FIELD OF THE INVENTION

The invention relates to a method for manu ^¬ facturing plywood from wood veneers. The invention al ^¬ so relates to plywood comprising a plurality of veneer layers stacked on top of another and glued together with a thermosetting adhesive. 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.

SUMMARY OF THE INVENTION

The invention is characterized by the fea ^¬ tures of the independent claims.

In one embodiment of the invention, plywood is manufactured from wood veneers using a method that comprises :

- applying an adhesive on some of the ve ^¬ neers, the adhesive comprising a thermosetting adhe ^¬ sive or an adhesive composition including water,

laying the veneers on top of another to produce a plywood billet which comprises a surface ve ^¬ neer at the bottom and on the top of the plywood bil ^¬ let, 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 hav- ing 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 av ^¬ erage 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/m ³ , preferably at least 570 kg/m ³ .

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%, pref- erably at least 4%, more preferably at least 5% higher than the average density of the veneers used as inte ^¬ rior 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 con ^¬ crete forms.

In another embodiment of the method, the num ^¬ ber of veneers included in the plywood billet is from 15 to 29, preferably from 17 to 25. This kind of ply- wood 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 ve ^¬ neers 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 .

In an embodiment of the method, the adhesive comprises a thermosetting resin.

In an embodiment of the method the adhesive includes water.

In an embodiment of the method the adhesive comprises 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 - 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 thermoset ^¬ ting 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.

In an embodiment of the invention, plywood comprises a plurality of veneer layers stacked on top of another and glued together with a thermosetting adhesive 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 direc- tion, which is perpendicular to said first direction;

- a plurality of interior veneer layers hav ^¬ ing their grain running alternately in the first di ^¬ rection and the second direction so that the interior veneer layers form a cross-bonded structure;

wherein the average density of the oriented veneer layers is higher than the average density of the plurality of interior veneer layers.

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/m ³ , preferably at least 650 kg/m ³ , more preferably at least 670 kg/m ³ .

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, com ^¬ prises a plywood board formed of plywood panels ac- cording to the present invention or plywood panels manufactured by a method according to the present in ^¬ vention .

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 embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to ^¬ gether 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, illus ^¬ trate 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 ve ^¬ neers 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 lay- ers 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 illus ^¬ tration, the first grain direction (indicated with vertical stripes) is perpendicular to the plane of pa- per, 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 phenol ^¬ ic 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 res ^¬ in. 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 sub ^¬ jected to hot pressing which is carried out between heated metal plates. During hot pressing the veneers are consolidated and the thermosetting adhesive be- tween them is cured. After hot pressing plywood is sanded to reach a desired thickness. Finally, a coat ^¬ ing 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 evapo- rates and penetrates into the veneers. The veneers first swell and then, after the vapor starts to re ^¬ lease from the plywood billet, shrink.

Hot pressing can be carried out with a step ^¬ wise 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 es ^¬ cape 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 prede ^¬ termined 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 finish ^¬ ing step.

One problem of the prior art is that the ply ^¬ wood 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 ve- neer 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 ve ^¬ neers 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 compressi ^¬ bility of randomly selected interior veneers.

The compressibility of individual veneers has been found to depend on their density and their loca ^¬ tion 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 ve- neers, it is possible to reduce the thickness varia ^¬ tions in plywood. When the thickness variation is re ^¬ duced, less sanding is needed to reach the desired fi ^¬ nal 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 re ^¬ duced . Plywood according to the present invention is preferably manufactured from veneers with a density of at least 550 kg/m ³ , preferably 570 kg/m ³ (measured at moisture content of 3%) . Suitable wood species in- elude, 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 aver ^¬ age density which is at least 3%, preferably at least 4%, more preferably at least 5% higher than the aver- age density of the veneers used as interior veneers.

The density of the plywood according to the present invention can be at least 630 kg/m ³ , prefera ^¬ bly at least 650 kg/m ³ , more preferably at least 670 kg/m ³ (measured at moisture content of 10%) . The aver- age 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 inte ^¬ rior veneer layers.

Figure 1 shows one possible embodiment of plywood structure. It has to be noted that the struc ^¬ ture need not be fully symmetrical if warping of ply ^¬ wood 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, hor ^¬ izontal strokes denote veneer layers having their grain direction oriented parallel to the plane of pa- per, and vertical strokes denote veneer layers having their grain direction oriented at perpendicular to the plane of paper.

Figure 3(a) schematically shows plywood hav ^¬ ing a surface veneer layer A on each side, four ori- ented veneer layers B next to each surface veneer lay ^¬ er 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 aver ^¬ age 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 ve ^¬ neer layer A is three. The total number of veneer lay ^¬ ers 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 ve ^¬ neer 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 di ^¬ rection. 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 ve ^¬ neer layer A is three. The total number of veneer lay- ers 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 ve ^¬ neer 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 need ^¬ ed to produce a cross-bonded plywood structure. Conse ^¬ quently, 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 ve ^¬ neer layers A, thus the total thickness of plywood is also reduced. As a result of thinning, the surface ve ^¬ neer layers A tend to form an unstable base for coat ^¬ ing. 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 de- lamination of the coating 2. If the thickness of the surface veneer layer A can be kept as intact as possi ^¬ ble 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 ve- neers. Consequently, the compressibility of the ply ^¬ wood billet comes up to expectations, less extra ve ^¬ neers 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 mate ^¬ rial 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 an ^¬ other similar freight transport unit of a freight transport vehicle is loaded and unloaded with a fork- lift, 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 frame- work, 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.