Characteristics of Dry and Semi-dry Electrospray-printed Polymeric Films

In electrospray printing, high electric potentials are used to atomize a print material solution into a spray of charged droplets. The droplets follow electric field lines from the emitter to the target surfaces to create thin, conformal films. The extent of in-flight solvent evaporation determines the microstructure and function of the film. Dry, particulate films are formed when the solvent completely evaporates, while for incomplete evaporation, residual solvent is delivered with the print material to create a dense film. We report on the microstructural differences of these two print regimes defined as dry and semi-dry electrospray printing. Here, the print solution consisted of polyimide dispersed in dimethylformamide. The degree of solvent evaporation was controlled via the print solution flow rate and the flow rate of co-flowing air introduced during printing. Low solution flow rates or high co-flowing air resulted in dry films of polyimide, while semi-dry films were printed at high solution flow rates and no co-flowing air. The films were evaluated for their dielectric strength, optical characteristics, and corrosion resistance. Compared to the dry printed films, the semi-dry films had high optical transparency and superior dielectric strength, with breakdown field strengths of nearly 400 V/µm. Both print regimes showed excellent corrosion resistance when used to protect thin (30 µm) metal wires bonded to metallic pads. However, the semi-dry printed films performed better at low print times.