EP0000798A1 - Pulverulent polyurethane varnishes - Google Patents

Abstract

The subject matter is powdery polyurethane lacquers made from hydroxyl-containing polyesters, polyacrylates or epoxy resins and -caprolactam-blocked polyisocyanates, the polyisocyanate component being the triisocyanato-isocyanurate of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate and optionally oligomers together with monomeric 3-isocyanatomethyl-3,5,5 - Trimethylcyclohexyl isocyanate is used. The isocyanate / isocyanurate mixture prepared in situ is particularly advantageously used directly.

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, ε-caprolactam, in particular, has established itself as a blocking agent for the isocyanate groups with regard to the odor nuisance and blistering (DT-OS 19 57 483).

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

Polyurethane powder coatings based on IPDI blocked with C-caprolactam are particularly good compared to polyurethane powder coatings based on other polyisocyanates Weather stability, flow characteristics and thermal stability. However, they have the disadvantage - compared with, for example, epoxy resin powders - that the curing conditions, ie the curing temperature and the curing time, are relatively high.

There has been no shortage of attempts to reduce these disadvantages, e.g. by adding catalysts that reduce curing times. 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 disadvantages. 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 leads usually to decrease 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 either do not have the abovementioned disadvantages, or do so in a very weakened form, when they are polyisocyanate, the trilsocyanato isocyanurate from 3-isocyanatomethyl -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 composed of hydroxyl-containing polyesters, polyacrylates or epoxy resins and ε-caprolactam-blocked polyisocyanates as the curing component, characterized in that the polyisocyanate component is the triisocyanato isocyanurate of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate and optionally oligomers together with monomeric 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate is used.

This aliphatic triisocyanate isocyanurate can be prepared by the process of DT-OS 23 25 826. A further suitable trimerization catalyst is also described in DT-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 test the reaction at a abort certain MCO content of the mixture, preferably when 30-60% of the NCO groups have reacted with trimerization. The unreacted isocyanate is then separated from the friisocyanurate by thin-layer distillation. The accessible triisocyanatoisocyanurate is used in a mixture with isocyanurate-free diisocyanate for blocking with ε-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 triisocyanato-isocyanurate mixture prepared in, among others, can still contain other oligomers of diisocyanate, used directly with ε-caprolactam in a mixture with the monomeric diisocyanate.

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

The isocyanurate groups which are not completely blocked with ε-caprolactam are also suitable for the preparation of the powder-like crossover agents according to the invention, but not more than one isocyanate group per molecule must be free, since crosslinking occurs only when the components are mixed.

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

The compounds to be used which contain hydroxyl groups contain, as essential components, condensed:

• 1. Cyclic polycarboxylic acids, such as rhthalic acid, isophthalic acid, terephthalic acid, benzene-1,3,5-tricarboxylic acid, trimellitic anhydride, dimethyl terephthalate (DMT);
• 2. Diols, for example glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanedioi, 2,2-dimethylpropanediol, 1,6-hexanediol, 4,4'-dihydroxydicyclohexylpropane-2,2, cyclohexanediol, diethylene glycol and bis-ethoxylated 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. Benzoic acid and acyclic polycarboxylic acids, such as adipic acid, 2,4,4- (2,2,4) -trimethyladipic 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 agents according to the invention can be produced in suitable mixing units, for example in stirred tanks or mixing screws. Conventional additives, such as pigments, leveling agents, plasticizers, fillers and catalysts, can also be used in a simple manner without using solvents be added. The powdery coating compositions can easily be processed in the usual way, 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 ε-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 mol (1323 g) dimethyl terephthalate, 2.25 mol (373.5 g) terephthalic acid, 6 mol (624 g) 1,3-dimethylpropanediol, 1 mol (134 g) trimethylol propane and 3 , 0 moles (432 g) of 1,4-dimethylolcyclohexane were combined in a 5 1 glass flask and heated with the aid of an oil bath. After the majority of the substances had melted, 0.1% by weight at a temperature of 160 ° C. % Of dibutyltin oxide was added as the esterification catalyst, and the bottom temperature was slowly increased to 185 ° C. in the course of 3 hours, and the temperature was raised further to a maximum of 230 ° C. within a further 8 hours, the heating rate being based on the methanol / water separation The mixture was then cooled to about 210 ° C. and largely volatile components were removed by evacuation at about 1 mn Hg. The bottom product was stirred gently during the entire condensation time. A nitrogen stream of about 30 l / h ensured better discharge of metha nol 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 subjected to the esterification as described in A-1 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. Manufacture of the triisocyanatoisocyanurate / isocyanate mixture

B-1 100 parts by weight of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate 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, 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 minutes the temperature of the reaction medium was again 60 ° C. The NCO content was 28.5%. This mixture of trimerized and monomeric IPDI was subjected to working up in a thin-film evaporator. The oligomer mixture, which was 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 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate 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-Diazabicyclooctan- (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 30 torr for 15 minutes. During this time the NCO content of the piaction mixture changed to 27%.

C. Blocked isocyanate component

C-1 The solid trimerized 3-isocyanatomethyl-3,5,5-triasethylcyclohexyl isocyanate according to B-1 was dissolved in a 2 1 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 ε-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 had dropped to approx. 110 ° C, the reaction was completed by heating 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.

C-2 To 100 parts by weight of the isocyanato-isocyanurate mixture prepared according to B-2 and B-3, 76.5 and 72.6 parts by weight of ε-caprolactam were added in portions at 100 ° C. so that the reaction temperature not rise above 120 ° C. To complete the reaction, the reaction mixture was kept at 120 ° C. for a further 2 h.

The composition and the chemical and physical data of Hardener.C-2.1 and C-2.2 are summarized in the.

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 treated 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 - Polyurethane 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 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 of <100 μ, 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 hardening.

Example 2 - Comparative Example

Two polyurethane powder coatings are described in this example. The results clearly show the differences between the lacquers according to the invention and the coatings based on the tsocyanaturethane adducts.

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:

Recipe 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.

Example 4 - clear coat

100 parts by weight of polyester A-2 were mixed with 46.8 parts by weight of the ε-caprolactam blocked isocyanate-isocyanurate mixture of IPDI, 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 on a RorngrcSBe <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.

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

• SD = Schichtstärke in µm
• HK = Härte nach König in sec (DIN 53 157)
• HB = Härte nach Buchholz (DIN 53 153)
• GS = Gitterschnitt (DIN 53 151)
• KS = Kugelschlag in inch·1b
• ET = Tiefung nach Erichsen in mm (DIN 53 156)
• GG = Glanz nach Gardner (ASTM-D-523)
• Imp.rev. = Impact reverse in inch·1b

Explanation of the abbreviations used in Tables 3 - 8

• SD = layer thickness in µm
• HK = hardness according to König in sec (DIN 53 157)
• HB = hardness according to Buchholz (DIN 53 153)
• GS = cross cut (DIN 53 151)
• KS = ball impact in inches1b
• ET = Erichsen cupping in mm (DIN 53 156)
• GG = Gardner gloss (ASTM-D-523)
• Imp.rev. = Impact reverse in inch.1b

Claims (1)

Fulver-shaped polyurethane lacquers made of hydroxyl-containing polyesters, polyacrylates or epoxy resins and ε-caprolactam-blocked polyisocyanates, characterized in that the triisocyanato isocyanurate of 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexyl isocyanate and optionally oligomers together with monomeric 3-isocyanatomethyl-3,5 as polyisocyanate component , 5-trimethylcyclohexyl isocythate is used.