GB2129807A - Cationic electrocoating paint compositions - Google Patents

Abstract

A novel cationic electrocoating composition comprises from 8 to 50% by weight finely powdered carbon, based on the total solids content, and a basic group-containing water-miscible resin as the main vehicle. This can be used to coat a metal surface by electrocoating and then, following baking, a second film can be applied to give a coated product having satisfactory properties.

Description

SPECIFICATION Cationic electrocoating paint compositions It is known to apply an electro-conductive paint film to a metal; the film is then electro-coated.

Such a method is described, for example, in JP-A-1 15757/1978 JP-A-47074/1977, JP-A40035/1978, JP-A-9194/1978, JP-A-115757/1978 and JP-A-58397/1980. The electroconductive paint film which is used in the particular methods described is not an electro-coating paint film.

It has generally been considered impossible to deposit two films on a metal by sequential electo-coating, because the first film is an insulator. However, in one instance, JP-A24566/1974, the following method is described: an anionic electro-coating paint composition containing a finely powdered black pigment is applied, using a vehicle, to form an electroconductive film on a metal; after baking, a protective film of an anionic electro-coating paint is formed on the film using the same vehicle. There is also a statement to the effect that articles having a black iron oxide-containing coating are highly adherent to substrates and have good electro-conductive, surface and anti-corrosive characteristics.It is also stated that coatings contining carbon black as the electro-conductive substance are inferior to those containing black iron oxide, having poorer adaptability to voltage, hiding power and resistance to salt water spraying, and less acceptable appearance. However, it has been found that the use of black iron oxide in electro-coating compositions has serious defects on an industrial scale.

Black iron oxide has a high specific gravity and low oil absorption. Electro-coating paints are used as a dilute aqueous solution (solids content is 8% by weight in the example of the above patent application), and settling is therefore very marked. Particularly when the shape of materials to be coated is complicated, the paint film, its throwing power and its electroconductivity are non-uniform. Anti-corrosive properties may not always be satisfactory.

According to the present invention, a cationic electrocoating composition comprises from 8 to 50% by weight finely powdered carbon, based on the total solids content, and a basic groupcontaining water-miscible or dilutible resin as the main or amine vehicle. A coating method according to the present invention comprises applying a composition of the invention to a metal by electrocoating, baking the composition to form a first layer of a paint film (primer coat), and forming a second layer of a paint film, on the primer coat, by electrocoating.

The finely powdered carbon used in the present invention may be, for example, carbon black or graphite. Carbon black may be produced by, for example, the thermal black, acetylene black, channel, furnace or lamp black method. The finely powdered carbon preferably has an oil absorption of 45 to 11 5 mI/ 1 00g and a particle diameter of 1 5 to 85,um, thereby optimising its dispersibility as well as the finish of the electrocoated film. The content of finely powdered carbon in the composition is preferably 1 5 to 30% w/w. Below 8%, the electro-conductivity of the paint film may be insufficient, causing non-uniformity of the second electrocoating film.

When the content is more than 50% by weight, the primer coat surface may be rough.

The or each electrocoating composition may comprise a lead compound, or a metal in finelydivided form. Further, one or more conventional additives such as non-ionic or cationic activators, anti-forming agents or pigments may be included.

The finely powdered carbon is preferably used in dispersion in a conventional resin vehicle in which materials can be ground. An example of such a vehicle which is preferred for use in the invention is a quaternary ammonium base-containing resin, since such resins can provide good dispersibility and stability.

Conventional cationic resin may be used as the main vehicle of the cationic electrocoating paint for the first layer. By "main" is meant, for example, that the resin is the largest single resin component in the composition, e.g. at least 50% by weight (solids). Examples are the urethane-curing systems of amine-modified bisphenol A type epoxy resins described in JP-B51924/1973, the onium base-containing resin systems described in JP-B-47143/1978, the acryl-vinyl polymerisation resin systems described in JP-B-25422/1974, and the polybutadiene systems described in JP-A-16048/1978. The basic group may be an amine, and it is particularly preferred from the standpoints of adhesion and throwing power as well as long-term stability, that the resin composition comprises an amine-modified bisphenol A-type epoxy resin compound of the kind now popularly used in practice, and an at least partially blocked polyisocyanate.

According to its intended use, a composition of the present invention may be modified by the use of other resin components and pait materials, examples being amino resins, phenol resins, polyamides, polyethers and polyesters.

Although fairly large amounts of electro-conductive finely powdered carbon are present.in a paint composition of the present invention, such a composition exhibits, not high electroconductivity, but rather a fairly high film resistance on electrocoating, without any reduction in throwing power. On baking such a film, e.g. at 100 to 250"C e.g. for 10 to 30 minutes, electro-conductivity is obtained. As a result, it is only possible, in practice, to electrocoat the second layer when the first layer paint film has been baked. A possible theoretical explanation of this phenomenon is that moisture and organic solvents in the unbaked paint film are dispersed by baking, to improve adhesion between the metal to be coated and the finely powdered carbon.

The paint used in providing the second layer may be of the anionic or cationic type. In order to provide better anticorrosive properties, and in view of the tendency for a second layer of an anionic paint to have lower curability then when used to coat a metal directly, cationic-type paints are preferred. The second layer-forming composition is preferably an acrylic resin system, especially when the first layer is formed from epoxy resin.

The metal to be coated may be, for example, steel-aluminium or copper-magnesium, a surface-treated, e.g. zinc-plated, steel plate, or composite thereof.

The invention will now be illustrated with reference to the following Examples and comparative examples.

Preparation of resin, paste and electrocoating paint used, for example: (1) Anionic vehicle resin A (used in comparative examples): In the same manner as described in Example 1 of Japanese Patent Publication No.

24566/1974, 652.89 of a maleinized drying oil resin, as made from linseed oil modified with about 15 weight % of maleic anhydride, are blended with 111 ml of 29% ammonium hydroxide, 145 ml of n-butoxyethanol and 36.6ml of deionized water, to prepare a vehicle resin having a solid content of 70% and pH of 7.8.

(2) Main vehicle resin B: This resin is produced by charging into a reactor 970 parts by weight of EPON 1001 (polyglycidyl ether of bisphenol (epoxy equivalent, 508), a commercial product of Shell Chemical Company) and 265 parts by weight of polycaprolactonediol (a trade name, PCP 0220; a commercial product of Union Carbide Corporation). This product may be considered to be a polymer having a molecular weight of about 543 produced through ring-opening of caprolactone by ethylene glycol. This mixture in the reactor is heated to 100"C in a nitrogen atmosphere, and 0.46 part by weight of benzyldimethylamine is added.This reaction mixture is further heated to 130"C, and after keeping it at the same temperature for about 1.5 hours, it is cooled to 110"C. Thereafter, to the reactor are added 110 parts by weight of methyl isobutyl ketone, 39.8 parts by weight of 73% non-volatile solution of ketoxime (produced by reacting methyl isobutyl ketone with diethylenetriamine in an isobutyl ketone solvent), and 100 parts by weight of additional methyl isobutyl ketone in this order. The mixture is cooled to 73"C, and after adding 53.1 parts by weight of diethylamine, it is heated again to 120"C, kept at the same temperature for 3 hours and then taken out.

576 parts by weight of this polycaprolactonediol chain-extended polyepoxide are mixed with 310 parts by weight of a polyurethane crosslinking agent and 1 3.2 parts by weight of of a dibutyltindilaurnte catalyst, neutralized with 1 2.3 parts by weight of glacial acetic acid, and then diluted with a sufficient amount of deionized water, enough to prepare a dispersion having a resin solid content of 40%.

The polyurethane crosslinking agent is prepared by gradually adding 218 parts by weight of 2-ethylhexanol to 291 parts by weight of 2,4-/2,6-toluene diisocyanate (80:20 mixture of the 2,4- and 2,4-isomers) with stirring in a dry nitrogen atmosphere, during which the reaction temperature is kept at about 38"C by external cooling. After keeping the mixture at 38"C for further 30 minutes, it is heated to 60"C, and 75 parts by weight of trimethylolpropane and then 0.08 parts by weight of a dibutyltindilaurate catalyst are added thereto. After initial generation of heat, the contents are kept at 121"C for about 1.5 hours until consumption of substantially all the isocyanate is confirmed by infrared absorption scanning, The contents are then diluted with 21 9 parts by weight of ethylene glycol monoethyl ether.

(3) Grinding vehicle resin C for pigment: A grinding vehicle resin for pigment is prepared by the following recipe: Component Parts by Solids weight content EPON 829 710.0 681.2 Bisphenol A 289.6 289.6 Toluene diisocyanate half-capped with 406.4 386.1 methyl isobutyl ketone in 2-ethylhexanol Quaternarizing agent (a) 496.3 421.9 Deionized water 71.2 - Butyl cellosolve 567.6 (a) Tertiary amine salt of organic acid prepared by the following recipe: Component Parts by Solids weight content Toluene diisocyanate half-capped with 320.0 304 methyl ethyl ketone in 2-ethylhexanol Dimethylethanolamine 87.2 87.2 Aqueous lactic acid solution 117.6 88.2 Butyl cellosolve 39.2 Diisocyanate half-capped with 2-ethylhexanol is added at room temperature to dimethylethanolamine in a proper reactor.The mixture generates heat, and it it stirred at 80"C for 1 hour.

Lactic acid and then butyl cellosolve are added. The reaction mixture is stirred at 65"C for about 1 hour to prepare the quaternarizing agent.

In order to prepare a resin for pigment dispersion, EPON 829 and bisphenol A are charged into a reactor in a nitrogen atmosphere, and on heating the mixture to 150"C to 160"C, heat generation begins. After heat is generated for 1 hour, the reaction mixture is cooled to 120"C, and toluene diisocyanate half-capped with 2-ethylhexanol is added. The reaction mixture is kept at 110"C to 120"C for 1 hour, and butyl cellosolve is then added.

The reaction mixture is cooled to 85"C to 90"C, made homogeneous and, after adding water, the quaternarizing agent is added. The temperature of the reaction mixture is kept at 85"C until the acid value reached 1.

(4) Black paste D: Component Part by weight Grinding vehicle resin C 71.4 Deionized water 203.6 Carbon black 100.0 The above components were mixed with 375 parts by weight of glass beads of 2mm in average diameter and pulverized for 2 hours to make the particle diameter 15,u. The glass beads removed by filtration to obtain a black 25 paste D.

(5) Black paste E: Component Parts by weight Grinding vehicle C 71.4 Deionized water 203.6 Carbon black 90.0 Lead silicate 10.0 The above components are treated in the same manner as in (4) to obtain a black paste E.

(6) White paste F: Component Parts by weight Grinding vehicle resin C 71.4 Deionized water 203.6 Titanium dioxide 50.0 Aluminium silicate 40.0 Lead silicate 10.0 The above components are treated in the same manner as in (4) to obtain a white paste F.

(7) White electrocoating paint G: Component Parts by weight Main vehicle resin B 350.0 White paste F 150.0 Deionized water 1500.0 After mixing the resin B and the paste F, deionized water is gradually added to the mixture with mild stirring to obtain a white paint having a solids content of 20% and a pH of 6.4.

This paint is used as an electrocoating paint for producing the second layer in the examples and comparative examples described later.

Examples 1 to 5 Example No. 1 2 3 4 5 Content of carbon 10 25 20 30 40 black (%) Main vehicle resin 637.5 581.25 525.0 412.5 300.0 B (part by weight) Black paste D 112.5 168.75 225.0 337.5 450.0 (part by weight) Deionized water 1250.0 1250.0 1250.0 1250.0 1250.0 According to the recipe shown in the above table, five kinds of electrocoating paint, of which the carbon black content is within a range of 10 to 40 weight % based on the total solid content, are prepared in the same manner as in (7) of preparation of sample. The solids content of every electrocoating paint is 15%, and the pH has a value between 6.2 and 6.6.

Zinc phosphated steel plate is electrocoated with these paints at a bath temperature of 28"C under a condition of 150-250V x 3min. to prepare the first-layer paint having a thickness of 20,u in every case.

Using the white paint G previously prepared, the second layer is applied to every first layer by electrocoating to obtain the results as shown in the following table.

Table 1 Example No. 1 2 3 4 5 Content of carbon black 10 15 20 30 40 First-layer Film thickness ( ) 20 20 20 20 20 electrocoating Finished appearance Good Good Good Good Surface roughness film Salt spray test Pass Pass Pass Pass Fail for 500 hours Film thickness ( ) 32 40 42 43 45 Secondlaxyer elec- Finished Slightly non- Uniform Uniform Uniform A little affected trocoating appearance uniform; the and and and by the surface film first-layer good good good roughness of the is seen first layer.

through the second layer.

Salt spray test Pass Pass Pass Pass Pass for 800 hours Particularly, little rust was generated at the edte portion.

The width of blister of not more than 3mm from the cur portion is regarded as "pass".

The above result shows that the electro-conductivity improves with increasing carbon black content, thus showing a tendency for the second-layer paint film to deposit easily, but that too large contents cause surface roughness and a reduction in the performance of paint film. From this, a practical carbon black content in the total solids content is judged to be 15 to 30 weight %.

Various tests are carried out with Example 2 (carbon black content, 15%) as a typical example, and the results are shown below: (1) The film thickness of the first layer is fitted to 5y, 10y and 20y, and the second layer is applied to every first layer by electrocoating. As a result, it is found that a paint film 20,u in thickness is added to every first layer, giving no difference owing to the film thickness of the first layer.

(2) After electrocoating at 200V X 3 min., the paint film is washed with water, and the second layer is applied thereto without baking by electrocoating. As a result, deposition of film is hardly observed.

When said paint film is baked at 150"C x 10 min. or more, a film of 20y in thickness deposited on the paint film.

(3) A throwing power is measured at 200V X 3 min. (film thickness, 20,u) by the Ford Pipe method. As a result, a good result of 21cm is obtained. This value is the same as that of the throwing power of the white paint G prepared for the second layer.

(4) A L-shaped plate is made and electrocoated. As a result, the thickness of the electrocoated film shows no difference between the vertical portion and the upper surface and the lower surface of the horizontal portion, a good result.

Further, the second layer is applied to said film by electrocoating, and a good paint film uniform at every portion is obtained.

(5) An acryl-melamine type anionic electrocoating paint (PAWARON #1000, produced by Nippon Paint Co.) is coated on a film of 20p in thickness (baked at 175"C X 2 min.). The resulting paint film is inferior in curability to the paint film obtained by direct coating of zinc phosphated steel plate when both are baked under the same condition (170"C X 30 min.) (fails in the rubbing test with MINK).

But, when a cationic electrocoating paint is used for making the second layer, reduction in the curability is not observed.

(6) A conventional melamine-alkyl type top-coating paint is coated in a thickness of 30,u onto a two-layer electrocoated film (total thickness: 40,u comprising 20y of the paint film in Example 2 and 20,u of the paint film of the white paint G. For comparison, a two-coat two-bake coating system (total thickness, 50y) comprising the paint film of the white paint G (20,u) and that of the top-coating paint directly coated thereon, and a three-coat three-bake coating system (total thickness, 80cm) having the paint film of a surfacer (oil-free melamine-alkyd type) inserted between two said paint films, are prepared.On comparing the finished appearance of every coating system, the electrocoating system is clearly superior to the two-coat two-bake system and the same as the three-coat three-bake system.

Example 6 Component Part by weight Main vehicle resin B 525 Black paste E 225 Deionized water 1250 An electrocoating paint is prepared according to the above recipe. The solids content is 15% and the pH is 6.4.

The results of various tests are shown below: (1) A zinc phosphate steel plate is electrocoated at a bath temperature of 28"C under a condition of 200V X 3 min. and baked at 175"C x 3 min. The paint film obtained has a thickness of 20y and a good surface state.

The second layer of the white electrocoating paint G is applied to the above paint film by electrocoating. As a result, a paint film of 2011 in thickness deposited, and it has a good and uniform surface.

(2) The salt water spray rest is applied to the above paint film. As a result, the simple firstlayer electrocoating film (20,u) passes the 800 hours' salt spray test, and the two-layer electrocoating film (40 ) passes the 1200 hours' salt spray test.

(3) A L-shaped plate is electrocoated. As a result, the finished of the paint film shows no large difference at any of the vertical portion and the upper surface and lower surface of the horizontal portion. Further, the second layer is applied to said paint film by electrocoating, giving a good result.

(4) A throwing power is tested at 200V X 3 min. by the Ford Pipe method to obtain a good result of 21cm.

Comparative example 1 Component Parts by weight Main vehicle resin B 693.75 Black paste D 56.25 Deionized water 1250.0 Using the above recipe and additional carbon black, an electrocoating paint, of which the carbon black content is 5 weight % based on the total solid content, is prepared in the same manner as in (7) of Preparation of sample. The solids content is 14%, and the pH is 6.2.

A zinc phosphate steel plate is electrocoated with this paint at a bath temperature of 28"C under a condition of 150-250V X 3 min. and baked at 175"C for 20 minutes to form the firstlayer paint film 201l in thickness. The finished appearance of this film is good, and the film passes the salt spray test for 500 hours.

Thereafter, the second-layer paint film of 20eel in thickness is formed using the previously prepared white paint. The appearance shows slight deposition at the edge portion, etc., and fails in the salt spray test for 800 hours.

Comparative Example 2 Component Parts by weight Anionic vehicle resin A 163.5 Finely powdered black iron 51.0 oxide (0.5,u) 6% cobalt naphthenate 0.7 6% manganese naphthenate 0.7 24% lead naphthenate 5.9 Deionized water 1849.5 According to Example 1 of Japanese Patent Publication No. 24556/1974, a paint is prepared using the above recipe.

The solids content is 8% and the pH is 8.0. On stopping stirring, however, the finely powdered black iron oxide shows a property to deposit very easily.

The results of various tests are shown below: (1) A zinc phosphated steel plate is electrocoated at a bath temperature of 24"C under a condition of 50 V x 60 sec. and baked at 150"C X 30 min. The paint film obtained is 20y in thickness, and its surface state is slightly non-uniform.

On electrocoating the white paint G used in the foregoing example on this paint film, a paint film of 20,u in thickness deposit The surface state of the film is relatively uniform and good.

(2) The salt spray test is applied to the above paint film, and as a result, the simple firstlayer electrocoating film (20,u) failed in the test in 200 hours, and even the two-layer electrocoating film (40,u) failed in the test in 400 hours.

(3) A L-shaped plate is electrocoated, and as a result, a large difference in finish and film thickness is observed among the vertical portion and the upper surface and lower surface of the horizontal portion. Particularly, the amount of black iron oxide is large at the upper surface of the horizontal portion and small at the lower surface thereof, showing non-uniformity of the film.

On electrocoating the second layer on the above film, both the finish and film thickness are non-uniform.

(4) A throwing power is tested by the ford Pipe method to obtain a very low value of 5cm.

Comparative Example 3 Component Parts by weight Anionic vehicle resin A 228.6 Carbon black 40.0 6% cobalt naphthenate 1.0 6% manganese naphthenate 1.0 24% lead naphthenate 8.2 Deionized water 1721.2 According to the above recipe, an anionic electrocoating paint with carbon black as electroconductive substance is prepared. The solids content is 10% and the pH was 8. The results of various tests are shown below: (1) A zinc phosphated steel plate is electrocoated at a bath temperature of 24"C under a condition of 50 V X 60 sec. and baked at 150 X x 30 min. The paint film obtained has a thickness of 1 OIL and a uniform and good surface state (but, film 20,u in thickness showed surface roughness).

On coating the second layer of the white paint G on the above film, a paint film 20,u in thickness further deposited, and its surface state is uniform and good.

(2) The salt spray test is applied to the above paint film, and as a result, the simple firstlayer electrocoating film (10,u) fails in the test in 100 hours, and even the two-layer electrocoating film (3OIL) fails in the test in 400 hours.

(3) A L-shaped plate is electrocoated, and as a result, no large difference in finish and film thickness is observed among the vertical portion and the upper surface and lower surface of the horizontal portion.

Further, on electrocoating the second layer on the film to a thickness of 10 jum, no difference between the portions is observed.

(4) Throwing power is tested by the Ford Pipe method; a very low value, 5 cm, is found.

Use of the method of this invention has various inherently desirable qualities, and can provide various advantages. For example there can be low settlement and yet high throwing power. Film thicknesses can be large, thereby avoiding the need for a surface coating in the three-layer coating system presently used for coating automobile bodies. Different electrocoating compositions can be selected for use in the first and second layers. The primer coat can itself have good anti-corrosive properties, and the increase in thickness provided by application of the second layer increases this effect.

Claims (11)

1. A cationic electrocoating composition which comprises from 8 to 50% by weight finely powdered carbon, based on the total solids content, and a basic group-containing water-miscible resin.

2. A composition according to claim 1, which comprises from 1 5 to 30% by weight of the finely powdered carbon, based on the total solids content.

3. A composition according to claim 1 or claim 2, in which the basic group is an amine.

4. A composition according to claim 3, in which the resin comprises an amine-modified bisphenol A-type epoxy resin and an at least partially blocked polyisocyanate.

5. A composition according to any preceding claim, in which the carbon is used in dispersion in a quaternary ammonium salt-containing resin.

6. A composition according to any preceding claim, which additionally comprises a lead compound.

7. A composition according to claim 1, substantially as described in any of the Examples.

8. A coating method which comprises applying a composition according to any preceding claim to a metal by electrocoating, baking the composition to form a first film, and forming a second film on the first film by electrocoating.

9. A method according to claim 8, in which the first film is formed by baking at from 100 to 250"C.

10. A method according to claim 8 or claim 9, in which the second film is formed by cationic electrocoating.

11. A method according to any of claims 8 to 10, in which the second film is formed from a composition comprising an acrylic resin.

1 2. A method according to claim 7, substantially as described in any of the Examples.