Figure 3 shows the chemical characteristics of the studied woods. The highest holocellulose content was observed for Eucalyptus grandis wood (69.51%), followed by Cedrela fissilis wood (69.02%) and Ochroma pyramidale wood (69.02%). The highest lignin content was observed for Tectona grandis wood (38.11%), followed by Araucaria angustifolia wood (34.90%) and Pinus elliottiii wood (30.27%). High extractives contents were found for Ochroma pyramidale wood (6.35%), Tectona grandis wood (5.25%) and Cedrela fissilis wood (4.90%). These results can be attributed to the wood natures since the hardwoods presented higher holocellulose and extractives contents, while the softwoods presented higher lignin contents, except for the Tectona grandis wood. These results are similar to those reported in the literature, i.e., 60–70% range for holocellulose content, 20.5–38.5% range for lignin content, and 1–10% range for extractives content (Severiano et al. 2010; Esteves et al. 2013; Borrega et al. 2015; Lengowski et al. 2020; Acosta et al. 2021b).
Figure 4 shows cross-sectional images of the studied woods. The Pinus elliottii wood shows some resin canals, which are elongated, tube-shaped intercellular spaces surrounded by epithelial cells.
The optical micrographs also confirmed the large differences in terms of diameter of the main liquid conducting anatomical elements from softwoods (c.a. tracheids) and hardwoods (c.a. vessels), i.e., the latter were 10–20 times wider. Table 1 shows anatomical characteristics acquired from the aforementioned optical micrographs. Among the studied softwoods, Araucaria angustifolia showed the largest tracheid diameter (ØF) and lumen diameter (ØFL). And the Cedrela fissilis wood was the hardwood with the widest vessels (ØV), in a decreasing order, followed by Ochroma pyramidale, Eucalyptus grandis and Tectona grandis. Regarding the radial ray width, the Ochroma pyramidale wood stood out compared to the other woods. These ducts are not found in hardwoods. The presence of vessels in the hardwoods shown here is an important anatomical difference between hardwoods and softwoods. Softwoods have a simpler morphological structure, which can be explained by the biological evolution of these plants, being botanically classified as primitive vegetables.
Table 1
– Anatomical properties of the woods.
Wood specie | ØF (µm) | ØV (µm) | ØLF (µm) | ØLV (µm) | R (µm) | LF (mm) |
Pinus elliottii | 25.88 (5.6) c | - | 22.01 (2.80) d | - | 21.67 (5.93) c | 2.39 (0.51) d |
Araucaria angustifolia | 46.86 (4.6) d | - | 41.57 (8.01) e | - | 18.46 (4.03) b | 2.67 (0.69) e |
Cedrela fissilis | 7.77 (1.5) b | 482.78 (13.1) c | 6.01 (0.30) c | 446.13 (53.38) c | 12.89 (4.27) a | 0.79 (0.27) ab |
Ochroma pyramidale | 4.88 (0.59) a | 372.30 (40.3) b | 4.12 (0.80) b | 367.83 (52.38) b | 95.76 (12.12) e | 1.29 (0.23) c |
Tectona grandis | 3.20 (0.85) a | 214.24 (10.18) a | 2.68 (0.48) a | 200.73 (61.57) a | 89.04 (17.15) d | 0.59 (0.08) a |
Eucalyptus grandis | 4.70 (1.15) a | 223.72 (51.6) a | 4.12 (0.80) b | 201.88 (43.31) a | 21.69 (5.93) c | 0.94 (0.23) b |
where: ØF is fiber diameter, ØV is vessel diameter, ØLF is fiber lumen diameter, ØLV is vessel lumen diameter, R is ray width, LF is fiber length (note: different letters represent significant differences in the column).
Table 2 shows the apparent density, basic density, moisture content and porosity results for the studied woods. The Tectona grandis wood showed the highest apparent and basic densities, justified by the small diameter of its anatomical elements, especially vessels and fiber lumens, and the consequent high lignin content since it is mostly likely located in the middle lamella between fibers. The Ochroma pyramidale wood presented the smallest density, which is attributed to its wide fibers and vessels, as well as its high porosity. The values found in this study are in agreement with those reported in the literature for all woods, 0.15–0.60 g/cm3 for apparent density and 0.11–0.55 g/cm3 for basic density (Valério Alvaro, Watzlawick et al. 2008; Zanella et al. 2016; Mahdian et al. 2020; Acosta et al. 2021a).
Table 2
– Physical properties of the woods.
Wood specie | ρa (g/cm³) | ρb (g/cm³) | MC (%) | Ø (%) |
Pinus elliottii | 0.59 (0.050) cd | 0.49 (0.041) d | 9.97 (1.99) ab | 46.55 (2.25) a |
Araucaria angustifolia | 0.54 (0.015) c | 0.47 (0.008) cd | 10.67 (1.16) ab | 49.85 (1.98) b |
Cedrela fissilis | 0.48 (0.083) b | 0.44 (0.073) c | 11.16 (0.30) ab | 58.31 (1.81) c |
Ochroma pyramidale | 0.18 (0.022) a | 0.15 (0.017) a | 9.18 (2.41) a | 73.88 (1.49) d |
Tectona grandis | 0.61 (0.027) d | 0.54 (0.062) e | 11.35 (0.90) b | 44.96 (1.59) a |
Eucalyptus grandis | 0.45 (0.017) b | 0.39 (0.018) b | 10.62 (0.53) ab | 56.47 (2.11) c |
where: ρa is apparent density, ρb is basic density, MC is moisture content, £ is porosity (note: different letters above the bars represent significant differences).
Among the studied woods, Tectona grandis and Eucalyptus grandis woods presented the largest water contact angles (Fig. 5a). These two woods were followed by Cedrela fissilis wood, Araucaria angustifolia wood, Pinus elliottii wood, and Ochroma pyramidale wood in a decreasing order of water contact angle. These contact angles can be ascribed to the fiber length of the woods since the smaller was the fiber length, the higher was the contact angle. On the other hand, all woods presented more similar contact angles results when soybean oil or furfuryl alcohol were used (Figs. 5b-5c), with a slightly higher value again for Tectona grandis and Eucalyptus grandis with soybean oil. In all cases, the contact angles for all liquids decreased up to 10–15 s, mostly stabilizing after that. Previous studies reported significant correlations of contact angle values and chemical (Kishino and Nakano 2004; Rossi et al. 2012), anatomical (Piao et al. 2010) (Oberhofnerová and Pánek 2016), and other physical properties (Amorim et al. 2013), which did not occur in the present study.
Figure 6 shows the height of the flow front in the capillary experiments for the studied woods. The values progressively increased until about 72 h, with little change after that until 160 h. These same three woods stood out in terms of both the soybean oil and furfuryl alcohol capillary pressures. In the case of the softwoods, these results can be ascribed to their high tracheid length and tracheid diameter. (Ahmed and Chun 2011) stated that the capillary pressure of a softwood is inversely proportional to its tracheid lumen diameter, which corroborates the findings of the current work.
On the other hand, the Ochroma pyramidale wood presented high capillary pressure probably due to its high porosity, which indicates that this physical property is a better permeability indicator than vessel diameter or length. In hardwoods, the porosity is dependent on both anatomical properties and presence of some anatomical elements, especially vessels, which explains the obtained result.
For water as the infiltrating fluid, the Pinus elliottii wood presented the highest value (Table 3), followed by Ochroma pyramidale wood and Araucaria angustifolia wood. For soybean oil as the infiltrating fluid, the Ochroma pyramidale wood presented the highest value, followed by Pinus elliottii wood and Araucaria angustifolia wood. For furfuryl alcohol as the infiltrating fluid, the Pinus elliottii wood presented the highest value, followed by Ochroma pyramidale wood and Araucaria angustifolia wood.
Table 3
– Capillary pressure of woods.
| Water | Soybean oil | FA |
Wood specie | Sample I | Sample II | Sample I | Sample II | Sample I | Sample II |
Pinus elliottii | 1716.75 | 1618.65 | 1499.26 | 1583.04 | 1829.07 | 1839.09 |
Araucaria angustifolia | 1177.20 | 1137.96 | 1146.49 | 1631.55 | 2061.86 | 1696.05 |
Cedrela fissilis | 588.60 | 539.55 | 749.63 | 811.36 | 1030.93 | 1053.10 |
Ochroma pyramidale | 1422.45 | 1451.88 | 1631.55 | 1675.64 | 1884.50 | 1828.07 |
Tectona grandis | 735.75 | 686.70 | 617.34 | 705.53 | 587.52 | 609.69 |
Eucalyptus grandis | 441.45 | 490.50 | 811.36 | 793.72 | 1053.10 | 1030.93 |
The capillary pressure results did not show a direct correlation with the previous contact angles results, which suggests that it is probably affected by bulk anatomical and chemical characteristics and not only surface ones.
Due to their differences (anatomical and chemical), only three wood species were selected for permeability studies. The Ochroma pyramidale wood was chosen due to its large vessel diameter and high porosity, and Eucalyptus grandis due to its large availability in Brazil. Among the softwoods, Pinus elliottii was selected since Araucaria angustifolia is a native wood in Brazil and there are restrictive laws regarding its exploitation.
Figure 7 shows the flow front position (Xff) and Fig. 8 shows the squared flow front position squared (Xff2) as a function of the injection time data for the selected woods. The Ochroma pyramidale wood showed the fastest flow front for both liquids, which can be partly explained by its significantly higher porosity compared to the other woods. Although the infiltration is affected by capillary pressure, this is a comparatively small driving force in the permeability experiment and indeed it did not show a direct correlation.
Compared to furfuryl alcohol, the soybean oil was approximately 5 times slower to permeate the wood, which is due to its higher viscosity (Zhang and Cai 2008). The lower chemical affinity with the wood, since these two compounds have a high degree of polarities, may also be partly responsible. Besides, the pattern of the flow front evolution for the soybean oil seems to be clearer than that of the furfuryl alcohol. In that context, (Trindade et al. 2019) affirmed that, when the fluid flows more slowly, the determination of permeability is more accurate, depending on the balance between injection pressure and capillary pressure.
Figure 9 shows the calculated mean permeability values. Compared to the literature, the used methodology showed a smaller variability (maximum CV of 22.25%), which suggests that this method is more precise than those found in the literature (Table 4). The results obtained for the water were similar to those related to the other fluids, although they are not comparable since were measured at different pressure levels. This also indicates that other infiltrating fluids could be applied for measuring wood permeability by vacuum infusion.
Table 4
– Values of wood permeability reported in the literature.
Wood specie | Permeability range (× 10− 11 m2) | CV (%) | Infiltrating fluid | Reference |
Pinus radiata | 0.36–0.60 | 48–50 | Water | (Booker 1990) |
Eucalyptus grandis | 1.2 | 55–57 | Water | (Rezende et al. 2018) |
Populus nigra | 11–12 | 32 | Water | (Emaminasab et al. 2015) |
Pinus sp. | 6–13 | 50 | Water | (Leggate et al. 2021) |
E. grandis e E. citriodora | 5–6 | 25–28 | Water and preservative | (Rogério et al. 2010) |
The reported CV values ranged from 25–55%, depending on the selected wood specie, infiltrating fluid and measurement apparatus. The high variation in wood permeability has already been reported in the literature and could be attributed to the inhomogeneous wood morphology, including diameter, distribution and grouping of porous anatomical elements. Nevertheless, it may be seen that the values reported in the literature are in the same range of those found in the present study, considering the similarity between the viscosities of FA and water.