PROCESS FOR PARTIAL DELIGNIFICATION AND FILLING OF A LIGNOCELLULOSIC MATERIAL, AND COMPOSITE MATERIAL STRUCTURE ABLE TO BE OBTAINED BY THIS PROCESS

Abstract

The invention relates to a process for treating a lignocellulosic material, e.g., wood, comprising the following steps: providing a lignocellulosic material; removing at least some but less than all lignin from the lignocellulosic material to yield a delignified structure; and densifying the delignified structure to yield the delignified, densified material, wherein the delignified, densified material is equal in size or is smaller in size relative to the lignocellulosic material provided; where densifying may include contacting said delignified structure, at least in part, with at least one fluid at a pressure greater than atmospheric pressure. The invention also relates to a composite structure able to be obtained in this way, and to any part comprising at least one such structure.

Claims

1. A method for generating a delignified, densified material, comprising: (a) providing a lignocellulosic material; (b) removing at least some but less than all lignin from the lignocellulosic material to yield a delignified structure; and (c) densifying the delignified structure of (b) to yield the delignified, densified material, wherein the delignified, densified material is equal in size or is smaller in size relative to the lignocellulosic material provided in (a); and, wherein densifying in (c) comprises contacting said delignified structure, at least in part, with at least one fluid.

2. The method of claim 1, wherein the delignified, densified material comprises a three-dimensional structure of cellulose and lignin, and wherein the three-dimensional structure comprises hydrogen bonds.

3. The method of claim 1, wherein the delignified, densified material has increased resistance to an axial compression force than the lignocellulosic material in (a).

4. The method of claim 1 wherein the removing in (b) removes at least 40% by weight and at most 85% by weight of the lignin from the lignocellulosic material.

5. The method of claim 1, wherein the removing in (b) comprises soaking the lignocellulosic material with at least one organic fluid.

6. The method of claim 6, wherein the at least one organic fluid comprises a polarity agent.

7. The method of claim 6, wherein the lignocellulosic material is soaked for at least 6 hours.

8. The method of claim 1, wherein the removing in (b) comprises soaking the lignocellulosic material with at least one aqueous fluid having an alkaline pH.

9. The method of claim 8, wherein the soaking is performed under a vacuum.

10. The method of claim 8, wherein the alkaline pH is about 12.

11. The method of claim 8, wherein the at least one aqueous fluid comprises sodium hydroxide.

12. The method of claim 8, wherein the at least one aqueous fluid comprises sodium sulfite.

13. The method of claim 8, wherein the soaking is performed for at least 30 minutes.

14. The method of claim 8, wherein the temperature of the at least one aqueous fluid exceeds 60° C.

15. The method of claim 8, wherein the removing in (b) further comprises washing the lignocellulosic material at least once with water.

16. The method of claim 15 wherein the water has a temperature of at least 30° C.

17. The method of claim 8, wherein the removing in (b) further comprises washing the lignocellulosic material at least once with an organic solution.

18. The method of claim 1, wherein the delignified structure in (b) has a water content of from 0% to 30%.

19. The method of claim 1, wherein the delignified structure in (b) or the delignified, densified material has improved fire resistance relative to the lignocellulosic material in (a).

20. The method of claim 1, wherein the delignified, densified material has a density at least 5% greater than that of the lignocellulosic material in (a).

21. The method of claim 1, wherein densifying comprises removing air from said delignified structure

22. A delignified, densified material, comprising: (a) cellulose; (b) hemicellulose; and (c) lignin; wherein the delignified, densified material is made from a natural wood; wherein at least 40% and at most 85% by weight of the lignin present in the natural wood is removed by a chemical treatment, wherein the delignified, densified material is equal in size or is smaller in size relative to the lignocellulosic material prior to chemical treatment.

23. The delignified, densified material of claim 22, wherein the delignified, densified material comprises a three-dimensional network of the cellulose and the lignin.

24. The delignified, densified material of claim 22, wherein the three-dimensional structure comprises hydrogen bonds.

25. The delignified, densified material of claim 22, wherein the delignified, densified material has a higher ductility than the natural wood.

26. The delignified, densified material of claim 22, wherein the delignified, densified material comprises hydrogen bonds.

27. The delignified, densified material of claim 22, wherein the lignin of the delignified, densified material is from 5.25% to 10.8% by weight of the delignified, densified material.

28. The delignified, densified material of claim 22, wherein the delignified, densified material has (i) a maximum stress at breaking and a bending deflection greater than the natural wood as determined by a bending measurement, or (ii) a maximum stress at breaking and deformation greater than the natural wood as determined by an axial compression measurement.

29. The delignified, densified material of claim 22, wherein the delignified, densified material is fire resistant.

30. The delignified, densified material of claim 22, wherein the delignified, densified material comprises essentially no air.

Description

DRAWINGS

[0246] The invention will be better understood in the light of the accompanying drawings in which:

[0247] FIG. 1 diagrammatically represents the principle of the treatment process according to the invention;

[0248] FIG. 2 diagrammatically represents an example of partial implementation of a treatment step of the process according to the invention, said step comprising the immersion of the structure of lignocellulosic material in a liquid;

[0249] FIG. 3 diagrammatically represents an example of implementation of the step of placing under pressure of the process according to the invention;

[0250] FIG. 4 diagrammatically represents an example of full implementation of the filling step (3) and of the finishing step (4) of the process according to the invention, for the case in which these steps comprise the immersion of the structure of the lignocellulosic material in a liquid according to FIG. 2;

[0251] FIG. 5 diagrammatically represents a view by scanning electron microscope (SEM) of a longitudinal cut of wood in the natural state;

[0252] FIG. 6 diagrammatically represents a macroscopic view in three dimensions of a structure of wood in the natural (or native) state, before treatment according to the invention;

[0253] FIG. 7 diagrammatically represents a microscopic view in three dimensions of the structure of wood of FIG. 6 in the natural (or native) state, before treatment according to the invention;

[0254] FIG. 8 diagrammatically represents a view at an intermediate scale of the structure of wood of FIGS. 9 and 10, after the steps of soaking (1) and washing (2) according to the invention;

[0255] FIG. 9 diagrammatically represents a macroscopic view in three dimensions of the structure of wood of FIGS. 6 and 7 after the steps of soaking (1) and washing (2) according to the invention;

[0256] FIG. 10 diagrammatically represents a microscopic view in three dimensions of the structure of wood of FIG. 9;

[0257] FIG. 11 diagrammatically represents a macroscopic view in three dimensions of the structure of wood of FIGS. 9 and 10 after the filling step by a filling compound (3);

[0258] FIG. 12 diagrammatically represents a microscopic view in three dimensions of the structure of wood of FIG. 11;

[0259] FIG. 13 diagrammatically represents a macroscopic view in three dimensions of the structure of wood of FIGS. 11 and 12 after the finishing step (4), that is to say of the structure of composite wood obtained by the treatment process according to the invention;

[0260] FIG. 14 diagrammatically represents a microscopic view in three dimensions of the cut of composite wood of FIG. 13;

[0261] FIG. 15 reproduces three photographs taken by scanning electron microscope (SEM) at a semi-microscopic scale of part of a structure of fir at different steps of the treatment process according to the invention, namely from left to right respectively before delignification treatment, after delignification and before impregnation by a monomer compound, and after polymerization of the monomer compound so impregnated;

[0262] FIG. 16 reproduces two photographs taken by scanning electron microscope (SEM) after partial enlargement of the part of a fir structure of FIG. 15, that is to say at a microscopic scale, namely from left to right respectively before treatment and after polymerization of the impregnated monomer compound;

[0263] FIG. 17 represents a diagram of the principle of measuring the bending of a structure of wood treated by a process according to the invention;

[0264] FIG. 18 illustrates the result of measuring the bending of a fir structure, before and after treatment;

[0265] FIG. 19 represents a diagram of the principle of measuring the axial compression of a structure of wood treated by a process according to the invention;

[0266] FIG. 20 illustrates the result of measuring the axial compression of a fir structure, before and after treatment;

[0267] FIG. 21 represents a diagram of the principle of measuring the axial traction of a structure of wood treated by a process according to the invention;

[0268] FIG. 22 illustrates the result of measuring the axial traction of a fir structure, before and after treatment; and

[0269] FIG. 23 reproduces ten photographs taken by Zeiss LSM710 Upright microscope after treatment of six different kinds of wood.

[0270] FIG. 1 diagrammatically represents the treatment process according to the invention, by a succession of sub-steps, each being represented by a box. Each of the boxes corresponds to the step bearing the same reference number of the process according to the invention, it being understood that references (5) and (6) are optional steps as shown by the arrows in dashed line linking boxes 4, 5 and 6.

[0271] In order, there can thus be distinguished a first step (1) which is a soaking step of the structure of lignocellulosic material. Step (1) performs partial extraction of the lignin of that structure. It is followed by a second step (2) which is a step of washing the structure resulting from step (1), to discharge the dissolved lignin resulting from step (1). This washing step (2) is followed by a third step (3) of filling the partially delignified structure resulting from the washing step (2), by at least one filling compound. The last and fourth step (4) is a step of fixation of the filling compound within the structure resulting from the filling step (3). This makes it possible to obtain a composite material structure formed by a three-dimensional network of transformed filling compound incorporated in a network of cellulose and lignin. This fourth step may be followed by a fifth step (5) of placing under pressure the structure resulting from the finishing step (4), possibly itself followed by a sixth step (6) of surface finishing of the structure (10) resulting from step (5).

[0272] FIG. 2 diagrammatically represents an example of partial implementation of a treatment step of the process according to the invention, said step comprising the immersion of a structure of lignocellulosic material (10) in a liquid. The structure of lignocellulosic material illustrated is a wood structure, for example fir. It is immersed in a treatment solution (11), which may be an organic solution of the soaking step (1), an organic solution of the washing step (2) or a solution comprising at least one filling compound of the filling step (3). The assembly rests on a mounting (15), for example of teflon, which is itself fastened into a tank (12), for example of stainless steel.

[0273] FIG. 3 diagrammatically represents an example of implementation of the optional step of placing under pressure (5) of the process according to the invention. In this case, the composite material structure (10′) is compressed in a compression apparatus (13, 14) composed of two symmetrical jaws (13) and (14) able to be brought towards each other while sandwiching the structure (10′) as a vise. On each of the parts (13) and (14) an axial force is applied which is opposite the force applied on the other part (the two parts being represented by arrows) so bringing them towards each other.

[0274] FIG. 4 diagrammatically represents an example of full implementation of the filling step (3) and of the finishing step (4) of the process according to the invention, for the case in which these steps comprise the immersion of the structure of the lignocellulosic material in a liquid according to FIG. 2.

[0275] In this Figure, the assembly presented in FIG. 2 is disposed inside a chamber (25). More specifically, the tank (12) is fastened within the chamber (25) by means of a metal mounting (16). The chamber (25) is such that it makes it possible to control the conditions of pressure and temperature which are present within it. These conditions mainly depend on the nature of the filling compound. The chamber (25) may be a vacuum oven or an autoclave.

[0276] A pipe (22), which divides into a pipe (22a) and a pipe (22b), makes it possible, by means of respective valves (17) and (18), respectively to create a vacuum or introduce dinitrogen N2 into the chamber (25).

[0277] A pipe (23), which divides into a pipe (23a), a pipe (23b), a pipe (23c) and a pipe (23d), makes it possible, by means of respective valves (19), (20), (21) and (26), to discharge or introduce a respective solution (24a), (24b), (24c) or (24d) into the chamber. A pure solution (24a), (24b), (24c) or (24d), or possibly a mixture of at least two of these solutions (24a), (24b), (24c) and (24d), thus constitutes the treatment solution (11) (which may vary according to the step or sub-step of the process considered), in which bathes the wood structure (10). For the carrying out of the filling step (3), this makes it possible to perform a treatment of the wood structure (10) by series, each series comprising successive immersions for example in the order of use of the solutions. The treatment step (3) usually comprises several series, typically from 2 to 6 series, for example 4 series.

[0278] It is possible by way of variant to provide the device presented in FIG. 4 with another device comprising as many organic treatment solutions as necessary, each treatment solution being associated with a pipe on which is present a valve, linked to the pipe (23) in direct relation with the chamber (25).

[0279] FIG. 5 diagrammatically represents a view from a scanning electron microscope (SEM) of the longitudinal cut of a wood structure, for example walnut, in the natural state. The micro-architecture of the wood can be seen therein. At this scale, it is possible to distinguish the cavities (or “lumens”) (28) of cellulose delimited by the walls (27) formed by cellulose fibers, as well as the transverse perforations forming pores or channels (29) between the cell cavities. These cavities have a transverse dimension of 30 to 60 μm for softwoods and 70 to 350 μm for hardwoods. The transverse perforations (29) represent bordered pits for softwoods or substantially circular orifices for hardwoods, of which the transverse dimension varies from approximately 6 to approximately 30 μm, and are on average approximately 15 μm. FIG. 5 substantially corresponds to a cut in the longitudinal direction of the structure represented diagrammatically in FIG. 6.

[0280] FIGS. 6 and 7 diagrammatically represent two views in three dimensions, which are respectively macroscopic and microscopic, of a wood structure in the natural (or native) state, that is to say before treatment according to the invention. Found therein are the cavities (28) of wood of average size in the transverse direction of approximately 75 μm, with disparities according to the nature of the wood, i.e. of approximately 30 to approximately 60 μm for softwoods, and approximately 70 to approximately 350 μm for hardwoods.

[0281] These cavities (28) are delimited by cell walls of average thickness of approximately 2 to approximately 10 μm for hardwoods and for softwoods.

[0282] As can be seen in FIG. 7, the micro-architecture of the wood from which arises the mechanical strength in the natural state comes from the ensemble of the walls of the cell walls (28) which are constituted by tubes or microfibrils (45), themselves formed by bundles or macrofibrils (47). The macrofibrils (47) are linked by chemical links to hemicellulose structures of transverse bracing (46), linked by transverse chemical links to lignin structures of longitudinal bracing (44), this ensemble (46, 44) serving as bracing for the cellulose macrofibrils (47)

[0283] FIGS. 8, 9 and 10 diagrammatically represent three views in three dimensions, respectively at the intermediate scales between the macroscopic and microscopic, of a structure of wood after the steps of soaking (1) and washing (2) according to the invention.

[0284] The cell wall (49) can be distinguished, which is thinned relative to that represented in FIGS. 6 and 7. The thickness of the cell wall (49) is approximately 2 to approximately 10 μm, on average approximately 6 μm. The 10 microfibrils (45), the hemicellulose (46) and the macrofibrils (47) are substantially unchanged relative to FIGS. 6 and 7. The lignin (44′) is still present, but “lightened”, that is to say in lower quantity relative to the lignin (44) represented in FIGS. 6 and 7.

[0285] FIG. 8 details the cell wall 49, at a wall junction (see FIG. 9). The cellulose cell wall (49) comprises a middle lamella (61) of thickness approximately 0.2 to approximately 1 μm, as well as two walls, the primary wall (55) of approximately 0.1 μm thickness, and the secondary wall (60), itself constituted by three sub-layers respectively (52), (53) and (54) in the direction from the cavity (28) towards the outside, the first sub-layer (52) being of approximately 0.1 to approximately 0.2 μm thickness, the second sub-layer (53) being of approximately 1 to approximately 5 μm thickness and the third sub-layer (54) being of approximately 0.1 to approximately 0.2 μm thickness. The transverse perforation (29) can also be distinguished, which is constituted by an actual orifice (50), of average dimension approximately 0.02 to 4 μm, surrounded by a perforation (51), of average dimension approximately 6 to 30 μm.

[0286] At the time of the partial delignification carried out by the soaking step (1) associated with the washing step (2), the primary wall (65) and the third sub-layer (54) of the adjoining secondary wall (60) have been the most delignified, themselves being the layers or sub-layers most charged in lignin, the third sub-layer (52) itself being very little charged with lignin, having practically not been delignified. This explains the differences in dimensional variations within the structure of the lignocellulosic material which occur on partial delignification according to the invention.

[0287] It is to be noted that, by adapting the dimensions indicated above, FIG. 8 could equally well illustrate the material prior to treatment during the treatment steps which are the steps of soaking (1) and washing (2) of the method according to the invention. To be precise, during these steps, the only modification identified arises from the thickness of some of the layers and sub-layers as explained above, which is lessened progressively with the treatment.

[0288] FIGS. 11 and 12 diagrammatically represent two views in three dimensions, respectively macroscopic and microscopic, of the wood structure of FIGS. 9 and 10 after the filling step by a filling compound (3). There can be found therein the microfibrils (45), the lightened structure of lignin (44′), the hemicellulose structure (46), the macrofibrils (47), and the walls (49) of the cavities (57). These cavities (57) are now filled with filling compound, forming a three-dimensional filling network (58). In practice, this amounts to filling the macrofibrils and their interstices with this filling compound, and possibly, the filling of the microfibrils with the filling compound according to the degree of penetration of the filling compound into the material.

[0289] FIGS. 13 and 14 diagrammatically represent two views in three dimensions, respectively macroscopic and microscopic, of the wood structure of FIGS. 11 and 12 after the filling step (4) (generally consisting of polymerization). They thus diagrammatically illustrate the composite wood structure obtained by the treatment process according to the invention. There can be found therein the microfibrils (45), the lightened structure of lignin (44′), the hemicellulose structure (46), the macrofibrils (47), and the walls (49) of the three-dimensional network (59) for filling.

[0290] FIGS. 15 to 23 are explained in the examples below.

[0291] The invention will be better understood in view of the following example embodiments, with reference to the accompanying drawings.

EXAMPLES

[0292] The following examples illustrate the invention without however limiting the scope thereof.

Example 1

Process According to the Invention for Treating a Structure of Fir

[0293] A parallelepiped sample of fir wood of dimensions 0.5 cm×4 cm×8.5 cm (b×l×h) was subjected to the treatment process according to the invention, which, in the context of laboratory experiments, enabled a composite specimen to be obtained of dimensions 0.45 cm×3.6 cm×8.2 cm (b×l×h).

[0294] The chamber used was a vacuum oven (25).

[0295] Thus, the sample was treated during a first soaking step (1) by means of three identical successive sub-steps, each consisting of an immersion of the sample in a solution of 6% sodium chloride and 0.05% sodium hydroxide, under a vacuum, at a constant temperature of 70° C. for 5 hours.

[0296] The washing step (2) of the sample was next implemented, by immersion of the sample resulting from the preceding soaking step (1), by means of 4 identical successive sub-steps, each consisting of immersion of the sample resulting from the preceding step or sub-step in a solution of 99% ethanol, under a vacuum, at constant temperature of 60° C. for 4 hours, followed by 3 second identical successive sub-steps, each consisting of immersion of the sample resulting from the preceding sub-step in a solution of 99% hexane under a vacuum, at constant temperature of 50° C. for 3 hours.

[0297] The sample resulting from the washing step (2) was next left to rest, such that the hexane still present in the wood sample evaporates, for a period of 2 hours. The steps of filling (3) and finishing (4) of the sample so obtained were carried out by means of the device represented in FIG. 4.

[0298] The filling step (3) was carried out according to the second embodiment, by impregnation under a vacuum. Thus, a “primary” monomeric solution was produced, composed of one part butyl methacrylate and three parts styrene, after purification of these compounds using a filtration powder made from diatomite. The primary monomeric solution was mixed for a first series, in a ratio of 50% by volume for 50% ethanol. The primary monomeric solution was mixed, in a ratio of 75% for 25% ethanol, for a second series. The primary monomeric solution (at 100%) constituted the solution of the third series. The primary monomeric solution (to 95%), added to 0.05 part catalyst (asoisobutyronitrile), constituted the solution of the fourth series.

[0299] The filling step (3) thus comprised four series, each series comprising four successive sub-steps, successively by the following solutions, made from solutions of fourth serious solution (monomeric solution+catalyst) (24a), ethanol (24b), hexane (24c) and monomeric solution (24d), without manipulation of the structure (10) and without contact with the air. The treatment of this step (3) was carried out under vacuum and at ambient temperature, for a duration of 24 hours per series.

[0300] At the end of the filling step (3), the solution (11) was evacuated by releasing the vacuum (17) and by blowing dinitrogen (18) to saturate the volume of the chamber (25), which advantageously prevented the evaporation of the monomers present in the structure (10).

[0301] The finishing step (4) which followed was a step of polymerizing the butyl methacrylate and styrene monomers filling the sample resulting from the filling step (3). This polymerization, leading to the formation of the styrene-butyl methacylate copolymer, was carried out under a vacuum, for a time of 20 to 24 hours for 500 mL of monomeric solution (11) at a temperature of 80° C. for the first two hours then 50° C. for the following part of the step.

[0302] This finishing step (4) was followed by a step of placing under pressure (5) which was carried out in the device of FIG. 3. The sample resulting from the finishing step (4) was enveloped between two sheets of latex each of 0.7 mm thickness, the 2 sheets fully covering the sample and therefore overlapping the four edges. They thus create a pocket conforming to the sample. This pocket was then placed under a vacuum using a valve, the container-contents assembly thus forming the structure (10′). The combination was placed in an oven at a temperature of 40° C. for a time of 24 hours.

[0303] The step of placing under pressure (5) was followed by a step of demolding the wood composite structure so obtained from the pocket, then surface finishing during a surface finishing step (6) using a light jet of ethyl acetate.

[0304] The composite fir sample so obtained was translucent.

[0305] It is also possible to provide a variant in which the chamber (25) is an autoclave (25), in which case a single series may be sufficient to perform the filling step (3), the finishing step (4) is carried out by immersion in a bath and the steps of placing under pressure (5) and surface finishing (6) then not being necessary. Example 1 was carried out several times, so as to obtain several composite fir samples which were evaluated in destructive mechanical tests and in non-destructive optical tests, as is explained in example 2 below.

Example 2

Evaluation of the Composite Fir Structure Resulting from the Treatment Process According to the Invention and Obtained in Example 1

[0306] The fir sample forming a composite fir structure obtained in Example 1 was evaluated not only for its properties of mechanical strength but also for its optical properties.

Mechanical Tests Illustrated by FIGS. 17 to 22

Bending Measurement Illustrated by FIGS. 17 and 18

[0307] This measurement was carried out on samples of fir of size 0.7 cm×2.5 cm×10 cm (b×l×h), according to a method developed by the applicant comprising three identical pulleys (39), of 3 cm diameter, applying bending to the structure. These three pulleys (39) were spaced pairwise by 3.5 cm, the distance between the furthest two pulleys (39) being 7 cm.

[0308] FIG. 17 represents the diagram of the principle of measuring the bending of the fir sample forming a composite fir structure (38) according to the invention. As can be seen in FIG. 17, a force F was applied to the sample, of increasing value, perpendicularly to its main plane, and the maximum stress just prior to breaking (arrow f) was measured.

[0309] FIG. 18 illustrates the result of this bending measurement of the fir sample, before treatment (curve A) and after treatment (curve B).

[0310] For the sample of natural (or native) fir (curve A), the maximum stress at breaking was measured at 175 kgf/cm.sup.2, and the bending deflection was 0.53 cm.

[0311] By contrast, for the translucent composite fir sample according to the invention (curve B), the maximum stress at breaking was measured at 350 kgf/cm.sup.2, and the bending deflection was 0.65 cm.

[0312] Thus, the cell densification of the fir by virtue of the treatment according to the invention enabled an increase of 200% in resistance to a bending force. Furthermore, in contrast to native fir, the breaking is more progressive in the case of the composite fir material according to the invention. Therefore, the material gained in ductility by virtue of the treatment according to the invention. Without wishing to be limited by any theory, the inventor thinks that this is probably due to a high adhesion between the fibers and the polymer matrix.

Axial Compression Measurement Illustrated by FIGS. 19 and 20

[0313] This measurement was carried out on samples of fir of dimensions 1 cm×3.5 cm×10 cm (b×l×h), according to a method developed by the applicant.

[0314] FIG. 19 represents the diagram of the principle of measuring the axial compression of the fir sample forming a composite fir structure (41) according to the invention.

[0315] As can be seen in FIG. 19, a compression force F was applied to the sample (41) by a crushing plate (40) of 7 cm diameter, of increasing strength, parallel to its main axis, and the maximum stress just before breaking (distance da) was measured.

[0316] FIG. 20 illustrates the result of this axial compression measurement of the fir sample, before treatment (curve A) and after treatment (curve B).

[0317] For the sample of natural (or native) fir (curve A), the maximum stress at breaking was measured at 254 kgf/cm.sup.2, and the deformation just before breaking was 0.959 cm.

[0318] By contrast, for the translucent composite fir sample according to the invention (curve B), the maximum stress at breaking was measured at 430 kgf/cm.sup.2, and the deformation just before breaking was 0.978 cm.

[0319] Thus, the cell densification of the fir by virtue of the treatment according to the invention enabled an increase of 170% in resistance to an axial compression force. Furthermore, in contrast to native fir, the breaking is more progressive in the case of the composite fir material according to the invention. Without wishing to be limited by any theory, the inventor thinks that this is probably due to the nature of the composite fir, which leads to the occurrence of a fissure in the matrix being stopped by fibers of wood reinforced by the polymer.

Axial Traction Measurement Illustrated by FIGS. 21 and 22

[0320] This measurement was carried out on samples of fir of dimensions 0.2 cm×3 cm×7.5 cm (b×l×h), according to a method developed by the applicant.

[0321] FIG. 21 represents the diagram of the principle of measuring the axial traction of the fir sample forming a composite fir structure (43) according to the invention.

[0322] As can be seen in FIG. 21, two identical traction forces F of opposite direction were applied to the sample via two identical grips (42), increasingly strongly, parallel to its main axis, and the maximum stress before breaking was measured. The dimensions of each grip (42) were 1.5 cm×2.5 cm×1 cm (b×l×h).

[0323] FIG. 22 illustrates the result of this axial traction measurement of the fir sample, before treatment (curve A) and after treatment (curve B).

[0324] For the sample of natural (or native) fir (curve A), the maximum stress at breaking was measured at 125 kgf/cm.sup.2, and the extension just before breaking was 0.7 cm.

[0325] By contrast, for the translucent composite fir sample according to the invention (curve B), the maximum stress at breaking was measured at 165 kgf/cm.sup.2, and the extension just before breaking was 0.7 cm.

[0326] Thus, the plastic deformation recorded was approximately 6% before breaking.

[0327] The behavior of the material according to the invention in relation to axial traction is thus substantially identical to that of the material before treatment. Degradation of this value by use of the process according to the invention was not observed.

Optical Tests Illustrated by FIGS. 15, 16 and 23

[0328] The composite fir structure obtained in Example 1 was translucent.

[0329] FIGS. 15 and 16 reproduce, at two different scales, photographs taken by scanning electron microscope (SEM) of part of a structure of fir wood treated in Example 1, at different steps of the treatment process according to the invention.

[0330] FIG. 15 reproduces three photographs made by scanning electron microscope (SEM) of part of a structure of fir wood at different steps of the treatment process according to the invention, namely from left to right respectively before delignification treatment (natural or native wood), after delignification and before impregnation by a monomer compound, and after polymerization of the monomer compound so impregnated. FIG. 16 reproduces two photographs taken by scanning electron microscope (SEM) after partial enlargement of the part of a structure of fir wood of FIG. 6, from left to right respectively before treatment (natural or native wood) and after polymerization of the impregnated monomer compound.

[0331] The light-colored parts which fill after impregnation with the filling compound, are different once the impregnation has been carried out, in particular whiter and wider (see the comparison between parts 30 and 31 of FIG. 15, as well as between parts 36 and 34 of FIG. 16).

[0332] The dark parts (32 for the second photograph of FIG. 15, 33 for the third photograph of FIG. 15, 37 for the first photograph of FIG. 16 and 35 for the second photograph of FIG. 16) correspond to air and could be notably reduced by later optimization of the process. These photographs show the disordered state of the wood before delignification, which does not make it possible to distinguish the cavities of the cells, the more ordered state of the wood after delignification, in which it is possible to clearly distinguish the cavities of the cells, and the state of the wood cavities partially filled with the polymer, after the treatment process according to the invention. It can thus be seen that the composite wood obtained after the treatment process according to the invention is highly structured in accordance with the original micro-architecture of the wood: the walls of the cells which were mainly constituted by cellulose are transformed into walls in which are inserted balls of polymer and the polymer at least partly fills the cell cavities.

[0333] FIG. 23 reproduces two photographs taken by Zeiss LSM710 Upright microscope with a ×20 lens of the composite fir structure, in longitudinal radial cut (LRC) and in transverse cut (TC).

[0334] The ambient light is 257 lux.

[0335] The first photograph is in the plane of a longitudinal radial cut (LRC) obtained with 14 lux of direct light transmission for the ambient light considered (i.e. 5.5% light transmission), while the second photograph of this sample is in the plane of a transverse cut (TC) obtained with 27 lux of direct light transmission for the ambient light considered (i.e. 11% light transmission).

[0336] These two photographs show that the composite wood is a structure in three dimensions, that is to say that whatever the cutting plane, the translucent character of the composite fir appears.

Example 3

Optical Evaluation of Composite Structures of White Pine, Pedunculate Oak, Mahogany, Tilia and Ash Resulting from a Treatment Process According to the Invention Obtained as in Example 1

[0337] The treatment process of Example 1 was reproduced on other kinds of wood, i.e. on samples forming composite structures of five kinds of wood: white pine, pedunculate oak, mahogany, tilia and ash. The optical properties of optical transmission of the samples of these five different kinds of woods were evaluated.

[0338] FIG. 23 reproduces eight photographs taken by Zeiss LSM710 Upright microscope with a ×20 lens of composite wood structures produced from these five kinds of wood.

[0339] The first photograph of the sample of white pine is in the plane of a longitudinal radial cut (LRC) obtained with 20 lux of direct light transmission for the ambient light considered (i.e. 8% light transmission), while the second photograph of this sample is in the plane of a transverse cut (TC) obtained with 65 lux of direct light transmission for the ambient light considered (i.e. 25% light transmission). The only photograph of the sample of pedunculate oak is the transverse cut (TC) obtained with 22 lux of direct light transmission for the ambient light considered (i.e. 9% light transmission).

[0340] The only photograph of the sample of mahogany is the transverse cut (TC) obtained with 33 lux of direct light transmission for the ambient light considered (i.e. 13% light transmission).

[0341] The first photograph of the sample of tilia is in the plane of a longitudinal radial cut (LRC) obtained with 30 lux of direct light transmission for the ambient light considered (i.e. 12% light transmission), while the second photograph of this sample is in the plane of a transverse cut (TC) obtained with 70 lux of direct light transmission for the ambient light considered (i.e. 30% light transmission).

[0342] The first photograph of the sample of ash is in the plane of a longitudinal radial cut (LRC) obtained with 11 lux of direct light transmission for the ambient light considered (i.e. 4.3% light transmission), while the second photograph of this sample is in the plane of a transverse cut (TC) obtained with 33 lux of direct light transmission for the ambient light considered (i.e. 13% light transmission).

[0343] In each case, these photographs show that the composite wood is a structure in three dimensions, that is to say that whatever the cutting plane, the translucent character of the composite wood appears.

[0344] It would be possible to notably improve the light transmission of the composite wood considered by refinement of the process by the person skilled in the art, in particular with regard to the control of the delignification, the depth of saturation/filling and the nature of the filling compound of which the refractive index once transformed must be practically homogenous with the refractive index of the composite lignocellulosic substrate.