Process for the acetylation of wood

Abstract

Disclosed is a process for the acetylation of lengthy pieces of solid wood or veneers. The process involves impregnation by submerging the wood in liquid acetylation fluid, and completing the impregnation under elevated pressure. Unexpectedly high acetylation contents are obtained with relatively low impregnation pressures.

Description

[0001] Title: PROCESS FOR THE ACETYLATION OF WOOD

[0002] Field of the Invention

[0003] The invention relates to a process for the acetylation of wood. Particularly, the invention relates to the acetylation of wood pieces, such as boards, beams and veneers, having a length of at least 60cm.

[0004] Background

[0005] Acetylation of wood has long been recognised as a method to improve the durability of otherwise non-durable wood species. Also, it is known as a method to improve wood properties, such as hardness and dimensional stability.

[0006] For decades many different processes for the acetylation of wood have been proposed, frequently based on lab-scale experiments. Today, the most successful commercial wood acetylation process is of the type disclosed in WO 2009 / 095687. This essentially is a non-catalytic process, based on impregnating wood with liquid acetylation fluid comprising acetic anhydride, and possibly also acetic acid, and subsequent heating, preferably in stages.

[0007] In the field of wood acetylation, generally two types of substrates for acetylation are distinguished, viz. solid wood and wood elements. The latter refers to smaller pieces of wood, generally ranging from larger elements, such as chips or strands, to small elements, such as fibres. The term “solid wood” is generally employed to indicate relatively large wood forms, such as beams or boards. Another representative relatively large wood form is that of veneers. Whilst essentially thin in one dimension (thickness), veneers come in relatively large dimensions of length, and possibly width.

[0008] A three-dimensional piece of wood cut from a tree, in whichever direction, will always have three recognizable directions with reference to the direction of growth of the tree. These directions are “longitudinal”, “radial”, and “tangential.” As a result, on such a piece of wood three different grains can be recognized, viz., face-grain (having longitudinal and tangential sides), side-grain (having longitudinal and radial sides), and endgrain (having tangential and radial sides). Taking into account the structure of wood, the pores directly accessible for unsupported impregnation of a submerged piece of wood will be mainly found in the end-grain. The natural limitation of such impregnation is generally about 30 cm. As a result, the impregnation of a piece of wood longer than 60cm, with liquid will be limited according to this so-called “end-effect”. This implies that with, e.g., a wood beam of 1.5m in longitudinal direction, impregnation starting from either end of such beam (which correspond to the end grains) will not proceed beyond at most 30 cm from either end. I.e., in the event of a beam of 1.5 m length, a central beam length of 90 cm will not, or hardly, become impregnated.

[0009] A high impregnation loading, however, is needed to ensure acetylation down to the core of the wood. This, in turn, is needed for the acetylated wood to satisfy a durability according to EU Class 2 or, preferably Class 1, of the entire board. Particularly the latter is essential for the acetylated wood to be accepted for outdoor applications with high durability demand and dimensional stability. Without a sufficiently high impregnation loading, the acetylation of larger pieces of wood on industrial scale may result in a so-called “envelope treatment”. Envelope treatment is e.g., the acetylation of wood at its surfaces, leaving the interior wood at a less effective level of acetylation or with no acetylation at all. Water and fungi may travel through the acetylated surface, reaching the less, or nonacetylated core, leading to swelling and potential decay of the in-sufficiently acetylated parts. Acetylated wood allows moisture to pass into and out of the entire cross section. In the acetylated parts, which are acetylated sufficiently, moisture encounters protected cell walls and cannot be used to support fungal growth.

[0010] Accordingly, for solid wood and veneers, exceeding in longitudinal direction a distance that could become impregnated by merely the endeffect, the impregnation needs to be forced. The standard method in the art is to do so by substantially increasing pressure. This refers to the external pressure on the liquid in the situation of wood being submerged in liquid acetylation fluid. To this end, WO 2009 / 095687 describes the submersion of boards in acetylation fluid at increased pressure. This pressure is disclosed to be generally 2 to 20 bara, and more specifically 10 to 15 bara. Prior to submerging the wood in liquid acetylation fluid, the method of WO 2009 / 095687 optionally includes applying initial vacuum (0.05 to 0.5 bara, typically 0.1 - 0.2 bara). The acetylation reaction is typically conducted at or modestly above atmospheric pressure. In WO 2009 / 095687 this is 1.1 to 1.9 bara. Another background reference directed to the acetylation of high- density wood, typically also requiring an impregnation pressure of 2-20 bar, more typically 10 bar, is WO 2013 / 117641. Herein it is disclosed to apply vacuum impregnation, at pressures below -0.5 barg, preferably below -0.7 barg.

[0011] The acetylation of non-durable wood species, results in greatly improving durability. Thus, non-durable softwood species, can be rendered into wood products satisfying a durability according to EU Class 2 or, preferably Class 1. Thereby, this durability can be essentially obtained without requiring hardwood from tropical rainforests, and without environmentally unfriendly impregnation or coating. A drawback, however, is that the resulting wood is relatively expensive. The acetylation process as it is known, requires investments in expensive high-pressure equipment, and high operational costs. Also, the process, generally involving hot acetylation fluid, has inherent safety risks. Particularly acetic anhydride is a hazardous chemical, requiring substantial safety precautions.

[0012] Therefore, a need exists in the art to provide a process for acetylating wood that allows using less expensive equipment and / or enables reduced operational costs, as well as reduces safety risks.

[0013] Summary of the Invention

[0014] In order to better address one or more of the aforementioned issues, the invention presents, in one aspect, a process for the acetylation of wood, the process comprising providing wood to be acetylated, the largest dimension of said wood being 60cm to 900 cm in longitudinal direction and said wood comprising less than 10 wt.% of moisture; subjecting the wood to impregnation in a vessel, wherein the impregnation comprises submerging the wood into liquid acetylation fluid comprising acetic anhydride and / or acetic acid at a filling pressure of below 0.5 bara, completing the impregnation at an impregnation pressure higher than the filling pressure so as to provide impregnated wood; removing excess acetylation fluid from the vessel; and heating the impregnated wood in order to allow wood acetylation to occur, wherein the impregnation pressure is lower than 2 bara.

[0015] Detailed description

[0016] The invention concerns the acetylation of wood having a relatively large size in the longitudinal direction of the wood. This lengthy wood particularly concerns solid wood and veneers. Since the largest dimension of the wood to be acetylated is 60cm to 900 cm in longitudinal direction, it is insufficient for impregnation to rely on liquid uptake by capillary forces from the end-face of the wood (and some hydrostatic pressure of the liquid in the vessel). The invention is based on the unexpected finding that, in the aforementioned lengthy wood, high acetyl contents can nonetheless be obtained when applying impregnation pressures well below the standard in the art. Particularly surprisingly, acetyl percentages can be achieved that reflect Class 1 durability, despite the relatively large dimensions of the wood subjected to the acetylation.

[0017] Said wood is selected from the group consisting of solid wood and veneers, the largest dimension of said wood being 60cm to 900 cm in longitudinal direction. The wood thus has a size which exceeds a distance which would merely accommodate suitable impregnation via the abovediscussed end-effect. Hereby the wood is generally selected from the group consisting of solid wood and veneers. Preferably, the wood has a length (L) of 600 to 6000 mm, more preferably 900-6000mm, such as 900-5400mm, such as 1200-5400mm, most preferably 2400-4800 mm; a thickness (T) of 1 - 200 mm, preferably l-150mm, more preferably 2-150mm, such as 5-150mm, most preferably 10- 100mm, and a width (W) of 20 - 300 mm, preferably 100- 200 mm. Most preferably, the wood is solid wood, preferred dimensions of which are a length of 1200 to 6000 mm preferably 2400 to 4800, a thickness of 25 to 100 mm, and a width of 100 to 200 mm. Most preferred lengths are 2.4m, 3.0m, 3,6m, 4,2m, and 4.8m. Most preferred thicknesses are 25mm, 32mm, 38mm, 50mm, 63mm, 75mm, and 100mm; most preferred widths are 100mm, 125mm, 150mm, and 200mm.

[0018] The wood comprises less than 10 wt.% of moisture. Although in the art higher moisture percentages are sometimes advocated, it is preferred to dry the wood prior to acetylation, since water will react with acetic anhydride, thereby making the acetylation fluid less effective. Accordingly, the wood to be acetylated preferably comprises at most 5 wt.% of water, such as at most 4 wt.% of water, preferably at most 3 wt.% of water, such at most 2 wt.% of water, more preferably 0.5 -4 wt.% of water, and still more preferably 1-3 wt.% of water. The process comprises submerging the wood into liquid acetylation fluid. The acetylation fluid comprises acetic anhydride and / or acetic acid. Preferably, this fluid comprises acetic anhydride in a range of from 70 wt.% to 100 wt.% and acetic acid in a range of from 0 wt.% to 30 wt.%. Although acetic anhydride is the main acetylation fluid, the initial acetylation fluid preferably comprises an amount of acetic acid. This amount is preferably 1 wt.% to 20 wt.%, more preferably 3 wt.% to 12 wt.%. The amounts indicated refer to the liquid acetylation fluid used for the impregnation, i.e., the initial acetylation fluid. It will be understood that the reaction of acetic anhydride with wood essentially yields acetic acid as a by-product. Accordingly, the composition of the acetylation fluid during the acetylation reaction will necessarily change.

[0019] The impregnation starts with submerging the wood in the liquid acetylation fluid. Preferably before initiating the submersion, the wood is subjected to a reduced filling pressure of below 0.5 bara. Preferably, this filling pressure is at most 0.4 bara, more preferably below 0.3 bara. More preferably the reduced pressure is in a range of from 0.005 to 0.4 bara, still more preferably 0.05 to 0.1 bara. The reduced filling pressure preferably is applied continuously during filling of the reactor with acetylation fluid.

[0020] Depending on the exact size and shape of the wood to be acetylated, and the configuration of the vessel in which the submersion of the wood is conducted, not all of the wood present may become submerged at the same moment. E.g., a preferred way of submerging wood is to first introduce a pile of wood into a vessel. Generally, the layers of boards in the piled wood will be separated from each other, typically using stickers, so as to facilitate the access of liquid acetylation fluid and gas to all parts of the wood, at all sides. Next, the wood is subjected to the aforementioned reduced pressure. Liquid acetylation fluid will then be added to the vessel, preferably starting at the bottom, and gradually submerging the pile of wood from bottom to top.

[0021] After submersing the wood, the impregnation can be completed by increasing the pressure to which the wood is subjected to above the filling pressure. Preferably, this is done at or after completion of the submersion, as this better ensures an effective completion of the impregnation.

[0022] Following the preferably complete submersion of the wood, the present process comprises completing the impregnation by subjecting the wood to a pressure above the filling pressure, and below 2 bar absolute. Preferably the pressure is elevated to a level at or above atmospheric pressure. As mentioned, this is a pressure substantially below that conventionally applied in the field. With general wood impregnation pressures typically being 6-10 bara, the impregnation pressures applied in actual practice for wood acetylation typically are substantially higher. Particularly in the event of acetylating lengthy wood such as herein concerned, industrial production plants for wood acetylation apply impregnation pressures in a range of from 10 to 15 bara.

[0023] The duration of the step of completing the impregnation, can also be referred to as “impregnation time.” As noted above, not all of the wood may experience the same duration of the submersion. Therefore, an acceptable point in the process to start determining the impregnation time, is when the pressure has been increased from the reduced submersion pressure to the elevated pressure of completing the impregnation (which, accordingly, can also be indicated as being the “impregnation pressure”). The impregnation time generally is in a range of from 1 min to 500 min, preferably 3 -300 min, more preferably 5 - 90 min, still more preferably 7-45 min, most preferably 10-25min.

[0024] In an embodiment, multiple submersion, impregnation and acetylation steps can be conducted in series. Generally, this involves a first sequence of conducting the steps of reducing the pressure of the wood to filling pressure, submerging the wood at said reduced filling pressure, completing the impregnation at an impregnation pressure above the filling pressure, and removing excess acetylation fluid, e.g., by vacuum and perform a first acetylation cycle. Thereupon the pressure is reduced again to filling pressure, and the same sequence is repeated. Releasing the pressure after the first acetylation cycle advantageously causes a substantial amount of acetic acid formed to evaporate from the wood, leaving space for new anhydride. Such a double sequence thus allows to supply more anhydride than possible in one go.

[0025] It will be understood that the aforementioned filling and impregnation pressures can be achieved in one or more suitable vessels, configured to be drawn to vacuum, respectively to apply pressure. Generally, the vessel used for the submersion will be the same vessel in which the increased pressure is applied to complete the impregnation. Preferably, one and the same vessel is used for submersion under reduced pressure, completion of the impregnation under increased pressure, and conducting of the acetylation reaction under atmospheric pressure, or increased pressure, the latter being preferred. Suitable vessels, such as chemical reactors, are familiar to the person skilled in the art. With a view to accommodating the relatively lengthy pieces of wood that are impregnated and acetylated with the present invention, the vessel used is preferably cylindrically shaped. An advantage of the present process is that the requirements for the vessel used for impregnation can be less stringent in terms of pressure-resistance, which generally implies lower costs. Also, the surprisingly low impregnation pressure applied in the process of the invention is less prone to result in leakage of the hazardous acetylation fluid, which is a general risk in conventional industrial wood acetylation plants. This enhances the possibilities for pre-existing chemical plants to conduct the acetylation of lengthy wood as defined above, without making substantial investments in dedicated wood acetylation equipment.

[0026] Accordingly, the invention beneficially applies an impregnation pressure (i.e., to complete the impregnation) of below 2 bar absolute (bara). With a view to broadening the types of reactors, and particularly the construction materials thereof, it is preferred to apply an impregnation pressure of < 1.9 bara, such as 1.2 bara to 1.8 bara, such as 1.3 bara to 1.7 bara, such as 1.4 bara to 1.6 bara, such as about 1.5 bara.

[0027] During the submersion and the completion of the impregnation, generally a temperature is applied in a range from at or above ambient temperature to below the boiling temperature of acetic anhydride at the applicable pressures. Preferably, the temperature is in a range of from 20°C to 120°C, preferably of from 25°C to 75°C, more preferably 50-65°C.

[0028] The acetylation reaction itself is generally initiated with increasing the temperature, preferably in stages. Acetylation reactions are generally conducted at temperatures of from 120°C to 200°C, such as 140°C to 160°C. The duration of the acetylation treatment, i.e., after the step of completing the impregnation, generally ranges from 30 minutes to 3 hours. The skilled person will be able, for a given reactor equipment and depending on the shape and size of the wood to be acetylated, to optimize the time and temperature conditions to achieve a level of acetylation of the wood that would be desirable.

[0029] Prior to initiating the actual acetylation reaction, excess impregnation fluid is preferably removed from the vessel in which the impregnation has been completed. This removal can be done, generally, via suitable drain at a bottom of the vessel. Subsequently, as an aid to initiate the acetylation reaction, an inert heating fluid is preferably introduced into the reaction vessel. Preferably this inert fluid is gaseous nitrogen. The nitrogen is heated to the desired acetylation reaction temperature. Preferably this is in stages, with further heating after a first exotherm has been observed. The process thus preferably comprises introducing into the vessel an inert fluid, circulating and heating the fluid until the internal temperature of the wood begins to show an exotherm, controlling the supply of heat to the wood until the exotherm is complete and maintaining the internal temperature of the wood below 180°C; then heating the circulating fluid to a temperature of 85°C to 160°C for a time of 10 to 120 minutes to initiate a second exothermic reaction, controlling the supply of heat to the wood until the exotherm is complete and maintaining the internal temperature of the wood below 180°C. Thereafter the circulating fluid is removed, generally by evaporation under vacuum, and the wood is allowed to cool to room temperature. The inert fluid is a fluid that does not react with acetic anhydride or acetic acid and can typically be gaseous nitrogen, gaseous carbon dioxide or flue gas. In this step, the acetylation fluid is preferably heated to a temperature of from 60 to 150°C, more preferably from 70 to 120°C, for a time of 10-120 minutes, preferably of 15-60 minutes. The commencement, duration and completion of the exotherm is preferably detected and monitored by thermocouples located in the wood. In some instances, the inert fluid, e.g. nitrogen, may be partially or fully saturated with acetic anhydride and / or acetic acid. This can range from 20% to full saturation (100%) by volume.

[0030] The process of the invention surprisingly is capable of achieving an acetyl content of at least 20% by weight, preferably 22%-25% by weight. This refers to the acetyl content determined in a sample obtained from the core of a board, minimally 30cm from the end of the board, and the geometrical centre of the width and thickness of the board. In order to reach the geometrical centre, it will be understood that material needs to be removed (by drilling or sawing) until said geometrical centre can be reached so as obtain a sample therefrom with drilling.

[0031] In order to determine the acetyl content of the wood, samples are ground to wood particles. From these samples residual trace amounts of acetic acid and / or acetic anhydride are removed, by washing with water and subsequent drying at 103±2°C for between 14-24 hrs. After weighing these dried samples the acetyl groups are released from the wood in the form of acetate ions by saponification with sodium hydroxide solution at elevated temperature. This saponification reaction runs for 4 hrs, with stirring every 15 minutes. The acetate ions are quantified by means of high-pressure liquid chromatography (HPLC), after calibrating this HPLC with standard acetate solutions and using sodium butyrate as internal reference. This gives a direct measure of acetyl content that is preferred over general weight percentage gain (WPG) frequently used in other studies.

[0032] The wood to be acetylated with the present process could be of any species known, but preferably belongs to non-durable wood species such as soft woods, for example, coniferous trees, typically spruce, pine or fir, or to non-durable hardwoods. Non-limiting examples of suitable types of wood are spruce, sitka spruce, maritime pine, scots pine, radiata pine, ponderosa pine, eucalyptus, red alder, European alder, beech, birch, loblolly pine, lodgepole pine, pitch pine, red pine, Southern yellow pine, Japanese cedar (sugi), and hemlock. Also suitable are monocots, such as palm, and other hardwoods, such as Paulownia, rubberwood, teak, maple, oak, white oak, and the like.

[0033] The process of the present invention is preferably applied to wood species having a density below 650 kg / m3. More preferably, the wood density is in a range of from 250 kg / m3to 550 kg / m3. Preferred wood species are radiata pine, loblolly pine, ponderosa pine, southern yellow pine, beech, alder, lime, maple, and poplar.

[0034] In sum, a process is disclosed for the acetylation of lengthy pieces of solid wood or veneers. The process involves impregnation by submerging the wood in liquid acetylation fluid, and completing the impregnation under elevated pressure. Unexpectedly high acetyl contents are obtained with relatively low impregnation pressures.

[0035] The invention is illustrated with reference to the following, nonlimiting example.

[0036] Example 1

[0037] About 40 m3 of Radiata Pine boards (38 mmxl50 mmx 4800 mm) with an average moisture content of 2.8% are separated with 15 mm stickers vertically. The wood is loaded into a 150 m3 Equid capacity reaction pressure vessel. The vessel is equipped with a gas loop.

[0038] A vacuum is applied to -0.92 barg. The reactor is filled with acetylation fluid (95% acetic anhydride and 5% acetic acid, 60° C.) while maintaining the vacuum below -0.80 barg, that results in each board being submerged. The reactor is pressurized to 1,7 bara for 25 min.

[0039] Nitrogen gas, saturated with acetic anhydride, at about 90° C, is used as a heating medium for the acetylation reaction. During the acetylation the pressure in the gas loop increases to 2.5 bara and is kept between 2.4 and 2.6 bara. The acetylation takes about 4 hours in total, including the heating up time.

[0040] At the end of the acetylation period, more of the unused acetic anhydride and the by-product acetic acid are condensed from the circulating gas. The temperature is increased gradually to about 140° C. The boards are dried to a point where they contained less than 0.40 wt % of free acetyls (acetic acid and acetic anhydride).

[0041] The acetyl level is determined for 19 samples of wooden boards. Samples are taken at least 60 cm away from the end face of a board, and from the geometrical center (with respect to thickness and width)

Claims

Claims1. A process for the acetylation of wood, the process comprising providing wood to be acetylated, the largest dimension of said wood being 60cm to 900 cm in longitudinal direction and said wood comprising less than 10 wt.% of moisture; subjecting the wood to impregnation in a vessel, wherein the impregnation comprises submerging the wood into liquid acetylation fluid comprising acetic anhydride and / or acetic acid at a filling pressure of below 0.5 bara, completing the impregnation at an impregnation pressure higher than the filling pressure so as to provide impregnated wood; removing excess acetylation fluid from the vessel; and heating the impregnated wood in order to allow wood acetylation to occur, wherein the impregnation pressure is lower than 2 bara.

2. A process according to claim 1, wherein the impregnation pressure is at most 1.9 bara, preferably 1.2 bara to 1.8 bara.

3. A process according to claim 1 or 2, comprising subjecting the wood to a reduced filling pressure of below 0.5 bara, preferably in a range of from 0.03 to 0.4 bara, before and / or during submerging it.

4. A process according to any one of the preceding claims, wherein the impregnation time is in a range of from 5 - 90 min, preferably 10-45 min.

5. A process according to any one of the preceding claims, wherein the wood has a density below 650 kg / m3, preferably, in a range of from 250 kg / m3to 550 kg / m3.

6. A process according to any one of the preceding claims, wherein the wood has a length of 600 to 6000 mm, a thickness of 1 - 200 mm, and a width of 20 - 300 mm.

7. A process according to claim 6, wherein the wood has a length of 900-5400mm, preferably 2400-4800 mm.

8. A process according to claim 6 or 7, wherein the wood has a thickness of preferably l-150mm, preferably 10-100mm.

9. A process according to any one of the claims 6 to 8, wherein the wood has a width of 20 - 300 mm, preferably 100-200 mm.

10. A process according to any one of the preceding claims, wherein the wood to be acetylated comprises at most 5 wt.% of water, preferably 0.5 - 4 wt.% of water.