CA2440349C - Method of reducing the emission of formaldehyde from formaldehyde laden layered products - Google Patents

Abstract

A method of reducing the emission of formaldehyde from formaldehyde laden layered products having at least 2 layers, at least one of which being a boa rd or veneer, wherein prior to bonding the layers together, at least one side surface of said board or veneer is treated with a solution comprising an inorganic sulphur containing salt. The invention also relates to a board, a veneer and a flooring material obtainable by the method, to an aqueous solution comprising ammonium sulphite or bisulphite and urea for use in the method and to a process for preparation of the aqueous solution.

Description

METHOD OF REDUCING THE EMISSION OF FORMALDEHYDE FROM
FORMALDEHYDE LADEN LAYERED PRODUCTS

The present invention relates to a method of reducing the emission of formaldehyde from formaldehyde laden layered products having at least two layers, at least one of which being a board or veneer. It also relates to a board, a veneer and a flooring material obtainable by the method, to an aqueous solution comprising ammonium sulphite or bisulphite and urea for use in the method and to a process for preparation of the aqueous solution. The method, according to the present invention, comprises treatment prior to bonding the layers together of at least one of the surfaces of said board or veneer with a solution comprising an inorganic sulphur containing salt.
Aldehydes, especially formaldehyde based resins, are widely used in adhesive compositions, which are used in the manufacture of construction materials such as panelling, decking, etc.; home furnishings such as furniture, flooring material, etc.
Typically, these adhesives compositions contain a substantial molar excess of formaldehyde. Some of this excess formaldehyde is released upon curing of the resin during the manufacture of the product. However, it is well known that formaldehyde continues to be released from these products even after the manufacturing process is completed. Thus, formaldehyde in the indoor air has been a major concern for many years.
Several attempts have been made for many years for reducing formaldehyde emission, but all entail significant mechanical, chemical, environmental or economical disadvantages.
EP 0 027 583 discloses a method of reducing the emission of formaldehyde from particleboard bound with carbamide. resin. The boards are treated with a thermally decomposable ammonium compound, such as ammonium carbonate, subsequent to pressing. The thus treated boards are then stacked and stored at temperatures from 40 to 70 C, so that ammonia can be released.
Moreover, layered flooring material, such as three-layer parquet floor can emit formaldehyde due to the different wood layers being normally bonded together by gluing with formaldehyde based adhesives. The top layer usually consists of hardwood, the middle layer of softwood or a board, . for example, MDF, HDF or particle board and the bottom layer of a veneer. The top layer is usually treated with a formaldehyde free UV-lacquer or an oil to protect the surface, which also may function as a barrier to the formaldehyde emission. The different parts of a parquet sample, such as the front, the back, and the edges, give different emission rates. For example, the formaldehyde emission from the backside can be 20 times higher than that from the front side.

Furthermore, there is an increasing demand for parquet flooring which can meet the required El value obtained in a chamber test according to the Swedish standard SS
270236 (SS 1988). In cases where the emission test for parquet flooring has been performed with samples having unexposed backsides and sealed edges, the obtained emission values have been very low, often under 0.05 mg/ m3 (the limit value for El being 0.13 mg/ m).
Additionally, there is proposed a new European standard, EN 717-1, for formaldehyde emission determination, according to which all samples to be measured will have exposed backside and partially exposed edges. This makes it more difficult for the flooring producers to achieve extremely low formaldehyde emission values.
Furthermore, there will be demands on meeting the Japanese standard JAS
SIS20, which involves a desiccator measurement where the backside and the edges of the sample cannot be sealed, but they are totally exposed.
Thus, technical solutions are still sought for the need to decrease the emission of formaldehyde from wood-based products containing formafdehyde-based resins, meeting the new more strict standards for formaldehyde emission from these products.
Accordingly, the present invention provides a method of reducing the emission of formaldehyde from wood-based products containing formaldehyde based resins, by which the above mentioned problems are overcome.
The method according to the invention comprises treatment of at least one of the surfaces of a board or veneer, comprised in a layered product having at least two layers, prior to bonding the layers together, with a solution comprising an inorganic sulphur containing salt.
By "formaldehyde laden layered products" is herein meant a layered product containing formaldehyde based resins, wherein the layers are bonded to each other by a formaldehyde based adhesive and in case one or more of the layers is a board that the board as such may also be bonded with a formaldehyde based adhesive.
Suitably, the concentration of the inorganic sulphur containing salt in the solution is from about 1 to about 30 weight %, preferably from about 5 to about 20 weight %, and most preferably from about 8 to about 13 weight %. Although application of the salt in the form of a solution is preferred, according to the invention, it may also be applied in the form of a powder, if so desired. For environmental reasons, the solution is suitably an aqueous solution, although other solvents than water may be used. The salt solution may be applied by using any conventional coating technique, such as roller coating, curtain coating, or, spray coating. The amount of salt solution applied, when only one of the surfaces is treated, is suitably from about 30 to about 90 g/mZ, preferably from about 40 to about 80 g/m2, and most preferably from about 55 to about 65 g/m2. If both of the surfaces are to be treated, then the applied amount per side is suitably half of the amount if only one of the surfaces would be treated. In order to avoid disturbance of the moisture balance of the treated product large amounts of salt solution are undesirable.
The inorganic sulphur salts according to the invention have a very high water-solubility, whereas carbonate salts have a rather limited water-solubility.
Thus, in order to obtain an effective salt solution resulting in the desired reduction of formaldehyde emission, a larger amount of carbonate salt solution comparing to that of sulphite salt solution, is required to be applied onto the surface to be treated, which can affect negatively the moisture balance of the treated product and therefore even of the finished layered product.
Moreover, the inorganic sulphur salts according to the invention do not discolour the finished layered product, especially its outer surface, whereas carbonate salts may have a discolouring effect on the outer layer of the finished product and especially on oak surfaces.
Suitable inorganic sulphur containing salts include sulphite or bisulphite salts such as, for example, alkali metal sulphites or bisulphites or ammonium sulphites or bisulphites. Water-soluble sulphite or bisulphite salts are preferred.
Preferably, ammonium sulphite or bisulphite is used, and most preferably ammonium sulphite.
The method of reducing the amount of formaldehyde liberated by materials and products prepared with formaldehyde based resins is applicable to all formaldehyde based resins, for example, it is applicable to urea-formaldehyde, melamine-formaidehyde, phenol-formaldehyde resins and the like as well as copolymers, blends and mixtures made therefrom.
In a preferred embodiment of the invention, urea is added to the inorganic sulphur containing salt solution. Even though the inorganic sulphur containing salt does alone effectively reduce the emission of formaldehyde, the addition of urea to the salt solution improves further the reduction of formaldehyde emission. Suitably, the weight ratio of urea to sulphur-containing salt is from about 1:10 to about 1:1, preferably from about 2:10 to about 8:10, and most preferably from about 3:10 to about 6:10.
The board to be treated according to the invention may be any kind of board such as particle board, chip board, or, fibre board, and it may be bound with adhesives based on formaldehyde based resins or other non-formaldehyde based resins.
The veneer to be treated according to the invention may be any kind of wood veneer, preferably untreated veneer.
The boards or veneers may be conditioned after the application of the salt solution to desired moisture content before the bonding stage. Furthermore, although only one of the surfaces of the veneer or board need be coated with the salt solution to achieve the benefits of the invention, depending on the application method both of the surfaces may be coated if so desired.
The layers can be bonded by using any of the known conventional techniques, for example, by gluing and pressing.
Preferably, the board or veneer treated by the method according to the invention constitutes the backside surface or layer of the layered product. This gives rise to a significant reduction of the emission of formaldehyde from the backside of the layered product.
The layered product, according to the invention, may be any wood-based layered product. Suitably it is a flooring material, preferably a parquet flooring and most preferably a three-layer parquet flooring.
The aqueous salt solution, according to the present invention, comprises an ammonium sulphite or bisulphite and urea. Preferably, it comprises ammonium sulphite and urea.
The weight ratio between the urea and the salt is suitably from about 1:10 to about 1:1, preferably from about 2:10 to about 8:10, and most preferably from about 3:10 to about 6:10.
Suitably, the concentration of the ammonium salt and urea in the solution is from about 1 to about 30 weight %, preferably from about 5 to about 20 weight %, and most preferably from about 8 to about 13 weight %.
The process for preparation of the salt solution, according to the invention, comprises mixing of the salt, the urea and the water in a mixing ratio, which gives the desired concentration of salt and urea in the solution.
In a preferred embodiment of the process for preparation of the salt solution according to the invention, the ammonium salt is in the form of an aqueous solution before mixing with the urea and required additional water for receiving a solution with the desired salt concentration.
In a further preferred embodiment of the process for preparation of the salt solution, according to the invention, both the ammonium salt and the urea are in the form of aqueous solutions before mixing with each other for preparing an aqueous solution according to the invention with the desired salt concentration.
The invention is further illustrated by means of the following non-limiting examples. Parts and percentages relate to parts by weight respectively percent by weight, unless otherwise stated.

Examcles The ammonium sulphite solution used in the examples according to the present invention was prepared by mixing 19.7 kg of ammonium sulphite solution (35-36 weight %) and 3.0 kg urea with water to obtain a 10 weight % aqueous salt solution.

5 The ammonium carbonate solution used in the comparison examples was prepared by mixing 31.6 kg urea, 40.0 kg ammonium bicarbonate and 28.4 kg ammonium carbonate with water to obtain a 27.8 weight % aqueous salt solution.
Example 1(reference): Veneers of spruce (2 mm thick) were glued on both sides of the core material of pine with a thermosetting urea-formaidehyde resin and a hardener.
Example 2 (comparison): Veneers of spruce (2 mm thick) were pre-treated with an ammonium carbonate solution, on both sides and then left to dry for 4 hours before gluing and pressing in the same way as in example 1.
Example 3: Veneers of spruce (2 mm thick) were pre-treated with an inorganic sulphite solution, on both sides, left to dry and then glued and pressed in the same way as in example 2.
Example 4 (comparison): The procedure in example 1 was repeated, except that after gluing and pressing, the sample was left 15 minutes at ambient temperature and then the backside (the side facing away from the glue line) of the veneers was sprayed with an ammonium carbonate solution.
Example 5: The procedure in example 4 was repeated, except that the backside of the veneers was sprayed with an ammonium sulphite solution.
The formaldehyde emission from samples prepared according to the above examples was measured with a Field and Laboratory Emission Cell (FLEC) after 24 hours conditioning. The results for Examples 1 to 3 are shown in Table 1 below.
Table 1 Example Treatment of the Concentration of Applied amount Emission g/m h veneers the salt solution, salt solution, g/m2 after 24 hours % by weight per treated side 1 Without treatment 733 2 Pre-treated with 27,8 31 44 an ammonium carbonate solution 3 Pre-treated with 10 29 <10 an ammonium suiphite solution As can be seen from table 1, the formaldehyde emission from the samples containing treated veneers was generally significantly lowered compared to that of the samples containing untreated veneers. In addition, treatment of the veneers with a salt solution according to the invention gives rise to an even greater lowering of the formaldehyde emission comparing to that when the veneers are treated with an ammonium carbonate solution. This improvement becomes even more evident considering the fact that the concentration of the ammonium sulphite solution used was only about one third of that of the carbonate solution and that the applied amount (g/m2) of the ammonium sulphite solution was less than that of the applied ammonium carbonate solution.
Example 6: A spruce veneer for use in the manufacture of edge-glued 14 mm 3-layer parquet flooring was pre-treated on both sides with an ammonium sulphite solution.
The veneer was conditioned to about 7 % moisture content before gluing to the core material of pine, with a thermosetting urea-formaidehyde resin and a hardener.
The face lamella of ash was glued to the top of the core material with the same urea-formaidehyde resin/hardener mixture.
Example 7 (comparison): Example 6 was repeated except for that the spruce veneer was pre-treated with an ammonium carbonate solution.
Example 8 (reference): A parquet flooring sample was prepared in the same way as in examples 6 and 7, except for that the spruce veneer used was not treated with any salt solution.
Samples prepared according to all of the examples 6, 7 and 8 after being pressed and cooled to ambient temperature and the top layer was treated with a lacquer or oil, were wrapped in formaldehyde impermeable plastic. The formaldehyde emission was measured in a 1 m3 chamber according to the Swedish standard SS 270236 (SS
1988) with exposed front and backside and sealed edges. The results are shown in table 2 below.

Table 2 Example Treatment of the Concentration of Applied amount Emission, mg veneers the salt solution, salt solution, g/m2 formaldehyde/m3 % by weight per treated side air 6 Pre-treated with 10 29 0,02 an ammonium sulphite solution 7 Pre-treated with 27,8 31 0,02 an ammonium carbonate solution 8 Untreated 0,05 Evidently, pre-treatment of the veneers with an inorganic sulphite solution according to the invention unexpectedly reduces the emission of formaldehyde more effectively without need of using large amounts of a rather concentrated salt solution.
Use of large amounts of the salt solution results in that a larger amount of water is supplied onto the veneers, which can have a negative influence on the moisture balance of the product.

Claims (15)

CLAIMS:

1. A method of reducing the emission of formaldehyde from a formaldehyde laden layered product having at least 2 layers, at least one of which is a board or veneer, wherein prior to bonding the layers together, at least one surface of said board or veneer is treated with a solution comprising an inorganic sulphur containing salt.

2. A method according to claim 1, wherein said solution is an aqueous solution.

3. A method according to claim 1 or 2, wherein said solution has an inorganic sulphur salt content in the range of from about 5% to about 20% by weight.

4. A method according to any one of claims 1 to 3, wherein the inorganic sulphur salt is ammonium sulphite or ammonium bisulphite.

5. A method according to any one of claims 1 to 4, wherein the solution further comprises urea.

6. A method according to claim 5, wherein the weight ratio of urea to inorganic sulphur containing salt is from about 3:10 to about 6:10.

7. A method according to any one of claims 1 to 6, wherein the board or veneer treated by the method according to any one of claims 1 to 6 constitutes a backside surface or layer of the layered product.

8. A method according to any one of claims 1 to 7, wherein the layers are bonded together.

9. A method according to any one of claims 1 to 8, wherein the layered product is a flooring material.

10. A method according to claim 9, wherein said flooring material is a parquet flooring.

11. A flooring material board obtainable by the method according to claim 8.

12. A flooring material according to claim 11, which is a parquet flooring.

13. An aqueous solution useful in the method according to any one of claims 1 to 10, comprising:
a) ammonium sulphite or ammonium bisulphate; and b) urea.

14. An aqueous solution according to claim 13, comprising ammonium sulphite.

15. An aqueous solution according to claim 13 or 14, wherein the weight ratio of urea to sulphite or bisulphite is from about 3:10 to about 6:10.