Patented Sept. 14, 1954 2,689,187

UNITED STATES PATENT OFFICE 2,689,187 METHOD AND COMPOSITIONS FOR FORM ING 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 . 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 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 ------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. The washed ma be of little value in reflecting light. In the mak terial may then be broken up by stirring in a ing of aluminized Screens, it has been found that hot distilled water suspension with the water 50 the best aluminized films are formed when the kept near the boiling point. The suspension nitrocellulose film shrinks about 10 per cent lipOn Inay be filtered and rinsed with hot distilled Solidifying. Water, after which it should be dried for several Another factor which must be carefully con hours. This treatment is preferable in order to trolled during the process of film formation in prepare films which are completely homogeneous 55 cathode-ray tubes is the film thickness. Prefer and free of fine holes. ably, the film should have a fairly uniform thick The Selection of the proper viscosity grade of neSS and the thickness must not be either too nitrocellulose is of some importance since this is Small or too large. If the nitrocellulose film is One of the factors in determining the viscosity of too thick, when it is baked out after aluminiz the prepared solution. Although nitrocellulose 60 having a Viscosity grade of A-second is pre ing, air cannot gain complete access to the film ferred for the compositions of the present in and charring of the nitrocellulose occurs rather Vention, material of lower viscosity grade can be than complete burning. When charring occurs, used. For example, material having a viscosity the Screen becomes darkened. If the film is too of '4-Second is satisfactory and even lower vis 65 thin, of course it will be too fragile for easy Cosity grades, down to 33 c. p.s., form films hav handling. Experience has shown that the film ing sufficient strength for the intended purpose. should not be more than 2,000 A thick nor should On the other hand, if dilute solutions of nitro it be thinner than about 200 A, although the cellulose are used, the viscosity grade may be in absolutely lowest limit of film thickness is still creasingly higher, depending on the degree of 70 lower than this figure. Although variations of dilution. thickness in the film are permissible, the film In developing the compositions for use in the should be reasonably uniform. As a practical present invention, it was found that the forma matter, the thickness portions of the film should tion of nitrocellulose films suitable for making preferably not be more than about 1,000 A. aluminized screens for cathode-ray tubes, in 75 One of the essential features of the present 2,689,187 5 6 invention is in the formulation of compositions As previously pointed out, a certain perceratage which enable the above discussed factors in the of shrinkage is desirable in making nitrocellulose aethod to be satisfactorily controlled. The Sol films suitable for aluminizing. Where the plas Weat combination was selected to provide desired ticizer is dioctyl phthalate, the ratio of plasticizer spreading characteristics for the film. In order to nitrocellulose is about one-fourth in Order to to have the film spread over a sufficiently wide obtain a film shrinkage of about 10 percent. Al area, a solvent having a relatively high boiling though the One-fourth ratio is preferred, a cer point is desirable. As the boiling point of the tain range of proportions is permissible, depend solvent increases, the spreading area of the solu ing upon the particular plasticizer selected. The tion on an open dish of Water increases. Octyl O range of plasticizer which may be used is from acetate has been found to have a sufficiently about one-tenth to one-half the weight of the high boiling point to enable films to be spread nitrocellulose. For making nitrocellulose films Dyer the area of the Screen of the largest cathode suitable for aluminizing purposes, the plasticizer ray tubes in ordinary commercial use. Octyl should be substantially insoluble in Water. acetate is not the only solvent which could be 5 Otherwise, the water which serves as the spread used for this purpose. Any Water-insoluble Sol ing base will quickly leach it out of the thin went for nitrocellulose having a boilirag point film. Some plasticizers, such as dimethyl Within the range of about 150 to 250° C. is suit phthalate or camphor, for example, are too high able. Other typical examples are ethylbutyl in water solubility to function as film plasticizers acetate and ethylamyl acetate. If only this type 20 for the present purpose. Although the plasticizer of solvent is used, however, the time required is also a solvent for the nitrocellulose, it may ge for the film to harden is too long for practical distinguished from the other high boiling sol purposes. As previously stated, if the hardening vents used in these compositions. The plasti period is too long, imperfections may appear in cizers used herein cannot be removed from the the film, due to motion occurring in the Water 25 film by blowing an air stream through the tube base. To retain the good spreading character for a few minutes; i. e., 5 to 10 minutes. On istics of the high boiling Solvent and, at the the other hand, the high boiling solvents which Sanae time, to have the film harden more quickly, are not termed plasticizers in these composi it has been found desirable to use a certain pro tions are readily removed from the film in this portion of a solvent which can be gotten rid of 30 manner. Suitable plasticizers, other than the rapidly. This solvent may be one which evapo One mentioned in Example 1, are dibutyl phthal rates rapidly when exposed to the air at Ordinary ate and triglycol dihexoate. These are merely temperatures or it may be one which is ap additional examples of a fairly large number that preciably soluble in Water, or it may be both could be used. highly volatile and water soluble. If water Solu The thickness of the nitrocellulose film can be ble, its solubility is preferably about 10-30 per controlled by the amount of spreading solutiora cent by weight. When the film-forming solu tof a given concentration used to cover a partic tion is placed on the surface of the water, this ular surface area. In using, the composition of solvent is quickly removed from the film, either Example 1, when aluminizing a 4 or 5 inch tube, iby evaporation into the air or dissolving in the 40 one drop from an average dropper is sufficient water, or both. This causes the film to set to to form a film having a satisfactory thickness. a high viscosity quickly and prevents darnage to A drop of this nature occupies a volume of about the film due to any motion in the water. In the O5 ml. For a 7 inch tube, two drops are required example previously given, ethyl acetate is the and six drops are required for a 12 inch tube. solvent which is rapidly removed from the 45 Using these amounts, the thickest portions of the spreading solution after the solution is applied film which is formed will be about 1,000 A in to the water surface. Any other easily dispersed thickness and the thinnest portions will still be solvent for nitrocellulose can, however, be used safely above the minimum which is desirable. equally well. Other examples are cyclohexanone, Another example of composition suitable for nethyl isobutyl ketone and many other organic 50 forming a nitrocellulose film for aluminizing is and ketones. as follows: It will thus be seen that the combination of Eadnple 2 the two types of solvents is used, first, for the purpose of providing a solution having initially Percent hy a viscosity low enough for applying from a 55 Jingredient Weight; dropper and for rapid spreading and, second, for the purpose of maintaining the viscosity of the 30 film constant during the time the water cushion Octv Acetat 27.5 layer is being poured off. It will also be seen Apsitw)xid 50 that the type of combination selected enables Pronano...... 0 the film to spread properly, even over large areas, and yet to harden rapidly enough to prevent In this exaraple, the propanol may be omitted formation of imperfections of certain types. entirely, if desired. The mesityl oxide has been The amount of the non-volatile solvent may found to produce good results as the easily dis vary from about 2 to 4 times the weight of the persed solvent because of its relatively low vola nitrocellulose, and the easily dispersed solvent tility. One of its particular advantages is that, may be present in the alaaount of about 5 times due to its lower volatility, the shelf life of the the Weight of the nitrocellulose or higher. This solution is lengthened. It disappears from the solvent may be used to dilute the compositions firin guickly, however, due to its water solubility. to almost any extent desired. The compositions which are a part of the pres The function of the plasticizer in the present ent invention may be used for forming films as compositions is to control the percentage of described. More specifically, a particular exam shrinkage of the nitrocellulose film. It has been ple of a method of forming a film is as follows: found that, as the percentage of plasticizer is A tube, after having its phospor screen applied, increased, the percentage of shrinkage decreases. 5 is dried and placed on a pouring table. Without 2,689,187 7 8 Wetting the sides of the bulb, water is poured 7. A composition according to claim 1 in which into the tube to form a spreading base. For a Said third Solvent ingredient is ethyl acetate. short 4 inch tube, about 50 cc. of water are desir 8. A film-Spreading composition consisting es able. For a short 12 inch tube, 1,000 cc. may be Sentially in per cent by weight of nitrocellulose used and, for the long 12 inch variety, 1,500 cc. 10%, dioctyl phthalate 2.5%, octyl acetate 27.5%, are suitable. One or more drops of the film solu and mesityl oxide 60%. tion are then delivered about 1 cm. above the 9. A method of forming a stretched film of Water Surface. If more than one drop is used, nitrocellulose within a vessel comprising covering the drops should be delivered not slower than One the bottom of the vessel with a layer of water, or two per second in order to make uniform films. O floating on the Surface of said water layer a After one or two minutes, the neck of the tube solution consisting essentially of not more than is tilted to begin the pouring action, and a siphon about 20 percent nitrocellulose by weight and the is inserted beneath the film. As the tube is remainder a liquid mixture which is constituted tilted, water is removed through the Siphon. as follows: (1) a Substantially non-volatile, The rate of pouring should be slow enough so 15 water-insoluble plasticizer for said nitrocellulose that the area of the screen from which the Water in an amount equal to about is to 42 the weight meniscus has just receded is not glossy. As of said nitrocellulose, (2) a substantially water pouring progresses, the speed may be increased. insoluble solvent for said nitrocellulose having The pouring should normally take 8 to 12 min a boiling point of about 150° to 250° C., present utes, depending upon tube size and face contours. 20 in an amount equal to about 2 to 4 times the After pouring, the tube should be left for a few weight of Said nitrocellulose, and (3) a nitrocel minutes With the face nearly vertical to encour lulose solvent which is readily removable from age drainage of the water layer between the film Said Solution as it spreads over Said water Sur and the screen. Next, the film is dried in a slow face, permitting said solution to spread over said air stream to evaporate the high boiling solvent 25 Water surface to form a thin layer, permitting which is present and to dry the tube in prepa said solvent which is readily removable to leave ration for aluminizing. Said Solution whereby to cause said Solution to There has thus been described an improved partially set to form a thin film, removing said method of forming a Smooth Surfaced film of water layer from beneath said film, and rapidly nitrocellulose over a relatively rough surfaced 30 evaporating said water-insoluble solvent ingredi material. An essential part of the method is the ent from said film. use of improved solutions for spreading the nitro 10. A method according to claim 9 in which cellulose film which will automatically maintain Said plasticizer is dioctyl phthalate, said water the viscosity of the film Constant at a desired insoluble Solvent is octyl acetate and said easily value during the period of pouring off the water 35 removable solvent is ethyl acetate. from beneath the film. The use of a phosphor 11. A method according to claim 10 in which screen is not an essential factor in making the Said dioctyl phthalate is initially present in said films of the present invention. In fact, no other Solution of nitrocellulose in an amount of about film need be present at all beneath the nitro 2.5% by weight, said octyl acetate is initially cellulose film. 40 present in an amount of about 27.5% by weight I claim as my invention: and said ethyl acetate is present in an amount 1. A film-Spreading composition consisting eS of about 60% by weight. Sentially of nitrocellulose and liquid in which said nitrocellulose constitutes not more than References Cited in the file of this patent about 20% by weight and said liquid constitutes 45 the remainder and in which the liquid consists UNITED STATES PATENTS essentially of (1) a substantially non-volatile, Number Name Date water-insoluble plasticizer for said nitrocellulose, 826,781 Gregory ------July 24, 1906 present in an amount of about to A, the weight 1,309,981 Clarke ------July 15, 1919 of said nitrocellulose, (2) of substantially water 1,434,634 Sease ------Nov. 7, 1922 insoluble solvent selected from the class consist 1,653,010 Humphrey ------Dec. 20, 1927 ing of octyl acetate, ethylbutyl acetate and ethyl 1702,181 Wan Schaack ------Feb. 12, 1929 in an amount equal to about 2 to 4 2,015,077 Lawson ------Sept. 24, 1935 times the weight of the nitrocellulose, and (3) a 2,064,802 Edgar ------Dec. 15, 1936 Solvent selected from the class consisting of ethyl 55 2,086,714 Hucks ------July 13, 1937 acetate, cyclohexanone and methyl isobutyl ke 2,088,052 Ensminger ------July 27, 1937 tone constituting the remainder. 2,098,534 Church ------Nov. 9, 1937 2. A composition according to claim 1 in which 2,485,372 Farrell ------Oct. 18, 1949 said nitrocellulose constitutes about 10% and said 2,562,373 Arnold ------July 31, 1951 liquid about 90% of the composition. 60 2,631,334 Bailey ------Mar. 17, 1953 3. A composition according to claim 1 in which said plasticizer is dioctyl phthalate. FOREIGN PATENTS 4. A composition according to claim 3 in which Number Country Date said dioctyl phthalate is present in the amount 387,534 Great Britain ------Feb. 9, 1933 of about A the weight of said nitrocellulose. 65 217,455 Switzerland ------Feb. 16, 1942 5. A composition according to claim 1 in which OTHER REFERENCES said water-insoluble solvent is Octyl acetate. 6. A composition according to claim 5 in which Simonds et al.: "Handbook of Plastics,' 1943, said octyl acetate is present in an amount equal pp. 244, 245 and 248. to about 2.75 times the weight of said nitrocel 70 lulose.