Conductive Inks: FAQs
Conductive inks contain pigments that provide function, beyond that of color and substrate hiding power of graphic inks. Conductive components contained in conductive inks may be comprised of silver, carbon, graphite, or other precious metal coated base material. Depending on the application, more exotic conductive fillers exist.
Conductive inks can be applied in several ways including screen print, flexographic or rotogravure, spray, dip, syringe dispense, and stencil print. Conductive inks are formulated for a specific method of application. Conductive inks for screen printing tend to be solvent based because water based inks evaporate too quickly and the viscosity tends to be too low to screen effectively. Water- based inks are available for high speed printing and coating applications.
Silver is approximately 9 times denser than the surrounding binder and solvent(s). Left in storage, the silver settles to the bottom. Settling occurs faster in inks stored at higher temperatures.
In the event that your ink has settled, it is imperative that all of the silver is remixed thoroughly to re-suspend the silver particles into the ink. Failure to do so will result in prints with higher than normal resistance readings in the short term. In the long term, resistance values may improve because the silver to binder ratio has changed from the original ink.
There are two factors that may be getting noticed. The first is the flotation of nanoparticles on the ink surface. These nanoparticles have a slightly higher affinity for the air than the ink so they appear to float on the surface. When stirred, the particles reincorporate but then separate to show a mirror like finish on the ink surface. This is normal and will not cause conductivity issues.
The second involves the silver flakes. When stirred, the ink flows around the stirring implement and quickly changes the direction of flow from laterally along the blade surface to rolling around the edge and filling in the area behind the blade. This change in flow direction results in some of the flakes being oriented in different planes. The flake orientations result in regions that reflect light to the eye and other regions that appear darker because the ink is reflecting light in another direction. This is normal for silver laden conductive inks and will not cause conductivity issues.
Conductive carbon inks tend to be higher viscosity than their silver counterparts because of the properties that allow the carbon to conduct. Conductive carbons are often nanometer sized particles with high structure. The high structure and small particle size requires more resin to achieve similar viscosities to silver inks. A balance must be struck between acceptable viscosity and acceptable conductivity levels.
The rate at which a solvent evaporates is directly proportional to the total surface area so large exposed areas can experience large swings in viscosity, percent solids, and resistance values. The best way to handle the ink in this case is to work the ink into a corner of the screen into the smallest mound possible to minimize the surface area of the ink which is exposed to the atmosphere.
This depends on the chemistry of the system chosen and the solvent in question. In general, reactive inks tend to be more solvent resistant than non-reactive inks. Kayaku Advanced Materials, Inc. will happily work with customers to determine solvent compatibility for their individual needs.