US 3688738 A
Band-shaped layer supports can be coated at high speeds and with liquids of high viscosity by means of an apparatus comprising elements for casting and elements for wiping off excess-casting solutions.
Description (OCR text may contain errors)
United States Patent Eichler et a1.
15 3,688,738 1 1 Sept. 5, 1972  CASTING APPARATUS WITH FLEXIBLE WIPER FILM  Inventors: Wolfgang Eichler, Leverkusen; Rolf Briick, Opladen, both of Germany  Assignee: Agfa-Gevaert Aktiengesellschaft,
Leverkusen, Germany  Filed: Oct. 15, 1969  Appl. No.: 866,562
 US. Cl. ..118/62, 118/126, 118/401, 118/419, ll8/DIG. 2
 Int. Cl ..B05c 11/02  Field of Search ..118/106, 62, 126, 52,419, 118/401, DIG. 2
 References Cited UNITED STATES PATENTS 1,741,382 12/1929 Stokes ..118/62 X 2,289,798 7/1942 Nadeau et al ..1 18/DIG. 2 3,063,868 11/1962 Brandsmaet a1......l18/407X Smith et a1 ..118/126 X 3,081,191 3/1963 3,273,535 9/1966 Krikorian ..118/413 x 3,518,965 POSChel ..1 18/106 Primary Examiner--John P. McIntosh AttorneyConnolly and Hutz ABSTRACT Band-shaped layer supports can be coated at high speeds and with liquids of high viscosity by means of an apparatus comprising'elements for casting and elements for wiping off excess-casting solutions.
The layer support is deflected bya rigid or moving v deflecting roller and thereby is immersed in the casting liquid. Excess casting liquid is wiped off and the thickness of the layer is controlled by a flexible wiper film which initially makes tangetial contact with and is partly adapted to the curvature of the deflecting roller and which can be pressed against said roller by various means.
8 Claims, 6 Drawing Figures PATENTEDSEP 5 m2 3 6 8 8 7 3 8 sum 1 BF 2 FIG. 1
mvsmom WOLFGANG EICHLEIgRy, ROLF BRUCK.
PATENTEU SE? 5 3.688.738 SHEET 2 OF 2 INVENTORS WOLFGANG EICHLER, ROLF BRECK wkw M A-3,
CASTING APPARATUS WITH FLEXIBLE WIPER FILM The present invention relates to an apparatus for coating band-shaped layer supports.
The processes already known for coating moving films and paper webs or similar materials with liquid media can in practice be divided into two groups. The first includes a large number of mechanical devices such as coating knives, applicator rollers, pressure rollers, extruders etc., in which the thickness of the layer is automatically determined by the dimensions of the apparatus. These apparatuses necessitate considerable expenditure and require high standards of accuracy of the layer forming elements, especially as the layers to be applied are very thin and amount to only a few thousandths of a millimeter in thickness.
In the second group one may include those processes in which the viscosity of the liquid plays a part and the nature of the apparatus is not so critical because the forces which come into play are more of an elastic nature. These include the use of dipping processes and modifications thereof, air brushes and elastic wipers. Since the forces that come into play are relatively small, it has hitherto not been possible by these methods to work up liquids of high viscosity at high speed. This is because the liquid media can not be casted sufficiently uniformly, because the apparatuses themselves are too unstable or because the uniformity of the layer thickness is strongly affected by faults in the layer support. This applies, for example, to the arrangement described in French Pat. specification No. 1,348,216, in which a wiper blade of elastic material is employed. The layers obtained with this arrangement do not meet practical requirements, especially if high standards of uniformity in the layer thickness are demanded.
The object of the present invention is to provide a casting apparatus by means of which layers of uniform thickness can be produced at high speeds.
It has now been found that layer thicknesses of excellent uniformity can be achieved by use of a casting apparatus in which the casting liquid is first applied to a band-shaped layer support, e.g., by immersion of the support therein, and the layer support is then guided over a deflecting roller, the support carrying with it an excess of casting liquid. In the apparatus according to the invention, there is provided a combination of a rigid or moving deflecting roller and a partly curved, flexible wiper film which tangentially contacts the deflecting roller and bears against the roller over an area which adapts itself to the curvature of the roller. This film terminates before the moving, now coated, band-shaped layer support leaves the deflecting roller in the form of a straight, taut band.
In the simplest embodiment of the arrangement according to the invention, the flexible wiper film is simply contacted with the moving layer support by the forces of adhesion of the layer of casting liquid so that a certain thickness of layer is established. This simplest embodiment can be used for casting liquids of low viscosity of up to about cp. For a given viscosity, the thickness of the layer depends primarily upon the speed at which the layer support leaves the deflecting roller.
The invention will now be more fully described with reference to FIGS. 1 to 6 of the accompanying drawings.
FIG. 1 shows the above described simplest embodiment of the invention. FIGS. 2 to 6 illustrate preferred embodiments which contain additional parts.
In the apparatus according to FIG. 1, a band-shaped layer support 1 is drawn over a rigid or moving deflecting roller 2 through the casting liquid 3 contained in a casting vat, the liquid initially being retained on the surface of the band 1 and then being adjusted to the desired thickness by means of a flexible wiper film 4. The latter comprises a straight portion which makes tangential contact with the band on the roller and a curved portion which terminates before the layer support leaves the roller in a straight line.
This last mentioned condition is especially important if one is to obtain a layer of uniform thickness. If the wiper film is so long that it extends over'the part of the coated layer support which has already left the deflecting roller in a straight line, any faults in the layer support, such as longitudinal folds or particles of dust between deflecting roller and layer support have a very disturbing effect on the coated layer, especially in the case if thin films are used as supports.
As mentioned above, thin films, especially those with thicknesses below 50 t, give rise to particular problems in casting technique. The difficulties in casting liquids on such supports mainly arise out of the fact that the tension in the position where casting takes place frequently causes the formation of folds which immediately seriously impair the quality of the cast layer. This also applies if the surface of the deflecting roller is uneven, owing, for example, to the presence of particles of dust.
These difficulties, which are still liable to arise with the simplest embodiment of the casting apparatus according to the invention under unfavorable conditions, may be avoided by the-apparatus shown in FIG. 2. This differs from the basic apparatus shown in FIG. 1 in that the deflecting roller 2 is rigidly mounted and additional elements produce a film of liquid or an air cushion between the rigid deflecting roller and the rear of the band-shaped layer support 1, on which film or air cushion the band-shaped layer support slides. This eliminates the effect of unevenness of the roller or folds in the layer support.
FIG. 2 shows diagrammatically how a film of liquid can be produced between the deflecting roller and the layer support. The deflecting roller is a stationary barrel 24. Air or fluid is issued under pressure from a manifold 20 through a number of radiating passages 21 to form a fluid layer 22 as a film of liquid or air cushion.
In order that the apparatus according to the invention should function satisfactorily, it is important that the flexible wiper film should operate properly. This film should be made of a material that is, as far as possible, not affected by the casting liquid and which will not swell or even be dissolved by the action of the casting liquid. When choosing material for the wiper film, one should also take care that it does not allow the casting liquid to dry, especially at the edges of the film, and form firmly adhering incrustations. These deposits could easily produce stripes in the layer, which would be harmful. Particularly suitable are films of chemically inert synthetic resins, such as polymers of halogenated, preferably fiuorinated and in particular perfluorinated vinyl monomers, or copolymers of these products, for
Film thicknesses of 100 to 200 p. have generally been found to be suitable.
In the case of band-shaped layer supports of greater width, say above 1 m, it has been found that the quality of the cast layer depends increasingly on the quality of the wiper film. Even slight absorption of solvents causes swelling and warping of the wiper film.
To overcome these difficulties, it has proved advantageous to subject that part of the wiper film which lies against the curved part of the deflecting roller to a lateral tension. This tension evens out deformations and counteracts any swelling. The stretching must lie within the elastic range of the film and must, of course, be greater than any dimensional change due to the swelling. The force applied depends, of course, on the above mentioned properties of the individual wiper film. In the case of copolymers, e.g., the above mentioned perfluorinated copolymers, tensions of up to about 2 percent may be used without plastic deformation taking place.
As has already been explained, the simplest embodiment of the apparatus according to the invention, which is shown in FIG. 1, can be used without any difficulty with liquids of low viscosity. For high casting speeds, e.g., of more than 10 m per min., this simple arrangement is no longer satisfactory. The same applies when casting liquids of relatively high viscosity, e.g., casting liquids containing pigments are used, or when relatively thin layers are to be prepared. In this case, the efficiency of the arrangement can be considerably improved by exerting an additional pressure on to the curved part of the wiper film from above. This pressure should act as uniformly as possible only on those parts of the wiper film which lie on the deflecting roller.
A preferred embodiment of the apparatus according to the invention which meets this requirement is illustrated in FIG. 3. In this arrangement, uniform pressure is applied on the curved part of the wiper film by means of an additional film 8 stretched in a frame. The device for applying the pressure is shown in more detail in cross-sectional view in FIG. 4. It consists of the pressure film 8 itself, frame parts 9 and a tentering element 10 consisting of a beam and a tentering screw. The pressure film is attached to the lower part of the lefthand frame 9 and the lower part of the tentering frame 10. The pressure film can be stretched by means of the tentering screw of the element 10 and the contact pressure can thereby be altered.
The contact pressure can be varied as desired. Pressures between g and 1 kg per cm are generally sufficient. Casting liquids having viscosities between 50 and 4,000 cp can thus be cast at speeds of between about .10 and 50 m/min., and the thickness of the liquid layer can be adjusted as desired, e.g., in the range between 30 and 3,000 ,1.
When using the contact pressure element illustrated in FIGS. 3 and 4, the contact pressure against the wiper film is adjusted by moving the frame in the direction of the deflecting roller. Even if liquid is cast in a layer of considerable width, layer thicknesses of exceptional uniformity can be achieved, which fluctuate by not more than :2 percent.
The same materials may be used for the pressure films as for the wiper films, e.g., films 25 to 50 p. in thickness. However, since the pressure film does not come into contact with the casting liquid, a wider choice of materials is available. Thus, one may also use polyester films, especially films of stretched polyethylene terephthalate having thicknesses of about 6 to 12 p. or elastic textile fabrics made of all sorts ,of different elastomeric materials.
If the casting liquids have relatively high viscosities, e.g., several thousand cp, high pressures should be exerted on the curved part of the wiper film. These may amount to 1 kg per cm or more. Special care should be taken to ensure that this pressure acts as uniformly as possible over the whole width of the layer being cast. It has, therefore, been found advantageous to employ surface elements of rubber-like material, especially foamed elastic materials, i.e., so-called foam rubber, as the material which applies the contact pressure.
One possible embodiment of such a contact pressure element is shown in FIG. 5. The foam rubber surface is indicated by the reference numeral -l4. It is fixed to a base 15. Both elements can be forced against the web 4 by means of tightening screws 12 acting on lateral guides 11 which may at the same time serve as bearings for the shaft of the deflecting roller 2. The tightening screw at the center is guided by a bar 13 which prevents the foam rubber base 15 from bending in the middle. Fine adjustment may be made by means of small additional thrust screws 16 which are inserted in the foam rubber base 15.
In the apparatus according to the invention, high casting speeds together with relatively high viscosity of the casting liquid cause relatively high pressure heads to be built up in the space formed between the straight portion of the wiper film and the deflecting roller. This space is indicated by the reference 17 in FIG. 6. The pressure head acts on this part of the wiper film and causes deformation thereof. This produces variations in thickness of the applied layer. This pressure head can easily be avoided by supporting the straight part of the wiper film so that the force produced by the pressure head is compensated by the counterpressure on the other side of the wiper film.
One possible embodiment is illustrated in FIG. 6, in
which an additional elastic film or elastic cloth l8 supports the wiper film 4 from behind. The wiper film and support element 18 may, of course, be combined into a single unit. Alternatively, the contact pressure element illustrated in FIG. 3 may be so designed that it simultaneously serves as the support element of FIG. 6.
The thickness of the layer produced is influenced by the mechanical properties of the support element 18 such as elasticity, strength and stretch. The thickness of the applied layer may also be controlled continuously during the casting process in known manner, for example by use of an instrument which measures the thickness and which controls the thickness of the layer cast and at the same time corrects the tension or contact pressure. For the support element 18 one may use highly elastic materials such as films of woven elastic synthetic resin fibers or plates of rubber, or the synthetic resins mentioned above for the contact pressure element 8 or the wiper film.
The apparatus according to the invention is especially suitable for use in the production of pigment-containing layers. It has been found to be especially satisfactory when used in the production of magnetizable layers which contain ferro-magnetic particles, e.g., ferromagnetic iron oxides or chromium oxide, dispersed in a binder.
The advantages of the apparatus according to the present invention are summarized again below:
Liquids, the viscosities of which lie within a wide range of 50 to 4,000 cp, can be worked up.
The casting speed can be varied by up to 50 rn/min, without affecting the quality of the layer produced.
The width of casting can be over 100 cm. Difficulties do not arise even when-using still greater widths.
When using pigmented casting liquids, e.g., pigmented lacquers, the pigment is intensively dispersed in the narrow gap formed between the moving film and the contacting web, so that the layer becomes very homogeneous.
The thickness of the layer cast can be varied within a wide range regardless of the viscosity of the liquid and the casting speed. Layers of liquid of 30 to 3,000 p. in thickness can be obtained. The thickness can be readjusted automatically as a whole during the casting process.
The uniformity of thickness of liquid over the width of casting can be readjusted in localized positions or in larger areas. Variance in thickness of only i 2 percent may be obtained.
Films of thicknesses varying, for example, between 8 and 175 p., can be coated. Since the wiper film is held elastically under pressure, the thickness of the layer support has practically no influence on the thickness of the layer cast. The thickness is controlled solely by the contact pressure applied.
Deformations in the layer support have practically no effect on the quality and uniformity of layers formed. Thickened portions of the film, e.g., portions where the film is stuck together, run smoothly through the apparatus without requiring attention. The elastic contact pressure temporarily yields accordingly.
The layer supports so cast can be utilized right up to their outermost edges.
1. A coating apparatus comprising a curved surface, a movable band-shaped'layer support movably pressed in contact against the curved surface,
a bath of film-forming liquid immediately adjacent to the curved surface and receiving said layer support in direct contact with the surface of said bath for applying the film-forming liquid to the layer support, partly curved flexible wiper film means having a portion spaced away from the curved surface and extending to a point adjacent to the curved surface, and a curved portion of the film means adapted to conform to and extend over an area coaxially with the curved surface from said point of adjacency to the film end, at least said curved ortion bein ent'rel ou ide said bath said urved portic m being on t e exit side of said curved surface, said film end lying within the area of contact of the layer support on the surface.
2. Apparatus according to 1, wherein additional elements produce a fluid cushion between the layer support and the curved surface.
3. Apparatus according to claim 1, wherein the wiper film consists of a perfluorinated polymer or copolymer and has a layer thickness of to 200 p..
4. Apparatus according to claim 1, wherein a contact pressure device exerts pressure externally perpendicularly to the surface of the wiper film.
5. Apparatus according to claim 4, wherein the contact pressure device consists of a stretched elastic film or a fabric and externally exerts a uniform pressure on the curved portion of the wiper film.
6. Apparatus as claimed in claim 5 wherein a tensioning device for applying tension on the film means comprising a tentering frame, a pressure element stretched on the frame and adjustable means for stretching the pressure film whereby contact pressure of the element on the film means is varied.
7. Apparatus according to claim 5, wherein an elastic web of rubber or elastic fabric is employed as the contact pressure device, which device is arranged behind the portion of the wiper film spaced away from the curved surface and which loops round the curved portion of the film and thus acts partly as support for the wiper film and partly as means for applying pressure to the wiper film.
8. Apparatus according to claim 4, wherein a strip of foamed elastic rubber is used for the contact pressure device to exert an adjustable overall pressure and additional means for adjusting the thickness of the layer cast consists of means for varying the stretch of said contact pressure device.