Coating processes – Immersion or partial immersion – Cellulosic base
Reexamination Certificate
2001-06-18
2003-02-11
Bareford, Katherine A. (Department: 1762)
Coating processes
Immersion or partial immersion
Cellulosic base
C427S297000
Reexamination Certificate
active
06517907
ABSTRACT:
FILED OF THE INVENTION
The present invention relates to impregnation of wood using a supercritical fluid as carrier for the substance impregnated into the wood.
More particularly the invention relates to impregnating treatment of resin-containing wood and enables an expansion of the field of wood treatments using fluids in supercritical state.
BACKGROUND OF THE INVENTION AND PRIOR ART
Use of fluids under supercritical conditions offers substantial advantages in operations involving perfusing of a porous material for impregnation.
The advantages of using fluids under supercritical conditions over conventional processes using organic solvents or water as carrying medium at conditions of temperature and pressure in which the liquid stage is maintained, can be important and include the following features.
Supercritical fluids, possibly including minor amounts of co-solvents, are able to perfuse or penetrate porous materials quicker and more efficient than liquids, thereby enabling a more uniform impregnation in the interior of the material being treated and also enabling impregnation of materials regarded as a nearly impermeable to liquids.
The fact that supercritical fluids are almost as dispersible as gases facilitates an even contact with the porous substrate to be treated. Further, the fact that the solubility of several substances in super-critical fluids is highly pressure dependent enables an efficient deposition of such substances in the interior of the porous substances by pressure reduction following impregnation with supercritical solutions at higher pressures.
Supercritical fluids have also been suggested for the extraction, and especially the impregnation, of wood where the potential advantages include not only improved efficiency of the treatments but also involve substantial environmental improvements both in the performing of the treatment and possible post conditioning and in the subsequent use and disposal of the treated wood articles.
For further description of supercritical fluid treatments of wood materials reference is made to the following.
An article of Morrell & Levien: “Development of New Treatment Processes for Wood Protection” Conference Report from “Conference on Wood Preservation in the '90s and Beyond”, Savannah, Ga., USA, Sep. 26-28, 1994, which deals with impregnation of wood species normally resistant to impregnation, by using supercritical carbon dioxide to deliver and deposit biocides into said wood. The potential for completely impregnating virtually all wood species also with biocides not previously regarded as suitable, is discussed. The supercritical fluid treatments are described as representing the first truly revolutionary improvement in treatment in this century, although it is admitted that a substantial amount of research and testing will be required before these systems become commercially feasible.
Also a paper by Herve van Oost, Philippe Eymard and Michel Gastiger: “Traitement de l'épicéa en milieu supercritique”, Info Critt No. 6, 1995, provides a general description of the use of supercritical fluids for conservating treatment of wood, especially spruce. Based on laboratory experiments using carbon dioxide as supercritical fluid with possible addition of alcohol it is expected that the technique could be developed into commercial scale not only for introducing pesticides, but also for impregnation of wood with a view of improving physical characteristics thereof.
U.S. Pat. No. 5,094,892 comprises a review of prior art methods utilizing supercritical fluids for various purposes comprising deposition of various materials into a porous substrate or extraction of materials from such substrates. The latter process may be performed to recover valuable extracts or to improve characteristics of the substrate. The patent concentrates on the improvement obtainable by using co-solvents when perfusing wood, using typically carbon dioxide as supercritical fluid. Among the advantages also this patent emphasizes a uniformly impregnating of otherwise difficultly permeable materials.
Similar information can be found in U.S. Pat. No. 4,992,308 which i.a. describes impregnation using monomers which are polymerized in situ.
U.S. Pat. Nos. 5,364,475 and 5,476,975 both deal with the extraction of organic toxic contaminations from wood using supercritical carbon dioxide.
Also delignification of wood has been suggested in the above mentioned U.S. Pat. No. 4,992,308 and in U.S. Pat. No. 5,041,192.
WO-A-95/34360 describes an analytical extraction process using a solvent under high temperature and pressure but not in the supercritical state. After the extraction a purging or flushing step is performed which transports the extraction fluid into a collection chamber. The purging or flushing step utilize another fluid than the one used in the extraction process.
U.S. Pat. No. 5,785,856 describes an apparatus that is particular adapted to perform the extraction process described in WO-A-95/34360 in an automated fashion, but which also may be operated under supercritical conditions. An example of suitable purging fluid is nitrogen under high pressure. It is notes that beside transporting the extraction fluid to a collection chamber the purging step has the further advantage of drying the extracted material.
In spite of the fact that treatment of various materials by perfusion thereof to perform or impregnation, in principle may advantageously be carried out using a supercritical fluid as carrier in the perfusion process, such processes have hitherto not found commercial application, at least not at the level which could be expected in view of their potential advantages.
Especially within one of the largest potential application areas, namely in the treatment of wood substrates, these processes have, to the best knowledge of the present inventors, not achieved large-scale commercial use.
The present inventors have conducted extensive research with a view of developing and improving processes of the discussed type, especially for treatment of resin-containing wood substrates.
In the present specification and the attached claims the term “wood substrate” designates a substrate for the impregnation process which may typically be a shaped or partially shaped wood article, structural wood, timber, poles etcetera, but encompasses also materials comprising comminuted wood such as chips or building plates etcetera.
By said research and experiments it has turned out that an important feature which may be at least partly responsible for the lacking or very restricted commercial application of perfusion processes using supercritial fluids in wood products, is the contents of resin in most of such wood products. Such resin may under the influence of the supercritical fluid cause deterioration of the resulting products and/or operational complications.
In this context the term “resin” denotes the high viscous liquid of lipophilic or hydrophobic character present in amounts of typically some percent by weight in most types of wood, especially in wood from coniferous tres. Such resin is a very complex mixture of various substances including relative volatile components such as terpenes, whereas the main component is a mixture of non-volatile, partly unsaturated compounds including esters and free acids. The resin forms an extremely sticky gum which is capable of undergoing a certain slow hardening when exposed to the air.
The resin is normally present as small drops within the cells forming the wood structure.
Most of the substances coming into consideration as supercritical fluid in the wood perfusing processes, coming into consideration herein, including primarily carbon dioxide and hydrocarbons, such as ethane, propane and buthylene, as well as certain auxiliary substances suitable as adjuvants in the fluid, are soluble in resin and during the impregnative perfusion processes a substantial amount thereof is dissolved in the resin present in the interior of the wood.
As can be shown in experiments using samples of pure resin extracted from wood, the viscosity and s
Bareford Katherine A.
FLS Miljo A/S
Ostrolenk Faber Gerb & Soffen, LLP
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