Thixotropic Agents

So far it has been assumed that the non-Newtonian substances discussed are relatively insensitive to the time scale of flow. It is assumed that if the rate of deformation is ramped up and then down, that there will be a superposition of both stress responses. This may not be the case, as will be demonstrated for toothpaste formulations, where the introduction of thixotropes proves quite useful.

As reviewed earlier in this section, viscosity describes the resistance of a liquid to flow and pseudoplasticity relates to the decrease in viscosity observed with increasing shear rates. Thixotropy, however, is a time-dependent phenomenon, defined as:

• The ability of the substance to exhibit lower viscosities as a function of shear rate and duration.

• And its ability to have its structure reformed over a period of time.

For toothpaste, a great effort has been directed towards optimisation of toothpaste physical attributes. These attributes are strongly dependent on rheological characteristics of the toothpaste system, such as viscosity, pseudoplasticity, thixotropy and low shear yield stress.

Various types of rheological additives find their utility in toothpaste formulations. To perform adequately, they must exhibit a strong three-dimensional structure in lean solvent systems while providing the optimum rheological characteristics described above. Toothpaste exhibiting combined properties such as thixotropic behaviour and high yield value are particularly useful.

The main function of thickening and binding agents in toothpaste systems is to impart adequate paste texture and rheology during preparation, storage and utilisation, good stability with no phase separation or syneresis, smooth and shiny aspect, and improved mouthfeel, foamability, and rinsability. These are directly linked with the rheologi-cal characteristics of viscosity, pseudoplasticity, thixotropy, and yield stress.

The thixotropy of a toothpaste system can be described by a rheogram representing a plot of shear stress against shear rate. The hysteresis area between the up curve and down curve is defined, as the energy required to break the network structure of the toothpaste. It gives an indication of the degree of thixotropy of the system as given in Figure 6.

Contact Dermatitis Silicone
Figure 6 Flow behavior of a commercial gel toothpaste.
Viscosity Toothpaste
Figure 7 Thixotropy of medium water content cream toothpaste.

Five major rheological additive types are currently used in toothpaste systems. They are generally classified into four main categories: 1) natural, 2) modified natural, 3) synthetics, and 4) inorganic. These classes are represented respectively by 1) xanthan gum, carrageenan; 2) cellulose ethers; 3) crosslinked polyacrylic acids; 4) clays and amorphous silicone dioxide.

Figure 7 shows the thixotropic index (TI) of various gums in a cream toothpaste formulation. The thixotropic index is defined as the ratio of the up-curve viscosity to the down curve viscosity measured at the same shear rate. The higher the index, the more thixotropic is the dispersion. For reference a TI of 1.0 means that the dispersion is not thixotropic.

The figure clearly shows CMC 1 gives the most thixotropic structure to this formulation. For cellulose gums, the thixotropic index is shear rate dependent; the extent to which the structure rebuilds is dependent on the shear history to which the gums were subjected.

In comparison with the other gums, xanthan is not thixotropic. The thixotropic index of this formulation is not dependent on the shear rate. The structure is recovered almost instantaneously. Carrageenan has a higher thixotropic index than seen with xanthan gum, but it also recovers its initial structure very quickly.

Interacting fillers such as clay, fumed silica, and aluminum-magnesium hydroxide are also used as thixotropic modifiers in personal care products [19]. These materials tend to form complex networks or gels that show time-dependent rheological properties.

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