Drying and gas or vapor contact with solids – Material treated by electromagnetic energy – Microwave energy
Reexamination Certificate
2003-05-08
2004-06-01
Lazarus, Ira S. (Department: 3742)
Drying and gas or vapor contact with solids
Material treated by electromagnetic energy
Microwave energy
C034S382000, C034S396000, C219S678000
Reexamination Certificate
active
06742278
ABSTRACT:
The present invention relates to the treatment of wood, and is particularly concerned with a method for increasing the permeability of wood, especially moist wood.
The treatment of wood by impregnating it with preservatives and other agents, such as for fire-proofing, is very well known. One of the problems of the impregnation treatment is ensuring that the agent has impregnated the wood fully or at least sufficiently.
It is also known to prepare wood for impregnation by using steam to raise the temperature and pressure in the wood and break down some of the wood cells (after the pressure is rapidly reduced) and thereby to improve the natural permeability of the wood. Updated steaming specifications for, for example, round wood radiata pine steam conditioning recommend the use of steam at a temperature of 127° C. and pressure of 138 kPa for a period of from 1.5 to 18 hours depending upon the thickness of the wood before the pressure is rapidly reduced. While this treatment does help to improve the impregnation process, it has several disadvantages, particularly the duration of the treatment and the requirement for high pressure steam.
It is also known to dry wood and to form fibres by destroying the wood structure using microwave energy. It is important in drying the wood using microwave energy not to damage the wood and special drying schedules have been proposed for different wood species. In all of these schedules the microwave radiation intensity is very low, below 5 to 10 W/cm
2
, in order to avoid damage to the wood.
Destruction of wood using microwave energy to form fibres is performed at a considerably higher radiation intensity, for example up to 150 kW/cm
2
, with the aim of heating the moisture in the wood to form steam very quickly and in sufficient quantity to entirely break down the structure of the wood.
According to one aspect of the present invention there is provided a method for increasing the permeability of wood which comprises subjecting wood with a moisture content (based on dry weight) of at least 15% to microwave radiation at a frequency (f) in the range of from about 0.1 to about 24 GHz with a power intensity (p) from about 10 W/cm
2
to about 100 kW/cm
2
for a duration of from about 0.05 to about 600 seconds to cause water in the wood to vaporise resulting in an internal pressure in the wood such that the permeability of the wood is increased by partial or complete destruction of ray cell tissue, softening and displacement of wood resin, formation of pathways in the radial direction of the wood and/or by creating, on the base of destroyed rays, cavities in the wood, said cavities being primarily in radial-longitrudinal planes of the wood, and wherein the overall integrity of the wood is substantially maintained.
There is also provided wood whenever treated by the method described in the immediately preceding paragraph.
Using the method of the invention, it is possible to selectively or randomly treat wood and produce wood having regions of differing permeability. Therefore, according to another aspect of the invention there is provided wood having regions of differing permeability, wherein at least one of said regions has been treated to increase the permeability thereof relative to the untreated wood.
The microwave treatment of the present invention increases the permeability of the wood typically by vaporising water contained in the wood to create an internal pressure and a temperature above 100° C. resulting in the modification of the wood structure by any one or more of the destruction of ray cells in the wood, softening and mobilising of the resin in the wood and replacing it at least partially with open pores, and the creation of thin radial checks, resulting in cavities disposed mainly in radial-longitudinal planes. There may be no substantial drying of the wood during the process of the invention. The present invention may facilitate in-line processing of wood such as for impregnation or fast drying. That is, the thin, radial checks and cavities formed by the method of the present invention may allow more effective impregnation of certain treatment agents. The thin radial checks or cavities may also allow faster subsequent drying treatments than compared with the drying techniques conventionally employed in the absence of the treatment of the present invention. For example, hard wood species (i.e. refractory woods), such as eucalypts, may experience internal checking and collapse upon fast drying without the prior treatment of the present invention due to excessive pressure build up within the wood.
The microwave treatment of the present invention is not necessarily limited to subjecting the wood to a single microwave frequency or power intensity. The method may comprise, for example, subjecting the wood to various microwave frequencies and power intensities in a single treatment. Preferably, the power intensity, whether a single power intensity or a number of different power intensities are used, is maintained at not less than 10 W/cm
2
. In a preferred embodiment, however, the power intensity is maintained above 10 W/cm
2
.
The microwave treatment of the present invention may also be used to increase the permeability of a portion or portions of the wood either selectively or randomly. For example, energy impulses of predetermined duration and separated by set time intervals or random impulses may be used to treat the wood. Alternatively, microwaves may be directed at a portion or portions of wood to be treated. If a plurality of portions are to be treated, these may be selected randomly or in a predetermined manner with the proposed use of the final product in mind. That is, depending on the required flexibility, strength, permeability and other required characteristics of the product. Wood produced by either this selective or random treatment comprises regions of differing permeability wherein a treated portion of the wood constitutes at least one region and an untreated portion constitutes another region, and wherein the treated portion has greater permeability than the untreated portion.
The process of the present invention is preferably applied to the treatment of moist wood. As used herein and throughout the specification the term “moist wood” refers broadly to wood which is “green” after sawing as would be understood by a person skilled in the art. The amount of water present in the moist wood will, of course, vary depending on the species of plant, but it is considered that moist wood will generally have a moisture content in the range of from about 30 to about 200% based on the dry weight of the wood. The process of the present invention is also applicable to the treatment of wood having lower moisture contents, such as from 15% to 30%.
Wood is capable of absorbing very high quantities of microwave energy. The microwave energy causes the water in the cells of the wood to heat up and boil, creating steam pressure in the cells which results in the destruction of cell walls. The ray cells have thinner walls than the cells of the main wood tissues (tracheids, libriform) and ray cells are destroyed by the microwave energy before cells of the main wood tissue. The destroyed ray cells form paths in the radial direction for the easy transportation of liquids and vapours inwardly from the outer surface. Ray cells form from about 5 to about 35% of the wood volume, so their destruction may increase the wood permeability substantially.
The treatment, therefore, advantageously results in the destruction of ray cells while substantially maintaining the overall integrity of the wood. That is, the destruction of the ray cells may occur without significant destruction of cells of the main wood tissues (commonly referred to as the grains or fibres of the wood) resulting in treated wood which, as discussed hereafter, will generally have decreased tortional strength, but substantially unaffected flexural strength in the radial direction. In a treatment of approximately four seconds duration, for example, a low frequency (f) of about 0.4 GHz is prefer
Romero Francisco Javier
Torgovnikov Grigory
Vinden Peter
Brezner, Esq. David J.
Dorsey & Whitney LLP
Kosslak, Esq. Renee M.
Lazarus Ira S.
O'Malley Kathryn S.
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