ASPECTS OF THE ELONGATIONAL FLOW BEHAVIOR OF COATING COLORS

A technique for evaluating the extensional flow properties of mineral suspensions used for the coating of paper and board is presented. The technique, which has earlier been applied to polymer solutions, is bas ed on forcing the suspension through a converging channel with a known geometry. The shape of the channel and the Reynolds number for the fl ow are such that the suspension is extended or strained at a constant rate. The suspensions studied here contain a blend of kaolin and calci um carbonate particles, which are common pigments in the coating of pa per, a styrene-butadiene copolymer latex as binder and carboxymethyl c ellulose (CMC) as a water retention aid.The amount of CMC in the, colo ur was varied in order to produce suspensions with different rheologic al properties. The extensional viscosity obtained with this technique was found to decrease : with increasing strain rate and to increase wi th increasing CMC content. The influence of the strain on the extensio nal viscosity is discussed. Very few investigations on the elongationa l flow behaviour of paper coating suspensions have hitherto been repor ted. During blade as well as roll coating, the coating suspension is s ubjected to elongational deformations. It may be that the elongational properties are of importance for the runnability of the coating proce ss and the quality of the final coated paper.