Comparison of stiffness and strength properties of untreated and heat-treated wood of Douglas fir and alder

Abstract

This paper investigates the effect of heat treatment temperature on the stiffness and strength properties of Douglas fir (Pseudotsuga menziesii Franco) and common alder (Alnus glutinosa Gaertn.) woods. Two temperatures of heat treatment were used: 165 and 210 °C. The effects of dynamic elasticity modulus, static elasticity modulus, impact toughness, bending strength, and density were evaluated. It is already understood that the mechanical properties, primarily the bending strength, decreases with increasing temperature. In contrast to the favorable stability in shape and dimension that was achieved, the changes in the woods’ properties with temperature were mostly negative. Higher heat treatment temperatures corresponded with lower stiffness and strength properties. For higher temperature treatments, above 200 °C, deterioration of the tested properties was noticable as a result of the significant changes in the wood chemical structure. Even the positive effect of the equilibrium moisture decrease was not able to counterbalance the unfavorable changes. Moreover, it was observed that as the hemicellulose content is higher in alder wood, density, static bending strength, and toughness all decreased steadily at high temperatures, compared to Douglas fir wood.

Full Article

Comparison of Stiffness and Strength Properties of Untreated and Heat-Treated Wood of Douglas Fir and Alder

Vlastimil Borůvka, Aleš Zeidler,* and Tomáš Holeček

This paper investigates the effect of heat treatment temperature on the stiffness and strength properties of Douglas fir (Pseudotsuga menziesii Franco) and common alder (Alnus glutinosa Gaertn.) woods. Two temperatures of heat treatment were used: 165 and 210 °C. The effects of dynamic elasticity modulus, static elasticity modulus, impact toughness, bending strength, and density were evaluated. It is already understood that the mechanical properties, primarily the bending strength, decreases with increasing temperature. In contrast to the favorable stability in shape and dimension that was achieved, the changes in the woods’ properties with temperature were mostly negative. Higher heat treatment temperatures corresponded with lower stiffness and strength properties. For higher temperature treatments, above 200 °C, deterioration of the tested properties was noticable as a result of the significant changes in the wood chemical structure. Even the positive effect of the equilibrium moisture decrease was not able to counterbalance the unfavorable changes. Moreover, it was observed that as the hemicellulose content is higher in alder wood, density, static bending strength, and toughness all decreased steadily at high temperatures, compared to Douglas fir wood.

Keywords: Heat treatment; Thermowood properties; Dynamic and static elasticity moduli; Impact toughness; Bending strength; Density

Contact information: Department of Wood Processing, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 – Suchdol, 16521 Czech Republic; *Corresponding author: zeidler@fld.czu.cz

INTRODUCTION

Heat-treated wood is a well known material that is commonly utilized in industry. The heat treatment method changes, by means of appropriate action of the heat, some of the wood’s properties that are important from the view of its specific utilisation. The process does not use any chemicals, which is one of its many advantages. This aids in retaining the natural character of the wood. Thermowood has been utilized for several years, primarily in Western Europe. The most extensive and complete study of the wood heat treatment process took place in Finland. Heat-treated wood typically has a longer lifetime; therefore, it is a material suitable for outdoor exposure with no ground contact (ITA 2003; Barcík et al. 2014).

The influencing factors that affect the chemical reactions of the wood components during the heat treatment process (Fig. 1, Kačíková and Kačík 2011) include the wood species, ambient pressure, heat supply intensity, oxygen access to wood, and the wood’s initial moisture content (Reinprecht and Vidholdová 2008, 2011). The heat treatment process (for Thermowood production), utilizing only wood and steam, changes the wood’s internal structure (Gaff and Gašparík 2013; Gašparík and Gaff 2013). This causes a significant reduction in the hygroscopicity (up to 50%). Wood treated with this method achieves high structural and dimensional stability (Bekhta and Niemz 2003; ITA 2003; Calonego et al. 2012; Kvietková et al. 2015). On the other hand, the mechanical properties tested in these studies mostly reveal the negative impacts of increasing temperature during the heat treatment process (Borrega and Kärenlampi 2007; Shi et al. 2007; Sonderegger et al. 2007; Johansson 2008; Barcík and Gašparík 2014; Schneid et al. 2014). The changing in mechanical properties of wood mainly depends on the species and treatment degree. In the above-listed literature it is pointed out that at highest degrees of heat treatment, decreases of bending strength of conifer wood by approximately 20% (or by 40% in the case of decidious wood) is observed. The decrease in strength for the dynamic type of such load, which means toughness, can be even more significant. The decrease of static elasticity moduli can be 5%. A partial increase in most of the property values at lower heat treatment degree (160 – 170 ºC), or their preservation at the level of untreated wood cause the less significant chemical changes in wood, which lead only to the limitation of wood ability to bind the water.

Fig. 1. Chemical changes of the main components in wood during the heat treatment

The aim of this study was to extend the database of stiffness and strength characteristics for the thermally modified wood of Douglas fir and common alder, and their mutual comparison. From the view of wood utilisation, these characteristics are essential to set limiting conditions for proper usage and protection of individual timbers, including the proper degree of heat treatment to ensure required properties.

EXPERIMENTAL

Materials

The testing material originated from tree trunks from the Školní Lesní Podnik (Forest Establishment) of the Czech University of Life Sciences in Kostelec nad Černými Lesy. Douglas fir (Pseudotsuga menziesii) and common alder (Alnus glutinosa Gaertn.) wood were processed into rectangular solids with dimensions of 25 mm 50 mm 1000 mm (RTL). Six test pieces with dimensions of 20 mm 20 mm 300 mm were cut from each piece to determine the longitudinal parallelism of the test samples with the samples chosen for two degrees of the heat treatment.

Transversal parallelism for two sets of tests (always a sample designed for the determination of density and toughness, underneath a sample on dynamic elasticity modulus, static elasticity modulus, and bending strength) enables mutual comparison. See the cutting diagram in Fig. 2.

Fig. 2. Cutting diagram for testing sample preparation

A total of 180 test sections of the samples were divided into sets of 30 (two treatment temperature degrees and two wood species). The physical requirements for the samples were as follows: no knots, cracks, or reaction wood, as well as minimum angle of fiber declination in the bending plane ( 5).

Subsequently, the samples were conditioned, to stabilize the equilibrium moisture content, inside of a Climacell 707 conditioning chamber (BMT Medical Technology Ltd., Czech Republic) at 20 2 C and  a relative humidity of 65 5%.

Afterwards, one third of the test samples was subjected to the first degree of heat treatment, in an air atmosphere at 165 °C, and the second third of the samples were heat-treated at 210 °C, in accordance with the Finnish technology for the wood heat treatment (Pat. EP-0759137 in Viitaniemi et al. 1998). The preparation of the testing samples took place inside a lab high-temperature chamber A type KHT (Katres Ltd., Czech Republic) (Fig. 3).

Fig. 3. Samples in position before the treatment in the chamber (left) and their conditioning after heat treatment (right)

A detailed preparation procedure is shown in Fig. 4. Subsequently, the test samples were conditioned again, to stabilize the equilibrium moisture content at 20 2 C and a relative humidity of 65 5% (Fig. 3).

Fig. 4. Diagram representation of the heat treatment procedure at 165 °C (left) and at 210 °C (right)

Douglas fir and alder woods were chosen intentionally for the scope of this research. These species are relevant representatives of conifer and deciduous wood species with similar densities (Table 1) (Podrázský et al. 2013).

Table 1. Properties of Douglas Fir and Alder Woods

¹Moisture content was between 12% and 15% (Wagenführ 2000)

Methods

The impact toughness (breaking power) is defined as the ability of wood to absorb the power of impact bending. The aim of this test was to determine the power consumed for the wood rupture (breaking point) under controlled conditions. Charpy’s hammer (CULS, Czech Republic) was used for this determination. The hammer impact direction was tangential.

The following formula was used to calculate the impact toughness,

 (1)

where A_w is the impact toughness at the moisture content during the test time in J.cm^-2, W is the power consumed for the wood rupture in J, and b and h are the wood transversal dimensions in cm.

The wood bending strength is the stress corresponding to the test sample rupture caused by the combined forces with momentum at the plane perpendicular to the cross section. For the action of a single force in the center of the supports, the bending strength was calculated according to the following formula,

 (2)

where _pohw is the bending strength at the moisture content during the test time in MPa, F_max is the force corresponding to the breaking strength in N, l₀ is the distance between supports in mm, and b and h are the width and height dimensions, respectively, in mm.

A theoretical basis for the determination of the bend elasticity modulus is the differential equation of the bending curve, as follows (Požgaj et al. 1997),

 (3)

where M is the bending momentum, E is the elasticity modulus, and I is the inertia moment.

For the action of a single force in the center of the supports, the static elasticity modulus was calculated according to the following formula,

 (4)

where E_ohw is the elasticity modulus at the moisture content during the test time in MPa, F is the difference between the forces at maximum and minimum load limits in N, l₀ is the distance between the supports in mm, b and h are the width and height dimensions, respectively, in mm, and y is the test sample deflection in the area of pure bending, equal to the difference between the bending values corresponding to maximum and minimum load limits, in mm.

The static bending tests were carried out on a Tira 50 kN testing machine (Tira GmbH, Germany) (Fig. 5) with support distances of 240 mm, i.e., 12-fold greater than the sample height.

Fig. 5. TIRA 50 kN testing machine (left), a rupture of heat-treated wood under the bending load (top right), and Fakopp Ultrasonic Timer instrument (bottom right)

The dynamic elasticity modulus was calculated as follows (Požgaj et al. 1997),

E_d = c² (5)

where E_d is the dynamic elasticity modulus in MPa, c is the speed of sound in m.s^-1, and is the wood density in kg.m^-3. We used a Fakopp Ultrasonic Timer instrument (Fakopp Enterprise Bt., Hungary) (Fig. 5).

A reading for the wood density determination was taken from each test sample after the experiment. The density was calculated as follows,

 (6)

where _w is the wood density at the moisture content during the testing time in g.cm^-3, m_w is the wood mass at the moisture content during the testing time in g, and V_w is the wood volume at the moisture content during the testing time in cm³.

After the samples were dried to zero percent moisture in a Binder FD 115 lab kiln (Binder Inc., Germany) at 103 2 C, the wood moisture content was calculated according to the following formula,

 (7)

where w_a is the sample’s moisture content in %, m_w is the sample’s mass at a certain moisture content in g, and m₀ is the sample’s dry mass in g.

All of the test samples were conditioned under standardized conditions in a conditioning chamber with a relative humidity of 65 5% and a temperature of 20 2 °C to obtain 12% equilibrium moisture content for the solid, untreated wood. The heat-treated wood exhibited a lower moisture content under these conditions depending on the degree of the heat treatment. All tests were carried out completely in accordance with the testing standards (ČSN 49 0103 (1979), ČSN 49 0108 (1993), ČSN 49 0115 (1979), ČSN 49 0116 (1982), and ČSN 49 0117 (1980)), and the determination of the dynamic elasticity modulus was based on the methodology specified in the Fakopp instrumentation manual.

For statistical analysis ANOVA (two-factors) was used to evaluate the significance of individual factors. Linear regression model was used to set the degree of correlation of selected factors. For all analysis the same significance level α = 0.01 (alternatively α = 0.05) was employed.

RESULTS AND DISCUSSION

Tables 2 and 3 summarize the statistical data for the physical and mechanical properties of both wood species after the heat treatment. It is also evident from Table 1 that the evaluated properties of native wood corresponded to those presented in literature for the tested properties. The data obtained from this study were subjected to the statistical analysis (Tables 4 to 8; Figs. 6A to 6E). The end of this section contains more detailed analysis; however, it is worth mentioning that a drastic decrease in the bending strength was observed at higher treatment temperatures for alder wood, while an insignificant impact of the heat treatment temperature was observed for the static eleasticity modulus. This paper includes the results of a two-factor analysis; however, the wood species impact was obvious, i.e., the results of a single-factor (heat treatment) analysis would be sufficient. At the same time, the existence of correlation between static and dynamic elasticity moduli has been confirmed (Dinwoodie 2000), taking into account the relation of all samples (both the reference and the heat-treated samples); see Fig. 6F.

Table 2. Basic Statistical Analyses of the Mechanical Properties for Untreated and Heat-Treated Douglas Fir Wood

Heat treatment degree: 1 = reference, with no treatment, 2 = heat treatment at 165 °C,

3 = heat treatment at 210 °C

Generally a more significant influence of heat treatment was shown with respect to the properties of alder wood than for Douglas fir wood (Table 9). The most significant changes by using heat treatment were achieved for toughness by using a higher temperature of heat treatment.

A decrease in the case of Douglas fir wood by 34% and for alder wood by 63% was observed in comparison to untreated wood. Further significant changes were achieved at bending strength while using higher heat treatment degree, specifically the decrease by 45% was observed for alder wood and only by 8% for Douglas fir wood in comparison to untreated wood. The general trend corresponds with the results specified for example in ITA 2003 and Johansson 2008. Factors explaining the difference between the two tree species are described below.

Table 3. Basic Statistical Analyses of the Mechanical Properties for Untreated and Heat-Treated Alder Wood

Heat treatment degree: 1 = reference, with no treatment, 2 = heat treatment at 165 °C,

3 = heat treatment at 210 °C

Table 4. Analysis of Variance for Wood Density

Significance was accepted at P < 0.01

Table 5. Analysis of Variance for Impact Toughness

Significance was accepted at P < 0.01

Table 6. Analysis of Variance for Dynamic Elasticity Modulus

Significance was accepted at P < 0.01

Table 7. Analysis of Variance for Static Elasticity Modulus

Significance was accepted at P < 0.01

Fig. 6. Graphic visualization of the effect of wood species and heat treatment temperature on A) wood density, B) impact toughness, C) dynamic elasticity modulus, D) static elasticity modulus, and E) bending strength, at a 95% significance level. The relationship between static and dynamic elasticity moduli is shown in F. X-axis: D = Douglas fir and A = alder

Table 8. Analysis of Variance for Bending Strength

Significance was accepted at P < 0.01

Table 9. Changes in Wood Property of Heat-treated Wood in Comparison to the Reference (untreated) Wood in %

2/1 = heat treatment at 165 °C vs. reference, with no treatment

3/1 = heat treatment at 210 °C vs. reference, with no treatment

It is crucial, with respect to the heat treatment impact on the wood properties, to know that hemicelluloses are the most affected chemical components within this process (ITA 2003). Hemicelluloses consist of mostly pyranose structures. Coniferous species consist mostly of glucomannans, and deciduous species contain mostly xylans. Coniferous species, according to Požgaj et al. (1997), contain less hemicelluloses (approximately 23% to 25%) than deciduous species (approximately 26% to 35%). Mannan fractions, which are prevailing in softwood, behave more like a skeleton than filling material of wood. Moreover they have stronger bonds with cellulose than xylans in hardwoods. From the higher content of hemicelluloses in alder wood in comparison to Douglas fir wood, as well as their generally lower strength, the more significant decrease in density, static bending strength, and toughness at alder wood relative to Douglas fir wood was obvious at higher degrees of heat treatment (Figs. 6A and 6E). The differences in trend of these properties between two species at lower heat treatment degree, as well as elasticity moduli at higher treatment degree was not shown.

The disagreement between the static and dynamic moduli, not from the perspective of correlation, which was pretty high (r = 0.78), but from the perspective of increasing rate of parameters‘ values of measured dynamic moduli and static moduli at heat-treated wood was observed. It was shown that there was a greater increase of parameters for dynamic moduli than for static moduli, which can be explained by the differences in wood moisture content, while measuring the dynamic moduli and the static moduli. The passage rate of ultrasonic waves through wood in longitudinal direction is about 5000 m/s and through the water, bound water in this case, is about 1500 m/s (Kollmann and Côte 1968; Bucur 2006). Thus, in case of linear conversion of moduli values on uniform moisture, different conversion moisture coefficients should be applied for these two methods. The verification of this point needs further experiments and investigation. Another factor, influencing the difference in property values between the dynamic and static moduli, is the existence of shear stress in static three-point bending, which is irrelevant for the dynamic modulus set on base of the speed rate passage of ultrasonic waves. The combination of these two factors have a synergistic effect at thermowood.

Overall, it is generally understood that especially the strength properties of wood decrease with increasing heat treatment temperature. Increase of values, mostly insignificant, of majority of the properties at lower temperature degree of the treatment is related to the fact that the chemical changes are not yet so important and thus causing only a reduction of the ability of wood to bind water (Reinprecht and Vidholdová 2008, 2011). A decreased water content in wood in the same environmental conditions is the reason of the above-mentioned finding. This was, obviously, least evident for the dynamic impact test. For higher treatment temperatures, above 200 °C, a reduction in equilibrium moisture did not have a notable influence on the structural properties of wood because the greatest changes occurred in the wood’s chemical structure and therefore affected the mechanical properties of wood.

CONCLUSIONS

1. Partial increase in the wood properties at lower temperatures were similar to those of untreated wood. This was related to the fact that the chemical changes have not yet occurred to a significant degree, thus causing only a reduction of the ability of wood to bind water.
2. Higher heat treatment temperatures resulted in lower stiffness and strength properties of heat-treated wood. For heat-treatement temperatures above 200 °C, the decrease of the tested properties was noticable as a result of the significant changes in the wood chemical structure and even positive effect of the moisture content decrease was not able to counterbalance this changes.
3. Apparently, wood with a higher hemicellulose content, i.e. a lower overall resistance, exhibits less density, static bending strength, and toughness. Therefore, a more significant decrease was observed for the common alder than for the Douglas fir at higher treatment temperatures. The decrease of toughness by about 63% at alder wood with treated temperatures of 210 ºC was observed in comparison to untreated wood. Bending strength at heat treated alder wood decreased by 45%.
4. Higher strength resistance was observed in Douglas fir than for alder wood with an increasing temperature treatment.
5. There was a significant correlation between static and dynamic elasticity moduli, which accounted for both sets of samples (untreated and heat-treated samples).
6. From the view of wood utilisation, the knowledge of above mentioned facts is important for determination of limiting values to ensure proper and save application of individual timbers, including the appropriate degree of heat treatment to ensure required properties. From the achieved results it is clearly seen that usage of wood, treated with high temperatures, especially wood of decidious species, is not suitable for construction purposes, because of significant decrease at bending strength and toughness.

ACKNOWLEDGMENTS

The authors are grateful for the grant provided by the National Agency for Agricultural Research project No. QJ1520299. The authors are also grateful for the support of the Internal Grant Agency of the Faculty of Forestry and Wood Sciences, Czech University of Life Sciences, Prague, Project No. 20143127.

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Article submitted: August 13, 2015; Peer review completed: October 5, 2015; Revised version received and accepted: October 12, 20115; Published: October 28, 2015.

DOI: 10.15376/biores.10.4.8281-8294