1. Field of the Invention
This present invention relates to electrocoating resin technologies. More specifically, it relates to a novel electrocoating process which enables the rapid uniform thin film deposition of a continuous (free of pin holes) coating onto one substrate and enables the subsequent rapid removal and transfer of said coating onto another substrate. This invention discloses compositions of electrodepositable polymers useful in the new process. Novel electrodeposited coating copolymer blends for a transfer process are disclosed enabling copolymers displaying high adhesion to metals, and which normally cannot be contact transferred, to be made transferable as thin continuous barrier films.
2. Description of the Prior Art
Electrodeposition of coatings is a process that is used in a significant variety of industrial applications from the lacquering of food cans to the painting of appliances and commercial vehicles. In electrodeposition, the coating materials are deposited from a water-based or solvent medium onto a substrate. Sometimes known as electrophoretic painting, the technology of electrodeposition of coatings encompasses the deposition of film when a d.c. current is applied to an article which is immersed in generally an aqueous dispersion stabilized with a colloidal electrolyte. In the electropainting process, it is generally the case that the particles in the disperse phase carry a negative charge and migrate towards the anode when an electrical potential difference is applied to the system. However, both anodic and cathodic polymers are known.
When polymers are being electrodeposited with aqueous systems, generally it is necessary for the polymer to be hydrophillic so as to be soluble in bulk but the polymer must also be capable of producing continuous and water insoluble films on air drying or storing.
Conventional electrocoating technology employs two general classes of polymer systems: (1) anodic polymers such contain carboxylic poly-anion functional groups and (2) cathodic polymers which contain amine salt poly-cation functional groups.
In conventional anodic/cathodic electrodeposition of coatings technology a carboxylic anion functional polymer in solution will migrate and deposit onto a positively charged substrate (anode) while a cation (amine quaternary salt) functional polymer in solution will migrate and deposit onto a negatively charged substrate (cathode).
The polymer systems in their anion or cation states are solvent soluble or dispersable but when a current is passed through the coating solution, electroneutralization takes place and the polymers become insoluble at the electrode/substrate interface or precipitate out onto the substrate surface. ##STR1##
Various patents such as U.S. Pat. Nos. 3,304,250, 3,444,064 and 3,444,063 are directed to maintaining the operational stability of various electrodeposition baths. Other patents such as U.S. Pat. Nos. 3,230,162, 3,434,988 and 3,450,660 describe specific coating formulations suitable for electrodeposition. While the larger portion of electrodeposition formulations described are aqueous dispersed systems, patents such as U.S. Pat. No. 3,450,655 describe nonaqueous colloidal suspensions in organic solvents.
The processes of electrophoretic painting or electrodeposition yield coated articles, the coatings of which have excellent metal adhesion or bonding, but once deposited onto a substrate these coatings are not readily removable or transferable to any other substrate. This invention teaches how to make selected normally non-transferable copolymers to be transferable as thin continuous films.
Electrocoating resins typically display excellent adhesion to metal substrates. This excellent adhesion to a primary substrate is not always desirable as, for example, for forming, by a transfer process, articles having thin film barrier coatings. Commercial production of thin film coated articles is hindered by the absence of suitable formulations and methods for forming or transferring uniform continuous thin films to articles. By continuous is meant being free of pin holes, i.e., able to function as a barrier coating. Certain fine films useful as barrier coatings are costly and commercialization is prevented by absence of suitable formation and transfer technology. It has been found that only certain types of additives (elastomeric releasing agent) can be mixed with selected conventional electrocoating resin systems (electrodepositable polymer) to allow their release from an electrodeposit-coated metal surface.
Thin uniform continuous films are desirable for use in barrier coatings and related applications, however, technology to produce and transfer thin (submicron), continuous (free of pin holes) films has been absent until the present invention.
U.S. Pat. No. 3,753,885 describes a process for producing "coherent organic polymeric films". The process in U.S. Pat. No. 3,753,885 is essentially a flocculation process involving countercurrent flow and uses conductive particles to form a wet conductive polymeric film. The resultant films of the process in U.S. Pat. No. 3,753,885 are typically 1 mil or thicker and the preferred thicknesses of the process in U.S. Pat. No. 3,753,885 are 0.5 to 2.0 mils. Films produced according to the process in U.S. Pat. No. 3,753,885 are coherent but they are not continuous (see Example 23 herein). The resultant films of the process in U.S. Pat. No. 3,753,885 have a significant water content.