Rheology is one of the most important characteristics of a personal care cleanser in terms of consumer appeal; although achieving the ideal rheological properties for a given product can prove difficult. If the rheology is not ideal, if the product is too thin or too viscous for its application, the consumer can be turned off before even using it. Have you ever purchased a cleanser, went to use it in the shower, and had it run through your fingers and down the drain? Certainly, many of us have experienced this, and as a result, never purchased this product again. Rheology modifiers, a variety of which exist in the market, solve this problem.
Most people, including many formulators, often mistake rheology to mean viscosity. It is actually much more than that. Rheology is the study of the deformation and flow of materials under the influence of external forces.1 In other words, rheology refers to the way in which a material behaves when force is applied to it. It also plays an essential role in the aesthetics of personal care formulations. There are four key elements to consider when analyzing the rheological behavior of a material, namely shear flow, shear stress, shear rate and viscosity. In formulating cleansing products, shear stress and viscosity are the two most important parameters to consider. The action of rubbing a body wash onto the hands, massaging shampoo onto the hair, or even applying sunscreen or lotion to the skin, demonstrates the concept of shear stress.Viscosity is generally referred to as the “thickness” of a material. It is a measure of the resistance of a material to flow, and its dimensions are Pascal-seconds (Pa∙s). Another common unit for measuring viscosity is centipoises (cps) (1cP = 10-3 Pa.s). Overall, rheology illustrates what occurs when a substance is flowing and how it recovers when it stops flowing.
The relationship between the rheological parameters described here differs for materials that exhibit either Newtonian or Non-Newtonian behavior. A material for which the viscosity is constant due to a linear relationship between shear stress and shear rate is described as Newtonian. For example, water is Newtonian as it remains fluid no matter how much force is exerted on it. In contrast, the viscosity of non-Newtonian fluids such as shampoos, body washes and other cleansers is not constant. The viscosity changes as a function of the shear rate. Some non-Newtonian fluids exhibit a decrease in viscosity as the shear rate increases. This phenomenon is referred to as shear thinning.
This shear thinning rheology enables the consumer to use a product that is nice and viscous in a bottle, able to extrude through a small opening, and then retain its thickness in your hand. During use, the product will spread nicely onto the skin or hair.On the other hand, a shear thickening material is one in which viscosity will increase with increasing shear rate.
Rheology modifiers, materials that can affect the flow behavior of a formulation in the ways described above, are generally grouped into the following categories: natural gums, modified naturals, synthetics and inorganics.2 Although these materials are used to modify the flow characteristics of a surfactant formulation, they are often used in conjunction with another thickener. They are rarely used as the primary ingredient to build viscosity in a formulation.
As one of the common categories of rheology modifiers, natural gums, such as xanthan gum and guar gum, are typically used at very low use levels because higher concentrations of these products in a formula would negatively impact its aesthetics, resulting in a pituitous feel and appearance. In addition, these materials can be difficult to use, as they need to be dispersed in water and hydrated; they cannot simply be dropped into a formulation.
Another commonly used thickener, sodium chloride, common salt, is an inexpensive and accessible ingredient. However, its performance in different surfactant systems can be problematic and unpredictable. For example, salt will thicken a system up to a certain electrolyte concentration, but once the salt curve is reached, further additions of salt will cause the viscosity to plummet. In addition, some primary surfactants (i.e. sulfates) already contain a level of salt in them, which can vary from one supplier to the next.Therefore, it can be difficult to predict when the formula will reach its salt curve. Salt can also negatively impact certain aesthetic properties such as clarity and rheology. Too much salt added to a surfactant formula can result in haziness, or an unappealing gel-like or pituitous/stringy consistency. Furthermore, salt has very little effect on sulfate-free systems, which can be very difficult to thicken.
Although there are many rheology modifiers currently used in the personal care cleansing market, many have a negative impact in one aspect or another of the surfactant formulation. Some of these shortcomings can be related to the ease of dispersion of the ingredient or other undesirable aesthetics. Some of the ideal attributes that a rheology modifier should exhibit include the ability to easily control viscosity and other rheological properties of the formulation, ease of use, good performance across a wide range of surfactant systems, a broad pH range, and high active content enabling the formulator to use lower concentration levels and still achieve positive end results.
An Ideal Rheology Modifier
In an effort to meet all the characteristics of an ideal rheology modifier for surfactant systems, Croda has developed Versathix (INCI: PEG-150 pentaerythrityl tetrastearate (and) PPG-2 hydroxyethyl cocamide (and) water). Versathix is a high performance thickener that provides efficient viscosity building and desirable rheology across a wide variety of surfactant combinations relevant for shampoos and other cleansing systems. The material is supplied as a 70% active liquid product, with a use pH range of 4 to 9, and recommended usage levels between 1% and 5% w/w. Versathix is covered under US patent No. 6,531,443.
Versathix builds viscosity efficiently in both traditional, as well as sulfate-free systems, which formulators often find challenging to thicken. Rheological and thickening properties of Versathix and other commonly used liquid thickeners on the market were evaluated in four different surfactant bases at various concentrations. These systems were also tested with and without salt, at different levels. Table 1 illustrates the systems that were evaluated.
Normally, sulfate-based surfactant systems are easier to thicken than other chassis. Salt is often used as the sole viscosity builder in such bases. In the traditional ALES/ALS base shown in Table 1, Versathix greatly outperformed the other thickeners tested, with and without the presence of salt, as seen in Figure 1. A significant thickening effect is seen with the addition of just 0.75% salt to the system containing 2% Versathix. The viscosity of the base more than doubled from 15,500cps to 36,000cps, illustrating the synergistic effect between Versathix and salt in this traditional base.
In the SLES base evaluated, the addition of 0.75% salt to the system with 2% Versathix resulted in an impressive 15-fold increase in viscosity, once again highlighting the synergy between Versathix and salt in traditional surfactant systems (Figure 1).The various thickener responses obtained when formulating with Versathix demonstrate the versatility of this material in traditional bases. It is possible to obtain ideal shampoo or body wash viscosities with use levels of less than 1% active Versathix in combination with low levels of salt.
The Scoop n’ Soap Scoopable Bubble Bath formula highlights the synergistic effect of Versathix and salt in a sodium laureth sulfate base system to obtain a formula with very high viscosity. The ability of this highly viscous bubble bath formulation to shear thin on application makes it a very unique, niche product (Table 2).
Sulfate-free systems are known to be difficult to thicken. Versathix has been shown to perform consistently well in these formulations. At 2% active levels, Versathix outperformed all the other market thickeners tested, with and without salt, in both the APG base and the Croda shampoo base shown in Figure 2. Although the impact of salt addition was not as significant as in the traditional systems containing Versathix, the sulfate-free systems with Versathix did exhibit an increase in viscosity that was more substantial than the viscosity response seen with the other thickeners tested.
Versathix is able to significantly increase the viscosity of sulfate-free bases at concentrations greater than 2% without the addition of salt. At 3% active level in the Croda shampoo base, Versathix thickens the system to 11,000cps, a viscosity that is ideal for personal care cleansers. At 4% active, Versathix yields a viscosity of 29,000cps.
The Shear Suds Sulfate Free Shampoo with Versathix utilizes the same Croda shampoo base described in Table 1. The formula contains 3% w/w of Versathix, yields an aesthetically-pleasing, shear-thinning rheology, and imparts nice, voluminous foam that is difficult to attain in sulfate-free shampoos (Table 3).
Although the viscosity response of a thickener is important, the rheological behavior of that material in a surfactant system is also a crucial criterion of consumer perception.Rheology not only plays a role in the flow behavior under shear stress but also contributes to the sedimentation and thermal stability of a formula. Versathix exhibits excellent shear thinning behavior compared to other rheology modifiers in all the systems evaluated.
Figure 3 compares the shear thinning properties of Versathix to the other liquid thickeners that were tested in a sulfate base. The flow curves illustrated on the graph are a measure of the change in the shear stress as a result of the change in shear rate. This relationship is plotted on a log-log scale. Versathix begins to shear thin slightly before 10/s, which is ideal for surfactant formulas, such as shampoos and body washes. The slight dip, which is circled on the graph, represents the point where the viscosity of the system decreases as a result of the shear applied, such as when a shampoo is squeezed out of a bottle during use. Although the graph shows that the control also begins to shear thin at the same point as Versathix, the viscosity of the Versathix system is much greater. It is the outstanding thickening efficiency of Versathix coupled with its superior shear thinning performance that makes it a more effective rheology modifier than all of the other materials tested.
In addition to the viscosity building and rheological advantages of Versathix, this product has the added benefit of mitigating the irritation potential of sodium lauryl sulfate (SLS) on human skin. A 17-panelist clinical study was conducted to determine inflammation of the skin using a Laser Doppler, and the results can be seen in Figure 4. The image shows that 1% SLS solution causes more blood perfusion to the skin, which translates into severe inflammation compared to water, which acts as the control. The addition of 1%, 2.5% and 5% w/w of Versathix to 1% SLS decreases the irritation potential.Levels of 2.5% and 5% show no significant difference from water.
Versathix is an economical liquid thickener that shows excellent versatility across a wide range of surfactant systems. It exhibits rheological properties that are consistent with consumer expectations and preference. The product has the benefits of cold processability, low pH stability and salt tolerance. It produces clear systems and counter-irritancy benefits, without negatively impacting in use attributes, such as foam and feel. In summary, by providing all these attributes, Versathix is an ideal, versatile rheology modifier for surfactant systems.
1. Hemi N. Naé, “Introduction to Rheology,” in “Rheological Properties of Cosmetics and Toiletries,” edited by Dennis Laba, Marcel Dekker, Inc., New York, NY (1993), p. 9
2. Mary T. Clarke, “Rheological Additives,” in “Rheological Properties of Cosmetics and Toiletries,” edited by Dennis Laba, Marcel Dekker, Inc., New York, NY (1993), p. 57
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