WO2011136734A1

WO2011136734A1 - Wood protection method and wood product produced using the same

Info

Publication number: WO2011136734A1
Application number: PCT/SE2011/050548
Authority: WO
Inventors: Nasko Stoyanov Terziev; Dmitri Panov
Original assignee: Nasko Stoyanov Terziev; Dmitri Panov
Priority date: 2010-04-30
Filing date: 2011-05-02
Publication date: 2011-11-03

Abstract

The present invention relates to a wood protection method, as well as to the resulting wood product. The present invention provides in particular non-metal impregnation fluids based on plant oils and processes for modification of wood to protect it against biological decay, to improve the physical properties and ensure dimensional stability of the material.

Description

WOOD PROTECTION METHOD AND WOOD PRODUCT

PRODUCED USING THE SAME

FIELD OF TECHNOLOGY

The present invention relates to a wood protection method as well as to the resulting wood product. The present invention provides in particular non-metal impregnation fluids that can be based on any plant oil and process for modification of wood to protect it against biological decay, to improve physical and mechanical properties and ensure dimensional stability of the material.

BACKGROUND

Protection of wood is an important technological step aiming at durability

improvement of the material. Timber has been and still is impregnated with chemical impregnation fluids that in many cases have been or are toxic and constitute a serious hazard for the human's health and environment. The most toxic chemicals, e.g. arsenic, have been replaced, but very few environmentally friendly alternatives for wood protection have been suggested in the recent decades.

Various plant oils have previously been used in methods devised for protection of wood. These oils provide somewhat better wood durability and are environmentally friendly, but in general all plant oils lack fungicidal properties and must therefore be supplemented with an agent with biocide action in applications with a high biological load and moisture content. This is even valid for tall oil and its derivates.

Moreover, the use of a large amount of oil in order to fully impregnate the wood and mechanically stop water does not ensure a significant hydrophobicity of the wood cell wall after long exposure at high relative humidity. Another negative feature of all oils is their ability to leach.

Examples of previously disclosed methods are referred to below.

JP 02206613 discloses a method of protection by impregnating the wood with a protective chemical composition. In said method, COOH groups of polymerisable monobasic acids, e.g. (meth)acrylic acid, react with the epoxy groups of epoxidized soybean or linseed oil (ring-opening addition). After that radical polymerisation initiator BZ O2 is added to the reaction mixture to ensure polymerisation of the (meth)acrylic acid. As a result, the main polymeric chain of the product is formed by polymerized acid, not oil. The purpose of this patent is to obtain epoxidized oil having low viscosity and fast curing rate and to be used as a coating or

impregnating agent for building material, furniture, etc. by carrying out addition reaction of a polymerizable monobasic acid to epoxidized higher fatty acid esters.

JP 02045103 discloses a method of protecting wood by impregnating the wood with a protective chemical composition. In the first of two impregnation steps, the wood is impregnated with phosphoric acid. The wood is dried and impregnated with epoxidized oil with addition of boric acid in the second step. By this process, the epoxy groups undergo ring-opening addition so as to form an ester linkage by the catalytic action of the phosphoric acid or the like.

The epoxidized oil prevents leaching of the toxic boric acid during exploitation of timber. The epoxidized oil is linked to the cell wall ("the epoxy group undergoes ring-opening addition so as to form an ester linkage by the catalytic action of the phosphoric acid or the like") while the boric acid ensures toxicity, i.e. prevents against fungi. The patented method thus encompasses double impregnation steps and drying in between. Boric acid is not soluble in epoxidized oil and does not contribute to the process of polymerisation, i.e. it is not a hardener. Phosphoric acid can react very quickly with the epoxidized oil, resulting in epoxide hardening in the surface layer of the wood. Thus, it can be difficult to obtain deep penetration of epoxidized oil in the wood structure. Thus, there is a need for a wood protection method that is friendly to the

environment, workers involved in the wood protection process, as well as to the users of the wood product. Moreover, there is an increasing need for an

impregnated wood product that is even suitable for indoor use. For indoor use, a minimal leakage of impregnation fluids from the impregnated wood material would thus be required.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide biological protection of wood, while maintaining its dimensional stability, by means of chemical modification of the wood cell wall. Plant oil(s) is used as protective agent. In this way a protection method that is friendly to the environment and workers involved in the wood protection process is achieved. The present invention also relates to a wood protection method as well as to the final wood product. The new wood product is friendly to the environment and users thereof. Moreover, the wood product is suitable for indoor use.

This objective is achieved by providing new non-metal fluids and processes for modification of wood to protect it against biological decay and insect infestation, to improve the physical and mechanical properties and ensure dimensional stability of the material. This is achieved by chemical immobilisation of plant oil(s) in the wood cell wall, modification of said oil and polymerisation of said oil into a polymer matrix. The hydroxyl groups in the wood (i.e. in cellulose, hemicellulose and lignin) are the target for modification by means of epoxidized highly fatty esters and mineral and/or organic acids or mixtures thereof as hardeners. This is performed without using any solvent. The method of using direct acid hardener and solvent- free wood-derivatization may therefore be classified as environmentally friendly since no toxic hazardous organic solvents or chemicals are necessary for the reaction. The impregnation fluids developed according to the invention are easily prepared stable solutions that are characterized by sufficient penetration in wood, lack of odour, no exudation of impregnated fluid on the surface of the treated wood during increased temperatures, excellent water repellence properties and high dimensional stability of the treated timber.

A first aspect of the present invention relates to a method of treating wood or wood products comprising the steps:

-drying the wood or wood product to a moisture content of less than 20% (w/w), preferably less than 16% or starting with a wood or wood product having a moisture content of less than 20% (w/w), preferably less than 16% (w/w);

-contacting the wood material with an epoxidized oil and a hardener; and

-curing the impregnated wood material. In one embodiment the hardener is selected from acetic acid or any mono- or polybasic carboxylic acids, or anhydrides thereof.

In one embodiment the oil is selected from linseed, soybean, catalpa seed, corn, rapeseed, hempseed, sunflower, cottonseed, tung, perilla or safflower oil.

In another embodiment the oil is selected from linseed, soybean, corn, sunflower or tung oil. In another embodiment the method further comprising the step of exposing the wood or wood product to a pre-pressure of above atmospheric pressure prior to the contacting with the oil and hardener.

In another embodiment the curing is done during heating.

In another embodiment no intermediate drying step is performed between the addition of the oil and hardener.

Another aspect of the present invention relates to a mixture comprising epoxidized oil and a hardener wherein the hardener is selected from acetic acid or any mono- or polybasic carboxylic acids, or anhydrides thereof.

In one embodiment the mixture further comprises a biocide and/or fungicide. In another embodiment the oil is selected from linseed, soybean, catalpa seed, corn, rapeseed, hempseed, sunflower, cottonseed, tung, perilla, safflower oil.

In another embodiment the mixture comprises 70-80 weight parts of the oil and 20- 30 weight parts of the hardener.

In another embodiment the hardener comprises a mixture of two organic and/or inorganic acids. In another embodiment the hardener comprises carboxylic groups and wherein the ratio of said groups to the epoxy groups in the oil is 0.5- 1.05: 1 , preferably 0.6- 1.05: 1 , or more preferred 0.8- 1.0: 1 or most preferred 0.95-1.0: 1. A yet another aspect of the present invention relates to an oil composition comprising the mixture defined above and additional oil.

In one embodiment the additional oil is creosote. Another aspect relates to the use of the method or the mixture defined above to protect wood or wood products.

Another aspect relates to the use of the method or the mixture defined above to increase the hydrophobicity of wood or wood products.

A yet another aspect of the present invention relates to wood or a wood product treated with the method or with the mixture defined herein.

In a preferred method the method comprises the following steps:

- Drying a wood material to be impregnated such that the moisture content is below approximately 16% (w/w) or starting with a wood material having a moisture content below approximately 16% (w/w);

- Contacting the wood material with an epoxidized oil containing hardener (e.g. impregnation step);

- Curing the impregnated wood material.

According to one embodiment, the wood that is going to be impregnated is dried by contact with ordinary plant oil (oil drying). In other embodiments, the wood is dried by known drying methods, e.g. conventional, vacuum, high-temperature drying, etc or combinations thereof. If dried in ordinary plant oil, said oil may be drained off, or be removed by other conventional means, such as vacuum treatment, or treated by a combination of said methods.

In an optional step, subsequent to the drying step, the wood material is pre- pressured to a pressure above atmospheric pressure; this may suitably be used when low retention of the impregnation fluid must be achieved. In one embodiment, the dried wood is inserted in an autoclave and pre-pressure is applied. This step is intended to create an air cushion (i.e. pressurised air) inside the wood. Subsequently, the wood material is contacted with impregnation formulation that consists of epoxidized oil and hardener. Said oil may be applied on the wood by any conventional method, such as brushing, spraying, dipping, etc. The use of these methods may also be combined with exposing said wood material to a pressure higher than that of the pre-pressurising step, such that the impregnation fluid is pressed into the wood. The epoxidized oil is based on plant oil. In one embodiment, the plant oil is selected from plant oils with a high degree of unsaturation.

After the epoxidized oil and hardener are applied to the wood structure, said oil is cured, preferably under elevated temperature. The catalyzed reaction between the epoxidized oil and the functional reactive groups of the wood material are initiated by heating the impregnated wood in the presence of saturated vapour of the hardener. In one embodiment, wood is heated at an elevated temperature (i.e.

higher than in the previous steps) in saturated vapour of the hardener. In another embodiment, this is performed in the presence of humid air with high relative humidity, i.e. 90- 100%. Both embodiments prevent evaporation of the hardener. Unlike prior art the present method does not need an intermediate drying step between the addition of the oil and the hardener, the addition of the impregnation formulation is done in one step. The method according to the present invention may use 70-80 weight parts of epoxidized plant oil, that is for example 72-78 weight parts or 74-76 weight parts, and 20-30 weight parts of hardener, that is for example 22-28 weight parts or 24- 26 weight parts. For certain applications, such as outdoor applications, it may be required to add a component with biocide action. In such a case, the impregnation mixture may contain 0. 1 -5 weight parts of such a component.

The impregnation step may be followed by a step of draining of the non-reacted impregnation fluid (epoxidized oil and hardener). In one embodiment this is performed in an autoclave. Superfluous non-reacted impregnation fluid may also be wiped off or left on the wood surface. The draining step may also be followed by a step of vacuum treatment. In one embodiment vacuum is applied after draining off the non-reacted impregnation fluid. This normally takes place only when wood has been impregnated under pressure and is aimed at extracting part of the redundant impregnation fluid from the surface of wood material, i.e. to partly dry and clean the wood surface. This step can be done prior to curing of the impregnation fluid in wood.

According to another aspect of the invention, there is provided a wood product protected by using the present invention method. The modified wood has somewhat slightly darker color than the untreated wood. No other visible differences appear.

In the process described in the JP 02206613 patent, the COOH group of the polymerizable monobasic acid (such as (meth) acrylic acid) reacts with an epoxy group of the epoxidized oil (e.g. soybean or linseed oil). After that, a radical polymerisation initiator BZ O2 is added to the reaction mixture in order to ensure polymerisation of the (meth)acrylic acid. As a result, the main polymeric chain of the product is formed by polymerized acid, not oil.

CH,

H₂C-CH H₂C=C

COOH COOH

acrylic acid methacrylic acid

In the present wood protection method and wood product, a saturated organic acid provides the ring-opening addition and polymerization reactions of the epoxidized oil. No components intended for radical polymerization are used in the present method.

SHORT DESCRIPTION OF THE FIGURES

Figure 1 : Moisture content fluctuations of lap-joints exposed above ground for 27 months according to ENV 12037. Each measurement is an average of 10 samples. Retention of epoxidized (ELO) and linseed oil (LO) in parentheses. DESCRIPTION OF THE INVENTION

According to one aspect of the invention, there is provided a method for protection of wood. Specific features of said method are described below.

In the present application the term "wood product" means a product or a

construction made out of wood.

Oil and its epoxidation

Any plant oil could be used for wood modification that is susceptible to epoxidation. Oils with high number of double bonds, i.e. high degree of unsaturation, are preferred because they have more reactive sites and thus bind harder to the wood and ensure higher degree of cross-linking. Preferably the oil should have an iodine number of at least 100, or more preferably more than 120 or even more preferably more than 140. Linseed and soybean oils are examples of feasible oils for wood modification because they are widely spread and commercially available. In one embodiment, the oil used for the epoxidized oil is selected from the group

comprising the plant oils linseed, soybean, catalpa seed, hempseed, sunflower, cottonseed, tung, perilla, safflower oil. These oils are preferred from an

environmental point of view as they are non-toxic and degradable. In another embodiment, any other conventional oils (i.e. fatty acids) with high degree of unsaturation, e.g. tall oil and its derivates can be used.

Epoxidation is a well known process and can be achieved by oxidation of the named oils by various methods. For example, oxidation with peracetic acid or hydrogen peroxide could be involved. The epoxidation of these plant oils is performed by oxidising the double bonds of the fatty acids to oxirane groups (the simplest example of an epoxide), to modify/ increase their reactivity. These processes are used in large scale production, thus the epoxidized oils used according to the invention for impregnation of wood are commercially available products.

Hardeners

In one embodiment, the hardener in the curing step is acetic acid because it can activate the opening of epoxy rings and provide bonding of oil to the natural structural polymers of the wood cell wall. In a further embodiment, the hardener is selected from the group comprising any mono- or polybasic carboxylic acids, or anhydrides thereof, because they can react with the epoxy groups of the oil and provide an increase of molecular mass (condensation, polymerisation) and viscosity of the oil. In case of polybasic acids or anhydrides, they can act as cross-linkers between oil molecules. In a further embodiment, the hardener is a mixture of organic and inorganic acids. For example the hardener could be a mixture of acetic acid and orthophosphoric acid. Small addition of strong mineral acid enhances the activity of hardener. Two simultaneously competing processes are present: 1 ) ring- opening addition of an organic acid to the epoxy groups and 2) acid-catalyzed ring- opening polymerisation of the epoxy groups. Addition of monobasic acid is not a favourable process since it results in a small increase of the product molecule mass. Addition of a dibasic acid to two oil residues results in cross-linking of said oil molecules. Sterically, the reaction between two oil molecules is more favourable than reaction between two fatty acid residues in the same molecule.

The strength of the used acid affects the rate of epoxy oil polymerisation and polymerisation/ addition ratio. Strong mineral acids ensure immediate oil polymerisation, while weak long-chain organic acids ensure only ring-opening addition to epoxy ring. Preferably the hardener should have a pKa of at least 3, preferably between 3 and 5.

Mixing epoxidized oil and hardener

In one embodiment, the epoxidized oil for impregnation is mixed with acetic acid in equimolar quantities with the epoxy groups of the oil. 1 kg of epoxidized linseed oil with 5.6 mol content of epoxy groups are mixed with 336 g of acetic acid as hardener.

In another embodiment, said oil is mixed with acetic and maleic acid in equimolar quantities with the epoxy groups of the oil. The hardener may contain 2 to 8 parts of acetic acid per part of maleic acid by mass. Bifunctional acids, such as maleic acid, act as cross-linker between two molecules of epoxidized oil.

Maleic acid regulates the speed of polymerisation and hardness of the polymer. The higher the amount of maleic acid, the faster the cross-linking between the oil molecules proceeds and the harder the polymer gets. This leads to less oil molecules attached to the wood hydroxy groups and, consequently, an increased swelling of wood. A typical proportion between acetic and maleic acids is 2: 1 to 4: 1 , e.g. 1 kg of epoxidized linseed oil, 260 g of acetic acid and 130 g of maleic acid to 1 kg of epoxidized linseed oil, 300 g of acetic acid and 72 g of maleic acid.

In a further embodiment, any epoxidized plant oil for impregnation step is mixed with acetic and any other polybasic organic acid in equimolar quantities with the epoxy groups of the oil. Very good results were achieved when the molar amount of carboxylic groups in the hardener was equivalent to molar amount of epoxy groups in the oil. However, the ratio could be 0.5- 1.05: 1 ; preferably 0.6- 1.05: 1 ; or more preferred 0.8- 1.0: 1 or most preferred 0.95- 1.0: 1. Minimal swelling of the impregnated wood and minimal leaching of the impregnation composition were achieved with this ratio between the components.

In a further embodiment 10-20 weight parts of mineral acid as orthophosphoric acid is mixed with 80-90 parts of acetic acid. Drying wood prior to the impregnation

Wood with a moisture content of less than approximately 16% facilitates the impregnation process, thus the impregnation can be performed after initial drying. The initial drying can be performed by means of any already known method

(conventional, vacuum, high-temperature drying, etc.). Since oil is used in the impregnation process, drying in oil seems to be the most feasible drying method for industrial scale production.

Impregnation of wood with the impregnation fluid

Depending on the desired retention of the impregnation fluid, i.e. kg impregnation fluid per m³ wood, any known method (full cell and empty cell impregnation but even spraying, brushing or dipping) can be applied to introduce certain amount of the impregnation fluid into the wood structure. Most typically, the process is carried out in an autoclave that can maintain pressure and vacuum. The

technology is as follows: - Timber is inserted in the autoclave. Pressure is applied to the timber (pre-pressure step). The pressure is in the range of 0.5-5 bar above atmospheric pressure depending on the timber dimensions. The duration of pre-pressure step is in the range of 20-30 min. This step could be omitted, i.e. is optional. This step is intended to create an air cushion of pressurised air inside the wood cell lumens. Said pressurised air ensures quick evacuation of the impregnation fluid from the wood cell lumens, and thus only impregnation of the wood cell wall is ensured. Thus, the practical implications of the pre-pressure are:

1. The amount of retained impregnation fluid is regulated by the size of the air cushion, i.e. by the magnitude of the initial pre-pressure.

2. No additional or significantly less drying is necessary since no impregnation fluid is left in the cell lumens. Thus, the drying time of wood after

impregnation is omitted or significantly reduced. - The impregnation fluid is introduced into the autoclave.

- The impregnation step comprises applying and keeping pressure, above that in the pre-pressure step, to timber and impregnation fluid. The typical values are in the range of 1.5-15 bars, duration of impregnation in the range of 60-180 min

(depending on the timber dimensions).

- The wood is drained of the residual impregnation fluid.

- Vacuum is applied to extract part of the redundant impregnation fluid from the surface of the timber, i.e. to partly dry and clean the wood surface (e.g. 90% vacuum, 20-30 min.). This part is optional. Curing of the impregnation fluid in the wood

In order to polymerise the impregnation fluid, heat is preferably applied but other known sources to initiate curing could also be used such as light or microwaves. Simultaneously, the relative humidity of air should be kept in the range 90- 100% to avoid evaporation of the hardener. The process could be carried out in any conventional drying kiln. An exemplary method is described below:

- plant oil is introduced in the autoclave;

- optionally, the impregnated wood is heated at atmospheric pressure in the ordinary plant oil, that may have a temperature of 60-80°C, for 2-6 h;

- the wood is drained of residual plant oil; and - optionally, vacuum is applied to extract part of the redundant plant oil from the surface of timber, i.e. to partly dry and clean the wood surface (e.g. 90% vacuum, 20-30 min).

- the impregnated wood is cured at atmospheric pressure in saturated vapour of the hardener or humid air (90-100% relative humidity) under elevated temperature, e.g.

80- 100°C, for 2-6 h.

Addition ofbiocides or fungicides

The wood impregnated according to the present invention method is not completely fungicide proof, but can ensure reasonable protection against decay by fungi above ground in an outdoor environment, while being ecological from an environmental perspective. The method and wood material according to the present invention may be combined with a biocide. The biocide component is optional, but may be required in some applications where the biological load may be high and there is a potential risk of rot or insect infestation. Fungicides that may be used in the protective method according to present invention may be selected from conventional fungicides with low water solubility. Exemplary fungicides are oil soluble copper salts, tolylfluanid, dichlofluanid, 3-iodo-2-propynyl butylcarbamate, thiabendazole, sorbic acid, potassium (E,E)-hexa-2,4-dienoate, 4-5-dichloro-2-octyl-2H-isothiazol- 3-one, propiconazole, tebuconazole, cyproconazole, dazomet, potassium salt of cyclohexylhydroxydiazene- 1 -oxide, N-(3-aminopropyl)-N-dodecylpropane- 1 ,3- diamine, didecyldimethylammonium chloride, quaternary BKC, DDAC or TMAC, benzyl-C i2 i6-alkyldimethylammonium chloride, N,N-didecyl-N-methyl-polyoxyethyl- ammonium propionate, fenpropimorph and creosote.

Explanation of the chemical mechanisms

During the above-described steps, the following chemical reactions take place:

Double bonds of oil are modified to epoxy groups that react with hydroxyl groups of cellulose, hemicellulose and lignin.

The wood is impregnated with the epoxidized oil and hardener to activate the reactive groups and is then heated under defined time and temperature to provide the cross linking between the epoxy groups and the hydroxyl groups of the wood. Simultaneously, the epoxidized oil is polymerized and solidified in the ultra- and nanostructure of wood.

General reactions are:

Reaction with wood cell wall components bearing hydroxyl groups.

2. Polymerisation reaction between two chains containing epoxy groups.

3. Ring-opening addition of carboxylic acid to epoxy group.

4. Cross-linkin with maleic acid.

Wood items that can be treated

Both hard- and softwoods can be treated according to the invention. Further, the method of the present invention could be applied to various wood products such as timber, paper, carton, veneer, fibres and particles, saw dust, strands, or other solid particles of wood material. The wood product according to the present invention may be used for various purposes, such as outdoor items (garden furniture, decks, railings and stairs, walkways, boardwalks, playground equipment), building parts (fascia, cornice, siding, sills, frames, millwork), bridge parts (beams, railings, decking), railway sleepers, utility poles, heavy construction timber, fence and vineyard posts, stakes, highway items (guard rail posts, guard rail plates, sign posts, light poles), flooring and containers (tanks, buckets), as well as door and window frames, furniture details, and other outdoor and indoor items.

Behaviour and appearance of the treated wood

The wood product is defined by high dimensional stability, improved physical and

mechanical properties and durability. The wood surface is brownish and its appearance does not differ much from the surface of the untreated wood. The proposed method ensures fixation of the used plant oil to the wood cell wall. Treated wood becomes dimensionally stable, hydrophobic and oil leaching is eliminated. For instance, lap-joint samples never exceeded moisture content of 25% during a three-year period of exposure above ground. The volumetric swelling decreases to 6-8% compared to 12-13% of the untreated wood. At the same time the mechanical properties of the wood increased. Although modified, the oils are not fungicide proof, but can ensure reasonable protection against decay fungi when exposed above ground. EXAMPLES

Example 1 - Treatment process Scots pine sapwood (Pinus sylvestns L.) was used throughout a laboratory and field studies presented herein. This species is broadly used in the Nordic countries, but solid wood or other solid wood products of any other species can be protected using the method according to the present invention. Other wood products (veneer, fibres, particles, saw dust and strands) may be also modified individually, or be combined in glued or board products after protection.

In the following experiment Scots pine wood blocks with thickness of 30 mm was used. Drying was carried out in linseed oil at 80°C to decrease the wood moisture content below 16% prior to impregnation.

The wood was then pre-pressurised at 1.0 bar above atmospheric pressure and 80°C.

Impregnation was carried out without decreasing the pre-pressure. The

impregnation fluid, containing 75 weight parts epoxidized linseed oil and 25 weight parts of acetic acid, was inserted into the autoclave and the pressure was increased to 4 bars. The duration of the impregnation was 60 min. After the impregnation, the pressure was decreased to atmospheric pressure and the residual impregnation fluid was drained off.

In order to polymerise the impregnation fluid, heat was applied. This was done in two steps. Firstly, ordinary linseed oil was introduced in the autoclave and the wood heated and kept at 60°C for 2 h. Secondly, the impregnated timber was cured using elevated temperature. The temperature of wood was kept at 80°C for 6 h. After the curing, the residual linseed oil was drained off. Finally, vacuum was applied to extract part of the redundant linseed oil from the surface of the wood, i.e. to partly dry and clean the wood surface (90% vacuum for 20 min). Example 2 (General behaviour of the treated wood)

The proposed method ensured fixation of the epoxidized plant oil to the wood cell wall. Treated wood became dimensionally stable, hydrophobic and any oil leaching was eliminated. The wood hydrophobicity is demonstrated by the next example. Lap-joint samples never exceeded moisture content of 25% during a three-year exposure period above ground. The volumetric swelling decreased to 6-8%

compared to 12- 13% of the untreated wood. The mechanical properties of the treated wood increased. Ultimate bending strength and modulus of elasticity increased with 10-15% compared to the untreated wood. Hardness increased with 10% when the retention was 100 kg/m³. Hardness can be improved further if the retention is increased additionally, i.e. a property that can be used for treatment of timber for heavy duty floors, railroad sleepers and poles. In order to achieve higher hardness some parameters of the impregnation schedule, i.e. pressure and duration, can be adjusted to ensure retention higher than 100 kg/ m³. In general, both pressure and duration must be increased.

Example 3 (Behaviour of the treated wood)

Epoxidized plant oils showed very good hydrophobic properties after being introduced in the wood cell wall. The epoxidized oils occupy some of the hydroxyl sites of the main structural polymers and simultaneously seal pores and rays, thus almost eliminating the transport of free water. The anti swelling efficiency of epoxidized linseed oil (ELO) treated wood was found in the range of 50-60%, thus being comparable with that of thermally modified wood. The above mentioned is a good prerequisite for using ELO treated wood in above ground conditions. Since no laboratory tests are available to simulate such conditions, a standard lap-joint test is on-going to reveal the efficacy of the treatment.

The field performance of lap-joints treated with linseed oil (LO) and ELO after 27 months of exposure is demonstrated in Fig. 1. The climate in Uppsala, Sweden is characterized by an average annual precipitation and temperature of 562 mm and 7 °C. These climatic conditions guarantee the untreated Scots pine sapwood lap- joints a median decay rating of 3.0 after 6 to 7 years of exposure and an average service life in the range 6.5-8.5 years (Bergman et al. 2008). The above ground field exposure of treated and reference samples demonstrated that the moisture content fluctuations are significantly higher in the untreated samples (above 100% in winter) than in those impregnated with oil (Fig. 1). ELO showed best performance keeping the average annual moisture content to less than 20%; the moisture content never exceeded 25%. Although having higher retention, LO was less effective than the ELO showing 10 to 25% more moisture in summer and winter than ELO. Both LO and ELO treated samples have less checks than the untreated ones. The LO treated samples are discoloured by staining fungi both externally and in the lap while ELO treated samples are less discoloured and totally free of stain in the lap. The results support the conclusion that ELO treated timber can thus be used for claddings with or without painting.

The above ground field test bears some interesting ideas.

Considering the properties that an eventual alternative protective formulation to creosote should fulfill, some very important ones could be mentioned namely, high dimensional stability, high protective efficacy, unchanged or improved wood mechanical properties and non-corrosive. Nowadays creosote alternatives do not seem to be viable, especially when cost is taken into account. The use of creosote impregnated wood is restricted to railway sleepers and electrical poles, even though it counts for an important quantity of wood consumption in Europe. A future ban on the use of creosote would threaten the continuous use of wood in these certain applications and would instead promote the use of alternative materials such as concrete. The use of non-renewable materials (i.e. concrete) does not represent any improvement in terms of environment when comparing life cycle analysis. Table 1 : Corrected mass loss of creosote and creosote + ELO treated samples in EN 113 test. Each measurement is an average of 5 samples. Standard deviation in parentheses.

Treatment and

Trametes Coniophora Postia Lentinus

average

versicolor puteana placenta lapideus

retention

Creosote (83. 1

kg/m³) 0 (2.8) 0.2 (0.1) 0.4 (0.4) 1.1 (0.5)

(Hazard class 3)

Creosote (1 12.0

kg/m³) - 1.4 (0.9) 0.7 (0.7) 0.4 (0.4) 1.3 (0.6)

(Hazard class 4)

Creosote + ELO

(30.7 + 73.7 -0.9 (0.1) 1.3 ( 1.6) 1.8 (2.5) 1.7 (5.0)

Creosote + ELO

(39 + 93.5 -3.6 (0.7) -2.9 ( 1.3) -4.1 (0.3) 2.8 (4.1) The results of laboratory decay test (EN 1 13) are shown in Table 1. The mixture of creosote and ELO is as effective as creosote at similar retention approved by the Nordic Wood Protection Council for hazard class 4. It should be emphasized that the mixtures are effective against Lentinus sp. - a fungus used when creosote and oily preservatives are tested. The test demonstrated also that the amount of creosote in the mixture can be reduced to approximately 30%, i.e. 30 kg/m³. It is probable that the creosote retention in the mixture can be further optimized at lower retention for above ground use (hazard class 3). As a step this should have a positive impact on the environment and can probably be a step to retain creosote as preservative. The authorities responsible for the use of creosote in Sweden classify railway sleepers in hazard class 3, i.e. above ground. Although debatable, this decision opens an option for using ELO as a substitute for creosote. Table 2: Corrected mass loss of LO, ELO, ETOE, fenpropimorph + ELO and only fenpropimorph treated samples in EN 1 13 test. Each measurement is an average of 5 samples. Standard deviation in parentheses.

Treatment and Trametes Coniophora Postia Gloeophylum Lentinus average retention versicolor puteana placenta trabeum lapideus

Untreated 19.3 (3.3) 28.3 (4.8) 32.2 (3.6) 28.5 (6.1) 27.3 (6.4)

LO (116 kg/m3) 12.0 (4.2) 29.6 (5.3) 35.0 (5.6) - 39.2 (8.5)

ELO

14.4 (4.4) 24.7 (2.2) 20.7 (7.8) 24.3 (5.2) 33.4 (6.6)

(99.8 kg/m³)

ETOE

(85.4 kg/m³) 6.8 (4.6) 28.6 (14.4) 22.3 (10.3) 8.9 (4.2) 27.3 (3.9)

Fenpropimorph +

ELO

2.0 (1.3) 13.7 (5.0) 6.5 (2.2) - 13.3 (4.4)

Fenpropimorph +

ELO

-0.4 (2.1) 6.8 (7.2) 4.7 (3.6) -0.1 (3.9)

(105.4 g/m³ + 126

kg/m³)

Fenpropimorph

3.4 (3.2) 36.6 (8.5) 11.4 (5.0) - 13.2 (3.3) (88 g/m³) An example of the synergic action between an organic fungicide (fenpropimorph) and ELO based on the results of laboratory decay test (EN 1 13) is shown in Table 2. The treated samples had a similar retention in the range of approximately 85- 120 kg/ m³. Comparison between untreated, LO, ELO and epoxidized tall oil ester (ETOE) treated samples showed no significant improve of the wood durability. In general no difference can be monitor even between the epoxidized linseed and tall oil ester treated samples. Fenpropimorph is a systemic morpholine fungicide with water solubility of 4.32 mg/L at 20 °C and pH 7. Fenpropimorph is hydrolytically stable at low pH which is essentially important for the used formulation. Fenpropimorph itself is not effective at the retention tested (88 g/m³). However, the combination of ELO and fenpropimorph improved significantly the decay resistance; the retention of 126 kg/m³ ELO and addition of 105.4 g/m³ demonstrated an efficiency that is reasonable when tested by EN 1 13.

It was proven previously (Panov et al. 2010) that ELO ensures water repellence and dimensional stability of the treated wood. Despite the short exposure of the treated wood in an above ground test (28 months) there is a good prerequisite for long lasting effect on the durability of ELO treated lap-joints consisting of very small moisture content fluctuations (±5%).

The present study showed that although the improved hydrophobic behaviour of the wood treated with ELO, it has no effect on the durability when tested by laboratory EN 1 13 test that mimics in ground conditions. The study demonstrated the synergy between epoxidized oil and conventional biocide to develop more environmentally-friendly wood protecting products.

The role of the synergist consists of regenerating or strengthening another substance. In the case of fungicides, recent studies indicate that inclusion of antioxidants gives a preventive mechanism against free radicals produced by fungi in wood and protects the cellular wall from colonisation of fungi in the initial stage (Green, et al. 2003 and Schultz et al. 2005a). Another type of synergy is that the antioxidant may protect the biocide from becoming deteriorated by the formation of radicals of microbial origin - in other words, antioxidants increase the natural resistance of the biocide by defending it against microbial degradation (Schultz et al. 2005b).

The approach in the present study is to combine the hydrophobic oil treatment with organic fungicide. Many plants oils could be a source of natural substances to be used as synergic bioproducts thus expanding the biocidal capacity of synthetic fungicides and enabling their concentration to be reduced. Table 3: Selected mechanical properties of untreated and ELO treated samples. Each measurement is an average of 20 samples. Standard deviation in parentheses.

11899 100.5

Untreated - - 53.1(11.4) - 2.11(0.35) - (2616) (28.9)

ELO (103.7 13078 99.6

9.9 0 60.0(11.7) 13.0 2.56(0.41) 21.3 kg/m³) (1999) (20.1)

ELO (196.5 16655 142.6

39.9 42.4 60.0(12.9) 13.0 3.86(0.62) 82.9 kg/m³) (84) (4.7)

The results of the selected mechanical tests (Table 3) showed no decrease of any of the measured properties. An average retention of 103.7 kg/m³ ELO increased slightly the MOE and impact bending strength as well as the hardness. The properties can further be improved if higher retention (196.5 kg/m³) is applied. Such an approach can be of interest when wood is used for internal joineries, e.g. flooring, windows and doors.

Claims

1. A method of treating wood or wood products comprising the steps:

-contacting the wood material with an epoxidized oil and a hardener; and

-curing the impregnated wood material.

2. The method according to claim 1 wherein the hardener is selected from acetic acid or any mono- or polybasic carboxylic acids, or anhydrides thereof.

3. The method according to any of the preceding claims wherein the oil is selected from linseed, soybean, catalpa seed, corn, rapeseed, hempseed, sunflower, cottonseed, tung, perilla or safflower oil.

4. The method according to any of the preceding claims wherein the oil is selected from linseed, soybean, corn, sunflower or tung oil.

5. The method according to any of the preceding claims wherein the method further comprising the step of exposing the wood or wood product to a pre-pressure of above atmospheric pressure prior to the contacting with the oil and hardener.

6. The method according to any of the preceding claims wherein the curing is done during heating.

7. The method according to any of the preceding claims wherein no intermediate drying step is performed between the addition of the oil and hardener.

8. A mixture comprising epoxidized oil and a hardener wherein the hardener is selected from acetic acid or any mono- or polybasic carboxylic acids, or anhydrides thereof.

9. The mixture of claim 8 further comprising a biocide and/or fungicide.

10. The mixture of any one of claims 8 and 9 wherein the oil is selected from linseed, soybean, catalpa seed, corn, rapeseed, hempseed, sunflower, cottonseed, tung, perilla or safflower oil.

1 1. The mixture of any one of claims 8- 10 wherein the mixture comprises 70-80 weight parts of the oil and 20-30 weight parts of the hardener.

12. The mixture of any one of claims 8- 1 1 wherein the hardener comprises a mixture of two acids.

13. The mixture of any one of claims 8- 12 wherein the hardener comprises carboxylic groups and wherein the ratio of said groups to the epoxy groups in the oil is 0.5-1.05: 1 , preferably 0.6- 1.05: 1 , or more preferred 0.8- 1.0: 1 or most preferred 0.95- 1.0: 1.

14. An oil composition comprising the mixture according to any of claims 8- 13 and an additional oil.

15. The composition according to claim 14 wherein the additional oil is creosote.

16. The use of the method according to claims 1 to 7 or the mixture according to claims 8 to 13 to protect wood or wood products.

17. The use of the method according to claims 1 to 7 or the mixture according to claims 8 to 13 to increase the hydrophobicity of wood or wood products.

18. Wood or a wood product treated with the method according to any one of claims 1 to 7 or with the mixture according to claims 8 to 13.