CA2064290A1

CA2064290A1 - Coating compositions in powder form

Info

Publication number: CA2064290A1
Application number: CA 2064290
Authority: CA
Inventors: Dietmar Fink; Claus Godau; Christoph Just; Helmut Plum
Original assignee: Individual
Current assignee: Hoechst AG
Priority date: 1991-04-18
Filing date: 1992-03-27
Publication date: 1992-10-19
Also published as: EP0509392A1; DE4112688A1; JPH05132634A

Abstract

HOECHST AKTIENGESELLSCHAFT - Werk KALLE-ALBERT

91/F 113 16 April 1991 WL-Dr.Ot.-ui ABSTRACT

Coating compositions in powder form A coating composition in powder form which is composed of (A) a copolymer containing glycidyl groups, (B) an aliphatic or cycloaliphatic dibasic acid, its anhydride or a polyol-modified anhydride of a dibasic acid, (C) optionally, pigments and other additives.
The copolymer (A) has a molecular weight (Mn) of 1,000-10,000 and a glass transition temperature of 30-90°C and is a mixed polymer composed of a) at least 10% by weight of glycidyl acrylate or glycidyl methacrylate, b) 5-50% by weight of tert.-butyl acrylate or tert.-butyl methacrylate, c) 20-60% by weight of one or more alkyl esters of aliphatic unsaturated monocarboxylic or dicarboxylic acids and d) 0-50% by weight of one or more other olefinically unsaturated monomers.

Description

2~6'~'~9D

HOECHST AKTIENGESELLSCHAFT - Werk XALLE-ALBERT

91/F 113 16 April 1991 WL-Dr.Ot.-ui Coating compositions in powder form The invention relates to coating compositions in powder form, also called powder coatings, which represent mixtures of epoxide group-containing acrylate resins and various curing agents.

Powder coatings which contain as the essential binder an epoxide group-containing acrylate copolymer are well known. They are described, for example, in the following patents: US 3,730,930, US 3,752,870, US 3,781,379, US 3,787,521, US 4,091,049, US 4,091,048, US 3,939,127, US 3,932,367, US 3,991,132, US 3,991,133, US 4,092,373, US 4,044,070, US 4,374,954 and US 4,346,144, DE 2,353,040, DE 2,423,886, DE 2,441,753 and DE 2,509,410. Dibasic acids, their anhydrides, or substances which form a dibasic acid under curing conditions, are used as curing agents. According to EP 299,420, the curing agent can also be a reaction product of a polyanhydride and a polyol.

~he copolymers described in the above patents contain up to 30% by weight of glycidyl acrylate or glycidyl meth-acrylate only; the remainder of thè copolymer consists of other unsaturated monomers. A large number of compounds of this type are suitable as such unsaturated compounds, including inter alia styrene and acrylates, without express reference being made to the particular suit-ability of acrylates containing tert.-butyl groups.
According to the examples of the prior art relatively high temperatures (above 140C~ are necessary in most cases in the curing of the powder coatings described therein. However, in many applications, for example in the finishing of wood and plastics or when used as a topcoat over a temperature-sensitive basecoat in 2 ~ 9 0 automotive finishing, it is advantageous that the coating compositions in powder form cure at the lowest possible temperatures, for example as low as 120C.

The object of the present invention was to provide powder coating systems which cure at temperatures as low as 120C even in the absence of catalysts or in the presence of the smallest possible amounts of catalysts to furnish coatings having satisfactory properties. Surprisingly, this object can be achieved if the epoxide group-containing acrylate copolymer contains tert.-butyl (meth)acrylate as comonomer. Why the reactivity of the acrylate copolymers should depend on the tert.-butyl (meth)acrylate content is not clear. Although, as explained above, a very large number of epoxide group-containing acrylate powder coatings have been described, this surprising effect has not been mentioned anywhere.

The invention relates to coating compositions in powder form which are composed of (A) a copolymer containing glycidyl groups, (B) an aliphatic or cycloaliphatic dibasic acid, its anhydride or a polyol-modified anhydride of a dibasic acid, (C) optionally, pigments and other additives, the copolymer (A)- having a molecular weight (Mn) of 1,000-10,000 and a glass transition temperature of 30-90C and being a mixed polymer composed of a) at least 10% by weight of glycidyl acrylate or glycidyl methacrylate, b) 5-50% by weight of tert.-butyl acrylate or tert.-butyl methacrylate, c) 20-60% by weight of one or more alkyl esters of aliphatic unsaturated monocarboxylic or dicarboxylic acids and d) 0-50% by weight of one or more other olefinically unsaturated monomers.

Copolymers A having the following composition:

2~&~?~0 14-43% by weight of glycidyl methacrylate, 15-35% by weight of tert.-butyl acrylate or tert.-butyl methacrylate, 12-51~ by weight of alkyl acrylates or methacrylates and 0-40% by weight of other olefinically unsaturated compounds, and copolymers having the following composition:

14-43% by weight of glycidyl methacrylate, 20-40~ by weight of styrene, 0 10-30% by weight of a dialkyl ester of an olefinically unsaturated dicarboxylic acid, 0-36% by weight of alkyl acrylates or methacrylates and 5-20% by weight of tert.-butyl acrylate or tert.-butyl methacrylate are preferred.

Suitable alkyl esters of unsaturated carboxylic acids are acrylic or methacrylic esters derived from monohydric alcohols, preferably those having 1-18 carbon atoms, particularly preferably those having 1-12 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec.-butyl, n-pentyl, n-hexyl, 2-ethylhexyl, n-octyl, n-nonyl, isononyl, n-decyl, n-dodecyl, n-tridecyl, isotridecyl, myristyl, cetyl, stearyl, eicosyl and isobornyl acrylate or methacrylate. It is also possible to use small amounts, ie. up to 5% by weight, of a diacrylate or dimethacrylate of a dihydric or trihydric alcohol, such as hexanediol diacrylate or butanediol diacrylate or dimethacrylate or trimethylol-propane triacrylate or trimethacrylate. Other monomers which can optionally be used in admixture with the acrylic or methacrylic esters are esters of ~, ~-unsaturated dicarboxylic acids such as maleic or fumaric acids and saturated monohydric alcohols, for example dimethyl maleate, diethyl fumarate, dibutyl maleate, dibutyl fumarate, etc.. Other suitable comonomers are acrylamide or methacrylamide, styrene, 2~642~0 vinyltoluene, ~-methylstyrene, tert.-butylstyrene, vinyl chloride, acrylonitrile, methacrylonitrile or vinyl acetate (component d).

The acrylate resins can be prepared by known polymer-ization processes such as solution, emulsion, bead or bulk polymerization. Particularly preferred acrylate resins are those prepared by solution polymerization or by a bulk polymerization process as described, for example, in EP 56,971.

The acrylate resins have a glass transition temperature of 30-90C. The preferred glass transition temperature is in the range of 30-60C. The molecular weights (number average based on polystyrene standard) are generally 1,000-10,000, preferably 1,000-5,000.

The aliphatic dibasic acids employed in the invention as curing agents - component (B) - are, for example, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, malonic acid, succinic acid, glutaric acid, 1,12-dodecanedioic acid, etc.. The anhydrides of these acids can also be used, for example glutaric anhydride and succinic anhydride as well as the polyanhydrides of these dicarboxylic acids. These polyanhydrides are obtained by intermolecular condensation of the cited aliphatic dibasic dicarboxylic acids. Examples are adipic (poly)anhydride, azelaic (poly)anhydride, sebacic (poly)anhydride, dodecanedioic (poly)anhydride, etc.. The polyanhydrides have a molecular weight (weight average based on polystyrene standard) of 1,000-5,000. The poly-anhydrides can also be modified with polyol, as described in EP 299,420. The polyanhydrides are solids at room temperature. The preparation of t~e polyanhydrides is carried out by reacting the dicarboxylic acids with acetic anhydride at temperatures of 120-200C, preferably 120-170C. In this reaction acetic acid is split off. The removal of the acetic acid can be speeded up by distil-lation in vacuo.

2 ~ ~ ;1 ?~0 The polyanhydrides can also be used as curing agents in a mixture with the aliphatic dibasic dicarboxylic acids or in a mixture with hydroxycarboxylic acids which have a melting point between 40C and 150C, for example 12-hydroxystearic acid, 2- or 3- or 10-hydroxyocta-decanoic acid, 2-hydroxymyristic acid, etc.

The amount of the anhydrides and acids, used as curing agents - component B -, based on the acrylate resin, can vary within a wide range and is governed by the number of the epoxide groups in the acrylate resin. In general, a molar ratio of carboxyl groups or anhydride groups to epoxide groups of 0.4-1.4:1, preferably of 0.8-1.2:1, is chosen.

The powder coating can contain the usual pigments and fillers. In addition, it can also contain a catalyst in order to increase the rate of crosslinking and to lower the curing temperature. Suitabl~ catalysts are tetra-alkylammonium or phosphonium salts, imidazoles, tertiary amines, metal salts of organic carboxylic acids or phosphine. However, in the majority of cases the presence of a catalyst is not necessary.

The powder coating can furthermore contain various additives such as those conventionally used in powder coatings, in partic~lar degassing agents such as benzoin, generally employed in amounts of 0.1-3% by weight~
Furthermore, it is possible to use flow control agents, for example oligomeric poly(meth)acrylates such as polylauryl ~meth)acrylate, polybutyl (meth)acrylate, poly-2-ethylhexyl (meth)acrylate or fluorinated polymers or polysiloxanes. In order to improve the weathering resistance, known UV absorbers and antioxidants can be added.

The components A, B and C of the powder coating are first mixed dry and then extruded using a twin screw extruder at a temperature of 80-130C, preferably 80-100C. After - 6 - 2~ 6~ 0 being cooled and comminuted in a mill, the extrudate is ground, aiming at an average particle size of 20-90 nm, preferably of 40-70 nm. Any oversize particles which may be present can be removed by sieving.

The powder coating is applied using one of the conventional methods, for example by electrostatic spraying or tribospraying. After application the curing is effected at a temperature of 120-200C, the curing temperature preferably being 130-160C.

The powder coating is particularly suitable for use as a clearcoat for aqueous basecoats. 2-coat finishes having exceptional surface smoothness, gloss and resistance to chemicals and weathering are obtained.

~xamples 5 1 Preparation of epoxide qrou~-containin~ acrylate resins 1.1 Resins la-le Solvesso 100 was heated to 150C under nitrogen. The mixture of monomers together with the initiator was then added at a uniform rate at 150C over 7 hours. After the addition, the reaction mixture was kept at 150C for a further 2 hours and the Solvesso was then distilled off, first at 150C and normal pressure, finally at 170C and 18 mbar. A solid, colorless resin was obtained. The formulations and parameters are summarized in Table 1 (parts by weight = pbw):

2 ~ , 9 ~

Table l la lb lc ld le Solvesso 100 15.00 pbw 15.00 p~ 15.00 pbw 15.00 pbw 15.00 pbw methacrylate 31.64 " 31.64 ~ 31.64 ~ 31.64 " 31.64 "
t-butyl acrylate 15.56 " _ 15.56 " _ 7.78 "
Methyl methacrylate 12.80 " 12.80 " 32.80 ~ 32.80 " 32.80 "
Styrene 40.00 " 40.00 " 20.00 " 20.00 ~ 20.00 n-butyl methacrylate _ 15.56 " _ 15.56 " 7.78 Di-tert.-butyl 1.50 " 1.50 ~ 1.50 " 1.50 " 1.50 . , Viscosity tubbelohde~
50% solu-tion in butyl ace-tate, 20C 390 290 190 300 230 mæa.s Glass tempr erature 54 47 45 41 42C
Mw 20,400 13,000 14,500 9,300 9,800 ~ide equivalent weight 500 495 500 480 480 g/mol 1.2 Resins 2a, 2b Di-isopropyl maleate was preheated to 175C. The mixture of monomers together with the initiator was then added at a uniform rate at 175C over 7 hours. The reaction mixture was then kept at this temperature for a further 1 hour and the volatile components (initiator decom-position products) were then distilled off in vacuot18 mbar). A solid, colorless resin was obtained. The formulations and parameters are summarized in Table 2:

2~6~90 Table 2 2a 2b .
Di-isopropyl maleate22.00 g 22.00 g Glycidyl methacrylate 28.50 g 28.50 g Methyl methacrylate 5.00 g t-butyl acrylate _ 5.00 g Styrene 44.00 g 44.00 g Di-tert.-butyl peroxide 0.50 g 0.50 g Viscosity (plate, cone, DF 100 s, 170C) 2,1003,900 mæa.s Glass temperature 45 43C
Mw 6,600 19,000 E~de e~uivalent weight 535 535 2. Preparation of dodecanedioic polyanhydride 69.27 pbw of dodecanedioic acid and 30.73 pbw of acetic anhydride were heated to 150C. In this operation acetic acid was removed by distillation. As soon as no more acetic acid distilled off, the temperature was raised to 170C and more acetic acid was distilied off, first under normal pressure, then in vacuo. The vacuum was controlled in such a way that only acetic acid distilled off and no acetic anhydride. The reaction mixture was then kept for a further 3 hours at 170C/20 mbar and then cooled. The residue had a melting point of 84C.

3. Preparation and testing of the powder clearcoats ~, 737 g of the resins 2a, 2b, 224 g of dodecanedioic polyanhydride (or 726 g of resins la-le and 271 g of dodecanedioic polyanhydride) and 3 g of benzoin were first mixed dry. This mixture was then dispersed in the melt in a laboratory extruder at temperatures of 2~6~ '~'90 g 80-120C. After being cooled and subjected to a preliminary comminution, the extrudate was ground in a blower mill to an average particle size of 50 ~m to form a powder coating. Particles having a size larger than 90 ~m were removed by sieving. Using an electrostatic powder spraygun at 60 kV, the powder coating was sprayed onto degreased, earthed steel panels in such a way that a film thickness of 60 ~m resulted after baking at 140C/30 min. The test results are summarized in Table 3:

Table 3 la lb lc ld le 2a 2b Gel time (140C) 175 195 200 235 230 240 110 s Flow distance at 140C
~DIN 16916a)) 65 80 117 123 136 130 78 mm Gloss (60, DIN 67530) 103 107 106 108 107 102 107 Fl~w-out very very very very very very very good good good good good good good Erichsen indentation (DIN 53156)10.4 10.8 11.9 12.3 11.9 10.0 9.3 m~
Crosshatch (DIN 52151) O 0 0 0 0 0 O
I~pact test (ASTM D 2794;
reverse side) 160 80 160 20 80 20 160 i.p.

a) Amountweighed:0.2 g;substrate:degreasedsteelpanel;l min.
horizontally, then at a 60 incl;n~tian The t-butyl acrylate-containing products exhibit a greater reactivity and a much better impact indentation value than the resins without t-butyl acrylate.

Claims

1. A coating composition in powder form which is composed of (A) a copolymer containing glycidyl groups, (B) an aliphatic or cycloaliphatic dibasic acid, its anhydride or a polyol-modified anhydride of a dibasic acid, (C) optionally, pigments and other additives, the copolymer (A) having a molecular weight (Mn) of 1,000-10,000 and a glass transition temperature of 30-90°C and being a mixed polymer composed of a) at least 10% by weight of glycidyl acrylate or glycidyl methacrylate, b) 5-50% by weight of tert.-butyl acrylate or tert.-butyl methacrylate, c) 20-60% by weight of one or more alkyl esters of aliphatic unsaturated monocarboxylic or dicarboxylic acids and d) 0-50% by weight of one or more other olefinically unsaturated monomers.

2. The coating composition in powder form as claimed in claim 1, the copolymer (A) being composed of 14-43% by weight of glycidyl methacrylate 15-35% by weight of styrene tert.-butyl acrylate or tert.-butyl methacrylate 12-51% by weight of alkyl acrylates or methacrylates and 0-40% by weight of other olefinically unsaturated compounds.

3. The coating composition in powder form as claimed in claim 1, the copolymer (A) being composed of 14-43% by weight of glycidyl methacrylate, 20-40% by weight of styrene, 10-30% by weight of a dialkyl ester of an olefinically unsaturated dicarboxylic acid, 0-36% by weight of alkyl acrylates or methacrylates and 5-20% by weight of tert.-butyl acrylate or tert.-butyl methacrylate.

4. The coating composition in powder form as claimed in claim 1, the copolymer (A) being prepared by solution or bulk polymerization.

5. The coating composition in powder form as claimed in claim 1, the component (B) being a saturated aliphatic dicarboxylic acid having 4 to 12 carbon atoms.

6. The coating composition in powder form as claimed in claim 1, the component (B) being a monomeric anhydride of an aliphatic dicarboxylic acid.

7. The coating composition in powder form as claimed in claim 1, the component (B) being a polyanhydride of an aliphatic dicarboxylic acid.

8. The coating composition in powder form as claimed in claim 1, the component (B) being a reaction product of the polyanhydride of an aliphatic dicarboxylic acid and a polyol.

9. The coating composition in powder form as claimed in claim 1, the component (B) being present in an amount corresponding to 0.4 to 1.4 carboxyl and/or anhydride groups per epoxide groups of the copolymer (A).

10. Method of using the coating composition as claimed in claim 1 for the production of coatings.

11. Method of using the coating composition as claimed in claim 1 as a clearcoat over aqueous basecoats.