CA2010525C

CA2010525C - Two-phase cathodic electrocoat

Info

Publication number: CA2010525C
Application number: CA002010525A
Authority: CA
Inventors: Thomas Schwerzel; Hans Schupp; Klaus Huemke; Ulrich Heimann
Original assignee: BASF Lacke und Farben AG
Current assignee: BASF Farben und Fasern AG
Priority date: 1989-02-28
Filing date: 1990-02-21
Publication date: 1999-12-14
Anticipated expiration: 2010-02-21
Also published as: JP2975038B2; AU620697B2; DE59006089D1; KR0156225B1; CN1025043C; ES2056268T3; ZA901465B; EP0385292A3; CN1045272A; CA2010525A1; JPH0320371A; EP0385292B1; DE3906145A1; EP0385292A2; ATE107336T1; AU5050090A

Abstract

A two-phase cathodic electrocoat is obtainable by coating an electrically conducting substrate with a mixture containing A) an aqueous cationic dispersion of a polycondensation or polyaddition product, B) an aqueous cationic dispersion of a polycondensation or polyaddition product having a glass transition temperature of from -80 to 20°C, the polymers of dispersions (A) and (B) being mutually incompatible, and C) optionally a crosslinking agent, a pigment paste, auxiliaries and/or additives, and then baking.

Description

Two-phase cathodic electrocoat The present invention relates to a two-phase cathodic electrocoat obtainable by depositing a mixture of two aqueous cationic dispersions of mutually incom-patible polymers on an electrically conductive substrate and then baking' .
Many of the present-day cathodic coating systems take the form of a dispersion comprising an ionically charged mixture: of an organic binder and a crosslinking agent. Instead of this resin mixture it is of course also possible to use a self-crosslinking ionically charged resin. The binders for cathodic electrocoating composi-tions are usually basic nitrogen resins converted into dispersible polymers by protonation with an acid. Resins which have su:lfonium or phosphonium groups are also known. Besides these basicity characteristics, the resin must also have functional groups which are capable of reacting with the crosslinking component. Examples of such groups are acidic hydrogen atoms, eg. NH and OH
groups, unsaturated olefin_c double bonds, dienophiles and dienes . The resins can be prepared by polymerizing or copolymerizing unsaturated compounds which carry at least Partly basic atoms. A frequent choice are polyaddition compounds formed by reacting epoxy resins with compounds which are reactive toward oxirane groups. The basic groups are frequently introduced by using primary, secondary or tertiary amines or permanent charges in the form of quaternary ammonium salts. Furthermore, the epoxy resins thus modified are reacted with flexibilizing compounds.
The addition of dispersed polymers has also been described. For instance, EP-A-70 550 published on 26 January 1983 (MORIARITY et al.) describes dispersions obtained from reaction products of polyepoxies with polyoxyalkylenE~polyamines. These reaction products confer good flow and higher flexibility on the coating composi-tions and also a higher cratering resistance. WO 86/05196 published in x.986 describes a reaction product of a polyoxyalkylenepolyamine with a monoepoxy for use in cathodic electrocoating.
EP-A-253 404 published on 20 January 1998 McIntyre et al" describes a mixture of a cationic epoxy resin formed from a reaction product of a polyol diglycidyl ether with at least one bifunctional phenol and an optional blocking reagent with subsequent conversion of some excess oxirane groups into cationic groups and a further epoxy-based cathodic electrocoating resin. The advantage of these mixtures is that they give full control of the paint film thickness, in particular in relation to high-build systems.
It is <~ feature common to all prior art systems that the flexibilizing component which increases the elasticity of the paint film also reduces the glass transition temperature of the baked paint film. This has a disadvantageous effect on the temperature-dependent properties. For instance, corrosion protection is much impaired, in particular following a severe cyclical exposure test, for example the cyclical exposure test of VDA 621-415, which is carried out at a maximum tempera-ture of 400C, or the GM scab test (Fisher Body Division TM 54-26), which is carried out at a maximum temperature of 60°C.
It is an object of the present invention to develop coatings which ensure good corrosion protection in cyclical exposure tests, even at elevated temperature, but which also are highly elastic.
,' i It has been found that this object is achieved~by coat with a two-phase structure.
The present invention as broadly disclosed hereinafter provides a two-phase cathodic electrocoat obtainable by depositing a mixture containing:
A) an aqueous cationic dispersion of a polycondensa-tion or polyaddition product, B) an aqueous cationic dispersion of a polycondensa tion or polyad~~ition product having a glass transition temperature of from -80 to 20°C, the polymers of dispersions (A) and (B) being mutually incompatible, and C) optionally a crosslinking agent, a pigment paste, auxiliaries and./or additives, on an electrically conduc-tive substrate and then baking.
However, the invention as claimed is restricted to a two-phase cathodic electrocoat obtained by depositing a mixture comprising:
A) 10-90~ by weight of an aqueous cationic dispersion of an aminoepoxy resin having a glass transition temperature of from 20 to 100°C an average molecular weight of from 200 to 20.000 and an amine number of from 30 to 150.
B) 10-50~ by weight of an aqueous cationic dispersion of an aminoepoxy resin which is obtained by reacting a butadiene acrylonitrile copolymer which contains primary and/or secondary amino groups with an epoxy resin having an average molecular weight of from:
40 to 10,000 and containing on average from 1.5 to 3 epoxy groups p~sr molecule, the aminoepoxy resin having a glass transition temperature from 80 to 20°C and the polymers of dispersions A) and B) being mutually incompatible, and C) 0-50'~ by weight of a crosslinking agent, a pigment paste, auxiliaries and/or additives, on a electrically conductive substrate and then baking.
As afores,~id component (A) is an aqueous dispersion of a polyconde~nsation or polyaddition product. This product may pos:cess hydrophilic groups such as quaternary ammonium groups and sulfonium or phosphonium groups, rendering it water-soluble or water-dispersible. Advanta-geously, however, it is possible to use the art recog-nized polycondE~nsation or polyaddition products which contain primary and/or secondary hydroxyl groups and primary, secondary and/or tertiary amino groups and have an average molecular weight of from 200 to 20,000, such as aminoepoxy resins, aminopoly(meth)acrylate resins and/or aminopolyurethane resins. The use of aminoepoxy resins is preferred for basecoats intended to confer a high level of corrosion protection. The amino resins advantageously have amine numbers of from 30 to 150. The lower limit of the amine number should be 35, preferably 50, and the upper limit should be 120, preferably 100.
Examples of aminoepoxy resins are reaction products of epoxy-containing resins having preferably terminal epoxy groups with saturated and/or unsaturated secondary and/or primary amines ~~r aminoalcohols. These reaction products can be modified at the alkyl moiety by at least one primary and/or secondary hydroxyl group, by a mono- or dialkylamino group and/or by a primary amino group which is at least temporarily protected by ketiminization.
The epe~xy resins used can be any desired mate-rials provided they have an average molecular weight of from 300 to 6000 and contain on average from 1.0 to 3.0 epoxy groups per molecule, preferably compounds having 2 epoxy groups per molecule. Preference is given to epoxy resins having average molecular weights of from 350 to A

4a 5000, in part:Lcular from 350 to 2000. Particularly preferred epoxy resins are for example glycidyl ethers of polyphenols which on average contain at least two phen-olic hydroxyl groups in the molecule and which are preparable in a conventional manner by etherification with an epihalo:hydrin in the presence of alkali. Aromatic polyepoxies having a higher epoxy equivalent weight can be prepared from those having a lower epoxy equivalent weight and pol~~phenols or else by a suitable choice of the ratio of ph,enolic OH groups to epihalohydrin.
The amino groups can be introduced in a conven-tional reaction as known to those skilled in the art and as described for example in EP-A-134 983, published on 27 March 1985 (S~UMPP et al.), EP-A-165 556 published 27 December 1986 (KEMPER et al.) and EP-A-166 314 published 2 January 1986 (KEMPTER et al.).
By introducing groups which are capable of crosslinking, e.g. blocked isocyanates, it is possible to render the amino resins self-crosslinking, as described for example in US-A-4 692 503 published 8 September 1966 (SURYYA et al.), US-A-3 935 087 published 27 January 1976 (JERABEK et al.. ) and EP-A-273 247 published 6 July 1988 (KARAIN et al.).
The gla:~s transition temperatures of the amino resins are advantageously within the range from 20 to 100°C, preferably from 20 to 80°C, particularly prefer-ably from 25 to 45°C.
The amino resins can be converted into a water-soluble or wager-dispersible form by protonation with an acid. A suitable acid is phosphoric acid, but it is 4b 2 0 1 0 5 2 5 preferable to use an organic acid, e.g. formic acid, acetic acid, propionic acid or lactic acid. It is also possible to ad.d the resin to a water/acid mixture. In general, the dispersions used have a solids content of from 20 to 45$ by weight, preferably from 30 to 40 o by weight.
As aforesaid, component (B) is an aqueous disper-sion of a poly~~ondensation or polyaddition product which has a glass transition temperature of from -80 to 20°C, preferably frorl -70 to 0°C, particularly preferably from -60 to -10°C, and is incompatible with the polymer of dispersion (A). The word "incompatible" means that a ,Y

- 5 - 0.2. 0062/02083 mixture of tlhe polymers of dispersions (A) and (B) will separate and form two phases.
Suitable polyaddition products are for example the reaction products of butadiene/acrylonitrile copoly mers which contain primary and/or secondary amino groups with epoxy resins. Suitable butadiene/acrylonitrile copolymers have acrylonitrile contents of from 5 to 45%
by weight, preferably from 10 to 30% by weight, and butadiene contents of from 55 to 95% by weight, prefer-ably from 70 to 90% by weight, and contain on average from 1.4 to 3.0 primary and/or secondary amino groups per molecule with or without tertiary amino groups. The average molecular weight (Mn) of the copolymers is advantageously from 500 to 15,000, preferably from 2000 to 8000.
The ~unino-containing copolymers are obtainable for example lby reacting carboxyl-containing butadiene/
acrylonitrile copolymers with diamines. Such copolymers are commercially obtainable, for example under the designation HYCAR' 1300 x ATBN and HYCAR 1300 x 16 ATBN
(from B.F. Goodrich), containing 10% by weight and 16.5%
by weight of acrylonitrile respectively.
It is also possible to obtain amino-containing butadiene/acrylonitrile copolymers by partial hydrogena tion of butadiene/acrylonitrile copolymers or by addition of primary amines to epoxy-containing butadiene/acrylo-nitrile copolymers .
Suitalble epoxy resins are those which have an average molecular weight (Mn) of from 140 to 10,000 and contain on avE~rage from 1.5 to 3 epoxy groups, preferably 2 epoxy group~a, per molecule.
Preferred epoxy compounds are glycidyl ethers of aliphatic dials such as butanediol or hexanediol or polyetherols or glycidyl ethers of polyphenols which contain on average two or more phenolic hydroxyl groups and are preparable in a conventional manner by etherifi-cation with an epihalohydrin in the presence of alkali.

Examples of suitable phenol compounds are 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), 4,4'-dihydroxy-benzophenone, 1,,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)iaobutane, 2,2-bis(4-hydroxy-tert-butyl-phenyl)propane, bis(2-hydroxynaphthyl)methane and 1,5-dihydroxynaphthalene. It is desirable in some cases to use aromatic epoxy resins having a higher molecular weight. They can be obtained by reacting the abovemen-tioned diglycid:yl.ethers with a polyphenol, for example 2,2-bis(4-hydro:Kyphenyl)propane, and then further react-ing the resulting product with epichlorohydrin to prepare polyglycidyl etlhers.
The weight ratio of the butadiene/acrylonitrile copolymer to the epoxy resin is in general determined in such a way that: from 1.05 to 20 moles, preferably from 1.2 to 4.0 mols~s, of NH groups of the amino-containing butadiene/acryl~~nitrile copolymer are used per mole of epoxy group of 'the epoxy resin.
The reaction is in general carried out by react-ing the butadiene/acrylonitrile copolymer with an epoxy resin in an organic solvent or solvent mixture which is inert not only 'toward amino groups but also toward epoxy groups, at 20-1:50°C, preferably 50-110°C. The reaction time can be up to 20 hours.
The po7.ymeric reaction products thus obtained have amine numbers of from 20 to 150 mg of KOH/g of solid substance, preferably 40-100 mg of KOH/g of solid sub ' stance.
By introducing groups capable of crosslinking, e.g.
blocked isocyanates, it is possible to render the amino resins self-crosslinking, as described for example in the above mentioned US-A-4 692 503, US-A-3 935 087 and EP-A-273 247.
A

The amine groups of these acids can be wholly~or partly neutralized with an acid and the protonated resin dispersed by the addition of water. A suitable acid is phosphoric aci~~, but it is preferable to use an organic acid such as formic acid, acetic acid, propionic acid or lactic acid. It is also possible to add the resin to a water/acid mixture. Thereafter the organic solvents may be distilled o:Ef .
In general, the dispersions used have a solids content of from 5 to 40~ by weight, preferably from 15 to 25o by weight.
Optional component (C) comprises customary cross-linking agents, pigment pastes, auxiliaries and/or additives.
Suitable crosslinking agents are for example aminoplast resins such as urea-formaldehyde resins, melamine resins and benzoguanamine resins, blocked isocyanate type crosslinking agents, crosslinking agents which cure by ~steraminolysis and/or transesterification, e.g. (3-hydroxyalkyl ester type crosslinking agents as described in :EP-A-40 867 published on 2 December 1981 (KLEIJMANS et al.) and carbalkoxymethyl ester type crosslinking agents as described in DE-A-3 233 139 published on 27 January 1983 (HIJPTE et al). Further possible crosslinking agents are phenolic Mannich bases as described f:or example in DE-A-3 422 457 published on December 19, 1985 (SCHUPP et al) .
If either or both of components (A) and (B) is <
self-crosslinking, no crosslinking agent need be used.
~:, 7a 2010525 Besides customary pigment pastes it is also possi-ble to use auxiliaries and/or additives such as solvents, flow control accents, defoamer or curing catalysts.
Components (A), (B) or (C) can be used within a wide weight ratio. In accordance with the invention as claimed, use :is made of: from 10 to 90% by weight, preferably front 40 to 75~ by weight, of component (A), from 10 to 50$ by weight, preferably from 5 to 20 o by weight, of component (B) and from 0 to 50o by weight, preferably from 20 to 40~ by weight, of component (C), the weight percentages of components (A), (B) and (C) adding up to 1000 by weight.
Besides using components (A) and (B) as aqueous cationic dispersions it is also possible to use their ~4~~~
- 8 - O.Z. 0062/02083 starting resins for preparing the mixture by for example neutralizing an amino-containing resin wholly or partly with an acid and dispersing the protonated resin by the addition of water. The second protonated resin can then be dispersed in the dispersion of the first protonated resin. However, it is also possible to protonate the two resins and to disperse them together. Suitable acids are the acids mentioned for the preparation of the cationic dispersions (.A) and (B) .
It is also possible to disperse the resins individually in succession or together in an acid/water mixture.
Component (C) can be added before, during or after the preparation and mixing of components (A) and (B).
For cathodic electrocoating, the solids content of the electrocoating bath is in general set to 5-30$ by weight. Deposition customarily takes place at from 15 to 40°C in the course of from 0.5 to 5 minutes and at a pH
of from 4.0 to 8.5, preferably a neutral pH, at a voltage of from 50 to 500 volts. In cathodic electrocoating, the electrically conducting object to be coated is connected as the cathodte. The deposited film is cured at above 100°C in the course of about 20 minutes.
The cured films have two phases and, from DSC
measurements, two glass transition temperatures. The glass transition temperature which is assignable to the crosslinked polymer of starting component (A) is in general within the range from 50 to 150°C, preferably from 70 to 1:10°C, particularly preferably from 80 to 110°C. The glass transition temperature which is assign able to the polymer of starting component (B) is in general within the range from -80 to +20°C, preferably from -70 to 0°C, particularly preferably from -50 to -10°C.
The advantage of these systems, which on baking form two phasers, is that the hard polymer matrix, which - 9 - O.Z. 0062/02083 is responsible for such important properties as for example corrosion protection and sandability, is pre-served, whereas the soft phase distinctly improves the elasticity of the coatings.

Preparation of polymeric reaction products ac<:ording to the present invention General method In al:l the Examples, the butadiene/acrylonitrile component used was a copolymer having an average molecu lar weight Mn of 3500-3800, obtainable by reacting amino ethylpiperazine with carboxyl-terminated butadiene/
acrylonitrile copolymers having an acrylonitrile content of 16% by wei<~ht (Hycar 1300 x 16 ATBN).
In Examples 1 to 5, the epoxy resin used was a polytetrahydrofuran diglycidyl ether having an epoxy equivalent weight (EEW) of 840. In Example 6, a polypro pylene oxide diglycidyl ether (EW - 330) was used. In Example 7, a bisphenol-A diglycidyl ether (EEW = 188) was used.
The butadiene/acrylonitrile copolymer was dis-solved in toluene, admixed with the epoxy resin and stirred at 80°C for several hours (see Table I) until the epoxy value was virtually zero. Thereafter, the reaction mixture was diluted with ethylene glycol monobutyl ether and isobutanol. and cooled down to 40°C. Acetic acid was then added, followed by deionized water in the course of an hour.
Thereafter, some of the organic solvents and water was distilled off under reduced pressure, and the solids content: listed in the table was then set with deionized water.
The ds~tails of these experiments are given in Table I.

- 10 - O.Z. 0062/02083 TABLE I

Exa~le 1 2 3 4 5 6 7 Bu/AN co-polymer [g] 359.8 359.8 359.8 359.8 359.8 172.9 359.8 Epoxy resin [g] 23.1 46.3 52.9 61.7 70.7 27.4 9.2 Toluene [g] 164.1 174.0 176.9 180.6 184.5 85.8 158.1 Ethylene glycol monQbutyl et~x [g] 6:2.9 68.4 68.2 62.4 59.4 41.4 86.0 Isobut:arbl [g] 9'7.1 116.0 119.0 112.0 111.0 73.2 125.3 Acetic acid [g] ..i.9 6.4 6.3 5.8 5.5 3.8 5.5 Water [ml] 146:3 1588 1588 1452 1452 960 2000 Reaction time [h] 12 10 10 10 11 10 19 Solids con-gent [wt.%] li' 19 17 18 18 18 17 Amine mun-ber [mg of ICCH/g of solid sub-stance] 65.0 61.2 60.3 58.4 57.4 59.8 59.4 Electrocoating baths a) Preparation of the base resin al) A mixture of 5800 g of hexamethylenediamine, 250 7 g of dimeric fatty acid and 1400 g of linseed oil fatty acid was slowly heated to 195 C
while the water formed (540 g) was distilled off. The mixture was then cooled down to 100C and diluted with 5961 g of toluene to a solids content: of 70% by weight. The product had an amine number of 197 mg of KOH/g.

a2) In a second stirred vessel, of a 10 equivalents diglycidyl ether based on bisphenol A and epichlorohydrin having an equivalent weight of 485 were dissolved in a - 11 - O.Z. 0062/02083 solvent mixture of 1039 g of toluene and 1039 g of isobutanol b~,~ heating. The solution thus formed was cooled down to 60°C and admixed with 300.4 g of methyl-ethanolamine and 128 g of isobutanol, the temperature rising to 78 °C in the course of 5 minutes . Thereafter, 1850 g of the condensation product obtained by al) were added, and th~a mixture was heated at 80°C for 2 hours.
b) Preparation of the pigment paste To 525.8 g of the binder obtained by a) were added 168.7 g of butylglycol, 600 g of water and 16.5 g of acetic acid. This was followed by 800 g of titanium dioxide, 11 c~ of carbon black and 50 g of basic lead silicate, and the mixture was ball-milled to a particle size of less 'than 9 gym. Thereafter, a solids content of 47% by weight was set with water.
c) Preparat:Lon of the crosslinking agent A mixture of: 1.32 kg of toluene, 0.42 kg of tri-methylolpropane and 0.72 kg of bisphenol A was stirred at 60°C until a homogeneous solution had formed. This solution was added to a hot mixture of 3.45 kg of isoph-orone diisocyanate, 0.86 kg of toluene and 0.0034 kg of dibutyltin dil.aurate at 60°C. The mixture was maintained at 60°C for a hours and then admixed with 2.0 kg of dibutylamine, the rate of addition being such that the temperature of the reaction mixture did not exceed 80°C.
1.11 kg of to7.uene were then added, and the mixture was maintained at 80°C for a further hour.
d) Preparation of the electrocoating baths 700 g of the binder obtainable by a) and 300 g of crosslinking agent c) were dispersed by the addition of 19 g of acetic acid with sufficient water to form a dispersion Naming a solids content of 31% by weight.
Organic solvents were then distilled off azeotropically, and thereafter the dispersion was adjusted with water to a solids content of 35% by weight.
The dispersion thus obtained was mixed with 775 g of the pigment paste obtainable by b) and with varying - 12 - O.Z. 0062/02083 amounts of i~he dispersion according to the present invention and made up with water to a volume of 5000 ml.
The e:Lectrocoating baths were stirred at 30°C for 168 hours. Cathodes comprising zinc phosphatized test panels of steel were coated with paint films in the course of 120 seconds. These paint films were then baked at 160°C for 20 minutes.
The compositions of the baths, the coating condi-tions and the test results are summarized in Table II.

- 13 - O.Z. 0062/02083 r, ~U
a N

H
N
I I~f11~1uM11t~~~uNllyi a r, 40 ri M N tC r-I 01 01 N N
~rl r-i rl rl r-1 ri O O H .~
-, tl1 t~ t~ 01 ri ri t~ N .-1 O O O O ri e-i O ~-~1 r~~ ~
O
O
fJ7 ~ ~
ri rl r~ 1l~ M lfl .-~ rl M ri cV O O M N ~ 00 OD ri Cp .-i rl ,-I ri ri rl .-I ~ N
V /~ n r, H _ H
th tf1 d' 01 C1 ~G ~C1 d' Il1 10 Lf1 l~ l0 ~O ~O l0 O O ~ p w r~i r~1 U~
W1 M tr1 N M O vf1 d~ tp O
fV N N N N N N N N N
i~
aD ~a~oo°~o°~~apoo~o~a°o O o CV N N M N N N N N N
I '-1 N M d~ d~ d' Lf1 t0 n O .~ +.I t~
r-1 C1 ~ c't~~ M cn~7 ~ f~ ~p ~, .,., r, ~'M'M~tf~MMMM
a mw ui ~ O
w ~ ~ a rte, e~-~ .~-1 .~-1 ~~-i ~ ~ r~l ~ ~ ° ~r ..
~~ N
..
.. ..
rl N M d~ 1l1 ~D l~ CD 01 O ~~.~. ~ ~ p '"~ d~

Claims

1. A two-phase cathodic electrocoat obtained by depositing a mixture comprising:
A) 10-90% by weight of an aqueous cationic dispersion of an aminoepoxy resin having a glass transition temperature of from 20 to 100°C an average molecular weight of from 200 to 20.000 and an amine number of from 30 to 150.
B) 10-50% by weight of an aqueous cationic dispersion of an aminoepoxy resin which is obtained by reacting a butadiene acrylonitrile copolymer which contains primary and/or secondary amino groups with an epoxy resin having an average molecular weight of from:
40 to 10,000 and containing on average from 1.5 to 3 epoxy groups per molecule, the aminoepoxy resin having a glass transition temperature from 80 to 20°C and the polymers of dispersions A) and B) being mutually incompatible, and C) 0-50% by weight of a crosslinking agent, a pigment paste, auxiliaries and/or additives, on a electrically conductive substrate and then baking.

2. A two-phase coat as claimed in claim 1, wherein the crosslinked phase A) has a glass transition temperature of from 50 to 150°C and the phase B) has a glass transition temperature of from -80 to +20°C.

3. A two-phase coat as claimed in claim 1 or 2, wherein the crosslinked phase A) has a glass transition temperature of from 70 to 110°C and the phase B) has a glass transition temperature of from -70 to 0°C.

4. A two-phase coat as claimed in any of claims 1 to 3, wherein the crosslinked phase A) has a glass transition temperature of from 80 to 110°C and the phase B) has a glass transition temperature of from -60 to -10°C.