US3082144A

US3082144A - Extrusion coating under reduced pressure

Info

Publication number: US3082144A
Application number: US695341A
Authority: US
Inventors: Haley Floyd Cresswell
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1957-11-08
Filing date: 1957-11-08
Publication date: 1963-03-19
Anticipated expiration: 1980-03-19
Also published as: FR1227231A; GB855216A; BE572753A

Description

March 19, 1963 F. c. HALEY I I xx'rausxou comm; UNDER REDUCED PRESSURE Filed'Nov. 8', 195? 3 Sheets-Sheat- 1 Ill/I/l/j/l/II/l/l/I/l F GQI STATIC EQUILIBRIUM PRESSURE CURVE B OCCLUSION PRESSURE APPARENT EQUILIBRIUM PRESSURE 2400 FIG.2

. 3: 22. mmnmmwt E382 COATING SPEED FTJMIN) INVENTOR FLOYD CRESSWELL HALEY BY 6M ATTORNEY F. c. HALEY 3,082,144 EXTRUSION Comma UNDER REDUCED PRESSURE Filed Nov. 8, 1957 s Sheets-Sheet 2 STATIC EQUILIBRIUM PRESSURE CURVE A 31 22v I wmzmmmma mks 5mm COATING SPEED (FT./ MIN) INVENTOR FLOYD CRESSWELL HALEY ATTORNEY 3,082,144 mcrausron coumcuuom amucm PRESSURE c. HALEY March 19, 1963 s Sheets-Sheefi'ii Filed we. 1957 FIG. 5

IF J TEMPERATURE (F) I Q mvsmox FLOYD CRESSWELL HALEY BY W ATTORNEY Patented Mar. 19, 1963 3,082,144 EXTRUSION COATING UNDER REDUCED PRESSURE Floyd Cresswell Haley, Middletown, N.J., assignorto E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Nov. 8, 1957, Ser. No. 695,341 Claims. (Cl. 156-444) This invention relates to a process of coating and drymg viscous fluids on a flexible web. More particularly, it relates to a process of applying a coating of a viscous fluid onto a continuous, moving flexible web at reduced pressures and at high speeds. Still more particularly, this invention relates to a process of coating aqueous water-permeable colloid silver halide dispersions onto base materials such as films, foils and papers as layers in photographic elements.

Various processes of coating viscous fluids and particularly viscous, aqueous, photographic emulsions onto continuous, flexible webs have been proposed. Among these processes are those wherein the coating and drying chambers are at a reduced pressure. Early attempts to coat and dry such materials at reduced pressures were carried out at moderate pressures in the' range of 10 to inches of mercury absolute. At such pressures, however, it was found that high coating speeds could not be -maintained and were generally limited up to about 100 or 120 feet per minute. 1

One of the principal reasons why higher coating speeds could not be obtained to produce a coated film of satisfactory quality was due to the occurrence of gas occlusion,

1 sometimes called air draw-under, between the flexible web and the fluid material being coated onto the web. Air or other non-condensable gas drawn into the nip formed by the flexible web support and the material being coated was entrapped as objectionable bubbles. This was par-' ticularly true and particularly objectionable as coating speeds were increased. 1

A recent attempt to overcome this problem is disclosed in Johns U.S. Patent No. 2,795,522. Johns teaches the introduction of a condens'able vapor into a reduced pressure coating atmosphere, so that entrapment of any ambient vapor will not result in on-condensable bubbles, but rather any vapor which-may be entrapped between the web and the coated layer will condense in an unobjectionable manner and not remain in the form of bubbles. in practicing the process taught by the Johns patent, it has been found that some difficulty may be experienced in, maintaining a completely saturated, instantaneously condensable atmosphere which is free from non-condenthe coating zone had to be above the equilibrium pressure of the material being coated. In other words, the

' total pressure in the coating zone could not be permitted to fall below the pressure at which the viscous fluid being coated would boil at the coating temperature. It is well known that, when a thin film is placed in a vacuum chamber and the total pressure within the chamber reduced until the total pressure reaches the static equilibrium pressure (referred to as static since the thin film is not moving), the thin film of material will commence to bubble and boil violently, obviously ruining the quality of any product and making the article completely unacceptable for use. The static equilibrium pressure, therefore, has heretofore been considered an operable minimum absolute pressure, below which the coating zone pressure should not and could not be reduced.

Viscous coating fluids within the scope of this invention which have been coated onto flexible webs according to methods known in the prior art, have been dried chilled and then exposed to high velocity air which has sable gasses in the immediate vicinity of the coating station, and at the same time preventing the formation on the coated product of droplets of liquid resulting from condensation of the particular vapor introduced into the coating zone. v

The Johns process is further disclosed in Beck U.S. application Serial No. 375,404, filed August 20, 1953 (US. Patent 2,815,307, December 3, 1957), wherein there is-described process and apparatus for coating viscous solutions and dispersions as layers in photographic elements and drying such layers, said coating and drying taking place at reduced pressures.

In another proposed process, described in Beguin U.S. Patent No. 2,681,294, an attempt has been made to reduce air draw-under by applying adjacent the coating device a pressure differential across the material being coated on the moving web. a

In the prior art coating processes it has been generally accepted that, although reduced pressures in the coating zone will reduce or eliminate objectionable air drawunder and air bubble entrapment, the total pressure in previously been conditioned to supply the necessary heat for vaporization and for removing the carrying medium,

'e.g., the solvent, from the coated material. The aforementioned Beck U.S. Patent No. 2,815,307 discloses the application of external heat, as by electrical platens, to provide sutficient heat of vaporization to remove the solvent from the newly coated material, the drying proc ess also taking place at greatly reduced pressures.

' An object of this invention is to provide a novel process of coating viscous fluids, and in particular photo-t graphic materials, at speeds heretofore unattainable and at pressures below the static equilibrium pressure. Another object is to provide such a method wherein the applied coatings are of uniform thickness and free from entrapped bubbles betweenthe coatingand the supporting web. Still another object is to provide such a method wherein extremely thin coatings of high quality can be made. Yet another object is to provide a method of coating viscous water-permeable colloid dispersions and especially aqueous colloid silver halide dispersions'onto continuous film or paper at high rates of speed. An additional object is to provide such a method which can be carried out in a relatively simple apparatus.

A further object of this invention is to provide a process for applying a viscous fluid by extrusion as a coating onto a flexible web whereby the extruded material is chilled and set without the application of external refrig' eration. A still further object isto provide a process for coating and drying a viscous fluid without the application of external heat. Yet a further object is to provide a drying process wherein the carrying medium in a newly applied coating of a viscous fluid is removed from the coated layer as a liquid without the necessity of vaporization. Another object is to provide a process which permits drying films at low temperatures and also low pressures without the necessity of having different pressures in the coating and drying areas which can only be achieved by a two-chamber system separated by vacuum sealing arrangements. A particular object is to provide a continuous coating and drying process to produce commercially acceptable coated film elements at speeds up to 2,400 feet a minute and even higher. Still other objects will be apparent from the following description of the invention;

I have found, surprisingly, that with increased coating speed in carrying out reduced pressure coating of the general type described above, the coating material upon extrusion does not boil at pressures considerably below the static equilibrium pressure. .In other words, there is an apparent equilibrium pressure lower than the static equilibrium pressure, and the apparent or actual equilibrium pressure decreaseswith increasing coating speed. Still more surprisingly, it has been observed, when applying a fluid coating material onto a moving flexible web in a low pressure atmosphere below the static equilibrium pressure of the coating material, that the coating material sets without external cooling promptly after the extruded material effects contact with the web. The solvent and/ or other carrying medium in the material separates from the solute and/or other solids of the extrudate and appears as a clear supernatant liquid on the surface of the layer of coated material. Without further treatment, the surface of the layer of coated material is extremely smooth, uniform and resistant to abrasion.

-It has furthermore been observed, following coating according to this invention at a pressure below the static equilibrium pressure, that if the coated web is maintained for an extremely short time in the low pressure coating atmosphere below the equilibrium pressure, the supernatant liquid vaporizes from the surface of the web and the coated layer thereby dries in a very few seconds.

The present invention in its broadest aspects therefore comprises the web coating process of extruding in a coating zone a coating material in the form of a fluid (which can be a solution or a dispersion in a solvent or other suitable carrying medium) of high viscosity through a narrow orifice onto a-moving web traveling at a higher linear speed than the fluid being extruded, while the coating zone is at an absolute pressure above the apparent equilibrium pressure of the coating material at the speed of the moving web and below the static'equilibrium pressure of the coating material and furthermore below the air occlusion pressure, i.e., the pressure at which air drawunder occurs, said coating material at extrusion being at a temperature above the set point of the coating material and below a temperature at which damage due to heat occurs to the coating material, and removing the carrying medium from the coated material.

In its broadest aspect, the invention can be restated as being the process of extruding a high viscosity fluid through a narrow orifice in the form of a thin sheet onto a moving web traveling at a higher linear speed than the fluid being extruded, at a temperature above the set point of the fluid and below a temperature at which the coating material would be damaged due to heat, in a low pressure atmosphere and at a coating speed, said atmosphere and speed being within the limits indicated by the shaded area between curve A and curve B of FIGURE 2 of the drawing, and removing the carrying medium from the coated material.

As used herein, the expression carrying medium" of the coating material is intended to mean the solvent, when the coating material is in the form of a solution. The expression is also intended to encompass the fluid medium in which particulate solids are dispersed or suspended in a dispersion or suspension or similar system. Combinations of two or more carrying mediums can of course be used in the same coating material. The preferred carrying medium is water.

By the expression high viscosity" as used herein is meant a viscosity of the coating material in the range from 1,000 to 100,000 centipoises, and preferably in the range from 4,000 to 10,000 centipoises. In the extrusion of dispersions, particular advantages have been found to be obtained when the dispersion'has a solids content in the range from to 65% by weight, and preferably 48 to 55%. In some types of dispersions it is possible to obtain the required viscosity at lower or higher solids contents. With such dispersions the critical factor is the viscosity.

In a preferred aspect of the invention, a viscous aqueous dispersion of light-sensitive silver halides in a waterpermeable, natural or synthetic colloid, at a temperature above the set point (solidification temperature), and preferably at least about 8 F. above the set point, of the dispersion and below a temperature at which the lightsensitive silver halides would be damaged by heat (maximum of about 150 F.), is extruded in a coating zone in the form of a thin sheet onto a continuous flexible web, e.g., a thin film base or paper that is traveling at a linear speed from 2 to 200 times faster than the speed of extrusion of said dispersion, said dispersion having a viscosity in the range from 1.000 to 100,000 centipoises, and preferably 4,000 to 10,000 eentipoises, and a solids content in the range from 20 to 65%, and preferably from 48 to 55%, while maintaining the atmosphere in the coating zone at an absolute pressure above the apparent equilibrium pressure of the dispersion at the speed of coating and at an absolute pressure below the static equilibrium pressure of the dispersion at that temperature and furthermore below the pressure at which air draw-under occurs, and effecting removal of water from the newly coated sheet.

It has. been found unexpectedly that carrying out the above described process of this invention eliminates the necessity for introducing into the coating atmosphere any critical amount or kind of vapor, since the presence of air in the coating atmosphere at pressures below the static equilibrium pressure has been found to be unobjectionable.

In practicing this invention, the material to be coated is prepared at the preselected viscosity and temperature by any suitable procedure. Photographic materials (lightsensitive or not) to be coated can be prepared in dilute emulsion form in a conventional manner. Outgassing, concentration to the preselected viscosity and to the desired solids content where appropriate, and temperature adjustment of the emulsion can be readily effected, for example, in apparatus which is described in Haley U.S. application Serial No. 525,570, filed August 1, 1955 (Patent No. 2,866,499, December 30, 1958). Suitable coating aids and other additives can be admixed with the emulsion, as will be understood in the art, which is then ready to be extruded under the conditions according to this invention. I

The effect of extruding according to the present invention at a pressure below the static equilibrium pressure but above the apparent or actual equilibrium pressure can be illustrated by the following example. Typically, it may be desired to extrude a photographic emulsion from a hopper which is heated to about 105. F. At this temperature, an aqueous emulsion normally boils at pressures below the static equilibrium pressure, that is to say, below about 58 millimeters of mercury absolute. However, according to this invention, when the pressure is reduced to about 33 millimeters below the static equilibrium pressure (that is to say reduced to a pressure of about 25 millimeters of mercury absolute), it is unexpectedly observed that no boiling occurs if the web speed (the speed of the base material) is maintained at about 400 feet per minute or higher. By way of comparison, a thin film sample of the same material placed in an open container and held at F. in the same coating chamber boils violently. As previously indicated, boiling does not occur when coating according to the process of this invention.

Upon extrusion and draw-down of the materials of high viscosities suitable for coating according to the processes of this invention, it is observed that rapid cooling of the free unsupported extruded sheet of coating material (being drawn down prior to contact of the coating material with the flexible web) takes place. This is particularly unexpected since a working (stretching) of plastic materials is generally accompanied by a rise in temperature. The rapid cooling is believed to be essentially independent of the ambient temperature. Therefore, the cooling is not due to rapid evaporation of the solvent or other carrying medium as might be expected. Temperature decreases of as much as 40 F. have been observed when the draw-down ratio is about 10 to 1.

ing medium in a very few seconds.

tional means.

When-the temperature of the material being extruded is,

for example, about F. above the set-point, a tem-' perature decrease of F. upon extrusion effects a net cooling of 30 F. belowthe set point or solidification temperature of .the coating material. of the material is not instantaneous but requires a small amount of time even' at this temperature, 'the material does-not setup until it has'contacted the support.

Although I do not wish to'be. limited by the following explanation, there is some reason tobelie've that the material upon extrusion is supercooled'in'the sense that Because the setting and (2). the path which the coated article, i.e. the web it remainsa liquid or'fluid at a temperature well below I that at which-it would normally solidify until contact is madewith the flexible web.- Under these conditions, it is believed that the flexible web furnishes a surface on which the solid components of the coating material solidify in much the same manner that a seed crystal causes rapid crystallization from a'supersaturated solution.

As stated above, following the effecting of contact of the self-supporting extruded sheet of coatingmaterial with the flexible web underthe conditions of the proeessof this inventiongthere is an almost immediate separation of the coating material into a solid layer and a supernatant liquid layer on the surface of the solid layer. ,At the same time, it has been observed that partial vaporization of the carrying medium will take place. At this point in the process, the newly applied coating material is ready to be dried, -i.e.,' removal'of the carrying medium can be effected. j 1

In general, thecarrying medium, e.g., the-solvent such as water, can be removed'by any one of several methods whichjwill now be described, as well as any other suitable method or combination of methods.

.(I) It has been found that merely maintaining the newly coated web in the low pressure atmosphere below 'the static'equilibriu'm pressure for a very few seconds will effect the removal of the carrying medium by vaporization from the surface of the web. This result is believed to stem partially from the sensible heat in the flexible web and partially from the heat supplied'upon setting (heat of setting). A period of up to 5 seconds is sufficient for most coating materials, and 1 to 3 seconds is preferred.

(2) Alternatively, or incombination with the just-mentioned method of drying the coating material, there can be used a novel process, heretofore impossible, which accomplishes the removalof excess-solvent or other carry- According to this procedure,-the solvent can be removed by mechanical means, e.g., wi'ped or squeegeed from thesurface of the coated material by sponges on rotating rolls; air-knif'ed from the surface by apparatus described for example in US. Patent No. 2,139,628, removed by a simple vacuum device, or-the like.

(3) The coated material can also be dried by-conventhe aforementioned- Beck US. patent. In a preferred embodimentof this'in'vention the external application of heat'as-by electrical platens of conventional drying meth ods (3) is combined with the drying process -(1 )wherein the coated Web-is maintained for a few seconds below thestatie equilibrium pressure. In a drying process utilizing electrical heaters; the wattage supplied to the heaters will he obviously selected to provide sufficient heat to evaporate the solvent which will be removed at a temperature corresponding to the pressure selected. The

effective heater area should be. sufficient to handle this wattage without reaching an objectionabletemperature as will be obvious'kto those skilled in theart.

- In carrying out the process according tothis invention,

. it has been found that, with increasing coating speeds,

certain other variable process, conditions also increase One'sueh suitable means is described in fined critical ranges described below.

The invention will be further understood by reference to the accompanying drawings which form a part of this application. Referring now to'the drawings:

FIG. l is a schematic representation of a suitable apparatus for carrying out this invention.

:FIG. 2 is a graph showing the coating pressure-web speed relationship necessary according to the present invention. FIG. 3 is a detailed schematic view illustrating the coating operation according to this invention, with parts greatly enlarged for purposes of-clarity and not necessarily drawn to scale;

FIG. 4 is a graph similar to that ofFIG. 2 illustrating the effect of a variation of process conditions; and

FIG., 5 is a composite graph showing the relationship between the viscosity, temperature and solids content for particular coating fluids which can be coated satisfactorily according to this invention.

Referring now to thedrawings, a suitable apparatus is shown schematically in FIG. 1 wherein 1 is a chamber into which a web of paper or film 2 passes through inlet or feed rolls 3, downwardly and around guide rollers 4 and 5,.and over coating roller 6 which constitutes the point of coating, illustrated in greater detail in FIG. 3.

'Afterreceiving a layer of coating material from extrusion hopper 7, the coated web passes overadjustable roller 8, thence around

guide rollers

9,, 10 and 11, and travels outwardly from chamber 1 through .exit or outlet rolls 12 Any or all of the web guiding rollers-including the coating roller 6 and the adjustable roller 8 can be driven or. idler rolls as desired. An alternate apparatus can be'used, of course, in which the web supply roll and take-up roll are in the vacuum chamber and new web supplies are introduced and coated stock removed through vacuum locks.

Extrusion hopper 7 can be supported by any suitable means (not shown) and can conveniently be connected,- as by. means of a suitable pipe.l3,,to a continuous source or supply (not shown) of coating material, as will bereadily understood. The temperature of the extrusion,

hopper can be controlled by any suitable means, such as internal channels or ducts positioned asdesired within the device. adjacent the extrusion lips and/or elsewhere throughoutthe body of the hopper, and connected to a'suitable source of heating or cooling gas or liquid medium.- Alternatively, electric resistance heaters can be located in the hopper and lip structures. The wall of the chamber can be conveniently provided with a window so-that the coating operation can be observed.

. Opening 14 in the chamber 1 is connected by any conin criticality; Two such factors include (1)."t he'path the self supporting' extruded sheet of coating material takes as itjeffectscontact with the flexible web support,-

'venient meansto a v'acuum pump or aspirator (not shown) for reducing the pressure within the chamber.

Referring now to FIG. 2 of the drawings, the horizontal dotted line represents the static equilibrium pressure of the coating material at the coating temperature, conveniently expressed in millimeters of-mercury. It is this level of pressure which has heretofore been considered the absolute minimum below which coating could not be satisfactorily accomplished. The static equilibrium pressure will not be the same for all materials being coated, and will'varywith the ingredients of the coatingmaterial, e.g., the solvent, type of solids, etc., as

well as the temperature of the material. The static equilibrium pressure can be conveniently found for a given set ofconditions (emulsion or solution type, percent solids,

viscosity, temperature, etc.) by observing the pressure at which a thin film of the coating material will begin to boil when placed in a vacuum chamber and the pressure gradually reduced.

As explained above, I have unexpectedly found that, with increased coating speeds, the actual or apparent equilibrium pressure for a given coating material gradually decreases with increasing coating speeds, as illustrated by curve A. Below curve A is an area of pressure and speed at which boiling will occur, but 'at pressures and speeds above curve A boiling of the coating material does not occur.

The gas occlusion or air draw-under pressure has likewise been found to vary with the coating speed. As can be seen from FIG. 2, the occlusion pressure decreases below the static equilibrium pressure at a gradual rate, much less than the rate of decrease of the apparent equilibrium pressure. Above curve B, gas occlusion or air draw-under will take place, while below curve B but 7 ing speeds at more or less indefinitely rapid coating speeds.

Referring now to FIG. 3 of the drawing, the extrusion hopper 7 is shown extruding a thin film of coating material 15, which moves in a self-supporting manner until it effects contact with the oncoming uncoated web 2 as the latter passes around coating roller 6. Layers 2 and 15 move into laminar contact with each other, and readily adhere to each other with or without the aid of interposed adhesives as will be understood in the art. The coated article moves onward and around adjustable roller 8, as more clearly shown in FIG. 1.

In FIG. 3 the dotted line 16 shows the path the free self-supporting layer 15 would take as it is being extruded onto the moving web 2 if tension were the only force acting on the material upon extrusion. However, the force due to the adhesion between layer 15 and web 2 causes the point of contact of the coating material with the web to be positioned away from point of tangency 17 at a position where the force of adhesion equals the force resulting from tension. The presence of any other forces acting to affect the forces of adhesion and tension will upset the balance, effecting the movement of the point of contact forward toward the point of tangency 17 and thus allow gas from the coating atmosphere to be trapped between layer 15 and layer 2.

It will therefore be understood that it is desirable to control the point of first contact between the coating material and the web support so that the contact occurs at some point prior to; tangential point 17. This condition can be accomplished in several ways, controlled either separately or simultaneously.

For example, the pressure buildup in the nip of the layers 15 and 2 can be controlled at a minimum by rerucing the density of the atmosphere surrounding the nip to a level as low as operably possible... Thus, operating within the satisfactory coating region of the shaded area of FIG. 2 of the drawing, a direction of travel of self-supporting film 15 more further removed from the tangential will be obtained at pressures closely adjacent to curve A.

A second way of maintaining the non-tangential relationship of the coating layer at the coating roller can be accomplished by positioning the lips .or extrusion orifice of the hopper as close to the coating roller 6 as possible. The actual distance which the self-supporting film 15 will travel from the extrusion orifice to the point of contact with the flexible web 2 will depend on the structural limitations of the coating apparatus.- It has been found that particularly advantageous results are obtained when this distance is less than about one-eighth of an inch.

A third way of maintaining the aforementioned relationship is by elimination of centrifugal force acting on the material being coated. Thus, referring to FIG. 3, it can be seen that the direction at which the outrunning coated article leaves the coating roller 6 can be effective ly adjusted and controlled by positioning adjustable roller 8 about a pivot as desired. This has been found to be particularly essential at high coating speeds on the order of 600 or 700 feet per minute and higher in order to overcome the adverse effects of centrifugal force acting on the newly applied coating material. However, the effect of the angle can be demonstrated at speeds about feet per minute. The angle of adjustment of the adjustable roller 8 can be conveniently described as angle theta having its apex at tangential point 17, as illustrated in the drawing.

The-undesirable effects of the centrifugal force can be reduced and even eliminated by maintaining the angle theta within a preselected range. Since the criticality of this variable increases gradually with increasing coating speed and will vary depending on the rheological and other properties of the coating material, it"will be understood that limits on the angle theta are not intended to to be precise but rather are intended only to provide guidance for the persons in the art within which optimum operating conditions for any given material can be readily ascertained. 1

With this in mind, therefore, it can be pointed out that' for coating speeds in excess of about 200 feet a minute the angle theta should be maintained preferably within the range from 178 to 184", while at coating speeds in excess of about 400 feet per minute the angle theta should be maintained preferably within the range from 178 to 182, and at coating speeds in excess of about 600 feet per minute, the angle theta should be maintained preferably within the range from 178 to 181.

From the foregoing discussion, it can be seen that, in the process of this invention, extrusion of the self-supporting sheet of coating material is in a plane non-tangential-to the coating roller or equivalent support, and not in the same plane as the outgoing coated products, and not necessarily in a plane parallel to the plane of the outgoing product.

FIG. 4 represents the pressure-speed relationship wherein the coating process takes place utilizing the apparatus shown in FIG 3 except that the angle theta adjustment of the adjustable roller 8 is greater than 184. It will be noted that this improper setting of the angle theta causes curve A and curve B to approach each other with increase in coating speed. It has also been found that this relationship between these two curves A and B, wherein the acceptable coating region is reduced to a negligible amount or completely eliminated, will result when the coating material has properties outside the ranges defined in the appended claims.

Referring now to FIG. 5, the composite graph shows the relationship between viscosity, temperature, and solids content for typical suitable coating materials within the scope of this invention. On the graph is plotted the varying viscosity at the indicated temperatures of typical solutions and dispersions of silver halides in aqueous gelatin and synthetic polymer systems of the type described in the examples which appear below.

Specifically, curve C of FIG. 5 represents the viscositytemperature characteristics for an aqueous gelatin solution containing 20% by weight solids. Curve D represents the viscosity-temperature characteristics for an aqueas the latter is extruded through the orifice.

em'ulsions having solids contents'of 55% and 60% by weight, respectively. Curve G represents the viscositytemperature characteristics for an aqueous cyan color- .forming polyvinyl acetal silver halide emulsion of 32% solids content.

Graphsof the type plotted in FIG. can be prepared conveniently for other fluids to'be coated according to this invention. Referring to the graph, the set point or solidification temperature is indicated as the point at which the slope of the curve approaches infinity. This is shown, for example, with respect to curve D as the two vertical' dotted lines indicating that the setpoint for a particular 45% solids aqueous gelatino silverhalide photo'graphic emulsion is about 95 F.

From the above disclosure, it is apparent that the pressure in the coating'zone can be varied over a fairly wide range for any given coating speed, and likewise the coat- .ing speed can be varied over a fairly wide range for any 1 out the process of this invention extends from about 40 millimeters of mercury absolute at 100 feet per minute to slightly below 20 millimeters of mercury at speeds in ex-' cess of 500 feet per minute. Atan intermediate pressure of about 35 millimeters of mercury absolute, excellent coating quality without objectionable draw-under of air curve and the air draw-under ourve (curves 'A and B of FIG. 2) will generally follow the pattern illustrated in FIG. 2.

The determination of these curves for any given coating material can be-easily accomplished and recorded. As an exemplary illustration, assume that an aqueous dispersion of photographic. materials'having a viscosity of 5,600 centipoises at 105 F. and a 50% solids content is to be coated onto a web utilizing apparatus of the type shown in FIG. 1 wherein the extrusion hopper orifice is 0.0l0inch and the web speed is 2,000 feet per minute.

The emulsion supply system, which may conveniently include a metering pump or similar device, and the web speed are paced together so that the web moves at a linear speed of about l0-times that of the coating material The'coating material at the orifice is maintained at about 105 R, which has been determined from the curve of the type shown in FIG. 5 to be about 8 to 10 F. above the set point of the coating material. The total pressure of the atmosphere in the coating chamber is reduced by a vacuum pump or other suitable means. While the web is runningat a low speed, the coating chamber pressure is reduced to about 60 millimeters of mercury absolute, and the coating speed is increased to approximately 200 feet per minute whereupon air draw-under occurs. The pressure is then reduced to about 58 millimeters of mercury absolute (static equilibrium pressure) and air draw-under ceases.

'Consecutively, the coatingspeed is increased to about 380 feet per minute whereupon air draw-under commences, and next the pressure is dropped to about 56 millimeters of mercury absolute at which time air draw- Thus, in order to determine the acceptablerang'e of pressures for a particular given coating material, at various-speeds the total pressure in the coating chamber can be reduced until boiling occurs-and increased until feet per minute and above.

can be obtained over. a range from about 150 to 2,400 It will be understood that once the operable ranges required for obtaining satisfactory coating quality have been determined for a particular coating material, this information can be recorded, and in subsequent coating operations the information can be utilized to preset the pressure conditions at the desired level, and he web brought up to speed immediately.

The process of this invention is applicable to the simultaneous coating of more than one layer onto a supporting web. Suitable apparatus for such multiple coating is described, for example, in Beck U.S. application Serial No. 506,458, filed May 6, 1955 (Patent No. 2,901,770, Sept. '1, 1959). In carrying out such multiple coating, it is important that at least one of the coating materials have a viscosity within the range defined in the appended claims. It is not necessary however, that more than one such layer being coated, or that all of the layers being coated, have characteristics within the limits specified. Actual examples of multiple layer coating are set forth as Examples 8 and 9 below.

The invention is further described but not intended to be limited by the following examples:

Example 1 A light-sensitive aqueous gelatino silver halide photographic emulsion of the medical X-ray type having a viscosity of 6,800 centipoises at the coating temperature of F. and a solids content of 50% by weight was coated onto a gelatin sublayer on a cellulose acetate web, using apparatusof the type shown in FIG. 1. The coating chamber was provided withan extrusion hopper of the type describedin Velvel U.S. Patent No. 2,765,492, t-he orifice of which was adjusted to 0.006 inch. Electrical heating platens of the type described in the aforementioned Beck U.S.-application Serial No. 375,404 were positioned within the coating chamber adjacent the coatface defects. The drying time after coating until the web was wound on a roll satisfactorily was about two seconds." 7 The coated web furthermore was lacking in boiling streaks and objectionable. entrapped bubbles due to air drawunder.

Example 2 Example 1 was repeated except thatt he viscosity of the emulsion was 5,800 centipoises, and the coating-temperature was increased to F. The coating conditions maintained in Example 1 were repeated to a web speed of about 380 feet per minute and a pressure of 30 millimeters of mercury absolute, after which the web speed was increased slowly to 750 feet per minute. Excellent quality coated material was obtained throughout the range of coating speeds from 380 to 750 feet per'minute.

Example 3 Example 1 was repeated except that the emulsion was concentrated to 52% solids by weight and had a viscosity of 11,600 centipoises at 105 F. Extrusion coating took place at a chamber pressure of 47.5 millimeters of mercury absolute and'a web speed of 220 feet per minute.

Example 4 Example 3 was repeated except that the speed of the web was gradually increased from 220 feet per minute to 600 feet per minute and the pressure reduced to 41.5 millimeters ,of mercury absolute. Excellent coating results were obtained throughout this entire range.

Example 5 Example 3 was-repeated except that the coating chamber pressure was reduced to 35 millimeters of mercury absolute as the web speed was increased to 550feet per minute. Excellent coating quality was obtained throughout this entire' range.

Example 6 Example 1 was repeated except that the emulsion had a solids content of about 55% by weight and a viscosity of 21,000 centipoises at 105 F. The extrusion coating took place at a web speed of about 400 feet per minute and the coating chamber pressure was 39 millimeters of mercury absolute. The same excellent results of Example 1 were obtained.

Example 7 A light-senitive aqueous gelatino silver halide photographic emulsion having a viscosity of 5,600 centipoises at 105 F. and a solids content of about 52% by weight, to which a conventional emulsion hardener was added in the amount usually employed in the art for the weight of gelatin in the emulsion, was coated onto a polyethylene terephthalate supporting web made in the manner described in Alles and Saner US. Patent 2,627,088 employing .a vinylidene chloride/eo-methylacrylate/co-itaconic acid copolymer coating which was overcoated with a conventional gelatin sub. The lips of the extrusion hopper were spaced 0.010 inch apart. A coating roll inch in diameter was used. The pressure in the coating chamber was reduced to 58 millimeters of mercury absolute and coating was started. Excellent coating quality was obtained as the speed of the web was increased to 360 feet per minute, at which speed air draw-under started.

The speed of the web was maintained at 360 feet per minute and the pressure was reduced to 21 millimeters of mercury absolute at which pressure boiling was observed to begin. Excellent coating quality, however, was obtained throughout the entire range from 58 to 21 millimeters of mercury absolute at the coating speed of 360 feet per minute. The obtained coating weight was 81.4 milligrams of silver per square decimeter.

Example 8 A light-sensitive aqueous gelatino silver halide photographic emulsion of the medical X-ray type was prepared and concentrated to a solids content of about 50% by weight using apparatus described in Haley U.S. application Serial No. 525,570 filed August 1, 1955 (Patent No. 2,866,499). The emulsion had a viscosity of 7,500 centipoises at 110 F. Separately, an anti-abrasion gelatinous coating solution was concentrated in the aforementioned Haley apparatus to a solids content of about 35% by weight, and the resultant concentrated solution had a viscosity of about 3,500 centipoises at 110 F. The two fluids were coated simultaneously from the dual extrusion hopper described in Beck U.S. application Serial No. 506,458 filed May 6, 1955 (Patent No. 2,901,770), onto a gelatin subbed polyethylene terephthalate web of the type described in the preceding example. The lips of the extrusion hopper were set at 0.006 inch and 0.002 inch apart respectively for the emulsion and anti-abrasion layers in such manner that the emulsion layer was in contact with the film support. The coating chamber was of the type described in Example 1. The pressure in the coating chamber was maintained at 58 millimeters of mercury, and the web speed was feet per minute. Examination of the resultant composite structure showed two smooth layers of excellent quality had been applied.

Example 9 Example 8 was repeated except that the materials coated from the dual extrusion hopper were (1) a cyan color-forming polyvinyl acetal silver halide aqueous emulsion of the type described in Martin US. Patent 2,538,257, and .(2) amagenta color-forming polyvinyl acetal silver halide aqueous emulsion of the type described in Martin US. Patent 2,680,732. The emulsions were each concentrated separately to a viscosity estimated to be 6000 centipoises at a temperature of 110 F. The coating chamber pressure was adjusted to 60 millimeters of mercury absolute. The extrusion orifice openings were each .006 inch and the web speedwas maintained at about feet per minute. Examination of the resultant composite structure showed two clearly defined, smooth layers of excellent quality.

Example 10 A light-sensitive aqueous gelatino silver halide photographic emulsion of the positive type having a solids content of about 53% by weight and a viscosity of 5,600 centipoises at 110 F. was coated as described in Example 1, except that the coating chamber pressure was maintained at 48 millimeters of mercury absolute, the web speed was 200 feet per minute, and the heaters were removed. The coated web was wound up without any external drying air. Upon inspecting the coated web, the usual glossy smooth surfacewas observed. The coated surface of the web was not tacky and was substantially free from solvent.

Example 11 A light-sensitive aqueous gelatino silver halide photographic emulsion of the positive type having a solids content of about 49% by weight and a viscosity of 4,400 centipoises at 110 F. was coated as described in Example 1, except that the hopper opening was increased to 0.010 inch, and the coating chamber pressure was reduced to 40 millimeters of mercury absolute as the web speed was brought up to about 500 feet per minute. The pressure was maintained at 40 millimeters, and the web speed gradually increased to 1,650 feet per minute. The coated web was wound up without application of external heat for drying. Upon inspecting the coated web, the surface was found to be substantially dry and the coating quality satisfactory.

. Example 12 A light-sensitive aqueous gelatino silver halide photographic emulsion of the negative type having a solids content of about 55% and a viscosity of 7,600 centipoises at 110 F. was coated as described in Example 1, except that the coating chamber pressure was reduced from the static equilibrium pressure (about 67 millimeters of mercury absolute) to 60 millimeters of mercury absolute as the web speed was increased to 200 feetper minute. The

. chamber pressure was further reduced to 45 millimeters of mercury absolute as the web speed was increased to 2,000 feet per minute. The coated web was wound up without application of external heat for drying. Examination of the coated 'web showed a complete lack of objectionable air occlusions, and generally excellent coating characteristics.

Example 13 A light-sensitive aqueous gelatino silver halide photographic emulsion of the negative type to which a hardenspeeds and pressures.

Speed feet per minute: I

ing agent had been added, having a solids content of about,5 7% by weight and aviscosity of" about 7,000

centipoises at 110 F., was coated as described in Example, 1, except that the-coatingehamber pressure was progressively reduced from a' static equilibrium pressure of about 67 millimeters. of mercury absoluteuat the starting coating speed, as shown in the following table.

' of mercury absolute so '66 200 -t 62 270 52 330 51 400 46 The web was dried as described in Example 1. The

coated ,web exhibited a smooth hard surface free from coating defects. Sensitometricjdata were equivalent to those of thesame emulsion in dilute form'coated by the skim coating process.

Example 14- Example 1-3 was repeated except that the coating hopper lip opening was increasedto 0.0l2 inch, and the coating and drying was'accornplished atthe following web Pressure in millimeters of mercury absolute Pressure in. millimeters -14 of such alcohols with each other or with ltctones, e.g., acetone, methyl ethyl ketone, methyl propyl ketonc,

.methyl isobutyl ketone, or in certain chlorinated hydro carbons, e.g., trichloroethylene etc.

The invention is usefulin preparing a product or article wherein it is desired to apply one or moresurface layers'of a coating material to a supporting web. It will thereforebe understood that the invention is useful in 'preparing' 'photographic film elements including multicolor films, photographie films of a positive or negative type, printing papers, X-ray films, motion picture films,

portrait'films, photographic papers, transparent films and papers, etc. It is alsouseful in the production of matrices for printing or dye transfers or for use in the graphic arts in general. The products can also have utility in fields outside of photography, e.g., in special adhesive uses and in the application of surface layers to textiles, paper, etc. Coating'materials can also include conventional additives, aswill be understood in the art, suchas chemical hardeners, 'e.g., mucochloric acid and chrome alum. It

will be recognized that theaddition of such additives may the appended claims.

so efiect the properties of the coating material as to require for optimum operation minor adjustments in the coating speed and pressure within the limits asdefined in It is believed that the major limiting factor, so far as coating speeds are concerned, is the mechanical and physical capacity of the apparatus utilized No critical upper The same exeellentresults ot the preceding example were obtained.

Example 15 A positive type light-sensitivesilver halide photographic.

emulsionwith a polyvinyl alcohol binder to silver halide chamber pressure was reduced from a static equilibrium pressure of about 67 millimeters of mercury absolute to a pressure of millimeters of mercury absolute as the limit is believed to exist, and the only speed limitation encountered in a large amount of experimentation in this field has been the limit on the capability of the web handling equipment.

' An advantage of this invention resides in the fact that viscous-fluid material and in particular photographic material can becoatedat speeds and coating pressures heretofore unattainable without deleterious effect on the quality and especially the photographic properties of the coat- 7 ed article. Another advantage is that photographic films web speedwasj increased to 1,200 feet per minute. The

coated web from which a large amount of clear, supernatant liquid could be removed mechanically was wound up without application of external heat for drying. A coated article of acceptable quality was obtained.

' The process of this invention is useful in the coating of of natural and syntheic water-permeable cooloids'which have been used as binding agents for the silver halide grains, including gelatin, albumen, agar-agarfwater-permeable polyamides, polyvinyl alcohol, partially hydrolyzed polyvinyl esters, hydrophilic polyvinyl acetals including those containing color-former nuclei, and othertypes of viscous binder materials. it is particularly useful in I the coating of aqueous gelatino silver'halide emulsions.

4 The process is not limited to the coating of silver halide emulsion layers; The process-can also be used in the coating of gelatin sub-layers, gelatin filter layers,"antihalation layerswhich contain dyes and pigments which absorb preselected wavelengths of light, anti-abrasion layers, water proofinglayers', sublayers, etc. Additionally, the process is useful in the coating of dispersions of magnetizable materials or phosphors in suitable carrying mediums. 1 I 4 While it is preferred that the carry-ing medium be aqueous' such as water, aqueous ethanol, etc.,. it will be understood that 'theprocess is useful in the applicati'oriof coating'materials inorganicsolvents, e.g., alcohols such as methyl, ethyl, propyl, and butyl alcohols, and mixtures light-sensitive silver halide dispersions in the various types 40 ratio of 065, having a solids content of about 11.8% by weight and a viscosity of about 1,200 centipoises at 110 1 F.,' was coated as described in Example 12.' The coating cal than prior art and papers are obtained having improved surface characteristics and an absence of objectionable air occlusion beneath the coating. .Still another advantage is that the process is readily controllable and permits coating at a wide range of speeds and pressures.

A further advantage resides in the fact that the process can be carried out in apparatus which does not require the maintenanceof differential pressures,not require the introduction of condensable vapors, nor require precise control of the temperature in the coating zone. A still truding at a speed from $6 to 1200 feet per minute a coating material comprising an aqueous solution of a waterpermeable organic colloid having a viscosity in the range from 1,000 to 100,000 centipoises and a solids content in the range from 20% to by weight, in the form of a thin sheet, into a low pressure coating zone and onto a moving flexible web traveling at a linear speed from to 2400 feet per minute and 2 to 200 times faster than the speed of extrusion, while maintaining said coating zone at a temperature above the set point of said solution and below aboutlS O F. and at an absolute pressure above the apparent equilibrium pressure of the coating material, below the static equilibrium pressure of the coatingmaterial and below the air occlusion pressure of the said coating material, while continuously removing water vapor fromthecoating zone, the condensable vapors in the coating zone consisting essentially of those evolved from the said aqueous solution.

2. The process as set forth in claim 1 wherein the temperature is from about 105 F. to about 150 F.

3. The process as set forth in claim 1 wherein the coated web is maintained in the coating zone for a period of 1 to 5 seconds.

4. The process as set forth in claim 1 wherein the absolute pressure in the coating zone is maintained within the limits indicated by the shaded area between curve A and curbe B of FIG. 2 of the drawing and the pressure delineated by the curves are the total pressures of all gases in the coating zone.

5. The process as set forth in claim 1 wherein water from said aqueous solution is removed from said thin sheet at least partially by Wiping.

6. The process as set forth in claim 1 wherein said sheet after being moved into contact with said web is passed through a heated drying zone for at least one second, said drying zone being maintained at a pressure below the static equilibrium pressure of said coating material.

7. The process as set forth in claim 1 wherein said thin sheet moves into first contact with said web as said web is moved around a coating support, said first contact being at a position non-tangential to said support.

8. The process which comprises extrudingan aqueous dispersion of silver halide grains in a water-permeable colloid, said dispersion having a viscosity in the range from 1,000 to 100,000 centipoises and a solids content in 16 the range from 20% to by weight, at a temperature above the set point of said dispersion and below about F., through a narrow slot orifice in the form of a sheet, into a low pressure zone and onto a moving flexible web traveling at a linear speed from 2 to 200 times 'faster than the speed of extrusion of said dispersion, said linear speed of said web and the absolute pressure in said zone being maintained within the limits indicated by the shaded area between curve A and curve B of FIG. 2 of the drawing, while continuously removing water from the coating zone, the condensable vapors in the coating zone consisting essentially of those evolved from the said aqueous solution.

9. The process as set forth in claim 8 wherein said temperature is more than about 8 F. above said set point.

10. The process as set forth in claim 8 wherein said colloid is gelatin.

References Cited in the file of this patent UNITED STATES PATENTS 1,180,255 Cossitt et a1. Apr. 18, 1916 1,861,918 Hickman June 7, 1932 2,175,125 Mack Oct. 3, 1939 2,681,294 Beguin June 15, 1954 2,795,522 Johns June 11, 1957 2,815,307 Beck Dec. 3, 1957

Claims

1. THE PROCESS FOR COATING WEBS WHICH COMPRISES EXTRUDING AT A SPEED FROM 1/2 TO 200 FEET PER MINUTE A COATING MATERIAL COMPRISING AN AQUEOUS SOLUTION OF A WATERPERMEABLE ORGANIC COLLOID HAVING A VISCOSITY IN THE RANGE FROM 1,000 TO 100,000 CENTIPOISES AND A SOLIDS CONTENT IN THE RANGE FROM 20% TO 65% BY WEIGHT, IN THE FORM OF A THIN SHEET, INTO A LOW PRESSURE COATING ZONE AND ONTO A MOVING FLEXIBLE WEB TRAVELING AT A LINEAR SPEED FROM 100 TO 2400 FEET PER MINUTE AND 2 TO 200 TIMES FASTER THAN THE SPEED OF EXTRUSION, WHILE MAINTAINING SAID COATING ZONE AT A TEMPERATURE ABOVE THE SET POINT OF SAID SOLUTION AND BELOW ABOUT 150* F. AND AT AN ABSOLUTE PRESSURE ABOVE THE APPARENT EQUILIBRIUM PRESSURE OF THE COATING MATERIAL, BELOW THE STATIC EQUILIBRIUM PRESSURE OF THE COATING MATERIAL AND BELOW THE AIR OCCLUSION PRESSURE OF THE SAID COATING MATERIAL, WHILE CONTINUOUSLY REMOVING WATER VAPOR FROM THE COATING ZONE, THE CONDENSABLE VAPORS IN THE COATING ZONE CONSISTING ESSENTIALLY OF THOSE EVOLVED FROM THE SAID AQUEOUS SOLUTION.