Mineral Coating Pigment Packing Micro Geometry and Pore Volume Distribution
Pigment micro-geometry and particle packing dictate coat pore volume distribution, controlling ink vehicle absorption rate, print gloss, and coat strength.
Blade Coater Rheology
High Shear Hydrodynamics
High-shear fluid deformation governing mineral pigment suspensions beneath a rigid or bent doctor blade determines liquid coat weight application and runnability. Operating at shear rates exceeding one million reciprocal seconds, blade coater rheology dictates the hydrodynamic lift force exerted against the metering edge. Coating formulations must exhibit controlled pseudoplastic or shear-thinning flow profiles to avoid excessive hydraulic pressure that forces the blade away from the base paper.
Dilatant formulations generate severe normal forces at the metering boundary, resulting in blade bleeding, web scratching, and catastrophic web breaks. The viscosity curve across high shear regimes defines the target coat weight window for paperboard machines.
Viscoelastic Balance
Elastic recovery and extensional viscosity within the aqueous suspension control streak generation and misting at the exit nip of the blade. Additives such as carboxymethyl cellulose, polyvinyl alcohol, and associative synthetic thickeners alter the structural breakdown and recovery kinetics under blade coater rheology constraints. Rapid structural rebuilding immediately after the blade prevents formulation penetration into the sheet while eliminating micro-ridges along the machine direction.
Excessive elasticity induces viscoelastic normal stress differences, causing the trailing edge of the suspension to form spatters and scratches on lightweight coated papers.
Formulation Limits
Mineral solids loading directly bounds the hydrodynamic stability of the coating application. Elevating suspension solids beyond sixty-five percent shifts the suspension close to its maximum packing fraction, where slight shear rate fluctuations produce dramatic spikes in apparent blade load. Temperature variations across the coating supply loop further destabilize fluid viscosity, shifting applied coat weights across the machine profile.
Rheological control ceases to determine runnability when base paper surface roughness exceeds the nominal wet coating thickness beneath the blade.