APPARATUS FOR MANUFACTURING CELLULOSE ACETATE FILM

Abstract: Apparatus for casting cellulose acetate film from a hot solution is described. The apparatus includes a casting wheel (12), means (13, 14) for passing heated liquid through the wheel to heat the wheel, means (15) for extruding a band of solution onto the wheel, a take-off roller (16) for taking the band, in the form of a film, off the wheel and further rollers (18, 20, 22) for leading the film (26) to a drier (24) after which, when the solvent has been largely removed from the film, the film is wound up into a roll. The take-off roller (16) and the further rollers (18, 20, 22) are concave whereby a defect in the form of surface imperfections is avoided. The imperfections take the form of short sinuous lines oriented generally parallel to the length of the film.

Description: APPARATUS FOR MANUFACTURING CELLULOSE ACETATE FILM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the manufacture of 5 cellulose acetate film 2.Description Relative to the Prior Art It is well known to manufacture cellulose acetate film by extruding a band of solution of cellulose acetate onto a heated support which often 10 is a wheel but sometimes is an endless belt. When enough solvent has been evaporated that the band can be taken off the wheel as a self-sustaining film, it is taken off the wheel or belt around an idler take-off roller. The film is then led over idler 15 ladder rollers on its way to the driers. In the driers, the film is led through a serpentine path, over a plurality of rollers, during which hot air is impinged on the film. Solvent is evaporated from the band on the wheel or belt and from the film during 20 its travel to the driers, as well as from the film in the driers. After leaving the wheel or belt and prior to entering the driers, the film has a high solvent content, it being termed in the art "green". It is, of course, desirable to have as high 25 a production rate as possible. Obviously the film cannot be taken off the wheel or belt until it is self sustaining. However, as production speeds have been increased, a type of defect has been encountered which renders portions of the film containing the 30 defect, unsatisfactory for such demanding purposes as are encountered in the photographic industry. Such portions have had to be slit off and discarded. The defect which has been encountered is a non-uniformity in the planarity of the film and is visible at the 35 surface of the film as small wrinkles elongate ') parallel to the length of the film. Closer inspection shows the wrinkles to be concavities or convexities. The defect has its greatest frequency adjacent the side edges of the film and the frequency decreases towards the middle of the width of the film. It has been referred to by some as rice paper mottle. Japanese Patent Kokais nos . 62-046625; 62-046626 and 62-115035 have proposed solutions to the problem. In those published specifications there are described various ways of gripping the edges of the film as it leaves the casting wheel, or after it has passed over a stripping roller, and maintaining the film in the gripped condition into and through the driers. The gripping may solely maintain the width of the film, or it may be such as to cause the width of the film to increase (as in tentering) or the film may be allowed to controlledly decrease in width. The means for gripping and applying laterally directed forces to the film to achieve the desired width maintenance, increase or controlled decrease, are very elaborate and are costly to maintain. The aforementioned Kokais state that rollers cannot provide the lateral forces necessary to control the width and it is for this reason that they have proposed the use of the very elaborate gripping and tensioning means. It is an object of the present invention to overcome the above—described problem of wrinkles in an effective manner which is cheaper and simpler in both installation and maintenance, than the solutions proposed in the aforementioned Kokais. SUMMARY OF THE INVENTION It has been discovered that the object of the invention can be achieved if the take-off roller and such further rollers downstream of the take—off roller as are necessary to overcome the problem, are concave, that is, each such roller is substantially circular cylindrical but has a slightly smaller diameter at the middle of its length than at its ends. It is believed that the defects are due to heterogeneous shrinkage of the film due to heterogeneous evaporation of solvent and that such heterogeneous shrinkage leads to the non-planarity of the film which is visible as the elongate concavities and convexities on both surfaces of the film. The concave form of the rollers is believed to oppose the lateral shrinkage. If the defect is found to persist with just the take-off roller being of concave form, other rollers which the film passes over after leaving the take—off roller, may also be given a similar concave shape. In one embodiment of the present invention, the take-off roller has a length of about 65 inches and a nominal diameter of 4 inches, the concavity being such that the diameter at the middle of the length of the roller is 0.021 inch less than at the ends of the roller. In an instance where it was found that the problem persisted even when the take-off roller was formed in accordance with the present invention, the ladder rollers were also of concave form. The ladder rollers had a length of 65 inches and a nominal diameter of 3 inches, with the diameter at the middle of their lengths being 0.015 inch less than at the ends of the roller. The roller or rollers which are concave in accordance with the present invention advantageously have a finish, such as a 5 — 10 microinches finish formed by blasting with glass beads. In some situations in which the problem is found to persist when the take-off and ladder rollers are concave in accordance with the present invention, at least some, starting from the upstream end, of the rollers in the drier may also be concave, in accordance with the present invention. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic representation of a side elevation of a part of apparatus for casting cellulose acetate film, including an extrusion die, a casting wheel, a take-off roller and ladder rollers, in accordance with the present invention; Fig. 2 is a section, in a plane including the axis, of one of the rollers, showing its concave profile, in accordance with the present invention; and Fig. 3 is a computer generated representation of one surface of a portion of cellulose acetate film having a surface defect sometimes known as rice paper mottle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 1 there is illustrated a portion of apparatus 10 for casting cellulose acetate film. The apparatus 10 includes a casting support, which, in the present embodiment, is in the form of a casting wheel 12 of known form. The wheel 12 is mounted for rotation, counter—clockwise in the present embodiment, about a horizontal axis 14. As is well known, the wheel is heated, by a flow of heated liquid or air through its interior, so that solvents are evaporated from the solution of cellulose acetate cast on its surface. Means 13 for passing heated liquid or air through the wheel are provided. High on the left side of the wheel 12 is an extrusion die 15 for extruding a band 17 of a solution of cellulose acetate onto the surface of the wheel 12 as it is rotated. High on the right hand side of the wheel 12 is an idler take-off roller 16 which is in close proximity to the surface of the wheel 12. The take-off roller serves to change the path of the film and, therefore, the film is wrapped around a portion of the roller. As the film is under tension, the film is in forced contact with the roller. Above and spaced from the take-off roller 12 are first, second and third ladder rollers 18, 20 and 22 respectively. The ladder rollers 18, 20 and 22 are so disposed that the film is trained about a small portion of their surfaces. Thus, each ladder roller causes a deflection of the path of the film and, hence, causes the film , which is under tension, to be in forced contact with the surfaces of the rollers. After leaving the last ladder roller 22, the film enters a first drier 24 which serves to remove solvent from the film. The drier 24 includes a plurality of rollers 25 about which the film is threaded and which cause the film to follow a serpentine path. After being dried to a desired extent, perhaps by passage through further driers, the film is wound up into a roll by means not shown. As is well known, the residence time of the solution of cellulose acetate on the wheel 12 is such as to allow the band of solution to be heated such that sufficient solvents have been evaporated that the film 26 taken off the wheel at the take-off roller 16, is self—sustaining. For a given size of wheel, the necessary residence time on the wheel is determinative of the rate of production of the film. Fig. 3 is a computer generated three dimensional plot of a series of profile measurements made on a portion of one surface of a cellulose acetate film having the rice paper mottle defects. The measurements were made by interference means on a WYKO T0P0-3D surface profiling microscope. The maximum peak to valley height found was 5.13 microinches and the RMS value of the peak to valley heights found was 0.413 microinches. It is to be understood that the other surface of the film represented in Fig. 3 would also have the defects, the defects being deformations of the planarity of the film and not just of one surface. In one embodiment of the present invention it has been found that the rice paper mottle defects can be avoided if the take-off roller 16 and the first and second ladder rollers 18 and 20 are concave in accordance with the invention. Fig. 2 is a section of a representative one of the take-off and ladder rollers 16, 18, and 20 with the concavity greatly exaggerated for the sake of illustration. The roller has a normally rectilinear axis 30 and a concave surface 32 which contacts the film 26. The plane of the section represented in Fig. 2 contains the axis 30. In a section plane containing the axis 30, the concave surface 32 has a circular arcuate form, convex towards the axis and there are two short cylindrical lands 33 at the ends of the concave surface.. The dimensions of the rollers, when manufacturing a film 63 inch wide, are as follows: Length of roller Length of arcuate surface Nominal diameter Difference between end and middle diameters These dimensions have been found to be effective but other dimensions are also known to be effective. The best dimensions for a particular manufacturing condition should be determined by experiment. The surfaces of the rollers all have been treated by blasting with glass beads to give a finish of 8 - 12 microinch. While in the embodiment particularly described above, there are three rollers with concave surfaces, in accordance with the present invention, it is to be understood that other numbers of rollers, starting with the take—off roller, may have concave surfaces, as necessary in particular situations. The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims: What is claimed is: 1. Apparatus for manufacturing cellulose acetate film, including: a casting support; means for passing heated fluid through the support for heating the support; means for extruding cellulose acetate dope onto the casting support; a take-off roller located adjacent the casting support and adapted to guide film immediately after it is stripped from the support; drier means including rollers for guiding the film along a serpentine path; ladder rollers adapted to guide the film after it leaves the take-off roller and before it enters the drier means; the take-off roller being substantially circular cylindrical and having a concave profile. 2. Apparatus as claimed in claim 1, wherein: further said rollers including at least the first ladder roller encountered by the film after the take-off roller, are substantially circular cylindrical and have concave profiles. 3. Apparatus as claimed in claim 1 or 2, wherein the surface of the roller or each roller having a concave profile, considered in a plane containing the axis of the roller, conforms to a circular arc. 4. Apparatus as claimed in claim 1 or 2, wherein the surface of the roller or each roller having a concave profile, considered in a plane containing the axis of the roller, conforms to a parabolic curve. 5. Apparatus as claimed in claim 1 or 2, wherein the surface of the roller or each roller having a concave profile, considered in a plane containing the axis of the roller, conforms to a sine curve. 6. Apparatus as claimed in claim 1, wherein the ' take-off roller has a diameter of about 4 inches and the diameter at the middle of its length is about 0.02 inch less than the diameter at the ends of the roller. 7. Apparatus as claimed in claim 2, wherein each of said ladder rollers having a concave profile has a diameter of about 3 inches and the diameter at the middle of its length is about 0.015 inch less than the diameter at the ends of the roller.

Inventors: STEWART ARTHUR M, TAGLIALATELA MAR A, STEWART, ARTHUR, M., TAGLIALATELA, MAR, A.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: B29D7
IPC codes: B29C37, B29C41, B29C55, B29D7, B29K1, B29L7