Process for coating electrically conductive substrates, a water-

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

1994-02-09

1995-09-19

Niebling, John

Chemistry: electrical and wave energy

Processes and products

Electrostatic field or electrical discharge

2041811, 523402, 523404, C25D 1310

Patent

active

054513057

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a process for coating electrically conductive substrates, in which electro-dipping primer
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 leading to electro-dipping primers of high chloride ion contents, i.e. 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.


SUMMARY OF THE INVENTION

The object on which the present invention is based is the provision of crosslinking agents based on MDI for water-based electro-dipping primers, in the use of which electro-dipping primers stable on storage are obtained.


DESCRIPTION OF THE PREFERRED EMBODIMENTS

Surprisingly, this object is achieved by the provision of masked polyisocyanates which are obtainable by reacting depressed, by partial reaction with a monool and/or polyol or a mixture of monools and/or a mixture of polyols, so far that it is liquid at room temperature and
In principle, any organic compound containing, as a statistical average, at least two hydroxyl groups per molecule can be employed as component (a). It is also possible to employ mixtures of such organic compounds. Linear, optionally branched, aliphatic diols having 2 to 12 C atoms in the molecule, poly-(ethylene oxide), poly(propylene oxide) and poly-(ethylene oxide)(propylene oxide) having number average molecular weights of 100 to 1000, preferably 200 to 400, and linear, optionally branched, aliphatic triols having 2 to 12 C atoms in the molecule are mentioned as examples of polyols which can be employed. Examples of suitable polyols are ethylene glycol, propylene glycols, butanediols, neopentyl glycol, hexanediols, dimethylolcyclohexane, glycerol, trimethylolethane, trimethylolpropane, trimethylolhexane, pentaerythritol, dipentaerythritol, pentaerythritol mono and diallyl ethers and trimethylolpropane allyl ether. It is preferable to employ a trimethylolpropane as component (a).
Diphenylmethane 4,4'-diisocyanate, the melting point of which has been depressed, by partial reaction with a monool and/or polyol or a mixture of monools and/or a mixture of polyols, so far that it is liquid at room temperature, is employed as the component (b). In addition to the reaction products from diphenylmethane 4,4'-diisocyanate and monools or polyols, the diphenylmethane 4,4'-diisocyanate employed as the component (b) can also contain, if desired, diphenylmethane 2,4'-diisocyanate and/or reaction products thereof with monools or polyols and/or di

REFERENCES:
patent: 4892897 (1990-01-01), Redman
patent: 5324402 (1994-06-01), Ott et al.

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