Known methods of treating metal surfaces to improve paint adhesion and corrosion resistance of the painted metal surfaces include two general classes of chemistries. The first class is based on traditional conversion coating types of chemistries, such as zinc phosphate, iron phosphates, chromium chromate, chromium phosphate, etc. The second class is based on more recent developments in the metal pretreatment industry and is characterized by what is now referred to as “dried-in-place” technology. Traditional conversion coating chemistries require rinsing of the metal substrate to remove applied pretreatment solution. Dried-in-place chemistries allow for the applied solutions to be dried on the metal substrate to which they are applied, without subsequent rinsing.
Chromium compounds have been used as traditional conversion coatings to treat metal surfaces. Such chromium compounds show toxicological effects and have been determined by the Environmental Protection Agency to be a risk to the environment and by the Occupational Safety and Health Agency to be a health risk. Moreover, hexavalent chromium compounds such as are used in some of these systems are classified as carcinogens by these agencies.
As a result, much effort has been expended in developing compositions and methods for producing chromium-free pretreatments. For example, the owner of the present application has commercialized products that are successful in improving the corrosion resistance and paint adhesion of metal surfaces. Such compositions are disclosed in, for example, U.S. Pat. No. 5,859,106 to Jones et al, directed to an aqueous composition including a polymer system having carboxylic functional groups and hydroxyl groups and a compound of a group IV-B element.
One application for conversion coatings is the pretreatment of metal coils. In a pretreatment coating operation, a coil (roll) of metal is unwound, cleaned, pretreated with the conversion coating to improve corrosion resistance and/or paint adhesion, and then decorated (e.g., painted and baked). The painted stock is then rewrapped and ultimately used to create articles which, already having been painted, require no additional decorating step. Thus, a convenient and economic means for producing painted articles is achieved.
Because the prepainted coil must be cut, bent, impacted, and shaped into a desired article, good adhesion of the paint to the metal surface is important. Further, corrosion resistance can be of importance since an article produced from painted coil can be required to perform in severe weather. Articles such as siding and rainwear (e.g., gutters and spouts) are some examples. Thus, improved conversion coating materials and methods for applying them are constantly sought.