FR2508837A1 - PROCESS FOR THE MANUFACTURE OF PLYWOOD - Google Patents

Abstract

The improvement of the tackiness (so-called cold pre-adhesive force) of veneer glues containing amino resins is possible with the aid of dispersions containing, in particular, acrylic acid incorporated by polymerisation.

Description

 During the manufacture of plywood, it has become of general use to subject glued veneers, in a pile comprising several panels, to a prior cold pressing. After cold pressing, the veneers must adhere firmly enough to each other so that the various panels can be conveyed, by automatic feeding devices, without the use of sheet metal shims, to the press with heating plates.

This automatic process saves costs, it ensures a better distribution of humidity in the panel and defective points, for example points where excess glue was produced, can be removed even before hot gluing.

 The property of the adhesives to confer a certain cohesion even before the setting proper is called the resistance of prior bonding or aptitude for cold bonding.

 When gluing plywood with phenol-formaldehyde resins (phenoplasts), cold pressing of the veneers succeeds in these problems; even after a relatively short preliminary pressing time, very good cohesion of the veneers is obtained.

 AIinoplast resins (that is to say in particular urea-formaldehyde condensates) have, by their nature, a significantly less good ability to cold bonding than phenolic resins. To obtain a pre-bonding of previously pressed panels allowing their transport, the viscosity of the glue bath has hitherto been increased by lengthening the latter with wheat flour or by adding it with powdered plasma albumin or casein. despite these measures, cold pre-bonding is generally of rather poor quality. It is in particular when sizing with mixed urea-melamine and urea-melamine-phenol condensates that the products in the dition cannot be added in sufficiently high quantities since they reduce the temperature stability of the sizing. In fact, plywood glued with mixed condensates must generally be weatherproof, or even boiling water.

 The essential object of the invention, which emerges from the foregoing, consists in improving the strength of prior cold bonding of (mixed) condensates of aminoplasts, by addition of an appropriate product.

 However, it has been found that plywood is advantageously obtained by prior cold pressing of sheets glued with curable aminoplast resins and hardening, under pressure and hot, of the packet of sheets previously pressed, by mixing with the glue a secondary quantity of an aqueous dispersion of acrylate resin containing, in polymer bond, free or neutralized carboxyl groups.

 By secondary quantity in the sense of the invention is meant, for example, an effective amount of 0.5 to 10 * by weight (dry extract of the dispersion relative to the dry extract of the adhesive).

 As aqueous dispersions of acrylate resins which can be used in the process according to the invention, having free carboxyl groups, that is to say not esterified but optionally neutralized (in the form of a salt), mention may be made, for example, of copolymers (meth) acrylic ester (meth) acrylic acid generally containing - based on the molar sum of the monomer units - up to 70 * (moles) of (meth) acrylic acid units or equivalent amounts of maleic acid. '(meth) acrylic acid or maleic acid, they most often contain, mainly or exclusively, esters of (meth) acrylic acid with 1 - 6 carbon atoms in the alcohol residue, preferably 1 or 2, and optionally secondary quantities of other polymerizable olefinically unsaturated compounds, such as styrene or vinyl compounds, in each case in the form of polymer bonded units.

 Where appropriate, these dispersions of acrylate resins can be supplemented with other anionic emulsifiers, for example carboxylates or sulfonates.

 Said dispersions, which are generally free of plasticizers and solvents, are certainly on the market, but have not hitherto been added to glues of aminoplast resins.

 As for the addition operation, it will be noted that the pH of the adhesive bath is lowered to approximately 3.5 to 6.0, by addition of a weak, free acid, as a hardener, therefore for example a carboxylic acid or a mixture of carboxylic acids, and the dispersion is added only afterwards.

 When the pH value of the adhesive is too high, greater than pH 6, the initially insoluble polymer in the strip swells so strongly that there is a too strong increase in viscosity. This is why the use of ammoniacal (or latent) hardeners is not recommended.

 The dispersion used in the following examples is an aqueous dispersion at approximately 25 ffi of a copolymer, with approximately equal molar parts, of acrylic ester and (meth) acrylic acid.

 The quantities indicated in the following examples (relating to glue, hardeners, fillers and polymer dispersions) are parts by weight.

Example 1 urea resin adhesive, at 65 * 100.0 100.0 wheat flour 30.0 30.0 hardener A) ++ hardener B) ++ 10.0 polymer dispersion - 5.0 viscosity after let sit
10 min at 2O0C [mPa.s 13300 6250 2 quantity of glue applied per m
single sided [gj 180 180 dwell time after application
glue [min 1 15 15 cold pressing time under
0.5 mm pressure [min] 10 10% open joints (average value
3 x 3 x 2.5 mm panels,
beech veneer) after
5-minute stay in the open 20 14) + A suitable resin is in the liquid state in
trade, for example under the name of Colle de
Kaurit 285.

) ++ Hardener A is a 16% aqueous solution of
technical mixture of C3 to C5 dicarboxylic acids.

Hardener B is a 15% aqueous solution of
ammonium chloride.

 By reducing the dose of hardener A, a higher viscosity and a better cold bonding ability are obtained. Normally, the quality of bonding obtained (see table above) should however be satisfactory.

Example 2
Mixed condensate based on melamine resin (at 63%) + 100.0 100.0 100.0
Infusil) ++ 12.0 12.0 12.0
Vavanite) ++ 10.0 10.0 10.0 wheat flour 2.0 2.0 2.0 hardener A 4.0 4.0 4.0
Dispersion as in Example 1 2.5 5.0 viscosity after leaving to stand
10 min at 20 C [mPa.s] 4400 7600 3900 quantity of glue applied per m
single sided g] 180 180 180 dwell time after application
glue Lmn] 15 15 15 cold pressing time under
0.5 mm pressure [min] 10 10 10 * of open joints (value
average of 3 panels 3 x 2.5 mm,
beech veneer) after
5-minute stay uncovered 13 7 20) + An appropriate resin adhesive is present
commercially available under the name of Glue
Kauramine 541.

) ++ Commercial type charges.

With the melamine-urea-phenol-formal condensate used in Example 2 above, the effect of the dispersion is clearly stronger than with the urea-formaldehyde resin used in Example 1. By adding, for example, in
2 the preparation / above, only 2 parts by weight of hardener A instead of 4 parts by weight, a pH of 6.4 is established; from a rest time of 15 minutes, the adhesive bath becomes too viscous to be able to be worked. The effect of the desired cold bonding ability can therefore be adapted exactly to the requirements by adjusting the pH value by adding variable amounts of hardener.

 The addition of the dispersion according to the invention does not affect either the resistance to boiling water or the resistance to alternating tests.

Claims (1)

 CLAIM  Process for the production of plywood by pre-cold pressing of endlit veneers of curable aminoplast resins as glue, and hardening of the packet of veneers previously pressed, under pressure and hot, characterized in that a glue is mixed with a secondary quantity of an aqueous dispersion of acrylate resin containing, in polymer connection, free or neutralized carboxyl groups.