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.
BET Surface Area Analysis
Porosity Measurement
Gas adsorption techniques calculate the specific surface area of finely divided or porous solids by measuring the amount of physical gas adsorption on the material surface. In paper coating formulations, bet surface area analysis helps evaluate the structure of calcium carbonate and clay pigments. The method measures the volume of nitrogen gas adsorbed at liquid nitrogen temperatures across a range of relative pressures.
Pigment Characterization
Microscopic void spaces and surface irregularities affect how liquid binders are absorbed into the dried coating layer. When a pigment has a high specific area, as shown by bet surface area analysis, it demands a greater proportion of adhesive to achieve the same mechanical strength. Formulators must adjust the latex or starch ratio to compensate for this demand, ensuring the dry coating does not lose its structural integrity during printing.
Ink Absorption
Liquid vehicle drainage from offset printing inks occurs rapidly on highly porous coatings. This absorption behavior is related to the pore volume determined through bet surface area analysis. Excessive absorption rates can lead to low ink gloss and poor rub resistance because the binder is drawn away from the pigment on the surface of the paper, meaning that controlling the pigment pore structure is a requirement for high print quality.