Run-time Ink Stability in Pneumatic Aerosol Jet Printing Using a Split Stream Solvent Add Back System

Aerosol Jet printing is a non-contact process capable of printing nano-ink patterns on conformal and flexible surfaces. Aqueous or solvent nano-inks are pneumatically atomized by the flow of nitrogen gas. The flow of atomizing gas into and out of the cup leads to evaporation and removal of volatile solvent(s). As the solid loading fraction of the ink increases, the rheological changes eventually lead to instabilities in print output. A potential solution to this problem is to moisten the atomizing ink by running it through a bubbler. In this study, neat co-solvent solutions of ethanol and ethylene glycol at 85: 15 and 30:70 mixing ratios were atomized using nitrogen flow rates ranging from 600 to 1000 ccm. It was observed that ethanol, being the more volatile solvent, was depleted from the neat solution. When using a bubbler solvent add-back system, an excessive amount of ethanol was returned to the neat solution. The rate of solvent loss from an ethanol rich neat solution (80%) was higher compared to an ethylene glycol rich neat solution. A mixture of dry and wet (ethanol moistened) nitrogen gas was used to equalize the rate of ethanol evaporation. Ethanol equilibrium in neat solutions with higher ethylene glycol loading (70%) was achieved with a 40-60% wet nitrogen component while neat solutions with higher ethanol loading (85%) were stable with 85 -90% wet nitrogen gas. The results were validated with copper nano ink with similar co-solvent ratios. The solid content of the ink remained constant over four hours of printing when the optimal dry: wet nitrogen gas ratios were used. Copper ink with 85% ethanol being atomized at 1000 ccm exhibited increase in copper loading (3%) despite the dry: wet solvent add back system.