Coating processes – Coating by vapor – gas – or smoke – Wood base
Reexamination Certificate
2000-11-13
2002-06-18
Beck, Shrive P. (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Wood base
C427S297000, C427S351000, C427S393000, C427S399000, C427S408000
Reexamination Certificate
active
06406749
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process of preparing a modified lignocellulosic material, such as wood, or wood chips or particles, or other fibrous plant material, and to impregnating compositions for use in the process.
It is well known to modify wood with anhydrides to esterify the cellulose and hemi-cellulose hydroxyl groups in order to minimise swelling when the wood is wetted.
It is also known to modify wood with resins in order to improve mechanical properties such as hardness and abrasion resistance, but also to contribute to stabilisation and improved performance on wetting. Generally, the resins used must be thermoset, but may also be chemicals which propagate a chemical reaction which goes to completion at elevated temperature. Some of the essential requirements for both successful and commercially viable modification of wood are that there should be no binder related toxic volatiles generated, that the level of the modifying chemical added to the wood and the level of resination added to the wood and the relative proportions between them should be infinitely variable and under accurate control, that the carrier solvent or water present must be capable of total removal from the modified matrix, and that the solvent must be non-reactive, inert, preferably have a low boiling point, have a high evaporative rate, and be easily recoverable and preferably non-flammable. It is however undesirable that the impregnating composition is in the solvent medium because of the danger of explosion, the cost of solvent recovery, control of solvent emissions to the atmosphere, the possibility of solvent interference with the chemical reactions, the danger of toxicity, the cost of specialist plant required and the cost of the solvents themselves. In addition, when the composition is in the water phase, the wood or fibres are easier to handle in the post impregnation phase of the process. The modification system must have true latency in the sense that its chemical make-up must not change over an extended period of time. There should be no change in viscosity and any resin, where present, must not prematurely partly polymerise or condense or cross-link.
When wood chips or plant fibres are modified for the purpose of producing a composite board it is an advantage to have a technology that is sufficiently flexible to allow for the continuous production of boards after the impregnating composition has been added to the material, as is the case with conventional particle board. In this case, the chemicals in the impregnating composition are concentrated to allow for a maximum application of up to 25% of impregnating composition to treated material on a weight basis. In a discontinuous process, the chips or fibres are treated with the impregnating composition, the material is thoroughly dried and may then be pressed to form a board at a later time or different place. In other words, it is only when the heat and pressure are applied that the chemical reactions in the composition go to completion to form the board.
An additional problem with lignocellulosic material products is their susceptibility to attack by various organisms such as fungi and insects.
Copper/chrome/arsenic treatments are used in the wood industry as wood preservatives. However, these are now under environmental scrutiny, particularly the chrome and arsenic components.
In addition, it has been known that boron, borax, boric acid, sodium borate and other boron compounds set as wood preservatives, having both fungicidal and termiticidal properties. The fire retardation properties of such boron compounds are an added advantage.
However, the reason for the limited use of boron compounds in wood preservation is the ready solubility of these compounds resulting in their rapid leaching from impregnated wood when exposed to water wetting, or to high humidity conditions. In these circumstances, the boron rapidly migrates to the surface of the treated wood by osmotic pressure as well as capillarity.
There is thus a need for an improved wood treatment process.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a process of chemically modifying a lignocellulosic material by:
(1) impregnating the lignocellulosic material with an impregnating composition comprising:
(i) a dicarboxylic anhydride or a tricarboxylic anhydride;
(ii) a partially or fully hydrolysed polyvinyl alcohol; and
(iii) water.
The impregnating composition preferably includes one or more of:
(iv) a water soluble boron compound;
(v) a compound which acts as an ammonia donor on heating;
(vi) a high boiling point tar acid; and
(vii) a formaldehyde donor or a precursor of formaldehyde.
The lignocellulosic material, when in particle or fibre form, may also have added thereto a thermosetting resin in finely divided dry powder form.
The process of the invention preferably includes the step, prior to step (1) of: impregnating the lignocellulosic material with ammonia gas.
The process, when the lignocellulosic material is solid wood, preferably includes the steps of:
prior to step (1):
(a) placing the wood in a suitable container such as a pressure cylinder and subjecting the wood to a vacuum;
(b) optionally
(i) introducing into the container an amount of ammonia gas to impregnate the wood;
(ii) removing all of the residual ammonia as by releasing the vacuum and expelling the residual ammonia gas from the container; and
(iii) subjecting the ammonia gas impregnated wood to a vacuum;
in step (1):
(c) introducing the impregnating composition into the container;
(d) impregnating the wood with the impregnating composition by applying pressure, for example hydraulic or pneumatic pressure;
after step (1):
(e) removing the impregnating composition from the container;
(f) removing free water from the impregnated wood; and
(g) optionally heating the wood to cause the chemical modification of the wood to go to completion.
The process, when the lignocellulosic material is wood chips or plant fibres, preferably includes the steps of:
prior to step (1):
(A) placing the wood chips or plant fibres into a suitable mixer;
in step (1):
(B) adding the impregnating composition to the wood chips or plant fibres in the mixer with mixing;
(C) optionally adding a thermosetting resin in finely divided dry powder form to the wood chips or plant fibres in the mixer with mixing;
after step (1):
(D) removing the water from the treated wood chips or plant fibres; and
(E) optionally heating the treated wood chips or plant fibres to cause the chemical modification of the lignocellulosic material to go to completion.
According to a second aspect of the invention there is provided an impregnating composition for chemically modifying a lignocellulosic material, the impregnating composition comprising:
(i) a dicarboxylic anhydride or a tricarboxylic anhydride;
(ii) a partially or fully hydrolysed polyvinyl alcohol; and
(iii) water.
DESCRIPTION OF EMBODIMENTS
The invention is a process of modifying a lignocellulosic material by impregnating it with an impregnating composition.
By a lignocellulosic material there is meant any plant material emanating from the photosynthetic phenomenon.
The lignocellulosic material may be wood, or wood chips or particles, paper, paper waste or pulp, or other fibrous plant material. However, the process of the invention is of particular application to solid wood products, such as wood in sawn, sliced, peeled or veneer form, or paper sheeting.
The lignocellulosic material is chemically modified by impregnating the lignocellulosic material with an impregnating composition comprising a dicarboxylic anhydride or a tricarboxylic anhydride, a partially or fully hydrolysed polyvinyl alcohol, various optional components, and water, as the solvent.
The dicarboxylic anhydride may be selected from the group consisting of maleic anhydride, phthalic anhydride, succinic anhydride and tetrahydrophthalic anhydride, and the tricarboxylic anhydride may be trimellitic anhydride. The preferred anhydride is maleic anhydride.
The impregnating co
Beck Shrive P.
Crockford Kirsten A.
Windsor Technologies Limited
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