CHRISTIANE CASSINI, DR. SYLVIE DOULUT*
*Corresponding authorRohm and Haas Europe Services Aps, a wholly owned subsidiary of the Dow Chemical Company, 371 rue L. van Beethoven, 06560 Valbonne, France

Abstract

As more importance is given nowadays to the use of soft preservation (1-3) there is an obvious need to formulate personal care products, including cleansing products such as shampoos or shower gels, at lower pH (pH around 5 and below). Lower pH is required to ensure the stabilization and maintain the efficacy of organic acids/salts used as preservatives. In these kinds of systems, polymers and surfactant micelles are building complex structures involving hydration forces, steric stabilizations, ionic interactions, hydrophobic interactions etc. The nature of the surfactant system and the polymer design (its composition and own structure) determine the structures built in the product, but as ionic interactions are involved, the formulation pH will impact the polymer performance. The behavior at low pH of the new HASE suspension aid is reviewed and both suspending properties and clarity of formulations containing the new polymer are presented.

INTRODUCTION

The apparent viscosity of a personal care cleansing formulation (shower gel, shampoo, etc.) is mainly due to the surfactant system and the ability of surfactants to build micellar structures. Inorganic salt (NaCl etc.) can be used to increase the apparent viscosity if required, but the corresponding rheological profile exhibits a Newtonian behavior. However, a pseudoplastic profile can be advantageous for the product handling, as the energy required for pumping is reduced at higher flow (shear) rates and this pseudoplastic behavior is mandatory whenever suspension properties are required. Indeed, higher viscosities at lower flow rates are needed to prevent the migration of suspended material and this low-shear viscosity should be independent of the apparent viscosity.
Associative thickeners such as Hydrophobically-modified-Alkali-Swellable-Emulsion (HASE) polymers, thanks to their dual hydrophobic and hydrophilic nature, interact with surfactant micelles and build appropriate structures that will impact both the apparent and the low-shear viscosity and change the rheological profile of a cleansing formulation from Newtonian to more ...