CH620461A5 - Process for the preparation of pulverulent melamine-formaldehyde condensates - Google Patents

Description

The invention relates to a method for producing powdered melamine-formaldehyde condensates.

In addition to solutions, powdered condensates are also used for the most diverse areas of application of melamine-formaldehyde condensates. They have the advantage over the solutions that they can be stored for much longer and that the transport of water is saved when shipping.

Powdered melamine-formaldehyde condensates can be prepared by drying the aqueous condensate solutions obtained in the condensation of melamine and formaldehyde. In practice, the aqueous solution is usually converted into the powder form by spray drying, with the aqueous condensate solutions being sprayed into a warm air stream in a fine distribution by means of compressed air or a rapidly rotating disk, in which the water or solvent is evaporated. The performance of the drying systems depends to a large extent on how quickly the water can be removed from the aqueous condensate solutions to be dried without the already dried powder particles sintering at the drying temperatures in the water vapor atmosphere. Condensates that sinter slightly lead to heavy deposits in the drying systems. These deposits, to which the melamine-formaldehyde condensates are particularly prone, become insoluble and therefore unusable if the drying units are left for a longer period of time. In addition, the deposits in the separators of the drying plants, the cyclones, lead to blockages, so that the plants often have to be shut down.

A targeted further condensation of the melamine-formaldehyde condensates can partially eliminate the sintering or deposition during the spray drying of the condensation products. As a result of the further condensation, however, the viscosity of the solutions prepared from the powder products increases to such an extent that they are no longer suitable, in particular, for the purposes of soaking paper. If the products are further condensed, the water compatibility (water dilutability) decreases significantly, so that the use of such resins is no longer possible for purposes in which dilute solutions are used.

It has been found that powders made from at least limited water-soluble melamine-formaldehyde condensates, which have little or no tendency to deposit or sinter in the drying plants during spray drying, by condensing melamine and formaldehyde in aqueous solution and spray drying the condensate solution can be obtained if the condensation in the presence of 0.01

Mol to 0.1 mol of an alkali sulfite, bisulfite, dithionite or - (formaldehyde) sulfoxylate.

Due to the improved sintering resistance of the sulfite-modified, powdered melamine-formaldehyde resins at the drying temperatures, significantly higher amounts of resin solutions can be implemented in the drying systems.

In itself is - e.g. from German Patent No. 883 652 - the modification of melamine resins and also other aminoplast resins, e.g. using sodium bisulfite,

already known or was considered in these descriptions and e.g. in the case of urea resins, modification with sodium bisulfite is also customary for special resins; however, a relatively high amount of sulfite is condensed in, which also influences the water solubility of the condensed (thermoset-hardened) resins.

The special areas of application of melamine resin powders, especially for the production of surface coatings or laminates, exclude the deliberate increase in water sensitivity. These Harzé, used as impregnating resins and as glue resins for boil-proof gluing, are said to be largely waterproof.

They generally have a molar ratio of melamine to formaldehyde between 1: 1.3 and 1: 6 and are e.g. condensed at pH 7.5 to 10 to a water compatibility of 1:20 to 1: 2 with a solids content of the aqueous solutions between 40 and 75%. It is surprising that the addition of sulfite, like the other suitable modifiers, does not reduce the water resistance of the cured resins in the small amount used, but does practically remedy the difficulties mentioned at the outset.

The following comparison shows the improved drying ability of the melamine-formaldehyde resin solutions modified with the alkali sulfites:

A resin solution made from 1 mole of melamine and 3 moles of formaldehyde, which was condensed at a temperature of 95 ° C and a pH of 8 to 9 until 1 part of the resin solution was diluted with 5 parts of boiling hot water and cooled to 42 ° C beginning turbidity was processed in a drying apparatus customary for aminoplast resins by means of warm air at 150 ° C on powder. The amount of resin solution that could be passed through the drying system per minute was 23 liters. When larger quantities of resin solution than 23 liters were passed through, there were heavy deposits, particularly in the separating organs of the apparatus, which led to blockages and considerable malfunctions.

In comparison, melamine-formaldehyde condensates which were condensed in the same molar ratio of melamine: formaldehyde and under otherwise identical conditions, but with the addition of 0.01 mol to 0.1 mol of alkali sulfite per mol of melamine, could be dried under the same conditions the following quantities of resin solutions are passed through the drying apparatus:

with the addition of 0.025 mole of alkali sulfite per mole of melamine 26 liters.

with the addition of 0.05 mole of alkali sulfite per mole of melamine 32 liters.

with the addition of 0.1 mole of alkali sulfite per mole of melamine 35 liters.

Compared to the throughput of 23 liters in the resin solution without sulfite, the increase in throughput is s

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with addition of 0.025 mol of alkali sulfite 17.4% with addition of 0.050 mol of alkali sulfite 39.1%

with the addition of 0.1 mol of alkali sulfite 52.2%.

By adding more than 0.1 mol of alkali sulfites to melamine-formaldehyde condensates, a certain additional improvement in the ease of spraying of the melamine-formaldehyde condensates and thus a somewhat higher throughput of resin solution during spray drying could be achieved. A content of more than 0.1 mole of alkali sulfite per mole of melamine already has a noticeable effect on the water resistance of the finished products, especially when the resins are used for waterproof gluing.

Melamine and formaldehyde can be condensed in aqueous solution to a solution of a curable condensate in the customary manner, wherein melamine and formaldehyde in a molar ratio of 1: 1.3 to 1: 6, preferably 1: 2 to 1: 4, are advantageously used. The condensation is advantageously carried out by heating the reactants in aqueous solution at temperatures between 60 and 100 ° C. and at pH values from 7.5 to 9. The condensation at the temperatures indicated is advantageously carried out until a part of the resin solution, after the addition of 5 parts of boiling hot water, shows turbidity when cooling to temperatures between 20 and 60 ° C.

According to the invention, an aqueous solution of the type described above is used for the spray drying, which were prepared with the addition of 0.01 mol to 0.1 mol of alkali sulfite, biosulfite, dithionite or (formaldehyde) sulfoxylate per mole of melamine . 0.015 to 0.05 mol of sodium sulfite or bisulfite per mol of melamine is preferably used. Sodium (formaldehyde) sulfoxylate is understood to mean an addition compound of sodium sulfoxylate with formaldehyde, which has also become known under the trade name “Rongalit” and forms easily. The other oxygen acids in sulfur can also form such addition compounds and are effective in this form. The addition can take place at the start of the condensation. However, it is also possible to add the additives during the condensation.

The process is also useful for mixed condensates that contain minor amounts of other substances that react with formaldehyde, such as urea, thiourea, phenol, dicyandiamine, mono- or polyhydric alcohols, etc., in addition to melamine. "Subordinate quantity" should e.g. represent a proportion of 0.01 to 0.5 mol per mol of melamine.

The spray drying of the aqueous melamine-formaldehyde condensates is generally carried out in the customary drying apparatus, the air temperature expediently being 100 to 250 ° C., in particular 130 to 180 ° C.

The melamine-formaldehyde-condensate solutions used according to the invention produce powders during spray drying which do not tend to sinter or deposit in the drying units at the drying temperatures. The insensitivity of the powders produced according to the invention to the higher water vapor concentration in the drying air allows drying of significantly larger amounts of candate solutions per unit of time in comparison to condensate solutions which are prepared without addition.

Despite the good solubility in water of the powders produced according to the invention, when used, for example in the waterproof gluing of plywood or as impregnating resins, materials are obtained which in their water resistance are equivalent to pure melamine-formaldehyde condensates. In contrast, melamine-formaldehyde condensates

sate, which were produced with the addition of more than 0.1 mole of alkali sulfite per mole of melamine, already noticeably reduced water resistance after processing on:

Test specimens which consisted of three-ply beech plywood and which were produced in accordance with DIN 53 255 using non- and sulfite-modified melamine-formaldehyde resins under the same conditions resulted, for example, when testing the gluing in accordance with test AW 100 in accordance with DIN 68 705, sheet 1 , the following shear strengths:

when using a glue resin without added sulfite 2.37 N / mm2

when using a glue resin with 0.05 mole sulfite additive per mole melamine 2.42 N / mm2

when using a glue resin with 0.1 mole sulfite addition per mole

Melamine 1.92 N / mm2.

The condensates according to the invention also have the advantage that their processing products, in particular resin-impregnated dry papers, also when stored in high layers, i.e. Do not bake when stacked and in high humidity.

The amounts in parts and percentages given in the examples relate to the weight.

example 1

A mixture of 3,676 parts of a 40% aqueous formaldehyde solution, 2000 parts melamine and 50 parts sodium bisulfite is adjusted to a pH of 8.8 by adding a 50% aqueous potassium hydroxide solution and heated to 95 ° C. with stirring. At this temperature, the condensation is continued until a sample, to which 5 parts of boiling hot water has been added, shows turbidity when cooling to 40 ° C. After adding 1600 parts of water, the solution is cooled to about 50 ° C. The condensate solution obtained is transferred at 140 to 160 ° C in a spray dryer into a resin powder that is easily soluble in water at room temperature and is very suitable in this form as glue for the production of waterproof plywood gluing.

Example 2

A mixture of 2940 parts of 40% aqueous form aldehyde solution, 64 parts of water, 2400 parts of melamine and 120 parts of sodium bisulfite is adjusted to pH 8.7 by adding 25% sodium hydroxide solution and heated to 96 ° C. with stirring. The condensation is continued at 96 ° C., the pH falling from 8.7 to 8.1 to 8.2 in the course of the further condensation. At the time when 1 part of the solution, when diluted with 5 parts of boiling hot water, turns to turbidity when cooled to 38 ° C, 1860 parts of water are added and the solution is cooled to 40 ° C. The product obtained is processed at a temperature of 130 ° C in the spray tower for powder. The resin powder produced in this way is easily soluble in water and very well suited for the surface treatment of chipboard.

Example 3

A mixture of 2549 parts by weight of 40% aqueous formaldehyde with a pH of 8.0 and 180 g of urea is heated to 95 ° C. and kept at this temperature for 10 minutes. The pH of the solution is then adjusted to 4.5 by adding formic acid and the Ge5

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mix heated to 90 to 95 ° C for a further 40 minutes. After cooling the solution to 50 ° C

1010 g melamine, 26 g sodium bisulfite and

43.5 g of sodium dithionite were added.

The mixture is then heated again to 96 ° C., a pH of 8.8 being established. The condensation is continued at temperatures between 90 and 95.degree. C. until 1 part of the solution, when diluted with 5 parts of boiling hot water, becomes cloudy when it cools to 44.degree. After adding 500 parts by weight of water, the solution is cooled to room temperature.

The clear resin solution obtained is converted at 120 to 130 ° C. by spray drying into a resin powder which dissolves easily in water and in this form is well suited as a binder for the production of waterproof chipboard.

Claims (3)

620461 1. A process for the preparation of powders from at least limited water-soluble melamine-formaldehyde condensates by condensing melamine and formaldehyde in aqueous solution and spray drying the condensate solution, characterized in that the condensation in the presence of 0.01 to 0.1 mol an alkali sulfite, bisulfite, dithionite or - (formaldehyde) sulfoxylate, based on the melamine present. 2. The method according to claim 1, characterized in that one carries out the condensation with a molar ratio of melamine to formaldehyde from 1: 2 to 1: 4 in the presence of 0.015 to 0.05 mol, based on the melamine, sodium sulfite or bisulfite. 2nd PATENT CLAIMS 3. Use of the powders produced by the process according to claim 2 as impregnating resins or wood glues.