Study on the Longitudinal Permeability of Oil Palm Wood - American

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Study on the Longitudinal Permeability of Oil Palm Wood Adrian C. Y. Choo,† Tahir M. Paridah,*,† Alinaghi Karimi,†,‡ Edi S. Bakar,§ Khalina Abdan,∥ Azmi Ibrahim,⊥ and Fatomer A. B. Balkis† †

Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia ‡ Department of Wood Science & Technology Faculty of Natural Resources, University of Tehran, Tehran, Iran § Faculty of Forestry, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia ∥ Aerospace Malaysia Innovation Centre, Level 7, Tower Block, Faculty of Engineering, Universiti Putra Malaysia, 43400, Serdang, Selangor, Malaysia ⊥ Division of Structural and Computing, Faculty of Civil Engineering, Universiti Teknologi MARA, 41450 Shah Alam, Selangor, Malaysia ABSTRACT: In this research, variations in longitudinal permeability of oil palm (Elaeis guineensis Jacq) wood were investigated. Panels were prepared from bark to pith with the study carried out on 3 parts of the transverse surface: outer, middle, and inner. Microscopic observations were done to determine the anatomical properties to establish its theoretical permeability using Poiseuille’s equation. Results showed that the middle part of the transverse surface of oil palm wood had the highest theoretical, water, and gas permeability values in the longitudinal direction followed by the inner and outer parts. A decrease in the length of samples resulted in an increase in the permeability of the samples. For all parts, theoretical permeability values were the highest followed by water and gas permeability Lower gas permeability values in comparison to water permeability indicates that oil palm wood is prone to drying defects and is more difficult to treat with chemicals after drying.

1. INTRODUCTION The oil palm has been cultivated for palm oil production in Malaysia since 1917 and is one of the most important economic crops in Malaysia. Its economic contribution to the country is significant with recorded export earnings of RM59.8 billion in 2010.1 The total planting area of the oil palm has been increasing steadily year by year with 4.85 million hectares of land being used for oil palm in 2010 which is an increase of 3.4% from the previous year. It is estimated that nine million oil palm trees are harvested yearly for replanting programs as the trees reach between 20 and 30 years of age.2 This would result in approximately 14.4 million cubic meters of oil palm woody material produced each year which can be utilized as raw material in the wood panel industry. It is a potential raw material for plywood3 and compressed lumber4 production in Malaysia. Loh et al.5 and Choo et al.6 found that oil palm wood is very hygroscopic and has a large variation in moisture content (MC) and density in the transverse surface and anisotropic in nature from the bark to the core in the radial direction. These characteristics are well-known as major disadvantages in lumber and veneer drying, modification and treatments such as preservation with chemicals and also the removal of lignin in the pulp and paper industry. It is known that permeability is a key factor that predicts the anisotropic behavior of wood in drying and preservation. According to Dinwoodie,7 permeability is simply the quantitative expression of the bulk flow of fluids through a porous material. Siau8 remarked that one of the principal benefits of research in the field of wood permeability measurements would be the ability to use flow measurements to describe anatomical structure by the determination of the © XXXX American Chemical Society

sizes of pit openings, tracheids, and vessels and the number of openings by which the various cell types and interconnected. Both Siau8 and Dinwoodie7 concurred that moisture content is an important factor that affects permeability in wood. In some species, pit aspiration occurs after drying and depending on the degree of aspiration, the permeability of some species can reach zero. Various researchers have conducted studies on the effect of length on the permeability of wood and monocots, and the general consensus is that permeability decreases with increasing length (Zaidon;9 Perng;10 Bramhall;11 Siau;12 Karimi;13 Perrè and Karimi;14 and Taghiyari et al.15). The variation in density and moisture content in different parts of the transverse surface of oil palm wood (Choo et al.6 and Bakar et al.16) suggests that permeability will be affected by both factors. Currently there is little or no information on the permeability of oil palm wood, and this research was done to obtain objective information on this property of the wood. Therefore the objective of this study was to determine the water, gas, and theoretical permeability of oil palm wood.

2. MATERIALS AND METHODS 2.1. Specimens. The oil palm trees were obtained from University Putra Malaysia’s oil palm plantation site. The average age of the trees was 30 years old which is within the range of the replanting age of commercial oil palm trees. The average height and diameter at breast height of the trees were Received: March 22, 2013 Revised: May 31, 2013 Accepted: May 31, 2013

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dx.doi.org/10.1021/ie4009259 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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sectional area of specimen perpendicular to flow direction (m2), and ΔP = pressure differential (dyne/(m2)). 2.4. Gas Permeability Measurement. Various techniques have been developed to measure permeability in solid woods and wood-composite materials (Shi;17 Pokki et al.;18 Pokki et al.;19 Dermoe et al.20). In the present study, the longitudinal gas permeability measurements were taken using an apparatus (Figure 1) designed and built by Taghiyari et al.21 The

14 m and 40 cm, respectively. Oil palm discs were cut and stored in sealed plastic bags and kept in a cold room to preserve its natural state until laboratory tests were conducted. For the water permeability tests, green samples 20 cm lengths were taken from 3 parts of the transverse surface of the oil palm discs, namely the outer, middle, and inner parts. The permeability of the samples were tested, and the samples were progressively cut to 15 cm, 10 cm, and 5 cm, and the permeability was tested again at each length to determine the effect of length. For gas permeability samples, oil palm lumber was carefully dried in a kiln to about 15% MC. Specimens from the outer, middle, and inner parts of the transverse surface were taken from the discs for the gas permeability study. All specimens were cut into cylinders with a diameter of 19 mm using a computer numerical control (CNC) machine. The samples tested were 5 cm in length and were then tested again when they were cut to 3.2 and 1.6 cm lengths, respectively, to determine the effect of length and parts. The surface areas of the transverse surface of the specimens were sliced with a scalpel to ensure that all the vessels were opened. The entire surface except the transverse surface of the samples was coated with liquid silicone and left to dry for a minimum of 16 h. The purpose of this was to ensure that they were airtight so that the flow occurs only in the longitudinal direction. 2.2. Density, Moisture Contect (MC), and Porosity Measurement. The density and moisture content of the samples used in this study was measured using the oven-dry method. 2 × 2 × 2 cm samples were cut across the center (bark to pith to bark) of the oil palm discs, and the weight of the samples were measured using a Fisher Scientific Series 320FSMB Type B-220C digital balance with a precision to 0.0001 g. The dimensions of the samples were measured using a veneer caliper. The porosity of the specimens was calculated using the following equation (eq 1) Porosity (%) = (1 − 0.667ρo)*100

(1)

where ρo = density (g/cm ) of the specimens in an oven-dry condition*. 2.3. Water Permeability Measurement. The longitudinal water permeability measurements were taken using an apparatus designed and built by Perrè and Karimi.14 The apparatus is a combination of various parts which include an air compressor that provides a stated maximum of 10 kg/cm2 of air pressure to the system, a series of tubes that is interconnected with valves that stops or release air or water to the different parts of the apparatus, an air pressure regulator that regulates the pressure of the air coming into the system from the air compressor, an electronic balance to weigh the water exiting the specimens, and a computer with a program installed for recording the data from the balance in a fixed interval (3 s). Distilled water dyed with Safranin-O was used to stain the pathways in the specimens and filtered using a microfilter of 0.2 μm to prevent particulate matter from clogging minute openings in the water pathways of the samples. Water permeability was measured using Darcy’s equation (eq 2) for steady state flow 3

K=

QLη AΔP

Figure 1. Overview of the gas permeability apparatus (USPTO No. US 8,079,249, B2; Pub. No. 2010/0281951 A1) equipped with singlestorey millisecond precision electronic time measurement device (Taghiyari et al.;22 Taghiyari23) (approved by The Iranian Research Organization for Scientific and Technology under certificate No. 47022).

apparatus has a floating marker to mark the movement of water, two motion sensors, and an electronic time measurement device to measure the time taken for the marker to pass the motion sensors (1 ms precision). Measurements were done using falling water displacement volume method described by Siau.24 Four measurements were taken from each specimen. The superficial permeability coefficient was then calculated using Siau’s equations24 (eqs 3 and 4). To calculate the specific permeability (K = kgη), the superficial permeability coefficients were multiplied by the viscosity of air (η = 1.81 × 10−5 Pa s)

(2) 2

where K = specific water permeability (m ), Q = volumetric flow rate (m3/s), L = length of specimen in the flow direction (m), η = viscosity of fluid ((dyne * s)/(m2)), A = crossB


Industrial & Engineering Chemistry Research kg =

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0.076 m Hg VdCL(Patm − 0.074z) × tA(0.074z)(Patm − 0.037z) 1.013 × 106 Pa

C=1+

Vr(0.074Δz) Vd(Patm − 0.074z)

(3)

(4) 3

where kg = longitudinal specific gas permeability (m /m), Vd = πr2Δz [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), Patm = atmospheric pressure (m Hg), 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), Δz = change in height of water during time t (m), and Vr = total volume of apparatus above point 1 (including volume of hoses) (m3). 2.5. Theoretical Permeability Measurement. Samples from the water permeability tests were observed anatomically. The water entrance and exit areas at each length in the transverse surface of the samples were observed under an image analyzer to determine the size and number of vessels used to transport the dyed water. Poiseuille’s equation (eq 5) of viscous flow12 was used to determine the theoretical or maximum permeability of the specimens kL =

nπR4 × 1.013 × 106 8η

Figure 3. MC of oil palm wood across the transverse section of the stem.

the MC increasing as the measurements were taken progressively toward the center section of the stem. The highest MC values were found in the center of the stem with an MC of 310%, while the lowest values were found in samples near the bark with an average value of 105%. Based on the findings above, it was expected that the porosity of oil palm wood shows a declining trend from the outer to the core sections. This can be observed in Figure 4. The higher void space in the core of the stem allows a higher amount of water to be stored there.

(5)

where kL = longitudinal permeability, m (fluid) m atm−1 s−1), 1 atm = 1.013 × 106 dyne m−2, R = radius of vessels, m, n = N/A = no. of vessels per m2 of transverse surface, and η = viscosity of fluid, (dyne s) m−2. 2.6. Statistical Analysis. An analysis of variance (ANOVA) was carried out on the data, and it was then further analyzed using the Least Significant Difference method (LSD). In this analysis, a simple random design was used to determine significant effects of the (1) parts and (2) lengths of samples on the permeability values. 3

−1

3. RESULTS AND DISCUSSION 3.1. Density, MC, and Porosity. It can be seen from Figure 2 that the density of the oil palm wood decreases toward the core section of the stem. The highest densities were recorded for samples near the bark with an average density of 0.556 g/cm3, while the lowest density was found at the center of the stem with a density of 0.242 g/cm3. The MC of the oil palm wood can be observed in Figure 3. The results showed an inverse nature compared to density with

Figure 4. Porosity of oil palm wood across the transverse section of the stem.

3.2. Longitudinal Water Permeability. Figure 5 shows that the middle part of the transverse surface of the specimens

Figure 2. Density of oil palm wood across the transverse section of the stem.

Figure 5. The effect of parts of the transverse surface and length of specimens on the longitudinal water permeability of oil palm wood. C



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All parts had significantly different (P ≤ 0.05) permeability values from each other. This might be due to several reasons such as the occurrence of extractives occluding the pathways in the outer part. As mentioned in the results of the water permeability, the presence of leaf traces in the outer part might also be a contributing factor to the lower permeability values found in the outer part of the oil palm stem compared to the middle and inner parts of the transverse surface. It is also very clear that length plays a very important role in the longitudinal gas permeability of oil palm wood. 3.4. Theoretical Permeability. It was observed that the density of vascular bundles was clearly different at different parts of the transverse surface of the oil palm wood. Figure 7 shows the stained vessels (active vessels) of the different parts of the transverse surface of the oil palm wood. The density of vascular bundles was clearly higher in the outer part followed by the middle and inner parts. Generally, the vascular bundles in the outer part contained just a single vessel, but sometimes two vessels can be found in a single vascular bundle. Two or more vessels were usually found in the vascular bundles of the middle part. Vascular bundles in the inner part usually contained 2 or 3 vessels and in some rare cases up to 6 vessels. This is in agreement with the discovery by Shirley (2002). It is clear from Table 1 that the percentage of active vessels contributing to the water flow decreases as the length of the specimens increase. This explains the drop in permeability values as the length of the specimens increase. As displayed in Table 2, the theoretical permeability value of the middle part was the highest followed by the outer and inner parts in the transverse surface. According to Poiseuille’s equation, the permeability of wood is proportional to the fourth power of the radius (r4) of the flow openings in wood. This implies that a slight increase in the radius of vessels will result in a large increase in the permeability value (Taghiyari et al.22). The middle part had the highest radius which explains the high theoretical permeability value. It was also observed that the vessels in the outer part of the transverse surface had a smaller radius which explains its low permeability according to Poiseuille’s equation of viscous flow. 3.5. Comparison between the Theoretical, Water, and Gas Permeability Values. There is a noticeable difference between the theoretical, water, and gas permeability values of the samples as illustrated in Figure 8. The theoretical values gave the highest permeability values followed by the water and gas permeability in specimens. The permeability values of the outer, middle, and inner theoretical values is bigger than the outer, middle, and inner water permeability values by a factor of approximately 5, 4, and 3 respectively. In this study, the decreasing trend of density from the outer to the inner section does not show a similar trend with permeability as the middle section which has the highest permeability values have density values that were between the inner and outer sections (Table 3). Again, no similar trend was found between the permeability of the oil palm wood with its MC. As with the density, the middle section which gave the highest permeability had MC values that were between the outer and inner parts of the samples. The overall actual water and gas permeability values are much lower than the theoretical value. This might be due to the length effect and also the presence of possible blockages in the vessels. The vessels elements in oil palm wood are connected via scalariform and simple perforation plates. Scalariform

has the highest permeability values at all lengths followed by the inner and outer parts. The permeability values were significantly different between parts. Decrease in the permeability of the samples with their increasing length was also reported in previous work on other species (Taghiyari and Sarvari Samadi;21 Karimi;13 Perre and Karimi14), and in this study, the length of samples significantly affects the longitudinal water permeability values of the oil palm wood. The water permeability values of the oil palm wood dropped drastically as the length of the sample increases (Figure 1) with one 20 cm sample from the outer section showing zero flow even after one and a half hours of testing. Similar adverse correlations were also reported between the gas and water permeability of solid woods (Taghiyari23). This phenomenon took place regardless of the area in which the permeability is measured. The highest longitudinal water permeability value was recorded in the 5 cm samples from the middle part with 7.89 darcys. The lowest longitudinal water permeability value belonged to the 20 cm samples from the outer parts with 0.01 darcys. The presence of leaf traces in the outer section of the oil palm wood might be a contributing factor to the low permeability values found in the outer section especially for longer specimens. The water permeability values of oil palm obtained in this study was considerably lower than 3 species of rattan: Calamus manan, Calamus ornatus, and Calamus peregrines, measured by Zaidon and Petty.25 The water permeability of oil palm wood is comparable to some species of softwood such as Tsuga Canadensis (Comstock26) and hardwood such as Fagus orientalis and Fagus silvatica (Perre and Karimi14). 3.3. Longitudinal Gas Permeability. The middle part of the transverse surface of oil palm wood gave the highest longitudinal gas permeability values regardless of length of specimen (Figure 6). The highest values were given by the 1.6

Figure 6. The effect of parts of the transverse surface and length of specimens on the longitudinal gas permeability of oil palm wood.

cm specimens from the middle part with an average permeability of 2.87 darcys, and the lowest were given by the 5 cm specimens from the outer part with an average permeability value of 0.63 darcys. As can be seen in Figure 6, the longitudinal gas permeability of the oil palm changes according to the length of the specimen. As the specimens were progressively cut shorter, its longitudinal gas permeability value increases. The 1.6 cm specimens had an average permeability value of 1.98 darcys followed by the 3.2 cm specimens with 1.36 darcys and the 5 cm specimens with 0.96 darcys. D



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Figure 7. An example of stained vessels (active vessels) after the water permeability test for different parts of the transverse surface of the oil palm wood: (a) outer, (b) middle, and (c) inner.

perforation plates significantly reduce the permeability of wood (Thomas27 and Petty28), and this might also explain the lower water and gas permeability values in comparison to the theoretical values. The actual water permeability values of the inner part are higher than the outer part which contradicts the theoretical values. Shirley29 found that the length of vessel elements for the outer part of the oil palm was significantly shorter than those from the middle and inner parts. Shorter vessel elements would mean an increase in the number of perforation plates in a vessel which in turn increases the resistance to liquid flow. This coupled with the fact that the average radius of the outer vessels is also smaller than the average radius of the inner values contributes to the lower permeability values found in the outer part in comparison with the inner part. The water permeability values for the 5 cm specimens from the outer, middle, and inner specimens were 7, 6, and 6 times higher than the 5 cm specimens from the outer, middle, and inner gas permeability values, respectively. According to Siau (1984), kiln or air drying of wood results in high capillary forces at the surface of the free water, and these forces can cause aspiration in the pits of cells of woods. This might explain the large difference between the permeability values found between the water and gas specimens. The large difference between the water and gas permeability values indicates that the oil palm wood is highly susceptible to drying defects. This agrees with the findings of Bakar et al. (2008).

Table 1. Percentage of Active Vessels Contributing to Flow at Different Lengths of Specimens in Different Parts of the Transverse Surfacea active vessels (%) at different lengths

a

part

total no. of vessels entrance (0 cm)

5 cm

10 cm

15 cm

20 cm

outer middle inner

83 72 41

22.9 34.7 51.2

6.0 31.9 41.5

3.6 29.2 39.0

0 27.8 36.6

Total length of specimens: 20 cm.

Table 2. Average Number of Vessels in 1 cm2 and Theoretical Permeability Value Using Poiseuille’s Equation in Different Parts of the Transverse Surface transverse surface

no. of vessels (in 1 cm2)

ave radius (μm)

theoretical permeability (darcy)

outer part middle part inner part

120 98 71

78.21 92.33 85.12

20.93 31.23 17.70

4. CONCLUSIONS The oil palm wood is relatively permeable in comparison to some hardwood species. The longitudinal theoretical permeability values are much higher compared to the longitudinal water and gas permeability values. This is due to the effect of length of the specimens. The gas permeability values are considerably lower than the water permeability values. This might be due to the effect of pit aspiration during the drying of the oil palm wood that causes defects. The lower gas permeability values also indicate that the oil palm wood is prone to drying defects and is much more difficult to treat with preservatives. The present study proves that there is a large variation in permeability between different parts of the transverse surface. Therefore, for plywood industries, separation of veneers for drying and final utilization is recommended.

Figure 8. Comparison between theoretical, water, and gas permeability values taken at different parts of the transverse surface (The length of the water and gas samples were 5 cm.).

Table 3. Density, MC, and the Longitudinal Permeability of Oil Palm Wood in Different Parts of the Transverse Surfacea permeability (darcy) 3

outer middle inner a

density (g/cm )

MC (%)

theoretical

water

gas

0.463 0.339 0.252

138 212 303

20.93 31.23 17.7

4.28 7.89 5.56

0.63 1.38 0.88

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

The length of the water and gas samples are 5 cm.

Notes

The authors declare no competing financial interest. E



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(19) Pokki, J. P.; Laakso, V. V.; Tikka, P.; Aittamaa, J. Specific permeability of wood to water part 1: Perpendicular specific permeability of steamed, impregnated, and Kraft-cooked wood. Ind. Eng. Chem. Res. 2010, 49, 2155. (20) Dermoe, D.; Zillig, W.; Carmeliet, J. Variation of measured cross-sectional cell dimensions and calculated water vapor permeability across a single growth ring of spruce wood. Wood. Sci. Technol. 2012, 46, 827. (21) Taghiyari, H. R.; Sarvari Samadi, Y. Ultimate length for reporting gas permeability of Carpinus betulus wood. Special Topics Reviews in Porous Media, 2012, 1 (4). (22) Taghiyari, H. R.; Efhami, D.; Karimi, A. N.; Pourtahmasi, K. Effect of initial spacing on gas permeability of Populus nigra var. betufolia. J. Trop. For. Sci. 2011, 23 (3), 305. (23) Taghiyari, H. R. Correlation between gas and liquid permeabilities in some nano-silver-impregnated and untreated hardwoods. J. Trop. For. Sci. 2012, 24 (2), 249. (24) Siau, J. F. Wood: Influence of moisture on physical properties, Dept. of Wood Science and Forest Products, Virginian Polytechnic Institute and State University, Blacksburg, 1995. (25) Zaidon, A.; Petty, J. A. Steady-state water permeability of rattan (Calamus spp.). Part I. Longitudinal permeability. J. Trop. For. Sci. 1998, 4 (1), 30. (26) Comstock, G. L. Factors affecting permeability and pit aspiration in coniferous sapwood. Wood. Sci. Technol. 1968, 2, 279. (27) Thomas, R. J. Anatomical features affecting liquid penetrability in three hardwood species. Wood Fiber 1976, 7, 256. (28) Petty, J. A. Fluid flow through the vessels of birch wood. J. Exp. Bot. 1978, 291, 1463. (29) Shirley, M. B. Cellular Structure of Stems and Fronds of 14 and 25 Year-Old Elaeis guineensis Jacq. Dissertation, University Putra Malaysia, 2002.

ACKNOWLEDGMENTS The authors wish to thank the Ministry of Science, Technology and Innovation Malaysia and also the Ministry of Higher Education Malaysia for funding a part of this project. We would also like to thank Dr. Hamid Reza Taghiyari from Shahid Rajaee Teacher Training University, Iran for his invaluable assistance in providing the parts for the gas permeability apparatus.

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REFERENCES

(1) MPOB. Overview of the Malaysian Oil Palm Industry 2010. 2011. http://econ.mpob.gov.my/economy/Overview%202010_final.pdf (accessed May 10, 2013). (2) Anis, M.; Kamarudin, H.; Astimar, A. A.; Mohd. Basri, W. Treatment of Oil Palm Lumber. MPOB Information Series. 2008; ISSN 1511-7871. MPOB TT No. 379. (3) Yeoh, B. H.; Loh, Y. F.; Norhafizah, A. W.; Paridah, M. T.; Jalalludin, H. Development of a new pilot scale production of high grade oil palm plywood: Effect of pressing pressure. Mater. Des. 2011, 36, 215. (4) Nurjannah, S.; Rokiah, H.; Othman, S.; Mazlan, I.; Sato, M.; Hiziroglu, S. Optimum manufacturing parameters for compressed lumber from oil palm (Elaeis guineensis) trunks: response surface approach. Composites, Part B 2012, 93, 988. (5) Loh, Y. F.; Paridah, M. T.; Yeoh, B. H.; Bakar, E. S.; Anis, M.; Hamdan, H. Resistance of phenolic-treated oil palm stem plywood against subterranean termites and white rot decay. Int. Biodeterior. Biodegrad. 2011, 65, 14. (6) Choo, A. C. Y.; Paridah, M. T.; Karimi, A.; Bakar, E. S.; Khalina, A.; Ibrahim, A.; Loh, Y. F. Density and humidity gradients in veneers of oil palm stems. Eur. J. Wood Wood Prod. 2011, 69 (3), 501. (7) Dinwoodie, J. M. Timber: Its nature and behavior, 2nd ed.; E & FN Spon: 2000. (8) Siau, J. F. Transport Processes in Wood; Springer-Verlag: Berlin, 1984. (9) Zaidon, A. The Structure and Properties of Rattan in relation to Treatment with Boron Preservatives. Ph.D. Thesis, University of Aberdeen, 1995. (10) Perng, W. R. Studies on flow in wood I. Permeability and axial structural variation. Mokuzai Gakkashi 1980, 26, 132. (11) Bramhall, G. The validity of Darcy’s law in the axial penetration of wood. Wood. Sci. Technol. 1971, 5, 121. (12) Siau, J. F. Flow in Wood; Syracuse University Press: New York, 1971. (13) Karimi, A. Macroscopic and Microscopic Characterization of Flow in Beech Wood (Fagus orientalis and Fagus sylvatica), Interpretation by a Percolation Model. PhD Thesis, Ecole Nationale Genie Rural, des Eaux et des Forets (ENGREF), Nancy-France, 1995. (14) Perrè, P.; Karimi, A. Fluid migration in two species of beech (Fagus silvatica and Fagus Orientalis): A percolation model able to account for macroscopic measurements and anatomical observations. Maderas: Cienc. Tecnol. 2002, 4 (1), 50. (15) Taghiyari, H. R.; Karimi, A. N.; Parsapajouh, D.; Pourtahmasi, K. Study on the longitudinal gas permeability of juvenile and mature wood. Special Topics Reviews Porous Media 2010, 1 (1), 31. (16) Bakar, E. S.; Sahri, M. H.; H’ng, P. S. Anatomical Characteristics and Utilization of Oil Palm Wood. In The Formation of Wood in Tropical Forest Trees: A Challenge from the Perspective of Functional Wood Anatomy; Nobuchi, T., Sahri, M. H., Eds.; Penerbit Universiti Putra: Malaysia, Serdang, 2008. (17) Shi, S. H. Q. Diffusion model based on Fick’s second law for the moisture absorption process in wood fiber-based composites: is it suitable or not? Wood Sci. Technol. 2007, 41, 645. (18) Pokki, J. P.; Laakso, V. V.; Tikka, P.; Aittamaa, J. Specific permeability of wood to water part 1: Longitudinal specific permeability of steamed, impregnated, and Kraft-cooked wood. Ind. Eng. Chem. Res. 2010, 49, 2144. F


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