The present invention relates to a method for producing a wood based product and a method for reducing aldehyde emissions from a wood based product. It also relates to an aqueous composition for use in the methods and a wood based product.

Background

Emission of aldehydes, especially formaldehyde, from products such as flooring materials, furniture materials and building materials is a growing concern. Mostly, the emission of aldehydes originates from a glue line between two layers in a layered product, or, from a material such as fibre board where a formaldehyde emitting binder has been used in its production.

When applying an aldehyde scavenger onto a surface of a wood based product the following problems may arise: the aldehyde scavenger is frequently present dissolved in water leading to wetting which is followed by drying. This may cause problems such as warping due to the change of moisture content in the wooden material. There may also be a problem with remaining solid aldehyde scavenger which many times precipitate out after the aqueous medium has dried off leaving particles as a powder on the surface. The solid particles may then fall off during handling and may also be aesthetically disturbing. The object of the present invention is to minimise and/or solve the problems described above.

Also, a further object of the present invention is to provide a method which shows a long-lasting effect in aldehyde emission reduction.

JP 2001-149456 discloses an aldehyde deodorant composition. JP 10-237257 discloses a deodorant aqueous emulsion. JP 10-252247 discloses a building board where a formaldehyde catching agent is applied to its rear surface. JP 2002-273145 discloses a method of treating a wooden material with a composition comprising hydrazine.

The inventjon The object of the present invention is reached by a method for producing a wood based product comprising applying to at least one surface of a wood based product an aqueous composition comprising a film former and a compound (a) reactive to an aldehyde, wherein the compound (a) is to at least some extent present as solid matter in the composition. The object of the present invention is also reached by a method for reducing emission of an aldehyde from a wood based product comprising applying to at least one surface of a wood based product an aqueous composition comprising a film former and a

compound (a) reactive to the aldehyde, wherein the compound (a) is to at least some extent present as solid matter in the composition

By "to at least some extent" is herein meant at least 1 weight % of total weight of compound (a), suitably from about 1 to about 100 weight %, preferably from about 5 to about 95 weight %, more preferably from about 10 to about 90 weight %, even more preferably from about 25 to about 90 weight %, most preferably from about 50 to about

90 weight %.

By "wood based" is herein meant that the wood based product can be made of any material derived from wood such as solid wood, veneer, fibreboard, particleboard, and chipboard. The wood based product is preferably a layered product comprising at least two wood based layers glued together. The wood based product is suitably a flooring material, a furniture material, a construction element for walls or roofs, or plywood. In the case where there is a front- and backside of the product, for example a flooring material, preferably, the surface treated is the backside of the layered product. The flooring material is suitably parquet flooring or veneered flooring.

In the case the wood based product is a layered product, the layers are preferably glued together with an adhesive which is preferably a urea-formaldehyde-, phenol-formaldehyde-, melamine-formaldehyde- or melamine-urea-formaldehyde resin based adhesive. The amount composition applied to the surface can be any amount suitable to achieve a desired effect, suitably from about 1 to about 300 g/m ² , preferably from about 5 to about 100 g/m ² , most preferably from about 10 to about 60 g/m ² .

The amount component (a) applied to the surface can be any amount suitable to achieve a desired effect, suitably from about 0.1 to about 50 g/m ² , preferably from about 1 to about 25 g/m ² , most preferably from about 3 to about 15 g/m ² .

The temperature at application of the composition is suitably from about 0 to about 60 ⁰ C, preferably from about 15 to about 40 ⁰ C.

The present invention further relates to an aqueous composition comprising a film former and a compound (a) reactive to an aldehyde, wherein the compound (a) is to at least some extent present as solid matter in the composition.

Suitably, the solid matter is particles. The particles are suitably evenly distributed in the composition.

Various compounds can be used as compound (a) in the present invention.

Examples of such compounds are: compounds comprising an -NH- structural unit such as amines, amides such as urea, imides, derivatives of hydrazine such as hydrazides, hydrazidines, and azoles. Examples of other compounds are azines, hydrazones, sulphites and carbonates. Suitably, compound (a) is a hydrazide, preferably, adipic acid

dihydrazide. The hydrazide content in the composition is suitably from about 5 to about

60 weight %, preferably from about 10 to about 50 weight %, more preferably from about 15 to about 45 weight %, most preferably from about 20 to about 40 weight %.

The dry content film former in the composition is suitably from about 10 to about 75 weight %, preferably from about 15 to about 55 weight %, most preferably from about 20 to about 35 weight %.

The film former is suitably a polymer, preferably a polyvinyl ester, a polyacrylate, or a polyvinyl alcohol, most preferably a homo- or copolymer of vinylacetate such as polyvinylacetate. Suitably, the film former is present in the composition as a dispersion or solution.

If the film former is present in the composition as a dispersion, the composition preferably comprises an agent lowering the minimum film forming temperature of the film former. One preferred such agent is butyl diglycol acetate, which is preferably added in an amount of from about 0.5 to about 2 weight %. The aldehyde is suitably formaldehyde.

As a measurement of long-term effect, the emission of aldehyde from a wood based product treated according to the present invention suitably increases less than about 100 %, preferably less than about 50%, more preferably less than about 10%, most preferably less than about 5% when comparing an emission value 12 months after treatment with an emission value directly after treatment.

In one embodiment of the invention, an aqueous composition comprising from about 40 to about 60 weight % of a polyvinyl acetate dispersion, having a dry content of from about 40 to about 65 weight %, from about 15 to about 30 weight % of adipic acid dihydrazide, and from about 0.5 to about 2 weight % of butyl diglycol acetate, the rest being water, is applied to a veneered core which is a flooring material. The applied amount is from about 15 to about 60 g/m ² .

The present invention also relates to a wood based product obtainable by the method of producing a wood based product and method for reducing emission of an aldehyde herein described. Preferably, the wood based product is a flooring material, a furniture material, a construction element for walls or roofs, or plywood.

The present invention will now be described in the following examples, which, however, should not be interpreted as limiting the scope of the invention.

Examples

Example 1 : A veneered flooring material was made having a core of HDF board with 0.6 mm oak veneer glued on both sides with a UF adhesive. The upper

surface intended to be visible of the flooring material was lacquered with a UV lacquer, thereby forming a wear resistant layer which also acts as a barrier for emission. The backside surface was applied with an aqueous composition comprising 56.5 weight % of PvAc dispersion, the dispersion itself having a dry content of 50 weight %, 25 weight % adipic acid dihydrazide, 1 weight % butyl diglycol acetate and 17.5 weight % additional water added to adjust the viscosity before application. The solubility of adipic acid dihydrazide in the composition is about 4.5 weight %. An untreated reference was measured (Ref. 1). A reference sample was also made by treating a backing surface with pure PvAc dispersion (Ref. 2). The amount spread of the compositions was 25 g/m ² .

Pieces (15x15 mm) were cut out and the emission of formaldehyde was measured according to JIS A 1460 (2001).

A treated sample was stored for 12 months and thereafter the emission was re- measured to determine the long-term effect.

Table 1.

It is concluded from the results that the present invention leads to significant reduction of emission and good surface appearance. Also, the long-term effect of the treatment is very good.