CA2064292A1

CA2064292A1 - Coating compositions in powder form

Info

Publication number: CA2064292A1
Application number: CA002064292A
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: EP0509393B1; EP0509393A1; DE4112687A1; JPH05112743A; ES2099177T3; ATE148150T1; DE59207917D1

Abstract

HOECHST AKTIENGESELLSCHAFT - Werk KALLE-ALBERT

91/F 114 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 tMn) of 1,000-10,000 and a glass transition temperature of 30-90°C and is a mixed polymer composed of a) at least 20% by weight of glycidyl acrylate or glycidyl methacrylate, b) 35-50% by weight of styrene, c) 10-45% 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

23~4~2 HOECHST AKTIENGESELLSCHAFT - Werk KALLE-AL~ERT

91/F 114 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 3,787,52~, 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.

The copolymers described in the above patents contain up to 30% by weight of glycidyl acrylate or glycidyl meth-acrylate only; the remainder of the 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 alkyl esters of ali-phatic unsaturated monocarboxylic and dicarboxylic acids.
Catalysts are required for the curing of the powder coating in each of the examples of prior art cited above in which the copolymers used contain styrene. The catalyst used is exclusively tetraalkylammonium bromide.
However, salts of this type considerably impair the resistance of the coatings to water, acids and alkalis.
In addition, according to the examples of prior art 2~6-~92 relat~vely high temperatures (above 140C) are necessary in the curing of powder coatings based on copolymers containing ~tyrene in most cases. 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 automotive finishing, it is advantageous that the coating compositions in powder form cure at the lowest possible temperatures, for example as low as 120C.

Accordingly, 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 cata-lysts to furnish coatings having satisfactory properties.
Surprisingly, this object can be achieved if the epoxide group-containing acrylate copolymer has a certain styrene content. Why the reactivity of the acrylate copolymers should depend on the styrene 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 20% by weight o glycidyl acrylate or glycidyl methacrylate, b) 35-50% by weight of styrene, c) 20-45% by weight of one or more alkyl esters o~
aliphatic unsaturated monocarboxylic or dicarboxylic 2~2~2 acids and d) 0-50% by weight of one or more other olefinically unsaturated monomers.

Copolymers A having the following composition:
14-43% by weight of glycidyl methacrylate, 15-45% by weight of styrene, 12-51~ by weight of alkyl acrylates or methacrylates and copolymers having the following composition:

14-43% by weight of glycidyl methacrylate, 40-50% 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 are preferred.

Suitable alkyl esters of unsaturated carboxylic acids are those 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 trimethylolpropane 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 alco-hols, for example dimethyl maleate, diethyl fumarate, dibutyl maleate, dibutyl fumarate, etc.. Other suitable comonomers are acrylamide or methacrylamide, styrene, ` _ 4 _ 2~642~2 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 i~
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 the 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.

2~6~292 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.

The amount of the anhydrides and acids, used as curing agents - component tB) -, 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. Suitable 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 particular 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 W 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 2~29~

being cooled and comminuted in a mill, the extrudate i5 ground, aiming at an average particle size of 20-90 nm, prefe~ably 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.

Examples 1. Preparation of dodecanedioic polyanhydride 69.27 parts by weight of dodecanedioic acid and 30.73 parts by weight 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 distilled 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 about 84C.

2.1 Preparation of epoxide group-containing acrylate resins Solvesso 100 was heated to 150C under nitrogen. ~he 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, 29~ 92 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):

Table 1 la lb lc ld Solvesso 10015.00 pbw 15.00 pbw 15.00 pbw 15.00 pbw Glycidyl methacrylate 31.64 " 31.64 " 31.64 " 16.00 "
t-butyl methacrylate 15.56 " 15.56 ~' 15.56 " 15.56 "
Methyl methacrylate 42.80 " 32.80 " 12.80 " 28.44 "
Styrene 10.00 " 20.00 ~' 40.00 " 40.00 "
Di-tert.-butyl peroxide 1.50 " 1.50 " 1.50 " 1.50 "
Viscosity (U~belohde, 50%
solution in butyl acetate, 20C 460 300 390 490 mæa.s Gaass temperature 44 41 54 48C
Mw 10,000 9,300 20,400 17,000 E~oxide equivalent weight 490 500 500 825 g/mol 2.2 Preparation and testing of the powder clearcoats 737 parts by weight of the resins la-lc, 260 parts by weight of dodecanedioic polyanhydride (or 773 parts by weight of resin ld and 224 parts by weight of dodecanedioic polyanhydride) and 3 parts by weight of benzoin were first mixed dry. This mixture was then dispersed in the melt in a laboratory extruder at temper-atures of 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 um to form 2a6~

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 2:

Table 2 la lb lc ld Gel time (140C) 450 235 195 245 s Flow distance at 140C (DIN 16916a)) 125 123 80 70 mm -Gloss (60, DIN 67530) 104 108 104 105 Flow-out gcod very very good gcod gcod Erichsen indentation (DIN 53156) 13.0 11.9 10.8 11.3 mm Crosshatch test (DIN 52151) 0 0 0 ~mpact test (AS~M D 2794;
reverse side~ 20-40 20-40 80 ~ 4 i.p.
a) kmcuntweighed:0.2 g;substrate:d~ sedsteelp~nel;l min.
horizo~ly, then at a 60 inclination With a low styrene content reactivity is low and the Lmpact indentation test yields poor values. The resin ld with a glycidyl methacrylate content of below 20% by weight has an entirely inadequate impact indentation value. Furthermore, in the case of resins with a low styrene content the flow distance at 140C is so long that fat edges and drips can form at the lower edge of the panel during vertical baking of the painted panels.

9 2~292 3.1 Preparation of epoxide group-containing acrylate resins:

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 vacuo (18 mbar). A solid, colorless resin was obtained. The formulations and parameters are summarized in Table 3 (parts by weight: pbw):

Table 3 2a 2b 2c Di-isopropyl maleate26.89 pbw 25.00 pbw 15.00 pbw Glycidyl methacrylate 28.29 " 28.29 " 2~.29 "
Nethyl methacrylate0.70 "5.66 " 31.88 ' Styrene 43.62 " 40.55 " 24.33 "
Di-tert.-butyl peroxide 0.50 i 0.50 0.50 Viscosity (plate, cone, DC 100 s, 170C) 880 7~0810 mæa.s Viscosity (U~belohde 50% solution in xylene, Glass tenperature 36 33 25C
Mw 4,800 4,9905,000 Epoxide e~uiv21ent weight 535 535 535 g/mol 3.2 Preparation and testing of the powder clearcoats:

Powder coatings were prepared, and applied, from 737 parts by weight of the resins 2a-2d, 224 parts by weight of dodecanedioic polyanhydride and 3 parts by weight of benzoin, as described in 1.2. A film thickness of 60 nm resulted after baking. The test results are summarized in Table 4.

-10- 2a~ s2 Table 4 2a 2b 2c Gel time (140C) 185 225 330 s Flow distance at 140C (DIN 16916a~) 158 193 204 mm 92king conditions 30 min./130C
Gloss (60, DIN 67530) 106 112 98%
Flow-out very very very good gDod g~od Erichsen indentation (DIN 53156) 12.43 12.7 12.4 mm Crosshatch test (DIN 52151) 0 0 0 I~pact test (AS~M D 2794;
reverse side) 80 20 < 4 i.p.
EaXing condi~ions 30 mIn./140C
Gloss (60, DIN 67530) 109 104 107%
Flcw-out very vçry very good good good Erichsen indentation (DIN 53156) 12.0 11.9 12.5 mm Crosshatch test (DIN 52151) 0 0 O
Impact test (AS~M D 2794;
reverse side) 160 160 20 i.p.
a) AmLunt weighed: 0.2 g; substrate: degreasedsteel panel; 1 mun.
horizontally, then at a 60 inclination It can be clearly seen that reactivity increases with increasing styrene content. The same order of reactivity is obtained if dodecanedioic acid is used as curing agent instead of dodecanedioic polyanhydride.

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 20% by weight of glycidyl acrylate or glycidyl methacrylate, b) 35-50% by weight of styrene, c) 10-45% 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 a) 20-50% by weight of glycidyl methacrylate b) 35-50% by weight of styrene and c) 12-45% by weight of alkyl acrylates or methacrylates.

3. The coating composition in powder form as claimed in claim 1, the copolymer (A) being composed of a) 20-43% by weight of glycidyl methacrylate, b) 40-50% by weight of styrene, c) 10-30% by weight of a dialkyl ester of olefinically unsaturated dicarboxylic acid d) 0-35% by weight of alkyl acrylates or methacrylates.

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.