Material Property
A numerical value that describes how much a fluid viscosity decreases when subjected to increasing mechanical shear strain characterizes its flow behaviour. In textile chemistry, the shear-thinning index indicates the ease with which a thickened dye liquor flows under the action of the pad mangle rollers. This property helps determine the pick-up consistency of chemical mixtures on fast-moving fabrics.
System Mechanism
When a polymer-thickened liquor enters the high-pressure nip of the pad mangle, the long polymer chains align parallel to the direction of flow. This alignment reduces the internal friction of the liquid, causing a drop in viscosity that allows the mixture to penetrate the yarn spaces quickly. A liquid with a high shear-thinning index undergoes a significant reduction in viscosity under shear, which improves fabric wetting at high production speeds.
Once the fabric exits the roller nip and the shear force drops, the viscosity rises again to prevent the dye from migrating to the fabric surface.
Operational Control
Measuring the viscosity at two or more different rotation speeds on a rotational viscometer allows technicians to calculate this parameter. When preparing pad baths, operators adjust the polymer concentration to maintain the shear-thinning index within a specified process window. Using a higher concentration of polymeric thickeners can increase this value to optimize performance.
Technicians monitor the temperature of the bath, as thermal energy can alter the polymer alignment and change the flow characteristics.
Process Constraint
If the value is too high, the rapid drop in viscosity can cause the liquor to splatter off the rollers at high speeds. This constraint requires careful selection of thickeners to match the specific speed of the production line.
