Rheology of Toothpaste

Abstract

In order to investigate the interaction between the ingredients of toothpaste, we measured the rheological properties of toothpaste using the Brookfield Synchro Lectric Viscometer. And we found that: (1) At low shear rate, the relation of shear stress versus shear rate in toothpaste has been obtained in general form of the well-known Williamson's empirical equation for pseudoplasticity. Here G is the shear rate. η* represents the limiting viscosity of toothpaste at G→∞. η* does the viscosity of toothpaste at G. The C1 and C2 are the constants. As can be seen from the equation form, the viscosity of toothpaste consists of sum of the first term which is independent of shear rate and the second term which decreases with increasing shear rate. The second term indicates the viscosity by structure. We take C1 to be the magnitude of contribution to the structural viscosity. So we attach no small importance to C1. (2) The rheological properties of suspensions are generally determined by the property of dispersing medium and the volume of concentration of the dispersed phase. We varied the property of the dispersing medium by varying the concentration percentage and kinds of Na-CMC. We varied also the volume concentration of the dispersed phase by varying the volume fraction of Dicalcium phosphate dihydrate (DCPD). First we measured C1 with modified Na-CMC in concentration percentage and kind. When C1/N is plotted as a function of concentration of Na-CMC (N), a straight line is obtained. Thus we have obtained the following empirical equation a, b are constant. The constant a is greatly variable according to the kinds of Na-CMC used. We think that it has been caused by the nature of Na-CMC. We think also that it is based on the difference between D.P. and D.S. Na-CMC is difficult to dissolve in water with increasing D.P. and decreasing D.S.. We have introduced D. P./D. S. to know the solubility of Na-CMC, and found that the constant a is proportional to D.P./D.S.. We have also determined the constant a by varying the concentration percentage of glycerine. We have found that the constant a indicates the measure of interaction between the Na-CMC molecules, because the entanglement of Na-CMC is proportional to the solubility of Na-CMC. (3) We have measured C1 by varying the volume fraction of powder (ø). When log C1 is plotted as a function of the volume fraction of (ø), two straight lines of volume concentration are obtained, locating the critical point with the larger slope of the straight line beyond it than below it. When the concentration percentage of Na-CMC increases, the position of the critical point is shifted to enlarge the side of the higher concentration of ø. It is thought that the structural viscosity of toothpaste is mainly the result of the network of Na-CMC below the critical point, and is as mainly the result of interaction of the powder beyond the critical point. © 1970, The Society of Materials Science, Japan. All rights reserved.