*2.1. Specimen Procurement*

Five discs from different beech trees (*Fagus orientalis* Lipsky) were cut at breast-height and air dried. From each disk, 80 longitudinal cylindrical specimens were prepared. The diameter of specimens was 17 mm, and the length was 30 mm. Specimens were checked not to have any knots, fissures, or cracks. They were first air dried for eight months, and then they were kept for four weeks in a conditioning room (25 ± 2 ◦C, and 40% ± 3% relative humidity) to avoid any undesirable effects of kiln-drying [23]. Specimens were divided into two equal groups of control (C) and nanosilver-impregnated (NS) groups. Each group was again divided into four subgroups of unheated, heat-treated at 145 ◦C (HT-145), heat-treated at 165 ◦C (HT-165), and heat-treated at 185 ◦C (HT-185). Gas permeability of all specimens was measured in the first phase of the research project before any heat treatment and impregnation. The NS specimens were then impregnated with a 400 ppm aqueous nanosilver suspension. All specimens were again kept in a conditioning room for two more months. Gas permeability was again measured. They were painted with three unpigmented resins of sealer-clear, polyester, and lacquer with organic solvent, produced by Pars-Eshen Co., as to their grea<sup>t</sup> popularity in the local market. A dolly was stuck to one end of the specimens for the paint-adhesion testing. Moisture content of specimens was 8% ± 0.5% in all treatments when permeability and pull-off tests were carried out because wood has a thermo-hygromechanical behavior and the properties relating to its deformation depend on the same factors, including moisture content, temperature, and relative humidity [24].

#### *2.2. Pull-O*ff *Adhesion Strength Testing*

Adhesion strength testing provides the force needed to pull a test diameter of coating away from the substrate. Adhesion tests were carried out in accordance with ASTM D4541-02 [25]. In the present study, an automatic PosiTest® pull-off adhesion tester (DelFesko, NY, USA) was used (Figure 1). This was a self-aligning spherical dolly-head tester (Type V according to the ASTM standard). The diameter of the dolly used was 20 mm. The greatest tensile pull-off force for which the coating could adhere to the substrate was evaluated in terms of mega Pascal. Breaking points occurred along the weakest plane of the whole structure. The adhesion strength (*X*) was calculated in terms of MPa (Equation (1)).

$$X = \frac{4F}{\pi d^2} \tag{1}$$

where *F* is the rupture force (Newton), and *d* is the diameter of the experiment cylinder (mm) (ASTM D4541-02).

**Figure 1.** PosiTest® AT-A automatic pull-off adhesion tester.

The moisture content of the specimens was 8% ± 0.5% when the pull-off adhesion tests were carried out, and the temperature was 25 ± 3 ◦C. In order to have an estimate of the pull-off strength of the substrate material for comparison purposes, a set of specimens was also prepared without paints.

#### *2.3. Gas Permeability Measurement*

Many methods and apparatuses have been used and invented to measure permeability in solid woods and wood-composite materials as porous materials [1,26–29]. In the present study, longitudinal gas permeability was measured with an apparatus equipped with a 7-level electronic device; time measurement was carried out with millisecond precision [26,30,31]. Falling water was applied to measure and calculate the specific longitudinal gas permeability values. For each single specimen, gas permeability values were separately measured at seven different vacuum pressures. In every single run, the seven time measurements were carried out to finally calculate seven specific permeability values for each specimen. The glass tube had an internal diameter of 13 mm. Water level was put to more than 15 cm above the first time measurement section (Gas 1). A fully airtight connection was made between the specimen and holder. The pressure difference (Δ*P*) was monitored by a pressure gauge connected to the permeability apparatus. The pressure difference could be read at any particular time and height. This provided monitoring of the viscose flow [22]; the gauge had a precision of millibar. Vacuum pressures at the starting and stopping levels were also measured [30].

Each specimen was tested three times to finally calculate the permeability. Then, the superficial permeability coefficient was calculated (Equations (2) and (3)) [32,33]. The superficial gas permeability coefficients (*k*g) were corrected by the viscosity of air (μ = 1.81 × 10−<sup>5</sup> Pa s) for the calculation of the specific gas permeability (*K*g = *<sup>k</sup>*gμ).

$$K\_{\rm g} = \frac{V\_{\rm d} \mathbb{C}L(P\_{\rm atm} - 0.074\overline{z})}{tA(0.074\overline{z})(P\_{\rm atm} - 0.037\overline{z})} \times \frac{0.760 \text{mHg}}{1.013 \times 10^6 \text{Pa}}\tag{2}$$

$$C = 1 + \frac{V\_{\rm r}(0.074 \Delta z)}{V\_{\rm d}(P\_{\rm atm} - 0.074 \overline{z})} \tag{3}$$

where:

*k*g = superficial gas permeability (m<sup>3</sup> m<sup>−</sup>1);

*V*d = π*r2*Δ*z* [*r* = radius of measuring tube (m)] (m3);

*C* = correction factor for gas expansion as a result of change in static head and viscosity of water;

*L* = length of wood specimen (m);

*P*atm = atmospheric pressure (mHg);

*z* = average height of water over surface of reservoir during period of measurement (m); *t*= time(s);

*A* = cross-sectional area of wood specimen (m2);

Δ*z* = change in height of water during time *t* (m);

*V*r = total volume of apparatus above point 1 (the volume of hoses was included) (m3).

## *2.4. Nanosilver Impregnation*

A 400 ppm aqueous nanosilver suspension was produced via the electrochemical technique in cooperation with Mehrabadi Mfg. Co. (Tehran, Iran). The nanoparticles ranged 30–80 nm in size; the pH was 6–7. Anionic and cationic surfactants were used to stabilize nanoparticles in the suspension. Specimens were impregnated using an empty-cell process in a pressure vessel. The pressure was set at 2.5 bars for 25 min. Before conducting the tests on the specimens, they were kept for three months at room temperature.

The nanosuspension was prepared by transferring the silver metal ion from the aqueous phase to the organic phase, where it reacted with a monomer. The formation and size of the nanosilver was carefully monitored by transmission electron microscopy (TEM). Samples for TEM were prepared by drop-coating the Ag nanoparticle suspensions on to carbon-coated copper grids. Micrographs were obtained using an EM-900 ZEISS transmission electron microscope (Carl Zeiss AG, Jena, Germany). Two kinds of surfactants (anionic and cationic) were used in the suspension as stabilizer. The concentration of the surfactants was two times the nanosilver [34].

#### *2.5. Heat Treatment Process*

Specimens to be heat-treated were randomly placed in an oven. Heat treatment was carried out at atmospheric pressure and with normal air. The starting moisture content of specimens was 8% ± 0.5%. Heat treatment of each thermal modification temperature level was carried out in a single run. All HT and NS-HT specimens were heat-treated at 145 ◦C for 12 h. Heat treatment for the HT-145 and NS-HT-145 specimens was discontinued. Then, HT-165 and NS-HT-165, as well as HT-185 and NS-HT-185 specimens continued to be heated at 165 and 185 ◦C for four more hours, respectively. During the heat treatment process, no specimen was in touch with the metal parts of the oven, to prevent extra overheating at one spot. Once the heat treatment process was completed, the silicone adhesive around all specimens was checked to make sure there was no failure in them. The gas permeability was then measured.
