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.
Latex Film Formation
Polymer Particle Coalescence
Progressive physical transition of discrete, aqueous-dispersed polymer spheres into a continuous, void-free adhesive film binds pigment particles to one another and to the base paper web. During the drying of waterborne paper coatings, latex film formation proceeds through three successive thermodynamic and mechanical stages: evaporative concentration, close packing of polymer particles, and particle deformation under capillary pressure. Once the particles touch, capillary forces exert compressive stresses that flatten the spheres into a continuous honeycomb matrix.
Interdiffusion of polymer chains across particle boundaries then establishes mechanical cohesion and structural strength throughout the coating layer.
Thermal Boundaries
Successful particle coalescence depends on the relationship between processing temperature and the glass transition temperature of the polymer. If drying occurs below the minimum film-forming temperature, latex spheres resist plastic deformation, resulting in a powdery, unbonded mineral layer plagued by severe dusting and low wet-pick resistance. Operating significantly above the glass transition temperature ensures rapid polymer chain interdiffusion, yielding high dry pick resistance and elastic flexibility during board creasing.
Formulators optimize latex film formation by selecting core-shell polymer morphologies that balance low film-formation temperatures against high blocking resistance in wound rolls.
Cohesion Mechanics
Water evaporation rate directly dictates the time available for polymer chain interpenetration before the system immobilizes. Premature surface drying traps uncoalesced particles beneath a dense skin, which impairs internal coating strength and invites blister defects during heatset offset printing. Plasticizing additives and coalescing aids lower the effective transformation temperature, facilitating particle deformation under high-speed industrial drying tunnels.
Film formation terminates once polymer auto-adhesion fully eliminates inter-particle interfaces.