to the high values of shrinkage and swelling, then the strength properties will not be a barrier to the use of this species in structural applications.
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Regardless of the information available, several aspects of heat effects on wood still need to be investigated further. One of these aspects is the temperature range that will favor the maintenance of the wood’s original structure. The variation of properties was related to the intensity of the heat treatment and the corresponding mass loss, but significant improvements could already be obtained for a mass loss of 3–4% without impairing the mechanical resistance.[50,51]
19. 20.
21.
22.
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