GB2333476A - Coating method and apparatus - Google Patents

Abstract

In a system for air bubble-free deposition of a coating solution 4 such as photochemical emulsion on to a substrate, the solution is supplied from a container 2 to a delivery module 6 and then to a means 14 for supplying the coating to a substrate. The solution is recirculated via lines 10 and 8 until the delivery module, which includes de-bubbling apparatus, is substantially free of bubbles. A valve is then switched to allow the solution to pass from the delivery module via line 12 to the means 14, thus minimising wastage of the solution. If the coating process is interrupted, the delivery module is switched into recirculation mode to keep the solution bubble-free until coating can be resumed. A plurality of selectively interchangeable containers 2 and modules 6 can be provided. Water may be flowed through the line 12 to remove gas bubbles before solution is supplied to the substrate.

Description

COATING METHOD AND APPARATUS Field of the Invention This invention relates to the supply of solution from a container onto a substrate. It relates particularly, though not exclusively, to the supply of a substantially bubble-free photographic emulsion onto a substrate for forming photographically-responsive film or paper.

Background of the Invention The invention finds application in the food industry and in blood transfusion apparatus, but for convenience, and by way of example only, the invention will be described with reference to the production of photographically-responsive products.

The term "solution" as used herein is to be understood as encompassing any liquid material in flowable form, not being restricted to a chemical definition of the term. Thus, it is to be understood as including, for example, not only photographic solutions but also emulsions.

Photographic film is manufactured by coating a plurality of layers of emulsion onto a polymeric substrate. It is important that the coating is substantially free of bubbles of gas, usually air entrapped during its initial preparation. This is achieved by passing the emulsion through debubbling apparatus and subsequently through a bubble detector until the required minimum level, preferably zero, is achieved. Bubbles also usually reside in the apparatus that is used to deliver the emulsion to the substrate, and these bubbles have to be removed before the emulsion is supplied to the substrate. This is done by circulating water under pressure through the relevant pipes, valves and pumps. A residue of water remains in the apparatus. Before coating can begin, the bubble-free emulsion is then passed through the delivery apparatus to the coating station and directed to a drain until all the residual water that it accumulates has been purged. A significant amount of emulsion is wasted in this manner because of the large volume of apparatus that has to be washed out.

Furthermore, if coating is interrupted, the flow of emulsion from its container is stopped, and the water purging repeated prior to restart, with associated further wastage of emulsion. This also involves a significant time delay, whose length depends on the reason for the interruption, before coating can be restarted.

Problem to be Solved by the Invention It is one object of the present invention to provide for supply of bubble-free solution with reduced wastage.

It is another object of the present invention to provide for delivery of bubble-free solution on demand, in response to an interruption due to a fault or to change the coating material, for example.

Summary of the Invention In accordance with one aspect of the present invention, there is provided a method of supplying a solution that is substantially free of gas bubbles from a container through a delivery system onto a surface of a substrate, wherein the delivery system comprises debubbling apparatus, and a plurality of valve arrangements, and wherein the method comprises: priming the delivery arrangement by passing the solution along a first path that extends from the container through the filter arrangement, through a first valve arrangement, through the debubbling apparatus in a reverse direction, and via a second valve arrangement back to the container until the delivery arrangement is substantially free of bubbles; changing the setting of the first valve arrangement; and supplying the solution along part of the first path from the container, through the first valve arrangement, through the debubbling apparatus in a forward direction, and then via a third valve arrangement and along a supply line to the substrate.

Thus, the delivery arrangement is primed with the solution that will be used for coating the substrate.

All that remains to be debubbled is the pipe, or other conduit, that transports the solution to the substrate. This may be carried out by passing coating solution therethrough to a drain until no bubbles remain, but this wastage can be avoided by arranging for water to flow via the third valve arrangement and along the supply line so as to remove any gas bubbles therefrom prior to supplying the solution to the substrate. By making the supply line of sufficiently small bore and feeding the water into it under sufficiently high pressure, the bubbles can be removed efficiently with very little water remaining. The water remnant can be cleared with minimum wastage of solution, which is directed to drain immediately prior to starting the coating process.

Advantageously, the delivery arrangement comprises a bubble detector, and the solution is passed through the bubble detector prior to changing the setting of the first valve arrangement. By suitable choice of debubbling apparatus, preferably the efficient ultrasonic apparatus disclosed in WO 97/20612, the time spent recirculating the solution to prime the delivery arrangement is minimised.

When supply of the solution to the substrate needs to be interrupted, the settings of the second and third valve arrangements are changed so that the solution is recirculated through said first path. In this way, not only is the solution maintained in its bubble-free condition, but since the delivery apparatus is kept full of solution, there is no need subsequently to purge with water as is the case in conventional systems.

Preferably, the delivery arrangement is provided as a module so that if a fault develops in one module it can quickly be replaced by another with minimum downtime of the coating process. The faulty module can then be repaired at a convenient time, and any other maintenance carried out, thus avoiding any further interruption of the coating process.

Advantageously, at least two of said containers are provided, and when one becomes empty supply of solution to the module is switched from one container to another. Each module may have a small buffer reservoir, so that the coating process is not interrupted at this stage.

In a preferred embodiment, the solution comprises a photographic emulsion, which is coated onto a substrate to produce photographically responsive paper or film.

In accordance with another aspect of the present invention, there is provided apparatus for supplying a solution that is substantially free of gas bubbles for coating onto a surface of a substrate, comprising: a container for the solution; means for supplying the solution to the substrate; a module for delivering the solution from the container to the supply means, the delivery module comprising debubbling apparatus, and a plurality of valve arrangements, wherein the valve arrangements are arranged to provide recirculation of the coating solution between the container and the delivery module until the delivery module is substantially free of bubbles and are then arranged to pass the solution to the supply means for coating the substrate; and a transfer station for connection to a plurality of said containers and to a plurality of said modules, the transfer station being arranged for selectively interconnecting one of said containers to one of said modules.

Preferably, the apparatus comprises means for passing water through the supply means to remove bubbles therein. Advantageously, the delivery module includes a bubble detector.

The supply means may comprise a narrow bore pipe into which the solution is supplied under high pressure for coating the substrate. The supply pipe may terminate in a hopper that is arranged to direct the solution onto the substrate that passes thereby.

Advantageously, the delivery module comprises a filter arrangement, a flow meter, a pump for the solution, and a buffer reservoir.

The apparatus may comprise a plurality of said containers and a plurality of said modules, the containers and modules being selectively interchangeable for supply of solution to the substrate.

Advantageous Effect of the Invention Priming the delivery arrangement with the solution that will be used for coating the substrate avoids the problem of having to remove contamination left by a water residue as a result of flushing the arrangement with water under high pressure to remove the bubbles. Water is needed only to remove bubbles from the supply line, and this can be made of narrow bore so that flushing under high pressure reduces to a minimum the volume of water remaining, and thus of the initial flow of debubbled solution that has to be wasted. The coating system can be switched to a recirculation mode if coating has to be interrupted, without the need subsequently to carry out debubbling or total re-priming.

When the delivery arrangement is of modular construction, minimum coating time is lost if any component thereof needs attention.

Brief Description of the Drawings Coating method and apparatus, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a coating system for supplying a single photographic emulsion to a polymeric substrate to form a photographic film; Figure 2 is a schematic diagram similar to that of Figure 1 for supplying a triple coating to form a photographic film; and Figure 3 is a schematic diagram showing detail of a delivery module of the coating systems of Figures 1 and 2.

Detailed Description of the Invention Referring to Figure 1, a supply kettle 2 at an upper level contains heated photographic emulsion 4 in its molten state. A delivery system module 6 is located at an intermediate level below the kettle 2 and receives emulsion down a feed pipe 8. As described below in more detail with respect to Figure 3, the module initially operates in a recirculation mode to remove air bubbles from the emulsion, which is fed back into the kettle 2 up a recirculation pipe 10.

When the emulsion 4 is confirmed to be free of bubbles, the module 6 switches to coating mode and the emulsion 4 is dispensed through a narrow bore downpipe 12 to a hopper 14 at a lower coating level, from which it is supplied to a photographic substrate (not shown). Operation of the equipment at each level is controlled by a respective one of three programmable logic controllers (PLCs) 16.

Figure 2 shows a delivery system that is more complicated than that of Figure 1 in that three emulsions are to be deposited as a single coating onto the photographic substrate. Suffixes a,b,c are used to differentiate the respective coating systems whilst maintaining the same basic reference numerals as in Figure 1 for the corresponding components. Thus, supply kettle 2a supplies a first emulsion 4a down feed pipe 8a to module 6a, and thence through narrow bore downpipe 12a to the hopper 14, whilst emulsions 4b and 4c are supplied to the hopper 14 via respective downpipes 12b and 12c. The kettles 2a,b,c and modules 6a,b,c are controlled by respective PLCs 16, whilst a common PLC 16d controls the coating level. A transfer station 20 acts as an interface for the flow lines between the kettles 2a,b,c and the modules 6a,b,c to facilitate replacing the kettles when they become empty and to facilitate replacement of the modules when they need maintenance, whilst minimising impact on the coating process. A coating transfer station 22 acts as an interface between the downpipes 12a,b,c and the hopper 14.

Operation of the coating system will now be described in more detail with reference to Figure 3.

A kettle 30 at an upper level contains an emulsion 32.

A delivery module 34 is parked in a docking bay of a transfer station 36 at an intermediate level, and receives the emulsion 32 from the kettle 30 and supplies it to a hopper 38 for coating onto a polymeric photographic substrate 40 moving along a track. The coated substrate 40 is then transported to a drying station 42.

Upon start up of the coating process, the module 34 is primed to ensure no gas bubbles are present in the emulsion 32 when it is delivered to the hopper 38.

To this end, a kettle valve 50 is set to allow the emulsion 32 to flow into the kettle pump 52. From there, the emulsion 32 flows via a first two-way valve 54 and pump 56 to a further valve 58 that is set to recirculation mode. The path of the emulsion continues to a valve 60 that is closed and thence through a second two-way valve 62 and back to the kettle 32 at recirculation inlet 64. The emulsion is discharged through a beak-shaped pipe down the wall of the kettle 30 using it as a weir to ensure that all bubbles in the emulsion flow to the surface of the contents of the kettle. The pump 52 is then switched off, and the second stage of priming changes valve 58 to module mode so as to send the emulsion 32 upwards through a filter 66 and into a buffer reservoir 68.

The upward flow of the emulsion 32 ensures that no airlocks or bubbles are generated by the filter 66.

When the reservoir 68 is full, a module pump 70 is started after a short delay, to ensure it is primed and thus to reduce the amount of bubbles generated, and the emulsion then flows rapidly to a third two-way valve 72. The valve 72 is set to direct the emulsion 32 through a flowmeter 74 and thence to fill a debubbling apparatus 76 in the reverse direction. The debubbling apparatus 76 acts by directing a beam of ultrasound axially upwards, towards its inlet in operational mode, along the vertical axis of a chamber containing the solution from which bubbles are to be removed. The debubbling apparatus is disclosed in WO 97/20612, the entire contents of which are included herein by this reference. At this stage, the emulsion 32 follows two paths from bubble eliminator 76. One path is through a narrow bore vent pipe 78 to the recirculation side of the valve 58 and thence via the recirculating outlet of the valve 60 back to the kettle inlet 64. The other path is out through the (operational) inlet of the bubble eliminator 76, back through the valve 72, through an ultrasonic bubble detector 80, and then via the valve 60, which is now open, back to the recirculation inlet 64 of the kettle 30.

By purging the system in this way, air bubbles are driven upwards from the bottom of the bubble eliminator 76, ensuring that its walls are thoroughly wetted by the emulsion. Furthermore, since the venting pipe 78 from the bubble eliminator 76 leads to the recirculation outlet of the valve 58 and then to the recirculation outlet of the valve 60, it is ensured that there is always a small flow of emulsion in this pipework, which would otherwise solidify during the coating mode.

During the priming described above to purge air from the emulsion and from the major part of the equipment, a two-way valve 82 is set to recirculation so that deaerated demineralised water from a supply 84 is flushed through a downpipe 86 that extends from the module 34 down to the coating hopper 38. The downpipe 86 is of narrower bore than the other pipework of the module 34 so that (a) the water can be fed therethrough under high pressure to remove air bubbles efficiently, and (b) its relatively small volume ensures that the quantity of water remaining after purging is small. The latter feature means that when the module 34 is switched to coating mode, very little emulsion is wasted in the initial flow for purging remnant water from the downpipe 86 and the hopper 38.

Purging continues by switching the valve 72 to forward flow so that air is driven out upwards through the flowmeter 74, thus further ensuring wetting of the components of the module 34. During both the reverse and forward purging by the emulsion, the valve 60 is opened and closed several times so that pressure pulses assist in dislodging bubbles from surfaces.

After the valve 72 is finally set to forward flow, the system is left to stand to allow free bubbles in the chamber of the bubble eliminator 76 to rise up above the tip of the ultrasonic horn. The ultrasonic generator is then switched on to produce bubble-free emulsion in the eliminator 76, and then the valve 73 is opened to attain a flow of 1.5 times the desired coating flow rate. After a short time to allow flushing out of bubbles in the path to the valves 60 and 82, the flow rate is reduced to the coating rate.

The module PLC (not shown) checks at intervals for the presence of bubbles in the emulsion and accuracy of flow control. Upon satisfactory performance, the PLC signals that the module 34 is ready to begin the coating process.

When the PLC (not shown) controlling the coating operation demands emulsion to be supplied to the hopper 38, valve 60 is closed just before valve 82 is switched from its recirculation to its coating setting, thereby preventing "suck bacN at the hopper 34 when changing over from recirculation to coating.

Coating emulsion 32 is then supplied from the kettle 30 into the debubbled module 34, where it flows from the valve 58 through the filter 66, reservoir 68, in a forward loop through the bubble eliminator 76, through the bubble detector 80, and via the valve 82 down the pipe 86 to the hopper 38 and onto the substrate 40.

If it becomes necessary to interrupt the coating of the substrate 40, the module 34 can be switched back temporarily into recirculation mode by reversing the settings of the valves 60 and 82. In this way, the module 34 can be switched between recirculation and coating modes as often as required without upsetting the flow of bubble-free coating emulsion.

Should the quantity of emulsion 32 in the kettle 30 be insufficient to coat the entire substrate 40, "kettle swing" may take place to switch to a standby kettle 90, which is connected to the supply and recirculation control valves 54 and 62 respectively of the module 34. The module reservoir 68 contains a sufficient quantity of emulsion 32 to act as a buffer during this changeover.

In a similar manner, should the filter 66 become blocked, or otherwise defective, flow of emulsion may be switched through a second filter (not shown). The filter 66 can be accessed from the exterior of the module 34.

The system described above provides coating of the substrate 40 with a single emulsion 32. Should one or more further layers, of emulsion or other material, need to be coated onto the substrate 40, then this may be achieved by employing further purged modules with their individual narrow bore downpipes to the hopper 38, as described above with reference to Figure 2.

It is to be understood that washing of the kettle 30 and module 34 can be carried out under control of the module PLC (not shown) by supplying suitable solutions and by appropriate operation of the various valves within the module 34.

It will be appreciated that purging the coating system, or at least the major part thereof, with the solution that is eventually used to coat the substrate, rather than with water as is conventional practice, results in a much more efficient coating process, in terms of, for example, reduction in the quantity of the solution that is wasted and also the time taken.

By providing the supply pipe to the hopper as a narrow bore pipe, the purging water can be flushed through at high pressure, and the coating solution may also be at high pressure. The resulting high velocities in the downpipe prevent entrapment of bubbles and help to keep the pipe wall clean. It is important to choose the pipe bore and solution pressure so that the solution is gradually reduced to atmospheric pressure at the exit, in order to avoid bubble "flashing".

The provision of the equipment in modular form facilitates its replacement in the event of a fault developing. Thus, if a problem occurred in the module 34, rather than spend time repairing it on-line, with corresponding downtime of the coating process, it can be replaced simply with a similar module, and the initial one repaired with less urgency at a more convenient time.

Claims (13)

ClAIMS:

1. A method of supplying a solution that is substantially free of gas bubbles from a container through a delivery system onto a surface of a substrate, wherein the delivery system comprises debubbling apparatus, and a plurality of valve arrangements, and wherein the method comprises: priming the delivery arrangement by passing the solution along a first path that extends from the container through the filter arrangement, through a first valve arrangement, through the debubbling apparatus in a reverse direction, and via a second valve arrangement back to the container until the delivery arrangement is substantially free of bubbles; changing the setting of the first valve arrangement; and supplying the solution along part of the first path from the container, through the first valve arrangement, through the debubbling apparatus in a forward direction, and then via a third valve arrangement and along a supply line to the substrate.

2. A method according to claim 1, wherein water is arranged to flow via the third valve arrangement and along the supply line so as to remove any gas bubbles therefrom prior to supplying the solution to the substrate.

3. A method according to claim 1 or claim 2, wherein the delivery arrangement comprises a bubble detector, and the solution is passed through the bubble detector prior to changing the setting of the first valve arrangement.

4. A method according to any one of the preceding claims, wherein supply of the solution to the substrate is interrupted and the settings of the second and third valve arrangements are changed so that the solution is recirculated through said first path.

5. A method according to any one of the preceding claims, wherein the delivery arrangement is provided as a module, wherein at least two of said containers are provided, and wherein supply of solution to the module is switched from one container to another.

6. A method according to any one of the preceding claims, wherein more than one modular delivery arrangement is provided, and wherein supply of solution to the substrate is switched from one module to another.

7. A method according to any one of the preceding claims, wherein the solution comprises a photographic emulsion, which is coated onto a substrate to produce photographically responsive paper or film.

8. Apparatus for supplying a solution that is substantially free of gas bubbles for coating onto a surface of a substrate, comprising: a container for the solution; means for supplying the solution to the substrate; a module for delivering the solution from the container to the supply means, the delivery module comprising debubbling apparatus, and a plurality of valve arrangements, wherein the valve arrangements are arranged to provide recirculation of the coating solution between the container and the delivery module until the delivery module is substantially free of bubbles and are then arranged to pass the solution to the supply means for coating the substrate; and a transfer station for connection to a plurality of said containers and to a plurality of said modules, the transfer station being arranged for selectively interconnecting one of said containers to one of said modules.

9. Apparatus according to claim 8, comprising means for passing water through the supply means to remove bubbles therein.

10. Apparatus according to claim 8 or claim 9, wherein the delivery module comprises a bubble detector.

11. Apparatus according to any one of claims 8 to 10, comprising a narrow bore pipe for supplying the coating solution to the substrate.

12. Apparatus according to any one of claims 8 to 11, wherein the delivery module comprises a filter arrangement, a flow meter, a pump for the solution, and a buffer reservoir.

13. Apparatus according to any one of claims 8 to 12, comprising a plurality of said containers and a plurality of said modules, the containers and modules being selectively interchangeable for supply of solution to the substrate.