US2369484A - Method and apparatus for making film - Google Patents

Description

1945- G. F. NADEAU METHOD AND APPARATUS FOR MAKING FILM Filed March ,51 1943 FIG.I.

FIG.2.

GALE F. NADEAU INVENTOR fim BY ATTORNEYS Patented Feb. 13, 1945 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR MAKING FILM Gale F. Nadeau, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application March 31, 1943, Serial No. 481,341

(Girls-) 7 Claims.

The present invention relates to the art of making film base or sheeting by coating solu tions of cellulosic material onto a smooth moving surface, such as a belt or wheel, and more particularly to an improved process for maintaining the proper conditions at the beginning of the coating operation and to an improved type of apparatus for accomplishing this result.

As is well known to the art, cellulosic film or sheeting is formed by spreading a thin coating of the desired material, generally a solution of a cellulosic ester, such as cellulose nitrate or cellulose nitrate in suitable solvents, onto an approprlate surface and permitting or causing the solvent to evaporate therefrom. The method usually employed consists of feeding the solution or dope from an appropriate feeding device onto the polished metallic surface of a slowly rotating wheel or drum. The solvent evaporates from the film more or less progressively as the wheel turns. In less than a complete revolution sufilcient solvent has been removed from the film base or sheeting so that it can be stripped from the wheel and conveyed to a wind-up or carried over other rolls or drums for a curing treatment.

The device usually employed for feeding the solution to the casting wheel comprises a V- shaped hopper provided with spaced apart blades mounted at an angle to one another at the apex of the V to form a coating slot. Usually one or both of these blades is adjustable with respect to the other so as to provide a means of controlling the thickness of the dope stream and, therefore, the sheet being formed.

Such hoppers are very satisfactory for coating many kinds of cellulosic solutions. However, when an attempt was made to coat what is herein termed a "gel dope" from such a hopper onto a cold wheel, 1. e., one which is maintained below the gelation temperature of the dope, in place of a hot wheel, various unforeseen difficulties arose, Some of the incompatibilities associated with the idea of coating a warm dope of thischaracter on a cold casting surface have capable of setting to a rigid gel at a temperature of approximately 75 F. The temperature of the coating surface is maintained at approximately 60 F., thus permitting the dope to gel immedidiately upon contacting the coating surface. One particular gel dope which the present invention is adapted to coat may be formed by dissolving at elevated or moderately elevated temperatures certain cellulose organic acid esters such as cellulose acetate and the like in a solvent mixture, consisting of propylene chloride and ethylene chloride or propylene chloride and a lower aliphatic alcohol such as methyl alcohol to give solutions or dopes which are susceptible of gelation by a rapid lowering of temperature.

If one views the point of contact of the dope stream with the casting surface, he sees a series of small droplets of solvent and a general fogged condition in the area between the point where the dope strikes the surface of the wheeland the near-by baflle. This condition iscaused by the low temperature of the casting surface as compared with the temperature of the dope. The sudden chillingof the air in this general area produces a fog, and the formation of these droplets of solvent on the casting surface'approaching the dope stream giverise to a defect in the film base or sheeting which has become known in the art as dope pits. These pits, it is believed, are the result of small droplets of solvent being dragged under the coating and producing surface irregularities in the interface of the dope and the casting surface. Of themany attempts to eliminate this trouble by means of temperature control, more complete bailling, change in design of the baflies, and manipulation of dope and wheel temperatures, none has been satisfactory and, in fact, none has made a real improvement over the condition as originally experienced.

One object of the present invention is the provision of a method of making film or sheeting by feeding a flowable cellulose derivative solution onto a casting surface which has a temperature lower than the solution by means of which the formation of "dope pits in the surface of the film or sheet engaging the casting surface is greatly reduced, or entirely eliminated, by the elimination of the formation of droplets of solvent on the film coating surface before it is contacted by the solution.

Another object of the present invention is the provision of apparatus for carrying out my improved method of making film. base or sheeting.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itmethods of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawing-in which,

Fig. 1 is a sectional end view of the coating apparatus constructed in accordance with one embodiment of the present invention and for carrying out my novel method of coating film and sheeting,

Fig. 2 is a sectional view taken'substantially on line 2-2 of Fig. 1, and,

Fig. 3 is a partial sectional end view of hopper and associated baiiie, and showing how the baille.

may be constructed to house the condenser tube to constitute a trough forthe condensate.

Like reference characters refer tocorresponding parts throughout the drawing. I

It has been shown in studies on vapor pressure that in a closed system the vapor pressure within that system is very largely influenced, if not determined, by the vapor pressure corresponding to the temperature of the coldest part of the system. By way of example, if ethylene dichloride is considered as the predominant solvent, its vapor pressure varies from 230 mm. at"122 F., a condition comparable to that of the warm dope as it leaves the coating hopper, to50 mm. at 60 F., the temperature of the coating-surface. It is obvious that any sudden change in temperature of this degree would give rise to'the formation of a fog should the other necessary conditions be present, such as the presence of minute dust etc., in the air.

According to the present invention, a further application of the principle underlying the formation of the fog is used as a practical solution to this problem. I have found that the formation of the fog at the interface between the stream of dope and surface of the casting wheel can be greatly reduced, if not eliminated, by bringing about a condensation of the solvent'vapor at a point spaced from this interface, and conducting the condensate from the area in question.

Referring now to the drawing, and particularly Fig. 1, there is shown a double walled coatinghopper I I having outside walls Ii and i2 and inside walls It and i4. The space ll between the double walls may be employed as a heating chamber for the coating solution, if desired. .At .the apex of the slanting walls and II there is a slot it through which the coating solution flows to the casting surface i1, preferably a wheehwhich is capable of being moved past said slot by means not shown.

To regulate the thickness of the sheet or film being coated, blades 25. and 28 are positioned on the walls It and I4, respectlvely.- As shown in Fig. 1, blade 25 is fixedly attached to wall it by screws 21, while blade 26 is movably mounted on wall II and can be movedtowards or away from the blade .25 by meansof adjusting screws 21' which engage membersv 29 mounted on wall.

ll. By moving'blade 28 up and down, the'width ofthe coating slot 30 may be adjusted to give a flow of solution from the hopper-of desired thick- JESS.

Asshown, the-casting surface vlI-is preferably mountedsome distance from the discharge slot Ill so that there is a substantially long dope stream between the hopper. and the surface. For known reasons, inciuding'the prevention of slugs on the ends of the hopper blades, and

particles,

asses self, however, both as to its organization and its.

the prevention of contamination or deformation of the dope stream by circulating air, etc., it is desirable to substantially enclose the space between the discharge slot 30 of the hopper and the surface of the casting wheel. As shown, the closur of this space may be accomplished by .baiiles 28 extending from the bottom of the hop per to points adjacent the casting wheel. The ends of this space maybe enclosed by end bafies 28' and 29' extending from the bottom of the hopper to points adjacent and below the casting surface and connecting the side bailies 283 as clearly illustrated in Fig. 2. The bafies 2!; which extend across the width of'the casting wheel are spaced from the wheel only enough to permit the free passage of the wheel therebeneath with and without the film deposited thereon, as shown.

When 'gel dunes" are coated with this apparatus the coating solution is maintained at a higher temperature than the surface of the casting wheel, and'the stream of solution sets, or

in their relative temperatures and the composition of the solution being coated. For'reasons set forth above, when the warm dope strikes the cold casting surface 9, fog is formed in the enclosed space between the casting surfaceand the discharge slot of the hopper. This fogis most concentrated at the point where the dope stream strikes the casting surface, and this fog tends to condense and form droplets of solvent on the-exposed portion of the casting surface which passes through the enclosed space upon approaching the dope stream. It is the formation of these droplets of solvent on the casting surface which are undesirable because they aredragged under the dope stream and result in dope pits at the interface between the deposited film and the casting surface.

According to the present invention, the formation of fog at the surface of the casting'wheel approaching the dope stream, and the formation of droplets of solvent thereon is greatly reduced, if not eliminated, by incorporating a metal condenser surface in the enclosed space at a point spaced from the exposed casting surface within the enclosed space and maintaining the temperature 'of the condenser surface at a temperature below the temperature of the casting surface. As illustrated. the metal condenser surface may comprise a cooling coil 3| extending the length of the discharge slot, and through which cold water or other suitable refrigerant may be circulated.

. Since the fog is most concentrated to the right of the dope stream, looking at Fig. 1, and it is the exposed portion of the wheel in the enclosed space which is to be protected from. droplets of solvent, the condenser II is preferably situated in the enclosed space to the right of the dope stream,

as shown, or in proximity to the exposed portion 1; of the wheel surface in the enclosed space.

. Since it is not desirable to use a simple condenser surface because of the condensed solvent dropping onto the wheel surface,-I mount the I condenser tube within a perforated tube 32. In

;,this way, the solvent vapor in the enclosed space can'reach the surface of the condenser tube ii,

and upon condensation the condensate willdrop to the bottom of. the tube which. in fact, is per-= ll may be then be drawn coffat theendof the hopper through spine. 1

In Fig. 311 havezshowna second embodiment of the apparatus 'whereinone-of the babies 28, preferably theone to theright of the dope stream, is designedito enclose the condenser tube ll and constitute a trough for carrying away the condensate As shown in Fig. 3, the baffle 35 may be provided with a recess 38 extending throughout its length and within which the condenser tube suspended inany suitable manner. not shown, so that it will not touch the bane and cool the same by conductiom The recess 30 is placed in communication with the enclosed space by a slot 81 situated at the top of'the recess so that the solvent vapor can reach the condenser tube and the bottom of the recess-will constitute a trough It to collect the condensate C dropping from the tube. The bottom of the trough 38 may be inclined towards one or both ends of the hopper sothat the condensate may be drawn of!..

The following simple experiment is indicative of the manner in which the rate of condensation on a surface can be influenced by a surface of a different temperature in the same area, and serves to clearly point out the theory on which the present invention is based. About 500 cc. of ethylene dichloride were placed in a 2000 cc.- beaker and two test tubes were held in the vapor space above the solvent. Ice water was added to one test tube and cold water was added to the second. The rate of condensation of the vapor on the warmer tube was observed by' measuring the time required for a drop to form and fall from the tube. This rate was first measured with the warmer tube only, and then the cold tube was immersed in the vapor and its effect on the rate of condensation on the warmer tube was noted. The following data show a gradual increase in the time of drop formation due to a gradual increase in the temperature of the tube. They do show, however, very conclusively that the rate of drop formation on the warmer tube is greatly affected by the presence of the cold tube.

of solvent, 135 F.;'temperature of In the above data, the variation in time for the drops to fall when the cold tube was placed in position and removed is due to the fact that thermal equilibrium of the tube surfaces is not immediaiely realized. The last value in each series more nearly represents the correct value.

From the dataobtained in this simple experiment, it can be readily appreciated that the rate of condensation of the solvent vapor on the surface of one temperature can be greatly controlled, or eliminated, by the presence of a colder surface in the same vapor and in proximity thereto. Then assuming that the warmer tube in the above experiment is the casting surface in the present invention. and the colder tube is the condenser tube in the present invention, both with space enclosed by baiiles, the manner in which the present invention operates to eliminate the formation of drops of solvent on the casting surface prior to the application of the dope thereon should be readily apparent.

While I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications theerof are possible. My invention, therefore, is not to be limited to the precise details of construction shown, but is intended to cover all modifications coming within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Pat ent of the United States is:

1. In the process of making film or sheeting by feeding a fiowable cellulose derivative solution from a container provided with a discharge orifice through which the solution flows onto a casting surface having a temperature below that of the solution and located within a substantially closed space, whereby an atmosphere of solvent vapor is present in said space, the steps of condensing the solvent vapor in said space at a point removed from said film forming surface, and conducting the condensate from said space. w

2. In the process of making film or sheeting by feeding a fiowable cellulose derivative solution from a container provided with a discharge orifice through which the solution flows onto a casting surface having a temperature below that of the solution and located within a substantially closed space, whereby an atmosphere of solvent vapor is present in said space in the vicinity of the exposed film formingsurface, the steps of condens ing the solvent vaporin said space in proximity to but spaced from the exposed film casting surgu face, and conducting the condensate from said space.

3. In the process of making film or sheeting by feeding a fiowable cellulose derivative solution from a container provided with a discharge orifice through which the solution flows onto a casting surface having a temperature below that of the solution and located within a substantially closed space, whereby an atmosphere of solvent vapor is present in said space in the vicinity of the exposed film forming surface, the steps of reducing the temperature of an area of said space in the vicinity of the exposed film casting surface below the temperature of said film casting surface to condense the solvent vapor in said space, and collecting and conducting said condensate from said space.

,4. In from a fiowable cellulose derivative solution the combination with a reservoir for the solution and having a discharge orifice through which the solution is adapted to flow, a movable film forming surface spaced from, and in alignment with, said discharge orifice, onto which the solution emerging from said orifice is deposited, means fixed to said reservoir on opposite sides of said orifice and extending toward said film forming surface to provide a substantially enclosed space between the reservoir and the film forming surface including said orifice and a length of the film forming surface, of a condenser located in said enclosed space in spaced relation to said film forming surface for condensing the solvent vapor confined therein, and means for collecting the condensate and conducting it from said enclosed space.

5. A film or sheeting making apparatus. accordan apparatus for making film or sheeting ing to claim 4, in which the condenser is a surface extending substantially the fulllength of the discharge orifice and includes means for cooling we surface to, and maintaining it at, a temperature below that of the film forming surface.

6. A film or sheet making apparatus according to claim 4. in which the condenser is a surface extending substantially the full length of the discharge orifice and includes means for cooling said surface to, and maintaining it at, a temperature below that of the film forming surface, and the condensate collecting and conducting means comprises a conduit concentric with said surface and spaced therefrom, and including openings through which the solvent vapor has access to the surface of the condenser but through which the condensate will not flow, said conduit inclined so that the condensate wiliiiow therealong and out of said enclosed space.

7. In an apparatus for making film or sheeting from a fiowable cellulose derivative solution, a

hopper for the solution slot-through which the solution is adapted to fiow, a movable film forming surface spaced from said discharge orifice ontowhich the solution emerging from said orifice is deposited, baiiies fixed to the hopper in surrounding relationship to said discharge orifice and extending toward and adjacent to said film forming surface to provide and having a discharge a substantially enclosed space in which a length GALE F. NADEAU.

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