Aqueous coating composition from a mercaptan containing polymer and a bis-maleimide

DETAILED DESCRIPTION OF THE INVENTION The coating composition contains a polymer or resin selected according to final desired use from a wide variety of known polymers in the coatings and electrocoating art.
The polymers contain pendant mercaptan groups.
Mercaptan groups can be attached to the polymer by esterification of free hydroxyl groups on the polymer (for example, a polyester) with a mercaptan-terminated acid, such as mercaptopropionic acid.
Similarly, mercaptan groups can be introduced into the polymer by reacting pendant primary or secondary amine groups on a polymer with a mercaptan-terminated acid or by reacting the free isocyanate group on a mono-isocyanate terminated polymer with a mercaptan-terminated acid ester having at least two pendant mercaptan groups.
Mercaptan groups can be introduced into the polymer by numerous other methods which are well known in the art and as further exemplified in the Examples of this application.
The mercaptan groups are pendantly attached to the polymer.
For purposes of this application pendant mercaptan groups include terminal mercaptan groups.
By pendantly attached is meant that such mercaptan groups are attached to the polymer chain or to a pendant side chain of the polymer.
The polymer containing pendant mercaptan groups should contain at least about 5% by weight of such pendant mercaptan groups, and up to about 50% if desired.
Representative polymers containing pendant mercaptan groups can be derived from epoxy and epoxy-modified diglycidyl ethers of bisphenol A structures, various aliphatic polyethylene or polypropylene glycol (diglycidyl ether) adducts, and glycidyl ethers of phenolic resin.
Other useful polymers containing pendant mercaptan groups include polyamide resins, for example, condensation products of dimerized fatty acids coreacted with difunctional amine, such as ethylene diamine, followed by reaction with 3-mercaptopropionic acid.
Polyamide resins generally are between about 500 and about 5,000 molecular weight.
Further useful electrocoating polymers containing mercaptan groups include acrylic resins having molecular weight of about 1,000 to about 100,000, polyester resins and polyurethane resins both having a molecular weight range of about 500 to about 5,000, vinyl resins, and amine resins