Wood laminate material and method for manufacturing same

Abstract

Provided is a strand board with improved strength and water resistance. Reduction in productivity is prevented and characteristics of the strand board can be varied as desired while achieving certain strength and water resistance of the strand board. A strand board B is formed by stacking and laminating five strand layers 1 each formed by a large number of strands 5. The strand board B has substantially constant density distribution in the lamination direction of the strand layers 1. Three of the five strand layers 1 of the strand board B are high-density strand layers 1a having a higher density than the other strand layers 1, and the other strand layers 1 are low-density strand layers 1b.

Claims

1. A wood laminate material, comprising: a stack of laminated woodbased material layers, each of the laminated woodbased material layers being a strand mat formed of multiple strands that are cut pieces of wood, wherein fibers of the strands extend in the same direction in each laminated woodbased material layer, the fibers of the strands in adjoining ones of the laminated woodbased material layers extend in directions crossing each other, the wood laminate material as a whole has a substantially constant density distribution in a lamination direction of the laminated woodbased material layers, and the substantially constant density distribution is such that the density does not vary more or less than 60 kg/m.sup.3 from an average density of the stack of laminated woodbased material layers.

2. The wood laminate material according to claim 1, wherein the strands have a density of 300 kg/m.sup.3 or more and 1100 kg/m.sup.3 or less.

3. The wood laminate material according to claim 1, wherein the laminated woodbased material layers are composed so that a thickness of the woodbased material layers gradually increases from the middle woodbased material layer in the lamination direction of the wood laminate material to the top and bottom woodbased material layers.

4. The wood laminate material according to claim 1, wherein of the multiple woodbased material layers, fibers of the strands in the top and bottom woodbased material layers extend in the same direction.

5. The wood laminate material according to claim 1, wherein the number of woodbased material layers is odd.

6. The wood laminate material according to claim 1, wherein the laminated woodbased material layers are laminated so that overall density distribution provided by the laminated woodbased material layers is plane symmetric with respect to a center in the lamination direction.

7. A wood laminate material, comprising: a stack of three or more laminated woodbased material layers, each of the laminated woodbased material layers being a strand mat formed of multiple strands that are cut pieces of wood, wherein: the stack of three or more laminated woodbased material layers includes one or more high-density woodbased material layers, the remainder of the laminated woodbased material layers each being a low-density woodbased material layer, and each of the one or more high-density woodbased material layers has a higher density than each of the low-density woodbased material layers, fibers of the strands extend in the same direction in each woodbased material layer, the fibers of the strands in adjoining ones of the woodbased material layers extend in directions crossing each other, the woodbased material layers located at both ends in the lamination direction of the woodbased material layers are the low-density woodbased material layers, and one or more of the woodbased material layers located in an intermediate part in the lamination direction of the woodbased material layers is the one or more high-density woodbased material layers.

8. The wood laminate material according to claim 7, wherein of the stack of three or more laminated woodbased material layers, fibers of the strands in the top and bottom woodbased material layers extend in the same direction.

9. The wood laminate material according to claim 7, wherein the number of woodbased material layers is odd.

10. The wood laminate material according to claim 7, wherein the stack of three or more laminated woodbased material layers is laminated so that an overall density distribution provided by the stack is plane symmetric with respect to a center of the stack in the lamination direction.

11. The wood laminate material according to claim 7, wherein the strands have a density of 300 kg/m.sup.3 or more and 1100 kg/m.sup.3 or less.

12. The wood laminate material according to claim 7, wherein the stack of three or more laminated woodbased material layers is composed so that a thickness of the woodbased material layers gradually increases from the middle woodbased material layer in the lamination direction of the wood laminate material to the top and bottom woodbased material layers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic perspective view of a multi-layered structure of a strand board according to a first embodiment of the present invention.

(2) FIG. 2 is a schematic perspective view of a first example of a strand board according to a second embodiment of the present invention.

(3) FIG. 3 is a schematic sectional view showing a laminate of strand layers in the first example of the strand board according to the second embodiment.

(4) FIG. 4 is a sectional view corresponding to FIG. 3, showing a second example of the strand board according to the second embodiment.

(5) FIG. 5 is a sectional view corresponding to FIG. 3, showing a third example of the strand board according to the second embodiment.

(6) FIG. 6 is a sectional view corresponding to FIG. 3, showing a fourth example of the strand board according to the second embodiment.

(7) FIG. 7 is a sectional view corresponding to FIG. 3, showing a fifth example of the strand board according to the second embodiment.

(8) FIG. 8 is a sectional view corresponding to FIG. 3, showing a sixth example of the strand board according to the second embodiment.

(9) FIG. 9 is a table illustrating specific configurations of the first to sixth examples of the strand board according to the second embodiment.

(10) FIG. 10 is a sectional image of a strand board of Example 1 of the first embodiment.

(11) FIG. 11 is a table showing the test results of Examples 1, 2 and Comparative Examples 1, 2.

(12) FIG. 12 is a graph showing density distribution in the strand board of Example 1 according to the first embodiment, in which the uniform short-dashed line represents an average density of the woodbased material layers in the thickness direction (lamination direction) and the long-dash short-dash line represents a constant density value for reference.

(13) FIG. 13 is a graph showing density distribution in a strand board of Comparative Example 1 as compared to the first embodiment, in which the uniform short-dashed line represents an average density of the woodbased material layers in the thickness direction (lamination direction) and the long-dash short-dash line represents a constant density value for reference,

(14) FIG. 14 is a table showing the results of a bending test for Examples 1, 2 and Comparative Example 1 of the second embodiment, along with their other physical properties.

(15) FIG. 15 is a graph showing density distribution in the thickness direction (lamination direction) of Examples 1, 2 and Comparative Example 1 according to the second embodiment.

(16) FIG. 16 is a table showing the results of a bending test and a boiling test for Example 3 and Comparative Example 2, along with theft other physical properties.

(17) FIG. 17 is a graph showing density distribution in the thickness direction (lamination direction) of Example 3 and Comparative Example 2 according to the second embodiment.

(18) FIG. 18 is a table showing the results of a bending test and a boiling test for Example 4 and Comparative Example 3, along with their other physical properties.

(19) FIG. 19 is a table showing the result of a nail pull test for Example 4 and Comparative Example 4, along with their other physical properties.

(20) FIG. 20 is a graph showing density distribution in the thickness direction (lamination direction) of Example 4 and Comparative Example 3 according to the second embodiment.

DESCRIPTION OF EMBODIMENTS

(21) Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended in any way to limit the invention, its applications or uses. Specific numerical values in the embodiments are shown merely by way of example in order to facilitate understanding of the invention and are not intended to limit the scope of the invention and materials of the invention.

First Embodiment

(22) FIG. 1 schematically shows a strand board B as a wood laminate material according to a first embodiment of the present invention.

(23) As shown in FIG. 1, the strand board B has an odd number of (in FIG. 1, five) strand layers 1, 1, . . . as woodbased material layers, and all of the strand layers 1, 1, . . . have the same thickness. That is, FIG. 1 shows an example in which, with the upper side in FIG. 1 being the top and the lower side being the bottom, the thickness w1 of the top and bottom strand layers 1, 1 is the same as the thicknesses w2, w3, w2 of the three intermediate strand layers 1, 1, . . . . The strand board B need not necessarily have an odd number of strand layers 1. The strand board B may have an even number of strand layers 1. The number of strand layers 1 is not limited to five. The number of strand layers 1 may be four or less or six or more.

(24) Each strand layer 1 is a mat made of laminated multiple strands 5, 5, . . . (woodbased materials) that are cut pieces of wood etc. Multiple mats of strands 5, 5, . . . are stacked together to form multiple strand layers 1, 1, . . . .

(25) For example, the strands 5 are strands or flakes that are about 150 to 200 millimeters long in the fiber direction, about 15 to 25 millimeters wide, and about 0.3 to 2 millimeters thick.

(26) Wood species that are used for the strands 5 are not particularly limited. For example, tropical wood species or broadleaf trees may be used, or other wood species may be used. Specific examples include Cedar (Cryptomeria japonica), Cypress (Chamaecyparis), sort of first such as Douglas fir (Pseudotsuga menziesii), Acacia (Acacia spp.), Aspen (Populus spp.), Poplar (Populus spp.), Pine (Pinus spp.) (Hard pine (Pinus spp.), Soft pine (Pinus spp.), Radiata pine (Pinus radiata), etc.), Birch (Betula spp.), and Rubber tree (Rubber wood (Hevea brasiliensis)). However, the wood species that are used for the strands 5 are not limited to these, and various other wood species may be used. Examples of the various other wood species include: Japanese wood species such as Sawara cypress (Chamaecyparis pisifera), Japanese elkhorn cypress (Thujopsis dolabrata), Japanese nutmeg-yew (Torreya nucifera), Southern Japanese hemlock (Tsuga sieboldii), Podocarp (Podocarpus macrophyllus), Pinus spp., Princess tree (Paulownia tomentosa), Maple (Acer spp.), Birch (Betula spp.) (Japanese white birch (Betula platyphylla)), Chinquapin (Castanopsis spp.), Japanese beech (Fagus spp.), Live oak (Quercus spp.), Abies firma, Sawtooth oak (Quercus acutissima), Oak (Quercus spp.), Camphor tree (Cinnamomum camphora), and Japanese zelkova (Zelkova serrata); North American wood species such as Port Orford cedar (Chamaecyparis lawsoniana), Yellow cedar (Callitropsis nootkatensis), Western redcedar (Thuja plicata), Grand fir (A. grandis), Noble fir (A. procera), White fir (A. concolor), Spruce (Picea spp.), Western hemlock (Tsuga heterophylla), and Scots pine (Pinus sylvestris); tropical hardwood species such as Agathis (Agathis spp.), Terminalia (Terminalia spp.), Lauan (Shorea spp.), Meranti (Shorea sect.), Sengon laut (A. falcataria), Jongkong (Dactylocladus stenostachys), Kamerere (Eucalyptus deglupta), Kalampayan (Anthocephalus chinensis), Amberoi (Pterocymbium beccarii), Yemane (Gmelina arborea), Teak (Tectona grandis), and Apitong (Dipterocarpus spp.); and other foreign wood species such as Balsa (Ochroma pyramidale), Cedro (Cedrela odorata), Mahogany (Swietenia spp.), Lignum-vitae (Guaiacum spp.), Acacia mangium, Aleppo pine (Pinus halepensis), Bamboo, Sorghum (Sorghum nervosum Bess.), and Kamerere (Eucalyptus deglupta). Any material can be used.

(27) Regarding physical properties of the strands 5, the strands 5 preferably have a density of about 300 to 800 kg/m.sup.3, more preferably 430 to 700 kg/m.sup.3. If the density is less than 300 kg/m.sup.3, a multi-layered mat with a larger thickness is required to form a strand board B of the same density and strength, and a higher pressure need be used for hot pressing in a press process described later.

(28) The strands 5 may have a density higher than 800 kg/m.sup.3, but it is difficult to obtain such strands 5. Namely, if strands 5 having a density higher than 800 kg/m.sup.3 can be easily obtained, the upper limit of the density is not limited to 800 kg/m.sup.3 and may be higher than 800 kg/m.sup.3.

(29) The moisture content of each strand 5 is preferably about 2 to 20%, more preferably 2 to 8%. If the moisture content is less than 2%, it takes more time to soften the strands 5 in the hot pressing of the press process. Namely, the press time is increased, which may cause reduction in strength.

(30) If the moisture content of the strands 5 is higher than 20%, it takes more time to heat and compress the strands 5 in the hot pressing, and the strands 5 tend to be blown. Moreover, curing of an adhesive is inhibited, which may cause reduction in strength.

(31) In each strand layer 1, the strands 5, 5, . . . are oriented such that the fiber direction (longitudinal direction of the strands 5), which is the direction in which fibers (not shown) of the strands 5, 5, . . . extend, is a predetermined direction. As also shown in FIG. 1, the fibers of the strands 5, 5, . . . in each strand layer 1 need not necessarily extend in exactly the same direction. In other words, the fiber directions of the oriented strands 5, 5, . . . do not have to be parallel to each other. Namely, the fiber directions of a part of the strands 5, 5, . . . may be tilted to some extent (e.g., by about 20°) with respect to a predetermined reference direction.

(32) The multiple strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5, 5, . . . in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. That is, in FIG. 1, the fiber direction of the strands 5, 5, . . . in the top strand layer 1 (uppermost layer in FIG. 1) is the same as that of the strands 5, 5, . . . in the bottom strand layer 1 (lowermost layer in FIG. 1).

(33) The first embodiment is characterized in that the strand board B has substantially constant density distribution in the lamination direction of the strand layers 1 (the thickness direction of the strand board B), namely the direction in which the strand layers 1 are laminated. Specifically, the multiple strand layers 1, 1, . . . are laminated so that overall density distribution provided by the multiple strand layers 1, 1, . . . is plane symmetric with respect to the center in the lamination direction of the strand board B.

(34) Next, a method for manufacturing a strand board B according to the first embodiment will be described. This manufacturing method includes a strand producing process, a strand pretreatment process, an adhesive coating process, a stacking process (mat forming process), and a press process.

Strand Producing Process

(35) In the method for manufacturing a strand board B, the strand producing process is first performed in which a large number of strands 5, 5, . . . (cut pieces of wood etc.) are produced. In this process, green wood such as logs or thinnings is cut with, e.g., a cutting machine to produce strands 5, 5, . . . . The strands 5, 5, . . . may be produced from wood scraps, wood wastes, etc. that are generated at construction sites etc. or may be produced from waste wood pallets.

Strand Pretreatment Process

(36) After the strand producing process, it is preferable that a large number of strands 5, 5, . . . produced be subjected to at least one of various strand pretreatment processes described below. This pretreatment is performed in order to allow low-pressure pressing using a pressure as low as, e.g., about 4 N/mm.sup.2 to be performed in the later press process. At least one of a physical treatment method, a high-frequency treatment method, a high-temperature high-pressure treatment method, a high-water pressure treatment method, a repeated deaeration and dehydration treatment method, and a chemical treatment method is used.

(37) The physical treatment method is a method in which the strands 5 are physically compressed. Examples of the physical treatment method include roll pressing, beating, and flat press pressing. The roll pressing is a linear compression method in which, although not shown in the figures, a large number of strands 5, 5, . . . (woodbased materials) are placed in a heat roll press machine such that the strands 5, 5, . . . evenly drop thereon, and the strands 5, 5, . . . are then compressed with heat. For example, this roll pressing is performed under the following conditions: heating temperature: room temperature to 200° C.; clearance between heat rolls: about 0.1 to 0.4 mm; feed rate: about 50 m/min; and compression ratio: about 20 to 60%. The strands 5 are thus compressed without being smashed, whereby high-density strands 5 are produced.

(38) The beating is a point compression method in which, as in metal forging, strands 5 are compressed and deformed by hitting with multiple continuously installed spring hammers etc. As in the roll pressing, the strands 5 are thus compressed without being smashed, whereby high-density strands 5 are produced.

(39) The flat press pressing is a surface compression method in which strands 5, 5, . . . (woodbased materials) are placed in a flat heat press machine and compressed with heat. For example, the flat press pressing is performed at a temperature of 120° C. and a pressure of about 4 N/mm.sup.2 for about five minutes. In the flat press pressing as well, the strands 5 are compressed without being smashed, whereby high-density strands 5 are produced.

(40) The high-frequency treatment method is a method in which strands 5 as dielectrics (nonconductors) are irradiated with high-frequency electromagnetic waves (high-frequency waves) between electrodes etc. and thus dielectrically heated from the inside and softened. This method allows low-pressure pressing using a low pressure to be performed in the later press process without increasing the density of the strands 5 as in the above physical treatment method.

(41) The high-temperature high-pressure treatment method is a method in which strands 5 are placed in a pressure vessel where the strands 5 are subjected to high temperature and high pressure so that cell walls of the strands 5 (woodbased materials) are damaged and the strands 5 are softened. For example, this method is performed at a temperature of 180° C. and a pressure of about 10 Bar for about two minutes. This method also allows low-pressure pressing using a low pressure to be performed in the later press process without increasing the density of the strands 5 as in the above physical treatment method.

(42) The high-water pressure treatment method is a method in which strands 5 are uniformly formed within a mesh material such as metal wire mesh and the surfaces of the strands 5 are finely scratched by high-pressure water of, e.g., about 200 MPa through the mesh material. This produces fine fractures in the strands 5. The softened strands 5 are thus obtained.

(43) The repeated deaeration and dehydration treatment method is a method in which strands 5 are first saturated with water and then placed in a batch type of vessel, and with the vessel being evacuated to vacuum, moisture is removed from the strands 5 to facilitate damage to cell walls of the strands 5 (woodbased materials) and thus soften the strands 5. This method also allows low-pressure pressing using a low pressure to be performed in the later press process without increasing the density of the strands 5 as in the above physical treatment method.

(44) The chemical treatment method is a method in which, for example, sodium hydroxide etc. is added to strands 5 for alkaline treatment to facilitate plasticization of the strands 5 themselves and thus soften the strands 5. This method also allows low-pressure pressing using a low pressure to be performed in the later press process without increasing the density of the strands 5 as in the above physical treatment method.

(45) In the high-frequency treatment method, the high-temperature high-pressure treatment method, the high water pressure treatment method, the repeated deaeration and dehydration treatment method, and the chemical treatment method, the state of the strands 5 after the treatment is maintained by drying the strands 5 as necessary after the treatment.

Adhesive Coating Process

(46) After a large number of strands 5, 5, . . . are thus produced, the adhesive coating process is performed in which the strands 5, 5, . . . are coated with an adhesive. For example, the adhesive may be an isocyanate adhesive or may be an amine adhesive such as a phenol resin, urea resin, or melamine resin.

Stacking Process

(47) Thereafter, the stacking process (mat forming process) is performed in which a large number of strands 5, 5, . . . are oriented and stacked to form strand mats and the strand mats are stacked in multiple layers to form a multi-layered mat.

(48) Specifically, with a mat forming machine etc., a large number of strands 5, 5, . . . coated with the adhesive are oriented such that fibers extend in a predetermined reference direction, and are stacked to a thickness of, e.g., about 7 to 12 mm to form a strand mat with a certain thickness. The thickness of the strand mat is not limited to the above values. The thickness of the strand mat may be less than 7 mm or more than 12 mm.

(49) After the strand mat with a certain thickness is thus formed, strands 5, 5, . . . oriented such that the fiber direction of the strands 5, 5, . . . is, e.g., perpendicular to that of the strands 5, 5, . . . in the strand mat are stacked on top of the strand mat to form another strand mat with a certain thickness.

(50) Subsequently, an additional strand mat is repeatedly stacked in a similar manner until the stack has a desired number of layers (e.g., five layers). At this time, the strand mats are stacked so that the fiber directions of the strands 5, 5, . . . in adjoining ones of the strand mats are perpendicular to each other. A multi-layered mat is formed in this manner. As shown in FIG. 1, in the case of the strand board B having the five strand layers 1, 1, . . . , the thickness of the five-layered mat is, e.g., about 35 to 60 mm.

(51) The number of strand mats in the multi-layered mat is determined based on the number of layers in the strand board B. Accordingly, the number of strand mats in the multi-layered mat may be four or less or six or more.

(52) The density of the strands 5, 5, . . . of the strand layer 1 may be either about the same or different between or among the multiple strand layers 1, 1, . . . .

(53) (Press Process)

(54) After the multi-layered mat is thus formed by stacking multiple strand mats, hot pressing is performed at a predetermined pressure and temperature with a hot press machine to compress or compact the multi-layered mat. This hot pressing is performed at a pressure of, e.g., 2 to 4 N/mm.sup.2 for, e.g., about 10 to 20 minutes. The press time varies depending on the thickness of the strand board B (finished product). Accordingly, in some cases, it may take less than 10 minutes to complete the hot pressing, and in other cases, it may take 20 minutes or more to complete the hot pressing. Pre-heat treatment with a heater may be performed before the hot pressing with the hot press machine.

(55) A strand board B having a density of 750 to 950 kg/m.sup.3 and flexural strength of 80 to 150 N/mm.sup.2 is thus formed as a laminate by these processes.

(56) In the first embodiment, the pressure for the hot pressing in the press process is as low as 2 to 4 N/mm.sup.2. A high density, high strength strand board B can thus be produced without using a special high pressure press machine.

(57) In the strand board B, the fiber direction of the strands 5, 5, . . . in the top strand layer 1 of the strand board B is the same as that of the strands 5, 5, . . . in the bottom strand layer 1 of the strand board B. Performance such as load resistance and impact resistance in the top part of the strand board B is therefore about the same as that in the bottom part of the strand board B. That is, this configuration allows the strand board B to have similar performance in its top and bottom parts. This is advantageous in that the user can use the strand board B without having to worry about which side is the top and which side is the bottom.

(58) The multiple strand layers 1, 1, . . . have about the same thickness. This allows the strand board B to have uniform board performance such as strength properties and water resistance properties in the thickness direction.

(59) Density distribution in the thickness direction of the strand board B formed by the strand layers 1, 1, . . . is plane symmetric. This allows the strand board B to have similar performance in its top and bottom parts. The user can therefore use the strand board B without having to know (worry about) which side is the top and which side is the bottom.

(60) In the case where the number of strand layers 1, 1, . . . is odd, the strand board B has similar performance in its top and bottom parts, as described above.

(61) As described above, it is preferable that the strands 5, 5, . . . produced in the strand producing process have a density of 430 to 700 kg/m.sup.3 and a moisture content of 2 to 20%. However, the strands 5, 5, . . . produced in the strand producing process can be used even if their properties are out of these preferred ranges.

(62) Specifically, the strands 5, 5, . . . having desired properties may be separated from strands cut (or sliced) from logs by a screening machine etc., and the strands 5, 5, . . . thus separated may be subjected to the subsequent processes, namely the strand producing process, the strand pretreatment process, the adhesive coating process, the stacking process (mat forming process), and the press process.

(63) The substantial moisture content and density of the strands 5, 5, . . . may be adjusted by changing, e.g., the composition, the coating method, etc. of the adhesive that is used in the adhesive coating process as desired. A predetermined pressing process may be performed, e.g., during or before the hot pressing in the press process. Specifically, a pressing process (including a compressed process) may be divided into in multiple stages to adjust the substantial moisture content of the strands 5, 5, . . . or increase the substantial density of the strands 5, 5, . . . for the hot pressing.

Second Embodiment

(64) FIGS. 2 to 8 shows a second embodiment of the present invention (the same portions as those in FIG. 1 are denoted with the same reference characters and detailed description thereof will omitted). FIGS. 2 to 8 show examples of a strand board B that is a wood laminate material according to the second embodiment. FIGS. 2 and 3 show a first example of the strand board B. FIG. 4 shows a second example, FIG. 5 shows a third example, FIG. 6 shows a fourth example, FIG. 7 shows a fifth example, and FIG. 8 shows a sixth example.

(65) In each of the first to sixth examples, the strand board B includes strand layers 1, 1, . . . as multiple (an odd number of) woodbased material layers. Each strand layer 1 is made of a mat of a large number of strands 5, 5, . . . (woodbased materials) that are cut pieces. Multiple mats of strands 5, 5, . . . are stacked together to form multiple strand layers 1, 1, . . . .

(66) In the second embodiment, the upper side in FIGS. 3 to 8 is the top of the strand board B and the lower side is the bottom thereof, and the strand layers 1, 1, . . . are sequentially numbered from top to bottom as the first strand layer 1, the second strand layer 1, the third strand layer 1, . . . . The strand layers 1, 1, . . . are thus marked with circled numbers in the FIGS. 3 to 8.

(67) In the second embodiment, the strands 5 preferably have a density of about 300 to 1100 kg/m.sup.3. If the density is less than 300 kg/m.sup.3, a multi-layered mat with a larger thickness is required to form high-density strand layers 1a, and a higher pressure need be used for hot pressing in a press process.

(68) The strands 5 may have a density higher than 1100 kg/m.sup.3, but it is difficult to obtain such strands 5. Namely, if strands 5 having a density higher than 1100 kg/m.sup.3 can be easily obtained, the upper limit of the density is not limited to 1100 kg/m.sup.3 and may be higher than 1100 kg/m.sup.3.

(69) In the second embodiment as well, the strands 5, 5, . . . in each strand layer 1 are oriented such that the fibers of the strands 5, 5, . . . extend in a predetermined direction. As also shown in FIG. 2, the fibers of the strands 5, 5, . . . in each strand layer 1 need not necessarily extend in the same direction. Namely, the fiber directions of the oriented strands 5, 5, . . . in each strand layer 1 do not have to be parallel to each other. In other words, the fibers of a part of the strands 5, 5, . . . in each strand layer 1 may be tilted to some extent with respect to a predetermined reference direction. For example, a part of the strands 5, 5, . . . in each strand layer 1 may be oriented so as to be tilted by about 20° with respect to the reference direction.

(70) The second embodiment is characterized in that, unlike in the first embodiment, at least one of the odd number of strand layers 1, 1, . . . in the strand board B is a high-density strand layer 1a having a density higher than the remaining strand layers 1b, and the remaining strand layers 1b are low-density strand layers. The “density of the strand layer” as used in the second embodiment does not refer to the density of the strands 5 but refers to the density of the strand layer 1 itself made of a mat of the strands 5.

(71) Each example of the strand board B will be specifically described in detail. In FIGS. 3 to 8, the layers shaded with dense dots represent the high-density strand layers 1a and the layers shaded with sparse dots represent the low-density strand layers 1b.

First Example

(72) FIGS. 2 and 3 show the first example of the strand board B according to the second embodiment. This strand board B has five strand layers, namely first to fifth strand layers 1, 1, . . . . These strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5, 5 in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. The fiber direction of the strands 5, 5 in the first strand layer 1 located at the top of the strand board B, namely in the uppermost strand layer 1 in FIG. 3, is the same as that of the strands 5, 5 in the fifth strand layer 1 located at the bottom of the strand board B, namely in the lowermost strand layer 1 in FIG. 3.

(73) Two of the five strand layers 1, 1, . . . are high-density strand layers 1a having a density higher than the other three strand layers, which are low-density strand layers 1b. The two high-density strand layers 1a, 1a have the same density, which is, e.g., about 1000 kg/m.sup.3 (average value). The three low-density strand layers 1b, 1b, . . . have the same density, which is, e.g., about 800 kg/m.sup.3. The density of these low-density strand layers 1b is about the same as that of common strand boards.

(74) Specifically, the first strand layer 1 located at the top of the strand board B, the fifth strand layer 1 located at the bottom of the strand board B, and the third strand layer 1 located in the middle part in the thickness direction of the strand board B are low-density strand layers 1b. Both the second and fourth strand layers 1, 1 located in the parts of the strand board B other than the top and bottom and the middle part in the thickness direction of the strand board B are high-density strand layers 1a.

(75) The five strand layers 1, 1, . . . have three different thicknesses. The thickness of each of the first and fifth strand layers 1, 1 (low-density strand layers 1b) is, e.g., 25% of the total thickness of the strand board B, the thickness of each of the second and fourth strand layers 1, 1 (high-density strand layers 1a) is, e.g., 20% of the total thickness of the strand board B, and the thickness of the third strand layer 1 (low-density strand layer 1b) is, e.g., 10% of the total thickness of the strand board B. The total thickness of the high-density strand layers 1a is therefore, e.g., 40% of the total thickness of the strand board B. The five strand layers 1, 1, . . . are laminated so that overall density distribution provided by the strand layers 1, 1, . . . is plane symmetric with respect to the center in the lamination direction, namely in the thickness direction, of the strand board B. The total thickness of the strand board B is, e.g., 28 mm.

(76) A method for manufacturing a strand board B according to the second embodiment will be described. This manufacturing method is applied not only to the strand board B of the first example but also to the strand boards B of the second to sixth examples.

(77) The manufacturing method of the second embodiment is basically the same as the first embodiment. Description of the same parts as those in the first embodiment is omitted, and only the differences will be described in detail.

(78) This manufacturing method has a strand producing process, a strand pretreatment process, an adhesive coating process, a stacking process (mat forming process), and a press process. Of these processes, the strand pretreatment process, the adhesive coating process, and the press process are the same as those of the first embodiment.

(79) In the second embodiment, when forming a multi-layered mat in the mat forming process by stacking another strand mat on top of a strand mat, the strand mat that is to be a high-density strand layer 1a is formed by the strands 5 having a density higher than the strands 5 of the strand mat that is to be a low-density strand layer 1b is. This allows both high-density and low-density strand layers 1a, 1b to be stacked together.

(80) For example, the following two kinds of strands are prepared in advance in the first process of the manufacturing method, namely in the strand producing process: strands with densities in a common range; and strands with densities higher than the common range. The strands with densities in the common range are used as the strands 5 of the strand mat that is to be a low-density strand layer 1b. The strands with densities higher than the common range as a result of compression etc. may be used as the strands 5 of the strand mat that is to be a high-density strand layer 1a.

(81) The wood species etc. of the strands 5 may be different between the strand mat that is to be a high-density strand layer 1a and the strand mat that is to be a low-density strand layer 1b. A wood species having a higher density may be used to produce the strands 5 of the strand mat that is to be a high-density strand layer 1a than a wood species that is used to produce the strands 5 of the strand mat that is to be a low-density layer strand layer 1b.

(82) After the multi-layered mat is formed, the press process is performed in which hot pressing is performed at a predetermined pressure and temperature with a hot press machine to compress or compact the multi-layered mat. In the press process, the hot pressing is performed at a pressure of, e.g., 2 to 4 N/mm.sup.2 as in the first embodiment, but the press time is, e.g., about 10 to 30 minutes. In the second embodiment as well, the press time varies depending on the thickness of the strand board B (finished product). Accordingly, in some cases, it may take less than 10 minutes to complete the hot pressing, and in other cases, it may take 30 minutes or more to complete the hot pressing. Pre-heat treatment with a heater may be performed before the hot pressing with the hot press machine.

(83) As described above, it is preferable that the strands 5 produced in the strand producing process have a density of 300 to 1100 kg/m.sup.3 and a moisture content of 2 to 8%. However, the strands 5 produced in the strand producing process can be used even if their properties are out of these preferred ranges.

Second Example

(84) FIG. 4 shows the second example of the strand board B. As in the first example, this strand board B has five strand layers, namely first to fifth strand layers 1, 1, . . . . These strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5 in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. The fiber direction of the strands 5, 5 in the first strand layer 1 located at the top of the strand board B, namely in the uppermost strand layer 1 in FIG. 4, is the same as that of the strands 5, 5 in the fifth strand layer 1 located at the bottom of the strand board B, namely in the lowermost strand layer 1 in FIG. 4.

(85) Two of the five strand layers 1, 1, . . . are high-density strand layers 1a, and the other three strand layers are low-density strand layers 1b having a density lower than the high-density strand layers 1a. The two high-density strand layers 1a, 1a have the same density, which is, e.g., about 1100 kg/m.sup.3 (average value). This density is higher than that of the high-density strand layers 1a of the first example. The three low-density strand layers 1b, 1b, . . . have the same density, and this density is lower than that of the low-density strand layers 1b of the first example (because the product density of the strand board B is lower than in the first example).

(86) Unlike in the first example, the first strand layer 1 located at the top of the strand board B and the fifth strand layer 1 located at the bottom of the strand board B are high-density strand layers 1a. The remaining strand layers, namely the second to fourth strand layers 1, 1, . . . located in the intermediate part in the thickness direction of the strand board B, are low-density strand layers 1b.

(87) The five strand layers 1, 1, . . . have the same thickness. The thickness of each strand layer 1 is, e.g., 20% of the total thickness of the strand board B. The total thickness of the high-density strand layers 1a is therefore, e.g., 40% of the total thickness of the strand board B. The five strand layers 1, 1, . . . are laminated so that overall density distribution provided by the strand layers 1, 1, . . . is plane symmetric with respect to the center in the thickness direction of the strand board B. The total thickness of the strand board B is, e.g., 9 mm.

Third Example

(88) FIG. 5 shows the third example of the strand board B. Unlike in the second example, the strand board B has seven strand layers, namely first to seven strand layers 1, 1, . . . . These strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5 in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. The fiber direction of the strands 5, 5 in the first strand layer 1 located at the top of the strand board B, namely in the uppermost strand layer 1 in FIG. 5, is the same as that of the strands 5, 5 in the seventh strand layer 1 located at the bottom of the strand board B, namely in the lowermost strand layer 1 in FIG. 5.

(89) Two of the seven strand layers 1, 1, . . . are high-density strand layers 1a. The other five strand layers are low-density strand layers 1b having a density lower than the high-density strand layers 1a. The two high-density strand layers 1a, 1a have the same density, which is, e.g., about 1000 kg/m.sup.3 (average value). This density is the same as that of the high-density strand layers 1a of the first example. The five low-density strand layers 1b, 1b, . . . have the same density, and this density is lower than that of the low-density strand layers 1b of the first example (because the product density of the strand board B is lower than in the first example).

(90) Specifically, the first strand layer 1 located at the top of the strand board B and the seventh strand layer 1 located at the bottom of the strand board B are high-density strand layers 1a. The remaining strand layers, namely the second to sixth strand layers 1, 1, . . . located in the intermediate part in the thickness direction of the strand board B, are low-density strand layers 1b.

(91) The seven strand layers 1, 1, . . . have two different thicknesses. The thickness of each of the first and seventh strand layers 1, 1 (high-density strand layers 1a) is, e.g., 15% of the total thickness of the strand board B, the thickness of each of the second, third, fifth, and sixth strand layers 1, 1, . . . (low-density strand layers 1b) is, e.g., 15% of the total thickness of the strand board B, and the thickness of the fourth strand layer 1 (low-density strand layer 1b) is, e.g., 10% of the total thickness of the strand board B. The total thickness of the high-density strand layers 1a is therefore, e.g., 30% of the total thickness of the strand board B. The seven strand layers 1, 1, . . . are laminated so that overall density distribution provided by the strand layers 1, 1, . . . is plane symmetric with respect to the center in the thickness direction of the strand board B. The total thickness of the strand board B is, e.g., 12 mm.

Fourth Example

(92) FIG. 6 shows the fourth example of the strand board B. Unlike in the second and third examples, this strand board B has three strand layers, namely first to third strand layers 1, 1, . . . . These strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5 in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. The fiber direction of the strands 5, 5 in the first strand layer 1 located at the top of the strand board B, namely in the uppermost strand layer 1 in FIG. 6, is the same as that of the strands 5, 5 in the third strand layer 1 located at the bottom of the strand board B, namely in the lowermost strand layer 1 in FIG. 6.

(93) One of the three strand layers 1, 1, . . . is a high-density strand layer 1a. The other two strand layers are low-density strand layers 1b having a density lower than the high-density strand layer 1a. The density of the high-density strand layer 1a is, e.g., about 800 kg/m.sup.3 (average value), which is lower than that of the high-density strand layers 1a of the second example. The two low-density strand layers 1b, 1b, . . . have the same density, and this density is the same as that of the low-density strand layers 1b of the first example.

(94) Specifically, only the second strand layer 1 located in the middle part (intermediate part) in the thickness direction of the strand board B is a high-density strand layer 1a, and the first and third strand layers 1, 1 located at the top and bottom of the strand board B are low-density strand layers 1b.

(95) The three strand layers 1, 1, . . . have two different thicknesses. The thickness of each of the first and third strand layers 1, 1 (low-density strand layers 1b) is, e.g., 20% of the total thickness of the strand board B, and the thickness of the second strand layer 1 (high-density strand layer 1a) is, e.g., 60% of the total thickness of the strand board B. The thickness of the high-density strand layer 1a is therefore, e.g., 60% of the total thickness of the strand board B. The three strand layers 1, 1, . . . are laminated so that overall density distribution provided by the strand layers 1, 1, . . . is plane symmetric with respect to the center in the thickness direction of the strand board B. The total thickness of the strand board B is, e.g., 18 mm.

Fifth Example

(96) FIG. 7 shows the fifth example of the strand board B. As in the fourth example, this strand board B has three strand layers, namely first to third strand layers 1, 1, . . . . Unlike in the first to fourth examples, these strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5 in adjoining ones of the strand layers 1 extend in directions parallel to each other. That is, the fiber direction of the strands 5, 5 in the first strand layer 1 located at the top of the strand board B, namely in the uppermost strand layer 1 in FIG. 7, is the same as that of the strands 5, 5 in the third strand layer 1 located at the bottom of the strand board B, namely in the lowermost strand layer 1 in FIG. 7. The fiber direction of the strands 5, 5 in the second strand layer 1 located in the middle part in the thickness direction of the strand board B is also the same as that of the strands 5, 5 in the first and third strand layers 1.

(97) Unlike in the fourth example, two of the three strand layers 1, 1, . . . are high-density strand layers 1a. The remaining one strand layer is a low-density strand layer 1b. The two high-density strand layers 1a, 1a have a density of, e.g., about 800 kg/m.sup.3 (average value). This density is the same as that of the high-density strand layer 1a of the fourth example. The density of the low-density strand layer 1b is lower than that of the low-density strand layers 1b of the first example (because the product density of the strand board B is lower than in the first example).

(98) Specifically, the first and third strand layers 1, 1 located at the top and bottom of the strand board B are high-density strand layers 1a, and only the second strand layer 1 located in the middle part in the thickness direction of the strand board B is a low-density strand layer 1b.

(99) The three strand layers 1, 1, . . . have two different thicknesses. The thickness of each of the first and third strand layers 1, 1 (high-density strand layers 1a) is, e.g., 40% of the total thickness of the strand board B, and the thickness of the second strand layer 1 (low-density strand layer 1b) is, e.g., 20% of the total thickness of the strand board B. The total thickness of the high-density strand layers 1a is therefore, e.g., 80% of the total thickness of the strand board B. The three strand layers 1, 1, . . . are laminated so that overall density distribution provided by the strand layers 1, 1, . . . is plane symmetric with respect to the center in the thickness direction of the strand board B. The total thickness of the strand board B is, e.g., 15 mm.

Sixth Example

(100) FIG. 8 shows the sixth example of the strand board B. As in the first example, this strand board B has five strand layers, namely first to fifth strand layers 1, 1, . . . . These strand layers 1, 1, . . . are stacked and laminated such that the fibers of the strands 5 in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. The fiber direction of the strands 5, 5 in the first strand layer 1 located at the top of the strand board B, namely in the uppermost strand layer 1 in FIG. 8, is the same as that of the strands 5, 5 in the fifth strand layer 1 located at the bottom of the strand board B, namely in the lowermost strand layer 1 in FIG. 8.

(101) Three of the five strand layers 1, 1, . . . are high-density strand layers 1a. The other two strand layers are low-density strand layers 1b having a density lower than the high-density strand layers 1a. The three high-density strand layers 1a, 1a, . . . have the same density, which is, e.g., about 1000 kg/m.sup.3 (average value). This density is the same as that of the high-density strand layers 1a of the first example. The two low-density strand layers 1b, 1b, . . . have the same density. This density is the same as that of the low-density strand layers 1b of the first example.

(102) Specifically, as opposed to the second example, the second to fourth strand layers 1, 1, . . . located in the intermediate part in the thickness direction of the strand board B are high-density strand layers 1a. The remaining strand layers, namely the first strand layer 1 located at the top of the strand board B and the fifth strand layer 1 located at the bottom of the strand board B, are low-density strand layers 1b.

(103) The five strand layers 1, 1, . . . have three different thicknesses. The thickness of each of the first and fifth strand layers 1, 1 (low-density strand layers 1b) is, e.g., 30% of the total thickness of the strand board B, the thickness of each of the second and fourth strand layers 1, 1 (high-density strand layers 1a) is, e.g., 15% of the total thickness of the strand board B, and the thickness of the third strand layer 1 (high-density strand layer 1a) is, e.g., 10% of the total thickness of the strand board B. The total thickness of the high-density strand layers 1a is therefore, e.g., 60% of the total thickness of the strand board B. The five strand layers 1, 1, . . . are laminated so that overall density distribution provided by the strand layers 1, 1, . . . is plane symmetric with respect to the center in the thickness direction of the strand board B. The total thickness of the strand board B is, e.g., 28 mm.

(104) FIG. 9 shows specific configurations of the first to sixth examples.

(105) In the second embodiment, the strand board B has multiple strand layers 1, 1, . . . , and a part (one to three) of the multiple strand layers 1, 1, . . . is a high-density strand layer 1a having a density higher than the other strand layers 1, 1, . . . . The high-density strand layer 1a provides high strength and high water resistance of the strand board B, whereby the strand board B having high strength and high water resistance is obtained.

(106) In the case where the density of the strand layer 1 is increased to use this strand layer 1 as a high-density strange layer 1a in the strand board B, the density of only the strands 5 of this high-density strand layer 1a need be increased, and it is not necessary to increase the density of the strands 5 of all the strand layers 1, 1, . . . . The press time with a press machine is therefore reduced accordingly and the pressure to be used is also reduced. This improves productivity and reduces or eliminates the risk of delamination in the press process.

(107) Moreover, one to three of the odd number of strand layers 1, 1, . . . of the strand board B are high-density strand layers 1a. Accordingly, as shown in the first to sixth examples, a layer(s) to be used as a high-density strand layer(s) la can be selected from the multiple strand layers 1, 1, . . . as necessary. Characteristics of the strand board B can therefore be varied as desired by changing the position(s) of the high-density strand layer(s) 1a, so that the strand board B has advantageous effects specific to each example.

(108) That is, in, e.g., the first example shown in FIGS. 2 and 3 (the sixth example shown in FIG. 8 is substantially similar to this configuration), the second and fourth strand layers 1, 1 located in the parts of the strand board B other than the top and bottom and the middle part in the thickness direction of the strand board B are high-density strand layers 1a, and the remaining layers, namely the first, third, and fifth strand layers 1, 1, . . . located at the top and bottom and in the middle part in the thickness direction of the strand board B, are low-density strand layers 1b. This structure is advantageous in that the use of the low-density strand layers 1b in the top and bottom parts reduces the pressure to be used in the press process and the high-density strand layers 1a provide increased nail pull resistance (force) for a nail that is a fastener to be driven into the strand board B. Especially, the sixth example shown in FIG. 8 further improves productivity.

(109) In the second example shown in FIG. 4 and the third example shown in FIG. 5, the strand layers 1, 1 located at the top and bottom of the strand board B are high-density strand layers 1a, and the strand layers 1, 1, . . . located in the intermediate part of the strand board B are low-density strand layers 1b. In this structure, the high-density strand layers 1a in the top and bottom parts increase flexural strength of the strand board B and improve water resistance in the top and bottom parts of the strand board B.

(110) In the fourth example shown in FIG. 6, the strand layer 1 located in the intermediate part in the thickness direction of the strand board B is a high-density strand layer 1a, and the strand layers 1, 1 located in the remaining part of the strand board B are low-density strand layers 1b. In this structure, the strand board B has a higher density in its intermediate part due to the high-density strand layer 1a, and the strand board B has uniform overall density distribution in the thickness direction. Since the high-density strand layer 1a is formed in the intermediate part in the thickness direction of the strand board B and the low-density strand layers 1b are formed in the top and bottom parts of the strand board B, this structure effectively reduces or eliminates the risk of delamination in the press process and improves productivity.

(111) In the fifth example shown in FIG. 7, the strand layer 1 located in the middle part in the thickness direction of the strand board B is a high-density strand layer 1a, and the first and third strand layers 1, 1 located at the top and bottom of the strand board B are low-density strand layers 1b. Moreover, the fiber direction of the strands 5, 5, . . . is the same in all of the first to third strand layers 1, 1 . . . . This structure improves flexural strength in the fiber direction and also improves shear strength.

(112) In the first to fourth examples of the strand board B of the second embodiment, the fibers of the strands 5, 5, . . . extend in the same direction in each strand layer 1, and the fibers of the strands 5 in adjoining ones of the strand layers 1 extend in directions perpendicular to each other. This structure has high strength against forces acting in various directions as compared to the case where the fibers of the strands 5 extend in the same direction in all of the strand layers 1, 1, . . . as in the fifth example. The larger the number of strand layers 1 is, the more significant the difference in strength of the strand board B due to the difference in fiber direction between the strand layers 1 is.

(113) On the other hand, in the case where the strands 5, 5 are oriented in the same direction along the entire thickness in the lamination direction of the strand board B as in the fifth example, the strand board B has high strength against a force acting in a specific direction, as described above.

(114) In the second embodiment as well, density distribution in the thickness direction of the strand board B formed by the strand layers 1, 1, . . . is plane symmetric. This allows the strand board B to have similar performance in its top and bottom parts. The user can therefore use the strand board B without having to know which side is the top and which side is the bottom.

(115) Moreover, the strand board B has an odd number of strand layers 1, 1, . . . . This allows the strand board B to have similar performance in its top and bottom parts.

Other Embodiments

(116) The present invention is not limited to the first and second embodiments. In the first embodiment, the thicknesses w1 to w3 of the multiple strand layer 1, 1, . . . are the same. However, the present invention is not limited to this, and the thicknesses W1 to W3 of each layer 1 can be set as desired.

(117) For example, the multiple strand layers 1, 1, . . . may be composed so that the thickness of the strand layer 1 gradually increases from the middle strand layer 1 in the thickness direction (lamination direction) of the strand board B to the top and bottom strand layers 1. That is, the thicknesses of the multiple strand layers 1, 1, . . . in FIG. 1 may have a relation of w1>w2>w3. The strand layers 1 on the outer sides (top and bottom) of the strand board B, which are more likely to be subjected to load and impact and are more susceptible to humidity etc., have a larger thickness than the remaining strand layer(s) 1. This allows the strand board B to have improved performance regarding influences from the external environment.

(118) One or more of the strand layers 1, 1, . . . may have a different thickness from the remaining strand layer(s) 1. For example, the thickness w1 of the top and bottom strand layers 1, 1 may be different from the thicknesses w2, w3 of the three intermediate strand layers 1, 1, . . . . Although not shown in the figures, all of the five strand layers 1, 1, . . . may have different thicknesses from each other.

(119) In the first embodiment, the fiber direction of the strands 5, 5, . . . in every strand layer 1 is perpendicular to that of the strands 5, 5, . . . in its adjoining strand layer 1. However, the present invention is not limited to this. For example, the fiber direction of the strands 1, 1, . . . in a part of the multiple strand layers 1, 1, . . . may be the same as that of the strands 5, 5, . . . in its adjoining layer 1. For example, in the case where the strand layers 1, 1 that are different in form of the strands 5, 5, . . . such as length or density of the strands 5, 5, . . . from each other are formed so as to adjoin each other, the fiber directions of the strands 5, 5, . . . in these adjoining strand layers 1, 1 may be the same.

(120) In the first embodiment, the density or thickness of the strands 5 (woodbased materials) may be different between or among the strand layers 1, 1, . . . of the strand board B.

(121) For example, in the stacking process (mat forming process), the multiple mats of strands 5, 5, . . . may be stacked so that the relative density of the strands 5 of the mat gradually increases from the top and bottom strand layers 1 to the middle strand layer 1 in the thickness direction of the strand board B. When the multi-layered mat is pressed in the press process, the relative density of the outer strand layers 1 that are directly subjected to the pressure of a press machine typically tends to become higher than that of the inner strand layer(s) 1. Since the relative density of the strands 5 of the inner strand layer(s) 1 is thus made higher than that of the strands 5 of the outer strand layers 1 prior to the pressing, the strand board B formed by the pressing has uniform density distribution in the lamination direction. In this case, the wood species of the strands 5 may be different or the same between or among the strand layers 1.

(122) That is, in a part of the strand layers 1 or all of the strand layers 1, the wood species, thickness, density, etc. of the strands 5 of the strand layer 1 can be selected as appropriate according to required characteristics, cost, etc.

(123) In the stacking process (mat forming process) of the first embodiment, the strand mats may be stacked so that at least one of the multiple strand layers 1, 1, . . . is formed by the strands 5, 5, . . . with a high density. This strand layer 1 is a layer formed by the strands 5 having a relatively higher density than the other strand layers 1. Specifically, in the case where the strand board B has, e.g., an odd number of strand layers 1, the strand mats may be stacked so that an odd-numbered strand layer 1 from the top or bottom of the strand board B is formed by the high-density strands 5. The strand mats may be stacked so that a specific one (at least one) of the multiple strand layers 1, 1, . . . are formed by the high-density strands 5, 5, . . . according to, e.g., the use of the strand board B, required strength properties and other performance of the strand board B, etc. In the case where there are multiple strand layers 1 that are formed by the high-density strands 5, 5, . . . , these strand layers 1, 1, . . . may be different in density and thickness from each other.

(124) In the second embodiment, the strand board B has an odd number of strand layers 1, 1, . . . . However, the strand board B may have an even number of strand layers 1. It is preferable that the strand board B have an odd number of strand layers 1, 1, . . . because this allows the strand board B to have similar performance in its top and bottom parts.

(125) In the second embodiment, the fibers of the strands 5 extend in the same direction in each strand layers 1, and the fiber directions of the strands 5, 5 of adjoining ones of the strand layers 1 are either perpendicular or parallel to each other. However, the present invention is not limited to this. The fiber direction of the strands 5 of each strand layer 1 may be determined as desired.

(126) The first and second embodiments are described with respect to the strand board B formed by stacking and laminating the mats of the strands 5 into the shape of a board. However, the present invention is not limited to this strand board B. For example, multiple strand layers having a rectangular section (in the shape of squared timber) and having no significant difference between their thickness and width may be stacked and laminated. In this case, a strand material (wood laminate material) can be used as a joist, pillar, etc. formed by stacking and laminating multiple strand layers.

(127) The first and second embodiments are examples of the strand board B formed by stacking and laminating the multiple strand layers 1, 1, . . . each formed by laminated multiple strands 5, 5, . . . . However, the present invention is also applicable to, e.g., plywood and laminated veneer lumber (LVL). Specifically, veneers may be used instead of the mats of strands 5. That is, in the case of plywood and LVL, each woodbased material layer is formed by at least one veneer.

(128) In the case where the wood laminate material is plywood or LVL, a common manufacturing method is used to manufacture plywood or LVL. Specifically, green wood such as logs or thinnings is cut with a cutting machine to produce veneers. Multiple veneers are then stacked together with an adhesive therebetween such that the fiber directions of adjoining ones of the veneers are the same in the case of LVL and the fiber directions of adjoining ones of the veneers are perpendicular to each other in the case of plywood. Subsequently, the staked veneers are formed by cold pressing or hot pressing to cure the adhesive.

(129) In the case where density distribution in the lamination direction of the woodbased material layers is to be made substantially uniform as in the first embodiment, the density, thickness, etc. of each veneer are adjusted before, e.g., the stacked veneers are formed in the press process.

(130) In the case where the woodbased material layers consist of a combination of high-density and low-density woodbased material layers as in the second embodiment, the density of the woodbased material or woodbased materials forming each woodbased material layer is made higher in a part of the woodbased material layers than in the remainder of the woodbased material layers by wood species etc. before, e.g., the stacked veneers are formed in the press process.

EXAMPLES

(131) Next, specific examples of the strand boards according to the first and second embodiments will be described. It should be noted that “examples” and “comparative examples” of the first embodiment are different from “examples” and “comparative examples” of second embodiments even though their numbers are the same. The examples and the comparative examples are specified for each embodiment.

First Embodiment

Example 1

(132) Mats of a large number of cypress strands were stacked into a multi-layered mat having five strand layers and a thickness of 37 mm. The strands were 150 to 200 mm long in the fiber direction, 15 to 25 mm wide, and 0.8 to 2 mm thick and had a density of 500 to 600 kg/m.sup.3. The multi-layered mat was then subjected to hot pressing at 140° C. and 4 N/mm.sup.2 for 10 minutes, whereby a strand board with a density of 818 kg/m.sup.3 and a thickness of 12.4 mm was obtained. This strand board was used as Example 1.

(133) FIG. 10 shows an image of Example 1. In FIG. 10, reference character “B” indicates the strand board and “1” indicates the strand layers. FIG. 11 shows the results of a bending test, a dimensional change test, and a water absorption test for Example 1. FIG. 12 shows the density distribution in the thickness direction (lamination direction) of the strand board measured with a density profile analyzer (“DENSE-LAB X” made by ELECTRONIC WOOD SYSTEMS GMBH).

Example 2

(134) Mats of a large number of Douglas fir strands were stacked into a multi-layered mat having five strand layers and a thickness of 36 mm. The strands were 150 to 200 mm long in the fiber direction, 15 to 25 mm wide, and 0.8 to 2 mm thick and had a density of 450 to 550 kg/m.sup.3. The multi-layered mat was then subjected to hot pressing at 140° C. and 4 N/mm.sup.2 for 10 minutes, whereby a strand board with a density of 832 kg/m.sup.3 and a thickness of 12.2 mm was obtained. This strand board was used as Example 2. FIG. 11 shows the results of the bending test, the dimensional change test, and the water absorption test for Example 2.

Comparative Example 1

(135) Mats of a large number of cypress strands were stacked into a multi-layered mat having five strand layers and a thickness of 42 mm. The strands were 150 to 200 mm long in the fiber direction, 15 to 25 mm wide, and 0.8 to 2 mm thick and had a density of 400 to 500 kg/m.sup.3. The multi-layered mat was then subjected to hot pressing at 140° C. and 8 N/mm.sup.2 for 10 minutes, whereby a strand board with a density of 779 kg/m.sup.3 and a thickness of 12.7 mm was obtained. This strand board was used as Comparative Example 1. FIG. 11 shows the results of the bending test, the dimensional change test, and the water absorption test for Comparative Example 1. FIG. 13 shows the density distribution in the thickness direction (lamination direction) of the strand board measured with the density profile analyzer (“DENSE-LAB X” made by ELECTRONIC WOOD SYSTEMS GMBH).

Comparative Example 2

(136) Mats of a large number of Douglas fir strands were stacked into a multi-layered mat having five strand layers and a thickness of 35 mm. The strands were 150 to 200 mm long in the fiber direction, 15 to 25 mm wide, and 0.8 to 2 mm thick and had a density of 350 to 450 kg/m.sup.3. The multi-layered mat was then subjected to hot pressing at 140° C. and 8 N/mm.sup.2 for 10 minutes, whereby a strand board with a density of 812 kg/m.sup.3 and a thickness of 12.4 mm was obtained. This strand board was used as Comparative Example 2. FIG. 11 shows the results of the bending test, the dimensional change test, and the water absorption test for Comparative Example 2

(137) The results in FIG. 11 show that Example 1 is higher in density, flexural strength, modulus of rupture (MOR), and modulus of elasticity (MOE) than Comparative Example 1. Percentage dimensional change and water absorption of Example 1 are about the same as those of Comparative Example 1. Example 2 has a higher density than Comparative Example 2, approximately the same flexural strength and MOR as Comparative Example 2, and a higher MOE than Comparative Example 2. Percentage dimensional change and water absorption of Example 2 are about the same as those of Comparative Example 2.

(138) The results in FIGS. 12 and 13 show that Example 1 has substantially constant density distribution in the lamination direction of the multiple strand layers as compared to Comparative Example 1. The substantially constant density distribution includes such density distribution that, in the case where the measurement result of the density distribution varies as shown in, e.g., FIGS. 12 and 13, the median shown by dashed line as shown in each figure does not vary significantly but is substantially constant. For example, as can be seen from comparison between the dashed line shown in FIG. 12 (Example 1) and the dashed line shown in FIG. 13 (Comparative Example 1), the median of the density distribution shown in FIG. 12 varies less than the median of the density distribution shown in FIG. 13, and the median of the density distribution shown in FIG. 12 is substantially constant.

(139) Since the density distribution is substantially constant, the strand board has uniform density distribution and overall water resistance and strength (shear strength etc.) of the strand board are improved. Specifically, a portion with a low density has lower water resistance and strength than a portion with a high density. Accordingly, if the density distribution is uneven, the overall performance of the strand board is governed by the water resistance and strength of the portion with a low density. However, in the case where the density distribution is substantially constant, such a portion that becomes a bottleneck for performance can be eliminated.

(140) The above bending test was conducted in accordance with IICL_Floor_Performance TB001 Ver. 2. The dimensional change test and the water absorption test were conducted in accordance with the cyclic boiling test of Japanese Agricultural Standard for plywood.

Second Embodiment

Example 1

(141) Mats of a large number of aspen strands were stacked into a multi-layered mat having five strand layers and a thickness of 53 mm. The strands had a thickness of 0.8 mm and a density of 300 to 600 kg/m.sup.3. As in the second example (see FIG. 4) of the strand board of the second embodiment, strands with common densities (average value: 393 kg/m.sup.3) were used for the second to fourth strand layers located in the intermediate part in the lamination direction out of the five strand layers. Strands with higher densities (average value: 557 kg/m.sup.3) than the common densities were used for the first and fifth strand layers located at both ends in the lamination direction.

(142) The multi-layered mat was then subjected to hot pressing at 160° C. and 4 N/mm.sup.2 for 8 minutes. The strand board thus obtained was used as Example 1. The time required to achieve a target thickness, namely the time required to press the multi-layered mat to a target thickness, was 24 seconds.

Example 2

(143) A multi-layered mat having five strand layers and a thickness of 52 mm was formed in a manner similar to that in Example 1. Strands having a density (average value: 805 kg/m.sup.3) higher than Example 1 were used for the first and fifth strand layers located at both ends in the lamination direction out of the five strand layers. The multi-layered mat was then subjected to hot pressing under conditions similar to those in Example 1. The strand board thus obtained was used as Example 2. The time required to achieve a target thickness was 12 seconds. Example 2 is otherwise the same as Example 1.

Comparative Example 1

(144) A multi-layered mat having five strand layers and a thickness of 62 mm was formed in a manner similar to that in Example 1. Strands with common densities (average value: 393 kg/m.sup.3) were used for all of the five strand layers. The multi-layered mat was then subjected to hot pressing under conditions similar to those in Example 1. The strand board thus obtained was used as Comparative Example 1. The time required to achieve a target thickness was 33 seconds. Comparative Example 1 is otherwise the same as Example 1.

Test A

(145) A normal-state bending test (bending test span: 225 mm) was conducted on each of Examples 1, 2 and Comparative Example 1. FIG. 14 shows the test results along with other physical properties.

(146) Density distribution in thickness direction (lamination direction) of each strand board was measured with the density profile analyzer (“DENSE-LAB X” made by ELECTRONIC WOOD SYSTEMS GMBH). FIG. 15 shows the measurement results.

(147) The results in FIG. 14 show that, as can be seen from comparison between Examples 1, 2 and Comparative Example 1, the use of high-density strand layers as the first and fifth strand layers located at the top and bottom out of the five strand layers allows the multi-layered mat before hot pressing to have a smaller thickness (bulk height) and thus facilitates compression of the multi-layered mat by hot pressing, thereby reducing the press time required to press the multi-layered mat to a target thickness (time to achieve the target thickness). Regarding flexural properties in the normal-state bending test, MORs and MOEs of Examples 1, 2 are about the same as those of Comparative Example 1.

Example 3

(148) Mats of a large number of aspen strands were stacked into a multi-layered mat having five strand layers and a thickness of 70 mm. The strands had a thickness of 0.8 mm and a density of 300 to 600 kg/m.sup.3. As in the first example (see FIG. 3) of the strand board of the second embodiment, strands with common densities (average value: 393 kg/m.sup.3) were used for the first, third, and fifth strand layers of the five strand layers, namely the strand layers other than the second and fourth strand layers located in the intermediate part in the lamination direction. Strands with higher densities (average value: 933 kg/m.sup.3) than the common densities were used for the second and fourth strand layers.

(149) The multi-layered mat was then subjected to hot pressing at 140° C. and 4 N/mm.sup.2 for 10 minutes, whereby a strand board having a density of 846 kg/m.sup.3 and a thickness of 12.5 mm was obtained. This strand board was used as Example 3. The MDI content or dozing was 12%.

Comparative Example 2

(150) A multi-layered mat having five strand layers and a thickness of 78 mm was formed in a manner similar to that in Example 3. Strands with common densities (average value: 393 kg/m.sup.3) were used for all of the five strand layers. The multi-layered mat was then subjected to hot pressing at 140° C. and 8 N/mm.sup.2 for 10 minutes, whereby a strand board having a density of 846 kg/m.sup.3 and a thickness of 12.6 mm was obtained. This strand board was used as Comparative Example 2. Comparative Example 2 is otherwise the same as Example 3.

Test B

(151) A normal-state bending test and a boiling test were conducted on Example 3 and Comparative Example 2. The boiling test was conducted in accordance with the cyclic boiling test defined in Japanese Agricultural Standard for Plywood. After the boiling test was conducted twice, thickness swelling TS, water absorption WA, and internal bond strength IB were measured. FIG. 14 shows the measurement results along with other physical properties.

(152) FIG. 17 shows density distribution in the thickness direction (lamination direction) of each strand board measured with the density profile analyzer as in Test A.

(153) The results in FIG. 16 show that, regarding Example 3 in which the second and fourth strand layers located in the intermediate part in the thickness direction out of the five strand layers are high-density strand layers and Comparative Example 2 in which all of the five strand layers are low-density strand layers, flexural strength and internal bond strength IB after the boiling tests of Example 3 are either the same or higher than Comparative Example 2. Namely, the flexural strength and internal bond strength IB after the boiling tests of Example 3 are not lower than Comparative Example 2.

(154) The results thus show that the use of high-density strand layers as the second and fourth strand layers of the five strand layers allows a strand board with performance similar to that of Comparative Example 2 to be formed by using a lower pressure of 4 N/mm.sup.2 instead of such a high pressure (8 N/mm.sup.2) as used in Comparative Example 2.

Example 4

(155) Mats of a large number of aspen strands were stacked into a multi-layered mat having five strand layers and a thickness of 130 mm. The strands had a thickness of 0.8 mm and a density of 300 to 600 kg/m.sup.3. As in the sixth example (see FIG. 8) of the strand board of the second embodiment, strands with common densities (average value: 413 kg/m.sup.3) were used for the first and fifth strand layers of the five strand layers, namely for the strand layers other than the second to fourth strand layers located in the intermediate part in the lamination direction. Strands with higher densities (average value: 1100 kg/m.sup.3) than the common densities were used for the second to fourth strand layers.

(156) The multi-layered mat was then subjected to hot pressing at 160° C. and 8 N/mm.sup.2 for 60 minutes, whereby a strand board having a predetermined density and thickness (see FIG. 18) was obtained. This strand board was used as Example 4.

Comparative Example 3

(157) A multi-layered mat having five strand layers was formed in a manner similar to that in Example 4. Strands with common densities (average value: 413 kg/m.sup.3) were used for all of the five strand layers. The multi-layered mat was then subjected to hot pressing at 140° C. and 8 N/mm.sup.2 for 60 minutes, whereby a strand board having a predetermined density and thickness (see FIG. 18) was obtained. This strand board was used as Comparative Example 3. The processes were otherwise the same as those of Example 4.

Comparative Example 4

(158) A multi-layered mat having five strand layers was formed in a manner similar to that in Example 4. Strands with common densities (average value: 413 kg/m.sup.3) were used for all of the five strand layers. The multi-layered mat was then subjected to hot pressing at 160° C. and 8 N/mm.sup.2 for 30 minutes, whereby a strand board having a predetermined density and thickness was obtained. This strand board was used as Comparative Example 4. In Comparative Example 4, hot pressing was performed at a higher temperature than in Comparative Example 3 in order to avoid insufficient curing of an adhesive during winter time. Comparative Example 4 is small in size, and the press time was shorter than in Example 4 and Comparative Example 3. The processes were otherwise the same as those of Example 4.

Test C

(159) A normal-state bending test, a boiling test, and a bond durability test were conducted on Example 4 and Comparative Example 3. FIG. 18 shows the test results along with other physical properties. In FIG. 18, “Elastic Limit Pmax” refers to elastic limit load, “Ratio of ELP” refers to the ratio of Elastic Limit Pmax to maximum load (Pmax), and “Inside Share Strength” refers to internal shear fracture strength. Regarding the bending direction, “longitudinal” refers to the longitudinal direction of the board, “lateral” refers to the lateral direction of the board, and “N=2 (N=3)” means that the number of test pieces was 2 or 3. Moreover, “TS” indicates thickness swelling, “WA” indicates water absorption, and “IB” indicates internal bond strength.

(160) A nail pull test was conducted on Example 4 and Comparative Example 4. In the nail pull test, a lead hole with an inside diameter of 2 mm and a depth of 25 mm was formed in advance in each sample of Example 4 and Comparative Example 4. Three samples of Example 4 and four samples of Comparative Example 4 were tested, and the average value of the samples was calculated for each of Example 4 and Comparative Example 4. FIG. 19 shows the results.

(161) FIG. 20 shows density distribution in the thickness direction (lamination direction) of each strand board measured with the density profile analyzer as in Test A.

(162) The results in FIG. 18 show that, regarding Example 4 in which the second to fourth strand layers located in the intermediate part in the lamination direction out of the five strand layers are high-density strand layers and Comparative Example 3 in which all of the five strand layers are low-density strand layers, flexural strength of Example 4 is about the same as that of Comparative Example 3, and internal bond strength after the boiling test of Example 4 is higher than that of Comparative Example 3.

(163) These results show that the use of high-density strand layers as the second to fourth strand layers of the five strand layers allows a strand board with performance similar to that of Comparative Example 3 to be formed.

(164) The results of FIG. 19 show that the use of high-density strand layers as the second to fourth strand layers located in the intermediate part in the thickness direction out of the five strand layers increases nail pull resistance (force) and achieves improvement in performance.

INDUSTRIAL APPLICABILITY

(165) The present invention is suitable for use as flooring materials for containers, watercraft, vehicles, etc. The present invention is extremely useful as new building materials that are suitable for use as flooring materials and structural bracing boards for buildings such as houses. The present invention thus has high industrial applicability.

DESCRIPTION OF REFERENCE CHARACTERS

(166) B Strand Board (Wood Laminate Material) 1 Strand Layer (Woodbased Material Layer) 1a High-Density Strand Layer (High-Density Woodbased Material Layer) 1b Low-Density Strand Layer (Low-Density Woodbased Material Layer) 5 Strand (Cut Piece)