j. Cosmet. sci., 52, 51-65 (January/February 2001) Quantitative methods for evaluating optical and frictional properties of cationic polymers WENJUN WU, JOY ALKEMA, GREG D. SHAY, and DAVID R. BASSET, Union Carbide Corporation, Cary, NC 27511. Accepted for pz/blication Jam/ary 15, 2001. Presented in part at the annual Scientific Seminar of the Society of Cosmetic Chemists, Chicago, May 6-7, 1999. Synopsis This paper presents three quantitative methods to examine gloss, opacity, and friction of cationic polymers. The adsorption of cationic polymers onto hair and skin can be regarded as a thin film coating. Therefore, optical and frictional properties of polymer films are of significant relevance to the applications of cationic polymers in hair care products. Such properties reflect the desirable hair condition attributes consumers seek in shampoo and conditioner products. Using these test methods, polyquaternium-10 and cationic guar samples of varying molecular weight and cationic substitution were compared. The effect of an artionic surfactant, sodium dodecyl sulfate (SDS), on polymer film properties was also investigated. Neat guar hydroxypropyl trimonium chloride imparts less friction than polyquaternium-10 but dulls the substrate employed in this study. The optical data show that polyquaternium-10 provides greater film clarity and gloss than cationic guars. In the presence of SDS, polyquaternium-10 also displays similar or lower friction than cationic guar. The comparative optical and frictional results are in good agreement with the visual assessment of the cationic polymer films. These results clearly demonstrate that polyquaternium-10 exhibits superior film properties in the forms of both neat polymer and polymer/surfactant complex. In addition, microscopic techniques such as scanning elec- tron microscopy (SEM) and atomic force microscopy (AFM) provide powerful explanations for the differences noted between the two popular classes of cationic polymers. The test methods described in this paper can be utilized to differentiate the upper performance potential of cationic polymers. These objective and standardized test methods derived from the coatings industry are not affected by the variability of hair or the formulation complexity of end products. They can be useful tools in the product development process in quickly screening the relative performance of different polymers. INTRODUCTION All hair care products are subject to consumers' sensory assessment. A common practice is to evaluate the hair conditions after product usage. For example, to differentiate cationic polymers, hair tresses are treated with straight polymer solutions or formulated shampoo products containing polymer. The hair condition attributes such as feel, ap- pearance, and ease of combing are then rated subjectively by a group of panelists. 51

52 JOURNAL OF COSMETIC SCIENCE Cationic polymers are formulated into hair care products to impart and to promote conditioning of hair. These polymers are highly substantive to hair because the hair surface bears a net negative charge at pH above its isoelectric point of 3.67 (1). Cationic polymers in general improve combability, provide curl retention in high-humidity conditions, and afford long-lasting static reduction. Due to the difficulty and subjec- tivity of hair measurements, objective screening methods are of great interest to the cosmetic industry. Cosmetic scientists have developed many test methods in attempts to translate the consumer-perceived hair conditions into physically measurable properties. This paper describes three such methods to quantify gloss, opacity, and friction of cationic polymer films in the absence and presence of surfactant. These test methods are used in a comparative study to differentiate the performance characteristics of polyquar- ternlure-10 and cationic guar samples. GLOSS OR SHINE Gloss or shine is one of the most sought-after hair properties, as it implies health and beauty. Several instruments have been developed to measure the shine of human hair. The method of Thompson and Mills (2) measured the reflectance from an assembly of hair fibers. Reich and Robbins (3) employed a photogoniometer to measure the gloss of a single hair or aligned hair fibers. They monitored the changes caused by soiling, cleaning, and interaction on the hair surface. The results showed excellent correlation with the subjective evaluations of hair shine. A computer-interfaced photogoniometer utilized by Guiolet eta/. (4) yielded information regarding the structural parameters of hair surface, luster, and pigmentation. Unfortunately, fiber curvature, light scattering, degree of alignment, and hair color all contribute to the discernment of gloss. All these variables must be considered when taking gloss measurements on hair. COMBING PROPERTY AND FRICTION OF HAIR Combability can be defined as the subjective perception of the relative ease or difficulty encountered when human hair is combed. Fiber properties such as curvature, friction, stiffness, diameter, length, and cohesion all affect combing ease. Combing difficulty arises with increased fiber curvature, friction, or static charge. On the other hand, increasing fiber stiffness, diameter, or cohesive forces makes hair easier to comb. Hair feel is usually judged subjectively by sliding the fingers over hair or by rubbing fibers between the fingers. Scientists have translated this subjective evaluation into an objec- tive measurement of friction, with the assumption that hair variability has no conse- quence. Friction is the resistance of two contacting surfaces to relative motion. It is proportional to normal force, and the proportionality constant is defined as the coefficient of friction. Friction measurements can usually differentiate the performance of similar shampoo formulations because they are more sensitive and quantitative than subjective combing tests. Schwartz and Knowles (5) utilized a torsion balance to record the frictional force of a single fiber as a cylinder moved against the hair. The hair fiber was draped over a cylinder with equal weight applied to both ends. Another method, introduced by Scott