RHEOLOGICAL AND SKIN SENSORY PROPERTIES OF COSMETIC EMULSIONS 69 PREPARATION OF THE FORMULATIONS Six emulsions containing the previously mentioned polymers were prepared. The emul- sion formula is shown in Table II. To be consistent with practical use levels, the concen- tration for HEC, HMHEC, HPMC, and PAA Na was 1% 5% for PVP and 0.5% for PAA (adjusted to pH 6.5 with NaOH). One control sample containing no thickeners was also prepared for comparison. RHEOLOGY All rheological measurements were carried out with a MCR 101 (Anton Paar GmbH, Graz Austria) Rheometer, equipped with a cone plate (CP 50-1) at 25°C. A time of 5 min was set for all tests. Continuous fl ow tests were conducted at a shear rate range from 0.01 to 100 1/S. Amplitude sweeps scanned strain deformations from 0.1% to 100% at a constant frequency of 1 Hz for the emulsions. SENSORY ANALYSIS A panel of 10 college students (age 19–24) of both sexes was recruited from Beijing Tech- nology and Business University. Sensory evaluation protocols based on well-accepted guidelines for the skin feel analysis of creams and lotions (ASTM, 2003) (8) were used to train the panelists. All of the sensory attributes evaluated in this study are shown in Table III. For each sensory attribute, four to fi ve commercial products were used as standard reference points to defi ne the scales (0–10) during the training. STATISTICAL ANALYSIS Several statistical analysis methods were applied using JMP 12.0.1 software (Cary, NC), with the confi dence level set as 95%. For each sensory attribute, A one-way analysis of variance (ANOVA) of the Kruskal–Wallis tests was conducted to determine the overall differences among the seven emulsions, and the Tukey’s honest signifi cant difference (HSD) tests were conducted to compare the mean between each pair of the seven emulsions to categorize them. Table II Emulsion Formula Used in This Study Trade name INCI name Supplier % (w/w) Distilled water Water Local q.s. 100 Glycerin Glycerin Local 2 Hydrophilic polymers Listed in the text Ashland 0.5/1/5 GTCC Caprylic (and) Capric Triglyceride Local 4 Brij™ 72a Steareth-2 Croda 2.55 Brij™ 721a Steareth-21 Croda 1.65 Liquid Germall™ Plus preservative Propylene glycol (and) diazolidinyl urea (and) iodopropynyl butylcarbamate Ashland 0.5 a Trademark owned by a third party.

JOURNAL OF COSMETIC SCIENCE 70 Principal component analysis (PCA) was used to analyze the relationships among samples, rheological parameters, and sensory attributes. Pearson’s correlation coeffi cients (PCC) were calculated to analyze how the rheological properties infl uence the sensory attributes. RESULTS AND DISCUSSION THICKENING MECHANISM As shown in Table I, three of the polymers, HEC, HPMC, and HMHEC are polysaccha- rides. They derive from a natural polymer—cellulose—and have the benefi ts of both natural and synthetic polymers. Celluloses are linear polymers consisting of (1-4) β-D-glucan with 1-4 glycosidic linkages. They can be substituted with hydroxyethyl and hydroxypropyl methyl to produce HEC and HPMC, respectively. HEC and HPMC thicken aqueous so- lutions mainly through H-bond interaction. HMHEC, which is hydrophobically-modifi ed HEC, was also selected. Its thickening mechanism is through the combination of a hy- drogen bond interaction (the same as traditional cellulosic derivatives) and a hydrophobic interaction of long alkyl chains. As synthetic polymers, slightly crosslinked PVPs thicken aqueous solutions mainly through interlinks among polymer chains and steric interactions. While PAA and PAA Na are polyacrylates with anionic groups, they thicken the systems mainly through electrostatic interactions—the polymer chain becomes uncoiled due to the rejections among the same electric charge groups. RHEOLOGICAL PROPERTIES The fl ow curves of six emulsions and the control sample are shown in Figure 1. All of the samples showed a non-Newtonian and shear-thinning behavior, with the viscosity Table III Sensory Attributes Evaluated in the Study Factors Sensory attributes Defi nition Appearance Gloss The degree of light refl ected from the product Pick-up Firmness Force required to fully compress the product between the forefi nger and the back of the hand Ease of pick-up The amount of product picked up by fi ngers Peak after pick-up The degree to which products stands up after fi nger pick-up Rub-in (after 15 circles) Spreadability The ease of spreading the product Hydration feel The degree of hydration felt while rubbing in Oil feel The degree of oiliness felt while rubbing in Absorbencya Total circles used to disperse the samples until full absorbency is reached. Limit: 120 circles. After-feel (after 5 m) Gloss The degree of gloss Slipperiness The ease of sliding fi ngers across the skin Greasiness The degree of feeling of greasiness/product residue Tackiness The degree to which fi ngers adhere to residue product Moisture The amount of moisture perceived when moving fi ngers across the skin a Scales are defi ned by the equation: 10 - (total circles/12).