Process for coating electrically conductive substrates, water-ba

Details Process for coating electrically conductive substrates, water-ba Process for coating electrically conductive substrates, water-ba

1993-02-04

1994-06-28

Gorgos, Kathryn

Chemistry: electrical and wave energy

Processes and products

Electrostatic field or electrical discharge

2041817, 523404, C25D 1310

Patent

active

053244028

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for coating electrically conductive substrates, in which
(1) the electrically conductive substrate is dipped into a water-based electro-dipping primer
(2) the substrate is connected as the cathode
(3) a film is deposited on the substrate by means of direct current
(4) the coated substrate is removed from the electro-dipping primer and
(5) the deposited film coating is baked.
The invention also relates to water-based coatings and crosslinking agents containing masked NCO groups.
The cathodic electro-dipping process described above is known (cf., for example, German Offenlegungsschrift 3,518,732, German Offenlegungsschrift 3,518,770, EP-A 4090, EP-A 12,463 and EP-A 262,069) and is employed, in particular, for priming motor vehicle bodies, especially automobile bodies.
Coatings of excellent quality can be obtained by means of the cathodic electro-dipping process--particularly if electro-dipping primers containing cationic, amine-modified epoxide resins are used as binders.
The electro-dipping primers employed for carrying out the cathodic electro-dipping process preferably contain masked polyisocyanates as crosslinking agents. It is known that if masked diphenylmethane 4,4'-diisocyanate (MDI) is used as the crosslinking agent, the troublesome discoloration (yellowing) of topcoat layers which is observed when other aromatic polyisocyanates are used does not occur. MDI can, however, not be employed without difficulty, because it exhibits a strong tendency to crystallize and thus leads to the destabilization of the electro-dipping primers. It is stated in EP-A 236,050 that stable electro-dipping primers should be obtainable using MDI containing at least 5% by weight of the 2,4'-isomer. However, commercially obtainable MDI grades meeting this condition as a rule have, disadvantageously, a high content of hydrolyzable chlorine, which results in electro-dipping primers of high chloride ion contents, i.e. electro-dipping primers of increased corrosivity. It is stated in EP-A 293,088 that stable electro-dipping primers should be obtainable using MDI containing a uretonimine. However, this suggestion for solving the crystallization problem unfortunately does not lead to success in all cases. The electro-dipping primers described in EP-A 236,050 and EP-A 293,088 should be improved, not least in respect of baking temperatures and leveling properties. In addition, it is desirable to provide a process for reducing the viscosity of MDI crosslinking agents, because this makes it possible to reduce the viscosity of the electro-dipping primers and increase the layer thickness of the resulting film coatings.
The object on which the present invention is based is the provision of crosslinking agents for water-based electro-dipping primers in the use of which stable electro-dipping primers are obtained which have improved properties compared with comparable electro-dipping primers of the state of the art.
Surprisingly, this object is achieved by the provision of masked polyisocyanates which are obtainable by
(I) reacting, in a first stage, product (A) containing isocyanate groups, and
(II) reacting this intermediate product (A) in a second stage with containing hydroxyl groups which
(III) is reacted in a third stage either with of such polyisocyanates and with
Stable electro-dipping primers of low chloride ion content can be prepared by means of the crosslinking agents according to the invention. When electro-dipping primers containing the crosslinking agents according to the invention are used, troublesome discolorations of topcoat layers do not occur. Electro-dipping primers containing the crosslinking agents according to the invention instead of the crosslinking agents described in EP-A 236,050 and EP-A 293,088 can be baked at lower baking temperatures and exhibit improved leveling proper, ties. In addition, in comparison with comparable crosslinking agents which have been prepared using only MDI as the polyisocyanate component, the crosslinking agents according to the invention are distingui

REFERENCES:
patent: 4285789 (1981-08-01), Kobayashi et al.
patent: 4296010 (1981-10-01), Tominaga
patent: 4668360 (1987-05-01), Mels et al.
patent: 4892897 (1990-01-01), Redman

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