606 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS facial preparations is known. Spray-on bandages represent another ap- plication of films to the skin. The water vapor permeation of these films is an important characteristic of all films as it is an indication of the "breathing" quality of the film. Breathing films, unlike occlusive fihns, will not impair the normal functions of the skin. As the use of these fihns in cosmetic products increases, the water vapor transmission be- comes increasingly important and should be determined. There has been an increased interest in the use of different polymeric agents for the formulation of a spray-on bandage. These polymeric agents must be capable of forming a continuous film when either sprayed or otherwise applied to the skin surface. These films have found wide- spread use due to their ease of application. Several have been used in surgical procedures as reported by Wallgren (1). He described a fihn consisting of an acrylic resin which had a water vapor permeation of I4 mg/cm2/day at 32øC when applied at a thickness of 100 •. A fluid surgi- cal dressing (2) of hydroxylated vinyl acetate-vinyl chloride copolymer, •t-butyl methacrylate polymer, ethyl cellulose, and other polymer systems has also been reported and said to provide a nontoxic, sterile, trans- parent, and water-resistant sprayable film. Miller et al. (3) have men- rioned the use of cellulosic compounds plasticized with 2-ethoxyldi- phenyl phosphate as a sterile plastic dressing in general surgery. In ad- dition, a number of new spray-on-tissue adhesives, consisting of cyano- acrylate resins and derivatives, have been reported (4-6). Lange and Gonet (7) and Lange and Fang (8) have also reported 'the use of a number of aqueous topical spray-on bandages consisting of a copolymer of poly- vinyl alcohol/polyvinyl acetate or acrylic resin and other water-soluble resins. The selection of a polymeric fihn for application as a protective film to the skin is determined by a consideration of various physicochemical and biological properties. Sciarra and Gidwani (9) reported some of these properties for several films including hardness, modulus of elas- ticity, alkali resistance, water vapor transmission, and stability to degra- dation from exposure to ultraviolet radiation. These properties were evaluated in order to determine the use of certain film plasticizers which could be used with films and applied to the body via an aerosol spray. A film-forming polymer which is suitable for application to injured skin should be permeable to water vapor so as to decrease the possibility for the growth of anaerobic bacteria in the vicinity of the wound. On the other hand, there are conditions which may require .the use of an

WATER VAPOR TRANSMISSION OF FILM-FORMING AGENTS 607 occlusive dressing. Many reports have been published concerning the permeability of single polymeric free films (10-13). Components added to film-forming agents as a part of formulation may affect the rate of water vapor transmission. The effects of several plasticizers such as diacetin, diethylphthalate, triethylcitrate, and acetyl triethyl citrate on the water vapor transmission of cellulose acetate films have been reported The rate of water vapor transmission (WVT) of a film between two specified parallel surfaces is dependent upon the film, the plasticizer, and its concentration, as well as the thickness of the film. It has been reported that some materials, such as polystyrene (which adsorbs little water), be- have in accordance with Fick's law and the water permeation rate is in- versely proportional to the film thickness (14). According to the ASTM Test No. E96-53T method (15) for materials in sheet form, the water vapor transmission can be calculated from the following: where: WVT = (g) (24/0 (a) g = weight of loss or gain, in grams t = time in hours during which loss or gain "g" was observed a = exposed area of the specimen, in in. 2 WVT -- rate of water vapor transmission, expressed in g/in2/24 hr Flux (F) can be calculated from WXT F- AX0.1 where: W = weight of water vapor permeating in mg T = thickness of the film A = effective area of the exposed film The permeability coefficient (P0 is defined as the milligrams of water vapor that permeate through a film 0.1-mm thick, per unit area in cm =, per unit pressure drop every 24 hours, following a steady state of diffu- sion under the experimental conditions of temperature and pressure. Mathematically, this can be expressed as F R e --