Powder coatings are used throughout industry to paint a myriad of metallic objects. This method of coating has gained popularity because of its conservation of materials and elimination of volatile organic compounds (VOCs). Resins used in powder coatings are traditionally chosen from those that have low melting points (polyesters, acrylics, urethanes, epoxies, etc.). These resins are used because they can melt and flow into a smooth coating before curing to a durable surface. High-performance resins such as Teflon, nylon, and polyimide have not found use in powder coatings because of their high melting points.

A number of low-melt polyamic acids have been developed for use as the resin in a powder coating. These resins, when sprayed onto metal surfaces, could then be cured in conventional powder coating ovens to deliver high-performance polyimide coatings that would offer superior heat and electrical stability, as well as superior chemical resistance over other powder coatings.

Coatings manufacturers are continually searching for new techniques and formulations to increase protection of metallic surfaces and other systems. These coatings are, in many cases, an improvement on traditional liquid-based coatings with respect to oxidation/corrosion protection, and have the added advantage of being much more environmentally friendly. Polyimides are one example of a high-performance resin that has been used in some coating applications (i.e., wire insulation). This type of coating has to be applied as a film wrap because of the difficulty encountered in working with this polymer.

Polyimide is a high-performance plastic. One of its outstanding features is its high-temperature property. Because of this, however, this plastic cannot be processed or extruded using typical industry techniques. Precursor polyamic acids are soluble in some organic solvents, thus allowing this precursor to be cast as a thin film, which can then be heat-cured into the final polyimide. Recent research has identified some polyamic acids that have low melting points. These melting points are low enough that the polymer can melt and flow before cure to the polyimide is complete.

Given these low melting points, it now becomes possible to prepare this type of polymer in a form that can be applied to metal surfaces, heated to induce melting and flow, and then cured to the final polyimide. A number of potential benefits of this technology are envisioned. One such use would be as a high performance powder coating for metallic surfaces to provide superior protection from heat, oxidation, corrosion, etc. Another potential use would be as a primary electrical wire insulation. Low melt polyamic acids could be applied in powder form, melted and cured to a final insulation in a “continuous” polyimide coating, an achievement not possible at the time of this reporting for traditional polyimide films.

This work was done by Scott Jolley of Kennedy Space Center. For more information, contact the Kennedy Space Center Technology Transfer Office at 321-867-7171. KSC-12777

NASA Tech Briefs Magazine

This article first appeared in the January, 2015 issue of NASA Tech Briefs Magazine.

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