Studies on Relationship Between Porosity of Filler for Paper and Opacity After Printing, 1993 Papermakers Conference Proceedings

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In recent years the basis weight of newsprint paper has decreased. However, when the basis weight of the paper decreases, its opacity undesirably reduces. In order to prevent a reduction in the opacity, especially after printing, precipitated silica has been widely used as a filler in Japan. We have developed a new aluminum silicate compound for this purpose.

In general, the ink is composed of pigment and a vehicle. According to our extensive experiments,

1) The pigment is absorbed in the pores of the filler between 3,000 and 40,000 A.

2) The vehicle is absorbed in the pores of the filler below 8,000 A.

3) Pulp of the paper absorbs the ink, but it does not adsorb the pigment. However, the ink is absorbed and adsorbed in the filler.

The method to increase the opacity after printing is to decrease the thickness of the layer in which the ink is impregnated. Although the most suitable filler for this purpose needs to have a wide range of pore-sizes to adsorb both the pigment and the vehicle (l00-40,000 A), it is difficult to produce a filler with such a wide pore-size distribution. We have developed a new aluminum silicate compound with two peaks in the pore-size distribution i) l00-1,000 A, for the absorption of the vehicle, ii) 3,000-40,000 A for the adsorption of the pigment. This filler improves remarkably the opacity after the printing of newsprint compared with conventional fillers.