Whether you are working with printed electronics, touch displays (OLEDs) or any number of applications including conductive inks, coatings, paints and pastes, viscosity is one of the key parameters that should be considered during manufacturing. Paints and inks are non-Newtonian fluids, which means their viscosity depends on the shear rate. Viscosity testing has shown us that inkjet inks need to be tested at higher shear rates than paints and coatings to simulate the inkjet process. Compared with paints and coatings, the range of shear rates is much higher and can even reach 100,000 1/s. When using viscosity as a function of shear rate testing, we are able to determine the behavior of inks, paints and coatings.
Thixotropy is a common rheological behavior in ink and coating formulations. Thixotropy refers to the time-dependent shear thinning properties of non-Newtonian fluids (Mewis 1979). This is a reversible process in which certain materials are thick or viscous under static conditions but thin or liquefy when shaken, stirred or otherwise disturbed . Then, after being allowed to sit for a fixed period of time, the materials reset or return to their more viscous state. One of the common thixotropic materials is ketchup .
ink
Viscosity measures a liquid's resistance to flow, which can have a significant impact on manufacturing when ink viscosity is used. Surface tension and viscosity determine the printing quality of the ink. These factors affect the formation of ink droplets, which in turn affect the quality of the final printout. Throughout the printing process, the viscosity properties of inks can have multiple effects on ink absorption, color strength and drying (Thai and Tupe, 2011).
The viscosity of the ink depends on the components of the ink, including resins, pigments and binders. Temperature also affects the viscosity of the ink. A 5.5°C increase in temperature can reduce the viscosity of printing inks by 50% or more (DeJidas and Destree, 2005). Since such small changes in temperature can have such a large impact on ink viscosity and print quality, the importance of maintaining a stable temperature while printing cannot be overemphasized. High viscosity materials can clog and possibly damage inkjet nozzles.
Conductive and ceramic inks are fast-growing applications utilizing advancements in inkjet technology. With inkjetable conductive inks, accurate liquid dispensing and patterning for RFID (radio frequency identification) is now possible. Inkjetable ceramic inks allow finer, finer patterns to be printed on tiles.
paint
Viscosity measures a paint's resistance to spreading, which "plays an important role in its application" (Zenith Home Finishes). The viscosity of the paint will need to be adjusted according to the application method you plan to use. The viscosity of a paint is a key indicator of the intended use of the paint and its ability to provide an even coating on the desired surface (The House Painting Guide, 2013). Artists have been known to thicken acrylic paint to the point that it needs to be applied to the canvas with a metal spatula, rather than a traditional paint brush. For example, if you will be using a brush or roller, your paint will have a higher viscosity than if you were using an airbrush. If you try to use a Spray Gun on highly viscous paint, your application will be uneven and it could even clog or damage your equipment.
Like any other liquid, the importance of measuring the viscosity of inks, paints and coatings cannot be overemphasized. The VROC ® Dynamic Viscometer provides accurate and reliable viscosity results that can be measured quickly and easily from a QC perspective with high shear rates and small sample capabilities for R&D development.
