EP0699510B1 - Method for reducing the formaldehyde emission from lignocellulosic products - Google Patents

Abstract

Redn. in HCHO generation in aminoplast-bonded, pressed lignocellulose articles (esp. wood- or fibre-board) is effected by: (a) adding condensed tannin to the lignocellulose before, during or after applying the aminoplast; and (b) applying an HCHO-reactive material after pressing.

Description

The invention relates to a method for reducing the Formaldehyde release from aminoplast-bound pressed lignocellulose products, in particular chipboard and fiberboard, by gluing wood fibers and wood chips with condensation products from urea and / or melamine and / or phenol and formaldehyde, optionally in a mixture with another Aldehyde.

For the production of chipboard for furniture construction and that Construction becomes wood shavings or other lignocellulose-containing Particles with a urea-formaldehyde resin in quantities of usually from 8 to 10% by weight, based on the dry Chip material, glued, scattered to chip mats and under heat and Pressure pressed. The still hot chipboard is made after the Presses are usually stacked.

Instead of a urea-formaldehyde resin, other binders can be used such as alkaline curing phenol formaldehyde resins, polymers based on diisocyanates (PMDI) etc. can be used. So far, these binders have only been comparatively small Importance because they are either expensive or have other disadvantages have.

Particle boards, the urea-formaldehyde resins or other amino resins contain as a binder, give after production formaldehyde over a relatively long period of time. The release of formaldehyde this type of plate depends on various factors how molar ratio of urea to formaldehyde in the used resin, pressing time, binder expenditure, age of the plates etc. The formaldehyde released can reach values that the Well-being of people in rooms where with urea-formaldehyde resin bound chipboard are built in, strong affect. For the determination of formaldehyde release from Chipboard offers several methods, including these the perforator method (DIN EN 120), the gas analysis method (DIN 52368) and the WKI bottle method (Roffael 1975 / Pr EN 717). The emission of formaldehyde from chipboard can also be with the help of test rooms, in which the temperature, the number of air changes and the room load are fixed. After the Hazardous Substances Ordinance of October 1993 or the Chemicals prohibition ordinance are not allowed in the wood materials Placed on the market when their emission of formaldehyde in a defined climatic chamber is more than 0.1 ppm.

To reduce the subsequent release of formaldehyde from chipboard, has already been proposed urea formaldehyde resins with a low content of condensed formaldehyde use or the molar ratio formaldehyde: urea belittling. The use of these resins does decrease the subsequent release of formaldehyde is considerable, but can on the physical-technical properties of the manufactured Chipboards have a negative impact.

It is also known that by admixing urea and / or ammonia to the urea-formaldehyde resin the formaldehyde release can be reduced because these substances the formaldehyde can chemically set. The addition of such substances Resin is only possible to a limited extent since the content of such substances, the setting speed of the resin is slowed down and the physical-technological properties can deteriorate with constant pressing time.

It has also been suggested that the wood chips after the Glue to add finely divided substances with formaldehyde react chemically (DE-OS 1 653 167). One uses such Substances that react with the formaldehyde more slowly than it does corresponds to the rate of condensation of the resin. Such Fabrics are e.g. Melamine, urea, polyacrylic acid amide, Guanidine, hydroxylamine, ethylenediamine, phenol, resorcinol, Diphenylmethane diisocyanate (MDI).

The urea-formaldehyde resin has also been proposed Molecular compounds of various types to mix the To reduce formaldehyde emissions (Kubitzky, INDUSTRIE-ANZEIGER No. 53, Edition Plastics - Processing and Application, Göppingen 1972).

It has also been proposed (DE-OS 2 317 884), in which Manufacture of chipboard made of urea-formaldehyde resin Add tannins or tannin derivatives to the formaldehyde release to reduce the wood materials or chipboard produced. This disclosure, however, does this no quantitative examples given. In an article by Cammeron and Pizzi (ACS Symposium Series 316, p. 198-200, 1986) it is found that the addition of tannins during the gluing the release of formaldehyde from wood chips with urea-formaldehyde resins only temporarily reduced, so that tannic and Wood chipboard not containing tannins after a short period of had approximately the same formaldehyde release for about a week.

It is also known that the subsequent release of formaldehyde can be reduced by having the chipboard with a paint system based on aqueous plastic emulsion paint certain composition strokes on all sides. This contains formaldehyde-binding paint for particle board Ammonia, lactic acid casein and hydrazine hydrate in certain Composition (DE-PS 1 188 750).

In EP-A 0012 169 multi-layer, predominantly aminoplast-bound Particle or fiberboard described in an interior, preferably in the middle layer, as Binder not belonging to the aminoplast group Glue, e.g. Diisocyanate, and which are characterized are that only such binders for this interior can be used, the curing of which by the presence of additional formaldehyde-reactive substances are not affected is, the binder these additional substances in the form of ammonia, ammonium carbonate, ureas, thioureas, Melamine or dicyandiamide are mixed in with the after the pressing of the plate still existing or released Formaldehyde under the influence of moisture and / or heat directly, possibly also indirectly via fission products, setting it react.

EP-PS 0 006 486 describes a method for reduction the release of formaldehyde from aminoplast-bound chipboard or Fiberboard known in which on the surface of the plates in hot state immediately or at short intervals their removal from the heating press urea or other ammonia releasing substances applied in the form of an aqueous solution will. A process which is similar in principle, in which the plates already cooled to a conditioning temperature an ammonium salt solution is applied from the EP-PS 0 027 583 known.

The present invention is in relation to the cited state based on the task, plates with extreme produce lower formaldehyde release, their formaldehyde release hardly differs from that of natural wood differs.

This object is characterized by that in claim 1 Procedure solved.

It has surprisingly been found that the release of formaldehyde can be reduced to a minimum by giving the chips before or during or after gluing with aminoplast resins, for example with a molar ratio F: NH ₂ of about 0.6: 1 or 0.5: 1 , condensed tannin in amounts based on the weight of the wood chips from about 0.1% to a maximum of 5%, preferably 1% to 0.1%, and the plates are treated in a manner known per se with formaldehyde-reactive (basic) agents are, for example in the manner known from EP-PS 0 006 486 or EP-PS 0 027 583.

The method described in EP-PS 0 006 486 alone is sufficient not for this, since the formaldehyde release level reached is significantly higher than that according to the method according to the invention. The method described in DE-OS 2 317 884 is also suitable not for this, because without subsequent spraying of the plates after pressing the condensed tannin has no effect on the The formaldehyde release of the plates unfolds.

The invention is explained in more detail in the following example.

Wood chips are coated with a urea-formaldehyde resin before gluing (e.g. 8% solids based on the weight of the wood chips) the formaldehyde-urea molar ratio F: U of 1: 1 with tannin mixed in amounts of approximately 0.5% by weight, based on the Weight of the wood chips. A distinction is made between hydrolyzable Tannins and condensed tannins, the latter one much higher reactivity to formaldehyde have (Holz-Zentralblatt 1994, No. 6, pp. 90-93) and therefore at the method described here are preferred. That condensed Tannin is preferably obtained from quebracho wood, in itself known way by boiling the wood, whereupon the obtained Extract can be dried and pulverized. The tannin can added to the chips in powder or liquid form will. Then the glued wood chips become chip mats manufactured and this in the usual way to chipboard hot pressed. After pressing, the plates are still hot Condition in the manner already known with treated with basic, i.e. formaldehyde-reactive substances, e.g. With sprayed with an aqueous urea solution. The chipboard produced in this way are in the table below with "Type 1" designated.

For comparison, chipboard with urea-formaldehyde resin produced without the addition of tannins and then with Sprayed formaldehyde-reactive substances when hot (type 2). Furthermore, chipboard with urea-formaldehyde resins and the chips were made with urea-formaldehyde resin before gluing with tannins in quantities of 0.5% (parts by weight based on the weight of the chips) mixed. The chipboard were not sprayed after production (type 3). Furthermore, chipboard with urea-formaldehyde resin as Binder made in quantities of 10% (parts by weight based on the weight of the wood chips). The wood chips were neither condensed tannins were added, nor were the particle boards treated with formaldehyde-reactive substances after production (Type 4).

From the tabulated information it can be seen that the addition of tannins to the chips before gluing with the urea-formaldehyde resin and subsequent pressing has no appreciable effect on the formaldehyde release of the chipboard produced, as long as the chipboard produced is not post-treated (see type 3 and 4). In contrast, the effect of spraying on the formaldehyde release of the chipboard that does not contain tannins can be clearly determined (type 2). However, this effect is far less than if the chipboard contains tannins (type 1). In this respect, the process described here differs fundamentally from all known processes, since the effect of the condensed tannins as a formaldehyde-reducing agent only comes into play after spraying. Formaldehyde release according to the WKI bottle method 3h in mg per 100g plate TYPE 1 0.05 TYPE 2 0.3 TYPE 3 0.4 TYPE 4 0.5

Claims (10)

1. Method of reducing the emission of formaldehyde from aminoplastic-bonded, pressed lignocellulosic products, particularly woodchip or fibre boards, characterised in that before, during or after the gluing with aminoplastic resins condensed tannin is added to the lignocellulosic materials and after the pressing process a substance which reacts with formaldehyde is applied to the manufactured lignocellulosic product.
2. Method as claimed in Claim 1, characterised in that the tannin is added to the lignocellulosic materials in powder form.
3. Method as claimed in Claim 1, characterised in that the tannin is added to the lignocellulosic materials in liquid form.
4. Method as claimed in Claim 1, 2 or 3, characterised in that the tannin is added in amounts of at least 0.1% with respect to the weight of the lignocellulosic materials.
5. Method as claimed in Claim 1, 2 or 3, characterised in that the tannin is added to the lignocellulosic materials in amounts of at most 5% with respect to the weight of the lignocellulosic materials.
6. Method as claimed in Claim 1, characterised in that the lignocellulosic board is sprayed with the substance which reacts with formaldehyde.
7. Method as claimed in one of the preceding claims, characterised in that urea or another substance which liberates ammonia is applied as the substance which reacts with formaldehyde.
8. Method as claimed in one of Claims 1 to 7, characterised in that the tannin is introduced only into the surface layers of multilayer chip boards.
9. Method as claimed in one of Claims 1 to 7, characterised in that the tannin is introduced only into the middle layer of multilayer chip boards.
10. Method as claimed in one of Claims 1 to 7, characterised in that the tannin is introduced both into the middle layer and also into the surface layers of multilayer chip boards.