The use of an anionic polymer to enhance coating performance by in situ modification of clay particle size distribution

A method to preferentially flocculate fine clay particles with a low to medium molecular weight anionic polymer is presented. Clay particles with equivalent spherical diameter of less than 0.2 microns are preferentially flocculated to create particles with diameters of 0.5 to 5 microns. This resulting clay has a narrower particle size distribution and enhanced performance properties. The polymer can be used to treat a clay which is then used in a paper coating, or it can be added to a completed paper coating with equal effect. Approximately twice the polymer addition level is required, to achieve a comparable effect, if the polymer is added to the coating instead of the clay alone. Several pilot coater trials are described which demonstrate that this method can be used to increase printed ink gloss by as much as 6 points. In addition, a method is described for making a structured, bulking pigment by flocculating a clay with an anionic polymer with moderately high molecular weight. The scattering coefficient of the clay was doubled by the addition of the polymer. This polymer call also be added to a completed coating with equivalent effect. The structured pigment used in a coating or structured coating gave increases in coating brightness and opacity by as much as 1 to 1.5 points.