Continuous Photographic Emulsion Processing

Abstract

A continuous preparation of photographic emulsions by adding a plurality of individual components, the individual components being introduced continuously and successively by metering pumps. Each individual component is completely mixed with the main stream of emulsion before the next component is introduced. Mixing is carried out in a static mixing zone or alternatively by producing a secondary turbulent flow in a particularly designed mixing zone.

Description

The present invention relates to a process for the continuous preparation of photographic emulsions which are ready for casting in which the individual components are introduced together into a closed, tubular body with the starting emulsion and dispersed in one another, and to an apparatus for carrying out this process.

It is known that metering pumps can be used to carry out reactions in which reactants cannot all be immediately combined. By means of such metering pumps, the different reactants can be delivered in graduated quantities to a reaction vessel. It is possible in this way for the reaction to be controlled quantitatively and for its duration to be controlled also.

Various processes for the continuous preparation of emulsions are already known. Unfortunately, particular problems arise where even small quantities of certain additives have an appreciable effect upon the photographic properties of the emulsion. For this reason, any process in which two or more reactants are continuously delivered to a reaction vessel and a quantity equal to the sum total of all the reactants is continuously run off from the reaction vessel, cannot be considered for use in this case. Through calculations well known in process technology based upon the residence time in finite reaction vessels, it is possible to show that the parts by volume removed from the vessel contain parts by volume which have a non-uniform residence time in the vessel.

Processes in which the usual sequences of events takes place successively in different apparatus, are equally unsuitable.

Even in cases where these ideas are further developed, it is of course not possible with simple formulations to mix the liquids, for example, to satisfy the stringent requirements made of the process as to accuracy which are determined by the characteristics of the photographic emulsion

The particular recipe for preparing photographic emulsions makes it necessary for the additives to be individually introduced because otherwise certain properties cannot be achieved. Accordingly, it is extremely important to only introduce one additive at a time into the mixture, rather than to combine several additives, because of the mixing effect at high speeds.

Two liquids can be effectively admixed with one another. However, suitable apparatus for such admixture has the disadvantage that it includes mechanically moving components and, if several additives are to be introduced, it becomes unweildy and unsuitable for the purpose envisaged.

In another known process, the basic component is carried along in an ascending stream, whilst the additional components are superimposed in continuous succession upon the basic component rotationally symmetrically in a cross-stream. One disadvantage of this process is that admixture is always incomplete and that it is impossible to establish consistent mixing times for different parts of the emulsion.

The object of the invention is to react the smallest possible proportions by volume of the emulsion and additives as directly and as quickly as possible. An apparatus which provides the reactants with temporary protection from the mixture, is thus unsuitable in principle.

The problem which the invention seeks to solve is discussed in more detail in the following:

Hitherto, photographic emulsions have been provided in accordance with certain recipes with a number of additives which impart to these emulsions advantages such as for example sensitivity in certain spectral regions, stability during casting, stability during storage, and favourable fogging characteristics. These substances have to be added in a certain order and in a certain chronological sequence in order to achieve the required objective. This is associated with the fact that, in order to become active, these substances have to be adsorbed onto the silver halide grain. A place on the grain surface which is occupied by substance A either cannot be taken up by substance B at all, or can only be taken up after a desorption process. These processes are subject to known physical laws. In general, attempts were made to allow a small quantity of an active substance to act upon a large proportion of emulsion for a certain period of time and, in this way, to ensure that equilibrium is adjusted in the mixture. One disadvantage of this procedure is that the active substance is introduced in concentrated form in a relatively small quantity by volume into a large quantity of emulsion and subsequently has to be dispersed therein by stirring. The fact that the substances are insoluble in water is often particularly unfavourable so far as effective distribution over the grain surfaces of the silver halide grains is concerned. Accordingly, the idea behind the process was to rapidly bring one unit by volume of the emulsion into direct contact with one unit by volume of the active solution so that not only does rapid admixture in the accepted sense occur, but also the rate at which adsorption takes place is comparable with the rate at which precipitation takes place.

Accordingly to the present invention, there is provided a process for the continuous preparation of photographic emulsions which are ready for casting, wherein individual emulsion components are introduced into a stream of starting emulsion in a closed tubular body and dispersed in the stream, the individual components being introduced continuously and in succession to one another by means of metering pumps, each individual component being completely admixed with the stream in a mixing zone downstream from where it is introduced before the next component is introduced.

Admixture advantageously takes place in a static mixing zone. In a mixing zone of this kind, a particularly intense mixing effect can be achieved by generating a secondary turbulent flow in the mixing zone.

In a further development of the process according to the invention, the rate of flow of the main stream is selected in such a way that the average residence time of a particular component in its respective mixing zone is at most 50 seconds.

Accordingly to the present invention, there is also provided an apparatus for the continuous preparation of photographic emulsion which are ready for casting, comprising a tubular body having inlets arranged along its length which each communicate through metering pumps with one or more individual emulsion component storage vessels, a mixing zone which has no parts adapted to be mechanically moved being arranged between each adjacent pair of inlets.

In one advantageous embodiment of the apparatus, the mixing zone consists of two tubes arranged one behind the other in hairpin-fashion in each of which tubes a spiral is arranged whose diameter is substantially identical to the internal diameter of the tube. The spiral is best dimensioned in such a way that the ratio of its lead to the distance between its core and the inner surface of the respective tube is 2:1.

The inlets for the individual components are simply formed by tangential tubes or, in preferred embodiment, are in the form of sprinkler heads.

To make cleaning easy, the hairpin-like mixing zones are provided with quick closures, the spirals being locked in position by holders when the mixing zones are closed.

Preferably, the metering pumps are not driven through separate motors, but instead have a common drive.

In order to keep both the pH-value and the viscosity of the completed mixture constant, suitable measuring sensors and regulating devices are preferably provided at the end of the mixing zone. In addition, measuring control vessels are incorporated both in the upstream end of the tubular pipe and between the inlets for the additives and their supply vessels for monitoring the mixing ratio.

By virtue of the process according to the invention, and additives are quickly and completely admixed with the main stream.

By comparison with the conventional process for the continuous preparation of emulsions, the time required to complete the adsorption of photographically active substances from a partly precipitated heavily diluted solution is greatly reduced. The effect of guiding flow in the manner described is that the solution of the photographically active substances of the emulsion is delivered in small proportions by volume and, through the intensive admixing effect, passes directly to the silver halide grain before precipitation can take place. The effect of this is that there is no need for the hitherto necessary digestion times during preparation of the emulsion. In the past, it has for the same reason not been possible to use a large number of very promising photographically active substances because they change their physical and chemical qualities during the digestion period. Accordingly, the process according to the invention also opens up the use of a new group of photographically active additives.

Another advantage of the process according to the invention is that it is possible to quickly change from production of one emulsion of one composition to an emulsion of a different composition. In this respect the low overall volume achieved through the special hairpin-like configuration of the mixing zones has a favourable effect.

It is also readily possible to provide regulating means for continuously regulating the additives at the individual inlets. In this case, the installation can be centrally controlled from a process computer to which all regulating systems are linked.

The installation according to the invention is extremely easy to maintain and clean. In addition, its reliability in operation is extremely high because there are no mechanically moved parts in the emulsion feed zone as a whole.

Referring to the accompanying drawings;

FIG. 1 diagrammatically illustrates the installation as a whole;

FIG. 2 is a partial cross-actional view which shows the hairpin-like mixing zone in detail;

FIG. 2A is a cross-sectional view taken through FIG. 2 along the line 2A--2A;

FIG. 3 is an enlarged fragmental cross-section which shows how the secondary eddy current is generated;

FIGS. 4 and 5 are cross-sectional views which show special embodiment of the inlets.

FIG. 1 shows how the stream of emulsion is guided. A starting emulsion is continuously introduced into the installation at an inlet 1, flows through a measuring control vessel 2 and is then pumped by a mainstream metering pump 3 into the hairpin-like mixing zones 4 which are arranged one behind the other. Additives A.sub.1 to A.sub.7 are successively introduced into the main stream 1 by reciprocating metering pumps 5 through inlets 6. The main stream pump 3 and all the metering pumps 5 have a common drive. All the pumps are best adjusted in phase in such a way that they deliver the additives in synchronism with one another. The throughput in the mixing zone can be adjusted substantially as required by a rotational speed adjustment (not shown), or alternatively can be automatically regulated in dependence upon their levels and thus adapted to consumption. The mixing ratio, and hence the composition of the emulsion, remains constant irrespective of the througput. The mixing ratio is additionally monitored at fixed time intervals by the measuring control vessel 2 and by further measuring control vessels (not shown) which are installed in the feed pipes for additives A.sub.1 to A.sub.7. Incorrect metering, if any, can be immediately detected in this way.

The average residence time of any one unit of volume of the components added in the mixing zone 4 is governed by the throughput. In the practical operation of the installation, residence time of b 50 seconds are generally not exceeded because otherwise the emulsions can change their properties.

Additives A.sub.1 to A.sub.7 can include dyes for optical sensitisation, organochemical substances for stabilisation, wetting agents, hardeners, dye components, optical brighteners, pH-regulators, etc. The mixing effect in the zone 4 is so intense that complete admixture of a given component with a volume of the main stream always takes place before a new additive is introduced into that volume. Spirals 7, which generate a secondary turbulent flow, are shown in the mixing zones 4.

The last two mixing zones are provided with detectors 8, 9 and control systems 8', 9' which are adapted to keep the pH-value and the viscosity, respectively, constant. These values can be influenced by the additives A.sub.6 and A.sub.7. For this purpose, the last two metering pumps 5' are provided with an adjustable piston stroke which is controlled by the regulating systems 8' and 9'. In certain cases, it can be of advantage to distribute an additive through several metering pumps and mixing zones 4 rather than introducing it through a single metering pump 5. The emulsion, ready for casting, is removed from an outlet 10 of the installation and can be directly delivered to a coating machine.

FIG. 2 and FIG. 2A show hairpin-like mixing zone 4 in detail. It consists of two tubes 11 which are arranged one behind the other and in which the spirals 7 are fixed by means of holders 12. The ends of the tubes 11 are provided with quick closures 13. When the mixing zones 4 are closed, the spirals 7 are locked in position by the holders 12. When the quick closure 13 is released, the spirals 7 can be readily removed so that the tubes can be cleaned. The mixing zones are surrounded by a jacket 14 and can be thermostatically maintained at a temperature of from 30.degree. to 60.degree.C by means of a tempering liquid which is introduced through the jacket opening 15. The inlet 6 through which additives are introduced is situated at the lower end of a tube 11 of the mixing zone.

FIG. 3 shows a detail of the mixing zone on a larger scale. The tempering vessel 14, which is filled with a heating medium, concentrically surrounds the tube 11 of the mixing zone. A particularly intense mixing effect is achieved if the spiral 7 is dimensioned in such a way that the ratio of lead a to internal space b is about 2:1. With these dimensions, secondary turbulent flow 16 is developed in the spiral 11, as shown in the drawing.

FIGS. 4 and 5 show embodiments of the inlet 6. FIG. 4 shows an inlet in the form of a simple tangentially arranged tube. The component to be added flows through an inlet tube 17 substantially perpendicularly of the direction of the main stream at the lower end of the mixing tube 11.

An improved mixing effect is obtained where the inlet 6 is in the form of a sprinkler as shown in FIG. 5. In this case, the component to be added flows through the inlet tube 17 into an annular duct 18 and through openings 19, perpendicularly of the direction of the main stream, into the mixing tube 11.

The static form of mixing zone is provided by omitting spirals 7 from mixing zones 4, and a complete static mixing zone apparatus is as shown in FIGS. 1, 2 and 2A without any spirals 7.

Claims

What we claim is:

1. A process for the continuous preparation of photographic emulsions which are ready for casting, wherein individual emulsion components are introduced into a stream of staring emulsion at different locations in a series of closed tubular bodies and dispersed in the stream, the individual components being introduced continuously and in succession to one another by means of metering pumps, and a succession of changes in the direction of flow of the stream are caused by channeling it through a nonlinear static flow channeling element in a mixing zone in the closed tubular body downstream from where each individual component is introduced whereby each individual component is completely admixed with the stream before the next component is introduced.

2. A process according to claim 1, wherein the admixture takes place in a static mixing zone.

3. A process according to claim 1, wherein a secondary turbulent flow is generated in the mixing zone.

4. A process according to claim 1, wherein the average residence time in the mixing zone of an introduced component is at most 50 seconds.

5. An apparatus for the continuous preparation of photographic emulsions which are ready for casting, comprising a series of tubular bodies having inlets arranged at different locations between them, each of the inlets communicating through metering pumps with one or more individual emulsion component storage vessels, a mixing zone which as no parts adapted to be mechanically moved being arranged between each adjacent pair of inlets, and a spiral flow channeling element in each of the mixing zones for causing a succession of changes in the direction of flow in the mixing zones whereby each component is completely admixed before the next is introduced.

6. An apparatus according to claim 5, wherein the inlets are in the form of sprinklers.

7. An apparatus according to claim 5, wherein each metering pump has a common drive shaft which also drives a main stream pump.

8. An apparatus according to claim 5, comprising sensors adapted to sense the pH value and viscosity of the substantially completed emulsion and regulating system adapted to maintain these values constant.

9. An apparatus according to claim 5 comprising a measuring control vessel located before the first tubular body in the series.

10. A mixing zone for introducing a liquid additive into a flow stream comprising at least one tubular conduit, removable heads connected to each end of the tubular conduit for connecting it as part of a tubular body, an additive inlet connected to each of the heads, a spiral lead screw having helical flutes disposed about a longitudinal core rod being removably inserted within each of the tubes for generating a secondary turbulent flow within each of the tubes, and the removable heads being constructed and arranged to permit removal of the spiral lead screws for cleaning.

11. A mixing zone according to claim 10, wherein the outer dimension of the flutes is slightly less than the inside dimension of the tube to form a spiral flow channel within the tube between the core and the inner wall of the tube.

12. An apparatus for the continuous preparation of photographic emulsions which are ready for casting, comprising a series of tubular bodies having inlets arranged at different locations between them, each of the inlets communicating through metering pumps with one or more individual emulsion component storage vessels, a mixing zone which has no parts adapted to be mechanically moved being arranged between each adjacent pair of inlets, and each mixing zone comprises two adjacent tubes arranged in hairpin-fashion in each of which is arranged a spiral flow channel having a diameter substantially identical in the internal diameter of the respective tube.

13. An apparatus for the continuous prepartion of photographic emulsions which are ready for casting, comprising a series of tubular bodies having inlets arranged at different location between them, each of the inlets communicating through metering pumps with one or more individual emulsion component storage vessels, a mixing zone which has no parts adapted to be mechanically moved being arranged between each adjacent pair of inlets, each of the mixing zones includes at least one tubular conduit, removable heads connected to each end of the conduit for connecting it as part of the tubular body, one of the inlets being connected to each of the heads, a spiral lead screw having helical flutes disposed about a longitudinal core rod being removably inserted within each of the tubes for generating a secondary turbulent flow within each of the tubes, and the removable heads being constructed and arranged to permit removal of the spiral lead screws for cleaning.

14. An apparatus according to claim 12, wherein the spiral flow channel has a core and a lead along the core, and the ratio of the lead of the spiral to the distance between the core of the spiral and the inside of the tube is about 2 : 1 .

15. An apparatus according to claim 13, wherein each of the heads includes a pair of openings for connection to a pair of the tubes, one of the heads connecting one end of each of the tubes to each other, and the other of the heads including an inflow connection to one of the tubes and an outflow connection to the other of the tubes.

16. An apparatus according to claim 13, wherein the outer dimension of the flutes is slightly less than the inside dimension of the tube to form a spiral flow channel within the tube between the core and the inner wall of the tube.

17. A mixing zone for introducing a liquid additive into a flow stream comprising at least one tubular conduit, removable heads connected to each end of the tubular conduit for connecting it as part of a tubular body, an additive inlet connected to each of the heads, spiral lead screw having helical flutes disposed about a longitudinal core rod being removably inserted within each of the tubes for generating a secondary turbulent flow within each of the tubes, and the removable heads being constructed and arranged to permit removal of the spiral lead screws for cleaning, each of the heads includes a pair of openings for connection to a pair of the tubes, one of the heads connecting one end of each of the tubes to each other, and the other of the heads including an inflow connection to one of the tubes and the outflow connection to the other of the tubes.

18. An apparatus for the continuous preparation of photographic emulsions which are ready for casting, comprising a series of tubular bodies having inlets arranged at different locations between them communicating through metering pumps with one or more individual emulsion component storage vessels, a mixing zone which has no parts adapted to be mechanically moved being arranged between each adjacent pair of inlets, and the inlets comprising tubes arranged tangential to the series of tubular bodies.