Zeta Potential of Printing Inks Offers Insights into Performance
Inks for high-quality applications are frequently applied using a process by which a series of rollers acquire ink selectively according to a pattern formed on their surfaces and then transfer this ink to the printable substrate. This process is controlled by a combination of mechanically and electrostatically enhanced contacts.
Two ink samples were provided. One of these had performed well in a test and the other had shown undesirable characteristics. With the idea that particle-particle interactions may be critical in achieving the desired performance, we examined both in the Brookhaven ZetaPALS instrument. The results given below were obtained.
As an evaluation test of the Brookhaven ZetaPALS, a sample which contained dispersed water droplets with surfactant in a silicone oil was provided. The relative dielectric constant for the oil was 2.3 and the viscosity of the continuous phase (silicone oil) was 3.7 cP. These values imply a relative mobility of 0.007 in the context of the above table. It was of interest to verify that the Brookhaven ZetaPALS in it’s standard configuration with a 30mWatt Laser at 670 nm could accurately measure such a low mobility. The result of three replicate measurements is given in the summary presentation below.
The ZetaPALS instrument has clearly shown that these two samples differ in two important respects:
1. A zeta potential which is greater in magnitude than about 20 mV is normally considered to be sufficiently high to stabilize a dispersion. The fact that the “good” ink has a zeta potential of -33.2 mV whereas the “bad” sample is only 14.2 mV is significant and indicates that the latter material is much more likely to be unstable with respect to aggregation. Our additional observation that the “bad” sample was significantly more viscous than “good” supports this inference and indicates that aggregation has already occurred and the distribution of resulting larger particle sizes has affected the mechanical properties of the material.
2. The fact that the two samples have zeta potentials which are opposite in sign has important consequences for the application process which uses an electric field to enhance the transfer of the ink from the roller to the subject surface. Clearly the actual size and magnitude of the charge on the particles is critical in this and the difference found here would explain the unsuitability of the second sample.
The ZetaPALS instrument has identified two important aspects of the dispersion of ink particles in their media. That this medium (toluene) is an organic liquid of very low dielectric constant (2.38) is highly significant because in this the mobility of the particles is very low. Values of -8.4 and 3.6 m2/V.s x 10-10 were measured here. Only the Brookhaven Instruments ZetaPALS instrument can determine mobilities as low as these, and thus, only the ZetaPALS could have found the reasons for the relative behavior of these samples.