This article discusses how a visual microfluidic rheometer can be used for general ink characterization, using a visual microfluidic rheometer called FluidicamRHEO.
The transfer technique of each ink will varies depending on the printing process, and consequently different pressures will be applied. An inkjet printer will require ink which is very liquid and thin (1 - 5 mPa.s, <0.5 μm) whilst offset printing will need a viscous and thick ink layer (40 - 100 mPa.s, >0.5 μm).
Other ingredients give specific properties to the ink, in addition to main color molecules: drying time, pH, viscosity, etc. These will allow optimization of printing process and quality.
Nonetheless, on a daily basis, ink properties like viscosity can differ between batches and can cause printing problems in some cases. It is crucial to fully understand and control formulation and printing processes to guarantee high quality printing.
Understanding the behavior of the ink is an assurance of high quality results and avoids process issues such as bleeding or clogging the printing head ink that is too viscous.
To measure viscosity, FluidicamRHEO utilizes a co-flow microfluidic principle. Sample and reference standard are pushed together to the microfluidic channel (typically 2.2 mm X 150 μm) under controlled flow rates. Interface position between reference and sample relates the viscosity ratio between the two to the flow rates, in this laminar flow.
Figure 1: FluidicamRHEO measuring principle.
Images acquired during the measurement permit the calculation of the position of the interface and the direct plotting of an interactive flow curve.
To reproduce printing conditions, various ink formulations were tested at several temperatures (25, 35 and 50 °C). Shear rates vary from one sample to another depending on the final application and viscosity achieved. Typically, shear rate range was between 103 - 104 s-1. For some samples when a shear thinning behavior was seen at lower shear rates the minimum conditions reached 100s-1.
Ink Viscosity and Temperature Sweeps
Two cases of ceramic inks measured at high shear rates between 20 and 70 °C are presented below. FluidicamRHEO provides a smart protocol design to automatically measure viscosity as a function of shear rate and temperature.
Only a small volume needs to be thermos-controlled, as the instrument operates with microvolumes. Therefore, full shear and temperature dependencies can be analyzed in minutes.
Figure 2. Viscosity of ceramic ink measured at 20 - 50 °C with 15 °C automatic step.
Different Ink Formulations Compared
A single ingredient adjustment during ink development can often have a huge impact on printing performance in real life conditions. With FluidicamRHEO it is possible to measure several formulations easily within minutes and, thanks to high precision, characterize viscosity differences as small as 0.5 mPa.s.
Figure 3. Viscosity of 2 inkjet ink formulations at 35 °C.
A significant difference of flow behavior can be observed in both cases:
- In the first case one of the inks presents slightly stronger shear thinning behavior and lower viscosity overall
- In the second case, ink viscosity at high shear rate is very similar (less than 1 mPa.s of difference) but low shear viscosity vary strongly
FluidicamRHEO permits high precision viscosity measurements. Working with microfluidic chips enables achieving high shear rates and consumes only a slight sample volume. The feature is especially attractive for the ink industry, where FluidicanRHEO can aid formulators in understanding ink properties in real life conditions, without the need to extrapolate the data, therefore saving a lot of time.
This information has been sourced, reviewed and adapted from materials provided by Formulaction.
For more information on this source, please visit Formulaction.