Increasing naturality level: COSSMA

This article demonstrates, how to effectively lower the level of fossil-derived polymers by combining them with a naturally derived cellulosic polymer. This will allow the formulator to benefit from properties of both polymers and use the synergy.

Hydrophobically Modified Hydroxyethylcellulose (HMHEC) was selected as the naturally derived polymer due to its hydrophilic and hydrophobic interactions. Acrylates polymers are used quite extensively in emulsions¹. Their popularity stems from their relatively low cost and their high efficiency when it comes to thickening and supporting stability. Over the years, acrylates have been modified to give the chemist a wide array of textures, rheological properties, and hydrophobicities. Even with all of the modifications that have been instituted, this family of polymers still lacks good electrolyte tolerance and stability at low pH levels. Acrylates are in the process of being classified by regulators as microplastics. Cellulosic thickeners are rooted in nature as they are traditionally derived from cotton fibres and/or trees². Cellulose is then reacted with various functional groups to produce several polymeric variants. These variants have been used very successfully as thickeners and rheology modifiers for various applications in personal care. The polymer HMHEC contains both hydrophilic and hydrophobic domains. The presence of hydrophobic and hydrophilic chains on this thickener not only allows it to interact with water but allows it to interact with the oil phase of an emulsion. The dual functionality of this polymer makes it a great candidate for use in emulsions. An idealised chemical structure of HMHEC is displayed in figure 1. HMHEC is supplied in two different commercial forms, Natrosol Plus 330 CS and Polysurf 67 CS; both have the INCI name Cetyl Hydroxyethylcellulose^3,4. In this paper, we studied the effect of adding relatively low levels of HMHEC polymers on formulations thickened with fossil-based polymers. A variety of formulations with varying oil content and made with different emulsifiers was selected. The effect of these polymers on viscosity, stability, texture and rheology was studied.

fig. 1: Idealised chemical structure of HMHEC. figures: Ashland

Formulations

The prototypes selected encompass four different formulations ranging from gel creams with low internal phase to emulsions with more substantial ones.

Formulation A is a gel cream containing a low level of emulsifier (0.4% w/w) and relatively low oil phase (~8% w/w). The internal phase is emulsified using a polymeric emulsifier/ thickener, namely Acrylic Acid/ VP Crosspolymer at 1% w/w. 0.1% (w/w) HMHEC was added to the formulation. Formulation B is a clear gel that contains suspended waxy flakes. When dispensed from a pump, the formulation comes out as a smooth cream. The aqueous gel contains an anionic thickener namely, PVM/MA Decadiene Crosspolymer. 0.15% (w/w) HMHEC was added to the formulation. Formulation C is an anionic o/w emulsion containing potassium Cetyl Phosphate as the primary emulsifier. The emulsion has a relatively low internal phase (14% w/w) and does not contain fatty alcohols or waxes. The formulation is thickened with 0.45% w/w sodium polyacrylate. 0.4% w/w HMHEC was added to the formulation.

Formulation D is a non-ionic o/w emulsion containing a lamellar gel forming ingredient. The formulation contains a rather high internal phase (21% w/w) designed to give richness and body to the texture. The formulation is stabilised with a relatively low level (0.15% w/w) of an anionic thickener namely, PVM/MA decadiene crosspolymer. 0.5% (w/w) HMHEC was added to the formulation.

Methods

Viscosity was measured at 25°C after one-minute rotation at 5 RPM using a Brookfield RVDV-E viscometer. Spindles B, C, and D were used for formulations A, C and D respectively. Viscosity of formulation B was not measured due to the presence of large wax particles that interfered with the measurement. The emulsion samples were place on a glass side and immediately inspected with the optical light microscope, utilising non-polarised light to access emulsion quality. Polarisation microscopy was used to identify Maltese cross-like structures as an indicator of liquid crystal components in the formulation. Stability of the formulations at 45°C, 4°C and room temperature was monitored for three months. In addition, the formulations were subjected to five freeze/thaw cycles. The formulations were evaluated for their viscosity, pH, colour, odour, and state untreated, one week (1W), 2W, 4W, 8W and 12W. Rheological properties of the prototypes were characterised using a strain-controlled ARES G2 Rheometer. Rheological tests were carried out at 25 ± 0.1ºC. Dynamic Strain Sweeps were applied using 25 mm stainless-steel plates (R = 12.5 mm) with smooth surfaces.

The Laos experiments were conducted with parallel plates. Essentially, the rheometer twists the lower plate to defined strain rates (specific angles), first in one direction and then in the other direction. The experiment starts with small twists that become progressively larger with each successive twist. When the experiment is finished, the plates separate, and one may observe the physical (usually broken) state of the formula. This data is recorded in the form of Lissajous plots. By generating this type of rheological profile for each sample, a fingerprint for each formula is resolved, illustrating the effects of high shear rub-in on the physical properties of the structured formulation⁵.

Results and Discussion

Viscosity increased 1.4, 50 and 9.5 times in formulations A, C, and D, respectively when HMHEC was added. The lowest increase was noticed in the gel cream which contained the lowest level of HMHEC and very low oil phase. The highest effect was observed in Formulation C that did not contain any waxes and fatty alcohols and HMHEC interacted with the oil phase to structure that completely. The effect on formulation D was also pronounced due to its high internal phase and wax matrix. It is interesting to note that the viscosity of 0.5% (w/w) HMHEC in water is less than 100 cps. This indicates that the increase in viscosity in emulsion D is not due to HMHEC thickening of the water phase alone but must be due to an interaction with the lamellar crystals of the emulsion and/or a synergistic interaction with PVM/MA decadiene crosspolymer. One can deduce that the presence of HMHEC, and its hydrophobic character, is the main contributor to viscosity in this formulation. The presence of HMHEC in regular O/W emulsions improves the particle size of the emulsions and makes them more uniform. In the case of non-traditional emulsions where non-ionic lamellar gel forming ingredients are used in lieu of traditional emulsifiers, the presence or absence of HMHEC did not affect the particle size of the emulsion to a great extent. Inspection of the polarized images indicates that the presence of HMHEC may have improved the structure of the liquid crystals as they appear to be more prominent and well defined.

The addition of HMHEC had a pronounced effect on the stability of formulations. Three formulations (B, C and D) without HMHEC failed the stability protocol whereas those containing the hydrophobically-modified polymer passed the threemonths stability protocol. This indicates that HMHEC has a synergistic effect with fossil-derived polymers, enhancing the suspension properties of the system and improving the overall stability of emulsions. Lissajous plots of all four formulations were generated in the presence and absence of HMHEC and displayed in figure 2 formulations A, B, C and D, respectively. Formulations A and C which were devoid from waxes and relied on polymers to create the texture, the Lissajous plots showed a more elastic behaviour that became more pronounced when HMHEC was used. The elastic behaviour contributed to more cushion upon application. Formulations B and D contained waxes, the Lissajous plots displayed a wax-like character and thixotropic behaviour that was accentuated in the presence of HMHEC. The presence of HMHEC in these formulations contributed tremendously to the structure and texture behaviour.

fig 2: Lissajous plots of the formulations with and without the addition of HMHEC. figures: Ashland

Conclusion

The data suggests that the addition of HMHEC to oil-in-water emulsions or gel creams has a great impact on the viscosity and stability of the formulation as well as the rheological behaviour of such systems. In oil-in-water formulations with relatively high internal phases, the effect of the addition of HMHEC was dominated by the interaction of its hydrophobic groups with the oil phase to create rich, and rigid textures with thixotropic character. On the other hand, in gel creams with a relatively low oil phase, the addition of HMHEC mostly enhanced stability and cushion due to the synergy with the primary thickener. This synergy could enable the formulator to lower the use of primary rheology modifiers leading to a reduction of fossil-derived rheology modifiers.

References

^1. H. Epstein, “Skin care products” in Handbook of Cosmetic Science and Technology, 4th ed., Eds. A.O. Barel, M. Paye, and H.I. Maibach, CRC Press: Boca Raton, FL, Chapter 9, pp. 103- 114 (2014).

^2. R.C. Pettersen, “The chemical composition of wood” in The Chemistry of Solid Wood, Ed. R.M. Rowell, Advances in Chemistry Series 207, Washington, D.C.: American Chemical Society; 1984: Chapter 2.

^3. Ashland Specialty Ingredients, Product Data Sheet, Number 4032-7, Natrosol Plus CS, Grade 330, cetyl hydroxyethylcellulose.

^4.Ashland Specialty Ingredients, Product Data Sheet, Number 4084-2, Polysurf 67, cetyl hydroxyethylcellulose.

^5. T. Gillece, R.L. McMullen, H. Fares, S. Ozkan, L. Senak, and L. Foltis, “Probing the textures of composite skin care formulations using large amplitude oscillatory shear,” J. Cosmet. Sci., submitted (2015).