WO1997023328A1

WO1997023328A1 - Method and apparatus for densifying ligno-cellulosic material

Info

Publication number: WO1997023328A1
Application number: PCT/US1996/020407
Authority: WO
Inventors: Micail N. Raskin; Vladimir V. Zlatev; Yuriy J. Iossel; Azariy Z. Pukis; Yevgueniy N. Petrov
Original assignee: Cellutech, L.L.C.
Priority date: 1995-12-22
Filing date: 1996-12-20
Publication date: 1997-07-03
Also published as: AU1686697A; CA2241254A1

Abstract

A method for changing the physical structure of a ligno-cellulosic work piece having a selected grain and a first structural configuration. The first structural configuration preferably includes those structural and physical properties, including density and hardness, that are associated with an unplasticized ligno-cellulosic product. The method includes appropriate stucture for subjecting the ligno-cellulosic workpiece to electromagnetic radiation in the range of about 5 MHz and about 3 GHz to plasticize, e.g., soften, the lignin component of the material. The plasticized workpiece is then compressed along an axis transverse to the grain of the workpiece and stabilizes the workpiece by allowing said softened lignin components to bond together to attain a second structural configuration. The workpiece is compressed only when the moisture content is 30 % or less.

Description

METHOD AND APPARATUS FOR DENSIFYING LIGNO-CELLULOSIC MATERIAL

Background of the Invention This invention relates generally to the treatment of ligno-cellulosic material, and more particularly to the process and associated apparatus for densifying ligno-cellulosic material. As used herein, the term ligno-cellulosic material is intended to include logs, lumber, wood particles, wood chips, wood flakes, wood wafers, wood fibers, wood veneer and other wood products and parts thereof, as well as other lignin and cellulose containing matter, such as woody plants, foliage, roots, shells, pot, nuts, husks, fibers, straw, vine, grass, bamboo, and reeds.

Prior art methods of treating ligno-cellulosic material, such as wood, are known and exist. Prior art methods for treating ligno-cellulosic material include heating/drying techniques for removing moisture and impregnating techniques for impregnating the material with one or more chemical agents. Conventional heating/drying techniques include air drying and kiln drying. The kiln drying process employs known structure, e.g., a kiln, to effect high temperature drying of the wood. A drawback of kiln drying is that it generally requires upwards of six days for most wood types to reduce the water content thereof to within acceptable limits, e.g., less than 30% moisture. Conventional air drying processes involve the prolonged storage of wood for extensive periods of time, such as between three months and three years, to reduce the moisture content. Thicker articles of wood require even longer periods of time to dry, since the drying period typically increases with the square ofthe thickness.

In addition to the extensive times required for present air drying and kiln drying techniques to heat/dry, such techniques result in a significant amount of wood degradation. This damage includes splitting and cracking caused by the destructive temperature and moisture gradients formed within the wood during the drying process, as well as splitting and staining that arises as a result of exposure to the environment. Prior methods for accelerating the heating/drying process include thermodrying processes, such as the use of hot platen plates and high temperature steam, and electromagnetic energy. One prior technique includes employing hot platen plates to heat the wood. The wood is placed between the plates by any conventional means and the plates are placed in contact with the outer stratum of the wood. Since wood is a poor thermal conductor, the heat from the plates initially heats the outer portions ofthe material and then heats the inner portions. A drawback of this technique is that it requires considerable amounts of time and energy to heat the inner portions ofthe material. Additionally, if the applied energy is increased beyond a selected level, scorching or charring ofthe outer surface ofthe material occurs.

The use ofhigh temperature steam to heat the ligno-cellulosic material is accomplished by placing the material in a steam chamber and exposing the material to steam having elevated temperatures, typically in the range between about 120°C and about 220°C. Drawbacks of this technique include the amount of time necessary to heat the wood, the amount of energy consumed by the heating process, the quantity of moisture that is added to the material, the cost ofthe heating equipment, the risk of injury to personnel from the use ofhigh temperature steam, and the creation of destructive temperature and moisture gradients.

Other techniques include the use ofhigh frequency or microwave energy to heat the ligno-cellulosic material. According to one prior practice, a waveguide delivers microwave energy generated by an appropriate microwave source to the cellulosic material. When the material is irradiated by the microwave energy, the energy vaporizes pockets of moisture present in the wood, which in turn heats the surrounding material. One example of this practice is shown and described in U.S. Patent No. 4,469,156 of Norimoto et al., where the wood material is heated to temperatures in excess of 100° C. Furthermore, the uneven moisture dispersement throughout the wood results in uneven heating ofthe wood upon the application of microwave energy. This uneven heating can result in cracking and other unwanted breaches in the integrity ofthe material. Other problems include those associated with the use of microwave energy, such as the creation of standing waves. The structure used to apply the microwave energy to the wood is also expensive and unsuitable to process large-sized commercial loads. The foregoing techniques are typically used in systems that manufacture plywood, particle board and other formed parts composed of cellulosic particulate matter. In these systems, the cellulosic particles are mixed with an adhesive and the admixture is heated to cure the adhesive and to attain a selected level of pliability for pressing. Subsequently, the heated mixture is compressed to form the final product. This technique is well known in the art as disclosed in U.S. Patent No. 4,933,125 of Reiniger, U.S.

Patent No. 4,517,147 of Taylor et ai, U.S. Patent No. 5,154,968 of DePetris et ai, and U.S. Patent No. 4,726,881 of Schultz. All of these systems suffer from the above¬ mentioned drawbacks.

Prior compressing techniques can also be used in conjunction with microwave heating to pre-treat the wood prior to conventional bending and shaping. According to these techniques, the ligno-cellulosic material, e.g., wood, is heated in excess of 100°C and is then compressed axially along the length ofthe wood fibers, i.e., compressed along the grain, while suitable structure prevents deflection of the wood in a direction transverse to the direction of compression. The pressure is then released and the wood expands to about its original length. This compression and subsequent expansion ofthe wood creates axial folds in the wood fibers, thus rendering the wood suitable for bending and shaping.

The application of electromagnetic fields, e.g., RF or microwave, are also used to soften wood. This wood pretreatment transforms the wood into a malleable medium, which can be subsequently shaped. One such method of softening is described in U.S. Patent No. 5,555,642 of Rem et al. Rem describes a process for upgrading low quality wood. The process includes first providing relatively wet wood, e.g., wood having a moisture content between 67% and 150% on dry weight, which is ohmically heated at a temperature between 140° C and 240° C for a selected period of time. The softened wood is then subjected to a two-stage drying process, where it is simultaneously heated and compressed at pressures ranging between 1 bar and about 20 bar. The wood is then heated and pressed again during a second heating stage in order to compress the wood to a smaller volume. During this compression, the applied pressures vary between about 0.1 bar and about 10 bar.

A drawback ofthe foregoing system is that it requires multiple steps and procedures to ensure softening and drying ofthe wood article. Additionally, the multi¬ step process employs low-yield and relatively expensive equipment to perform the multiple processes. This equipment is also expensive to operate and maintain, and the process consumes relatively large amounts of energy.

Due to the foregoing and other shortcomings ofthe foregoing prior art technologies, an object of this invention is to provide a novel system and method for treating and compressing wood to attain relatively dense wood products.

Another object ofthe invention is to provide a system and method for manufacturing relatively dense wood products from relatively non-dense wood types, including 'scrap' wood. Still another object ofthe invention is to provide a relatively inexpensive system and method for producing dense wood products.

Other general and more specific objects ofthe invention will in part be obvious and will in part appear from the drawings and description which follow. Summary of the Invention

The present invention attains the foregoing and other objects ofthe invention by providing a process for changing the physical structure of a ligno-cellulosic work piece having a selected grain and a first structural configuration. The first structural configuration preferably includes those structural and physical properties, including density and hardness, that are associated with an unplasticized ligno-cellulosic product. The method includes the steps of subjecting the ligno-cellulosic workpiece to electromagnetic radiation, e.g., either a high frequency electric field or microwave energy, in the range between about 5 MHz and about 3 GHz to plasticize, e.g., soften, the lignin, hemicellulose and other components ofthe material. Structure suitable for subjecting the workpiece to the radiation includes a pair of electrodes and a high frequency generator. A compressing element compresses the plasticized work piece along an axis transverse to the grain ofthe work piece to attain a second selected structural configuration, e.g., increased hardness or density. According to one practice, the workpiece is only compressed when the moisture content is about at the fiber saturation level or less, e.g., at about 20-30% or less. The compressing ofthe dried ligno- cellulosic material decreases the time necessary to plasticize the lignin, as well as decreases the amount of time necessary to stabilize the material at the second structural configuration. The compressing element further stabilizes the work piece by maintaining a selected degree of compression to attain this second structural configuration. The second structural configuration preferably includes the polymerization or polycondensation process that the softened lignin and hemicellulose components ofthe work-piece undergo (cross-linking and bonding ofthe components) when placed in relatively close proximity to each other for a sufficient period of time. According to one practice, the second structural configuration ofthe work piece exhibits an increase in density and hardness that is substantially greater than the density/hardness associated with the first structural configuration. More specifically, the compressed work piece has a second density value that is substantially greater than the density value associated with the original work piece.

According to one aspect ofthe invention, the work piece includes a plurality of adjacent plasticized lignin components that bond together, when compressed, according to a polycondensation process to attain the second structural configuration. According to still another aspect, the compressing element, when stabilizing the work piece, prevents substantial spring-back ofthe compressed work piece to at least close to its initial cross-sectional dimension. According to still another aspect, the electromagnetic radiation is either high frequency radiation, e.g., between about 5 MHz and about 300 MHz, or microwave radiation, e.g., between about 300 MHz and about 3 GHz. the workpiece is exposed to this radiation for a time that is in the range between about 0.5 min and about 5 min. According to one practice, exposing the workpiece to the radiation heats the workpiece, and particularly at its core, to a temperature between about 60° C and about 140° C.

According to another aspect, the workpiece is stabilized by maintaining the compression ofthe workpiece. The pressure applied by the compressing element is, according to one practice, greater than or significantly greater than 20 bars, and according to another practice, is in the range between about 25 bars and about 250 bars, preferably between about 70 bars and about 250 bars, and more particularly between about 100 bars and about 200 bars.

According to still another aspect, the workpiece is compressed without the application of additional heat, such as from any conventional heat source or from the heating ofthe platen plates of a compressing machine. The ability to plasticize and compress the lignin components of a dry ligno-cellulosic workpiece to attain a workpiece having improved density and hardness characteristics is a significant feature ofthe present invention. The present method further provides for a cost-effective and relatively simple method for densifying ligno-cellulosic material, without employing burdensome multi-step processes and energy consuming equipment.

According to another aspect, the step of compressing for stabilizing the work piece further includes the step of maintaining the work piece in a selected condition of compression for attaining said stabilization.

The invention also includes a densified ligno-cellulosic work piece having a selected grain, a first cross-sectional dimension, and an associated first density value. The work piece further includes a main body having a cellulose and lignin component, such that the lignin component is plasticized by applying to the work piece an alternating electromagnetic field having a frequency between about 5 MHz and about 3 GHz from a suitable power source. The plasticized work piece is compressed along an axis transverse to the grain and stabilized, such that the compressed work piece has a second cross- sectional dimension smaller than the first cross-sectional dimension and a second density value substantially greater than the first density value. According to one practice, the workpiece is only compressed and stabilized when the moisture content thereof is about 30% or less. Other general and more specific objects ofthe invention will in part be obvious and will in part be evident from the drawings and description which follow. Brief Description of the Drawings

The foregoing and other objects, features and advantages ofthe invention will be apparent from the following description and apparent from the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings illustrate principles ofthe invention and, although not to scale, show relative dimensions.

FIGURE 1 is a perspective view of one embodiment of a ligno-cellulosic material treatment assembly according to the invention. FIGURE 2 A is a side view of a two-dimensional pressing machine suitable for use in the practice ofthe present invention having an uncompressed, plasticized work piece loaded therein.

FIGURE 2B is a side view of the two-dimensional pressing machine of

FIGURE 2A showing the wood article during compression.

Description of Illustrated Embodiments

The present invention provides for a system and method for increasing the density and/or hardness of ligno-cellulosic material. The present invention is particularly suitable for converting lightwood into composites having markedly increased mechanical and resistive properties. Examples of trees yielding such lightwood material include but are not limited to spruce, poplar, willow, beech, pine, rubber tree, basswood, alder and eucalyptus.

FIGURE 1 illustrates the ligno-cellulosic treatment system 10 of the present invention. For the sake of simplicity, the term wood will be used herein in place ofthe term ligno-cellulosic material, and is not to be construed in a limiting sense. The illustrated system 10 includes a wood drying stage 12, a wood plasticizing stage 16, and a compressing and stabilizing stage 18. Other permutations ofthe above processing sequence are apparent to those of ordinary skill in the art.

The wood drying stage 12 ofthe invention prepares the wood for further treatment by drying the wood and by homogenizing the moisture content in the wood.

The wood drying stage 12 includes a housing 24 having side walls 26, 28, and 30, a floor portion 32, a top portion 34, and a door (not shown). A set of spaced apart and substantially parallel plates or electrodes 36A, 36B, and 36C are seated within the chamber 38 formed by the housing 24. The electrodes can be coated with a thin dielectric film, such as polyethylene, to maintain the uniformity ofthe electric field created therebetween and to prevent unwanted airing and other ionization effects during the drying operation. Appropriate electrical conductors connect the electrodes to the output terminals of a generator 40 that is electrically coupled to the housing 24. A movable support stand or carriage 42 seats within the chamber 38 to support one or more wood work pieces W between two ofthe electrodes 36A-36C. The electrodes thus form the terminal ends of a capacitor, and the wood articles disposed therebetween form a portion of the capacitor dielectric.

The illustrated drying stage 12 further includes a diffuser (not shown), preferably disposed between the generator 40 and the electrodes 36A-36C, to help diffuse power between the individual electrodes 36A-36C. An exhaust fan 44 coupled to the top portion 34 ofthe housing and a second fan 46 coupled to the side wall 28 control the internal temperature ofthe housing by circulating air through the chamber 38.

The illustrated drying stage 12 dries the wood W, as well as homogenizes the moisture content therein. This homogenization process is effected as follows. The wood work piece W typically contains large pockets of moisture that are randomly dispersed throughout the wood structure, as well as other smaller moisture pockets. The size and number of moisture pockets is a function ofthe total moisture content ofthe wood, the ambient pressure and temperature, and other environmental factors. Typically the moisture pockets are located towards the center of the work piece since wood dries from the outside towards the center ofthe work piece. When the wood article is placed between two of the electrodes 36A-36C and is exposed to a high frequency electric field, the field penetrates deep into the wood and transfers energy into the internal or interior portions of wood, and specifically to the water contained in the moisture pockets.

The electromagnetic field absorbed by the wood, and particularly by the water, causes molecules, and particularly those with a relatively high electric dipole moment such as water molecules, to vibrate and to rotate back and forth as a result ofthe torque placed thereon by the field. The mechanical energy created by this dipole rotation and/or molecular vibration is transferred to the surrounding wood structure as internal energy or heat. Adjacent wood structure containing lower levels of moisture absorb less energy and thus generate less heat. The difference in heat absoφtion between adjacent locations effectively creates a thermal gradient within the wood, causing heat from high temperature areas to diffuse to lower temperature areas.

The thermal gradient created by the absorbed high frequency energy also causes water to migrate out ofthe high moisture regions and into surrounding regions containing lower moisture. By way of example, the electromagnetic energy supplied by the generator 40 at a selected frequency initially, selectively targets the water within the wood without unnecessarily heating adjacent wood structure. By maintaining the adjacent wood structure below a selected temperature, the capillaries and pores ofthe wood remain open. The selective heating ofthe water in the larger moisture pockets vaporizes a portion ofthe water, creating water vapor. Since the surrounding cellulosic wood structure is at least semi-permeable to the resultant water vapor and is heated to a lesser extent because of its lower water content, internal pressure differences cause the water to migrate into the open capillaries and pores ofthe adjacent wood structure. This resultant process uniformly disperses the water throughout the wood during drying. This selected water migration within the wood structure creates a work piece having a homogeneous moisture content. The homogenization of the moisture content in the wood provides for uniform heating ofthe wood article during the drying stage 12. This occurs since the homogenized wood absorbs similar levels of energy throughout nearly the entire work piece, thus providing for relatively even and uniform drying ofthe wood article.

The quantity of heat generated within and removed from the wood during the drying stage 12 can be quantitated and is known as the specific absorption of wood , e.g., the heat power created per unit volume of medium, and can be determined by the following formula:

P₀ = 2 π ε ε₀ f E^ tg δ

where P₀ is the specific heat absoφtion of wood, f is the frequency ofthe applied field, ε ₀ is the absolute dielectric constant, ε is the dielectric permeability of wood, E is the intensity ofthe electric field, and tg δ is the tangent ofthe dielectric loss angle of the wood. Both ε and tg δ depend on the moisture content ofthe wood, and both increase in value when the moisture content ofthe wood increases. Consequently, those wood portions containing the most moisture, e.g., the large moisture pockets, experience more intensive heating than surrounding areas containing less moisture. Because of this phenomena, the core portions ofthe wood experience more intense heating and thus are disposed, at least initially, at a higher pressure. The heat absoφtion of wood P₀ is also directly proportional to the frequency and the intensity ofthe applied electric field. The degree of heating uniformity thus depends upon the frequency and voltage ofthe electric field. For example, the higher the frequency, the greater the amount of heat generated in the wood structure, and thus the greater the heat absoφtion. Additionally, the frequency can be selected or varied to provide for optimum heating ofthe wood to attain a homogeneous moisture content by initiating thermo-osmotic processes in the work piece. According to one practice, the generator 40 produces an electric field between the electrodes 36A-36C having a frequency in the range between about 5 MHz and about 300 MHz. The system 10 can also include a control system having dedicated hardware with resident software that controls the intensity ofthe electric field applied by the generator 40 to the chamber 38 based upon a number of specific parameters, including the temperature ofthe wood, the moisture content ofthe wood, the frequency produced by the generator, the size and volume ofthe wood, and the type of wood used. Thus, the control system can automatically vary the intensity of the electric field based upon one or more of the foregoing parameters during the drying process to dry effectively and uniformly the wood work piece.

According to a preferred practice, the generator 40 produces an electric field between electrodes 36A-36C in the range between about 5MHz and about 300MHz, and preferably less than about 27 MHz, when the moisture content in the wood exceeds 30%. Conversely, when the moisture content is below 30%, the generator 40 produces an electric field at about 27 MHz, or higher. The generator, as controlled by the dedicated hardware and software, preferably applies different frequencies and voltage to the wood to optimize the drying process. The software can be constructed in accordance with principles known to those of ordinary skill in software design to instruct the dedicated hardware to control the generator 40 and other components ofthe system 10 as a function of one or more of the foregoing parameters.

The generator 40 is preferably a high frequency generator that generates a frequency in the foregoing range, and supplies a voltage to the electrodes 36A-36C in the range between about 1KV and about 15KV.

The illustrated wood drying stage 12 preferably dries the wood to the fiber saturation level or less, typically between 20-30% moisture content or less prior to transfer to the plasticizing stage 16. The moisture content of wood is defined as the weight of water in wood given as a percentage of ovendry weight. As is known, moisture (e.g., water) exists in wood either as bound water that is held in the cell walls, or as free water that is located within the cell cavity or volume. Hence, the moisture content at which the cell walls are still saturated but virtually no water exists in the cell cavities is called the fiber saturation point. This utility level of dryness allows fast and cost-efficient plasticizing, e.g. softening, ofthe lignin during the plasticizing stage 16.

According to another practice, one of the sidewalls of the housing 24 can mount a microwave generator and a waveguide, which are adapted to deliver microwave energy to the interior of the housing 24. The microwave generator preferably generates an electromagnetic field having a frequency between about 300 MHz and about 3 GHz. The foregoing moisture level can also be achieved by first dehydrating, e.g., dewatering, the naturally occurring or impregnated ligno-cellulosic material through an electro-osmotic mechanism prior to drying. The electro-osmotic mechanism or unit can include a DC source that applies a potential across the ligno-cellulosic material 51, which is placed within a selected solution. A first electrode can be placed within the solution, and a second selected electrode can contact an upper end ofthe material. In order to make contact with substantially an entire cross-section of one end ofthe ligno- cellulosic material, the second electrode can be a wire gauze electrode. The DC source generates a positive potential across one electrode and a negative potential across the other electrode. The electro-osmotic unit draws moisture out ofthe ligno-cellulosic material and replaces it, if desired, with some ofthe solution. Following treatment with the electro-osmosis unit, the material is dried through any conventional means, such as by the above described high frequency and microwave units, or by other means, such as kiln drying, vacuum drying, infrared drying, or by using adsorbents and absorbents, or by any combination of these methods.

The wood can be impregnated with raw materials dissolved in a suitable solvent, such as an organic solvent, water or a mixture thereof. The raw materials can include natural latexes, resins, fats, waxes, lignin and cellulose products and derivatives and furfurol. These materials impart additional desired properties to the wood. As set forth above, an electro-osmosis unit can be used as an initial dewatering mechanism. When used in this way, the source can deliver to the electrodes a constant voltage in the range between about 100 V/cm and about 150 V/cm. The unit expends during operation between about 0.2 and 0.3 kWh to remove approximately 1 kg of moisture. This is about three times more efficient than the above-described drying process, which requires much higher expenditures of energy for water removal.

A significant advantage ofthe drying stage of the invention is the ability to heat and to dry uniformly the wood work piece by selectively targeting areas having high moisture content. The areas can be targeted by exposing the work piece to electromagnetic energy having a selected frequency depending upon the moisture content ofthe wood. This selective targeting homogenizes the moisture content in the wood to effect uniform heating, and thus drying. The even heating ofthe wood greatly diminishes the mechanical stresses and strains which result from uneven heating/drying ofthe wood. This in turn reduces or even eliminates the occurrence of fiber ruptures which manifest as cracks, thus preserving the integrity ofthe wood article during heating, and thereby greatly reducing the occurrence of waste product. Additionally, the ability to heat the wood uniformly decreases the overall drying time from two weeks to about ten to twenty - 1 1 -

hours, as a function of moisture content, thickness and wood species, thus increasing the overall efficiency of the illustrated system. Those of ordinary skill will appreciate that the specific amount of time required to properly dry the wood is a function ofthe type of wood used, the size ofthe wood, the moisture content ofthe wood, and the frequency applied by the generator 40, as well as other parameters obvious to those of ordinary skill. Referring again to FIGURE 1, the dried wood article can be transferred to the plasticizing stage 16 along conveyor rollers 52 by the dedicated control system. The plasticizing stage 16 includes a pair of electrodes 54A and 54B which communicate electrically with a second high frequency generator 56. The electrodes and generator are similar in construction to those described above in relation to the drying stage 12. The wood can be automatically positioned by any suitable means between the electrodes 54A and 54B, where the wood is exposed to a high frequency electric field. The electric field heats the wood to plasticize the components ofthe wood. The plasticizing stage can also employ, instead ofthe high frequency generator, a microwave generator to produce microwave energy. The frequency ofthe generators is preferably in the range between about 5 MHz and about 3 GHz.

Wood is a vascular material that is composed of, among other things, elongated cells having cell walls which surround an inner cell cavity. The cell wall is composed of a fibrous cellulose armature. The armature is typically a long chain polymer that comprises a plurality of linked monomers, e.g., up to 10,000, bound together by amoφhous lignin and hemicellulose. Hemicellulose and lignin are smaller chained polymers, and thus have smaller molecular weights.

Lignin is a complex aromatic compound that contains methoxylated and nonmethoxylated phenyl propane chains, which are connected together by various types of bonds. The chemical structure of lignin varies between plant species, but it is believed that, in general, the monomeric unit (or units) of lignin includes a substituted styryl functionality, i.e., a substituted vinyl benzene unit, such as coniferyl alcohol, p-coumaryl alcohol, and sinapyl alcohol (see, e.g., S. Budavari, ed. (1989) "The Merck Index", 11th edition, Merck & Co., Rahway, New Jersey, p. 864, and references cited therein). It is presently understood that exposing wood to electromagnetic radiation plasticizes the lignin and hemicellulose components. Additionally, the quantity of moisture present in the wood at the time of heating affects the transformation of lignin into a viscous condition, i.e., plasticized state, and specifically affects the temperature at which this occurs. Hence, it is desirable to treat wood in this stage having a moisture content of about 30% or less. The temperature at which lignin plasticizes according to the principles of the present invention is defined as that temperature that causes destabilization ofthe lignin structure such that polymerization or polycondensation occurs between adjacent lignin components. The type of wood used and the moisture content affects this temperature, which is in the range between about 60°C and about 140°C. Lignin at temperatures within this temperature range diffuse and exhibit increased mobility and tackiness. When disposed in this plasticized state, the lignin is inclined to autoadhesion, e.g., cross-linking and bonding to adjacent lignin components. It is believed that bonding between lignin units is, at least in part, the result of free radical polymerization of lignin monomers, e.g., free radical polymerization of vinyl functionalities, to form new covalent bonds. Analogous free radical polymerization processes are well known in the art, e.g., in the polymerization of styrene to form polystyrene. When wood is treated according to the methods ofthe invention, the formation of free radicals in the wood has been detected by electron paramagnetic resonance (EPR) techniques. The skilled artisan will appreciate that other modes of bond formation, both covalent and noncovalent, may be operative. It is believed that bonding of adjacent lignin components results in an increase in the hardness ofthe wood work piece, as a result of the formation of cross-links between adjacent lignin components that stabilize the structure ofthe wood to form a strong, hard and stable work piece.

It is believed that the lignin components do not bind together until the core ofthe wood attains a selected temperature within the same range of temperatures as defined above, e.g., between about 60°C and 140°C. Consequently, heating the wood to within this range results in softening (plasticizing) ofthe lignin, as well as bonding and cross-linking between adjacent lignin components.

The high frequency or microwave treatment ofthe dried ligno-cellulosic material serves to heat and eventually soften the lignin components ofthe material. Since the treatment heats the interior portion ofthe material first, creating positive (from the interior outwards) temperature gradients, the outer surfaces or regions remain at a lower temperature, and even after treatment, can be manipulated by hand.

The viscous condition ofthe lignin-hemicellulose matrix created by temperatures in the softening temperature range is reversible. Specifically, the plasticized lignin cellular matrix re-solidifies upon cessation of temperatures in this range.

The heating of relatively dry wood, e.g., wood having a moisture content less than about 30%, requires the application of greater amounts of energy in order to extract the remaining moisture in the wood and thus to plasticize the wood components. This occurs since much ofthe bound moisture is believed to be trapped by hydrogen bonding ofthe water molecules to the hydroxyl groups ofthe lignin components, and because this water is trapped within the cell walls ofthe ligno-cellulosic material. The additional energy absorbed by the wood breaks these bonds and removes the water from the wood. In operation, the wood is exposed to the high frequency field generated by generator 56 for a selected period of time, such as between about 0.5 min and about 5.0 min. Those of ordinary skill will recognize that the selected time period can differ according to wood type, moisture content, and volume of wood. According to another practice, the power supplied by the generator 56 can be varied during the plasticizing process in accordance with the exigencies ofthe particular situation. For example, the amplitude ofthe voltage can be automatically varied by the control system based upon a number of specific parameters, including the temperature ofthe wood, the moisture content ofthe wood, the frequency produced by the generator, the size and volume ofthe wood, and the type of wood used, in a range of about 30% during the plasticizing stage 16 to attain the appropriate softening and connecting temperatures.

Those of ordinary skill will appreciate that the drying stage 12 can also be employed to plasticize the wood, rather than providing a separate plasticizing stage 16. The advantage of this approach is the utilization of a single apparatus for drying the wood to a suitable moisture content, e.g., 30% or less, and for plasticizing the wood to promote autoadhesion of adjacent lignin components.

A significant advantage ofthe plasticizing stage ofthe invention is that the internal temperatures ofthe wood are in the range between about 60°C and about 140°C, and preferably between about 60°C and about 120°C, and are sufficient to heat the wood to plasticize the lignin components. This is in contrast to prior art methods, including steam heating, which require the wood to be heated to temperatures in excess of 140°C. The ability to plasticize wood at lower temperatures provides for a relatively efficient wood heating/plasticizing system that consumes less energy without compromising the integrity ofthe wood, e.g., cracking the wood. Furthermore, the relatively short time necessary to heat the wood further reduces processing time and labor cost.

The plasticizing ofthe lignin components ofthe wood article prepares the wood for the compressing stage 18. Referring to FIGURES 1, 2A and 2B, conveyor rollers 58 can transfer automatically the plasticized wood to the compressing stage 18. The illustrated compressing stage 18 includes a hydraulic pressing machine 60 that can include a pair of matched upper and lower dies 62 and 64. The dies are preferably in registration with one another and are hydraulically coupled to a hydraulic press 66. A power supply 68 supplies the operating power to the press 60. The plasticized wood article W can be positioned between the matched dies by any suitable method, such as by hand or by known automated assembly techniques operated by the control system ofthe invention. Those of ordinary skill will recognize that any conventional pressing machine can be used in the practice ofthe present invention. Additionally, although a two- dimensional press is shown, those of ordinary skill will recognize that other types of pressing machines can be employed, including one-dimensional presses. Prior to compression, the wood article W has a first cross-sectional dimension D 1 , a first hardness and density, and a first anatomical structure consistent with plasticized lignin. The term "density" as used herein is meant to include the weight ofthe ligno-cellulosic material per unit volume, which is expressed according to the Cl system in gm/cn This term is further meant to include other definitions of density that are standard or in common usage in the fields of forestry and wood, such as volume- weight. Hydraulic pressure is then applied to the top die 62 by way of suitable hydraulic conduits by the hydraulic press 66. When the hydraulic source is actuated, the top die 62, which is coupled to the press, moves towards the bottom die 64. The dies 62, 64 capture the wood article W therebetween and compresses the wood W to a smaller cross-sectional dimension D2, Figure 2B. The plasticized wood is preferably compressed in a direction transverse to the grain ofthe wood. The compression of the wood can be controlled by controlling the hydraulic pressure applied to the wood work piece.

The compression ofthe plasticized wood reduces the voluminous mass and intercellular space ofthe wood. This reduction in wood air mass compresses together the microstructure ofthe wood, allowing adjacent plasticized lignin to bond together. If the lignin is disposed adjacent other lignin components for a sufficient period of time, the strands bond together. The bonding ofthe adjacent lignin components forms a three- dimensional, stable and strong wood structure where the different portions ofthe lignin bond to adjacent lignin components to form this stable structure. The wood is preferably compressed substantially immediately after being plasticized by the plasticizing stage 16. The compressing machine 60 compresses the wood at a pressure greater than or significantly greater than 20 bars, and according to another practice, is in the range between about 25 bars and about 250 bars, preferably between about 70 bars and about 250 bars, and more particularly between about 100 bars and about 200 bars. Those of ordinary skill will recognize that even higher pressures can be used, but must be controlled so as not to destroy the wood. Lower pressures can be used for prolonged stabilization, where time and throughput are not the systems primary concern. The amount of pressure (compressing force) supplied by the compressing machine depends upon the wood type, the internal temperature ofthe wood (plasticizing temperature), the moisture content, and the final specifications ofthe wood product. According to one practice, the compression time is in the range between about 1 sec and 5 minutes.

The compressed wood can be stabilized for a sufficient period of time to allow polymerization/polycondensation to occur by restricting or preventing the spring- back ofthe wood. This can be accomplished by any suitable device designed to maintain the compressed wood in the reduced cross-sectional shape by maintaining any suitable pressure that prevents spring-back ofthe wood at or close to the original diameter DI . Thus, stabilization ofthe wood work piece decreases the elastic deformation ofthe wood to attain a stable, densified and hardened work piece. The wood is preferably stabilized for between about 10 min and about 60 min. This time range depends upon the type of wood and the desired hardness and density of the resultant stabilized wood work piece. The illustrated pressing machine or any other suitable constraining structure, such as press rollers, can be used to stabilize the compressed wood article. The compressing machine can utilize cold or hot plates during the compressing stage. According to a preferred practice, the compressing machine 60 also stabilizes the wood product, and the machine utilizes cold plates to reduce the amount of energy consumed by the overall process.

Those of ordianry skill will recognize that the pressure imparted by the compressing machine 60 during the compression stage can be different from the pressures imparted during the stabilization stage.

Different wood types typically require different pressures to compress the wood, and preferably presses capable of achieving pressures of between 25 bars and about 250 bars and even higher are used. The one and two dimensional presses can be used to form conventional shapes, such as square and rectangular cross-sections. However, modified dies can be used to create relatively complex shapes from the plasticized wood. The stabilized and compressed work piece ofthe invention preferably has an increased hardness and density, as well as a decreased diameter D2. The specific hardness and density ofthe resultant compressed work piece can be selected by subjecting the wood work piece to a selected compression at a selected pressure for a selected period of time, preferably in excess ofthe time required to stabilize the work piece.

The practice ofthe present invention can produce stable wood having increased hardness and density. The following non-restrictive examples exemplify these features. Examples

The longitudinal and end grain hardness were measured using the known and conventional Brinell Hardness test and the resultant hardness readings are defined in Brinell units (BHN).

Example 1

A piece of Douglas fir wood having a longitudinal hardness of 1.0 BHN and an end grain hardness of 2.95 BHN was plasticized, compressed and stabilized according to the foregoing systems and methods. The resultant compressed wood product had a longitudinal hardness of 3.44 BHN and an end grain hardness of 7.09 BHN. Consequently, it is apparent that the foregoing systems and methods increase at least the hardness of the wood work piece.

Example 2 A piece of pine wood having a longitudinal hardness of 1.28 BHN and an end grain hardness of 4.17 BHN was plasticized, compressed and stabilized according to the foregoing systems and methods. The resultant compressed wood product had a longitudinal hardness of 3.88 BHN and an end grain hardness of 6.78 BHN. Consequently, it is apparent that the foregoing systems and methods increase at least the hardness of the wood work piece.

Example 3

A piece of alder wood having a longitudinal hardness of 1.49 BHN and an end grain hardness of 4.21 BHN was plasticized, compressed and stabilized according to the foregoing systems and methods. The resultant wood product had a longitudinal hardness of 2.42 BHN and an end grain hardness of 6.92 BHN. Consequently, it is apparent that the foregoing systems and methods increase at least the hardness of the wood work piece.

Example 4

A piece of aspen wood having a longitudinal hardness of 1.08 BHN and an end grain hardness of 3.8 BHN was plasticized, compressed and stabilized according to the foregoing systems and methods. The resultant compressed wood product had a longitudinal hardness of 2.47 BHN and an end grain hardness of 6.59 BHN. Consequently, it is apparent that the foregoing systems and methods increase at least the hardness ofthe wood work piece. Example 5

A piece of birch wood having a longitudinal hardness of 1.36 BHN and an end grain hardness of 4.72 BHN was plasticized, compressed and stabilized according to the foregoing systems and methods. The resultant compressed wood product had a longitudinal hardness of 2.20 BHN and an end grain hardness of 6.27 BHN.

Consequently, it is apparent that the foregoing systems and methods increase at least the hardness ofthe wood work piece.

The wood article, treated and processed as described above, exhibits a number of desirable properties, including increased fire-resistance and strength, increased resistance to environmental and biological degradation from fungi, termites, and other detrimental organic and inorganic forces, the achievement of odd shape stabilization, and an improvement in the plastic and elastic properties of wood.

A further advantage ofthe present invention is that relatively soft wood and 'scrap' wood can be used to form hardened wood product having higher hardness and density characteristics. Furthermore, the process can be used to remediate damaged, e.g., cracked, wood by reducing the size ofthe fiber ruptures.

Example 6

Eight (8) pieces of Appalachian Poplar measuring 3^lA in. x 4 in. x 12 in. and having a moisture content of under 6% were placed between electrodes spaced VA inch apart, and exposed to an electromagnetic field of 27 MHz, 5.4 kV, and 0.8 ampere for a period of 300 seconds. The ends ofthe pieces attained a temperature of between about 140°F and 150°F.

Five ofthe heated pieces were then compressed on all four faces peφendicular to the longitudinal axis (grain) at 3000 psi (210 bar) pressure for 15 seconds. The final dimension ofthe pieces was approximately 2-3/8 in. x 3 in. x 12 in. Three other of the heated pieces were compressed on all four faces peφendicular to the longitudinal axis (grain) at 2800 psi (200 bar) pressure for 15 seconds. The final dimension ofthe pieces was approximately 2-1/2 in. x 3-1/4 in. x 12 in. The pieces were then analyzed for mechanical strength using ASTM

Standard D 143, Part II, Sections 247 - 254), and for hardness using ASTM Standard D 143, Part II, Sections 285 - 288. For the first five samples, the maximum load ofthe treated samples averaged 718 lbs. compared to 389 lbs. for untreated samples of poplar. Similarly, the average modulus of rupture increased to 22933 psi for the treated samples, from 12736 psi for untreated poplar, and the average modulus of elasticity increased to 2.08 million psi from 1.61 million psi. The average hardness (tangential surface) of the treated samples was 1512 lb. compared to 722 lb for the untreated samples, and the average hardness (radial surface) ofthe treated samples was 1992 lb. compared to 547 lb for the untreated samples.

For the set of three samples, the maximum load ofthe treated samples averaged 646 lbs. compared to 389 lbs. for untreated samples of poplar. Similarly, the average modulus of rupture increased to 20355 psi for the treated samples, from 12736 psi for untreated poplar, and the average modulus of elasticity increased to 1.81 million psi from 1.61 million psi. The average hardness (tangential surface) of these five treated samples was 1254 lb. compared to 722 lb for the untreated samples, and the average hardness (radial surface) ofthe treated samples was 1472 lb. compared to 547 lb for the untreated samples.

Example 7

Three (3) pieces of basswood measuring 3-1/16 in. x 4-1/16 in. x 12 in. and having a moisture content of under 6% were placed between electrodes spaced 4-1/8 in. apart, and exposed to an electromagnetic field of 27 MHz, 5.8 kV, and 0.8 ampere for a period of 240 seconds. The ends ofthe pieces attained a temperature between 135° and 140°F.

The heated pieces were compressed on all four faces peφendicular to the longitudinal axis at 2300 psi (160 bar) pressure for 15 seconds. The final dimension of the pieces was approximately 2-1/4 in. x 3 in. x 12 in. The pieces were then analyzed for mechanical strength using ASTM Standard D 143, Part II, Sections 247 - 254), and for hardness using ASTM Standard D 143, Part II, Sections 285 - 288.

The maximum load ofthe treated samples averaged 692 lb. compared to 336 lb. for untreated samples of basswood. Similarly, the average modulus of rupture increased to 20301 psi for the treated samples, from 10410 psi for untreated basswood, and the average modulus of elasticity increased to 2.02 million psi from 1.29 million psi. The average hardness (tangential surface) ofthe treated samples was 1018 lb. compared to 382 lb for the untreated samples, and the average hardness (radial surface) of the treated samples was 951 lb. compared to 479 lb for the untreated samples.

Example 8

Four (4) pieces of basswood measuring 3-1/16 in. x 4-1/16 in. x 12 in. and having a moisture content of under 6% were placed between electrodes spaced 4-1/8 in. apart, and exposed to an electromagnetic field of 27 MHz, 4.9 - 5.9 kV, and 0.8 ampere for a period of 240 seconds. The ends of the pieces attained a temperature between 135° and 145°F.

The heated pieces were compressed on all four faces peφendicular to the longitudinal axis at 1400 - 2200 psi (96 - 150 bar) pressure for 15 seconds. The final dimension ofthe pieces was approximately 2-3/8 in. x 3-3/8 in. x 12 in. The pieces were then analyzed for mechanical strength using ASTM Standard D 143, Part II, Sections 247 - 254), and for hardness using ASTM Standard D 143, Part II, Sections 285 - 288.

The maximum load ofthe treated samples averaged 512 lb. compared to 336 lb. for untreated samples of basswood. Similarly, the average modulus of rupture increased to 15541 psi for the treated samples, from 10410 psi for untreated basswood, and the average modulus of elasticity increased to 1.54 million psi from 1.29 million psi. The average hardness (tangential surface) ofthe treated samples was 963 lb. compared to 382 lb for the untreated samples, and the average hardness (radial surface) ofthe treated samples was 870 lb. compared to 479 lb for the untreated samples.

Example 9

Samples of White Pine measuring 4"x2"xl2" in size and having 10% moisture content were placed between the electrodes spaced 2.25" apart, and treated (e.g., softened) with an electromagnetic field having a voltage of 3.1 kV between the electrodes and an anodic current of 0.7 Amps for 3 minutes and 45 seconds. The preheated sample were then pressed in a press-form in a direction peφendicular to the longitudinal axis for 5 seconds under 75 bar pressure, and held in the press-form for 15 minutes to cool the wood. After the treatment, the density ofthe White Pine was enhanced in an average from between about 0.330 to 0.850 kg/m . From the pressed wood the standard samples with definite dimensions were cut out for the investigation ofthe mechanical strength according to industry standards ASTM D143, part II, sections 247-254 and ASTM D143, part II, sections 285- 288. The untreated samples showed strength (hardness) in the tangential surface of 403 lbs on average, and in the radial surface 307 lbs on average, while the pretreated samples showed the enhancement of strength (hardness) up to 2916 lbs on average in the tangential surface and up to 1381 lbs on average in the radial surface.

For the other samples, the hardness test for the pretreated samples showed the strength (hardness) in tangential surface was 1672 lbs in average versus 351 lbs for untreated samples, and in radial surface 1319 lbs versus 307 lbs on average. According to the other indicators of mechanical strength (hardness), for example, the results of a static bending tests, the pretreated samples were also considerably superior to the untreated samples. Thus the pretreated samples showed the next results versus the untreated samples: 621 versus 270 lbs by max. load, 18547 versus 8336 PSI by Modules of Rupture and 1.71 versus 1.01 million PSI by Modulus of Elasticity by average values from 10 measurements. The moisture content was also different: for the pretreated samples it was

8.1% in average, while for the untreated samples it was 1 1.9%, so the pretreated samples had a lower equilibrium moisture.

Example 10 Preparation of a relatively high quality product from a rubber tree, containing natural latex, could be considered as an example ofthe use of natural compounds, such as those contained in the wood.

A rubber tree sample measuring 120x120x300 mm in size with -40% moisture was placed in an electromagnetic field having a frequency of 27 MHz, and an output power of 1.5 KW between the two electrode plates, which were spaced 130 mm apart.

The treatment process was 20 min long and the anode voltage was maintained at 2.1 kV and an anodic current at 0.6 A. Then the sample was pressed at 83 bar pressure and kept in a press-form for 12 h for cooling. The pressed wood had dimensions 85x85x300 mm, that is the density of wood became 2 times higher (from 400 3 to 800 kg/m ) after pressing. The strength ofthe sample was 3.2 BHN and was higher than the strength ofthe oak (-3.0 BHN) with density -670 kg/m .

The stability ofthe sample and its resistance to water action was measured within a year after it's production by boiling the sample in water for 4 hours, the eater resistamce ofthe sample was very high. The linear dimensions ofthe investigated sample (60x20x5 mm) underwent a change in size during boiling of not more than 1%. It will thus be seen that the invention efficiently attains the objects set forth above, among those made apparent from the preceding description. Since certain changes may be made in the above constructions without departing from the scope ofthe invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be inteφreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are to cover all generic and specific features ofthe invention described herein, and all statements ofthe scope of the invention which, as a matter of language, might be said to fall therebetween. Having described the invention, what is claimed as new and desired to be secured by Letters Patent is:

Claims

1. Method for densifying a ligno-cellulosic workpiece having a selected grain and a first value of density, comprising subjecting the workpiece to an electromagnetic field having a selected frequency to render at least the lignin component ofthe workpiece to a selected plastic condition, compressing the workpiece when the lignin component is in said plastic condition along at least one axis thereof transverse to the grain ofthe workpiece, and only when said workpiece has a moisture content of about 30% or less, for increasing the density thereof at least to a second value of density greater than the first value, and stabilizing the workpiece at said second value of density.

2. Method in accordance with claim 1, further comprising the step of electromagnetically drying the ligno-cellulosic workpiece.

3. Method in accordance with claim 2, wherein said step of electromagnetically drying comprises the step of drying the ligno-cellulosic workpiece to about the fiber saturation level or below.

4. Method in accordance with claim 1 , wherein said step of subjecting comprises the step of exposing the workpiece to a frequency between about 5 MHz and about 3 GHz.

5. Method in accordance with claim 1, further comprising the step of varying the frequency as a function of a selected wood condition.

6. Method in accordance with claim 1 , wherein said step of subjecting comprises the step of heating the ligno-cellulosic workpiece to a temperature between about 60 °C and about 140 °C to plasticize the workpiece.

7. Method in accordance with claim 1 , wherein said step of subjecting comprises the step of exposing the ligno-cellulosic workpiece to an electric field having a frequency between about 5 MHz and about 300 MHz, for a time between about 0.5 min and about 5 min.

8. Method in accordance with claim 1 , wherein said step of subjecting further comprises the step of exposing the ligno-cellulosic workpiece to microwave energy having a frequency between about 300 MHz and about 3 GHz, for a time between about 0.5 min and about 5 min.

9. Method in accordance with claim 1, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure significantly greater than 20 bars.

10. Method in accordance with claim 1, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure between about 70 bars and about 250 bars. [Please give range]

1 1. Method in accordance with claim 1 , wherein said step of stabilizing includes the step of stabilizing said compressed workpiece for a time sufficient to allow adjacent lignin components in said workpiece to bond together, thereby maintaining said second density value.

12. Method in accordance with claim 1, wherein said steps of compressing and stabilizing comprises the step of maintaining the workpiece in a selected condition of compression for attaining said stabilization.

13. Method in accordance with claim 1, wherein said step of subjecting further comprises the step of selecting said frequency as a function of one or more parameters of the workpiece.

14. Method in accordance with claim 1 , wherein said step of stabilizing is free of additional heating when attaining said second density value.

15. Method in accordance with claim 1 , wherein said step of compressing further comprises the step of compressing the workpiece from a first cross-sectional dimension to a second smaller cross-sectional dimension, and wherein said step of stabilizing further comprises the step of preventing spring-back ofthe compressed workpiece at least substantially close to said first cross-sectional dimension.

16. Method for densifying a ligno-cellulosic workpiece having a selected grain and a first value of density, comprising electromagnetically drying the ligno-cellulosic workpiece, subjecting the workpiece to an electromagnetic field having a selected frequency to render at least the lignin component ofthe workpiece to a selected plastic condition, compressing the workpiece when the lignin component is in said plastic condition along at least one axis thereof transverse to the grain ofthe workpiece, and only when said workpiece has a moisture content of about 30% or less, for increasing the density thereof at least to a second value of density greater than the first value, and stabilizing the workpiece at said second value of density.

17. Method in accordance with claim 16, wherein said step of electromagnetically drying comprises the step of drying the ligno-cellulosic workpiece to said moisture content of about 30% or below.

18. Method in accordance with claim 16, wherein said step of subjecting comprises the step of exposing the workpiece to a frequency between about 5 MHz and about 3 GHz.

19. Method in accordance with claim 16, wherein said step of subjecting comprises the step of heating the ligno-cellulosic workpiece to a temperature between about 60 °C and about 140 °C to plasticize the workpiece.

20. Method in accordance with claim 16, wherein said step of subjecting comprises the step of subjecting the workpiece to said electromagnetic energy for a time between about 0.5 min and about 5 min.

21. Method in accordance with claim 16, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure greater than 20 bars.

22. Method in accordance with claim 16, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure between about 25 bars and about 250 bars.

23. Method in accordance with claim 16, wherein said step of stabilizing is free of additional heating when attaining said second density value. - 24 -

24. Method for densifying a ligno-cellulosic workpiece having a selected grain and a first value of density, comprising subjecting the workpiece to an electromagnetic field having a selected frequency to render at least the lignin component of the workpiece to a selected plastic condition, compressing the workpiece at a pressure between about 25 bars and about 250 bars when the lignin component is in said plastic condition along at least one axis thereof transverse to the grain ofthe workpiece, for increasing the density thereof at least to a second value of density greater than the first value, and stabilizing the workpiece at said second value of density.

25. Method in accordance with claim 24, further comprising the step of electromagnetically drying the ligno-cellulosic workpiece.

26. Method in accordance with claim 25, wherein said step of electromagnetically drying comprises the step of drying the ligno-cellulosic workpiece to about the fiber saturation level or below.

27. Method in accordance with claim 24, wherein said step of subjecting comprises the step of exposing the workpiece to a frequency between about 5 MHz and about 3 GHz.

28. Method in accordance with claim 24, wherein said step of subjecting comprises the step of heating the ligno-cellulosic workpiece to a temperature between about 60 °C and about 140 °C to plasticize the workpiece.

29. Method in accordance with claim 24, wherein said step of subjecting comprises the step of exposing the workpiece to said electromagnetic field for between 0.5 min and about 5 min to plasticize the lignin.

30. Method in accordance with claim 24, wherein said step of stabilizing includes the step of stabilizing said compressed workpiece for a time sufficient to allow adjacent lignin components in said workpiece to bond together, thereby maintaining said second density value.

31. Method in accordance with claim 24, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure between about 70 bars and about 250 bars.

32. Method in accordance with claim 16, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure between about 25 bars and about 100 bars.

33. Method in accordance with claim 16, wherein said step of compressing further comprises the step of compressing the workpiece at a pressure significantly greater than about 25 bars.

34. A densified ligno-cellulosic work piece having a selected grain, comprising a main body having a cellulose and lignin component wherein the lignin component is plasticized by applying to the work piece an electromagnetic field in the range between about 5 MHz and about 3 GHz, the work piece having a first cross- sectional dimension and an associated first density value, the work piece being compressed along at least one axis transverse to the grain thereof only when said moisture content is about 30% or below while the lignin component is in said plastic condition, and stabilizing the compressed work piece such that the compressed work piece has a second cross-sectional dimension smaller than the first cross-sectional dimension and a second density value substantially greater than the first density value.