DE3110574A1 - Process for the preparation of water-dilutable (water-thinnable) thermocurable paint binders of high reactivity - Google Patents

Abstract

Novel products of the etherification of hexamethylolmelamine are obtainable if hexamethylolmelamine having a content of bonded formaldehyde of from 5.7 to 7.5 mol of formaldehyde per mol of hexamethylolmelamine is etherified in a reaction medium containing, before commencement of the reaction, from 12 to 20 mol of methanol per mol of hexamethylolmelamine and from 25 to 60% by weight of water, based on methanol, at a pH electrode value of between 4.0 and 5.0, until a degree of etherification of from 62.5 to 75% of the methylol groups present at the beginning has been reached.

Description

Process for the production of water-dilutable,

thermosetting lacquer binders high reactivity -The present one The invention relates to a method for the production of water-thinnable, thermosetting Paint binders of high reactivity, which improved the paint films produced from them Give permanently elastic properties, adhesive strength and top-flammability.

Etherified melamine-formaldehyde resins have been around for a long time used as a binder for paint systems. They give the cured paint films good adhesive strength on most substrates, high hardness and chemical resistance.

They develop these excellent properties in combination with the most diverse alkyd resins or saturated, oil-free polyesters. An application etherified melamine-formaldehyde resins as the sole binder would at their high Brittleness fail.

The butanol-etherified melamine-formaldehyde resins used so far come for products that meet the ecological requirements that have increased in recent years should be sufficient, in several respects not considered. For one thing, they're through the high number of butoxyl groups in the preferred environmentally friendly solvent, the water, practically insoluble. Second, there are generally solids contents over 75%, as required for high-solid systems, because of their high degree of condensation and the resulting high viscosity are out of the question. Moreover, corresponds to also the emission of butanol as a cleavage product during the hardening process is not understood the environmental friendliness required today.

The properties mentioned are, however, of melamine-formaldehyde resins provided that are etherified with shorter-chain mono- or polyhydric alcohols and whose condensation has not progressed too far. The widest application have achieved types etherified with methanol.

Low molecular weight, methoxylated melamine-formaldehyde resins are already manufactured for many years and widely used in the textile and paper industry, as well as in the paint industry.

They can be represented by the etherification of melamine-formaldehyde addition compounds with methanol in a weakly acidic, sometimes also in an alkaline environment in the case of increased Temperature. When applied lower temperatures must be in The presence of larger amounts of acids can be used (see Houben-Weyl, "Methods of organic chemistry ", Volume XIV / 2, pages 361-371, 4th edition (1963), G. Thieme Verlag, Stuttgart).

In contrast to butanols or others, they are immiscible with water The water of reaction which forms during the etherification cannot azeotropically alcohols distilled off and also not used to determine the progress of the reaction will. If it is to be removed, the etherification product must be practically perfect evaporated and then redissolved in a suitable solvent.

As is well known, such aminoplasts show their degree of condensation is not yet too advanced and has practically no free NH functions have more, the best mechanical properties. The presence of higher proportions of free NH functions increases the reactivity of the resin very strongly, but the reaction leads mainly to self-crosslinking of the amino resin while the co-condensation with the free hydroxyl or carboxyl groups of the combination binder The polyester used only plays a subordinate role (see e.g. W.J. Blank and W.L. Hensley, J. Paint Technol. 46 (593), 46 to 50 (1947), or R. Seidler and H. G. Stolzenbach, Farbe und Lack 81 (4), 281 to 288 (1975)). High proportions self-crosslinking of the lacquer binders give the lacquer system probably high thermal reactivity, but the cured paint films show due to lack of elasticity, poor adhesion to the substrate and a strong tendency to increase Embrittlement. A high NH content of the amino resin therefore generally leads also lead to reduced storage stability and higher viscosities, so that a use such products in highly concentrated form is problematic.

They therefore have particularly good mechanical and elastic properties Etherification products of hexamethylolmelamine, which practically no free Have more NH functions. A major disadvantage of the long-used one However, hexamethoxymethylmelamine is its extremely low reactivity, so that a Use of catalysts and the application of high stoving temperatures are necessary are. In general, the use of dried hexamethylolmelamine leads to highly etherified ones Products with excellent elasticity, but at medium stoving temperatures from 110 to 1409C do not have sufficient reactivity.

Added in the etherification of hexamethylolmelamine or lower Melamine-formaldehyde condensation products with methanol in an aqueous medium (see US Pat 2 998 411, FR-PS 1 086 826, AT-PS 163 630), a larger water content of the Reaction system up to now felt rather annoying than favorable. U.S. Patent 2,998,411 is a two-stage etherification process of filter-moist hexamethylolmelamine specified, whereby in the first stage the product is pre-etherified to the extent that it is that a subsequent evaporation of the water and excess methanol becomes possible without increasing the degree of condensation, and in the second stage that Product is completely etherified.

In AT-PS 237 901 an etherification in the presence of water is recommended after precondensation in an alkaline environment. FR-PS 1 086 826 teaches also an etherification of polymethylolmelamines in the presence of water, but - Resins which are only partially soluble or not at all soluble in water are obtained. In none In the cases mentioned, however, products are added to those produced according to the invention Coating resins obtained equivalent properties.

Surprisingly, it has now been found that an increase in reactivity of etherification products of hexamethylolmelamine by lowering the degree of etherification while maintaining the good elastic properties and the low Degree of condensation is possible. For this purpose, the etherification of hexamethylolmelamine in the presence of precisely determined amounts of water and in a weakly acidic medium carried out. The prejudice that had existed up to now had to be overcome that In an acidic environment, larger amounts of water always accelerate the condensation reaction act (see AT-PS 237 901).

A process for the production of paint binders has now been found that characterized is that you can hexamethylolmelamine with a bound formaldehyde content of 5.7 to 7.5 moles of formaldehyde per mole of hexamethylolmelamine in a reaction medium which before the start of the reaction 12 to 20 mol of methanol per Moles of hexamethylolmelamine and 25 to 60% by weight of water, based on methanol, etherified at a pH electrode value between 4.0 and 5.0 to a degree of etherification from 62.5 to 75 9 of the methylol groups present at the beginning is reached.

The preparation of the necessary for the process according to the invention Starting product hexamethylolmelamine, the formaldehyde bound per mole from 5.7 to 7.5 moles can be carried out according to methods known per se (see e.g. Houben-Weyl loc. and U.S. Patent 2,998,411 and references cited therein).

It should be noted that according to the various in the literature The procedure described often results in products that differ in terms of their Content of bound formaldehyde, its degree of condensation or its molecular weight Distinguish uniformity. For the method according to the invention it is essential that that the hexamethylolmelamine used ~ per mole of at least 5.7 moles of bound formaldehyde contains. A hexamethylolmelamine is preferably used, which per mole over 6, for example 6.2 to 7.5, preferably 6.5 to 7 mol of bound formaldehyde contains. If necessary, it can be determined by simple preliminary examinations which method known from the literature is one for the method according to the invention suitable Starting product can be obtained.

Starting products that have more than 6 moles of formaldehyde bound to 1 mole Containing hexamethylolmelamine can contain formaldehyde in excess of 6 mol in the form of hemiacetals of methylols.

The use of such products is preferred because among the Under acidic conditions for the etherification according to the invention, formaldehyde is split off from methylol groups or acetals can occur and at such a high level of the starting product in bound formaldehyde can be ensured that the etherification product still contains at least 6, now partially etherified methylol or contains methylene ether groups and no noticeable increase in NH functionality occurs with its undesirable side effects. For example, it was found that when using about 6.8 to 7 moles of bound formaldehyde containing hexamethylolmelamine the ratio of formaldehyde to melamine after etherification according to the invention is still about 6.1: 1.

It is also advantageous to use such starting products, whose condensation has not progressed too far, which occurs, for example, when if it has been dried too forcefully.

The amount of methanol to be used according to the invention is 12 to 20 moles per mole of hexamethylol melamine.

Preferably, 15 to 20 moles of methanol are used per mole of hexamethylene melamine used.

It is an essential feature of the method according to the invention, that the methanol used contains 25 to 60% by weight of water before the start of the reaction. This water content is preferably from 30 to 45% by weight.

This water content is important for controlling the degree of etherification and thus the reactivity of the lacquer binders accessible according to the invention.

In contrast to the previously known methods in which the water content the methylolmelamine resin solution to be etherified does not play a significant role plays (see AT-PS 237 901) or is no longer determined or taken into account at all (see AT-PS 163 630), the inventive method is based on an accurate Knowledge of the water content of the system and its specific use within certain limits.

The water that must be present according to the invention can be different Manner can be introduced into the reaction mixture. It is possible to do it all over or partially as fresh water to be added. But it can also, for example in Form of methanol distillates, wholly or partially recycled from previous approaches will. Furthermore, the water can be introduced together with the hexamethylolmelamine be, for example in the form of mother liquor or washing liquid, the hexamethylolmelamine from its manufacture still adheres.

In order to obtain products of high reactivity, the etherification must not be carried out to progress too far. According to the invention, this criterion can be down to ideal Way by controlling the water balance c) of the system. Additionally to the effect of regulating the equilibrium of the etherification reaction With increasing water content, water can reduce the amount necessary for etherification Acid can be chosen lower.

Another essential feature of the method according to the invention is compliance with a pH electrode value between 4.0 and 5.0. The expression "pH electrode value" means that a corresponding potential value at one in pH units calibrated glass electrode is read, which is immersed in the reaction mixture. There the reaction mixture contains large proportions of organic substances, would be the indication a pH value according to the usual definition does not make sense.

It is preferable to work with one within the stated range pH electrode value that is about 0.2 to 0.5 units below the value that to completely dissolve the hexamethylolmelamine in the other present Conditions would be necessary.

The pH electrode value can be adjusted by adding acid be made. A wide variety of acids are suitable for this, for example organic Acids such as formic acid, acetic acid or oxalic acid or mineral acids such as phosphoric acid, Boric acid or sulfuric acid.

Oxalic acid is preferably used to adjust the pH electrode value used.

The choice of the reaction temperature is generally of minor importance Meaning. For example, at temperatures in the range from 450C up to Working temperature at which the respective reaction mixture is refluxed at normal pressure boils. It is generally advantageous to be within the stated temperature range to work at relatively high temperatures, as the reaction time is then shortened. Temperatures in the range from 60 to 70 ° C. are particularly preferred.

The reaction according to the invention is carried out until there is a degree of etherification from 62.5 to 75% of the methylol groups present at the beginning is reached. Preferably the reaction is carried out to a degree of etherification of 63 to 74%. the Etherification has generally progressed to a suitable degree of etherification, when all of the hexamethylolmelamine used has gone into solution.

However, Mann can also lengthen the reaction time a little, for example by 10 to 30 minutes beyond the time of clearing up. As a result, minor changes in the properties of the products are still possible. In general, the reaction time until clear is about 10 to 20 minutes, depending on which reaction temperature is used.

When the reaction mixture has cleared and, if necessary, still has reacted somewhat, the reaction is generally terminated by neutralization. Bases can be added for this, for example high-boiling alkylamines.

Triethanolamine or dimethylethanolamine, for example, can be used as bases in question. Dimethylethanolamine is preferably used.

After completion of the reaction, the product can be used up to the desired Solids content can be concentrated, for example by adding methanol in vacuo and separates water. A clear liquid is obtained in this way, which is generally with Water and solvents such as isopropanol can be mixed in any ratio.

The paint binders produced according to the invention are distinguished by an increased reactivity of the etherified hexamethylolmelamine and good elastic Properties of the paints that can be produced from it.

The amino resins obtained according to the invention have a low viscosity on. They are already highly reactive at stoving temperatures from 1100C.

They give the films made from them a out of the ordinary high permanent elasticity, overburnability and adhesive strength, as they normally are are not found with melamine resins of the same reactivity. The products are generally miscible with water in any proportion and also in most organic Solvents, with the exception of gasoline hydrocarbons, soluble. Depending on the The quality of the hexamethylolmelamine used can sometimes be in the range of a solids content between 90 and 85% - there is a slight miscibility gap with water.

The present invention further relates to new etherification products of hexamethylolmelamine, which are characterized in that they are available, by adding hexamethylolmelamine with a bound formaldehyde content of 5.7 to 7.5 moles of formaldehyde per mole of hexamethylolmelamine in a reaction medium which before the start of the reaction, 12 to 20 moles of methanol per mole of hexamethylolmelamine and 25 Contains up to 60% by weight of water, based on methanol, at a pH electrode value between 4.0 and 5.0 etherified until a degree of etherification of 62.5 to 75% of the to The beginning of existing methylol groups is reached '.

Products which can be used as stoving enamels can be selected from the inventive accessible lacquer binders are produced by converting them into known Way with alkyd resins or oil-free polyesters and, if necessary, with other customary ones Mixing additives, e.g. solvents, pigments, etc.

Examples The following examples 1 to 6 illustrate the invention Procedure without limiting it in any way. Examples 7-11 are for comparison, with example 7 having a lower water content of the reaction mixture was applied as required according to the invention and the Examples 8 to 11 were carried out according to the state of the art.

The test results of paints obtained in the same way from the products of Examples 1 to 11 are given after the examples (See in particular Tables .1 and 2).

Example 1 1000 g of hexamethylolmelamine (7.0 mol of formaldehyde bound), which still contained 60.3% water from the manufacturing process, were mixed with 756 g of methanol (- 20 moles per mole of hexamethylolmelamine) added, resulting in a water content, based on methanol ', of 44.4%. This suspension was with oxalic acid on a Adjusted pH electrode value of 4.5 and heated to 65 "C until the solution had become clear. After another 10 minutes, during which the temperature is still at 650C was held, was neutralized with dimethylethanolamine. The degree of etherification was then 68.3% of the methylol groups present at the beginning. In a water jet vacuum was then concentrated to a solids content of 95% and the like then redissolved with isopropanol to a resin content of 90%.

Example 2 1000 g of hexamethylolmelamine of the same quality as in Example 1 was given with 567 g of methanol (= 15 mol per mol of hexamethylolmelamine) added, resulting in a water content, based on methanol, of 51.5%. The further procedure was as given in Example 1. The degree of etherification of the product thus obtained comprised 62.5% of the methylol groups present at the beginning.

Example 3 1000 g of hexamethylolmelamine (7.0 mol of formaldehyde, bound) which still contained 47.9% water from the manufacturing process, were mixed with 744 g of methanol (= 15 moles per mole of hexamethylolmelamine) added, resulting in a water content, based on methanol, of 39.2%. The suspension was adjusted to a pH electrode value with oxalic acid set of 4.5 and heated to 65 ° C. 10 minutes after the solution has cleared up was neutralized with dimethylethanolamine. The concentration and redissolving of the resin solution took place as indicated in Example 1. The degree of etherification of the product thus obtained was 69.4% of the methylol groups present at the beginning.

Example 4 1000 g of hexamethylol melamine of the same quality as at Example 3 was given with 992 g of methanol (= 20 mol per mol of hexamethylolmelamine) added, resulting in a water content, based on methanol, of 32.6%. The further procedure was as described in Example 3. The degree of etherification of the product thus obtained comprised 68% of the methylol groups present at the beginning.

Example 5 1000 g of hexamethylolmelamine of the same quality as in Example 3 was mixed with 595 g of methanol (= 12 mol per mol of hexamethylolmelamine), resulting in a water content, based on methanol, of 44.6%. The pH electrode value the suspension was adjusted to 4.0 with oxalic acid. After heating to 650C this temperature was maintained until 10 minutes after the solution had cleared up. It was then neutralized with dimethylethanolamine. After constricting and releasing in isopropanol, the solution had a solids content of 90%. The degree of etherification of the product thus obtained comprised 69.5% of the methylol groups present at the beginning.

Example 6 1000 g of hexamethylolmelamine (6.5 mol of formaldehyde bound), which still contained 37.5% water from the manufacturing process, were mixed with 935 g of methanol (= 15 mol per mol of hexamethylolmelamine) added, resulting in a water content of 28.6% adjusted (based on methanol). The suspension was made with oxalic acid on set a pH electrode value of 4.5 and heated to 650C. 10 minutes after the clearing of the solution was neutralized with dimethylethanolamine.

The resin was concentrated and redissolved as in the example 1 specified. The degree of etherification of the product thus obtained was 73.3% methylol groups present at the beginning.

Example 7 (for comparison) The same hexamethylolmelamine which Used in Example 3 was air dried until there was a water content of only 3.6%. 1000 g of this air-dried hexamethylol melamine (7.0 mol of formaldehyde bound) were with 1377 g of methanol (= 15 mol per'Mol .. Hexamethylolmelamin ) added, resulting in a water content of 2.5% (based on methanol). The further procedure was as indicated in Example 3.

As can be seen from Tables 1 and 2, this amino resin probably products with excellent elastic properties, but without the necessary Reactivity can be obtained. This is particularly evident from the low stoving temperatures determined pendulum hardness (see Table 1).

Example 8 According to AT-PS 163 630, Example 2, the procedure was followed and the product obtained dissolved 90 g in isopropanol.

Example 9 The procedure described in US Pat. No. 2,998,411, Example 4, was followed and the product obtained was dissolved 90% in isopropanol.

Example 10 According to AT-PS 237 901, Example 1, the procedure was followed and the product obtained dissolved in isopropanol for 90 days.

Example 11 The same procedure as AT-PS 23i 901, Example 4 was followed and the product obtained dissolved 90% in isopropanol.

The amino resins prepared according to Examples 1 to 11 were tested in a white pigmented, water-thinnable stoving topcoat.

A water-thinnable, oil-free polyester was used as the binder partner in 75% solution, with an OH number of 80 and an acid number of 20.

The test recipe was as follows: polyester / amino resin ratio (solid / solid) 75:25 binder / pigment ratio 1: 0.8 Composition of the lacquer: binder (solid) 36.97% pigment (TiO2) 29.55% organic solvent 9.78% water 23.27 % Auxiliaries 0.43% 100.00% The stoving conditions were as follows: 1. 30 minutes at 1200C (see Table 1) 2. 30 minutes at 1500C (see Table 2) The test results The stoving top coats produced in this way are shown in Tables 1 and 2. Tabel 1 stoving condition: 30 min 120 ° C amine resin from example 1 2 3 4 5 6 7 8 9 10 11 1st test after 2 h after stoving Gardner gloss 20 ° angle of reflection 75% 76 % 72% 71% 69% 72% 74% 66 $ - 66% 73 Pendulum hardness according to König (s) 139 151 144 153 141 142 79 177 sticks 176 137 Adhesive strength acc.

DIN 53 151 Gt Gt Gt Gt Gt Gt Gt 1) - 1) 1) 1 1 - 2 0 - 1 1 - 2 1 1 - 2 1 Erichsendehnung (mm) 5.5 7.3 7.0 4.7 6.9 8.2 9.3 1.1 sticks 1.2 3.2 2nd test after aging 72 h 70 ° C gloss acc. To Gardner 20 ° angle of reflection 70% 70% 70% 64% 69% 67% 68% 1) - 1) 1) König pendulum hardness (s) 153 154 147 163 152 143 113 1) - 1) 1) Adhesive strength n.

DIN 53 151 Gt Gt Gt Gt Gt Gt Gt 1) - 1) 1) 1-2 1-2 1 2 1-2 1-2 1-2 Erichsen elongation (mm) 5.8 6.6 7.1 5.0 7.2 5.6 7.8 1) - 1) 1) 1) due to the bad Elasticity behavior no longer checked Table 2 stoving conditions: 30 min 150 ° C amino resin from example 1 2 3 4 5 6 7 8 9 10 11 1st test after 2 h after baking, Gardner gloss 20 ° angle of reflection 58% 59% 59% 59% 54% 62 % 56% 53% 76% 50% 57% Pendulum hardness according to König (s) 172 174 167 169 178 172 158 179 28 175 166 Adhesive strength acc.

DIN 53 151 Gt Gt Gt Gt Gt Gt Gt 1) 1) 1) 1) 1 1-2 1 1-2 1-2 1-2 1 Erichs elongation (mm) 4.4 4.3 5.6 3.3 5.5 4.1 6.7 0.9 9.0 0.8 1.2 2nd test after aging 72 h at 70 ° C Gardner gloss 20 ° angle of reflection 54% 57% 61% 60% 53% 59% 58% 1) 1) 1) 1) König pendulum hardness (s) 170 181 163 175 182 172 158 1) 1) 1) 1) Adhesive strength n.

DIN 53 151 Gt Gt Gt Gt Gt Gt Gt 1) 1) 1) 1) 1 1-2 1 1-2 1-2 1-2 1 Erichsen elongation (mm) 3.8 4.2 5.1 1.7 3.9 4.4 6.5 1) 1) 1) 1) 1) due to the bad Elasticity and / or reactivity behavior no longer checked.

Comments on the test results from Tables 1 and 2: A comparison of the values of the amino resin from Example 7 with those of the amino resin from Example 3 shows in a very impressive way how the addition of water in of the type according to the invention with otherwise the same reaction conditions to one at 1200C significantly increased reactivity, without significant loss of elasticity or to cause overburnability.

The amino resins used for comparison from Examples 8, 10 'and 11 prove to be under the highly selective test conditions used in terms of their elasticity behavior as not competitive. An examination after 72 hours of aging at 700C could be achieved shortly after the burn-in process noticeable high brittleness of the paint films.

The amino resin from Example 9 was found to be completely unsuitable for Manufacture of stoving top coats.

The amino resins produced by the process according to the invention from Examples 1 to 6, on the other hand, show excellent reactivity as well high level of elasticity and adhesive strength of their paint films.

Claims (10)

Claims 1) Process for the production of paint binders, characterized in that one hexamethylolmelamine with a content of bound Formaldehyde from 5.7 to 7.5 moles of formaldehyde per mole of hexamethylolmelamine in one Reaction medium containing 12 to 20 moles of methanol per mole of hexamethylolmelamine before the start of the reaction and contains 25 to 60% by weight of water, based on methanol, at a pH electrode value etherified between 4.0 and 5.0 until a degree of etherification of 62.5 to 75 9 of the to The beginning of existing methylol groups is reached. 2) Method according to claim 1, characterized in that the hexamethylolmelamine a bound formaldehyde content of 6.2 to 7.5 moles of formaldehyde per mole of hexamethylolmelamine exhibit. 3) Process according to claims 1 and 2, characterized in that the reaction medium before the start of the reaction 15 to 20 moles of methanol per mole of hexamethylolmelamine contains. 4) Process according to Claims 1 to 3, characterized in that the reaction medium before the start of the reaction 30 to 45 wt .-%, based on water Methanol. 5) Process according to Claims 1 to 4, characterized in that one works at a pH electrode value which is about 0.2 to 0.5 units below of the value that is necessary for the complete dissolution of the hexamethylolmelamine necessary is. 6) Process according to Claims 1 to 5, characterized in that the pH electrode value is adjusted by adding acid. 7) Process according to Claims 1 to 6, characterized in that one works at temperatures in the range from 450C up to the temperature at which the respective reaction mixture boils at normal pressure. 8) Process according to Claims 1 to 7, characterized in that the reaction is carried out at least until all of the hexamethylolmelamine used has gone into solution. 9) Process according to Claims 1 to 8, characterized in that the reaction is terminated by adding bases. 10) New etherification products of hexamethylolmelamine, characterized in that that they are obtainable by hexamethylolmelamine containing bound Formaldehyde ven 5.7 to 7.5 moles of formaldehyde per mole of hexamethylolmelamine in one Reaction medium, 'which before the start of the reaction 12 to 20 Moles of methanol contains per mole of hexamethylolmelamine and 20 to 60% by weight of water, based on methanol, etherified at a pH electrode value between 4.0 and 5.0 until a degree of etherification from 62.5 to 75% of the methylol groups present at the beginning is reached.