Consumer acceptance of personal care products depends greatly on their consistency or "feel" when they are dispensed, spread in the hands, or rubbed on the skin or hair. Differences in behavior can by measured by rheometry to quantitate resistance to flow. These measurements are essential to proper formulation and quality control of cleansers, lotions, gels, and similar personal care products.

A controlled-stress rheometer from TA Instruments was used to illustrate such applications. Two grades of hair styling gel with different "hold" ratings were tested at flow conditions that include those of their applications.

Figure 1

Flow of Gels in Processing, Dispensing, and Spreading

The viscosity of two gels is shown in Figure 1, over a range of shear (flow) rates. The measurements were made while increasing the shear rate (up curves) in three minutes, then decreasing it (down curves) in the same time.

At the higher shear rates, used in some plant processes or in spray dispensing, the viscosities are relatively low, which facilitates these operations. In the mid-range of shear rates, typical of pouring, the gels have higher viscosities, especially for the Mega Hold product. This gives them a rich feel.

Spreading on the hands and in the hair is easy, because of the reasonably low viscosity at the corresponding shear rate range of approximately 10/s. After application to the hair, shear rates are very low and the much higher viscosities, seen in Figure 1, serve to hold the hair in place. Differences between the two products are very clearly shown by this test and agree with the hold rankings on the labels (higher for Mega Hold). Agreement of the up and down curves in Figure 1 indicates a lack of thixotropic thinning due to the high shearing rate excursion. This is an important contributor to rapid hold when applied to the hair. Upon drying, the viscosity naturally increases further and its rate could be measured by separate tests.

Deformation Rates (Hold) of Freshly Applied Gels

Figure 2 shows vastly different rates of deformation for the two gels under the same constant shearing stress for 600 seconds (the "Creep" test mode). This simulates low forces on the gel after application to hair, but before drying.

Figure 2

The Mega Hold gel quickly sheared about two percent, then barely increased with very low slope (high viscosity). The Extra Strong Hold gel quickly sheared about one hundred times more and continued to flow readily, indicating much less initial holding power.

At 600 seconds the stress was removed, allowing "creep recovery" due to any elasticity in the deformed gels. The Mega Hold gel recovered elastically to 0.2% strain (90 % recovery), while the weaker gel had insignificant recovery. This indicates a bouncy initial response of the hair with Mega Hold gel, and easy reshaping with the other gel before it dries.

This dramatic difference in elasticity can be shown by a third mode of testing, dynamic shearing at controlled frequency and amplitude. The elastic and viscous components of the viscosity or modulus can be measured. Results, not shown here, confirm results above for relative elasticity of the gels. At a frequency of one cycle per second, approximating that of combing, the Mega Hold gel has eight times greater elastic modulus than the other gel. Again, we have a sensitive test of performance differences for two products with high, but different, hold ratings.

Impact Analytical has the expertise and facilities to help solve your problems involving rheological behavior. Our rheometers and thermal-mechanical instruments operate over wide ranges of forces, deformation rates and temperatures. Viscosities from 10 to 10¹² times that of water (0.01 to 10⁹ Pa s) can be measured. Excellent reproducibility and accuracy of the viscosity or viscoelastic results are obtained.