EP-4735219-A1 - PROCESS FOR PRODUCING DENSIFIED WOOD PRODUCT

Abstract

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.

Inventors

• KAVAKKA, Jari
• KAHELIN, JUKKA

Assignees

• Stora Enso Oyj

Dates

Publication Date

20260506

Application Date

20240626

Claims (1)

1. Claims 1. 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 Kraft cooking process conditions; d) densifying the delignified wood veneers; wherein the ISO Brightness of the delignified wood veneers obtained in step c) is in 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. 2. A process according to claim 1, wherein the pre-hydrolysis is carried out by steaming the veneers for 15-180 minutes. 3. A process according to claim 2, wherein the pre-hydrolysis is carried out by steaming the veneers for 30-180 minutes. 4. A process according to claim 1, wherein the pre-hydrolysis is carried out by immersing the veneers into water at a temperature of 140-200 °C for 15-180 minutes. 5. A process according to claim 4, wherein the pre-hydrolysis is carried out by immersing the veneers into water at a temperature of 140-200 °C for 30-180 minutes. 6. A process according to any one of claims 2-5, wherein the pre- hydrolysis is carried out in the presence of acidic catalyst. 7. A process according to any one of claims 1-6, wherein the delignification step is carried out using white liquor and/or a mixture comprising water, sodium hydroxide and sodium sulfide. 8. A process according to claim 7, wherein the temperature in the delignification step is in the range of from 150°C to 200°C. 9. A process according to any one of claims 1-8, wherein the wood veneers are from hardwood. 10. A process according to any one of claims 1-8, wherein the wood veneers are from softwood. 11. A process according to any one of claims 1-10, wherein the density of the densified material is in the range of from 400 to 1500 kg/m 3 . 12. A process according to claim 11, wherein the density of the densified material is in the range of 750 to 1400 kg/m 3 . 13. Densified wood veneers obtained in the process according to any one of claims 1-12. 14. Use of densified wood veneers according to claim 13 in cladding, decking, window and door profiles, light poles, jetties, joinery, furniture, car interiors or wind mills.

Description

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 ven