Coatings

A coating has to stick to its substrate, cure into a continuous film, and then keep its properties through heat, weather, chemicals and wear. Silicon chemistry helps at each of those steps. The Si–O and Si–N bonds at the heart of silicone and polysilazane binders are stronger and more heat-stable than the C–C backbone of most organic resins, and silanes make the interface between coating and substrate chemically bonded rather than merely adhered.

Polysilazane binders

Organopolysilazanes react with moisture and with the hydroxyl groups on metal, glass and ceramic surfaces, curing into dense films with pencil hardness of 6H or more and water contact angles of roughly 100–110°. Depending on the grade they cure at room temperature, with heat, or by UV and free radicals, and they tolerate service temperatures up to about 500 °C. Typical uses are scratch-resistant clear coats for automotive trim, easy-clean and water-repellent coatings, high-temperature and anti-corrosion coatings on metals, and non-stick coatings on moulds. See polysilazane resins for a comparison of the grades.

Silicone resins for heat-resistant paints

Methylphenyl silicone resins keep their integrity at temperatures that break down epoxies and alkyds, which is why they are the binders of high-temperature paints, heat-resistant powder coatings and class H insulating varnishes. The phenyl content improves compatibility with organic resins, so they are also used to upgrade the heat resistance of organic coatings.

Silanes as adhesion promoters and crosslinkers

A few tenths of a percent of silane at the interface can turn a coating that peels into one that holds. Epoxy and methacryloxy silanes are built into epoxy, acrylic and polyurethane coatings to bond them to glass, metal and mineral fillers, and epoxy silane oligomers crosslink water-based systems with less alcohol release than monomers. Fluoroalkyl silanes go the other way: they leave a surface that water, oil and dirt do not wet, for easy-clean glass and protective films on electronics.

Choosing a system

• Can the part be heated? If not, choose a room-temperature-curing polysilazane or a moisture-curing silane system.
• What must the film survive? For scratch and chemical resistance, polysilazane; for sustained high temperature, phenyl silicone resin or a vinyl-functional polysilazane.
• Is adhesion the problem? Match the silane's organic group to the resin: epoxy or amino silanes for epoxy and polyurethane, methacryloxy silanes for acrylics.

For a recommendation for your formulation, a COA, an SDS or a quotation, contact our technical sales team.