The present invention relates to a multiply wood product. The expression "multiply wood product" is here mainly meant to include plywood and laminated veneer lumber.

Plywood industries all over the world have in re¬ cent years been compelled to face the fact that it is becoming increasingly difficult to obtain wood of an app¬ ropriate quality. The supply of birch wood for wood plies has decreased to such an extent that it is today diffi¬ cult to maintain the traditionally high quality of ply¬ wood; birch wood is very well suited for the production of plies as it is compact and possesses a high wear re¬ sistance. Due to the decreased supply it has become ne¬ cessary to replace birch with other kinds of wood, e.g. softwood, to an increasing degree. Wood plies made of softwood cause problems both for the manufacturer and the user of plywood. Softwood plies have a coarser surface than birch plies. They fracture more easily, and the gross- grained pieces of wood at knot points do not properly stay in place. The traditional production processes of plywood can utilize only about 25 per cent of the obtained raw material. The rest requires new special measures in order to maintain a reasonable quality.

Deterioration in the quality of plywood has caused reactions everywhere in the world. Plywood industries in e.g. Sweden, the Soviet Union and France have made earnest appeals for improvement in this matter. For example, it has been found out that plywood is today more liable to fracture than previously. Neither is it suited for va¬ rious purposes as well as previously. Plywood causes prob¬ lems e.g. in the moulds of concrete casting. It does not withstand the strains caused by the concrete flow or the vibration of concrete but is broken so that freezing water

gets into the cracks, and fractures are formed. Nor does it absorb water and, -consequently, water remains between the concrete and the mould, and the formed air bubbles leave marks in the hardened concrete. It has also been found that the impact strength of plywood is lower than previously.

Various efforts have been made to solve the prob¬ lems. Superficial defects, such as recesses and cracks, affecting the appearance of the plywood, have been evened with a filler. This may be acceptable if the defects are small and not too numerous. As compared with the surround¬ ing wood surface, a defect nevertheless forms an area of discontinued homogeneity which may be the root of problems, depending on the use. The filler does not eliminate the fracture effect caused by the defect; furthermore, the filler often comes off partially or wholly so that mois¬ ture gets in between the plies, and a stripping process is started. Defects caused by knots have been repaired by means of a circular piece of homogeneous woodply which is fitted accurately in a hole made round the knot and fastened in the woodply beneath in the single pressing' of the plywood sheet. After the pressing, the sheet is ground. With respect to the fracture effect, this way of repairing is better, because the defect is confined with¬ in smoothly curving contours. However, the circular piece does not always stay in place. When the plies are apart from each other before the pressing step, the piece is glued in the ply only at the edges thereof. When handling the ply, the piece is often displaced or comes off com¬ pletely. A further problem arises in that an unnecessarily accurate control is required to provide the circular piece with similar shrinking and swelling properties as the surrounding wood ply. It often happens that the cir¬ cular piece is curved or cracked due to the greater swelling thereof, or comes off at the edges thereof due

to greater shrinking. In general, all the repairs are carried out manually. In view of the manufacturing costs, this is a drawback. The repairs do not notably improve the wear resistance of the plywood, nor is its resistance to weather improved at the repaired points. The appea¬ rance, either, is not improved to any greater degree by the local repairings. Thus, it is obvious that this kind of problems are further aggravated as the quality of wood used for plies is deteriorating everywhere.

As to the plywood products in which the preserva¬ tion of the natural wood pattern of the outer surface of the plywood is of primary importance, the outer surface of the plywood is covered with a transparent membrane which is bonded to the wood surface by means of a resin by hot-pressing. The membrane is a thin film manufactured in advance by pressing, and it is pressed in place in con¬ nection with the single pressing of the plywood plate. It covers the defects occuring in the wood surface at least to such an extent that they are not too easily noticeable, including possible local repairings. The wood surface may then be rather coarse and of inferior quality. U.S. Patent Specification 4,084,996 deals with this kind of prior art, and it teaches that a thin cellulose film manufactured in advance is hot-pressed on the outer surface of the plywood by single pressing, with phenol formaldehyde re¬ sin as a bonding agent. It can be said that the publica¬ tion represents the known technique for the manufacture of coated plywood sheets at present on the market, ir¬ respective of whether the membrane is transparent or not. An example of this kind of plywood sheets would be e.g. plywood sheets coated with a so called painting base paper or a so called concreting membrane. Essential is that the fibre structure of the membrane used in the pro¬ duction gets its final shape at an earlier production stage. Consequently, the fibres of the membrane are not

any longer able to follow the contours of the defects of the wood surface-±n ^~ the- fixing pressing. —

It is also known to coat the wood surface in one way or another e.g. with a layer of glass fibre which is bonded by means of a resin, such as a polyester. The coating is usually constructed manually layer by layer. The wood surface may be rather coarse and of inferior quality. The coating adheres properly to the surface of the wood if the surface is clean, and it gives a good protection against any disadvantageous outer effects. Also, it considerably improves the strength of the struc¬ ture. However, the method is too slow to be applied to mechanical production of plywood sheets, in which the desired final qualities have to be achieved by a single pressing.

A common drawback of known coating methods is that the adhesion between the coating and the surface of the wood is not sufficient for all applications. In such cases as disclosed in the above-mentioned publication, this is due to the unsufficient mouldability of the mem¬ brane. Although it is elastic and resilient, it does not stretch into every corner of the recesses. Due to the pre¬ forming of the membrane, the fibres are so oriented in the direction of the plane of the membrane that they are bent over the sharp corners of the recesses when pressed against the wood surface, so that an empty space remains under the membrane. Besides macroscopic recesses, there are numerous sharp microscopic recesses on the surface of the wood, especially in pine plies. The dense membrane structure does not reach into these recesses. As a re¬ sult, the thin intermediate layer between the coating and the surface of the wood is not homogeneous. The shearing stresses are thereby in some places transmitted over areas which are not reinforced with fibres. This, of course, affects adversely the strength of the coated

plywood sheet as a whole, as compared with a plywood sheet having a homogeneous adhesion layer. A coating which is not pressed on to the surface of the wood is even more greatly affected by this adhesion drawback.

The object of the present invention is to obviate the above drawback s.

Thus, the invention is concerned with a multiply wood product which is mainly characterized by what is disclosed in the claim 1. Preferred embodiments are de¬ scribed in more detail in the claims 2 to 10.

According to the invention, the multiply wood pro¬ duct is provided with a synthetic ply which is bonded to a wood ply or wood plies by means of a resin by a single step of hot-pressing in the production of the multiply wood product. As distinct from the prior art, the syn¬ thetic ply, according to the invention, changes in struc¬ ture when being pressed in to final position. The synthe¬ tic ply initially has a low density and is relatively thick. In this stage, the ply is here called a ply blank. In other words, this ply blank is an elastic fibre com¬ posite, light in structure and comprising at least sub¬ stantially unpressed fibre material, a bonding agent and possibly additives and/or filler materials. The blank is manufactured in advance and is either positioned as such to form the outer surface of the multiply wood product, or is positioned between wood plies before the pressing of the product. At the pressing stage of the multiply wood product, the ply blank is pressed into a synthetic ply in a single hot-pressing step so that it in the fi¬ nished wood product forms a structural part the thick¬ ness and function of which is similar to those of the wood plies. Since the fibres of the ply blank are hapha¬ zardly oriented, they reach into every corner of the recesses. The coating is homogeneous and, consequently, the strength of the entire multiply wood product is higher

than that of a structure coated with a thin preformed fibre film. Due to the homogeneity,-the fracture initia- ties are considerably less numerous in spite of the fact that the number of the wood ply surface defects may be much greater, i.e. the quality of the wood ply may be much lower than in known coating applications. By virtue of the better adhesion, the coating is also superios in that the wood product is less exposed to the effects of weather. The resistance of the surface is increased by the mere thickness of the synthetic ply as it has roughly the same thickness as the wood plies. But even if the synthetic ply would fracture due to mechanical strains, possible water damage is decisively slowed down by virtue of the high homogeneity and adhesion in the are around the fracture point.

The ply blank is treated according to the require¬ ments of intended purposes. Its basic structure comprises a combination of a fibre material^ preferably mineral wool (a glass fibre, mineral fibre, slag fibre, or cera¬ mic fibre) and a resin. In special applications, the mi¬ neral wool can, if required, be replaced with a cellulose fibre or some other organic fibre or, more generally, with a fibre which can be opened by machining so that fibres adhering to each other form a firm net. The bonding agent is preferably a resin of high fire-resistance. In special applications, the ply blank is treated according to the requirements in each particular case. Such a spe¬ cial requirement may be e.g. an improved fire-resistance, a required hygroscopit , a required suppression of rever¬ beration, a low natural vibration spectrum, etc. The pri¬ mary object, however, is to provide an even, integral, continuous synthetic ply which at least maintains the traditionally good qualities of plywood even though the quality of wood plies would deteriorate considerably. This object is achieved by means of a ply blank having

the basic structure comprising at least mineral wool and resin. ^~

The aforementioned aspects relating to plywood essentially apply for laminated veneer lumber as well. In principle, laminated veneer lumber differs from ply¬ wood in being thicker, i.e. having more plier, up to about 60 to 160 millimeters, and in that the grain di¬ rection of the wood plies are at least essentially in parallel, lengthwise. Laminated veneer lumber is used e.g. for girders in building construction. The U.S. Pa¬ tent 3,908,725 represents the prior art in this field.

The Finnish Patent Specification 842,695 discloses a composite membrane which is composed of broken mineral fibres bonded to each other with a polymerized bonding agent and which is intended e.g. for building plates. This composite membrane and the fibres thereof, however, are hot-pressed in advance, and the fibres are bonded by means of resin. The use of the composite membrane requires two separate hot-pressing steps: one when the membrane is manufactured and the other when it is fixed in place.

In the following the invention will be described in more detail with reference to the attached schematical drawing.

Figure 1 is an exaggerated cross-sectional view of a plywood sheet according to the prior art.

Figure 2 is a cross-sectional view of a multiply wood product (plywood) according to the invention.

Figure 3 illustrates a preferred embodiment of laminated veneer lumber.

In Figure 1, the reference numerals 1, 2 and 3 de¬ signate three wood plies which are glued one upon another so as to form a plywood plate. Defects in the wood plies, such as recesses 4 and 6, are covered with a membrane 5 bonded with a resin. Due to the insufficient resiliance of the membrane, it does not follow the contours of the

recesses but bends over them, as e.g. at a point 7. The membrane may also swell under the - influence of the pres¬ sure created by steam or gases generated during the hot- pressing, as e.g. at a point 8. Thus, the coating is not homogeneous. The fragile coating may fracture at a cavity, and the recesses may still bring about a further fracture effect. The surface of intermediate plies may be repaired as described above e.g. by means of a circular piece 9 positioned at a knot 10, or by means of a filler 11; how- ever, the result is hardly satisfactory. In the embodiment according to Figure 2, i.e. that according to the inven¬ tion, the fibres of coating 14 reach into to the bottom of the recesses. If an unusually high strength is required, one or more wood plies are replaced with a synthetic ply. The synthetic ply 15 of Figure 2 generally illustrates such a replacement.

In principle, the mineral wool of the synthetic ply is qualitatively similar to the wool obtained immedia¬ tely after the wool chamber of a conventional mineral wool production line. Thus, it has a low density. The density

3 of the ply blank preferably varies from 50 to 200 kg/ .

If heavy filler material are used, the density may be higher. The ply blank may also contain an original bonding agent, but this is not necessary. If required, the ply blank is pressed lightly into an advantageous initial thickness. However, it is important that the initial length of the fibres, i.e. ≤IOO mm, with mineral wool preferably 5l0 mm, remains unchanged as well as the hapha¬ zard orientation of the fibres. At the final stage, i.e. when the ply blank is pressed to form a synthetic ply of

3 a multiply wood product, its density is 100 to 5,000 kg/m , and the length of the fibres 5l0 mm, for mineral wool pre¬ ferably 5300 urn. So called technical wool may well be used, such wool is not especially intended for heat in¬ sulation applications. The expression "crude fibre ply

blank" here refers to the stage before the hot-pressing step.

A preferred bonding agent for use in ply blanks here contemplated is such, that it is activated at the required pressing temperature, i.e. within the range from 50 to 350 C. If the application requires the use of a bonding agent which requires an unusually great amount of heat to be transmitted through the structure to be pressed, the temperature is maintained at an essentially constant value within said range while the pressing time is made longer. If no special requirements exist, a PVC- type or a generally chlorine containing resin is used as a bonding agent for improving the fire-resistance. In general, thermoplastic resins are advantageous, because they melt at a moderately raised temperature and are restored to their initial state when they cool. Ther o- setting resins lose their bonding properties if the tem¬ perature raises too much. However, an advantage thereof is that they resist higher temperatures than thermoplas¬ tic resins. Having a greater elasticity, they are more suitable for the transmission of the shearing forces generated in sandwich structures. In addition, they have a higher resistance to tension. A compromise has to be resorted to in the choice of resin; i.e. such a resin is chosen which meets the requirements of the primary function of the synthetic ply in the best possible way.

If a special application requires the choice of an organic fibre or, more generally, a fibre which can be opened by machining, a preferred opening degree is 10 to 30°SR. Such fibres are e.g. cellulose fibres, animal furs, asbestiform fibres, etc.; more generally, fibres which can be made to adhere to each other by machining so that a firm fibre net is formed.

The above-mentioned hygroscopity and the suppres¬ sion of reverberation can be adjusted by an advantageous

porousity of the synthetic ply. The porousity here means that portion-of- he volume of the synthetic ply which does not contain solid substances. The desired porosity can be obtained e.g. by the use of a polycondensation resin such as phenol formaldehyde resin as a bonding agent. The natural vibration spectrum of the multiply product can be shifted to lower frequencies by adding an advanta¬ geous amount of a heavy filler material, such as a mineral powder or components based on barium, tungsten and/or lead, to the ply blank.

An improved fire-resistance is achieved by adding to the ply blank e.g. antimony trioxide or dihydrate gypsym.

The above shows only limitedly how a ply blank provides a multiply wood product with desired properties. All possibilities cannot, of course, be described in this connection. A primary object is to make plywood sheets made of wood plies of low quality such, that they meet the expected requirements. This is achieved according to the invention in such a manner that an advantageous number of the wood plies of the multiply wood product is replaced with synthetic plies comprising a fibre/bonding agent com¬ posite as well as additives and filler materials possibly added thereto. The synthetic ply is positioned, in its initial form, as a ready blank, between the wood plies to be pressed into a sheet and obtains its final form as a synthetic ply of the multiply product in the single hot-pressing step for forming the multiply wood product. It is thereby self-evident that the invention also re¬ lates to all those numerous special properties which are required from a plywood product and which can be achieved by means of the preferred composition of the ply blank.

In the following a few examples are set forth to illustrate the possibilities offered by the method ac¬ cording to the invention. The strength of the sheets is

used as a criterion in the comparison, because this is the most important criterion of the usability of plywood.

Example 1

A strength test was carried out in which the strength values of geometrically identical wood plies and pieces of synthetic plies manufactured according to the invention and by means of conventional pressing pressures were com¬ pared separately under identical conditions and under iden¬ tical stress.

The fibre material of the synthetic ply was glass

3 fibre having the density of 1,500 kg/m , and a phenol for¬ maldehyde resin having a concentration of 30 per cent by weight was used as a bonding agent. The porousity of the synthetic ply was 25 per cent per volume. The following results were obtained:

Example 2

Three birch plies having the thickness of 1.3 mm were treated according to the conventional plywood pro¬ duction practices, and were positioned one upon another into a hot press. A synthetic ply blank manufactured ac¬ cording to the invention and having the thickness of about 15 mm and the density of 100 kg/m was positioned on the uppermost wood ply and under the lowest wood ply. The ply blank comprised mineral wool and phenol formaldehyde re¬ sin having the concentration of 35 per cent by weight calculated on the weight of the blank. The whole package was pressed during 4 minutes at a temperature of 130 C

and a pressure of 1.6 MPa.

After the- pre-s-sing- the thickness o-f the synthetic plies was 1 mm, the density 1,400 kg/m , and the porousi¬ ty 25 per cent per volume, and the thickness of the entire coated plywood plate was 6 mm.

For the strength comparison, another similar sheet was made of the same plies, the outer surfaces of which sheet were coated only with a thin layer of resin; and still another, conventional plywood plate, i.e. a plate the outer surface of which were not treated. The plies were chosen accurately so that the patterns of the de¬ fects in the plies of the test plates were as identical as possible both with respect to the position and the geometry thereof. All three test plates were subjected to identical bending tests, using a number of strips cut off the plates in an identical manner. The following re¬ sults were obtained:

As appears from the foregoing, the most essential part of the multiply product according to the invention is the synthetic ply and its initial form, a ply blank. It mainly comprises mineral wool which can be glass wool, mineral wool or slag wool, and the multiply product is preferably formed by means of a single pressing.

The single pressing has four distinct advantages. Firstly, the single pressing, of course, reduces costs because there is no other pressing. Secondly, the single pressing is advantageous in that bonds of a thermosetting type contained in the wood structure are strained only once,

Thirdly, the pressure exerted on the wood structure in the production of plywood may approach the break limit of the wood grains. Particularly in the conventional method of two pressings, the wood material is weakened. The method according to the invention substantially reduces this dis¬ advantage. Fourthly, an adhesive layer of phenol formalde¬ hyde resin between the wood plies is not at all strained when it is in the polymerized state, because it is formed during the single pressing.

The properties of a mineral wool coating is superior to that of other fibres; no dela ination deteriorating the propertie occurs under the coating. The coating prevents substances possibly emanating from the wood plies from forming bubbles and breaches on the surface. Traditionally, phenol formal¬ dehyde resins are known for being capable of adhering both on wood and mineral fibres, which property can be fully utilized.

By the use of a suitable pressing technique, a plate of accurate dimensions can be provided by means of the mi¬ neral wool coating, which is not possible by means of the prior technique for the manufacture of plywood because of the nonexistent deformability of the layers to be lami¬ nated.

The properties of mineral fibre plies are superior to those of wood plies; in other words, the ply blank strengthens the wood ply structure so that the traditional strength properties can be obtained with a wood material of lower quality. As the mineral ply blank is able to follow the irregularities of the underlying surface and nevertheless form a homogeneous coating, it is possible to use a ply material of a lower quality than previously, and at the same time avoid the manual repairing of plies, the surface plies in particular. The ply material accord¬ ing to the invention has always a higher strength than a knotty and/or repaired wood ply.

The properties of the synthetic ply blank can be varied to _a gr_e_at extent by_ changing the components of the ply blank and the proportions of the components. This can be effected similarly as e.g. in the case of plastic composites reinforced with broken fibres and alloyed plas¬ tic composites.

In the following a few special advantages are de¬ scribed which can be obtained by means of the synthetic ply blank with a suitable combinations of the raw materials and which substantially widen the use of plates coated with the coating according to the invention. The basic structure of the synthetic ply is mainly formed of mine¬ ral wool, which has the above-described splitting and reorienting properties during the pressing of the final laminate layer.

The structure and the covering properties of the laminate layer can be affected by varying the different pressing parametres during the pressing (pressing velo- . city, cycling) .

The special properties of the laminate layer (sound, heat) can be affected substantially by varying the porou¬ sity of the surface. A porous laminate layer can be ob¬ tained by a suitable polycondensation resin or by means of different kinds of propellants, or by using insuffi¬ cient amounts of the bonding agent.

The coatability of the laminate layer can be af¬ fected e.g. by the porousity, because a porous surface is easier to coat than a completely tight surface.

The fire resistance of the laminate layer, which layer by virtue of the large amount of organic material contained therein reduces the fireload of the wood plate, can be affected by

- reducing the amount of the bonding agent,

- using less flammable bonding agents,

- using fire prevention chemicals,

- using inorganic bonding agents,

- adding inorganic filler materials to the bonding agent.

- adding water and foam forming additives to the bonding agent.

The electric, magnetic and thermal properties of the laminate can be modified by means of metal and coal fibres.

The use of traditional textile fibre based coatings becomes even more advantageous because. the adhesion to the surface according to the invention is extremely good.

The coating can be coloured throughout. plate coated according to the invention has stabile dimensions and is homogeneous in all directions.

The manufacture of different kinds of profile pieces of the coating according to the invention is fully pos¬ sible as well as the profiling- of the coating e.g. for im¬ proving the friction.

By means of the coating according to the invention a surface considerably harder and more wear-resistant than previously can be obtained.

The coating is completely inert with respect to mois¬ ture.

In principle, the multiply wood product according to the invention competes with all kinds of coated plywood products which are used today. If the preservation of the wood surface is required, it would be advisable to coat a synthetic ply blank with a wood ply. The price of a mul¬ tiply wood product coated with the synthetic ply according to the invention is competitive as compared with prior plywoods, and besides the traditional applications the product may even find new applications.

This material coated with synthetic ply has at least the following practical applications:

-motor and transport industries

-surface plates of building industries

- casting mould materials - - - -

- boat building and ship building (intermediate walls)

- prefabrication industries

- farm building

- packing materials.

Figure 3 illustrates an embodiment of the invention, when applied to laminated veneer lumber. Reference numeral 20 designates wood plies, 21 designates synthetic plies according to the invention, 22 indicates an opening which according to prior art would require a separate filling piece, 23 indicates a notch in the wood ply, 24 and 25 indicate joints which are displaced lengthwise in order to improve the bend strength of the laminated veneer lumber. The joints of the wood plies 20 and the synthetic plies 21 may be bevelled, as indicated by reference numerals 26 and 27.