Shear Rate
Rate of change describes the variation in fluid velocity across the thickness of a flowing liquid or boundary layer. In textile finishing, yarn sizing, and coating applications, velocity gradient measures the shear rate within the applied chemical paste or sizing solution as it is pressed onto the yarn or fabric surface. This gradient determines the shear stress experienced by the starch or polymer emulsion, which directly influences its viscosity and depth of penetration into the yarn core.
Sizing Absorption
Sizing absorption relies on maintaining a controlled shear rate between the sizing roll and the moving warp sheet. When the warp sheet runs through the sizing box, a high velocity gradient occurs at the nip point of the squeeze rolls, which reduces the viscosity of the shear-thinning starch mixture. This allows the protective size to coat the yarn evenly.
Coating Thickness
Coating thickness is governed by the spacing between the doctor blade and the fabric surface on a continuous coating line. As the fabric passes under the blade, the velocity gradient across the liquid polymer layer determines how much coating is deposited and how much is sheared off. This mechanism is critical when applying flame-retardant or water-resistant finishes, where too low a gradient results in an excessively thick, brittle coating that will crack during subsequent handling.
Technicians adjust the blade angle and the gap to maintain the required shear profile for the specific coating viscosity used in production.
Viscous Heating
Viscous heating occurs when high shear rates generate internal friction within the coating compound. In high-speed continuous operations, this heat can break down the emulsion, leading to spotty application and reduced wash-fastness on the finished synthetic fabrics.
