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Title:
IMPROVED PROCESS FOR THE TREATMENT OF A POROUS MATERIAL
Document Type and Number:
WIPO Patent Application WO/2010/121624
Kind Code:
A2
Abstract:
Improved methods for the treatment of a porous material, such as wood, more specifically a method in which an active ingredient to be deposited within a porous material in a first step is kept in a mobile phase and in a second step is kept in a stationary phase.

Inventors:
HENRIKSEN OLE (DK)
KJELLOW ANDERS WESTH (DK)
FERNANDES JOAEO LUIS BEJA (DK)
Application Number:
PCT/DK2010/050089
Publication Date:
October 28, 2010
Filing Date:
April 21, 2010
Export Citation:
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Assignee:
VKR HOLDING AS (DK)
HENRIKSEN OLE (DK)
KJELLOW ANDERS WESTH (DK)
FERNANDES JOAEO LUIS BEJA (DK)
International Classes:
B27K3/08
Foreign References:
DE19852070A12000-05-18
FR2747697A11997-10-24
US5137760A1992-08-11
Other References:
ACDA, M; MORRELL, J J; LEVIEN, K L: "Effect of process variables on supercritical fluid impregnation of composites with tebuconazole", WOOD AND FIBER SCIENCE, vol. 29, no. 3, 1997, pages 282 - 290
ACDA, M N; MORRELL, J J; LEVIEN, K L: "Supercritical fluid impregnation of selected wood species with tebuconazole", WOOD SCIENCE AND TECHNOLOGY, vol. 35, no. 1-2, 2001, pages 127 - 136
ANDERSON, M E; LEICHTI, R J; MORRELL, J J: "The effects of supercritical C02 on the bending properties of four refractory wood species", FOREST PRODUCTS JOURNAL, vol. 50, no. 11-12, 2000, pages 85 - 93
DRESCHER, M; JOKISCH, A; KORTE, H; PEEK, R D; STEINER, R: "Differential pressure characteristics of wood impregnated with compressed gases, liquids and supercritical fluids", HOLZ ALS ROH-UND WERK- STOFF, vol. 64, no. 3, 2006, pages 178 - 182, XP019397126, DOI: doi:10.1007/s00107-005-0060-1
KANG, S M; RA, J B; LEVIEN, K L; MORRELL, J J: "Developing diffusion coefficients for SCF impregnation of douglas fir heartwood with cyproconazole", JOURNAL OF WOOD CHEMISTRY AND TECHNOLOGY, vol. 26, no. 2, 2006, pages 111 - 124
LUCAS, S; GONZALEZ, E; CALVO, M P; PALENCIA, C; ALONSO, E; CO- CERO, M J: "Supercritical C02 impregnation of Radiata pine with organic fungicides - Effect of operating conditions and two-parameters modeling", JOURNAL OF SUPERCRITICAL FLUIDS, vol. 40, no. 3, 2007, pages 462 - 469
SAHLE-DEMESSIE, E; LEVIEN, K L; MORRELL, J J: "Impregnation of Wood with Biocides Using Supercritical Fluid Carriers", ASC SYMPOSIUM SERIES, vol. 608, 1995, pages 415 - 428
SCHNEIDER, P F; LEVIEN, K L; MORRELL, J J: "Effect of wood characteristics on pressure responses during supercritical carbon dioxide treatment", WOOD AND FIBER SCIENCE, vol. 38, no. 4, 2006, pages 660 - 671
SCHNEIDER, P F; MORRELL, J J; LEVIEN, K L: "Internal pressure development during supercritical fluid impregnation of wood", WOOD AND FIBER SCIENCE, vol. 37, no. 3, 2005, pages 413 - 423
See also references of EP 2421685A2
Attorney, Agent or Firm:
HAUGE, Sidsel et al. (Rigensgade 11, København K, DK)
Download PDF:
Claims:
P A T E N T C L A I M S

1. A method for the treatment of a porous material constituting a solid phase using a carrier fluid constituting a mobile phase comprising the steps of pressurising and depressurising the porous material charac- terized in that the carrier fluid comprises at least one active ingredient and that the at least one active ingredient during the pressurisation step is kept in the mobile phase and that the at least one active ingred ient during the depressurisation step is kept in the solid phase.

2. Method according to claim 1, wherein the porous material is selected from the group consisting of cellulose, silica and wood .

3. Method according to claim 1 or 2, wherein the at least one active ingredient is selected from the group consisting of water repellents, modifiers of the building blocks of the porous material, organic and inorganic biocides, aromas, colorants and organic or inorganic salts. 4. Method accord ing to cla im 3, wherein the mod ifier of the building blocks of the porous material is selected from the group consisting of polymers and co-polymers.

5. Method according to claim 3 wherein the biocide is an organic biocide selected from the group consisting of triazoles, pyrotroides, car- bamates and salts of organic acids.

6. Method according to any one of the preceding claims wherein the carrier fluid is a supercritical fluid having a critical point at a temperature of 8-1000C and a pressure of 5-100 bar under a pressure of at least 8 bar and a temperature below 1000C. 7. Method according to claim 6, wherein the supercritical fluid is carbon dioxide.

8. Method according to any of the preceding claims wherein the density of the carrier fluid is increased during the pressurisation step.

9. Method according to any one of the preceding claims wherein the treatment is selected among impregnation, extraction, dying or drying and any combination thereof.

10. Method according to any of the preceding claims wherein the treatment time is 1 - 9 hours.

11. Method for the treatment of wood comprising the steps: a) charging a vessel with wood to be treated; b) pressurising the vessel using the carrier fluid until the treatment pressure is reached; c) depressurising the vessel to ambient temperature; and d) removing the treated wood from the vessel, wherein the required amount of active ingredient is placed in a mixing vessel prior to step b) or added during or after step b); and wherein the steps b) and c) are performed according to claim 1 and optionally at least one of the claims 2-10.

Awapatent A/S International Patent-Bureau A/S

Description:
Improved process for the treatment of a porous material

The present invention relates to an improved method for the treatment of a porous material, such as wood, more specifically a method in which an active ingredient to be deposited within the porous material in a first pressurisation step is kept in a mobile phase and in a second depressurisation step is kept in a stationary phase.

Background Minimizing treatment times is important for the economic viability of the supercritical treatment processes of porous materials, such as wood treatment. Therefore, a lot of research has been focused on examining the effects of different pressurization and depressurization rates on the mechanical properties of particularly impregnated wood in order to try to establish just how fast wood can be pressurized and depressur- ized without being damaged .

The present inventors have previously disclosed a method in which exudation of resin from the wood during impregnation is avoided, and EP 1501664 discloses a method for treating wood having a certain length and being susceptible to damage using a supercritical fluid . Even though these methods have proven effective for solving the problems addressed therein they still fail, when used in combination with an active ingredient, to result in a satisfactory distribution of said active ingredient within a time span that is economically viable. During the past two decades supercritical carbon d ioxide has repeatedly been investigated as a possible solvent for wood impregnation . The majority of the research has focused on the effects of supercritical treatment on the physical properties of wood and a wide variety of wood species have been treated with an equally wide array of organic biocides (e.g . Anderson et al . 2000, Acda et al . 2001) . Researchers often report of a concentration gradient between biocide concentrations in the inner and outer parts of supercritical impregnated samples (e.g . Sahle- Demessie et al . 1995, Acda et al . 1997, Kang et al . 2006) . Despite these observations, the issue of biocide movement and deposition has received little attention and most studies do not discuss these gradients beyond noting their existence.

From differential pressure measurements carried out in situ dur- ing supercritical wood impregnation, it is known that the carbon dioxide penetrates the wood during treatment to the extent that pressure inside and outside of the sample equilibrates (Schneider et al. 2005, Schneider et al. 2006, Drescher et al. 2006). The biocide concentration gradient is therefore not the result of incomplete penetration of wood samples by the CO2/biocide mixture. Probable causes for the resulting active biocide gradients in the wood could then be that either 1) part of the biocides gets re-extracted from the wood as the carbon dioxide leaves the wood during depressurization, or 2) the biocides get filtered from the carbon dioxide as the mixture fills up the wood matrix. Understanding biocide movement and the deposition mechanisms is an important part in the continued development of the supercritical wood impregnation process especially when impregnating wood of larger dimensions.

Impregnation or other treatments are often combined with the use of e.g. water repellents, biocides or other suitable active ingredients in the wood production and there is still a need for effective methods of treating wood wherein both the manufacturing time is economically feasible and the resulting piece of wood has a satisfactory distribution of active ingredient within this time span. Therefore, the aim of the present invention is to provide an improved method for treating porous materials, which will result in improved distribution of active ingredient in the material without increasing treatment times.

Summary of the invention

This object is solved by a method for the treatment of a porous material using a carrier fluid comprising the steps of pressurising and depressurising the porous material characterized in that the carrier fluid comprises at least one active ingredient and that the at least one active ingredient during the pressurisation step is kept in the mobile phase and that the at least one active ingredient during the depressurisation step is kept in the stationary phase. Treatment of a porous materials using a carrier fluid, e.g. a supercritical carrier fluid, can be regarded as a chromatographic process in which the fluid is considered the mobile phase and the porous material, such as wood fibre, is considered the stationary phase. During treatment an active ingredients held in the mobile phase is absorbed or adsorbed in/by the stationary phase, i.e. the wood matrix. As most chromatographic processes this causes a concentration gradient through the "column" i.e. piece of wood due to the continuing equilibrium of the active ingredient between the mobile and stationary phase. What has presently been found is that by retarding the active ingredient in the mobile phase during the pressurisation step of the treatment, this gradient will within a shorter treatment time decrease, and the resulting treated piece of wood will have an improved distribution of the active ingredient all through to the centre of the wood. This has a dramatic effect on the quality of the resulting wood manufactured in a shorter time pe- riod as compared to wood treated by traditional methods within the same time span.

The results presented here also indicate that the rate limiting factor in treatment of porous materials might not, as previously believed, be how fast the material can be pressurized without causing fail- ures but rather how fast active ingredients can be delivered to the center of treated samples. What the inventors have found is that the active ingredients have a surprisingly higher affinity towards the porous material than the fluid, why prior art methods have failed in providing active ingredient to the centre of the piece of wood. Therefore, to ensure a movement, such as a rapid movement, of active ingredient through the porous material, the equilibrium should be moved as far as possible towards the fluid/mobile phase during the pressurization of the treatment cycle and towards the stationary phase, e.g. wood, during depressurisa- tion .

Ways of doing this include increasing the gas phase concentration of active ingredient by adding more active ingredient to the system or controlling process parameters, such as flow rate, pressure and tem- perature and/or adding excipients to the carrier fluid in order to increase the affinity of the active ingredients towards the carrier fluid . The carrier fluid/active ingredient system is determinative for the specific process and depends on the affinity of the active ingredient to the fluid and porous material, e.g . wood matrix, respectively. Specific determination of the process parameters is within the skill of the art once having recognized as the present inventors have that the determinative factor for a rapid even distribution is obtained by using the method of the present invention .

According to the results obtained in the various experiments performed by the present inventors, the rate at which active ingredients move through wood, either by bulk flow or diffusion, can be manipulated .

Wood is an example of a porous material behaving like a chromatographic column, any other material, which is constituted of a po- rous material such as cellulose, silica etc. will behave in the same way.

Without limitation in the following description the invention will partly be described using wood as an example of a porous material .

In the following, the resulting equilibrium between active ingredient concentrations in the porous material and in the carrier fluid will be described by the partition ratio. Thus, the partition ratio is defined as the concentration of active ingredients in the porous material/wood divided by the concentration of active ingredients in the carrier fluid .

Throughout the description and claims the concentration is calculated on weight basis i.e. weight/weight unless otherwise indicated . A low partition ratio for a specific active ingredient is defined as a value, which is lower than the partition ratio of the same active ingredient using the prior art method . The partition ratio should only be low as defined in the present context during the pressurisation step. The magnitude of the partition ratio is dependent on the physical conditions. The partition ratio decreases as the affinity of the actives to the solvent increases. To ensure a rapid movement of the active ingredients through e.g. the wood structure, the pressure and tempera- ture cond itions during the pressurization and treatment parts of the treatment cycle must be chosen so that the corresponding partition ratios favour rapid active ing redient movement, and at the end of the treatment cycle, the process conditions are altered in a way that shifts the affinity of the actives towards the porous material, i.e. increasing the partition ratio.

Hence, the conditions should be chosen in such a way that the thermodynamic equilibrium favour the interaction between the active ingredients and the fluid rather than with the porous material. This represents pressure and temperature conditions where the partition ratios show their lowest values i.e. conditions where the solubility of the active ingredients in the fluid is high. Thus, on their way towards the centre of the porous material, such as wood, the active ingredients will be less retained by the material, and the chromatographic effect will be diminished. Therefore, according to the present invention an even distribution of active ingredient in the porous material is obtained within a shorter period of time as compared to the prior art methods.

Additionally, when more than one active ingredient is used the present invention provides an even deposition of each of the ingredients within a shorter treating time. Thus, when using different active ingredients having different affinities for the porous material these will within a shorter time be evenly deposited in the material. Thus, where the prior art methods would either use a longer treatment time or add more of the low affinity ingredients in order to obtain a 1 : 1 deposition, such measures may be avoided when using the method of the present invention.

With the substances tested by the inventors, the partition ratio was proportional to the fluid density. For other fluids/active ingredients/ substrate combinations there might be other relations.

A porous material is in this context defined as a solid (often called frame or matrix) permeated by an interconnected network of pores (voids) filled with a fluid (liquid or gas), and more specifically the porous material is selected from cellulose, wood, cork, silica, membranes and polymers. A presently preferred porous material is wood, cork or cellulose.

The treatment may be any type of treatment and is preferably selected among deposition of active ingredients, impregnation, extrac- tion, dying, drying and any combination thereof.

Treatment of e.g. wood depends on the specific application of the piece of wood, thus in a preferred embodiment the active ingredient is selected from the group consisting of water repellents, modifiers of the building blocks of the porous wood matrix and biocides or a combi- nation thereof. Preferred water repellents are oils, co-polymers, waxes and silicones.

The modifier of the building blocks of the porous wood matrix is selected from the group consisting of polymers or copolymers having bulky functional groups. The polymers/co polymers react with the porous media eliminating e.g. any free hydroxyl groups in the cellulose and filling the open spaces. Specific examples of wood modifiers are, fire retar- dants, inorganic salts, aromas, colorants, etc.

The purpose of modifying the porous material is to enhance preferred properties of the material, such as moisture and dimensional sta- bility, or adding new properties to the material, such as adding fire re- tardants, to increase the resistance against fire, thermal insulating agents to improve the insulation power of the material etc.

Preferred of the at least one active ingredient(s) are selected from the group of water repellents, modifiers of the building blocks of the porous material, organic and inorganic biocides, aromas, colorants and organic or inorganic salts, such as salts of organic acids and combinations of all of the above. Any other active ingredient suitable for being deposited in a porous material using the method of the invention is con- templated .

In a preferred embodiment the biocide is an organic biocide selected from the group consisting of triazoles, pyrotroides, carbamates, organic or inorganic salts, salts of organic acids, essential oils having disinfectant properties and combinations of one or more of these.

Examples of specific active ingredients are selected from but not limited to the triazoles propiconazole and tebuconazole, iodopropynylbu- tylcarbamate (in the following IPBC), carbon dioxide, chlotianidin, di- chlofluanid, difenacoum, difethialone, etofenprox, K-HDO, slufuryl fluo- ride, thiabendazole, thiamethoxam and any conmbination of thereof. The ratio of active ingredient to carrier fluid is typically in the range of 1 ppm to 30 % (weight/weight) depending on the active ingredient. When the active ingredient is a fungicide the preferred content is lOppm - 20 % (weight/weight) . In a presently preferred embodiment the carrier fluid is a supercritical fluid having a critical point at a temperature of 20-50 0 C and a pressure of 5-100 bar under a pressure of at least 20 bar and a temperature below 65°C. A presently preferred carrier fluid is carbon dioxide however other suitable carrier fluids, without limitation, encompass pro- pane, ethane and R134a. Carbon dioxide is a fluid, which has a relatively low critical pressure, low cost, is non-toxic and non-flammable. Additionally, carbon dioxide is easy to recover again from the treatment process for repeated use in the method . Carbon dioxide is particularly preferred when the active ingredient is a non-polar chemical . Another aspect of the present invention is a piece of wood obtainable by the method of the present invention characterised in an even distribution of the active ingredient throughout the wood .

Figures Figure 1 is a diagram showing the estimated partition ratios for propiconazole in a wood/CO 2 system at 4O 0 C and varying pressures (referred to in example 1) .

Figure 2 is a diagram showing the estimated partition ratios for propiconazole in a wood/CO 2 system at 50 0 C and varying pressures. Experiments at 80 bar were done but the retention time could not be recorded at this pressure (referred to in example 1).

Figure 3 is a graph showing the partition ratios of propiconazole as a function of CO 2 density (referred to in example 1).

Figures 4a - c are the time/temperature/pressure curves for three treatment processes (referred to in example 2).

Figure 5 is a schematic illustration of sample collection from a treated wood piece (referred to in example 2). Figure 6 is a graph showing deposition of active ingredient in the wood. Diamond/short-dotted line is an active ingredient with a low partition ratio a high feed, the square/long-dotted line is an active ingredient with a low partition ratio, and a triangle/solid line is an active ingredient with a high partition ratio, i.e. a wood prepared according to the prior art method (referred to in example 2).

Figure 7 is a graph showing the ratio of deposition of two active ingredients (propiconazole and tebuconazole) in the wood. Diamond/short-dotted line is an active ingredient with a low partition ratio, the square/long-dotted line is an active ingredient with a low partition ratio and a triangle/solid line is an active ingredient with a high partition ratio, i.e. a wood prepared according to the prior art method (referred to in example 3).

Figure 8 is a graph showing the partition ratio of tebuconazole, propiconazole and IPBC respectively as a function of carbon dioxide den- sity.

Detailed description of the invention

A porous medium or a porous material is defined as a solid (often called frame or matrix) permeated by an interconnected network of pores (voids) filled with a fluid (liquid or gas). Usually both the solid matrix and the pore network (also known as the pore space) are assumed to be continuous, so as to form two interpenetrating continua such as in a sponge. Many natural substances such as rocks, soils, biological tissues (e.g. bones), and man made materials such as cements, foams and ceramics can be considered as porous media.

The concept of porous media is used in many areas of applied science and engineering : mechanics (acoustics, geomechanics, soil mechanics, rock mechanics), engineering (petroleum engineering, construc- tion engineering), geosciences (hydrogeology, petroleum geology, geophysics), biology and biophysics, material science, etc. Fluid flow through porous media is a subject of most common interest and has emerged a separate field of study. The study of more general behaviour of porous media involving deformation of the solid frame is called poro- mechanics.

Preferred porous materials in the context of the present invention are selected from the group of silica, cellulose, gels, aerogels, cork and wood. A presently preferred porous material is wood, cork or cellulose. Throughout the claims and specification the term partition ratio

(p) is defined as the concentration of active ingredient in the porous material divided by active ingredient in the fluid, i.e. p = c WO od/Cfi U id- Thus, the partition ratio is defined as the ratio of the active compound concentration in the porous material to its concentration in the fluid phase at equilibrium. The value of the partition ratio depends on the material treated, the active ingredient and on the carrier fluid. In the applications shown in the experiments, the partition ratio for propiconazole can be as high as 38.3 or as low as 2.6. While for tebuconazole, the partition ratio can be as high as 80.4 or as low as 5.9. The same values are seen for IPBC, presenting partition ratios as high as 16 or as low as 2.9. The partition ratios given as examples above are for supercritical carbon dioxide and wood. If the carrier fluid is changed to another fluid, tebuconazole and propiconazole and IPBC are changed to another active ingredient(s) and/or the wood exchanged with another porous medium, the partition ratios would have completely different absolute values. But the principle remains the same.

Without the wish to be bound by any theory it is believed that the results presented herein can explain the concentration gradients of- ten reported by wood impregnation researchers. As the porous material fills up with the fluid/active ingredient mixture while adding the loaded carrier fluid it separates chemically the active ingredient from the carrier fluid. The concentration of active ingredient in the treatment solution will thus decrease from the surface of the porous material towards the centre. When the porous material is saturated with the carrier fluid and there is no longer a net flux of carrier fluid into the porous material the active ingredient is left to move by diffusion towards the centre of the porous material. The velocity by which the active ingredients moves by diffusion will be dependent on the K D -value (partition ratio). During the removal of the carrier fluid there is, of course, a net flux of carrier fluid out of the porous material. But now the high porous material affinity of the active ingredients becomes beneficial to the porous material preserver as the active ingredients are held back in the matrix as the carrier fluid exits the porous material.

Minimizing treatment times is important for the economic viability of the treatment process. Therefore, a lot of research has focused on examining the effects of different pressurization and depressurization rates on the mechanical properties of impregnated wood in order to try to establish just how fast wood can be pressurized and depressurized without being damaged. However, the results presented here indicate that the rate limiting factor in wood impregnation might not be how fast wood can be pressurized without causing failures but rather how fast active ingredients can be delivered to the center of impregnated samples. To ensure a rapid movement of active ingredients through the wood structure, the equilibrium should be moved as far as possible towards the carrier fluid during the pressurization and impregnation part of the treatment cycle. Ways of doing this include 1) increasing the gas phase concentration of active ingredients by adding more of these to the sys- tern, 2) controlling process parameters, 3) adding excipients to the carrier fluid.

Control of the process parameters during the movement through the wood is preferably done by increasing the pressure or lower- ing the temperature, this increases the density of the carrier fluid, such as carbon dioxide, and consequently the solubility of active ingredients in the carrier fluid . This will ensure that the active ingredient is maintained in the mobile phase. The relationship between carbon dioxide density and the partition ratio of three active ingredients is illustrated in figure 8. This clearly shows that a means for obtaining a low partition ratio is increasing the density of the carrier fluid, in this figure illustrated with carbon dioxide.

Examples of excipients are alcohols, preferably with low volatil- ity, for example, propylene glycol or triethylene glycol . The excipients are preferably used as a formulation adjuvant and facilitates the loading of the active compounds to the process. Other purposes of the excipient can be to facilitate flowability of the active ingredient in the carrier fluid or simply to handle the active ingredient in liquid form for safety rea- sons.

In addition, it is likely that the choice of active ingredient and/or carrier fluid has an impact on the K D -value.

The treatment may be an impregnation process where one or more active compounds are deposited in the wood . These active com- pounds may be biocides, fungicides, insecticides, colorants, fire retarding compounds, strength improving compounds etc.

The treatment may also be an extraction process where particular compounds are extracted from the wood, such as resin, terpenes etc., or it may be toxic compounds that have to be removed from wood before disposal of the wood .

Examples of carrier fluids are carbon dioxide, ethane, ethylene, propane, propylene, cyclohexane, isopropylene, benzene, toluene, p- xylene, chlorotrifluoromethane, trichlorofluoromethane, ammonia and water. A presently preferred carrier fluid is carbon dioxide. The method of the present invention may comprise but is not limited to the following steps : a) a vessel is loaded with wood to be treated; b) the required amount of active ingredient is placed in a mix- ing vessel c) the vessel is pressurised using the carrier fl u id u ntil the treatment pressure is reached; d) a holding period where the pressure is essentially constant or the pressure changes at a low rate; e) depressurising the vessel to ambient pressure followed by removal of the treated wood .

In the context of the present invention during steps c) and d), the partition ratio is lowered i.e. the active ingredient equilibrium mobile phase <-> stationary phase is kept to the left, and d uring step e) the partition ratio is increased, i.e. the equilibrium is forced to the right.

Alternatively, step b) may be omitted and the active ingredient added either during or after step c) when the solubility in the solvent is high enough to ensure that the active ingredient is maintained in the carrier fluid .

As preferred pressures and temperatures can be mentioned : A treatment, wherein the supercritical treatment pressure in step c) is in the range of 85-300 bar, preferably in the range of 100-200 bar, more preferred in the range of 120-170 bar and most preferred in the range of 140-160 bar. A treatment, wherein the temperature of the carrier fluid in the wood is above 1O 0 C, preferably above 2O 0 C, preferably above 25 0 C, preferably above 3O 0 C, more preferred above 32.5 0 C and most preferred above 35 0 C. A treatment, wherein the temperature of the carrier in the wood is in the range of 25-65 0 C, preferably in the range of 31-55 0 C in step b) and d) when the pressure is above 30 bar. A treatment, wherein the temperature during step b) and d) is below 65 0 C, preferably below 6O 0 C, preferably below 55 0 C, more preferred below 5O 0 C and most preferred below 45 0 C. A treatment, wherein the temperature during step d) is above 45 0 C, preferably above 5O 0 C, preferably above 55 0 C and more preferred above 6O 0 C when the pressure is above 30 bar. When the pressure has been reduced to 10-30 bars it may be released to atmospheric pressure without further measures (step (d)) .

Treatment periods vary with the properties of the porous me- dia, such as porosity, permeability, affinity of the carrier fluid and the porous material to the active ingredients. In the example of impregnation of, wood, such as Norway spruce (Picea Abies), with organic fungicides and using carbon dioxide as a carrier fluid, the treatment can take between 2 and 9 hours, such as 2.5-4 hours, depending on the actual dimensions and properties. It is the experience of the inventors that other wood species such as birch {Betula), in spite of having a much higher permeability, may require longer treatment times due to the chromatographic effect of that species of wood. The invention will now be described in more details in the following non-limiting examples.

Examples

Example 1 Interaction of propiconazole and wood in a supercritical carbon dioxide atmosphere was examined by measuring the retention times of propiconazole in a wood filled column mounted on a supercritical chro- matograph.

Materials

Propiconazole >96% purity was supplied by Janssen Pharmaceutical, Belgium . Sawdust with a particle size < 125μm was prepared from boards of Norway spruce {Picea abies) supplied by Vida Wood, Sweden. The supercritical chromatog raph was a HP G 1850A ChemStation equipped with a HP 1050 UV detector. Carbon dioxide was from a pressurized bottle, Air Liquide E290.

Method

A column was built by filling a metal cylinder with sawdust. The column had a length of 294 mm and an inside diameter of 6.4 mm .

Sawdust was prepared from Norway spruce and passed through a 125 μm mesh. 3.4 g of the fraction passing the mesh was transferred to the column. The ends of the column were packed tight with metal sieves and cellulose filters on top of glass wool to prevent the sawdust from leaving the column during the experiments. Thus the column build-up was as follows: metal sieve - cellulose filter - glass wool - sawdust - glass wool - cellulose filter - metal sieve. The metal sieves were intended for col- umn preparation and labeled "fine" but the actual mesh size was unknown.

The column was mounted on the chromatograph and subjected to a flow of carbon dioxide at various supercritical conditions. For each run the propiconazole was dissolved in ethanol and the ethanol- propiconazole mixture was injected into the pre-column flow path of the carbon dioxide. The propiconazole was then moved downstream through the column to the detector. The retention time was then established from the resulting chromatogram and the procedure was repeated . All system parameters i.e. pressure, temperature, flow rate, injection vo- lume, and detector wavelength were controlled via system software (HP ChemStation). Temperature was set to either 40 or 5O 0 C and the pressures were set to 80, 100 or 150 bar when running at 40 0 C, and 100, 110 or 150 bar when running at 5O 0 C.

It was not possible to record a retention time at 5O 0 C and 80 bar and therefore an extra set of measurements at 110 bar were made instead . For all runs the flow rate was set at 2 ml Cθ 2 /min and the detector was set to measure the absorbance at 204 nm based on the measured UV spectra of propiconazole in ethanol.

Results

The measured retention times are reported in Table 1. Also reported are the retention factors and the partition ratios calculated as follows:

and k'V [Biocide] m

K n = M, 'ood

^D (2)

F s, [Biocide] CO ',2 J

where k ' is the retention factor, t R is the recorded retention time, t M is the hold-up time, K D is the partition ratio (IUPAC has discontinued the term partition coefficient), V M is the volume of the mobile phase (i.e. the hold-up volume), and V 5 is the volume of the solid phase (i.e. the volume of the sawdust).

K D is essentially a measure of the relationship between biocide concentration in the solid phase and the fluid phase. A value higher than 1, would signify a higher affinity of propiconazole for wood than for CO 2 .

The table also lists the hold-up times i.e. the retention times of the carbon dioxide. The hold-up times were calculated under the assumption that the CO 2 did not interact with the sawdust. Under this assumption the hold-up time can be calculated from the flow rate into the pump (F IN ) and the volume of the mobile phase (V M ) taking into account the difference in fluid specific volume at either side of the pump (υ IN and υ OUT) :

t _ = V ' M 11 O

1 M IN (3)

^ IN^ OUT

The pump input was liquid CO 2 at 57 bars cooled to 5°C soυ IN was a constant 1.09 cm 3 /g- υ 0U τ varied depending on the physical conditions of the run between 1.28 cm 3 /g and 3.60 cm 3 /g- Table Ia. Retention times of propiconazole in the wood column under the indicated physical conditions. Corresponding retention factors and partition ratios are shown together with the calculated hold-up times of CO 2 .

T P Molecule Retention time 1 Retention Partition

(°C) (Bar) R t (i) (min) factor (k ) ratio (K D )

Propiconazole 16.0 5.2 43.3

80 CO 2 (hold-up 1.07* time)

Propiconazole 6.4 1.7 5.1

40 100 CO 2 (hold-up 2.42* time)

Propiconazole 5.5 0.8 2.6

150 CO 2 (hold-up 2.99* time)

Propiconazole 19.5 12.4 38.3

100 CO 2 (hold-up 1.46* time)

Propiconazole 10.0 4.2 12.9

50 110 CO 2 (hold-up 1.93* time)

Propiconazole 5.4 1.0 3.1

150 CO 2 (hold-up 2.68* time)

*) Calculated value

Fig. 1 and Fig. 2 show the estimated partition ratios at the examined physical conditions. First, experiments were run at a constant temperature of 4O 0 C and three different pressures 80, 100 and 150 bar. Next, the temperature was increased to 50 0 C and the measurements were repeated at the same pressures. However, a retention time at 80 bar was not recordable, because the affinity of propiconazole to wood at these conditions was so large that a peak could not be established from the chromatogram. The efficiency of the sawdust column was very low meaning that peak broadening increased considerably with time. At 5O 0 C and 80 bar, the chromatogram did not show a peak but rather a very long low hill, which made a correct establishment of the retention time impossible. A measurement was made at 110 bar instead.

The results show that the experimental conditions had a large influence on the wood/propiconazole interactions. Partition ratios decreased isothermally with decreasing pressures. Isobarically, partition ratios decreased with increasing temperatures. These results indicate a correlation between the CO 2 density and the partition ratios. Figure 3 shows the partition ratios as a function of CO 2 density. As can be seen from figure 3 there is a strong relationship between the density of the carbon dioxide and the partition ratio. This is because at the higher densities there are more molecules of CO 2 available for interaction with the propiconazole, which leads to a higher solvat- ing capacity, which in turn shifts the equilibrium Biocide wood <-> Biocide 2 to the right. However, the magnitude of propiconazole interactions with wood is surprising. All partition ratios were above 1.0 signifying that the affinity of propiconazole for wood was significantly higher than for CO 2 at all conditions. Even at carbon dioxide densities approaching 0.8 g/ml, propiconazole still has a higher affinity for wood than for CO 2 . At lower densities the affinity for wood is such that the equilibrium is shifted almost completely to the side of the wood.

These results show that deposition of biocides in the supercritical wood impregnation process is controlled by adsorption/desorption behaviour rather than being a precipitation process. The results could explain the biocide concentration gradients often reported to exist in supercritical impregnated samples. The concentration gradient likely develops because the wood has a chromatographic effect on the biocides as they are moved through the wood structure by the carbon dioxide. The same experiment was performed with tebuconazole and IPBC, the compiled results are shown in the table below: Table Ib) Retention times, retention factors, and partition ratios of three different biocides in a wood filled column subjected to a flow of carbon dioxide at various supercritical conditions.

-t5.4 ι.±l 9 I 1? 5 !±'3.4 ) ±61) j,2 r±0 J

4.3 L- i

5S.4tO4i 27.2.-±'0.31 8αS L±l »

ΪUH04! 5.4.-±'0.31 l&i L±ϋϊf

21 OI0SI IM liSJ ?.3.? r±oa J r±oa oit io n r±o; r ±00

Example 2

A piece of wood (spruce) was treated according to the Super- wood TM process, i.e. generally comprising the steps:

• The wood is placed in an impregnation vessel

• The required amount of active ingredient is placed in a mixing vessel

• Carbon dioxide is added, and the pressure and temperature are adjusted to the desired condition, whereby the active ingredients are dissolved in the carbon dioxide and maintained in the fluid phase

• Carbon dioxide together with active ingredients is circulated through the impregnation vessel for a suitable period of time, thus ensuring an even distribution of the active ingredients in the wood

• The vessel is depressurised and the conditions are adjusted so that the active ingredient is forced into the wood phase, whereby any excess active ingredients are deposited from the gas, enabling both carbon dioxide and the active ingredients to be recycled • The wood is removed from the impregnation vessel and is ready to be used.

Each test sample was specifically treated under the following conditions:

Method according enabled to dissolve the active ingredients so that the to the invention partition ratios obtained were (PROPI - 43.3 and IPBC - 16.0). The increase in pressure continued until t = 120 min. At this point the partition ratios reached their minimum values (PROPI - 2.6; TEBU - 5.9; IPBC - 2.9). After this point the temperature was raised to 60°C and the partition rations also showed a small increase (PROPI - 3.1; TEBU - 8.4; IPBC - 3.6). At t = 150 min the pressure was decreased and the partition ratios increased again to (PROPI - 38.3; TEBU - 80.4; IPBC - 16.0). When the pressure was reduced to below 80 bar, the active ingredients became almost insoluble in the CO 2 . The time/pressure/temperature treatment curve is shown in figure 4c.

After the treatment one or more pieces are selected from each batch to be analyzed, and each piece is cut out in samples of minimum

500 mm in axial direction from the end. The outer 20 mm in each side of the tangential direction of the board are removed, and the sample is subdivided into 2 mm samples and labelled as shown in Figure 5.

Each subdivided sample is grinded in a rotary grinder with a 0.5 mm mesh, and extracted with methanol. Duplicates of approximately 1 g of grinded wood samples were extracted with 20 ml_. methanol and treated for about 30 minutes in an ultrasonic bath followed by extraction on a rotary table for additional 16 hours.

The extract was filtered through a 0.45 μm disposable filter and analysed using gas chromatography using a FID-detector (flame ionization detector) for determination of propiconazole and tebuconazole con- tents and an ECD-detector (electron capture detector) for determination of the IPBC content. The detection limits were as follows: Using a FID- detector with a probe having a net weight of 1 g the detection limit was 1 - 2 mg/kg depending on the active ingredient. Using a ECD-detector with a probe having a net weight of 1 g the detection limit was 0.1 - 0.2 mg/kg IPBC. Each extract was analyzed twice and the average of these results was used.

Calibration was performed externally with a standard obtained from Riedel de Haen (propiconazole and tebuconazole) and from Sigma Aldrich (IPBC). For each of the tested active ingredients a calibration was performed before and after measuring the sample extract. The two calibrations for each active ingredient were substantially identical which means that all measurements of the sample extracts could be used for deducing a function for the calculation of active ingredient concentration in the sample extracts.

As the wood properties have a broad variation even with wood of the same origin, the values for each depth were determined as an average value taken from a series of batches of the same wood type and origin impregnated with the same impregnation program. In batch #314 96 samples were used, in batch #338 28 samples were used and in batch #335 7 samples were used. The number of samples were chosen based on experience with homogeneity of the results.

Three treatment programs were analyzed. Two using the method of the present invention represented by the batch numbers 335 and 338 fed at a normal and high ratio respectively and one using the prior art method, represented by the batch number 314.

The results can be seen in figure 6 where the relationship between depth of the wood piece and the active ingredient content is depicted. The measurement points were extrapolated and the resulting equations are as follows:

335 : y = -123.67x + 714.83; R 2 = 0.483

338 : y = -96.864x + 866.71; R 2 = 0.3028

314: y = -162.42x + 1288.6; R 2 = 0.3699 where Y is the content of active ingredient (ppm) measured in the wood sample and x is the depth (mm) of the wood sample taken in the piece of wood that has been treated.

The active ingredient content (AI) gradient was extrapolated and the equations describing the linear relationships clearly showed a substantially steeper gradient in the prior art method wood as compared to wood prepared according to the present invention . Additionally, it was noted that comparing the methods according to the present invention a longer treatment time resulted in a gradient with a lower inclination. The most important observation is that two samples impregnated in comparable time spans using the prior art (314) and the present invention method (335) differs significantly in the distribution of active ingredient.

Example 3 A piece of wood was treated according to the present invention both with a short impregnation time (approximately 1 hour) denoted 335, and a long impregnation time (approximately 2 hours) denoted 338 and another piece of wood was treated according to the prior art method for approximately IVi hour, and was denoted 314. All wood pieces were treated with a 1 : 1 mixture of two fung icides (propiconazole and tebu- conazole) and were analyzed for an even distribution of the two fungicides in the wood piece.

Test samples were taken from the wood pieces as described in example 2. The results can be seen in figure 7 where the relationship between depth of the wood piece and the ratio propiconazole:tebuconazole content is depicted . The measurement points were extrapolated and the resulting equations are as follows :

335 : y = 0.0636x + 0.7433; R 2 = 0.1511 338 : y = 0.0485x + 0.8046; R 2 = 0.3271

314 : y = -0.0979x + 2.3583; R 2 = 0.1373 where Y is the ratio propiconazole :tebuconazole measured in the wood sample and x is the depth (mm) of the wood sample taken in the piece of wood that has been treated . As the fung icides were added in a 1 : 1 ratio the optimal result should be a 1 : 1 distribution in the wood samples taken . As can be seen from the figure samples taken from wood pieces prepared according to the present invention showed a substantially even distribution of the two fungicides through the wood piece, i.e. the graphs centre around the y- value 1. The results from the prior art method show a significant faster propiconazole uptake as compared to tebuconazole uptake, i.e. a ratio (y-value) above 2. It was estimated that in order to reach an even dis- tribution of the fungicides using the prior art method the impregnation time would under these condition have had to be around 7-8 hours. This is a further indication that a faster and yet more reliable and even distribution of any active ingredient is obtained when using the method of the present invention .

References

All references are hereby incorporated by reference.

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Acda, M N, Morrell, J J, Levien, K L (2001) : Supercritical fluid impregnation of selected wood species with tebuconazole. Wood Science and Technology, 35(1-2), 127-136.

Anderson, M E, Leichti, R J, Morrell, J J (2000) : The effects of supercritical CO2 on the bending properties of four refractory wood species. Forest Products Journal, 50(11-12), 85-93.

Drescher, M, Jokisch, A, Korte, H, Peek, R D, Steiner, R (2006) : Differential pressure characteristics of wood impregnated with compressed gases, liquids and supercritical fluids. HoIz AIs Roh-und Werk- Stoff, 64(3), 178-182.

Kang, S M, Ra, J B, Levien, K L, Morrell, J J (2006) : Developing diffusion coefficients for SCF impregnation of douglas fir heartwood with cyproconazole. Journal of Wood Chemistry and Technology, 26(2), 111- 124. Lucas, S, Gonzalez, E, Calvo, M P, Palencia, C, Alonso, E, Co- cero, M J (2007) : Supercritical CO2 impregnation of Radiata pine with organic fungicides - Effect of operating conditions and two -para meters modeling . Journal of Supercritical Fluids, 40(3), 462-469. Sahle-Demessie, E, Levien, K L, Morrell, J J (1995) : Impregnation of Wood with Biocides Using Supercritical Fluid Carriers. ASC Symposium Series, 608, 415-428.

Schneider, P F, Levien, K L, Morrell, J J (2006) : Effect of wood characteristics on pressure responses during supercritical carbon dioxide treatment. Wood and Fiber Science, 38(4), 660-671.

Schneider, P F, Morrell, J J, Levien, K L (2005) : Internal pressure development during supercritical fluid impregnation of wood. Wood and Fiber Science, 37(3), 413-423.