EP3152016B1

EP3152016B1 - Method for manufacturing plywood and plywood

Info

Publication number: EP3152016B1
Application number: EP14739497.7A
Authority: EP
Inventors: Simo Koponen; Jani KIURU
Original assignee: UPM Plywood Oy
Current assignee: UPM Plywood Oy
Priority date: 2014-06-04
Filing date: 2014-06-04
Publication date: 2018-11-14
Anticipated expiration: 2034-06-04
Also published as: WO2015185789A1; EP3152016A1; PL3152016T3

Description

FIELD OF THE INVENTION

The invention relates to a method for manufacturing plywood from wood veneers. The invention also relates to plywood comprising a plurality of veneer layers stacked on top of another and glued together with a thermosetting adhesive including water or an adhesive composition including water. Further, the invention relates to the use of plywood according to the present invention.

BACKGROUND OF THE INVENTION

In general, plywood consists of three or more veneer layers, most of which are laid with their grain at right angle to their adjacent veneer layers so that the veneer layers form a cross-bonded structure. In distinction from plywood, laminated veneer lumber (LVL) is a strongly orientated wooden product, having normally the grain direction of all the veneer layers in parallel to each other.
Document EP 2 705 950 A1 discloses plywood and a method for manufacturing plywood. The method comprises forming veneer layers of the plywood by laying veneers of wood material on top of the other such that each veneer layer of the plywood comprises at least one veneer, the number of the veneer layers in the plywood being 12 to 44, applying an adhesive between the veneer layers when the veneers are laid on top of the other, and hot-pressing the veneer layers together, wherein the veneers are laid such that in 25 to 38.4% of the veneer layers a grain direction in the veneer layers is perpendicular to the first principal strength direction of the plywood and the grain direction of the veneer(s) in at least one veneer layer among three topmost veneer layers or among three bottorn veneer layers is crosswise to a grain direction of the veneer(s) in at least one other veneer layer among the three topmost or three bottom veneer layers.

SUMMARY OF THE INVENTION

The invention is defined by the features of the independent claims.
According to the invention, plywood is manufactured from wood veneers using a method that comprises:

applying an adhesive on some of the veneers, the adhesive comprising a thermosetting resin including water or an adhesive composition including water,
laying the veneers on top of another to produce a plywood billet which comprises a surface veneer at the bottom and on the top of the plywood billet, the surface veneers having their grain running in a first direction; from two to four oriented veneers next to each surface veneer, the oriented veneers having their grain running in a second direction which is perpendicular to said first direction; and a plurality of interior veneers having their grain alternately in the first direction and the second direction so that the interior veneers form a cross-bonded structure; wherein the veneers are selected so that the average density of the oriented veneers is higher than the average density of the plurality of interior veneers,
hot pressing the plywood billet to glue the veneers together, in which connection water from the adhesive vaporizes, penetrates into the veneers and finally escapes therefrom, which causes compression of the veneers, and
sanding plywood to reach a desired plywood thickness.

In an embodiment of the method, the average density (as measured before adhesive application and hot pressing) of the veneers included in the plywood billet is at least 550 kg/m³, preferably at least 570 kg/m³.
In an embodiment of the method, the veneers are sorted so that the average density (as measured before adhesive application and hot pressing) of the veneers used as oriented veneers is at least 3%, preferably at least 4%, more preferably at least 5% higher than the average density of the veneers used as interior veneers.
In an embodiment of the method, the number of veneers included in the plywood billet is at least 9, preferably at least 13, more preferably at least 15.
In an embodiment of the method, the number of veneers included in the plywood billet is from 11 to 15. This kind of plywood is suitable for use in concrete forms.
In another embodiment of the method, the number of veneers included in the plywood billet is from 15 to 29, preferably from 17 to 25. This kind of plywood is suitable for use in floorings of the cargo space of trucks, trailers and railway carriages.
In an embodiment of the method, the number of oriented veneers laid next to each surface veneer is identical. The number of oriented veneers laid next to each surface veneer can be four, three or two, most preferably three.
In an embodiment of the method, all the veneers included in the plywood billet are of the same wood species. This wood species can be, for instance, birch or beech, most preferably birch.
In an embodiment of the method, each veneer of the plurality of interior veneers is laid so that it has a grain direction which is perpendicular to the grain of both its adjacent veneers. This structure is normally used when the number of interior veneers is odd.
In an alternative embodiment, two veneers of the plurality of interior veneers are laid so that they have a grain direction which is parallel to that of an adjacent veneer, whereas all the other veneers of the plurality of interior veneers are laid so that they have a grain direction which is perpendicular to the grain of their adjacent veneers. This structure is normally used when the number of interior veneers is even.
The adhesive may comprise a thermosetting resin.
The adhesive includes water.
The adhesive may comprise a thermosetting resin including water.
In an embodiment of the method the adhesive is a phenolic resin in liquid form. The water content of the liquid binder composition used for gluing together the veneers can be 40 - 60%, preferably 43 - 55%.
In an embodiment of the method, a coating is provided on at least one side of plywood, preferably on both sides. Plywood can be coated with a thermosetting resin, or a cellulose film impregnated with a thermosetting resin.
In an embodiment of the method, the coating comprises thermosetting resin. The thermosetting resin can comprise, for instance, a phenolic resin.
The plywood comprises a plurality of veneer layers stacked on top of another and glued together with a thermosetting adhesive including water or an adhesive composition including water, the veneer layers comprising:

a surface veneer layer on both opposite sides of plywood, the surface veneer layers having their grain running in a first direction;
from two to four oriented veneer layers next to each surface veneer layer, the oriented veneer layers having their grain running in a second direction, which is perpendicular to said first direction;
a plurality of interior veneer layers having their grain running alternately in the first direction and the second direction so that the interior veneer layers form a cross-bonded structure;

In an embodiment of plywood, the number of oriented veneer layers on both sides of plywood is identical. The number of oriented veneer layers next to each surface veneer layer can be four, three or two, most preferably three.
In an embodiment of plywood, the density of plywood is at least 630 kg/m³, preferably at least 650 kg/m³, more preferably at least 670 kg/m³.
In an embodiment of plywood, the density of the oriented veneer layers is at least 2%, preferably at least 4%, more preferably at least 5% higher than the average density of the interior veneer layers.
In an embodiment of the invention, a flooring for a cargo space of a freight transport vehicle or a freight transport unit of a freight transport vehicle, which flooring is fastened to a chassis of the freight transport vehicle or to a chassis of the freight transport unit of a freight transport vehicle, comprises a plywood board formed of plywood panels according to the present invention or plywood panels manufactured by a method according to the present invention.
In an embodiment of the invention, a flooring for a cargo space of a freight transport vehicle or a freight transport unit of a freight transport vehicle, which flooring is fastened to a chassis of the freight transport vehicle or to a chassis of the freight transport unit of a freight transport vehicle, comprises a plywood board formed of two or more plywood panels manufactured by a method according to the present invention.
In an embodiment of the invention, in a concrete form comprising plywood board fastened to a framework, the plywood board consists of two or more plywood panels according to the present invention or plywood panels manufactured according to the present invention.
The adhesive may be a thermosetting resin. In an embodiment of plywood, the adhesive is an adhesive composition including water. In an embodiment of plywood, the adhesive is a thermosetting adhesive including water. In an embodiment of plywood, the adhesive is a phenolic resin in liquid form.
In an embodiment of plywood, a coating is provided on at least one side of plywood, preferably on both sides of plywood. In an embodiment of plywood, the coating comprises thermosetting resin. In an embodiment of plywood, the thermosetting resin is impregnated in a cellulose film. In an embodiment of plywood, the coating comprises phenolic resin.
In an embodiment of plywood, the thickness of plywood is at least 10 mm, preferably at least 15 mm, more preferably at least 17 mm. In an embodiment of plywood, the thickness of plywood is from 15 mm to 20 mm. In an embodiment of plywood, the thickness of plywood is from 20 mm to 40 mm, preferably from 25 mm to 35 mm.
The plywood can be manufactured by a method according to the present invention.
The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Figure 1 schematically shows an embodiment of plywood.
Figure 2 illustrates the compression of veneers at different positions during hot pressing of plywood.
Figures 3(a) to 3(f) schematically show six other embodiments of plywood.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Herein, the term "veneer" is used to refer to a thin sheet of wood material peeled from a log. In case of hardwood veneers, the veneer thickness is normally 0.8-2.0 mm, more preferably 1.0-1.8 mm, most preferably 1.2-1.6 mm. A typical thickness of birch veneer is 1.4 mm.
The "average density" of veneers has been measured from a plurality of veneers dried into a moisture content of about 3%, before gluing and hot pressing. That is to say, the average density represents the density of dried veneers before they are stacked into a plywood billet. It is well known that the density of individual veneers peeled from the same log can vary widely.
Herein, the term "veneer layer" is used to refer to a veneer layer in plywood after gluing and hot-pressing. Consequently, the density of veneer layers is influenced by the adhesive applied between the veneers and compression of the veneers during hot pressing.
Herein, the term "cross-bonded structure" is used to denote a plywood structure in which either all the veneers are cross-bonded or all but one pair of veneers are cross-bonded. When the number of veneers is even and the goal is to reach a nearly symmetric structure, there has to be one pair of veneers with parallel grains in the otherwise cross-bonded plywood structure.
Herein, the terms "oriented veneers" and "oriented veneer layers" are used to refer to a group of adjacent veneers or veneer layers having a similar orientation of grain.
Figure 1 schematically shows one embodiment of plywood. Plywood 1 comprises, from face to center X, a surface veneer layer A in which the grain is in a first direction, three oriented veneer layers B in which the grain is in a second direction, which is perpendicular to the grain of the surface veneer layer A, and a plurality of interior veneer layers C in which the grain is alternately in the first direction and the second direction so that the interior veneer layers C form a cross-bonded structure. In this illustration, the first grain direction (indicated with vertical stripes) is perpendicular to the plane of paper, and the second direction (non-striped area) is parallel to the plane of paper. Further, there is thermosetting adhesive between the veneer layers. The thermosetting adhesive can be, for instance, a phenolic resin. Both faces of plywood are provided with a coating 2, which can be, for instance, in the form of a cellulose film impregnated with a thermosetting resin including water. The thermosetting resin can be, for instance, a phenolic resin.
Plywood is manufactured from veneers which have been peeled from a log and dried to a desired moisture content. Some of the veneers are passed through a glue spreader where a thermosetting adhesive is applied on both sides of the veneer. Veneers are stacked on top of another in a preselected order and orientation to produce a laid-up assembly of veneers, so called plywood billet. The plywood billet is subjected to hot pressing which is carried out between heated metal plates. During hot pressing the veneers are consolidated and the thermosetting adhesive between them is cured. After hot pressing plywood is sanded to reach a desired thickness. Finally, a coating can be applied on one or both surfaces of plywood. The coating material can be applied in liquid form or in the form of a cellulose film, or paper, impregnated with the coating material. Further, at least one of the coated surfaces can be provided with a surface profiling to increase the friction of the surface.
Hot pressing has two main objectives: to press the glue into a thin layer over (and into) each veneer, and to activate the thermosetting adhesive. At the same time, water contained in the adhesive evaporates and penetrates into the veneers. The veneers first swell and then, after the vapor starts to release from the plywood billet, shrink.
Hot pressing can be carried out with a stepwise or a steplessly lowering pressure. In general, there are three main steps in hot pressing. First, a high peak pressure is applied for a short time to bring the veneers together and to spread the adhesive between the veneers. Then, the pressure is reduced to about half of the peak pressure. Finally, a low end pressure is applied in order that water vapor can escape from the plywood billet.
In hot pressing of birch veneer, the temperature of press plates is typically around 130°C. The pressure and duration of pressing depend on the number of veneers and, thus, the thickness of plywood. The target is to raise the temperature in the center of the plywood billet to at least 100°C so that all the free water can be made to evaporate.
The number of veneer sheets stacked into a plywood billet is generally selected so that a predetermined plywood thickness can be reached after hot pressing. The fine adjustment of plywood thickness is then to be carried out by sanding plywood in a finishing step.
One problem of the prior art is that the plywood thickness tends to vary after hot pressing even though the number of veneers included in the plywood billet is kept constant. For example, when producing a plurality of birch plywood panels consisting of 13 veneer layers, the thickness of the plywood panel after 30 minutes of hot pressing was discovered to vary in a range from 16.9 mm to 18.4 mm.
Figure 2 shows how a veneer's location in a plywood lay-up influences on the compression of the veneer during hot pressing. The test was carried out with a plurality of cross-bonded plywood lay-ups, each consisting of 13 birch veneers. The target thickness of plywood after hot pressing was 18 mm. Figure 2 clearly shows that the second, third and fourth veneers from the surface are compressed the most. In this test all the veneers were cross-bonded. If the second, third and fourth veneers were oriented (with the same grain direction), their compression would have been even higher.
High density veneers are able to absorb more water vapor than low density veneers. On the other hand, high density veneers compress less during the high pressure stage (pressure over 1 MPa) when they dry. The compressibility of the selected high density veneers is also more predictable than the compressibility of randomly selected interior veneers.
The compressibility of individual veneers has been found to depend on their density and their location in the veneer lay-up. Hence, if veneers of low density and veneers of high density are randomly used at a position where the compression of the veneer tends to be high, the variation of plywood thickness after hot pressing can be expected to be high.
By selecting the oriented veneers disposed next to a surface veneer so that their average density is higher than the average density of the interior veneers, it is possible to reduce the thickness variations in plywood. When the thickness variation is reduced, less sanding is needed to reach the desired final thickness. Hence, surface veneer layers A can be kept more intact, and the risks of cracking of the surface veneers A or delamination of coating 2 are reduced.
Plywood according to the present invention is preferably manufactured from veneers with a density of at least 550 kg/m³, preferably 570 kg/m³ (measured at moisture content of 3%). Suitable wood species include, for instance, birch and beech. Preferably all the veneers of plywood are of the same wood species. The veneers used as oriented veneers can have an average density which is at least 3%, preferably at least 4%, more preferably at least 5% higher than the average density of the veneers used as interior veneers.
The density of the plywood according to the present invention can be at least 630 kg/m³, preferably at least 650 kg/m³, more preferably at least 670 kg/m³ (measured at moisture content of 10%). The average density of the oriented veneer layers can be at least 2%, preferably at least 4%, more preferably at least 5% higher than the average density of the interior veneer layers.
Figure 1 shows one possible embodiment of plywood structure. It has to be noted that the structure need not be fully symmetrical if warping of plywood can be prevented by other means. It is of higher importance that the number of oriented high-density veneer layers is the same next to each surface veneer layer.
Figures 3(a) to 3(f) schematically show six further embodiments of plywood. In these figures, horizontal strokes denote veneer layers having their grain direction oriented parallel to the plane of paper, and vertical strokes denote veneer layers having their grain direction oriented at perpendicular to the plane of paper.
Figure 3(a) schematically shows plywood having a surface veneer layer A on each side, four oriented veneer layers B next to each surface veneer layer A, and 11 interior veneer layers C which are cross-bonded. The grain of the oriented veneer layers B is perpendicular to the grain of the surface veneer layers A. Each interior veneer layer C has a grain direction which is perpendicular to the grain direction of its two closest veneer layers. The average density of the oriented veneer layers B is higher than the average density of the interior veneer layers C. The total number of veneer layers is 21.
Figure 3(b) schematically shows plywood which differs from plywood of Figure 3(a) in that the number of oriented veneer layers B next to each surface veneer layer A is three. The total number of veneer layers is 19.
Figure 3(c) schematically shows plywood which differs from plywood of Figure 3(a) in that the number of oriented veneer layers B next to each surface veneer layer A is two. The total number of veneer layers is 17.
Figure 3(d) schematically shows plywood which differs from plywood of Figure 3(a) in that the number of interior veneer layers C is an even number. Consequently, there needs to be two interior veneer layers next to each other arranged to have parallel grain direction. The veneer layers having parallel grain direction need not be the middlemost veneer layers. The total number of veneer layers is 20.
Figure 3(e) schematically shows plywood which differs from plywood of Figure 3(d) in that the number of oriented veneer layers B next to each surface veneer layer A is three. The total number of veneer layers is 18.
Figure 3(f) schematically shows plywood which differs from plywood of Figure 3(d) in that the number of oriented veneer layers B next to each surface veneer layer A is two. The total number of veneer layers is 16.
The number of interior veneer layers C can be different from those shown in Figure 1 or Figures 3(a) to 3(f). A minimum of three interior layers C is needed to produce a cross-bonded plywood structure. Consequently, the number of veneer layers is at least 9, preferably at least 13, more preferably at least 15. In plywood that is suitable for use in concrete forms the number of veneer layers can range from 11 to 15. In plywood that is suitable for use in cargo space floorings the number of veneer layers can range from 17 to 25.
Sanding reduces the thickness of surface veneer layers A, thus the total thickness of plywood is also reduced. As a result of thinning, the surface veneer layers A tend to form an unstable base for coating. When plywood is repeatedly exposed to wear and tear, the instability of the surface veneer layer can lead to cracking of the surface veneer layer A or delamination of the coating 2. If the thickness of the surface veneer layer A can be kept as intact as possible by minimizing the need for sanding, a uniform and stable coating 2 can be obtained.
The predictability of plywood thickness after hot pressing can be improved by selecting the oriented veneers so that their average density is higher than the average density of the plurality of interior veneers. Consequently, the compressibility of the plywood billet comes up to expectations, less extra veneers are needed to reach a desired plywood thickness, and sanding of plywood to reach a desired nominal thickness can be kept at a minimum.
The present invention allows minimizing material losses due to sanding. Normally the sanding loss is about 0.6-2 mm, but the new method enables lowering the sanding loss to about 0.3-1 mm.
Plywood according to the present invention is suitable for use as flooring in the cargo space of a freight transport vehicle or a freight transport unit of a freight transport vehicle, which flooring is fastened to a chassis of the freight transport vehicle or to a chassis of the freight transport unit of a freight transport vehicle. Typically, a trailer or another similar freight transport unit of a freight transport vehicle is loaded and unloaded with a forklift, which is driven on and off the cargo space of the trailer. The surface of the flooring is subjected to repeated abrasion, wear and tear and alternating stresses. Further, the surface of the flooring, which consists of two or more plywood panels joined side by side, must be free of steps or dents.
Plywood according to the present invention is also suitable for use in the manufacture of concrete forms in which plywood panels are fastened to a framework, which supports plywood against the pressure of concrete poured on the opposite side of plywood.
It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

Claims

Method for manufacturing plywood from wood veneers, the method comprising:
- applying an adhesive on some of the veneers, the adhesive comprising a thermosetting resin including water or an adhesive composition including water,

- laying the veneers on top of another to produce a plywood billet which comprises a surface veneer (A) at the bottom and on the top of the plywood billet, the surface veneers (A) having their grain running in a first direction; from two to four oriented veneers (B) next to each surface veneer (A), the oriented veneers (B) having their grain running in a second direction which is perpendicular to said first direction; and a plurality of interior veneers (C) having their grain alternately in the first direction and the second direction so that the interior veneers (C) form a cross-bonded structure; wherein the veneers are selected so that the average density of the oriented veneers (B) is higher than the average density of the plurality of interior veneers (C),

- hot pressing the plywood billet to glue the veneers together, in which connection water from the adhesive vaporizes, penetrates into the veneers and finally escapes therefrom, and the veneers compress, and

- sanding plywood to reach a desired plywood thickness.
A method according to claim 1, wherein the veneers are sorted so that the average density (as measured before adhesive application and hot pressing) of the veneers used as oriented veneers (B) is at least 3%, preferably at least 4%, more preferably at least 5% higher than the average density of the veneers used as interior veneers (C).
A method according to any one of claims 1 to 2, wherein all the veneers included in the plywood billet are of the same wood species.
A method according to claim 3, wherein all the veneers included in the plywood billet are of birch.
A method according to any one of claims 1 to 4, wherein each veneer of the plurality of interior veneers (C) is laid so that it has a grain direction which is perpendicular to the grain of both its adjacent veneers, or all but two interior veneers are laid so that they have a grain direction which is perpendicular to the grain of both their adjacent veneers, in which case two veneers of the plurality of interior veneers (C) are laid so that they have a grain direction which is parallel to the grain of one of their adjacent veneers.
A method according to claim 5, wherein each veneer of the plurality of interior veneers (C) is laid so that it has a grain direction which is perpendicular to the grain of both its adjacent veneers.
A method according to claim 5, wherein two veneers of the plurality of interior veneers (C) are laid so that they have a grain direction which is parallel to that of an adjacent veneer, whereas all the other veneers of the plurality of interior veneers (C) are laid so that they have a grain direction which is perpendicular to the grain of their adjacent veneers.
Plywood comprising a plurality of veneer layers stacked on top of another and glued together with a thermosetting adhesive including water or an adhesive composition including water, the veneer layers comprising:
- a surface veneer layer (A) on both opposite sides of plywood (1), the surface veneer layers (A) having their grain running in a first direction;

- from two to four oriented veneer layers (B) next to each surface veneer layer (A), the oriented veneer layers (B) having their grain running in a second direction, which is perpendicular to said first direction;

- a plurality of interior veneer layers (C) having their grain running alternately in the first direction and the second direction so that the interior veneer layers (C) form a cross-bonded structure;
characterized in that the average density of the oriented veneer layers (B) is higher than the average density of the plurality of interior veneer layers (C), wherein the average density represents the density of dried veneers before they are stacked into a plywood billet.
Plywood according to claim 8, wherein all the veneer layers (A, B, C) are of the same wood species.
Plywood according to claim 9, wherein all the veneer layers (A, B, C) are of birch.
Plywood according to any one of claims 8 to 10, wherein the density of plywood (1) is at least 630 kg/m³, preferably at least 650 kg/m³, more preferably at least 670 kg/m³.
Plywood according to any one of claims 8 to 10, wherein the average density of the oriented veneer layers (B) is at least 2%, preferably at least 4%, more preferably at least 5% higher than the average density of the interior veneer layers (C).
Plywood according to any one of claims 8 to 12, wherein each veneer layer of the plurality of interior veneer layers (C) has a grain direction which is perpendicular to the grain of its both adjacent veneer layers, or all but two of the plurality of interior veneer layers (C) have a grain direction which is perpendicular to the grain of their both adjacent veneer layers, the two anomalous veneer layers having a grain direction which is parallel to the grain of one of their adjacent veneer layers.
Plywood according to claim 13, wherein each veneer layer of the plurality of interior veneer layers (C) has a grain direction which is perpendicular to the grain of its both adjacent veneer layers.
Plywood according to claim 13, wherein two veneer layers of the plurality of interior veneer layers (C) have a grain direction which is parallel to a veneer layer next to it, whereas all the other veneer layers of the plurality of interior veneer layers (C) have a grain direction which is perpendicular to the grain of their both adjacent veneer layers.
A flooring for a cargo space of a freight transport vehicle or a freight transport unit of a freight transport vehicle, which flooring is fastened to a chassis of the freight transport vehicle or to a chassis of the freight transport unit of a freight transport vehicle, wherein the flooring comprises a plywood board formed of two or more plywood panels according to any one of claims 8 to 15.
A concrete form comprising plywood board fastened to a framework, wherein the plywood board consists of two or more plywood panels according to any one of claims 8 to 15.