GB2051742A

GB2051742A - Gelatin solution dispensing

Info

Publication number: GB2051742A
Application number: GB8020931A
Authority: GB
Original assignee: Ciba Geigy AG
Current assignee: Novartis AG
Priority date: 1979-06-28
Filing date: 1980-06-26
Publication date: 1981-01-21
Also published as: GB2051742B

Abstract

A method for dispensing liquids such as gelatin solutions at just above their setting temperature is performed by determining the viscosity of the liquid just above its setting temperature, controlling the temperature of the liquid so as to obtain this desired viscosity and dispensing the liquid only whilst it is at the desired viscosity. The liquid is fed by a pump 8 whose downstream pressure is detected by a transducer 17 to determine the viscosity and cause heating or cooling of a supply vessel 1 to change the viscosity to the required value before a dispensing valve 15 is opened. <IMAGE>

Description

SPECIFICATION Dispensing liquids This invention relates to a method and apparatus for dispensing at or just above their setting point thermally reversible sol-gel system materials which set at ambient temperature.

Often it is required when dispensing liquids which set at ambient temperature that they are dispensed as close to their setting point as possible. This is to ensure that the liquid sets and can be handled as a solid as soon as possible after it has been dispensed.

It is particularly desirable that liquids which set at ambient temperature and which contain particulate material can be handled as solids as soon as possible after dispensing them. Bulk liquids containing particulate material can be kept homogeneous by stirring, but as soon as stirring stops the particles tend to sediment and form agglomerations. Thus it is desirable that liquids which set at ambient temperature and which contain particulate material are dispensed as liquids with the particles distributed homogeneously in the liquid and that as soon as possible after the liquid has been dispensed it sets to trap the particles before sedimentation occurs, so producing a homogeneous distribution of the particles in the set mass.

By thermally reversible sol-gel system materials which set at ambient temperature are meant liquids which contain a setting or gelling agent and which at ambient temperature are set to form a jelly-like mass but which at higher temperatures flow like a liquid.

The gelling agent may be a natural polymer such as gelatin or agar-agar. The temperature at which the liquid begins to flow depends on the gelling agent present and on its concentration in the liquid. Also a liquid which contains a setting agent experience a gradual transformation from the liquid state to the set state and vice versa. Most liquids having therein a setting agent of a particular type at a particular concentration do no have a very clearly defined setting temperature because to some extent the setting temperature depends on whether the temperature of the liquid is being raised or lowered and on how quickly this change in temperature is effected. This is because the liquid has to some extent a memory and also because the setting process releases latent energy in the form of heat which delays cooling when the liquid sets.Thus it is very difficult to predict the behaviour of a liquid near its setting point as a very small change in temperature can cause a free-flowing liquid to change into a firm jelly.

Furthermore, the temperature at which this change occurs will depend on the composition of the liquid, e.g. the grade of gelatin used therein, and on the degree of stirring to which it is subjected, shouid it have thixo-tropic properties. There is thus a band of temperatures anywhere wherein the setting-point may be expected to lie. This makes it impracticable to dispense into a storage vessel liquid at a predetermined temperature at or only just above the setting point, because it is too difficult to define the setting temperature which is to be used as a datum and in relation to which to control the temperature of the liquid.

The viscosity of a liquid which sets at room temperature when fully liquid gradually increases as the temperature is lowered and then it suddenly increases, reaching infinity as the liquid sets. This sudden increase in viscosity is when the liquid is in the partially set state. The temperature range over which the viscosity suddenly increases, that is to say, from when the liquid is in the fully liquid state to when it is set, is very small and may be less than 1"C.

Moreover the actual temperature at which this sudden increase in viscosity occurs is indeterminate for the reasons just set forth.

We have discovered a method and apparatus for dispensing liquid which sets at ambient temperature.

According to the present invention there is provided a method of dispensing a thermally reversible sol-gel system material which sets at ambient temperature which comprises selecting a viscosity of the material above the transition point as hereinafter defined, adding or subtracting heat to or from the material to obtain the selected viscosity and dispensing the material only while it is at the desired viscosity.

The choice of a suitable viscosity at which to dispense the material does not require a detailed knowiedge of the characteristics of the material.

Figure A shows the viscosity of gelatin/water solutions of various gelatin concentrations (expressed in percentages by weight) plotted against solution temperature. It will be see that as the solution temperature is reduced the viscosity of any such solution increases slowly at first and then very steeply as the setting temperature of that solution is approached. The setting temperatures for the different solutions shown vary over the range 20 to 30"C approximately, but for the grade of gelatin to which Figure A relates all the solutions of concentration 8% or above attain viscosities above 100 Centipoises before they set. It would be appropriate, therefore, to control the viscosity of any of these solutions to a value of 125 Centipoises during dispensing.

If it is required to minimise the viscosity at which dispensing is carried out, a preliminary test may be needed to determine the viscosity at which the transition occurs from the fully liquid state to the viscous state associated with the onset of setting.

Figure B shows the data of Figure A replotted as logarithms of viscosity (in Centipoises) against 1000 + Absolute Temperature (Kelvin) of the solution.

Plotted in this way, the characteristic for each solution approximates two straight lines: a solid line representing the fully liquid condition and a broken line representing the viscous condition associated with the onset of setting. The intersection of these two lines will hereinafter be referred to as the "transition point".

The method therefore comprises dispensing the material at a viscosity adjusted to a value above the transition point as described with reference to Figure B.

The transition point may be determined experimentally by measuring the viscosity of the material at a minimum of four different temperatures, two of which correspond to the fully liquid state and two of which correspond to the viscous state associated with the onset of setting.

Preferably the material is dispensed at a viscosity twice that corresponding to the transition point Thus in spite of the fact that the viscosity varies greatly with change in temperature when the viscous material is in the partially set state it has been found possible to choose a viscosity of the material when it is in the partially set state and by altering the heat content, i.e. by slightly cooling or slightly heating the material while in a holding vessel, to control the viscosity of the bulk material to the required viscosity. It is not possible to dispense the material at a controlled temperature because the viscosity changes too rapidly if it is attempted to control only the temperature of the material.Thus if it is attempted to dispense the material at a controlled temperature without attempting to control the viscosity of the material sometimes the liquid being dispensed will be too liquid and othertimes itwill be almost set and thus controlled dispensing is not possible.

The method of the present invention is of particular use for dispensing controlled amounts of material which sets at ambient temperature and in this case a metering pump can be used to dispense a predetermined volume of material, the dispensing taking place only when the material is at the desired viscosity.

The method of the invention is of particular use when the material which sets at ambient temperature is a liquid which contains as the setting or gelling agent gelatin. The method is of especial use when the gelatin solution contains a particulate material. An example of such a solution is a gelatino silver halide emulsion as used to coat photographic material base to form silver halide sensitised material. Gelatino silver halide emulsions are prepared and coated as liquids but are usually stored as gelled solids. The silver halide crystals are heavy particles and very quickly sediment and form agglomeration in a gelatin solution unless the solution is continuously stirred.However when a gelatin solution which contains a homogeneous dispersion of the silver halide crystals therein set quickly a gelled mass is formed in which the silver halide crystals are still homogeneously dispersed. If sedimentation or agglomeration of the silver halide crystals has occured it is often very difficult to reform a homogenous distribution of the crystals in the gelatin solution when the gelled mass is heated to reform a gelatin solution. If a silver halide gelatin solution in which the silver halide crystals are not homogenously dispersed is coated on base the coated material could not be used as photographic material because it would exhibit too many sensitometric faults.

According to another aspect of the present invention there is provided an apparatus for dispensing controlled amounts of a thermally reversible sol-gel system material which sets at ambient temperature which comprises a material holding vessel, means to exchange heat with the material in the material holding vessel, a metering pump connected to the vessel and which leads to a liquid dispensing means, and means to determine the viscosity of the material leaving the holding vessel together with means to allow only material which is at a desired viscosity to be fed to the liquid dispensing means.

Preferably the metering pump is a continuous positive displacement rotary pump, for example a gear pump.

Preferably the material holding vessel comprises a scraper means which continuously or periodically scrapes the sides of the vessel to prevent any material setting on the sides of the vessel.

When the material to be dispensed by the apparatus comprises particulate material it is specially desirable that the vessel comprises additional stirrer means to ensure that the particles are homogenously dispersed throughout the material.

In one embodiment of the invention the viscosity of the material is determined by pumping the material through a Hagan-Poisseuille restriction tube. This causes a pressure build-up which is directly related to the viscosity of the material being pumped through the restriction tube. This pressure build-up is recorded by a pressure transducer.

Preliminary experiments determine the requisite pressure build-up which is equivalent to the required viscosity.

According to a preferred embodiment of the present invention there is provided an apparatus for dispensing controlled amounts of a thermally reversible sol-gel system material which sets at ambient temperature which comprises a material holding vessel, means to pass heat into or out of the material in said holding vessel, an outlet therefrom connected to a continuous positive displacement metering pump the outlet of which is connected to a Hagan-Poisseuille restriction tube, the outlet end of which is connected to a bifurcated pipe, one of which branches leads back to the holding vessel via a valve and the other of which branches leads to a liquid dispensing means via a valve, there being present a pressure transducer located between the restriction tube and the metering pump, the output from the transducer being fed to a heat input control means which acts to adjust the heat content of the material in the material holding vessel so as to maintain the pressure registered by the transducer to a preset value, means to record the revolutions made by the metering pump when the valve leading to the dispensing means is open, means to close the valve leading to the dispensing means when a preset number of such revolutions has been recorded and means to open the valve in the branch leading back to the holding vessel whenever the valve in the branch leading to the dispensing means is closed.

The apparatus of the present invention is of particular use in dispensing aqueous gelatin solutions of silver halide emulsions. When the apparatus is so used it is preferred that both a scraper and a stirrer are present in the holding vessel.

In one embodiment the gelatin solutions are dispensed by the dispensing means into storage containers, a predetermined volume being dispensed into each container. The viscosity of the aqueous gelatin solution is chosen so that the solution sets in a very short time after entering the container.

The accompanying drawing will serve to illustrate both the method of the present invention and the apparatus of the present invention.

Figure C is a flow diagram of an apparatus according to the present invention.

The apparatus comprises a solution holding vessel 1 which is enclosed in a jacket 2 through which cold or hot water can be circulated from a water storage container 3.

Present in the holding vessel 1 is a scraper 4 which also acts as a solution stirrer.

At the bottom of the vessel 1 is an outlet pipe 6, in which is set a valve 7, leading to a positive displacement metering pump 8. The pump 8 is connected to a T-piece 9 by a Hagan-Poisseuille restriction tube 10. The upper pipe branch 12 connected to the T-piece 9 leads back into the vessel 1 via a valve 13.

A lower pipe branch 14 connected to the T-piece 9 leads via a valve 15 to a liquid dispenser nozzle 16.

Present between the pump 8 and the restriction tube 10 is a pressure transducer 17. Signals from the pressure transducer 17 are fed to a water temperature controller 19 which acts to alter the temperature of the water in the water storage container 3.

Valves 13 and 15 are fitted with actuators and are linked together mechanically so that one valve is opened when the other is closed. Pump 8 is fitted with a device to count shaft revolutions, each revolution being equivalent to a certain volume of liquid.

The temperature controller 19 into which the signal from the pressure transducer 17'is fed has a scale with two pointers. One pointer displays the measured pressure and the other indicates the set-point chosen. The pressure scale can be calibrated in terms of viscosity: thus the liquid to be dispensed can be maintained at any chosen viscosity by positioning the set-point indicator.

The output from the pressure transducer 17 is also used to provide interlocks such that valve 15 cannot be opened until the viscosity of the liquid corresponds to the pre-set value on the controller 19.

If, however, valves 13 and 15 are not mechanically interlinked, then means are provided to ensure that whilst the pump 8 is in operation whenever valve 15 is closed valve 13 is opened, but as soon as valve 15 is opened valve 13 is closed.

Before commencing the liquid dispensing operation the liquid is tested to ascertain a suitable viscosity above the transition point. This viscosity figure is pre-set on the dial of the controller 19. The liquid is then poured into the vessel 1 from a liquid store where it was maintained at a temperature above the setting point. The scraper/stirrer 4 is started. Valve 7 is then opened and the pump 8 is started. The pre-set revolution counter on the pump 8 is set to deliver the requisite volume of liquid. A container for the liquid is placed under the nozzle 16.

As it is unlikely that the liquid will be at the predetermined viscosity valve 15 will remain shut but valve 13 will be opened so that the liquid will be recirculated via pipe 12.

The transducer 17 will register the pressure and depending on the viscosity will control the temperature of the water in the water container 3 which is circulated in the jacket 2 enclosing the vessel 1. As soon as the liquid passing through the restriction tube 10 reaches the required viscosity this fact will be communicated from the transducer 17 to the valve 15 which will open and simultaneously the revolution counter on the pump 8 will come into operation.

The output from the high resolution counter, e.g.

100 counts per revolution, on the pump 8 is fed to a batch counter upon which the desired number of counts is set. Upon receipt of a start signal, the batch counterwill cause valve 15to open, valve 13to close, and will commence counting pump revolutions. When the total count is equal to the set count, then the batch counter will cause valve 15 to close, valve 13 to open and will reset to await the next start signal. This operation will continue until eitherthe vessel is drained of liquid or a sufficient number of containers has been filled with the liquid.

Lower pipe branch 14 is surrounded with a heating jacket 21. This is to keep the liquid passing therethrough at a temperature just above its setting point to prevent setting in this branch when no liquid dispensing is taking place.

It is important that the vessel 1 and all the pipe connections and the pump are drained and washed out after use because the whole system would become blocked with the gelled liquid if it were allowed to stand and cool down.

Preferably all pipework and components are wound with an electrically resistive tape through which a current is passed to provide a power dissipation slightly exceeding thermal losses. This maintains the product in the pipes and components in a liquid state without materially affecting performance once the liquid is being circulated.

The pressure drop through valves 13 and 15 is chosen to be small compared with the pressure drop through the restriction tube 10 so as to avoid any significant momentary pressure fluctuations when valves 13 and 15 open and close being received by the pressure transducer 17. Preferably the pressure drop through branch 12 and valve 13 should be the same as that through branch 14 and valve 15.

It is a special feature of this apparatus that the choice of set-point on the controller determines the viscosity at which the liquid is maintained. Because of this, the metering pump is always operating under the same conditions of delivery pressure and fluid viscosity for that particular set-point. This ensures that the volume of liquid dispensed for each revolution of the pump is highly reproducible. Thus by use of this apparatus it is possible very accurately to dispense pre-setvolumes of liquid into containers virtually automatically. Furthermore the viscosity of the liquid being dispensed can be so chosen that almost as soon as the liquid has been dispensed into the containers it sets to a gel led mass.

The dispensing of a liquid close to its setting point when its viscosity is high has additional advantages in that problems associated with dispensing lower viscosity liquids, such as splashing, are reduced or eliminated, enabling dispensing rates to be greatly increased.

The apparatus as just illustrated is of particular use in dispensing pre-set volumes of aqueous gelatin silver halide emulsions into containers which set very shortly after being received in the containers and well before any sedimentation or agglomeration of the silver halide crystals in the liquid has occurred.

Claims

1. A method of dispensing a thermally reversible sol-gel system material which sets at ambient temperature which comprises selecting a viscosity of the material above the transition point as hereinbefore defined, adding or subtracting heat to or from the material to obtain the selected viscosity and dispensing the material only while it is at the desired viscosity.

2. A method according to claim 1 wherein controlled amounts of liquid are dispensed using a metering pump.

3. A method according to claim 1 or claim 2 wherein the material which sets at ambient temperature contains gelatin as the setting agent.

4. A method according to any one of claims 1 to 3 wherein the material which sets at ambient temperature is a gelatino silver halide coating solution.

5. An apparatus for dispensing controlled amounts of a thermally reversible sol-gel system mterial which sets at ambient temperature which comprises a material holding vessel, means to exchange heat with the material in the material holding vessel, a metering pump connected to the vessel and which leads to a liquid dispensing means, and means to determine the viscosity of the material leaving the holding vessel together with means to allow only material which is at a desired viscosity to be fed to the liquid dispensing means.

6. An apparatus according to claim 5 wherein the metering pump is a continuous positive displace ment rotary pump.

7. An apparatus for dispensing controlled amounts of a thermally reversible sol-gel system material which sets at ambient temperature which comprises a material holding vessel, means to pass heat into or out of the material in said holding vessel, an outlet therefrom connected to a continuous positive displacement metering pump the outlet of which is connected to a Hagan-Poisseuille restriction tube, the outlet end of which is connected to a bifurcated pipe, one of which branches leads back to the holding vessel via a valve and the other of which branches leads to a liquid dispensing means via a valve, there being present a pressure transducer located between the restriction tube and the meter ing pump, the output from the transducer being fed to a heat input/output control means which acts to adjust the heat content of the material in the material holding vessel so as to maintain the pressure registered by the transducer to a pre-setfigure, together with means to allow the dispensing means valve to open only when the pressure exceeds the pre-set figure, means to record the revolutions me by the metering pump when the valve leading to the dispensing means is open, means to close the valve leading to the dispensing means when a pre-set number of such revolutions has been recorded and means to open the valve in the branch leading back to the holding vessel whenever the valve in the branch leading to the dispensing means is closed.

8. An apparatus according to any one of claims 5 to 7 wherein the material holding vessel comprises a scraper means.

9. An apparatus according to any one of claims 5 to 8 wherein the material holding vessel comprises a liquid stirrer means.

10. An apparatus for dispensing controlled amounts of a viscous material which sets at ambient temperature substantially as hereinbefore described with reference to the accompanying drawings.