Rubber Substitutes as Coatings for Balloon Fabrics

A brief discussion of the materials and balloon fabrics in general is included. The permeabilities to hydrogen of balloon fabrics coated with rubber substitutes and rubber-like materials are given. All have much lower permeabilities than rubberized fabric. T N its simplest form a balloon fabric consists of a •*closely woven cotton cloth coated with a material which serves to render it gas-tight. In common with all structural materials employed in lighter-than-air craft, emphasis is placed upon optimum propertiesstrength, durability, impermeability—per unit weight of the fabric. Because of the uncommon combination of necessary qualities, many of the readily available materials which have been suggested can not be utilized successfully as coating materials. In most cases they lack the essential flexibility. A balloon fabric must withstand an unavoidably rigorous treatment involved in handling—placement in the ship, inflation, deflation —without impairment of its strength or its gas-retaining ability. The problem of providing a suitable coating consists essentially in developing a material that will withstand severe flexure under varying conditions of temperature and humidity and which will be virtually impermeable to lifting gas. Work on this problem is being conducted for the Bureau of Aeronautics, Navy Department. It is generally recognized that the mechanism of the passage of gas through a continuous film takes place by a process of solution and not of flow through small openings. The chemical composition of the film is then a largely controlling factor in determining the rate of permeation. It is unfortunate that rubber, which so adequately meets the requirements of strength and flexibility, should possess an inherently high permeability to gases. Within relatively ineffective limits the permeability of rubber is little altered by variations in compounding or curing. One of the most effective means of lowering the permeability of rubberized fabric has been the simple expedient of adding a coating of paraffin wax. By this means it is possible to lower the permeability about 60 percent, but the accompanying increase in weight may be as much as 20 percent. Furthermore a film of paraffin wax lacks continuity and permanence. To afford the high degree of impermeability desired in gas cells, it has been customary to use, either as direct coatings or to supplement rubber coatings, films of hydrophilic materials. These include such substances as goldbeaters' skin, regenerated cellulose, and gelatin. All these substances are very impermeable to hydrogen and helium. In order to maintain such substances in a flexible condition it is necessary that they be allowed to absorb a certain amount of moisture. For this purpose a hygroscopic substance such as glycerol is added. The use of a hydrophilic material presents the disadvantage that it is subject to physical changes under varying atmospheric humidities. In high humidities it may become wet, and lose strength and adhesion to the fabric, and in prolonged exposure to high humidity there is danger of loss of glycerol by leaching. In low humidities such a material may shrink and become brittle. Other types of commonly employed coating materials, which include varnishes, cellulose derivatives, and a variety of synthetic resins, also present difficulties when applied to this problem. Aside from the fact that many of them are not particularly impermeable they do not have the desired pliability. It is customary to increase the flexibility of varnishes by including a non-drying or semi-drying oil in their composition. To cellulose derivatives and synthetic resins, liquids of low vapor pressure, referred to as plasticizers, are added. For a number of purposes such coatings are satisfactory but in attempting to apply them to balloon fabrics, the large amount of plasticizer added to obtain flexibility tends to produce excessive tackiness. Furthermore, these materials are characterized by extensibility but are not capable of corresponding retraction. While rubber is a material which is unique in its combination of great strength and elastic extensibility, it is deficient in certain properties, which renders it unsuitable for many purposes. One of these properties is its lack of resistance to the swelling action of mineral oils. The necessity of obtaining a material which is superior to rubber in this respect has led to the development of a number of synthetic rubber-like materials. Thus the treatment of vinylacetylene with hydrogen chloride yields chloroprene, having the formula CH2 — C (CI) — CH = CH2, This substance has a high rate of polymerization and yields a material whose properties more closely approach those of rubber than any material hitherto produced. An analogous material, vinyl chloride, having the formula CH2 = CHCl, is capable of a high degree of polymerization and yields rubber-like gels when suitably plasticized. Both of these materials are much less permeable to 1 Publication approved by the Director of the National Bureau of Standards of the U. S. Department of Commerce. 2 T. P . Sager, Permeability of Synthetic Film-Forming Materials to Hydrogen, BS J. Research, 13, 879 (1934) R P 750.