Wood treatment

Abstract

The present invention relates to a method and apparatus for treating wood, as well as to the treated wood and uses thereof. The involves the steps of: contacting the wood with a solution containing a carboxylic acid or carboxylic anhydride and a polyol; and subsequently contacting the wood with oil at a temperature of 110 - 160ºC for 30 minutes to 48 hours.

Description

[0001] Wood Treatment

[0002] Field of the Invention

[0003] The present invention relates to a method and apparatus for treating wood, as well as to the treated wood and uses thereof.

[0004] Background to the Invention

[0005] Wood is a popular building material due to its durability and strength. However, wood is susceptible to decay, rot, fire and insect infestation, which can compromise its integrity and shorten its lifespan. To overcome these problems, wood is often treated to increase its resistance to decay, fire, weathering and insects.

[0006] Various different wood treatments are known. One such treatment involves treating the wood in an aqueous solution of citric acid and sorbitol, followed by hot air drying to cure the citric acid and sorbitol, thereby producing a wood / polymer matrix. This process works well, but can be time and energy consuming. It is always desirable to provide an improved wood treatment with additional qualities.

[0007] Summary of the Invention

[0008] According to a first aspect, the invention relates to a method of treating wood, the method involving the steps of: contacting the wood with a solution containing a carboxylic acid or carboxylic anhydride and a polyol; and subsequently contacting the wood with oil at a temperature of 110 - 160°C for 30 minutes to 48 hours.

[0009] According to a second aspect, the invention relates to apparatus configured to treat wood, the apparatus including: a first bath for contacting wood with a solution containing a carboxylic acid or carboxylic anhydride and an polyol; a second bath for contacting wood from the first bath with oil; a heater for heating oil in the second bath; a finishing area; and a conveyor system for conveying wood into the first bath, from the first bath to the second bath, and from the second bath to the finishing area.

[0010] According to a third aspect, the present invention relates to treated wood, which is obtainable by the method of the first aspect of the invention. According to a fourth aspect, the present invention relates to the use of the treated wood in the marine environment, in construction, in furniture, for decorative purposes, or in contact with soil.

[0011] The first step of the method, which involves the wood being contacted with a solution, leads to the wood becoming impregnated with the carboxylic acid or carboxylic anhydride and polyol. The second step of the method involves subsequently contacting the wood with oil at an elevated temperature of 110 - 160°C for a sustained period of time of 30 minutes to 48 hours, (the “hot oil” step). This leads to water within the wood evaporating and to curing of the carboxylic acid or carboxylic anhydride and a polyol.

[0012] The inventors have surprisingly discovered that using a hot oil step to effect curing leads to a very even and efficient distribution of carboxylic acid or carboxylic anhydride and polyol in the wood, resulting in wood of high quality and performance. In particular, the wood can have excellent dimensional stability, can be flexible and less brittle. Without wishing to be bound by theory, this is thought to be due to the fact that oil has a high specific heat capacity, and so the whole thickness of the wood heats up to the temperature of the oil bath quickly. On heating there are essentially two processes taking place, water evaporates from the wood, and the carboxylic acid or anhydride and polyol cure with themselves and the wood. When water evaporates from the wood, it tends to take some of the carboxylic acid or anhydride and polyol with it, leading to migration from the centre of the wood to the surface. When using an oil bath, the curing reaction occurs quickly, effectively fixing the carboxylic acid or anhydride and polyol in place through the whole thickness of the wood. In this way, there is less migration to the surface, leaving an even and efficient distribution of carboxylic acid or carboxylic anhydride and polyol in the wood. This can improve the mechanical properties, and lead to the wood being particularly suitable for use in harsh environments.

[0013] In addition, the hot oil step can be faster and more efficient than a traditional hot airdrying method, resulting in reduced energy consumption or can mean that less of the carboxylic acid or carboxylic anhydride and polyol need to be used to achieve the same result.

[0014] A further advantage is that the wood ends up with a layer of oil on its outside surface which leads to increased water resistance, leading to a higher dimensionally stable material also meaning that less carboxylic acid or carboxylic anhydride and polyol need to be used to achieve the same durability compared to a traditional hot air method, due to the additional protection from the surface layer of oil.

[0015] The invention has particular potential for applications that are especially harsh for wood, including in the marine environment, in construction, in furniture, or in contact with soil.

[0016] Detailed Description of the Invention

[0017] The invention relates to a method of treating wood. Any type of wood can be used, such as a hardwood or softwood. The method is particularly suitable for use with Scots Pine, Norway spruce or Downey Birch. After being harvesting, the wood is usually pre-treated to remove some of the water, which means that it can more easily be impregnated with the solution. This can be through simple air drying for days, weeks, months, or years or, to speed up the drying process, the wood can be placed in a kiln. The water content of wood being treated, i.e. after pre-treatment by air or kiln drying, is typically 10-20% wood moisture content, with 12 to 18% being typical. Any size and shape of wood can be used, but the wood would typically be square for cladding, decking or construction, or it can also be in the shape of poles for fencing, marine applications, telegraph poles and the like.

[0018] The first step of the method of treating the wood, is to contact the wood with a solution containing a carboxylic acid or carboxylic anhydride and a polyol. The purpose of this step is to impregnate the wood with the solution. By impregnate, we mean that the solution is taken up into the wood. Usually it is desirable for the solution to impregnate the whole thickness of the wood, but it can just impregnate the wood to a small extent at one or more surfaces, for example all surfaces of the wood. The optimal impregnation can depend on the type of wood, with pine wood ideally being impregnated throughout the sapwood, whereas spruce only needs to be impregnated to a depth of 4-5mm from the surface. The contact can involve spraying the solution onto the surface of the wood, or dipping the wood into the solution, but, for increased uptake, preferably involves immersing the wood in the solution. The immersion typically lasts for 30 minutes to 10 hours, or 1 to 5 hours, preferably 2-3 hours.

[0019] Methods of impregnating wood with chemical solutions are known in the art, and can involve a vacuum-pressure followed by overpressure to ensure effective impregnation. In the present method, the wood can be contacted with the solution under a vacuum of 20 mbar to a pressure of 15 bars. This step is generally not heated and would usually be carried out at ambient temperature, which could be between - 5 °C and 40°C depending on the location, and would generally be 0 to 30 °C.

[0020] In a preferred embodiment, the wood is initially contacted with the solution under a vacuum of 20 mbar to 200 mbar, or 30 to 100 mbar and then contacted with the solution under elevated presume of 1 to 15 bar, or 5 to 10 bar. The initial vacuum stage is for any suitable length of time, such as 10 minutes to 3 hours, or 20 to 40 minutes, and the elevated pressure stage is also for any suitable length of time, such as 30 minutes to 10 hours, or 30 minutes to 3 hours. This has been found to optimise impregnation. After being taken out of the solution, a post-vacuum treatment can be applied, where the wood is placed under a vacuum of 20 mbar to 200 mbar. This has the effect of removing the excess solution, leaving the surface substantially dry.

[0021] The solution in the first step contains a carboxylic acid or carboxylic anhydride and a polyol. The carboxylic acid or carboxylic anhydride is preferably a polycarboxylic acid, i.e. , has more two or more carboxyl groups or is a polycarboxylic anhydride i.e. , is an anhydride which is derived from a carboxylic acid having more two or carboxyl groups. The carboxylic acid or carboxylic anhydride usually has a relatively low molecular weight, or low number of carbon atoms, by which we mean a molecular weight or number of carbon atoms which is low enough to penetrate wood fibres, such as of up to 15, 20 or 25 carbon atoms. Usually a carboxylic acid is used but, as explained below, curing is thought to proceed initially via two carboxyl groups from the carboxylic acid forming an anhydride and releasing a water molecule. Accordingly, without wishing to be bound by theory, the anhydride could replace the carboxylic acid as the initial reagent.

[0022] The carboxylic acid or anhydride can be acetic acid or acetic anhydride. In a preferred embodiment, the carboxylic acid is selected from the group consisting of: 1 , 2, 3, 4 - butanetetracarboxylic acid, citric acid, maleic acid, succinic acid, ita-conic acid, transaconitic acid, cis-aconitic acid, tricarballylic acid, 1 , 2, 3-benzenetricarboxylic acid, 1 , 2, 4-benzenetricarboxylic acid, 1 ,2, 3, 4- cyclobutanetetracarboxylic acid, tetrahydrofuran- 2, 3, 4, 5-tetracarboxylic acid, 1 ,2, 4, 5- benzenecarboxylic acid, oxalic acid, malonic acid, glutaric acid, adipic acid, fumaric acid, isocitric acid, malic acid, oxalosuccinic acid, trimesic acid, trimellitic acid, hemimellitic acid, or a mixture thereof. The most preferred carboxylic acid is citric acid. The preferred polycarboxylic anhydride is a derivative of one or more of the polycarboxylic acids selected from the group above, and can be maleic anhydride or citric anhydride.

[0023] The solution also contains a polyol, i.e., a compound with two or more hydroxy groups. The polyol is preferably selected from the group consisting of: hexanediol, xylitol, mannitol, glycerol, sorbitol, ethylene glycol, propylene glycol, 1 ,3-propanediol, 1 ,2-butylene glycol, 1 ,3-butylene glycol, 1 ,4-butanediol, 2-methyl-1 ,3-propanediol, pentaerythritol, neopentyl glycol, trimethylolpropane, 2,2,4,4-tetramethyl-1 ,3- cyclobutanediol, 3-methyl-1 ,5-pentanediol, 1 ,4-cyclohexane-dimethanol, 1 ,3- cyclohexanedimethanol, diethylene glycol, dipropylene glycol, triethylene glycol, 1 ,6- hexanediol, tripropylene glycol, tetraethylene glycol, glucose, dextrines, xylitol, saccharose, fructose, erythritol, lactitol, isomalt, maltitol, hydrated starch hydrolysate (HSH), threitol, adonitol, arabitol and inosite or a mixture thereof. Preferably the polyol is: hexanediol, xylitol, mannitol, glycerol, or sorbitol or a mixture thereof. Most preferably the polyol is sorbitol.

[0024] Citric acid and sorbitol have been used in the past to treat wood. It is expected that the carboxylic acids or carboxylic anhydrides and polyols listed above would work in a similar manner. The solution is typically an aqueous solution, with the carboxylic acid or carboxylic anhydride and polyol being added to water as dry ingredients, usually powders. The water is used as a vehicle for impregnating the wood with the carboxylic acid or carboxylic anhydride and a polyol, rather than as a useful component itself, so the solution is as concentrated as possible, while maintaining a suitably low viscosity. In a preferred embodiment, the weight ratio of dry components (i.e. the carboxylic acid or carboxylic anhydride and polyol) to water is 1 :1.5 to 3:1 , preferably 1.5:1 to 3:1 , most preferably about 2.5:1. Alternatively a concentrated solution of dry components in water can be made with a weight ratio of dry components to water of 1 :0.5 to 1 : 1.5, preferably 1 :1.7 to 1 :1 , or about 1 :0.8, where 10g of solid dry components is dissolved per 8g of water. The concentrated solution can then be diluted in water to give, for example, a 10, 20, 30, 40, 50, 60, 70 or 80% solution, preferably a 20, 40 or 60% solution. This results in a ratio of dry components to water of around 1 :1 or <1 :1 , for example 1 :1.5 to 1 :10, or about 1 :2, 1 :3.5 or 1 :8.

[0025] The molar ratio of dry components is also important. In a preferred embodiment, the molar ratio of carboxylic acid or carboxylic anhydride to polyol in the solution is 1 :5 to 5:1 , 1 :1 to 5:1 , or 2:1 to 4:1 , and is preferably about 3:1.

[0026] After being contacted with the solution, the wood is contacted with oil at a temperature of 110 - 160°C for 30 minutes to 48 hours. The purpose of this step is to cure the carboxylic acid or carboxylic anhydride and polyol with each other and with the wood. This leads to improved dimensional stability and biological durability of the wood. This is a critical step of the method, and different from existing citric acid / sorbitol wood treatments.

[0027] Without wishing to be bound by theory, curing of the wood is thought to proceed via a mechanism in which two carboxyl groups from the carboxylic acid form an anhydride and release a water molecule, and then the anhydride esterifies a hydroxyl group of cellulose which is naturally present in the wood and / or the polyol. Repetition of this reaction leads to the carboxylic acid crosslinking cellulose with cellulose, and cellulose with polyol. This reaction occurs spontaneously at a temperature of 110 - 160°C. The duration of 30 minutes to 48 hours is selected to allow sufficient time for all the possible reactions to be substantially completed, while keeping the energy usable to a minimum. The exact temperature of the oil and the duration of immersion can be selected depending on the type and concentration of carboxylic acid or carboxylic anhydride and polyol used, the wood species and size, and other factors.

[0028] As above, carboxylic acids and polyols have previously been used for treating wood, particularly citric acid and sorbitol. The exact mechanisms are still to be investigated, but “Katarzyna Kurkowiak, David Hentges, Stephane Dumarqay, Philippe Gerardin & Holger Militz (2023) Understanding the mode of action of sorbitol and citric acid (SorCA) in wood, Wood Material Science & Engineering, 18:1, 67-75, DOI" proposes that during curing the reaction proceeds through the mechanism as follows. This shows that the citric acid (CA) forms an anhydride; the anhydride esterifies a hydroxyl group of cellulose which is naturally present in the wood, or sorbitol, and a further anhydride is formed which reacts with other hydroxy groups of cellulose or sorbitol. Accordingly, citric acid and sorbitol become crosslinked with the wood. However, the existing treatments usually involve curing with hot air, as in “Katarzyna Kurkowiak, David Hentges, Stephane Dumargay, Philippe Gerardin & Holger Militz (2023) Understanding the mode of action of sorbitol and citric acid (SorCA) in wood, Wood Material Science & Engineering, 18:1, 67-75, DOI". Hot steam has also been mentioned as in EP 3 967 468 A1. The hot oil method of the present invention can achieve curing in less time than the hot air or hot steam method, such as around half the time, leading to time and energy savings. In addition, using a hot oil step to effect curing can lead to very even and efficient distribution of carboxylic acid or carboxylic anhydride and polyol in the wood, resulting in wood of high quality and performance with good mechanical properties as set out above. A further advantage is that the wood ends up with a layer of oil on an exterior surface (or surfaces) which leads to increased water resistance, meaning that less carboxylic acid or carboxylic anhydride and polyol can be used compared to a traditional hot air or steam method to achieve the same durability, due to the additional protection from the surface layer of oil.

[0029] A hot oil bath has been used previously in wood treatment methods but not in connection with a first step involving a solution of carboxylic acid or carboxylic anhydride and polyol, and particularly not in connection with a citric acid / sorbitol treatment as is preferred in the present application. Instead a hot oil bath has been used following impregnation of preservatives such as copper. In this context, the oil used is to displace water and effectively seal the wood, but not for curing, particularly involving esterification of hydroxy groups on the polyol and cellulose, which is key to in the present method.

[0030] The inventors have found that heat from the oil penetrates the wood quickly due to the high heat capacity of oil, and cures the carboxylic acid or carboxylic anhydride and polyol within it quickly. This means that it is fixed in place throughout the thickness of the wood, and is less likely to migrate to the surface of the wood with the evaporating water, resulting in a more even and efficient crosslinking which improves the properties of the treated wood. This is not the case where curing is not taking place, for example where the wood has been impregnated with preservatives such as copper. The contact can involve spraying the oil onto the surface of the wood, but preferably involves immersing the wood in the oil. The inventors have found that 30 minutes to 48 hours of contact leads to optimal curing. In a preferred embodiment, the wood is contacted with oil for 1 to 20 hours, 2 to 10 hours, 3 to 8 hours, or about 5 hours.

[0031] To optimise curing, the wood is contacted with oil at a temperature of 110 - 160°C. In a preferred embodiment, the temperature is 20 - 150°C, preferably 130 to 150°C, most preferably about 140°C.

[0032] The hot oil step is usually carried out at atmospheric pressure, advantageously meaning that there is no need to provide pressurising equipment.

[0033] Any oil can be used, but an oil with a high heat density is particularly advantageous, as is an oil that is environmentally sustainable. In a preferred embodiment, the oil is mineral oil and / or organic oil, preferably silicon oil, linseed oil, mineral oil or a mixture thereof.

[0034] The apparatus used to treat wood can include a first bath for contacting wood with a solution containing a carboxylic acid or carboxylic anhydride and a polyol. This is particularly useful when the wood is immersed in the solution. The apparatus can also include a second bath which can contain oil, a heater for heating the oil, and usually a temperature controller. The apparatus will include a finishing area, where treated wood can be stored after being taken out of the second bath. In order to move the wood through the baths, the apparatus can include a conveyor system for conveying wood into the first bath, from the first bath to the second bath, from the second bath to the finishing area.

[0035] The baths can be made of any suitable material, such as stainless steel or other non- corrosive materials including fibre glass, or coated steel. The heater can be any type of heating element, such as electric, gas, bio, or oil-fired. The temperature controller can be a thermostat or other suitable device that regulates the temperature of the hot oil bath to maintain a consistent temperature throughout the process. The conveyor system can include a belt, rollers, or any other suitable mechanism and can be operated manually or automatically and can include a timer or other control mechanism to ensure that the wood is immersed in the hot oil bath for the desired duration. The conveyor system can also be equipped with a drying system to remove any excess oil from the wood surface after the treatment.

[0036] The treated wood, which is obtainable by the method above, will have the carboxylic acid or carboxylic anhydride and polyol cured inside it, with a more even distribution, which is advantageously achieved by the hot oil step. This can be seen by a similar density though a cross-section of the wood. It will also have oil impregnated at the surface. Usually the oil is impregnated to a depth of 1-3 mm from the surface of the wood.

[0037] Due to its beneficial properties, the treated wood can advantageously be used in the marine environment, in construction, in furniture, or in contact with soil, particularly as telegraph poles, cladding, decking, fence posts, railway sleepers, bridges, on buildings or on boats.

[0038] Figures

[0039] Figure 1 shows a schematic diagram of the apparatus for treating wood.

[0040] Figure 2 shows the density profile for pine samples treated and then cured in air (comparative) and treated and then cured in oil (according to the invention).

[0041] Figures 3 and 4 shows citric acid leaching for samples treated and then cured in air (comparative) and treated and then cured in oil (according to the invention).

[0042] In Figure 1 there is a chamber (1) with a bath (2) of hot oil (3) in it. Eight pieces of wood (4) are immersed in the hot oil (3). This is step 2 of the process, applied after step 1 (not shown) in which the wood (4) has been contacted with a solution containing a carboxylic acid or carboxylic anhydride and a polyol. The bath (2) is heated to a temperature of 110 - 160°C by (A) a series of internal heaters (5) at the bottom of the bath (2) and / or (B) an external oil heater (6). The external oil heater (6) has a thermometer (7) and a temperature controller (8). The hot oil (3) flows from the bath (2) into the external oil heater (6) through the “in” pipe (9), and from the external heater (6) back into the bath using the “out” pipe (10). Oil can be returned from the bottom to the top of the tank via the “oil return” (11) if needed. Water, oil and foam can be removed from the top of the bath. The wood (4) is left in the bath (2) for between 30 minutes and 48 hours.

[0043] During this time, the hot oil (3) rapidly heats the wood (4) all the way through to the centre. This results in water from the wood (4) evaporating. The water is removed from the top of the bath (2). The carboxylic acid or carboxylic anhydride and a polyol within the wood (4) react with themselves and the wood during this time to become cured.

[0044] Examples

[0045] Example 1

[0046] Powdered citric acid (VWR Chemicals, CAS 77-92-9) and D-Sorbitol (VWR Chemicals, CAS 50-70-4) were used in a 3:1 molar ratio to achieve complete esterification of the citric acid. These solids were dissolved in deionised water with stirring at 20°C. 8 g of water was used to dissolve each 10 g of solids. The liquid solution exhibited a pH of 1.2. This solution was then reduced to 60% solution by adding water.

[0047] Two samples for the test were produced from Scots pine sapwood (Pinus sylvestris) with a sample size of 100x100x18mm. Pine wood is the predominant wood species used for wood modification processes intended for outdoor use in northern Europe, hence Norwegian grown pine was chosen for this experiment.

[0048] All wood samples were impregnated with the solution of citric acid and sorbitol by immersion for 30 min under a pre-vacuum of 40 mbar followed by 1 hour at 8 bars of pressure. For the hot oil treatment a hot oil bath from I KA was used. The bath was filled with a mixture of linseed and mineral oil and heated up to 140°C. The temperature was controlled with both digital and analog thermometers. The samples were in the oil bath for 5 hours, which is approximately the half of the time used in dry curing. The samples were rigorously boiled for around 1 hour, which was the time needed for the removal of water. During the last 4 hours the samples were left in the hot oil for the curing of the chemicals inside.

[0049] The samples were after treatment cut to 50x50mm and profiled in a GRECON Density profiler to see the distribution of chemicals inside.

[0050] Results - Ex 1

[0051] The samples had an average weight before impregnation of 99g. After impregnation, the samples had an average weight of 240g. After the hot oil treatment, the samples had a weight of 138g. These results are similar to what one would expect from the dry curing but have been achieved in only half the time.

[0052] From the density profile it is evident that the samples were uniformly cured and had a 1-2mm layer of oil in the surface area.

[0053] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and / or groups thereof.

[0054] Example 2

[0055] Further tests were undertaken on a total of 63 samples from two types of wood, Scots pine and Norway spruce. Each sample measured 45x45x28 and was sealed on four sides, leaving only the top and bottom surfaces unsealed. The sides were sealed with a coating in order to simulate a long wooden board, so penetration of solution occurred on the larger top and bottom surfaces, but not on the smaller side surfaces.

[0056] After the sealing, the density of each sample is measured using a Grecon densitometer, and they were grouped into sets of three for identical impregnation.

[0057] A concentrated solution of citric acid and sorbitol was produced using 10g of dry (solid) components (citric acid and sorbitol) per 8g of water. The molar ratio of citric acid to sorbitol in the solution was 3:1. The concentrated solution was then diluted with water to make a 60%, 40%, and 20% solution which were used for the impregnation process. The samples were all impregnated with 30 minutes prevacuum of 40mbar and a pressure phase of 2 hours at 8 bar.

[0058] The curing process took place either at 140°C for 2, 4, 6, or 9 hours in air to provide comparative results, or by boiling in oil at 140°C for 2, 4, or 6 hours. The curing was conducted the day after impregnation, or on day 2. The samples were wrapped in plastic between the impregnation and curing stages to maintain their condition. The oil used for the process was boiled linseed oil from Gjoko, a thick non-penetrating oil with a density of 925kg / m3.

[0059] For the oil curing phase, three samples were placed in a rack in the oil, which had been heated to 140°C. This resulted in a lot of foaming for about an hour while the water was boiling out of the wood. After the water had boiled off, the foaming stopped.

[0060] The next set of three samples was introduced two hours after the first. This also resulted in foaming, and care was taken to prevent the oil from boiling over. The final three samples were added after the first three had boiled for 4 hours, again resulting in a substantial amount of foam for around an hour. All nine samples were removed from the oil bath simultaneously. Curing at 140°C in air in the oven was conducted simultaneously with the oil boiling process.

[0061] Density measurements and results The samples were weighed before impregnation, after impregnation, before curing, and after curing. After curing, all samples are again measured in the density profile.

[0062] Fig 2 shows the density profile for pine samples which has been impregnated with the 40% concentration solution and cured in air for 6 hours (P406A) and 9 hours (P409A) or cured in oil for 2 hours (P402O) and 6 hours (P406O). We can see that the oil curing led to a denser profile than the air cured, which means there is a more event and efficient distribution of citric acid and sorbitol in the wood, and ultimately a higher dimensional stability. This is thought to be because oil heats the wood up quicker, thereby curing the citric acid and sorbitol to fix them in place, and reduce the level being boiled off with the water. We can see that just 2 hours of oil curing performs better than 9 hours of air curing.

[0063] The increased density of the oil samples at the edges of the wood show that oil has absorbed into the edge region. This advantageously leads to increased water resistance.

[0064] Leaching measurements and results

[0065] Samples were tested for leached citric acid (CA). The leaching procedure was according to EN 84:2020 (Durability of wood and wood-based products. Accelerated ageing of treated wood prior to biological testing. Leaching procedure). Three samples were placed in each tray (a total of 21 trays for each type of wood). The volume of each tray was calculated to be 850.5 ml of water in each tray.

[0066] Samples were then taken from the water surrounding the wood to determine the amount of CA that had leached out of the wood after 2 hours, on day 1 , day 2, day 3, and on day 10. New water was added after each sampling so the values represent the amount of CA leached since last water change. The sampled water was analyzed using High-Performance Liquid Chromatography (HPLC) for CA content.

[0067] Fig 3 shows how much citric acid has leached out after 2 hours and 1 / 3 / 10 days for samples that had been impregnated with the 20, 40 and 60% solutions and then subjected to either air curing as above for 2, 4, 6, and 9 hours or oil curing as above for 2, 4 and 6 hours. The results were averaged across curing times. As would be expected, higher impregnation solution concentrations led to more citric acid leaching but much lower levels of leaching was observed with the oil curing than with the air curing. This shows that oil curing is more effective at curing the citric acid and sorbitol in the wood, thereby fixing them in place. This ultimately means that the oil-cured wood will have higher dimensional stability and be longer lasting than the air-cured wood.

[0068] Similarly, Fig 4 shows how much citric acid has leached out after 2 hours and 1 / 3 / 10 days for samples that had been impregnated with either the 20, 40 or 60% solution and then subjected to either air curing as above for 2, 4, 6 or 9 hours or oil curing as above for 2, 4 or 6 hours (with oil method curing was complete after 6 hours so the 9 hours experiment was not carried out). The results were averaged across 20 / 40 / 60% samples. As would be expected, generally higher curing times led to less citric acid leaching but lower levels of leaching was observed with the oil curing than with the air curing, demonstrating that oil curing is more effective than air curing. Indeed, 2 hours of oil curing is can be similar to or more effective than 9 hours of air curing. This shows that oil curing is also more time efficient and often also more energy efficient.

[0069] While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.

Claims

Claims1 . A method of treating wood, the method involving the steps of: contacting the wood with a solution containing a carboxylic acid or carboxylic anhydride and a polyol; and subsequently contacting the wood with oil at a temperature of 110 - 160°C for 30 minutes to 48 hours.

2. The method of claim 1 , wherein the carboxylic acid or carboxylic anhydride is a polycarboxylic acid or polycarboxylic anhydride, preferably wherein the polycarboxylic acid is selected from the group consisting of: 1 , 2, 3, 4 - butanetetracarboxylic acid, citric acid, maleic acid, succinic acid, ita-conic acid, transaconitic acid, cis-aconitic acid, tricarballylic acid, 1 , 2, 3-benzenetricarboxylic acid, 1 ,2, 4-benzenetricarboxylic acid, 1 ,2, 3, 4- cyclobutanetetracarboxylic acid, tetrahydrofuran- 2, 3, 4, 5-tetracarboxylic acid, 1 ,2, 4, 5- benzenecarboxylic acid, oxalic acid, malonic acid, glutaric acid, adipic acid, fumaric acid, isocitric acid, malic acid, oxalosuccinic acid, trimesic acid, trimellitic acid, hemimellitic acid, or a mixture thereof; most preferably wherein the polycarboxylic acid is citric acid; and the polycarboxylic anhydride is a derivative of one or more of the polycarboxylic acids selected from the group, preferably wherein the polycarboxylic anhydride is maleic anhydride or citric anhydride.

3. The method of claim 1 or 2, wherein and the polyol is selected from the group consisting of: hexanediol, xylitol, mannitol, glycerol, sorbitol, ethylene glycol, propylene glycol, 1 ,3-propanediol, 1 ,2-butylene glycol, 1 ,3-butylene glycol, 1 ,4- butanediol, 2-methyl-1 ,3-propanediol, pentaerythritol, neopentyl glycol, trimethylolpropane, 2,2,4,4-tetramethyl-1 ,3-cyclobutanediol, 3-methyl-1 ,5- pentanediol, 1 ,4-cyclohexane-dimethanol, 1 ,3-cyclohexanedimethanol, diethylene glycol, dipropylene glycol, triethylene glycol, 1 ,6-hexanediol, tripropylene glycol, tetraethylene glycol, glucose, dextrines, xylitol, saccharose, fructose, erythritol, lactitol, isomalt, maltitol, hydrated starch hydrolysate (HSH), threitol, adonitol, arabitol and inosite or a mixture thereof; preferably wherein the polyol is: hexanediol, xylitol,mannitol, glycerol, or sorbitol or a mixture thereof; most preferably wherein the polyol is sorbitol.

4. The method of any preceding claim, wherein the wood is contacted with the solution containing carboxylic acid or carboxylic anhydride and polyol under a pressure of 20 mbar to 15 bars and a temperature of up to 40°C; preferably wherein the wood is initially contacted with the solution under a vacuum of 20 mbar to 200 mbar, preferably 30 to 100 mbar and then contacted with the solution under elevated presume of 1 to 15 bar, preferably 5 to 10 bar; optionally wherein between contact with the solution and contact with oil, the wood is the placed under a vacuum of 20 mbar to 200 mbar.

5. The method of any preceding claim, wherein the weight ratio of dry components of the solution including carboxylic acid or carboxylic anhydride and polyol to water is 1 :3 to 3:1 , preferably 1.5:1 to 3:1 , most preferably about 2.5:1 , or where the weight ratio of dry components to water around 1 :1 or <1 :1 , preferably 1 :1.5 to 1 :10, or about 1 :2, 1 :3.5 or 1 :8.

6. The method of any preceding claim, wherein the molar ratio of carboxylic acid or carboxylic anhydride to polyol in the solution is 1 :5 to 5:1 , preferably 1 :1 to 5:1 , preferably 2:1 to 4:1 , most preferably about 3:1.

7. The method of any preceding claim, wherein the wood is contacted with oil at a temperature of 120 - 150°C, preferably 130 to 150°C, most preferably about 140°C; preferably wherein the oil bath is at atmospheric pressure.

8. The method of any preceding claim, wherein the wood is contacted with oil for 1 to 20 hours, 2 to 10 hours, 3 to 8 hours, or about 5 hours.

9. The method of any preceding claim, wherein the oil is mineral oil and / or organic oil, preferably wherein the oil is silicon oil, linseed oil, mineral oil or a mixture thereof.

10. The method of any preceding claim, wherein the wood is selected from a hardwood or softwood, preferably wherein the wood is Scots Pine, Norway spruce or Downey Birch.11 . Apparatus configured to treat wood, the apparatus including: a first bath for contacting wood with a solution containing a carboxylic acid or carboxylic anhydride and a polyol; a second bath for contacting wood from the first bath with oil; a heater for heating oil in the second bath; a finishing area; and a conveyor system for conveying wood into the first bath, from the first bath to the second bath, and from the second bath to the finishing area.

12. Treated wood, which is obtainable by the method of any of claims 1 to 10.

13. Treated wood according to claim 12, wherein the wood has oil impregnated at the surface, preferably wherein the oil is impregnated to a depth of 1-3 mm from the surface of the wood.

14. Use of treated wood according to claim 12 or 13 in the marine environment, in construction, in furniture, or in contact with soil.

15. Use of treated wood according to claim 14 as telegraph poles, cladding, decking, fence posts, railway sleepers, bridges, on buildings or on boats.

Images