GUMS IN COSMETIC FORMULATIONS By GERH^Rt) LEVY* Presented •7une 4, 1958, New York City HYDROPHILIe OVMS are widely used by the cosmetic industry in the manufacture of lotions, creams, jellies, emulsions, tooth pastes and many other products. In the past, these gums could not always be used to their best advantage, since little was known of their properties under different conditions. The additional knowledge that has been gained durin• the last several years has made it possible to extend the usefulness of i W- drophilic gums consider ably. In the following, the effect of a number of variables on the performance characteristics of certain water-soluble or dispersible gums are discussed. These variables include methods of preparation of the solutions or disper- sions, degree of polymerization, presence of impurities, preservatives, sequestering agents or polyhydroxy compounds, as well as pH and tempera- ture. METHODS OF PREPARATION Gums are usually dissolved or dispersed in water by one of two methods: direct addition of the gum to water subjected to high shear agitation, or, pre-mixing of the gum with water-soluble or water-miscible substances and subsequent addition of water. Differences in methods of preparation, such as length and intensity of agitation, the order of mixing, and the use of heat will affect the viscosity, stability and rheologic properties of many gums. Intensive shear may bring about partial degradation of a hydrophilic polymer. Asbeck and Baxter (1) have subjected aqueous solutions of poly- ethylene oxide to uniform shear. They found the extent of depolymeri- zation to be proportional to the intensity of the shear field and the original molecular weight of the polymer. The shear intensity is affected by the viscosity of the solution which in turn is influenced by the concentration of the polymer and the temperature of the solution. * The University of Buffalo School of Pharmacy, Buffalo 14, New York. 395

396 JOURNAl. Of THE SOCIETY OF COSMETIC CHEMISTS The mechanical degradation of another hydrophilic polymer, sodium alginate, has been observed by Schwarz and Levy (2). The viscosity of sodium alginate solutions decreased permanently by 50 per cent or more as a result of prolonged shearing. Many other reports in the literature de- scribe the chain rupture of polymers when subjected to shear. The shear rates employed in the preparation of gum solutions may also affect rheologic characteristics other than the coefficient of viscosity. Using a type of sodium carboxymethylcellulose, deButts and co-workers (3) could demonstrate that aqueous solutions prepared at a low shear rate de- veloped little gel strength whereas those prepared at a high shear rate de- veloped considerable gel strength. With the aid of high power input shear- ing, these workers prepared CMC solutions that were highly thixotropic. The effect of the method of preparation on the stability of a hydrophilic gum is exemplified by tragacanth. Levy and Schwarz (4) compared the effect of heat, homogenization and simple agitation on the hydration of this gum. Tragacanth hydrates very slowly so that solutions of the gum tend to increase in viscosity for some time subsequent to preparation unless the gum has been fully hydrated. This hydration can be accomplished by repeated homogenization with relatively little simultaneous degradation. On the other hand, hydration by heating is accompanied by extensive deg- radation. The viscosity of fully hydrated tragacanth solutions decreases rapidly with time while the viscosity of partially hydrated tragacanth solu- tions will increase for varying periods of time before it also begins to de- crease. By controlled partial homogenization, tragacanlh solutions pos- sessing a wide range of stability characteristics may be prepared. DEGREE OF POLYMERIZATION The polymeric nature of hydrophilic gums accounts for the variations in their average molecular weight. These variations may occur in a natural gum due to differences in geographic origin or harvest year and necessitate the modification of manufacturing formulas in order to obtain products of a desired viscosity. Instead of using definite quantities of a gum for a given product, the concentration of gum is adjusted to account for the differences in thickening capacity resulting from variations in the degree of polymeri- zation. Most synthetic gums a•e available in a number of different average molec- ular weight ranges. This is also true of some processed natural hydrocol- loids. For example, the molecular weight of commercial grades of sodium alginate is reported to range between 32,000 and 200,000 with a degree of polymerization between 180 and 930 (5). Most gums may be fractionated by conventional methods, using electrolytes or organic solvents. Thus it is possible to prepare products containing either small concentrations of a gum of high molecular weight, or higher concentrations of the same gum of a