Effect of Microwave Treatment on Strength and Permeability of Wood: A Snapshot Review

Microwave treatment is a promising technique for modifying the properties of wood. It has gained popularity as an alternative method to conventional treatments for improving the properties of wood. Microwave drying has shown that it reduces the drying stresses and improve the quality of wood compared to traditional drying methods. This efficient heating reduces the risk of deformation, cracking, and other defects that can occur with prolonged exposure to high temperatures during traditional drying and also helps in increasing the permeability of wood. This snapshot review provides an overview of the effect of microwave treatment on the strength properties and permeability of wood with variations in parameters such as the frequency, power, and treatment time along with the potential benefits and drawbacks of this method. Overall, the review indicate that microwave treatment is an effective method for modifying the properties of wood and has the potential to improve its performance in various applications.

Wood is an organic material made up of cellulose, hemicellulose, lignin and extractives. Due to the existence of secondary metabolites like phenolic compounds in hardwoods and terpenes in softwoods, a living tree confers resistance against wood decaying agents like fungus and insects. However, once the tree is felled, it begins to lose this resistance. Natural durability of wood can be understood as it is the ability of wood to withstand the combined effects of rot, insects, and decay [1].

One of the defining characteristics of a species is its innate ability to withstand external factors. The sapwood of all species serves as a conduit for sap to flow between the roots and canopy and also contains starch that attracts insects and fungi. On the other hand, heartwood, which is made up of dead cells, is primarily composed of toxic substances such as gums, resins, tannins, phenolic extractives, and lignin. This feature exhibits significant variability, indicating the anisotropic nature of wood, not only within species but even among trees from the same location. The presence of extractives and mechanical barriers such as tylosis and aspirated pits are two primary factors that can either increase or decrease a species' natural durability [2]. Wood can deteriorate physically, mechanically, chemically, as well as due to bacterial growth (wet wood), fungal growth (brown rot, white rot, soft rot, stain), insect infestation (borer, powder post beetle, termites), and marine borer activity. Protection against these elements is crucial to extending the lifespan of wood [3].

Freshly cut wood from the forest must be treated carefully before any additional usage. Two crucial treatments, preservation and drying, must be carried out before performing any further operations on wood. This is because the recently cut wood has a significant amount of moisture, with water occupying mainly the cell walls and lumens [4]. Wood has two types of water: free water and bound water. Free water is water that is present in cell lumens or cavities but is not chemically linked to wood. It is simple to remove and has no impact on the characteristics of the wood [5]. Since humans first began utilizing wood, they have endeavored to dry it before using it. Initially, wood was dried solely by exposing it to air and sunlight. However, in the past century, significant progress has been made in the field of artificial drying using various heat methods. Throughout this period, several types of kilns have been developed, such as solar and steam-heated kilns, vacuum kilns, electrical heating kilns, dehumidifying kilns, and others. High-frequency waves, including microwaves and radiofrequency, are effective in drying highly and moderately refractory wood species, which usually develop drying defects during standard kiln drying. Nonetheless, they are not commonly used.

Dielectric heating

Materials that are either excellent or extremely poor conductors of electricity are known as dielectric materials. A material that can become polarized under the influence of an electric field is referred to as a dielectric. Unlike metals, where free or loosely bound electrons move through the substance, no current flows through dielectrics when they become polarized initially [6]. As a result, electric charges are displaced from their usual equilibrium positions, causing polarization of the dielectric material. Polar compounds possess a dipole moment. When these polar molecules are subjected to an electric field, they have a tendency to align themselves in the direction of the field. As the applied electric field oscillates, these molecules move and rotate to maintain their alignment with the field. This process is referred to as "dielectric rotation." When the direction of the electric field changes, the molecules also reverse their direction [7,8].

The kinetic energy of the molecules affects the temperature of the molecules. The molecules' kinetic energy increases during dielectric rotation, which leads to an increase in temperature for the molecules. These molecules collide when they come into contact with one another, transferring energy to other parts of the material and heating it up. And this material heating caused by dielectric rotation is frequently referred to as dielectric heating. In this method, the heating is carried either via electromagnetic fields or high-frequency electric fields. The frequency and wavelength of the generated field affect how well the dielectric heating system works [8-10]. Heat is transferred through thermal conductivity in conventional heating methods and moves from source to vessel first, followed by the vessel to the solution. Different thermal conductivities impair the ability to manage temperature and prolong the establishment of thermal equilibrium in this sluggish and inefficient technique of heat transmission. Microwaves quickly heat any solvent, solute, or substance in solution through dipolar rotation and rather than depending on thermal conductivities, making this method of heating more effective, precise, and secure [9].

Although microwave heating was discovered in the late 19th century, it wasn't widely used on an industrial or commercial scale until the development of methods for producing significant power at high frequencies in the late 1930s. In regulated environments, wood is dried using dielectric heating. In the realm of wood science, aspects including heating-based wood modification and Phyto sanitization are being investigated [11]. Electric current of high frequency can be classified into two distinct ranges: Radio Frequency (RF) and microwave frequency Radio Frequency (RF) and microwave frequency. The former employs an open wire circuit for frequencies below 100 MHz, while the latter utilizes a waveguide to transfer power to materials at frequencies beyond 500 MHz [12]. Polarization of the insulating substance is possible using high frequency electromagnetic waves. The microwave can provide a greater amount of heat, but it is limited in its ability to heat wood-based items uniformly or deeply enough.

The water molecules are reorganized in the microwave electromagnetic field, causing movement and friction from the fast molecular motion and heat as a result. As the temperature of the water rises, so does the temperature of the wood. Due to its strong polarity, water absorbs microwave energy more efficiently than wood. As a result, areas with higher moisture content experience higher temperatures [13]. When Vermaas HF [14] investigated the mechanism, he reported that the frequency, voltage, and two characteristics of the wood's substance all affected how hot the material became. The dielectric constant and power factor are the two material characteristics that are referred to as dielectric qualities. In order to obtain moisture homogeneity in the final product within allowable limits, it is necessary to avoid over drying wood by hot air. Instead, microwave heating is being used to dry the timber from 15% to 6% moisture content [15].

Microwave and wood: Effects and applications

The main processing, which involves the two key procedures of wood drying and wood preservation, has become crucial in order to retain the wood for a longer period of time. The seasoning of wood is a crucial step in extending the life of the wood or providing protection against deteriorating chemicals. In addition, seasoning has other advantages that help with better exploitation of timber under various conditions. Due to its hygroscopic nature, wood tends to absorb water when it is kept in moist environment. Before using the wood for any purpose, the moisture level of freshly cut wood must be brought in between 8 and 12 percent.

Water that is chemically bonded to wood and found in cell walls is known as bond water. The removal of bond water from wood causes a multitude of changes in the properties of the wood. During the drying process a theoretical point known as Fiber Saturation Point (FSP) was reached as water continued to evaporate from wood. At this point, the cell lumens becomes completely empty and the cell wall remains fully saturated with bonded water [16]. Wood shrinks as it begins to dry below the FSP, when the bond water begins to evaporate from the wood, or when dry wood attempts to absorb water into the cell wall. When there is moisture present, the wood's shrinkage, swelling, and dimensional instability cause issues with the finished product [17]. Due to the anisotropic structure of wood, the shrinkage/swelling varies depending on the wood and can even occur in various directions inside the same wood. The wood becomes more stable and durable and the shrinkage and swelling are decreased once the wood has been dried below FSP till the moisture content is 12 to 8%. Additionally, the growth of undesirable alien organisms is not permitted at this moisture content [18,19]. Mechanical properties also improve in this MC range, but below 8%, wood starts to lose strength and increase brittleness, which should be avoided. As a result, seasoning has become a crucial step in the use of wood. Using traditional techniques in accordance with IS 1141: 1992 for seasoning. Dielectric heating is among the new techniques being tested to shorten the drying time of some refractory timber that is difficult to dry. Dielectric heating significantly accelerates the seasoning process and minimizes drying defects such as warping, collapse, and uneven drying. Microwave treatment produces seasoned wood of high quality and with fewer drying defects [11].

To maintain the biological balance of forests and alleviate the pressure from timber demand, by extending the lifespan of available wood and promoting the effective utilization of less durable timber species, wood preservation plays a crucial role in environmental sustainability, and generating foreign exchange through increased exports and decreased imports [20]. In order to increase the service life, wood must be treated with a chemical that prevents deterioration. These chemicals can be categorized as fixed or leachable, water or oil borne, and either harmful or environmentally beneficial (Indian Standard, IS: 401: 2001), each having its own set of advantages and disadvantages in comparison to the others.

Drying low permeability woods poses a challenge in terms of impregnating them with preservatives. However, several strategies have been developed and tested to enhance the treatability of such woods. One of these methods includes pressure-based treatment techniques, which have been developed to improve preservative uptake in wood [21]. Although the results of these treatment techniques have been considered satisfactory, they do have some drawbacks. Heating wood with microwaves can enhance its permeability by splitting and breaking down its anatomical microstructures at their weak points. Excessive microwave treatment can lead to substantial structural deterioration in the microstructures of the wood, ultimately leading to reduced strength properties [22]. Therefore, it is essential to continue exploring new and innovative options that can be skillfully employed in wood preservation. One such effort in this area is pretreating wood using microwave before wood preservation. The permeability of the wood, which increased following microwave pretreatment, enhances treatability [23]. Utilizing microwaves to enhance the permeability of refractory wood is a practical and effective approach to accelerate the drying process without causing degradation, further it does carry a risk of strength loss. The extent of strength loss in the wood increases with higher intensity and duration of exposure to electromagnetic fields. However, compared to control samples, the microwave-treated specimen exhibit superior penetration and retention levels, as the obstructions that previously impeded vessel passage have been eliminated [22,24].

Impact of microwave treatment on strength properties

Presently microwave heating is not very popular, but it can be an excellent alternative to conventional drying because it has the potential to dry wood more quickly while maintaining quality [25]. When the moisture content gradient is larger, this method can be quite helpful [26]. High frequency current can be used to evaporate and transport heat from wood depends on its dielectric attributes. Along with electric current, these characteristics are affected by wood's moisture level, temperature, and density [27]. High moisture content wood that is above the Fiber Saturation Point (FSP) has a tendency to absorb a lot of electromagnetic energy from microwaves. The material's specific heat capacity value influences how much energy is needed to increase the temperature [26]. By means of high Microwave (MW) intensity or extended exposure to MW radiation may result in an improved permeability of Norway spruce ripe wood. However, such an approach could also result in a notable alteration of the wood's mechanical properties [28]. Hansson L, et al. [29] studied microwave drying and conventional kiln drying method And report that comparative study shows that MW drying took significantly less time than that of conventional method of wood drying (Figure 1), however, the outcome demonstrated that there is no way to distinguish between both the two drying techniques in terms of the wood's strength.

Figure 1: Drying times for conventional and microwave drying [29].

Sethy AK, et al. [30], utilized microwave drying to uniformly dry the Pinus radiata sample (20 × 20 × 20 mm3) across its thickness, resulting in a decreased level of drying stress. Furthermore, even at low moisture content levels as low as 6%, no surface or end checks were observed in the dried samples. In other study, Eucalyptus macrorhyncha samples having dimensions of 30 × 90 × 2900 mm3 with average Moisture Content (MC) of 70.5% (ranging from 65 to 85%) showed better results when treated with low microwave treatment, which also reduced several drying defects as collapse (20%), internal check length (50%), and internal check width (70%). However, there were considerable decreases in shear strength (13%) and compressive strength of timber perpendicular to grain (10%) further, the study also compared the mechanical properties of wood samples treated with different levels of microwave radiation. The results showed that the high level microwave group had the lowest compression and shear strength values of 58.3 MPa and 14.92 MPa, respectively, while the control group had the highest values of 65.18 MPa and 18.76 MPa. The microwave treatment did not significantly affect the stiffness or bending resistance of the wood (Figure 2). However, it is possible to make certain changes to low-power microwave drying that can lead to improved results without compromising the strength of the wood [31].

Figure 2: Mechanical properties tested, compression strength parallel to grain, shear strength, Modulus of Elasticity (MOE) and Modulus of Rupture (MOR) on static bending. CTL is untreated wood, LMW is wood treated with low level of microwave (89 kWh/m3) and HMW is wood treated with high level of microwave (95 kWh/m3) adapted from [31].

The utilization of microwaves encompasses fast drying of refractory wood, preservation treatment, and stress release for growth and drying. The business industry presently has access to three fundamental Microwave (MW) processing parameters. This technology significantly minimizes energy and material costs, while also introducing novel concepts into the product development process within established industries [32]. Moreover, microwaves are employed to treat wood that has already been dried considerably using conventional techniques, with the heat focused on the moist regions [33].

Freshly cut wood contain a large quantity of water before drying to an equilibrium moisture content of 8 to 12 percent. This reduces weight for transport, improves stability, makes wood easier to treat, and increases mechanical strength depending on the degree of treatment. Increased in microwave treatment duration and intensity can result in a 60 percent reduction in strength attributes of Caribbean pine timber [34]. The gradient in moisture content and the temperature of the wood both affect how quickly wood dries [35,36]. Similar to the microwave the radio frequency heating, moisture loss, and degree of internal and surface inspection are all directly correlated. The drying rate is particularly noticeable for moisture level below the Fibre Saturation Point (FSP). Energy usage directly correlates with the durations of microwave/RF heating and kiln drying [37]. When drying wood, microwave conditioning improves the drying performance of 25 mm thick wood of Eucalyptus urophylla and Eucalyptus tereticornis. As we extend the duration of the microwave radiation, the reduction in drying stress increases [13]. The impact of various MW intensities on the permeability of Eucalyptus and Radiate pine wood was investigated by Vinden P, et al. [38] and found that radiation penetration depth is influenced by the material's dielectric properties.

The drying technique followed has substantial impact on the mechanical properties of the timber (Figure 3) and thereby affects the final products strength [34]. High internal temperature gradients will dramatically alter the color of wood [39]. Norway spruce and Radiata pine wood structures change as a result of microwave modification at frequencies between 0.922 and 2.45 GHz. High moisture wood exposed to high-intensity microwaves underwent some structural changes, including pit membrane fracture, pit opening, disintegration of the ray cell wall, and weakening and rupture of the middle lamella [24].

Figure 3: Summary of strength properties for a moisture content of 12% [34].

Heat degradation leads to a reduction in the mechanical strength of wood and samples treated at a power density of 5 mW mm-3 and did not experienced a substantial loss of strength, however samples treated at a power density of 10 mW mm-3 did show some loss in strength [40]. As far as there is free water in wood, temperatures in non-refractory or permeable wood cannot rise above the boiling point of water. The temperature begins to climb as bond water replaces free water, meaning that the timber is now below the fiber saturation point. A prolonged period of high temperature can weaken wood. The splits and checks in the wood may be caused by the weaker resistance to pressure [41]. The quantity of moisture lost during treatment rises linearly with microwave exposure period. At 2 kW of microwave power, water was lost at a rate of 13 g per minute from Grevillea robusta (Silver oak) samples of three different thickness. Additionally, regardless of how long the wood is exposed to the microwave, the pace of subsequent air drying is the same [42]. Microwave treatment was performed on Populus alba and Eucalyptus regnans before solar drying, resulting in a significant reduction in drying time without compromising the wood's quality [43].

Impact of microwave treatment on permeability of wood

Low wood permeability leads to a variety of issues during the production of lumber, including drying and preservation of the wood. It is very challenging to impregnate a low permeability timber with resin or a preservative substance. The microwave treatment destroys some of the wood structures and increases the permeability of both liquids and gases [24]. Microwave was used to improve the impregnability of Picea orientalis and found the increase of 47.5% and 70% for the samples with initial moisture contents of 55% and 83%, respectively when compared with control samples [44]. The microwave treatment procedure is more energy and environmentally friendly because to the microwave treatment, which also results in a cleaner woodworking environment. With the combination of preservation and seasoning treatment, a large amount of money can be spared in the form of energy usage for handicraft and other small-scale wood-based companies [23]. Another crucial component of impregnation regarding the effectiveness of the treatment to ensure the long-term durability of the treated wood material, is fixation of wood preservatives in wood. Preservative formulation, wood characteristics, treatment and post-treatment circumstances, and preservative use are the main factors of fixation [45].

Heartwood permeability is increased as a result of Microwave (MW) wood treatment, which also enhances preservative distribution and assimilation. After being treated with microwave, wood with a higher percentage of heartwood demonstrate a noticeable increase in permeability and preservative uptake of Eucalyptus tereticornis. Wood which is difficult to treat and dry is treated using a microwave at 2.4 GHz frequency, varying the radiation time and intensity. Further it was reported that treatment intensity and duration are increased, longitudinal wood permittivity increases significantly [46]. Quercus infectoria (Oak) heartwood's tyloses structure was altered by microwave drying techniques, enhancing air permeability and diffusion coefficients. The steaming pretreatment, however, stops steam from penetrating the specimens' interiors, resulting in a constant diffusion coefficient [47]. The density and moisture level of the wood being utilized determine the amount of microwave energy needed [48]. Thermal alteration in the case of birch wood results in a decrease in the rate of water absorption in both the radial and tangential surface. Additionally, greater tangential surface changes result in a reduction in the anisotropy of the transverse absorption rates of the wood. The rate of water absorption through into the radial surface of pine wood was marginally influenced by thermal change. Higher modification temperatures cause a greater rise in the rate of moisture absorption through the tangential surface, which, in contrast to birch wood, leads in enhanced anisotropy. Because thermal modification slows down the drying process in both birch and pine wood, it may be advantageous for the fixation of preservatives in thermally treated wood [49]. It has been reported that when wood is exposed to microwave radiation at various intensities and for various time intervals, the pH of the wood decreases, but the retention of preservative in the wood increases with increasing microwave treatment intensity and duration, while the resistance of the wood to termites was unchanged [50]. The effectiveness of the fixation process is important in determining the durability and effectiveness of the preservative treatment. When preservatives are well-fixed in wood, they provide long-term protection against decay and insect attack. Microwave treatment can be an effective method for fixing preservatives in wood. When wood is exposed to microwaves, the electromagnetic radiation causes the water molecules within the wood to vibrate and generate heat. This heat can then accelerate the diffusion of the preservative into the wood, leading to deeper and more uniform penetration [51,52]. Additionally, it can reduce the amount of preservative required, thereby reducing costs and environmental impact [11]. The middle section of the maple wood sample was found to have undergone a higher level of butyrylation compared to its exterior. Furthermore, when subjected to microwave treatment, the wood exhibited a greater degree of yellowing compared to untreated wood. Subsequent to the modification, the volumetric stability of the timber was also improved [53].

Microscopic fissures appeared at the extremities of the openings within the cross-field pits, extending towards the cellular boundaries of tracheids. Additionally, the central lamella separating the ray parenchyma cells and the longitudinal tracheids exhibited fractures resulting it high permeability [54]. The authors found that 99% of chromium can be fixed in just 30 minutes, along with arsenic and copper. This approach is significant for the wood treatment industry, which faces increasing environmental concerns. Hexavalent chromium will be entirely fixed after being exposed to temperatures of 110°C at a frequency of 13.56 MHz for five hours without having any negative impacts on the quality of the wood. With the aid of a better chamber design, fixation time can also be cut more drastically [55]. Copper is fixed in wood as a result of conversion of Cu-II (divalent copper) to Cu-I (monovalent copper) [45]. At high temperatures (105°C to 120°C), red pine is treated with Alkaline (amine) Copper Quaternary (ACQ) Cu-II that has been significantly reduced to Cu-I. Wood treated with a microwave after being microwaved at 85W for 30 minutes or 165W for 15 minutes demonstrated a significant improvement in resistance to leaching of ACQ-D (Ammoniacal Copper Quaternary Ammonium Salts Type D) treated wood, which will be achieved by air drying in a few days. Additionally, the post-microwave treatment of grains has a minor impact on grain compression [56].

Heartwood from Eucalyptus tereticornis, a refractory class wood and is extremely difficult to treat. When heartwood of Eucalyptus tereticornis that has been microwave-treated at various intensities and times exhibited a noticeably increased permeability to air and preservative absorption and suggests that microwave drying can be applied commercially to the wood preservation industry to increase the permeability of refractory wood species [48]. The impregnation results revealed that microwave-treated heartwood samples of both pine and Eucalyptus exhibited enhanced water absorption properties. Notably, among the microwave-treated specimens, only Eucalyptus heartwood showed a reduction in compression strength parallel to the grain compared to the control group. Consequently, microwave treatment holds promise for expanding its applications within the wood industry [57]. Microwave treatment significantly enhanced the permeability of Eucalyptus delegatensis. This is attributed to the boiling and expansion of moisture within the wood, which forms channels through the voids and wood structures, resulting in some modifications to the wood's structure. Consequently, these changes lead to an increase in the wood's permeability [40].

The application of a microwave treatment with an intensity of 112.65 W cm^-2 for three distinct time periods- 4 minutes, 4.5 minutes, and 5 minutes-followed by dipping for 5 minutes had a favorable effect and improved preservative retention and absorption percentage as well as wood permeability [46]. Heartwood permeability is influenced by microwave treatment, which also increases the uptake of preservatives in wood (Figure 4). The microwave treatment of Populus deltoides Bartr results in a striking increase in permeability at a frequency of 2.4 5 GHz and an intensity of 64.4 W cm^-2 [58]. Fir (Abies alba L.) wood after microwave pretreated at a frequency of 2450 MHz for 10, 12, 14, and 16 for four distinct radiation treatments. The impregnation of the preservative was significantly enhanced after microwave pretreatment, except for leaching. Therefore, the performance of refractory timber species can be improved with microwave pretreatments, and wood species that are difficult to treat can be used with ease on an industrial scale. By enhancing wood retention and penetration, the treatment will increase the treatability classes of the wood and make it useful for a variety of purposes [52].

Figure 4: Mean value of different properties of wood specimens and effect of M W treatment [58].

Research on the use of dielectric heating in the wood sector in India is still in its infancy. But, this technology might be able to give the wood sector new advantages. Wood can be modified by Microwave (MW) drying to enhance its resistance to decay and dimensional stability, which will ultimately increase the serviceability of wooden products. Dielectric heating has a number of benefits over conventional drying techniques, including improved drying uniformity, increased permeability for preservative treatment, less degradation during drying, speedy drying, and environmental friendliness. The system's design has a significant impact on how effectively dielectric heating works.

1. Scheffer TC, Morrell JJ. Natural durability of wood: A worldwide checklist of species. Forest Research Laboratory, Oregon State University. Research Contribution 22. p.58.
2. Taylor AM, Gartner BL, Morrell JJ. Heartwood formation and natural durability: A review. Wood and Fiber Science. 2002;34(4):587-611.
3. Shupe TF, Lebow ST, Ring DR. Causes and control of wood decay, degradation & stain. Louisiana State University Agricultural Center, Louisiana Cooperative Extension Service. 2008.
4. Perré P. Fundamentals of wood drying, European COST, AR BO. LOR: Nancy, France. 2007.
5. Desch HE, Dinwoodie JM. Seasoning of Wood. In: Timber structure, properties, conversion and use. Palgrave, London; 1996. p.144-158.
6. Jones PL. High frequency dielectric heating in paper making. Drying Technology. 1986;4(2):217-244. doi: 10.1080/07373938608916325.
7. Mijović J, Wijaya J. Review of cure of polymers and composites by microwave energy. Polymer Composites. 1990;11(3):184-191. doi: 10.1002/pc.750110307.
8. Grant E, Halstead BJ. Dielectric parameters relevant to microwave dielectric heating. Chemical society reviews. 1998;27(3):213-224. doi: 10.1039/A827213Z.
9. Menéndez JA, Arenillas A, Fidalgo B, Fernández Y, Zubizarreta L, Calvo EG, Bermúdez JM. Microwave heating processes involving carbon materials. Fuel Processing Technology. 2010;91(1):1-8. doi: 10.1016/j.fuproc.2009.08.021.
10. El Khaled D, Novas N, Gazquez JA, Manzano-Agugliaro F. Microwave dielectric heating: Applications on metals processing. Renewable and Sustainable Energy Reviews. 2018;82:2880-2892. doi: 10.1016/j.rser.2017.10.043.
11. Sharma R, Kumar R, Ray S. Dielectric heating: An eco-friendly alternative for drying of wood. Plant. 2022;4:933-940.
12. Ramaswamy H, Tang J. Microwave and radio frequency heating. Food Science and Technology International. 2008;14(5):423-427. doi: 10.1177/1082013208100534.
13. He Q, Wang X. Drying stress relaxation of wood subjected to microwave radiation. BioResources. 2015;10(3): 4441-4452.
14. Vermaas HF. High frequency heating of wood: Part 1: A review of general principles. South African Forestry Journal. 1972;81(1):13-16. doi: 10.1080/00382167.1972.9629268.
15. Resch H. Drying of incense cedar pencil slats by microwave power. Journal of Microwave Power. 1967;2(2):45-49. doi: 10.1080/00222739.1967.11688644.
16. Gezici-Koç Ö, Erich SJ, Huinink HP, Van der Ven LG, Adan OC. Bound and free water distribution in wood during water uptake and drying as measured by 1D magnetic resonance imaging. Cellulose. 2017;24(2):535-553. doi: 10.1007/s10570-016-1173-x.
17. Mantanis GI, Young RA, Rowell RM. Swelling of wood. Wood Science and Technology. 1994;28(2):119-134. doi: 10.1007/BF00192691.
18. Brischke C, Rapp AO. Dose-response relationships between wood moisture content, wood temperature and fungal decay determined for 23 European field test sites. Wood Science and Technology. 2008;42:507-518. doi: 10.1007/s00226-008-0191-8.
19. Johansson P, Bok G, Ekstrand-Tobin A. The effect of cyclic moisture and temperature on mould growth on wood compared to steady state conditions. Building and Environment. 2013;65:178-184. doi: 10.1016/j.buildenv.2013.04.004.
20. Evans PD, Matsunaga H, Preston AF, Kewish CM. Wood protection for carbon sequestration: A review of existing approaches and future directions. Current Forestry Reports. 2022;8(2):181-198. doi: 10.1007/s40725-022-00166-x.
21. Milton FT. The preservation of wood. Minnesota Extension Service; 1995. p.110.
22. Ganguly S, Balzano A, Petrič M, Kržišnik D, Tripathi S, Žigon J, Merela M. Effects of different energy intensities of microwave treatment on heartwood and sapwood microstructures in Norway spruce. Forests. 2021;12(5):598. doi: 10.3390/f12050598.
23. Ganguly S, Tripathi S. Study on effect of microwave treatment on wood permeability and preservative retention in imported timber. J Agroecology Nat Res Management. 2018;5(1):34-40.
24. Terziev N, Daniel G, Torgovnikov G, Vinden P. Effect of microwave treatment on the wood structure of Norway spruce and Radiata Pine. BioResources. 2020;15(3):5616-5626.
25. Mascarenhas FJR, Dias AMPG, Christoforo AL. State of the art of microwave treatment of wood: Literature review. Forests. 2021;12:745. doi: 10.3390/f12060745.
26. Hansson L. Microwave treatment of wood (Doctoral dissertation, Luleå tekniska universitet); 2007.
27. Resch H. High-frequency electric current for drying of wood-historical perspectives. Maderas. Ciencia y tecnología. 2006;8(2):67-82. doi: 10.4067/S0718-221X2006000200001.
28. Dominik H, PETR P, Jakub D, Jan B. Permeability and mechanical behavior of microwave pre-treated Norway spruce ripe wood. Wood Research. 2021;66(4):569-581. doi: 10.37763/wr.1336-4561/66.4.569581.
29. Hansson L, Antti AL. The effect of microwave drying on Norway spruce woods strength: A comparison with conventional drying. Journal of Materials Processing Technology. 2003;141(1):41-50. doi: 10.1016/S0924-0136(02)01102-0.
30. Sethy AK, Vinden P, Torgovnikov G, Militz H, Mai C, Kloeser L, Przewloka S. Catalytic acetylation of Pinus radiata (D. Don) with limited supply of acetic anhydride using conventional and microwave heating. Journal of Wood Chemistry and Technology. 2012;32(1):1-11. doi: 10.1080/02773813.2011.573121.
31. Balboni BM, Ozarska B, Garcia JN, Torgovnikov G. Microwave treatment of Eucalyptus macrorhyncha timber for reducing drying defects and its impact on physical and mechanical wood properties. European Journal of Wood and Wood Products. 2018;76(3):861-870. doi: 10.1007/s00107-017-1260-1.
32. Torgovnikov G, Vinden P. Microwave wood modification technology and its applications. Forest Products Journal. 2010;60(2):173-182. doi: 10.13073/0015-7473-60.2.173.
33. Afzal MT, Thomson FC. Moisture equalization of Kiln-wet Lumber using microwave heating. 2004 ASAE. 2004. doi: 10.13031/2013.16964.
34. Oloyede A, Groombridge P. The influence of microwave heating on the mechanical properties of wood. Journal of Materials Processing Technology. 2000;100(1-3):67-73. doi: 10.1016/S0924-0136(99)00454-9.
35. Li X, Zhang B, Li W, Li Y. Research on the effect of microwave pretreatment on moisture diffusion coefficient of wood. Wood Science and Technology. 2005;39(7):521-528. doi: 10.1007/s00226-005-0007-z.
36. Kollmann FF, Kuenzi EW, Stamm AJ. Principles of wood science and technology: II wood based materials. Springer Science & Business Media. 2012. doi: 10.1007/978-3-642-87931-9.
37. Abubakari A. Radio frequency heating pre-treatment of sub-alpine fir to improve kiln drying. 2010.
38. Vinden P, Romerio F, Torgovnikov G. Method for increasing permeability of wood. 2003.
39. Sandoval-Torres S, Jomaa W, Marc F, Puiggali JR. Causes of color changes in wood during drying. Forestry Studies in China. 2010;12(4):167-175. doi: 10.1007/s11632-010-0404-8.
40. Aitken L. Factors affecting microwave modified wood permeability and strength. 2013.
41. Anderson AB, Fearing WB, Wilke CR. Solvent drying of California redwood. Forest Products Journal. 1962;12(10):493-496.
42. Aggarwal PK, Chauhan SS. Microwave drying of planks of Grevillea robusta A. Cunn. ex R. Journal of the Indian Academy of Wood Science. 2011;8(2):84-88. doi: 10.1007/s13196-012-0050-y.
43. Brodie G. Microwave treatment accelerates solar timber drying. Transactions of the ASABE. 2007;50(2):389-396.
44. Kol HŞ, Çayır B. Increasing the impregnability of oriental spruce wood via microwave pretreatment. BioResources. 2021;16(2): 2513-2523.
45. Tascioglu C, Cooper P, Ung T. Effects of fixation temperature and environment on copper speciation in ACQ treated red pine. Holzforschung. 2008;62:289-293. doi: 10.1515/HF.2008.035.
46. Poonia PK, Tripathi S, Sihag K, Kumar S. Effect of microwave treatment on air permeability and preservative impregnation of Eucalyptus tereticornis wood. Journal of the Indian Academy of Wood Science. 2015;12(2):89-93.
47. Dashti H, Shahverdi M, Taghiyari HR, Salehpur S, Heshmati S. Effects of steaming and microwave pretreatments on mass transfer characteristics of Aleppe oak (Quercus infectoria). BioResources. 2012;7(3):3262-3273.
48. Vinden P, Torgovnikov G, Hann J. Microwave modification of radiata pine railway sleepers for preservative treatment. European Journal of Wood and Wood Products. 2011;69(2):271-279. doi: 10.1007/s00107-010-0428-8.
49. Cirule D, Kurnosova N, Vervkins A, Kuka E, Antons A, Andersone I, Andersons B. Penetration of wood preservatives into thermally modified birch and pine wood. Pro Ligno. 2018;14(4):23-30.
50. Poonia PK, Hom SK, Sihag K, Tripathi S. Effect of microwave treatment on longitudinal air permeability and preservative uptake characteristics of chir pine wood. Maderas. Ciencia Y Tecnología. 2016;18(1):125-132. doi: 10.4067/S0718-221X2016005000013.
51. Poonia PK, Tripathi S. Effect of microwave heating on pH and durability of Eucalyptus tereticornis wood. Journal of Tropical Forest Science. 2017;389-394. doi: 10.26525/jtfs2017.29.3.389394.
52. Samani A, Ganguly S, Kanyal R, Tripathi S. Effect of microwave pre-treatment on preservative retention and treatability of melia composite wood. Journal of Forest Science. 2019;65(10):391-396. doi: 10.17221/39/2019-JFS.
53. Chang HT, Chang ST. Improvements in dimensional stability and lightfastness of wood by butyrylation using microwave heating. Journal of Wood Science. 2003;49(5):455-460. doi: 10.1007/s10086-002-0504-8.
54. Avramidis S, Ruddick JN. CCA accelerated fixation by dielectric heating. Forest Products Journal. 1996;46(7/8):52-55.
55. Yu LL, Gao W, Cao JZ, Tang ZZ. Effects of microwave post-treatments on leaching resistance of ACQ-D treated Chinese fir. Forestry Studies in China. 2010;12(1):1-8. doi: 10.1007/s11632-010-0008-3.
56. Mascarenhas FJR, Pereira Geraldes Dias AM, Christoforo AL, Dos Santos Simões RM, Palos Cunha AE. Effect of microwave treatment on drying and water impregnability of Pinus pinaster and eucalyptus globulus. Maderas. Ciencia Y Tecnología. 2023;25. doi: 10.4067/s0718-221x2023000100406.
57. Poonia PK, Tripathi S. Effect of microwave treatment on permeability of Populus deltoides bartr. Wood. Indian Forester. 2015;141(5):528-532. doi: 10.36808/if/2015/v141i5/70232.
58. Ramezanpour M, Tarmian A, Taghiyari HR. Improving impregnation properties of fire wood to Acid Copper Chromate (ACC) with microwave pre-treatment. iForest-Biogeosciences and Forestry. 2015;8(1):89. doi: 10.3832/ifor1119-007.