J. Cosmet. Sci., 69, 67–75 ( January/February 2018) 67 The Rheological and Skin Sensory Properties of Cosmetic Emulsions: Infl uence of Thickening Agents YANAN LI, ZHAOQING ZHOU, XIAOMIN ZHAO, HUA ZHAO, and XIN QU, ASI Shanghai Technical Centre, Ashland Inc., Shanghai 200233, China (Y.L., X.Z., X.Q.) and School of Science, Beijing Technology and Business University, Beijing 100048, China (Z.Z., H.Z.) Accepted for publication September 17, 2017. Synopsis Hydrophilic polymers are widely used in the cosmetics industry as thickening agents/rheology modifi ers. These thickening agents have different chemical structures which affect the rheological properties, as well as the sensory attributes of the formula. Systematic study is important to determine the relationship among them. Six commonly used hydrophilic polymers, including cellulose derivatives and synthetic polymers, were used as thickening agents in a series of oil-in-water emulsions. The rheological properties were evaluated in relation to the thickening mechanism and polymer structures. Comprehensive skin sensory studies were carried out to test factors such as the pick-up, rub-in, and after-feel of these emulsions and the control sample. Results showed that all the samples demonstrated a non-Newtonian and shear-thinning behavior, and synthetic polymer-based formulas were more viscous than cellulose derivative-based ones. All eight attributes for the factors of appearance, pick-up, and rub-in showed statistically signifi cant differences (p  0.05), whereas all fi ve attributes for the after-feel factor exhibited no statistically signifi cant differences (p 0.05) for different thickening agents. According to the results calculated using Pearson’s correlation coeffi cients, four sensory attributes were mostly correlated with the rheological parameters. INTRODUCTION In recent years, the infl uence of specifi c raw materials on the rheological and sensorial properties of cosmetic formulas has become an important topic in the cosmetics industry. For example, Ozkan et al. (1) used steady fl ow and Large Amplitude Oscillatory Shear (LAOS) to characterize the yield stress and its correlation with sensory attributes. Lukic et al. (2) studied the effect of four emollients on the textural, sensorial, and in vivo skin performance of water-in-oil (w/o) hand creams. Tamburic et al. (3) investigated the application of thermo- rheology and textural analysis in the evaluation of w/o creams stabilized with a silicone emul- sifi er. Bekker et al. (4) studied mineral-based and wax-based cosmetic emulsions and jellies, relating their rheological measurement to their primary and secondary attributes when Address all correspondence to Xin Qu at xqu@ashland.com.

JOURNAL OF COSMETIC SCIENCE 68 applied to skin. Wang et al. (5) looked at the effect of the rheological properties of 12 mois- turizing creams/lotions on their effi cacy and attributes. Savary et al. (6) combined sensory and instrumental characterization to study the impact of emollients on the spreading properties of cosmetic products. The same authors (7) also conducted a large study on the impact of eight hydrophilic polymers on the textural properties of cosmetic emulsions. However, the aforementioned literature only evaluated limited sensory attributes, and few articles have correlated the skin sensory and rheological/mechanical properties of formulas with the chemical structure of raw materials. Hydrophilic polymers are widely used in the cosmetics industry as thickening agents/ rheology modifi ers for gels, shampoos, emulsions, color cosmetics, etc. These thickening agents have completely different chemical structures, including natural and synthetic polymers, crosslinked and noncrosslinked polymers, homopolymers, copolymers, etc. The difference in polymer structure affects both the rheological properties and the sensory attributes of the formula. Systematic study is important to determine the relationship among them. In this study, six commonly used hydrophilic polymers were used as thickening agents in a series of oil-in-water (o/w) emulsions. The rheological properties were evaluated and discussed in relation to the thickening mechanism and polymer structures. Comprehen- sive skin sensory studies were carried out, evaluating factors such as the appearance, pick-up, rub-in, and after-feel of these emulsions and the control sample. The results illuminated how, and to what extent, the polymer structure difference and rheological properties can affect the fi nal skin sensory attributes of the emulsions. EXPERIMENTAL SETUP MATERIALS Six commonly used rheological modifi ers from Ashland were the object of this study. The International Nomenclature of Cosmetic Ingredients (INCI) names, codes, trade names, and chemical structures are illustrated in Table I. Table I INCI Names, Codes, Trade Names, and Chemical Structures of the Polymers Used Code INCI name Trade name Chemical structure HEC Hydroxyethyl Cellulose Natrosol™ HEC 250 HHR Graft, natural derivative, and nonionic HPMC Hydroxypropyl Methylcellulose Benecel™ HPMCE10M Graft, natural derivative, and nonionic HMHEC Cetyl Hydroxyethyl Cellulose Natrosol Plus 330 HMHEC Graft, natural derivative, long alkyl chains, and nonionic PVP Polyvinyl Pyrrolidone FlexiThix™ polymer Synthetic, crosslinked, and nonionic PAA Na Sodium Polyacrylate RapiThix™ A-100 polymer Synthetic, linear, anionic, and neutralized PAA Carbomer Ashland™ 980 carbomer Synthetic, crosslinked, anionic, and unneutralized