Particle board and particle board manufacturing method

Abstract

In a particle board 10 which has a three-layer structure in which wood crushed pieces 2, ..., 2 finer than a core layer 11 are used for surface layers 12, a large number of flaky wood cut pieces 3, ..., 3 having a uniform thickness of 0.05-0.35 mm are included in the core layer 11.

Description

Particle board and method for manufacturing particle board

[0001] The present invention relates to a particle board having a three-layer structure and a method for manufacturing a particle board having a three-layer structure.

[0002] Traditionally, tropical plywood such as lauan plywood has been widely used. However, in recent years, tropical plywood has become difficult to obtain due to the depletion of raw materials and concerns about environmental destruction, and efforts are being made to replace it with other wood boards.

[0003] Patent Document 1 discloses that particle board, formed by applying adhesive to crushed wood chips and depositing them into a mat-like structure and then hot-pressing them with a hot platen, can be used as a substitute for plywood. Patent Document 1 also discloses a three-layer particle board in which crushed wood chips are classified, and coarse crushed wood chips are used as the core material and fine crushed wood chips are used as the surface layer. Particle boards can be manufactured relatively inexpensively because crushed wood chips, such as waste wood and recycled materials, can be used as components. Furthermore, even if no waste wood or recycled materials are used, three-layer particle boards can be manufactured relatively inexpensively because all of the crushed wood chips produced by crushing wood materials are used without waste. Thus, three-layer particle boards are excellent as a substitute for plywood because they can be manufactured relatively inexpensively.

[0004] WO 2005 / 115705

[0005] However, because three-layer particle boards use fine ground wood chips in the surface layer, they have improved surface smoothness compared to single-layer particle boards made of coarse ground wood chips, but their bending strength is lower. Increasing the density of the surface layer to ensure a certain level of bending strength in three-layer particle boards increases the weight and the amount of adhesive used. Furthermore, particle boards have low screw pull-out strength, making them unsuitable for use as load-bearing subfloor surfaces for screw installation, and they also suffer from poor dimensional stability due to moisture absorption.

[0006] The present invention has been made in view of the above points, and its object is to provide a three-layer particle board that has excellent surface properties, bending strength, and screw pull-out strength, and that exhibits little dimensional change, without increasing cost or weight.

[0007] In order to achieve the above object, in the present invention, a core layer of a three-layer particle board contains a large number of extremely thin wood shavings.

[0008] Specifically, the first invention is based on a particle board having a three-layer structure in which the surface layer is made of finer pulverized wood chips than the core layer.

[0009] The first invention is characterized in that the core layer contains a large number of thin wood shavings having a uniform thickness of 0.05 mm or more and 0.35 mm or less.

[0010] Here, "ground wood chips" refers to wood chips of various shapes and of non-uniform thickness, formed by finely grinding wood material using a known grinding device until the material passes through a sieve with a specified mesh size installed at the outlet of the grinding device. Also, "wood shavings" refers to thin, uniformly thick wood chips formed by cutting wood material into a thin, uniform thickness using a known cutting device with a blade that cuts the material into a thin, uniform thickness. Note that "wood shavings having a uniform thickness" allows for thickness variations due to processing errors in the cutting device, for example, a thickness variation of about ±20%.

[0011] In the first invention, the core layer of a three-layer particle board contains not only the usual ground wood chips, but also wood shavings with an extremely thin, uniform thickness of 0.05 mm to 0.35 mm, which are machined to increase the bending strength of the core layer. This configuration eliminates the need to increase the density of the surface layer or use large amounts of adhesive to ensure the particle board's bending strength. Furthermore, compared to a core layer composed solely of ground wood chips, the presence of thin wood shavings makes it easier to bond the wood chips (ground wood chips and wood shavings) together with a smaller amount of adhesive. Therefore, the three-layer particle board of the first invention can reduce the amount of adhesive used while maintaining bending strength comparable to conventional particle boards.

[0012] Furthermore, in the particle board according to the first invention, the surface layer is made up of relatively fine wood chips, just like conventional three-layer particle boards, and therefore has excellent surface properties (smoothness).

[0013] Furthermore, in the particleboard according to the first invention, if some of the ground wood chips in the core layer are replaced with thinner wood shavings, the core layer will contain more wood shavings than the original ground wood chips. The wood shavings have a uniform thickness and a flat surface. Therefore, if the core layer contains a large number of wood shavings (if some of the ground wood chips in the core layer are replaced with wood shavings), the total surface area of ​​the numerous wood chips that make up the core layer will be significantly increased, making screws less likely to slip out than in conventional three-layer particleboards whose core layer is composed solely of ground wood chips. Therefore, in the particleboard according to the first invention, the inclusion of wood shavings in the core layer reinforces the core layer and significantly improves the screw pull-out strength (wood screw holding power), making it suitable for use as a load-bearing subfloor surface material for screw installation.

[0014] The inventors of the present application conducted a test to verify the effect of incorporating extremely thin wood shavings, between 0.05 mm and 0.35 mm thick, into the core layer of a three-layer particleboard. The results showed that particleboards containing wood shavings in the core layer had improved modulus of resistance (MOR), length change due to water absorption (LE), and wood screw holding strength (WS) compared to conventional particleboards that did not contain wood shavings in the core layer. While the thickness swelling due to water absorption (TS) was not significantly improved, it did show some improvement, and the absolute value of TS fully met the standard (12% or less) required for structural particleboards.

[0015] As described above, according to the first invention, a three-layer particle board having excellent surface properties, bending strength, and screw pull-out strength and small dimensional change can be provided without increasing cost or weight.

[0016] A second invention is the first invention, characterized in that the surface layer contains a large number of the wood shavings.

[0017] In the second invention, extremely thin wood shavings, measuring 0.05 mm to 0.35 mm in thickness, are incorporated into not only the core layer but also the surface layers of a three-layer particle board. This configuration increases the bending strength of both the core layer and the surface layers. Therefore, there is no need to increase the density of the surface layers or use large amounts of adhesive to ensure the particle board's bending strength. Furthermore, by incorporating wood shavings into both the core layer and the surface layers, the amount of adhesive used can be further reduced compared to when wood shavings are incorporated only into the core layer.

[0018] Furthermore, in the particle board according to the second aspect of the present invention, the wood chips are extremely thin, with a thickness of 0.05 mm to 0.35 mm, so that even if they are included in the surface layer, they are unlikely to cause irregularities on the surface, and therefore the surface properties (smoothness) are as excellent as those of conventional particle boards.

[0019] Furthermore, in the particle board according to the second invention, by including wood chips not only in the core layer but also in the surface layer, screws become more difficult to pull out. Therefore, the second invention can provide a three-layer particle board with excellent screw pull-out strength (wood screw holding power).

[0020] Furthermore, in the particleboard according to the second invention, by replacing some of the relatively fine wood chips in the surface layer with wood shavings of uniform thickness and flat surfaces, the total surface area of ​​the numerous wood chips that make up the surface layer is significantly increased. A significantly increased total surface area of ​​the numerous wood chips that make up the surface layer increases the bonding area between the numerous wood chips, and also increases the number of bonding points and makes them more evenly distributed. Therefore, the particleboard according to the second invention is more resistant to moisture penetration and has excellent water resistance compared to conventional particleboards whose surface layer is made solely of wood chips.

[0021] Furthermore, in conventional particle boards whose surface layers are made only of crushed wood chips, the crushed wood chips, which are not cut to a thin thickness, are crushed during press molding, and when moisture penetrates the interior, the crushed wood chips absorb the moisture and tend to swell, resulting in significant dimensional changes (changes in length and thickness).In contrast, the particle board of the second invention, as described above, has a structure that makes it more difficult for moisture to penetrate the interior than conventional particle boards, and because the numerous replaced wood chips are cut to a thin thickness, they do not absorb moisture and therefore do not undergo significant dimensional changes (changes in length and thickness).

[0022] The present inventors conducted a test to verify the effect of incorporating extremely thin wood shavings, 0.05 mm to 0.35 mm thick, into both the core and surface layers of a three-layer particleboard. The results showed that particleboards containing wood shavings in both the core and surface layers exhibited improved modulus of resistance (MOR), wet modulus of resistance (wet MOR), moisture absorption length change (LE), moisture absorption and desorption length change (HLE), and wood screw holding strength (WS) compared to conventional particleboards containing no wood shavings in either the core or surface layers. While the moisture absorption thickness swelling (TS) did not show a significant improvement, it did show some improvement, and the absolute value of the moisture absorption thickness swelling (TS) fully met the standard (12% or less) required for structural particleboards.

[0023] As described above, according to the second invention, a three-layer particle board having excellent surface properties, bending strength, screw pull-out strength, and water resistance, and having little dimensional change, can be provided without increasing cost or weight.

[0024] The third invention is characterized in that, in the first or second invention, the core layer contains first wood chips as the wood pulverized pieces for the core layer, which pass through a first sieve with a mesh size of 16 mm or less but do not pass through a second sieve with a mesh size of 0.5 mm or more and smaller than the first sieve, and the surface layer contains second wood chips as the wood pulverized pieces for the surface layer, which pass through both the first and second sieves.

[0025] In the third invention, the core layer uses a large number of first wood chips that pass through a first sieve with a mesh size of 16 mm or less but do not pass through a second sieve with a mesh size of 0.5 mm or more that is smaller than the first sieve, and the surface layer uses fine second wood chips that pass through both the first and second sieves. By using finer ground wood chips than the core layer in this way and including wood shavings in the core layer, it is possible to provide a three-layer particle board that has excellent surface properties, bending strength, and screw pull-out strength, and that exhibits little dimensional change, without increasing cost or weight.

[0026] The fourth invention is characterized in that, in the first or second invention, the wood cuttings in the core layer are mixed in a ratio of 20% by weight to 90% by weight of the total weight of the wood pieces for the core layer, which is composed of the wood crushed pieces and the wood cuttings contained in the core layer.

[0027] From the results of the above-mentioned verification tests, the inventors of the present application have found that by mixing wood cuttings into a large number of core layer wood pieces in a proportion of 20% by weight to 90% by weight of the total weight of the core layer wood pieces, the normal bending strength (MOR), water absorption length change (LE) and wood screw holding strength (WS) are improved, and that the higher the proportion of wood cuttings mixed in the core layer, the greater the improvement rates of normal bending strength (MOR) and water absorption length change (LE).

[0028] Therefore, in the fourth invention, wood shavings are mixed into the multiple core wood pieces at a ratio of 20% to 90% by weight of the total weight of the core wood pieces. By mixing the multiple core wood pieces with wood shavings in this ratio, it is possible to provide a three-layer particle board with excellent surface properties, bending strength, and screw pull-out strength, and with little dimensional change, without increasing cost or weight.

[0029] The fifth invention is a method for manufacturing a particle board having a three-layer structure in which finer wood pulverized chips are used in the surface layer than in the core layer, the method comprising: a first crushing step of crushing a first wood material to produce a large number of wood chips; and a second crushing step of sieving the large number of wood chips through a first sieve having a mesh size of 16 mm or less and a second sieve having a mesh size of 0.5 mm or more and smaller than that of the first sieve to produce a large number of first wood chips that pass through the first sieve but not the second sieve; a large number of second wood chips that pass through both the first and second sieves; and a large number of wood chips that pass through the first and second sieves. and a large number of third wood chips that do not pass through either of the first and second wood chips, and the first wood chips are used as wood pulverized pieces for the core layer and the second wood chips are used as wood pulverized pieces for the surface layer. A cutting process is performed to cut the second wood material so that the fibers appear linearly on the surface to produce a large number of thin wood flakes with a uniform thickness of 0.05 mm to 0.35 mm. A second crushing process is performed to crush the large number of wood flakes and the large number of wood flakes so that they are broken along the fiber direction, thereby producing a large number of thin, elongated small wood flakes, and a second crushing process is performed to crush the large number of small wood flakes. a selection step of selecting at least one type of the numerous first to third small wood pieces obtained by a second classification step using a third sieve and a fourth sieve smaller than the third sieve as numerous core layer wood shavings; a wood piece mixing step of mixing the numerous core layer pulverized wood pieces with the numerous core layer wood shavings to form numerous core layer wood pieces; an adhesive application step of applying adhesive to the numerous surface layer wood pieces including the numerous surface layer pulverized wood pieces and the numerous core layer wood pieces; and an adhesive application step of applying adhesive to the numerous surface layer wood pieces. The method is characterized by comprising a mat-forming step of forming a three-layered mat by piling wood chips to form a first mat, piling the numerous wood chips for the core layer coated with adhesive on the first mat to form a second mat, and piling the numerous wood chips for the surface layer coated with adhesive on the second mat to form a third mat, and a hot pressing step of forming the particle board by subjecting the three-layered mat to a hot pressing process to harden the adhesive of the first to third mats.

[0030] Here, "crushing the first wood material" refers to using a known crushing device to finely crush the first wood material until it passes through a sieve with a specified mesh size installed at the outlet of the crushing device, thereby forming wood chips of various shapes and with non-uniform thicknesses. Furthermore, "cutting the second wood material" refers to using a known cutting device with a blade that cuts material to a thin, uniform thickness to form thin, flake-like wood chips with a uniform thickness. Note that "wood chips with a uniform thickness" allows for thickness variations due to processing errors, for example, a thickness variation of approximately ±20%.

[0031] According to the fifth invention, a three-layer particle board having excellent surface properties, bending strength, and screw pull-out strength and small dimensional change can be provided without increasing cost or weight.

[0032] The sixth invention is characterized in that, in the fifth invention, the cutting process, the second crushing process, and the second classification process are included in a method for manufacturing a wood board in which a large number of first small wood flakes that pass through the third sieve but do not pass through the fourth sieve are glued together in a collective state.

[0033] The seventh invention is the sixth invention, characterized in that in the selection step, the large number of first small wood slices are selected as the large number of wood cutting pieces for the core layer.

[0034] The eighth invention is characterized in that, in the sixth invention, in the selection process, the numerous second small wood flakes that pass through both the third and fourth sieves and the numerous third small wood flakes that do not pass through both the third and fourth sieves are selected as wood cutting pieces for the core layer.

[0035] In the sixth to eighth inventions, the wood shavings produced when the second wood material is processed to form another wood board are used as the particle board. Therefore, even if a material (wood shavings) different from the material (ground wood chips) used in ordinary particle boards is used, it can be easily procured because it is the material produced when processing the material for the other wood board. Therefore, according to the sixth to eighth inventions, it is possible to easily provide a three-layer particle board with excellent surface properties, bending strength, and screw pull-out strength, and with little dimensional change, at a relatively low cost.

[0036] In particular, in the eighth invention, the wood shavings contained in at least the core layer are selected from the first small wood flakes, which are obtained by classifying the small wood flakes. In this way, waste wood generated during the production of other wood boards, which is not used for other wood boards, is used as the wood shavings for the core layer of the three-layer particle board, making it possible to produce the three-layer particle board more inexpensively. Furthermore, since the second wood material of the other wood boards is used without waste, this leads to the effective use of wood resources.

[0037] The ninth invention is characterized in that, in any one of the fifth to eighth inventions, in the wood chip mixing process, the multiple core layer wood pulverized chips and the multiple core layer wood chips are mixed so that the multiple core layer wood chips account for 20% by weight or more and 90% by weight or less of the total weight of the multiple core layer wood chips.

[0038] In the ninth invention, in the wood chip mixing step, wood shavings are mixed with the numerous core wood chips in a ratio of 20% by weight to 90% by weight of the total weight of the core wood chips. By mixing the numerous core wood chips with wood shavings in this ratio, it is possible to provide a three-layer particle board with excellent surface properties, bending strength, and screw pull-out strength, and with little dimensional change, without increasing cost or weight.

[0039] As explained above, according to the present invention, the core layer of a three-layer particle board contains a large number of thin, uniformly thick wood shavings, making it possible to provide a three-layer particle board that has excellent surface properties, bending strength, and screw pull-out strength, and that exhibits little dimensional change, without increasing cost or weight.

[0040] FIG. 1 is a cross-sectional view showing a portion of a particle board according to embodiment 1. FIG. 2 is a diagram showing the process of producing pulverized wood chips. FIG. 3 is a diagram showing the process of producing shaved wood chips. FIG. 4 is an enlarged perspective view schematically showing a first small wood chip. FIG. 5 is a diagram showing the manufacturing process of a particle board. FIG. 6 is a partially enlarged cross-sectional view showing a comparison between a mat before a heat-pressing process and a particle board formed after the heat-pressing process. FIG. 7 shows the results of an experiment to verify the effects of embodiment 1. FIG. 8 is a cross-sectional view showing a portion of a particle board according to embodiment 2. FIG. 9 shows the results of an experiment to verify the effects of embodiment 2.

[0041] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following description of the embodiments is merely exemplary in nature and is not intended to limit the present invention, its applications, or uses.

[0042] 1 shows a particle board 10 according to a first embodiment of the present invention. The particle board 10 can be used, for example, as a base material for building materials such as flooring, or as a component material for furniture.

[0043] Particle board 10 is made up of a large number of wood pieces 1, ..., 1 glued together in a collected state. Particle board 10 contains a large number of wood pieces 1, ..., 1 and an adhesive. The feature of the present invention is the constituent material (wood pieces 1) of particle board 10, and there are no particular limitations on thickness or density. However, particle board 10 is, for example, a particle board specified in JIS A5908 with a thickness of 5 mm to 40 mm and a density of 400 kg / m ３ More than 900kg / m ３As will be described in detail below, in this embodiment 1, by using not only ground wood chips 2 but also wood cuttings 3 as the numerous wood chips 1, 1, the bending strength is superior to that of conventional particle boards that contain only ground wood chips 2, 2. For example, even if the thickness is 3 mm or more but less than 5 mm, sufficient bending strength can be ensured, and the density can be increased to 400 kg / m ３ Even if the thickness is less than this, sufficient bending strength and peel strength can be ensured.

[0044] <Wood Pieces> In this embodiment 1, the numerous wood pieces 1, 1 that make up the particle board 10 are made up of numerous pulverized wood pieces 2, 2 and numerous shaved wood pieces 3, 3. The particle board 10 has a three-layer structure including a core layer 11 and two surface layers 12, 12 that sandwich the core layer 11 and form the surface. The core layer 11 and the surface layers 12 differ in the configuration of the wood pieces 1, 1 that they contain.

[0045] Both the core layer 11 and the surface layer 12 contain a large number of wood fragments 2,...,2 as wood pieces 1,...,1, but the wood fragments 2,...,2 used in the core layer 11 and the surface layer 12 are different in size, with finer wood fragments 2,...,2 used in the surface layer 12 than in the core layer 11. Also, the core layer 11 contains not only a large number of wood fragments 2,...,2 but also a large number of wood shavings 3,...,3. In other words, in this embodiment 1, the wood fragments 1,...,1 of the core layer 11 are made up of wood fragments 2,...,2 and wood shavings 3,...,3, and the wood fragments 1,...,1 of the surface layer 12 are made up only of wood fragments 2,...,2 that are finer than those in the core layer 11.

[0046] In this embodiment 1, the ground wood chips 2 are those used as wood chips in conventional particle boards, while the wood shavings 3 used in the core layer 12 are those used as a constituent material for another wood board 50, which will be described later. That is, in this embodiment 1, the core layer 11 uses a large number of wood chips 1 that are a mixture of ground wood chips 2 used in conventional particle boards and ground wood chips 3 used as a constituent material for another wood board 50, and the surface layer 12 uses a large number of wood chips 1 that are the ground wood chips 2 used in conventional particle boards. In the core layer 11, the wood shavings 3 are mixed in a ratio of 10% by weight to 90% by weight of the total weight of the wood chips 1.

[0047] The ground wood chips 2, . . . , 2 and shaved wood chips 3, . . . , 3 used in the particle board 10 will be described in detail below.

[0048] [Wood Chips] As shown in Fig. 2, the wood chips 2, ..., 2 are obtained by finely crushing wood material A using a known crushing device such as a crusher, hammer mill, pin mill, jet mill, ring flaker, or cutter mill until the chips pass through a sieve with a predetermined mesh size provided at the outlet of the crushing device. In the particle board 10 of the first embodiment, as shown in Fig. 2, wood chips 2a, ..., 2a obtained by crushing wood material A are classified using two sieves, one large and one small. For classification, a first sieve with a mesh size of 16 mm or less and a second sieve with a mesh size of 0.5 mm or more but smaller than the first sieve are used. The first wood chips 2b, ..., 2b that have been sorted by classification (i.e., pass through the first sieve but not the second sieve) are used as wood pulverized pieces 2, ..., 2 for the core layer 11, and the second wood chips 2c, ..., 2c that have passed through both the first and second sieves (i.e., are finer than the first wood chips 2b, ..., 2b) are used as wood pulverized pieces 2, ..., 2 for the surface layer 12.

[0049] As the first sieve having a mesh size of 16 mm or less, for example, a sieve having a nominal mesh size of 2 mm to 16 mm as specified in JIS Z8801-1:2019 can be used, preferably a sieve having a nominal mesh size of 2.8 mm to 8 mm, and particularly preferably a sieve having a nominal mesh size of 4 mm to 5.6 mm. As the second sieve having a mesh size of 0.5 mm or more and smaller than the first sieve, for example, a sieve having a nominal mesh size of 0.5 mm to 2 mm as specified in JIS Z8801-1:2019 can be used, preferably a sieve having a nominal mesh size of 0.5 mm to 1.4 mm, and particularly preferably a sieve having a nominal mesh size of 0.5 mm to 1 mm. In this embodiment 1, the wood chips 2a, ..., 2a are sorted by classifying them using a sieve (first sieve) with a nominal mesh size of 4 mm and a sieve (second sieve) with a nominal mesh size of 1 mm (i.e., the chips pass through the first sieve but do not pass through the second sieve), and the sorted chips are used as the first wood chips 2b, ..., 2b.

[0050] As shown in Figure 2, the third wood chips 2d, ..., 2d that do not pass through both types of sieves (i.e., are coarser than the first wood chips 2b, ..., 2b) due to the above classification are not used as wood pulverized pieces 2, ..., 2, but are returned to the grinding device and ground again.

[0051] The species of wood material A is not particularly limited, and any species of tree can be used as wood material A. For example, fir wood such as cedar, cypress, and Douglas fir, acacia, aspen, poplar, pine (hard pine, soft pine, radiata pine, etc.), birch, rubber (rubber tree), etc. can be used as wood material A. In addition, domestic timber such as Abies sachalinensis, larch, Ezo spruce, sawara, cypress, kaya, Japanese larch, Japanese cypress, various pines, paulownia, maple, birch (white birch), chestnut, beech, oak, fir, oak, camphor, and zelkova; North American timber such as American cypress, American cypress, American cedar, American fir, spruce, American larch, and redwood; South Sea timber such as agathis, terminalia, lauan, meranti, Junconobium, camellere, carampayan, ambeloi, melina, teak, apitong, and sengonlaut; and other imported timber such as balsa, cedarwood, mahogany, lignum vitae, acacia mangium, Mediterranean pine, sorghum, and camellere. In addition to logs and thinned wood, scraps and waste wood generated at construction sites, waste pallet materials, recycled materials, etc. may also be used as the wood material A.

[0052] [Wood shavings] The wood shavings 3 used in the core layer 11 are produced from the wood material B during the formation of a wood board 50 different from the particle board 10 of the present invention. As shown in Figure 3, the wood board 50 is made by cutting the wood material B into a thin, uniform thickness using a known cutting device with a blade that cuts the material into a thin, uniform thickness, resulting in a large number of wood flakes 3a having a uniform thickness t (0.05 mm to 0.35 mm). These wood flakes are then further crushed using a known crushing device to form a large number of smaller wood flakes 3b, which are then classified using two types of sieves, one large and one small, and the first smaller wood flakes 3c selected by the classification are glued together in a consolidated state.

[0053] In the particle board 10 of this embodiment 1, when forming the wood board 50, a number of wood flakes 3a, ..., 3a having a uniform thickness t (thickness of 0.05 mm to 0.35 mm) obtained by cutting the wood material B are further crushed to produce a number of small wood flakes 3b, ..., 3b, and the remaining wood flakes 3b, ..., 3b are selected by classification and used in appropriate amounts as the wood shavings 3, ..., 3c to be included in the core layer 11. For classification, a third sieve with a mesh size of 35 mm or less and a fourth sieve with a mesh size of 0.5 mm or more and smaller than the third sieve, are used. By classifying the numerous small wood flakes 3b, 3b using the third and fourth sieves, numerous first small wood flakes 3c, 3c are selected, and also the third small wood flakes 3e, 3e that pass through both the third and fourth sieves (i.e., the second small wood flakes 3d, 3d that are finer than the first small wood flakes 3c, 3c) and the third small wood flakes 3e, 3e that do not pass through both the third and fourth sieves (i.e., the first small wood flakes 3c, 3c) are obtained. In this embodiment 1, the second small wood flakes 3d, 3d and the third small wood flakes 3e, 3e that are not selected by this classification are used as the wood shavings 3 to be included in the core layer 11. In other words, in this embodiment 1, among the small wood flakes 3b, ..., 3b obtained by cutting and crushing the wood material B, those that are not classified and are not used in the wood board 50 can be used as wood cutting pieces 3, ..., 3 to be included in the core layer 11.

[0054] As the third sieve having a mesh size of 35 mm or less, for example, a sieve having a nominal mesh size of 5.6 mm to 31.5 mm as specified in JIS Z8801-1:2019 can be used, preferably a sieve having a nominal mesh size of 5.6 mm to 16 mm, and particularly preferably a sieve having a nominal mesh size of 5.6 mm to 8 mm. As the fourth sieve having a mesh size of 0.5 mm or more and smaller than the third sieve, for example, a sieve having a nominal mesh size of 0.5 mm to 2 mm as specified in JIS Z8801-1:2019 can be used, preferably a sieve having a nominal mesh size of 0.5 mm to 1.4 mm, and particularly preferably a sieve having a nominal mesh size of 0.5 mm to 1 mm. In this embodiment 1, of the small wood flakes 3b, ..., 3b, the second small wood flakes 3d, ..., 3d and the third small wood flakes 3e, ..., 3e that were not selected by classification using a sieve with a nominal mesh size of 5.6 mm (third sieve) and a sieve with a nominal mesh size of 0.5 mm (fourth sieve) are used as wood cutting pieces 3, ..., 3 to be included in the core layer 11.

[0055] The thickness t of the wood flakes 3a is 0.05 mm or more and 0.35 mm or less, preferably 0.10 mm or more and 0.30 mm or less, more preferably 0.15 mm or more and 0.25 mm or less, and even more preferably 0.15 mm or more and 0.20 mm or less. The thickness t of the wood flakes 3a can be changed by changing the thickness setting (e.g., the material feed speed) of a known cutting device. The wood flakes 3a cut by the cutting device will have a uniform thickness t. Furthermore, a large number of wood flakes 3a, ..., 3a cut by a cutting device whose thickness setting (e.g., the material feed speed) is set so that the thickness of the wood flakes 3a becomes t will have the same thickness t. Furthermore, since the small wood flakes 3b and the first to third small wood flakes 3c to 3e that are classified from the small wood flakes 3b are crushed wood flakes 3a, they have a uniform thickness t, just like the wood flakes 3a, and the numerous small wood flakes 3b, ..., 3b, the numerous first to third small wood flakes 3c, ..., 3c, the numerous second small wood flakes 3d, ..., 3d and the numerous third small wood flakes 3e, ..., 3e each have the same thickness t.

[0056] In addition, "the wood flakes 3a (and similarly the small wood flakes 3b, the first small wood flakes 3c, the second small wood flakes 3b and the third small wood flakes 3e) have a uniform thickness t" means that thickness variations due to processing errors of the cutting device are allowed, for example, a thickness variation of about ±20% is allowed. Furthermore, "the numerous wood flakes 3a, ..., 3a (as well as the numerous small wood flakes 3b, ..., 3b, the numerous first to third small wood flakes 3c, ..., 3c, the numerous second small wood flakes 3d, ..., 3d, and the numerous third small wood flakes 3e, ..., 3e) have the same thickness t" does not only mean that all of the wood flakes 3a, ..., 3a actually have the same thickness, but also includes cases where cutting is performed using a cutting device in which the thickness setting (for example, the material feeding speed) is adjusted so that the numerous wood flakes 3a, ..., 3a have the same thickness t, and where there is variation in thickness of, for example, about ±20% due to biological variation in the wood material B and processing errors in the cutting device. In addition, since the wood material B is a natural material, biological characteristics such as the part of the wood material B (heartwood or sapwood), specific gravity (hardness), and fiber direction affect (variation in) the thickness of the numerous wood flakes 3a, ..., 3a formed by contact with the blade. Therefore, thickness variation in the numerous wood flakes 3a, ..., 3a can be caused not only by processing errors but also by the biological characteristics of the wood material B.

[0057] The first small wood flakes 3c used to form the wood board 50 will now be described in detail. As shown enlarged in Figure 4, the first small wood flakes 3c have an elongated (striped) shape in which the fiber direction dimension d1 in the direction along the fibers a of vessels, tracheids, etc. is longer than the fiber-orthogonal direction dimension d2 in the direction perpendicular to the fiber direction. If the fiber direction dimension d1 is the length and the fiber-orthogonal direction dimension d2 is the width, the length d1 is 40 mm or less, preferably 35 mm or less, more preferably 30 mm or less, even more preferably 25 mm or less, and even more preferably 10 mm to 20 mm. The width d2 is 35 mm or less, preferably 15 mm or less, and more preferably 0.5 mm to 5 mm. The length d1 and width d2 of the first small wood flakes 3c are both average values. The length d1 and width d2 (both average values) of the first wood flakes 3c are calculated by photographing a large number of first wood flakes 3c, ..., 3c, approximating the image of each first wood flake 3c to an ellipse using publicly known image analysis software, and averaging the length of the long side of the approximated ellipse as the fiber direction dimension and the short side as the fiber perpendicular direction dimension d.

[0058] As described above, the first small wood flakes 3c, ..., 3c of such thickness, shape and size are selected by classifying a large number of small wood flakes 3b, ..., 3b using two types of sieves (third and fourth sieves) appropriately selected, and this classification also results in the second small wood flakes 3d, ..., 3d and the third small wood flakes 3e, ..., 3e to be used in the core layer 11.

[0059] In the first embodiment, as described above, after the numerous wood flakes 3a are crushed, the wood flakes other than the first small wood flakes 3c (the second small wood flakes 3d and the third small wood flakes 3e) selected by classification can be used as the wood shavings 3 in the core layer 11 of the particle board 10. However, the wood shavings 3 constituting the core layer 11 of the particle board 10 are not limited to those described above. The first small wood flakes 3c used in the wood board 50 may also be used as the wood shavings 3 constituting the core layer 11. Furthermore, the wood flakes 3a before crushing after crushing or the numerous small wood flakes 3b before crushing and classification after crushing may also be used as the wood shavings 3 constituting the core layer 11. Furthermore, all, two, three, or four of the wood flakes 3a, the small wood flakes 3b, the first small wood flakes 3c, the second small wood flakes 3d, and the third small wood flakes 3e may be combined and used as the wood shavings 3 that make up the core layer 11. In other words, at least one type of wood flakes 3a, the small wood flakes 3b, the first small wood flakes 3c, the second small wood flakes 3d, and the third small wood flakes 3e may be used as the wood shavings 3 that make up the core layer 11. Each of the thin pieces 3a to 3e selected as the wood cutting pieces 3 has a uniform thickness t, and the core layer 11 contains a large number of wood cutting pieces 3, ..., 3 having the same thickness t.

[0060] The species of wood material B is not particularly limited, and any species of tree can be used as wood material B. For example, fir wood such as cedar, cypress, and Douglas fir, acacia, aspen, poplar, pine (hard pine, soft pine, radiata pine, etc.), birch, rubber (rubber tree), etc. can be used as wood material B. In addition, domestic timber such as Abies sachalinensis, larch, Ezo spruce, sawara, cypress, kaya, Japanese larch, Japanese cypress, cypress, various pines, paulownia, maple, birch (white birch), chestnut, beech, oak, fir, oak, camphor, and zelkova; North American timber such as American cypress, American cypress, American cedar, American fir, spruce, American larch, and redwood; South Sea timber such as agathis, terminalia, lauan, meranti, Junkon, camellere, carampayan, ambeloi, melina, teak, apitong, and sengon laut; and other imported timber such as balsa, cedar, mahogany, lignum vitae, acacia mangium, Mediterranean pine, sorghum, and camellere. In addition to logs and thinned wood, scraps and waste wood generated at construction sites, waste pallet materials, recycled materials, etc. may also be used as the wood material B.

[0061] The numerous wood pieces 1, . . . , 1 that make up the particle board 10 are made up of the above-described crushed wood pieces 2, .

[0062] Regarding the physical properties of the wood pieces 1 (ground wood pieces 2, wood cutting pieces 3), their density is preferably 250 kg / m ３ More than 800kg / m ３ More preferably, it is 300 kg / m or less. ３ More than 500kg / m ３ Density is 250 kg / m or less. ３ If the density of the wood pieces 1 is less than 800 kg / m, it is possible to form particle boards 10 with the same density and bending strength by increasing the mixing ratio of wood flakes, but stable production becomes difficult. ３ However, it is difficult to easily obtain such a wood piece 1. ３ If it is possible to easily obtain wood pieces 1 exceeding this value, the upper limit of the density is 800 kg / m ３ It is not limited to this and may be a higher value.

[0063] Furthermore, the moisture content of the wood pieces 1 (ground wood pieces 2, shaved wood pieces 3) is preferably 2% or more and 20% or less, and more preferably 2% or more and 8% or less. If the moisture content is less than 2%, it may take a long time to soften the wood pieces during the hot pressing process in the press molding step, which may increase the pressing time and reduce the bending strength. On the other hand, if the moisture content of the wood pieces 1 exceeds 20%, it may take a long time to heat and compress the wood pieces during the hot pressing process, and furthermore, the hardening of the adhesive may be inhibited, which may reduce the bending strength.

[0064] <Adhesive> The adhesive used for the particle board 10 is not particularly limited as long as it is a thermosetting adhesive. For example, an isocyanate-based adhesive can be used. Other adhesives that can be used include amine-based adhesives such as phenolic resin, urea resin, and melamine resin, and natural adhesives. The adhesive is applied to the wood pieces 1, ..., 1 before they are stacked, and hardens when the mat on which the wood pieces 1, ..., 1 are stacked is hot-pressed. This allows the numerous wood pieces 1, ..., 1 to be bonded together in a collective state.

[0065] -Method for manufacturing particle board- Next, we will explain the method for manufacturing particle board 10. The method for manufacturing particle board 10 includes a wood piece manufacturing step S1, an adhesive application step S2, a mat forming step S3, and a heat pressing step S4.

[0066] <Wood Piece Producing Step S1> The wood piece producing step S1 includes a wood crushed piece producing step S11, a wood shavings producing step S12, and a wood piece mixing step S13.

[0067] [Wood Chip Production Step S11] The wood chip production step S11 includes a material preparation step S111, a crushing step S112, and a classification step S113.

[0068] (Material preparation step S111) First, prepare a wood material A. When using raw wood such as logs or thinned wood, the logs are cut into short pieces and the bark is removed to form the wood material A. When using scrap wood, waste wood, waste pallet wood, recycled wood, or the like generated at a construction site, the logs are cut into short pieces to form the wood material A.

[0069] (Crushing step S112) Next, the prepared wood material A is finely crushed into small pieces using a known crusher such as a crusher, hammer mill, pin mill, jet mill, ring flaker, or cutter mill until the pieces pass through a sieve with a specified mesh size provided at the outlet of the crusher. By crushing the wood material A, wood chips 2a, ..., 2a are obtained. The wood chips 2a, ..., 2a may have various shapes, such as powder, granules, rods, columns, fibers, or flakes.

[0070] (Classification step S113) The wood chips 2a, ..., 2a obtained by grinding are classified into three sizes using two types of sieves (first and second sieves), one large and one small. Specifically, the wood chips 2a, ..., 2a are divided into first wood chips 2b, ..., 2b of a desired size that pass through the first coarse sieve but not the second fine sieve, second wood chips 2c, ..., 2c that are finer than the first wood chips 2b, ..., 2b that pass through both the first and second sieves, and third wood chips 2d, ..., 2d that are coarser than the first wood chips 2b, ..., 2b and do not pass through either the first or second sieve. The first wood chips 2b, ..., 2b classified and selected in this way are used as the ground wood pieces 2, ..., 2 for the core layer 11, and the second wood chips 2c, ..., 2c obtained at the same time are used as the ground wood pieces 2, ..., 2 for the surface layer 12. The third wood chips 2d, ..., 2d are not used as the ground wood pieces 2, ..., 2 but are returned to the grinding device and ground again.

[0071] [Wood Shavings Production Process S12] The wood shavings production process S12 includes a material preparation process S121, a cutting process S122, a crushing process S123, and a classification process S124. As described above, in the first embodiment, the second wood flakes 3d, ..., 3d and the third wood flakes 3e, ..., 3e generated during the formation of the wood board 50 are used as the wood shavings 3, ..., 3 for the core layer 11. Therefore, the wood shavings production process S12 is a part of the method for manufacturing the wood board 50.

[0072] (Material preparation step S121) First, prepare a wood material B. When using raw wood such as logs or thinned wood, the logs are cut into short pieces and the bark is removed to prepare the wood material B. When using scrap wood, waste wood, waste pallet wood, recycled wood, or the like generated at a construction site, the logs are cut into short pieces to prepare the wood material B.

[0073] (Cutting step S122) Next, the prepared wood material B is cut into a thin, uniform thickness using a known cutting device having a blade that cuts material into a thin, uniform thickness, to form a large number of wood flakes 3 a, ..., 3 a with a uniform thickness t. Specifically, the thickness setting of the cutting device (for example, the speed at which the wood material B is fed) is adjusted to form a large number of wood flakes 3 a, ..., 3 a with a uniform thickness t, with fibers a appearing in straight lines on the surface.

[0074] In addition, by producing the wood flakes 3a, ..., 3a having a thickness of t in this cutting step S122, the thickness of the small wood flakes 3b, ..., 3b obtained in the subsequent crushing step will also be thickness t.

[0075] (Crushing step S123) Next, the wood flakes 3a, ..., 3a obtained by cutting are crushed using a known crushing device such as a crusher, hammer mill, pin mill, jet mill, ring flaker, or cutter mill to reduce the size of the wood flakes to a size smaller than that immediately after cutting. For example, if the wood flakes 3a, ..., 3a after cutting are crushed in the width direction using a jet mill so as to split along the fiber direction (length direction), thin and long wood flakes 3b, ..., 3b can be produced. In other words, applying a force (impact) to the wood flakes 3a, ..., 3a in the direction perpendicular to the fiber direction makes them less likely to split, but applying a force parallel to the fiber direction makes them more easily split. Even if the wood flakes 3a, ..., 3a are curled, they can be split into flat pieces by crushing. The direction of the force applied to the wood flakes 3a, ..., 3a is not limited to the direction parallel to the fiber direction. When force is applied to the wood flakes 3 a, ..., 3 a in a random direction, the wood flakes 3 a, ..., 3 a usually crack from the part where the force is weaker (they tend to crack in the direction of the weaker force). Since the force connecting the fibers of the wood flakes 3 a, ..., 3 a (the force perpendicular to the fibers) is overwhelmingly weaker than the force in the fiber direction, if the wood flakes 3 a, ..., 3 a are crushed in a random direction using the crushing device (when force is applied), the wood flakes 3 a, ..., 3 a will crack along the fiber direction and shorten in the direction perpendicular to the fiber, resulting in long, thin wood flakes 3 b, ..., 3 b.

[0076] Furthermore, even if the wood flakes 3a, . . . , 3a contain dead knots, the dead knots are more brittle than the other parts, so the dead knots are turned into powder by crushing and are removed in the subsequent classification step.

[0077] In this way, by adding the crushing step S123 after the cutting step S122 and going through two processing steps, rather than just one step, it is possible to easily produce small, high-strength wood flakes 3b, ..., 3b of the required size without dead knots.

[0078] (Classification step S124) The small wood flakes 3b, ..., 3b obtained by pulverization are classified into three sizes using two types of sieves (third and fourth sieves), one large and one small. Specifically, the small wood flakes 3b, ..., 3b are divided into first small wood flakes 3c, ..., 3c of the desired size that pass through the coarse third sieve but not the fine fourth sieve, second small wood flakes 3d, ..., 3d that pass through both the third and fourth sieves (i.e., finer than the first small wood flakes 3c, ..., 3c), and third small wood flakes 3e, ..., 3e that do not pass through either the third or fourth sieve (i.e., coarser than the first small wood flakes 3c, ..., 3c). The second small wood flakes 3d, ..., 3d and the third small wood flakes 3e, ..., 3e other than the first small wood flakes 3c, ..., 3c classified and selected in this manner are used as the wood cutting pieces 3, ..., 3 for the core layer 11.

[0079] [Wood chip mixing process S13] In the wood chip mixing process S13, the numerous wood chips 2, ..., 2 (first wood chips 2b, ..., 2b) for the core layer 11 produced in the wood chip production process are mixed with the numerous wood chips 3, ..., 3 (second small wood chips 3d, ..., 3d and third small wood chips 3e, ..., 3e) for the core layer 11 produced in the wood cutting chip production process so as to achieve the desired mixing ratio (the proportion of the wood cutting chips 3, ..., 3 to the total weight of the wood chips 1, ..., 1 is 10% by weight or more and 90% by weight or less).

[0080] In the wood piece production process S1, the above-mentioned wood crushed piece production process S11, wood shavings production process S12, and wood piece mixing process S13 are carried out to produce wood pieces 1, ..., 1 for the core layer 11 (a mixture of first wood chips 2b, ..., 2b and second small wood flakes 3d, ..., 3d and third small wood flakes 3e, ..., 3e) and wood pieces 1, ..., 1 for the surface layer 12 (second wood chips 2c, ..., 2c).

[0081] <Adhesive Application Step S2> The wood pieces 1, ..., 1 for the core layer 11 and the wood pieces 1, ..., 1 for the surface layer 12 produced in the wood piece production step S1 are each carried into an adhesive application device and adhesive is applied. Examples of adhesives that can be used include isocyanate-based adhesives, as well as amine-based adhesives such as phenolic resins, urea resins, and melamine resins, and natural wood (tannin) adhesives. A commonly used water repellent agent may also be used together with the adhesive.

[0082] <Mat Forming Process S3> In the mat forming process S3, as shown on the left side of Figure 6, first, a large number of wood pieces 1, ..., 1 for the surface layer 12, each coated with adhesive, are stacked together in the thickness direction until they reach a predetermined thickness (height), forming a first mat 9a of wood pieces 1, ..., 1. Next, a large number of wood pieces 1, ..., 1 for the core layer 11, each coated with adhesive, are stacked on top of the first mat 9a, each stacked together in the thickness direction until they reach a predetermined thickness (height), forming a second mat 9b of wood pieces 1, ..., 1. Furthermore, a large number of wood pieces 1, ..., 1 for the surface layer 12, each coated with adhesive, are stacked on top of the second mat 9b, each stacked together in the thickness direction until they reach a predetermined thickness (height), forming a third mat 9c of wood pieces 1, ..., 1. In this manner, the three-layer mat 9 is formed.

[0083] At this time, the thicknesses of the first to third mats 9a to 9c are adjusted to achieve the desired ratio of the thickness of the core layer 11 and each surface layer 12 to the overall particle board 10. For example, when forming a 9-mm particle board 10 in which the surface layer 12 accounts for 15% of the overall particle board 10 thickness (surface layer 12: core layer 11: surface layer 12 = 15:70:15), the mats 9 are formed so that the thickness (height) of the first and third mats 9a, 9c, which will form the surface layers 12, is approximately 13.5 mm, and the thickness (height) of the second mat 9b, which will form the core layer 11, is approximately 63 mm. Note that the thickness ratio of the core layer 11 and each surface layer 12 to the overall particle board 10 is not limited to the above ratio and may be any ratio.

[0084] <Hot Pressing Step S4> The mat 9 formed in the mat forming step S3 is carried into a hot press device and set between heating platens. The mat 9 is then compressed by hot pressing at a predetermined pressure and temperature. At this time, the adhesive hardens, bonding the numerous wood pieces 1 together. As a result, the particle board 10 shown on the right side of Figure 6 is formed.

[0085] At this time, as shown in Figure 6, the mat 9 having a thickness of about 90 mm as exemplified above is compressed into a particle board 10 having a thickness of, for example, 9 mm, and the thickness is compressed to 1 / 10. The pressing temperature for the hot pressing process is not particularly limited, but is, for example, 160 to 220°C. The pressing pressure for the hot pressing process is, for example, 2 to 4 N / mm ２ The pressing time is, for example, 1.5 to 3 minutes (for example, 10 to 20 seconds per mm of thickness). The pressing time varies depending on the thickness of the particle board 10, and may be completed in less than 1 minute or may require more than 3 minutes. Furthermore, a preheating treatment using a heating device may be performed before the hot pressing treatment using the hot pressing device.

[0086] Effect of First Embodiment In the first embodiment, the core layer 11 of the three-layer particle board 10 contains not only the wood pulverized chips 2 (first wood chips 2b), which are typically included, but also wood shavings 3 (3) with an extremely thin, uniform thickness of 0.05 mm to 0.35 mm, which are obtained by cutting. This configuration increases the bending strength of the core layer 11 of the particle board 10. Therefore, it is not necessary to increase the density of the surface layer 12 or use a large amount of adhesive to ensure the bending strength of the particle board 10. Furthermore, compared to when the core layer 11 is composed only of the wood pulverized chips 2, the presence of the thin wood shavings 3 makes it easier to bond the wood pieces 1 (the wood pulverized chips 2 and the wood shavings 3) together with a small amount of adhesive. Therefore, according to the particle board 10 having a three-layer structure of the first embodiment, it is possible to reduce the amount of adhesive used while maintaining bending strength comparable to that of conventional particle boards.

[0087] Furthermore, in the particle board 10 of this embodiment 1, as with conventional particle boards with a three-layer structure, the surface layer 12 is composed of relatively fine wood pulverized pieces 2, ..., 2 (second wood chips 2c, ..., 2c), so that the surface properties (smoothness) are excellent.

[0088] Furthermore, in the particle board 10 of the first embodiment, if some of the wood shavings 2,...,2 contained in the core layer 11 are replaced with wood shavings 3,...,3 that are thinner than the wood shavings 2,...,2, the core layer 11 contains more wood shavings 3,...,3 than the wood shavings 2,...,2 before replacement. The wood shavings 3,...,3 have a uniform thickness and a flat surface. Therefore, if the core layer 11 contains a large number of wood shavings 3,...,3 (if some of the wood shavings 2,...,2 contained in the core layer 11 are replaced with wood shavings 3,...,3), the total surface area of ​​the many wood shavings 1,...,1 that make up the core layer 11 increases significantly, making screws less likely to slip out than in a conventional three-layer particle board whose core layer is composed only of granular wood shavings. Therefore, according to the particle board 10 of this embodiment 1, by including wood shavings 3, ..., 3 in the core layer 11, the core layer 11 is reinforced and the screw pull-out strength (wood screw holding power) is significantly improved, making it suitable as a load-bearing subfloor surface material for screw installation.

[0089] As will be described later, the inventors of the present application conducted tests to verify the effects of including extremely thin, flaky wood shavings 3, 3, with a thickness of 0.05 mm to 0.35 mm, in the core layer 11 of a three-layer particle board 10. As a result, it was found that particle board 10 including the thin wood shavings 3, 3 in the core layer 11 had improved bending strength (MOR), length change due to water absorption (LE), and wood screw holding strength (WS) compared to conventional particle boards not including the thin wood shavings 3, 3 in the core layer 11, and that the thickness swelling due to water absorption (TS) was somewhat improved, although not significantly, and that the absolute value of the thickness swelling due to water absorption (TS) fully met the standard (12% or less) required for structural particle boards.

[0090] As described above, according to the first embodiment, a three-layer particle board 10 having excellent surface properties, bending strength, and screw pull-out strength and small dimensional change can be provided without increasing cost or weight.

[0091] In this embodiment, the core layer 11 is made of a large number of first wood chips 2b, ..., 2b that pass through a first sieve with a mesh size of 16 mm or less but do not pass through a second sieve with a mesh size of 0.5 mm or more that is smaller than the first sieve, and the surface layer 12 is made of fine second wood chips 2c, ..., 2c that pass through both the first and second sieves. By using finer wood chips 2, ..., 2 than the core layer 11 for the surface layer 12 and including wood shavings 3, ..., 3 in the core layer 11, a three-layer particle board 10 with excellent surface properties, bending strength, and screw pull-out strength and minimal dimensional change can be provided without increasing cost or weight.

[0092] Incidentally, based on the results of the above-mentioned verification tests, the inventors of the present application have found that by mixing wood cutting pieces 3, ..., 3 (second wood thin pieces 3d, ..., 3d and third wood thin pieces 3e, ..., 3e) into a large number of wood pieces 1, ..., 1 for the core layer 11 at a ratio of 20% by weight or more and 90% by weight or less of the total weight of the core layer 11, the normal bending strength (MOR), water absorption length change rate (LE) and wood screw holding strength (WS) are improved, and that the higher the mixing ratio of wood cutting pieces 3, ..., 3 in the core layer 11, the greater the improvement rate of normal bending strength (MOR) and water absorption length change rate (LE). Therefore, by mixing wood cuttings 3, ..., 3 with a large number of wood pieces 1, ..., 1 for the core layer 11 in a ratio of 20% by weight to 90% by weight of the total weight, it is possible to provide a three-layer particle board with excellent surface properties, bending strength and screw pull-out strength, and little dimensional change, without increasing cost or weight.

[0093] In addition, in this embodiment 1, the wood shavings 3, 3 are produced when the wood material B is processed to form the other wood board 50 and are used as the wood shavings 3, 3 for the particle board 10. Therefore, even if a material (wood shavings 3, 3) different from the material (ground wood chips 2, 2) used in ordinary particle boards is used, the different material can be easily procured because it is produced when the material is processed to form the other wood board 50. Therefore, according to the manufacturing method of this embodiment 1, a three-layer particle board 10 with excellent surface properties, bending strength, and screw pull-out strength and little dimensional change can be easily and relatively inexpensively provided.

[0094] In particular, in this embodiment 1, the wood shavings 3, 3 included in at least the core layer 11 are those other than the first small wood flakes 3c, 3c (the second small wood flakes 3d, 3d and the third small wood flakes 3e, 3e) selected by classifying the small wood flakes 3b, 3b. In this way, waste wood not used in the other wood boards 50, generated during the formation of the other wood boards 50, is used as the wood shavings 3, 3 for the core layer 11 of the three-layer particle board 10, thereby enabling the three-layer particle board 10 to be formed more inexpensively. Furthermore, since the wood material B of the other wood boards 50 is used without waste, this leads to the effective use of wood resources.

[0095] - Verification of Effects - To verify the above-mentioned effects of including the wood shavings 3 in the core layer 11 of the particle board 10, the inventors prepared the following five types of particle boards 10 (specimens X0 to X4) and conducted bending strength tests, water absorption thickness swelling tests, water absorption length change tests, and wood screw holding strength tests in accordance with JIS A5908 and JIS A5905 to determine the normal bending strength (MOR), water absorption thickness swelling (TS), water absorption length change (LE), and wood screw holding strength (WS). The results are shown in Figure 7. Note that specimen X0 was constructed similarly to a conventional particle board that did not include the wood shavings 3, while specimens X1 to X4 differed in the mixing ratio of the wood shavings 3 in the core layer 11, but were otherwise constructed similarly to the particle board 10 of embodiment 1. 7 shows the test results of specimens X1 to X4 in relative proportions, with the test result of specimen X0 being set at 100. Specimen X0: The wood shavings 3, ..., 3 are mixed in the core layer 11 at 0% by weight Specimen X1: The wood shavings 3, ..., 3 are mixed in the core layer 11 at 20% by weight Specimen X2: The wood shavings 3, ..., 3 are mixed in the core layer 11 at 40% by weight Specimen X3: The wood shavings 3, ..., 3 are mixed in the core layer 11 at 60% by weight Specimen X4: The wood shavings 3, ..., 3 are mixed in the core layer 11 at 80% by weight

[0096] 7, particle boards 10 (specimens X2 to X4) in which the core layer 11 contained 40% or more by weight of wood shavings 3, ..., 3 showed improved normal bending strength (MOR) compared to the conventional particle board (specimen X0) in which the core layer 11 did not contain wood shavings 3, ..., 3. Furthermore, the higher the mixing ratio of the wood shavings 3, ..., 3 in the core layer 11, the higher the normal bending strength (MOR).

[0097] Furthermore, with regard to dimensional change, the particle boards 10 (specimens X1 to X4) containing the wood shavings 3 in the core layer 11 showed some improvement in water absorption thickness swelling (TS) compared to the conventional particle board (specimen X0) not containing the wood shavings 3 in the core layer 11, although not a significant improvement. The absolute value of the water absorption thickness swelling (TS) fully satisfied the standard (12% or less) required for structural particle boards. On the other hand, the particle boards 10 (specimens X1 to X4) containing the wood shavings 3 in the core layer 11 showed an improvement in water absorption length change (LE) compared to the conventional particle board (specimen X0) not containing the wood shavings 3 in the core layer 11. Furthermore, the higher the proportion of wood shavings 3 in the core layer 11, the greater the improvement in water absorption length change (LE).

[0098] Furthermore, the particle boards 10 (specimens X1 to X4) containing wood shavings 3, ..., 3 in the core layer 11 also showed improved wood screw holding strength (WS) compared to the conventional particle board (specimen X0) that did not contain wood shavings 3, ..., 3 in the core layer 11. When the mixing ratio of wood shavings 3, ..., 3 in the core layer 11 was 40% by weight or more, the improvement rate of wood screw holding strength (WS) was 40% or more.

[0099] As described above, particle boards 10 (specimens X2 to X4) containing wood shavings 3, 3 in the core layer 11 had higher normal bending strength (MOR) and wood screw holding power (WS) and lower water absorption length change (LE) compared to conventional particle boards (specimen X0) that did not contain wood shavings 3, 3 in the core layer 11. These results show that by including wood shavings 3, 3 in the core layer 11 of three-layer particle board 10, it is possible to provide a three-layer particle board 10 that not only has excellent surface properties like conventional particle boards, but also has excellent bending strength and screw pull-out strength and exhibits little dimensional change, without increasing cost or weight.

[0100] Second Embodiment of the Invention A particle board 10 according to a second embodiment of the invention is a particle board 10 obtained by partially modifying the configuration of the particle board 10 according to the first embodiment. Specifically, as shown in Figure 8, the particle board 10 according to the second embodiment contains wood shavings 3, 3 not only in the core layer 11 but also in the surface layer 12. In other words, in the second embodiment, both the core layer 11 and the surface layer 12 contain not only a large number of pulverized wood chips 2, 2 but also a large number of wood shavings 3, 3.

[0101] In the first embodiment, among the small wood flakes 3b, ..., 3b formed by cutting and crushing the wood material B to form the wood board 50, those not classified for use in the wood board 50 (second small wood flakes 3d, ..., 3d and third small wood flakes 3e, ..., 3e) were used as the wood shavings 3 to be included in the core layer 11, but in the second embodiment, the first small wood flakes 3c, ..., 3c used in the wood board 50 are used as the wood shavings 3 that make up the core layer 11. Furthermore, in the second embodiment, the first small wood flakes 3c, ..., 3c used in the wood board 50 are also used as the wood shavings 3 to be used in the surface layer 12, just like the wood shavings 3 to be used in the core layer 11.

[0102] As in the first embodiment, the wood shavings 3 used in the particle board 10 (core layer 11 and surface layer 12) are not limited to those described above. The second small wood flakes 3d and the third small wood flakes 3e, which are not used in the wood board 50, may be used as the wood shavings 3 used in the particle board 10. Furthermore, the wood flakes 3a after cutting and before crushing, and the numerous small wood flakes 3b after crushing and before classification, may be used as the wood shavings 3 used in the particle board 10. Furthermore, all, two, three, or four of the wood flakes 3a, the small wood flakes 3b, the first small wood flakes 3c, the second small wood flakes 3d, and the third small wood flakes 3e may be combined and used as the wood shavings 3 used in the particle board 10. In other words, at least one of the wood flakes 3a, the small wood flakes 3b, the first small wood flakes 3c, the second small wood flakes 3d, and the third small wood flakes 3e may be used as the wood shavings 3 used in the particle board 10.

[0103] The other configurations are the same as those in the first embodiment.

[0104] The method for manufacturing the particle board 10 is different from that of the first embodiment only in the wood chip mixing step S13, and the other steps are the same as those of the first embodiment.

[0105] In embodiment 2, in the wood piece mixing process S13, wood crushed pieces 2, ..., 2 and wood cutting pieces 3, ..., 3 are mixed to produce wood pieces 1, ..., 1 for the core layer 11 and wood pieces 1, ..., 1 for the surface layer 12.

[0106] Specifically, the production of the wood pieces 1,...,1 for the core layer 11 is substantially the same as the wood piece mixing step S13 of embodiment 1. The wood pieces 1,...,1 for the core layer 11 are produced by mixing a large number of wood pulverized pieces 2,...,2 for the core layer 11 (first wood chips 2b,...,2b) produced in the wood pulverized piece production step with a large number of wood shavings 3,...,3 for the core layer 11 (first wood thin pieces 3c,...,3c) produced in the wood shavings production step, at a desired mixing ratio (the proportion of the wood shavings 3,...,3 relative to the total weight of the wood pieces 1,...,1 is 10% by weight or more and 90% by weight or less).

[0107] On the other hand, the wood pieces 1, ..., 1 for the surface layer 12 are produced by mixing a large number of wood pulverized pieces 2, ..., 2 (second wood chips 2c, ..., 2c) for the surface layer 12 produced in the wood pulverized piece production process with a large number of wood shavings 3, ..., 3 (first small wood flakes 3c, ..., 3c) for the surface layer 12 produced in the wood shavings production process to a desired mixing ratio (the wood shavings 3, ..., 3 relative to the total weight of the wood pieces 1, ..., 1 is 10% by weight or more and 90% by weight or less).

[0108] Effect of the Second Embodiment In the second embodiment, extremely thin wood shavings 3, 3, with a thickness of 0.05 mm to 0.35 mm, are included not only in the core layer 11 of the three-layer particle board 10 but also in the surface layer 12. This configuration increases the bending strength of not only the core layer 11 but also the surface layer 12 of the particle board 10. Therefore, it is not necessary to increase the density of the surface layer 12 or use a large amount of adhesive to ensure the bending strength of the particle board 10. Furthermore, by including the wood shavings 3, 3 in both the core layer 11 and the surface layer 12, the amount of adhesive used can be further reduced compared to the particle board 10 of the first embodiment, in which the wood shavings 3, 3 are included only in the core layer 11.

[0109] Furthermore, in the particle board 10 according to the second embodiment, the wood chips 3 are extremely thin, with a thickness of 0.05 mm to 0.35 mm, so that unevenness is unlikely to occur on the surface even when they are included in the surface layer 12. Therefore, the surface properties (smoothness) are excellent, similar to those of conventional particle boards.

[0110] Furthermore, in the particle board 10 according to the second embodiment, wood shavings 3 are included not only in the core layer 11 but also in the surface layer 12 (part of the wood pulverized pieces 2 included in the surface layer 12 is replaced with wood shavings 3). By replacing part of the wood pulverized pieces 2 (second wood pulverized pieces 2c) for the surface layer 12, which are finer than the wood pulverized pieces 2b (first wood pulverized pieces 2b) for the core layer 11, with wood shavings 3 having a uniform thickness and flat surfaces, the total surface area of ​​the numerous wood shavings 1 making up the surface layer 12 is significantly increased. Therefore, the particle board 10 according to the second embodiment makes screws less likely to come out. Therefore, the second embodiment provides a three-layer particle board 10 with superior screw pull-out strength (wood screw holding power).

[0111] Furthermore, in the particle board 10 of the second embodiment, some of the relatively fine wood pulverized chips 2 of the surface layer 12 are replaced with wood shavings 3 of uniform thickness and flat surfaces, significantly increasing the total surface area of ​​the numerous wood pulverized chips 1 that make up the surface layer 12. A significant increase in the total surface area of ​​the numerous wood pulverized chips 1 that make up the surface layer 12 increases the bonding area between the numerous wood pulverized chips 1, and also increases the number of bonding points and makes them more evenly distributed. Therefore, the particle board 10 of the second embodiment is more resistant to moisture penetration and has excellent water resistance compared to conventional particle boards whose surface layer 12 is composed only of wood pulverized chips 2.

[0112] Furthermore, in conventional particle boards whose surface layers are made only of crushed wood chips, the crushed wood chips, which are not cut to a thin thickness, are crushed during press molding. When moisture penetrates the interior, the crushed wood chips are more likely to absorb moisture, which can result in significant dimensional changes (changes in length and thickness). In contrast, the particle board 10 of the second embodiment, as described above, has a structure that makes it more difficult for moisture to penetrate the interior than conventional particle boards, and because the numerous replaced wood chippings 3,...,3 are cut to a thin thickness, they do not absorb moisture and therefore do not undergo significant dimensional changes (changes in length and thickness).

[0113] As described above, according to the second embodiment, a three-layer particle board 10 having excellent surface properties, bending strength, screw pull-out strength, and water resistance, and having little dimensional change, can be provided without increasing cost or weight.

[0114] In the second embodiment, the first small wood flakes 3c, 3c selected by classifying the small wood flakes 3b, 3b are used as the wood shavings 3 contained in the core layer 11 and the surface layer 12. The first small wood flakes 3c, 3c selected by classification for use in other wood boards 50 have smaller dimensional variations (length and width) than the second small wood flakes 3d, 3d and the third small wood flakes 3e, 3e used as the wood shavings 3 in the first embodiment. Therefore, by using such first small wood flakes 3c, 3c as the wood shavings 3, 3, a three-layer particle board 10 can be provided that has excellent surface properties, bending strength, and screw pull-out strength and that experiences less dimensional change.

[0115] --Verification of Effects-- In order to verify the above-mentioned effects of including extremely thin, flaky wood shavings 3, ..., 3 with a thickness of 0.05 mm to 0.35 mm not only in the core layer 11 but also in the surface layer 12 of the three-layer particle board 10, the inventors prepared the following three types of particle board 10 (specimens X10, X11, and X12). They conducted bending strength tests, wet bending strength tests, water absorption thickness swelling tests, water absorption length change tests, and wood screw holding strength tests in accordance with JIS A5908 and JIS A5905 to determine the normal bending strength (MOR), water absorption thickness swelling (TS), water absorption length change (LE), and wood screw holding strength (WS). They also conducted a moisture absorption and desorption test to determine the moisture absorption and desorption length change (HLE), and the results are shown in FIG. 9. In the moisture absorption / desorption test, three types of particle boards 10 (specimens X10, X11, and X12) were placed under conditions of 20°C and 65% relative humidity, high temperature and high humidity, and high temperature and low humidity. The length change rate under the high temperature and high humidity conditions was measured, with the length under the 20°C and 35% relative humidity condition used as the reference. The difference in length change rate was calculated as the moisture absorption / desorption length change rate (HLE). Specimen X10 was constructed similarly to a conventional particle board that did not contain wood shavings 3, while specimens X11 and X12 differed in the mixing ratio of wood shavings 3 in the core layer 11 and surface layer 12, but were otherwise constructed similarly to the particle board 10 of embodiment 2. Figure 9 shows the test results for specimens X10, X11, and X12, expressed as a relative ratio when the test result for specimen X10 was set to 100. Specimen X10: The mixture ratio of wood shavings 3, ..., 3 in the core layer 11 and the surface layer 12 is 0% by weight. Specimen X11: The mixture ratio of wood shavings 3, ..., 3 in the core layer 11 is 40% by weight, and the mixture ratio of wood shavings 3, ..., 3 in the surface layer 12 is 0%. Specimen X12: The mixture ratio of wood shavings 3, ..., 3 in the core layer 11 is 40% by weight, and the mixture ratio of wood shavings 3, ..., 3 in the surface layer 12 is 40%.

[0116] As shown in Fig. 9, the test specimen X12 had significantly improved MOR and wet MOR compared to the test specimens X10 and X11. The improvement rates of the test specimen X12 over the test specimen X11 were 48% for MOR and 51% for wet MOR, which were significantly higher than the improvement rates of the test specimen X11 over the test specimen X10 (MOR 28%, wet MOR 26%).

[0117] Regarding dimensional change, the water absorption thickness swelling coefficient (TS) of specimen X12 was not significantly improved compared to specimens X10 and X11, but it was somewhat improved, and the absolute value of TS fully met the standard (12% or less) required for structural particleboard. On the other hand, the water absorption length change coefficient (LE) of specimen X12 was improved compared to specimens X10 and X11.

[0118] Furthermore, the wood screw holding strength (WS) of specimen X12 was improved compared to specimen X10, and the improvement rate was equivalent to the improvement rate of specimen X11 compared to specimen 10.

[0119] Furthermore, the moisture absorption / desorption length change rate (HLE) of specimen X12 was significantly improved compared to specimens X10 and X11. The improvement rate of specimen X12 over specimen X11 was 26%, which was higher than the improvement rate of specimen X11 over specimen X10 (22%).

[0120] Furthermore, with regard to water resistance, the particle board 10 (specimen X12) containing wood shavings 3, ..., 3 in the surface layer 12 showed an improved water resistance index (wet bending strength (wet MOR) / normal bending strength (MOR)) compared to the particle boards (specimens X10 and X11) that did not contain wood shavings 3, ..., 3 in the surface layer 12.

[0121] Furthermore, although not shown in FIG. 9 , the root mean square height Sq and arithmetic mean height Sa (indicators of surface property) of the surfaces of specimens X10 and X12 were measured and compared, and the results showed that both were almost the same, with only a difference of about 1 to 4 μm.

[0122] As described above, the particle board 10 (specimen X12) containing the wood shavings 3,...,3 in the core layer 11 and surface layer 12 had higher normal bending strength (MOR), wet bending strength (wet MOR), wood screw holding power (WS), and water resistance, and lower water absorption length change (LE) and moisture absorption / desorption length change (HLE), compared to the conventional particle board (specimen X10) that did not contain the wood shavings 3,...,3. Furthermore, the particle board 10 (specimen X12) containing the wood shavings 3,...,3 in the surface layer 12 had similar root mean square height Sq and arithmetic mean height Sa (indicators of surface properties) to the conventional particle board (specimen X10) that did not contain the wood shavings 3,...,3 in the surface layer 12. These results show that according to the second embodiment, in which the core layer 11 and surface layer 12 of the particle board 10 contain wood shavings 3, ..., 3, it is possible to provide a three-layer particle board 10 that not only has excellent surface properties like the conventional particle boards, but also has excellent bending strength, screw pull-out strength, and water resistance, and exhibits little dimensional change, without increasing cost or weight.

[0123] Furthermore, the particle board 10 (specimen X12) containing the wood shavings 3 in the core layer 11 and the surface layer 12 had higher normal bending strength (MOR), wet bending strength (wet MOR), and water resistance, and lower water absorption length change (LE) and moisture absorption / desorption length change (HLE), compared to the particle board 10 (specimen X11) containing the wood shavings 3 only in the core layer 11. These results show that embodiment 2, in which the core layer 11 and the surface layer 12 of the particle board 10 contain the wood shavings 3, 3, can provide a three-layer particle board 10 that is superior in bending strength, screw pull-out strength, and water resistance and has smaller dimensional change than embodiment 1, in which the core layer 11 of the particle board 10 contains the wood shavings 3, 3. This can be achieved without increasing cost or weight.

[0124] Other Embodiments In the above-described embodiments 1 and 2, the two surface layers 12, 12 are configured to be thinner (height) than the core layer 11, but the two surface layers 12, 12 may also be thicker than the core layer 11.

[0125] In addition, in the above embodiments 1 and 2, examples have been described in which wood shavings generated during material processing of other wood boards 50 are used as wood shavings 3, ..., 3, but the wood shavings 3, ..., 3 may not be generated during material processing of other wood boards 50, but may be procured to form the above-mentioned particle board 10.

[0126] The present invention is useful for a particle board having a three-layer structure and a method for manufacturing a particle board having a three-layer structure.

[0127] 2 Crushed wood pieces 3 Cut wood pieces 3a Wood thin pieces 3b Small wood pieces 3c First small wood pieces 3d Second small wood pieces 3e Third small wood pieces 10 Particle board 11 Core layer 12 Surface layer 50 Wood board

Claims

1. A particleboard having a three-layer structure, in which the surface layer is made of finer ground wood chips than the core layer, and the core layer contains a large number of thin wood chips having a uniform thickness of 0.05 mm to 0.35 mm.

2. The particle board according to claim 1, characterized in that the surface layer contains a large number of the wood shavings.

3. A particle board as claimed in claim 1 or 2, characterized in that the core layer contains first wood chips as the ground wood pieces for the core layer, which pass through a first sieve with a mesh size of 16 mm or less but do not pass through a second sieve with a mesh size of 0.5 mm or more and smaller than the first sieve, and the surface layer contains second wood chips as the ground wood pieces for the surface layer, which pass through both the first and second sieves.

4. A particle board as claimed in claim 1 or 2, characterized in that in the core layer, the wood shavings are mixed in a ratio of 20% by weight to 90% by weight of the total weight of the wood pieces for the core layer, which is composed of the crushed wood pieces and the wood shavings contained in the core layer.

5. A method for manufacturing a particleboard having a three-layer structure in which fine ground wood chips are used for the surface layer, the method comprising: a first grinding step of grinding a first wood material to produce a large number of wood chips; and a first classification step of classifying the large number of wood chips using a first sieve having a mesh size of 16 mm or less and a second sieve having a mesh size of 0.5 mm or more and smaller than the first sieve, into a large number of first wood chips that pass through the first sieve but not the second sieve, a large number of second wood chips that pass through both the first and second sieves, and a large number of third wood chips that do not pass through both the first and second sieves, and using the first wood chips as ground wood chips for the core layer and the second wood chips as ground wood chips for the surface layer. a cutting step of cutting the second wood material so that the fibers are linearly present on the surface to produce a large number of thin, flake-like wood flakes having a uniform thickness of 0.05 mm to 0.35 mm; a selection step of selecting at least one of the large number of wood flakes, the large number of small wood flakes having an elongated shape produced by a second crushing step in which the large number of wood flakes are crushed so as to break along the fiber direction, and the large number of first to third small wood flakes obtained by a second classification step in which the large number of small wood flakes are classified using a third sieve and a fourth sieve smaller than the third sieve, as a large number of wood shavings for a core layer; a wood piece mixing step of mixing the large number of core layer crushed wood flakes and the large number of core layer wood shavings to produce a large number of core layer wood pieces; and an adhesive application step of applying an adhesive to each of the large number of surface layer wood flakes including the large number of surface layer crushed wood flakes and the large number of core layer wood pieces. The method for manufacturing a particle board comprises a mat forming step of forming a three-layered mat by piling the numerous adhesive-coated wood pieces for the surface layer onto which the adhesive has been applied to form a first mat, piling the numerous adhesive-coated wood pieces for the core layer onto the first mat to form a second mat, and piling the numerous adhesive-coated wood pieces for the surface layer onto the second mat to form a third mat, and a hot pressing step of forming the particle board by subjecting the three-layered mat to a hot pressing process to harden the adhesives of the first to third mats.

6. A method for producing particle board as described in claim 5, characterized in that the cutting process, the second crushing process and the second classification process are included in a method for producing a wood board in which a large number of the first small wood flakes that pass through the third sieve but not the fourth sieve are glued together in a collective state.

7. A method for producing a particle board as set forth in claim 6, characterized in that in the selection step, the numerous first small wood pieces are selected as the numerous wood shavings for the core layer.

8. A method for producing particle board as described in claim 6, characterized in that in the selection process, the numerous second small wood flakes which pass through both the third and fourth sieves and the numerous third small wood flakes which do not pass through both the third and fourth sieves are selected as the numerous wood shavings for the core layer.

9. A method for manufacturing a particleboard as claimed in any one of claims 5 to 8, characterized in that in the wood chip mixing step, the multiple crushed wood chips for the core layer and the multiple wood chips for the core layer are mixed together so that the multiple wood chips for the core layer account for 20% by weight or more and 90% by weight or less of the total weight of the multiple wood chips for the core layer.

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