Extensional Viscosity
Extensional viscosity defines the resistance of polymer melts and concentrated solutions to elongation during high-speed coating and filament stretching operations on paper converting lines. Extensional viscosity describes the ratio of tensile stress to the rate of extension in a flowing fluid. Extensional viscosity governs how barrier coatings behave when drawn into thin liquid films before contacting moving paper substrates.
Extensional viscosity also dictates edge tearing limits during rapid extrusion coating applications where polymer webs leave dies at elevated line speeds. Extensional viscosity rises sharply above shear viscosity in long-chain polymer solutions because polymer chains stretch under uniaxial tension. Extensional viscosity stops applying when molecular relaxation rates exceed deformation rates in low-molecular-weight mixtures.
Elongational Behavior
Elongational behavior measures the ratio of extensional viscosity to shear viscosity under standardized processing conditions for aqueous packaging coatings. Elongational behavior predicts necking stability during high-speed extrusion coating where molten polyethylene draws down onto paperboard. Elongational behavior limits maximum line speeds because excessive drawing tensions cause polymer webs to fracture before web contact occurs.
Elongational behavior prevents pinhole formation in water-based dispersion barriers by maintaining film integrity under mechanical stretch. Elongational behavior drops sharply if molecular weight distributions narrow beyond specific processing tolerances set by converters.
Flow Resistance
Flow resistance quantifies the force required to pull polymer melts through slot die coating heads during multilayer liquid packaging production. Flow resistance determines the thermal stability window for fluoropolymer dispersion application on grease-resistant food service paperboards. Flow resistance spikes when polymer chain entanglement increases in high-solid coating formulations designed for heavy containerboard grades.
Flow resistance declines rapidly as fluid temperatures rise inside pressurized coating delivery systems. Flow resistance defines operational limits for high-speed curtain coaters handling high-molecular-weight starch solutions.
