UV-STABLE AND SUPERHYDROPHOBIC WOOD SURFACE

Abstract

The invention relates to a method for treatment of wood material. In a first treatment step comprises the provision of a solution comprising zinc oxide or titanium(IV) isopropoxide and immersion of the wood material into the solution. After drying of the wood material a second treatment step is following in case of the first solution comprising zinc oxide. The second treatment step comprises the provision of a solution comprising a zinc compound and immersion of the wood material from the first treatment step into the solution followed by drying of the wood material. In a second aspect the invention relates to a wood material characterized by the visibility of the natural appearance of the surface and at least one other property relating to UV resistance, weathering resistance, mechanical resistance or hydrophobic properties.

Claims

1. A method for treatment of wood material comprising a) a first treatment step comprising the steps of i. provision of a first solution comprising zinc oxide in aqueous solution with a pH>7; and ii. immersion of said wood material into said first solution; and iii. drying of said wood material at a temperature of 80 C. to 110 C., particularly 90 C. to 110 C. more particularly 110 C.; b) a second treatment step comprising the steps of: i. provision of a second solution comprising a zinc compound; and ii. immersion of said wood material into said second solution, wherein said second solution has a temperature of 70 C. to 110 C., in particular 70 C. to 90 C., more particular 85 C.; and iii. drying of said wood material.

2. The method for treatment of wood material according to claim 1, wherein the first solution has a pH 8-14, more particular a pH 9-13, most particular a pH 10 to 12, and/or the second solution has a pH7, in particular a pH 7-14, more particular a pH 8-13, most particular a pH 10 to 12.

3. The method for treatment of wood material according to claim 1, wherein the first solution comprises zinc oxide with a concentration of 30 to 60 mM ZnO, in particular 40 to 55 mM ZnO, more particular 40 to 50 mM ZnO and most particular 50 mM ZnO.

4. The method for treatment of wood material according to claim 1, wherein. said wood material is immersed in the first treatment step into the first solution for 30 seconds up to 5 minutes, in particular 3 minutes to 5 minutes, more particular 5 minutes, and/or said wood material is immersed in the second treatment step into the second solution for at least 5 minutes, in particular 5 minutes to 240 minutes, more particular 30 minutes to 150 minutes, most particular 120 minutes.

5. The method for treatment of wood material according to claim 1, wherein drying of said wood material is performed in the first treatment step for at least 2 minutes, in particular 2 to 30 minutes, more particular 10 minutes, or in the second treatment step at a temperature of 50 C. to 80 C., in particular 60 C. to 80 C., more particular 65 C. for at least 2 hours, in particular 2 h to 16 h, more particular 4 h to 12 h.

6. The method for treatment of wood material according to claim 1, wherein the zinc compound of said second solution is selected from zinc oxide, zinc nitrate and zinc acetate, particular zinc nitrate.

7. The method for treatment of wood material according to claim 1, wherein: in case said zinc compound is zinc oxide the second solution additionally comprises ammonium hydroxyl and/or at least one ammonium salt, and in case said zinc compound is zinc nitrate or zinc acetate, the second solution additionally comprises hexamethylentetramine and/or at least one ammonium salt.

8. The method for treatment of wood material according to claim 1, wherein the at least one ammonium salt is selected from ammonium citrate [(NH.sub.4).sub.3(C.sub.6H.sub.5O.sub.7)], ammonium nitrate [NH.sub.4NO.sub.3], ammonium acetate [CH.sub.3COONH.sub.4] and ammonium chloride [NH.sub.4Cl], particularly the at least one ammonium salt is selected from ammonium citrate [(NH.sub.4).sub.3(C.sub.6H.sub.5O.sub.7)].

9. The method for treatment of wood material according to claim 1, wherein the second solution comprises: zinc oxide with a concentration of 40 to 65 mM, in particular 40 to 55 mM, more particular 50 mM, or zinc nitrate with a concentration of 40 to 65 mM, in particular 40 to 55 mM, more particular 50 mM, or zinc acetate with a concentration of 40 to 110 mM, in particular 50 to 100 mM.

10. The method for treatment of wood material according to claim 1, further comprising a stabilization step after step b) comprising the steps of: i. provision of a stabilizing solution comprising a precursor of SiO.sub.2, ii. immersion of said wood material into said stabilizing solution, wherein the duration of immersion into the stabilizing solution is 1 h to 6 h, in particular 2 h to 5 h and more particular 3 h to 4 h.

11. The method for treatment of wood material according to claim 10, wherein the stabilizing solution comprises: tetraethyl orthosilicate, and/or ammonium hydroxide, and/or water and organic solvent, in particular water and alcohol, more particular water and ethanol.

12. The method for treatment of wood material according to claim 9, wherein the volume ratios in the stabilization solution of: i. water to organic solvent ranges from 20:100 to 20:70, and/or ii. water to ammonium hydroxide ranges from 20:0.5 to 20:1.5, and/or iii. water to tetraethyl orthosilicate range from 20000:10 to 20000:60.

13. A method for treatment of wood material comprising the steps of i. provision of a first solution comprising titanium(IV) isopropoxide (TTIP) and an organic solvent, in particular an alcohol, more particular ethanol or isopropanol; and ii. immersion of said wood material into said first solution; and iii. drying of said wood material;

14. The method for treatment of wood material according to claim 13, wherein the first solution additionally comprises Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 and particularly steps ii and iii are repeated up to 15 times, in particular 5 to 10 times.

15. The method for treatment of wood material according to claim 13, wherein the first solution comprises TTIP with a concentration from 1 wt % to 10 wt %, in particular 2 wt % to 8 wt %, more particular 4 wt % to 6 wt %, and in case of the first solution comprising Ce(NH.sub.4).sub.2(NO.sub.3).sub.6, with a molar ratio of TTIP to Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 ranging from 220:1 to 220:30.

16. The method for treatment of wood material according to claim 13, wherein said wood material is immersed in the first treatment step into the first solution: for 12 hours up to 36 hours, in particular 14 hours to 24 hours, more particular 24 hours, and in case of the first solution comprising additionally Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 for 6 hours to 60 hours, in particular for 24 hours to 48 hours, more particular 48 hours.

17. The method for treatment of wood material according to claim 13, wherein in the first treatment step drying of said wood material is performed in three steps with increasing temperatures after each step with temperatures between 50 C. and 200 C. and a drying time between 15 minutes and 3 hours in each of the three steps; or in case of the first solution comprising additionally Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 drying of said wood material is performed at 50 C. to 80 C., in particular at 65 C.

18. The method for treatment of wood material according to claim 12, wherein the wood material of step ii. is pre-treated by soaking the wood material into an aqueous salt solution for least 12 h.

19. The method for treatment of wood material according to claim 18, wherein a drying process at room temperature for more that 24 h or an accelerated drying in an oven at 65 C. for more than 6 h is applied to the pre-treated wood material.

20. The method for treatment of wood material according to claim 18, wherein the aqueous salt solution comprises at least one of the salts ions selected from FeCl.sub.2, FeCl.sub.3, Fe(NO.sub.3).sub.2, Fe(NO.sub.3).sub.3, Ce(NO.sub.3).sub.3, CeCl.sub.3, Ce(NH.sub.4).sub.2(NO.sub.3).sub.6, CuCl, CuBr, Cu(NO.sub.3).sub.2, in particular from Ce(NO.sub.3).sub.3 and Fe(NO.sub.3), more particularly from Ce(NO.sub.3).sub.3.

21. The method for treatment of wood material according to claim 18, wherein the concentration of the salt in aqueous salt solution is ranging from 0.5 mM to 50 mM, in particular 1.5 mM to 20 mM, in more particular 10 mM.

22. The method for treatment of wood material according to claim 1, further comprising a hydrophobization step comprising the steps of: i. provision of a hydrophobic solution comprising a hydrophobic molecule, in particular a long alkyl chain thiols, organic silanes, fatty acids, aromatic azide or fluorinated organic silane molecules, in particular the hydrophobic molecule is selected from: 1H,1H,2H,2H-Perfluorooctyltriethoxysilane, Trichloro (1H,1H,2H,2H-perfluorooctyl) silane, methyltrichlorosilane, decyltrichlorosilane, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate, 1-Dodecanethiol, Trimethoxy(octyl)silane and stearic acid; ii. immersion of said wood material into said hydrophobic solution, iii. heat treatment of said wood material.

23. The method for treatment of wood material according to claim 22, wherein: the immersion time is between 0.5 h to 12 h, in particular 2 h to 12 h more particular 4 h to 12 h, most particular 12 h and/or the heat treatment is performed at a temperature of 50 C. to 80 C., in particular 60 C. to 70 C., more particular 65 C., for a duration of 0.5 hours to 3 hours, in particular 3 hours

24. A wood material characterized by the visibility of the natural appearance of the surface of said wood material and at least one of the following properties: I. the resistance to UV induced changes in perceivable colour, wherein the said wood material has no decrease in lightness (L*) after at least 100 hours of UV exposure, and/or; II. the resistance to UV induced changes in perceivable colour, wherein the said wood material has a change of red/green opponent colours (a*) of less than 2, in particular less than 1, after at least 100 hours of UV exposure, and/or III. the resistance to UV induced changes in perceivable colour, wherein the said wood material has a change of yellow/blue opponent colours (b*) of less than 5, in particular less than 3, after at least 100 hours of UV exposure, and/or IV. the resistance to UV induced changes in perceivable colour, wherein the said wood material has a total colour change (E) of less than 4, in particular less than 3, after at least 100 hours of UV exposure, and/or V. the resistance to weathering induced changes in perceivable colour, wherein the said wood material has a total colour change (E) of less than 4, in particular less than 3, after at least 100 hours of accelerated weathering as defined in table 1 and/or; VI. the absence of X-ray diffraction peaks at degrees two-theta having a margin of error +/0.2 degrees 2-theta: 16.4 and 22.4; and/or VII. the counts for the X-ray diffraction peak at degrees two-theta 22.4 having a margin of error +/0.2 degrees 2-theta is below 20000, in particular below 15000, more particular below 13000; and/or VIII. the presence of X-ray diffraction peaks at degrees two-theta having a margin of error +/0.2 degrees 2-theta: 31.77, 34.42, 36.25, 47.54 and 62.86, and/or; IX. the resistance to UV induced degradation of lignin, wherein the band in the FT-IR spectrum at 1507.3 cm.sup.1 is not significantly changed after at least 100 h of UV exposure, and/or; X. the resistance to an UV induced increase in the band in the FT-IR spectrum at 1723.1 cm.sup.1 after at least 100 h of UV exposure, and/or XI. superhydrophobic properties, wherein the advancing contact angle of a water drop is between 150 and 165, in particular between 155 and 160, most particular 160 and the receding contact angle of a water drop is between 145 and 165, in particular between 150 and 160, more particular between 155 and 160, most particular 156, the rolling off angle of water drop is 1.6, and/or XII. wood material comprising a coating, wherein the coating is resistant to mechanical damage and retains at least one of the properties I. to X after a blade scratch test or a tape-and-peel test.

25. A wood material particularly comprising the features of claim 24 obtainable by a method for treatment of wood material comprising: a first treatment step comprising the steps of provision of a first solution comprising zinc oxide in aqueous solution with a pH>7; and immersion of said wood material into said first solution; and drying of said wood material at a temperature of 80 C. to 110 C., particularly 90 C. to 110 C. more particularly 110 C.; a second treatment step comprising the steps of: provision of a second solution comprising a zinc compound; and immersion of said wood material into said second solution, wherein said second solution has a temperature of 70 C. to 110 C., in particular 70 C. to 90 C., more particular 85 C.; and drying of said wood material.

Description

SHORT DESCRIPTION OF THE FIGURES

[0171] FIG. 1 shows a schematic illustration of the chemical bath deposition process for wood surface modification. a) Norway spruce, b) seeding solvent: ZnO, NH.sub.3.H.sub.2O, c) baking at 110 C., d) growth at 80 C.; Growth solution 1: ZnO, NH.sub.3.H.sub.2O, Ammonium citrate and NH.sub.4NO.sub.3; Growth solution 2: Zn(NO.sub.3).sub.2, HMTA

[0172] FIG. 2 shows the X-ray diffraction pattern of Norway spruce with a dense ZnO coating (a), Norway spruce with a seed layer (b), and Norway spruce without any treatment (c). The reference powder diffraction pattern of ZnO (JCPDS 36-1451) is shown for comparison (d). Asterisk (*) denotes the position of characteristic cellulose peaks.

[0173] FIG. 3 shows (a) SEM images of the radial plane of Norway spruce acquired from the early wood region; (b) SEM images of the wood cell structures with ZnO coating; (c) High magnification SEM image acquired from the squared area shown in b; (d) The SEM image of transverse plane of spruce wood with ZnO coating on the tangential plane; (e) SEM acquired from the tangential plane of spruce wood with ZnO coating, and (f) the high magnification SEM image acquired from the squared area shown in b.

[0174] FIG. 4 shows (a) Low magnification TEM image of wood lamina with ZnO coating prepared by focus ion beam; ML=middle lamella, S1=secondary wall S1 1, S2=secondary wall S2 2, S3=Secondary cell wall S3. (b) Selective area electron diffraction (SAED) pattern from the square area noted in the inset; (c) High resolution TEM (HRTEM) image from the ZnO and wood cell wall interface; (d) HAADF-STEM image acquired from the ZnO and wood cell wall interface.

[0175] FIG. 5 shows (a) Applied parameters and feedback curves of the micro scratch test. (b) Optical microscopy image acquired from the dense ZnO coating wood surface after scratch test. The scratch profile started from left to right end was marked with black notes.

[0176] FIG. 6 shows (a) Dependence of lightness factor (L*) on the irradiation time. (b) The variation of the redness factor (a*) of spruce versus the irradiation time. (c) The variation of yellowness (b*) of the samples as a function of irradiation time. (d) Total color change difference E as a function of irradiation time. (.square-solid.) spruce with dense ZnO coating; (.circle-solid.) spruce with ZnO nanorod array coating; (.box-tangle-solidup.) spruce without any modification.

[0177] FIG. 7 shows FT-IR spectra of spruce wood before (b) and after (a) UV irradiation, as well as the dense ZnO coating spruce wood after UV irradiation (c); inset: zoom of the carbonyl and aromatic absorption region.

[0178] FIG. 8 shows (a) FT-IR spectra acquired from the cross section of spruce wood. The spectra were acquired along middle lamella from the most top (point 1) to the third cell layer (point 5) as noted in (b). (c) FT-IR spectra acquired from the cross section of ZnO coating spruce wood after 102 h UV treatment. The spectra were acquired along middle lamella from the most top (point 1) to the third cell layer (point 5) as noted in (d)

[0179] FIG. 9 shows (a) Changes in the appearance of modified and non-modified spruce wood during the accelerated weathering test. Notably the bare spruce shows marked cracks in the surface after weathering. In contrast, no cracks can be observed in the treated wood material after weathering. (b) Difference of lightness (L*), redness (a*), yellowness (b*) as well as total colour changes (E) in the surface of spruce wood with or without modification during weathering test.

[0180] FIG. 10 shows water repellency. (a) Snapshots from the measurement of advancing and receding contact angles and roll-off of a water droplet measured on the ZnO nanorods arrays modified wood surface. (b) Extremely low water adhesion confirmed by push and pull of a water droplet to the superhydrophobic surface. (c) Behavior of coffee droplets on the ZnO modified superhydrophobic wood surface (left) and bare wood surface (right), respectively.

[0181] FIG. 11 shows changes of a) lightness (L*), b) redness (a*), c) yellowness (b*) in the surface of spruce wood with (TiO.sub.2 coating, .square-solid.) or without modification (spruce reference, .circle-solid.) during UV exposure for 144 hours. d) Changes in the appearance of modified (TiO.sub.2 coating) and non-modified spruce wood (spruce reference) during UV exposure for 144 hours. Notably the bare spruce shows much higher total colour changes (E=15.9) as compared to modified spruce (E=6.4).

[0182] FIG. 12 shows a transparent TiO.sub.2 Gel generated by adding Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 into the ethanol TTIP solution. a) is the UV-Vis spectra of the free standing TiO.sub.2 gel. It indicates that the TiO.sub.2 gel is transparent to visible light, but absorbs almost all of the UV light compare to glass. The insert in a) displays that the logo of EMPA could be clearly see though the light yellow TiO.sub.2 xerogel. b) is the x-ray diffraction patterns of spruce wood and the TiO.sub.2 xerogel coated spruce. It demonstrated that the TiO.sub.2 xerogel on wood surface is amorphous. The intensity of the diffraction peaks of the crystallized cellulose in wood is suppressed because of the coating on the surface.

[0183] FIG. 13 shows a transparent TiO.sub.2 Xerogel on wood surface. a) and b) show the microstructure of the cut opened wood cell structure; c) shows the TiO.sub.2 xerogel coating on the tangential section of the wood surface; d) shows the TiO.sub.2 xerogel coating on wood surface observed from cross section, from which the thickness of the coating could be measured.

[0184] FIG. 14 shows TiO.sub.2 Xerogel inside wood (on wood lumen surface). a) is the schematic illustration of the SEM sample preparation by cutting into the wood. b) show the back scattering electron image of TiO.sub.2 xerogel coating on the lumen surface of the cells inside wood. The distribution of the TiO.sub.2 xerogel can be distinguished based on the contrast. The exhibition of TiO.sub.2 inside wood was further check by Energy Dispersive X-ray Spectrum (EDX) as shown in c). Spectra A and B are acquired from the regions marked in b). It indicates that TiO.sub.2 xerogel is mainly coating on the lumen surface. d) the cross section of the same sample, from which the coating on the lumen surface of the wood cells inside the wood could be seen.

[0185] FIG. 15 shows a schematic of the shape selective deposition of ZnO nanostructures on the wood surface. SEM image of an unmodified Norway spruce surface (a), ZnO nanorod array coating formed in the presence of aluminium nitrate (b) and the dense ZnO coating with platelet like structure (c). The insets in (b) and (c) are the corresponding low magnification SEM images.

EXAMPLES

[0186] The technical problem in wood surface modification is how to cover the rough wood surface (uneven topography with lumen and cut-open cells wall) with a continuous and complete coating of ZnO. A strong adhesion between the inorganic coating and wood surface is necessary to prevent the separation of the coating. Finally, the inorganic coating should possess a high degree of crystallinity to achieve a certain transparency for aesthetic reasons and to avoid etching under water spraying.

[0187] In this specification, a novel seeding method is disclosed to modify the wood surface with a thin seeding layer by dipping wood material into a basic solution of Zinc Oxide before drying. This liquid based seeding method enables to seed the rough wood surface completely and to form a strong interface between the seeding layer and the wood surface. By adjusting the amount of ammonium citrate in the growth process, the synthesis of ZnO nanorod arrays or dense coatings can specifically be selected. The chemical bath deposition process is illustrated in FIG. 1.

Example 1

[0188] The process of wood treatment with ZnO in basic solvent according to the invention comprises the following steps:

[0189] I. ZnO Coating [0190] 1) Preparation of basic solution of zinc oxide containing 50 mM ZnO and 5M NH.sub.3 by magnetic stirring assisted solubilisation (dissolving solid zinc oxide in aqueous solution of ammonia), the as-prepared solution was denoted as seeding solvent. Ammonium hydroxide (28 wt %; Alfa Aesar) was used to prepare basic solution. [0191] 2) Preparation of the growth solution containing NH.sub.4NO.sub.3 (8.02 g, Sigma Aldrich, p.a. grade), ammonium citrate (0.195 g, Alfa Aesar, p.a. grade) and ZnO powder (8.145 g, Sigma Aldrich, p.a. grade) and 132 ml of NH.sub.4OH (28 wt %, Alfa Aesar). The components were loaded into a plastic bottle before pouring 18 M cm deionized water until the total volume reached 2 L. The growth solution was stirred for 12 hours and stored at room temperature until usage. 200 mL growth solution was filtered (1 m glass fiber filters) into a glass flask followed by adding 0.1356 g of ammonium citrate. [0192] 3) Spruce wood slices (radial (R)tangential (T)longitudinal (L) dimensions: 52540 mm) were firstly ultrasonically cleaned in a mixed solution of ethanol and acetone for 30 min, and then stored in the artificial climate room with a temperature of 20 C. and a relative humidity of 65%. [0193] 4) Dipping the wood slice into the seeding solvent described in step 1) for 3 minutes before transferring it into the oven at 110 C. for 2 min. In order to obtain a dense and continuous ZnO coating, the seeding procedure was repeated for three times to ensure that the wood surface was completely seeded. [0194] 5) After pre-heating the growth solution at 90 C. for 15 min, the seeded spruce slices were immersed into the growth solution. Chemical bath deposition was carried out at 90 C. for 150 minutes. After the chemical bath deposition, the wood samples were washed with water and dried in an oven at a temperature of 65 C. for 12 hours.

[0195] Another typical procedure for a chemical bath deposition is described below:

[0196] NH.sub.4NO.sub.3 (8.004 g, p.a. grade, Sigma-Aldrich, Belgium), ZnO powder (8.139 g, p.a. grade, Sigma Aldrich, the Netherlands) and 132 ml of NH4OH (28% in water, Alfa Aesar, Karlsruhe, Germany) were loaded into a plastic bottle before pouring deionized water until the total volume reached 2 I. The precursor solution was stirred for 12 h and stored at r.t. until usage. For the formation of dense ZnO coating, 200 ml of precursor solution was filtered (1 m glass fibre filters) into a glass flask followed by adding 0.170 g of ammonium citrate (p.a. grade, Alfa Aesar, Karlsruhe, Germany). The pH value of the precursor solution was about 11.4. After pre-heating at 90 C. for 15 min, the seeded spruce slices were immersed into the growth solution. Chemical bath deposition was carried out at 90 C. for 150 min. After the chemical bath deposition, the wood samples were removed from the solution, washed with water, and dried in an oven at a temperature of 65 C. for 12 h. For the formation of ZnO nanorod coating, 200 ml precursor solution was filtered into a glass flask followed by adding aluminium nitrate nonahydrate (0.008 g, Sigma Aldrich, Germany) to form a precursor solution with an aluminium ions concentration of 0.1 mM. Then, the seeded spruce slice was immersed into the growth solution. Deposition was carried out at 90 C. for 30 min. After the chemical deposition, the wood samples were washed with water and dried in an oven at a temperature of 65 C. for 1 h.

[0197] II. UV and Weathering Stability Assessment

[0198] A UV curing chamber (UVACUBE 400, Honle Goup) was used as a light source for the short-time irradiation to assess the UV protection efficiency of the coating.

[0199] Exposure of the wood coating to artificial weathering using fluorescent UV lamps and water was carried out in the QUV Accelerated Weathering Tester (Q-LAB, United State) following the European standard EN 927-6:2006 (E). An exposure cycle of one week consists of a condensation period followed by a sub-cycle of water spray and UV-A 340 irradiation as given in Table 1.

TABLE-US-00001 TABLE 1 Exposure cycle Step Function Temperature Duration Condition 1 Condensation (45 3) C. 24 h 2 Subcycle 144 h consisting Step 3 + 4 of 48x cycles of 3 h consisting of steps 3 and 4 3 UV (60 3) C. 2.5 h Irradiance set point 0.89 W/(m.sup.2nm) at 340 nm 4 Spray 0.5 h 6 L/min to 7 L/min, UV off

[0200] Colour evaluation of the wood samples before and after UV exposure and water spraying were determined in the CIE L*, a* and b* system with the Chroma Meter CR-200 (MINOLTA, Japan). L* represents the lightness from black (0) to white (100), while a* and b* were the chromaticity indices where +a* was the red, a* was the green, +b* was the yellow, b* was the blue directions. The total colour change was defined according to equation (1):

E={square root over (L.sup.2+a.sup.2+b.sup.2)}(equation 1) Characterization of total colour changes

[0201] The ZnO coating of the wood surface was characterized by x-ray diffraction (XRD) (FIG. 2). Spruce wood exhibits a couple of strong diffraction peaks at 16.4 and 22.4, which can be attributed to the cellulose crystals inside the wood structure. The XRD pattern of wood with a seeding layer, has no diffraction peak belonging to ZnO. However, the diffraction peaks of cellulose crystals are suppressed. It reveals that the wood surface was covered with an amorphous ZnO seeding layer after dipping into the zinc oxide dissolved ammonia solution and heat treatment. After the chemical bath deposition process, a series of diffraction peaks assigned to hexagonal ZnO (JCPDS 36-1451) were observed. On the other hand, the diffraction peaks of cellulose crystals almost disappeared, because the wood surface was covered by a continuous, dense and highly crystalline ZnO film. The high crystallinity of ZnO coating contribute to their excellent durability under weathering conditions.

[0202] The dense and continuous ZnO film coating on the wood surface was further investigated by SEM from radial section and cross section. It demonstrates that the wood surface is covered with a conformal ZnO film which also covers the lumen surface of the first cell layer (FIGS. 3a and b). A similar result can be observed from the SEM image acquired from the transverse plane which indicates that ZnO film follows the topography of the rough wood surface (FIG. 3c). The higher magnification SEM images demonstrate that the wood surface is covered by a dense and continuous ZnO layer with an average thickness of about 3.5 m, while the surface of the inner, intact lumen of the most top cell layer is also covered by ZnO film with a thickness of about 600 nm (FIG. 3d). The latter indicates that the precursor solution penetrated through the outermost wood cell wall during the CBD process. The morphology of the ZnO coating surveyed from the tangential plane demonstrates that the ZnO crystal is highly crystalline with a flat end face (FIG. 3e,f). ZnO crystals are merging together with twinned boundaries.

[0203] The dense and continuous ZnO coating proved to be beneficial for the weathering resistance of the wood surface, which is discussed below. The coating obtained by ZnO platelets was characterised in more detail by TEM presented in FIG. 4. A lamina for S/TEM analysis was cut by FIB to avoid mechanical artifacts. Low magnification TEM imaging demonstrates that ZnO coating is tightly covering the cell wall surface without any crevices (FIG. 4a). SAED pattern indicates that the dense ZnO film grows, as expected, along the [001] wurtzite direction (FIG. 4b). From the HRTEM image of the cell wall and ZnO interface (FIG. 4c), (100) lattice planes of hexagonal ZnO as well as the diffraction contrast image from cellulose crystal in the wood cell wall were observed. Because in the presence of ammonium citrate the growth of ZnO along the [001] direction is suppressed, the ZnO crystals on wood surface have the tendency to grow along the [100] direction, i.e. parallel to wood surface. This controlled crystal growth leads to the formation of a dense film by adjoining single crystals with a twinned boundary. The lattice-resolved image indicates that the wood cell wall and ZnO coating interface is fused. From the HAADF-STEM image (FIG. 4d), the seeding layer that is in direct contact with the cell wall is made of dendritic crystal whiskers, which allows for the strong adherence to the wood cell wall.

[0204] The platelet-like ZnO is highly crystalline and accumulates on the wood surface in a compact fashion, from which the hexagonal structure with clear crystal boundary could be observed on the top.

[0205] A micro scratch tester was used to characterize the practical adhesion failure of the ZnO coating with a 10 m tip in 1 mm length (FIG. 5). The friction coefficient increased sharply in the first 50 m, where the tip penetrated into a depth of about 25 m. In this stage, the tip was mainly in contact with the ZnO coating. After that, the increment in the friction coefficient is minor, because the tip had cut through the coating and mainly contacted with the wood substrate which was soft compared to the ZnO coating. The micro tip penetrated into a depth of about 160 m as the loading increased from 3 mN to 1000 mN. However, the optical microscopy image of the wood surface after the scratch test, reveals no obvious peeling off from the ZnO coating. As discussed above, the ZnO coating had a strong interaction with the wood substrate because of the coordination effect. Therefore, the ZnO coating will be shoved off along the grain boundary following substrate when the tip penetrated into wood instead of peeling off. It reveals that the coating has a strong adhesion to the wood surface, and is robust to mechanical scratch. A non-quantitative tape-and-peel test and a blade scratch were used to evaluate the coating adhesion at the macro scale. The ZnO coating followed the surface topography of the wood surface, which displayed almost no change after 100 cycles of the tape-and-peel test. In the harsher blade scratch, the continuous and dense ZnO coating was grinded into rough particles and the surface became white. However, the ZnO coating was still on the wood surface after the brushing with compressed air. The mechanical property characterization demonstrates that the ZnO coating has strong adhesion and can withstand most of the mechanical damage in outdoor application.

[0206] The UV protection efficiency of the modified wood was assessed by alternating UV exposure and color measurement (FIG. 6). During the UV exposure, the unmodified sample lost lightness from 77.9 to 69.5, while the modified sample experienced a minute increment from 73.9 to 75.2 (FIG. 6a). The redness index (a*) of the unmodified sample underwent a decrease in the first 6 h and then increased up to 8.0 (FIG. 6b). In comparison, the value of a* decreased from 0.34 to 0.54 and then remained rather constant during the irradiation for the dense ZnO coating sample. FIG. 6c demonstrates that the non-modified spruce has an apparent yellow shift with an increment in the value of b* from 25.8 to 32.9. However, for the sample with dense ZnO coating, b* slightly decreased from 14.8 to 11.36 in the first 18 h, and further experienced a slow increment to finally reach 12.3. The total color difference E as a function of the irradiation time is given in FIG. 6d. E shows a systematic trend of increase with increasing irradiation time for unmodified spruce, which shows a total color change of 11 after 102 hours of UV exposure. In comparison, the total color change of the modified sample is clearly limited (about 3.0). These results demonstrate that a wood surface with a dense ZnO coating is sufficiently protected against color changes induced by UV exposure. For comparison between the two nanostructure morphologies the UV-stability of spruce wood with a ZnO nanorod array is shown.

[0207] The change in chemical structure of wood was further analyzed by FT-IR spectroscopy (FIG. 7). FIGS. 7a and b indicates that the absorption of UV light yields significant intensity changes in the FT-IR spectra of the wood surface without coating. The characteristic aromatic lignin band at 1507.3 cm.sup.1 was absent after 102 h UV exposure, which is linked to the strong formation of the new carbonyl absorption centered at 1723.1 cm.sup.1 (non-conjugated aliphatic carbonyls). It reveals that UV irradiation induced the photooxidation of lignin on the spruce wood surface without modification. In order to analyze the change in the chemical structure of the ZnO coating spruce after UV exposure, an experiment was carried out by removing the coating layer via ultrasonic treatment in 2 wt % HCl. The FT-IR spectrum was acquired from the surface after drying in the oven at 65 C. for 3 h. Although, the aromatic lignin band at 1507.3 cm.sup.1, as well as the CO stretch of conjugated or aromatic ketones (between 1600-1700 cm.sup.1) have a slight decrease, there is no increment in the intensity between 1700-1750 cm.sup.1 (FIG. 7). It reveals that the UV induced photooxidation is suppressed by the ZnO coating. The decline in the intensity of aromatic peak only happened in the outmost layer. It was further confirmed by FT-IR microscopy which was applied to study the lignin degradation on the cross section (FIG. 8).

[0208] Accelerated weathering tests were carried out based on the European standard: EN 927-6:2006. As a result, there is not only a colour change but also cracking observed from the bare spruce slice after the weathering test (FIG. 9a). However, the samples with surface coating, experience almost no colour change except a slight surface haze. This indicates that the ZnO coating forms a transparent protective coating, which serves not only as a UV blocking, but also as a waterproofing layer which reduces dimensional changes of the wood and as a result the occurrence of cracks due to weathering. Accurate colour change was quantified by the CIE L*, a* and b* system with the Chroma Meter (FIG. 9b).

[0209] III. Concept Behind the Invention

[0210] Without wishing to be bound by theory the disclosed invention takes advantage of the coordination effect of Zn[(OH).sub.x(NH.sub.3).sub.y].sup.2-x complexes to hydroxyl, carbonyl groups of lignocellulose to coat the uneven wood surface with conformal ZnO film via chemical bath deposition. The morphologies of ZnO coatings can be selectively controlled from rough nanorods to a continuous and dense film using different capping agents. The high crystallinity and continuity of the ZnO crystals in the dense film not only endows the coating with effective UV blocking properties, but also makes it more stable during weathering. The dense film renders the coating more transparent for preserving the aesthetic appeal of the wood surface. The study with advanced S/TEM indicates a wood surface interlocking with ZnO dendritic crystal whisker at the interface, which results in the strong interaction between the inorganic coating and the wood biopolymer.

Example 2

[0211] The process of wood material preparation using Zinc nitrate hexahydrate and hexamethylenetetramine (HMTA) and including a hydrophobization step according to the invention comprises the following steps:

[0212] I. ZnO Coating [0213] 1) Preparation of basic solution of zinc oxide containing 50 mM ZnO and 5M NH.sub.3 by magnetic stirring assisted solubilisation (dissolving solid zinc oxide in aqueous solution of ammonia), the as-prepared solution was denoted as seeding solvent. Ammonium hydroxide (28 wt %; Alfa Aesar) was used to prepare basic solution. [0214] 2) Preparation of the growth solution containing 80 g Zn(NO.sub.3).6H.sub.2O and 35 g hexamethylenetetramine (HMTA) were loaded into a stainless steel container before pouring deionized water until the total volume reach 6 L. [0215] 3) Spruce wood slices (radial (R)tangential (T)longitudinal (L) dimensions: 151.565 cm) were cut and stored in the artificial climate room with a temperature of 20 C. and a relative humidity of 65%. [0216] 4) Dipping the wood slice into the seeding solvent described in step 1) for 5 minutes before transferring it into the oven at 80 C. for 10 minutes. In order to obtain a dense and continuous ZnO coating, the seeding procedure was repeated for three times to ensure that the wood surface was completely seeded. [0217] 5) After pre-heating the growth solution to 45 C., two seeded spruce slices were immersed into the growth solution. Chemical bath deposition was carried out at 85 C. for 120 minutes. After the chemical bath deposition, the wood samples were washed with water and dried in an oven at a temperature of 60 C. for 2 hours. The same procedure was repeated one more time to obtain a high quality ZnO coating.

[0218] II. Preparation of Hydrophobic Surface-Layer [0219] 1) Preparation of a hydrophobic solution containing 5 mM stearic acid in ethanol solution, [0220] 2) Immersing the wood slices obtained from process I. into the hydrophobic solution for 12 h. [0221] 3) The wood slices were subsequently put in to an oven and heated up to 65 C. for 2 h to secure the strong bonding of hydrophobic molecules.

Example 3

[0222] The process of wood material preparation with ZnO including the addition of a SiO.sub.2 blocking layer and a hydrophobization step according to the invention comprises the following procedures:

[0223] III. ZnO Coating [0224] 1) Preparation of basic solution of zinc oxide containing 50 mM ZnO and 5M NH.sub.3 by magnetic stirring assisted solubilisation (dissolving solid zinc oxide in aqueous solution of ammonia), the as-prepared solution was denoted as seeding solvent. Ammonium hydroxide (28 wt %; Alfa Aesar) was used to prepare basic solution. [0225] 2) Preparation of the growth solution containing NH.sub.4NO.sub.3 (8.02 g, Sigma Aldrich, p.a. grade), ammonium citrate (0.195 g, Alfa Aesar, p.a. grade) and ZnO powder (8.145 g, Sigma Aldrich, p.a. grade) and 132 ml of NH.sub.4OH (28 wt %, Alfa Aesar). The components were loaded into a plastic bottle before pouring 18 M cm deionized water until the total volume reached 2 L. The growth solution was stirred for 12 hours and stored at room temperature until usage. 200 mL growth solution was filtered (1 m glass fiber filters) into a glass flask followed by adding 0.1356 g of ammonium citrate. [0226] 3) Spruce wood slices (radial (R)tangential (T)longitudinal (L) dimensions: 52540 mm) were firstly ultrasonically cleaned in a mixed solution of ethanol and acetone for 30 min, and then stored in the artificial climate room with a temperature of 20 C. and a relative humidity of 65%. [0227] 4) Dipping the wood slice into the seeding solvent described in step 1) for 3 minutes before transferring it into the oven at 110 C. for 2 min. In order to obtain a dense and continuous ZnO coating, the seeding procedure was repeated for three times to ensure that the wood surface was completely seeded. [0228] 5) After pre-heating the growth solution at 90 C. for 15 min, the seeded spruce slices were immersed into the growth solution. Chemical bath deposition was carried out at 90 C. for 150 minutes. After the chemical bath deposition, the wood samples were washed with water and dried in an oven at a temperature of 65 C. for 12 hours.

[0229] IV. Preparation of SiO.sub.2 Blocking Layer [0230] 1) Preparation of stabilizing solution containing tetraethyl orthosilicate (32 L), water (80 mL), ethanol (20 mL) and ammonium hydroxide (1 mL, 25%). [0231] 2) Immersing the wood slices obtained from process I. into the stabilizing solution for a duration of 3 hours at room temperature.

[0232] V. Preparation of Hydrophobic Surface-Layer [0233] 1) Preparation of a hydrophobic solution containing 5 mM stearic acid in ethanol solution, [0234] 2) Immersing the wood slices obtained from process II. into the hydrophobic solution for 12 h. [0235] 3) The wood slices were subsequently put in to an oven and heated up to 65 C. for 2 h to secure the strong bonding of hydrophobic molecules.

[0236] Compared with Existing Technologies, the Advantages of this Invention Include the Following Points: [0237] 1) The treatment of wood material as described is facile, mild and economic. Both the seeding and growth process can be finished in a short time compared to current technologies. [0238] 2) The treatment method via a facile aqueous based chemical bath deposition process was carried out in a non-toxic and mild condition (without exceeding temperature of 120 C. at any step). [0239] 3) The morphology of the ZnO coating can be tailored by adjusting the concentration of ammonium citrate, to selectively synthesize ZnO nanorods array coatings or dense ZnO film coatings. [0240] ZnO nanorod is generated when ammonium citrate is absent or the concentration of ammonium citrate is lower than 0.5 mM. Alternatively, the morphology of ZnO is controlled by adding Al(NO.sub.3).sub.3 in the second solution yielding we could get ZnO nanorod coating. [0241] If the concentration of ammonium citrate is higher than 3 mM and lower than 6 mM, dense ZnO coating, particularly comprising a patelet structure, is generated. The coating is mechanically robust and hardly separated from the wood surface. [0242] 4) Both UV protection and superhydrophobic wood surface modification are combined in this invention. [0243] 5) The disclosed method is efficient and can be easily scaled-up for large scale surface modification, in order to make wood more reliable in building facade applications.

[0244] According to the invention, a ZnO seeding layer is provided, which is strongly attached to the wood surface allowing for the growth of ZnO on the seeded wood surface by applying the growth solution. Thus, providing a unique complete coverage on the wood surface with a strong binding strength of the layer.

[0245] By controlling the morphology of ZnO in the second step, particularly by adding Al(NO.sub.3).sub.3, a ZnO nanorod coating is provided comprising a good UV protection property. Moreover, the nanorod structures enable controlling the wood surface roughness. A superhydrophobic wood surface ca be provided by modifying the surface of ZnO nanorod with hydrophobic molecules. The ZnO nanorod coating is particularly good for indoor application.

[0246] By inducing ammonium citrate ions into the growth solution a dense and highly crystalline ZnO layer is provided. The citrate ions will cap onto the {0001} facet of the ZnO, thus, providing such a layer. The dense and continuous ZnO layer cannot scatter visible light, therefore, it is transparent. Furthermore, the layer is highly crystalline, which makes it very stable under rain washing in outdoor applications.

Example 4

[0247] Protection of Wood Surface by TiO.sub.2

[0248] The coating process contains the hydrolysis initiated by the nucleophilic ligands in wood and polymerization reactions of the precursor, which is titanium tetraisopropoxide (TTIP) in this report. 2 mL TTIP was injected into 20 mL ethanol solution. Then, 2 pieces of spruce sample (50505 mm in longitudinaltangentialradical in dimension) were immersed into the mixed solution for 24 hours at room temperature. The samples were removed from the solution followed by drying in the oven at 65 C. for 2 hours, 120 C. for 1 hour and 180 C. for 0.5 hour.

[0249] TTIP treatment results in an amorphous TiO.sub.2 which covers wood the surface continuously, but without any boundary. It is a continuous thin film, which is generated by the hydrolysis of TTIP molecules initiated by the moisture in wood or the hydroxyl groups in wood. Moreover, because of the permeation of the TTIP and alcohol precursor solution into the wood, the resulted TiO2 layer not only covers the wood surface but also covers the lumen surface inside the wood.

[0250] UV Stability

[0251] The UV protection effect of the modified sample was assessed by UV exposure in the UV chamber combined with color measurement. As shown in FIGS. 11a and b, the bare spruce exhibits larger change in lightness and becomes more red compared to the modified counterpart. The most obvious difference was observed in the yellowing index (FIG. 11c). The yellowing index (b) increases from 20 to 32 after 144 hours UV exposure in the unmodified sample, while it increase from 19.5 to 21.5 in the modified one. The appearance and total color change of the samples with or without UV exposure was shown in FIG. 11d.

Example 5

[0252] Transparent TiO.sub.2CeO.sub.2 UV Protection Coating

[0253] Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 was dissolved in 200 mL of isopropanol solution with a concentration of 1.5 mM. A total volume of 20 mL of Titanium isopropoxide (C.sub.12H.sub.28O.sub.4Ti) was added rapidly to this solution to produce a yellow precursor solution. The precursor solution was filled in a box (polypropylene) with a lid on the top.

[0254] A spruce wood panel was placed in the precursor solution. The box was put on the shaker and kept at room temperature for 48 h.

[0255] The precursor solution was removed from the box and stored in a conical flask. The wood panel was kept in the box. 4.5 mL precursor solution was dropped onto the wood surface followed by a drying step in the box at room temperature for 45 min. This process was repeated up to 5-10 times to increase the thickness of TiO.sub.2CeO.sub.2 coating on the surface of wood panel. After that the wood panel was dried in the oven at 65 C.

Advantages

[0256] 1) The small amount of Ce(IV) (with a molar ratio of 220:1=Ti:Ce) prevents the hydrolysis of Titanium isopropoxide caused by the moisture from the air and the wood; [0257] 2) The stable precursor solution can be reused for several modification; [0258] 3) The stable precursor solution allows for the buildup of a thick and transparent coating on the surface for efficient UV protection. [0259] 4) The precursor solution helps to avoid particle formation during the coating process.