Abstract
Heat treatment changes some physical, mechanical, and chemical properties of wood. Inorganic borates have been used as wood preservatives for many years. The aim of this study was to investigate the effects of impregnation chemicals on some mechanical properties (bending strength (MOR), modulus of elasticity (MOE), tensile strength parallel to the grain (TS), compression strength parallel to the grain (CS), and shear strength parallel to the grain (SS)) of heat-treated oak (Quercus petraea Liebl.). For this purpose, the oak wood specimens were impregnated with 5% aqueous solution of boric acid (BA) and borax (BX). Then specimens were heat-treated at 160, 190, and 220 °C for 2 and 4 h. According to the results of the study, borax retention value was higher than boric acid. The bending strength, modulus of elasticity in bending, tensile strength parallel to the grain, and shear strength parallel to the grain decreased due to heat treatment. The highest mechanical strength losses were determined in samples heat treated at 220 °C for 4 h. Generally the mechanical strength losses of samples impregnated with borax were lower than non-impregnated controls and specimens impregnated with boric acid.
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Effects of Boron Impregnation and Heat Treatment on Some Mechanical Properties of Oak (Quercus petraea Liebl.) Wood
Osman Percin,a,* Sait Dundar Sofuoglu,a and Oguzhan Uzun b
Heat treatment changes some physical, mechanical, and chemical properties of wood. Inorganic borates have been used as wood preservatives for many years. The aim of this study was to investigate the effects of impregnation chemicals on some mechanical properties (bending strength (MOR), modulus of elasticity (MOE), tensile strength parallel to the grain (TS), compression strength parallel to the grain (CS), and shear strength parallel to the grain (SS)) of heat-treated oak (Quercus petraeaLiebl.). For this purpose, the oak wood specimens were impregnated with 5% aqueous solution of boric acid (BA) and borax (BX). Then specimens were heat-treated at 160, 190, and 220 °C for 2 and 4 h. According to the results of the study, borax retention value was higher than boric acid. The bending strength, modulus of elasticity in bending, tensile strength parallel to the grain, and shear strength parallel to the grain decreased due to heat treatment. The highest mechanical strength losses were determined in samples heat treated at 220 °C for 4 h. Generally the mechanical strength losses of samples impregnated with borax were lower than non-impregnated controls and specimens impregnated with boric acid.
Keywords: Borax; Boric acid; Heat treatment; Oak wood; Mechanical strength
Contact information: a: Dumlupinar University, Faculty of Simav Technology, Department of Wood Works Industrial Engineering, 43500 Simav/Kutahya, Turkey; b: Cankırı Karatekin University, Technical and Business College, 18200, Cankiri, Turkey;
* Corresponding author: osman.percin@dpu.edu.tr
INTRODUCTION
Environmental awareness has led to increased interest in developing new, alternative wood modification methods. Heat treatment is one of the processes used to modify the properties of wood (Mazela et al. 2004) and can be considered an environmentally friendly technique (Ates et al. 2009). Heat treatment, as a wood modification method, serves to improve the natural properties of the wood, such as dimensional stability (Esteves et al. 2008b; Todorovic et al. 2012) and resistance to bio-deterioration, as well as bestowing the wood material with new properties (Altinok et al. 2010). The modification treatment is always performed in the temperature range of 180 to 240 °C (Niemz et al.2010). Heat treatment, especially at temperatures over 150 °C permanently changes the physical, mechanical, and chemical properties of wood (Mitchell 1988; Yildiz et al. 2006; Gunduz et al. 2008). Cellulose and lignin degrade more slowly and at higher temperatures than hemicelluloses. Thermal degradation starts by deacetylation of hemicelluloses, and the released acetic acid acts as a depolymerization catalyst, which accelerates the decomposition of polysaccharides (Tjeerdsma et al.1998; Nuopponen et al. 2004).
It is known that organic acids, such as formic and acetic acid, are formed during the heat treatment process and that these may affect the properties of wood (Garrote et al. 2001; Feng et al. 2002; Manninen et al. 2002). In addition, heat treatment also reduces wood’s pH (Boonstra et al. 2007a). According to Salim et al. (2008), depending on the acid concentration and applied temperature, hemicellulose, the most reactive wood component, will be hydrolysed into oligomeric and monomeric structures (Carrasco and Roy 1992).
The strong correlation between strength reduction and acidity in wood has been reported in previous studies. Treatments between 200 and 260 °C can cause significant degradation in hemicellulose content of wood, which releases a large amount of acetic acid (Weiland et al. 1998). The loss of strength is associated with changes in wood acidity (Hodgin and Lee 2002). Sundqvist et al. (2006) found that when birch was heated to 180 °C for 1 to 2.5 h, it lost considerable strength and hardness. Losses in mechanical properties can be linked to the loss of mass and increase in concentrations of formic and acetic acid. Cellulose degradation can contribute to the loss of mechanical strength in wood materials under high temperature treatment (Sundqvist 2004). Boron compounds are used as wood preservatives as they are both fungicides and insecticides, relatively inexpensive, and environmentally acceptable (Thévenon et al. 2010). According to Tjeerdsma et al. (1998), the degradation of the hemicelluloses results in the formation of acetic acid, which serves as the catalyst in further carbohydrate decomposition (Awoyemi and Westermark 2005).
Wang et al. (2012) investigated that the impact of pH on chemical and mechanical properties of thermally modified Cathay poplar (Populus cathayana Rehd.) wood. As a result, disodium octoborate tetrahydrate (DOT) and buffering solutions decreased mass loss of heat-treated wood and increased MOR and MOE. Winandy (1997) determined that adding boron-based buffers to the fire retardant treatment chemicals appeared to significantly reduce subsequent thermal degradation. Awoyemi (2008) reported that pre-impregnation of borate as an alkali-buffering medium decreased the severity of strength loss during heat treatment. This was invariably due to the buffering effect of alkali on wood acidity. Awoyemi and Westermark (2005) suggested that a preliminary borate impregnation of wood before heat treatment may reduce the severity of mechanical strength loss during heat treatment. Kartal et al. (2008) determined that effects of boron impregnation and heat treatment on chemical and strength properties of wood and discovered that boric acid (BA) and di-sodium octoborate tetrahydrate (DOT) clearly changed the pH value of wood, which made the decrease in MOE in the untreated wood slightly higher than that in the treated specimens. The objective of this study was to determine the effect of the impregnation with different boron compounds (borax and boric acid) that have different pH value before heat tretment on mechanical strength of oak (Quercus petraea Liebl.) wood.
EXPERIMENTAL
The oak (Quercus petraea Liebl.) wood samples used in this study were selected randomly. The wood was parallel to the grain and sawn into specimens measuring 50×80×800 (tangential, radial, longitudinal) mm in dimensions. Afterwards, the lumber was air dried for two months until it reached approximately 12% moisture content.
In the literature wood samples were impregnated with different concentrations (1 to 13%) of boron compounds (Peker et al. 1999; Toker 2007; Awoyemi 2008; Özçifçi 2008). In conclusion, mechanical strength losses increased with increasing concentration of boron compounds. Therefore, 5% concentration was preferred in this study, and the oak wood specimens were impregnated with 5% aqueous solution of boric acid (BA) (H3BO3 is 56.30% 1/2 B2O3, 43.70% H2O with a molecular weight 61.84, density 1.435 g.cm-3 and melting point 171 °C) and borax (BX) (Na2B4O7.5H2O content is 21.28 Na2O, 47.80% B2O3 and 30.92% H2O with a molecular weight of 291.3, density 1.815 g.cm-3, and melting point 741 °C) (Örs et al. 2006). The samples were dipped in the impregnation pool for 16 h. Prior to the impregnation, the dimensions respectively were measured by a digital caliper and their weights were recorded on a digital scale. After the impregnation, specimens were weighted again. All treated and control specimens were then reconditioned at 20 ±2 °C and 65 ±5% relative humidity for three weeks.
The impregnation was carried out at 20±2 °C. The retention content for each treatment was calculated by the following formula (Eq. 1),
where G is the amount of impregnation solution absorbed by the sample, T2 is the sample weight after the impregnation, T1 is the sample weight before the impregnation, C is the concentration (%) of the impregnation solution, and V is the dry volume of the samples.
Afterward, impregnated specimens of oak wood were cut into smaller specimens that were free of defects for determination of air-dry density according to (ISO3131, 1975), compression strength parallel to the grain (CS) according to (ISO3787, 1976), bending strength (MOR) according to (ISO3133, 1975), modulus of elasticity in bending (MOE) according to (ISO3349, 1975), shear strength parallel to the grain (SS) according to (TS3459, 2012) , and tension strength parallel to the grain (TS) according to (ISO3345, 1975).
The samples were heat-treated at 160, 190, and 220 °C for 2 and 4 h. Heat treatment was carried out under atmospheric pressure, with water vapour as a shielding gas. The total duration of the heat treatment (pre-treatmet period + actual heat treatment period + cooling and moisture conditioning period) was 30 h and actual times at the high temperatures were 2 and 4 h. After the heat treatment, specimen dimensions were re-measured with the digital caliper, and their weights were recorded on a digital weight scale. All treated (impregnation and heat treated) and control specimens were then reconditioned at 20 ±2 °C and 65 ±5% relative humidity for 3 weeks.
Multiple variance analysis was used to determine the difference between the physical and mechanical properties of impregnated and heat-treated woods. When there was a significant difference between the groups, the P ≤0.05 confidence level was compared with Duncan’s test.
RESULTS AND DISCUSSION
Properties of the chemicals (23 ±2 °C, 5% solution concentration) used in the impregnation process are given in Table 1.
Table 1. Properties of Impregnation Chemicals
DW: Distilled water, SD: Standard deviation, BI: Before impregnation, AI: After impregnation
According to Table 1, there was no important change in the acidity and density of the solutions before and after the impregnation.
Averages of the mechanical properties of impregnation chemical, heat temperature, and heat treatment duration are given in Table 2.
Table 2. Average of Mechanical Properties of Impregnation Chemical, Heat Temperature, and Heat Treatment Duration
MOR: Bending strength, MOE: Modulus of elasticity, TS: Tensile strength parallel to the grain, CS: Compression strength parallel to the grain, SS: Shear strength parallel to the grain,HG: Homogenety groups
The retention value of boric acid (8.27) was higher than borax (6.73). Results of variance analyses are given in Table 3. According to Table 3, the effect of the impregnation chemical, heat treatment temperature, and heat treatment duration on bending strength (MOR), modulus of elasticity (MOE), tensile strength parallel to the grain (TS), compression strength parallel to the grain (CS), and shear strength parallel to the grain (SS) were found to be statistically significant (P<0.05). The Duncan test was used to determine the differences between means at a prescribed level of α= 0.05.
The results of the Duncan test are given in Table 4. Table 5 shows the percentage decrease or increase of values in relation to the control for each impregnation and heat treatment and each measured parameter.
Table 3. Results of Variance Analyses
Factor A: Borax, Boric acid; Factor B: Heat treatment temperature; Factor C: Heat treatment duration, MOR: Bending strength, MOE: Modulus of elasticity, TS: Tensile strength parallel to grain, CS: Compression strength parallel to grain, SS: Shear strength parallel to grain
Table 4. Results of the Duncan Test
HG: Homogenety groups, SD: Standard deviation, SV: Statistical values, MOR: Bending strength, MOE: Modulus of elasticity, TS: Tensile strength parallel to the grain, CS: Compression strength parallel to the grain, SS: Shear strength parallel to the grain
The mechanical strengths of impregnated and heat treated oak wood were shown Figs. 1, 2, 3, 4, and 5.
Fig. 1. Bending strength of impregnated and heat-treated oak wood
Fig. 2. Modulus of elasticity of impregnated and heat-treated oak wood
Fig. 3. Tensile strength parallel to the grain of impregnated and heat-treated oak wood
Fig. 4. Compression strength parallel to the grain of impregnated and heat-treated oak wood
Fig. 5. Shear strength parallel to the grain of impregnated and heat-treated oak wood
Density
According to Table 4, the highest density was obtained in specimens impregnated with borax and untreated samples (0.7430 g/cm3), and the lowest density was obtained in non-impregnated and untreated samples (0.6463 g/cm3). The density of the heat-treated wood specimens decreased significantly more than that of the the unheat treated samples. Densities of all the specimens decreased depending on the temperature and duration of the heat treatment. Boonstra et al. (2007b) reported a 10% and 8.5% decrease in density for heat-treated Scots pine and Norway spruce, respectively. The results are parallel to previously published reports for different species (Yildiz et al. 2006; Korkut et al. 2008; Gunduz et al. 2009).
Bending strength (MOR)
According to Table 4, the highest MOR was obtained in samples impregnated with borax and heat-treated at 160 °C for 2 h (116.60 N/mm2) and the lowest in samples impregnated with boric acid and heat-treated at 220 °C for 4 h (80.69 N/mm2).
Table 5. Percentage Decrease or Increase of Mechanical Properties in Oak Wood Following Impregnation and Heat Treatment for Different Durations
The bending strength (MOR) values showed a small increase in the samples impregnated with borax and heat-treated at 160 °C for 2 h. The highest bending strength (MOR) loss compared to the control was 27.58% (Table 5). Results showed that boron treatments before heat treatment increased the MOR values of wood specimens compared to non-impregnated and heat-treated specimens, except for those impregnated with Boric acid and heat-treated at 220 °C for 2 and 4 h (Table 4). The effects of impregnation chemicals, heat treatment, and heat treatment duration on MOR are given in Table 3. According to Table 3, no statistical difference was found in MOR values between untreated and borax-treated wood. However, there was a statistical difference in MOR values between borax-treated and boric-acid treated wood. The highest MOR was obtained in untreated samples. MOR of all the specimens decreased depending on the temperature and duration of the heat treatment. Toker et al. (2009) determined the effects of some boron compounds on modulus of rupture and modulus of elasticity of wood. They reported that MOR values of wood specimens treated with borates were lower than those of untreated control specimens. Yıldız et al. (2004) determined the effects of the wood preservatives on mechanical properties of yellow pine. They reported that 2.8% of Wolmanit CX-8 and 7% of ACQ-1900 increased the MOR compared to control specimens, while the other chemicals either decreased or did not affect the MOR. Colakoglu et al. (2003) detemined that MOR levels of laminated beech veneer lumber impregnated with 5% boric acid were reduced 5.12% compared to untreated control specimens.
Modulus of elasticity in bending (MOE)
According to Table 4, the highest MOE was obtained in samples impregnated with borax and heat-treated specimens at 160 °C for 2 h (12027 N/mm2) and the lowest in those impregnated with boric acid and heat-treated specimens at 220 °C for 4 h (8779 N/mm2). The modulus of elasticity (MOE) had a small increase in impregnated with borax and heat-treated specimens at 160 °C for 2 h, in addition to samples impregnated with borax and untreated specimens. The highest modulus of elasticity in bending loss compared to the control was 25% (Table 5). The modulus of elasticity (MOE) of the wood samples was reduced after the heat treatment. However, losses of MOR and MOE in the impregnated samples with boron compounds, especially borax, were less than the non-impregnated samples. The effects of impregnation chemicals, heat treatment, and heat treatment duration on MOE are given in Table 3. According to Table 3, there was a statistical difference in MOE values between non-impregnated, borax-treated, and boric-acid treated samples. Yıldız et al. 2004 reported that 2.8% of Wolmanit CX-8 and 2% of Tanalith E-3491 increased the MOE compared to the control samples. In this study, the MOE of all specimens decreased depending on the temperature and duration of the heat treatment. Can et al. (2010) determined the effects of boron impregnation and heat treatment on some physical and mechanical properties of spruce and pine wood. They reported that borax pre-impregnated wood samples provided more reasonable results than boric acid treated ones for strength losses. Colakoglu et al. (2003) detemined that MOE levels of laminated beech veneer lumber impregnated with 5% boric acid were reduced 3.75% compared to untreated control specimens. Wahab et al. (2012) reported that the increase in the hemicellulose and cellulose contents or holocellulose contents causes an increase in the strength (MOR and MOE) of acacia hybrid wood. On the other hand, the reduction of the hemicellulose and lignin contents causes a drop in strength of the hot oil treated acacia hybrid.
Tensile strength parallel to the grain (TS)
According to Table 4, the highest TS was obtained in impregnated with borax and untreated samples (95.25 N/mm2) and the lowest in impregnated with boric acid and heat-treated samples at 220 °C for 4 h (61.85 N/mm2). The highest tensile strength parallel to the grain (TS) loss compared to the control was 34.51% (Table 5). The effects of impregnation chemicals, heat treatment, and heat treatment duration on TS are given in Table 3. According to Table 3, there was a statistical difference in TS values between non-impregnated samples, borax-treated samples, and boric acid-treated samples. The TS of all specimens decreased depending on the temperature and duration of the heat treatment. The highest decrease in TS was observed in samples treated at 220 °C (26.56%). The TS of the heat-treated wood specimens decreased significantly compared to the untreated samples. Boonstra et al. (2007b) reported that two-stage heat treatment clearly affects the tensile strength of Scots pine specimens, which is strongly reduced (39%). When tensile stresses occur in wood, the cellulose microfibrils and/or fibrils slide and pull away from one another which requires the breaking of covalent bonds. Degradation of the cellulose polymer, decreasing the DP, was suggested as the main cause for losses in tensile strength (Kass et al. 1970; Esteves and Pereira 2009). Korkut et al. (2008) reported that the lowest tensile strengths perpendicular to the grain were at 180 °C for 10 h (2.131 N/mm2) in Red-bud maple (Acer trautvetteri Medw.) wood. The loss of tensile strength perpendicular to the grain was 46.563%.
Compression strength parallel to the grain (CS)
According to Table 4, the highest CS was obtained in samples impregnated with borax and heat-treated specimens at 160 °C for 2 h (84.78 N/mm2) and the lowest in those impregnated with boric acid and heat-treated at 220 °C for 4 h (72.06 N/mm2). The effects of impregnation chemicals, heat treatment, and heat treatment duration on CS are given in Table 3. According to Table 3, there was a statistical difference in CS values between non-impregnated wood, borax-treated wood, and boric acid-treated wood. The highest CS was obtained in samples impregnated with borax. The CS of all the specimens increased depending on the temperature of heat treatment compared to untreated specimens. The compressive strength parallel to the grain clearly increased after impregnation and heat treatments. Sahin Kol (2010) reported that the compressive strength increased by 4.2% for pine and 17% for fir with heat treatment. The compressive strength parallel to the grain clearly increased after heat treatment (28%), the radial compressive strength decreased (43%), and the tangential compressive strength slightly increased (8%) after heat treatment (Boonstra et al. 2007b; Kol 2010). Can et al. (2010) determined effects of boron impregnation and heat treatment on some mechanical properties of spruce and pine wood. For this purpose, spruce and pine wood specimens were treated with 4% aqueous solution of both boric acid (BA) and borax (BX), and then the basic thermowood method was applied to the samples as the heat treatment method. Heat treatment was carried out in an industrial furnace at 212 °C for 2 h. As a result, compression strength decreased depending on heat treatment except for the pine specimens impregnated with borax. Colakoglu et al. (2003) determined that compression strength in the longitudinal direction levels of laminated beech veneer lumber impregnated with 5% boric acid increased 1.38% compared to untreated control specimens. Vital et al. (1983) reported that the compression strength values generally deteriorated with increasing temperature or exposure time.
Shear strength parallel to the grain (SS)
According to Table 4, the highest SS was obtained in non-impregnated and untreated samples (17.16 N/mm2) and the lowest in samples impregnated with boric acid and heat-treated at 220 °C for 4 h (10.15 N/mm2). According to Table 3, the greatest decrease in SS was observed in samples treated at 220 °C. The highest shear strength parallel to the grain (SS) loss compared to the control was 40.85% (Table 5). The SS of the heat-treated wood specimens decreased significantly compared to the un-heat treated samples. According to Stamm (1964), degradation of the hemicelluloses reducing the load-sharing capacity between cellulose microfibrils/fibrils most probably has a negative impact on shear strength (Boonstra et al. 2007b). When wood is heated at a high temperature, it becomes more brittle and its mechanical strength decreases, depending on the level and duration of the treatment (Bekhta and Niemz 2003; Korkut 2008; Esteves and Pereira 2009).
In this study, bending strength (MOR), modulus of elasticity in bending (MOE), tensile strength parallel to the grain (TS), and shear strength parallel to the grain (SS) decreased due to heat treatment. The decreases in the mechanical strength can be explained by the rate of thermal degradation and losses of substance after heat treatments. Also, the decreases in mechanical strength may be due to the decomposition of wood components, especially hemicellulose. In accordance with Esteves et al. (2008), hemicelluloses are affected first, followed by cellulose and lignin. The decrease in mechanical strength is mainly due to the depolymerization reactions of wood polymers (Kotilainen et al. 2000). The primary reason for the mechanical strength loss is the degradation of hemicelluloses, which are less resistant to heat than cellulose and lignin. Changes in or loss of hemicelluloses play key roles in the strength properties of wood heated at high temperatures (Hillis 1984). There are chemical changes in the the wood during heating. Chemical changes start by deacetylation of hemicelluloses followed by depolymerization catalysed by the released acetic acid (Tjeerdsma et al. 1998; Sivonen et al. 2002; Nuopponen et al. 2004).
CONCLUSIONS
- The effects of impregnation chemicals (borax and boric acid) on density and some mechanical properties (bending strength (MOR), modulus of elasticity in bending (MOE), tensile strength parallel to the grain (TS), compression strength parallel to the grain (CS), and shear strength parallel to the grain (SS) of heat treated oak wood were studied.
- The values of density decreased with increasing heat treatment temperature and time.
- Compression strength increased with the impregnation and increasing temperatures and durations of the heat treatment. However, as the temperature increased, compression strength showed a declining trend.
- The bending strength (MOR), modulus of elasticity in bending (MOE), tensile strength parallel to the grain (TS), and shear strength parallel to the grain (SS) decreased due to heat treatment. Shear strength decreased more than tensile strength.
- The heat treatment temperature was more effective than the heat treatment time on mechanical strength, and borax impregnation had relatively more positive values.
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Article submitted: February 9, 2015; Peer review completed: March 22, 2015; Revised version received: March 31, 2015; Accepted: May 6, 2015; Published: May 13, 2015.
DOI: 10.15376/biores.10.3.3963-3978