INTRODUCTION
New developments in siloxane surface treatments have been reported in recent Military/Industry Technology Exchanges including RUST 2000-2, NRL Technology Exchange August 2001, the Air Force Coatings Removal Conference 2002, and the 9 th International Hard Anodizers Association Technical Symposium September 2002. This paper review the development of a specific class of reactive silane- siloxane coatings called Ambient Temperature Curing (ATC) glasses. Unlike most pure silane-silicate coatings that typically are baked to achieve a cohesive film, ATC glasses can be applied in field and industrial environments using common paint application tools and techniques. Case studies and test data will be presented to illustrate potential applications of ATC glass coatings in military applications, lessons learned, and current and future developments of the technology.
The backbone of most paint resins is the C-C bonds. Hydrocarbon resins tend to be high-molecular weight, low-density polymers that cross-link and condense upon evaporation of a solvent carrier. Films must be applied thinly enough to minimize solvent entrapment. Film adhesion to a substrate is principally mechanical with a few covalent bonds between OH groups in the metal oxide and active organic groups in the polymer. This requires the substrate to be free of grease, oils, and salt contaminants and to have a rough profile to facilitate mechanical adhesion. Over paints, the substrate must also either be tacky (that is, most of the solvent evaporated, but still with active organic groups) or fully cured, clean, and roughened. The development of silicone-based coatings is better described elsewhere 1-4. Silicones, more properly called polysiloxanes, have Si-O backbones that are 130% stronger than the C-C backbones of organic polymers (bond energies of 108-110 vs. 84-87 Kcal/mole, respectively) 5. The Si-C bond is also strong and stable provided the organic group is small like methyl (CH3). The non-polar structure and strength of polysiloxane make it very stable at both high and low temperatures.
Chlorine-terminated oligomers of smaller molecular weight produce silicone fluids or silicone oils are used today as wetting and flow agents in paints 1. Silanes also enjoy wide application as coupling agents 2. The low glass transition temperature of polysiloxanes (e.g., -127°C for poly(dimethylsiloxane)) represents the ease by which R-groups rotate around the Si-O backbone that "...leads to rapid orientation of methyl groups at the surface. ''1
Typically, siloxane films were baked to drive off organics and promote cross-linking ~. To be practical as a coating, siloxanes that cured at ambient temperature were needed. "Acid-hydrolyzed ethyl silicate combined with powdered zinc was.., one of the first self-curing zinc silicate products. '~ Zinc silicate coatings are the basis for the inorganic zinc primer and pre-construction primers used in Naval ship construction and repair to this day. The allowable volatile organic compound (VOC) content for such weld-through pre-construction primers is 650 g/1 (5.4 lbs./gal) per the National Emissions Standards for Hazardous Air Pollutants (NESHAP) for Shipbuilding & Repair (40CFR63:780-788).
Hybrid Organic-Inorganic Resins
Organic resins can be mixed with up to 30% silanes to produce hybrid coatings such as the silicone alkyds used in topcoating Navy ships. More recently, graft co-polymers have been formulated for electronics, medical tubing, and lens coatings. The Ameron PSX TM series of"engineered siloxanes" represents another hybrid organic-inorganic paint coating.
Until recently, pure reactive silanes were suitable only as additives or specializ
