Method for producing densified wood products

Abstract

The present invention relates to a method for producing a delignified and densified wood product. The wood is delignified in the form of a veneer using process conditions corresponding to the sulfate process. The wood veneer is subjected to a pre-hydrolysis treatment prior to a process step corresponding to the sulfate process. The invention also relates to a delignified and densified wood product produced using said method. The delignified and densified wood product according to the invention has particularly advantageous strength properties.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing a delignified and densified wood product. The wood is delignified in the form of veneer using process conditions corresponding to the Kraft process. Prior to the process steps corresponding to the Kraft process, the wood veneer is subjected to a pre-hydrolysis treatment. The present invention also relates to a delignified and densified wood product produced using the method. The delignified and densified wood product according to the present invention has particularly advantageous strength properties. BACKGROUND

[0002] It is highly desirable to have cellulose composites with excellent mechanical properties and which can be modified to obtain new functionalities.

[0003] Untreated wood has cellulose fibres with a predominantly parallel orientation. The cell wall of the wood fibres consists of lignin, cellulose and hemicellulose. The cellulose fibres contribute to the tensile strength, while the lignin contributes to the compressive strength and structural integrity. In order to maintain the structural integrity, the densification methods described in the prior art are characterised by partial removal of the lignin and replacement of the lignin with a resin. Typically, a mixture of hydrogen peroxide and acetic acid is used to achieve delignification.

[0004] WO2018191181 discloses a strong and tough densified wood structure formed by subjecting natural wood based on cellulose to a chemical treatment which partially removes lignin therefrom. The treated wood is then pressed in a direction transverse to the direction of extension of the lumina, such that the lumina collapse and any residual fluid within the wood is removed.

[0005] WO2018197222 discloses a method for obtaining a densified material comprising the steps of: a) providing a lignocellulosic material, b) delignifying the lignocellulosic material, providing a delignified material, wherein the delignification step is carried out 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) densifying the delignified material, providing a densified material. In particular, the delignification is carried out using hydrogen peroxide and acetic acid until a white sample is obtained.

[0006] Various possibilities for delignification of veneer have been discussed in the literature (Frey, M. et al., 2019, J. Vis. Exp. 153, 1-8).

[0007] It would be desirable to be able to use a process suitable for industrial production in the production of densified and delignified wood. Furthermore, it would be desirable to have improved mechanical properties of the densified and delignified wood product.

[0008] Thus, there is a need for an improved modified wood product. There is also a need for an improved method for preparing a modified wood product. SUMMARY

[0009] In contrast to the methods described in the prior art, the present invention relates to the production of highly densified cellulose composites which are almost free of lignin, which are suitable for industrial production and which have particularly advantageous mechanical properties.

[0010] The present invention relates to a method of preparing a delignified and densified wood veneer, comprising the following steps:

[0011] a) providing a wood veneer;

[0012] b) subjecting the wood veneer to prehydrolysis;

[0013] c) subjecting the wood veneer from step b) to delignification using alkaline sulphate cooking conditions;

[0014] d) densifying the delignified wood veneer;

[0015] wherein the ISO brightness of the delignified wood veneer obtained in step c) is in the range of 15% to 40% and wherein the Kappa value of the delignified wood veneer obtained in step c) is in the range of 5 to 40.

[0016] The present invention also relates to a delignified and densified wood product obtained in the method according to the present invention. DETAILED DESCRIPTION

[0017] The present invention relates to a method of preparing a delignified and densified wood veneer, comprising the following steps:

[0018] a) providing a wood veneer;

[0019] b) subjecting the wood veneer to prehydrolysis;

[0020] c) subjecting the wood veneer from step c) to delignification using alkaline sulphate cooking conditions;

[0021] d) densifying the delignified wood veneer;

[0022] wherein the ISO brightness of the delignified wood veneer obtained in step c) is in the range of 15% to 40% and wherein the Kappa value of the delignified wood veneer obtained in step c) is in the range of 5 to 40.

[0023] As used herein, the term "Kappa value" refers to the measurement of the Kappa value according to standard ISO 302:2015.

[0024] As used herein, the term "ISO brightness" refers to a measurement of brightness according to ISO standard 2470.

[0025] The wood veneer is prepared from wood using methods known in the art. Preferably, the wood veneer has a thickness of less than 10 mm, for example an average thickness in the range of 1 mm to 8 mm or 2 mm to 5 mm. The width of the veneer depends on the log used, for example 20 cm to 3 m. The length of the veneer depends on the log used, but is typically 20 cm to 10 m, for example 20 cm to 3 m. In one embodiment, the veneer is prepared from never-dried wood. Typically, the wood has a dry content in the range of 40% to 100%, for example 40% to 90%, for example 40% to 70% or 40% to 65%.

[0026] The wood can be any type of hardwood or softwood, for example but not limited to, basswood, oak, poplar, ash, alder, aspen, balsa, beech, birch, cherry, hickory, chestnut, rosewood, elm, pecan, maple, oak, rosewood, plum, walnut, willow, boxwood, bald cypress, cedar, Douglas fir, fir, hemlock, larch, pine, mahogany, spruce, larch, juniper, and yew. The wood can be a naturally occurring fibrous plant other than a tree, for example bamboo. Preferably, the wood veneer is from spruce and / or birch.

[0027] The pre-hydrolysis step b) is performed by steaming the veneer with or without an acidic catalyst or by immersing it in water and holding at a temperature of 140-200 °C for 15-240 minutes, for example 30-240 minutes. The pre-hydrolysis makes the material more susceptible to the subsequent alkaline sulphate cooking. During the pre-hydrolysis step, when a liquid is used, the pH is preferably in the range of 1 to 7, for example 2 to 5. The pre-hydrolysis step in combination with the subsequent delignification using alkaline sulphate cooking conditions reduces the amount of hemicellulose in the veneer. As the material is "activated" by the pre-hydrolysis stage, the subsequent alkaline sulphate cooking step can be milder, thus more of the cellulose fibres are preserved. In one embodiment, the pre-hydrolysis is performed in the presence of an acidic catalyst, in which case the pH is typically in the range of 1 to 3. Examples of acidic catalysts include organic acids, inorganic acids, sulphur dioxide and sulphuric acid. In one embodiment, the acidic catalyst is formic acid. If an acidic catalyst is used, the amount of acidic catalyst, for example formic acid, in the pre-hydrolysis liquor is 5-20 g / l.

[0028] The delignification step results in a significant reduction of the lignin and hemicellulose content in the wood veneer, but only a moderate reduction of the cellulose content. By removing the lignin / hemicellulose from the cell wall fraction, the wood becomes more porous and less stiff. The delignification is performed using alkaline sulphate conditions. More specifically, the veneer is treated with a mixture comprising water, sodium hydroxide and sodium sulphide. Chemicals such as surfactants, demulsifiers, antifoams and dispersants can also be used. Preferably, the temperature used in the delignification step is in the range of 140°C to 200°C. The duration of the delignification step is typically in the range of 30 minutes to 4 hours, such as 1 hour to 3 hours. The alkali charge or cooking chemical charge consists of white liquor and black liquor. The charge of white liquor is expressed as effective alkali (NaOH + ½ Na2S) based on oven dry wood (EA% / OD wood, based on NaOH), and it is in the range of 15% to 100%, more preferably 20% to 50%.

[0029] The ISO brightness of the delignified wood veneer is in the range of 15% to 40%, preferably in the range of 20% to 30%.

[0030] The Kappa number of the delignified wood veneer is in the range of 5 to 40, preferably in the range of 5 to 25. When using wood veneer from hardwood, the Kappa number of the delignified wood veneer is in the range of 5 to 15. When using wood veneer from softwood, the Kappa number of the delignified wood veneer is in the range of 10 to 25.

[0031] The method according to the present application is more effective in preserving the fiber length compared to a method using hydrogen peroxide and acetic acid for delignification compared to prior art methods.

[0032] In one embodiment, a washing step is performed between the delignification step and the densification step. The washing step is preferably performed by washing the delignified veneer with water.

[0033] The method according to the present application can be performed as a continuous method or batch-wise, preferably batch-wise.

[0034] After the chemical treatment, the wood is densified, which comprises pressing in a direction perpendicular to the direction in which the cellulose-based structures extend, i.e. in the direction in which the tree grew and the cavities of the natural wood extend.

[0035] For the densification, it is preferred to apply pressure and heat simultaneously, as this combination will improve the densification of the wood. The densification can be performed off-line, on-line or in-line, i.e. in-line with the method of the present application.

[0036] The densification can be performed stepwise. The densification is performed by applying a compression force at intervals until a predetermined or maximum degree of densification of the delignified material is obtained. For example, a delignified multi-layer material can have a height of 10 mm and the goal is a height of 3 mm. The compression is performed stepwise, for example by 7 cycles of 1 mm compression followed by a waiting time of 15 sec, wherein the compression force is not further increased. The compression force needed to reduce the height from 10 mm to 9 mm can be less than the compression force needed to reduce the height of the same material from 4 mm to 3 mm. This can be explained by the stronger resistance to compression of the 4 mm thick partially densified material compared to the 10 mm thick material without densification. Depending on the material and the degree of compression already obtained, varying compression forces can be applied.

[0037] In particular, a compression force of 2-20 MPa, for example 8-12 MPa, is applied.

[0038] If the volume (cavity) of the cellulose fibers is minimized and the contact surface between the cellulose fibers is maximized and the pores in the delignified cell wall are compacted, a maximum densification is achieved. The densification can be performed for example in repetitions of 1 mm compression, 15 sec waiting time.

[0039] The densification is performed by applying a vertical compression force and additional lateral vibrations causing a lateral shear force. This vibration causes entanglement of the cellulose fibrils and fibers. Entanglement is only possible if the lignin between the fibrils is sufficiently removed. The lateral vibrations result in a more compact stacking of the cellulose fibers.

[0040] The densification can be performed in a radial or tangential direction.

[0041] In one embodiment, at least two units of delignified material are combined together before the densification step is applied, such that the fibers of the units are oriented parallel to each other or differently oriented. If several delignified units are combined in such a way that the cellulose fibers of all units are aligned in one orientation, a densified material comparable to laminated timber will be obtained upon densification.

[0042] If several delignified units are combined in the way of alternating layers such that the cellulose fibers of one layer are aligned along one orientation and the cellulose fibers of another layer are aligned along a rotated orientation, in particular rotated by 90°, a densified material with layer orientation as in plywood will be obtained upon densification.

[0043] Several delignified units can be combined in such a way that the cellulose fibers of the units are in various orientations to each other. The combination of several delignified units can also be used to increase the size of the densified material by assembling smaller units of delignified material.

[0044] In certain embodiments, at least one thermoset resin or thermoplastic resin, such as an epoxy resin, is added to the delignified wood veneer before or after the densification step. The thermoset resin or thermoplastic resin fills the space between the cellulose fibers and cellulose fibrils created by the removal of lignin.

[0045] In one embodiment, at least one epoxide or thermoplastic material suspension is added to the delignified wood veneer before or after the densification step.

[0046] In one embodiment, a composite of multiple delignified and densified wood veneers can be prepared by attaching the multiple delignified and densified wood veneers to each other using, for example, a thermoset resin or thermoplastic resin.

[0047] The present invention also relates to a delignified and densified wood product obtained in the method according to the present invention. The delignified material is characterized in that the cellulose fibers and microfibers retain the structural directionality of the lignocellulosic material, and the lignin of the lignocellulosic material is almost completely removed.

[0048] The density of the densified material is increased compared to the delignified material before densification. Preferably, the density of the densified material ranges from 400 to 1500 kg / m 3 , in particular from 600 to 1500 kg / m 3 or from 800 to 1300 kg / m 3 . Preferably, the density of the densified material is at least 20% higher, preferably at least 60% higher, more preferably at least 80% higher, most preferably at least 100% higher than the material before it was densified.

[0049] The delignified and densified wood has improved strength and toughness compared to the natural wood before treatment and pressing. Furthermore, additional materials can be added to the wood before pressing or after pressing to form a hybrid structure. The added materials can increase functionality that the natural wood could not otherwise obtain, for example, by providing hydrophobicity or fire resistance, while enjoying the improved mechanical properties provided by the densified wood after chemical treatment and pressing.

[0050] The present invention also relates to products comprising veneers that have been delignified and densified according to the above method. The resulting products can be used to produce many different products, such as cladding, decking, window and door profiles, light poles, jetties, joinery, furniture, automotive interiors, windmills, etc.

[0051] Example

[0052] Example 1

[0053] Thick spruce veneers (3 mm thick) are loaded into a cylindrical reactor within a retainer, allowing liquid to transfer between the individual veneers. Due to reactor design limitations, the liquid-to-wood ratio is between 16 and 23:1, and the sulfidation degree of the cooking liquor is Na₂S / (Na₂S+NaOH). 100% = 27%. The steaming stage was performed at 170°C for 30 ± 50 min (heating + holding) (P factor 550). Next, an alkaline stage with an EA 75% alkali load / wood was performed for 30 ± 90 min (heating + holding, H factor 1600). After cooling, the veneer was repeatedly washed with water until pH < 10. The resulting air-dried veneer showed a yield of 41%, a Kappa value of 18.9, and a brightness of 26.2 ISO, indicating that 90-92% of the material was cellulose.

[0054] The resulting dry and brittle material was re-wetted from both sides by water spray until visually wetted, and then stabilized for 15 min. One or more veneers were then heated between 50-90°C for 15 min under no force in a press. Next, a pressure of 3 MPa was applied for 60 min at the same heating temperature. The densified veneers were treated with epoxy resin (EPIKOTE resin MGS RIMR 235, EPIKOTE curing agent MGS RIMH 238) in a 112.5 Torr pressure cylinder for 30 min, followed by a 30 min treatment at a pressure stage of 10 bar. Next, the veneers were laminated on a glass stage in a depressurized bag, where excess resin was removed, and the layers were pressed together at 0.85 atm and heated at 60°C for 24 h. Alternatively, composite samples could be post-treated in an oven at 70°C for 1 h to allow the resin to post-cur. The resulting composite exhibited an ultimate flexural strength of 342 MPa, a flexural modulus of 30 GPa, and a specific strength of 0.34 (σ). M MAX) / (kg / m 3 The specific modulus is 0.033 (GPa MAX) / (kg / m³). 3 Bending tests were conducted according to ISO 14125 standard.

[0055] Example 2

[0056] Thick spruce veneers (3 mm thick) were loaded into a cylindrical reactor positioned vertically within a retainer, allowing liquid transfer between the individual veneers. The veneers, measuring 350 × 140 mm, allowed for standard strength property testing as a composite material. Due to reactor design limitations, the liquid-to-wood ratio was 13:1, and the sulfurization of the cooking liquor was Na₂S / (Na₂S+NaOH). 100% = 38%. The steaming stage was performed at 170°C for 30 ± 15 min (heating + holding), resulting in a P factor of 200. Next, an alkaline stage with an EA of 39% alkali load / wood was used for 35 ± 95 min (heating + holding, H factor 1600). After cooling, the veneer was repeatedly washed with water in a barrel for 42 h until pH < 10 was achieved. The resulting air-dried plywood showed a wood yield of 41.1% and a Kappa value of 13.1, indicating that 91–93% of the material was cellulose.

[0057] The resulting dry and brittle material was re-wetted from both sides by water spray until visually wetted, and then stabilized for 15 min. One or more veneers were then heated between 50-90°C for 15 min under no force in a press. Next, a pressure of 3 MPa was applied for 60 min in a hydraulic hot press at the same heating temperature. The densified veneers were treated with epoxy resin (EPIKOTE resin MGS RIMR 235, EPIKOTE curing agent MGS RIMH 238) in a 112.5 Torr pressure cylinder for 30 min, followed by a 30 min treatment at a pressure stage of 10 bar. The veneers were then laminated on a glass stage in a depressurized bag, where excess resin was removed, and the layers were pressed together at 0.85 atm and heated at 60°C for 24 h. Alternatively, composite samples could be post-treated in an oven at 70°C for 1 h to allow the resin to post-cur. The resulting composite showed, for example, a lamination density of 1197 kg / m³. 3 The tensile strain is 0.5%, the ultimate flexural strength is 250 MPa, the flexural modulus is 48 GPa, and the specific strength is 0.30 (σ). M MAX) / (kg / m 3 ), Specific modulus 0.044 (GPa MAX) / (kg / m³) 3 Tensile testing was performed according to ISO 527-1 / 4. Bending testing was performed according to ISO 14125.

[0058] For reference, sulfate cooking without any pre-hydrolysis step prior to cooking was carried out with EA 28% alkali load / wood and cooking time of 35 ± 120 min (heating + holding), yielding an H factor of 2400. The same white liquor was used, and post-cooking washing was performed similarly to the examples above, i.e., after cooling, the veneer was repeatedly washed with water until the pH reached <10. The resulting air-dried veneer showed a yield of 45.7% and a Kappa value of 40.4, indicating that 86-88% of the material was cellulose.

[0059] The composite material was manufactured as described above. The resulting composite material exhibited, for example, a lamination density of 1301 kg / m³. 3 The tensile strain is 0.75%, the ultimate flexural strength is 273 MPa, the flexural modulus is 37 GPa, and the specific strength is 0.24 (σ). M MAX) / (kg / m 3 ), Specific modulus 0.033 (GPa MAX) / (kg / m³) 3 Tensile testing was performed according to ISO 527-1 / 4. Bending testing was performed according to ISO 14125.

[0060] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it will be apparent that such other modifications and variations can be made without departing from the spirit and scope of the present invention.

Claims

1. Method of producing delignified and densified wood veneer, comprising the steps of: a) providing a wood veneer; b) subjecting the wood veneer to pre-hydrolysis; c) subjecting the wood veneer from step b) to delignification using Kraft cooking process conditions; d) densifying the delignified wood veneer; wherein the ISO brightness of the delignified wood veneer obtained in step c) is in the range of 15% to 40% and wherein the Kappa number of the delignified wood veneer obtained in step c) is in the range of 5 to 40.

2. The method according to claim 1, wherein the pre-hydrolysis is performed by steaming the veneer for 15-180 minutes.

3. The method according to claim 2, wherein the pre-hydrolysis is performed by steaming the veneer for 30-180 minutes.

4. The method according to claim 1, wherein the pre-hydrolysis is performed by immersing the veneer in water at a temperature of 140-200 °C for 15-180 minutes.

5. The method according to claim 4, wherein the pre-hydrolysis is performed by immersing the veneer in water at a temperature of 140-200 °C for 30-180 minutes.

6. The method according to any one of claims 2-5, wherein the pre-hydrolysis is performed in the presence of an acidic catalyst.

7. The method according to any one of claims 1-6, wherein the delignification step is performed using white liquor and / or a mixture comprising water, sodium hydroxide and sodium sulfide.

8. The method according to claim 7, wherein the temperature in the delignification step is in the range of 150 °C to 200 °C.

9. The method according to any one of claims 1-8, wherein the wood veneer is from a hardwood.

10. The method according to any one of claims 1-8, wherein the wood veneer is from a softwood.

11. The method according to any one of claims 1-10, wherein the density of the densified material is in the range of 400 to 1500 kg / m 3 .

12. The method of claim 11, wherein the densified material has a density in the range of 750 to 1400 kg / m 3 .

13. Densified wood veneer obtained in the method according to any one of claims 1-12.

14. Use of the densified wood veneer according to claim 13 in cladding, deck, window and door profiles, lamp posts, breakwaters, joinery, furniture, automotive interiors or windmills.