Physical, mechanical, and chemical properties of steamed beech wood. (Fundamental Disciplines).
Steam treatment is often applied to beech (Fagus orientalis Lipsky) to improve the stability and permeability of the wood, obtain a desirable color, and soften the wood. This paper evaluates the effect of steaming on the physical, mechanical, and chemical characteristics of beech steamed for 20,50,70, and 100 hours at 80[degrees]C. The results show that steaming decreases the strength and physical properties of the wood. Following 100 hours of steaming, compression strength and modulus of elasticity decreased 13.2 and 16.5 percent, respectively. Steaming had a slight effect on radial and tangential shrinkage, and reduced density about 2 percent. Hemicelluloses in steamed wood decreased only 7 to 9 percent, but wood solubility was greatly decreased. Acetyl content also decreased with steaming as hemicelluloses decreased. The lignin content of steamed wood was slightly higher than that of untreated wood.
Wood is frequently subjected to thermal treatments such as drying and steaming. Because temperature affects the physical, mechanical, and chemical properties of wood during such practices, the influence of thermal processes in wood exposed to high temperatures has been studied extensively. Since thermal degradation of most wood components can be observed at elevated temperatures above 100[degrees]C, some effects might be less severe at lower temperatures, as a result of additional factors, such as exposure period, heating medium, wood moisture content (MC), and pressure (9). A previous study reported that approximately 53 days of heating at 100[degrees]C was required to reduce the modulus of rupture (MOR) of dry wood by 5 percent, whereas only 0.67 and 0.2 days were required to achieve the same effect by steaming at 100[degrees] and 120[degrees]C, respectively (23).
The chemical structure of wood during drying or steaming has been a major concern in this area of study. The relationship between wood chemistry and wood strength has been studied (4,12,14,19,20,27), and changes in wood components have been found to play a key role in the strength of wood exposed to elevated temperature (11). Generally, the process of thermal treatment of wood is accompanied by cleavage of the lignin-polysaccharide complex by organic acids released from hemicelluloses (15,17,19,33). LeVan et al. (19) and Winandy (32) showed that initial strength loss in wood appears to be related to the removal of sidechain hemicellulose components. According to Sweet and Winandy (27), progressive degradation of hemicelluloses between microfibrils is the primary mechanism of hydrolytic strength loss in wood. Sweet (26) also found that the correlation between strength loss and loss in mannose content was stronger than the correlation between strength and other carbohydrates.
Although previous studies on steaming of beech showed that the lignin in this wood is relatively resistant to hydrothermal treatment at 100[degrees]C (13,15,24), the behavior of other wood components, such as cellulose, hemicelluloses, and extractives, is a matter of theoretical and practical importance in terms of the strength properties of beech during the thermal process. A recent study showed that cellulose is thermally more stable than are hemicelluloses and that carbohydrates in pine and birch are more amenable to various degradation reactions than is lignin (33). In wood species other than beech, steaming caused mobilization and partial removal of extractives in wood; removal of extractives affected specific gravity, volumetric shrinkage, and longitudinal permeability (1,2,6,7, 16,18).
Beech is a major hardwood species in Turkey and has been widely used in the forest products industry. Subjecting beech to steaming is commonly practiced to reduce growth stress, improve dimensional stability, decrease drying time, achieve desirable color, and soften the wood. The objective of the study reported here was to determine the effect of steaming at low temperature (80[degrees]C) for varying durations on the physical, mechanical, and chemical characteristics of beech.
MATERIALS AND METHODS
Beech (Fagus orientalis Lipsky) wood was obtained from 10 trees. Logs from each tree were sawn into boards of the following dimensions: 100 cm by 15 to 20 cm by 6 cm. Some boards were steamed at 80[degrees]C [+ or -] 2[degrees]C (dry-bulb temperature) for 20, 50, 70, and 100 hours in a computer-controlled laboratory steaming and drying kiln. Kiln dry-bulb and wet-bulb temperatures were controlled with thermocouples and a hygrometer at selected points in the kiln that were connected to the computer. Wood was steamed in saturated conditions (100% relative humidity (RH)). After steaming, steamed and untreated boards were conditioned to 12 percent MC in a conditioning room at 23[degrees]C [+ or -] 2[degrees]C and 65 percent ([+ or -] 5%) RH. Small clear specimens were obtained from the boards for physical and mechanical tests. The number of specimens taken from each log was nearly equal. Tests of wood density, radial and tangential shrinkage, compression strength parallel to grain, bending strength, modulus of el asticity (MOE), and impact bending were carried out at 23[degrees]C [+ or -] 2[degrees]C and 65 percent ([+ or -] 5%) RH based on Turkish Standards (TSE) (31).
Untreated specimens and specimens steamed for 20 and 100 hours (minimum and maximum steaming periods, respectively) were used for chemical analyses. Wood from each group was ground and screened from 40 to 100 mesh.
Klason lignin content in wood was determined according to Runkel and Wilke (21) using 72 percent sulfuric acid and 40 percent hydrobromic acid. Klason lignin values were corrected for ash content gravimetrically following incubation of lignin at 575[degrees]C for less than 3 hours. Acid-soluble lignin was determined by the absorbance of the hydrolysate at 205 nm (25).
Holocellulose content was determined according to the sodium chlorite method. Approximately a 5-g sample was hydrolyzed with sodium chlorite solution in a water bath at 70[degrees]C for 4 hours; sodium chlorite solution and acetic acid were added to the wood sample every hour. After hydrolysis, the hydrolysate was filtered through glass crucibles, the residue was weighed, and the results were reported in terms of the original sample mass.
For determining pentosan content, 0.5 g of ground wood was refluxed with 200 mL 3.2 M hydrobromic acid in a soxhlet extraction thimble. The hydrolysate was diluted at desired concentrations and absorbance was measured using a spectrophotometer. Absorbance was compared to the standard furfural curve and furfural content in each sample was calculated. The percentage of pentosan in each sample was estimated using the furfural content in the standard curve.
Sugar content of the hydrolysates was determined by anion exchange high performance liquid chromatography (HPLC) using pulsed amperometric detection (8). Sugars were quantitated using an internal standard method and results were reported in terms of percentage of original sample mass.
To determine acetyl content, an approximately 100-mg sample was deacetylated by incubation for 2 hours at 60[degrees]C in 5 mL 100-mM sodium hydroxide with 0.134 mM propionic acid added as an internal standard. Hydrolysates were diluted hundredfold in deionized water and their acetate contents were determined by anion exchange HPLC using suppressed conductivity detection. The chromatographic separation was achieved with an IonPac AG-11 guard and AS-11 analytical columns connected in series by elution with 0.10mM sodium hydroxide at a flow rate of 2.0 mL/min.
Hot water, 1 percent sodium hydroxide, ethanol, and ethanol-benzene solubility of untreated and steamed wood was determined according to TAPPI Test Methods T207 om-93, T212 om93, and T204 om-88 (28), respectively. The loss in weight of extracted wood was determined and calculated as percentage of solubility.
To determine wood pH, an approximately l0-g wood sample was placed in 100 mL deionized water at 23[degrees]C [+ or -] 2[degrees]C for 24 hours and solution pH was measured with a digital pH meter. To determine the turning point of pH and change in pH, the effluent was diluted to 250 mL in a volumetric flask with deionized water. This solution was then titrated with 0.1 M sodium hydroxide and the change in pH was determined. The titration was continued until the solution pH reached the turning point.
For physical and mechanical properties, all multiple comparisons were first subjected to an analysis of variance (ANOVA) and significant differences between mean values of control and steamed samples were determined using Duncan's Multiple Range Test. For chemical analysis results, only mean values and standard deviations were given.
RESULTS AND DISCUSSION
The chemical composition of untreated and 20- and 100-hour steamed beech wood is given in Tables 1 and 2. Chemical analyses showed that 20 and 100 hours of steaming at 80[degrees]C had little effect on the macromolecular substance of the wood. Klason lignin content of untreated wood was 22.97 percent compared with 23.88 and 23.90 percent in wood steamed for 20 and 100 hours, respectively.
It is generally known that lignin is the most stable compound against thermal degradation in wood and that thermal degradation of lignin is observed at temperatures above 100[degrees]C (9,10,24). Klason lignin content was found to be 22.3 percent in untreated F. sylvatica wood compared with 23.8 and 24.0 percent in wood steamed at 135[degrees] and 150[degrees]C for 10 minutes, respectively (15). Lignin content of red oak steamed at 100[degrees]C for 96 hours increased from 15.4 to 25.8 percent (17). Thompson (29) found that steaming at 118[degrees]C for 20 hours caused a 15 percent increase in oak lignin and 14 percent increase in southern yellow pine lignin. On the other hand, Sweet and Winandy (27) found that Klason lignin content decreased 77.4 percent in southern pine with drying at 66[degrees]C for 560 days. These studies suggest that exposure time is as important as temperature in thermal degradation of lignin.
In our study, it appeared that after 20 hours of steaming, the 3.96 percent increase in Klason lignin was somehow linked to the 3.55 percent decrease in acid-soluble lignin. This correlation might lead to the conclusion that certain condensation reactions of lignin-causing acid-insoluble compounds in lignin may occur after as few as 20 hours of steaming at 80[degrees]C. However, this correlation disappeared in wood steamed for 100 hours. Funaoka et al. (10) concluded that the condensation of lignin increases in wood containing moisture as opposed to dry wood because water lowers the softening temperature of lignin; the increased lignin content might be due in part to condensation products (13,17).
Carbohydrate analysis showed that steaming at 80[degrees]C for 20 and 100 hours caused small decreases in hemicelluloses (Table 1). The decrease rates for arabinose, galactose, rhamnose, and mannose in 100-hour steamed wood compared with untreated wood were 9.26, 7.69, 7.69, and 4.84 percent, respectively. Sweet and Winandy (27) found that arabinose, galactose, xylose, and mannose content of southern pine dried at 66[degrees]C for 560 days was decreased about 50 percent or more compared with that of untreated wood. They concluded that the mechanism of strength loss during heat treatment is related to progressive degradation of hemicelluloses between microfibrils. Their findings showed that the MOE of untreated wood was unaffected; however, MOR and work to maximum load were decreased about 18 and 48 percent, respectively, in dried wood. Kosikova et al. (15) investigated the thermal stability of linkages between lignin and polysaccharides in beech. These researchers concluded that the splitting of the linkages accelerates at temperatures above 200[degrees]C and the partial removal of hemicelluloses in beech wood may result in lower strength properties during drying at 120[degrees]C for 12 hours.
The average physical and strength properties of untreated and steamed beech are given in Table 3. Figure 1 shows the percent reduction in strength properties after 20, 50, 70, and 100 hours of steaming compared with the strength properties of untreated wood. Our results suggest that steaming has an adverse effect on the strength properties of wood through its effect on chemical composition. The reduction in strength increased with the length of the steaming period. Of the properties tested, strength loss was greatest for compression and MOE after 100 hours of steaming. For 100-hour steamed beech, compression strength decreased 13.2 percent and MOE decreased 16.5 percent, compared with those properties of untreated wood (Fig. 1). Percent reductions in strength losses were greater than those in hemicellulose losses compared with the results of previous studies (11,17-19,26,27,29,30). Nevertheless, these studies agree that hemicellulose loss in thermally degraded wood is correlated with strength loss.
Beech wood steamed for 100 hours had a pH of 5.28; the pH of untreated wood was 4.73 (Table 2). Acetyl content of 100-hour steamed wood decreased 9.85 percent compared with that of untreated wood (Table 1). The decreased acidity of steamed beech wood is assumed to be caused by the removal and leaching of organic acids from the wood, especially acetic acid. Acetic acid is believed to originate from acetyl groups in the hemicelluloses (9,18).
Experiments that determined the effect of steaming on the solubility of wood indicated that steaming for 20 and 100 hours caused major changes in wood extractives (Table 2). Compared to that of untreated wood, the ethanol-benzene solubility of 20-hour steamed wood increased 54.4 percent and ethanol solubility of 100-hour steamed wood increased 82.8 percent. In addition, larger increases in hot water solubility were observed in steamed wood. It is clear that steaming in saturated conditions dissolved some extractives and degraded certain easily hydrolysable components of wood. Changes in wood extractives with heat treatment were observed in previous studies (1,2,5,7,22). Mobilization and partial removal of extractives during steaming probably allow greater access of water molecules to the cell wall, resulting in more rapid radial and tangential diffusion after drying (1,2). In studies by Alexiou et al. (1,2), steaming did not affect the shrinkage of blackbutt wood steamed at 100 [degrees]C for 3 hours. Shupe e t al. (22) found little difference in the shrinkage of extracted and nonextracted sweetgum during drying. However, Chafe (5), Kubinsky and Ifju (18), and Avramidis and Oliveira (3) found that steaming caused an important increase in shrinkage of other wood species. Our results showed that steaming caused slight radial and tangential shrinkage of the wood (Table 3).
In this study, we evaluated the effect of steaming on several chemical and mechanical properties of beech wood. Steaming at 80[degrees]C apparently has some effect on the strength properties and chemical characteristics of beech wood. Further research on steaming at elevated temperature and prolonged exposure is currently in progress to improve the understanding of the relationship between steaming and the strength and physical properties of wood. At elevated temperatures, the wood chemistry and strength relationship would be more predictive because of the possible higher decomposition rate of wood components.
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TABLE 1. Chemical properties of untreated and steamed beech. (a) Steam Klason Acid-soluble treatment lignin lignin Holocellulose Pentosan Acetyl (%) None 22.97 4.22 85.90 22.40 4.06 (0.50) (0.08) (0.65) (0.40) (0.06) 20 hr. 23.88 4.07 86.18 23.00 3.96 (0.40) (0.10) (0.70) (0.50) (0.02) 100 hr. 23.90 3.93 86.58 23.37 3.66 (0.40) (0.08) (0.60) (0.50) (0.08) Carbohydrate Steam treatment Arabinose Galactose Rhamnose Glucose Xylose (%) None 0.54 1.04 0.39 40.40 18.69 (0.03) (0.05) (0.03) (0.25) (0.40) 20 hr. 0.56 0.94 0.39 39.77 19.20 (0.04) (0.04) (0.03) (0.30) (0.45) 100 hr. 0.49 0.96 0.36 39.97 19.39 (0.04) (0.03) (0.02) (0.20) (0.47) Carbohydra te Steam treatment Mannose (%) None 1.24 (0.06) 20 hr. 1.40 (0.03) 100 hr. 1.18 (0.05) (a)Each value represents the average of four analyses in specimens from two trees; values in parentheses indicate standard deviations. TABLE 2. Effect of steaming on solubility, ash content, and wood pH. (a) Solubility Steam treatment Ash Ethanol benzene Ethanol Hot water 1% NaOH (%) None 0.41 1.25 0.29 0.57 13.82 (0.04) (0.01) (0.02) (0.02) (0.45) 20 hr. 0.45 1.65 0.30 1.15 14.98 (0.03) (0.02) (0.02) (0.02) (0.50) 100 hr. 0.52 1.93 0.53 2.21 16.15 (0.02) (0.03) (0.03) (0.02) (0.35) Steam Turning treatment Wood pH point of pH NaOH added (mL) None 4.73 8.48 510.10 (0.07) (0.35) (10.25) 20 hr. 5.39 9.03 300.10 (0.07) (0.30) (9.54) 100 hr. 5.28 8.70 330.10 (0.05) (0.40) (10.50) (a)Each value represents the average of four analyses in specimens from two trees; values in parentheses indicate standard deviations. TABLE 3. Physical and mechanical properties of untreated and steamed beech wood. (a) Compression Steam Density strength parallel treatment Unit Air dry (b) Ovendry to grain (kg/[m.sup.3] (MPa) None Avg. 0.655 0.638 54.231 SD 0.041 0.043 5.80 N 426 235 98 20 hr. Avg. 0.64 0.620 50.995 SD 0.039 0.041 5.71 N 450 272 110 50 hr. Avg. 0.637 0.620 49.033 (***) SD 0.043 0.045 5.05 N 387 258 121 70 hr. Avg. 0.644 0.625 47.955 (***) SD 0.047 0.047 4.50 N 345 241 106 100 hr. Avg. 0.641 0.623 47.072 (***) SD 0.049 0.051 5.43 N 380 224 109 Steam Impact Shrinkage treatment Bending strength MOE bending Radial (MPa) (GPa) (NM/[cm.sup.2]) (%) None 103.96 11.91 92.353 5.33 14.06 3.16 28.47 0.94 161 161 167 161 20 hr. 97.321 (***) 10.39 (***) 91.765 5.69 11.68 2.34 31.88 1.18 116 116 224 181 50 hr. 97.394 (***) 10.44 (***) 86.373 5.56 (*) 9.02 2.08 31.00 1.22 60 60 206 152 70 hr. 96.565 (***) 10.36 (***) 82.255 (**) 5.54 (*) 10.73 2.03 30.45 1.01 91 91 148 158 100 hr. 98.959 (***) 9.941 (***) 82.843 (**) 5.58 (*) 13.90 2.41 26.88 1.12 96 96 175 148 Steam Shrinkage treatment Tangential (%) None 11.37 1.00 161 20 hr. 12.07 (***) 1.35 181 50 hr. 11.57 1.29 152 70 hr. 11.92 (**) 1.30 158 100 hr. 11.58 1.34 148 (a)SD = standard deviation; N = number of specimens used in each test; 1 GPa = 1.45 x [10.sup.5] psi; asterisks denote significant difference compared with untreated control; (*)p = 0.05, (**)p = 0.01, and (***)p - 0.001. (b)Air-dry density values were determined in samples used for mechanical properties.
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|Author:||Yilgor, Nural; Unsal, Oner; Kartal, S. Nami|
|Publication:||Forest Products Journal|
|Date:||Nov 1, 2001|
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