CA1141071A - Epoxy resin powder primer compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Epoxy-resin powder primer compositions for metal substrates which have good flow, corrosion resistance, and adhesion to lacquer overcoats are provided by blending two epichlorohydrin-bisphenol-A
epoxy resins with a curing agent. One of the epoxy resins can be extended with phenol to increase its flow properties. The curing agent is a hydroxy-containing mixed aromatic-aliphatic polyether resin.

Description

Epoxy-Resin Powder Primer Compositions BACKGROUND OF THE INVENTION
Field Of The Invention This invention is related to thermoset-ting powder coating compositions and particularly to epoxy-based compositions that are useful to prime substrates in preparation for the application of a topcoat.
Description Of The Prior Art Primers are widely used in preparation for the topcoating of metal or steel substrates to form a smooth surface over which the topcoat is applied. Epoxy-based primer compositions have been used since it is generally known that epoxides give better corrosion resistance and chip resistance than, ~ for example, most alkyd resin primers. Epoxy-based ; primers of the past, however, have not adhered well to lacquer topcoats which are popular as automotive finishes.
With the current emphasis on reduction of solvent emissions, the use of powder-coating compo-sitions, including primers, which contain no solvent, has gained in popularity. Many powder coatings, however, require unacceptably high temperatures, and therefore high energy consumption, to cure. Many of tnose powder formulations that can be cured at lower temperatures produce a coating that can lose adhesion. Therefore, there remains a need for a powder coating composition that not only cures rapidly at commercially-acceptable temperatures but also exhibits good adhesion to lacquer topcoats and resistance to corrosion and chipping.
SUMMARY OF THE I~TVENTION
According to the present invention, there is provided a thermosetting powder coating composition which consists essentially of finely divided particles at least 90 percent by weight of which have a maximum particle size not exceeding 100 microns. The particles are a blend of a com-position of:
(A) 30-45 parts by weight of an epoxy resin of the formula 25_~--C~--O--CH2 ~H~H2 3, where n is sufficiently large to provide a resin having a Gardner-Holdt viscosity of O-S and an epoxide equivalent weight of 700-850;
(B) 18-40 parts by weight of (1) an epoxy resin of the general formula of (A) having a Gardner-Holdt viscosity of H-l, and an "~

:~ epoxide equivalent weight of 575-700, or (2) a phenol-modified epoxy resin which is a resin of the general formula of (A) which has been modified with bisphenol-A
and phenol to provide a phenol-modified epoxy resin having an epoxide equivalent weight of 550-675, or mixtures of (1) and (2);
(C) 15-30 parts by weight of a resinous curing :~ lO agent consisting essentially of:
: (1) a mixture of compounds of the general formula .
~ ~ 15 ~0-Ar- ~ 0-CH2 CH-C~2-0-Ar ~ -~H
- aH .. '~t-.~ , . , . where Ar- ~ C ~ .
: , - - - . 'C~

and where x is O or a positive number providing a mixture of compounds having ~ an e~uivalent weight of 230-1000, - and : (2) 0.2~5.0 percent by weight, based on the : weight of (1), of an accelerator which is a mono or diaIkyl imidazole, dialkyl amino methyl phenol, or mixtures of these;
(D) 0.3-2.5 parts by weight, per 100 parts of (A),(B)3 and (C), of a tertiary amine; and (E) 0.1-5.0 parts by weight, per 100 parts of (A),(3), ~ 35 and (C), of a flow control agent selected ~rom ; 3 7~
-the group consisting of polyalkyl acrylates polyalkyl methacrylates, fluorinated esters of polyethylene glycol~ fluorinated esters of polypropylene glycol r ethyl cellulose, silicones, and mixtures of these.
In a preferred embodiment, the present invention provides a thermosetting powder coating composition wherein there are 38-40 parts by weight of (A~i 25-27 parts by weight of (B); 19-21 parts by weight of (C);
the accelerator is 2~-methylimidazole and the tertiary amine is triethanol amine, and in which (B) is a phenol-modified epoxy resin of epoxide equivalent weight of 590-630 which is the reaction product of an epichlorohydrin-bisphenol-A epoxy resin of epoxide equivalent weight 15 180-192, bisphenol-A, and phenol in an epoxy-resin/
: bisphenol-A/phenol equivalent weight ratio of 1.82/1.0/0.5.
In another embodiment, the present invention provides a thermosetting powder coating composition of finely-divided particles at least 90 percent by weight of which have a maximum particle size not exceeding 100 microns wherein the particles of a blend consisting essentially of:
(~) 38-40 parts by weight of an epoxy resin of the formula CH2-CH-C~12- ~ ~ CH3 O-CH2-CH-CH2- ¦ -O-. ~ -CH2 - CH - CH2 where n is sufficiently large to provide a resin having a Gardner-Holdt viscosity of O-S and an epoxide equivalent weight of 700-850;

.,, ,;,. . .

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4a (B) 25-57 parts by weight of a phenol-modified epoxy resin of epoxide equivalent weight 590-630 which is the reaction product of an epichlorohydrin-bisphenol-A epoxy resin of epoxide equivalent weight 180-192, bisphenol-A, and phenol in an epoxy-resin/bisphenol-A/phenol equivalent weight ratio of 1.82/1.0/0.5;
(C) 19-21 parts by weight of a resinous curing agent consisting essentially of ~1) a mixture of compounds of the general formula Ho-Ar-(-o-cH2-cH-cH2-o-Ar-) -OH
OH

Where Ar= ~ , ~

and where x is O or a positive number, providing a mixture of compounds with an equivalent weight of 230-1000, and

(2) 0.5-0.7 percent by weight, based on the weight of (l), of 2-methylimidazole;
(D) 0.4-0.8 parts by weight of a flow control agent selected from the group consisting of fluorinated polyethylene glycol esters, fluorinated polypropy-lene glycol esters, and mixtures of these;
(E) 1-2 parts by weight of triethanol amine; and (F) 1-3 parts by weight of filler particles selected from the group consisting of talc, silica, barium sulfate, calcium carbonate, mica, and mixtures of these~
DESCRIPTION OF THE INVENTION
The coating composition of this invention is in the form of finely-divided powder particles at least 90 percent by weight of which do not exceed 100 microns in maximum dimension and preferably do not exceed 80 microns. In one particularly preferred formulation, the particles all have been passed through a standard 200-mesh screen (75 microns).

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4b Exclusive of pigments and other nonreactive components, the film-forming components of the present invention are epoxy resins and a hydroxy containing mixed aromatic-aliphatic polyether curing agent.
About 30-45 parts by weight, and pre-ferably 35-40 parts by weight, of the powder coating composition are an epoxy resin which is of the epichlorohydrin-bisphenol-A kind, of the formula / \ CH OH
CH2 -CH-CH2 - -0~ C ~ ' -O-~ CH3 J
CH3 ~\
~, ~ O--CH2 -CH--CH2 where n is sufficiently large to provide a Gardner-Holdt viscosity of-O-S and an epoxide equivalent weight of 700-850. As used herein, the epoxide equivalent weight is the weight in grams of epoxy resin that contains one gram equivalent of epoxide, and the Gardner-Holdt viscosity is neasured at 40 percent polymer solids in diethylene glycol mono-butyl ether at 25C.
Other epoxy resins constitute 18-40 parts by weight, preferably 24-36 parts by weight of the present invention. Thesé resins can be either (1) an epichlorohydrin bisphenol-A resin of the general formula shown above having a Gardner-Holdt viscosity of H-L and an epoxide equivalent weight of 575-700, or (2) a phenol-modified epoxy resin, having an epoxide equivalent weight of 550-575, which is an epoxy resin of the epichlorohydrin-bisphenol-A
kind which has been further reacted with phenol and bisphenol-A. Mixtures of these two resins can also be used.
In the production of phenol-modified epoxy resins, by varying the epoxide equivalent weight of the initial epichlorohydrin-bisphenol-A
epoxy resin and the weight ratios of the reactants, the viscosity, epoxide equivalent weight, and reactivity of the resultant resin can be varied.
A preferred phenol-modified epoxy resin has an epoxide equivalent weight of 550-675 and a viscosity (ASTM-D445) of 3500-7000 centistokes at 120C.
One such preferred resin is prepared by reacting an epichlorohydrin-bisphenol-A epoxy resin, having an epoxide equivalent weight of 180-192, with bisphenol-A and phenol in an epoxy-resin~bisphenol-A/

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phenol equivalent-weight ratio of 1.82/1.0/0.5 to provide a phenol-modified epoxy resin having an epoxide equivalent weight of 590-630.
In addition to the epoxy resins described above, another film-forming component of the present coating composition is a resinous curing agent. About 15-30 parts by weight of the curing agent are used in the composition. The curing agent consists essentially of:
1. a mixture of compounds of the general formula .
HO-.~.r- ~o-~H2-c~-cH2-o-Ar~ ~oH

' C~i ' uhere Ar= ~ C
.. 'C~

and x is 0 or a positive number, pro- -viding a mixture of compounds having an equivalent weight of 230-1000;
and 2. 0.1-5.0 percent by weight, based on the weight of 1, of an ac~elerator which is a mono or di-alkyl imidazole, a dialkyl amino methyl phenol, or mixtures of these.
Preferred imidazoles for use in the curing agent are those in which the alkyl groups have 1-4 carbon atoms. Most preferred are methyl and ethyl-~l~tl~37 imidazole. Also preferred for use in the curing agentare dimethyl amino methyl phenol and tris-dimethyl amino methyl phenol.
The composition also includes flow control agents in an amount ranging from 0.1-5.0% by weight, based on the weight of the film-forming components.
In general, the flow control agent should be a polymer having a number-average molecular weight of about 1000-20,090.
Typically useful flow control agents are polyalkyl acrylates and methacrylates (a suitable example of which is ~odaflow~
available from Monsanto Company), and fluorinated polymers such as esters of polyethylene glycol or polypropylene glycol (a suitable example of ~hich is Fluorad~, available from the 3M Company). Ethyl 1~ cellulose or silicones such as dimethyl polysiloxane or methyl phenyl polysiloxane can also be used.
About 0.3-2.5% by weight, based on the weight of the film-forming components, of the compo-sition is a tertiary amine of the general formula Rl wherein Rl, R2, and R3 are the same or different alkyl or hydroxyalkyl radicals having 1-6 carbon atoms. The presence of the tertiary amine is especially useful to make the coating resistant to chemical attack by llue gas in instances when the coating is cured in a gas-fired oven.
To improve sandlng properties of the coating, the composition can contain from 0.01-60% by weight ? based on the weight of the film-for~ing constituents, of ~iller particles. Conventional 35 * d~no~e~ trade mar~

particulate fillers that can be used are talc, silica, barium sulfate, calcium carbonate, aluminum silicate, mica, and the like. Preferably, the filler particles have a maximum dimension of 20 microns.
The composition can be pigmented, contain- -ing up to 100% by weight, based on the weight of the film-forming componentsJof pigment. Any conventional organic or inorganic pigments can be used. Examples are carbon black; metallic oxides such as titanium dioxide, zinc oxide, iron oxide, and the like; metallic powder; and metal hydroxides.
One method for forming the powder coating composition is to blend the components together and then to pass the mixture through a conventional melt-extruder. The extrudate is cooled and can then bereduced to a powder using conventional grinding equipment. After grinding, the powder is passed through a standard mesh screen to remove large par-ticles. Preferably, a sieve that eliminates par-ticles having maximum particle size greater than100 microns is used.
The powder primer of this invention can be applied directly to a metal or steel substrate or to one which has been pre-primed with, for example, a conventional cathodic or anodic electrodeposition primer. Application can be by using electrostatic spraying techniques or by using a fluidized bed which can be electrostatic. The preferable method is electrostatic spraying in which, for example, a voltage of 20-100 kilovolts is applied to the spray gun. The composition can be applied either in one pass or in several passes to provide variable thick-nesses, after cure, of 20-100 microns, pre~erably 20-50 microns.
The substrate to be coated can, optionally, be heated to any temperature up to 175C prior to the application of 7~

the powder. P-reheating the article provides better powder deposition and allows a more uniform coating After the application of the powder, the po~der-covered article is heated a~ 135-210C for 5-45 minutes to fuse and to cure the powder particlesinto a substantially continuous, uniform film. The quality of the film might be affected by the tempera-ture of the cure and the percentage of the accelerator present in the curing agent in that curing temperatures in the high end of the indicated range can sometimes reduce the flow properties of the coating when the weight percentage of accelerator in the curing agent exceeds 1.6~.
The powder coating composition of this in-vention is generally useful as a high quality surface-treatment for substrates of various kinds, but is particularly useful as a primer for automotive bodies - since it provides excellent corrosion resistance and interlayer adhesion between the substrate and topcoat.
The enhanced interlayer adhesion is especially evident when the topcoat is an acrylic solution or dispersion lacquer, or a powder enamel. Epoxy primers, which generally adhere well to liquid enamel topcoats, have'neretofore exhibited norn~ally unacceptable adhesion to lacquer and powder enamel topcoats.
Before application of the topcoat, the primer coating can be s&~ded, but this is not necessary. One of the benefits of the present invention is tnat no sanding is re-quired to promote adhesion to the topcoat.
The following examples illustrate the invention.
In the examples, the components will be referred to according to -the following numbering system:
(1) Epoxy resin having the formula , C~2-CR-CH2 ~0~ CH3 O-C:' -,~'.'~) O-CH3 n _~_ C ~.O-~H?-6~-CH2 . 3 where n is sufficiently large to provide a resin having a Gardner-Holdt viscosity of O-S and an epoxide equivalent weight of 700-850.
(2) Phenol-modified epoxy resin which is an epoxy resin (epoxide equivalent weight 186-192) of the formula of (1) above reacted with bisphenol-A
and phenol in an epoxy-resin/bis-phenol-A/phenol equivalent-weight ratio of 1.82/1.0/0.5 to provide a phenol-modif'ied epoxy resin having an epoxide equivalent weight of 590-630.
(3) Epoxy resin of the general formu7;a of (1) above having a Cardner-Xoldt viscosity of H-L and an epoxide equivalent weight of 575-700.

(4) Curing agent which is a combination of:
(a) a mixture of compounds of the general formula ' ~0-Ar-~-O-CH2-CH-CH2-0-Ar-)-OH
OH
: ~ C~3 where Ar= ~ t C~3 3~

and x is 0 or a positive num.ber, providing a mixture of compounds with an equivalent weight ol ?30-1000; and (b) 0.67p by weight, based on the wei~ht of (a), of 2-methylimidazole.
Example 1 The following components are blended 2S
described:
10 Portion 1 Parts by Weig.~t .
Component 1 38.4 Component 2 25.6 Component 4 20.7 "Fluorad~'(fiow control agent)0.6 15 Triethanol amine 1.5 ~ortion 2 Talc p2rticles, mzximum diameter of 20 microns 2.5 20 Titanium dioxide pigment 11.5 Carbon black 0.5 Portion 1 is char5ed into a Welex*grinder-mixer and mixed for 30 seconds. Portion 2 is then added and mixing is continued for one minute. This blend is then charsed into a melt extruder and exvruaed at 88-94C. The extrudate is chilled, broken into chips, and then charged into 2 grinding mi~ll where it is ground to a fine powder. The powder is then passed through a standard 200-mesh screen to remove particles of maximum dimension greater than 75 microns.
The powder is sprayed as a primer onto 20-gauge Bonderite*phosphatized panels using Ransbur~
electrostatic powder guns, and he coated panels are then heated ir. a gas-fired oven for approxim2tely 45 * denotes trade mark .~ .

!
~, ~ 12 minutes at 160-170C. The resultant cured lilm has 2 thic~ness of 25-35 microns and a ~ukon .~ardness OL
abGut 16 knoops.
The chip resistance of the primer film on the panels exceeds commercially-acceptable standards when measured at -18C using 2 QGR Gravelo-meter*available from the C Panel Company of Cleveland, Ohio.
Several of the primer-coated panels are scored to the metal with a nail and placed in a salt-spray cabinet where they are exposed to a mist of a solution of NaCl(5% by weight) in water for 336 hours at 38C. After this time,the panels are removed, dried, and inspected. There is no face corrosion on the primer film and no creepage ol ~he film from the score line.
Several more of the primer-coated panels are top-coated with a conventional white-pigmented automotive acrylic lac~uer finish. Both solution and dispersion lacquers are used. After the top-co2t is baked, providing a top-coat film of approximately 60-micron thickness, the coatings of these panels are tested for chip resistance as described above. The chip resistance exceeds commercially-acceptable standards, indicating excellent flexibility and inter-layer adhesion provided by the primer film. These same panels are then subjected to the salt~sp~ay test as described 2bove, after ~hich the chip resistance of the coztings of these panels is again tested. There 30 is no decline in the chip resistance.
xample 2 The following co~ponents are blended 2S
described:
Parts by Weight 35 Component 1 ~0 Com~onent 3 35 * denotes trade mark r~.~ "~
1:~ .

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Component 4 24.4 r~odaflow~ - o.6 Triethanol amine 1.5 Talc particles, maximum 3.0 diameter 20 ~icrons Titanium dioxide pigment 13.5 Carbon black 0.6 The components are charged into a Welex grinder-mixer and mixed for 60 seconds. The mix is 10 then charged into a melt extruder and extruded at 88-94C. The extrudate is chilled, broken into chips, and then char~ed into a grinding mill where it is ground to a fine powder. The powder is then passed through a standard 270-mesh screen to 15 remove particles larger than about 50 microns.
As in Example 1, the powder is sprayed onto 20-gauge Bonderite phosphotized panels and baked to yield a cured film with a thickness of 25-35 microns and a Tukon Hardness of about 16 knoops.
Testing as in Example 1 would expectedly show excellent corrosion resistance and commercially-~accep~table chip resista~nce.
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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A thermosetting powder coating compo-sition of finely-divided particles at least 90 percent by weight of which have a maximum particle size not exceeding 100 microns wherein the particles are a blend consisting essentially of (A) 30-45 parts by weight of an epoxy resin of the formula where n is sufficiently large to provide a resin having a Garaner-Holdt viscosity of 0-S and an epoxide equivalent weight of 700-850;
(B) 18-40 parts by weight of a phenol-modified epoxy resin of epoxide equivalent weight of 590-630 which is the reaction product of an epichloro-hydrin-bisphenol-A epoxy resin of epoxide equi-valent weight 180-192, bisphenol-A, and phenol in an epoxy-resin/bisphenol-A/phenol equivalent weight ratio of 1.82/1.0/0.5, (C) 15-30 parts by weight of a resinous curing agent consisting essentially of:
(1) a mixture of compounds of the general formula and where x is O or a positive number providing a mixture of compounds having an equivalent weight of 230-1000, and (2) 0.2-5.0 percent by weight, based on the weight of (l), of an accelerator which is a mono or dialkyl imidazole, dialkyl amino methyl phenol, or mixtures of these:
(D) 0.3-2.5 parts by weight, per 100 parts of (A), (B), and (C), of a tertiary amine of the general formula wherein R1, R2, and R3 are the same or different alkyl or hydroxyalkyl radicals having 1-6 carbon atoms; and (E) 0.1-5.0 parts by weight, per 100 parts of (A), (B), and (C), of a flow control agent selected from the group consisting of polyalkyl acrylates, polyalkyl methacrylates, flourinated esters of polyethylene glycol, fluorinated esters of polypropylene glycol, ethyl cellulose, silicones, and mixtures of these.

2. The powder coating composition of Claim 1 wherein there are 35-40 parts by weight of (A), and 24-36 parts by weight of (B).

3. The powder coating composition of Claim 2 wherein: there are 38-40 parts by weight of (A);
25-27 parts by weight of (B); 19-21 parts by weight of (C); the accelerator is 2-methylimidazole; and the tertiary amine is triethanol amine.

4. The powder coating composition of any one of Claim l, Claim 2 and Claim 3 additionally containing pigment in an amount up to 100 percent of the weight of (A), (B), and (C).

5. The powder coating composition of any one of Claim l, Claim 2 and Claim 3 additionally containing from 0.01-60 percent by weight, based on the weight of (A), (B), and (C), of filler particles.

6. A substrate coated with the composition of any one of Claim 1, Claim 2 and Claim 3.

7. A thermosetting powder coating composition of finely-divided particles at least 90 percent by weight of which have a maximum particle size not exceeding 100 microns wherein the particles of a blend consisting essentially of:
(A) 38-40 parts by weight of an epoxy resin of the formula where n is sufficiently large to provide a resin having a Gardner-Holdt viscosity of O-S and an epoxide equivalent weight of 700-850;
(B) 25-57 parts by weight of a phenol-modified epoxy resin of epoxide equivalent weight 590-630 which is the reaction product of an epichlorohydrin-bisphenol-A epoxy resin of epoxide equivalent weight 180-192, bisphenol-A, and phenol in an epoxy-resin/bisphenol-A/phenol equivalent weight ratio of 1.82/1.0/0.5;
(C) 19-21 parts by weight of a resinous curing agent consisting essentially of (1) a mixture of compounds of the general formula Where Ar= and where x is O or a positive number, providing a mixture of compounds with an equivalent weight of 230-1000, and (2) 0.5-0.7 percent by weight, based on the weight of (1), of 2-methylimidazole;
(D) 0.4-0.8 parts by weight of a flow control agent selected from the group consisting of fluorinated polyethylene glycol esters, fluorinated polypropy-lene glycol esters, and mixtures of these;
(E) 1-2 parts by weight of triethanol amine; and (F) 1-3 parts by weight of filler particles selected from the group consisting of talc, silica, barium sulfate, calcium carbonate, mica, and mixtures of these.

8. The powder coating composition of Claim 9 additionally containing pigment in an amount up to 100 percent of the weight of (A), (B), and (C).

9. A substrate coated with the composition of Claim 7 or Claim 8.