A METHOD FOR THE PRODUCTION OF WOOD PRODUCTS

Abstract: The present invention relates to a process for the production of wood products such as plywood and similar products. In the process a formaldehyde based curable adhesive is applied to a surface of a veneer layer and a separate hardener is applied to a surface of another veneer layer opposite to the one coated with adhesive. The separate hardener is applied in a zone along at least two edges of the surface which are opposite to each other. A number of veneers are coated in this manner and the veneers are then assembled to a package which is cold pre-pressed. The package is then cured by pressure and heat.

Description: A method for the production of wood products The present invention relates to a method for the production of wood products such as plywood or similar products. More particularly the invention relates to an improved cold pressing technique at pre-pressing a package of veneer- In the production of plywood, glue is applied to veneers and a number of glue coated veneers are assembled to a package. This package is usually cold pressed before being hot pressed for the final curing. The number of veneers in the assembly is determined by the final thick¬ ness of the product. Suitably at least some of the veneers are placed with their grain directions perpendicular to the grain direction of the neighbour veneers. After glue application the veneers remain in the package for a longer or shorter time before this is further treated. The veneers hereby absorb moisture from the adhesive, they swell and become still more uneven than they were from the beginning. The veneer package becomes larger than the available press table opening of the hot press. To decrease the thickness of the package this must be pre-pressed cold. The pre- pressing shall also result in a holding together of the veneer layers at the further handling of the package. Here it is important that the cold tack of the adhesive composi- tion (ie the bonding strength of the adhesive before this has been cured by heat) is sufficient for keeping the veneers together after the pre-pressing. The adhesives used in the manufacture of plywood are usually phenol resins (condensation products of phenol and formaldehyde) and amino resins (condensation products of formaldehyde and urea and/or melamine) . The amino resins do, however, give a high release of formaldehyde, both at pressing and from the finished products. If the molar ratio between formaldehyde (F) and comonomer (eg urea (U) ) in the adhesive is lowered, the formaldehyde emission will, however, be decreased. To fulfil the demands with regard to environment it has been necessary to reduce the molar ratio considerably. At low molar ratios (F:U < 1.8) the adhesive has an inferior cold tack and long cold press times are. then required to obtain acceptable results. For reasons of * , economy it is thus desired to have short assembly times and- cold press times. - It is common to try to improve the cold tack of the-; adhesive by different additives. Among others, starch and', gluten are used. In DE A 3125823 it is suggested to add * , acrylate resin dispersions with free carboxylic groups to increase the cold tack of amino resins. The cold press properties of the adhesive are de~. pendent on several factors, among others on the moisture content of the veneers and the type of wood in these. Even, such factors as the adhesive formulation, the amount of * adhesive and external climatic variations in the form of'. moisture and temperature changes are of importance. An: adhesive formulation which works well in an industry in one: place can be quite unsatisfactory in another location. ; Compensating for all these variations by different addi-:. tives and modifications of the adhesive is not a good;. solution in practice. It is both toilsome and costly. The present invention, as disclosed in the patent claims, is directed to an improved method at cold pressing of packages of veneers. In the method one surface of a- veneer layer is coated with a curable adhesive in a known' manner. The veneer layer to be placed against the thus . adhesive- coated veneer is coated with a separate hardener in a zone along at least two of its edges. If only two edges are coated, those opposite each other are chosen. The veneer package is then assembled with every second surface -. coated with adhesive and every second coated with a sepa- . rate hardener. The package obtained in this manner .is then . loaded in the cold press and the adhesive is hereby cured " in the zones along the edges which have been coated with a " separate hardener when this is mixed with the adhesive. In the cold pressed veneer package the veneer layers will be kept together by means of the cured adhesive along the edges. The requirement for sufficient cold tack of the adhesive has thus been eliminated. The time for the cold pressing can be kept short, since the separate hardener makes the adhesive cure rapidly. Hereby the need for long cold press times has also been eliminated. The cured adhesive along the edges gives as a result that also those veneers which before the cold pressing have undulating dented edges, due to moisture absorption, after the cold pressing will have straight edges which keep together well. The cold pressed veneer package is easy to handle and it is easy to introduce it into the often narrow space between the press platens in the hot press, where the final curing then takes place. It is easily understood that it in order to make the veneer layers keep together in a package it can be fully sufficient to coat these along only two edges which are opposite to each other. If a greater safety and a stronger holding together is desired, the veneers can be coated along all four edges. The coating of different veneer layers with either adhesive or separate hardener can be carried out in arbitrary order. For a veneer package with three plies the middle veneer can thus, for example, be coated on both its surfaces with a hardener solution along the edges and the cover veneers can be coated with adhesive on one surface. Just as well the center veneer can be coated with adhesive on both surfaces and the cover veneers be coated with hardener on one surface, along the edges. The separate hardener can be applied in a zone along the edges of the veneer layer. A suitable size for this zone can be determined by the man skilled in the art by simple tests. For large veneers a suitable width can be from 10 to 100 mm, preferably from 30 to 60 mm. The zone to which the separate hardener has been applied does not have to be entirely coated. It can be sufficient that the zone is coated in strings or in dots along its surface, among other things dependent on the application equipment. 10 mm broad strings can thus for example be applied respectively 15 mm, 25 mm and 50 mm from the edge. The method according to the invention can be used with conventional formaldehyde based, curable adhesives which are used in plywood production. Among these can be mentioned condensation products of formaldehyde and urea, melamine, phenol, resorcinol or mixtures thereof. It is preferred to use the method with urea-formaldehyde resins with a low F:ϋ value, ie < 1.6. The applied adhesive contains, as conventional, a slow-reacting hardener for the final curing in the hot press. The applied amount of adhesive can vary within wide limits. As a suitable amount can, for example, be mentioned from 100 to 300 g/m . As separate hardeners conventional rapidly reacting hardeners are used * With a rapid reacting hardener is meant a hardener which gives the adhesive such a gelling time that it cures during the cold press process. The hardener should give the adhesive a gelling time of less than 1 hour, preferably of from 5 to 20 minutes. It is not possible to use such a hardener as the sole hardener for the adhesive, since the adhesive would then get a much too short assembly time, which would result in a curing of the adhesive before the veneer package has reached the hot press. Using the present process the adhesive cures only along the edges at the cold pressing. When the adhesive is an amino resin the hardener can for example be an inorganic or organic acid, such as phosphoric acid, trichloroacetic acid, citric acid or maleic acid. Acid salts, such as for example aluminium sulphate and aluminium chloride, can also be used, when the adhesive is a phenol resin the edges of the veneer layers can be coated with a basic compound, * eg sodium carbonate. The amount of applied separate hardener can also -vary within wide limits. A suitable amount can be from 15 to 40 g/m 2 of a 20% aqueous hardener solution. To make certain of or to improve the adhesive technical properties the har¬ dener can be combined with for example tack giving substan¬ ces such as aqueous solutions or aqueous dispersions of poly inyl alcohol, polyvinyl acetate, polyacrylates. The press times required at cold pressing are of the same magnitude as for adhesives with good cold tack proper¬ ties, and thus in the range of from 5 to 20 minutes, preferably from 10 to 15 minutes. The invention is further illustrated in the following examples wherein parts and per cent relate to parts by weight and per cent by weight, unless otherwise stated. Example 1: At the production of plywood 3 Abachi veneers with the dimensions 300x300x2 mm were used. The moisture content of these was about 10%. The center ply was coated with a hardener solution consisting of an aqueous solution of citric acid and aluminium sulphate (20%) in an amount of 40 g/m 2 around the edges in a zone with a width of 10 mm. The hardener solution was coloured to give a better control of its spreading. The water was allowed to evaporate (this is, however, not strictly necessary). The other two veneers were coated with a urea-formaldehyde adhesive to which an hardener had been added. The adhesive had a F:U ratio of 1.16 (as adhesive was used urea resin 1203 from Casco Nobel, 100 parts by weight with 20 parts by weight of hardener, hardener 2545 from Casco Nobel, based on ammonium chloride) . The amount of adhesive was 180 g/m 2 for a single adhesive joint. Two tests were made with variation of the assembly time (ie the time before cold pressing) and the time between cold pressing and hot pressing. Each test was made with separate hardener (test a) and without separate hardener (test b) . Cold pressing Temperature: 23°C Press pressure: 0.8 MPas Press time: 10 minutes Hot pressing Temperature: 125°C Press pressure: l MPas Press time: 2.5 minutes Example 2: Example 1 was repeated but with 3 veneers of poplar having the dimensions 300x300x2 mm. The moisture content of these was about 5%. Assembly Time between cold Result time and hot pressing Test 3a 2 min 20 min closed edges Test 3b 2 min 20 min open edges Test 4a 20 min 5 min closed edges Test 4b 20 min 5 min open edges Both the cold gluing and hot gluing results were excellent for the plywood panels prepared according to the present method. The cold pressed veneer package had a good stability, despite the short press time. At the tests without addition of separate hardener the veneer layers did not keep together after the cold pressing.

Claims: Claims 1. A method for the production of wood products such as plywood or similar products by applying a formaldehyde based curable adhesive on veneer layers, assembling of the veneer layers to a package and cold pre-pressing of this, whereafter the package is cured by pressure and heat, characterized in that one surface of a veneer layer is coated with the curable adhesive and a surface opposite to this is coated with a separate hardener in a zone along at least two edges opposite to each other. 2. A method according to claim 1, characterized in that one surface of a veneer layer is coated with a sepa¬ rate hardener in a zone along two edges opposite to each other. 3. A method according to claim 1, characterized in that the zone is coated in dots or strings along its surface. 4. A method according to claim 1, characterized in that the separate hardener is a rapid-curing hardener. 5. A method according to claim 4, characterized in that the hardener is combined with a tackifying substance. 6. A method according to claim 5, characterized in that aqueous solutions or aqueous dispersions of polyvinyl alcohol, polyvinyl acetate or polyacrylates are used as tackifying substances. 7. A method according to claim 1, characterized in that the adhesive is a urea-formaldehyde adhesive with a molar ratio formaldehyde:urea < 1.6.

Inventors: LEHNERT EMIL, LEHNERT, EMIL
Applicants: CASCO NOBEL AB
Application year: 1988
Main classification: B27D1
IPC codes: B27D1, C09J5