JPS5820761B2 - treated wood - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to wood coated with a polymeric material film and having a controlled water vapor evaporation rate, and a method for producing the same.

Generally, raw wood contains moisture that is close to 30 to 200 inches of the weight of the wood itself.

When felled wood is left in the atmosphere, the moisture on the surface of the wood evaporates, and the moisture inside diffuses to the surface to compensate for this moisture.

However, as drying progresses, the diffusion of moisture from the interior cannot keep up with the rate of moisture evaporation from the surface, and a moisture gradient or a pressure gradient occurs between the surface and the interior.

This slope generally causes internal moisture to diffuse and move to the surface, but in the case of wood, internal diffusion is slow compared to surface evaporation, so it is necessary to strengthen the drying conditions or expose local areas to direct sunlight or air currents. , internal diffusion cannot follow surface evaporation,
It is presumed that only the surface dries excessively and shrinks, causing so-called dry cracks.

Yields in the lumber industry are extremely low for both softwood and hardwood, estimated at 60-80% and 50-60%, respectively.

Although various reasons for this low yield can be enumerated, the damage caused by the above-mentioned dry cracking is extremely high, and overcoming and resolving this is the biggest challenge in the wood industry, including the lumber industry. It is no exaggeration to say that there are.

Up to now, several proposals have been made to solve the above problems.

For example, A: The end of the wood is sealed in advance with a highly water-resistant resin, and the tannins contained in the wood are converted into natural resin by the catalytic action of a saturated aqueous solution of dibasic potassium phosphate, which is press-injected to make the structure dense. , a method of cutting off the end of the wood and drying it naturally (Special Publication No. 14437-1971);
B: A method of applying paint, wax, etc. to the surface of wood and impregnating it into the inside of the wood by heat and pressure treatment (Japanese Patent Application Laid-Open No. 49-13
No. 4804), a method in which melamine resin is applied to the end of the wood and the wood is forcedly dried using a high-frequency induction furnace (No. 4804).
JP-A No. 51-76405).

However, in method A, the tannin polymerization reaction due to the catalytic action of a saturated aqueous calcium phosphate solution is slow, and it takes several months or more to complete the reaction under normal weather conditions. It cannot be said to be a practically effective method for preventing wood cracking because it is not very effective for evaporation, the process involves multiple stages, and expensive high-pressure equipment is required.

Also, in method B, since it is necessary to impregnate the inside of the wood, there are problems like in method A, such as requiring expensive pressurizing and heating equipment.

Although method C is intended to dry the deep parts of the wood, the drying progresses from the surface due to forced drying by conductive heat, so it cannot achieve a decisive effect in preventing cracking, and even if it is effective, high-frequency dielectric Since it uses a furnace, it is economically disadvantageous and cannot be a practical method for preventing cracking.

In this way, with any of the above methods, in order to achieve the effect of preventing drying and cracking on the wood surface, the applicable wood products are extremely limited and expensive auxiliary equipment is required, so it is not widely practical. It has not yet been achieved.

The present inventors conducted intensive research in view of the above-mentioned current situation regarding technology for preventing the occurrence of dry cracks on the surface of wood.

As a result, when drying wood, especially wood with high moisture content,
It has been found that controlling the water vapor evaporation rate appropriately is essential from the viewpoint of preventing cracking.

Furthermore, it has been found that for this purpose, by covering most of the surface of the wood with a polymer film having a specific range of water vapor permeation, it is possible to obtain wood in which the water vapor evaporation rate is appropriately controlled as described above. .

That is, the present invention has a water vapor permeability of 10 to 100 g/rrI'·day (measurement conditions: temperature 37.5 g/rrI'. The present invention provides wood with a controlled water vapor evaporation rate, characterized by having a coating that is at a temperature of 8° C. and a relative humidity of 90%.

The wood with a controlled water vapor evaporation rate (hereinafter referred to as treated wood) of the present invention can be produced by an extremely simple method by applying the polymer emulsion on the surface of the wood to be treated at room temperature and then drying it naturally or forcefully. can do.

The present invention further provides a method for preventing drying and cracking of unseasoned wood.

That is, after applying a polymer emulsion with a minimum film formation temperature of 0°C to 40°C on the surface of undried wood with a moisture content of 30 to 200 at room temperature, the emulsion is dried to form a continuous film. Accordingly, there is provided a method for treating wood, which is characterized by controlling water vapor permeability and substantially preventing drying and cracking of wood.

One of the characteristics of the treated wood of the present invention is that most of the wood surface
In other words, a polymer film having a water vapor permeation amount within a specific range is provided in close contact over at least 60 parts of the total surface area, preferably 80 parts or more, and more preferably substantially the entire surface area.

In the present invention, the "total surface area of wood" refers to the total area of the exposed parts of the wood surface that are in direct contact with the atmosphere, and refers to the total area of the exposed parts of the wood surface that are in direct contact with the atmosphere. Excludes the surface area of the

As mentioned above, the ratio of the total coating area to the total wood surface area, that is, the coverage ratio, is usually preferably 10% for effectiveness, but considering the moisture content of the wood to be treated, economic efficiency, etc.
It is also possible to have a partially exposed portion within the range of the above ratio.

In this case, if there is a locally exposed part with an excessively large area, dry cracks may occur in that part.

Therefore, it is preferable that the area of the exposed portion be kept within 5 d at most.

It is necessary that the coating provided on the treated wood of the present invention has a water vapor transmission rate of 10 to 100 g/m'' under the above conditions.

When the water vapor permeability exceeds 10og/m, there is no effect of preventing cracking regardless of the type of wood or wood.

If the water vapor permeability is less than 10 g/m, the drying rate will be too low, resulting in quality problems due to insufficient drying and economic problems due to excessive drying time.

Although the mechanism of action for preventing wood surface cracking in the treated wood of the present invention is not clear to date, it is thought to be as follows.

In other words, in general, wood contains 30 to 20% of the weight of the wood itself.
It contains moisture in the range of 0.00%.

Of this water, 30 to 40% or less of the water is bound water that is bound to the wood tissue, and 30 to 40% or more of the water is free water that is not bound to the wood tissue.

When undried wood is left in the air, free water evaporates from the surface of the wood.

To compensate for this moisture, the moisture inside the wood diffuses to the surface.

However, if the evaporation of free water from the wood surface is extremely rapid, the supply of water from inside the wood cannot keep up with the evaporated water.

The moisture content of the wood surface layer approaches the bound water-free water boundary of 30 to 40 ratios, and then the bound water begins to evaporate from the surface layer, causing shrinkage of the surface layer.

If only the surface layer of the wood loses bound water and starts to shrink even though the center of the wood retains free water, the distribution of shrinkage stress becomes uneven, causing cracks in the surface layer of the wood. will occur.

This is because drying cracks are likely to occur on the wood surface when the wood is dried under strong artificial drying conditions, or when the wood is dried under direct sunlight or partially exposed to strong airflow. is also clear.

In the present invention, since the wood surface has the above-mentioned film, moisture does not evaporate rapidly from the wood surface, moisture is sufficiently replenished to the surface layer, and the center of the wood retains free water. Despite this, the bound water does not evaporate from the surface layer.

In other words, only the surface layer of the wood starts to shrink and dry cracks do not occur.

The coating amount of the surface film of the treated wood of the present invention is 5% by dry weight within the range that satisfies the requirements of claim 1.
g/m to 300 g/m, and this amount is variable depending on the water vapor permeability of the film formed from the polymer emulsion.

'Furthermore, the form of the wood to be treated in the present invention includes materials such as felled logs and logs from which the bark has been peeled off, as well as any form of sawn wood having an angular, cylindrical, or plate-like shape obtained by primary processing of these materials. The target tree species are not limited to domestic timber or imported timber, for example.

Furthermore, the wood to be treated is not limited to a single piece of wood, but may also be a collection of two or more pieces, and in particular may be one made by bundling pieces of wood in the shape of square timbers, boards, or the like.

In this case, as mentioned above, it is not necessarily necessary to prevent moisture evaporation from the parts where the pieces of wood come into contact with each other with the composition film, so at least If 60 or more parts of the total of both end faces and all side faces of the wood bundle are coated with the composition film, the effect of preventing drying and cracking is sufficiently recognized.

The film having the above-mentioned characteristics of the present invention is formed from a polymer emulsion that has film-forming properties at room temperature, that is, a polymer emulsion whose minimum film-forming temperature is 0°C to 40°C.

The term "polymer emulsion" as used in the present invention refers to an aqueous dispersion in which particles of a natural or synthetic polymer substance are stably dispersed in water, and includes all those commonly referred to as latex or emulsion.

Furthermore, compositions containing these polymer emulsions as a main component with other additive components are also included within the scope of polymer emulsions.

The polymeric emulsions of the present invention typically contain from 30 to 7070 parts by weight molecular solids.

Furthermore, if necessary, thickeners or dispersants such as methylcellulose, carboxylated methylcellulose, polyvinyl alcohol, polyacrylates, casein, starch, etc., water resistance agents such as melamine-formalin resin, urea-formalin resin, or anti-aging agents are added. There is no problem in adding agents, fungicides, fungicides, etc.

The minimum film-forming temperature of the polymer emulsion used in the treated wood of the present invention must be 40°C or lower.

If the minimum film formation temperature exceeds 40°C, it will be difficult to form a film at room temperature, and even if a film is formed, cracks will occur on the film surface, which will prevent the water vapor permeation rate through the film, which is the objective of the present invention, from being controlled. It is difficult to
The effect of preventing wood from drying out and cracking will no longer be recognized.

The minimum film forming temperature of the polymer emulsion of the present invention is Pr
Otzman et al.'s temperature gradient plate method (Journal of
Applied Polymer 5science
4, 81, (1960)).

The polymer emulsion having film-forming properties at room temperature, specifically, the polymer emulsion having a minimum film-forming temperature of 0°C to 40°C, or the composition mainly composed of the same, which is used in the treated wood of the present invention, includes: Natural rubber latex, styrene-butadiene copolymer latex (for example, copolymer latex consisting of 33% by weight of butadiene, 65% by weight of styrene, and 2 parts by weight of acrylic acid), acrylonitrile-butadiene copolymer latex (for example, 38% by weight of acrylonitrile) %, copolymer latex consisting of 62% by weight of butadiene), methyl methacrylate-butadiene copolymer latex (e.g. copolymer latex consisting of 42 parts by weight of methyl methacrylate, 8 parts by weight of butashe, and other polyisobutylene-based copolymers) Polymer latex, synthetic rubber copolymer latex such as polychloroprene copolymer latex, vinyl acetate polymer emulsion, ethylene-vinyl acetate copolymer emulsion (e.g. ethylene weight ratio, vinyl acetate weight type weight ratio) copolymer emulsion consisting of 45 parts by weight of methyl methacrylate and 55 parts by weight of ethyl acrylate), acrylic ester copolymer emulsion (e.g. copolymer emulsion consisting of 45 parts by weight of methyl methacrylate and 55 parts by weight of ethyl acrylate), styrene-acrylic ester copolymer emulsion (e.g. styrene 55 parts by weight,
copolymer emulsion consisting of 45 parts by weight of butyl acrylate), acrylic ester-vinyl acetate copolymer emulsion, vinyl chloride polymer emulsion,
Any polymer emulsion such as a vinyl chloride-bisillitin chloride copolymer emulsion may be used.

As a method for forming a film on the surface of the treated wood of the present invention, various coating methods such as dipping method, roll method, blade method, spray method, etc. can be adopted depending on the shape of the wood and the amount of treatment. .

To give an example of a specific embodiment of the wood treatment method of the present invention, 30 to 95 parts by weight of vinyl acetate and 5 to 70 parts of a monoolefin monomer that can be copolymerized with vinyl acetate are emulsion polymerized. A copolymer aqueous emulsion with a pH of 3.5 to 7.0 obtained by It is done by applying a substance to the wood surface.

Furthermore, according to another embodiment, 28 to 70 parts by weight of aliphatic conjugated diolefin, 30 to 70 parts by weight of alkenyl aromatic monomer
72 parts by weight and O to 42 parts by weight of other monoolefin monomers that can be copolymerized with these are emulsion polymerized to form a copolymer latex, which is the main film-forming component.A composition is applied to the wood surface. It is done by doing.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but these are merely illustrative and do not limit the scope of the present invention.

Example 1 33 parts by weight of butadiene, 65 parts by weight of styrene, 2 parts by weight of acrylic acid, 0.7 parts by weight of sodium persulfate, 100 parts by weight of ion-exchanged water, 1.0 parts by weight of sodium alkylbenzenesulfonate, and 0 parts by weight of t-dodecylmercaptan. ．．
After emulsion copolymerizing 2 parts by weight of the mixture at 80° C. for 6 hours in an autoclave, the pH was adjusted to 6.0 with caustic soda and the latex solid content was adjusted to 488 parts by weight to obtain a copolymer latex.

The minimum film forming temperature of this latex was 17°C.

This latex has a thickness of 200 mvt and a width of 250++ m.
.

A coating amount of 120 g/m' (
The coated wood specimen was coated so that the solid content (the same applies hereinafter) and the coverage area ratio of the entire surface of the wood was 80%.

The film thickness was 124μ. This film was coated on uncoated kraft paper under the same conditions and air-dried for 24 hours at a temperature of 23°C and a humidity of 60%. A test piece was prepared using the cup method (ASTM E-96-63T). The water vapor transmission rate measured under the conditions of 37.8°C and 90 fb relative humidity was 26 g/m'day.

This coated wood specimen was subjected to an outdoor exposure test under direct sunlight for three months in the summer (June to August), and then the state of deterioration such as discoloration and cracks on the film on the surface of the specimen was observed with the naked eye. As a result, no discoloration, cracks, etc. had occurred.

In addition, when the coated wood test specimens after the outdoor exposure test described above were observed with the naked eye for drying cracks such as butt end cracks and surface cracks on the wood surface layer, no drying cracks were observed at all.

Comparative Example 1 A coated wood specimen prepared in the same manner as in Example 1 except that the coverage area ratio of the wood surface was changed to 40% in Example 1, surface cracking occurred one month after the start of the outdoor exposure test.

Comparative Example 2 When the same hemlock dog split wood used in Example 1 was subjected to an outdoor exposure test without any treatment, end cracks and surface cracks occurred within 7 days from the start.

Example 2 In Example 1, the coating amount was 409/771. Coverage area rate 100%
(Full surface coating), film thickness 41μ, film water vapor permeability 8
With respect to the coated wood test specimen made under the same conditions as in Example 1 except that the exposure rate was 0 g/rrl·day, no drying cracks occurred even after 3 months from the start of the outdoor exposure test.

Comparative Example 3 In Example 1, the coating amount was 9/7711, the coverage area ratio was 40, the film thickness was 82μ, and the water vapor permeability of the film was 4 (1/1).
For the wood test specimens coated for 2 days, end cracks and surface cracks occurred 21 days after the start of the outdoor exposure test.

Example 3 The styrene-buccadiene copolymer latex of Example 1 had a thickness and width of 1105 nm and a length of 3.0007 nm and 71 mm, respectively.
11 Apply amount of 80 g/71 to the surface of cedar square timber with a moisture content of 65 using a roller brush. , the entire surface was coated to a film thickness of 82μ.

Regarding this coated test piece, no drying cracks occurred even after a three-month outdoor exposure test.

Comparative Example 4 In Example 3, the coating amount was 30g/rrl, the film thickness was 31μ, the water vapor permeability of the film was 104g/771″, and the coverage area ratio was 1
Regarding the coated test specimen prepared in the same manner except that the test specimen was coated with Coating No. 00, surface cracking occurred within two months after the start of the outdoor exposure test.

Comparative Example 5 When the same untreated cedar square timber used in Example 3 was subjected to an outdoor exposure test, end cracks and surface cracks occurred within 3 days after the start of the test.

Example 4 Bukudiene 50 stacked part, Metilef stacked part weight ratio, solid content 4
A latex of 9%, pH 7, and minimum film formation temperature of 5°C was applied to the same large hemlock wood used in Example 1 over the entire surface using a roller brush to a thickness of 80 g/m'' and a film thickness of 80 μm.

The water vapor permeability of this film was 35 g/rri''·day.

Regarding this coated test piece, no drying cracks occurred after 3 months of outdoor exposure test.

Example 5 Ethylene-vinyl acetate copolymer emulsion (Sumitomo Chemical Co., Ltd. Sumikaflex 500, solid content 55%, pH 5)
, 5, minimum film formation temperature 10°C) was applied in an amount of 2 with a roller brush to the same large hemlock wood used in Example 1.
597 yi', film thickness 22μ, and coating area ratio 90%.

The water vapor transmission rate of this film was 469/rrt·day.

Regarding this coated test piece, no drying cracks occurred at all even after a three-month outdoor exposure test.

Comparative Example 6 In Example 5, when the coverage area ratio was 30%, end cracks and surface cracks occurred 25 days after the start of the outdoor exposure test.

Comparative Example 7 In Example 5, coating amount log/77L, film thickness 8μ,
When the coating was applied to the entire surface and the water vapor permeability of the film was 110 g/m/day, surface cracking occurred within two months after the start of the outdoor exposure test.

Comparative Example 8 Vinylidene chloride-vinyl chloride copolymer latex (Asahi Dow Co., Ltd. Saran Latex X-302, solid content 50%)
, pH 5.0, and a minimum film formation temperature of 18°C were applied to the entire surface of the same hemlock wood used in Example 1 using a roller brush at a coating amount of 40 g/rl and a film thickness of 50 μm.

The water vapor permeability of this film was 4.0 g/rn'·day.

Regarding this coated specimen, no dry cracking occurred even after 3 months of outdoor exposure test, but mold was found to grow in some parts of the lower part of the wood, causing it to turn black.

Example 6 In Comparative Example 8, when the coating amount was set to tog/m, the film thickness was 13 μm, and a film with a water vapor transmission rate of 36 g/lri'day was applied, no dry cracking or mold formation occurred even after 3 months of outdoor exposure test. I was not able to admit.

Example 7 Natural rubber latex (solid content 60%, I)Hll, minimum film formation temperature 12°C) was applied to the same cedar square material as used in Example 3 at a rate of 50 g/rrt using a roller brush.
- The entire surface was coated to a film thickness of 50μ.

Is the water vapor permeability of this film 35g/n?・It was day.

Regarding this coated test piece, no drying cracks occurred at all even after 3 months of outdoor exposure test.

Comparative Example 9 A styrene-acrylic acid ester copolymer emulsion (solid content 50 parts, pH 8.3, minimum film formation temperature 61°C) consisting of 1 part by weight of methyl and 25 parts by weight of butyl acrylate was prepared as an example. The coating was applied to the same hemlock dog split material used in 1 using a roller brush so that the coating amount was 100 i/m and the coating thickness was 5 μm.

The water vapor permeability of this film is unmeasurable due to many cracks.
In addition, this coated test specimen developed end cracks and surface cracks within 3 days after the start of the outdoor exposure test.

Example 8 Seventy square cedar pieces, the same as those used in Example 3, were placed horizontally.
0 pieces are stacked in a row, width 25? l! +1! The styrene-butadiene copolymer latex of Example 1 was applied to both ends, both sides, and the upper side of a bundle of square cedar timbers with iron bands hung in three places in the longitudinal direction, using a one-fluid spray device in an amount of s o g/ The coating was applied so that the coating had a coating thickness of 82 μm and a coverage ratio of 74 μm, which was the sum of the areas of both end faces and side surfaces.

The water vapor permeability of this film was 409/m/day.

During the 3-month outdoor exposure test for this coating test wood bundle, no end cracks or surface cracks occurred on either the surface or internal square timbers of the wood bundle.

Example 9 The following experiment was conducted to clarify the effect of the present invention on how much the film of the present invention moderates the drying rate of wood.

The styrene-butadiene copolymer latex used in Example 1 was applied with a roller brush in an amount of 80 g/rrr' ( solid content)
The coated wood was used as a coated wood specimen.

For comparison, uncoated wood of the same size was used as an uncoated wood test piece.

When both of the above specimens were subjected to an outdoor exposure test for two months in summer, the results of the occurrence of surface drying cracks and changes in the moisture content of the wood are shown in Figure 1.

The water vapor transmission rate was measured using ASTM E-96-63T at a temperature of 37.8°C and a relative humidity of 90%.4
It was 0g/m'day.

As is clear from Figure 1, the moisture content of the comparative uncoated wood specimens decreased rapidly after the start of the outdoor exposure test, and surface drying cracks occurred 3 days after the start of the test, and surface drying cracks began to occur. The moisture content at that point was almost at the upper limit of the fiber saturation point.

On the other hand, for the coated wood specimens, the rate of decrease in moisture content was slow, and no surface drying cracks occurred even when the moisture content was below the lower limit of the fiber saturation point.

From the above results, by applying the slow moisture permeable film of the present invention to the surface of the wood specimen, the rapid evaporation of moisture from the wood surface is suppressed, and the shrinkage stress is prevented only in the wood surface layer from starting to shrink rapidly. It is thought that drying can be prevented by gradually drying so as to alleviate the dryness.

Regarding the coated wood test specimen, after two months of outdoor exposure testing, four sides in the length direction were planed and another outdoor exposure test was conducted, but surface drying cracks no longer occurred.

However, in the actual sawmilling industry, the drying speed of wood varies depending on the type, shape, size, and initial moisture content of the wood, and the drying speed also varies depending on external weather conditions. Although it is not possible to judge whether or not it is possible, the above experimental results show that once the moisture content of the wood is below the fiber saturation point, dry cracking due to shrinkage stress will no longer occur, so the wood treated by the method of the present invention has no fiber If the moisture content is below the saturation point, drying cracks will not occur after sawing, and even if the wood is sawn before the moisture content reaches the fiber saturation point or below, drying cracks that occur in sawn timber products can be prevented by applying the treatment of the present invention.

As is clear from the above results, the minimum film formation temperature is 0.
The water vapor permeability of a film formed from a polymer emulsion having a temperature of 0C to 40C or a composition containing the same as a main component under conditions of a temperature of 37.8C and a relative humidity of 90C. 10 g/m” per 24 hours
Wood with a controlled water vapor evaporation rate characterized by having a film of a composition of ~100 g/m' applied to at least 60 inches of the wood surface can withstand long-term outdoor exposure under direct sunlight. This is wood that has been given an extremely good effect of preventing wood cracking after treatment, and it is an epoch-making treated wood that can significantly improve the yield and has extremely high commercial value.

[Brief explanation of the drawing]

FIG. 1 of the accompanying drawings shows the drying rates of uncoated wood specimens and coated wood specimens.

Claims (1)

[Scope of Claims] 1. A polymer emulsion with a water vapor permeability of 10 to 100 g/m/day (temperature 37.8° C.) formed from a polymer emulsion with a minimum film formation temperature of 0° C. to 40° C. A wood with a controlled water vapor evaporation rate, characterized in that a coating layer made of a continuous film having a relative humidity of 90 parts or more is provided over 60 parts or more of the total surface area of the wood. 2 The weight per unit area of the coating layer is 5 to 300 g/m
3. The wood has a controlled water vapor evaporation rate according to claim 1. 3. The wood is a log that has been felled, a log from which the bark has been peeled,
Alternatively, the wood with a controlled water vapor evaporation rate according to claim 1 or 2, which is lumber that has been primarily processed into a square, cylindrical or plate shape, or an aggregate thereof. 4. The wood with a controlled water vapor evaporation rate as set forth in claim 3, wherein the wood is made of two or more square or plate-shaped sawn timbers that are integrally bundled. 5. The wood with a controlled water vapor evaporation rate according to any one of claims 1 to 4, wherein the coating layer is provided on substantially the entire surface of the wood. 6. By applying a polymer emulsion with a minimum film formation temperature of 0°C to 40°C at room temperature on the surface of undried wood with a moisture content of 30 to 200, and then drying the emulsion to form a continuous film. , a method for treating wood, which substantially prevents drying and cracking of wood by controlling water vapor transmission rate. 7. The treatment method according to claim 6, wherein the polymer emulsion is an aqueous emulsion of a polymer. 8. The method according to claim 7, wherein the high molecular weight polymer is a copolymer of vinyl acetate and a monoolefin monomer. 9. Claim No. 9 in which the high molecular weight polymer is a copolymer of an aliphatic conjugated diolefin and an alkenyl aromatic monomer, or a copolymer of these monomers and other monoolefin monomers. The method described in Section 7. 10. The method according to any one of claims 6 to 9, wherein the polymer emulsion is applied by dipping, coating or spraying. 11. Claims 6 to 10, in which the wood to be treated is a felled log, a log from which the bark has been peeled, a sawn lumber that has been primarily processed into a square, cylindrical or plate shape, or an aggregate thereof.
The method according to any of paragraph 10. 12. The method according to claim 11, wherein the wood to be treated is a bundle of two or more square or plate-shaped sawn timbers.