Crystalline Pigment
Mineral additives with exceptionally high refractive indices are used in paper coatings to scatter light and hide the underlying substrate. In the paper industry, the most effective of these pigments is rutile titanium dioxide, which has a refractive index of approximately two point seven. It is widely used in lightweight papers and high-opacity packaging boards where maximum hiding power is required with minimal coating weight.
The crystal structure of this mineral is highly compact, which allows it to scatter light more efficiently than other forms of titanium dioxide or calcium carbonate. This property makes it indispensable for premium printing papers.
Opacifying Efficiency
Light scattering depends on both the difference in refractive index between the pigment and the binder, and the particle size distribution of the pigment. When paper manufacturers use rutile titanium dioxide, they can achieve a high level of opacity at a much lower loading level compared to clays or calcium carbonates. This efficiency is due to the pigment’s ability to bend and scatter incoming light before it can pass through the sheet and reveal the print on the reverse side.
This reduction in the required pigment volume preserves the physical strength of the paperboard, as high filler loading can otherwise weaken the fiber-to-fiber bonds. The resulting sheets maintain high stiffness and tensile strength while remaining opaque.
Photolytic Stability
Exposure to ultraviolet light can trigger photocatalytic reactions in titanium pigments that degrade the surrounding paper fibers and polymer binders. Because rutile titanium dioxide has a more stable crystal lattice and is often treated with inorganic surface coatings, it is much less reactive than the alternative anatase form. This treatment prevents the yellowing and loss of strength that occurs when printed packaging is exposed to sunlight or store lighting for extended periods.
