Low Humidity Impact on Dispersion Barrier Coating Fold Integrity
Low relative humidity embrittles paperboard and dispersion coatings, causing fold micro-cracks that degrade barrier integrity unless creasing geometry adapts.
Latex Particle Coalescence
Microstructural Process
The merging of individual synthetic resin spheres into a homogeneous film during the drying of a coating color defines the transition from a liquid suspension to a solid barrier. Proper latex particle coalescence ensures that the pigment particles in a clay coating are bound securely to each other and the paper surface. This fusion is required for the gloss and the ink holdout of high quality printing papers.
Physical Requirement
Surface tension and capillary forces pull the particles together as the water evaporates from the wet layer. Successful latex particle coalescence requires the ambient temperature to exceed the glass transition point of the polymer. If the drying happens too quickly or at too low a temperature, the particles remain separate and create a dusty or porous surface.
This defect leads to picking where the tacky printing ink pulls the coating off the paper during the press run.
Coating Strength
Coating strength depends on the degree of interdiffusion between the polymer chains at the particle interfaces. When latex particle coalescence is complete, the coating resists the mechanical stresses of calendering and folding. A well formed film also prevents the fountain solution in offset printing from softening the binder.