Coatings are typically applied to substrates to provide protective and/or decorative qualities. In particular, coatings are often applied to metal surfaces to inhibit or prevent corrosion.
One effective technique for applying coatings includes an electrocoating process, which typically involves depositing a composition onto an electrically conductive substrate with an applied electrical potential. Electrocoating is popular because it provides improved corrosion protection and low environmental contamination compared to other coating processes.
The process of electrocoating is well known in the art. Commercial electrocoating processes sometimes use anionic electrocoating processes, where the substrate being coated serves as the anode. However, cathodic or cationic electrocoating processes tend to provide coatings with superior corrosion resistance, and today are the most prevalent methods of electrocoating. For example, the vast majority of automotive primer coatings are now produced by cationic electrodeposition.
Highly crosslinked coatings are desirable for corrosion resistance as well as aesthetic appeal. Typically, such coatings are formed by a reaction between a crosslinkable functional group and a blocked isocyanate group. A catalyst is typically used to promote crosslinking reactions.
A variety of catalysts are known in the art. Organotin compounds, such as dibutyltin oxide (DBTO), dioctyltin oxide (DOTO), dibutyltin dilaurate (DBTDL), are among the known catalysts used in electrocoating. The most commonly used of these commercial catalysts is DBTO, a solid material that can be easily incorporated into an electrocoat composition. However, the human health risks and environmental issues associated with tin compounds are increasingly scrutinized. Organotin compounds, including DBTO, are sometimes considered pollutants, and there is increased regulatory pressure to substantially reduce or eliminate use of such compounds. Therefore, replacements for organotin compounds, and specifically dibutyl tin compounds like DBTO, for example, are being sought in electrocoat systems.
A wide variety of non-organotin catalysts may be used in electrocoat systems, although not all such catalysts are as effective as DBTO. Certain bismuth-containing compounds have been suggested as a replacement for organotin catalysts. For example, organic bismuth salts of carboxylic acids have been described as electrodeposition catalysts. These compounds are, however, liquid, hydrophobic and immiscible in water. As a result, they cannot be easily incorporated into the pigment paste of the electrocoat bath and tend to exude to the surface creating undesirable float in the electrocoat bath.
Various other bismuth-containing compounds have been proposed as catalysts for electrocoating processes. These include, for example, metallic bismuth, bismuth trioxide, organosulfur-based bismuth compounds, and the like, and are typically used in conjunction with other catalytic compounds like metallic zirconium, organotin compounds, or heterocyclic compounds like the mercapto-functional compounds.
From the foregoing, it will be appreciated that what is needed in the art is an effective catalyst for electrodeposition that is substantially or even completely free of organotin compounds but can be easily incorporated into an electrocoat bath without causing undesirable float problems or losing desirable cured film properties such as corrosion resistance. Such catalysts, compositions containing such catalysts, and methods for preparing and using the catalysts and compositions are disclosed and claimed herein.