The objective of this project was to evaluate the performance of magnesium oxide as a possible pigment for use in paper coatings. The use of magnesium oxide as a pigment in coatings is not currently nor has it ever been practiced in a mill situation. The principle goal of using magnesium oxide was to increase brightness and opacity in coatings at the level of performance found with titanium dioxide at a far lower cost. The evaluation was done of the basis of rheological performance as well as optical testing. Coatings of various levels of magnesium oxide and titanium dioxide were applied to a recycled base sheet similar to that of a topliner of a coated board. The coatings were applied on the Dow Laboratory coater at low speeds with a blade applicator. All coatings were evaluated by the Brookfield viscometer as well as the Hercules high shear viscometer before coating was done so coating could be analyzed at similar viscosities.
The results during experimentation have given the following conclusions. Some shear thinning occurred with the substitution of magnesium oxide for titanium dioxide at high shear rates. Dispersion without any dispersant caused rheological problems with the magnesium oxide. At similar coat weights, the optical objectives of the experiment were satisfied with these results: equal brightness with 100% substitution of magnesium oxide for titanium dioxide, higher gardner gloss with magnesium oxide than titanium dioxide, equal opacity for pure substitution, with optimum opacity coming at an even 50/50 split of the pigments, improved smoothness with total replacement, as measured by Parker Print Surf, and a higher ink absorbtivity in the magnesium oxide coatings due to less dense coating structure containing more pores.
Overall, the use of magnesium oxide is warranted if the problems of dispersion and coater rheology are overcome or managed with the use of dispersant. The unknowns to be explored with further study are the runnability on a machine coater and the abrasiveness of the magnesium oxide on equipment
