DK153745B - PROCEDURE FOR JOINING WOOD - Google Patents

Description

DK 153745 B

The invention relates to a method of joining wood by applying formaldehyde-based curable condensation products and hardeners to the surfaces to be joined, and providing curing by high frequency heating.

5 Joining of wood using curable formaldehyde-based glue can be done in both cold and hot presses. Heat is often used to accelerate the curing of the glue, thereby shortening the pressing time.

The most common way to heat the glue is by transferring heat from hot press plates through the intermediate wood to the glue joint. This method requires relatively long pressing times and high pressing temperatures to provide curing of the adhesive in joints located relatively far from the heating surface.

A significantly faster heating of deep-seated glue joints in a structure can be achieved * if it is subjected to a high frequency alternating current (HF heating). The water molecules and other bipolar or charged atoms or atomic groups in the tree and especially in the glue joint are forced to oscillate with the high-freq. Quadrant current, thereby converting motion energy into heat which accelerates the curing of the glue.

Since HF heating requires relatively expensive appliances, and since the energy costs are high, it is desirable to shorten the heating ** and the pressing time, thereby saving energy and increasing the capacity of the machine.

Shortened heating time is also desirable from the point of view that the wood material becomes less strongly heated, which has a positive effect on its stability by reducing the drying of the wood. However, reduced heating time causes, 3.Q, a low temperature in the glue joint itself, which prolongs the final cure. So far, it has been tried to solve it- 2

DK 153745 B

tea problem using special high frequency hardeners containing electrolytes. These hardeners in combination with e.g. carbamide resin provides a glue with relatively low electrical conductivity and high dielectric losses in an electric high-frequency field. This causes a rapid heating and thus hardening of the glue.

A disadvantage of this adhesive is that the electrolyte addition in many cases shortens the useful life to approx. 1 hour at room temperature, while a working time of at least 3 hours is desirable with such an adhesive in which the components are mixed prior to application. Another and more serious disadvantage is that the glue, when compressed by the materials to be joined together, penetrates the joint surfaces, thereby easily causing the mixed salts to cause sparks and local fire which cause interruption and which can damage it. glued construction. In addition, local electrolyte concentrations in the adhesive joint can cause breakdowns between the electrodes, thus damaging the structure to be bonded.

To avoid this, the glue spread must in many cases be kept low and within narrow limits (60-75 g / m). If this limit is exceeded, fire is easily caused by glue penetration, and if the limit is exceeded, the bonding can be too poor. In many cases, however, it is desirable to have a glue spread of approx. 80-120 g / m. This amount of adhesive better manages to bridge the unevenness and thickness variations in the construction and therefore provides more secure bonding.

Said disadvantages according to the invention are eliminated in that the hardener containing an acid and having a reactivity such that the useful life of the condensation product and hardener mixture is less than 30 hours is applied to one surface to be joined and the formaldehyde based curable condensation product. containing carbamide resin and having a lower conductivity than the hardener, is applied separately to the other surface,

. DK 153745B

3

This will result in significant improvements. First, the separate application itself known per se gives a special and unexpected advantage in this context. The hardener contains more electrolytes and has greater conductivity than the resin.

By separate application, however, the hardener in the wood material is absorbed to a considerably higher degree than the glue. Therefore, the portion of the adhesive mixture which penetrates at the edges of the product during compression consists predominantly of the poorly conductive adhesive component and therefore causes no significant risk of coating.

Second, according to the invention, a rapidly reacting hardener is used to replace a high electrolyte concentration in the adhesive, thereby maintaining a high cure rate, even though the adhesive with admixed hardener 15 has significantly lower electrolyte content than in previous high frequency bonding methods. This reduced electrolyte concentration leads to a generally lower impact risk, so that not only does the risk of impact caused by extruded adhesive be reduced, but also the risk of other types of over-2Q impact.

Thirdly, an adhesive spread of over approx. 75 g / m, which means that a larger amount of adhesive can be applied than previously used in high frequency bonding. The amount of adhesive approaches that used by 25 I for other types of sizing, e.g. using conventional hot presses. The greater adhesive spread is made possible by the reduced risk of impact and leads to a better filling of the joint's bumps and therefore reduces the number of discards due to unsuccessful adhesions. so good that the pressing times and / or the pressing temperatures can be lowered and not only maintained unchanged in relation to bonding with the strongly conductive adhesives. The improvement

DK 153745 B

4 less load on the glued structures.

The hardener is applied in the form of a thin layer, e.g. by rolling or spraying suitably in an amount of 1-25 2 2 g / m, preferably 2-15 g / m.

For example, the hardener may e.g. be an organic or inorganic acid.

For example, carbamide resins include phosphoric acid, trichloroacetic acid, maleic acid, citric acid in combination with aluminum sulfate. For phenol resorcinol resins, formalin or paraformaldehyde-based hardeners are used.

1Q Separate application for utilization of fast curing adhesive system means, as mentioned, that lower temperatures can be used in the ffiEV heating to cure the adhesive. This results in an energy-saving effect and partly eliminates the risk of the wood material being transformed by vigorous drying out. A further reduced risk of overestimation is also obtained by the low temperature causing a smaller water vapor discharge, which partly implies a lower conductivity in the air closest to the glued structure and the electrodes and partly prevents the glue of the steam being driven out of the joint and forms 2Q electrically conductive. bridges.

Thus, the process allows one to operate at a temperature below 100 ° C, which is a great advantage for the aforementioned reasons, whereas with conventional HF bonding of the parquet one must operate at a temperature of about 110 ° C and pressurized. 25 set times of at least 70 sec. In contrast, in the process of the present invention, pressures of less than 60 sec can be obtained at a temperature of 80-85 ° C.

As formaldehyde-based glue may be mentioned condensation products of formalin, melamine, carbamide, phenol or mixtures of 3Q phenol and resorcinol. Carbamide resins are preferably used.

These may be without or with additives such as ammonium =

, DK 153745B

Sulfate, ammonium chloride, ammonium phosphate and buffers such as urea, tricalcium phosphate, hexamine, sodium acetate and filler, e.g. kaolin, wood flour, calcium sulfate and cereal types.

Example 1 5 To illustrate a known method for joining wood 2, a chipboard was placed on a chipboard. 130 g / m of a mixture between 100 parts by weight of a carbamide resin condensate, molar ratio formaldehyde: urea 1.83, resin content 67% and 15 parts by weight of high frequency hardener containing ammonium chloride, sodium chloride as electrolyte and pesticide. The electrical conductivity of the adhesive mixture was 600 micromho / cm. The useful life was approx. 1-1 1/2 hours at 20 ° C.

Beech wood stayer was placed on the glue-coated chipboard. The adhesive connection was pressed for 60 seconds, whereby a high frequency alternating current field was switched on.

Spark formation and fire occurred - unapproved bonding.

Example 2 2 On a chipboard was placed approx. 40 g / m of a 25% solution of a mixture containing organic acid and aluminum salt.

20 The coated plate was at room temperature for less than 60 minutes. and 2 were later applied with approx. 130 g / m of a carbamide resin precondensate having a molar ratio of formaldehyde: urea 1.83 and a resin content of approx. 67%. The electrical conductivity of the resin was 70 micromho / cm. The mixing time of the mixture was 25 50 min. at 20 ° C.

Beech wood sticks were later placed on the glue layer. The adhesive compound was pressed for 40 seconds, affected by HF alternating current of the same strength as in Example 1.

Claims (2)

A method of joining wood by applying formaldehyde-based curable condensation products and hardeners to the surfaces to be joined, and providing curing by high-frequency heating, characterized in that the hardener containing an acid and having such reactivity that the useful life of a mixture of condensation product and hardener is less than 1 hour, one surface is applied. which are to be joined together and that the formaldehyde-based curable condensation product containing carbamide hair pica1 and having lower conductivity than the hardener is applied separately to the second surface to be joined or on top of the hardener. Process according to claim 1, characterized in that the adhesive joint is heated to a maximum of 100 ° C.