The use of electrodepositable resin compositions in aqueous baths to coat objects is well known in the art. Typically, a resinous composition, which has been mixed with a cross-linking composition to form an electrodepositable coating composition, is salted with an acid so that the composition is solubilized in water. The aqueous coating composition is thought to be closer to a dispersion than a true solution and is typically described as solubilized rather than dissolved. The resins typically used are epoxide resins which have been either reacted with or adducted with amines. The amine is necessary so that a nitrogen atom is available to be salted by an acid to solubilize the composition in water. Solubilization is typically done by feeding an electrodepositable resin composition into a reactor vessel containing an organic or inorganic acid and water.
The electrodeposition bath generally contains an anode, immersed in the bath, which is connected to a DC electric circuit. An article to be coated must comprise an electrically conductive material such as metal. The article is connected to the previously mentioned DC circuit and functions as the cathode. The article is dipped into the bath and the circuit is closed wherein electrons flow through the cathode to the anode, i.e. conventional current flow from the anode to the cathode. Simultaneously, positively charged particles of the electrodepositable coating composition are transported and deposited on the surfaces of the cathodic article. The film thickness of the deposited film is a function of time, current flow, conductivity, etc. and is related to the resinous composition utilized in the bath. The current flow typically diminishes as the deposited film layer increases in thickness due to the electrically insulating properties of the coating. The coated article is then removed from the bath after the desired thickness is achieved and typically baked in an oven as a curing step resulting in the cross-linking or curing of the coating composition, thereby producing a hard, corrosion resistant coating. Although there are many patents covering compositions of this type, some of the more typical patents are described below.
U.S. Pat. No. 4,031,050 discloses cationic electrodepositable compositions comprising amine-epoxy resin adducts and blocked polyisocyanates.
Cationic epoxide-amine reaction products used as electrodepositable coating resins are disclosed in U.S. Pat. No. 4,182,833. This patent also discloses the use of blocked polyisocyanate cross-linking compositions.
U.S. Pat. No. 4,104,147 dicloses electrodepositable cationic chain extended polyepoxides, wherein the molecule is extended with an organic polyol, which is adducted with a secondary amine and cured with capped or blocked isocyanate derivatives.
U.S. Pat. No. 4,225,479 discloses electrodepositable resin compositions comprising the reaction product of a polyepoxide resin and a primary amine. This patent also discloses the use of cross-linking agents such as aminoplast resins, phenoplast resins and blocked polyisocyanates.
U.S. Pat. No. 4,093,594 discloses polyepoxide resins adducted with polyamines using an excess of amine. The resins are then reacted with a monoepoxide or a monocarboxylic acid, and, when reacted with an acid, form water soluble or dispersible resins useful to coat objects in cathodic electrodeposition processes.
An improved process for the preparation of cationic resins wherein polyepoxide resins are contacted with particular polyether polyols and then reacted with cationic base group formers such as an amine or an acid is disclosed in U.S. Pat. No. 4,419,467.
The cathodic electrodepositable coating compositions known in the art have several disadvantages associated with their use including inadequate film build and throwpower, high bake temperatures, and rough film surfaces.
Build is defined as film thickness. Throwpower relates to the capacity to coat areas of the cathodic article which are remote or shielded from the anode. Throwpower is defined as the rate of deposition of film relative to the position of the anode.
The coatings of the prior art typically have film thicknesses of about 0.6 mils, although certain high build films apparently produce 1.4 mils film thicknesses. Film thickness is related to corrosion resistance. In addition, the need for a primer coat may be eliminated in certain cases when using high build films. The coatings of the prior art typically require at least about 21/2 minutes to deposit as a coating and cure at temperatures of at least about 325.degree. F. A reduction of the deposition time and baking temperature will result in cost and energy savings.
While great strides have been made in the area of electrodepositable coating compositions, there is a constant search for improvements in this technology such as high film build, high throwpower, smooth film surfaces and low cross-linking or cure temperatures.
Accordingly, what is needed in the art are aqueous cathodic electrodepositable coating compositions which produce higher film builds under existing process parameters and have lower cure temperatures.