Distribution of preservatives in thermally modified Scots pine and Norway spruce sapwood
- First Online:
- Cite this article as:
- Ahmed, S.A., Hansson, L. & Morén, T. Wood Sci Technol (2013) 47: 499. doi:10.1007/s00226-012-0509-4
- 471 Views
Studying the impregnation and distribution of oil-based preservative in dried wood is complicated as wood is a nonhomogeneous, hygroscopic and porous material, and especially of anisotropic nature. However, this study is important since it has influence on the durability of wood. To enhance the durability of thermally modified wood, a new method for preservative impregnation is introduced, avoiding the need for external pressure or vacuum. This article presents a study on preservative distribution in thermally treated Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) sapwood using computed tomography scanning, light microscopy, and scanning electron microscopy. Secondary treatment of thermally modified wood was performed on a laboratory scale by impregnation with two types of preservatives, viz. Elit Träskydd (Beckers) and pine tar (tar), to evaluate their distribution in the wood cells. Preservative solutions were impregnated in the wood using a simple and effective method. Samples were preheated to 170 °C in a drying oven and immediately submerged in preservative solutions for simultaneous impregnation and cooling. Tar penetration was found higher than Beckers, and their distribution decreased with increasing sample length. Owing to some anatomical properties, uptake of preservatives was low in spruce. Besides, dry-induced interstitial spaces, which are proven important flow paths for seasoned wood, were not observed in this species.
Wood modification with heat is a widely investigated method to reduce equilibrium moisture content, water absorption, and to increase dimensional stability. Hydrophilic wood materials become more hydrophobic due to a large number of chemical changes including the esterification of hydroxyl groups (–OH) and a reduction in hemicelluloses and the number of accessible –OH groups within the wood (Alén et al. 2002; Bekhta and Niemz 2003). The reason for decay resistivity is the reduction in hemicelluloses content, moisture, and other wood components such as starch, fatty acids, and lipids which are essential for mold and fungal growth (Tjeerdsma et al. 1998). However, wood modification with heat is also reported to be vulnerable to different biodegrading agents, like marine borers (Westin et al. 2006), termites (Doi et al. 1999), and fungi (Jämsä and Viitaniemi 2001; Kamdem et al. 2002). Thermal modification could degrade wood components through the oxidation and/or hydrolysis process, and consequently, wood becomes easily attacked by fungi and/or insects after changing their composition (Doi et al. 1999). The use of thermally modified wood in ground contact is thus not recommended without further protection (Kamdem et al. 2002).
Thermal modification resulted in some anatomical changes like destruction of tracheid walls, ray tissues, pit deaspiration (Awoyemi and Jones 2011), and thus increase in the wood cell wall porosity (Andersson et al. 2005). Terziev and Daniel (2002) reported that some of the pit apertures are torn, and the appearance of micro (1–2 μm)- and nano-checks (10–20 nm) in the warty and S3 layers is noted, which provide direct penetration of the impregnated liquid into the S2 layer. They also suggested that thermal degradation of hemicelluloses makes the entire polymer complex more permeable to liquids. In this connection, increased wettability and water absorption of thermally modified pine sapwood are observed (Metsä-Kortelainen et al. 2006). Conventional kiln drying is also reported to have a distinct effect on the wood microstructure properties and hence an influence on liquid impregnation. In relation, Booker (1990) and Ahmed et al. (2012) studied the penetration paths of preservatives in dried wood. In addition to the important paths of penetration, they discussed the existence of secondary flow paths resulting from the collapse of thin cell wall layers of parenchymatous cells. Another report by Booker and Evans (1994) on the effect of air, kiln, and high temperature drying on radial gas permeability revealed that high temperature leads to increased gas permeability through the resin canal network, which is the predominant route for gas movement in radiata pine dried at high temperature. Furthermore, thermal modification does not limit the absorption of liquid in wood (Kamdem et al. 2002). Thus, it is postulated that thermally modified wood can be impregnated with preservative more easily. A previous study shows that thermally modified Scots pine and Norway spruce can uptake a considerable amount of oil-borne wood preservatives (Ahmed and Morén 2012).
Wood deterioration primarily results from the uptake of liquid water and moisture sorption, causing mechanical stresses and leading to colonization of wood decay fungi. The use of water repellents can be an effective tool to reduce wood deterioration (Sailer et al. 2000; Karlsson et al. 2011). These water-repellent compounds cause macro-pore blocking by depositing hydrophobic compounds in the lumens of vessel, tracheid, and ray cells (Weigenand et al. 2007). Thus, thermally modified wood in addition to water-repellent wood preservatives can provide a suitable method of protection against different biodegrading agents. However, the success of chemical wood preservation is dependent on the ability of the preservative to disperse throughout the wood cells, which is mainly related to the permeability of the different wood tissues, and the capability of the preservative to penetrate deeply into the wood structure (Rak 1976). Preservative penetration is more pivotal in the performance of treated wood than its retention in the wood tissue. Deep and uniform penetration of preservative is necessary rather than the least amount of retention. A lesser amount of retention and a shallow distribution of preservative, even at high concentration in the outer zone, do not provide adequate protection against wood-deteriorating organisms (Kollmann and Côté 1984; De Groot 1994). Successful impregnation with water-repellent wood preservative will protect thermally modified wood from attack of different decay fungi in outdoor application.
The literature discussed above draws attention to the susceptibility of thermally modified wood to different biodegrading agents. Since thermally modified wood is permeable, it can easily be impregnated with preservatives to ensure resistance against different biodegrading agents for use in outdoor applications. Thus, a very simple and effective method is used for secondary treatment of thermally modified wood (Ahmed and Morén 2012). As a part of the experiment, this paper provides information pertaining to preservative distribution in different cells of thermally modified Scots pine and Norway spruce sapwood. Observations were made mainly using computed tomography (CT) scanning, light microscopy (LM), and scanning electron microscopy (SEM).
Materials and methods
Wood species and thermal modification
Commercially treated thermally modified Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) boards (ca. 50 × 125 mm²) were collected from Thermoplus, Arvidsjaur, Sweden. Moisture content of the boards was reduced to 18 % by kiln drying prior to thermal modification. Boards were thermally modified at 170 °C for 2.5 h. Saturated steam was used during drying and thermal modification as a protective vapor to prevent the wood from burning. More about this process introduced by the Danish company Wood Treatment Technology (WTT), can be found in Dagbro et al. (2010). Treated boards were brought to the laboratory, and sapwood was separated from the heartwood. Separated sapwood samples were used for the secondary treatment with preservatives. Additional green pine and spruce boards (ca. 50 × 125 mm²) were also collected before thermal modification. Sapwood portions were separated from the heartwood and left in the laboratory. After attaining equilibrium moisture content at room condition, they were used as source of control samples.
Two different types of water-repellent preservatives were used viz. Elit Träskydd and pine tar. Elit Träskydd (Beckers, Stockholm, Sweden) is a water-miscible commercial product which contains additives such as propiconazole (0.6 %) and 3-iodo-2-propynyl butylcarbamate (IPBC) (0.3 %) in modified linseed oil as the binder and water as the solvent. In addition, supplement of 1.9 % (w/v) tert-butylhydroquinone (Sigma-Aldrich, St. Louis, USA) was added as antioxidant. On the contrary, commercially produced pine tar (Claessons Trätjära AB, Göteborg, Sweden) was mixed in boiled linseed oil (Claessons Trätjära AB). Turpentine (Claessons Trätjära AB) was added to thin the oil-based preservatives. The ratio of pine tar, linseed oil, and turpentine solution was 1:4:2. Preservative solutions were stirred properly before use. In the following, Elit Träskydd and pine tar will be referred to as Beckers and tar, respectively.
Computed tomography (CT)
A Siemens CT scanner (SOMATOM Emotion Duo, München, Germany) was used for measuring the density profiles in the treated wood. The 512 × 512 pixels CT images were obtained using scan settings of 130 kV, 250 mAs, and scan and delay time were 1.5 and 3.0 s, respectively. The intensity (gray scale) value of each pixel corresponds to the measured density (kg m−3) in that measuring volume. The volume or the voxel was 0.63 × 0.63 mm through the cross-section and 10 mm in the grain direction. A total of seven scans at 10-mm interval were performed in the grain direction for every sample. This covered almost half of the sample length. Scanning was performed before (oven dried) and after preservative treatment. Quantitative analysis of preservative distribution profile was calculated by subtracting the density profiles obtained from the preservative-treated wood from that of the oven-dried wood before impregnation using MatLab 22.214.171.1249 (The MathWorks, Inc., Natick, USA) software. Averaged data from the triplicate samples were used for each treatment profile.
Light microscopy (LM)
Sample blocks from preservative-treated wood with dimensions of ca. 5 (R) × 5 (T) × 5 (L) mm³ were prepared, and cross-, R, and T sections, ca. 15–20 μm thick, were obtained with a sliding microtome (Reichert, Vienna, Austria). The standard Sudan black procedure is proved to stain all available lipids (other than steroids) gray to blue-black (Chayen and Bitensky 1991). Thus, sections were treated with saturated Sudan black B (AppliChem GmbH, Darmstadt, Germany) in 70 % ethanol for 30 min, rinsed in 70 % ethanol to remove excess dye from the surface, washed well in water, and finally mounted in glycerin–gelatin jelly. The stained sections were examined under an optical microscope (Olympus SZH-ILLD, Tokyo, Japan) fitted with a USB uEye LE camera, UI-14xLE-C (IDS Imaging Development Systems GmbH, Obersulm, Germany). Digital images were captured and stored by means of uEye Demo, 3.70.0 software in a bitmap (BMP) file format.
Scanning electron microscopy (SEM)
Wood blocks measuring ca. 5 (R) × 5 (T) × 3 (L) mm³ were prepared from thermally modified wood. Blocks were soaked in water overnight for softening. Observed surfaces were finished on a microtome, dried, adhered to aluminum stubs with double-sided tape, and sputter-coated by gold using Desk II Cold Sputter (Denton Vacuum Inc., Moorestown, USA). At different magnifications, wood microstructures were then examined by using a JSM-5200 (JEOL, Akishima, Japan) electron microscope at an accelerated voltage of 15 kV. Six sample blocks from each of the thermally modified wood species were examined for the following quantified details: number of axial and radial resin canal per mm2, their tangential diameters delimited by epithelial cells, number of ray parenchyma per mm2 and cell wall thicknesses, and the number of bordered pits per longitudinal tracheid. For LM counting of number of bordered pits, the samples were split into matchstick-sized pieces and macerated in a solution of glacial acetic acid and hydrogen peroxide (1:1, v/v) at 60 °C for 18 h (Franklin 1945). A total of 35 measurements were carried out for each anatomical feature. Additional SEM observations were also carried out for the preservative impregnated samples.
To test whether there were any significant differences between the quantitative anatomical properties of pine and spruce, a Student’s two-tailed group t test (at α = 0.05, 0.01, 0.001) was performed using Excel 2003 program (Microsoft, Redmond, USA).
Results and discussion
Mean (±standard deviation) of the selected anatomical features of thermally modified pine and spruce
Level of significance
Number of axial resin canals per mm2
0.64 ± 0.11
0.27 ± 0.13
Number of radial resin canals per mm2
0.59 ± 0.22
0.40 ± 0.19
Diameter of axial resin canal (μm)
107.54 ± 15.91
106.73 ± 19.63
Diameter of radial resin canal (μm)
42.33 ± 5.70
41.75 ± 6.35
Number of ray parenchymas per mm2
23.00 ± 3.16
23.42 ± 6.92
Thickness of ray parenchyma cell wall (μm)
1.49 ± 0.29
2.67 ± 0.26
Number of pits per earlywood tracheid
90.37 ± 17.05
75.51 ± 15.24
Number of pits per latewood tracheid
39.97 ± 12.78
31.83 ± 10.48
Since the wood is preheated, any air contained within the cell cavities and voids is also hot and therefore expands. Immersing the hot wood in preservative solution at room temperature gives rise to a rapid contraction of the air within the cell cavities and voids, resulting in the solution being sucked into and retained in the void spaces.
LM and SEM analyses
Until, and unless, bordered pits are closed, longitudinal tracheids are considered the most important structure regulating the permeability of softwoods. Those longitudinal tracheids conducted preservative liquid through the permeable bordered pits that were open or partially open (Fig. 4c, f). In this regard, it is reported that all earlywood bordered pits are aspirated in air-dried pine and spruce sapwood (Liese and Bauch 1967). Bordered pit aspiration is considered to be caused by hydrogen bonding between the margo/torus and the adjacent cell wall (Thomas and Kringstad 1971). However, thermal modification can result in pit deaspiration (Awoyemi and Jones 2011) in which the hydrogen bonding in the aspirated pits may have been reduced. However, in dried wood, when bordered pits are aspirated, resin canals are reported the most important flow path (Booker 1990). It is also believed that axial and radial resin canals took active part in deep penetration of preservative liquid in thermally modified pine and spruce samples. Impregnated preservative through resin canal can diffuse to adjacent ray parenchyma and longitudinal tracheid (Fig. 4g). The overall preservative uptake was lessened through the length of the sample, and this trend is in agreement with Siau (1972). Thus, in middle of the sample, the deposition of preservative was found low (Fig. 5).
The number of pits per tracheid varied from earlywood to latewood. Even though latewood has fewer and smaller pits, they are reported more permeable than earlywood tracheid in dried material, while earlywood is more permeable than latewood in greenwood (Petty and Preston 1969). Distribution profiles of preservative obtained by CT scan also provide information concerning the higher permeability of latewood than the earlywood (Fig. 3). Gradual distribution of preservatives from latewood to earlywood was also apparent. When the wood is dried, the position of the pit membrane is changed by the process of aspiration in which the torus moves across the pit chamber to seal off either side of the pit aperture, thus preventing fluid flow through the pit pair. After thermal treatment, pit membrane disruption and open and/or partially open bordered pits were observed (Fig. 6f, h) through which preservative liquid can traverse to the neighboring tracheids (Fig. 6d, g, i). The highest preservative uptake was observed in control pine sample treated with tar. Thus, special attention was given to find out the reasons. Control wood samples were preheated at 170 °C for 1 h before immersing in preservative solution which resulted in some degree of pit deaspiration. It was revealed from the SEM observation (Fig. 6d, e) that higher frequency of open or partially open bordered pit and increased permeability of tar through half-bordered pit membrane are probably the explanation for the highest amount of preservative uptake.
Secondary treatment on thermally modified wood was performed successfully using a simple and effective procedure. But the performance of this kind of treated wood in field condition is yet to be conducted.
Secondary treatment of thermally modified pine and spruce was conducted with Beckers and tar preservative. Both species retained a considerable amount of preservative, especially tar, without applying any external vacuum or pressure. Preservative distribution in different cells of thermally modified wood was examined through CT scan and microscopy techniques. CT scan image analysis provided distribution profile of preservative solution in pine and spruce samples. On the other hand, LM and SEM techniques provided information about the reasons related to the variability of preservative uptake. Anatomical differences between pine and spruce were related to the variation in permeability. Disrupted and open bordered pit due to thermal modification took an active part in preservative uptake and distribution. The lower frequency of axial and radial resin canals and the inability to form secondary flow paths due to ray cell collapse restricted the preservative uptake and distribution in spruce. Besides, the compositions of preservative liquids were also responsible for the differences of penetration level. Using turpentine in the tar formulation increased the preservative impregnation in pine and spruce compared with Beckers. For both kinds of preservative, the uptake amount decreased with an increase in sample length. The investigation into preservative uptake and distribution using the CT scanning technique has been found to be a useful method. Further studies should preferably be extended to using different species with larger dimensions.
Financial support from the European Union and the European Regional Development Fund, the County Administration of Västerbotten, the municipality of Skellefteå, and TräCentrum Norr is gratefully acknowledged.