US2168972A - Method of removing air bubbles from film dope - Google Patents

Description

Aug. 8, 1939 E. K. CARVER ,168,97

METHOD OF REMOVING AIR BUBBLES FROM FILM DOPE Filed Feb. 2, 1957 FIG. 1.

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Emmett K. Carver Patented Aug. 8, 1939 METHOD OF REMOVING AIR BUBBLES FROM FIIM DOPE Emmett K. Carver, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application February 2, 1937, Serial No. 123,737

3 Claims.

This invention relates to the manufacture of transparent sheeting, and more particularly to the forming of a bubble-free transparent sheeting suitable for use as a photographic film base 5 which provides a support for the sensitized emul- SlOIlS.

In the manufacture of photographic film, a thin sheet or ribbon of transparent cellulose material is first formed, and this sheet is then coated with the proper sensitized emulsion.

This sheet thus provides a suitable base or support for the emulsion, all of which is well known to those in the art.

This film support is made from various ingreclients and solvents which, when properly mixed, form what is known in the trade as film dope. The materials and solvents from which this dope is made contain more or less dissolved air and other gases. In order to insure a high grade film, it is highly essential that all this air and other gases be removed from the film dope. If these gases are not removed from the dope prior to the coating or casting thereof to form the film support, bubbles will be formed in the support,

and these bubbles will then appear in the finished film. The disadvantages of such bubble formations are obvious to those familiar with the art.

To overcome these disadvantages, the present invention provides a new and simplified method for treating the film base or support. By means of this method, all of the air and other gases are thoroughly removed from the film dope so as to insure a clear bubble-free film support. Furthermore, this method enables the various processes of preparing and casting the film dope to be carried on under optimum temperatures.

The principal object of the invention is to produce a bubble-free film base or support. Another object of the invention is to provide a method of preparing a bubble-free film base in which the various processes can be carried at their optimum temperatures. A further object of the invention is to provide a method whereby the film dope is first heated so as to thoroughly remove the air and other dissolved gases, and then cooled so that the casting of the dope into thin sheets or ribbons may be done at the desired temperature. A still further object of the invention is to provide a method whereby both free air bubbles and dissolved air are removed from the film dope prior to the coating thereof into a thin sheet to form a transparent film base. An additional object of the invention is to provide v a method of preparing film dope so as to provide a bubble-free base, which is simple, relatively inexpensive, and highly efiective.

To these and other ends, the invention resides in certain improvements and sequence of steps, all as will be hereinafter more fully described, 5 the novel features being pointed out in the claims at the end of the specification.

In the drawing:

Figure lis a view, in diagrammatic form, of the arrangement of the apparatus used in carrying out the processes embodied in the present invention.

Figure 2 is a view similar to Figure 1, showing a slightly modified arrangement which includes the use of a storage tank in which a reserve supply 5 of film dope may be stored subsequent'to the removal of air.

The same reference numerals throughout the several views indicate the same parts.

The present invention is embodied in the present instance, by way of illustration, in a process of preparing a bubble-free photographic film support or base on which the sensitized emulsion may be deposited. This method comprises, in general the heating of the film dope to a temperature sufficiently high to remove enough of the dissolved and entrained gases and air so that no bubbles will form when the dope is cast onto a film forming surface such, for example, as a casting wheel. This gas-free dope is then preferably pumped through a heater to superheat the dope so as to reduce its viscosity for filtering. After filtration, the film dope is suitably cooled, in the line going to the coating machine, so that the dope may be cast into a thin sheet or ribbon at the desired temperature.

By means of this process of first heating and then cooling the film dope, not only is the air and gases removed from the dope, but the dope may also be coated at a coating temperature suitable for the particular dope involved. The term air is herein used in a generic sense to include air and any other entrained gases which may cause, or tend to cause, bubbles in the film base or support when the film dope is coated on a coating surface such, for example, as a coating wheel.

Referring now to the drawing, and particularly to Figure 1, there is shown the preferred arrangement of the apparatus for carrying out the process embodied in the present invention. A making mixer, generally indicated by the numeral I I, is provided for assembling and thoroughly mixing the various ingredients and solvents used in the preparation of the film dope. This mixer H may be of any desired construction, but preferably of the type illustrated in the drawing, in which suitable mixing wheels or paddles, not shown, are arranged to rotate in opposite directions on the shafts l5 and it which are suitably journaled in the sides of the mixer H, in the usual and well known manner. The rotation of these wheels thoroughly mixes the various ingredients and solvents which are used in the preparation of the dope, thus providing a dope of the proper composition and consistency. A suitable cover l'l may be provided for closing the top of the mixer H during the mixing operation, the cover being removable or displaceable to permit the placing of the material within the mixer;

After the ingredients have been mixed to the proper consistency in the making mixer II, a dope pump 20 is started to draw the dope from the mixer H through the pipes 2i and 22, and to force the dope through the pipe 23 to a heating mixer 25. This heating mixer 25 is similar in construction to the making mixer H, and is also provided with a pair of oppositely rotating paddles or wheels, not shown, mounted on the shafts 30 and 3i suitably journaled in the sides of the mixer 25. The rotation of these paddle wheels provides means for thoroughly mixing the ingredients which form the film dope.

To assure the positive and complete removal of the air, the present invention contemplates the heating of the dope while in the mixer 25. This heating, combined with the stirring, effectively removes both the entrained and dissolved air from the film dope.

To this end, the mixer 25 is preferably provided with a fluid tight jacket 35 arranged on the sides and bottom of the mixer 25. A supply of heating fluid, preferably hot water, is supplied to the jacket 35 through the inlet pipe 35 from a suitable heating source, not shown. A discharge pipe 31 returns the heating fluid to the heat source or to some suitable point of disposal. The temperature of the water in the jacket 35 is such that the dope is heated to a suificiently high temperature, preferably to its bubbling point by which is meant the temperature at which the dissolved air comes off in the form of bubbles. These bubbles being lighter than the dope rise to the surface and are released.

As the dope is composed largely of solvents, the bubbling point of the dope is that temperature at which the vapor pressure of the solvent or solvents plus the partial pressure of the dissolved air is substantially equal to the barometric pressure. At this point, therefore, the dissolved air which is invisible in the dope is converted into free air in bubble form. These bubbles, of course, contain air and solvent vapors in the ratio of their partial pressures. In order to free the dope from these free air bubbles, sufficient time must be allowed for them to rise to the top of the dope due to their buoyancy. Ordinarily this freeing of the air bubbles will occur in the heating mixer 25 and 40, but sometimes it is desirable to provide auxiliary means for removing these air bubbles. It is evident, of course, that the tank I00, hereinafter described, is admirably adapted for this purpose, but it is contemplated that other suitable auxiliary means may be utilized Without departing from the spirit of the invention.

As is well known, the bubbling point of the dope is variable depending on the amount of air present in the dope, and also on the type of dope involved. For example, dope made with acetone has a bubbling point of about degrees while the boiling point is approximately 133 degrees Fahrenheit, while dope made with methyl alcohol has bubbling and boiling points which are about 120 and degrees Fahrenheit respectively. It has been found, however, that a general range of 120 to degrees Fahrenheit will sufficiently deaerate the dope. The particular temperature, of course, depends on the type of dope and the solvent used.

It is contemplated that the entire heating and mixing operation may be completely performed in the heating mixer 25. To insure, however, the proper and thorough mixing of the dope and the complete removal of the air therefrom prior to its being pumped into the supply line, a second heating mixer 40 is preferably used. This second heating mixer is of the same construction as the mixer 25, and is provided with suitable mixing paddles, not shown, rotatively mounted on the shafts AI and i2 journaled in the sides of the mixer 40. This mixer is also provided with a suitable jacket 43, arranged on the sides and bottom of the mixer, and supplied with a heating fluid, such as hot water, through the pipe 45. A discharge pipe 46 returns the water to the heating source, or to a point of disposal. heating mixer thus assures that the dope will be properly and thoroughly mixed, and the air removed therefrom before the dope is fed to the coating or castng machine where it is formed into a thin sheet or ribbon.

The dope is fed to the second mixing heater 40 through a pipe 50 by means of a dope pump 5| arranged to draw the dope from the mixer 25 through the pipes 52 and 53 which are connected to the bottom of the mixer 25, as clearly illustrated in the drawing.

The use of separate making and heating mixers provides a continuous rather than a batch process. For example, while one portion is being heated and mixed in the mixer 35, a second portion is being treated in the mixer 25, and a new batch is being prepared in the making mixer II. By means of this arrangement, a substantially continuous flow of dope is furnished to the coating machine.

During the heating and stirring of the dope in the mixers 25 and 40, a portion of the solvents may be evaporated therefrom. These evaporated solvents may be removed from the heating mixers, and recovered by means of suitable condensers. If, however, these solvents are thus removed from the heating mixers, suitable allowance must obviously be made for such removal in the preparation of the dope ingredients in the making mixer II. In the preferred embodiment, however, these evaporated solvents are condensed within the heating mixers 25 and 40 and are thus immediately returned to and mixed with the dope within the heating mixers.

Therefore, each of the heating mixers 25 and 46 is provided with a cooling jacket 65 preferably arranged on the top of the mixer, and adapted to condense the evaporated solvents and to immediately return these solvents to the dope being prepared in the particular mixer. These jackets may be supplied with a suitable cooling medium such, for example, as cold water through an inlet pipe iii. A discharge pipe 62 carries the cooling water from the jacket 60 to a suitable point of disposal. The pipes BI and 62 are provided with unions 63, adjacent each of the jackets 60, which may be disconnected to permit removal or dis- This second placement of the jacket to afford access to the interior of the heating mixer.

After the mixing and heating operations have been completed in the heating mixer 40, the dope is discharged therefrom through the lines 10 and H to a third dope pump 13 which forces the dope through a feed line 15, in which are respectively arranged a heater [6, a fiter 11, and a cooler 18, all of which will be hereinafter more fully described. Beyond the cooler 18.the feed line 15 is provided with a plurality of take-off connections 86, each of which leads to a separate coating machine, not shown. Each of the various takeoff lines is provided with a valve 8| by which the line may be disconnected from the feed line 15, as is well known to those in the art. Valves are also preferably provided on both sides of each of the pumps 20, 5| and 13 so that the pumps may be disconnected from the system when desired. The excess dope is returned to the mixer 25 through the line 15 as illustrated in the drawmg.

The filter 1.1 may be of any suitable type which is adapted to thoroughly remove any suspended or foreign matter carried by the film dope, thus assuring the formation of a clear uniform sheet when the dope is cast on the coating machine. To expedite the filtering of the dope, however, it is desired to superheat the dope so as to reduce the viscosity thereof during the filtering operation. By super heat is meant, the heating of the dope above the temperature at which it was mixed in the mixers 25 and 40, and not necessarily above the boiling point of the solvents in the dope.

The superheating of the dope, prior to filtering, is accomplished, in the present embodiment, by means of the heater #5 arranged in the line 75 intermediate to pump 13 and the llter Tl. This heater may be of the usual or well-known construction, having an inlet pipe 86 connected to a source of heating fluid, not shown, and a discharge pipe 86a adapted to return the cooled or condensed fluid to a heater or other point of disposal. The heating mediums may be varied to suit the particular type of dope. It has been found, however, that either steam or hot water are admirably adapted for this purpose.

The dope is discharged from the filter H at a relatively high temperature. In coating film supports from certain types of dope, especially from those which tend to form a gel when coated on a coating surface, it may be desirable to coat these dopes at a temperature approximating room temperature, or even lower. To this end, the dope may be cooled before being cast into thin sheets or ribbons on the coating machine.

The dope is, therefore, brought to the desired coating temperature by means of the cooler 18, preferably of the tubular type, arranged in the line going to the coating machine, as is clearly shown in the drawing. A pipe so connects the cooler to a source of cooling fluid, such as cold water, and a discharge pipe 9| carries the cooling fluid, somewhat raised in temperature, to a cooling source or to some suitable point of disposal.

By reason of the use of such a cooler, the dope is reduced to the proper coating temperature, the advantages of which are obvious to those familiar with the art. While a cooler has been shown for reducing the temperature of the dope, this is by way of illustration only, as it is contemplated that the line 15 may be of sufiicient length or may be suitably cooled so as to reduce the dope temperature, in which case the cooler 18 may be omitted.

The casting surface on which the dope is coated to form the film base is usually maintained at a temperature of about to degrees Fahren- I heit, the exact temperature depending on the type of dope being cast. As the dope is cast at a temperature slightly above thatof the wheel, it is evident that, in general, it is necessary to cool the dope approximately 10 to 20 degrees Fahrenheit below that to which it was originally heated. However, dopes which boil at a high temperature and which would, therefore, be heated to a higher temperature, are obviously cooled more than 10 to 20 degrees, as is the case with the ordinary type of dope used. In all cases, however, the final dope temperature at the time of coating is slightly above that of the coating wheel, as is well known to those in the art.

In Figure 2, there is shown a modified arrangement which is the same as that illustrated in Figure 1 except that a storage tank I00 and a pump It]! are interposed between the pump 13 and a heater 16, all corresponding parts are designated by the same numerals as in Figure 1. This tank provides a suitable storage for the dope and this affords a continuous supply to the coating machine. The tank IOH may be vented as shown at [02. As the tank I00 is normally under atmospheric pressure, it is usually necessary to provide a pump Iill ahead of the heater so as to force the dope through the heater 16, filter I1, cooler 16, feed line 15, and take-off connection 80 to the coating machine. If however, sufficient static head can be secured, the pump I0l\ may be omitted, as is obvious.

If desired a by-path I05 may be provided for by-passing the tank I00, as clearly shown in Figure 2. Suitable valves I06 may be arranged, in a manner shown in Figure 2, so as to control the path of travel of the dope to the tank 100 or the by-path I05. When the tank I00 is not in use, it is evident that the pump 13 will provide the necessary pressure for forcing the dope through the system. Valves 85 are also provided on both sides of the pump IOI.

It is apparent from the above description that the present invention provides a method whereby not only is the air thoroughly removed from the film dope prior to the casting thereof; but the dope may also be coated substantially at room temperature, or even below. The exact coating temperature depends upon the type of dope being coated, as is well known to those in the art.

While certain embodiments of the invention have been disclosed, it is by way of illustration only as it is contemplated that the inventive idea may be carried out in a number of ways. This application is, therefore, not to be limited to the precise details shown, but is intended to cover all variations and modifications thereof falling within the spirit of the invention and the scope of the appended claims.

I claim:

1. A process for the manufacture of films from a dope comprising heating the dope to a sufiiciently high temperature to remove entrained and dissolved gases, super-heating said gas-free dope to reduce its viscosity, filtering said super-heated dope, cooling said dope, and finally casting the cooled gas-free dope into a thin sheet.

A process for the manufacture of films from a dope comprising heating the dope in a mixer nearly to its bubbling point by applying heat to a portion of said mixer so as to liberate the entrained and dissolved gases, condensing the solvent distilled oif when said dope is heated, superheating the gas-free dope to reduce the viscosity thereof, filtering the super-heated dope, cooling the dope substantially to room temperature in the absence of air, and finally casting the cooled gas-free dope in a thin sheet onto a coating surface.

3. A process for the manufacture of films from a dope comprising heating the dope in a mixer nearly to its boiling point by applying heat to this cooled gas-free dope in a thin sheet onto a 10 coating surface.

EMMETT K. CARVER.

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