Influence of Thermal Treatment on Properties of Ash Wood

Abstract

The objective of this study was to investigate some of the properties of ash (Fraxinus excelsior L.) wood treated at a temperature of 185 °C for 3 h. We tested the samples’ physical properties, including their dimensional stability in the form of water absorption and radial/tangential swelling and their surface roughness, as well as their mechanical characteristics such as Brinel hardness and bending strength. Our results show that the water absorption and swelling values of the samples improved, ranging from 27% to 32% and from 21% to 46%, respectively. Also, the Brinell hardness of the thermally treated wood registered a slight decrease from 19.5 N/mm² to 15.2 N/mm² in the radial direction and from 15.7 N/mm² to 12.2 N/mm² in the tangential direction. It appears that the bending resistance of the samples decreased, which is generally thought to be an adverse effect of heat treatment on any wood species.

1. Introduction

Wood resources in the form of timber are increasingly being used in all social and economic fields given that they are natural materials and can be processed and transformed into various shapes and sizes. Untreated wood materials have a major disadvantage: they absorb moisture from the surrounding environment, which can have severe effects on their dimensions and dimensional stability, leading to distortion; thus, they need to be treated before use [1,2,3]. The uptake of moisture can lead to the emergence of various defects and even intense degradation due to fungal and xylophagous insect attacks [4,5]. Consequently, in recent years, there has been increasing interest in developing techniques to protect wood from atmospheric moisture by inhibiting direct water absorption or moisture from the atmosphere, with the ultimate goal of increasing the dimensional stability and durability of wood over time. Typically, dimensional stability is quantifiable through water absorption and grain orientation swelling tests, and durability is determined through biological attack and fungicide tests [4,6,7,8].

There are several procedures for protecting wood, including the application of chemicals, acetylation, and thermal treatment. Wood thermal treatment can also be carried out with oil (OHT: Oil Heat Treatment in Germany), high frequency or high voltage currents, and steam (Plato Wood in the Netherlands, Huber Holz in Austria, Bois Perdure and Rectification in France, Thermo-wood in Finland, and WTT in Denmark) [9,10,11]. The use of dry thermal treatment processes has been declining rapidly because they can cause defects (such as cracks, structural collapse, and superficial cementation), especially when the speed of the treatment is high. Therefore, thermal treatment processes with steam are increasingly being used in order to obtain dimensional stability and prevent cracks in the wood [12,13]. Even the artificial drying of lumber is performed under saturated steam to avoid defects from the drying process.

One of the longest-established thermal modification systems is Thermo-wood^TM [6,14,15], a trademark of the Finnish Thermo-wood Association and the VTT Technical Research Centre in Finland [6]. It has been shown that this process is environmentally friendly because no chemicals are involved in the treatment; it only requires steam. The process is based on heating the wood to high temperatures of 170–225 °C under saturated steam in an oxygen-poor environment (with 3% oxygen) [6,16,17]. The thermal treatment takes place in steam cells with a horizontal cylindrical section in order to withstand the high pressure from the steam. The purpose of this thermal treatment is two-fold: to increase the stability of wood (by reducing water absorption and swelling) and to enhance its natural durability (by keeping its properties intact for a long time while preventing attacks from insects and xylophagous fungi) [18].

Defect-free wood without abnormalities (resin pockets, knots, etc.) should be used as the working material for thermal treatment. Raw timber should not be used because it contains defects that need to be eliminated before treatment. Wood species that usually undergo thermal treatment are soft hardwoods like poplar and birch, dense hardwoods like beech and hornbeam, and softwoods like pine and spruce. Because of its low affinity for humidity, thermally treated wood can successfully replace untreated wood with expensive finishes, which are necessary for bath/kitchen furniture or some products that are used outdoors [19,20].

Amid the increasing demand for lumber and other wood products in the construction industry and due to the increasing number of restrictions regarding the use of toxic wood protection substances, thermally treated wood can be used as good-quality timber. It was observed that by increasing the temperature of the thermal treatment [20,21], the biological durability of wood also increases but the resistance decreases slightly [6,22]. Other disadvantages of thermally treated wood include a change in the natural color of the wood to a blackish one [23,24,25] and a reduction in the wood’s sheen [23]. However, some authors believe that thermal treatment makes the color of the wood more uniform (there are no differences in color between sapwood and heartwood and between earlywood and latewood) and that the black silky color is an indisputable advantage of this type of treated wood [2,4].

Chemical analyses of thermally treated wood have revealed better thermal stability of resinous wood compared to hardwoods [26,27]. Hemicelluloses are the first chemical compounds to degrade at high temperatures of thermal treatment (starting from 170 °C), followed by cellulose (over 230 °C) and lignin (over 260 °C). Therefore, the thermal treatment temperature for wood stability is between 175 and 225 °C, with the aim of degrading wood hemicelluloses since they are the most hydrophilic substances in wood as they contain several hydroxyl groups at the end of their molecular chain [25,26]. Also, thermal treatment increases the crystallinity of the microscopic structure and decreases the amorphous areas [25,26,27]. Some studies that were conducted on thermally treated and finished wood did not report any serious problems regarding film adhesion or durability over time [28,29].

European ash wood (Fraxinus excelsior L.) is considered one of the most important species, after oak wood, in Central Europe, and in 2001, it was considered the tree of the year. It is distinguished by a uniform and light color as well as high resistance, machinability, strength, and durability. The density of European ash can vary by site, with a value of 689 kg/m³ at a certain site and 665 kg/m³ at another one [30,31].

Objectives of the paper. A review paper showed that thermally treated wood is a promising solution to the shortage of high-quality timber, which is necessary for finished wood products that are used in environments with high moisture content (both indoors or outdoors). It was also observed that one of the less commonly used species in thermal treatment is European ash, which is why the main objective of the paper is to determine the physical and mechanical properties of thermally treated ash wood. In this context, the aim of the paper is to increase the dimensional stability of the ash wood by thermal treatment at 185 °C for different pieces of timber; ash wood that was not thermally treated was used as the control for comparison. The resistance of three types of joints (commonly used in wood engineering) produced from thermally treated and untreated wood was also analyzed in this research.

2. Materials and Methods

Experimental materials. For the experiments, 20 fresh-cut timber pieces of ash wood (Fraxinus excelsior L.), with a tangential grain orientation and sizes of 2000 × 50 × 50 mm were purchased from the company Unilin Flooring Romania, Brasov, Romania. Of these, 10 pieces of thermally treated ash wood and 10 pieces of untreated ones were used, with all of them having sawn faces. The 10 pieces of heat-treated or untreated lumber were carefully chosen so that they were similar in structure, without major differences between them.

Prior to their acquisition, details about the pieces of timber were obtained from the manufacturing company to ensure that they were thermally treated at 185 °C for 3 h in a steam environment under atmospheric pressure. Thermal treatment was performed inside a metal autoclave. The thermal treatment period consisted of the initial heating period, the thermal treatment period at 185 °C, and the final cooling period. After cooling, the pieces of timber were removed from the autoclave and stored in a conditioning room for 10 days. Also, these semi-finished products were prepared before thermal treatment by eliminating all shape and structure defects.

Furthermore, the treated and untreated pieces of timber were cut into samples specific to each laboratory test, with the treated ash wood samples being marked as T and the untreated ones being labeled as N. Due to the particularities of non-destructive testing of wood, the Brinell hardness, density, and roughness tests were performed successively on the same specimen. Specifically, first, the test was performed on the untreated specimen; then, the specimen was treated; and finally, the test was performed on the thermally treated specimen.

Figure 1 shows the 3 periods of thermal treatment (heating–drying, heat treatment, and cooling–remoistening). During this process, a temperature of 185 °C was used to obtain high-dimensional stability [2,4,6]. It is also noteworthy that although the entire thermal treatment process takes more than 40 h, the duration of the actual thermal treatment was only 4 h, which is less than 8% of the total time.

Wood density. Regardless of whether or not the pieces of timber were thermally treated, 10 planed samples were cut for this test, conforming to ISO 13061-2:2014 and EN 325:2012 [31,32], with dimensions of 100 × 20 × 20 mm. These samples were conditioned in an ASLI-type air conditioning room (LiXin, Dong Guan, Lixin, China) at a temperature of 20 °C and relative humidity of 64%, which would favor equilibration at a moisture content of about 12% when a longer period of time was required [22].

During the test, the mass of each sample was determined using a Kern-type analytical balance, with an accuracy of one decimal point, and the dimensions were determined with an electronic caliper, with an accuracy of two decimal points. Based on the values of the dimensions and the mass of each sample, their density was determined using the following formula:

(1)${\rho }_w={\displaystyle \frac{m}{l·b·g}}·{10}^6\left[{\displaystyle \frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}}\right]$

where:

m—mass of samples, in g;

l—length of specimens, in mm;

b—width of samples, in mm;

g—thickness of samples, in mm.

Water absorption and tangential/radial swelling were measured according to ASTM D570-22 [33] and ASTM D143-21 (2021) [34,35]. In order to determine the influence of water on treated and untreated ash wood, 10 pieces of 100 × 20 × 20 mm were cut from the pieces of timber. The samples were weighed using the Kern 250 balance in order to obtain their mass, and their radial/tangential thickness was measured with the electronic caliper, with an accuracy of two decimal points. Then, the samples were placed in a laboratory environmental chamber (Memmert, Hamburg, Germany) to dry them at 103 ± 2 °C for 10 h to obtain the absolute dry mass of the samples. In order to cool them, the samples were kept in a desiccator (using anhydrous calcium chloride to prevent moisture) for 4 h. Then, the cooled samples were weighed, and the radial/tangential thickness and mass were determined. This procedure was used to obtain the absolute dry mass and the thickness of the samples. Then, the samples underwent the moisture absorption process through complete immersion in water. After obtaining the initial masses and thicknesses, the samples were placed in an immersion tank containing distilled water at room temperature (20 °C) and kept at 20 mm below the water level. The two types of samples (thermally treated and untreated (control)) were immersed in water for 2 h and 24 h in order to determine water absorption and tangential/radial swelling after both durations (ASTM D570-22) [33]. After 2 or 24 h, the samples were removed from the immersion bath, buffered with blotting paper in order to eliminate the excess water, and weighed with the same analytical balance. Then, the thickness of the samples was measured in both the tangential and radial grain orientations with the same electronic caliper.

Next, water absorption was determined after 2 h and 24 h using the following two formulas (Equation (3)):

(2)$A_{2\mathrm{h}}={\displaystyle \frac{m_{i2}-m_0}{m_0}}·100 [\mathrm{\%}]$

(3)$A_{24\mathrm{h}}={\displaystyle \frac{m_{i24}-m_0}{m_0}}·100 [\mathrm{\%}]$

where:

A_2h—water absorption after 2 h of immersion in water, in %;

m_i2—mass of the test sample immersed for 2 h, in g;

m₀—initial mass of the fully dry specimen, in g;

A_24h—water absorption after 24 h of immersion in water, in %;

m_i24—the mass of the sample immersed in water for 24 h, in g.

The decrease in water absorption following the thermal treatment of ash wood was obtained using the following formula (Equation (4)):

(4)$D={\displaystyle \frac{A_n-A_t}{A_n}}·100 \left[\mathrm{\%}\right]$

where:

D—decrease in water absorption after the thermal treatment of ash wood, in %;

A_t—water absorption of thermally treated ash wood, in %.

The swelling in the tangential/radial grain orientation of the samples was determined according to the variation in the thickness of the samples after 2 or 24 h using the following two formulas (Equation (5)):

(5)$S_{2\mathrm{h}}={\displaystyle \frac{g_{i2\mathrm{h}}-g_0}{g_{0i}}}·100 \left[\mathrm{\%}\right] S_{24\mathrm{h}}={\displaystyle \frac{g_{i24\mathrm{h}}-g_0}{g_0}}·100 [\mathrm{\%}]$

where:

S_2h—swelling (change in thickness) after immersion in water for 2 h, in %;

g_i2h—thickness of samples immersed in water for 2 h, in mm;

g₀—initial thickness of fully dry specimens, before immersion, in mm;

A_24h—swelling (change in thickness) after 24 h immersion in water, in %;

g_i24h—the thickness of the samples immersed in water for 24 h, in mm.

Determining the black-and-white color of wood in the CIELab color space. The color of the wood changed from light brown to light black during thermal treatment [24,25]. That is why the color was determined on both untreated and thermally treated samples. This was achieved using the luminance (denoted by L*) values obtained from the CIELab color system; the luminance represents the degree of white or black color of the untreated and thermally treated ash samples. A Tess-135A colorimeter (Tess Electrical, Electronic Corp., Taipei, Taiwan) and the color space CIELab were used to obtain these measurements. This coloristic system makes it possible to transform the visual aspects of color into a numerical value. Since the main difference between natural and heat-treated ash wood is the gray-black hue, the dominant effect of thermal treatment (from a light color to a dark one) can be simply represented by luminance; so, only the L* values were considered (Figure 2). For these measurements, 10 specimens, with dimensions of 20 × 20 × 100 mm, were used, with 4 tests being conducted on each side of the samples.

Brinell hardness was measured according to EN 1534:2003 [35] and ASTM D1474-98:2008 [36,37]. Brinell hardness expresses the resistance of the surface of a material when a 10 mm diameter ball is pressed against on the surface to try to deform or destroy it. The resistance of wood materials is higher at the cross-section surface than the tangential and radial ones. Figure 3 presents the shape of the Brinell device that was used to test the wood surfaces.

The main dependence parameter of the formula for the Brinell hardness is the diameter of the traces left by the ball on the surface of the wood material, as can be seen from the following formula (Equation (6)):

(6)$HB={\displaystyle \frac{2\times P}{\pi \times D(D-\sqrt{D^2-d^2)}}}[{\displaystyle \frac{\mathrm{N}}{{\mathrm{m}\mathrm{m}}^2}}]$

where:

P—applied force, related to wood density, in N;

D—diameter of the punch ball (which was 10 mm in this study);

d—diameter of the trace imprint on the wood, in mm.

Ten valid samples were used to test each ash wood type: treated or untreated. Using the same wood samples, 5 tests were performed on the radial surfaces and 5 more tests were carried out on the tangential surfaces. The tests were performed in a zigzag pattern in order to include both earlywood and latewood from the same section of the same specimen each time.

Roughness. The planed wooden piece of 20 × 20 × 30 mm was analyzed to determine its roughness; it had a moisture content of about 12% and was arranged on a perfectly horizontal support, with the working head 2 located above it (Figure 4). The working head, which was elastically fixed to an articulation in order to level out all the uneven parts of the piece, was adjusted in two directions, namely, vertically and horizontally. Then, we measured its roughness along a distance of 8 mm. The main parameters of roughness that were measured are Ra (arithmetic mean of irregularities), Rz (average height of irregularities), and Rq (mean square root of irregularities).

Ra is the most widely used roughness parameter, and Ra represents the arithmetic mean of the absolute deviations of the roughness profile related to the average line. For the latter parameter, it is necessary to know the mean line of the roughness profile and the heights and depths of this profile. Based on the values of the profile and the center line, several deviations are obtained, with some of them being located on the upper side of the central line and others under it (Figure 5).

If the deviations under the central line of the profile are considered to be negative, their modulus positive absolute values are used (Equation (7)).

(7)$Ra={\displaystyle \frac{{\sum }_{i=1}^n{a}_{si}}{n}}+{\displaystyle \frac{{\sum }_{j=1}^m\left|a_{ij}\right|}{m}}$

where:

a_si—current superior deviation, in μm;

a_ij—inferior current deviation, in μm,

n—number of superior deviations;

m—number of inferior deviations.

Rq represents the value of the mean square root of Ra. Rq is much more sensitive to the maximum and minimum points on the measured surface and tends to give a higher value than Ra, which is why it is often used in the optical industry. Based on the results of the statistical analysis, the values of Rq were more highly significant than those of Ra_. First, the values of Ra were calculated for every point, and then, the 2-grade radical was extracted from it (Equation (8)).

(8)$Rq=\sqrt{{\displaystyle \frac{1}{n}}{\sum }_{i=1}^n{y}_i^2}$

The average maximum height parameter Rz is based on the maximum height of the profile during the assessment. In order to determine Rz, the roughness profile is determined, and then, five areas of reference are considered, where the largest height and the depth are found for the profile. The differences between them are noted as R_z1, R_z2, R_z3, R_z4, and R_z5 (Figure 5b).

(9)$Rz={\displaystyle \frac{{{\sum }_{i=1}^{5}R}_{zi}}{5}}[\mathsf{\mu }\mathrm{m}]$

The standard EN ISO 4287:1997 [38] indicates that there are 14 classes of roughness, labeled as N0 to N13, where Ra varies from 0.012 to 100 μm (Table 1 and Figure 6).

Wood materials have values that fall in N7–N11 classes, with a range of 1.6–25 μm.

Modulus of rupture (MOR) and modulus of elasticity (MOE) in bending. To determine the MOR and MOE (ISO 13061-1:2014) [38,39,40] of the two categories of samples (untreated and thermally treated ash wood), a universal testing machine with hydraulic motion (type WE 10, Shanghai, China) was used. The machine was equipped with the bending devices, namely, the two supports of the samples and the force punch (Figure 7), both having a diameter of 30 mm and a width large enough (greater than 50 mm) to fit the samples. For the bending test, both the MOR and MOE were determined at static bending. For the determination of the MOR and MOE at static bending, sample sections of 20 × 20 mm × 290 mm (maintaining a distance of 240 mm between the supports) were used for untreated ash wood (control) and thermally treated ash wood in the perpendicular and radial directions of the grain. The formulas for the determination of the MOR and MOE at static bending (Figure 7) are as follows (Equation (10)), according to ISO 13061-1:2014:

(10)$\mathrm{M}\mathrm{O}\mathrm{R}={\displaystyle \frac{3}{2}}·{\displaystyle \frac{P_{max}·l}{b·g^2}}[\mathrm{N}/{\mathrm{m}\mathrm{m}}^2] \mathrm{M}\mathrm{O}\mathrm{E}={\displaystyle \frac{l_1^3·(P_1-P_2)}{4·b·t^3·(a_2-a_1)}} [{\displaystyle \frac{\mathrm{N}}{{\mathrm{m}\mathrm{m}}^2}}]$

where:

MOR—static bending modulus of rupture, in N/mm²;

P_max—maximum rupture force of the sample, in N;

l—distance between supports, in mm; b—width of the samples, in mm;

g—thickness of the sample, in mm;

MOE—static bending modulus of elasticity, in N/mm²;

l₁—distance between supports, in mm;

P₁—10% of the maximum rupture force, in N;

P₂—40% of the maximum rupture force, in N;

b—width of the samples, in mm (usually 50 mm);

t—thickness of the sample, in mm;

a₁—deformation recorded for force F₁, in mm;

a₂—deformation recorded for force P₂, in mm.

Joint strength. We used 3 types of joints (Figure 8), namely, finger joint, corner joint with a dowel for frames (frame joint), and corner joint for boxes (box joint), both for untreated and thermally treated wood. A total of 10 specimens were used for each category of joints for treated and untreated wood. The finger joints (specific to Glulam (glue-laminated timber) or CLT (cross-laminated timber)) were subjected to the tensile test until rupture [41]; the width of the samples was 80 mm, and their thickness was 30 mm. Their resistance (${\sigma }_t$) was determined as the ratio between the rupture force and the area of the rupture section (5 times the lateral surface of a tooth) using the following formula (Equation (11)):

(11)${\sigma }_t={\displaystyle \frac{F_{tmax}}{5\times b_t\times h_t}}[\mathrm{N}/{\mathrm{m}\mathrm{m}}^2]$

where:

F_tmax—maximum tensile rupture force, in N;

b_t—finger width (80 mm);

h_t—finger height (15 mm).

The last two categories of joints (specific to frames and boxes) were introduced in the universal test machine, and their resistance to rupture was tested according to the process shown in Figure 9.

The joint’s resistance was determined as the ratio between the rupture force and the area of the gluing surface using Equation (12). For the frame joints, the resistance was 2 times the lateral surface of the dowel, and for the box joint, it was 3 times the lateral gluing surface of the tooth.

(12)${\tau }_f={\displaystyle \frac{F_{max1}}{2\times b_d\times h_d}};{\tau }_b={\displaystyle \frac{F_{max2}}{3\times b_g\times h_g}}[\mathrm{N}/{\mathrm{m}\mathrm{m}}^2]$

where:

τ_f—joint shear resistance of the frame, in N/mm²;

F_max1—maximum shear force of the corner joint for frames, in N;

b_d—width of the dowel (80 mm);

h_d—height of the dowel (20 mm);

${\tau }_b$—shear resistance of the box joint, in N/mm²;

b_g—gluing surface width (80 mm);

h_g—surface height (15 mm).

Statistical data processing. All the obtained values were analyzed using descriptive statistics. Specifically, we determined the arithmetic mean of the values and the standard deviation, as well as the Pearson correlation coefficient and the limits of variation of the values using a confidence interval of 95%, i.e., with two standard deviations. Also, a probability plot was created for the 18 values of the modulus of rupture using the Minitab 18 statistical program.

3. Results

3.1. Wood Density

As expected, heat treatment led to a decrease in density due to a loss of mass (through the volatilization of substances and microcracks) such that the density of untreated ash wood was 652.4 kg/m³, whereas that of thermally treated wood was 605.8 kg/m³, representing a decrease of 7.1%. The standard deviation (showing the degree of variation between the values) of the density of the treated wood 37.2 kg/m³) was more than 50% higher than that of untreated ash wood (17.9 kg/m³), which means that the density of the thermally treated ash wood is more homogenized. Figure 10 presents the densities of the two categories of ash wood.

A one-way ANOVA was used to analyze the native and treated densities, with a significance level of ἀ = 0.05. The low values of coefficients F and P show that there was no significant difference between the two analyzed densities (Table 2).

3.2. Water Absorption

During the absorption tests, it was observed that the amount of water absorbed depended not only on the immersion period (2 or 24 h) but also on the type of material (untreated or thermally treated). The highest values of absorption observed for the untreated samples were 10.8% and 30.7% when the immersion times were 2 h and 24 h, respectively (Figure 11). The water absorption values for thermally treated ash wood were 37.9% and 32.5% lower than those of untreated ash wood after 2 h and 24 h of immersion. It has been shown that thermally treated wood is less hydrophilic than untreated wood [42,43,44].

3.3. Swelling in the Radial/Tangential Grain Orientation

Swelling related to grain orientation is another main parameter of dimensional stability that was determined. Figure 12 shows that the swelling of the thermally treated ash wood was lower than that of the untreated one, with a 21% decrease after 2 h of immersion and a 46% decrease after 24 h of immersion; this suggests that there was a larger decrease (10.05%) in the first part of the immersion than in the second part (1.91%).

Therefore, we observed a decrease in the hydrophilicity of thermally treated ash wood [42,43] as well as variations in swelling in the tangential/radial grain orientation in the 24 h range, with greater swelling in the first 2 h than the remainder of the time.

3.4. Color of Untreated and Thermally Treated Ash Wood in the CIELab Space

The luminance of the ash wood [43] showed that there was a substantial difference between the untreated ash wood, which was close to the standard white color (with a value of 87.2 points), and the thermally treated one, which was close to the standard black color, with a value of 7.7 points (Figure 13). Moreover, the large difference between the two types of ash wood (79.5 color points) shows that thermal treatment darkens the ash wood substantially, giving it a dark black color.

3.5. Roughness of Untreated and Thermally Treated Surfaces of Ash Wood

We observed that the roughness of the surface of the untreated and thermally treated ash wood was higher in the transverse direction than in the longitudinal one (Figure 14), which is largely due to the direction of the anatomical elements of the wood [45,46]. In this regard, the following differences were determined:

• -. For thermally treated ash wood, Ra increased by 16%, Rq increased by 20.4%, and Rz increased by 13.8%.

• -. For untreated ash wood, Ra increased by 51%, Rq increased by 44%, and Rz increased by 28%.

All the parameters of surface roughness of thermally treated ash wood were higher than that of untreated ash wood; the differences in surface roughness are shown below:

• -. Ra increased by 183% in the longitudinal direction and by 11.7% in the transverse direction.

• -. Rq increased by 195% in the longitudinal direction and by 146% in the transverse direction.

• -. Rz increased by 190% in the longitudinal direction and by 156% in the transverse direction.

Thermal treatment resulted in ash wood being moved from the N7–N8 classes to the N9 class.

3.6. Brinell Hardness

The Brinell hardness of the wood surface differed depending on the type of grain orientation (tangential or radial) and whether the wood surface was thermally treated or not (Table 3 and Figure 15).

The Brinell hardness in the tangential direction was lower than that in the radial direction, with values of 24% for untreated ash wood and 19% for thermally treated ash wood. Also, thermally treated ash wood had 22.29% lower Brinell hardness in the tangential direction and 22.05% lower Brinell hardness in the radial direction than untreated ash wood

3.7. MOR at Static Bending

Here, we studied one of the important mechanical properties of ash wood, namely, the MOR at static bending. For untreated ash wood (i.e., control samples), the MOR was 103.42 N/mm², and for thermally treated ash wood, the MOR was only 99.57 N/mm² (Figure 16). The MOR of thermally treated ash wood was 3.7% lower than that of the control ash wood, suggesting that the bending resistance of thermally treated ash wood was slightly lower than that of untreated wood, which indicates a slight decline in its mechanical properties. This decrease in bending resistance is explained by the slight degradation of the structure of thermally treated wood due to a decrease in the amount of hemicellulose in wood [36,37].

As shown in Figure 16b, the values of the two variables intersect, and the values of the statistical parameters (Anderson–Darling value and p-value) are small, representing a normal distribution of values. Also, the MOR values of treated and untreated ash wood were not significantly different.

3.8. Modulus of Elasticity (MOE) at Static Bending

We also measured the modulus of elasticity (MOE) at static bending, which is another important mechanical property of wood. For untreated ash wood, the MOE was 5792.59 N/mm², and for thermally treated ash wood, the MOE was 6973.33 N/mm² (Figure 17). The MOE of thermally treated ash wood was 20.3% higher than that of the control ash wood. To ensure that all samples were homogenous, the pieces of timber that were used did not have any defects such as knots, clogged bark, or inclined fiber. The slight increase in the MOE indicates that the elasticity of ash wood increased after thermal treatment at a temperature of 185 °C. This suggests that the degradation of the wood’s structure begins at a temperature as low as 185 °C [19,20].

3.9. Resistance of the Three Types of Joints

The resistance values of the three types of joints, determined using Equations (10) and (11), (commonly used in the wood processing industry) were different from each other (Figure 18).

As expected, thermally treated ash wood had lower resistance than untreated ash wood, with values of 50% for the finger joint, 50.8% for the frame joint, and only 19.6% for the box joint. Although box joints generally have low resistance [28,29], the difference in resistance between treated and untreated ash wood for this type of joint was due to the wood degradation during thermal treatment. There were small differences in resistance between the finger joints and the frame joints, even though the finger joints were subjected to traction efforts and the corner joints for the frames were subjected to compression-shear efforts.

4. Discussion

The values obtained in this study are summarized in Table 4 so that they can be compared to those obtained by other researchers.

In this study, we observed differences between the values of untreated and thermally treated ash wood, with a small decrease in some properties, such as density, MOR, and Brinell hardness (Table 4, column 5); these parameters were negatively affected by thermal treatment, whereas other properties, such as thickness, swelling, and water absorption, which also decreased, were positively affected by thermal treatment. Finally, other properties, such as the MOE, increased slowly but it was negatively affected by thermal treatment [46,47,48].

Our results are consistent with the values of other studies. Regarding the moisture content, many authors [3,23,27] found that 12% is the reference value for the analysis of woody species (which was also used in the present research). In [23,27], a density of 750 kg/m³ was reported for ash wood, whereas in [30], it was observed that the density of ash wood was 689 kg/m³ for untreated wood and 665 kg/m³ for thermally treated wood. The latter values are more similar to those of the present research.

Other authors [42] observed a decrease (from 15% to 12.8%) in water absorption, but these values were obtained using other species, such as beech wood, and different methodologies. The differences between these values and those from the present research (20.5%) are due to the differences between the two species, with beech being a deciduous tree with uniformly scattered pores and ash being ring-porous. The authors of [43] highlighted that thermal treatment reduced swelling (from 7.3% to 5.7%) in beech wood (Fagus sylvatica), representing an efficiency of 22%. Other researchers [21,27] studied the swelling of thermally treated wood by other methods with respect to its chemical composition. Some authors [22,26] studied the swelling of beech wood (Fagus orientalis Lipsky) and pine wood that were thermally treated at temperatures of 120, 160, and 210 °C for 2 and 6 h in an inert medium (nitrogen). Following the treatment, swelling in the tangential direction ranged from 13.5% (at 120 °C/2 h) to 12.8% (at 200 °C/6 h) for the treated samples, while the untreated samples had a swelling of 15.1% in the tangential direction. Tangential swelling ranged from 10.76% (at 120 °C/2 h) to 8.46% (at 200 °C/6 h), while the control samples had a tangential swelling of 11.85%.

Regarding the MOE [8,22,25], experimental research on beech wood was conducted in the presence and absence of oxygen, and it was found that the modulus of elasticity decreases significantly by 8−10%. Some authors [26] treated beech wood using the French Bois Perdure and Rectification method at temperatures between 200 °C and 260 °C and obtained a maximum decrease of 20% in the modulus of elasticity and a decrease of 40% in the bending resistance. Regarding the change in color of heat-treated wood, all authors [49,50,51,52] consider that the color darkens a lot. Another study [53] reported wood roughness (Ra) values similar to those obtained in the present research.

5. Conclusions

In this paper, we measured the physical (moisture content, water absorption, and radial/tangential swelling), mechanical (Brinell hardness, MOR, MOE, etc.), and coloristic (luminance) properties of thermally treated and untreated ash wood and obtained values that were different from those reported by other researchers in this field.

The thermal treatment of ash wood reduced its hydrophilicity (water absorption) by 28.9% after a 2 h immersion and by 37.2% after 24 h of immersion. This also shows that absorption is high during the first 2 h and decreases thereafter.

The dimensional stability of thermally treated ash wood, expressed by radial/tangential swelling, decreased by 21.1% after 2 h of immersion and by 46% after 24 h of immersion in water. Therefore, thermally treated ash is 21.1% more dimensionally stable after 2 h of immersion and 46% more dimensionally stable after 24 h of immersion.

The fact that the MOR at static bending of the thermally treated ash wood was only 3.7% lower than that of the untreated ash wood demonstrates that this thermal treatment process does not have a large influence on the mechanical properties of ash wood.

A 20.3% higher elasticity of thermally treated ash wood compared to untreated wood represents a major advantage, broadening its use in various wood products.

The roughness of samples, expressed by Ra, increased by 183% in the longitudinal direction and by 11.7% in the transverse direction.

The Brinell hardness of treated ash wood decreased by 22.29% in the tangential direction and by 22.05% in the radial direction.

Luminance decreased from 87.2 to 7.7, with the color of treated ash wood becoming blackish.

The resistance of the three types of joints for treated ash wood decreased by 50% for finger and frame joints and by 19.6% for box joints.

In conclusion, our results show that the thermal treatment of ash wood not only increases its dimensional stability (which occurs at a temperature of 185 °C) but also decreases its affinity for water and slightly increases its elasticity (by 20%).

Author Contributions

Conceptualization, A.L.; methodology, C.S.; software, A.L.; validation, A.L; formal analysis, A.L.; investigation, A.L.; resources, C.S.; data curation, A.L.; writing—original draft preparation, A.L.; writing—review and editing, A.L; visualization, C.S.; supervision, A.L.; project administration, A.L.; and funding acquisition, C.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The experimental data can be obtained from the authors upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Diagram of the thermal treatment process (for stability) for pieces of ash wood timber.

Figure 2. Color luminance in the CIELab space.

Figure 3. Brinell device: 1—support for attaching the test machine; 2—body of the device; 3—ball-fixing washer; and 4—ball with a diameter of 10 mm.

Figure 4. Mitutoyo SJ-201 roughness meter. The working principle of the instrument is as follows: 1—wooden piece; 2—Diamante tip; 3—horizontal displacement element; 4—vertical displacement columns; 5—top matching wheel; 6—digital viewer; and 7—electrical cable with transformer.

Figure 5. Ra (a) and Rz (b) of surface roughness.

Figure 6. Correlation between roughness classes and values of Ra.

Figure 7. Determination of MOR/MOE at static bending (3P): P—force, in N; l—distance between the supports, in mm; g—thickness of the sample, in mm; and b—width of the sample, in mm.

Figure 8. Joints produced from ash wood: left—untreated; right—thermally treated; upper right—finger joint; upper left—frame joint; and down—box joint.

Figure 9. Schematic of the process for testing corner joints: P—rupture force.

Figure 10. Density of untreated and thermally treated ash wood.

Figure 11. Water absorption in untreated (native) and thermally treated ash wood after 2 h and 24 h.

Figure 12. Swelling (measured as a change in thickness) in untreated and thermally treated ash wood after 2 and 24 h.

Figure 13. Luminance of untreated (native) and thermally treated ash wood.

Figure 14. Surface roughness of untreated ash wood (a) and thermally treated ash wood (b): Ra—arithmetic mean of deviations; Rq—average square mean; and Rz—mean of maximum heights.

Figure 14. Surface roughness of untreated ash wood (a) and thermally treated ash wood (b): Ra—arithmetic mean of deviations; Rq—average square mean; and Rz—mean of maximum heights.

Figure 15. Brinell surface hardness of untreated and thermally treated ash wood.

Figure 16. Modulus of rupture (MOR) at static bending for untreated and thermally treated ash wood (a) and probability plot of the MOR for treated ash (MOR T) and native ash (MOR N) (b).

Figure 17. MOE for untreated and thermally treated ash wood.

Figure 18. Resistance of the 3 types of joints produced from untreated (native) and thermally treated ash wood.

Table 1

Correlation between classes of roughness and Ra.

N0	N1	N2	N3	N4	N5	N6	N7	N8	N9	N10	N11	N12	N13
0.012	0.025	0.05	0.10	0.20	0.40	0.80	1.60	3.20	6.30	12.50	25.00	50	100

Table 2

Analysis of variance (ANOVA) (* value is very small, with less than 3 decimal places).

Source	DF	Adj SS	Adj MS	F-Value	p-Value
Treated	9	13,896	1544	*	*
Error	0	*	*
Total	9	13,896

Table 3

Statistical parameters for Brinell hardness. The ranges of values shown have a confidence interval of 95%.

No.		Mean	Median	Standard Deviation	Coefficient of Variance	Total Interval
Untreated ash	R	19.525	18.190	3.527	0.193	8.946–30.107
	T	15.775	15.775	2.885	0.182	7.121–24.431
Thermally treated ash	R	15.243	15.171	1.372	0.091	11.127–19.359
	T	12.228	12.731	2.105	0.172	5.913–18.543

Table 4

Characteristics of untreated and thermally treated ash wood.

No.	Characteristic	Untreated Ash Wood	Thermally Treated Ash Wood	Difference
1	Density, kg/m3	652	605	7.2%
2	Water absorption, 24 h, %	30.7	20.5	33.2%
3	Thickness swelling, 24 h, %, radial	8.2	4.4	46.3%
4	Thickness swelling, 24 h, %, tangential	6.3	3.4	46%
5	MOR, N/mm2	103	99	3.8%
6	MOE, N/mm2	5790	6970	2%
7	Brinell hardness, N/mm2, radial	19.5	15.2	22.1%
8	Brinell hardness, N/mm2, tangential	15.7	12.2	22.2%
9	CIELab, luminance	87	7	80 points
10	Roughness, Ra, µm, paralel	2.12	6.0	183%
11	Roughness, Ra, µm, perpendicular	3.21	6.9	114%
12	Finger joint strength, N/mm2	3.24	1.72	46.9
13	Frame joint strength, N/mm2	3.4	1.7	41.1
14	Box joint strength, N/mm2	1.22	0.98	19.6%

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