The global ink industry reached a value of more than $14 billion in 2006. In the global ink industry, the inkjet ink technology is growing rapidly and turning out be a key digital printing technology. The developments in the inkjet technology paved the way to new applications, including patterning for RFID, precise liquid dispensing, and much more.
Significance of Viscosity Measurement
Surface tension and viscosity are the two physical characteristics of inks that determine their printing quality as the droplet formation of the ink is affected by these two factors. Hence, it is essential to effectively control the viscosity of ink in order to ensure its printing quality. The viscosity of ink varies with its ingredients such as pigments, binding agents and resins. Since these formulas are complicated, ink has a very complex viscosity behavior that varies with the level of shearing or the inkjetting rate. The ink experiences a complex time dependent shearing when it passes through the inkjet nozzle, and the extent of shearing is very high especially near the nozzle.
The magnitude calculation suggests that the shear rate can range between 105 ~ 107 s-1. The calculation signifies that obtaining low shear rate viscosity data is not only sufficient to thoroughly analyze the behavior of inks during inkjetting. It is also important to consider the viscosity at high shear rates. Moreover, non-Newtonian inks exhibit drastically different viscosity that is related to the inkjetting rate (shear rate). Considering the physical complexity of inks, gaining insight through viscosity measurement over a broad range of shear rates is essential in order to obtain better control. This article discusses the testing of three inks using the VROC system from RheoSense.
VROC Chip Technology
VROC (viscometer/rheometer-on-a-chip) is the tiniest viscometer built in a chip that offers a complete viscosity measurement over the broad range of shear rates for any liquid. This chip is recognized for its precision and accuracy, and designed to perform viscosity characterization for high throughput systems. The VROC is ideal for ink rheology characterization thanks to the micron scale flow path in the chip that very much resembles the actual inkjet conditions. This feature facilitates the VROC to simulate actual inkjet conditions by enabling it to provide viscosity characterization beyond 105 s-1. With this range and accuracy, the VROC system becomes an essential device to perform ink characterization over the complete shear rate range in order to ensure ink quality.
The viscometer-on-a-chip performs viscosity measurements from pressure drop caused by the flow of a test liquid through a rectangular slit. The VROC chip features a rectangular slit, which is composed of glass and a monolithic Si pressure sensor array. The rectangular slit’s width is much higher than its depth. Moreover, its edges are a trivial contribution to the pressure drop.
Figure 2. Rectangular Slit
In this study, three ink samples were tested with VROC chip types C-05 and A-05 at a temperature range of 23 ~ 23.7°C. The syringe was loaded with the test sample and mounted onto a pump. Then the viscosity was determined as a function of shear rate using VROC_RateSweep program. True viscosity was extracted for non-Newtonians using 2D-Weissenberg-Rabinowitsch correction. Then, the measurements were stored and plotted to get viscosity curves, as shown in Figure 3. Using a suitable solvent, the flow path was cleaned after each test.
Figure 3. Viscosity vs. shear rates
The results demonstrate the complexity of the viscosity curves of inkjet inks tested. Inks exhibited different viscosity behavior even when the viscosity value was low. The viscosity measurements suggested that there is a difference in the viscosities of inks and their shear rates dependencies. Similar viscosities at low shear rates did not yield adequate data to detect whether the two inks delivered the same performance. The differences in inkjetting performance may be due to dissimilar viscosity at high shear rates.
These results emphasize the significance of complete viscosity characterization. Adequate information can be obtained by measuring the entire viscosity curve in order to achieve better control over ink quality. This measurement can be accurately and reliably performed with the VROC system from RheoSense.
This information has been sourced, reviewed and adapted from materials provided by RheoSense.
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