Light Attenuation
Mineral fillers used in papermaking absorb specific wavelengths of light depending on their chemical composition and crystalline structure. This property, known as kaolin clay optical absorption, describes how much light is lost as it passes through or reflects from the clay particles within the paper coating matrix. Lower values of this parameter are desirable because they indicate that the clay will not dull the appearance of the paper sheet.
It is determined by measuring the reflectance of the clay powder across the visible spectrum, especially in the blue region where iron and titanium impurities tend to absorb light. Controlling this characteristic is necessary for producing high-brightness coated papers.
Pigment Purity
Natural deposits of kaolin contain trace amounts of iron oxides and anatase titanium dioxide that can degrade the optical performance of the refined clay. These impurities increase the kaolin clay optical absorption in the short-wavelength blue region of the spectrum, which causes the clay to look cream-colored rather than a neutral white. To minimize this effect, clay processors employ physical and chemical purification techniques, such as magnetic separation and chemical leaching.
These processes reduce the concentration of light-absorbing minerals and improve the overall reflectance of the clay pigment. The resulting purified clay allows paper mills to formulate coatings that require fewer expensive whitening agents to achieve high brightness.
Coating Performance
Paper coatings that incorporate kaolin with low absorption properties demonstrate superior scattering efficiency and print gloss. When kaolin clay optical absorption is kept to a minimum, the light penetrates the coating layer, scatters off the pigment particles, and returns to the observer, maximizing the perceived opacity of the sheet. This scattering ability is particularly critical for lightweight coated papers where a thin coating must hide the underlying dark fibers.
