HK1068070A - Ammonia and organic amine catalysis of epoxy hybrid powder coatings - Google Patents

Description

Catalysis of ammonia and organic amines on epoxy hybrid powder coatings

Technical Field

The invention relates to the use of ammonia and organic amines as catalysts in epoxy hybrid powder coatings.

Technical Field

Powder coatings are dry polymer powders that are applied to a substrate and then heated to cause the particles to coalesce together to form the final coating. The technical requirements for such coatings are noteworthy: the powder must be capable of being coated and cured to form a firm and flexible smooth coating. The binder resin must have a fast cure rate at as low a temperature as possible to allow high production throughput with minimal energy consumption. In addition, the powder must also be non-tacky at temperatures up to 40 ℃ so that the particles do not agglomerate during long term storage prior to application.

Resin curing is typically catalyzed by the addition of quaternary ammonium or phosphonium salt catalysts. To be effective, these salts must be compatible with the resin and have sufficient fluidity in the resin melt to diffuse into and contact the reactive groups of the polymer component of the resin to catalyze the reaction. These catalysts carry mobile ions into the resin coating and can compromise hydrolytic and electrochemical stability. In addition, the incorporation of the catalyst into the resin composition generally by melt extrusion can induce premature curing of the composition. Finally, because the molecular weight of the catalyst is generally high, a very large amount of catalyst is required in the formulation.

An important part of the powder coating market is represented by carboxyl functional polymer-epoxy resin mixtures, which are mixtures of epoxy resins with oligomers terminated with carboxylic acids, such as polyester oligomers. The use of quaternary ammonium or amine or phosphonium salts as catalysts for polyester-epoxy films is well known. These materials are typically added to the article to be coated by melt mixing or dry compounding prior to coating. However, it is generally necessary to add relatively large amounts of amine or quaternary ammonium salt to fully cure the film, resulting in an excess of material to be used which then remains as a potential impurity in the cured film.

GB1,474,140 discloses the use of nitrogen-containing bases or their thermally decomposable salts to catalyze the reaction of oligoesters or polyesters containing free hydroxyl groups but substantially no free carboxyl groups with compounds containing at least two epoxy groups.

Japanese patent application JP50-85632 describes a heat-curable coating composition comprising a polyepoxy compound and a polycarboxy compound, at least one of which is a film-forming polymer in combination with ammonia or an organic amine or a film-forming polymeric compound having both epoxy and carboxyl groups; wherein at least part of the carboxyl groups are included in the form of salts with ammonia or organic amines. These compositions are prepared by conventional melt mixing of the components of the thermosetting composition at 80 to 150 ℃ followed by grinding of the composition to a particle size of no more than about 200 μm.

Summary of the invention

The invention relates to a process for preparing a heat-curable coating composition comprising subjecting a powder comprising a carboxy-functional polymer and a polyepoxide to the action of an amine selected from the group consisting of organic amines and ammonia under mild conditions.

The invention also relates to a product manufactured by the method described herein.

Detailed Description

It has been found that for powder coatings, the rate of crosslinking in carboxyl functional polymer-epoxy mixtures can be greatly increased by catalytically reacting the carboxyl end groups of the carboxyl functional polymer with ammonia or an organic amine. Surprisingly, it has also been found that this reaction does not require additional compounding, but can be carried out with pre-compounded epoxy-carboxy functional polymer powders by subjecting the powder to the action of ammonia or an organic amine or mixtures thereof.

Subjecting to this action is accomplished at a milder temperature, i.e., not so high as to cause premature crosslinking of the coating composition components. Suitable temperatures for constituting such mild conditions are between-30 and +50 ℃ and preferably between-10 and +10 ℃.

Suitable carboxy-functional polymers for use in the process of the present invention include carboxy-functional polyester resins, carboxy-functional polyacrylate resins, carboxy-functional polymethacrylate resins, carboxy-functional polyamide resins, carboxy-functional polyimide resins, and carboxy-functional polyolefin resins. Preferably the carboxy functional polymer is a carboxy functional polyester.

Suitable carboxyl-functional polyester resins are obtainable by polycondensation of dicarboxylic-or polycarboxy-containing monomers with dihydroxy or polyhydroxy monomers to give an excess of carboxyl groups. Suitable carboxyl group-containing monomers are terephthalic acid, isophthalic acid, trimellitic acid, adipic acid, sebacic acid, maleic acid, and the like. Useful hydroxy compounds include ethylene glycol, propylene glycol, 1, 4-butanediol, 1, 6-hexanediol, diethylene glycol, dipropylene glycol, 1, 4-cyclohexanedimethanol, neopentyl glycol, bis (hydroxyethyl) terephthalate, trimethylolpropane, glycerol, pentaerythritol, and the like.

Suitable carboxyl functional acrylic resins may be obtained by polymerizing or copolymerizing carboxyl group-containing monomers such as acrylic acid, methacrylic acid, and the like. Examples of monomers copolymerizable with monomers such as acrylic acid, methacrylic acid, and the like include acrylates, methacrylates, and other ethylenically unsaturated monomers such as acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl acetate, and the like.

Suitable carboxy-functional polyolefin resins can be obtained by a process of copolymerizing a carboxy-containing monomer such as acrylic acid, methacrylic acid, etc. with an olefinic monomer capable of copolymerization such as ethylene, propylene, butene, butadiene, chloroprene, vinyl chloride, styrene, etc.

Suitable carboxyl-functional polyamide resins are obtainable by polycondensation of dicarboxylic-or polycarboxy-containing monomers with diamino-or polyamino-monomers, whereby the carboxyl groups are in excess. Examples of the carboxyl group-containing monomer are terephthalic acid, isophthalic acid, trimellitic acid, adipic acid, sebacic acid, maleic acid, and the like. Useful amino monomers include ethylenediamine, hexamethylenediamine, dodecamethylenediamine, phenylmethylenediamine, metaphenylenediamine, paraphenylenediamine, and the like.

Suitable carboxyl functional polyimide resins are obtained by polycondensation of dicarboxylic anhydride containing monomers with di-or polyamino monomers to provide an excess of carboxyl groups. This can be done by using an excess of the bisanhydride monomer or by performing only partial condensation such that free carboxyl groups adjacent to amidated carboxyl groups remain non-imidized. Examples of the carboxylic anhydride-containing monomer include pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, and biphenyl tetracarboxylic dianhydride. Useful amino monomers include ethylenediamine, hexamethylenediamine, dodecamethylenediamine, phenylmethylenediamine, metaphenylenediamine, paraphenylenediamine, and the like.

Suitable polyepoxides include epoxy compounds or resins having two or more glycidyl groups in one molecule. These compounds, which are commonly used as curing agents, are the reaction products of phenolic compounds such as bisphenol a, novolaks, and the like, with epichlorohydrin; a reaction product of a cresol compound such as cresol novolak or the like with epichlorohydrin; glycidyl ethers obtained by reacting an alcohol compound such as ethylene glycol, propylene glycol, 1, 4-butanediol, polyethylene glycol, polypropylene glycol, neopentyl glycol, glycerin, or the like with epichlorohydrin; glycidyl esters obtained by reacting a carboxylic acid compound such as succinic acid, adipic acid, sebacic acid, phthalic acid, terephthalic acid, hexahydrophthalic acid, trimellitic acid, or the like with epichlorohydrin; 3, 4-epoxy-6-methylcyclohexanecarboxylic acid-3, 4-epoxy-6-methylcyclohexylmethyl ester, 3, 4-epoxycyclohexylcarboxylic acid-3, 4-epoxycyclohexylmethyl ester, and the like; triglycidyl isocyanurate (TGIC) and derivatives thereof; and so on. Preferred epoxy compounds are TGIC and the reaction product of bisphenol A and epichlorohydrin.

Organic amines suitable for the process of the invention include C₁～C₁₂Alkylamine, C₆～C₁₂Arylamine, C₇～C₁₂Alkylaryl amines and C₇～C₁₂An aralkyl amine. More specifically, suitable organic amines include primary amines such as methylamine, ethylamine, propylamine, butylamino, ethylenediamine, methanolamine, ethanolamine, aniline, cyclohexylamine, benzylamine, and the like; secondary amines such as dimethylamine, diethylamine, dipropylamine, dibutylamine, dimethanolamine, diethanolamine, diphenylamine, benzylamine, phenethylamine, dicyclohexylamine, piperazine, imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-isopropylimidazole, 2-phenylimidazole, 2-methylimidazoline, 2-phenylimidazoline, and the like; and tertiary amines such as trimethylamine, triethylamine, dimethylhexylamine, N-methylpiperazine, dimethylbenzylamine, xylidine, and the like. The organic amine or ammonia may be employed in gaseous form or dissolved in a suitable aqueous or nonaqueous solvent. Aqueous ammonia and triethylamine are preferred.

The advantage of catalysis by ammonia is that the cations are highly mobile from the standpoint of diffusion through the polymer melt and can eventually diffuse out of the polymer, leaving little or no catalyst residue in the polymer. Furthermore, the catalyst is present only where it is needed, i.e. where the carboxyl terminus is reacted with the epoxide; this makes the catalysis much more efficient than adding conventional quaternary ammonium salt catalysts which are much higher in molecular weight and which therefore require very large amounts of catalyst to be effective. Finally, there is no introduction of persistent ionic species to the coating that may compromise hydrolytic and electrochemical stability. An additional advantage of this approach is that it can enhance the "anti-blocking" effect of ammonia. Certain carboxyl functional polymers such as ammonium salts of polyesters have softening temperatures higher than that of the free carboxylic acids, which increases the temperature stability of the powder to caking (clouping). If the ammonium salt concentration at the surface of the particles is higher than that at the center of the particles, the anti-blocking effect should be enhanced for a given content of ammonia.

An illustrative example of a polyesterammonium salt-epoxy curing reaction is shown in equations 1 and 2.

Polyester resin

Ammonium salt of polyester resin

Reaction scheme 1

Epoxy resin

Cured coatings

Reaction formula 2

The same benefits of increased cure rate achieved by reacting an organic amine with the carboxyl groups of the polymer, as well as the benefits of reduced yellowing of the coating after curing, achieved by reacting the carboxyl groups with ammonia, are achieved. The conversion to the alkyl ammonium salt of the carboxy functional polymer can be accomplished by treating the powder of the carboxy functional polymer-epoxy mixture with an organic amine. Triethylamine is preferred.

The present invention therefore also provides a process for preparing a heat-curable coating composition by subjecting a powder comprising a carboxy-functional polymer and a polyepoxide to the action of a catalyst, i.e. a catalyst which is non-ionic or free or substantially free of ions such as cations or anions. The catalyst may be a nitrogen-containing compound, such as an organic amine or ammonia. The present invention also provides a cured coating composition prepared from a powder comprising or prepared from a carboxy functional polymer and a polyepoxide, wherein the curing of the composition is catalyzed by a nitrogen containing compound such as an organic amine or ammonia, and the composition is free or substantially free of catalyst residues, such as ionic catalyst residues, or free or substantially free of ions such as cations or anions. Articles can be coated with such compositions.

Although it is common practice in the present invention to subject the carboxy-functional polymer-epoxy material to the action of ammonia and/or amine prior to coating it on the substrate to be coated, it is contemplated that such powder may be coated onto the article and subsequently subjected to the action of ammonia and/or amine as described above.

Once a powder coating has been applied to a substrate by any conventional powder substrate coating method, including but not limited to spray or fluidized bed coating methods, it is typically baked to cause the powder to flow and cure. Typically, this curing is accomplished at temperatures of 150 ℃ and higher. These substrates are used in the manufacture of a variety of articles including, but not limited to, automotive and other initial equipment components for use.

Examples

Three resin powders were used in the following examples:

grey #1: 54: 46 Grey pigmented neopentyl glycol terephthalate carboxyl polyester (AEW. RTM. 740)/bisphenol A epoxy resin (EEW. RTM. 740) containing about 13pphTiO₂And d90 is 60 microns.

White #2: ammonium salt catalyzed 92: 8 neopentyl glycol terephthalate carboxyl polyester (AEW 1280)/TGIC resin containing about 35pph TiO₂And d9.0 is 60 microns.

Transparent #3: unpigmented 50: 50 carboxy polyester (AEW 700)/bisphenol a epoxy resin (EEW 700), d90 ═ 90 μm.

All three powder samples contained about 0.5-1% benzoin as a degassing aid. AEW is acid equivalent weight; EEW ═ epoxide equivalent weight.

Examples 1 to 9: ammonia treatment of polyester-epoxy hybrid resins in gaseous form

Samples of the industrial powder coating resin were stirred in the resin kettle with a paddle stirrer while passing gaseous ammonia through the powder for 60 minutes at room temperature. A slight exotherm was observed due to the heat of reaction of ammonia with COOH in the powdered resin. For samples above about 100 grams, external cooling with an ice water bath was necessary to prevent side reactions (premature solidification would result in higher melt viscosity).

The ammonia-treated industrial powder coating was then dry-roll mixed with various proportions of untreated powder to vary the amount of ammonia in the formulation. Samples of the mixture were subjected to a gel pellet test (also known as a tilted plate flow test) and a dynamic differential scanning calorimetry test (dynamic DSC) to determine the relative cure rate as a function of ammonia content.

Gel pellet test conditions: 200 ℃/15 minutes; the downstream surface of the static oven (without air circulation) is made of aluminum foil coated with polytetrafluoroethylene and tied on a steel guard plate by a belt; deviating from the horizontal by 42 degrees. In a standard (Parr) hand pellet press, 1.3 cm pellets were extruded with 0.70 grams of powder. The convection distance is measured from the front of the pellet; shorter melt flow distance before gelation means faster crosslinking rate.

Dynamic differential scanning calorimetry (Dynamic DSC) was performed using a DuPont model 912 differential scanning calorimeter. The amount of the sample is 10-15 mg of polymer powder; the heating rate was 10 deg.C/min. The cure rate of epoxy formulations can be correlated with the peak temperature of the exothermic peak in a dynamic DSC scan (see E.Turi, ed., Academic Press 1981 "Thermal Characterization of Polymeric Materials( Polymeric material Thermal characterization of the material) "r.prime article" Thermosets "in one book). Peak(s)Lower temperature values mean higher reactivity; i.e. the curing chemistry takes place at a lower temperature. Likewise, the heat of reaction (integrated area within the exothermic peak) can be correlated to the degree of cure (i.e., the number of reacted epoxy groups). A heat of reaction lower than the expected value means that pre-reaction occurs during the process (e.g., melt extrusion); i.e. a few residual reactive groups are chemically reacted during the curing reaction.

Table 1 summarizes the thermal data for various ammonia-treated powders and untreated control powders.

TABLE 1

Thermal analysis of powder coatings after Ammonia treatment

Example #

Coating powder

％NH3-treated powder

DSCTPeak value(10 ℃/min)

200 ℃ gel pellets (cm)

Control A	Untreated Gray #1 (control)	0	183	8.8
					1	Untreated gray #1+ NH3Treated Gray #1 (3: 1)	25	175	4.7
2	Untreated gray #1+ NH3Treated Gray #1 (1: 1)	50	173	3.3
					3	NH3Treated Gray #1	100	150	2.0

Control B	Untreated white #2 (control)	0	167	4.2
					4	Untreated white #2+ NH3Treated Gray #2 (3: 1)	25	168	2.0
5	Untreated white #2+ NH3Treated white #2 (1: 1)	50	163	1.1
					6	NH3Treated white #2	100	147	0.6

Control C	Untreated clarity #3 (control)	0	194	12.0
					7	Untreated clear #3+ NH3Treated clear #3 (3: 1)	25	139/189	4.5
8	Untreated clear #3+ NH3Treated clear #3 (1: 1)	50	138	3.0
					9	NH3Treated clear #3	100	147	1.7

The stepwise decreasing DSC peak exotherm temperature and shorter gel pellet melt flow versus ammonia content illustrate the catalytic effect of ammonia. The heat of reaction of these polyester-epoxy formulations after ammonia treatment was essentially unchanged, as determined by DSC, with only about a 10% change in heat of reaction, which is close to experimental error.

The zinc phosphated steel plates were electrostatically sprayed with a polyester-TGIC powder mixture (control B and examples 4 to 6) and baked at 200 ℃ for 20 minutes. The coatings were then tested for hardness (D3363), impact strength (astm D2794), flexibility (astm D1737), cure state (MEK rub), and adhesion. The glossy white coating has a thickness of 1 to 1.5 mils (25 to 40 microns). The coating adhesion was excellent in all cases; all panels passed the cross-cut bond strength test (astm d3359) with pipe tape (ducttape). Performing Methyl Ethyl Ketone (MEK) double friction by covering the ball end of the two-pound round-head hammer with six layers of coarse plain cloth wetted by methyl ethyl ketone; the hammer coated with the scrim is placed on the surface of the plate and stretched back and forth over it (creating a "double friction") using the weight of the hammer to apply pressure. The damage caused by methyl ethyl ketone to the coating was scored as follows: 1, no damage is caused; 2-slightly dull or scratched; medium scratch 3 ═ medium scratch; 4 ═ severe lesions; the coating was completely destroyed. This coating property is summarized in table 2 below.

TABLE 2

Plate Properties of Ammonia-treated polyester mixture

Example #	Hardness of pencil	Impact force Forward/reverse (kg/cm)	Zero T-bend	200 MEK double rubs (score)
					Comparative example B	3H	290/290	By passing	2
4	3H	290/290	By passing	2
					5	3H	290/290	By passing	3
6	3H	290/290	By passing	2

The improvement in thermal stability of the ammonia-treated powder is described below. A sample of the ammonia-treated powder coating (about 1 gram) was placed in a glass vial in a 40 ℃ oven for 24 hours. The powder was still completely free-flowing, so that it was left at 50 ℃ for a further 24 hours. After this additional 24 hours at 50 ℃, the ammonia-treated powder remained free-flowing. Comparative examples A and B (pure gray #1 and white #2 powders, both untreated with ammonia) remained free flowing after 40 deg.C/24 hour testing, but became a solid mass after 24 hours at 50 deg.C.

The ammonia-treated powders (examples 3, 6, 9) retained their curing activity after 24 hours at 40 ℃ followed by 24 hours at 50 ℃ as shown by dynamic DSC and gel pellet tests, which are summarized in Table 3.

TABLE 3

Dynamic DSC and gel pellet test after powder stability test

Example #	Heating at 40 deg.C/24 hr and at 50 deg.C/24 hr	DSC TPeak value(℃，J/g)	200 ℃ gel pellets (cm)
				3	Not heated	147，13	1.7
3	Heating of	148，10	1.5
				6	Not heated	147，12	0.5
6	Heating of	147，11	0.3
				9	Not heated	147，25	1.6
9	Heating of	148，18	1.8

Examples 10 to 11: triethylamine treatment of polyester-epoxy mixed resin

Example 10

A suspension of 14 g of clear #3 (the sample contained about 10 mmol COOH) in 50 ml of hexane was magnetically stirred under nitrogen and cooled in an ice-water bath. 1 g (10 mmol) of triethylamine was added and the mixture was stirred for 16 hours while it was heated to room temperature. The slurry was suction filtered through a sintered funnel, washed twice with 15 ml of hexane, and allowed to air dry for 6 hours with suction to give a fine powder. The powder was accelerated by means of triethylamine and did not appear yellow in the gel pellet test (see table 4).

Example 11

A suspension of 10 g of clear #3 (this sample contains about 7 mmol COOH) in 50 ml of hexane was magnetically stirred under nitrogen with 1 g (10 mmol) triethylamine for 90 hours at room temperature. The slurry was suction filtered through a sintered funnel, washed with 30 ml of hexane and air dried under suction to give a fine powder. The powder was accelerated by triethylamine and also did not appear yellow in the gel pellet test (see table 4).

TABLE 4

Thermal analysis of triethylamine-treated powder coatings

Example #	DSC TPeak value(℃，J/g)	190 ℃ gel pellets (cm)
			Control D	194，38	15.3
10	181，40	9.3
			11	159，37	4.4

Claims (20)

1. A method of preparing a heat-curable coating composition comprising exposing a powder comprising a carboxy-functional polymer and a polyepoxide to an amine selected from the group consisting of organic amines and ammonia under mild conditions.

2. The method according to claim 1, wherein the carboxyl functional polymer is selected from the group consisting of carboxyl functional polyester resins, carboxyl functional polyacrylate resins, carboxyl functional polymethacrylate resins, carboxyl functional polyamide resins, carboxyl functional polyimide resins, and carboxyl functional polyolefin resins.

3. The method according to claim 2, wherein said carboxyl functional polymer is a carboxyl functional polyester resin.

4. The process according to claim 1, wherein the organic amine is selected from C₁～C₁₂Alkylamine, C₆～C₁₂Arylamine, C₇～C₁₂Alkylaryl amines and C₇～C₁₂An aralkyl amine.

5. The process according to claim 4, wherein the organic amine is selected from the group consisting of methylamine, ethylamine, propylamine, butylamino, ethylenediamine, methanolamine, ethanolamine, aniline, cyclohexylamine, benzylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, dimethanolamine, diethanolamine, diphenylamine, benzylamine, phenethylamine, dicyclohexylamine, piperazine, imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-isopropylimidazole, 2-phenylimidazole, 2-methylimidazoline, 2-phenylimidazoline, trimethylamine, triethylamine, dimethylhexylamine, N-methylpiperazine, dimethylbenzylamine, and dimethylaniline or a mixture thereof.

6. The process according to claim 1, wherein the amine is in a state selected from vapor, liquid or dispersed in a solvent.

7. A method according to claim 1, wherein said powder is subjected to the action of ammonia.

8. The method of claim 1 wherein said powder is exposed to aqueous ammonia.

9. The process according to claim 5, wherein said organic amine is triethylamine.

10. The method of claim 1 wherein said mild conditions include a temperature between-30 ℃ and +50 ℃.

11. The method according to claim 10, wherein said mild conditions include a temperature between-10 ℃ and +10 ℃.

12. The method according to claim 1, wherein said action occurs before said powder is applied to a substrate.

13. The method according to claim 1, wherein said action occurs after said powder is coated on a substrate.

14. The product of the process according to claim 1, 11 or 12.

15. An article coated with the product of claim 1, 11 or 12.

16. A process for preparing a heat-curable coating composition comprising subjecting a powder comprising a carboxy-functional polymer and a polyepoxide compound to the action of a substantially ion-free catalyst.

17. The process according to claim 16 wherein said catalyst is a nitrogen-containing compound.

18. A cured coating composition prepared from a powder comprising a carboxy functional polymer and a polyepoxide, wherein curing of the composition is catalyzed by a nitrogen-containing compound, and the composition is substantially free of catalyst residues.

19. The composition according to claim 19, wherein the catalyst residue is an ionic residue.

20. An article coated with the composition of claim 18.