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UNITED STATES PATENT OFFICE 2,689,187 METHOD and COMPOSITIONS for FORM ING NITROCELLULOSE FILMS Soren M

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UNITED STATES PATENT OFFICE 2,689,187 METHOD and COMPOSITIONS for FORM ING NITROCELLULOSE FILMS Soren M Patented Sept. 14, 1954 2,689,187 UNITED STATES PATENT OFFICE 2,689,187 METHOD AND COMPOSITIONS FOR FORM ING NITROCELLULOSE FILMS Soren M. Thomsea, Pennington, N. J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application September 29, 1951, Serial No. 249,011 11. Claims. (CI. 106-184) 2 The present invention relates generally to an and then to add a few drops of the nitrocellulose improved method of forming a film of nitro solution. This solution was allowed to Spread cellulose, including the step of spreading a solu and to harden by loss of solvent. After the film tion of nitrocellulose over a water Surface, and had spread to the edges of the tube and had to improved compositions suitable for use in the 5 hardened, the water was slowly removed by improved method. More particularly, the in siphoning or pouring and the nitrocellulose film vention relates to an improved method of form was left resting on the phosphor Screen. With a ing a filin of nitrocellulose over the phosphor thin layer of water held between the film and screen of a cathode-ray tube where the nitro the Screen. After the tube had been dried with cellulose film is intended to have a bright film 10 a slow air stream, the nitrocellulose film Was of aluminum deposited thereon. ready to receive the aluminum deposit. After Cathode-ray tubes having aluminized phosphor the aluminum was deposited, the tube was baked. Screens have been known for Some time. Warious Although this procedure has proved commer ninethods of applying aluminuin films in such cially practical, it has certain disdvantages. tubes have been devised and have been used ex 15 When the solvent evaporates too rapidly, the film tensively. In general, it has been customary to hardens more quickly in one spot than in an cover the rough interior face of the phosphor other, and considerable differences in thicknesses screen with a film of nitrocellulose and then to of the final film often occur. Another disad deposit a thin layer of aluminuin on top of the vantage in the use of isoamyl acetate, alone, as nitrocellulose film. When the process is properly 20 a solvent, is that solutions of nitrocellulose in carried out, the aluminum surface is oright and this solvent do not spread well on water. In shiny, forming a good reflector for light, and orger to make the solution spread better, a sol when the nitrocellulose film is Subsequently vent, such as acetone, which is appreciably solu baked out of the tube, the aluminum remains as ble in Water, must also be added to the composi a satisfactory means for reflecting light gen- 25 tion, but, if too much acetone is used, an irregu erated within the phosphor, toward the face of lar mass of precipitated nitrocellulose results. the tube. The nitrocellulose also precipitates if acetone, Previous niethods of forming the phosphor alone, is used. Although it is possible to pre Screen and the Superimposed nitrocellulose film pare a solution containing isoamyl acetate and have included the steps of settling the phosphor 30 acetone in proportions such that the proper screen on the tube face and baking this in the spreading characteristics are obtained, this type usual rianner. Then, the tube is partially filled of Solution cannot be kept very long without With Water. A few drops of a solution of nitro- a good deal of trouble in its use. The highly cellulose in a volatile solvent are placed on the volatile acetone evaporates rapidly whenever the water surface and a film of nitrocellulose is per- 35 container is uncapped and the solution gradually mitted to spread over the surface of the water becomes more and more, viscous. Even during while the solvent volatilizes rapidly. Later, the the brief period consumed in the application of Water layer is removed, either by siphoning or a few drops of solution to the water surface, decantation and the film constituents are dried viscosity of the solution changes appreciably. in an air stream. Finally, the aluminum is de- 40 Thus, the flowing qualities of the Solution change posited Over the nitrocellulose film and the tube constantly and the use of the solution becomes is baked to remove all of the organic constituents difficult. of the fins. One object of the present invention is to pro In the past, various solvents have been used Wide an improved composition for Spreading a for making up the solutions of nitrocellulose used 45 film of nitrocellulose over a water surface. to form the films upon which the aluminum was Another object of the invention is to provide deposited. One of the most commonly used of an improved composition for use in forming a these Solvents was isoamyl acetate. The film- stretched film of nitrocellulose over a relatively forming Solution, for example comprised 5 per rough Surface. cent of 89-second viscosity grade nitrocellulose 50 Another object of the invention is to provide and the remainder solvent. One expedient for an improved composition for forming a nitro improving the spreading characteristics of this cellulose film upon which a good light-reflecting Solution was to drop a Small amount of the sol- film of aluminum is to be deposited. vent onto the Water Surface in order to presatu- Another object of the invention is to provide rate the air and water surface with the solvent 55 an improved nitrocellulose film-forming com 2,689,187 3 4. position for use in a process of aluminizing the which the aluminum had a satisfactory reflect screen of a cathode-ray tube. ing surface, was influenced by a number of fac Another object of the invention is to provide an tors which Will now be explained in more detail. improved composition for the formation of a One of the factors which influences the forma nitrocellulose film having a predetermined thick tion of nitrocellulose films on a water surface is neSS and Smoothness. the Spreading characteristics of the solution Still another object of the invention is to pro LSed to form the film. With some solvents, the vide an improved method of forming a film of area of spreading of a nitrocellulose solution ap nitrocellulose suitable for aluminizing. plied dropWise to the Water Surface is too limited These and other objects will be more apparent 0 in extent for practical use. More extensive spread and the invention will be more readily understood is prevented by the formation of a set or hardened from the following description of preferred em edge, even though the main body of the film re bodiments. mains liquid for Some time. This is the case An essential feature of the present invention is When isoamyl acetate, alone, is used as a solvent the use of a film-forming composition comprising 5 for the nitrocellulose. It is necessary that the not more than 20% by Weight nitrocellulose and Solvent used permit spreading of the nitrocellu the remainder solvent. The solvent is composed lose film over the entire area, which it is desired of three different types of ingredients. The first to cover. In the case of cathode-ray tubes, the of these ingredients is a non-volatile, water-in SC'een inay have a diameter of anywhere from Soluble plasticizer for nitrocellulose. This in 20 about 2 or 3 inches up to more than 20 and the gredient is present in an amount equal to about composition used should preferably be one which to to a the weight of the nitrocellulose. The can be utilized in either the Smallest or the Second Solvent ingredient is a substantially Water largest diameter tubes. Another complicating inSoluble solvent for the nitrocellulose. This factor, however, is that the solvents used for the Solvent has a boiling point of between 150° and 25 nitrocellulose must not cause the time required 250° C. and is present in an amount equal to for hardening to be unduly long. If the harden about 2 to 4 times the weight of the nitrocellulose. ing period is too long, the film remains highly The third Solvent ingredient is a solvent for fluid over a lengthy period. Motions in the water nitrocellulose having appreciable solubility in base produce Swirls and other patterns, and some Water. This ingredient may have relatively high 30 times cause open areas to form in the film. volatility and constitutes the remainder of the After a nitrocellulose film is placed in a cath composition. Ode-ray tube, it must be dried to remove all vola Eacample it tile constituents. This causes the film to shrink. If the film shrinks to much in area, it may pull A preferred spreading composition may be 35 loose from the screen. This is more likely to oc made up by preparing a solution of nitrocellulose cur in regions where the tube is highly curved. with the ingredients in the following proportions: In making up the improved compositions which are a part of the present invention, it was found ingredient PEpy that the percentage of shrinkage upon solidifying 40 of the film could be controlled by use of the proper plasticizers in controlled amounts. It Nitrocellulose, 34 sec- 0. Dioctyl Phthalate--- 2.5 Was also found that some shrinkage of the film OctylAcetate----- 27.5 is essential to produce the desired type of surface Ethyl Acetate-------------------- 60 for aluminizing. A film which is not caused to Shrink sufficiently will not be smooth enough to For use in the above composition, the nitrocellu 45 have a Surface suitable for aluminizing. If lose fiber, as received from the manufacturer, aluminum is deposited upon a film which is not should be thoroughly washed with distilled water Smooth enough, it will appear to be dark and will to remove all foreign matter.
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