Laboratory Studies of Simultaneous Use of Titanium Dioxide Pigment and Fluorescent Whitening Agent in Double Coated Printing Paper, 1998 Coating Conference Proceedings

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Titanium dioxide pigments (TiO2 pigments) are mainly used in paper products of high quality. TiO2 pigments yield opacity and brightness because of their high refractive index and small particle size. The superior optical efficiency of TiO2 pigments are manifested in lightweight grades, where high pigment loadings are prohibited by the inevitable deterioration in strength properties.

An improved brightness with a blue tone desired for paper can be achieved by using fluorescent whitening agents (FWAs). FWAs added in paper or in paper coating are able to convert the invisible UV part of light into a blue tone visible light.

The simultaneous use of TiO2 pigment and FWA in double coated printing paper has been studied in this paper. It is hypothesized that TiO2 pigment and FWA can be used together in paper on condition that they are placed in paper so that light first meets the FWA and then the TiO2 pigment.

The efficiency of FWA applied in the paper coating depends on the presence of a suitable carrier (co-binder). Because FWAs and carriers are water-soluble they follow water in the paper coating process. Thus the essential questions in the simultaneous use of TiO2 pigment and FWA are the adsorption of FWA-carrier mixtures on other components of paper and the control of FWA’s migration (bleeding).

In the experimental part of this paper, new laboratory test methods to study the adsorption and migration properties of FWA-carrier mixture are presented. These phenomena were studied using simplified methods which cannot be transferred to practical conditions in the coating process. However, they help to test and understand the phenomena. Based on the test results the hypothesis was proven: the optical efficiency of both TiO2 pigment and FWA in double coated paper can be utilized if TiO2 pigment is in the inner layer and FWA in the outer layer of the paper.