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Title:
METHOD FOR THE PRESERVATION OF WOOD BY CHEMICAL MODIFICATION
Document Type and Number:
WIPO Patent Application WO/2010/062180
Kind Code:
A1
Abstract:
Disclosed is a method for the preservation of wood by chemical modification. In the method wood is impregnated with a liquid preservation agent, and modified by activating the preservation agent. The preservation agent according to the invention is selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels –Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, such as maleic anhydride, and mixtures thereof. The activation comprises subjecting the oil to curing under the influence of heat during or after impregnation. Preferably the conjugated fatty acid chains involved are conjugated trienoic fatty acid chains, and particularly are based on calendula oil.

Inventors:
BLAAUW ROELF (NL)
VAN HAVEREN JACOBUS (NL)
VAN LOO EIBERTUS NICOLAAS (NL)
Application Number:
PCT/NL2009/050723
Publication Date:
June 03, 2010
Filing Date:
November 27, 2009
Export Citation:
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Assignee:
STICHTING DIENST LANDBOUWKUNDI (NL)
BLAAUW ROELF (NL)
VAN HAVEREN JACOBUS (NL)
VAN LOO EIBERTUS NICOLAAS (NL)
International Classes:
C07D307/89; B27K3/34
Domestic Patent References:
WO2004080673A12004-09-23
WO1992004166A11992-03-19
WO2007101910A12007-09-13
Foreign References:
EP1174231A12002-01-23
GB518461A1940-02-27
Other References:
TJEERDSMA ET AL, PROCEEDINGS OF THE SECOND EUROPEAN CONFERENCE ON WOOD MODIFICATION, 6 October 2005 (2005-10-06) - 7 October 2005 (2005-10-07), Göttingen, XP002528153, Retrieved from the Internet [retrieved on 20090514]
Attorney, Agent or Firm:
HATZMANN, M.J. (Johan de Wittlaan 7, JR Den Haag, NL)
Download PDF:
Claims:
Claims

1. A method for the preservation of wood by chemical modification, comprising impregnating the wood with a liquid preservation agent and modifying the wood by activating the preservation agent, wherein the preservation agent is selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels -Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, and mixtures thereof, and wherein the preservation agent is subjected to curing under the influence of heat during or after impregnation, so as to activate the preservation agent and modify the wood. 2 A method according to claim 1, wherein the preservation agent comprises the oil as such.

3. A method according to claim 1, wherein the preservation agent comprises the reaction product of the oil and a dienophilic unsaturated cyclic anhydride, preferably maleic anhydride. 4. A method according to claim 1 or 2, wherein the preservation agent comprises a conjugated trienoic fatty acid chain.

5. A method according to claim 4, wherein the preservation agent is selected from the group consisting of calendula oil, tung oil, pomegranate seed oil, dimorphoteca seed oil, oiticica oil, momordica oil, the adducts of any of the foregoing oils with a dienophilic unsaturated cyclic anhydride, and mixtures of. the foregoing oils and adducts.

6. A method according to any one of the preceding claims, wherein the preservation agent comprises a combination of a natural oil and a reaction product of a natural oil and a dienophilic unsaturated cyclic anhydride. 7. A method according to any one of the preceding claims, wherein the preservation agent is presented to the wood in the form of an oil-in-water emulsion, and the resulting impregnated wood is dewatered before or during the curing.

8. A product for the preservation of wood by chemical modification, comprising a preservation agent selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels -Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, and mixtures thereof, wherein the product is in the form of an oil-in-water emulsion.

9. The use, as an agent for the preservation of wood by chemical modification, of a compound obtainable by reacting an oil comprising a conjugated, conjugated trienoic, fatty acid with a dienophilic unsaturated cyclic anhydride, preferably maleic anhydride, under circumstances that allow the formation of a Diels-Alder adduct.

10. Wood, preferably spruce or pine, modified by chemical reaction with a preservation agent, the wood comprising a linkage with a preservation agent obtainable by heating wood when impregnated with the preservation agent., wherein the preservation agent is selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels -Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, and mixtures thereof,

11. Wood according to claim 10, wherein the linkage is in the form of an interpenetrating network of wood polymers and a polymerization product obtainable by heating wood impregnated with an oil comprising a conjugated fatty acid, preferably a conjugated trienoic fatty acid. 12. Wood according to claim 10, wherein the linkage is in the form of a bound resin obtainable by heating wood impregnated with the reaction product of an oil comprising a conjugated fatty acid, preferably a conjugated trienoic fatty acid, with a dienophilic unsaturated cyclic anhydride, preferably maleic anhydride.

13. Wood according to any one of the claims 10 to 12, comprising both a linkage as defined in claim 11 and a linkage as defined in claim 12.

Description:
Title: Method for the preservation of wood by chemical modification

Field of the Invention

The invention pertains to a method for the preservation of wood by chemical modification, comprising impregnating wood with a wood modifying preservation agent in the form of a vegetable oil-based liquid. The invention further pertains to the use of oils comprising conjugated fatty acid chains, and modifications of such oils, in the preservation of wood, and to wood thus preserved.

Background of the Invention

In the timber industry efforts are ongoing to make wood more durable. This refers, inter alia to the desire that wood is better capable of withstanding the influence of moisture, which is due to lead to dimensional instability and wood rot. The provision of more durable wood is of particular importance in order to be able to replace tropical hardwood by ecologically better acceptable wood, such as spruce or pine, which by nature are less durable.

One of these efforts is disclosed by B.F. Tjeerdsma et al. in Proceedings of the Second European Conference on Wood Modification (2005). Herein vegetable oil heat treatments are applied, using rapeseed oil, linseed oil, and a modified, maleinized linseed oil. The modified oil is made by maleinizing the oil in a reaction sequence which in itself is well-known in the field of organic chemistry, viz. an "ene" reaction, followed by a Diels-Alder cyclo-addition. A similar modified linseed oil is also disclosed in EP 1 174 231 (Dekker).

Amongst the oils mentioned herein are also so-called drying oils, which usually comprise poly-unsaturated fatty acids and which find use, inter alia, as or in wood varnish. Such drying oils, e.g. linseed oil, are applied, and allowed to harden under the influence of oxygen (the "drying"), and are then capable of providing some protection to outside influences, much like a paint. The method to which the invention pertains, different from paints and other wood finishes, is directed to providing a wood that is essentially modified by reaction with the wood preservation agent. This is generally accomplished in a reaction vessel by the combination of impregnation and heating steps.

In the Dekker reference, the efficacy of the wood preservation agent disclosed is largely based on oxidatively drying oils. This refers to a process of film formation, as a result of the influence of oxygen on a poly-unsaturated fatty acid. This typically is a process that occurs when the oil is subject to the influence of air, and is therewith applicable to treatment of wood on, or near, its surface. Dekker does not refer to a heat treatment after impregnation, and relies on a relatively quick reaction of the anhydride groups in the resin with hydroxyl groups in the wood, and on the aforementioned oxidative drying. Further, the preparation of the modified linseed oil as disclosed in the Dekker reference turned out to be difficult to reproduce.

In the Tjeerdsma reference, the impregnation of wood is followed by a dewatering treatment and a heat treatment. Frequently, the impregnation is conducted with the modified linseed oil, and the heat treatment is conducted with refined rapeseed oil or refined linseed oil. Sometimes the heat treatment and the impregnation are both done with the modified linseed oil, and in that event the impregnation is integrated in the heating phase.

Whilst the results as presented in the Tjeerdsma reference favor the use of the modified linseed oil, the latter is associated with drawbacks that render its practical use more difficult. Particularly, since linseed oil is not conjugated and hence cannot give direct Diels-Alder addition with dienophiles, in preparing the modified oil a very high temperature (200 0 C) is required. As described by Tjeerdsma et al., the nature of the reaction is such that a significant part of the linseed oil remains unmodified. This unmodified fraction leaches out of the wood upon curing at the temperatures used by Tjeerdsma et al. (180 0 C, 200 0 C, 220 0 C), resulting in a sticky surface of the treated wood. It is thus desired to provide a method for the preservation of wood in which a good degree of preservation can be obtained by chemical modification inside the wood, without unfeasible chemical modification of the oil. It is further desired to provide a wood preservation agent that is capable of forming an interpenetrating network with lignocellulosic polymers inside wood, rather than by oxidative drying. It is further desired to provide a wood preservative agent that combines both the capability of chemical bonding to wood, and of forming interpenetrating networks with lignocellulosic polymers inside wood.

References on Diels-Alder cyclo-additions on the basis of oils, chiefly relate to fatty acid esters isolated therefrom. One such reference is a publication by Biermann et al., Eur. J. Org.Chem., 2007, 3859-3862. Herein it is described how methyl calendulate, methyl α-eleostearate and punicic acid methyl ester are subjected to a Diels-Alder reaction with maleic anhydride.

Summary of the Invention

In order to better address one or more of the foregoing desires, the invention, in one aspect, is a method for the preservation of wood by chemical modification, comprising impregnating the wood with a liquid preservation agent and modifying the wood by activating the preservation agent, wherein the preservation agent is selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels - Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, and mixtures thereof, and wherein the preservation agent is subjected to curing under the influence of heat during or after impregnation, so as to activate the preservation agent and modify the wood. In another aspect, the invention presents a product for the preservation of wood by chemical modification, comprising a preservation agent selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels —Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, and mixtures thereof, wherein the product is in the form of an oil-in-water emulsion.

In yet another aspect, the invention pertains to the use, as an agent for the preservation of wood by chemical modification, of a compound obtainable by reacting an oil comprising a conjugated, conjugated trienoic, fatty acid with a dienophilic unsaturated cyclic anhydride, under circumstances that allow the formation of a Diels-Alder adduct.

In a further aspect, the invention pertains to wood modified by chemical reaction with a preservation agent, the wood comprising a linkage with a preservation agent obtainable by heating wood during or after impregnation with the preservation agent, wherein the preservation agent is selected from the group consisting of oils comprising at least one conjugated fatty acid chain, modified oils comprising a Diels-Alder adduct of a conjugated fatty acid chain and a dienophilic unsaturated cyclic anhydride, and mixtures thereof.

Detailed Description of the Invention

In its most simple, and already therefore advantageous form, the invention puts to use an unmodified natural oil, selected from those oils that comprise one or more conjugated fatty acids, preferably multiply conjugated fatty acids with more than two C=C bonds (particularly conjugated trienoic fatty acids), and mixtures thereof.

It will be understood that, in another aspect, the invention can also be carried out using synthetic oils comprising conjugated fatty acids, such as dehydrated castor oil. Other examples of synthetically obtained conjugated fatty acid (esters) include conjugated linoleic acids and conjugated linolenic acids.

The term oil is to be broadly understood as encompassing not only triglycerides, such as natural oils, but also diglycerides or glycol esterified with two fatty acids. The term "oil" also includes wax esters, generally fatty alcohols esterified with fatty acid.

The term "oil" also includes reconstituted oils, that can be prepared e.g. by first isolating conjugated fatty acids, and then reconstituting triglycerides. This can be carried out using, e.g., known lipase enzymes. The term "conjugated fatty acid" is known to the skilled person in organic chemistry, and refers to a fatty acid having two or more carbon-carbon double bonds (i.e. C=C bonds) that are covalently linked to each other via a single carbon-carbon bond. Or, in other words, both carbon atoms of a single covalent carbon-carbon bond are covalently attached to a further carbon atom through a double bond. A conjugated system, in its simplest form, can be depicted as -C=C-C=C-.

Natural oils generally comprise a mixture of fatty acids, including saturated and mono- and poly-unsaturated fatty acids. In the present invention, natural oils are selected that comprise a conjugated fatty acid, and preferably a conjugated trienoic fatty acid.

These oils are known and include, but are not limited to, calendula oil, tung oil, pomegranate seed oil, dimorphoteca seed oil, oiticica oil, oil of Momordica spec, but also genetically modified plant oils can be made and have been made that contain conjugated fatty acids (e.g. Glycine max (soybean)). Information on conjugated fatty acids is available widely, but for example also at http://www.cyberlipid.org/fa/acid0003.htm.

E.g. tung oil comprises in its fatty acid composition at least 60 wt.% and typically 75-80 % of eleostearic acid, i.e. (9E,llE,13Z)-octadeca-9,ll,13-trienoic acid, which has three conjugated double bonds. Calendula oil comprises 50- i.e. also a poly-unsaturated fatty acid having three conjugated double bonds. Pomegranate seed oil comprises about 65 wt.% of (9Z,11E,13Z)- octadecatrienoic acid (punicic acid). Dimorphoteca seed oil comprises about 60 wt.% of 9-hydroxy-(10E,12Z)-octadecadienoic acid (dimorphecolic acid). Oiticica oil comprises about 78 wt.% of 4-oxo-eleostearic acid (licanic acid).

In fact, it is preferred in the invention to make use of natural oils comprising a conjugated trienoic fatty acid, such as in calendula oil or tung oil (i.e. having three conjugated double bonds, for which a simplified formula is -C=C-C=C-C=C-). The oil preferably is selected from the group consisting of calendula oil, tung oil, and mixtures thereof.

The conjugated fatty acid-containing oils, preferably conjugated trienoic fatty acid-containing oils can be put to use in several ways.

In one embodiment, the oil-based liquid comprises the oil itself, i.e. the oil is used in an unmodified form. This embodiment is advantageous inter alia in that it does not require conducting a chemical reaction prior to providing the wood preservation agent, and thus this enables an easy to conduct, straightforward wood preservation method. This method comprises the aforementioned impregnation and heating (curing) steps. Herein the oil can be used unheated for impregnation, after which excess oil is removed and the impregnated wood is heated. The oil can also be pre-heated, and then used for impregnation, provided that the pre-heating is not done at a level resulting in crosslinking and/or polymerization. Preferably, to choose the optimal result rather than the optimal simplicity of the process, the oil is pre-heated before impregnation in order to reduce viscosity if desired, and, after excess oil is removed, the resulting impregnated wood is heated too, in order to allow for chemical reactions to take place inside the wood, i.e. a curing step. .

The impregnated wood is preferably heated to a temperature in the range of from 100 0 C to 200 0 C, preferably of from 120 0 C to 180 0 C, at atmospheric pressure. It can be preferred to apply elevated pressure so as to optimally prevent the oil from leaking out. If the oil is pre-heated, this is preferably to a temperature in the range of from 40 0 C to 90 0 C. A typical duration for the heat treatment is 30 minutes to 4 hours, preferably 1-3 hours, depending on the curing temperature.

As a result of the impregnation and heating steps, the conjugated, preferably conjugated trienoic, fatty acid chains interlink within the wood (i.e. interlinked with and in between wood polymers such as cellulose, hemicellulose and/or lignin).

Without wishing to be bound by theory, the inventors believe that the conjugated, preferably conjugated trienoic, fatty acid chains, as a result of their relatively high reactivity, upon heating will polymerize. As the polymerization takes place within the impregnated wood, the result is an interpenetrating network (IPN) of the polymerized fatty acid and the lignin network present in wood. This effectively leads to a desirable bonding of the impregnated wood preservation agent with the wood, and thus a fixed hydrophobicity in the wood, so as to render the wood more durable. It will be understood that an oil which does not comprise a conjugated fatty acid residue, but e.g. a non-conjugated poly-unsaturated fatty acid such as in linseed oil or soybean oil, will, upon the application of heat with temperatures not higher than proposed here, according to this theory not polymerize within the wood. In another embodiment, the oil is chemically modified before use so as to introduce chemical functionalities that are capable of chemical binding to wood. Without wishing to be bound by theory, the inventors believe that heating wood impregnated with the reaction product of an oil comprising a conjugated fatty acid, and preferably a conjugated trienoic fatty acid, with a dienophile, notably maleic anhydride, will lead to the in situ formation of a resin that binds to the wood's hydroxyl group- containing constituents.

This embodiment is advantageous inter alia in that the improved bonding of the wood preservation agent is believed to be associated with improved wood preservation. Moreover, as compared to the chemical modification of poly-unsaturated oils with dienophiles disclosed in the art, the presence of conjugated fatty acids, preferably conjugated trienoic fatty acids, allows for a better, more economical process, e.g. involving a considerably lower reaction temperature and with only a single reaction step required. Thus, other than as described in the aforementioned references (Tjeerdsma et al. and EP 1 174 231), the oil used in the present invention can directly undergo a Diels-Alder cyclo- addition with a dienophile such as maleic anhydride. This reaction occurs at relatively low temperatures (e.g. as of about 100 0 C), whilst the references teach to first conduct a step at much higher temperature (e.g. as of 200 0 C), as needed in the case of linseed oil, to undergo a first addition reaction. Moreover, these high temperatures lead to the formation of by-products such as CO2. Hence, the agent of the invention has a higher degree of purity, is better defined, and can be produced at lower energy costs.

Without the inventors wishing to be bound by theory, and necessarily in a simplified representation of complex chemistry occurring in the impregnated wood, it is noteworthy that, with a view to the wood-preservation use of the present invention, and the heat-curing step applied in the method of the invention, a big difference exists between a maleic anhydride adduct of a conjugated fatty acid (or mono-alkyl ester), and the maleic anhydride adduct of an oil comprising one or more conjugated fatty acid residues. For, the adduct of a single, isolated fatty acid chain as such is not likely to have the aimed effect of a wood preservation agent, which - in the modified oil embodiment - is due to both the presence of anhydride groups for binding to wood and remaining unsaturated (both conjugated and unconjugated) fatty acid residues for forming an interpenetrating network upon curing. Similarly, the modified conjugated trienoic fatty acid oil of the invention has this combined effect, which a modified linseed or soybean oil does not.

Whilst maleic anhydride is preferred, other dienophilic cyclic anhydrides are possible, as well-known to the skilled person, such as itaconic anhydride, citraconic anhydride, and aconitic anhydride. The dienophilic anhydride is preferably used in at most a stoichiometric amount calculated on the basis of the amount of conjugated fatty acid present in the oil.

The reaction of the dienophile with the oil comprising a conjugated fatty acid, can be conducted at temperatures well below 200 0 C, preferably between 100 0 C and 150 0 C, more preferably between 110 0 C and 130 0 C. The optimal temperature will to some extent depend on the exact combination of diene (i.e. the oil) and dienophile. The eventual choice of reaction circumstances is within the ambit of the skilled person's standard practice.

In accordance with this embodiment of the wood preservation method of the invention, the invention also provides a wood preservation agent obtainable by reacting an oil comprising a conjugated fatty acid, preferably a conjugated trienoic fatty acid, with a dienophile, under circumstances that allow the formation of a Diels-Alder adduct.

Whilst the above-mentioned Biermann reference discloses Diels-Alder reactions to conjugated fatty acids, it was surprisingly found that the modification of wood by heat treatment with conjugated oils and/or Diels-Alder derivatives there from gave preserved wood products without the shortcomings obtained when non-conjugated oils are used.

After impregnation with the so-modified oil of the invention, a curing step is conducted in which, generally, the wood is heated to a temperature within the range of from 100 0 C to 200 0 C, preferably of from 120 0 C to 180 0 C. A typical duration for the heat treatment is 30 minutes to 4 hours, preferably 1-3 hours.

In accordance with the invention, conjugated oils and modified conjugated oils can also be used in combination, in any ratio.

In the aforementioned natural oils, as well as in the synthetic oils comprising conjugated linoleic acids and conjugated linolenic acids, the conjugated fatty acids generally form the majority (i.e. more than 50 wt.%) of fatty acid residues present in these oils. This does not preclude the invention from using oils in which only a minority (i.e. less than 50 wt.%) of the fatty acid residues consists of conjugated fatty acids. It is also not precluded in the invention, to make use of mixtures of oils comprising (in majority or in minority) conjugated fatty acid residues, and oils that comprise (in majority or in minority) different fatty acid residues, such as saturated fatty acids, mono- unsaturated fatty acids, and poly-unsaturated fatty acids in which the double bonds are not conjugated (such as in linseed oil used by Tjeerdsma and Dekker).

If such oils, or mixtures of oils are used care should be taken that enough conjugated fatty acid residues are available in the oil impregnated into the wood, to allow the formation of the interpenetrating networks upon curing at the indicated temperature ranges, and with the modified oil product (i.e. the Diels-Alder adduct) that enough conjugated fatty acid residues and/or anhydride groups on the modified conjugated fatty acid residues are available in the oil impregnated into the wood, for the formation of the interpenetrating networks and/or the formation of covalent bonds with the hydroxyl-groups of the wood polymers. It is possible to use mixtures leading to a range of conjugated fatty acid residues in the oil impregnated into the wood of 5 to 100 %. Preferably, the proportion of conjugated fatty acid residues in the oil mixture impregnated in the wood (as % of total fatty acid moieties) is over 33 % with triglycerides, to - at least on average - have one conjugated fatty acid chain per triglyceride molecule, which allows most triglyceride molecules to be bound either to the interpenetrating network or to the wood polymers. Thus leakage of unreacted triglyceride molecules is prevented or reduced. Lower amounts of the conjugated fatty acids residues are possible and will still lead to the desired wood preservation effect and could be economically desirable, however at the cost of an increased risk of leakage of unreacted triglycerides. Even more preferred, the proportion of conjugated fatty acid residues plus modified conjugated fatty acid residues per triglyceride should be higher than one on average to increase the proportion of triglyceride molecules that will be linked at least twice to either the interpenetrating network or the wood polymer. In the case of oils of the wax ester type (an ester of a fatty alcohol and a fatty acid), it is also desirable to have at least one conjugated fatty acid or alcohol per wax ester molecule to reach the same goal of a high frequency of molecules linked to the IPN or to the wood polymers, but here this means at least 50 % conjugated fatty acids or alcohols to reach an average of one per molecule of impregnating oil in the wood; with wax ester oils, preferably both the fatty acid and fatty alcohol have conjugated bonds.

As a further guidance, if such oils, or mixtures of oils, or Diels-Alder adducts of oils are used, preferably the impregnated amount of the oil is sufficient to leave at least 5 wt % (calculated on the weight of the wood) of conjugated fatty acid. Preferably, the amount of conjugated fatty acid ending up in the impregnated wood is at least 10 wt.%.

In practice, the amount of impregnated oil fixed in the wood can easily be higher than 40 wt% (on basis of original wood weight), however, such a large amount of impregnation is considered uneconomical in practice.

In all of the embodiments of the invention, the impregnation can be conducted by simple immersion of the wood in the liquid preservation agent.

Thus the liquid is used in a generous amount, so as to saturate the wood with the liquid. Any remaining liquid (i.e. not introduced into the wood) can be re-used in a further impregnation.

Impregnation itself is a technique well-known to the skilled person. The skilled person will know how to select, in a specific situation determined by variables such as the type of wood, the desired degree of loading (the desired balance of durability and economics), the appropriate vacuum-presssure sequences in a conventional impregnation vessel.

The viscosity of the liquid wood preservation agent as used in the invention should be low enough to allow proper impregnation. Irrespective of whether the conjugated fatty acid-based oil in the liquid wood preservation agent of the invention is in the unmodified or modified form, the improved reactivity towards IPN formation and/or bonding with wood allows the viscosity requirements to be more relaxed than in the prior art.

With conventional impregnation agents, the viscosity needs to be sufficiently low when impregnating (e.g. by heating an otherwise too viscous oil), and sufficiently high when contained in the wood, so as to avoid leaking out of the agent. The IPN formation and/or bonding with wood will prevent the latter.

Nevertheless, for the purposes of impregnation, a relatively low viscosity, below 100 cP, is preferred, more preferably below 80 cP. It will be clear to the skilled person that this refers to the dynamic viscosity, the determination of which is well-known. If needed, the temperature will be adjusted, and/or solvent used, to secure an appropriately low viscosity

Besides the natural oil-based liquid, the wood preservation agent can also contain small quantities of non-aqueous solvent to facilitate penetration into the wood. The choice of non-aqueous solvent is not particularly critical as long as the non-aqueous solvent dissolves the conjugated oil and/or the anhydride functional resin. It is preferred to use a non-aqueous solvent that evaporates easily. The non-aqueous solvent preferably has a low boiling point. It is more preferred to use ketones with a relatively short carbon-chain. It is preferred to have a carbon-chain of between 3 and 7 carbon atoms. It is even more preferred to use methyl isobutyl ketone (4-methyl-2-pentanone).

In a particularly preferred embodiment of the invention, the wood preservation agent is presented in the form of an oil-in-water emulsion. This allows benefiting from certain advantages as compared to the use of undiluted oil, or oil diluted with non-aqueous solvent.

The emulsions can be prepared by emulsifying, in manners standard in the art, the oil (either unmodified or modified through the aforementioned Diels-Alder reaction), in water. Suitable emulsifiers are known to those skilled in the art, and are mentioned in e.g. Becher, P., Encyclopedia of emulsion technology, Marcel Dekker, New York, 1996, or Stauffer, CE. 1999. Emulsifiers Handbook, Eagan Press, 102pp.

In general the emulsions have a considerably lower viscosity than the oils, and thus do not require pre-heating to bring the viscosity to below 100 cP. The use of the emulsions allows a better control over the amount of oil introduced into the wood. In fact, the water serves as a temporary process tool (i.e. a carrier for the oil) and is removed after impregnation (by a dewatering step). The dewatering step is generally conducted by allowing the water to evaporate at ambient pressure and temperature. In order to speed up water evaporation, a drying chamber can be used operating at e.g. 50 0 C. The water can generally be removed before or during the curing step. Particularly if the heat-activation conducted to effect the curing involves conditions under which water will boil, it is preferred to remove the water before curing.

By virtue of using water as a carrier, the oil can be more easily applied on those spots where it is the most efficacious, viz. at the lignocellulosic cell walls. A further advantage in connection herewith, is that the amount of liquid overdosing is lower than if undiluted oil is used. Similarly, during use of the emulsion, less or no oil exudation will take place. The latter is a particular problem, as oil exudation results in a thin, fatty skin present at the wood's surface. This is visually less attractive, and for many daily uses of wood the layer will have to be removed (e.g. by sanding or planing) first. Some of the foregoing advantages are also attainable by lowering the viscosity by dissolving the impregnating agent in a non-aqueous solvent. This, however, has the drawbacks of having to use relatively large amounts of organic solvents that are generally undesired from a safety and environmental point of view. A benefit of using water is that, whilst it may be considered to be counter-intuitive to add water to a wood that is to be preserved against, inter alia, the influence of moisture, it does not additionally add anything to the wood that is not natural. Apart from the foregoing process advantages, it will be apparent that the result is considered to lead to a better preserved wood. Notably, in the event of an oil loading at similar level (calculated as weight percentage based on the weight of the wood), a better resistance against fungi is possible. The ratio of oil-to-water in the emulsion will generally be within the range of from 20:80 to 50:50 (on weight basis).

The oil droplets preferably have an average droplet size below 3 μm diameter, and preferably are as small as possible. Technically nanodroplets can be produced with sizes below 300 nm (Hielscher, T. (2005): Ultrasonic Production of Nano-Size Dispersions and Emulsions, in: Proceedings of

European Nanosystems Conference ENS'05). Practically, it is most feasible to use oil droplets on average of a size of 0.1-2.5 μm diameter, more preferably 0.5 to 1.5 μm, and most preferably around 1 μm diameter. However, the droplet size may vary, and the person skilled in the art will be able to determine, per type of oil and per type or piece of wood, the optimal size of the emulsion droplets.

The liquid wood preservation agent provided by the present invention can further contain additives and adjuvants, such as biocides, anti-oxidants, and oil-soluble catalysts to accelerate the curing of the agent after impregnation.

The method of the invention is generally applicable to any type of wood, particularly to natural, unmodified wood, with the greatest benefits attainable with species such as spruce, pine, beech, maple, birch.

The invention also pertains to the resulting treated, modified wood. As will be apparent from the above, this includes wood comprising an interpenetrating network of wood polymers and a polymerization product obtainable by heating wood impregnated with an oil comprising a conjugated, preferably conjugated trienoic, fatty acid. It also includes wood comprising a bound resin obtainable by heating wood impregnated with the reaction product of an oil comprising a conjugated, preferably conjugated trienoic, fatty acid with a dienophilic unsaturated cyclic anhydride, notably maleic anhydride.

It is to be understood that the invention is not limited to the embodiments as described hereinbefore. It is also to be understood that in the claims the word "comprising" does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

The invention will be further explained hereinafter with reference to the following, non-limiting Examples.

Example 1. Preparation of calendula oil — maleic anhydride adduct

450 parts of calendula oil and 50 parts of maleic anhydride were added to a four-necked flask provided with cooler, mechanical stirrer, thermometer (coupled to a heating mantle) and nitrogen inlet. The mixture was stirred in a nitrogen atmosphere at 120 0 C for 3 hours. The product is a dark yellow-brown oil. Characterisation by GC and NMR spectroscopy showed that maleic anhydride had reacted completely and that a Diels-Alder addition to the calendic acid side chains on the triglyceride molecules had occurred.

Example 2. Preparation of tung oil — maleic anhydride adduct 450 parts of tung oil and 50 parts of maleic anhydride were reacted and characterized in the manner described in Example 1. The product is a dark brown oil.

Example 3 Wood treatment with conjugated oil(s) or derivatives thereof Between 25 and 30 dried and weighed pine sticks (10 x 1 x 1 cm) were arranged inside a stainless steel reactor (1 liter). A vacuum was applied, and, if the viscosity of the oil was higher than 100 centiPoise, the reactor was heated to a temperature at which the viscosity of the oil was lower than 100 centiPoise. After 30 minutes, the conjugated oil (or derivative thereof), which was first heated to a temperature at which the viscosity of the oil was below 100 centiPoise, was added to the reactor. Subsequently, a pressure of 5 bar was applied to the reactor for 1 hour, while keeping the vessel at the required temperature. Then, the remaining oil was pumped out of the reactor, and pressure was reduced to 1 atmosphere. The vessel was opened, and excess oil was removed from the treated pine sticks by paper tissues. The sticks were transferred to an oven and cured at a temperature between 100 and 200 0 C for a period long enough to ensure full curing. The cured specimens were weighed in order to determine the amount of oil inside the wood. With calendula oil an average weight gain of 27% was determined. With the calendula oil / maleic anhydride adduct from Example 1 also a 27% average weight gain was determined.

Example 4. Preparation of emulsion of maleated calendula oil

A mixture of the modified calendula oil of Example 1 (40 parts), demineralised water (58 parts) and sodium dodecyl sulphate (2 parts) at a temperature between 60 and 80 0 C was emulsified with a high shear mixer (Ultra-Turrax T25) during 5 minutes (24000 rpm). In order to prevent oil droplets coalescence the obtained emulsion was cooled to a temperature lower than 25°C by immersing the container in which it was prepared in ice water immediately after emulsification.

Example 5. Treatment of radiata pine with maleated calendula oil emulsion Wood specimens (15 x 25 x 50 mm) were impregnated with the modified calendula oil emulsion of Example 4 in a reactor by subjecting the wood specimens to a vacuum for 45 minutes and then transferring the emulsion to the reactor, thereby submerging the wood in the emulsion. Subsequently, a pressure of 6 bar was applied for 1 hour. The remaining emulsion was then pumped out of the reactor. A vacuum was applied to the reactor for 30 minutes to remove any excess emulsion out of the wood specimens. The treated wood specimens were then transferred to a conditioning room for drying. Finally, the dried treated wood specimens were cured in an oven at 160 0 C for 4 hours.

Example 6. Treatment of radiata pine with maleated calendula oil

Wood specimens (15 x 25 x 50 mm) were impregnated with the modified calendula oil of Example 1 in a reactor by subjecting the wood specimens to a vacuum for 45 minutes and then transferring the modified oil to the reactor, thereby submerging the wood in the oil. Subsequently, a pressure of 8 bar was applied for 1 hour. The remaining oil was then pumped out of the reactor. A vacuum was applied to the reactor for 30 minutes to remove any excess oil out of the wood specimens. The treated wood specimens were then transferred to an oven and cured at 160 0 C for 4 hours.

Example 7. Determination of natural durability

The natural durability of radiata pine treated according to the method described in Example 6 to Basidiomycetes (Coniophora puteana, Gloeophyllum trabeum, Poria placenta and Coriolus versicolor) was tested according to standardised method EN 113. Results were compared with those of untreated wood. Prior to EN 113, the wood samples were pre-conditioned according to EN 84. From EN 113 it followed that treatment according to the method of the invention resulted in excellent resistance to Basidiomycetes; durability class improved from Class 5 (untreated wood) to Class 1.