PL233957B1 - Melamine-strengthened UF adhesives containing up to 0,9% of melamine for manufacturing fibreboard with average density - Google Patents

Description

Description of the invention

The present invention relates to a process for the production of melamine-urea-formaldehyde resins (MUF = melamine-urea-formaldehyde) by the three-stage reaction of formaldehyde, urea and melamine.

WO-A-2009/080798 discloses a continuous process for the production of melamine-urea-formaldehyde resins (MUF) with a melamine content of 0.1 to 15% by weight, in which a mixture of an amine compound (urea or melamine) is prepared in step a) ) with an aqueous formaldehyde solution, then the catalyst is added in step b) and condensation is carried out in step c) in a continuously operating reactor, e.g. a tubular reactor. In optional step d), a further amine compound can then be added, the same as in step a) or different. In order to produce MUF adhesives with a melamine content of up to 0.9% by weight in this process, the amine compound in step a) must be urea, and the amine component in step d) - melamine. adhesives cure slower. The negative effect of the later added melamine is known, for example, from G. E. Troughton, S. Chow, Holzforschung, 1975, pp. 214 to 217. In this way, MUF adhesives containing up to 0.9 wt. melamine in which the melamine would already be present in the condensation phase (step c).

From US-A-4 536 245, MUF resins with low formaldehyde emission are known which contain 0.15 to 40 wt. melamine and have a mole ratio of formaldehyde to urea from 1.3: 1 to 0.9: 1 However, at least 4% of melamine is required to significantly reduce the formaldehyde emission. The production of MUF resins is carried out in a process consisting of the first stage, in which urea and formaldehyde and possibly melamine are methylolated in an alkaline medium,

In the 2nd stage with condensation at a pH value of 6 to 8.3 with addition of urea and melamine and in the 3rd stage with addition of a scavenger, e.g. urea.

The disadvantage of this method is the amount of melamine to significantly reduce the formaldehyde emission.

The object of the present invention was therefore to eliminate the above-mentioned drawbacks.

The invention relates to a process for the production of melamine-urea-formaldehyde resins by the reaction where

a) reacting urea which contains from 0 to 10% by weight of one or more modified ureas from the group consisting of ethylene urea, ethylenediurea or dipropylene triurea, or guanamines such as benzoguanamine, or amides such as caprolactam, formaldehyde and melamine in the presence of bases at a pH value of 7.5 to 11, at a temperature of 20 to 120 ° C and a pressure of 0.1 to 10 bar,

b) the reaction is then carried out in the presence of an acid, which optionally contains urea and from 0 to 10 wt. one or more compounds A selected from classes of substances including modified ureas such as ethylene urea, ethylene diurea, or dipropylene triurea, or guanamines such as benzoguanamine, or amides such as caprolactam at a temperature of 60 to 180 ° C and a pressure of 0 1 to 10 bar and

c) then urea is added which contains from 0 to 10 wt. one or more compounds A selected from the class of ureas or guanamines or amides modified substances, characterized in that step b) is carried out at a pH of 4.1 to 5.5 and in that the melamine-urea-formaldehyde resins contain up to 0.9 wt.% melamine, based on the total weight of the resin.

A method for producing melamine-urea-formaldehyde resins is preferred, which is characterized in that the melamine-urea-formaldehyde resins according to the invention have a melamine content greater than or equal to 0.05% by weight. and less than / equal to 0.9 wt.%, based on the total weight of the resin. A method of producing melamine-urea-formaldehyde resins is preferred, which is characterized in that the process is carried out in reactors connected to one another.

Furthermore, a method for producing melamine-urea-formaldehyde resins is preferred, which is characterized in that the molar ratio of formaldehyde to urea in step a) is 1.5: 1 to 4: 1.

A process for the production of melamine-urea-formaldehyde resins is preferred, which is characterized in that the molar ratio of formaldehyde to melamine in step a) is from 3000: 1 to 50: 1.

A method for producing melamine-urea-formaldehyde resins is preferred, which is characterized in that the molar ratio of formaldehyde to NH2 groups [formaldehyde: (2xurea + 3xmelamine)] in step a) is from 0.8: 1 to 2: 1.

PL 233 957 B1

A process for the preparation of melamine-urea-formaldehyde resins is preferred, which is characterized in that the molar ratio of formaldehyde, optionally with the addition of one or more compounds A, to urea in step b) is 1.4: 1 to 4: 1.

Furthermore, a process for the production of melamine-urea-formaldehyde resins is preferred, which is characterized in that the molar ratio of formaldehyde to urea in step c) is 0.7: 1 to 1.5: 1.

A process for the production of melamine-urea-formaldehyde resins is preferred, which is characterized in that the acids are organic acids from the group consisting of formic acid, acetic acid or maleic acid, or inorganic acids from the group consisting of nitric acid or sulfuric acid, preferably organic acids. from the group consisting of formic acid, acetic acid or maleic acid, more preferably formic acid.

Furthermore, a process for the production of melamine-urea-formaldehyde resins is preferred, which is characterized in that alkali compounds, such as inorganic bases, are, for example, hydroxides of alkali metals or alkaline earth metals, such as sodium hydroxide, alkali metal or alkaline earth metal carbonates. or organic amines, for example tertiary amines such as tributylamine, triethylamine or tertiary alkanolamines such as triethanolamine, methyldiethanolamine or mixtures thereof, preferably aqueous alkali metal hydroxide solutions and triethanolamine, particularly preferably sodium hydroxide solution.

The invention can be implemented as follows:

In step a), a urea feed can be prepared that contains 0 to 10 wt. one or more compounds A (hereinafter referred to as "urea / mixture of ureas"), formaldehyde and melamine, or add them in any order and react at a pH of 7.5 to 11, preferably 8 to 10.5, particularly preferably 8 , 5 to 10. The pH can be adjusted by adding a base at one or more time points during this reaction. Typically the pH is adjusted at the start of the reaction and is optionally corrected by adding a base. Urea / urea mixture and formaldehyde can also be used as formaldehyde-urea precondensate / urea mixture.

In the process, in step a), a formaldehyde feed with a base can be prepared, and then the urea / urea mixture is added.

Moreover, in step a), a mixture of formaldehyde and base can be added to the prepared urea feed / urea mixture.

In both variants, in step a), the melamine can be introduced into the reaction mixture at any time.

The reaction in step a) is carried out usually at a temperature of 20 to 120 ° C, preferably 20 to 110 ° C, particularly preferably 20 to 95 ° C, especially 60 to 90 ° C, and a pressure of 0.1 to 10 bar, preferably 0 5 to 5 bar, particularly preferably 0.9 to 1.5 bar, in particular at normal pressure (atmospheric pressure). Conveniently, the formaldehyde, base or mixture of formaldehyde and base is heated to the above-mentioned temperature already before combining with the urea / urea mixture and melamine.

Then, in step b), the further reaction of the reaction mixture can be carried out in the presence of an acid at a pH value of 4 to 5, 9, preferably 4.1 to 5.5, particularly preferably 4.2 to 5.3, especially 4.5 to 5. , 2 and at a temperature of 60 to 180 ° C, preferably 90 to 180 ° C, particularly preferably 100 to 150 ° C, especially 110 to 140 ° C and a pressure of 0.1 to 10 bar, preferably 0.5 to 5 bar particularly preferably 0.9 to 1.5 bar, especially at normal pressure (atmospheric pressure). In this connection, the viscosity of the reaction mixture, which is usually from 10 to 5000 Pas, preferably from 100 to 2000 mPas, particularly preferably from 200 to 800 mPas, measured with an Anton Paar MCR 51 viscometer with a plate-cone measuring system at room temperature (Thomas Mezger, Das Rheologie-Handbuch, Hannover, Vincentz, 2000, p. 203 and DIN EN ISO 3219: 1994-10), it is possible to correct the reaction time, temperature and pressure.

Optionally, one or more portions of urea / urea mixture can be added at any point in time during step b), usually the amount of urea / urea mixture added during step b) is selected so that, based on the total amount of urea / urea mixture, is 0 to 20% by weight, preferably 0 to 15% by weight, particularly preferably 0 to 10% by weight.

Subsequently, a further conversion of the reaction mixture can be carried out after the addition of the base in step c) at a pH of 6.5 to 11, preferably 7 to 10, particularly preferably 7.5 to 9.5,

Especially 8 to 9, by adding urea / urea mixture at a temperature of 30 to 120 ° C, preferably 40 to 100 ° C, particularly preferably 45 to 95 ° C, especially 50 to 90 ° C and a pressure of 0 1 to 10 bar, preferably 0.5 to 5 bar, particularly preferably 0.9 to 1.5 bar, especially at normal pressure (atmospheric pressure). Typically, it is possible to optionally partially remove the diluent, such as water, in vacuo, and then add the urea / urea mixture again under the above-mentioned temperature and pressure conditions.

The addition can be carried out in such a way that the urea and optionally compound A are either mixed in solid form as a solution with the reaction mixture, or the resin is added, while stirring, to the urea solution, which optionally contains compound A. The two components can be mixed at a temperature of room temperature, or by mixing with urea a resin, the temperature of which is still up to 80 ° C. The mixture according to the invention may then be cooled to room temperature. Preferably, the pH of the cooled mixture is adjusted to a value of from 8 to 10.

The process is typically carried out such that the solids content of the MUF resin is 50 to 70 wt.%. with respect to an aqueous resin mixture. However, it is also possible to increase the solids content to a value of 60 to 80% by weight by distilling off the water at a temperature of 30 to 60 ° C under reduced pressure.

The method can also be carried out in reactors connected to one another, e.g. in a cascade of mixers. Here, for example, step a) may be carried out in the first reactor, step b) in the second and step c) in the third reactor.

The molar ratio of formaldehyde to the total amount of urea and optionally compound A in step a) is usually 1.5: 1 to 4: 1, preferably 2: 1 to 3: 1, more preferably 2: 1 to 2.6: 1.

The molar ratio of formaldehyde to melamine in step a) is usually 3000: 1 to 50: 1, preferably 2500: 1 to 100: 1, more preferably 2250: 1 to 150: 1.

The molar ratio of formaldehyde to NH2 groups [F: (2xU + 3xM)] in step a) is usually 0.8: 1 to 2: 1, preferably 1: 1 to 1.5: 1, particularly preferably 1: 1 to 1, 3: 1. The relationship A corresponds to U.

The entire amount of the optionally added urea / urea mixture added in step b) can be added in one, two or more portions. Typically, the amount of urea / urea mixture is selected so that the molar ratio of formaldehyde to the total amount of urea and compound A is 1.4: 1 to 4: 1, preferably 1.8: 1 to 3: 1, particularly preferably 1. 8: 1 to 2.6: 1.

The entire amount of urea / urea mixture added in step c) can be added in one, two or more portions. Typically, the amount of urea / urea mixture is chosen so that the molar ratio of formaldehyde to the total amount of urea and compound A is 0.7: 1 to 1.5: 1, preferably 0.75: 1 to 1.2: 1 particularly preferably 0.8: 1 to 1.05: 1, very particularly preferably 0.8: 1 to 0.89: 1.

Paraformaldehyde or 5 to 70 wt.% Is suitable as formaldehyde. aqueous formaldehyde solutions, for example preferably 30 to 60% by weight. aqueous solutions, particularly preferably 45 to 55 wt. aqueous solutions.

The urea is used either in solid form or as an aqueous solution, preferably as an aqueous solution, the urea concentration in the solution being 30 to 85%, preferably 40 to 80%, particularly preferably 50 to 75%.

Formaldehyde (F) and urea (U) can also be used at least partially in the form of aqueous formaldehyde-urea solutions and / or aqueous formaldehyde-urea precondensate solutions. The pH of the aqueous formaldehyde solution, the aqueous formaldehyde-urea solution and / or the aqueous formaldehyde-urea precondensate solution is preferably 4 to 5.5, in particular 4.5 to 5.

The concentration of the aqueous solution of formaldehyde and urea is preferably 50 to 80% by weight, and the weight ratio of urea to formaldehyde is preferably in the range from 10:90 to 70:30, in particular from 20:80 to 55:45, particularly preferably from 30 : 70 to 50:50. This solution may contain minor amounts of urea-formaldehyde addition products and low molecular weight condensation products.

Furthermore, an aqueous solution of formaldehyde-urea precondensate which is produced by reacting urea and formaldehyde at a pH above 7 can be used. The weight ratio of urea to formaldehyde is preferably in the range from 10:90 to 70:30, preferably from 20:80 to 55:45, especially from 30:70 to 50:50. The concentration of the aqueous formaldehyde-urea precondensate solution is preferably from 50 to 80%.

PL 233 957 B1

Urea, whether it is used as solid urea, as urea solution, as formaldehyde-urea solution or as formaldehyde-urea precondensate solution, can be partially replaced by compounds A that can react with formaldehyde. Suitable compounds A are modified ureas such as ethylene urea, ethylenediurea or dipropylene triurea or guanamines such as benzoguanamine, or amides such as caprolactam in amounts of from 0 to 10% by weight, preferably 0 to 8% by weight, particularly preferably 0 to 5 wt.%, Based on the total amount of urea. This means 100 to 90 wt.%. % urea and 0 to 10 wt. % of compound A, preferably 100 to 92 wt. % urea and 0 to 8 wt. % of compound A, particularly preferably 100 to 95 wt. % urea and 0 to 5 wt. % of compound A, i.e. pure urea (100% by weight) or a mixture of urea and compound A (> 100 to 90% by weight of urea and <0 to 10% by weight of compound A) for a total of 100% by weight.

Suitable melamine is powdered melamine or liquid or powdered melamine-formaldehyde resins (MF resins) or melamine-urea-formaldehyde resins (MUF resins), preferably MF and MUF resins, particularly preferably powdered MF and MUF resins, particularly preferably powdered MF resins. If MF or MUF resins are used as the source of melamine, the amounts of formaldehyde and urea they contain are taken into account when calculating the molar ratios.

Melamine formaldehyde resin powder is typically prepared by spray drying aqueous solutions of melamine-formaldehyde condensation products.

Suitable diluents are alcohols, such as C1-C4-alkanols, for example methanol, ethanol, n-propanol, isopropanol, mixtures of propanol isomers, n-butanol, isobutanol, sec-butanol, tert-butanol, mixtures of butanol isomers, water and mixtures thereof %, preferably water or 1 to 99 wt. aqueous alcohol solutions which are C1-C4-alkanols, particularly preferably water.

Suitable bases are all commonly used alkaline compounds, such as inorganic bases, for example alkali metal or alkaline earth metal hydroxides, such as sodium hydroxide, alkali metal or alkaline earth metal carbonates, or organic amines, for example tertiary amines, such as tributylamine, triethylamine, or tertiary alkanolamines such as triethanolamine, methyldiethanolamine or mixtures thereof, preferably aqueous alkali metal hydroxide solutions and triethanolamine, particularly preferably sodium hydroxide solution.

Suitable acids are organic acids, such as formic acid, acetic acid, maleic acid, or inorganic acids, such as nitric acid, sulfuric acid, preferably organic acids, particularly preferably formic acid.

Typically, the melamine content of the melamine-urea-formaldehyde resins prepared by the process of the invention is> (greater than / equal to) 0.05 wt.%. and <(less than / equal) 0.9 wt.%, based on the total weight of the resin, i.e. 0.05 to 0.9 wt.%, preferably 0.1 to 0.7 wt.%, particularly preferably 0, 2 to 0.5 wt.%

Before use, the melamine-urea-formaldehyde resins according to the invention may optionally be mixed with urea-formaldehyde condensation products having a formaldehyde to urea weight ratio of 2: 1 to 0.85: 1 and / or with solid or solid urea. as an aqueous solution. The solids content of the urea formaldehyde condensation product is usually 50 to 80%. The solids content can be determined by weighing a liquid resin (e.g. approx. 1 g) in a shallow sheet pan, then drying for two hours at 120 ° C and re-weighing (M. Dunky, P. Niemz, Holzwerkstoffe und Leime, Springer , Berlin, 2002, p. 458). Typically, the mixing with urea formaldehyde condensation products preferably takes place in a weight ratio of the melamine-urea-formaldehyde resin prepared by the process of the invention to urea-formaldehyde condensation products of 99: 1 to 10:90, in particular 95: 5 to 50:50. Typically, mixing with urea takes place in a ratio of the melamine-urea-formaldehyde resin produced by the process according to the invention to urea or urea solution from 99: 1 to 70:30, in particular 98: 2 to 80:20.

Other additives can be incorporated into these resins in an amount of up to 10 wt.%. This includes, for example, alcohols such as ethylene glycol, diethylene glycol or saccharides. Water-soluble polymers based on acrylamide, ethylene oxide, N-vinylpyrrolidone, vinyl acetate, as well as copolymers with these monomers can also be used. Fillers, such as, for example, cellulose fibers or mixtures thereof, can be added to the resins. In addition, they may contain carbonates or mixtures thereof.

PL 233 957 B1

In order to increase the water-dilutability of the melamine-urea-formaldehyde resins, sulfites, metabisulfites and bisulfites can be used which preferably contain alkali metal cations such as lithium, sodium or potassium, preferably sodium or potassium, particularly preferably sodium or ammonium, as cations. They can be used in an amount of 0.01 to 10% by weight, preferably 0.05 to 1% by weight, based on the weight of the liquid resin.

The resins according to the invention are usually stable under storage conditions at 20 ° C for several weeks.

The resins according to the invention are suitable binders, in particular for the production of lignocellulose-containing moldings, such as, for example, particle boards, fibreboards or oriented strand boards. In addition, the mixtures according to the invention are suitable for sizing wood surfaces, such as in the production of plywood, single and multi-layer boards, and glued laminated timber. The resins according to the invention are suitable in particular for the production of MDF (Medium Density Fiberboard), in particular when the application of the adhesive is carried out by the Blowline process. In the Blowline process, after the wood is defibrated in a cone mill, resin is injected into the high-speed stream of fibers through a nozzle. The sized fibers are then dried (M. Dunky, P. Niemz, Holzwerkstoffe und Leime, Springer, Berlin, 2002, p. 145).

The reactivity of the binder mixtures during curing can be increased by additionally adding a curing agent such as, for example, ammonium salts such as ammonium chloride, ammonium sulfate, ammonium nitrate, ammonium phosphate or carboxylic acids such as formic acid and oxalic acid, directly before processing, or Lewis acids such as aluminum chloride, or acid salts such as aluminum sulfate, or mineral acids such as sulfuric acid or mixtures thereof. Curing agents can be mixed with an aqueous binder solution ("sticky bath") and then sprayed onto the chips or fibers, or the curing agent can be applied to the substrate separately from the binder.

Lignocellulose-containing moldings, e.g. chipboards, OSB boards or medium-density fibreboards (MDF), can, for example, be produced by pressing under pressure at a pressing temperature of 120 to 250 ° C, from 5 to 30% by weight. solid resin with respect to a material containing lignocellulose. Additionally, the curing agents described above may be used. Under these conditions, the aminoplastic resin usually hardens quickly and wood materials with good mechanical properties are obtained.

In addition to the simple production method, the advantage of the melamine-urea-formaldehyde resins produced by the process according to the invention is that, compared to traditional resins with a comparable composition, an improvement in processing properties is obtained, in particular the resins produced by the method according to the invention are distinguished by a low swelling index and high strength with respect to low formaldehyde emission. Moreover, a low melamine content is advantageous.

Examples

P r z k ł a d 1

Adhesive with 0.33% melamine content and F: U ratio = 1.01 (melamine component = powdered MF resin)

1138 g (18.6 mol) of 49% aqueous formaldehyde solution was heated to 87 ° C and the pH was adjusted to 7.7 with 10% sodium hydroxide solution, 495 g (8.24 mol) of urea was added in 30 min. 11.5 g of powdered melamine resin (prepared from a solution of formaldehyde and melamine by alkaline condensation followed by spray drying, containing 60% by weight of melamine and 40% by weight of formaldehyde) were added and mixed for 15 min at 87 ° C at a pH value from 7.0 to 6.8. After heating to 92 ° C, the pH was adjusted to 5.0 with formic acid and condensation was carried out at 95 ° C until a viscosity of 550 mPas was reached (measuring the viscosity of a withdrawn sample at room temperature), then the pH was adjusted to 8.5 with 10% sodium hydroxide solution, cooled to 85 ° C, and 302 g (5.02 mol) of urea were added in 18 minutes. Then 230 g of water were distilled off under vacuum at 60 ° C and a further 309 g (5.14 mol) of urea was added in 18 minutes at 50 ° C. 2031 glue was obtained with the following physical properties:

- Dry substance content 66.9% (measured by weight difference after 2 h at 120 ° C in a drying oven)

- Viscosity 561 mPas (at a shear rate of 250 s ^-1 ),

- pH 8.2.

PL 233 957 B1

P r z k ł a d 2

Adhesive with a melamine content of 0.33% and a ratio of F: U = 1.01 (melamine component = solid melamine) 1148 g (18.8 mol) of a 49% aqueous formaldehyde solution was heated to 87 ° C and with 10% sodium hydroxide solution the pH was adjusted to 7.7, 495 g (8.24 mol) of urea was added in 30 min, then 6.7 g (0.053 mol) of melamine was added and stirred for 15 min. After heating to 92 ° C, the pH was adjusted to 5.0 with formic acid and condensation was carried out at 95 ° C until the viscosity was 550 mPas (viscosity measurement as in example 1), then the pH was adjusted to 8.5 with 10 % sodium hydroxide solution, cooled to 85 ° C, and 302 g (5.02 mol) of urea was added over 18 minutes. Then 230 g of water were distilled off under vacuum at 60 ° C and a further 309 g (5.14 mol) of urea was added in 18 minutes at 50 ° C. 2031 g of adhesive with the following physical properties was obtained:

- Dry substance content 67% (measured on the basis of the weight difference after 2 h at 120 ° C in a drying oven)

- Viscosity 532 mPas (at a shear rate of 250 s ^-1 ),

- pH 8.2.

The melamine content of the resin is 0.33 wt%, the F: U molar ratio is 1.01

Comparative example 1 (according to US-A-4 536 245, example n ° 1)

Adhesive with a melamine content of 3.9% and a ratio of F: U = 1.01

Prepared according to example 1 of US-A-4,536,245

Comparative example 2

A melamine-free adhesive with a ratio F: U = 1.01

1148 g (18.8 mol) of 49% aqueous formaldehyde solution was heated to 87 ° C and the pH was adjusted to 7.7 with 10% sodium hydroxide solution, 495 g (8.24 mol) of urea was added in 30 min and the mixture was stirred. for 15 min. After heating to 92 ° C, the pH was adjusted to 4.8 with formic acid and condensation was carried out at 95 ° C until the viscosity was 550 mPas (viscosity measurement as in example 1), then the pH was adjusted to 8.5 with 10 % sodium hydroxide solution, cooled to 85 ° C, and 302 g (5.02 mol) of urea was added over 18 minutes. Then 235 g of water was distilled off under vacuum at 60 ° C and a further 309 g (5.14 mol) of urea was added in 18 minutes at 50 ° C. 2019 g of resin with the following physical properties was obtained:

- Dry substance content 66.9% (measured by weight difference after 2 h at 120 ° C in a drying oven)

- Viscosity 528 mPas (at a shear rate of 250 s ^-1 ),

- pH 8.2.

Production of chipboard in the laboratory

a) Mixing of the starting materials

500 g of spruce chips were placed in the mixer. Thereafter, an adhesive bath of 100 parts and 4 parts of a 52% aqueous ammonium nitrate solution and 10 parts of water was applied. The amount of adhesive bath was chosen so that the degree of sizing was 10%. The degree of gluing is the quotient of the dry mass of glue and the dry mass of wood.

b) Pressing the stuck chips

The stuck shavings were pre-cold pressed in a 30x30 cm mold. Subsequently, they were pressed in a heated press (pressing temperature 200 ° C, pressing time 200 s). The thickness of the plates was 15.7 mm in each case.

Examination of wood materials

Density

Density determination was carried out according to EN 1058 24 hours after production. Transverse tensile strength

The determination of the transverse tensile strength was carried out according to EN 319. Degree of swelling

The degree of swelling was determined according to EN 317 after storage in water for 24 hours.

Formaldehyde emission (perforator method)

The determination of the formaldehyde emission was carried out according to EN 120.

PL 233 957 Β1

Melamine content [%] F: U molar ratio Density [kg / m ³ ] Transverse tensile strength [N / mm ² ] Swelling rate after 24 h [%] Perforator value [mg F / 100 g] Example 1 0.33 1.01 585 0.60 twenty 5.5 Example 2 0.33 1.01 582 0.61 19 5.3 Comparative example 1 3.9 1.01 588 0.49 twenty 5.5 Comparative example 2 0 1.01 584 0.55 25 6.1

Patent claims

Claims (10)

1. Method for the production of melamine-urea-formaldehyde resins by reaction, where a) reacting urea which contains from 0 to 10% by weight of one or more modified ureas from the group consisting of ethylene urea, ethylenediurea or dipropylene triurea, or guanamines such as benzoguanamine, or amides such as caprolactam, formaldehyde and melamine in the presence of bases at a pH value of 7.5 to 11, at a temperature of 20 to 120 ° C and a pressure of 0.1 to 10 bar, b) the reaction is then carried out in the presence of an acid, which optionally contains urea and from 0 to 10 wt. one or more compounds A selected from classes of substances including modified ureas such as ethylene urea, ethylene diurea, or dipropylene triurea, or guanamines such as benzoguanamine, or amides such as caprolactam at a temperature of 60 to 180 ° C and a pressure of 0 1 to 10 bar and c) then urea is added which contains from 0 to 10 wt. one or more compounds A selected from the class of urea modified substances or guanamines or amides, characterized in that step b) is carried out at a pH value of 4.1 to 5.5 and in that the melamine-urea-formaldehyde resins contain up to 0 .9 wt.% melamine, based on the total weight of the resin. Process for the production of melamine-urea-formaldehyde resins according to claim 1, characterized in that the melamine-urea-formaldehyde resins according to the invention have a melamine content greater than or equal to 0.05% by weight. and less than / equal to 0.9 wt.%, based on the total weight of the resin. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 2, characterized in that the process is carried out in reactors connected to one another. 4. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 3, characterized in that the molar ratio of formaldehyde to urea in step a) is 1.5: 1 to 4: 1. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 4, characterized in that the molar ratio of formaldehyde to melamine in step a) is from 3000: 1 to 50: 1. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 5, characterized in that the molar ratio of formaldehyde to NH2 groups [formaldehyde: (2 × urea + 3 × melamine)] in step a) is 0.8: 1 to 2 : 1. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 6, characterized in that the molar ratio of formaldehyde, optionally with the addition of one or more compounds A, to urea in step b) is 1.4: 1 to 4 : 1. PL 233 957 B1 Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 7, characterized in that the molar ratio of formaldehyde to urea in step c) is 0.7: 1 to 1.5: 1. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 8, characterized in that the acids are organic acids from the group consisting of formic acid, acetic acid or maleic acid, or inorganic acids from the group consisting of nitric acid or sulfuric acid, preferably organic acids from the group consisting of formic acid, acetic acid or maleic acid, more preferably formic acid. Process for the production of melamine-urea-formaldehyde resins according to one of claims 1 to 9, characterized in that alkali compounds, such as inorganic bases, are, for example, alkali metal or alkaline earth metal hydroxides, such as sodium hydroxide, alkali metal carbonates or alkaline earth metals or organic amines, for example tertiary amines such as tributylamine, triethylamine, or tertiary alkanolamines such as triethanolamine, methyldiethanolamine or mixtures thereof, preferably aqueous alkali metal hydroxides and triethanolamine, particularly preferably sodium hydroxide solution.