Flexographic Aqueous Latex Barrier Coating Viscosity Measurement Fundamentals
Flexographic aqueous latex barrier coating viscosity requires multi-shear rotational measurement to ensure accurate anilox transfer and barrier film continuity.
Thixotropy
Rheological Behavior
Fluid flow behavior where the viscosity of a material decreases over time under constant shear stress and then recovers slowly when the stress is removed is a critical property of high-quality inks and coatings. Press operators manage thixotropy in gel-based flexographic inks and heavy starch adhesives used in board laminating. This property prevents the liquid from dripping or spreading uncontrollably when applied on high-speed converting lines.
Recovery Mechanics
When a thixotropic fluid is pumped through an applicator nozzle, the mechanical shear breaks down the internal gel structure, which reduces the viscosity and allows it to flow easily. Once the fluid is deposited on the paperboard surface and the shear force stops, the structure begins to rebuild, but this process takes time. This delayed recovery is different from simple shear-thinning, as the viscosity depends on both the amount of force and the duration of that force.
This delay allows the ink or coating to flatten out and form a smooth, even surface before it sets.
Printing Application
Ink must remain fluid while being worked on the printing rollers but must quickly regain its body after transfer to the board to prevent bleeding and color smearing. If the recovery is too slow, the print becomes blurry and loses detail. If it is too fast, the ink does not level out, leading to a rough, uneven finish.
This time-dependent flow property ensures that high-speed printing presses can produce sharp, clear graphics on paperboard.