Silver inks contain particles of silver flake, typically smaller than 20 microns and sometimes as small as less than 1 micron. Compared to pigments used in traditional graphic inks, the silver flake is very dense. Conductive inks for screen printing are almost universally solvent based inks. Water based conductive inks tend to be too thin for screen printing. Making a UV cured conductive ink is difficult at best, because carbon, silver and other metals reflect UV energy so the polymer ink binder in the ink cannot get direct exposure to the UV energy and will not harden quickly.
Because silver is such a dense material compared to pigments used in graphic inks, silver ink will settle much more quickly in the container than graphic inks. It is essential that silver inks be remixed thoroughly before using. If the silver flake has settled into a hard pack on the bottom of the container, care must be taken to break it up completely and remix aggressively before using. Too often we see that someone will mix the ink by hand until it looks “silvery” and then start printing, not realizing that there is still a concentrated layer of silver particles left on the bottom of the container.
The swirling appearance of the ink is caused by the separation of large and small silver particles in the ink. Silver ink contains large flakes as well as very small flakes or particles to fill in the gaps between the large flakes to optimize conductivity. The very small particles tend to “float” in the ink binder, causing the appearance of a swirl pattern on the surface as the ink is mixed.
You will also see this swirling pattern more dramatically when silver ink is mixed with carbon ink.
It is important to note that the swirling appears in the wet ink, and will not be apparent on a wet printed trace, and on a printed trace after drying.
The carbon filler used in resistive inks is usually composed of carbon and graphite. The particles are all sub micron size. Because the particles are so small, carbon has a very large surface area by solid volume compared to other ink fillers and requires more aggressive effort to get the liquid binder to “wet out” the surface of all of the particles to optimize the ink properties. In general carbon inks tend to have much lower percent solids or more solvent content than silver inks, and are much higher in viscosity. If more solvent were added to drop the overall viscosity, the percent solids would be so low that it would be difficult to print and dry an ink trace that would have a suitable dry thickness.
Carbon inks are also highly thixotropic which means that if a carbon ink, with the consistency of wax, is mixed aggressively, or screened for a few cycles, the viscosity will drop considerably.
Generally it is not recommended to leave solvent based inks on a screen or in an open container for a long time because of the potential for loss of solvent to evaporation. When leaving inks on the screen for short periods, it is recommended that the ink be pooled to one end of the screen. This reduces the amount of surface area of the ink and will slow solvent evaporation. If the ink is to be left on the screen for more than 30 minutes, it is recommended that it be collected back in the container and sealed.
The majority of conductive inks utilize a “thermoplastic” polymer binder to hold the conductive particles together. A thermoplastic polymer is a material that is usually a solid at room temperature, and must be either heated to a high temperature or dissolved in solvent to make it a liquid. Once the thermoplastic is cooled or the solvent is dried off it returns to being a solid. This cycle can be repeated many times. Not all solvents will dissolve all thermoplastic materials so sometimes a suitable thermoplastic binder can be selected that will not dissolve in the solvent that it will be exposed to. The other option is to use a “thermoset” polymer like an epoxy for the ink binder. Unlike a thermoplastic, a thermoset is a liquid (usually called resin) that, when combined with another liquid or powder material (usually called hardener) causes a chemical reaction that crosslinks the molecules of both materials until a dense solid structure is left. Once crosslinked, it is not possible to dissolve a thermoset in solvent or heat it to turn it into a liquid again. However, it is important to understand that even though a thermoplastic may not dissolve in a particular solvent, and a thermoset will not dissolve in any solvent, they can still absorb solvents. This causes swelling and will weaken the mechanical properties of the polymer, and will reduce conductivity because the conductive particles are pushed apart by the solvent.
It is possible with some thermoplastic inks to add a hardener or crosslinker that will cause the ink binder to crosslink while the solvent is evaporating from it, thereby giving a more durable coating by turning the polymer into a thermoset. Our WB-101 water based conductive ink and Antistat 268 carbon coating can be provided with a crosslinker. HTC 300 carbon ink also is available with a crosslinking agent.
One drawback to using a thermoset binder system is that once the components are mixed they have a shelf life and must be used within a certain time or they will thicken to the point that they cannot be printed.
Crosslinking also tends to make an ink system less flexible, but more hard and with higher temperature resistant properties.