Nitrocellulose lacquers for Aircraft
As a result of this study, a n improved formulation was developed, and specification MIL-L-006805B (Aer) camouflage lacquer was written:
Formulations of Original Specification Lacquer and Improved Camouflage Lacquer Original Camouflage RS nitrocellulose, 1/2 sec. 30 ... RS nitrocellulose, 5-6 S ~ C . ... 26 Duraplex ND-78 60 58 Tricresyl phosphate 10 , . . Trioctyl phosphate , . . 16 Titanium dioxide
lacquer coating system, consisting of MIL-(2-15328 wash primer, MIL-P-8585 ( A x ) primer, and MIL-L7178 lacquer, had been used satisfactorily for years on propeller-driven aircraft to protect the exterior aluminum surfaces of the planes and to reduce their visibi!ity from the ground. This lacquer system was found to show sporadic cracking on j:t aircraft. Such failures occurr-d where the coating thickness was heavy in the regions of high stress and bzcause of the lower temperatures encountzred by jet aircraft in flight. Accordingly, the problem was (1) to determine the probable significant causes for the crazing of the lacquer topcoat; (2) to develop a laboratory procedure to simulate the conditions to which the jet aircraft are subjected; and (3) to modify the MIL-L-7178 lacquer to eliminate its deficiencies and yet retain its desirable characteristics. It was shown by conventional tests ( M I T folding endurance and mandrel) that the flexibility of the specification Iacquer was greatly reduced a t low temperatures. These tests, however, could not differentiate to any great extent the differences in flexibility due to various compositional changes in the lacquers. The Instron apparatus was used to measure tensile strength and elongation of free films and did show quantitative differences. How-vcr. this instrument is not readily availablc in coating laboratories and does not Eimulate servicr conditiors of a complete finish system. To overcome some 0,' the inherent limitations of these tests? the Krouse flexure fatigue apparatus was selected for this investigztion because it could differentiate flexibilitywise among various lacquer compositions. In addition, i t simulates aircraft conditions because the stresses that can be obtained with this instrument may actually be encountered in flight. This instrument is used by metallurgists to determine the fatigue strength of structural metals. Thr Krouse tester is a motor-driven, constantamplitude rnachine capable of reproducing a wide variety of bending stresses by means of a n eccentric variable throw crank. During the first half of a cycle when the sample is flexed downward, a tensile stress is produced on the upper surface of the specimen and a compressive stress on the lowrr suface. These stresses are reversed when the specimen is flexed upward during the second half of the cycle. Approximately 1750 cycles are obtained in one minute. For a cantilever-beam type of specimen designed for a constant stress, the bending stresses are calculated from the formula:
s = -6LP
Where S = stress in lb./sq. inch; L = constant (1.813); b = constant (0.5); d = thickness of test specimen in inches; and P = load in pounds. The results with this apparatus on the specification lacquer and various experimental lacquers showed that one of thecauses for the cracking of lhe specification lacquer on jet aircraft was its poorer flexibility when subjected to a sufficient and repeated stress a t low temperatures. There was a tendency for thicker topcoats to crack sooner than the same topcoat in thinner films. The substitution of higher viscosity nitrocellulose ( 5 to 6 seconds viscosity) for the l/2-second type in the MIL-L-7178 control formula improved the low temperature flexibility. The substitution of trioctyl phosphate for tricresyl phosphate gave still greater flexibility in conjunction with the higher viscosity nitrocellulose. The USE of a castor oil-modified alkyd resin in place of the short oil-modified coconut alkyd resin improved low temperature flexibility over that of the control lacquer. The initial adhesion (as measured by the wet tape test) of the lacquers, RS nitrocellulose, 5-6 second, short oil coconut alkyd and RS nitrocellulose l/zsecond, longer oil castor alkyd, to the MIL-P-8585 (Aer) primer was somewhat reduced as compared to the adhesion of the specification lacquer. However when the former lacquer was applied to MIL-P-7962 (Aer) lacquer-type primer: then adhesion comparable to that of the specification lacquer was obtained.
Talc 399 Celite 281
45 30 9
Krouse Flexure Fatigue Values at - 20 F Cycles at 50,000 Ib./sq. inch Topcoat film thickness 1.0 mil <440 ... 2.0 mils > 9,600 Cycles at 40,000 Ib./sq. 'inch Topcoat film thickness 2.5 mils <5,3OOa > 26,000 a Same lacquer did not crack in 26,000 cycles when flexed at room temperature.
The MIL-L-006805B (Aer) camouflage lacquer has replaced the MIL-L6805 camouflage lacquer as a low gloss lacquer for Naval aircraft. I t is also believed that use of the high viscosity type of nitrocellulose will decrease the tendency to apply very heavy film thickness on jet aircraft, especially in overhaul.
A. L. FALKOWITZ Naval Air Experiment Station, Philadelphia Naval Base, Pa.
F. E, PIECH Hercules Powder Co., Research Center, Wilmington, Del.
De-icing Lacquer for Stationary
of the difficulties encountered b>aircraft during the winter season is the frequent accumulation of ice on the wings and fuselage while the plane is parked on an air field or on the flight drck of a carrier ship. Before taking off on a scheduled flight or a n assigned mission. expenditure of time and labor would rhen be necessary for removal of such adherent ice. The Department of the Navy, Bureau of Arronautics, has been much concerned with this problem and was instrumental in seeking a suitable solution. Three possible approaches were studied :
INDUSTRIAL AND ENGINEERING CHEMISTRY
1. Applying large ice-phobic cover to the parked aircraft 2. Spraying the plane with antiicing and de-icing fluids 3. Applying a more or less permanent coating to the plane which would possess so little adhesion for ice that the removal could be accomplished very easily and speedily It was for the third method that the Bureau of Aeronautics gave a development contract to Interchemical Corp. covering a period of 2 years-namely, to develop a coating or finish which would
have this minimum adhesive property for ice. A list of ideal specifications was set up as a goal for this work. Probably the two most important ones were that the finish be capable of application on location with a fairly short drying period and with reasonable durability and adhesion to the finish already on the plane and, secondly, that it be effective for a minimum of 75 cycles of icing and deicing. It was therefore evident from the outset that only air drying type coatingsmost likely some form of lacquer-could be considered. Furthermore, any additive type ingredients used to promote ice release must necessarily be insoluble in water. Otherwise they would be leached out by rain or repeated freezings and hence become rapidly depleted, leaving the coating ineffective. The work required first the design and construction of suitable equipment for freezing a standard size block of ice (1inch diameter disk) onto panels coated with various lacquer to be tested and then removing them by the application of a measurable force. In this manner, a survey was made of various mating or film-forming materials in order to narrow the field to those showing the most promise for low ice adhesion. Such items as nitrocellulose, ethylcellulose, cellulose acetate, and polybutylene were examined both alone and in combination with resins, such as alkyds and vinyls, and/or plasticizers. A study was also made of a second, broader group of materials which were looked upon as additive agents. These were included in small amounts in the various films, in the expectation that they might have the property of promoting the ice release to an appreciable degree. This list included waxes, hydrophilic compounds such as high fatty acid derivatives, soaps, mineral oil, silicone oils, etc. The best of the various materials were then selected, and a systematic study was made of each one to determine the percentage to be used to produce the optimum results. T h e effect of varying the ratios of the different ingredients was illustrated by a series of graphs. As a result of this work, a lacquer was formulated based on nitrocellulose, 10 parts; alkyd and vinyl acetate resins, 20 and 2 parts, respectively; plasticizer, 3 parts; and silicone oil and microcrystalline wax, 1.5% and O.5y0,based on film solids. This met the requirements in the laboratory tests and is now undergoing field tests in various locations.
CLIFFORD J. ROLLE WILLIAM D. BARNES, JR. lnterchemical Corp. Research Laboratories, 4 3 2 West 45th Street, New York 36, N. Y.
Rapid Method of Evaluating Check Resistance of Furniture Lac
of the important properties desired in a finish is its resistance to checking failures. Furniture manufacturers, finishing materials suppliers, and the general consumer have been concerned with finish checking. Several methods of determining check resistance of finishes have been developed which are either time-consuming, expensive, or subject to considerable variation. As a result, laboratory quality control procedures for evaluating check resistance of finishing materials formulated to satisfy use requirements are not completely satisfactory. The checking tendencies of lacquer films have been correlated with the amount of deformation required to cause film failure. Furniture lacquer films can be formulated which vary in flexibility and resistance to checking. Flexibility tests were conceived as a means of predicting the check resistance of films applied to wood panels subjected to a cold-check cyclic test. A rapid method of determining the distortion
a t film failure, by using a range of bending mandrel sizes from ’/* to 11/4 inches over which the lacquer-coated metal sheets were bent, was correlated with cold-check tests of the lacquer on wood panels. Nine lacquers were selected representing a normal production range in coldcheck resistance. Two commercial lacquers of high and low check resistance were used and seven lacquers were prepared from two formulations of “hard” and “soft” lacquers supplied by the Lilly Co., High Point, N. C. Blends of the two specially formulated lacquers, on the basis of the percentage hard lacquer in the mix, produced seven lacquers which, when applied to Gardner laboratory tin plates, would check on bending over a range of mandrel sizes from ‘/8 to 11/4 inches. All lacquers contained 217, solids and viscosity of 21 seconds with a No. 4 Ford cup except the hard commercial lacquer which was 18 seconds. Cold-check resistance cyclic tests, using the nine lacquers, were conducted
Bending-mandrel test apparatus VOL. 48, NO. 8