US3038209A - Method of dope-casting film at high shear rate - Google Patents

Description

June 12, 1962 w. F; HUNTER, JR, ETAL 3, 38,

METHOD OF DOPE-CASTING FILM AT HIGH SHEAR RATE Filed July 10, 1958 2 Sheets-Sheet 1 Figz I 1y 14w 1 I MWMEHMM J: PR o R AR T Jerome S. Osmalofl INVENTORS METHOD OF DOPE-CASTING FILM AT HIGH SHEAR RATE Filed J y 10. 195

June 12, 1962 w. F. HUNTER, JR, ETAL 2 Sheets-Sheet 2 M /ZA WzlkerEHunlegJl: Jrome S. Osmalofl 1 N ENTORS TTOBNEYS United States Patent Ofiiee 3,h38,209 Patented June 12, 1962 3,038,299 METHQD OF DOPECASTING FILM AT HIGH SHEAR RATE Walker F. Hunter, Jr., and Jerome S. (lsrnaiov, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.'Y., a corporation of New Jersey Filed July 10, 1958, Ser. No. 747,694 3 Claims. (Ci. 18-57) This invention relates to the manufacture of film or sheeting from viscous compositions, and particularly to a method for the manufacture of film and sheeting which have a uniform thickness over their entire area and whose surfaces are free from optical defects.

This application is a continuation-in-part of our pending application Serial No. 485,988, filed February 3, 1955, and now abandoned.

In the manufacture of sheeting or film support, cellulose esters dissolved in suitable solvents are cast out of a slot hopper onto a continuously moving casting or receiving surface, i.e., a chromium plated wheel eighteen feet in diameter or an endless belt, where a sufficient quantity of the solvents are evaporated from the dope to allow the material to be stripped off the wheel. While it is common practice to cast the desired thickness of film at one time, it is also known to multiply cast a thick film by first forming and partially drying a thinner film on a casting surface and thereafter applying another coating of the same material to said preformed film to build it up to the desired thickness, see US. Patent 2,374,308. Imperfections in the optical quality in the sheeting or film support are not generally encountered at the first coating in this multiple casting technique since the dope has an opportunity to flow While on the wheel or other casting surface. When the second coating is applied, however, the dope attacks the first layer by solvent action, such that any imperfections in the second coating at the time of laydown are frozen in the resulting support. One of the major problems in this multiple casting technique, and sometimes in the single casting technique, has been the elimination of lininess (optically visible lines in the direction of the moving casting surface). A second problem has been the elimination of cross-lines (lines perpendicular to the direction of the moving casting surface). A third problem which is inherent in all dope hoppers having parallel wall extrusion slots is that they are inherently sensitive to temperature gradients in the dope and it is not possible to obtain a film thickness uniformity which is satisfactory or as good as indicated by the mechanical tolerance of the slot width without first homogenizing the dope to a constant temperature.

One object of the present invention is to provide a method for dope casting which eliminates, or greatly decreases, the appearance of lininess and cross-lines in the cast film or sheeting and greatly decreases variations in film thickness due to temperature gradients across the width of the dope stream as it is extruded from a hopper.

Another object of the present invention is to provide a method of casting film or sheeting which comprises feeding a film dope having non-Newtonian characteristics and a viscosity greater than 20 poises from a hopper provided with a relatively long discharge slot through which the dope flows toward a casting surface moving past the exit end of said discharge slot, including the step of subjecting said dope during its passage through said discharge slot to a shear rate sufiici-ently high to reduce the sensitivity of the apparent viscosity of the dope to variations in temperature and thereby reduce variations in uniformity of thickness of the extruded stream along the length of said slot because of any temperature variations in the dope along said slot.

And a further object is to provide a method of casting film or sheeting as set forth wherein the dope discharged from the hopper slot is accumulated in an elongated puddle extending along the length of said slot and from which puddle a layer of dope of uniform thickness is drawn by the casting surface moving at a linear velocity which is a fraction of the velocity of the dope passing through the discharge slot, a high shear rate in the slot being developed by this high velocity.

The novel features that we consider characteristic of our invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its mode of operation, together with additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic side elevation, partly in section and on a reduced scale, of a typical film making apparatus by which our invention maybe carried out, the hopper being exaggerated in relativesize for the sake of clearness;

FIG. 2 is an enlarged fragmentary sectional view of the mouth of a hopper constructedin accordance with the prior art in combination with a film casting surface;

FIG. 3 is an enlarged fragmentary sectional view of a hopper constructed in accordance with the present invention in association with a film casting surface, and showing the elongated puddle of dope from which the desired layer is drawn by, the casing surface and which decreases the distance to laydown of the dope stream; and

FIG. 4 is an enlarged fragmentary sectional view of the discharge end of a hopper constructed in accordance with the present invention and emphasizing the offset of the hopper lips.

Our invention is illustrated as applied to a film or a sheet casting machine of the rotary drum or wheel type in which a base it, supports a bearing 11 for the shaft 12 of the wheel or drum 13. The peripheral face 14 of the wheel 13 forms an endless dope casting surface and moves during rotation in the direction of the arrow 15 from the dope applying station 16 to a stripping station 17 and then back to the dope applying station 16. As is well known in the art, the dope is applied to the casting surface 14 in a thin layer of film 18 which dries as the wheel rotates and is later stripped off at the stripping station 17 and removed over the roller 19.

The dope 20 from which the sheet or film 18 is formed is customarily continuously fed into a hopper 21 disposed above the wheel surface and from which it flows in the form of a thin stream 22 through a discharge slot 23" whose length is equal to the width of the wheel surface. According to the present invention, the hopper 21 must be of the pressure type, as shown in FIG. 1, Where the dope is pumped into a closed chamber 122 under pressure through a pipe 123 and is forcefully extruded through the discharge slot. In prior art casting apparatus the stream of dope leaving the discharge slot of the hopper is deposited directly onto the wheel surface, see FIG. 2, and the width of the opening of the discharge slot more or less controls the thickness of the deposited film and the finish on the hopper blades is all important to the smootnness of the surfaces of the deposited film. An inspection of FIG. 2 will show that with prior art hoppers the velocity of the casting surface, which is equal to, or greater than, the dope velocity in the slot, acts to pull out the dope stream a considerable distance before it makes contact with the casting surface. This distance is commonly referred to as the distance to laydown and is indicated at X in FIG. 2 for prior art hoppers. This unsupported stream of dope has an opportunity to vibrate and cross'lines in the deposited layer result. This tendency is greatly increased in cases where coatings being laid down are very thin, i.e., 0.002 inch dry, and such cases are prevalent in multiple casting techniques where the final thickness of the film is built up in two or more successive layers. Furthermore, even if the walls of the extension slot are machined and/or adjusted as accurately as mechanically practical, the uniformity of thickness of the dope stream extruded therethrough may be objectionable due to changes in apparent viscosity of the dope resulting from temperature variations across the length of the discharge slot. It has been found that a 2 F. variation in dope temperature across the length of the slot produces a change in the apparent viscosity of a dope which will result in an objectionable variation in thickness of the dope stream.

We have invented a method for casting film or sheeting which eliminates or greatly reduces the above difficulties found in prior art procedures. Since cellulose ester dopes from which sheeting and film are formed are quite viscous, non-Newtonian materials, then their viscosities are not affected only by temperature, as in the case of water, but also by shear rate. Therefore, dope viscosities are generally termed apparent viscosities when the dope is in motion and are calculated at various temperatures from equations relating to flow rates, density, pressures, etc. The apparent viscosity of such a dope at a particular temperature and shear rate is not the same as the viscosity at the same temperature but another shear rate, or at the same shear rate and a different temperature. Generally, viscosity of dope changes rapidly with temperature, but this condition does not hold over the entire range of application of dopes. It has been shown experimentally that at high shear rates the apparent viscosity of the dope is generally less sensitive to temperature change than at lower rates. Thus, the volume rate of flow will be more uniform with a high shear rate slot than with a larger opening. The method of casting in accordance with the present invention makes use of this discovery by subjecting the dope to a high shear rate at the time of extrusion so as to make its apparent viscosity less sensitive to temperature variations across the length of the discharge slot of the hopper. A decided improvement in the thickness uniformity of the dope stream leaving the discharge slot is achieved by virtue of this technique.

The most practical way of subjecting the dope to a sufiiciently high shear rate in the discharge slot of the hopper is to restrict the width of the hopper slot as much as possible thus increasing the linear velocity or shear rate, for a given volume of dope passing through the slot over a given time interval. However, when the hopper discharge slot is reduced to achieve this high shear rate, then the dope stream leaving the hopper can no longer be extruded directly onto the casting surface because the thickness of the deposited layer would then be equal to, or less than, the width of the discharge slot, and which width would necessarily be less than the wet thickness of dope required to be laid down in casting film and sheeting. On the other hand, where the width of the discharge slot controls the wet thickness of the cast film, it is not possible to obtain the high shear rates required to make the apparent viscosity of the dope sufficiently insensitive to temperature variations along the slot to obtain a uniformly thick dope stream.

This problem is solved according to the present invention by feeding the film dope at a high shear rate into an elongated puddle or wave (an accumulation of dope extending along the entire length of the exit end of the hopper discharge slot and having uniform dimensions and/or volume at all points along its length) from which the casting surface pulls a layer of film. The thickness of this cast layer can then be a multiple of the width of the discharge slot and the thickness can be readily varied by merely changing the linear speed of the casting surface. All that is necessary for obtaining a cast film of uniform thickness with this method is that the dimensions of this elongated puddle be uniform throughout its length, or the volume of dope in any finite length of the puddle be the same as the volume in any other finite length of the same value at each and every point along the length of the puddle. This condition is achieved by reason of the fact that the discharge slot in our arrangement acts merely as a metering slot to feed dope to this puddle and because of the dope being subjected to a high shear rate whenpassing therethrough any inequalities in the volume of dope along the length of the slot which might be due to viscosity changes resulting from temperature variations along the length of the slot are greatly decreased.

This method of film casting depends upon the use of a film dope having suflicient film strength to puddle as it leaves the discharge slot of the hopper. Generally speaking, the film strength and viscosity of film dopes such as we are concerned with are analogous, so that a dope which will satisfy this requirement can be specified by its viscosity. If this method of film casting is attempted with solutions having too low a viscosity or film strength then the solutions would not puddle but would splatter or form rivulets upon being extruded through the hopper discharge slot so that there would be no puddle from which the casting surface could draw the desired layer of dope. We have found that a film dope must have a viscosity of 20 poises or greater in order to make this method of casting film or sheeting operative.

This method of metering dope to an elongated puddle from which it is pulled by the casting surface also eliminates, or greatly decreases, the appearance of lininess and cross-lines in the cast film or sheeting. Since the discharge slot of the hopper merely meters the dope into the puddle, any imperfections which might appear in the sheet due to inaccuracies in the edges of the discharge slot will disappear in the puddle by virtue of the fact that the dope has a chance to flow together or merge at this point before it is pulled out onto the casting surface. Furthermore, as explained below, with this method of casting, the distance to laydown of the layer of dope on the casting surface can be controlled to some extent so that the stream of dope between the puddle and casting surface is restrained to be relatively short and not be so apt to be subject to vibrations which tend to introduce cross-lines in the cast layer as mentioned above.

Referring now to the drawings, we will describe a preferred embodiment of an apparatus by means of which a new method of casting can be carried out. Looking at FIGS. 1, 3 and 4, the discharge slot 23 in the hopper is preferably relatively narrow compared to the discharge slot of prior art hoppers for casting the same thickness of film from the same dope in order to subject the stream of dope to a high shear rate (high rate of flow) as it passes therethrough. As shown in FIGS. 3 and 4, the discharge slot 23 is formed by straight parallel walls 25 and 26' on the front and rear blades 26 and 27, respectively, of the hopper. The blade of the hopper to which the casting surface 14 first approaches is designated the rear blade 27, while the blade 26 from which the casting surface last leaves is designated the front blade. According to the present invention, the dope must be passed through the discharge slot so as to subject it to a shear rate sufficiently high to reduce the sensitivity of the apparent viscosity of the dope to temperature variations along the length of said slot whereby equal volumes of dope will be supplied along the entire length of the slot despite any temperature gradient therealong. Shear rate through a slot can be defined by the equation which is analogous to Newtonian shear rate in a pipe as derived from velocity distribution equations, wherein V is the average velocity of the dope through the slot and a is the width of the slot opening. In accordance with the present invention, the shear rate to which the dope is subjected to derive this advantage of reducing the sensitivity of the apparent viscosity to temperature changesshould be such that is equal to at least 70 reciprocal seconds. We have found that this shear rate condition can be readily obtained if the width of the discharge slot 23 of the hopper has a maximum width of 0.015". Substituting the values 0.015 and 70 reciprocal seconds in the formula shear rate=% and solving for V it will be found that V=5.25 ft./-rnin. This :is the lower limit of linear velocity at which the dope must be passed through the hopper slot to obtain the shear rate necessary with the specified film dope to reduce the sensitivity of the apparent viscosity of the dope to temperature variations which might occur along the length of the slot so that equal volumes of dope will be metered at all points along the slot into the puddle. This size slot is satisfactory when casting films having a thickness of 0.030"-0.038". Obviously, a more narrow discharge slot can be used to advantage when casting thinner films.

Inasmuch as the dope is extruded from the hopper at a much greater linear velocity than it is being taken away by the casting surface, and the wet thickness of the cast dope is greater than the width of the discharge slot, some means must be provided for accumulating the dope as it leaves the discharge slot and accumulating it in such a way that a uniform layer of dope can be drawn therefrom by the casting surface. Providing the dope has suflicient viscosity or film strength, it could be allowed to balloon out of the discharge slot into a self-supporting elongated puddle from which the casting surface could draw a layer of dope. However, we prefer to provide a physical support for this puddle of dope and, therefore, have shown the hopper lips formed so as to provide such a physical support for this accumulation of dope and to aid in the formation of such a puddle which has velocity components which produce a quicker laydown of the dope stream.

Looking now at FIGS. 3 and 4, the lip 28 of the front blade of the present hopper is offset from lip 29 of the rear blade by a distance Z. We have found that for the best results this offset Z should be at least 0.030" and not more than 0.070", 0.050 being the offset most often used. Dope leaving the discharge slot 23 puddles (piles up in an orderly fashion or forms a wave), as shown at 30, on the exposed portion of the back blade 27 and/ or in the air prior to moving onto the casting surface. The layer of dope is then drawn from this elongated puddle of dope by the casting surface 14 by virtue of the adhesion of the dope to the casting surface and the film strength of the dope. In order to start a casting operation, the layer of dope must be drawn to the casting surface from the puddle by any suitable means to initially form the layer and fasten it to the casting surface, and from then on a uniform layer is pulled from this puddle by the casting surface.

If the discharge slot 23 meters the dope uniformly into the puddle 30 so as to maintain the dimensions or volume of the puddle equal at all points along its length, then the layer of dope pulled out of the puddle by the casting surface will be uniformly thick. If there are any imperfections in the stream of dope leaving the discharge slot due to blade finish, they have an opportunity to smooth out in the puddle 30 before reaching the casting surface. A further advantage obtained by virtue of the disclosed construction of the hopper discharge slot 23 is that a quicker laydown of the dope stream from the exit end of the discharge slot and the casting surface is obtained than with prior art hopper constructions so that there is less chance for this unsupported stream of dope to vibrate and cause cross-lines in the finished film or sheet. In use, the velocity component of that portion of the dope at the top of the puddle 30 is larger than that of the portion of the dope moving along the exposed portion of the rear blade 27 because of the resistance (friction) of the metal. As a result of this difierence in velocity, at the end of the puddle there is a velocity force tending to throw the dope stream leaving the puddle down onto the casting surface. This effect tends to counteract the pull-out of the dope stream caused by movement of the casting surface and a decided decrease in the distance to laydown Y is achieved as compared with the distance to laydown X characteristic of the prior art hoppers when using the same dope, pressures and casting speed. Shortening this distance to laydown eliminates to a large degree the cross-line pattern inherent in prior art hoppers. This eliminates the necessity of using a vacuum for the same purpose as disclosed in US. Patent 2,681,294.

In addition, the puddling characteristic of this hopper design allows very thin deposits, i.e., 0.005" wet (.001" cured thickness) or less, as well as heavy deposits, i.e., 0.040" wet (.008" cured thickness) or more to be made equally well.

The reduction in the sensitivity of the apparent viscosity of the dope due to temperature variations along the length of the discharge slot is another advantage derived from the present invention. Many non-Newtonian fluids exhibit this advantage when cast in accordance with the present invention. Exceptions are those non-Newtonian fluids having a viscosity below 20 poises or insuflicient film strength, because such fluids, due to their low viscosity or lack of film strength, would be delivered from the discharge slot of the hopper in a non-usable, uneven layer, or even splattered, rather than being extruded in a uniform stream in which case this advantage would be useless even if exhibited. Furthermore, such low viscosity fluids would not have the puddling characteristic necessary to carrying out the present invention. Fluids of such low viscosity, below 20 poises, do not come under the film dope category anyway. Examples of typical film dopes which can be, and have been, cast in accordance with the present invention are as follows:

Example 1 Cellulose acetate (43.5% acetyl) parts 100 Methylene chloride do 426 Cyclohexane-520 do- 42 Methyl alcohol do 52 Triphenylphosphate do 15 Viscosity at F poises 600 Example 2 Cellulose acetate butyrate (17% butyryl) parts Methylene chloride do 280 Cyclohexane350 do 35 Butyl alcohol do 35 Triphenylphosphate do 8 Viscosity at 80 F poises 520 The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be elfected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of casting film or sheeting comprising the steps of extruding a film dope having non-Newtonian characteristics and a viscosity greater than 20 poises under pressure through a narrow elongated metering slot having a length substantially equal to the width of the film to be cast at a linear velocity of at least 5.25 ft./min. to subject the dope to a shear rate sufficiently high to reduce the sensitivity of the apparent viscosity of the dope to temperature variations which may exist along the length of said slot whereby the volume of dope metered by said slot is substantially equal at all points therealong regardless of temperature variations, accumulating the dope metered through said slot into an elongated puddle substantially equal to the length of said slot, and drawing an unsupported layer of dope of the desired wet thickness out of said puddle and depositing it onto a casting surface spaced laterally from said puddle by moving said casting surface at a uniform linear speed transversely relative to said puddle; and stripping said layer of dope from said casting surface when it is sufficiently cured.

2. A method of casting film or sheeting according to claim 1 in which the film dope is accumulated in such a Way that the puddle is physically supported on the side from which the casting surface approaches and the side from which the casting surface leaves is unsupported, and that portion of the dope moving into and through this unsupported part of the puddle possesses a higher velocity than the portion of the dope at the supported side of the puddle which acts to reduce the distance to laydown of the unsupported layer of dope between the puddle and the casting surface.

3. A method of casting film or sheeting according to claim 1 in which the width of the metering slot is a fraction of the Wet thickness of the unsupported layer of dose deposited on said casting surface and has a maximum width of 0.015 inch.

References Cited in the file of this patent UNITED STATES PATENTS 1,921,953 Stephens Aug. 8, 1933 2,364,552 Reichel Dec. 5, 1944 2,659,103 Merritt Nov. 17, 1953 2,681,294 Beguin June 15, 1954 2,714,745 Kenyon Aug. 9, 1955 2,761,791 Russell Sept. 4, 1956

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