Electrophoretically overcoatable coatings applied by electrocoat

Details Electrophoretically overcoatable coatings applied by electrocoat Electrophoretically overcoatable coatings applied by electrocoat

1987-08-17

1989-11-21

Barr, Josephine

Compositions

Electrically conductive or emissive compositions

Elemental carbon containing

524495, 524496, 523468, H01B 106

Patent

active

048820903

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to cathodically depositable aqueous electropaints which contain cationic amine-modified epoxy resins and are pigmented with electroconductive carbon black and which, after deposition and crosslinking, provide electrophoretically overcoatable coatings.
Cathodic electrocoating is a very frequently employed coating method, whereby water-dilutable synthetic resins with cathodic groups are applied to electroconductive bodies.
Cationic amine-modified epoxy resins are particularly highly suitable for use as binders for aqueous cathodically depositable electropaints.
In standard electrocoating, the coating deposited is electrically insulating and no longer electrophoretically overcoatable.
To make it possible to use the technically very advantageous cathodic electrocoating process not just for base coating but for the production of multibuild coats as well, it has been tried to obtain electrophoretically overcoatable electropaint coats by depositing aqueous electropaints pigmented with conductive carbon blacks (GB No. 2,129,807).
In said GB No. 2,129,807, 8-50% by weight of finely divided carbon--for example graphite or carbon black having an oil absorption of 45-115 ml/100 g and a particle diameter of 15-85 .mu.m--are added to a cathodically depositable aqueous electropaint containing a cationic synthetic resin as binder.
However, the use of carbon black in electropaints leads to flaws in the surfaces of the deposited coats (roughness, pores, holes . . . ), which reduce the protection against corrosion and adversely affect the appearance and the physico-chemical properties of the electrophoretically applied second coat.
The surface flaws caused by the admixture of carbon black pigments decrease in intensity and frequency with decreasing carbon black concentration.
A further very important point to be observed in the industrial application of carbon black pigmented electropaints is the reliability with which the carbon black pigment can be applied. Application reliability can be said to be high when electrophoretically overcoatable coatings having a tolerable degree of surface flaws ar obtained even in the event of major fluctuations in the concentration of carbon black.
The invention has for its object to provide, for use in cathodically depositable aqueous electropaints which contain cationic amine-modified epoxy resins and are pigmented with electroconductive carbon black and which, after deposition and crosslinking, provide electrophoretically overcoatable coatings, carbon black pigments which not only can be employed in a very low concentration but also guarantee a high degree of application reliability.
This object is achieved according to the invention by employing as the electroconductive carbon black a carbon black which has an iodine absorption of 870-930 mg/g, a specific surface area (BET/N.sub.2) of 850-1,000 m.sup.2 /g, a pore volume(DBP) of 330-390 ml/100 g and an average particle size of 25-35 nm in an amount of 1.5-5.1% by weight--based on total solids.
The invention also relates to a process for preparing a multibuild coating by applying to an electroconductive substrate by electrophoresis a cathodically depositable aqueous electropaint which contains a cationic amine-modified epoxy resin and is pigmented with electroconductive carbon black, crosslinking, and overcoating, again by electrophoresis, wherein the electropaint used for preparing the basecoat contains as electroconductive carbon black a carbon black in an amount of 1.5-5.1% by weight, based on total solids--which has the following analytical data:


______________________________________ ASTM test method used ______________________________________ Iodine absorption 870-930 mg/g D 1510-79 Specific surface area (BET/N.sub.2) 850-1000 m.sup.2 /g D 3027-78 Pore volume (DBP) 330-390 ml/100 g D 2414-79 Average particle size 25-35 nm ______________________________________
The process is carried out as follows: The electroconductive substrate to be coated is dipped into the

REFERENCES:
patent: 4575523 (1986-03-01), Anderson et al.
patent: 4610811 (1986-09-01), Yamamoto et al.

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