EP0000798B1 - Pulverulent polyurethane varnishes - Google Patents

Description

It is already known to use powder coatings made of hydroxyl-containing polyesters, polyacrylates and epoxy resins and polyisocyanates, the isocyanate groups of which are blocked, in the paint sector for electrostatic application by spraying or spraying the substrates and subsequent curing.

After bad experience with phenol, _E- caprolactam, in particular, has established itself as a blocking agent for the isocyanate groups (DE-OS 19 57 483).

Because of a number of advantages, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, hereinafter called IPDI, is used as the polyisocyanate (DE-AS 21 05 777).

Polyurethane powder coatings based on IPDI blocked with e-caprolactam are distinguished from polyurethane powder coatings based on other polyisocyanates in particular by good weather stability, flow properties and thermal stability. But they have the disadvantage - compared e.g. B. with epoxy resin powders - that the curing conditions, d. H. the curing temperature and the curing time are relatively high.

There has been no lack of attempts to reduce these disadvantages, e.g. B. by the addition of curing times reducing catalysts. Such proposals could not prevail due to technological disadvantages.

Compared to the epoxy resin powder coatings, but also others, the weight ratio of resin (polyester) to hardener is clearly shifted to the side of the hardener. This fact not only affects the cost-effectiveness of powder coatings based on this resin group, but also requires special adjustment on the part of the production companies to these conditions.

There are two ways to address these drawbacks. Since in the case of this resin group a stoichiometric ratio of resin / hardener is required for setting the optimal property profile, the ratio resin / hardener can theoretically be shifted in favor of the resin by trying to increase the content of crosslinking isocyanate groups and / or To reduce the content of OH groups on the resin side. If you follow the first route, you will find that, based on the isocyanate urethane hardener used today, an increase in the NCO concentration in the hardener has a very negative effect on the storage stability of the finished powder coatings. The reduction in the OH number of the polyol component of the polyester usually leads to a reduction in the functionality of the polyol component. This results in a weakness of such paints with regard to chemical resistance, which should be compensated for by a higher functionality on the hardener side.

It has now surprisingly been found that powder coatings made of macromolecular compounds containing hydroxyl groups, such as polyesters, polyacrylates and epoxy resins, and ε-caprolactam-blocked polyisocyanates do not have the abovementioned disadvantages or do so in a very weakened form if they are the triisocyanatoisocyanurate from 3-isocyanatomethyl as the polyisocyanate -3,5,5-trimethylcyclohexyl isocyanate and optionally oligomers together with monomeric 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate.

The invention therefore relates to powdered polyurethane lacquers comprising hydroxyl-containing polyesters, polyacrylates or epoxy resins and s-caprolactam-blocked polyisocyanates as the curing component, characterized in that the triisocyanato isocyanurate of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate and optionally oligomers together with monomeric as the polyisocyanate component 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate is used.

This aliphatic triisocyanate isocyanurate can be prepared by the process of DE-OS 23 25 826. A further suitable trimerization catalyst is also described in DE-OS 26 44 684. The trimerization can be carried out in bulk or in inert solvents. To carry out the trimerization process, it is essential to stop the reaction at a certain NCO content of the mixture, preferably when 30-60% of the NCO groups have reacted with trimerization. The unreacted isocyanate is then separated from the triisocyanurate by thin-layer distillation. The accessible triisocyanatoisocyanurate is used in a mixture with isocyanurate-free diisocyanate for blocking with s-caprolactam. The addition of the isocyanurate-free diisocyanate allows the properties of the process products, in particular their melting point, to be varied in a desired manner in a simple manner.

It is particularly advantageous to use a triisocyanatoisocyanurate mixture prepared in situ, which u. a. can still contain other oligomers of diisocyanate, directly with s-caprolactam in a mixture with the monomeric diisocyanate.

Triisocyanatoisocyanurate and monomeric diisocyanate are used in a weight ratio of approximately 80:20 to 30:70.

The isocyanurate groups which are not completely blocked with s-caprolactam are also suitable for the preparation of the powdered coating compositions according to the invention, but not more than one isocyanate group per molecule may be free, since otherwise the Mixing of crosslinking components occurs.

• 1. cyclische Polycarbonsäuren, wie Phthalsäure, Isophthalsäure, Terephthalsäure, Benzol-1,3,5-tricarbonsäure, Trimellithsäureanhydrid, Dimethylterephthalat (DMT);
• 2. Diole, z. B. Glykol, 1,2-Propandiol, 1,3-Butandiol, 1,4-Butandiol, 2,2-Dimethylpropandiol, Hexandiol-1,6, 4,4'-Dihydroxydicyclohexylpropan-2,2, Cyclohexandiol, Diäthylenglykol und bisäthoxy- liertes4,4'-Dihydroxydiphenyl-2,2-propan, 1,4-Dihydroxymethylcyclohexan;
• 3. Polyole, wie Glycerin, Hexantriol, Pentaerythrit, Trimethylolpropan, Trimethyloläthan.

The hydroxyl group-containing compounds to be used contain the following as condensed components:

• 1. Cyclic polycarboxylic acids, such as phthalic acid, isophthalic acid, terephthalic acid, benzene-1,3,5-tricarboxylic acid, trimellitic anhydride, dimethyl terephthalate (DMT);
• 2. diols, e.g. B. glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 2,2-dimethylpropanediol, 1,6-hexanediol, 4,4'-dihydroxydicyclohexylpropane-2,2, cyclohexanediol, diethylene glycol and bisethoxy - gelled 4,4'-dihydroxydiphenyl-2,2-propane, 1,4-dihydroxymethylcyclohexane;
• 3. Polyols, such as glycerol, hexanetriol, pentaerythritol, trimethylolpropane, trimethylolethane.

Proportionally, the polyesters can also contain monofunctional carboxylic acids, e.g. B. benzoic acid, and acyclic polycarboxylic acids such as adipic acid, 2,4,4- (2,2,4) -trimethy! Adipic acid, sebacic acid, dodecanedicarboxylic acid. The polyesters are prepared in a manner known per se by esterification or transesterification, if appropriate in the presence of customary catalysts, end products being obtained by suitable choice of the COOH / OH ratio, the hydroxyl number of which is between about 40 and 240, preferably between about 40 and about 150 , lies.

The softening temperatures of the polyesters must be so low that they can be processed at temperatures between about 70.degree. C. and 120.degree. C. with the additives necessary for the preparation of the coating compositions according to the invention. On the other hand, the softening points must be so high that the coating compositions according to the invention obtained from the polyesters can be ground to form non-clumping, free-flowing powders with a particle size of approximately 20 to approximately 120 μm.

The coating compositions of the invention can in suitable mixing units, for. B. in stirred tanks or mixing screws. Conventional additives, such as pigments, leveling agents, plasticizers, fillers and catalysts, can also be added in a simple manner without the use of solvents. The powder coating agents can be easily in the usual manner, i.e. H. for example in a fluidized bed or by electrostatic spraying; by heating to temperatures above about 150 ° C, preferably between about 160 and 200 ° C, coatings with excellent properties are obtained.

• 1. Höherer Gehalt an NCO-Gruppen bei vergleichbarer oder verbesserter Lagerstabilität;
• 2. Erhöhung der Vernetzungsdichte im gehärteten Polyurethan und damit bessere Anpassung an Polyole mit niedriger OH-Zahl;
• 3. Reduzierung der Härtungszeiten;
• 4. Reduzierung der minimal notwendigen Härtungstemperaturen.

The technical advantages of the powder coating materials of hydroxyl-containing resins and s-caprolactam-blocked polyisocyanates containing isocyanurate structures compared to the prior art are as follows:

• 1. Higher content of NCO groups with comparable or improved storage stability;
• 2. Increase in the crosslinking density in the cured polyurethane and thus better adaptation to polyols with a low OH number;
• 3. Reduction of curing times;
• 4. Reduction of the minimum necessary curing temperatures.

Examples A. Production of the polyester

A-1 6.75 moles (1323 g) of dimethyl terephthalate, 2.25 moles (373.5 g) of terephthalic acid, 6 moles (624 g) of 1,3-dimethylpropane diol, 1 mole (134 g) of trimethylol propane and 3.0 Mol (432 g) 1,4-dimethylolcyclohexane was combined in a 5 glass flask and heated with the help of an oil bath. After most of the substances had melted, 0.1% by weight of dibutyltin oxide was added as an esterification catalyst at a temperature of 160.degree. The bottom temperature was slowly increased to 185 ° C. within 3 h. The further temperature increase to a maximum of 230 ° C. bottom temperature took place within a further 8 hours, the heating rate being based on the methanol / water separation. The polyester was then cooled to approx. 210 ° C. and largely freed of volatile components by evacuation at approx. 1 mm Hg. The bottom product was stirred gently during the entire condensation time. A nitrogen flow of approx. 30 l / h ensured better discharge of methanol and water.

Physical and chemical data:

A-2 9 moles (1746 g) of dimethyl terephthalate, 4 moles (416 g) of 1,3-dimethylpropanediol, 3.75 moles (540 g) of 1,4-dimethylolcyclohexane and 2.5 moles (335 g) of trimethylol propane were, as described in A-1, subjected to the esterification under the catalytic action of 0.05% by weight of dibutyltin oxide. The first elimination of methanol occurred at a bottom temperature of about 170 ° C. The transesterification was completed after about 14 hours, with transesterification at a maximum temperature in the final phase of 220 ° C. After the vacuum treatment (see A-1) and cooling, the following chemical and physical data were determined:

B. Preparation of the Triisocyanatoisocyanurate / Isocyanate Mixture

B-1 100 parts by weight of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI) were mixed with 0.75 parts by weight of triethylamine, 0.5 parts by weight of ethylene imine and heated to 60.degree. After 3 hours, the trimerization started with the development of heat. The temperature of the reaction medium was kept at a maximum of 105 ° C. by suitable cooling. After 75 min the temperature of the reaction medium was again 60 ° C. The NCO content was 28.5%. This mixture of austrimerized and monomeric IPDI was subjected to working up in a thin-film evaporator. The oligomer mixture largely freed from monomeric IPDI had an NCO content of 18.0% and still contained 1% monomeric IPDI.

B-2 100 parts by weight of IPDI were mixed with 0.5 part by weight of a catalyst system composed of 2 parts by weight of 1,2-propylene oxide and 1 part by weight of 1,4-diazabicyclooctane (2,2,2). Heated at 120 ° C for 3 h. During this time, the NCO content fell from 37.8% (100% IPDI) to 28.4% (50% IPDI sales). To deactivate the catalyst, the reaction mixture was cooled to 40 ° C. and stripped with nitrogen at this temperature for 1/2 h. The NCO content of the reaction mixture changed slightly to 28.2%.

B-3 100 parts by weight of IPDI were mixed with 0.75 part by weight of a catalyst system composed of 2 parts by weight of propylene oxide and 1 part by weight of 1,4-diazabicyclooctane (2.2.2) for 2 hours at 120 ° C heated. During this time, the NCO content fell from 37.8% to 29.4%. To deactivate the catalyst, evacuation was carried out at 120 ° C. and 3999 Pa (30 torr) for 15 minutes. During this time the NCO content of the reaction mixture changed to 27%.

C. Blocked isocyanate component

C-1 The solid trimerized IPDI according to B-1 was dissolved in a 2 l flat-bottom flask at temperatures of 110-120 ° C in monomeric IPDI with stirring. The solution was then cooled to 75 ° C and the melted e-caprolactam was added. The temperature in the reaction medium rose to about 150 ° C. within 10 minutes as a result of the exothermic addition reaction. After the temperature dropped to about 110 ° C, the reaction was completed
annealed at 100_110 ° C for 1 h. The composition and the chemical and physical data of the hardeners (blocked polyisocyanates) C-1.1 and C-1.2 are summarized in Table 1.

• 12 Mol (2664 g) monomeres IPDI und 6 Mol Diäthylenglykol (636 g) wurden in einem geeigneten Rührkolben gemischt und langsam auf ca. 70°C erwärmt. Bei dieser Temperatur setzte unter erheblicher Wärmetönung die Addition des Isocyanates an das Diol ein. Das Reaktionsgefäß wurde während der Addition im Eisbad gekühlt, so daß die Temperatur des Reaktionsgemisches nur auf ca. 100°C anstieg. Anschließend wurde noch 2 h bei 100°C zur Vervollständigung der Reaktion nachgeheizt. Der NCO-Gehalt betrug dann 15,1%.
• Nun wurde auf 80°C abgekühlt und die dem Gehalt an Isocyanat stöchiometrische Menge ε-Caprolactam zugegeben. Durch die ebenfalls exotherme Reaktion stieg die Temperatur auf 105°C an. Die hochviskose Schmelze wurde noch 5 h bei 100°C nachbehandelt und dann auf Raumtemperatur abgekühlt. Der nahezu farblose Feststoff hatte einen Erweichungspunkt von 83° C und einen NCO-Gehalt von 0,3%.

C-3 - comparison

• 12 moles (2664 g) of monomeric IPDI and 6 moles of diethylene glycol (636 g) were mixed in a suitable stirring flask and slowly warmed to approx. 70 ° C. At this temperature, the addition of the isocyanate to the diol commenced with considerable heat. The reaction vessel was cooled in an ice bath during the addition, so that the temperature of the reaction mixture only rose to approximately 100.degree. The mixture was then heated for a further 2 hours at 100 ° C. to complete the reaction. The NCO content was then 15.1%.
• The mixture was then cooled to 80 ° C. and the amount of ε-caprolactam stoichiometric to the isocyanate content was added. Due to the also exothermic reaction, the temperature rose to 105 ° C. The highly viscous melt was aftertreated for a further 5 hours at 100 ° C. and then cooled to room temperature. The almost colorless solid had a softening point of 83 ° C and an NCO content of 0.3%.

Example 1 - Poryurethane powder coatings

The powder coating was prepared in the manner corresponding to the prior art. It should therefore only be described briefly.

The composition corresponded to the following recipe:

First, a masterbatch of the leveling agent was made in the polyester. Masterbatch, polyester, hardener and pigment were intimately mixed in a solid mixer and then extruded at a melt temperature of 95-100 ° C. The solidified melt was ground to a grain size <100, and the powder coating was applied electrostatically to 1 mm steel test panels.

The paints were baked under various curing conditions and subjected to a paint test after 24 hours.

In this type of system, the elasticity, which is measured by the Erichsen indentation, is a good parameter for assessing the degree of curing.

Explanation of the abbreviations used in Tables 3-8

Example 2 - Comparative Example

In this example, two polyurethane powder coatings are described. The results make the differences between the paints according to the invention and the coatings based on the Isocyanaturethane adducts clearly.

With the technology described in Example 1, powder coatings of the following recipes were produced and applied to 1 mm steel sheets.

Recipe I - comparison:

Formula II _ Invention

Example 3 _ Polyurethane powder coating

The technology described in Example 1 was used to produce a polyurethane powder coating of the formulation below, applied to 1 mm steel sheet and cured under various conditions.

Recipe:

Example4 - clear coat

100 parts by weight of polyester A-2 were mixed with 46.8 parts by weight of the isocyanate-isocyanurate mixture of IPDI, blocked with s-caprolactam, prepared according to C-2.2, and 0.73 parts by weight of silicone oil in the Melt homogenized at temperatures of 120-140 ° C with an intensive stirrer. After cooling, the homogeneous melt was broken and then ground with a pin mill to a grain size <100 μ. The clear lacquer powder produced in this way was applied with an electrical powder spraying system at 60 kV to iron plates primed with zinc phosphate and baked in a forced-air drying cabinet at temperatures between 180 and 200 ° C.

Example 5 - clear coat

The polyester A-2 was reacted with the hardener described in C-2.1 in an equivalent amount and prepared as described in Example 4.

Recipe:

The clearcoat was applied as described in Example 4 and baked in a forced air drying cabinet at between 180 and 200 ° C.

Claims (1)

1. Powdery polyurethane lacquers from hydroxy group-containing polyesters, polyacrylates or epoxy resins and s-caprolactam-blocked polyisocyanates, wherein said polyisocyanate component is the triisocyanatoisocyanurate of 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexylisocyanate and, optionally other oligomers thereof together with monomeric 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocy- anate, and wherein the weight ratio of triisocyanatoisocyanurate to monomeric diisocyanate is between 80 : 20 to 30 : 70.