Quantifying Matting Agent Displacement in Package Printing Overprint Coatings
Quantifying matting agent displacement requires z-axis Raman depth profiling and strict viscosity recovery control to stop gloss drift and scuff failure.
Stokes Law
Stokes Velocity
Particle sedimentation velocity within liquid suspensions relies upon fluid viscosity alongside particle radius and density differentials. Stokes law calculates terminal settling rates for spherical particles falling through viscous media under constant gravitational acceleration. Liquid coatings applied to paper substrates contain mineral pigments suspended in aqueous binders.
Settling velocity dictates how rapidly these pigment particles agglomerate inside storage tanks prior to application. Larger diameter clay or calcium carbonate particles sediment faster than finer fractions because velocity scales with the square of particle radius.
Fluid Viscosity
Resistance to flow within coating suspensions dictates the friction retarding settling particles. Dynamic viscosity values establish the drag force exerted by the continuous aqueous phase upon suspended mineral pigments. Temperature fluctuations alter fluid viscosity significantly across production shifts.
Higher fluid temperatures reduce medium resistance and accelerate particle sedimentation inside coating kitchens. Operators control liquid viscosity through rheology modifiers to prevent premature pigment settling during prolonged storage periods.
Particle Size
Equivalent spherical diameter determines gravitational separation rates within coating formulations. Coarse mineral agglomerates exceed operational size thresholds and cause surface defects on coated paper webs. Centrifugal forces applied during high shear processing modify effective particle trajectories within application heads.
Fine fraction retention ensures uniform optical properties and gloss across finished print surfaces. Terminal settling velocity establishes maximum allowable storage durations for aqueous suspensions before mechanical agitation becomes necessary.