Method for imparting hydrophobicity to a wood material, and material obtainable therefrom

Abstract

Method for imparting hydrophobicity to a wood material, which comprises : applying to the surface of said material an aqueous suspension of at least one polyhydroxyalkanoate (PHA) in the form of particles with an average size (d50) of from 0. 5 μm to 100 pm, preferably from 0. 8 μm to 50 pm, more preferably from 2. 0 μm to 20 μm; maintaining the aqueous suspension of at least one PHA in contact with the surface of said material for a sufficient time to allow the PHA particles to partially deposit on the surface of the material and partially penetrate inside the same; drying the material thus treated at a temperature such as not to cause the PHA to melt. The wood material thus treated has PHA particles that are partially distributed on the surface and partially penetrate between the cellulose fibers, so as to impart hydrophobic and waterproofing properties both on the surface and by a certain thickness inside the treated material.

Description

[0001] METHOD FOR IMPARTING HYDROPHOBICITY TO A WOOD MATERIAL,

[0002] AND MATERIAL OBTAINABLE THEREFROM .

[0003] The present invention relates to a method for imparting hydrophobicity to a wood material , and to a material obtainable therefrom .

[0004] Wood materials , which are formed by cellulose fibers , hemicellulose , which acts as a binder for the cellulose fibers , and lignin, show a strong tendency to absorb water, both due to direct contact with liquid water and through the absorption of atmospheric humidity . This makes such materials unstable , both dimensionally, as the absorbed water causes swelling of said material , and for their mechanical properties .

[0005] Furthermore , the presence of water can cause problems of degradation of the cellulosic material due to chemical and biological phenomena . Indeed, the penetration of water inside the structure can lead to solubili zation of auxiliaries added to the cellulose fibers in order to impart speci fic properties to the material . The presence of water further allows the proli feration of microorganisms , with undesired ef fects also from a visual and ol factory point of view .

[0006] In general , to impart hydrophobic properties to wood materials , synthetic chemical products such as acrylic, epoxy, silicone or paraf fin resins are used . Such products do not meet the current requirements for the use of products with improved environmental sustainability, which be not derived from non-renewable fossil sources .

[0007] Furthermore , the surface treatment with such synthetic products results in the formation of an external coating layer, but without ensuring simultaneous penetration of the hydrophobic material between the cellulose fibers , so as to impart hydrophobic properties also inside the material . This leads to a loss of such properties over time due to wear and / or detachment of the surface layer .

[0008] Patent application US 2017 / 0260416 Al describes a method for applying a film of a polyhydroxyalkanoate ( PHA) on a substrate , in particular paper manufactured products , through treatment with an emulsion or suspension of the PHA and subsequent application of photonic energy, for example through a xenon flash lamp, in order to remove the solvent and melt the PHA so as to form a continuous film in a short time without modi fying the features of the substrate . The continuous layer of PHA thus obtained imparts barrier properties against water .

[0009] Patent application US 2023 / 0220155 Al describes a biodegradable aqueous mixture which comprises from 35% to 75% by weight of water and from 25% to 65% by weight of solids . The solids consist in turn of 40% to 99% by weight of polyhydroxyalkanoates ( PHA) , based on the weight of the total dry solids . PHAs , before being mixed with water, have a humidity content not lower than 1 % by weight and a particle si ze Dv ( 90 ) not exceeding 10 microns . Such aqueous mixture of PHA can be used to make coating layers on various substrates , for example on paper . To this purpose , after applying the PHA emulsion or suspension on the substrate , it is heated to a temperature from 105 ° C to 145 ° C in order to evaporate water and melt the PHA particles . In this way, the molten PHA spreads so as to form a continuous layer on said substrate .

[0010] Patent application US 2023 / 0383119 Al describes a composition which comprises 10-70% by weight of poly ( 3- hydroxybutyrate-co-4-hydroxybutyrate ) ( P3HB-4HB ) , and 1-10% of additives . The latter include a rheology modi fier and a surfactant or dispersant . Such composition can also be used as a coating agent for paper manufactured products . Also in that case , the drying of the paper manufactured product after application of the coating composition is carried out at high temperature ( speci fically at 170 ° C for 10 minutes ) .

[0011] The Applicant faced the technical problem of having a method for imparting hydrophobicity to a wood material that involves the employment of eco-compatible products , not derived from non-renewable fossil sources , and that are capable of imparting strong hydrophobic properties , which are not limited to the surface of the material , but also extend inside said material .

[0012] According to the prior art documents cited above , it is possible to impart hydrophobic properties to cellulose-based substrates by applying an emulsion or suspension of a polyhydroxyalkanoate ( PHA) and subsequently drying at high temperature , to evaporate water and melt the PHA, which then distributes uni formly on the treated surface forming a continuous layer .

[0013] However, the Applicant has found that , to impart hydrophobicity to a wood material , it is not necessary to form a continuous layer of PHA. On the other hand, a uni form layer of PHA constitutes a problem, as it tends to detach, thus compromising the durability of the treatment . Furthermore , as taught in the prior art , the formation of a continuous layer requires the application of relatively high temperatures , such as to cause the PHA to melt , which can damage the wood substrate .

[0014] The Applicant has found that the above-illustrated technical problems , and others that will be better highlighted below, can be solved through a method for imparting hydrophobicity to a wood material , which comprises applying to the surface of said material an aqueous suspension of at least one polyhydroxyalkanoate ( PHA) in the form of particles with an average si ze ( d50 ) of from 0 . 5 pm to 100 pm, preferably from 0 . 8 pm to 50 pm, more preferably from 2 . 0 pm to 20 pm, and subsequently drying the material thus treated at a temperature such as not to cause the PHA to melt .

[0015] The wood material thus treated has PHA particles that are partially distributed on the surface and partially penetrate between the cellulose fibers , so as to impart hydrophobic and waterproofing properties both on the surface and by a certain thickness inside the treated material , as will be better illustrated below . In particular, it has been found that the PHA particles , even without forming a continuous layer on the wood substrate , are capable of imparting excellent hydrophobic properties , which are maintained over time due to the fact that the PHA particles are capable of penetrating between the cellulose fibers forming the wood material , so as to obtain a deep hydrophobic ef fect which is maintained even after the removal of PHA particles present on the surface .

[0016] According to a first aspect , the present invention therefore relates to a method for imparting hydrophobicity to a wood material , which comprises : applying to the surface of said material an aqueous suspension of at least one polyhydroxyalkanoate ( PHA) in the form of particles with an average si ze ( d50 ) of from 0 . 5 pm to 100 pm, preferably from 0 . 8 pm to 50 pm, more preferably from 2 . 0 pm to 20 pm; maintaining the aqueous suspension of at least one PHA in contact with the surface of said material for a suf ficient time to allow the PHA particles to partially deposit on the surface of the material and to partially penetrate inside the same ; drying the material thus treated at a temperature such as not to cause the PHA to melt .

[0017] Preferably, the aqueous suspension of PHA contains PHA particles at a concentration comprised between 1 % w / v and 25% w / v, more preferably between 5% w / v and 20% w / v, the % being expressed by weight with respect to the overall volume of the suspension .

[0018] For the purposes of the present invention, in the following description and claims , the definitions o f the numerical ranges comprise the individual values within the range and its endpoints , unless otherwise speci fied .

[0019] For the purposes of the present invention, in the following description and claims , the term "comprising" also includes the terms "essentially consisting of" or "consisting of" .

[0020] The term "wood material" means a plant tissue that constitutes the stem of perennial plants having secondary growth, whose main components are : cellulose fibers , hemicellulose , and lignin .

[0021] As is known, polyhydroxyalkanoates ( PHA) are polymers produced by microorganisms isolated from natural environments or also by genetically modi fied microorganisms , and they are characteri zed by high biodegradability. They are produced and accumulated by various bacterial species under metabolic stress conditions and in the presence of an excess carbon source. The PHA are synthesized and accumulated by at least 300 different microbial species, comprised in more than 90 genera of Gram-positive and Gram-negative bacteria, such as Bacillus Rhodococcus , Rhodospirillum PseudomonasrAlcaligenesrAzotobacter Rhizobium.

[0022] In the cells, PHA is stored in the form of microgranules, having size and number per cell varying among different bacterial species. They appear under an electron microscope as refractive inclusions, with a diameter that can generally vary from 0.2 pm to 0.7 pm.

[0023] PHA is generally a polymer containing repeating units of the formula:

[0024] -O-CHRi- (CH2) n-CO- (I) wherein :

[0025] Ri is selected from: C1-C12 alkyls, C4-C16 cycloalkyls, C2- C12 alkenyls, optionally substituted by at least one group selected from: halogen (F, Cl, Br) , -GN, -OH, - COOH, -OR, -COOR (R = C1-C4 alkyl, benzyl) ; n is zero or an integer from 1 to 6, preferably is 1 or 2.

[0026] Preferably, Ri is methyl or ethyl, and n is 1 or 2.

[0027] PHA can be either homopolymers or copolymers or terpolymers. In the case of copolymers or terpolymers, they can consist of different repeating units of formula (I) , or of at least one repeating unit of formula (I) in combination with at least one repeating unit deriving from comonomers capable of copolymerizing with hydroxyalkanoates, for example lactones or lactams. In the latter case, the repeating units of formula (I) are present in an amount of at least 10 molt with respect to the total moles of repeating units.

[0028] Particularly preferred repeating units of formula (I) are those deriving from: 3-hydroxybutyrate, 3- hydroxyvalerate, 3-hydroxyhexanoate, 3- hydroxyoctanoate, 3-hydroxyundec-10-enoate, 4- hydroxyvalerate . Particularly preferred PHAs are: poly (3- hydroxybutyrate ) (PHB) , poly ( 3-hydroxyvalerate ) (PHV) , poly ( 3-hydroxyhexanoate ) (PHH) , poly(3- hydroxyoctanoate) (PHO) , poly ( 3-hydroxybutyrate-co-3- hydroxyvalerate ) (PHBV) , poly ( 3-hydroxybutyrate-co-3- hydroxyhexanoate) (PHBH) , poly ( 3-hydroxybutyrate-co-4- hydroxybutyrate ) , poly ( 3-hydroxyoctanoate-co-3- hydroxyundecen-10-enoate) (PHOU) , poly(3- hydroxybut yr ate- co- 3 -hydroxyvalerate- co- 4- hydroxyvalerate ) (PHBW) , or mixtures thereof. According to the objects of the present invention, the PHA is preferably a homopolymer, or a copolymer, or a terpolymer, more preferably a homopolymer, and even more preferably polyhydroxybutyrate (PHB) . Preferably, the PHA has a weight average molecular weight (Mw) which can vary from 10,000 to 1,000,000 Da.

[0029] As for the production of PHA, this is preferably carried out through microbial fermentation of an organic substrate (for example, carbohydrates or other fermentable substrates such as glycerol) by means of a strain of microorganisms capable of producing PHA, and subsequent recovery of the PHA from the cell mass. Then, the PHA obtained from the cell mass is generally subjected to purification and bleaching. For further details, see for example patent applications WO 99 / 23146, WO 2011 / 045625, and WO 2015 / 015395. Substrates suitable for producing PHA by fermentation can be obtained in particular by processing plants, for example juices, molasses, pulps deriving from the processing of sugar beet or sugarcane. Such substrates generally contain, in addition to sucrose and other carbohydrates, organic growth factors, nitrogen, phosphorus and / or other minerals useful as nutrients for cell growth. An alternative consists of glycerol, a low-cost source of organic carbon, being also available as a by-product of biodiesel production.

[0030] In the scope of the present description and the attached claims, "average particle diameter" means, unless otherwise indicated, the diameter (median value) d50, i.e., the value of the diameter below which there is 50% by weight of the particle population (see "A Guidebook to Particle Size Analysis" published by Horiba Instruments Inc. - 2016, available at https : / / www .horiba . com / fileadmin / uploads / Scientific / eMa g / PSA / Guidebook / pdf / PSA_Guidebook . pdf ) . It can be determined through laser diffraction technique according to ISO 13320:2009 standard.

[0031] The suspension of PHA in the form of particles can be prepared by dispersing powdered PHA in water under vigorous stirring. Alternatively, it may be advantageous to use the suspension of PHA obtained directly from the fermentation process and subsequent recovery and purification from the cell mass, without carrying out a final drying step. Such suspension has particularly advantageous features, as it is a highly stable and homogeneous suspension, having an average particle size suitable for carrying out the present invention. The application of the suspension of PHA on the wood material to be treated can be carried out by known techniques , for example by immersion, or by spraying, or by spreading .

[0032] In a preferred embodiment , the application step of the aqueous suspension of PHA is carried out by immersing the wood material in said suspension, at a temperature from 20 ° C to 95 ° C, more preferably from 40 ° C to 80 ° C . Preferably, such step is carried out at a pressure from 1 bar to 3 bar, more preferably from 1 bar to 2 bar . Generally, an aqueous suspension temperature above room temperature ( greater than 25 ° C ) is advantageous as it promotes the penetration of PHA particles between the cellulose fibers . However, with some types of PHA, a too high temperature ( > 80 ° C ) can compromise the stability of the suspension, with formation of PHA lumps deposited on the treated material .

[0033] As regards the time of maintaining the aqueous suspension of PHA in contact with the surface of the wood material to be treated, it can vary within wide limits depending on the features of the material , the temperature at which the treatment occurs , and the application technique employed . For example , in the case of application by immersion, the maintaining time can generally vary between 30 minutes and 48 hours , preferably between 1 hour and 24 hours .

[0034] After maintaining the aqueous suspension of PHA in contact with the wood material for the required time , the treated material is dried . The drying is carried out according to known techniques , under conditions adapted to preserve the features of both the wood material and the PHA, so as to ensure a satis factory result in terms of aesthetics and waterproofing properties. In order to promote drying, the material can be subjected to the dynamic action of an air flow at a controlled temperature, or the material can be maintained in static conditions at a predetermined temperature (maturation temperature) . Such temperatures can vary, for example, from 10°C to 80°C, preferably from 20°C to 60°C. The drying can generally be carried out at a pressure from 0.1 bar to 3 bar, preferably from 1 bar to 2 bar.

[0035] In the case where the cellulose-based material contains heat-sensitive products, the drying can be carried out at low temperature and reduced pressure (freeze-drying) , for example at a temperature from -50°C to 0°C and at a pressure from 10 mbar to 300 mbar.

[0036] The Applicant has observed that, at the end of drying, the treated material can have on its surface a more or less uniform layer of excess PHA particles that have not adhered to said material and which can be suitably removed by known techniques, for example by brushing. The PHA thus removed can then be recycled to produce the initial aqueous suspension, or reused for other purposes.

[0037] At the end of the treatment, after possible removal of excess PHA, the overall quantity of PHA particles partially adhered to the surface and partially penetrated inside is generally between 2% and 25% by weight, preferably between 5% and 20% by weight, with respect to the total weight of the treated material.

[0038] The wood material obtainable according to the method of the present invention has marked hydrophobic properties. In particular, it has a contact angle with water (measured according to the UNI EN 15802:2010 standard) greater than 90°, generally from 95° to 120°.

[0039] The following exemplary embodiments are provided solely for illustrative purposes of the present invention and should not be construed as limiting the scope of protection defined by the attached claims.

[0040] EXAMPLE .

[0041] An aqueous suspension of PHB with a concentration equal to 15% w / v was prepared. Such suspension was used to treat cedar wood blocks (size: 121x71x50 mm) by immersing the same in a tank filled with the PHB suspension, at different temperatures and for different times, as reported in Table 1.

[0042] Each block was weighed before and after treatment to determine the % by weight of PHB absorbed by the wood fibers. The data are reported in Table 1.

[0043] TABLE 1

[0044] The hydrophobic properties of the blocks thus treated were evaluated according to UNI EN 15802:2010 standard .

[0045] The treated block from Test 3 was brushed to remove possible PHB not adhered to the surface, and the contact angle with water droplets was determined on its surface. Subsequently, the same block was subjected to planing to remove about 1 mm of wood; the planed block was also subj ected to the test for measuring the contact angle with water . For comparison, the contact angle of an untreated cedar block was determined .

[0046] From a qualitative point of view, it was found that the untreated comparison block was substantially devoid of hydrophobic properties , as the water droplet applied to its surface quickly flattened, wetting said surface . On the other hand, the water droplet applied to the block treated according to the method of the invention has shown a spheroidal shape being maintained over time . On the same block after planing, the water droplet showed a similar behaviour, demonstrating that the PHB managed to penetrate between the wood fibers , thereby imparting deep hygroscopic properties .

[0047] The same tests were carried out by comparing an untreated block with a treated block from which 2 mm of wood have been removed by planing . The results were completely similar to the previous ones , demonstrating the penetration of the PHB inside the wood material .

Claims

CLAIMS1. Method for imparting hydrophobicity to a wood material, which comprises: applying to the surface of said material an aqueous suspension of at least one polyhydroxyalkanoate (PHA) in the form of particles with an average size (d50) of from 0.5 pm to 100 pm, preferably from 0.8 pm to 50 pm, more preferably from 2.0 pm to 20 pm; maintaining the aqueous suspension of at least one PHA in contact with the surface of said material for a sufficient time to allow the PHA particles to partially deposit on the surface of the material and to partially penetrate inside the same; drying the material thus treated at a temperature such as not to cause the PHA to melt.

2. Method according to claim 1, wherein the aqueous suspension of PHA contains PHA particles at a concentration comprised between 1% w / v and 25% w / v, preferably between 5% w / v and 20% w / v, the % being expressed by weight with respect to the overall volume of the suspension.

3. Method according to any one of the preceding claims, wherein said at least one PHA is selected from: poly-3-hydroxybutyrate (PHB) , poly-3-hydroxyvalerate (PHV) , poly-3-hydroxyhexanoate (PHH) , poly-3- hydroxyoctanoate (PHO) , poly ( 3-hydroxybutyrate-co-3- hydroxyvalerate ) (PHBV) , poly ( 3-hydroxybutyrate-co-3- hydroxyhexanoate) (PHBH) , poly ( 3-hydroxybutyrate-co-4- hydroxybutyrate) , poly ( 3-hydroxyoctanoate-co-3- hydroxyundecen-10-enoate) (PHOU) , poly(3- hydroxybut yr ate- co- 3 -hydroxyvalerate- co- 4-hydroxyvalerate) (PHBW) , or mixtures thereof.

4. Method according to any one of the preceding claims, wherein said at least one PHA has a weight average molecular weight (Mw) of from 10,000 to 1,000,000 Da.

5. Method according to any one of the preceding claims, wherein the aqueous suspension of PHA is prepared by dispersing the powdered PHA in water.

6. Method according to any one of claims 1 to 4, wherein the aqueous suspension of PHA is obtained directly from the fermentation process and subsequent recovery and purification from the cell mass, without carrying out a final drying step.

7. Method according to any one of the preceding claims, wherein the application of the aqueous suspension of PHA on the material to be treated is carried out by immersion, or by spraying, or by spreading .

8. Method according to claim 7, wherein the application step of the aqueous suspension of PHA is carried out by immersing the material in said suspension, at a temperature from 20°C to 95°C, preferably from 40°C to 80°C.

9. Method according to claim 8, wherein the immersion of the material to be treated in the aqueous suspension of PHA lasts from 30 minutes to 48 hours, preferably from 1 hour to 24 hours.

10. Method according to any one of the preceding claims, wherein the drying step is carried out by subjecting the treated material to an air flow at a controlled temperature.

11. Method according to any one of claims 1 to 9,wherein the drying step is carried out by maintaining the material in static conditions at a predetermined temperature (maturation temperature) .

12. The method according to any one of the preceding claims, wherein the drying temperature is from 10°C to 80°C, preferably from 20°C to 60°C.

13. The method according to claim 12, wherein the drying is carried out at a pressure from 0.1 bar to 3 bar, preferably from 1 bar to 2 bar.

14. Method according to any one of claims 1 to 11, wherein the drying step is carried out by freeze-drying, at a temperature from -50°C to 0°C and at a pressure of from 10 mbar to 300 mbar.

15. Method according to any one of the preceding claims, wherein at the end of drying the treated material is subjected to a step of removal of any excess PHA particles .

16. Method according to any one of the preceding claims, wherein at the end of drying and the possible removal of any excess PHA particles, the overall quantity of PHA particles partially adhered to the surface and partially penetrated inside is between 2% and 25 % by weight, preferably between 5% and 20% by weight, with respect to the total weight of the treated material.

17. Method according to any one of the preceding claims, wherein the obtained material containing cellulose fibers has a contact angle with water (measured according to the UNI EN 15802:2010 standard) greater than 90°, preferably from 95° to 120°.

18. Material containing cellulose fibers obtainable from a method according to any one of the preceding claims .

Images