EP1907178B1 - Method for production of wood material articles with low emissions of chemical compounds - Google Patents

Description

The invention relates to a method for producing a wood-based article with low emission of chemical compounds.

Wood-based products such as particleboard or fibreboard are made of chips or fiber material and a glue. Usually, in the manufacture of the wood-based material article, the glue is applied to the comminuted wood (chips) or the fiber material, and then the obtained, still moist wood-material articles are pressed together at elevated temperatures (hot pressing), whereby the glue hardens. Formaldehyde-containing resins have long been used as hardeners and / or binders in such glues.

Wood-based panels are often used inside closed rooms. Therefore, it is important that the clamping plates do not emit any interfering (e.g., strong odor compounds) or even damaging compounds. The emitted compounds can originate from the wood itself or from the glue used.

The publication JP 9019906 describes a process for producing a wood-based article according to the preamble of claim 1, wherein wood fibers glued together are treated with bisulfite prior to gluing. The aim is to provide a fiberboard that has a strong bond strength and that can be made with a reduced amount of resin.

The publication JP 10119010 also describes a process for producing a wood-based article, in which the wood is treated with bisulfite prior to gluing.

When using formaldehyde-containing resins in the manufacture of wood-based articles, the finished articles emit amounts of formaldehyde, the amount of which is limited by statutory regulations and voluntary requirements of the industry to extremely low limits

• Modifikation des Leimes durch Reduzierung des molaren Verhältnisses von Formaldehyd zu den anderen Komponenten des Harzes;
• Zugabe von Formaldehyd bindenden Substanzen, wie Carbamid;
• Mischen von formaldehydhaltigen Harzen mit anderen Harzen oder
• Verlängerung der Presszeit.

Various methods are known for reducing the formaldehyde content and in particular the emission of formaldehyde from the finished wood-based articles. Some of them are:

• Modification of the size by reducing the molar ratio of formaldehyde to the other components of the resin;
• Addition of formaldehyde-binding substances, such as carbamide;
• Mixing of formaldehyde-containing resins with other resins or
• Extension of press time.

However, these methods reduce the efficiency of the manufacturing process (increased costs, lower production capacity due to longer production times, etc.) or have negative physical or mechanical influences on the product (reduction of resin stability, reduction of strength and resistance of glue joints, etc.).

It was therefore an object of the present invention to provide an improved method for producing a wood-based article, wherein the obtained wood-based article in particular has a low emission of chemical compounds and the efficiency of the manufacturing process is not berintrüchtizt.

This object is achieved by a method for producing a wood-based material article according to Ansprunch 1 solved.

Surprisingly, it has been found that with the aid of the method according to the invention, improved wood-based articles can be produced which, in particular, show little or no unwanted emission of (volatile) chemical compounds which originate from the wood itself. Furthermore, the bisulfite-treated wood chips or wood fibers show a better reaction with the glue.

Further advantageous embodiments of the method are the subject of the dependent claims.

For the manufacture of wood-based articles conventional deciduous and coniferous woods are sorted or mixed in a preparation. First, the wood is debarked and crushed. Wood chips or shavings can be added to the shredded wood. Subsequently, this mixture is sieved and sifted, whereby impurities such as sand, splinters, stones or metal are removed. Subsequently, this mixture is washed, which in turn impurities such as sand and stones are removed. In the next step The shredded wood is heated and dehydrated. This is preferably done by pressing.

The thus treated crushed wood is then subjected to a steam treatment at a pressure of 6 to 12 bar in a saturated steam atmosphere. In addition, an aqueous solution of a bisulfite salt may be supplied to the vapor atmosphere. The bisulfite salts may be, for example, alkali metal and / or ammonium salts. Preference is given to using sodium and / or ammonium bisulfite salts. It is particularly preferred to use ammonium salts. Preferably, the amount of bisulfite salt is between 1 and 30 kg, preferably between 3 and 12 kg, per m ^{3 of} plate produced. The duration of treatment is according to the invention between 3 and 8 minutes. Higher levels of bisulfite require longer treatment times to complete the reaction so that the bisulfite can penetrate the fiber and ensure stable impregnation.

Preferably, in one embodiment, the wood is treated with bisulfite prior to the production of the wood chips or wood fibers, for example in the digester, in order to be able to carry out the process in a simple manner. A subsequent impregnation of the wood fibers with paraffin thus does not impair the effect of the method according to the invention.

According to another embodiment, the wood chips or wood fibers are treated with bisulfite, for example in the refiner, with bisulfite. As a result, the method can be carried out particularly efficiently.

Without wishing to be bound by this theory, the treatment of crushed wood with bisulfite is important in order to bind the volatile substances in the comminuted wood and to suppress the formation of new volatile substances. In bisulfite treatment, for example, the bisulfite adducts of aldehydes, methyl ketones or α-keto esters can be formed.

In addition, the bisulfite can develop its bleaching effect and react with the dyes present in the wood so that particularly bright wood-based articles can be obtained. The brightness of a plate is an important quality criterion.

Further, when treating the wood with bisulfite, a weakly acidic impregnation of the fiber takes place, so that in the case of using formaldehyde-containing resins in the glue complete polycondensation with the resin and thus a complete consumption of the free formaldehyde takes place. Both effects lead to wood-based panels, which have extremely low Perföratorwerte and accordingly an extremely low emission of formaldehyde.

A treatment time of at least 1.5 minutes with bisulphite ensures that the wood is almost completely dry and that subsequent treatment with the impregnating agent against moisture sources is not ineffective.

Subsequently, the impregnated pieces of wood are comminuted to wood shavings or wood fibers, glued, dried and pressed. This is done by conventional methods.

The glue used for gluing the wood chips or wood fibers is preferably an aminoplast resin. It is particularly preferred that a formaldehyde-containing resin is used. This is preferably selected from the group consisting of urea-formaldehyde resin, melamine-formaldehyde resin, urea-melamine-formaldehyde resin and melamine-urea-phenol-formaldehyde resin. Suitable resins are commercially available as solutions or powders and the preparation and use of suitable resins is also described in the prior art. In particular, urea-formaldehyde resins and urea-melamine-formaldehyde resins which are used in the customary amounts are preferred for the process according to the invention. Particularly advantageous formaldehyde-poor resins are used which have a low molar ratio of formaldehyde to urea. The The molar ratio of formaldehyde to urea is advantageously between 0.8: 1 to 1.05: 1.

However, by treating the shredded wood with bisulfite, glues having a higher molar ratio of formaldehyde to urea than the above can also be used. Even the wood-based articles produced with it have extremely low perforator values that meet all current standards. With these resins, which are more reactive and cheaper than the formaldehyde-poor resins, even E1 class plates can be made.

The process according to the invention also has the advantage that in the glue conventional resins without formaldehyde-binding additives which can have negative physical or mechanical influences on the final product can be used. However, if used, the glue used may additionally contain formaldehyde-binding substances.

In addition to the aminoplast resin, the glue may comprise further additives such as hardening accelerators, water repellents, inert salts, pH adjusters, stabilizers, fungicides or biocides.

The gluing is conveniently carried out by compressing the end-comminuted wood provided with the glue at temperatures of 120 to 250 ° C. Under these conditions, the aminoplast resin cures rapidly, resulting in wood-based articles with good mechanical properties, which are largely insensitive to moisture.

embodiment

Two fiberboards S1 and V1 were made by the above-described method using a formaldehyde-urea resin having a molar ratio of formaldehyde to urea of 1: 1.08, with the only difference that in the production of the fiberboard V1, the crushed wood was not treated with bisulfite before impregnation. The Treatment of the crushed wood with bisulphite was carried out at a pressure of 8.5 bar, with an amount of bisulphite of 3 kg per m ³ crushed wood and over a period of 3.5 minutes. The pretreatment of the wood is followed by wetting with a known moisture impregnating agent and then the compression.

Table 1 shows the perforator values (determined according to DIN EN 120 - wood-based materials - determination of the formaldehyde content) of the two fibreboards S1 and V1 . Table 1 fibreboard Perforator values in mg HCHO / 100g fibreboard S1 2.5 V1 6.5

The values in Table 1 show that by treating crushed wood with bisulfite, wood-based articles having extremely low formaldehyde emission are obtained. In addition, the fiberboard S1 emitted only extremely small amounts of other chemical compounds. In addition, the fiber board S1 was much brighter than the fiber board V1.

Claims (7)

1. Method for the manufacturing of a derived timber product article comprising wood particles or wood fibers bonded with each other, characterized in that the wood, prior to the bonding, is treated by means of bisulfite at a pressure of 6 to 12 bar in a saturated water vapor atmosphere or in water vapor saturated air over a time period of 3 to 8 minutes.
2. Method according to claim 1, wherein the wood is treated by means of bisulfite, for example in a cooker, prior to the production of the wood particles or the wood fibers.
3. Method according to claim 1, wherein wood particles or wood fibers with bisulfite are treated by means of bisulfite for example in a refiner.
4. Method according to one of the preceding claims, wherein the bisulfite is added in an amount of 1 to 30 kg per m³ of wood, preferably in an amount of 3 to 12 kg per m³ of wood.
5. Method according to one of the preceding claims, wherein the bisulfite is chosen from sodium bisulfite and ammonium bisulfite.
6. Method according to one of the preceding claims, wherein the wood treated by means of bisulfite is transformed into wood particles or wood fibers and the wood particles or wood fibers are bonded by means of an aminoplast resin.
7. Method according to claim 1, wherein the wood particles or wood fibers are bonded by means of an aminoplast resin that is chosen from the group comprising urea-formaldehyde resin, melamine formaldehyde resin, urea-melamine-formaldehyde resin and melamine-urea-phenol-formaldehyde resin.