Water Resistance vs. Rheology Control: An Exploration

Water resistance is a desirable property across a range of cosmetic product categories, including decorative cosmetics, leave-on hair products, sun protection and skin care (e.g. hand barrier creams). This work has focused on two formulation variables, the type of emulsifier system and the choice of film-forming polymer, both known to change the rheological profiles of semisolid systems. The aim of this study was to assess the effects of these two factors on the rheological and texture profiles, as well as the water resistance characteristics, of semisolid O/W emulsions.

A simple O/W emulsion formulation, containing glycerol, mineral oil, water, preservative and emulsifier system was used in this study. The basic emulsifier system consisted of two non-ionic emulsifiers (sorbitan stearate and polysorbate 60) in combination with a co-emulsifier cetearyl alcohol. The alternative emulsifier system had an anionic emulsifier sodium cetearyl sulphate added to the above emulsifier mixture.

A range of seven film-forming polymers were used in the study, representing acrylates, silicones, graft copolymers and block copolymers. Two groups of 8 emulsion samples, containing either basic or alternative emulsifier system and including a non-polymer control, were tested. Rheological measurements were carried out in both continuous flow and oscillatory mode and were complemented with texture analysis, performed using a spreadability test. In the absence of universally accepted in vitro water resistance test, a gravimetric method for measuring water resistance was developed and applied to all test samples.

The results have shown a variety of rheological profiles, which were both emulsifier- and polymerdependent. The addition of sodium cetearyl sulphate have resulted in softer (lower complex modulus) and more spreadable formulations (lower firmness), which coincided with lower water resistance scores. The block copolymer polyurethane-62 (and) trideceth-6 has produced specific rheological profiles, characterised by high internal resistance and high elasticity. The graft copolymer vinyl pyrrolidone/ eicosene was the best performer in terms of water resistance, showing the lowest weight change of 11.1% after the first 20-min immersion in water and 12.9% after the second immersion. The non-ionic emulsifier mixture based on sorbitan stearate and polysorbate 60 has provided superior performance in comparison to its combination with the ionic emulsifier sodium cetearyl sulphate.