The influence of fine kaolin and ground calcium carbonates on the efficiency and distribution of fluorescence whitening agents (FWA) in paper coating

Fluorescence whitening agent (FWA) distribution in paper coating is affected by paper and coating chemistry, coating and drying process. A pilot scale coating is a proper way to simulate a multi-layer coating. Fine kaolin blends with broad and narrow particle size distribution (BRD and NRW) ground calcium carbonates (GCC) were studied in top coat of double-coated paper from a high speed pilot coating. FWA efficiency was checked with TAPPI brightness and ISO CIE whiteness. Classic optical theory applications in light transmittance were further verified by another pilot scale coating with mixture design of experiment (DOE). This study found that when coating over a dark base stock containing no FWA, NRW GCC coating produced a higher light scattering coefficient, αscatt- as reflected in higher TAPPI brightness. However ISO CIE whiteness was lower than or equal to that of BRD GCC. An increase of αscatt of NRW GCC coating to UV light is due to the larger pore volume density, ρv and to a possible scattering cross-section, σs(λ) increase as wavelength reduces. In either case, the combination of ρv and σ s(λ) makes αscatt larger in NRW GCC than in BRW GCC to attenuate short wavelength UV light more, i.e. a reduced UV efficiency. The results of this study suggest that for FWA efficiency and uniformity it is desirable to design a carbonate and kaolin pigment system to produce a gradient FWA distribution in thickness direction with the highest FWA concentration towards surface. If wood free or ground wood base paper and board contain FWA for the whiteness and uniformity from multi-scattering properties, BRD GCC with fine kaolin can further improve CIE whiteness by allowing more UV light to transmit to the pre-coat and base paper to generate more blue light and to be backscattered through it.