Rheological Property
Non Newtonian behavior where the viscosity of a fluid decreases as the rate of deformation increases allows for easier flow under stress. Most pigment inks and printing pastes exhibit shear thinning to facilitate passage through fine nozzles or screen meshes. Once the force is removed, the fluid typically returns to a higher viscosity state.
Rapid thickening helps the ink stay in place on the fabric surface without spreading excessively.
Printing Mechanism
High shear forces are generated at the nozzle of an inkjet printer or under the squeegee of a rotary screen machine. Because of shear thinning, the thick paste or concentrated ink becomes thin enough to penetrate the textile structure or pass through a mesh. This reduction in resistance prevents the clogging of hardware during high speed production.
Precise control of the polymer chain length in the binder allows for a predictable drop in viscosity.
Surface Stability
Recovery of the original thickness after the shear force stops is necessary for preventing bleeding. A rapid return to high viscosity during the landing phase ensures that the design remains sharp. If the shear thinning effect lasts too long after impact, the ink will wick into the yarns.
Thixotropic agents are often added to the formulation to tune the timing of this recovery.
Viscosity Measurement
Rotational viscometers test the fluid at different speeds to map the flow curve. The degree of shear thinning is quantified by the slope of this curve.
