New star thickener for cleansing formulations

Commercial non- ionic associative thickeners in the market are hydrophilic polymers with 2 – 4 arms capped with fatty acids such as stearic acid and oleic acid. Their commercial trade names are - two arms - Antil 200, three arms - Glucomate VLT, and four arms - Versathix and Crothix Liquid. These products were developed many years ago primarily for sulfate-type surfactant cleansing formulations. Sorbeth 230 Tetraoleate is a new non-ionic associative thickener with six arms, designed for thickening modern sulfate-free and mild skin-friendly surfactant formulations. Figure 1 illustrates its molecular structure and the associated chemical reaction steps for its synthesis. It is initially derived from natural sorbitol with ethylene oxide to Sorbeth 230 and thus has six hydrophilic arms. Subsequent reactions with natural oleic acid form Sorbeth 230 Tetraoleate with approximately four hydrophobic interaction oleic groups at the end of the arms, leaving two unreacted hydrophilic arms. Further formulation with liquid carriers converts this solid thickener into a liquid, SorbiThix L 100, with an INCI name of Sorbeth 230 tetraoleate (and) Decyl Glucoside (and) Sorbitan Laurate.

The underlying principle explaining why this new star non-ionic associative thickener can thicken any surfactant class effectively is illustrated on the bottom of Figure 1. By bridging surfactant micelles with four hydrophobic fatty groups, it creates much larger bridged micellar structures than the original micelles, resulting in higher viscosities. In contrast, PEG 150distearate possesses only two hydrophobic arms and thus can only bridge two surfactant micelles, leading to a smaller bridged micellar structure with a small increase in viscosity. Other commercial thickeners with three or four arms can form bridged micelles of moderate size, resulting in an improvedbut still modest viscosity increase. Although these products are effective at thickening SLES type surfactants, they remain ineffective at thickening modern mild surfactants. SorbiThix L 100 is the most potent thickener because it has the largest bridging capacity to develop the largest bridged micelles, leading to the highest viscosity increase. In addition, the two un-reacted hydrophilic arms give SorbiThix L 100 an extra benefit – higher HLB and solubility in water, which improves the clarity of the personal cleansing formulation.

All three strategies reduce the curvature of the micelle structure by decreasing head group areas; they work very well for classic surfactant – SLES, because the physical volume of its head group is small. However, thickening mild, sulfate-free surfactants, such as amino acid derived surfactants – glycinate, sarcosinate, and glutamate, utilized in modern personal cleansing formulations with the classic strategies outlined above will inevitably lead to personal cleansing products without desired viscosity and clarity. This is because the hydrophilic head in sulfate-free surfactants is much larger than that of SLES type surfactants. In practice, commercial shampoos, shower gels, and other personal cleaning products must contain fragrance oils, extracts, actives, preservatives, chelators, etc. to differentiate a given brand from another and appease target consumers.

These additives may further interrupt the packing of surfactant molecules in water, resulting in an undesirable mixture of a large number of short rod and spherical micelles with a low inherent viscosity. Here, we demonstrate that combining this new star shape non-ionic associative thickener (Sorbeth230 TO) with salt and small hydrophobic thickeners easily delivers modern cleansing formulations with desired characteristics. This new approach works very well with mild skin-friendly amino-acid derived surfactants and as indicated in Table 1, the resulting shampoo and face wash formulations are clear with desired viscosity. In the Table 1 formulations, the hydrophobic thickeners are lauric acid and glyceryl laurate, and the salts are NaCl and magnesium sulfate.

Table 1: CLEANSING FORMULATIONS WITH AMINO-ACID SURFACTANT & STAR POLYMER

Fragrance is an important product attribute that differentiates a given personal cleansing product from others in a crowded market. However, it remains technically challenging to incorporate a high % of fragrance oil (= 2%) into shampoo or shower gel formulations without adverse effects on clarity, viscosity, and foaming. Figure 2 illustrates how the incorporation of additives to increase viscosity or improve clarity can have unintended consequences (the color shading within the bars correlates with clarity). The cleansing base formulation is composed of 17.5% surfactant actives - 20% SLES (70% conc.), 10% Cocamidopropyl Betaine, and 0.1% EDTA at pH 5.5. Salt increases the viscosity but causes the formulation to become opaque with or without 3% fragrance.

Alternatively, the addition of 4% Glyceryl caprylate/caprate (GCC), a hydrophobic thickener, increases the viscosity effectively while maintaining a desirable clarity. However, it again becomes opaque with the addition of 3% fragrance. The addition of fragrance solubilizers like 3% PEG-40 hydrogenated castor oil (HCO-40) or 3% Polysorbate 80 still cannot restore clarity. In short, many of the common core formulation tools in surfactant formulations, including salt, fragrance, hydrophobic thickener, and fragrance solubilizers cannot produce the desired clarity and viscosity together above a certain concentration of fragrance oil. For sulfate-free surfactants, these technical challenges become even more significant.

The arrival of this new star thickener – Sorbithix L-100 provides a solution to this formulation challenge. Figure 3 shows the thickening performance of the popular non-ionic associative thickeners using the above SLES model cleansing formulation. Overall, the number of arms on the non-ionic associative thickener is directly correlated with the efficiency to build up viscosity in surfactant formulations. While all formulations are clear, SorbiThix L-100 is markedly the most efficient. With this new six-arm non-ionic associative thickener, we can now develop practical shampoo formulations with both high fragrance oil and hair conditioner actives, while maintaining desired clarity and viscosity. Table 2 details two practical shampoo prototypes as examples, which maintain clarity with 3% fragrance oil, from two surfactants: SLES and AOS.

Table 2: CLEAR ‘N HIGH FRAGRANCE OIL SHAMPOOS