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.
Optical Scattering Efficiency
Light Dispersion
Ability of a paper sheet or mineral pigment coating to diffuse incident light and prevent its transmission through the substrate. In the production of white-lined paperboard, optical scattering efficiency determines the level of opacity achieved for a given basis weight. This optical property governs how effectively the top layer of the board hides the grey or brown recycled fibres beneath it.
Opacity Generation
Introducing titanium dioxide or calcined clay into the coating formulation increases the number of internal interfaces that bend and scatter light waves. The diameter and spacing of these pigment particles are optimized to match the wavelength of visible light, which maximizes the scattering of light. High optical scattering efficiency allows mills to reduce the coat weight of the white layer while maintaining high brightness and coverage.
If this efficiency is too low, the dark under-layers of the recycled board show through, which dulls the printed graphics and lowers the aesthetic value of the packaging.
Measurement Practice
Evaluating this parameter involves measuring the reflectance of the paper sheet over a black backing and a white backing using a spectrophotometer. This test yields a scattering coefficient that assists paper chemists in formulating cost-effective coatings. Optimizing the pigment blend minimizes the consumption of expensive whitening agents.