Meaning
Empirical dispersion diagrams plot the critical capillary number required to burst a suspended fluid droplet against the viscosity ratio between dispersed and matrix phases. The grace curve identifies processing windows where mechanical shear forces overcome interfacial surface tension during polymer blending in twin-screw extruders. The relationship governs dispersion efficiency for liquid additives and immiscible polymer phases.
Outside established shear rate windows, droplet deformation reverts to stable flow or simple elongation without achieving micro-dispersion.
Viscosity Ratio
Viscosity matching between matrix and dispersed polymer phases dictates whether shear or elongational flow field drives droplet breakup. The grace curve demonstrates that droplet breakup becomes difficult when the dispersed phase viscosity exceeds four times the matrix viscosity under pure shear flow. Matching phase viscosities at processing temperature reduces required mixing torque and prevents coarse domain formation.
Twin-screw extrusion profiles must generate elongational flow fields when processing high-viscosity dispersed additives.
Shear Breakup
Hydrodynamic drag forces deform spherical droplets into elongated threads that become unstable and snap into tiny droplets. Capillary forces resist deformation and attempt to restore spherical geometries. When shear stress exceeds critical capillary limits, viscous forces overpower surface tension.
Compounding Boundary
Processing conditions operating below critical capillary thresholds result in undispersed modifier agglomerates that cause mechanical weakness in moulded parts.
