U. S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS RESEARCH PAPER RP788 Part of Journal of Research of the N.ational Bureau of Standards, Volume 14, May 1935 FIRE.RESISTANT DOPED FABRIC FOR AIRCRAFT By Gordon M. Kline ABSTRACT Cellulose nitrate dope now commonly used to cover the fabric on the wings and fuselage of airplanes is very flammable and its replacement by a less-hazardous product is desirable. The development of a satisfactory nonflammable dope by the use of natural and synthetic resins or mixtures of synthetic resins with cellu­ lose nitrate and cellulose acetate has not proved to be feasible. An airplane cover­ ing with very good resistance to ignition may be obtained by the application of a 3:7 boric-acid-borax mixture to airplane cloth and subsequently doping it with cellulose acetate. Cellulose acetate dope is now only moderately more expensi ve than cellulose nitrate dope. The cost difference becomes insignificant when the lower fire hazard resulting from the use of the much less flammable cellulose acetate dope is considered. CONTENTS Page I. Introduction ______ ___ __________________________________________ 575 II. Test methods _____ ____ __________________________________________ 576 l. Tautness of doped fabl'ic __________________________________ 576 2. Weight of doped fabric _____________ __________________ ____ 577 3. Flammability of doped fabric _ _ _ _ _ _ _ _ _ _ _ __ _ __ _ _ __ _ _ _ _ _ _ _ _ _ _ 577 III. Experimental work ____________ ____ ._____________________________ 578 l. Resins, casein, and cellulose derivative;; as dope consiituents __ 578 2. Mixtures of resins and cellulose derivatives as airplane dopes __ 579 3. Fireproofing of cloth__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ _ _ _ _ 579 4. Effect of fireproofing airplane cloth on the fire resistance of the product covered with cellulosic dopes _ _ _ _ _ __ __ _ _ _ _ __ _ _ _ _ _ _ 581 5. Effect of mixed cellulose nitrate and cellulose acetate doping schemes on the fire resistance of the doped fabric ______ . __ 583 IV. Discussion of results ______________ _______________________________ 583 V. Summary and conclusions ___________________ ___ ____________ ______ 584 VI. Bibliography _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ __ _ _ _ _ __ __ _ _ _ _ _ __ _ _ _ _ _ _ _ _ 585 l. Airplane finishing _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ 585 2. Fireproofing ______ __________ _____ _______________________ _ 587 I. INTRODUCTION The rapid growth of the aviation industry in this country has brought to the fore the problem of eliminating the fire hazard inherent in fabric doped with cellulose nitrate, now commonly used to cover the wings and fuselage of airplanes. The destruction of costly aircraft because of the accidental ignition of the flammable covering by the backfiring of the motor, the careless toss of a lighted match or cigarette, or the chance settling of a spark from a nearby flue, has become too general an occurrence. The rapid spread of flames following a minor crash presents a formidable obstacle to the rescue of trapped survivors. It has been stated that the use of metal will eliminate this hazard, but 575 576 Journal oj Research oj the National Bureau oj Standards [Vol. H it is probable that, particularly for service airplanes, fabric will continue in use for some time to come because of its lower cost, avail­ ability, and ease of application, factors which facilitate rapid replace­ ment of losses in time of war. An investigation was, therefore, under­ taken by the National Bureau of Standards, with the financial assist­ ance of the National Advisory Committee for Aeronautics, to develop a nonflammable doped fabric for aircraft. The use of cellulose nitrate and cellulose acetate in airplane dopes during the war has been described by Esselen (3)/ Smith (5) Deschiens (10, 11, 12, 17,23,25), Drinker (18), and Britton (28,29). Very little work on the use of other materials for airplane dopes has been reported. Two Japanese investigators, Araki and N agamote, have described experiments with ethylcellulose (37) and benzyl cellulose (43). Des­ chiens (23) foresaw the day when "the formates, butyrates, oxalates, and benzoates of cellulose, the phenylcelluloses, the esters of the higher fatty acids, and many other cellulose derivatives that are now being explored will be of industrial importance." Gardner (39) recom­ mended a study of phthalic anhydride-glycerin resins for coating wing surfaces. Other resins and cellulose esters and ethers have been pro­ posed for airplane dopes in various patents, but no detailed study of the properties of these compounds relating to their satisfactory use for such purpose has been published. This paper presents the results of a comparative study of cellulose nitrate, cellulose acetate, and various natural and synthetic resins as constituents of airplane dopes, with particular emphasis upon the development of a nonflammable doped fabric. The effect of fireproofing airplane cloth on the fire resistance of the product covered with cellulosic dopes has also been determined. A satisfactory airplane dope should tauten the fabric to which it is appJied, dry relatively rapidly without "blushing", i. e., precipi­ tating the cellulose derivative in white patches because of moisture condensation, give a smooth, durable surface, and permit a low film weight. To these we believe should be added the requirement that the doped fabric should be fire resistant. The properties of the doped fabric which were measured in this study were weight, tautness, and flammability. Resistance to weathering, which necessarily requires long periods for its determination, was not investigated in this exploratory work, but was reserved for later study with those materials which satisfy the other requirements. II. TEST METHODS 1. TAUTNESS OF DOPED FABRIC Square panels 12 inches (30.5 em) inside dimension were prepared from 1.5 by 0.75 inch (3.8 by 1.9 cm) dressed white pine. 2 Grade A mercerized cotton airplane cloth, corresponding to U. S. Navy Depart­ ment specification 27c12, was applied to one face only of the panel and fastened by tacks along the sides at intervals of about 1.5 inches (3.8 cm). The various materials, dissolved in suitable solvents, were brushed on the cloth, usually by a four-coat application. After drying overnight at room temperature, the panels were placed in a I The numbers in parentheses refer to the hibliography at the end of this paper. , In later work not reported in this paper square panels 12.25 inches (31.1 cm) inside dimension prepared from 1.5 by 1 inch (3 .8 by 2.5 cm) spruce strips according to U. S. Naval Aircraft Factory proposed aero­ nautical process specification for covering of panels for exposure tests, SP-16, dated October 6, 1932, were used and are considered to be preferable. Kline] Fire-Resistant Fabric for Aircraft 577 room kept at about 21 0 C and 65 percent relative humidity. After approximately 48 hours the tautness was determined with a McGowan tautness meter (47) (see fig. 1). The values given in the tables for the tautness determinations are the deflections of the plunger, resting on the doped surface, when a load of 15 ounces (425 grams) is added. It is considered that for satisfactory tautness the deflection caused by the 15-ounce weight should not be greater than 0.130 inch (0 .33 em) for the size of panel used. 2. WEIGHT OF DOPED FABRIC The doped fabric was removed from the panels and three 2 by 4 inch (5. 1 by 10.2 cm) pieces were cut by a die from a strip at the center. These cut samples were weighed on a chemical balance in the con­ ditioning room. 3. FLAMMABILITY OF DOPED FABRIC Methods of determining the flammability of fireproofed fabrics have been discussed by Ramsbottom and Snoad (66), and of plasti­ cizers by Van Heuckeroth. ~ A vertical burning test was employed in each case; however, the cloth was ignited at the bottom by the former investigators, whereas in the work with the plasticizers, the lacquered paper was ignited at the top. In our work a horizontal burning test, which results in rates of burning intermediate between the two vertical tests mentioned above, was preferred because it accentuated any differences in the flammability of the various mate­ rials tested. The apparatus used is shown in figure 2 and was devel­ oped by C. R. Brown, of the Fire Resistance Section of the National Bureau of Standards. It consists of two parallel steel clamps sup­ ported by a steel frame, the distances between the clamps being adjustable. The specimen to be tested is fastened between the clamps and ignited at one end. The time required for the flame front to travel over a given distance as measured along the clamps is recorded with a stop watch and the rate of burning in inches per minute is calculated. Any length of specimen up to 24 inches (61 cm) and any width up to 7 inches (17.8 cm) can be burned in this apparatus. Brown found that no appreciable effect could be noted due to differ­ ences in length provided the specimens were well ignited before starting the time record.