FR3018716A1 - METHOD FOR THE TREATMENT BY DENSIFICATION OF A MATERIAL COMPRISING WOOD BY CHEMICAL REACTION BETWEEN CITRIC ACID AND PENTAERYTHRITOL - Google Patents

Abstract

A method of densifying a material comprising wood characterized in that it comprises the following successive steps: a) contacting a material comprising wood with a liquid aqueous treatment solution comprising pentaerythritol, citric acid and water at a temperature above 50 ° C, b) low temperature drying of the material comprising wood between 20 ° C and 100 ° C. c) heat treatment of the material comprising wood at a temperature of between 100 ° C and 200 ° C.

Description

- 1 - Method of densification treatment of a material comprising wood by chemical reaction between citric acid and pentaerythritol.

TECHNICAL FIELD The invention relates to a process for the chemical treatment of a material comprising wood, in order to obtain a modified impregnated material provided with specific properties, in particular mechanical, physical and chemical properties, that the material comprising wood, for example the wooden material does not have naturally. That is, a method of filling the natural open porosity of a material comprising wood. This filling treatment allows consolidation and stabilization of a material comprising wood, modified, provided with specific properties. This filling treatment of the porosity of a material comprising wood may be called densification in the rest of the text because it causes an increase in the density of a material comprising wood.

In the present description, generally means by wood, a material comprising wood, a material comprising a majority proportion of wood. By "majority proportion" is generally meant that the material in the dry state comprises at least 50% by weight of wood. Preferably, this material is made entirely of wood. In other words, the method of the invention aims to obtain a material comprising wood, for example wood, to give it physical properties in particular to increase its hardness and improve its mechanical strength. The method of the invention also aims to confer on the material comprising wood a better dimensional stability when the wood is in contact with a source of moisture. This dimensional stability means that the material comprising wood, such as wood, dilates less with moisture, or undergoes less shrinkage during drying, and has better resistance to attack by living organisms capable of degrading the material comprising wood, such as wood, for example molds or xylophagous insects. - 2 - Thus, in the field of wood preservation, coatings consisting of resins, stains, oils and varnishes are generally used, often polyurethane coatings [1] [2] [3]. The use of the aforementioned products is limited to a simple surface treatment, very superficial, the penetration into a substrate such as wood is very low, namely generally less than 1 mm. Due to the large size of these molecules and their hydrophobic character, there is no effective penetration, for example polyurethane, in the intimacy of the microstructure of the cell walls of the wood. To guarantee a superior resistance, namely at least ten years to satisfy the ten-year guarantee imposed by the building standards, it is necessary to resort to wood impregnation processes, in its volume, with preservatives such as copper-chromium-arsenic (CCA) salts, creosote or organochlorine compounds or other toxic substances which, unfortunately, all present disadvantages in relation to public health and respect for the environment. The recycling of wood products after such treatments is difficult to envisage. Furthermore, it is known, to improve the intrinsic mechanical properties of wood, that it is possible to densify the wood using physical methods to polymerize exogenous organic matter in the wood in-situ [4-5. ]. These treatments are carried out in two stages: a first stage of impregnation of a liquid resin of the precursor monomer of the final polymer, followed by the actual polymerization which can be carried out according to two approaches: i) gamma irradiation or Electronic irradiation are well known for cross-linking carbon chains by generating free radicals on carbon chains containing double bonds. ii) It is possible to pre-mix the monomer resin with a thermocatalyst (eg peroxide type) to initiate polymerization after a temperature rise of the monomer. With the aid of these densification processes, hydrophobic plastic material is introduced into the natural porosity of the wood. It is indeed possible to significantly improve the mechanical properties of such "composite woods"; on the other hand, these densification treatments are completely ineffective to modify the wood's sensitivity to moisture. Indeed, the hydrophobic resins are unable to penetrate into the cell wall (hydrophilic medium). Such densification slows down the penetration of water into the wood, but does not prevent it: the densified wood retains its initial instability and remains very sensitive to cracking during a humidification / drying cycle. more than the wood is more rigid, therefore more brittle. A method is also known for combining a wood-blowing treatment with polyethylene glycol and densifying the wood with a styrene-polyester resin [6]. The purpose of this process is indeed to stabilize and harden the wood at the same time. However, the disadvantage of the process is that the polyethylene glycol is soluble in water and leaches out when the substrate comprising wood is immersed in liquid water over long periods of time. In addition, it is mentioned in the literature [7] a method that combines the inflating of wood with an aliphatic fatty acid diacid with styrene-polyester densification. The main disadvantage of said process is that it requires the massive use of an expensive product such as azelaic acid. Furthermore, even if this treatment is of some interest for stabilizing the wood dimensionally vis-à-vis moisture, the process is not considered sustainable because the azelaic acid 30 remains free and susceptible to leaching during the aging process. prolonged contact with water. Finally, inventions [8-9] describe processes generally aimed at carrying out esterification reactions in situ in wood from polyols and polyacids. These inventions list a large number of possible polyalcohols that can react with a large number of polyacids. The reaction products obtained in the wood after esterification of polyols with polyacids are in most cases very hydrophilic, or even soluble in water. The documents do not specify the hydrophilicity of the products obtained from the esterification. This point is however important to ensure a good behavior of treated wood vis-à-vis moisture. Thus, densifying wood by creating in situ an even more hydrophilic material that wood is of little interest, especially if the reaction product obtained is soluble or partially soluble in water. It will have no resistance to leachate from rainwater. Similarly, if the wood is sufficiently hydrophilic to have a high rate of water retention (> 30% by mass), it will be more sensitive to fungal attack. Similarly, various combinations of polyols with polyacids proposed [8-9] have found in most cases a swelling and a very darkening of the wood, showing an attack "acid" of the cellulosic material of wood by polyacids used according to the inventions cited. Finally, in the inventions mentioned [8-9], the reactions are advantageously carried out using chemical catalysts such as persulfates, peroxides, metal salts (lead, cobalt, zirconium, zinc, or sodium hydrophosphite These products are for the most part harmful and greatly complicate the proposed treatment protocols.

Finally, the invention [10] describes a method of polymerization by esterification from an aqueous syrup comprising a mixture of glycerol with citric acid. After impregnation of the wood with an aqueous solution of glycerol and citric acid in equimolar proportion, the wood is heat-treated between 135 ° C and 230 ° C to initiate the polymerization reaction by esterification. Although this method is effective for stabilizing the wood, it has the disadvantage of allowing acid attack of the wood during the polymerization. Indeed, this process [10] proposes to perform the thermal esterification cycle when the wood is still waterlogged. Since the evaporation of water can not be instantaneous, especially for massive objects, it follows that the wood is forced to withstand citric acid for relatively long periods of several hours in the wet state. and at temperatures close to 100 ° C. The moisture present in the wood at the beginning of the polymerization cycle allows the acidity of the citric acid to be fully expressed at pH values below 3, which significantly alters the chemical integrity of the principal by hydrolysis. constituent polymers of wood (hemicellulose, cellulose, and lignin). On the other hand, high levels of citric acid tend to induce self-polymerization / pyrolysis of citric acid with itself, giving a very dark color to the formed polyester.

There is thus a need not yet satisfied for a method of hardening and stabilizing a material comprising wood, efficient and low cost, allowing treatment throughout the volume of the material, retaining the natural aesthetic appearance of the wood, conferring to wood a marked hydrophobic character, including a very good resistance to leaching against water, without using organic products such as reagents, organic solvents or expensive catalysts, harmful or toxic mentioned in the processes of the prior art. The object of the invention is to provide a method of densification and dimensional stabilization of a material comprising wood, such as a wood material, which satisfies all of the above all the needs mentioned above.

The invention will now be described in more detail in the following. The material treated according to the invention is a material comprising wood with any moisture content. The material is preferably made of wood, for example it may consist entirely of wood (taking into account the impurities naturally present in it and its initial moisture content).

The invention defines a method of densifying a material comprising wood characterized in that it comprises the following successive steps: a) contacting a material comprising wood with a liquid aqueous treatment solution comprising pentaerythritol, citric acid and water at a temperature above 50 ° C, b) drying at a temperature between 20 ° C and 100 ° C of the material comprising wood obtained in step a). c) heat treatment of the material comprising wood obtained in step b) at a temperature between 100 ° C and 200 ° C, for at least 1 hour. Advantageously, the treatment solution contains at least 80% by weight of a mixture comprising: 10% to 40% by mass of pentaerythritol, 10% to 60% by weight of citric acid, 0% to 80% by weight of water . More preferably, the treatment solution comprises: 15% to 35% by weight of pentaerythritol, 15% to 55% by weight of citric acid, 10% to 70% by weight of water.

According to a development of the invention, the treatment solution contains only the aforementioned 3 elements excluding other compounds. Advantageously, the molar proportion between pentaerythritol and citric acid is 1 to plus or minus 50 mol%, preferably plus or minus 20%. This stoichiometric ratio makes it possible to optimize the arrangement of the precursor reactant molecules of the polyester formed during the heat treatment c). Advantageously, the bringing into contact between the liquid treatment solution and the material comprising wood can be carried out by completely immersing the material comprising wood by introducing said substrate and the treatment solution defined above based on pentaerythritol and citric acid in a container at atmospheric pressure, above 50 ° C. - If the material comprising wood is dry according to a preferred embodiment of the invention, step a) of the process is carried out according to a "vacuum-pressure" technique to improve both the impregnation rate of the material comprising wood and the kinetics of impregnation. This "vacuum-pressure" technique consists of disposing the material comprising wood to be impregnated in a vacuum chamber, for example in an autoclave under a partial vacuum of 10 to 100 mbar, in order to degasify said material, then to immersing said material in the aqueous pentaerythritol and citric acid processing solution above 50 ° C. Then there is established in the autoclave overpressure on said treatment solution, for example an overpressure of 5 to 10 bar of a gas, to force the liquid to enter the wood. Advantageously, the contacting of the material comprising wood with the liquid treatment solution is carried out between 70 ° C. and 100 ° C. for a preferred period of between 1 h and 24 h, to allow a good dissolution of the pentaerythritol in the water, as well as a good mix with citric acid. Advantageously, the drying of the substrate according to step b) comprising wood is carried out at low temperature, for a preferred duration of between 1 h and 48 h, more precisely between 20 ° C. and 80 ° C. in the open air, in order to avoid an excessively fast water outlet expels the resin from the material comprising wood.

The wood is dry when its moisture content is less than 30% by mass (water mass divided by the mass of anhydrous wood). Advantageously, the material comprising wood impregnated and dried according to step b) is heated according to step c) between 150 ° C. and 180 ° C., for a preferred duration of between 1 h and 48 h, to carry out the reaction of in-situ esterification in wood between pentaerythritol and citric acid. According to a preferred embodiment of the invention, it is necessary to avoid exceeding 180 ° C., since the citric acid is altered beyond. In order to limit the pyrolysis of the wood at esterification heat treatment temperatures above 150 ° C., it is advantageously possible to carry out the esterification heat treatment in an oxygen-poor environment, ie under vacuum (pressure <10 mbar). ), either in a bath of refractory liquid fats (oils, paraffins) or in steam or in a neutral atmosphere (N2, Ar, CO2) or in any type of atmosphere with a lower oxygen content to that of the air. After the esterification heat treatment, the treatment described according to the present invention may be continued by a conventional finishing treatment known to those skilled in the art: oiling (drying oils), sanding, varnishing, sawing, painting, gluing, surface coloring of wood with lazures or paints. Surprisingly, the wooden articles treated according to the present invention are not attacked by citric acid during the wood polymerization phase at high temperature (> 150 ° C). In addition, the inventors were surprised to find that the treatment that is the subject of the present invention respects the natural aesthetic nature of the wood. Indeed, the use of pentaerythritol gives a virtually colorless polyester. Unpredictably, pentaerythritol also significantly reduces the polymerization time compared to other polyols, such as glycerol. Finally, for reasons that are not obvious, the polyester obtained according to the present invention is both hydrophobic and very hard, which makes it possible to significantly enhance the mechanical properties of the wood / polymer composite obtained. In summary, it can be concluded that, quite surprisingly, most of the different polycarboxylic acids and polyols mentioned in the literature do not yield polyesters having the specific properties obtained from citric acid and pentaerythritol. The process according to the invention is very simple, and it uses only known, available, very common and non-toxic products which can be prepared from low cost compounds; in fact, it is a process based on impregnations which uses highly specific molecules and which does not have the disadvantages of the impregnation processes of the prior art using other substances. The many advantages and surprising effects of the invention can therefore be enumerated as follows, without this enumeration being considered as limiting: - We obtain excellent stabilization of the substrate in a material comprising wood, such as the wooden substrate , vis-à-vis moisture. This stabilization can be expressed by the parameter called ASE ("Anti Shrinkage Efficiency"). ABE is a coefficient that compares the behavior of treated and untreated wood samples during humidification and drying cycles. The ABE is defined by the following formula: ASE (%) = 100 x (untreated wood treated) abois untreated with the following definition of S: treated or untreated (%) 100 X (HWWood moisture-Dry wood volume) It is also possible to define a partial ESA in which reference is made not to the volume of the wood sample but to one of the dimensions of the wood; for example in the case of a wood blade, it may be the length (L), the width (1) or the thickness (e). The partial ASE coefficient ASE1 relative to the width can thus be expressed by the definition: ASEi (%) = 100 x (Slbois untreated Slbois treated) albois untreated with the following definition of SI: Sibois treated or not (%) = 100X Wet width-wet width, 1 / dry-wood width According to the invention, the ASE is generally greater than 60%, even after several humidification / drying cycles -The compounds used according to the invention are little or not The polyester produced during the heat treatment according to the invention prevents significant water retention within the wood, which remains less than or equal to 30% by weight. -The process of the invention does not necessarily require reagents, catalysts, organic solvents, various adjuvants very harmful or dangerous whether to perform the esterification of wood, to transport the compound or compounds in accordance with the invention within the structure of the substrate of a material comprising wood or for cleaning the wood of all processing residues not fixed by the material comprising wood. The method of the invention makes it possible to treat the volume of the material comprising wood, such as wood, in depth, and is not limited only to its surface unlike the treatments with stain, varnish, oiling and other finishing products. The method makes it possible to use already proven techniques and installations for its implementation. It does not require any adaptation, modification of existing installations. In other words, it is possible to use a large number of industrial processes and installations already in use for impregnating wood, such as, for example, "pressure-vacuum" enclosures or ovens or ovens already installed, which make it possible to heat-treat high temperature wood and that could be used to perform the esterification step. -It is not useful to carry out deep cleaning of the substrate of a material comprising wood such as wood after the treatment defined according to the invention. The method according to the invention can be implemented indifferently on dry wood or moist wood, especially freshly cut wood still containing sap. The duration of the impregnation treatment can be chosen very short, only a few hours, if the "vacuum-pressure" technique is preferentially chosen during the first impregnation step a). The method according to the invention does not damage the material comprising wood, such as a wood material, and in particular the external appearance of the material is little modified. The process according to the invention makes it possible to significantly improve the mechanical properties of the material comprising wood, in particular its hardness. The synthesized polyesters are completely entangled in the wood fibers and hydrophobic, therefore the wood treated according to the invention remains insensitive to leaching by water, in particular when there is contact between the material comprising wood and liquid water over long periods. The process according to the invention is generally compatible with the various finishing operations of the wood used by the industry: oiling, sanding, varnishing, sawing, painting, gluing, coloring in the volume by pigments. One of the reagents used according to the invention is chosen from products of natural origin: citric acid. Thus formulated, the invention can be considered partly ecological because it uses a bio-sourced reagent.

The process according to the invention is particularly applicable to the field of wood preservation, particularly vis-à-vis the aggressions induced by moisture. Thus, the method according to the invention is particularly intended for woods kept outside, being part, for example, cladding, shutters, windows, doors, gates, stakes, signs, floors, poles, garden furniture, pots, barrels, furniture of urban spaces and other elements of exterior carpentry, etc. The method according to the invention can be applied to exotic woods or luxury wooden objects: musical instruments, jewelery, watchmaking, watch case, etc. The method according to the invention also has applications in the building, where the wood is part of the frame, interior furniture in contact with water (eg bathroom furniture or kitchen), stairs, floors, etc. The method according to the invention can be applied to wooden structures such as bridges, bridges, structures, frameworks and also to wooden objects used in maritime activities. The invention will now be described with reference to 2 examples, given by way of nonlimiting illustration. Example 1: A sample of ponderosa pine wood was selected in a geometry of 75 x 20 x 15 (fiber direction) mm. This wood sample is dry and has undergone the treatment defined according to the present invention with the following operating conditions: a) Impregnation of the sample by total immersion for 8 hours in a beaker containing 200 g of an aqueous treatment solution of which the composition is defined as follows: 80 g of water 50 g of pentaerythritol 70 g of citric acid The beaker is sealed with tight stretch film and placed in an oven at 100 ° C for 12 hours. - 12 - b) The wood sample is dried in an oven in the open air at 100 ° C for 12 hours. c) The sample is introduced into an oven in the open air at 170 ° C for 12 hours. Then it was taken out of the oven and cooled in free air at room temperature. The sample thus treated was subjected to a succession of humidification cycles (by immersion in pure water at room temperature for 24 hours) and by natural drying (in the open air at room temperature for 48 hours). After two humidification drying cycles, we find that the ASE remains above 60% and that the intake remains less than 10%. Moreover, the wooden test piece does not show any cracking of the wood during the humidification / drying cycles. The weightings and the dimensional readings are summarized in the following table: Steps Length Mass ASE Socket (mm) g (%) mass (%) Initial state 75.38 15.6 After 78.38 26.9 impregnation After drying 77.60 24.1 After 76.38 20.0 polymerization 1 "cycle 78.14 23.0 Humidification 1st cycle 76.94 21.0 60 + 9 Drying 2nd cycle 78.50 23.4 Humidification e» cycle 77, EXAMPLE 2: A sample of Ponderosa pine wood was selected in a geometry of 75 x 20 x 15 (fiber direction) mm. This wood sample is dry. and has undergone the treatment defined according to the present invention with the following operating conditions: a) impregnation of the sample by total immersion for 8 hours in a beaker containing 200 g of an aqueous treatment solution whose composition is defined as follows: 80 g of water 70 g of pentaerythritol 50 g of citric acid The beaker is sealed with stretch film and reed and placed in an oven at 100 ° C for 2 hours. b) The wood sample is dried in an oven in the open air at 80 ° C for 12 hours. c) The sample is introduced into an oven in the open air at 170 ° C for 12 hours. Then it is removed from the oven and cooled in the open air at room temperature. Mass taps and dimensional readings are summarized in the following table: Steps Length Mass ASE Socket of (mm) g (%) mass (%) Initial state 75.58 18.0 After 78.24 29.8 impregnation After drying 77.70 23.9 After 75.94 19.9 polymerization 1st cycle 77.96 27.2 Humidification 1st cycle 76.64 21.0 51 + 29 Drying 2nd cycle 78.04 27.9 Humidification 2nd cycle 76.68 20.9 49 + 33 Drying 3 '°' cycle 78.12 28.4 Humidification The sample thus treated has undergone a series of humidification cycles (by immersion in pure water at room temperature for 24 hours) and by natural drying (in the open air at room temperature for 48 hours).

After three cycles of humidification drying, we find that the ASE is less than 60% and the water intake remains higher than 10%. The results are less good than for example N ° 1. Indeed, the mixture used does not verify the equimolar condition between pentaerythritol and citric acid.

REFERENCES [1] Goland V.A., Khinskaya O.V., Mikhailova I.A., Procedure for treating and preserving wood to harden surface coating, including coating surface of wood with impregnating base composition on polymers, GB-A-62100288. [2] Matsushita Electric Works, Ltd., Preparation of inorganic board having finished surface by laying of flat core or core of urethane resin and binder, then shaped and cold pressing, JP 59088383 patent, 1984, Japan. 20 [3] Park S.B., Development of surface improvement technique of Japanese larch flooring, Journal of the Korean Wood Science and Technology, 1999, 27 (3), 31-38. [4] D. Lopez, G. Burillo, Gamma ray irradiation of polystyrene in the presence of the cross-linking agent Radiation Effects on Polymers, ACS Symposium Series 465, published by American Chemical Society, 1991, pp.262-270. [5] C. B. Saunders, A. Singh et al., Electron beam curing of aramid-fiber-reinforced composites, Radiation effects on polymers, ACS Symposium Series 475, published by American Chemical Society, 1991, pp. 251-261. [6] Albino C., Chaumat G. A method of consolidating and stabilizing a substrate of a material comprising wood. Patent FR 05 51383 (2005). [7] Chaumat G., Albino C. Wood stabilization and hardening treatment process. Patent 07 07149 (2007). [8] Kiljunen S. Koski A., Kuntu M., Volkonen T. Impregnation of chemicals into wood. WO 2011/042609 A1 [9] Wang Y. Polymeric composition for cellulosic materials binding and modification. WO 2009/006356 A1 [10] Gray J. Improvements in coating and impregnating materials and molded articles. GB 22,449 (Oct. 1912). 10 15 20 25 30 35

Claims (10)

REVENDICATIONS1. A method of densifying a material comprising wood characterized in that it comprises the following successive steps: a) contacting a material comprising wood with a liquid aqueous treatment solution comprising pentaerythritol, acid citric acid and water at a temperature above 50 ° C, b) drying at a temperature between 20 ° C and 100 ° C of the material comprising wood obtained in step a). c) heat treatment of the material comprising wood dried at the end of step b) at a temperature between 100 ° C and 200 ° C for at least one hour. 2. Method according to the preceding claim for which the treatment solution contains at least 80% by weight of a mixture comprising: * 10% to 40% by mass of pentaerythritol, * 10% to 60% by mass of citric acid, * 0% at 80% by mass of water. 3. Method according to the preceding claim wherein the molar proportion between pentaerythritol and citric acid is 1 to plus or minus 50 mol%. A method according to any one of the preceding claims wherein the contacting between the liquid treatment solution and the wood-comprising material is performed by completely immersing the wood-containing material by introducing said material and the defined treatment solution. above based on pentaerythritol and citric acid in a container at atmospheric pressure. 35 5. Method according to any one of claims 1 to 3 wherein step a) is performed according to a "vacuum-pressure" technique with a partial vacuum of between 10 and 100 mbar and a pressure of between 5 and 10 bar. .- 17 - 6. Method according to any one of the preceding claims wherein the contacting of the material comprising wood with the liquid treatment solution is advantageously carried out between 70 ° C and 100 ° C. 7. Method according to any one of the preceding claims wherein the drying of the substrate comprising wood is advantageously carried out at low temperature, more precisely between 20 ° C and 80 ° C. 8. Method according to any one of the preceding claims wherein the esterification heat treatment according to c) is preferably carried out at a temperature between 150 ° C and 180 ° C. 9. A process according to any one of the preceding claims wherein the esterification heat treatment according to c) is carried out in a low oxygen environment. 10. Process according to any one of the preceding claims, in which the treatment according to the invention is combined with finishing treatments for the wood. 30 35