Coating agent based on polymers containing carboxyl groups and c

Details Coating agent based on polymers containing carboxyl groups and c Coating agent based on polymers containing carboxyl groups and c

1992-01-24

1994-01-04

Marquis, Melvyn I.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

525165, 525530, 525531, 525533, 528112, 528297, C08L 6702

Patent

active

052760979

DESCRIPTION:

BRIEF SUMMARY
The invention relates to coating agents containing number-average molecular weight M.sub.n of 500-8,000 and an acid number of 15 to 200 mg of KOH/g as the essential binder component and agent.
Coating agents which contain polymers containing carboxyl groups as the binder and compounds containing epoxide groups as the crosslinking agent are known and are described, for example, in international PCT publication WO 87/02041, EP-A-103,199, EP-B-51,275, EP-A-123,793, DE-AS 2,635,177, Japanese Preliminary Published Specification 219,267/83, Japanese Preliminary Published Specification 76,338/77 and Japanese Preliminary Published Specification 53,145/79.
If these coating agents based on polymers containing carboxyl groups and epoxide compounds are to be hardened at low temperatures (that is to say .ltoreq.80.degree. C.), this requires the use of highly reactive epoxide compounds. However, such epoxide compounds based on aliphatic epoxides have usually proved to be mutagenic in the Ames test. This is to be attributed on the one hand to high contents of oligomeric constituents, but on the other hand also to the preparation process, that is to say the reaction with epichlorohydrin and the associated residual content of epichlorohydrin.
Epoxide compounds which are less toxicologically unacceptable are usually very high molecular weight compounds and are therefore less reactive, or they are built up on the basis of bisphenol A or bisphenol F and are unsuitable for use as a top or clear coat because of their tendency to yellow. Finally, acrylate copolymers which contain epoxide groups and have been prepared by conventional methods are likewise unsuitable for toxicological reasons, because of their high content of residual monomers.
The disadvantage of the known coating agents based on polymers containing carboxyl groups and epoxy crosslinking agents is moreover often the high viscosity of the coating agents and thus the low solids content of the coating agents at a viscosity which is advantageous for processing. This leads to high pollution of the environment with solvent during drying of the coating films.
Low-solvent stoving lacquers which consist of acrylate resin containing carboxyl groups, epoxy resin containing at least 2 oxirane groups per molecule, solvents and if appropriate pigment, catalyst and other auxiliaries are thus known from DE-PS 2,728,459. The acrylate resin containing carboxyl groups which is employed as the binder is prepared in bulk by means of free radical polymerization under pressures of 1.5 to 30 bar at temperatures of 160.degree. to 280.degree. C. and is distinguished by a low heterogeneity, measured by gel permeation chromatography, of between 1.5 and 2.0 and a number-average molecular weight of 1,500 to 3,000. However, since the epoxide compounds based on bisphenol A or compounds prepared using epichlorohydrin or acrylate copolymers containing epoxide groups which are usually employed are used as the crosslinking agent, these stoving enamels also have the abovementioned disadvantages caused by these crosslinking agents, such as a tendency to yellow and/or toxicity.
The preparation of acrylate copolymers containing functional groups, such as, for example, hydroxyl, carboxyl, epoxide, isocyanate or amino groups, and having only a low dispersity, a low number-average molecular weight of 500 to 6,000 and a low viscosity of the resulting solutions of the acrylate copolymers is also known from EP-A-96,901. The acrylate copolymers are prepared by means of a continuous bulk polymerization process, if appropriate under pressure, at polymerization temperatures between 180.degree. and 270.degree. C.
Coating agents which contain acrylate copolymers containing hydroxyl groups and prepared by the process of EP-A-96,901 as binders and urea resins, melamine resins or benzoguanamine resins as well as polyisocyanates or polyepoxides as crosslinking agents are also described in EP-A-96,901.
The present invention provides coating agents which are based on polymers containing carboxyl groups as crosslinking ag

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