FI91501C - Process for making wood products - Google Patents

Description

91501

Methods for making wood products

This invention relates to a process for the manufacture of wood products such as plywood or the like. More specifically, the invention relates to an improved cold pressing technique for pre-pressing a veneer sheet.

In the manufacture of plywood, glue is applied to the veneers and several glued-plywood veneers are stacked for stacking. Usually this bed is cold pressed before it is hot pressed for final curing. The amount of veneers in the lathing is determined by the thickness of the finished product. Suitably at least some of the veneers are positioned so that their causes are in a direction perpendicular to the direction of autumn of the adjacent veneers. The veneers of the adhesive application are divided into the stack for a longer or shorter time before its further processing. The veneers absorb moisture from the adhesive, swell and become even more uneven than they were from the beginning. The veneer lattice becomes larger than the available opening of the hot press table. To reduce the thickness of the barn, it must be pre-compressed cold. The result of the pre-compression must also be that the veneer layers remain together during the myShemm treatment steps of the stack. It is important that the cold adhesion of the adhesive composition (i.e., the bonding strength of the adhesive before it is cured) is sufficient to hold the veneers together with the pre-press fraction.

The adhesives used in the manufacture of plywood are usually phenolic resins (condensation products of phenol and formaldehyde) and amino resins (condensation products of formaldehyde and urea and / or melamine). However, amino resins cause high formaldehyde fasts both during compression and in finished products. However, if the molar ratio of formaldehyde (F) to comonomer (e.g. urea (U)) in the adhesive is reduced, the formaldehyde fasting will decrease. In order to meet the environmental requirements, it has been necessary to reduce the molar ratio considerably. At low molar ratios (F: U <1.8), the adhesive has poorer cold adhesion, and long cold compression times have been required to achieve acceptable results. For economic reasons, it would therefore be advantageous to provide short stacking times and cold pressing times.

The cold adhesion of the adhesive has generally been improved with various additives. Among others, monitoring and gluten have been used. DE application 3125823 proposes the addition of acrylate resin dispersions with free carboxyl groups to amino cold resins to improve cold adhesion.

The cold-pressing properties of the adhesive depend on several factors, including the moisture content of the veneers and the type of wood used in them. Factors such as the composition of the adhesive, the amount of the adhesive, and external climate variations caused by changes in humidity and lampB conditions are important to MyBs. An adhesive assembly that works well in an industry in one location can be quite unsatisfactory in another location. Compensating all of these variables with various adhesive additives and modifications is not a good solution. It is both tyBias and potassium.

. This invention is defined in the claims.

to an improved method for cold pressing veneer atoms. In the method, the second surface of the veneer layer is baked with a curable adhesive in a known manner. The veneer layer to be placed against the veneer etched with the adhesive is baked with a separate hardener from a zone running along at least two edges thereof. If only two edges are corrugated, those that are opposite each other are selected. veneer-. the lathe is then assembled so that every other surface is bonded with glue and every other separate with a hardener. The stack obtained in this way is then placed in a cold press and the adhesive hardens from the zones running along the edges, which are baked with a separate hardener when this is mixed with the adhesive. In a cold-pressed veneer laminate, the veneer layers are held together along the edges by the action of a cured adhesive.

N3in adhesive is no longer required to have sufficient cold adhesion. The cold pressing time can be kept short because a separate hardener causes the adhesive to cure quickly. N3in no longer needs long cold pressing times. The result is also achieved with the adhesive hardened along the edges that the veneers, which have wavy edges due to moisture absorption before cold pressing, also become straight edges during cold pressing, and the edges stick well together. The cold pressed veneer sheet is easy to handle and is easy to push into the silent, often narrow space of the hot press press plates, where the final curing then takes place.

It is easy to understand that in order to hold the veneer layers together in the stack together, it may be quite sufficient to fry them only along two opposite edges.

If greater security and stronger adhesion are desired, veneers can be veneered on all four edges; a. The baking of the different veneer layers with either glue or a separate hardener can be performed in any desired order. Thus, in a veneer sheet with three layers, for example, the center veneer can be etched along the edges on each surface with a hardener solution and the surface veneers can be etched on the other surface with adhesive. Just as well, the center veneer can be glued on both surfaces with glue and the surface veneers on one surface with a hardener along the edges.

A separate hardener can be applied to a zone running along the edges of the veneer layer. A person skilled in the art can, by simple experiments, determine the size of the belt. For large veneers, a suitable width may be 10 to 100 mm, preferably 30 to 60 mm. The zone on which the separate hardener has been brushed does not have to be completely corrugated. It may be sufficient for the zone to be baled along strips or patches along its surface, depending on, among other things, the coating equipment. Thus, for example, strips with a width delta of 10 mm can be used from 15 mm, 25 mm and 50 mm from the edge, respectively.

The method according to the invention can be used in connection with conventional formaldehyde-based, curable adhesives used in the manufacture of plywood. Among the N3, condensation products of formaldehyde and urea, melamine, phenol, resorcinol or mixtures thereof can be mentioned. It is recommended to use the method with urea-formaldehyde resins with a low F: U value, i.e. <1.6. The adhesive used, as usual, contains a slow-reacting hardener for the final curing in the hot press. The amount of adhesive used can vary within wide limits. As a suitable amount, for example, 100 to 300 g / m2 can be mentioned.

: Conventional fast-reacting hardeners are used as separate hardeners. By fast-reacting hardener is meant a hardener which gives the adhesive a gel time such that it hardens during cold pressing. The hardener should give the adhesive a gel time of less than 1 hour, preferably 5-20 minutes.

5 91501 It is not possible to use such a hardener as the sole hardener of the adhesive, since the adhesive would then have a far too short stacking time, which would lead to the adhesive curing before the veneer bed has reached the hot press. In the method of the invention, the adhesive cures in cold pressing only along the edges. When the adhesive is an amino resin, the hardener may be, for example, an inorganic or organic acid such as phosphoric acid, trichloroacetic acid, citric acid or maleic acid. Acidic salts of MyOs, such as aluminum sulfate and aluminum chloride, can be used. When the adhesive is a phenolic resin, the edges of the veneer can be embossed with an emixel compound, e.g. sodium carbonate.

The amount of separate hardener used can also vary widely. A suitable amount may be 15 to 40 g / m2 of a 20% aqueous solution of the hardener. To ensure or improve the technical properties of the adhesive, adhesives such as aqueous solutions or aqueous dispersions of polyvinyl alcohol, polyvinyl acetate and polyacrylates can be incorporated into the hardener.

The compression times required for cold pressing are in the same range as for adhesives with good adhesion properties, and thus in the range of 5-20 minutes, preferably 10-15 minutes.

The invention is illustrated in more detail by the following examples, in which parts and percentages mean parts by weight and percentages by weight unless otherwise stated.

Example 1

3 plywood veneers with dimensions of 300x300x2 mm were used for the production of plywood. Their moisture content was 6 91501 about 10%. The middle veneer was pSMl plated with a hardener solution consisting of an aqueous solution of citric acid and aluminum sulfate (20%), mSarSna 40 g / m 2 around the edges of a clear band of about 10 mm width. The hardener solution was concealed to better control its spread. The water was allowed to evaporate (however, this is not absolutely necessary). The other two veneers were baked with urea-formalde hydraulics to which a hardener had been added. The F: U ratio of the adhesive was 1.16 (the adhesive used was Casco Nobel urea resin 1203, 100 parts by weight with 20 parts by weight of hardener, Casco Nobel ammonium chloride-based hardener 2545). The amount of glue per glue joint was 180 g / m2.

Two tests were performed by varying the stacking time (i.e., time before cold pressing) as well as the silent time of cold pressing and hot pressing. Each test was performed with a separate hardener (test a) and without a separate hardener (test b).

stacking - cold and hot result time press change time test Sat 2 min 20 min closed edges test lb 2 min 20 min open edges test 2a 20 min 5 min closed edges test 2b 20 min 5 min open edges

Cold pressing lamp Mode: 23eC pressing pressure: 0.8 MPa 7 91501 pressing time: 10 minutes Hot pressing larap Mode: 125eC pressing bellows: 1 MPa pressing time: 2.5 minutes

Example 2

Example 1 was repeated but using 3 poplar veneers measuring 300x300x2 mm. Their moisture content was about 5%.

stacking- cold and hot result time pur. vcil.time 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 • · • «

The results of both cold gluing and hot gluing were excellent for plywood sheets made according to the methods of the invention. The stability of the cold-pressed veneer sheet was good despite the short pressing time. In tests without the addition of a separate hardener, the veneer layers did not stick together to the cold-pressed fraction.

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Claims (6)

1. Process for manufacturing wood products such as plywood or curing by applying a formaldehyde-based hardenable adhesive to veneer sheets, folding the veneer sheets into a pile and pressing it cold, after which the pile is cured by pressure and heat. the surface of a veneer sheet is coated with the curable adhesive and a surface of the opposing surface is coated with a separate hardener in a joint along at least two, opposite, edges, and the separate hardener is a quick hardener. The method of claim 1, characterized in that a surface of a veneer sheet is joined with a separate hardener in a sheet along two opposite edges. 3. A method according to claim 1, characterized in that the field is added in points or strands along its surface. 4. A method according to claim 1, characterized in that the hardener is combined with the tackifying agent. 5. A process according to claim 4, characterized in that, as the tackifying agent, aqueous solutions or aqueous dispersions of polyvinyl alcohol, polyvinyl acetate or polyacrylates are used. 6. A process according to claim 1, characterized in that the adhesive is an urea-formaldehyde adhesive with one. : mole Formaldehyde: urea ratio <1.6.