EP0110074B2 - Multiple coating process for moving webs - Google Patents

Description

The invention relates to a method for applying, in a substantially mixture-free manner, a plurality of layers to a web which is continuously guided past a coating location, using a bead-forming coating device with a sliding surface coating head.

A multilayer process which is important for the photographic industry is the cascade coating process, in which one or more layers flow downwards simultaneously on an inclined surface and are fed over a bead of the continuously moving web over a short distance between a coating edge and a web. Processes of this type are also referred to in the literature as bead coating processes.

Experience has shown that, according to today's demands, the cascade or bead coating process can only achieve casting speeds that are in many cases not sufficiently high from an economic point of view. As general experience shows, a drop in the pouring rate can be expected, especially when the viscosities of the coating compositions are higher and the solids concentration in the coating compositions is higher. On the other hand, however, high solids concentrations and the associated high viscosities are interesting in that they reduce the amount of water to be removed by drying and save drying energy, so that the plant can produce more cost-effectively. Last but not least, higher viscosities also result in better casting qualities, since these prevent the good casting quality achieved at the casting point from being reduced in the course of solidification and drying.

Efforts have therefore been made to overcome these disadvantages and to achieve a high pouring rate with high viscosities. DE-A 2 712 055 describes a method in which a lowermost layer with a low viscosity and low wet application is applied under a layer with a higher viscosity and a higher layer thickness. Any layer package can then be built up on these two lower layers. It is required that the lower two layers should be made of the same materials or materials that do not show photographic effects when mixed together. On the other hand, the mixing of these layers is consciously accepted during the pouring. According to the description, the viscosities of the first layer should be between 1-10 mPas and the second layer between 10-100 mPas and the layer thicknesses of the first layer should be between 2-12 micrometers, the second layer between 15-30 micrometers. In this method, mixing of the two layers due to vortex formation in the meniscus is disadvantageous, which means that there are possibilities of error in the photographic layer. Another limitation of the method arises from the requirement that the first and second layers should either consist of the same material or of materials that do not produce any effects photographically. With this method, speeds of up to 3.55 m per s or 210 m / min can be achieved.

The publication DE-A 2 820 708 refers to the disadvantages of the process according to DE-A 2 712 055 mentioned above, in particular to the fact that the layers become slightly unstable when the viscosity is very low. This instability can be avoided to a certain extent by applying a vacuum under the bead between the caster and the web, but these instabilities limit the web speed. In this publication it is therefore proposed to choose a material for the lower layer which is normally highly viscous and becomes low-viscosity and thin under shear stress and thus only has the desired low viscosity in the critical coating area, namely in the area of the bead or meniscus. However, this process requires a special choice of material for the bottom layer, which is not in any case compatible with the photographic purpose of the entire layer structure.

In the narrow patent specification 2 070 459, a further method is described which specifies the ratio of the viscosities of the first and second layers to one another within narrow limits, specifically the viscosities of the layers should have the ratio n, = (0.9-1.1) r ₁₂ have, the viscosities of these two layers should also change under the action of shear forces in different ways so that the viscosity of the first layer is reduced more than that of the second layer. With this method too, the choice of layer composition is no longer free.

Finally, it is known from US-A-3,573,965 to provide coating liquids with a viscosity of 20 mPas when casting layers with an extruder and between the edges of the individual outlet openings which are present on the extruder for each individual layer and the web provide a distance of approx. 300 f.1m. Such extrusion casters, however, do not allow closed layers to be applied to the web; rather, the layer build-up takes place there only on the train

The invention has for its object to provide a method of the type mentioned, with which it is easily possible to achieve a high coating speed without the layers are mixed together or the choice of substances for the layer structure is limited and the photographically active layer package of layers, which have a high solids content and a high viscosity and thus a particularly low wet coating and shortening of Trock _- opening time is made possible.

Starting from a method of the type mentioned at the outset, the object is achieved according to the invention in that an acceleration layer with a viscosity range of 1as20 mPas and a layer thickness of 2-30 µm is arranged under one or more higher-viscosity layers in such a way that the bead formation over a gap width between Sliding surface coating head and web to be coated of 100-200 µm is carried out, and that a vacuum of 1―8 mbar is applied under the coating bead.

In a preferred implementation of the method, an accelerating layer with a viscosity of 2-10 mPas, preferably of 2-3 mPas and a layer thickness of 2.5-10 µm, in particular of 2.5―5 µm, is selected.

The lower, low-viscosity, so-called acceleration layer flows between the photographically effective layer package and the coating device and takes over the connection between the layer package and the webs which are continuously guided past the coating point and are to be coated.

The narrow gap distance amazingly smoothes the bead into an arc at high web speeds in which no eddies are created. The web is evenly wetted, but mixing of the lowest acceleration layer with the layer or layers above is avoided.

This type of process implementation makes it possible to use highly viscous solutions with a high solids content and thus a low layer thickness at high pouring speeds in the layer package and thus to save energy in the drying system.

A method which is particularly advantageous in relation to very high coating speeds is characterized in that a further low-viscosity layer is arranged as a spreading layer over the low-viscosity acceleration layer and over the higher-viscosity layer or layers. This preferably has a viscosity of 1-10 mPas and a layer thickness of 5-20 µm. The spreading layer, which is known per se from US Pat. No. 2,932,855, covers the layer package as the uppermost layer during its formation, the coating and after the coating.

Surprisingly, it was found that the combination of the acceleration layer and the spreading layer enables excellent casting quality with layer packages that otherwise cannot be cast or can only be cast at low coating speeds. Furthermore, it was surprising that when the process was used in the cascade coating process, there was no mixing of the layers and therefore there was also no fear of a deterioration in the casting quality. It was also surprising that this acceleration layer can be set so thin with regard to layer thickness and viscosity that there were no adverse consequences in the further operations, such as when the layers solidified. Furthermore, it was surprising that by using a thin, low-viscosity spreading layer, highly viscous layer packages, which tend to contract, can be spread perfectly. Above all, however, it was surprising that when using the combination of acceleration layer and spreading layer with the cascade or bead coating process, pour rates of 400 mlmin (6-7 mls) and more could be achieved.

An explanation for this behavior can perhaps be given as follows:

At high coating speeds, the layer package running on the cascade is spread over the short distance of the bead or meniscus, i.e. the distance between the caster and the web accelerated very high and stretched in the process. Very large forces act on the layer package, which at higher speeds lead to a partial tearing of the layer package or to instabilities in the layer package. The first effect of the acceleration layer is that the speed of the layer package on the running surface of the cascade is greatly increased by the acceleration layer. In a second effect, the forces that occur when the webs that move past it are absorbed by the acceleration layer, or act only with a time delay. These effects could also explain the good casting quality, because the quality of the layer package, which determines the quality of the photographic material, is not disturbed by any influence in the meniscus or by contact with the web.

These relationships can be estimated by a calculation. This can be done, for example, with two layers, one of which has a viscosity of 500 mPas and the other of which has a viscosity of 2 mPas. The thicker layer should be applied to the web with a layer thickness of 100 µm at a web speed of 330 m / min. The thinner layer may be applied as a second layer with 2 µm. When running on the cascade, the thicker layer then has a speed of 7.9 m / min with a layer thickness of 5730 µm. Apply the layer package in reverse order, ie the layer with the viscosity 2 mPas as the lower layer and the layer with the viscosity 500 mPas as that upper layer, it can be seen that the upper layer of 500 mPas is accelerated by a factor of 2 and reaches a speed of 16.84 mlmin. This also explains the name of the acceleration layer.

Furthermore, the layer thickness of the highly viscous layer is reduced to 1809 µm, i.e. for example by a factor of 3. This means that this layer only has to be stretched by a factor of 18 in order to obtain the correct web application, whereas in the first case a stretching by a factor of 57 would have to be carried out. This high stretching then leads to the breakage of the layer

In the bead coating process, layer-specific and geometric requirements of the coating device must be combined. Particularly important for the bead coating process is the correct choice of the distance between the casting edge and the web and the choice of the vacuum under the bead. A small distance between the coating device and the web is necessary for the bead coating according to the prevailing method. This small distance is a prerequisite for achieving the good casting quality, as is the low vacuum that is possible and necessary. Furthermore, a certain length of the last run-off area must not be undercut in order to enable the acceleration layer to work. By choosing the small distance between the web and the pouring device, however, it is possible to freely choose the type of layer composition for the acceleration layer, i.e. any polymer solutions e.g. Gelatin, cellulose derivatives, polysaccharides or, in certain cases, wetting agent solutions are used. The layer thickness of these solutions can advantageously be chosen so that there is no adverse influence on the layer package - in the case of photographic materials, that is, the photographically active emulsion layers

The effect of the spreading layer, which allows a further increase in the coating speed, especially at high viscosities, can be explained as follows: Highly viscous casting solutions have the property of contracting under the influence of surface tension. This tendency can be reduced by the thin spreading layer that covers the surface of the highly viscous layers

In the following, the process for producing multilayer coatings is explained in more detail using a drawing using the example of coating photographic materials.

Fig. 1 shows a section through a cascade pourer for carrying out the bead coating process.

1 consists of several blocks 13, 14, which are arranged parallel to one another, screwed together and laterally bounded by end plates and include distributor chambers 5, through which the coating liquids 7, 8, 11 via feed channels and metering device (not shown) enter the caster and rise for distribution over the caster width in the column to the outlet slots 9.1 to 9.4.

The coating materials 8, 11 flow downward from the outlet slots on the inclined surfaces 3 and lie over the coating materials already flowing out one step below. The acceleration layer 7, which is characteristic of this method, emerges from the exit slit 9. 1, which is closest to the cast bead or meniscus 10. The layer package from the exit slots 9.2 to 9.4 rests on the acceleration layer 7 during the last part of the inclined surfaces 3. The spreading layer 8 is supplied from the top exit slot 9.4 above the planned layer structure from the slots 9.2 and 9.3 . It guarantees a perfectly spread position of the layer package by preventing the formation of an interface between the layer package and air. The entire structure of acceleration layer 7, layer package 11 and spreading layer 8 bridges the meniscus (application bead) of the foundry edge 4 and is guided in such a way that only a minimal suction depth for bead 10 is generated. It is thereby achieved that the photographically effective layer package 11 lies in a sheet on the substrate to be coated and the accelerating layer 7 between the coating edge 4 and the web 1 forms only a very small bead 10, which is completely sufficient to withstand even high coating speeds to achieve a very good wetting of the web 1 on the casting roll 6. During the wetting of the web 1, the acceleration layer 7 brings about a considerable reduction in the acceleration forces which act on the layer package 11 to be applied, so that the layer package 11 can be coated on the web 1 without qualitative impairment by the acceleration forces. The spreading layer 8 shields the upper side of the layer package 11 at the interfaces from the ambient air and prevents the highly viscous layers 11 from contracting and thus smoothes the surface of the web coating 2.

The considerable and surprising advantage of the method according to the invention for the cascade pouring lies in the unexpected increase in the pouring speeds for highly viscous coating materials 11, which is further increased by the additional spreading layer. Casting qualities of high quality are achieved

Furthermore, it surprisingly shows that surface-active substances in the photographically active Coating materials 11 are no longer required, whereby cost savings are possible.

Even the acceleration layer 7 and the spreading layer 8 require only small amounts of surface-active substances. In certain cases, these layers can also be used without surface-active substances.

With this method, webs with a variety of e.g. twelve or more layers can be coated with various coating materials.

In principle, the method of the invention can be used for coating coherent webs of paper, plastic, glass, wood and fabric. As already mentioned, the process is particularly suitable for casting photographic layer supports with photographic emulsions.

All conventional sheet materials can be used for the production of photographic materials, for example film sheets made of cellulose nitrate, cellulose triacetate, polyvinyl acetate, polycarbonate, polyethylene terephthalate, polystyrene and the like, as well as a wide variety of paper sheets with or without plastic coatings on their surfaces.

The process can be used to apply both photographic layers which contain silver halides as photosensitive compounds and those which contain photosensitive dyes or photoconductive zinc oxides and titanium dioxide. The layers may also contain additives other than those known in the field of photographic layer production, e.g. Carbon black, matting agents such as silicon dioxide or polymeric development aids and the like.

The photographic layers can also contain various hydrophilic colloids as binders. Examples of such colloids are, in addition to proteins such as gelatin, cellulose derivatives, polysaccharides such as starch, sugar, dextran or agar. Synthetic polymers such as polyvinyl alcohol or polyacrylamide or mixtures of such binders can also be used. In addition, the coating method of the invention is of course also applicable to the production of non-photographic layers, e.g. for the production of magnetic clay layers or other layers of paint and lacquer.

• ₁₁ = Viskosität [mPas]
• σ = Oberflächenspannung [m N/m]
• 8 = Nassauftrag auf der Bahn [pm]
• v = Bahngeschwindigkeit [mlmin]

Some examples of coatings are to be presented using examples. The examples are only a selection and can therefore only give an overview that does not claim to be complete. The tables shown in the following examples use symbols with the following meaning:

• ₁₁ = viscosity [mPas]
• σ = surface tension [m N / m]
• 8 = wet application on the web [pm]
• v = web speed [mlmin]

example 1

A coating device according to FIG. 1 was used for the bead application process. The layers to be applied showed the following composition:

The coating speed limit was already reached at 50 m / min using a vacuum of 7 mbar. The casting quality was bad. A paper web with a PE coating was used as the material to be coated

Example 2

A coating device according to FIG. 1 was used for the bead application process. The layers to be applied showed the following composition:

A pouring speed of 100 m / min was achieved, with a negative pressure of 6 mbar being applied. The distance from the caster to the web was 175 µm. The quality of the coating on the paper base with PE coating was good.

Example 3

1 was used for the bead application process. The applied layers showed the following composition:

The pouring speed was 130 m / min at a negative pressure of 7 mbar. The casting distance could be varied from 100-200 µm with good casting quality.

Example 4

1 was used for the bead application process. The applied layers showed the following composition:

The pouring speed was 200 m / min at a negative pressure of 4 mbar and a casting distance of 175 µm. The casting quality of the shift package was good.

Example 5

A 6 layer caster was used for the bead application process. The layers to be applied had the following composition:

The pouring rate was 100 mlmin at a negative pressure of 4 mbar and a pouring distance of 175 µm. The casting quality of the shift package was good.

Example 6

A coating device according to FIG. 1 was used as a pourer for the bead application process for a three-layer casting. The coating data for the individual layers were

Casting speeds of v = 400 m / min and more could be achieved. The negative pressure applied between the foundry edge 4 and the web 1 was only 1 mbar at a distance from the pouring edge 4 to the web 1 of 175 μm. Cellulose triacetate was used as the web material. The casting quality was good.

Example 7

For a coating as in Example 6, a paper coated with polyethylene was used as the web material. The pouring speed was v = 400 m / min. There was no tendency for the coating to tear off. The casting quality was good.

Example 8

A coating device according to FIG. 1 was used as a pourer for the wax application process for a four-layer coating. The casting dates for the individual layers were:

A pour rate of v = 400 mlmin could be achieved. The vacuum applied was 1 mbar at a pourer / web distance of 175 µm. Cellulose triacetate was used as the base. The casting quality was very good.

Example 9

A caster according to FIG. 1 was used as a caster for the bead application process for a three-layer cast. The casting dates of the individual layers were as follows:

A casting speed of v = 250 m / min could be achieved. The vacuum applied was 4.5 mbar at a pourer / web distance of 175 µm. PE paper was used as a base. The Cast quality was good.
the examples listed only show part of the scope. Other web documents, larger and smaller numbers of layers and other coating materials are possible by adjusting the acceleration layer with regard to viscosity and layer thickness.

The spreading layer enables a further improvement in the casting quality and an increase in the coating speed; no mixing occurred between the individual layers and between the accelerating layer and the layer above. The spreading layer also does not mix with the underlying photographic layer.

Claims (4)

1. Process for the application, substantially free from intermixing, of several layers to a web moved continuously past a coating point, using a coating device which has a sliding surface coating head and forms a bead, characterised in that an acceleration layer having a viscosity range of from 1 to 20 mPa.s and a layer thickness of from 2-30 µm is arranged under one or more relatively high viscosity layers, in that the bead formation takes place over a gap width between the sliding surface coating head and the web to be coated of from 100 to 200 µm and in that a vacuum of from 1 to 8 mbar is applied under the coating bead.

2. Process according to Claim 1, characterised in that the acceleration layer has a viscosity of from 2-10 mPa.s, preferably from 2-3 mPa.s, and a layer thickness of from 2.5 to 10 µm, preferably from 2.5 to 5 pm.

3. Process according to one of the preceding claims, characterised in that a further low viscosity layer is arranged as a spreading layer above the relatively high viscosity layer or layers.

4. Process according to Claim 3, characterised in that the spreading layer has a viscosity of from 1-10 mPa.s and a layer thickness of from 5-20 µm.

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