PAPERmaking! FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 1, 2016
a coating mixture is applied to the paper, the dewatering or water penetration into the base paper induces the immobilisation of the coating layer. The coating layer is further immobilised during the subsequent drying process, leading to shrinkage of the structure. When nano titanium pigment is incorporated in the coating mixture, the shrinkage of the coating layer during consolidation is counteracted. Apparently, this pigment act as spacers between the clay plates decreased the overall packing density (i.e., produce coated paper with loose packing), leading to high fibre coverage and a smooth surface of the coated paper (Gullichsen et al. 2000; Camilla et al. 1994). The same effect is shown for the three types of nano titanium dioxide pigments. The effect is more pronounced at 50% additional level and greatest effect was obtained with rutile pigment, the percentage decrease reached to 40, 24 and 25% for rutile, a mixture (Anatase + Brookite) and anatase, respectively.
Table 2 Optical and physical properties of coated papers having the prepared nano TiO 2 pigments The gloss of the clay-coated paper is much higher than the titanium dioxide-coated paper. This may be due to the disturbance of the platy structure of the clay upon the addition of TiO 2 . Table 2 shows the air permeability’s (porosities) of the coated papers for clay, and clay/nano TiO 2 systems. It is evident that blend of TiO 2 with clay with percent 70:30 has the lowest air permeance value. This may be due to the nanopigments fill in the narrow pores created between clay particles. As the percent of titanium pigments increased, the porosity significantly increased. In clay coating layer the particle integrity and void network extended uniformly throughout the entire coating. This void network is truly three- dimensional, interconnecting not just in the Z direction but in the machine and cross- machine directions as well. The high level of nano TiO 2 in clay coating mixture disturbs the high packing characteristic of clay pigment providing a loose coating layer packing consequently, imparting more air flow pathway.
Fig. 8 SEM micrographs of a clay and b clay/rutile coated paper at 50% additions
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Article 3 – Titanium Coating Pigments
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