[0001] The present disclosure concerns lightweight polyurethane wood composites and method for manufacturing lightweight polyurethane wood composites in particular panels/boards for furniture, comprising at least 10 % (w/w) of a foamable polyurethane, and at least 40 % (w/w) of wood particles/fibers or other lignocellulosic material in particle form obtainable by sieving, wherein the sieve hole is no more than 7 mm inclusive. BACKGROUND ART

[0002] For many years, the furniture industry used conventional wood-panels, such as particleboards, medium density fiberboard, plywood, etc., in the manufacture of furniture. The conventional particleboards have a density of about 550 - 900 kg/m3 that is responsible for several problems, such as: design limitation for high thickness elements, impracticality for the consumer to handle, transport and move furniture, high transportation costs both for consumers and industry.

[0003] Conventional wood-based particleboards are formed by three layers of wood particles (wood flakes, chips, shavings, sawdust and similar) and/or other lignocellulosic material in particle form (flax shives, hemp shives, bagasse fragments and similar) with the addition of an adhesive. The external face layers are composed by smaller particles, while the core is composed by larger particles. For producing the particleboards (or standard panels), wood particles are blended with the adhesive system normally composed by a urea-formaldehyde (UF) resin, paraffin, catalyst and water, in a blender. The particleboards (standard panels) are then pressed until reaching a final thickness between 3 mm and 60 mm in a hot-press, at temperature between 150 ^e C and 230 ^e C and for time between 70 and 250 s. This pressing stage enables the cure of the UF-resin which glues the wood particles. The final particleboards (standard panels) are characterized according to European standards for density (EN 323) and internal bond strength (EN 319). Depending on the size and shape of particles they are classified as, e.g., chipboard, panels with other particles, e.g. shives (flaxboard). According to the board’s structure they are classified as: single-layer boards; multi-layer boards; boards with gradient structure; extruded boards with hollow tubes.

[0004] Nowadays, the furniture industry worldwide has a high demand for wood-based materials, in particular for wood-based materials with low density, below 600 kg/m3 and with the appropriated mechanical features to enable the use on the furniture industry. Therefore, there is an increasing interest for lightweight panels. [0005] The lightweight particleboards are useful substitutes of the conventional wood- based particleboards, and their density is restricted, by a specific European standard (CEN/TS 16368:2014), to values below 600 kg/m3 instead of 550 - 900 kg/m3 of the conventional wood- based particleboards. [0006] Numerous approaches have been made to obtain lightweight panels. These approaches include using low density wood species (poplar) or other plant fibers (e.g. hemp), sandwich panels with foam or honeycomb core, or by incorporating polymeric particles during wood-based panels’ production. However, the reduced weight comes along with a decline of the mechanical properties, namely: bending strength, internal bond strength, resistance to screws withdrawal and machinability, and the increase in the porous edges with decrease in the resistance to axial withdrawal of edge screws, the decrease on edge machinability and edge banding ability.

[0007] The document US8304069 B2 is one of the attempts above-mentioned to obtain lightweight panels. This document discloses a light wood-based material comprising a density of less or equal to 600 kg/m3 and wood particles, a filler, a binder and preferably an antistatic coating for the filler before mixing it with the binder and/or the wood particles. Furthermore, the process herein unveiled involves the use of a pre-foamed solid polymer (polystyrene and/or styrene copolymers), which are mixed and molded at high temperature (150 ^e C to 230 ^e C) to give a wood-based material. Although, in one of the examples disclosed, the polyurethane is used as filler and the light wood-base material complies with the European standard for lightweight particleboards (CEN/TS 16368:2014), the internal bond strength is only of 0.19 [0008] The document US5554429 A is also an attempt to obtain polyurethane-wood composite panels, although being based on oriented strand boards (OSB), and not particleboards as in the present invention, and being directed towards flooring applications. In this disclosure, the inventors claim the production of oriented strand boards (OSB) by using a mixture of a foaming resin (e.g. polyurethane) and a non- foaming resin (e.g. urea resin, phenol resin). For pressing the panels up to the final thickness of 12 mm, high temperature is applied (210 ^e C or 160 ^e C). The final panels have densities slightly lower than 600 kg/m3 and great values of flexural strength. The process now disclosed has the advantage of being wholly operated at room temperature. In addition, the panels have mechanical values that are in accordance to the standard requirements for lightweight wood-based particleboards, even for densities values lower than 400 kg/m3. [0009] The current available strategies do not fulfil the desirable purposes for the lightweight panels: appropriated mechanical properties, namely bending strength, tensile strength perpendicular to the plane of the board (internal bond strength), and resistance to axial withdrawal of screws. As a consequence of the above-mentioned drawbacks, the lightweight panels already in the market are used for insulation and non- structural applications as the easier performed objectives, rather than as furniture, furniture components such as tables, chairs, bookshelves, kitchen worktops doors or ceiling panels and/or in the construction industry. The only exception are honeycomb core boards which are used in the furniture industry but impose many design restrictions.

[0010] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

[001 1] The furniture industry worldwide is continuously looking for strategies to reduce the weight of its products without disregarding the design of the products.

[0012] The growing importance of the takeaway furniture segment market worldwide and the use of attractive thick furniture pieces give rise to new demands, by the final consumer, on lighter furniture pieces, easy to handle, unpack, assemble and that having reduced transportation costs, and led to an increased pressure on reducing panels weight. For the furniture industry, the development of lightweight panels is also important as it allows to have cheaper raw material, have lower selling prices and higher profit margins as compared to pieces of furniture produced using conventional materials. There is also a need for the decrease of transportation costs, which are achievable either by reduction the weight of the total cargo or by increasing the number furniture units transported in each one (unassembled furniture).

[0013] The manufacture of lightweight panels has also environmental consequences since the manufacture of lightweight panels, can replace low density solid wood, and requires less particles less raw material, and, another environmental impact, is the reducing the fuel demands for transportation and the correspondent decrease of polluting gases emissions.

[0014] Therefore, lightweight panels have also a marketing effect on both the fu rniture industry and the lightweight panels manufactures.

[0015] Nevertheless, none of the approaches previously attempted to produce lightweight panels were able to produce lightweight panels with suitable and stable mechanical properties as desirable for furniture production such as, resistance to bending stress, tensile stress and screw extraction.

SUMMARY OF THE INVENTION

[0016] The present disclosure regards to lightweight panels with a polyurethane-wood composite core, from now on named lightweight polyurethane-wood composites and manufacture thereof. This disclosure overcomes the disadvantages of the current available solutions for the furniture industry previously listed.

[0017] The lightweight polyurethane-wood composites for panels/boards may also be applied to the construction industry as it can be used as building materials, for laminate floors, walls sheathings, doors, furniture and sound and/or temperature insulation. [0018] The panels/boards are characterized according to European standards for density (EN 323) and internal bond strength (EN 319). Depending on the size and shape of particles they are classified as e.g., chipboard, panels with other particles, e.g. shives (flaxboard). According to the board’s structure they are classified as: single-layer boards; multi-layer boards; boards with graduated structure; extruded boards with tubes.

[0019] The present disclosure relates to a lightweight composite for panels/boards, in particular panels/boards for furniture, comprising at least 10 % (w/w) of a foamable polyurethane , and at least 40 % (w/w) of wood particles/fibers or other lignocellulosic material in particle form obtainable by sieving, wherein the sieve hole is no more than 7 mm inclusive.

In a preferred embodiment the lightweight composite of the present invention comprising at least 10 % (w/w) of a foamable polyurethane, and at least 40 % (w/w) of wood particles/fibers or other lignocellulosic material in particle form obtainable by sieving.

[0020] In other embodiment, the particles/fibers of wood or other lignocellulosic size is between 0.18 to 7 mm preferably between 2-5 mm. To achieved said size the sieve hole can be from 0.18 to 7 mm, preferably between 2-5 mm.

[0021 ] In other embodiment, the composite comprises between 30 - 50 % (w/w) of foamable polyurethane and 50 - 30 % (w/w) of wood particles.

[0022The composite of the present invention has the following advantages:

the internal bond strength of the composite is 0.20 - 0.55 N/mm2;

the density at 20 °C of the composite is 300 - 465 kg/m3;

the resistance to axial withdrawal of screws of the composite is 200 - 550 N.

This behavior is explained by the formation of a strong covalent network between the wood particles and the cured polyurethane polymer, providing an exceptional mechanical performance to these lightweight panels, while the foamed structure and low wood content provide the low density. [0023] In other embodiment of the composite, the foamable polyurethane have an intrinsic density at 20 °C of 25 kg/m3.

[0024] In an embodiment, wherein the disclosed composite is coated with external layers, such as a wood veneer, a laminate based on resin impregnated papers, or a melamine-impregnated decor paper, among others.

[0025] The present subject-matter also relates to articles comprising the composite described previously, wherein said article is a board, a panel, a layer or a furniture. Furthermore, said furniture is a table, a chair, a bookshelf, a door, a kitchen worktop door, ceiling panel, laminate floor, a wall sheathing or sound and/or temperature insulation.

[0026] In an embodiment, the thickness of the board/panel is 10 - 60 mm, preferably 32 mm.

[0027] The present disclosure also relates to a method to produce a lightweight composite characterized in that it comprises the following steps:

a) sieving particles/fibers of wood or other lignocellulosic material in particle form wherein the sieve hole is no more than 7 mm inclusive;

b) mixing the wood particles obtainable by sieving in step (a) with at least 10 % (w/w) of a foamable polyurethane;

c) pouring the obtained mixture of step (b) into a closed mold at 20-25 °C; and d) wherein the molding step of the mixture of step (c) is performed for at least 5 or 30 minutes, preferably for at least 15 minutes at 20-25 °C

and followed by a second molding step of 5-30 minutes, preferably of 15 minutes, at a temperature of 80 °C;

or

molding the mixture of step (c) is performed at 20-25 ^e C for 1 - 3 hours, preferably 2 hours.

[0028] In a preferred embodiment of the method, the step (b) is mixing the wood particles obtainable by sieving in step (a) with foamable polyurethane. The sieved particles in combination with polyurethane form an adhesive system which guarantees the low density of the product and good cohesion between the wood particles and the foam. The foam is produced during the panel manufacture, providing separation of the wood particles, while simultaneously adhering to them.

[0029] In other embodiment, the average wood particles size or other lignocellulosic material in particle form is between 0.18 mm and 5 mm, preferably between 2 mm and 5 or between 0.18 mm and 2.

[0030] Other aspect of the present invention relates to the method to produce a panel or a board comprising a lightweight composite obtained by the method described before characterized in that it comprises applying at least one external layer to the lightweight composite with an adhesive. In a preferred embodiment, the adhesive is a resin or a polyurethane monocomponent adhesive, in particular urea formaldehyde. In a preferred embodiment the external layer is selected from a wood veneer, a laminate based on resin impregnated papers, and a melamine-impregnated decor paper.

[0031 ] Other aspect of the present invention relates to a panel or a board obtained by the method described before characterized in that it comprises lightweight composite described before and an external layer selected from a wood veneer, a laminate based on resin impregnated papers, and a melamine-impregnated decor paper.

[0032] Other aspect of the present invention relates to a method for manufacturing a furniture characterized in that it comprises the following steps:

a) providing a panel or a board obtained according to claim 7, said panel comprising at least 10% (w/w) of a foamable polyurethane, and at least 40% (w/w) of wood particles/fibers or other lignocellulosic material in particle form obtainable by sieving, wherein the sieve opening is no more than 7 mm inclusive; and

b) coating said panel with external layers selected from a wood veneer, a laminate based on resin impregnated papers, and a melamine-impregnated decor paper. [0033] This disclosure also relates to the use of lightweight composites described before for furniture, furniture components and construction materials, wherein the furniture, furniture components and construction materials are tables, chairs, bookshelves, doors, kitchen worktop doors, ceiling panels, laminate floors, walls sheathings or sound and/or temperature insulation.

[0034] The disclosure also relates to Furniture comprising at least a panel or a board described before. In a preferred embodiment the furniture is a table, a chair, bookshelves, a door, a kitchen worktop, ceiling panel, laminate floor, a wall sheathing or sound and/or temperature insulation.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention.

[0036] Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples, are provided by way of illustration and are not intended to be limiting of the present invention.

DETAILED DESCRIPTION

[0037] The present disclosure relates to lightweight panels with a polyurethane-wood composite and the development of a process for their manufacture.

[0038] Several approaches were attempted to produce lightweight panels with a polyurethane-wood composite. These approaches are now disclosed in detail.

[0039] The first approach to produce lightweight panels did not involve the use of polyurethane. The panels produced by this approach were composed by three layers of wood particles that had an irregular morphology, mostly pine wood, with a dry density in the range of 400 - 500 kg/m ³ . The wood particles were obtained directly from the furnish that was fed to the blender of an industrial line of a particleboard producer. The size distribution of the particles was determined by sieving and the resulting core and face layers had the following distributions: core layer, withheld in screen hole size in the range of 0.18 - 7 mm, and face layers, withheld in screen hole size in the range of 0.18 - 1 mm. The manufacture of these lightweight panels, with a density below 600 kg/m3, had the purpose of checking the possibility of reducing the density of standard particleboards just by removing, as much as possible, wood while still respecting the standard values for the mechanical properties. Lightweight panels with a target density in the range of 335 - 600 kg/m3 were obtained. However, the panels with the lowest densities had internal bond (IB) values below the standard requirements for LP2 type according to CEN/TS 16368:2014 (IB > 0.30 N/mm2 for panels with thickness of 16 mm).

[0040] The second approach attempted was based on using the larger wood particles (core layer) and pieces of foamable polyurethane. The pieces of foamable polyurethane were obtained by shattering a foamable polyurethane block attained after the expansion of a commercial polyurethane mixture. The commercial polyurethane is a combination of polymethylene polyphenyl isocyanate (p-MDI), 4,4'- methylene diphenyl diisocyanate (MDI) and a polyol and available in pressurized containers. The foamed polyurethane pieces had the dimensions of 10 10 10 mm3 and a density of 25 kg/m3. Lightweight panels with the target density of 135 - 640 kg/m3 were obtained, which corresponded to the incorporation of 2 % - 100 % (w/w) of pieces of foamable polyurethane. Only the incorporation of less than 5 % (w/w) of pieces of foamable polyurethane resulted in an acceptable value for the internal bond strength (IB = 0.48 N/mm2 for 2 % (w/w) incorporation), probably because the wood particles were still enough to guarantee good resistance to traction perpendicular to the plane of the board. All the other samples had an internal bond strength below 0.1 N/mm2. Therefore, besides being very challenging to ensure a good distribution of the wood particles in the panel core, there is not enough adhesion between the pieces of foamable polyurethane and wood particles, even when previously blended with the a UF-resin system.

[0041 ] Another approach attempted to obtain a lightweight panel with foamable polyurethane consisted in blending the wood particles (obtained from the fed of the core layer of a standard particleboard) directly with 50 % (w/w) of the commercial polyurethane mixture and keep the resulting blend at room temperature. After a period of 30 minutes, a rigid composite of wood species linked by the foamable polyurethane was formed. It is noteworthy that the foamable polyurethane mixture itself needed about 24 hours till its complete cure and it had much less rigidity than the lightweight panels with the polyurethane-wood composite.

[0042] In an embodiment, the rigid composite of wood species linked by the foamable polyurethane is named lightweight panel with a polyurethane-wood composite.

[0043] Based on the previous preliminary results, a new approach was attempted and herein disclosed.

[0044] In an embodiment, the lightweight panels were prepared and the standard UF- adhesive system of the core was replaced by the commercial foamable polyurethane mixture. For this preparation, the larger wood particles, withheld after sieving (in particular screen holes between 0.5 - 7 mm) were first blended with the foamable polyurethane mixture and the resulting mixture was laid in a mold. The external faces were veneers coated with a monocomponent PU - adhesive system on the core-side. The resulting mat was then shaped in a mold with a thickness predefined, in particular with a thickness of 32 mm. This first pressing stage (pre-pressing) was performed, in particular, for 15 minutes at 20 - 25 °C or room temperature conditions.

[0045] During the first pressing stage, a first expansion of the lightweight panel core occurred, maybe due to the carbon dioxide release during the reaction between the isocyanate groups of the foamable polyurethane mixture and water both from the wood and the surrounding atmosphere. Furthermore, the carbon dioxide diffuses into existing bubbles already nucleated in the liquid, causing the expansion of the foamable polyurethane till the restricted thickness. The foam has been expanded during pressing and then it is mixed with the particles. A second pressing stage is then initiated, in particular, at 80 ^e C for 15 minutes. In the second pressing step, the high temperature increases the hardening speed of the gelation reaction between the isocyanate groups and hydroxyl functionalities of the wood particles. In an embodiment, the entire process can be performed at room temperature, in particular for 2 hours till the expansion and cure of the PU foam.

[0046] The formation of a strong covalent network between the wood particles and the cured polyurethane polymer may justify the great mechanical performance of these lightweight panels, and the foamed structure and low wood content, its low density.

[0047] The lightweight polyurethane wood composites for a panel/broad comprise wood particles/fibers and foamed polyurethane without the need of adding binder, although binder may be added.

[0048] Throughout the description and claims the word "comprise" and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

[0049] Panels with different concentrations of polyurethane mixture in the core were prepared and characterized in terms of density and internal bond strength. Table 1 summarizes the results corresponded to the range of polyurethane mixture concentration of 34 - 50 % (w/w) (g of polyurethane mixture / g of core material c 100).

Table 1 . Density and internal bond of panels with a polyurethane-wood composite core and a thickness of 32 mm.

[0050] The panel P-3 had the lowest density, while maintaining the internal bond strength above the standard requirements of 0.20 N/mm2 for lightweight panels type LP2 (CEN/TS 16368:2014) and above the 0.30 N/mm2 required for the standard particleboards type P2 (EN 312). For that reason, a more detailed characterization in terms of bending strength (EN 310), formaldehyde content (EN ISO 12469-5) and resistance to axial withdrawal of screws (EN 320) was performed. Maintaining the polyurethane content, panels with less 20 % and 30 % of the total mass were prepared under the same pressing conditions (at room temperature for 2 hours). Just in the first case, the IB value respected the standard requirements.

[0051] For that reason, the characterization is referred to the panels with 45 % (w/w) of polyurethane and less 20 % of the total mass than the panels presented in table 1 . The faces were composed by laminates sheets with a thickness of 0.8 mm. The results correspond to average values of at least 3 determinations per test.

Table 2. Characterization of panels with 45 % (w/w) of foamable polyurethane in the core and a thickness of 32 mm and the corresponding standard values according to CEN/TS 16368:2014.

[0052] The lightweight panels with a polyurethane-wood composite have important mechanical features and low density, which are the unexpected result of a synergy effect between the wood particles and the foamable polyurethane.

[0053] The lightweight polyurethane-wood composites herein disclosed may be used in the construction industry as well as for furniture and furniture components. Some examples of the end products where lightweight polyurethane-wood composites can be used are tables, chairs, bookshelves, doors, in particular kitchen worktops doors, ceiling panels, laminate floors, walls sheathings, sound and/or temperature insulation.

[0054] Throughout the description and claims the word "comprise" and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. Furthermore, the present disclosure covers all possible combinations of particular and preferred embodiments described herein.

[0055] The disclosure is, of course, not in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof without departing from the basic idea of the invention as defined in the appended claims.

[0056] The above described embodiments are combinable. The following claims further set out particular embodiments of the disclosure.