In recent years, inkjet printing has evolved into one of the most efficient printing methods in the field of tile decoration, which enables the production of high definition patterns and images on a variety of non-planar ceramic substrates. To achieve such printing results, ceramic inks with specific rheological properties must be developed to accommodate ceramic inkjet printing processes. That is, when stored, the ink does not precipitate even if it is subjected to gravity; when printing, the ink is subjected to extremely high shearing action in the printing head, and the flow characteristics can be achieved.
Rotary rheometry is a widely used rheological characterization tool on the market that measures the shear viscosity of a material over a wide range of shear rates. However, a rotating rheometer is less suitable if the ultra-high shear rate conditions in the analog printhead are to be achieved. Relatively new microfluidic rheological measurement techniques can deliver significant value.
The importance of process-related rheological data
The shear rate of the ink in the print head ranges from 105 to 106 s-1. In order to achieve effective jet actuation and droplet deposition under such high shear conditions, the ink must have good fluidity and the viscosity is preferably maintained at 5 to 25 mPa.s. However, low viscosity is detrimental to the storage stability of the ink.
It is well known that ink is a suspension comprising a solvent, a binder and a surfactant, a suspended pigment and/or a dye. These suspended components are prone to precipitation under low viscosity formulations. Therefore, in order to maintain stability when stored in a bottle or container, the ink needs to have a relatively high viscosity under conditions of low shear stress (gravity only).
To meet these two conflicting requirements at the same time, researchers must accurately measure and control the viscosity of the ink formulation over a wide shear rate to achieve optimal ink performance.
Introduction to microfluidic rheological measurement technology
Rotating rheological measurement techniques are suitable for shear rate ranges from less than 1 s-1 to several thousand s-1. At higher shear rates, problems such as flow instability, edge cracking, shear heat generation, etc., are more pronounced for low viscosity formulations such as ink. Although the microfluidic rheological measurement technology has a short time to come, it can effectively compensate for the shortcomings of the rotating rheology technology and meet the measurement requirements at ultra-high shear rates.
In a microfluidic rheometer, liquid is passed through a narrow microchannel (typically 40-200 μm) at a known flow rate, using an embedded microelectromechanical system (MEMS) pressure sensor to measure the pressure drop along the flow direction. The viscosity of the sample can be obtained by finding the correspondence between the differential pressure and the volume flow. By varying the flow or geometry of the runner, the viscosity at different shear rates can be measured to obtain the flow curve of the ink, ie the shear viscosity versus shear rate. Like a rotating rheometer, a microfluidic rheometer can precisely control temperature and investigate the effect of temperature on ink fluid properties. The above two advantages are the value of the microfluidic rheometer for ink formulation development.
To illustrate how to combine a rotary rheometer with a microfluidic rheometer to provide important reference data for the development of ink formulations, we used the Malvern Kinexus rotational rheometer and the m-VROCi microfluidic rheometer in the experiment. A and B commercially available inkjet inks were measured. The two flow curves in Figure 1 represent the relevant data for Ink A and Ink B, respectively, and each of the flow curves can present data obtained using a Malvern Kinexus rotational rheometer and an m-VROCi microfluidic rheometer.
We used these two instruments to determine the viscosity of the ink at a shear rate of 0.5 s-1 to 100,000 s-1. Although the flow curves of both inks decreased slightly (indicating a slight non-Newtonian shear thinning behavior in the ink), the viscosity of both samples was relatively stable throughout the study. A ink has a viscosity of about 22 mPa.s at a shear rate of 1 s-1 and a viscosity of 17 mPa.s at a shear rate of 100,000 s--1, indicating that the ink is facing a higher shear rate. The viscosity will drop slightly. Although the viscosity drop is not large, it cannot be ignored. In many cases, this non-Newtonian property may be of great help in balancing the stability and high flow performance of the ink in the printhead.
Application prospects
With the increasing popularity of inkjet printing in tile decoration, there is an increasing demand for inks with advanced formulations. The so-called "advanced" means that the ink can perform at its best in all application stages. As a relatively new technology, microfluidic rheological measurement technology can measure the viscosity of ink under high shear rate in the print head, which is of great significance for the development of ink formulations. Using a Malvern microfluidic rheometer in combination with a rotary rheometer, researchers can simulate all the environments and conditions in which ink appears during printing, measuring ink viscosity over a very wide range of shear rates. These data can help researchers develop inks that have good storage stability, good flow performance, and good print quality.
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