1. Introduction
The heat and mass transfer properties of wood or wood-based materials are essential characteristics required for a variety of purposes, including the heat and mass transfer modeling during the densification process and characterization of densified wood as a building material. Among others, typical properties required are density, permeability, and thermal conductivity. A proper characterization of these properties is crucial for increasing the accuracy of model predictions. However, during the thermo-hygro-mechanical (THM) densification process, density, permeability, and thermal conductivity of wood are all time-dependent, which makes the characterization of these parameters difficult.
Bulk flow is the principal mechanism for the transport of fluids through wood, which occurs through the voids of the wood under a static or capillary pressure gradient [
1]. The bulk flow rate of fluids is determined by wood permeability. During the hot-pressing process of wood composite materials, the gas permeability controls the convective heat transfer from surface layers to the core layer and impacts the movement of the vapor from the core to the edges [
2]. Since gas permeability depends largely on the pore structure of the fiber or particle mat, the densification treatment should have a direct effect on the permeability. Comstock [
3] reported that the arrangement of wood principal directions has more impact on its gas permeability than any other parameter. In some species, the longitudinal permeability could be 10
6 higher than in the transverse direction, due to the arrangement of the wood cells.
A few researchers investigated the gas permeability of wood based panels using experimental methods [
4,
5,
6,
7,
8]. Almost all the methods described in the literature are based on measuring the amount of gas flow at a given pressure gradient applied across the sample. Defo et al. [
9] measured radial and tangential gas permeability values of sugar maple wood at 12% of moisture content (MC) (oven-dry density varied from 587 to 676 kg/m
3) between 2.04 × 10
−17 and 2.84 × 10
−17 . Von Haas et al. [
6] reported that the MC had almost no effect on gas permeability for low density samples and a slight effect on samples with a density above 900 kg/m
3. Moreover, air permeability was at least two orders of magnitude higher than steam permeability, which might be due to the swelling of wood and the viscosity of the fluid on the superficial permeability [
4,
10].
Thermal conductivity is an important material property in determining the heat transfer rate [
11]. The thermal conductivity of wood is affected by several basic factors: density, temperature, MC, extractive content, grain direction, structural irregularities, such as checks, and knots and microfibril angle, among which density and MC are predominant [
12]. Troppová et al. [
13] found that higher temperatures resulted in larger differences between the thermal conductivity values of wood-based fiberboards in the oven dry condition and at 14.2% MC. Thermal conductivity in the radial direction was reported to be about 5% to 10% higher than in the tangential direction [
14]. Thermal conductivity along the grain has been reported to be 1.5 to 2.8 times higher than across the grain but the reported values vary widely. For example, Maclean [
15] found that the thermal conductivity along the longitudinal direction is about 2.25 to 2.75 times higher than transverse thermal conductivity with an average of approximately 2.5.
The steady-state method is normally applied to measure the thermal conductivity of wood [
16]. A large number of empirical equations could be found in the literature, to describe the relationship between wood thermal conductivity, density, and MC [
1,
10,
15,
17]. However, these empirical equations are applicable only within a limited range of MC and density levels. Hence, these relations may not be appropriate to describe the variation of the thermal conductivity of wood undergoing THM densification because both MC and density vary continuously during the process. In addition, little empirical data were found for the thermal conductivity of sugar maple wood at different density levels. Therefore, the objective of the current study was to investigate the variation of density, gas permeability, and thermal conductivity of sugar maple wood during the THM densification process.
2. Materials and Methods
Sugar maple (
Acer saccharum Marsh) wood was selected for this study. This species is a diffuse-porous hardwood normally used for in-door applications such as flooring and furniture [
18]. Thin sawn strips of sugar maple wood were provided by a hardwood flooring plant (Lauzon, Distinctive Hardwood Flooring Inc., Papineauville, QC, Canada). Their average apparent density (20 °C and 65% relative humidity (RH)) was 734 (standard deviation: 8.4) kg/m
3 and their dimensions were 5.7 mm (radial) × 84.0 mm (tangential) × 695.0 mm (longitudinal). When they were received, the strips were stored in a conditioning room at 20 °C and 65% RH until an equilibrium moisture content of approximately 12% was achieved. Ten groups of 8 strips were densified for 0 min (control sample), 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, and 45 min, respectively. The strips were used for experimental determination of the evolution with time of density, gas permeability, and thermal conductivity during the THM densification process.
2.1. Thermo-Hygro-Mechanical Densification Process
A steam injection press (Dieffenbacher, Alpharetta, GA, USA) with dimensions of 862 mm × 862 mm was used for the densification treatment (
Figure 1). Steam injection holes with a diameter of 1.5 mm were distributed uniformly at 32 mm intervals on both the top and bottom platens of the press. To reduce wood surface carbonization and distribute the steam uniformly, both surfaces of the specimens were covered with a thin heat-resistant fabric permeable to steam made of Nomex
® Ш A manufactured by Dupont™ [
19]. The two press platens were preheated to the target temperature (200 °C) before treatment. The upper platen reached the specimens within 86 s during press closing.
For all of the treatments except for the control group, the densification process was pre-set in the computer control system. Steam was continuously injected during the whole process at a maximum manometer pressure of 550 kPa, while the specimens were pressed under an increasing mechanical manometer platen pressure up to 6 MPa [
20]. The evolution of steam pressure and platen pressure during the whole process is presented in
Figure 2 [
20]. The whole densification process was divided into ten steps according to the treatment time (0 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, and 45 min). The density, gas permeability, and thermal conductivity of all samples were determined for each treatment time in order to track their variation during the THM densification process. For the samples densified for 5 min, the control system stopped the process 5 min after the two hot platens closed, even though the maximum platen pressure had not been reached [
20]. For the other treatments, the control system stopped the process after 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min respectively. In these cases, both the maximum steam pressure and platen pressure were reached. All the treated samples were then stored in a conditioning room at 20 °C and 65% RH until their equilibrium moisture content was reached prior to their properties determination.
2.2. Properties Determination—Oven-Dry Average Density
Three specimens for each densification time with dimensions 50 mm × 50 mm were oven-dried and used to measure the density using an X-ray densitometer (Quintek Measurements Systems model QDP-01X, Knoxville, TN, USA) at intervals of 0.02 mm through the thickness. The average value (n = 3) was used as the final oven-dry density of each group.
2.3. Permeability Measurement
Three discs of 50 mm in diameter for each densification time were prepared for the gas permeability measurement. A special device developed in our laboratory by Lihra et al. [
7] was used to measure the transverse gas permeability of the wood samples. The gas permeability was measured in this study with air using the apparatus shown in
Figure 3. A cylinder of compressed air equipped with a pressure regulator was used to regulate the flow of gas at the desired pressure. In addition, a silicon seal was used on the edge of each disc in order to make a tight seal with a rubber sleeve surrounding it. A pressure of 600 kPa was applied to the rubber sleeve to prevent air leaks through the specimen edge. Two basswood discs (
Tilia americana) with high longitudinal gas permeability were placed both in the inlet and outlet sides of the specimen to distribute the air flow [
8]. Five flowmeters (
Figure 3) with increasing range were installed to measure the gas flow rate through the samples. For each measurement, the flowmeter with a larger range (flowmeter 5) was firstly used. If there was no value provided, it was closed and the next one was used. This procedure was repeated until the gas flow rate could be measured. Each disc (n = 3 for each group) was measured at four pressure levels (∆P—values measured from pressure gage B): 200 kPa, 250 kPa, 300 kPa, and 350 kPa, respectively.
The steady-state gas flow through wood can be characterized by Darcy’s law. It could be stated as:
where
is apparent gas permeability with slip flow (
), Q
g is the volumetric gas flow rate (m
3/s), L is the length in the flow directioncorresponding to the thickness of the sample (m), A
s is gas flow area (m
2), ∆P is the pressure differential between the inlet side and outlet side (Pa) (∆P = P1 − P2), P1 is the inlet air pressure (Pa), P2 is the outlet air pressure (Pa), P is the pressure at which Q
g was measured (Pa),
is the arithmetic average pressure (Pa),
.
The apparent gas permeability
from Equation (1) includes Knudsen diffusion, also called slip flow. When a gas flows through a capillary whose diameter is in the same order of magnitude as the average free path between the gas molecules, slip flow becomes significant and must be considered in the permeability measurement. The gas permeability corrected for slip flow could be obtained from the Klinkenberg equation [
21]:
where
is the apparent gas permeability corrected for slip flow (
Pa), s is the slip flow factor, λ is the average free path between gas molecules (m), r is the diameter of the capillary, R is the universal gas constant (8.31 J/mol/K), T is the absolute temperature (K),
is the molecular weight of air (kg/mol). K is the intrinsic gas permeability (
), µ is viscosity of fluid (Pa∙s) (for air at room temperature µ = 1.845 × 10
−5 Pa∙s). k
p represents the “true” gas permeability corrected for slip flow and can be determined graphically from the intercept of a plot of
against the reciprocal average pressure (1/
[
7,
21].
2.4. Thermal Conductivity Measurement
Four specimens for each densification time with dimensions 152.4 mm × 170.0 mm were prepared for thermal conductivity measurement using the apparatus LaserComp Fox 314 (TA instruments, New Castle, DE, USA) shown in
Figure 4a. The Fox 314 instrument was designed according to the ASTM C518-04 standard test method for steady-state thermal transmission properties by means of the heat flow meter apparatus. The specimen was placed between two heating plates (
Figure 4b) with different temperature for a sufficient length of time to obtain a uniform temperature gradient throughout the sample. The temperature of the upper heating plate was set at 10 °C and that of the base heating plate was set at 35 °C. The temperature equilibrium of the system is considered to be reached when the temperatures of the two plates are stable within ±0.2 °C after the set point has been reached. During the test, the auto thickness mode was selected, and the sample’s thickness was determined automatically by the instrument’s digital thickness measurement system.
The thermal conductivity is significantly affected by MC. To investigate the influence of MC on the thermal conductivity, each specimen (n = 4 for each group) was measured at three moisture content levels (0%, 6%, and 12%), respectively. Finally, the thermal conductivity was described as a function of densification time and MC.
2.5. Statistical Analysis
An analysis of variance (ANOVA) was performed to investigate the effect of densification time on the oven-dry density and gas permeability, and the effects of the densification time and MC on the thermal conductivity of densified sugar maple wood using SAS 9.4 (SAS Institute Inc., Cary, NC, USA) at significance level α = 0.05. Duncan’s test was conducted for multiple comparisons between average values obtained under different treatments.