Superprimer

#1. Field of the Invention

#The present inventions relates to corrosion protection and increased adhesion between substrates and a subsequent bonded material. More specifically, the present invention is related to primers, manufactured from at least one organofunctional bis-silane, having increased film thickness, chemical and scratch resistance, as well as being substantially chromate-free and comprising little to no VOCs.

#2. Background of the Invention

#FIG. 1 is a graphical representation of small angle X-ray scattering (SAXS) from tetramethylorthosilicate (TMOS) polymerized at three pH values;

#FIG. 2 is a dynamic mechanical analysis (DMA) relative to two-phase films with and without organosilane treatment;

#FIG. 3 is a chart listing metals and forms of corrosion where bis-silane films have been demonstrated to be very effective;

#Organofunctional silanes have been shown to be powerful systems for protecting a wide range of metals against corrosion. However, there exists a need for a coating which: a) adheres very well to the metal and is paintable by common paint systems such as, for example, epoxies, polyesters, acrylates, and polyurethanes, as well as improving adhesion of both soft (e.g. rubber adhesion to tire cord) and hard materials (structural adhesives); b) protects major engineering metals such as, for example, carbon steel, galvanized steel, stainless steel, aluminum alloys and magnesium alloys against common forms of corrosion, including localized attack such as pitting, stress corrosion cracking (SCC) and corrosion fatigue cracking (CFC); c) contains minimal additives such as dyes and inhibitors; d) is effectively devoid of chromates or other toxic components; e) has little or no volatile organic compounds (VOCs); and, f) is water soluble. The integrated organosilane system meeting these goals could replace the chromate system entirely including all chromate pretreatment and all corrosion-inhibiting chromate pigments. An exemplary application of such a coating might be directed to the sheet metal industry as a coil pretreatment prior to painting.

#The present invention, which, in the exemplary embodiment, provides a coating meeting the aforementioned goals, comprises at least one organofunctional silane, an organic resin and a nanoparticle filler. The novel primer, dubbed the “superprimer” is amenable to dipping, spraying, wiping or brushing onto any clean metal surface. No conversion coating, either phosphate or chromate, is required. One of the principles underlying the present invention is the hydrophobicity transition exhibited by organosilanes. While the unpolymerized silanes are hydrophilic and water soluble, they become highly hydrophobic on deposition, resulting in extremely low water-transmission rates.

#Optimization of silane films involves a number of seemingly contradictory requirements. It is desirable that the film precursors are hydrophilic to permit water-borne deposition. On the other hand, it is desirable that the film itself be very hydrophobic to assure superior protection. Also, it is desirable that the silane films be thin enough to be pore free yet thick enough to provide an adequate barrier to water penetration. Thin organosilane prior art films have been dense and have shown the required hydrophobicity, but lacked toughness. Thick films, on the other hand, were porous and susceptible to cracking and typically had reduced hydrophobicity.