PROCESS FOR PRODUCING DENSIFIED WOOD PRODUCT Field of the invention The present invention is directed to a process for preparing a delignified and densified wood product. The wood is delignified in the form of veneers, using process conditions corresponding to the Kraft process. The wood veneers are subjected to a pre-hydrolysis treatment before the process steps corresponding to the Kraft process. The present invention also relates to a delignified and densified wood product produced using said process. The delignified and densified wood product according to the present invention has particularly advantageous strength properties. Background Cellulosic composite materials which possess exceptional mechanical properties and can be modified for new functionalities are highly desired. Untreated wood has a mainly parallel orientation of cellulose fibers. The cell wall of the wood fibers consists of lignin, cellulose and hemicelluloses. Cellulose fibers contribute to tensile strength, whereas lignin contributes to compressive strength and structural integrity. To maintain the structural integrity, densification processes described in the prior art are characterized by partial removal of lignin and replacement of lignin by resins. Typically, a mixture of hydrogen peroxide and acetic acid is used to achieve delignification. WO2018191181 discloses a strong and tough densified wood structure which is formed by subjecting a cellulose-based natural wood material to a chemical treatment that partially removes lignin therefrom. The treated wood is then pressed in a direction crossing the direction in which the lumina extend, such that the lumina collapse and any residual fluid within the wood is removed.
WO2018197222 discloses 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. The delignification is in particular carried out using hydrogen peroxide and acetic acid until a white color of the samples is obtained. Various possibilities for veneer delignification have been discussed in the literature (Frey, M. et al., 2019, J. Vis. Exp.153, 1-8). It would be desirable to be able to use methods suitable for industrial production in the preparation of densified and delignified wood. Further, improved mechanical properties of the densified and delignified wood products would be desirable. There is thus a need for an improved modified wood product. There is also a need for improved processes for preparing modified wood products. Summary of the invention In contrast to the methods described in the prior art, the present invention relates to the production of an almost lignin-free, highly densified cellulosic composite material, suitable for industrial production and having particularly advantageous mechanical properties. The present invention is directed to a process for preparing delignified and densified wood veneers comprising the steps of a) providing wood veneers; b) subjecting the wood veneers to pre-hydrolysis; c) subjecting the wood veneers from step b) to delignification using alkaline kraft cooking conditions;
d) densifying the delignified wood veneers; wherein the ISO Brightness of the delignified wood veneers obtained in step c) is the range of from 15% to 40% and wherein the Kappa number of the delignified wood veneers obtained in step c) is in the range of from 5 to 40. The present invention is also directed to a delignified and densified wood product obtained in the process according to the present invention. Detailed description The invention is directed to a process for preparing delignified and densified wood veneers comprising the steps of: a) providing wood veneers; b) subjecting the wood veneers to pre-hydrolysis; c) subjecting the wood veneers from step c) to delignification using alkaline kraft cooking conditions; d) densifying the delignified wood veneers; wherein the ISO Brightness of the delignified wood veneers obtained in step c) is the range of from 15% to 40% and wherein the Kappa number of the delignified wood veneers obtained in step c) is in the range of from 5 to 40. As used herein, the term “Kappa number” refers to measurement of Kappa number according to the standard ISO 302:2015. As used herein, the term “ISO Brightness” refers to measurement of brightness according to the ISO standard 2470. The wood veneers are prepared from wood using methods known in the art. Preferably, the wood veneers have a thickness of less than 10 mm, such as an average thickness in the range of from 1 mm to 8 mm or from 2 mm to 5 mm. The width of the veneer depends on the log used, such as from 20 cm to 3 m. The length of the veneer depends on the log used, but is typically from
20 cm to 10 m, such as from 20 cm to 3 m. In one embodiment, the veneers are prepared from never-dried wood. Typically, the wood has a dry content in the range of from 40% to 100%, such as from 40% to 90%, such as from 40% to 70% or from 40% to 65%. The wood can be any type of hardwood or softwood, such as, but not limited to, basswood, oak, poplar, ash, alder, aspen, balsa wood, beech, birch, cherry, butternut, chestnut, cocobolo, elm, hickory, maple, oak, padauk, plum, walnut, willow, yellow poplar, bald cypress, cedar, cypress, douglas fir, fir, hemlock, larch, pine, redwood, spruce, tamarack, juniper and yew. The wood can be a naturally occurring fibrous plant other than a tree, such as bamboo. Preferably, the wood veneers are from spruce and/or birch. The pre-hydrolysis step b) is performed by steaming the veneer or by immersing it into water, with or without acidic catalyst, and holding at 140-200 °C temperature for 15-240 minutes, such as for 30-240 minutes. The pre- hydrolysis makes the material more susceptible towards the following alkaline kraft cooking. During the pre-hydrolysis step the pH, when liquid is used, is preferably in the range of from 1 to 7, such as from 2 to 5. The pre-hydrolysis step, in combination with the subsequent delignification using alkaline kraft cooking conditions, reduces the amount of hemicellulose in the veneers. As the material is “activated” by the pre-hydrolysis stage the subsequent alkaline kraft cooking step can be milder and hence preserve cellulose fibers more. In one embodiment, the pre-hydrolysis is carried out in the presence of an acidic catalyst, in which case the pH is typically in the range of from 1 to 3. Examples of acidic catalysts include organic acids, mineral acids, sulphur dioxide and sulphuric acid. In one embodiment, the acidic catalyst is formic acid. If acidic catalyst is used, the amount of acidic catalyst, such as formic acid, in the pre-hydrolysis liquid is 5-20 g/l. The delignification step leads to significant reduction of lignin and hemicellulose content in the wood veneers, but only a modest reduction of cellulose content. By partial removal of lignin/hemicellulose from the cell walls, the wood becomes more porous and less rigid. The delignification is carried out using alkaline kraft conditions. More specifically, the veneers are treated with a mixture comprising water, sodium hydroxide and sodium sulfide. Chemicals such as surfactants, emulsion breakers, defoamers and
dispersing agents may also be used. Preferably, the temperature used in the delignification step is in the range of from 140°C to 200°C. The duration of the delignification step is typically in the range of from 30 minutes to 4 hours, such as from 1 hour to 3 hours. The alkali charge, or cooking chemical charge, is composed from 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 as NaOH), and it is in a range of 15-100%, more preferably 20-50%. The ISO Brightness of the delignified wood veneers is in the range of from 15% to 40%, preferably in the range of from 20% to 30%. The Kappa number of the delignified wood veneers is in the range of from 5 to 40, preferably in the range of from 5 to 25. When wood veneers from hardwood are used, the Kappa number of the delignified wood veneers is in the range of from 5 to 15. When wood veneers from softwood are used, the Kappa number of the delignified wood veneers is in the range of from 10 to 25. In comparison to prior art methods, the process according to the present invention maintains fiber length more efficiently compared to methods using hydrogen peroxide and acetic acid for the delignification. In one embodiment, a washing step is carried out between the delignification step and the densification step. The washing step is preferably carried out by washing the delignified veneers with water. The process according to the present invention can be carried out as a continuous process or batch-wise, preferably batch-wise. After the chemical treatment, the wood is subjected to densification, which comprises pressing in a direction perpendicular to a direction in which the cellulose-based structure extends (i.e., in a direction in which the tree grew and in which the lumina of the natural wood extend). For densification, it is preferred to apply both pressure and heat, since this combination will improve the densification of the wood. The densification may
be done off-line, on-line or in-line, i.e. in-line with the process according to the invention. The densification can be performed stepwise. The densification is performed by applying a compression force in intervals until a predefined or the maximum degree of densification of the delignified material is obtained. For example, the delignified multi-layer material may have a height of 10 mm and a height of 3 mm is targeted. The compression is performed stepwise, for example by seven cycles of 1 mm compression followed by 15 sec waiting time in which the compression force is not further increased. The compression force required for a reduction from a height of 10 mm to 9 mm may be less than the compression force required for the height reduction from 4 mm to 3 mm of the same material. 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 degree of compression already obtained, varying compression forces may be applied. Particularly, compression forces such as 2-20 MPa, for example 8-12 MPa are applied. The maximum densification is reached if the volume (lumina) of the cellulosic fibers is minimized and the contact surface between the cellulosic fibers is maximized and the pores in the delignified cell walls are compacted. The densification may for example be performed in repetitions of 1 mm compression, 15 sec waiting time. The densification is performed applying a vertical compression force and an additional lateral vibration that causes a lateral shear force. This vibration causes an entanglement of the cellulose fibrils and fibers. The entanglement is only possible if the lignin is sufficiently removed between the fibrils. The lateral vibration causes a more compact stacking of the cellulose fibers. The densification may be performed in radial or tangential direction. In one embodiment, at least two units of the delignified material are combined in a way that the fibers of the units are in parallel orientation or in various
orientations to each other before the densification step is applied. 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 that is comparable to laminated timber will be obtained upon densification. If several delignified units are combined in alternating layers in such a way that the cellulose fibers of one layer are aligned in one orientation and the cellulose fibers of the other layer are aligned in one orientation rotated, in particular by 90°, densified material with a layer orientation like in plywood will be obtained upon densification. Several delignified units may be combined in such a way that the cellulose fibers of the units are in various orientation to each other. A combination of several delignified units can also be used to increase the dimensions of the densified material by an assembly of smaller units of delignified material. In certain embodiments, at least one resin, thermoset or thermoplastic, such as an epoxy resin, is added to the delignified wood veneers before or after the densification step. The resin, thermoset or thermoplastic fills the spaces between the cellulose fibers and the cellulose fibrils that result from the lignin removal. In one embodiment, at least one epoxide or thermoplastic suspension is added to the delignified wood veneers before or after the densification step. 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 by using for example a thermoset or thermoplastic resin. The present invention is also directed to a delignified and densified wood product obtained in the process according to the present invention. The delignified material is characterized by cellulose fibers and microfibrils that are maintained in the structural directionality of the lignocellulosic material and that the lignin of the lignocellulosic material is almost completely removed.
The density of the densified material is increased compared to the delignified material before its densification. Preferably, the density of the densified material ranges from 400 to 1500 kg/m3, in particular 600 to 1500 kg/m3 or 800 to 1300 kg/m3. Preferably, the densified material has a density which 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 its densification. The delignified and densified wood has improved strength and toughness as compared to the natural wood prior to the treatment and pressing. Moreover, additional materials can be added to the wood, either before pressing or after pressing, in order to form a hybrid structure. The added materials can add functionality not otherwise available with the natural wood, for example, by providing hydrophobicity or fire resistance, while enjoying the improved mechanical performance offered by the densified wood after the chemical treatment and pressing. The present invention also relates to products comprising the veneers that have been delignified and densified according to the process described above. The products obtained can be used for the production of many different products, such as cladding, decking, window and door profiles, light poles, jetties, joinery, furniture, car interiors, wind mills etc. Examples Example 1 Spruce veneers with 3mm thickness were loaded into a cylindrical reactor in a holder allowing liquid transfer between individual veneers. Due to reactor design limitations the liquid to wood ratio was between 16-23:1 and the sulfidity of the cooking liquor Na2S/(Na2S+NaOH)*100% = 27%. Steaming phase was 30+50 min (heat-up + holding) at 170 °C (P-factor 550). Next, alkaline stage with EA 75% alkaline loading per wood was used for 30+90 min (heat-up + holding, H-factor 1600). After cooling the veneers were washed repeatedly with water until pH < 10 was achieved. The resulting air-
dried veneers showed 41% yield, Kappa number 18.9 and brightness 26.2 ISO indicating 90-92% of the material to be cellulose. The resulting dry and fragile material was rewetted with water spray from both sides until visually wet followed by 15 min stabilization. Veneer or veneers were then heated between the press without force between 50-90°C for 15 min. Next, 3MPa pressure was applied under same heating temperature for 60 min. Densified veneers were epoxy resin (EPIKOTE Resin MGS RIMR 235, EPIKOTE Curing agent MGS RIMH 238) treated in pressure cylinder at 112.5 torr for 30 min followed by pressure phase at 10 bar for 30 min. Next the veneers were laminated on a glass table in a reduced pressure bag where excess resin was removed and the layers were pressed together at 0.85 atm under 60°C heating for 24h. Alternatively, the composite sample can be post- treated in oven at 70 °C for 1h to post-harden the resin. The resulting composite material showed ultimate flexural strength 342 MPa, and flexural modulus of elasticity 30 GPa, Specific strength 0.34 (σM MAX) / (kg/m3), Specific modulus 0.033 (GPa MAX) / (kg/m3). Bending tests were done according to the ISO 14125 standard. Example 2 Spruce veneers with 3mm thickness were loaded into a cylindrical reactor in vertical position in a holder allowing liquid transfer between individual veneers. Size of the veneers was 350x140 mm allowing standard strength properties testing as composite. Due to reactor design limitations the liquid to wood ratio was 13:1 and the sulfidity of the cooking liquor Na2S/(Na2S+NaOH)*100% = 38%. Steaming phase was 30+15 min (heat-up + holding) at 170 °C leading to P-factor 200. Next, alkaline stage with EA 39% alkaline loading per wood was used for 35+95 min (heat-up + holding, H- factor 1600). After cooling the veneers were washed repeatedly in a bucket for 42h with water until pH < 10 was achieved. The resulting air-dried veneers showed 41.1% yield on wood, Kappa number 13.1 indicating 91-93% of the material to be cellulose. The resulting dry and fragile material was rewetted with water spray from both sides until visually wet followed by 15 min stabilization. Veneer or veneers
were then heated between the press without force between 50-90°C for 15 min. Next, 3 MPa pressure was applied in the hydraulic hot press under same heating temperature for 60 min. Densified veneers were epoxy resin (EPIKOTE Resin MGS RIMR 235, EPIKOTE Curing agent MGS RIMH 238) treated in pressure cylinder at 112.5 torr for 30 min followed by a pressure phase at 10 bar for 30 min. Next the veneers were laminated on a glass table in a reduced pressure bag where excess resin was removed and the layers were pressed together at 0.85 atm under 60°C heating for 24h. Alternatively, the composite sample can be post-treated in oven at 70 °C for 1h to post- harden the resin. The resulting composite material showed e.g. laminate density 1197 kg/m3, tensile strain 0.5%, ultimate flexural strength 250 MPa, and flexural modulus of elasticity 48 GPa, Specific strength 0.30 (σM MAX) / (kg/m3), Specific modulus 0.044 (GPa MAX) / (kg/m3). Tensile tests were done according to the ISO 527-1/4 standard. Bending tests were done according to the ISO 14125 standard. As a reference, a kraft cook without any pre-hydrolysis step prior to the cook, was carried out with EA 28% alkaline loading per wood and cooking time 35+120 min (heat-up + holding) resulting in H-factor 2400. Same white liquor was in use, and washing after cook was carried out similarly as in the example above, i.e. after cooling the veneers were washed repeatedly with water until pH < 10 was achieved. The resulting air-dried veneers showed 45.7% yield, Kappa number 40.4 indicating 86-88% of the material to be cellulose. The composite was manufactured as described above. Resulting composite material showed e.g. laminate density 1301 kg/m3, tensile strain 0.75%, ultimate flexural strength 273 MPa, and flexural modulus of elasticity 37 GPa, Specific strength 0.24 (σM MAX) / (kg/m3), Specific modulus 0.033 (GPa MAX) / (kg/m3). Tensile tests were done according to the ISO 527-1/4 standard. Bending tests were done according to the ISO 14125 standard. 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 should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.