SOLVENTS, HUMECTANTS AND BLENDING AGENTS IN COSMETICS By E. CHADWICK and G. PEARS* AMONG THE various auxiliary products which play a relatively small but nevertheless important part in cosmetic formulations may be classified solvents, humectants and blending agents. In addition to any special properties necessary for their specific application they must all possess in general the following characteristics before they may be considered worthy of examination.: Freedom from toxic, allergic and sensitising action. Compatibility with the materials normally employed. Freedom from odour and taste. If, at the same time, the product also possesses emollient or other desirable properties, these are of advantage. Published information on the use of solvents, humectants and blend- ing agents appears to have been mainly concerned with the practical aspect of these materials in formu- lations and only occasionally with the more fundamental factors involved in selection of the most suit- able product for use in a particular formulation. In the following paragraphs the subject is reviewed in terms of the mechanisms involved in the action of various auxiliary agents, since it is on this basis alone that the most suitable ß agent can be correctly selected. SOLVENTS NAIL VARNISH SOLVENTS The consumption of solvents of usual type is probably greater in this field than in any other relating to cosmetics excepting lotion and per- fumery vehicles. It is not proposed to discuss in detail the formulation of nail varnishes since this is a complex sub- ject more relevant to the field of the lacquer technologist and has been reviewed from time to time fairly comprehensively •. The following comments, may, however, be made on the various types of solvent em- ployed. A properly formulated nail varnish employs solvents selected from pro- ducts with a wide range of evapora- tion rate from the very volatile, typified by ac. etone, to the non-vola- tile plasticisers. Volatile solvents are an essential component of nail lacquers owing to *British Industrial Solvents Ltd., Development Department, Carshalton, Surrey. 105

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the rapid drying required in this type of product, although many of them give rise to the attendant defect of "blushing ", due to condensation of atmospheric moisture as a result of the fall in temperature on evapor- ation. The water-miscible solvents, e.g. acetone or methyl acetate, generally are more prone to this defect, and the less water-miscible solvents, e.g. ethyl acetate and butyl acetate, are therefore to be preferred although they are some- what less volatile. Diluents or "thinnets", besides reducing the cost of the product effectively, reduce the viscosity of the solution and thus permit the use of higher solids contents leading to the deposition of heavier coatings. The three main types of diluent com- prise aliphatic hydrocarbons (viz. pertoleum distillates) aromatic hyd- rocarbons (toluene) and alcohols (ethanol, butanol). The hydrocar- bon thinners are true diluents. The alcohols function as diluents where the active solvents are ketones (acetone, methyl ethyl ketone) and as latent solvents where the active solvents arc esters (ethyl .acetate, butyl acetate). It will be apparent that the amount and evaporation rate of the diluent must bc so adjusted that on application of the film and evapora- tion of the volatile liquids present there is always an adequate amount of active solvent present to, prevent precipitation of the cellulose nitrate by the diluent. Should this not be the case poor strength and surface finish result. True diluents are not essential constituents of nail lacquers and many formulations are based •solely on solvents and latent sol- vents. The alcohols, particularly butanol, also serve the additional purpose of ,p r e v e n t i n g "blushing" and "bloom" Butanol forms a con- stant boiling mixture with water and thus is effective in removing deposited moisture. It is also an excellent blending agent for the solvent mixtures c o m m o n 1 y employed. Amounts of the order of 3-5% are sufficient to blend homo- geneously ethanol and petroleum hydrocarbons. Medium-boiling solvents are .re- quired to promote "secondary flow"--the process which elimin- ates brush marks and gives a smooth film of even thickness. They increase the drying time of the film (and should thus be used with discretion) and are an additional safeguard against precipitation of the resin by any diluents present. Amyl acetate and ethyl glycol ether are effective members of this class. The selection of the correct sol- vents is not sufficient to ensure a satisfactory product unless they are also correctly balanced. Solvent balance is determined not only by the evaporation rate but also by the existence of azeotropes between the various solvent components. These two factors permit the approximate determination of the correct solvent balance and the final adjustment is best made by trial. Only approxi- mate generalisations can be made on 106