Factors Affecting The Printing Strength Of Kaolin-Based Paper Coatings, 2007 Coating & Graphic Arts Conference

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The mechanical properties of paper coating layers are very important in printing and converting operations. These include stiffness, resistance to fold cracking, dusting and pick resistance.In this work we have studied the pick strength of kaolin coated papers as a function of the coating pigment size, morphology and binder level.

We found that dry pick strength using oil depended on the pigment particle size. Coarse clays gave much higher pick resistance than finer clays. A similar trend was observed using oil based inks. In the presence of water as a pre-damping fluid, the pick strength depended on the interval between pre-damping and printing. The coarse clay coatings had the greatest sensitivity to water, giving the lowest strength at short intervals, but the highest at long time intervals. Finer clay coatings were less sensitive to damping. The results indicate that the permeability of the coatings to water is the critical factor affecting wet pick resistance.

A new finding was that ink oils also reduce the strength of the coating layer. The extent of strength loss depended on the polarity of the fluid, being greatest with water and least with mineral oil. This effect of fluid may be a result of interruption of van der Waals bonding between latex and the kaolin surface. Hence the pick resistance of a coating layer is not a measure of its intrinsic strength, but is a result of the interactions between the ink and coating. Penetration of oil from the ink into the coating increases the applied stress through tack build, and simultaneously reduces the strength of the coating. Improving adhesion between mineral and binder may be the key to obtaining higher coating strength with offset inks.