METHOD FOR PRODUCING DENSIFIED CELLULOSIC COMPOSITE MATERIAL

Abstract

The present invention provides a method for obtaining densified material comprising the steps of

a. providing lignocellulosic material,

b. delignification of the lignocellulosic material providing a delignified material, wherein the delignification step is performed in such a way that the lignin of the lignocellulosic material is almost completely removed and wherein the structural integrity of the lignocellulosic material is maintained in the delignified material,

c. densification of the delignified material providing a densified material.

Furthermore, a densified material is provided. The fibers and fibrils are maintained in the structural directionality of the raw material and that the cellulosic material is whitish.

Claims

1. A method for obtaining densified material comprising the steps of a. providing lignocellulosic material, b. delignification of said lignocellulosic material providing a delignified material, wherein the delignification step is performed in such a way that the lignin of said lignocellulosic material is almost completely removed and wherein the structural integrity of said lignocellulosic material is maintained in said delignified material, c. densification of said delignified material providing a densified material.

2. The method according to claim 1, wherein the delignification step is performed for a certain time or repeatedly, in particular until the characteristic color of the lignocellulosic material is lost.

3. The method according to claim 1, wherein the delignification step comprises treating the lignocellulosic material with at least one acid, in particular an inorganic or organic acid, more particularly acetic acid, sulfuric acid, chloric acid, peracetic acid, or at least one oxidizing agent, in particular hydrogen peroxide, sodium chlorite, sodium sulfite, ozone, more particularly hydrogen peroxide, under alkaline or acidic conditions, in particular under acidic conditions or at least one base, in particular sodium hydroxide.

4. The method according to claim 1, wherein the delignification step comprises an incubation of the infiltrated lignocellulosic material at a temperature between 20 C. and 90 C., in particular between 60 C. to 90 C., more particularly between 75 C. and 85 C.

5. The method according to claim 1, wherein the densification is performed stepwise, in particular by applying a compression force in intervals until a predefined or the maximum degree of densification of the delignified material is obtained.

6. The method according to claim 1, wherein the densification is performed with additional lateral vibration, in particular with lateral vibration characterized by a frequency between 1 Hz and 1000 Hz.

7. The method according to claim 1, wherein the densification is performed in radial or tangential direction, in particular in radial direction.

8. The method according to claim 1, wherein at least two units of said delignified material are combined in a way that the fibers of said units are in parallel orientation or in various orientations to each other before the densification step is applied.

9. The method according to claim 1, wherein the lignocellulosic material is wood, in particular softwood or hardwood.

10. The method according to claim 1, wherein at least one resin, thermoset or thermoplastic in particular epoxide or thermoplastic suspension, is added after step b and before step c.

11. The method according to claim 1, wherein an additional modification step, in particular polymerization, mineralization, metallization or a combination thereof, is performed before and/or after step b.

12. A delignified material, in particular obtained by the method according to claim 1, characterized in that the cellulose fibers and microfibrils are maintained in the structural directionality of the lignocellulosic material and that the lignin of the lignocellulosic material is almost completely removed.

13. A densified material, in particular obtained by the method according to claim 1, characterized in that the cellulose fibers are maintained in the structural directionality of the lignocellulosic material, that the lignin of said lignocellulosic material is almost completely removed and that the density of the densified material is decreased compared to the delignified material, in particular the density of the densified material ranges from 400 to 1200 kg/m.sup.3, in particular 750 to 1150 kg/m.sup.3.

14. The densified material according to claim 13, wherein the elastic modulus ranges from 20 to 60 GPa, in particular from 40 to 50 GPa.

15. The densified material according to claim 13, wherein the tensile strength ranges from 100 to 300 N/mm.sup.2, in particular 225 to 275 N/mm.sup.2.

16. The densified material according to claim 14, wherein the tensile strength ranges from 100 to 300 N/mm.sup.2, in particular 225 to 275 N/mm.sup.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0085] FIG. 1 shows a schematic drawing of the densification protocol with a compressive force (F) and lateral shear (arrows). A delignified block (grey dotted area) is placed in a chamber that allows the efflux of water during densification. The delignified block is densified in lateral direction.

[0086] FIG. 2 shows stress strain curves of three delignified samples produced as described in Example 1

[0087] FIG. 3 shows a wood sample before treatment (left), a delignified sample (middle) and a delignified and densified sample (right). Samples were prepared as described in Example 2. The structural directionality of the cellulose fibers is maintained.

[0088] FIG. 4 shows twisted (a) and bent (b) samples. The upper samples shown in (a) and the samples shown in (b) are not densified. The lower sample shown in (a) has been densified. The fiber structure of the twisted and bent samples is maintained in all samples. Samples were treated as described in Example 2.

[0089] FIG. 5 shows delignified samples that have been gradually densified. Samples were treated as described in Example 2. The density decreases gradually from the thin to the thick end of the blocks, by applying different pressure along the block.

[0090] FIG. 6: Comparison of tensile properties of cellulose samples equilibrated at 65% relative humidity (RH), densified with or without additional shear; (a) stress strain curves of samples densified without shear loading (PO); (b) stress strain curves of samples densified with shear loading (PS); (c) Average elastic moduli (E) and specific elastic moduli (E_specific) for PO and PS samples +SD; (d) Average tensile strength () and specific tensile strength (_specific) for PO and PS samples +SD. Samples were prepared as described in Example 2.

[0091] FIG. 7 shows earlywood regions after densification without lateral shear forces (left image) and with lateral shear forces (right image). The densification with lateral shear forces results in a more compact packaging of the cellulose fibers compared to densification without lateral shear forces.

EXAMPLES

[0092] In the following, the general process for producing cellulose composite material including alternative or additional steps is described.

[0093] Material

[0094] Softwood or hardwood blocks with the dimensions 1001020 mm.sup.3 (longitudinalradialtangential).

[0095] Delignification

[0096] Delignification of wood samples can be achieved with a combination of acidic or alkaline and oxidation treatments in particular using hydrogen peroxide and acetic acid until a white color of the samples is obtained.

[0097] Unmodified samples had on average a density of 438 kg/m.sup.3, while samples after delignification treatments had an average density of 289 kg/m.sup.3, which is only 66% of the density of native unmodified material (34% less), which is due to a mass loss and not because of a volume increase. A reasonable figure for the lignin content of spruce is around 28%. Although one has to consider a partial removal of hemicelluloses and amorphous cellulose, one can conclude from these figures and the white color of the samples, that almost all lignin was removed by the treatment.

[0098] For further processing, the delignified wood can be kept at wet or at different relative humidity levels such as 65% or 95%, run through a solvent exchange cycle (e.g. ethanol, methanol) and/or be combined with a resin to obtain a compact fibre composite in the densification step.

[0099] Densification

[0100] The block of delignified material is densified in an apparatus, which restricts a lateral expansion. The densification is achieved by loading the sample from the top and a lateral shear movement (see FIG. 1, schematic drawing of the setup).

[0101] Depending on the sample treatment and the targeted sample dimensions it is possible to merge several smaller delignified blocks or sheets to one larger sample.

[0102] After the targeted state of densification is reached, the sample is allowed to dry, while the compressive force may be adjusted to still load the sample during shrinking of the sample.

[0103] For the densification, the following parameters may be chosen: The delignified wood sample is placed in a press mold which determines the final dimensions of the product. A fitting piston is used for the densification of the deliginfied wood sample. An additional lateral vibration is applied to build up a simultaneous shear force which allows for creating a tighter interaction between the cellulose fibres. Amplitude and frequency of the vibrational movement need to be adapted to the geometry of the sample. The vibrational densification takes place stepwise. After the first contact of the piston with the sample, a predefined distance for densification is covered by the piston. This is followed by a phase in which this distance is kept for a determined time period. This procedure is repeated until the specified sample thickness is reached. Finally the sample is allowed to dry, while the applied forces are kept (the piston follows the reduction of sample thickness, due to the shrinkage during drying).

Example 1: High-Density Densification of Wood Pieces in the Wet State

[0104] Material

[0105] Spruce wood blocks with the dimensions 1001020 mm.sup.3 (longitudinalradialtangential).

[0106] Delignification

[0107] Hydrogen peroxide (H.sub.2O.sub.2, 30% analytical grade, Merck) and acetic acid (99.8%, Sigma Aldrich) were mixed directly prior to the delignification at a volumetric ratio of 1:1. Wood bars were placed standing in a beaker and incubated in the acidic solution for 3 days and subsequently heated to 80 C. The delignification bath was kept at temperature for 12 h under stirring at 150 rpm before the wood was removed and intensively washed with deionized water. The washing procedure was continued for 5 days with 2 exchanges of the water per day before the subsequent densifying treatment.

[0108] Densification

[0109] The block of wet delignified material was densified in an apparatus, which restricts a lateral expansion. The densification is achieved by loading the sample from the top and a lateral shear movement (see FIG. 1, schematic drawing of the setup).

[0110] The delignified wood sample is placed in a press mold which determines the final dimensions of the product. A fitting piston is used for the densification of the deliginfied wood sample of up to 20 kN. An additional lateral vibration is applied to build up a simultaneous shear force which allows for creating a tighter interaction between the cellulose fibres. Amplitude and frequency of the vibrational movement need to be adapted to the geometry of the sample. The vibrational densification takes place stepwise. After the first contact of the piston with the sample, a predefined distance for densification is covered by the piston. This is followed by a phase in which this distance is kept for a determined time period. This procedure is repeated until the specified sample thickness is reached. Finally the sample is allowed to dry, while the applied forces are kept (the piston follows the reduction of sample thickness, due to the shrinkage during drying).

[0111] Tests on resin free sample revealed densities around 1150 kg/m.sup.3 and mechanical properties as follows: [0112] Sample 1: elastic modulus 47.0 GPa and a tensile strength of 250 N/mm.sup.2; [0113] Sample 2: elastic modulus 30.1 GPa and a tensile strength of 175 N/mm.sup.2; [0114] Sample 3: elastic modulus 31.6 GPa and a tensile strength of 111 N/mm.sup.2.

[0115] FIG. 2 shows stress-strain curves of the samples.

Example 2: Moderate-Density Densification of Lignification of Wood Pieces in Dry State (65% Relative Humidity)

[0116] Wood Samples

[0117] Wood pieces of Norway spruce (Picea abies) with the dimensions 1001020 mm.sup.3 (longitudinalradialtangential) were used. The pieces were stored at 20 C., 65% rel. humidity before treatment.

[0118] Delignification

[0119] Hydrogen peroxide (H.sub.2O.sub.2, 30% analytical grade, Merck) and acetic acid (99.8%, Sigma Aldrich) were mixed directly prior to the delignification at a volumetric ratio of 1:1. Wood bars were placed standing in a beaker and incubated in the acidic solution for 3 days and subsequently heated to 80 C. Wood pieces were infiltrated over night at RT under stirring at 150 rpm. The solution was then heated to 80 C. and the pieces were delignified for 6 h. The infiltration (over night) and delignification (6 h) steps were repeated once with a fresh peroxide-HAc solution. After delignification the samples were washed in water for 24 h. The washing solution was exchanged five times. After washing, samples were stored in climate cabinets until a constant mass was obtained. 3 different humidity conditions were used for densification: 20 C./65%, 20 C./95% and wet samples.

[0120] Densification of Delignified Wood Pieces

[0121] For densification the Zwick Roell 100 kN machine was used in compression mode. Delignified wood pieces were densified in radial direction in a mold (10020 mm.sup.2). The punch was pressed into the mold stepwise and position-controlled (1 mm compression, 15 s waiting time). The delignified wood pieces were compressed from an initial thickness of 10 mm down to 3 mm.

[0122] Shearing of the cellulose structure was obtained by applying a lateral vibrational movement to the punch during compression. The vibrational movement was induced by an air compressor gun (2 bar) connected to the punch.

[0123] After densification, the densified pieces were conditioned at 20 C./65%. While conditioning, the pieces were pressed with weights (15 kPa) to keep a pressure while shrinking of the samples.

[0124] In this example cellulose blocks were less strongly densified to reach a density around 750 kg/m.sup.3

[0125] Tensile Testing

[0126] Tensile properties of delignified and densified wood pieces were tested with the Zwick Roell 10 kN machine. The tested length was 25 mm. Strain was measured with a video extensiometer.

[0127] A comparison of mechanical properties of densified samples treated without additional shear and with additional shear indicate an increase in tensile strength due to shearing (FIG. 6), which might be further pronounced for higher shear loads and more strongly densified samples. The potential effect of the lateral shearing became also visible in light microscopy studies on the densified earlywood regions A comparison of densified samples treated without additional shear (FIG. 7, left image) and with additional shear (FIG. 7, right image) makes obvious, that the shearing can result in a beneficial stacking of the cellulose fibres. Higher shear forces may also lead to a fibrillation of the fibre's cell walls and an interlocking of cellulose microfibrils, which together with the stacking of fibers at a higher hierarchical level could further enhance the entanglement and lead to superior properties of the cellulose composites.