EP0031301A1 - Process and apparatus for coating photographic record carriers with photographic coating compositions - Google Patents

Abstract

A process for coating flexible photographic record carriers with liquid, highly viscous photographic coating compositions. The record carrier (6) to be coated is transported between two rollers (7 and 8), one (8) of which has a flexible surface and the other (7) a highly polished surface. The two rollers are pressed together with a pressure of approximately 5 to 50 kg per cm of roller width. One of the rollers (7) can be motor-driven and effects the transport of the record carrier, the other roller (6) being free running. The photographic coating composition is applied by means of an extruder (1) in the form of a liquid film, oriented transversely to the record carrier, in the inlet section of the roller nip to the side of the record carrier situated opposite the highly polished roller surface and then immediately pressed onto the record carrier in the roller nip. The surface of the highly polished roller is cooled to at least 20 DEG C, preferably 0 to 10 DEG C, so that the coating composition does not adhere to the highly polished roller and spontaneously solidifies on the record carrier. The chosen transport speed of the record carrier (6) is 2 to 50 times higher than the speed at which the coating composition emerges from the extruder slot. The considerable stretching of the highly viscous coating composition thereby produced yields a corresponding reduction in the layer thickness, so that dry layer thicknesses of only 2 mu m can be achieved with great precision, for example, with slot widths of 200 to 300 mu m. <IMAGE>

Description

The invention relates to a method for coating flexible photographic support webs with liquid photographic coating compositions and an apparatus for performing this method.

Various methods are currently available for coating supports with high-precision photographic layers. In addition to the usual application and immersion processes, in which the thickness of the layer applied is essentially determined by the viscosity of the layer mass and the running speed of the support, there is, for example, the slot casting process, as described, for example, in US Pat. No. 2,295,394. In this method, the liquid coating composition is transferred from a very precisely machined slit that extends across the width of the carrier via a free meniscus to the carrier passing by at a short distance. This method is suitable both for the application of a single layer or, e.g. in U.S. Patent No. 2,941,898 for multi-layer application in a single pass.

The cascade or sliding funnel method is particularly suitable for the simultaneous application of several layers, in which several liquid layers emerging from separate slots are superimposed on an inclined plane and, again via a free meniscus, are transferred to the passing carrier as a composite layer package. This process is described, for example, in U.S. Patents 2,761,791 and 2,761,417 / 418/419.

Another common method for coating carrier webs with one or more layers is the curtain casting method, as it is e.g. was described in DE-Auslegeschrippen 1 928 025 and 1 928 031 and also in other places. In this method, a substantially vertically free-falling curtain is generated by a slotted or sliding funnel device, which is applied at intervals of a few centimeters to the essentially horizontally guided support.

The air knife method should also be mentioned, in which an excess of the coating composition is applied to the carrier by a conventional application device; at a short distance from the application point, the excess of the coating composition is blown off by means of a sharply delimited, flat, "knife-like" air flow and in turn fed to the storage container. This procedure, e.g. described in GB 803 176 and 807 805 is, of course, only suitable for the application of a single layer.

A major disadvantage of all the coating processes mentioned so far is the fact that the viscosity of the coating composition is limited to a relatively narrow range. Compared to the application or immersion process and the slot casting process, the cascade casting process already brings a significant advance in that the usable viscosity range can be expanded from approx. 10 to approx. 100 cP (maximum viscosity). The situation is similar with the air knife method, but is of less technical value because of its comparatively lower precision and the limitation to single-layer methods.

The curtain casting process, which works with one or more layers at the same time, brings a further improvement Use of viscosities up to several hundred cP allowed.

In the case of gelatin-containing casting solutions, as are used in the vast majority of cases in photography, the dry contents of the coating materials are limited by the permissible viscosity. In the usual coating processes, they are between five and ten percent and can be increased to about 20% in extreme cases, such as are still permitted in curtain casting. Such low dry contents have various disadvantages: the solvent to be removed during the subsequent drying process, in most cases water, requires a high amount of energy and the use of very complex drying systems, both of which are largely responsible for the costs of the coating. Likewise, there is an additional effort which must be made when converting aqueous gelatin solutions to the gel state: the gelling of the coatings before the start of the drying process is absolutely necessary, in particular when coating with a low concentration and correspondingly low viscosity; a deformation of the layers by the blown drying air would otherwise be unavoidable. Now, however, gelatin solutions of a low concentration require a relatively long time to convert to the gel state and an increased cooling capacity compared to those with a high concentration.

It is therefore of great interest for various reasons to find a coating process which is also suitable for very highly viscous coating compositions and in particular also permits the production of the very precise layers with high uniformity and surface quality which are absolutely necessary for photographic materials. In this respect, the cascade casting process and, to an even greater extent, the curtain casting process mean technical progress, since both permit the use of coating compositions with a higher viscosity and thus also a higher concentration. So, for example - always counting on the same amount of dry matter - is the saving in expenditure Evaporation energy with an increase in the dry matter content from 5 to 10% 55 percent, with an increase in the dry matter content from 20% even 79 percent. Even higher savings can be achieved if the dry matter content can be increased to 40 7 or even 50% (.92.1 or 94.7 percent).

Gelatin-containing photographic coating compositions with dry contents of 20-50% have viscosities which are in the range from about 1000 to 100,000 cP or more at the usual coating temperatures of 35-45 ° C.

Such coating compositions can no longer be processed with the above-mentioned coating methods customary in photography. On the other hand, it is known, e.g. For the production of plastic films and plastic-coated papers, to process masses of similarly high viscosity into thin layers. The so-called extrusion process is used for this.

Externally, the devices used for the extrusion process differ only very little from the slot casters used for processing thinner materials. However, a relatively high pressure must be applied to convey the mass through the distribution channel and through the pouring slot; the high viscosity and the associated high pressure drop must also be taken into account when designing the distribution channels. So-called wide-slot extruders are well known from the production of plastic films and coated papers and can be constructed with the required accuracy.

An essential difference between the usual slot casting process and the extrusion process lies in the behavior of the extruded mass after it ver the slot of the extruder has left: Coating materials with a dry matter content of less than 20%, or viscosities of less than 1000 cP, can generally still be regarded as free-flowing liquids in which the effects of gravity outweigh the forces caused by the internal cohesion. The situation is different for coating compositions whose viscosity significantly exceeds 1000 cP.

If you try e.g. Applying such a liquid mass to a running carrier, similar to the slot casting process, does not form a meniscus with free circulation. Instead, strong tensile forces occur in the direction of flow, which can act from the point of contact with the running beam backwards over a considerable distance to the point where the coating compound emerges from the slot. It is thus possible to considerably increase the free distance between the beam and the slot exit, e.g. to a few cm without the liquid layer tearing. In addition, it is possible to exert tensile forces on the liquid film and thereby stretch it. This can be easily achieved by moving the carrier at a speed that. is significantly higher than the speed of the coating material when it emerges from the slot. Under suitable conditions, stretching by 50 times is easily possible without the coating composition tearing in its free-flowing part. On the one hand, this enables very thin coatings to be produced; on the other hand, the exit slot of the extruder can be kept in normal dimensions at the same time, which makes precise machining of the extrusion tool much easier.

In the production of plastic films by the extrusion process, a highly viscous solution of the plastic in a suitable solvent or, if appropriate, a melt at a higher temperature is applied to a highly polished surface, for example a metal roller or a metal strip. After solidification of the layer, either by cooling or by evaporation, possibly before If the solvent is used, the film can be pulled off freely and, if necessary, wound up after further post-treatment. By adjusting the belt speed, the stretching and thus the film thickness are brought to the desired value.

The procedure for producing plastic-coated papers, in which good adhesion between the base and the coating is sought, is somewhat different. Here, the extruded liquid film, which in this case practically always consists of a solvent-free melt, is brought into direct contact with the substrate to be coated in a roll nip, which is vertically a few centimeters below the exit slot of the extruder. The pad is covered with a certain wrap angle around an elastic, e.g. with a rubber-coated pressure roller. Opposite the pressure roller is a polished and cooled driven roller; the gap between these rollers corresponds to the total thickness of the coated carrier. The coating mass is fed into the nip so that it is on the side of the polished roller. The pressure is applied to the carrier in the nip; cooling avoids sticking to the polished surface. A detailed description of the technique for the production of polyethylene coated papers can be found e.g. in the article "Extrusion Coating Systems", VDI-Bildungswerk 902 (Association of German Engineers) or in the U.S. Patent No. 3,215,578.

When coating carriers with highly viscous gelatin-containing coating compositions, the problem arises in a somewhat different way. The coating specialist must assume that the highly viscous and extremely sticky coating compound must not be brought into contact with another surface before it dries. Attempts to freely extrude a gelatin-containing coating composition onto a moving support have so far always failed due to the problem of air inclusion. It is indeed possible to place the extruded film on the support, if necessary with simultaneous stretching. However, lie between the carrier and Coating always numerous air bubbles and remain trapped there. This problem cannot be solved using the usual means, for example removing the boundary layer by a barrier placed in front of the job site.

The amount of air bubbles trapped can be reduced by such a measure, but it is not possible to achieve a completely closed, uniform coating.

Surprisingly, it has now been found that an arrangement such as is used when coating with polyethylene melts can also be used to process a highly viscous and sticky gelatin-containing coating composition on the carrier to form a closed, uniform coating film free of trapped air bubbles, without the coating on the stuck polished roller.

Accordingly, the method according to the present invention is characterized in that the photographic coating composition is extruded from a slot die and, after a short, preferably essentially vertically directed, free-running section, it is applied to a photographic support which is passed through a driven, cooled, mirror-finished roller and a its opposite elastic, non-driven pressure roller formed nip runs such that the coating composition is applied directly in the nip, on the side of the carrier facing the polished roller. The coated carrier is then freed from the solvent by drying and rolled up in a conventional manner.

The method is particularly suitable for applying photographic layers with a particularly uniform layer thickness to supports made from coated or uncoated paper or from synthetic materials such as cellulose esters or high molecular weight polyamides, polyesters etc.

The polished roller is preferably cooled to 7-10 ° C. and a contact pressure of at least 5 kg per cm of roller width is maintained in the roller gap.

The nip, formed from the elastic pressure roller on the one hand and the polished cooling roller on the other, serves to press the extruded liquid film and the base evenly and with constant pressure so that no air can penetrate into the space between the substrate and the coating. There is no equalization of the layer in the longitudinal or transverse direction, since the high viscosity of the coating composition and the comparatively low contact pressure practically preclude the coating composition from flowing in the longitudinal or transverse direction. It is therefore necessary to extrude the extruded liquid film with the required accuracy by appropriate precision of the casting tool. The coating method according to the present invention differs fundamentally from the extrusion coating method as described in German Offenlegungsschriften 771 586 or 2 554 319. In these two cases, the carrier surface is only partially coated with the liquid to be extruded; the uniform distribution is then carried out in a roll nip by spreading the coating material laterally. The accuracy and uniformity of the layer thickness is determined in this case by the dimensions and the machining accuracy of the nip.

The process according to the invention is also outstandingly suitable for simultaneously coating a support with several layers by simultaneously extruding several layers of photographic coating compositions. This can be done, for example _, by forming a layer from two or more adjacent extruders and combining them with the carrier in the same nip, or by using an extruder in which two or more feed channels can be loaded with different coating compositions, for example the two liquid streams being combined into a multilayer shortly before a common slot exit.

Compared to conventional photographic coating processes, in which relatively low-viscosity coating compositions with viscosities of less than 1000, preferably even less than 100 cP are used, the process according to the invention has the advantage that the use of wetting agents in the layers is generally unnecessary. This is possible because both in the formation of layers and when a layer is brought together with the support or with different layers, the mechanical forces that occur far exceed the surface forces that are effective in other processes. Certain problems that otherwise occur in particular in the case of the usual multiple coatings in connection with the mutual wetting and adhesion between the layers can be largely avoided in this way.

The device for carrying out the new method is characterized according to the further invention in that it has at least one extruder with an extrusion slot extending transversely to the direction of movement of the carrier web, the length of which is at most equal to the width of the carrier web to be coated, means for uniformly metered supply of highly viscous Coating compounds in each extruder, a roller pair arranged essentially vertically below with at least one driven roller, one roller of the pair having an elastic coating and the other having a highly polished hard surface, means for the two rollers with a force between 5 and 50 kp to press against each other along their line of contact, includes means for driving the roller pair at a variable speed and means for cooling the high-gloss roller surface to a temperature below 20 ° C.

• Fig. 1 und 2 die beiden Ausführungsbeispiele in schematischer Dar_- stellung und
• Fig. 3 eine Detailvariante gleichfalls in schematischer Darstellung.

The invention is explained in more detail below with reference to two exemplary embodiments of the device according to the invention shown in the figures. Show it

• Fig. 1 and 2, the two embodiments in schematic Dar _- position and
• Fig. 3 also shows a detailed variant in a schematic representation.

The figures denote: with 1, 1 'or 10 extruders, with 8 the roller with the elastic surface and with 7 the roller with the highly polished surface. The roller 8 is free-running. The roller 7 is driven by a motor during the extrusion in the direction of the arrow P and transports the carrier web 6 to be coated, which after its coating is designated 9, through the coating zone.

The extruder 1 is fed with a photographic coating composition by a pump, not shown in the figures. The coating composition flows out of the distribution channel 2 through the extrusion slot 3, from the mouth 4 of which it is drawn into the gap formed by the two rollers 7 and 8 or the strip to be coated and the roller 7 with simultaneous stretching. The roller 7 is provided with a device for the supply of coolant, also not shown in the figure. The coating mass solidifies due to the cooling effect of the roller 7. The carrier web 6 is conveyed through the nip at the circumferential speed of the two rollers with partial wrapping of the roller 8 and leaves it as a coated belt 9 to a dryer, not shown in the figures. The two rollers 7 and 8 are operated by a device not shown in the figures, e.g. hydraulically or by spring force, radially compressed.

The device shown in FIG. 2 differs from that of FIG. 1 essentially only by an additional Ex truder 1 '. The two extruders 1 and 1 'are arranged in such a way that the liquid films 5 and 5' emerging from the orifices 4 and 4 'of their extrusion slots 3 and 3' combine in the gap between the rollers 7 and 8 with the carrier 6 and thereby become one double coated carrier 9 'is produced.

The variant of FIG. 3 shows an extruder 10 for producing a liquid film 16 consisting of two layers. Two different coating compositions are conveyed through a distribution channel 11 and 12 through a distribution slot 13 and 14 to the mouth 15 and leave it as a double-layer liquid film 16. which, as shown in FIGS. 1 and 2, can be applied to the carrier 6 through the nip. In an analogous manner, extruders can also be used which produce liquid films with three or more layers. A plurality of extruders arranged in parallel can also be used in a manner similar to that in FIG. 2, at least one of which produces a two or more liquid film.

• Ein 10 cm breites Trägerband (6) von mit Polyäthylen beschichtetem Papier mit einer Stärke von 180 µ wurde mit einer Geschwindigkeit von 2,5 Metern pro sec durch den von den beiden Walzen 7 und 8 gebildeten Spalt gezogen. Die Walze 7 wurde durch Zirkulation eines Kühlmittels derart gekühlt, dass die Temperatur ihrer polierten Oberfläche 10° C betrug. Durch die hydraulische Vorrichtung (nicht dargestellt) wurden die beiden Walzen 7 und 8 mit einer Kraft von 60 kp, entsprechend einem spezifischen Pressdruck von 6 kp pro cm Walzenbreite zusammengepresst.

The following coating was carried out using a device according to FIG. 1:

• A 10 cm wide carrier tape (6) of polyethylene-coated paper with a thickness of 180 μ was pulled through the gap formed by the two rollers 7 and 8 at a speed of 2.5 meters per second. The roller 7 was cooled by the circulation of a coolant so that the temperature of its polished surface was 10 ° C. By means of the hydraulic device (not shown), the two rolls 7 and 8 were pressed together with a force of 60 kp, corresponding to a specific pressing pressure of 6 kp per cm of roll width.

A photographic coating composition consisting of a solution of 40 percent by weight gelatin in 60% by weight water, the viscosity of which at 39 ° C. is 39,000 cP, was introduced into the extruder at 40 ° C. with a rate of 1.25 ml / sec 1, or its distribution channel 2 fed. Through the distribution slot 3, which at its Mouth 4 has a width of 250 microns, an extruded film of coating material was formed, which accelerates from an initial speed of 5 cm / sec on the free path (5) to the nip to 50 times the speed of the carrier and thereby from its original Thickness of 250 µ was stretched to 5 µ. In the nip, the liquid film combined with the carrier 6 without the inclusion of air bubbles. After leaving the nip, the coated tape was dried and then had a coating with a 2 μ thick dried gelatin film. The coating film had a completely smooth surface, was free of air pockets between the substrate and the coating and had excellent adhesion to the base. Experiments with the same coating composition, but with the addition of different conventional wetting agents, gave the same result and the same good adhesion.

Claims (12)

1. A method for coating flexible photographic support webs with liquid photographic coating compositions, characterized in that the photographic coating composition, in particular a highly viscous coating composition containing a volatile solvent, consists of at least one slit (transversely to the direction of movement of the support web (6) to be coated ( 3) with a width which is at most equal to that of the carrier web, and in a nip, which is formed by an elastic pressure roller (2) and an opposing mirror-polished roller (7), at least one of which is driven, with which joins along the circumference of the elastic roller with the circumferential speed of the carrier web moving the latter and presses and simultaneously cools through the polished roller opposite the layer side onto the latter. 2. The method according to claim 1, characterized in that one selects the speed of the carrier web (6) by 2 to 50 times the amount higher than the exit speed of the coating material from the extrusion slot (3), whereby a 2nd between the extrusion slot and the nip - Up to 50 times stretching and a corresponding reduction in layer thickness of the coating film takes place. 3. The method according to claim 1 or 2, characterized in that the surface of the polished roller (7) is cooled to a temperature between 0 and 10 ° C. 4. The method according to any one of claims 1 to 3, characterized in that between the polished roller (7) and the elastic pressure roller (8) generates a pressure between 5 and 50 kg per cm of roller width. 5. The method according to any one of claims 1 to 4, characterized in that several coating films are applied simultaneously from several adjacent extruders (1,1 ') in the nip on the carrier web (6). 6. The method according to any one of claims 1 to 5, characterized in that a multi-layer liquid film (16) consisting of at least two different coating compositions is produced by means of a multiple extruder (10) and this is applied to the carrier web (6) in the nip. 7. The method according to any one of claims 1 to 6, characterized in that the coating composition used is the aqueous solution of a hydrophilic colloid, which optionally contains further dissolved or dispersed constituents, and whose viscosity is between 10 ³ and 10 cP. 8. The method according to claim 7, characterized in that gelatin is used as the hydrophilic colloid, optionally in a mixture with other hydrophilic colloids. 9. The device for carrying out the method according to claim 1, characterized in that it has at least one extruder with an extrusion slot extending transversely to the direction of movement of the carrier web, the length of which is at most equal to the width of the carrier web to be coated, means for uniformly metered. Feeding highly viscous coating compositions into each extruder, a roller pair arranged essentially vertically below with at least one driven roller, one roller of the pair having an elastic coating and the other having a highly polished hard surface, means for the two rollers with a force between 5 and press 50 kp against each other along their contact line, means for driving the pair of rollers with a variable speed and means for cooling the high gloss roller surface to a temperature below 20 ° C, preferably below 10 ° C. 10. The device according to claim 9, characterized in that at least two extruders (1,1 ') with adjacent and parallel to each other slots (3,3') are arranged above the nip. 11. The device according to claim 9 or 10, characterized in that at least one of the extruders (10) is a multiple extruder, with at least two parallel feed systems (11, 12) for coating compositions which open into a common outlet slot (15). 12. The device according to one of claims 9 to 11, characterized in that the highly polished roller (7) is connected to a cooling system and can be cooled to 0 to 10 ° C.

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