UV protection process for wooden parts and UV-protected wooden parts

Abstract

UV protection process for wooden parts, the UV protection process comprising the following steps: a) supplying a wooden part comprising polymeric components onto which amine groups are grafted, b) chemically modifying at least one amine group by reaction with an aromatic aldehyde reagent, so as to graft at least one aromatic imine group onto at least one polymeric component. Figure for the abstract: 2

Description

Title of the invention: Method for protecting wooden parts from ultraviolet rays and wooden part protected against ultraviolet rays

[0001] The present invention relates to the field of wood and deals with its stability against ultraviolet radiation. The present invention relates in particular to a method for protecting wooden parts against ultraviolet radiation and to a wooden part protected from ultraviolet radiation by said method.

[0002] Due to its ability to store carbon and its reduced environmental impact compared to other materials, there is renewed interest in using wood as a building material. However, its low durability is an obstacle to its use for certain applications. For exterior cladding applications, the sensitivity of wood to water and attack by microorganisms can lead to its decomposition and limit its dimensional stability over time. Wood is also sensitive to irradiation in the ultraviolet range, which leads to degradation of lignin and the creation of a layer of gray cellulose on its surface.

[0003] The application of wood protection treatments is then necessary to improve its properties. The treatments can be applied on the surface, such as by coating or by applying paint, by impregnation in the wood, or consist of the chemical modification of at least part of its components. The modification of the wood has the advantage of being a more effective treatment in the long term. Indeed, the coatings or paints become damaged and must be replaced, an impregnated third material can be washed off following bad weather and come out of the wood in the short or medium term, simple impregnation not constituting a permanently fixed treatment.

[0004] Thus, there is a significant need to develop a method for fixing a protective agent to wood that is durable and effective against ultraviolet rays.

[0005] To this end, the present invention provides an anti-UV (anti-ultraviolet) protection method for wooden parts, the anti-UV protection method comprising the following steps:

[0006] a) providing a wooden part comprising polymeric components, such as lignin, cellulose and hemicelluloses, onto which amine groups are grafted,

[0007] b) chemical modification of at least one amine group by reaction with an aromatic aldehyde reagent, so as to graft at least one aromatic imine group onto at least one polymeric component.

[0008] Thanks to this process, the wooden part is functionalized by an aromatic group which gives it anti-UV properties in a lasting way over time. This process in fact generates the formation of a covalent bond between the aromatic imine group and the polymeric component of the wood, which constitutes a lasting modification unlike surface protection treatments such as paint or simple impregnation which emerges from the wood over time. The aromatic group advantageously allows UV to be absorbed in place of the natural components of the wood, thus increasing the longevity of the latter.

[0009] Furthermore, the existence of a simple covalent bond between the aromatic group and the wood components allows rotation of the aromatic group around this bond. The energy stored by the aromatic group by UV absorption can be dissipated at least in part during rotation, this makes it possible to increase the lifetime of this aromatic group compared to the case where no release of stored energy can be obtained.

[0010] According to one arrangement, the aromatic aldehyde reagent comprises an aromatic group substituted by an aldehyde function and at least one hydroxyl function. This increases the possibilities of electronic delocalization in this group, which helps to lower its energy level.

[0011] According to one possibility, the at least one hydroxyl function is arranged in the ortho position of the aldehyde function on the aromatic group. This configuration of the aldehyde reagent is advantageously preserved in the grafted imine. This allows the hydroxyl function to form a hydrogen bond with the nitrogen of the imine and to form a 6-atom ring. This ring is planar and approaches the configuration of an aromatic group. Its energy level is then lowered, which helps to further stabilize the graft. In addition, since it is necessary to break the hydrogen bond for the aromatic group to rotate axially around the covalent bond, the breakage consumes energy, it reduces that of the aromatic group and stabilizes it.

[0012] According to one embodiment, the aromatic aldehyde reagent is chosen from 1-hydroxy-2-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, 2,4-dihydroxybenzaldehyde, and / or a compound of formula (I) with R1, R2, R3 and R4 chosen independently of each other from hydrogen and an alkyl.

[0013] [Chem.l] fis'" 'Ÿ' 'R,

[0014] These reagents have given very good results both in the rate of imine grafting and in the effectiveness of the protective group against UV. Nevertheless, and for the reasons set out above, any aldehyde reagent comprising an aromatic group is capable of enabling the stabilization of the wooden part and limiting the appearance of the grayish layer on the surface.

[0015] According to one arrangement, step b) comprises the following steps: - placing the wooden part in a container, in particular an autoclave tank, - arrangement of the aromatic aldehyde reagent in the container, at a distance from the wooden part, i.e. the aromatic aldehyde reagent is not in contact with the wooden part, and - addition of carbon dioxide, - application of a pressure and a temperature such that the carbon dioxide is in the supercritical, or liquid, state, so as to react the aromatic aldehyde reagent with at least one amine group and lead to the grafting of at least one aromatic imine group onto at least one polymeric component of the wooden part.

[0016] This step b) makes it possible to create a durable covalent bond between at least one polymeric component of the wood and a UV-absorbing aromatic group. The advantageous use of scCO2 as a green solvent also facilitates the impregnation of aromatic aldehyde reagent into the heart of the wood, which promotes the reaction. The scCO2 acts as both solvent and reaction medium.

[0017] The wood must be at a distance from the reagent because otherwise, the penetration of the reagent is governed by a capillary phenomenon, which risks leading to non-homogeneity of the impregnation.

[0018] Concretely, the reagent can be added to the bottom of the container and the samples placed on an elevation whose height is higher than the liquid level. The CO2 passing into the supercritical phase will solubilize the reagent and carry it towards and into the wood. In this case, there is no surface tension phenomenon in the supercritical phase, which results in an absence of capillary forces.

[0019] In an industrial version, the process could be a little different: the reagent can be placed in an auxiliary autoclave in which supercritical CO2 circulates. This solubilizes the reagent and the mixture is introduced into the treatment reactor (the container for example) containing the wood samples. Thus, the entire treatment volume can be occupied with wood. The same embodiment can be applied when using liquid CO2.

[0020] According to one embodiment, the wooden part has a humidity level of less than or equal to 15%, preferably between 4% and 15% inclusive, for example a humidity level between approximately 4 and 7%, when the aromatic aldehyde reagent is placed in the container. The humidity level of the wood must be controlled so that it is at a humidity level close to that required by its final use (exterior, interior, etc.) in order to limit its dimensional variations. Indeed, below the PSF (or fiber saturation point), the wood undergoes dimensional variations which weaken it.

[0021] This humidity level is achieved in particular by drying the wood naturally or in an autoclave. Drying can consist of carrying out controlled temperature / humidity level cycles so as to gradually lower the humidity level of the wood and this in a homogeneous manner throughout the thickness.

[0022] In theory, we could go as far as dry wood (0%), but this has no industrial reality to date because the required drying process would be too expensive.

[0023] The humidity level can be measured by weighing the sample before and after drying at 103°C for 24 hours: by calculation the average humidity level is obtained.

[0024] On an industrial product, a point moisture meter is usually used: humidity values ​​are obtained depending on the measurement area, as well as the penetration of the points into the thickness.

[0025] According to one possibility, step b) is carried out under pressure, in particular a pressure in a range between 5 and 300 bars, for example in a range between approximately 210 and approximately 230 bars, in particular approximately 230 bars. Step b) carried out under pressure makes it possible to modify the carbon dioxide from the gaseous state to a liquid state, or even close to the supercritical state, and preferably to modify the carbon dioxide from the gaseous state to the supercritical state.

[0026] According to one arrangement, step b) is carried out with an increase in temperature, in particular to reach a temperature range of between 80 and 150°C, for example a temperature range of between approximately 100°C and approximately 130°C, in particular approximately 120°C. The application of temperature accompanies the effect of the pressure in order to allow the passage of carbon dioxide from the gaseous state to the supercritical state.

[0027] According to one embodiment, a grafting reaction i) is carried out prior to step a), comprising the grafting of at least one amine group by reaction of at least at least one amine reagent on at least one hydroxyl function of the polymeric components of the wood part. The hydroxyl function is in fact a function found naturally on polymeric components such as lignin, cellulose or hemicellulose derivatives. It represents a reaction site of choice for functionalization. It is easily accessible, reactive and capable of forming a covalent bond with reagents under appropriate reaction conditions.

[0028] According to one arrangement, the amine reagent comprises a primary amine, or even a secondary amine, so as to facilitate its subsequent modification in step b) into imine.

[0029] According to one possibility, the at least one amine reagent is chosen from urea, a diisocyanate, and / or a silane comprising a terminal amine group including Amino-Propyl TriethoxySilane (APTES). These amine reagents all having groups that are very reactive with respect to hydroxyls, their reactivities are sufficient to ensure grafting on the surface and sub-surface of the wood and the completion of the second step in the presence of the aldehyde reagent considered. For example, in the case of diisocyanates, one of the isocyanate groups attaches very quickly to the hydroxyl groups of the wood, the second isocyanate group after hydrolysis is transformed into a desired amine group -NH2, with release of CO2.

[0030] Preferably, the amine reactant is urea.

[0031] According to one arrangement, APTES is used in solution in ethanol at a concentration of between 10 and 20% by mass, mixed with an aqueous ammonia solution at pH 10, the volume of ammonia solution is in particular between 1 and 10% of the volume of the APTES solution in ethanol used.

[0032] The grafting rate on the wooden part is measured by evaluating a mass setting (on dry wood, so as not to count the water in the wood). To be effective, the mass settings in APTES for example are at least approximately 2% and for example approximately 7.5%, to ensure the viability of the second reaction. Indeed, a graft by an effective aromatic imine can consist of a surface and subsurface graft and not necessarily on the entire volume of the wooden part considered.

[0033] According to one embodiment, the grafting reaction i) of at least one amine group comprises the following steps: - arrangement of a wooden piece in a container, - placing the urea diluted in water in the container and / or the APTES solution in ethanol mixed with the aqueous ammonia solution, so as to submerge the wooden part, - application of a vacuum, in particular a vacuum between 200 and 400 mbar, for example several times, so as to promote the impregnation of the amine reagent into the heart of the wood, - drying of the wooden part, and - application of a heat treatment to reach a temperature between 100 and 150°C for a few hours, in particular for approximately 4 to 24 hours.

[0034] According to other characteristics, the protection method of the invention comprises one or more of the following optional characteristics considered alone or in combination:

[0035] Step b) comprises modifying a plurality of amine groups on a plurality of polymeric components of the wooden part.

[0036] Step b) comprises the modification of approximately 99% of the amine groups grafted onto the wooden part.

[0037] Step b) comprises modifying all of the amine groups on at least one or all of the polymeric components of the wooden part.

[0038] Generally speaking, almost all grafted amine functions react with the aromatic aldehyde. The mass gain observed during this reaction is at least 0.5%. It can exceed this value to reach, for example, 10%, in particular when the treatment applied penetrates beyond the surface of the wooden part. However, for reasons of efficiency and economy, it is not necessary to achieve such grafting in the depth of the wooden part.

[0039] According to another aspect, the invention provides a wooden part protected from UV rays, the wooden part comprising polymeric components onto at least one of which is grafted at least one aromatic imine group, capable of absorbing radiation in the ultraviolet range.

[0040] According to one possibility, a majority of the polymeric components of the surface and sub-surface of the wooden part are modified by the grafting of said aromatic imine group.

[0041] Thus modified, the wooden part is covalently associated with a UV-absorbing molecule enabling the polymer components to be protected from UV rays in a lasting manner.

[0042] According to one arrangement, the at least one aromatic imine group comprises an aromatic group having at least one hydroxyl substituent ortho to the imine function. This arrangement of the hydroxyl function makes it possible to stabilize the aromatic imine group, thus allowing greater and lasting absorption of UV radiation. As a result, the wooden part absorbs less energy from UV, which increases the duration of its protection. The gray coloring that appears over time will be less strong and slower to appear on the protected wooden part.

[0043] Other aspects, aims and advantages of the present invention will appear better on reading the following description of two embodiments thereof, given in as a non-limiting example and made with reference to the following figures:

[0044] [Fig-1] illustrates the grafting reaction of the amine derived from urea.

[0045] [Fig.2] illustrates the imine formation reaction.

[0046] [Fig.3] illustrates the reaction of imine formation starting from an amine derived from the APTES.

[0047] [Fig.4] illustrates the pseudo-absorption before and after imine formation.

[0048] [Fig.5] illustrates the measurement of the colorimetric evolution of wooden pieces after different UV accelerated aging treatments.

[0049] [Fig.6] illustrate some hydroxylated aromatic aldehyde reagents used in the invention.

[0050] With reference to Figures 1 and 2, the grafting of the aromatic imine group onto a hydroxyl function naturally present on the polymer components of the wooden part is illustrated. First of all, a first reaction i) begins by submerging a wooden part placed in an autoclave in urea diluted in water. A vacuum is applied at 200 mbar for 1 hour before returning to atmospheric pressure for another hour. This cycle is repeated at least twice so as to thoroughly impregnate the urea into the heart of the wood. The wooden part is removed from the autoclave and dried until it reaches a humidity level of less than 15%, the urea remaining impregnated in the wood.

[0051] Then, the wooden part is placed back in the autoclave and a heat treatment is applied to increase the temperature to a range of 120 to 150°C for 3 to 6 hours. This treatment allows the urea to be decomposed in situ. The decomposition leads to the formation of ammonia and an isocyanate which reacts with the hydroxyl function of the wood. The wooden part is then functionalized with an amine group. The ammonia is left to evaporate as a reaction by-product.

[0052] The second reaction consists of modifying the amine group by hydrolysis and condensation to form an aromatic imine group which is capable of absorbing UV (step b of the process). To do this, the wooden part and an aromatic aldehyde reagent, in particular 2-hydroxy-1-naphthaldehyde, are added to an autoclave, without being brought into contact. The autoclave is purged by introducing carbon dioxide three times at 10 bars. The air is thus expelled from the autoclave and replaced by CO2. The temperature of the reaction medium is increased and when it reaches approximately 35°C, the autoclave is pressurized by introducing CO2 at 50 bars. The temperature is raised to approximately 120°C and then the pressure is increased to 230 bars. The autoclave is kept closed for 2 to 6 hours under these conditions, then once depressurized, the wooden part functionalized by an aromatic imine group is extracted.It should be noted that this step carried out using supercritical CO2 is a promising way to increase the per . wood formations. The process is environmentally friendly for the extraction, drying, impregnation, modification or chemical conversion of wood.

[0053] According to a second embodiment of the anti-UV protection method, the urea is replaced by Amino-Propyl TriethoxySilane (APTES), as illustrated in [Fig. 3]. The wooden part is again submerged in an autoclave by a solution of APTES in ethanol mixed with an equivalent volume of ammonia in water. A vacuum of 400 mbar is applied for one hour, several times between two returns to atmospheric pressure for also one hour. These steps allow the good impregnation of the wooden part which is then dried. Once a humidity level less than or equal to 15% is reached, the wooden part is again placed in the autoclave to receive a heat treatment at approximately 100°C for approximately 24 hours. At the end of this treatment, an amine is grafted onto hydroxyl functions of the wooden part.

[0054] The second reaction consists of modifying the amine group by hydrolysis and condensation to form an aromatic imine group (step b). It is carried out in the same way as in the first embodiment using scCO2. It leads to a wooden part on which a graft of an aromatic imine group is obtained. Visual observations

[0055] The effectiveness of the process was evaluated by monitoring the color change during artificial weathering. The modified samples showed a lower color change during weathering than the unmodified wood.

[0056] To do this, a first natural wood piece and a second wood piece functionalized with an amine from urea were provided to apply the grafting reaction with an aromatic aldehyde reagent according to the conditions of step b). Observation with the naked eye of the two wood pieces treated in the same way showed that the first wood piece remained natural in color while the second darkened slightly. No graying appeared. The increase in the coloring of the second wood piece indicates that the grafting of the aromatic group that absorbs UV radiation took place on the amine group, unlike the first natural piece.

[0057] In a second step, the two wooden pieces previously obtained were soaked for 4 hours in an acetone bath in order to artificially and accelerate the leaching of wood in an external environment. Acetone being a better solvent than water, its use makes it possible to accelerate any leaching and dilution of the grafted group. Visual observation shows that the coloring does not vary after this treatment, which tends to prove the grafting of the aromatic imine group by creation of a covalent bond. Pseudo absorbance:

[0058] The pseudo-absorbance analyzed by reflectance of the second wooden piece is illustrated in [Fig.4]. The analysis method is carried out by UV-vis Diffuse reflectance spectroscopy (which can be translated as UV-visible spectroscopy with diffuse reflectance) on the tangential face with a Cary 5000 UV-vis-NIR spectrophotometer (Varian Inc., Palo Alto, USA) equipped with an integrating sphere in the range 200-800 nm. The reflectance R is converted into pseudo-absorbance via the Kubelka-Monk equation, where k is the light absorption coefficient and s is the light scattering coefficient (also known as the Anglo-Saxon terminology 'light scattering coefficient'):

[0059] [Math.l]

[0060] The spectrum obtained corroborates the observation made with the naked eye: the second wooden piece shows a higher absorption in the wavelength corresponding to UV radiation than that of a natural wooden piece. Compared to the reference curve, we see the appearance of absorption zones characteristic of the grafted molecule. The measurement is reproduced on the second wooden piece previously soaked in acetone: the same pseudo absorbance spectrum is obtained (curve slightly higher than the other obtained without leaching with acetone). These elements tend to prove that the graft is indeed obtained by non-leachable covalent bonding of the aromatic imine group on the wood. It should also be noted that the spectrum obtained is a signature of the nature of the group grafted on the wood; it varies in fact according to the chemical nature of the grafted aromatic imine group.

[0061] UV resistance:

[0062] Photo stability and resistance to climatic conditions were evaluated in following the color change after exposure of wooden pieces to UV light from a QSUN Xe-3-HSC Xenon lamp (Q-Lab, Westlake, USA). Irradiation was done at 89 W m 2 nm1 and a wavelength of 340 nm. The cycle consisted of a 2.5 h phase with light and 30 min without exposure at 45°C. Humidity was set at 50 and 90% for the phases with light and without exposure, respectively. The color changes of the wood surfaces related to irradiation were measured using a Color Guide 45 / 0 colorimeter (BYK Gardner GmbH, Geretsried, Germany). The color was expressed with CIE L*a*b* chromaticity coordinates and taken at the same position on the sample for each measurement. The AE color change was calculated from the parameters before irradiation (L0, aO, bO) and after each time period in the chamber (Li, ai, bi) with the following equation:

[0063] [Math.2] ⣠= - M2 + (¾ -¾)2 + (¼ - M2

[0064] Note that a visual difference on the wooden piece is only visible to the naked eye from an AE value greater than 4. Also, the higher the AE value, the more the color of the wood is modified.

[0065] The graft was applied to several wooden pieces in order to test the influence of different aromatic groups, in particular with the reagent phenyl trime-thoxysilane (or PhTMOS), benzaldehyde, and hydroxy-naphthaldehyde. All these pieces, as well as a reference piece (piece made of natural, non-functionalized wood) were treated with UV as previously described and the change in color AE was measured for each of the pieces by colorimetry over time.

[0066] The results illustrated in [Fig. 5] show that the reference part undergoes a change in AE color following a very significant slope from the beginning of the irradiation and during the first thirty hours approximately to end, after 300h, with an AE color change of approximately 14. The PhTMOS treatment attenuates the color change compared to the reference. The difference in behavior is especially visible after 100 hours of exposure. At 300 hours, the AE color change is less than 8, which is significantly lower than that of the reference.

[0067] Treatments carried out using an amine reagent (APTES or urea) and then an aldehyde reagent with aromatic groups having hydroxyl substituents in the ortho position give even better results, or even results in ranges without color change for functionalizations carried out using urea and aromatic aldehyde with a hydroxyl substituent in the ortho position. [Fig.5] indicates incomplete names of naphthaldehydes and benzaldehydes on the ordinate for reasons of clarity, but these are of course the p-hydroxylated compounds illustrated in [Fig.6].

[0068] The role of the groups grafted to the wood is to absorb UV (via the aromatic cycles), the aldehyde reagents carrying hydroxyl in ortho once grafted not only allow to absorb UV via the aromatic cycle, but they also seem capable of re-emitting radiation (other than UV) via a conjugation between the OH and the nearby imine, which would explain their better performance with respect to PhTMOS. Thus, the grafted molecules absorb UV radiation in place of the wood components, or even absorb them and then re-emit part of the energy in less aggressive wavelengths. It is assumed that the lifetime of the group grafted on the wood will be longer in the presence of hydroxyl functions in ortho. Experiments show that a graft comprising a simple aromatic cycle, not having the possibility of re-emitting the absorbed energy, will still induce a sub-delay significant in the degradation of wood compared to wood treated by other means.

[0069] Thus, thanks to the method of the invention, the stability of wooden parts to UV is increased, as illustrated above, and the grafting of UV-absorbing molecules has proven effective in accelerated and natural aging conditions.

Claims

Claims

1. A method of anti-UV protection for wooden parts, the anti-UV protection method comprising the following steps: a) providing a wooden part comprising polymeric components onto which amine groups are grafted, b) chemical modification of at least one amine group by reaction with an aromatic aldehyde reagent, so as to graft at least one aromatic imine group onto at least one polymeric component.

2. The UV protection method of claim 1, wherein the aromatic aldehyde reagent comprises an aromatic group substituted by an aldehyde function and at least one hydroxyl function.

3. UV protection method according to claim 2, in which the at least one hydroxyl function is arranged in the ortho position of the aldehyde function on the aromatic group.

4. A UV protection method according to one of claims 1 to 3, wherein the aromatic aldehyde reagent is selected from 1-hydroxy-2-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, 2,4-dihydroxybenzaldehyde, and / or a compound of formula (I) with R1, R2, R3 and R4 selected independently of each other from hydrogen and alkyl. [Chem.l] _.;O

5. rU" '"''a, R-,- (0 UV protection method according to one of claims 1 to 4, in which step b) comprises the following steps: - placing the wooden piece in a container, - placing the aromatic aldehyde reagent in the container at a distance from the wooden piece, - adding carbon dioxide - applying a pressure and a temperature such that the carbon dioxide is in the supercritical, or liquid, state so as to react the aromatic aldehyde reagent with at least one of ... aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in the container, - placing the aromatic aldehyde reagent in at least one amine group and lead to the grafting of at least one aromatic imine group onto at least one polymeric component of the wooden part.

6. UV protection method according to one of claims 1 to 5, in which step b) is carried out under pressure.

7. UV protection method according to one of claims 1 to 6, in which a grafting reaction i) is carried out prior to step a), comprising the grafting of at least one amine group by reaction of at least one amine reagent on at least one hydroxyl function of the polymeric components of the wooden part.

8. UV protection method according to claim 7, in which the at least one amine reagent is chosen from urea, a diisocyanate, and / or a silane comprising a terminal amine group including Amino-Propyl TriethoxySilane (APTES).

9. UV-protected wooden part, the wooden part comprising polymeric components onto at least one of which is grafted at least one aromatic imine group, capable of absorbing radiation in the ultraviolet range.

10. A UV-protected wooden part according to claim 9, wherein the at least one aromatic imine group comprises an aromatic group having at least one hydroxy substituent ortho to the imine function.