JP2509316B2 - High Speed Carten Coating Method and Equipment - Google Patents

Description

FIELD OF THE INVENTION The present invention relates generally to curtain coating processes, and more particularly to the use of curtain coating in the manufacture of photographic materials such as photographic film and paper. More specifically,
The present invention relates to an improved curtain coating process and an apparatus for use in such a process, which is particularly adapted to high speed manufacturing operations capable of achieving the high degree of precision required in the field of photography.

BACKGROUND OF THE INVENTION Of the many known material coating methods, two meet the strict requirements of the photographic industry for coating uniformity, layer ultrathinness, wide range coating speeds and especially the ability to coat multiple layers simultaneously. There is one. The first method, known as bead coating, is Beguin, US Pat. No. 2,681,294 issued June 15, 1954 and Russell 1956.
U.S. Pat. No. 2,761,791, issued September 4, 2014. The latter patent specification relates in particular to multilayer coatings in which the moving support is simultaneously coated with two or more layers of the coating composition in the manufacture of photographic materials. The second method, known as curtain coating, is Greiller, 1972.
U.S. Pat. No. 3,632,374 and Hughes issued Jan. 4, 2004, U.S. Pat. No. 3,508,94 issued Apr. 28, 1970
It is described in No. 7. The latter patent discloses a multi-layer curtain coating method in which the moving support is simultaneously coated with two or more layers of the coating composition by means of a free-fall curtain, especially in the production of photographic materials.

The curtain coating method is used in the production of photographic materials in which a coating material having a uniform thickness in the width direction and the length direction is applied to a continuously moving support material. It has many advantages. It is acknowledged that many of the advantages achieved by the curtain coating method result from the fact that a free-fall curtain can be formed by a slide hopper that is not very close to the application point of the moving support. The bead coating method has continued to be commonly used since it was so advanced that curtain coatings appeared in the manufacture of photographic materials. Research on bead coating for applying photographic materials
Controlling two stabilizing forces that influence bead formation is specifically directed to the coating zone, which has been found to be necessary to establish a very stable process. Controlling and stabilizing bead formation has made it possible to use a wide range of coating speeds, layer years and layer thicknesses. The stabilizing force is firstly the differential pressure (suction) applied across the coated beads to the point of application, as disclosed in US Pat. No. 2,681,294, and secondly the United States, issued Sep. 13, 1960. As described in Japanese Patent No. 2,952,559, an electrostatic charge difference (electrostatic charge) applied immediately before the application point is applied.
differential). Thus, both the differential pressure and the electrostatic charge help hold the beads within the coating zone. In the bead coating process, forces acting on the web, such as those imparted by differential pressure or electrostatic charges, help stabilize the beads and keep the beads in wet contact with the moving web. However, in the curtain coating method, beads are not formed and the mechanism of the coating action is obviously different. For example, in the curtain coating method, the curtain is allowed to fall freely and then impact with considerable momentum on the moving support to provide sufficient force to stabilize the application point and to provide a uniform wetting line on the moving support. Secure. The required momentum is obtained by proper selection of curtain flow velocity and free fall height.

In bead coating, the preferred method for obtaining a uniform electrostatic charge force at the point of application of the coating is to form a uniform level of bound polar charge on the support. The bound charge has equal and opposite electrostatic charges on the two sides of the support. The support is required to be a dielectric material having an extremely high resistance in order to keep the charge on the support until it reaches the coating application point. On a support where the charged support is passing over a grounded coating roll, the side of the support near the coating roll reflects the charge onto the coating roll surface, thereby contacting the coating roll surface. Effectively neutralize the charge on the substrate and then create an equivalent amount of net charge on the surface of the substrate being coated. This creates an electrostatic field at the coating application point between the support surface and the grounded hopper opening. When a moving support with a net charge on the surface to be coated passes over a grounded coating roll, the electrostatic field of the charge is effectively neutralized by the charge reflected on the grounded coating roll surface. It

However, it is extremely difficult to obtain a uniform electrostatic field at the point of application of the coating, and all subsequent patent disclosures since the disclosure of US Pat. No. 2,952,559 relate to the combined effect of attraction and electrostatic polar charge forces. For example, note US Pat. No. 3,206,323. Other patents mention the use of polar charge assist in bead coating of photographic materials at fairly low coating speeds. These include U.S. Pat.No. 3,470,471 and U.S. Pat.
3,670,203 and U.S. Pat. No. 3,671,806.
In addition, other patents disclose methods of measuring and controlling the electrostatic field so that the required amount of uniform charge is obtained. These include U.S. Pat.No. 3,531,314 and U.S. Pat.
30,753, U.S. Pat.No. 3,702,258, U.S. Pat.No. 3,757,1
63 and U.S. Pat. No. 3,549,406.

As briefly discussed above, the coating mechanisms involved in bead coating and curtain coating are completely different. In addition to the different forces used to stabilize the coating at the coating point, the effects of coating variables such as coating composition viscosity, flow rate per unit width of coating and substrate surface smoothness are Usually, the bead coating method is completely different from the curtain coating method. For bead coating, as disclosed in U.S. Pat. No. 4,001,024, the viscosity of the bottom layer is reduced (by dilution) to increase coating speed without affecting coating uniformity and thereby wet coating. We must increase the wet coverage. Also, rough support surfaces, such as textile or matte surfaces, are even more difficult to apply at high speeds. In all of these cases, as the coating speed increases, the coated beads tend to break or become unstable resulting in vertical and horizontal lines in the coating or in the coating at the substrate-coating boundary. Entrainment of bubbles spreads. For the curtain coating method, the opposite relationship to the above bead coating relationship is observed. When high coating speed curtain coating failures are prone to occur, they can often be avoided by increasing the viscosity of the coating composition, or by reducing the wet coverage of the bottom layer or coating a rough support. On the other hand, in the case of curtain coating at high speed, increasing the flow rate per unit width often causes the problem of "agglomeration" of the coating on the support, which usually occurs at the coating point on the support. Occurs when the curtain speed is higher than the speed of the substrate being coated. However, clumping also occurs when the substrate speed is greater than the curtain speed. Thus, the failure of curtain coating at high coating speeds is likely to occur because the momentum of the curtain at the coating application point is too high.

Given these very different requirements resulting from the radically different application mechanisms involved in bead coating and curtain coating methods, it is unexpected in curtain coating by having a high level of electrostatic polar charge on the support surface. The benefits are truly amazing. Although the use of low level electrostatic solutions is well known and developed for the bead coating process, the recognition that it may be useful in solving problems in high speed curtain coating of photographic materials is in the prior art. Not at all. On the contrary, it was believed that such forces would have no benefit as long as there was adequate curtain momentum. Failures seen in high speed curtain coatings occur because there is too much curtain coating momentum, and it is believed that any electrostatic force applied to the application site will make the situation worse. It was It may have been possible that such forces would be useful at low coating speeds where curtain momentum was inadequate, rather than when the curtain momentum was greater than adequate.

While not wishing to be bound by a theoretical explanation of how the present invention works, the electrostatic polar charge provides sufficient attraction to the falling curtain and the support to provide a uniform and defect-free application. It is believed to create between the body.

The coating defects that occur in curtain coatings when web speeds are increased to very high levels are partly referred to herein as "insufficient viscous friction".
ient viscous friction) "forces. The effect of" insufficient viscous friction "forces is that a large number of entrained bubbles between the coating and the substrate, and droplets of coating composition at the point of application. Characterized by the presence of an elongated band that occurs upstream and jumps out of the coating to form an elongated band The hypothesis of "insufficient viscous friction" force is that the coating solution is too viscous, the flow rate is too fast ,
Or it is suggested by the fact that the support surface is too smooth to be applied uniformly. Problems resulting from "insufficient viscous friction" forces appear only in curtain coatings at very high coating speeds, eg, web speeds above about 250 cm / sec. In contrast to the known use of electrostatic polar charges in bead coatings, which have a distinctly different purpose, namely the purpose of stabilizing the coated beads, according to the present invention, a given high level of Electrostatic polar charges are used to solve these problems of high speed curtain coatings. The high level of electrostatic polar charge apparently acts in the proper direction in the area where the curtain strikes the web, thereby substantially increasing the speed at which the curtain coating mechanism can be applied successfully. Contributes to the attractive force. More specifically, with the proper level of electrostatic polar charge, a particular combination of operating parameters is used, such as web smoothness, flow rate, coating composition viscosity and curtain height, while photographic It can operate at extremely high coating speeds while still successfully meeting the extremely high quality standards of coating technology.

As explained above, the high speed curtain coating method of the present invention is typically practiced at web speeds beginning at about 250 cm / sec and above. High speeds of about 1,000 cm / sec or more can be effectively used with the help of appropriate levels of electrostatic polar charge.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a curtain coating method and apparatus for coating moving objects at extremely high coating speeds.

The above object of the present invention is to advance the article to be coated (object) along the path through the coating zone at a speed of at least 250 cm / sec and to consist of one or more layers of liquid coating composition and A liquid coating composition for an article to be coated, wherein a free-falling curtain extending across the surface is made to collide with the surface of the moving article to be coated in the coating zone to form a coating film (coating article) having one layer or a plurality of layers thereon. It can be achieved by a method and a device for applying an object. The present invention, on the surface of the coating object for applying the coating film, at any point on the surface,
The amount of charge is selected according to the speed of the object to be applied so that the ratio of the charge measured in volts to the speed measured in cm / sec is at least 1: 1 to determine the electrostatic polar charge. It is aimed at applying.

Advantageously, the voltage is 200 volts greater than the voltage reached by said ratio.

The object to be coated is a web and a single or multiple layer liquid photographic coating composition is formed into a free-falling curtain extending across the web advanced along a path, and the curtain is then applied in the coating zone to the web. Advantages are obtained by the embodiments of the invention specifically implied for the manufacture of photographic elements that impinge on to form a single layer or multilayer coating. In a preferred embodiment, the web is at least 40
Move forward at a speed of 0 cm / sec.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an apparatus for applying a predetermined level of electrostatic polar charge to a web surface and then curtain coating the web surface with a liquid coating composition.

FIG. 2 is a graph relating electrostatic polar charge level to coating speed in a typical curtain coating process for making photographic materials.

FIG. 3 is a graph relating the coating composition flow rate and coating speed in a typical curtain coating process for the manufacture of photographic materials.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is described herein with particular reference to the application of photographic materials. The present invention is particularly useful in this field because the field of this application involves highly precise manipulations. However, the present invention is in no way limited to use in coating photographic materials and can be used to advantage in any curtain coating operation where it is desired to have a very uniform coating at very high speeds.
Both single-layer and multi-layer curtain coating methods benefit greatly from electrostatic polar charges.

The problem with curtain coatings solved by the present invention is that at very high coating speeds, typically 250 cm /
Only happens at web speeds above sec. In contrast to the prior art use of electrostatic charges in bead coatings, which includes the use of electrostatic charges for distinctly different purposes, namely for stabilizing coated beads.
According to the present invention, a predetermined high level of electrostatic polar charge is utilized to solve these problems in high speed curtain coating. The electrostatic polar charge at the coating point may be obtained by using a coating roll grounded to a support having a high level of bound polar charge. Such use of a predetermined high level of electrostatic polar charge in curtain coatings is herein conveniently referred to as "polar charge assist" in the sense that it directly assists in achieving effective application. ) ”Is often used. High level electrostatic polar charge
In the proper direction, the curtain clearly contributes to the substantial suction that acts in the area of impact with the web, thereby substantially reducing the speed at which the coating can be successfully applied by the mechanism of curtain coating. Increase. More specifically, using appropriate levels of electrostatic polar charge, the web fabric, flow rate, coating composition viscosity,
And operating at substantially higher coating speeds using a specific combination of operating parameters such as curtain height, while still successfully meeting the very high quality standards of the photographic coating art. It is possible.

As indicated above, the high speed curtain coating method of the present invention is typically performed at web speeds above 250 cm / sec. High speeds of about 1,000 cm / sec or more can be effectively utilized with the help of appropriate levels of electrostatic polar charges.

Any type of curtain coating device can be used to practice the present invention. So, for example,
The applicator may be an overflow weave type, a pressure extrusion type, a slide type, or a slide extrusion type curtain coating hopper. However, slide hoppers are especially preferred, especially for photographic coating operations. The applicator can be adapted to perform a single layer application or it can be of the type that applies multiple layers simultaneously. For example, Research Disclosure (Research D
isclosure), Item 17553, Volume 175, November 1978, for full width application or contact (abutti).
ng) or non-abutting striped coatings.

The method of the present invention can be utilized to apply any material or mixture of materials which can be in liquid form, for example in the form of a solution, dispersion or suspension. In many of the applications of the present invention, the coating composition is an aqueous composition, but other organic or inorganic liquid vehicles can also be utilized. When multiple layers are applied, each layer can be formed of the same or different liquid coating compositions, and these coating compositions can be miscible or immiscible with each other. May be.

As mentioned above, the method of the present invention is particularly useful in the photographic art for the production of multilayer photographic elements, i.e. elements comprising a support coated with a plurality of layers of photographic coating composition. The number of individual layers may be as high as 10 or more. In the photographic art, the liquid coating compositions used are usually aqueous compositions, but organic compositions can also be used. In the manufacture of photographic elements, the individual layers applied must be very thin, i.e. wet coverage up to about 0.015 cm, generally well below this value, and may be as thin as about 0.0001 cm. . In addition, the layers must be of very uniform thickness and the maximum variation in thickness uniformity is typically ± 2% and in some cases as small as ± 0.5%.

The method of this invention is suitable for use with any liquid photographic coating composition and can be used with any type of photographic support and is therefore used herein and in the appended claims. Thus, it is intended to encompass all coating compositions and supports as utilized in the photographic art within the scope of these terms.

Useful photographic supports include film bases such as cellulose nitrate films, cellulose acetate films, polyvinyl acetal films, polycarbonate films, polystyrene films, polyethylene terephthalate films and other polyester films; papers;
Glass; cloth; metal and the like. Paper supports coated with alpha-olefin polymers such as polyethylene and polypropylene, or with other polymers such as cellulose organic acid esters and linear polyesters may also be used if desired.

The term "photographic" usually refers to radiation-sensitive materials, but not all layers applied to a support in the manufacture of photographic elements are themselves radiation-sensitive. For example, subbing layers, pelloid protective layers, filter layers, antihalation layers, etc. are often applied separately and / or in combination, and these particular layers are not radiation sensitive. The present invention also relates to the coating of such non-radioactive layers and as used herein, the term "photographic coating composition".
Is intended to encompass the composition from which such layers are formed. Further, the present invention includes within its scope all radiation sensitive materials, including electrophotographic materials as well as materials sensitive to invisible radiation as well as those sensitive to visible radiation.

More specifically, the photographic layer coated by the method of the present invention is a light-sensitive material such as silver halide, zinc oxide, titanium dioxide, diazonium salt, photosensitive dye, and the like, and a technical field for use in the photographic layer. Other ingredients known in 1 can be included.

Various types of surfactants can be used to improve the surface tension and coatability of photographic coating compositions.
Useful surfactants include saponins; non-ionic surfactants such as polyalkylene oxides, (eg, polyethylene oxide), and water-soluble adducts of glycols and alkylphenols; alkylaryl polyether sulfates and sulfonates. Anionic surfactants; and amphoteric surfactants such as arylalkyl taurines, N-alkyl and N-acyl β-aminopropionates; betaine alkyl ammonium sulfonates and the like.

Aqueous photographic coating compositions typically contain a hydrophilic colloid. Examples of useful hydrophilic colloids are proteins,
For example, gelatin, protein derivatives; cellulose derivatives,
Polysaccharides such as starch; sugars such as dextran and vegetable gums; synthetic polymers such as polyvinyl alcohol, polyacrylamide and polyvinylpyrrolidone; and other suitable hydrophilic colloids as disclosed in US Pat. No. 3,297,446. Is mentioned.
Mixtures of the colloids may be used if desired.

The coating composition may be a gelatin silver halide emulsion. It is possible to use emulsions containing a wide variety of different silver salts such as silver bromide, silver iodide, silver chloride, or mixed silver halides such as silver chlorobromide, silver bromoiodide or silver chloroiodide. it can. Conventional additives such as curing agents, antifoggants,
Stabilizers, matting agents, plasticizers, developers and the like can be included in the emulsion. For use in color photography, the emulsions can contain color-forming couplers or can be emulsions adapted to be developed in a solution containing color-forming couplers or other color-generating substances.

In accordance with the present invention, devices that can be used to impinge a predetermined high level of electrostatic polar charge on a web surface are U.S. Pat. Nos. 3,470,417 and 3,730,753 and Research Disclosure, Item 16974, May 1978 (Ind.
Published by ustrial Opportunities Ltd., Homewell, Havant Ha
mpshire P09 1EF, UK), the disclosures of which are incorporated herein by reference. Equipment that can be used for single or multiple layer coatings by the curtain coating method is described in Hughes, U.S. Pat. No. 3,508,947 and Greiller, U.S. Pat. Included for reference.

Curtain coating hoppers used in the practice of the present invention typically include end guides for guiding and defining the width of the free fall curtain. Useful end guide methods include the use of end guides mounted on the web, as described in the aforementioned patents of Grailer and Hughes, and Research Disclosure, No. 17553.
And the use of the "liquid end guide" technique as described in Section, Volume 175, November 1978.

Under typical curtain coating conditions used in the manufacture of photographic materials, there is usually little gain from using polar charge assist at coating speeds below about 250 cm / sec. Because using polar assist at such speeds does not usually have a substantial effect on the uniformity of the coating, and other means can be used to achieve satisfactory coating uniformity. is there.

In addition to the level of electrostatic polar charge, the curtain coating method includes web speed, curtain height, viscosity of each layer, flow velocity of each layer, surface tension, application point on application roll and properties of web agent. There are many other factors that affect it. All of these factors interact in a very complex way, and all can have a significant impact on high speed applications.

References to the application point here refer to either positive or negative angles, which angle causes the surface defined by the axis of the application roll and the contact line of the free-fall curtain to intersect the axis of the application roll. Deviated from the axis along the apex of the coating roll supporting the containing surface and web. Too wide a distance from the center point can also cause undesirable obstructions to the curtain coating process and result in coating defects.

The method of the present invention provides an electrostatic polar charge at a predetermined high level that is sufficiently effective to enhance coating uniformity resulting from the action of a free-falling curtain of a liquid coating composition that impacts the surface to be coated. Is included. Therefore, this is clearly different from curtain coatings of webs or other objects that may get an irregular electrostatic charge pattern or low levels of electrostatic charge. Random and / or low level electrostatic charge storage can be obtained from a variety of sources. For example, transporting the web support over a series of rollers can produce an irregular electrostatic charge pattern on the web surface. Further, if the corona activation of the web surface is performed for the purpose of enhancing the coatability, the web surface will have an irregular electrostatic charge pattern. However, such charge patterns are not effective in overcoming the problems in curtain coatings solved by the present invention.
This is because in order to solve these problems it is necessary that there be an electrostatic polar charge on the surface to be coated. Preferably, the polar charge is at a level equal to or greater than what is known as an effective minimum level at substantially all points on the support surface to be coated-typically at a level of at least 400 volts-. Is.

In a preferred embodiment of the present invention, the web or other object to be coated is advanced through the coating zone at a speed of 400 cm / sec or higher and the electrostatic polar charge is applied to the surface to be coated on the surface. It is applied at a level such that the ratio of charge (measured in volts) to speed (measured in cm / sec) at any point above is at least 1: 1. Thus, for example, when applying at a speed of 400 cm / sec, one would use a charge level of at least 400 volts, but other factors,
For example, it could be well above 400 if required by using a support with a surface texture such that coating uniformity is particularly difficult to achieve. In a particularly preferred embodiment of the invention, the electrostatic polar charge level (in volts) is the web speed (cm / s).
ec) or above plus 200 volts, ie 6 for a web speed of 400 cm / sec
Have a polar charge level of 00 volts or higher.

In the method of the present invention, the upper limit of the level of polar charge that can be utilized is dictated by the practical considerations involved in the design and construction of the charge generating device and at a level that is not strong enough to adversely affect the photographic emulsion or other coating composition. Is determined by

In a preferred embodiment of the present invention, a level of polar charge is applied that is substantially uniform across the web surface. In other embodiments, it may be advantageous to provide some areas of the web surface with a higher level of polar charge than others to facilitate coating uniformity objectives. Thus, for example, in one curtain coating method,
The web extends beyond the end of the applicator roll, i.e. the web is wider than the applicator roll and thus modified so that it overhangs the roll. This may be done, for example, to reduce the tendency of the coating composition to adhere to the coating roll. Problems can occur with these methods in attempting to achieve a uniform application over the web of overhanging areas. Polar charges are less effective on the web of the overhang area than on the rest of the web surface for obvious reasons, especially in the overhang area, where there is no coating roll to provide electrical grounding behind the web. For this reason, it is desirable to apply a higher level of polar charge to the web in the area overhanging the coating roll than the rest of the web surface. In the method of the present invention, this can be easily achieved by providing replenishing charging means for imparting a replenishing polar charge only to the web in the area overhanging the coating roll. Thus, for example, only the web of areas that will overhang the coating roll after the web has passed below the charging means for applying a polar charge across its entire surface and before receiving the coating composition is polar. It is passed under a second charging means for applying an electric charge. The charge applied in these areas must be of the same polarity as that applied across the web surface and is preferably a voltage level of 2-3 times greater magnitude. For illustrative purposes, if a 800 volt polar charge is applied to the entire web surface, the replenishing polar charge applied to the flared area is about 1600 to
Advantageously, about 2400 volts.

An alternative technique used for web overhang from the applicator roll is that the entire web surface, including the overhang area, is substantially more uniform than is required for the particular application speed and application conditions without overhang. To provide a level of polar charge on the web of the overhang area that is adequate to ensure uniform application as on the rest of the web surface. This of course means that there will be higher levels of polar charge on the rest of the web surface than required, but this is usually not a problem.

The increase in coating speed that can be achieved using the method of the present invention is significant. For example, it is possible to double the coating speed as compared to the maximum speed that can be used without significant levels of polar charge assist while maintaining comparable coating quality.

Referring to the drawings, FIG. 1 is a schematic diagram of the coating method of the present invention in which a three-layer liquid coating composition is simultaneously coated onto a web support using a multi-slide hopper. As shown in FIG. 1, the web 10 is unwound from a supply roll 12 and then around a tension roll 14 and then over a grounded metal roll 16 where it receives an appropriate level of uniform electrostatic polarity charge. . The electrostatic polar charge generator is of the grid controlled ionizer type. The first such ionizer has a series of conductive elements 20: grid elements in combination with, for example, tungsten rods or wires.
Comprising 18. The grid 18 is connected to a DC power source 22, while the conductive element 20 is connected to a DC power source 24. The second ionizer comprises a grid 26 in communication with a variable DC power supply 28 and a conductive element 30 in communication with a variable DC power supply 32. First
The ionizer's function is to eliminate polar charge deviations on the surface of the incoming web. The high voltage series power supply provides the conductive element 20 with a voltage of 15,000 volts. This high voltage ionizes the atmosphere and the charged ions are then accelerated through the grid 18 toward the web 10. Web 10 grid
It is charged to a uniform level determined by the supply voltage to 18. Once the web reaches the grid potential, the ions are no longer accelerated between the grid and the web surface, and a uniform polar charge level exists on the web surface. Second
Ionizers help increase the uniform potential to high levels and control this level. This function is accomplished using a variable output power supply. This output voltage is electrically controlled using a feedback loop that includes an electrostatic voltmeter 34 that measures the voltage on the web. This voltage is compared to the reference voltage by the control circuit 36 and the power supply output voltage is automatically stepped up or down to maintain a constant voltage level on the web 10.

After the web 10 has reached the desired uniform level of electrostatic polar charge, the web passes over guide rolls 40, 42, 44, 46 and 48 onto a grounded applicator roll 50, where the proper feed is applied. Using a multiple slide curtain coating hopper 52 that receives emulsion from a source (not shown) and produces a free falling curtain 54 that impinges on the web,
The web is coated with a liquid coating composition, such as a three layer coating solution of three different photographic emulsions. After passing over the coating roll 50,
The web 10 is directed to a drying chamber 56 where the emulsion layer is contacted with hot air or other gaseous medium to dry and then wound around a guide roll 58 onto a take-up roll 60.

FIG. 2 correlates the coating speed for a typical curtain coating method of coating a gelatin silver halide photographic emulsion on the surface of the web with the level of electrostatic polar charge required to achieve uniform coating. The curves shown in Figure 2 are representative of the following coating conditions (although different combinations of coating conditions would cause curve movement): Curtain height-12.5 cm Curtain flow velocity- Curtain width 1.5 to 6 g / sec / cm Viscosity of coating composition -60 centipoise Web surface-Glossy polyethylene coating point--15 °.

Although uniform coating is achieved in the left hand side of the curve in FIG. 2, non-uniform coating occurs in the right hand side of the curve which appears not to be useful in photographic manufacturing operations. In other words, the curve represents the boundary line between satisfactory and failed coatings.

As shown in FIG. 2, at coating speeds up to about 250 cm / sec no polar charge assist is required to prevent unsuccessful coating. In the region of coating speeds between about 250 and about 400 cm / sec, the curves show a progressively increasing need for polar charge assist. However, normally, at speed levels between 250 and about 400 cm / sec, polar charge assist is not provided as other means may be used to achieve satisfactory coating uniformity. The slope of the curve decreases with increasing coating speed.
The ratio of charge (measured in volts) to speed (measured in cm / sec) should be at least 1: 1 to achieve uniform coating over the speed range of 400-800 cm / sec under the particular coating conditions used. It should be noted that it must be. In a preferred embodiment of the present invention, an additional electrostatic charge of the order of 200 volts may be needed, for example a polar charge level of 600 volts or higher for a web speed of 400 cm / sec.

FIG. 3 relates to the flow rate of the coating composition per unit width of curtain with respect to the coating speed. A uniform application area is shown, but this area is surrounded by a visible area where poor quality application is occurring. The size and shape of the uniform coating area depends on the height of the curtain, the viscosity of the coating composition, the point of application, the nature of the web surface and the level of electrostatic polar charge assist. The unstable curtain region is a region where the curtain flow velocity per unit width is too slow to form a stable curtain. In this area,
The curtain splits into numerous strands, which makes it impossible to form a uniform coating. Lump (puddlin
The area g) represents the state in which the curtain speed at the coating material application point is higher than the web speed. As a result, the coating composition accumulates behind the curtain and causes vertical stripes.

For any combination of operating conditions, the uniform coating area can be substantially extended using electrostatic polar charge assist, ie by applying a sufficiently high level of electrostatic polar charge to the web surface. Is a particularly important feature of the present invention. As described above, by using the electrostatic polar charge assist, the coating speed at which non-uniform coating starts to occur can be greatly widened. This allows a particular application machine to run at a much faster speed and thereby produce much more product without any sacrifice in application quality.

 The invention is further described by the following examples.

Example I-By Bead Coating Method A multiple slide bead coated hopper similar to that shown in U.S. Pat. No. 2,761,791 was used, except that the hopper was set to coat seven layers simultaneously. The coated product was Ektacolor Papar containing 7 separate layers. The wet coverage and viscosity of the 7 layers are listed in Table I.

The viscosity of the bottom layer was adjusted by diluting with water to obtain sufficient coating properties. Sufficient coatability is defined as a stable coat system without mottle, cross lines, microbubbles and other coat defects. Attempts to apply this product at high speeds (400 cm / sec or higher) were desired. Table II shows the maximum coating speeds achieved as a function of bottom layer viscosity and polar charge voltage. As shown in Table II, both matte (coarse) and glossy (smooth) surface supports were used.

For matte surface supports, Examples I (a) and I (b)
Dilution of the bottom layer from 6.5 to 4.6 centipoise results in an increase of the maximum coating speed from 300 to 350 cm / sec, as shown in, while for glossy surfaces the increase is 35
It was 0 to 400 cm / sec. As shown in Examples I (c) and I (d), when a 350 volt polar charge is applied to the support in front of the coating zone, the maximum coating speed for the matte side support is relative to the highly diluted bottom layer. Increased by 50 cm / sec to a maximum of 400 cm / sec. With a matte surface, an improved maximum coating speed of about 14% to 17% is reached depending on the viscosity of the bottom layer. It has been found that applying a level of polar charge above 350 volts does not improve the bead application speed to the extent that is seen.

For glossy backings, polar speeds were not required to achieve adequate coatability at speeds greater than 400 cm / sec. Gloss side support is generally about 50 than matte side
Can be applied at a speed as fast as cm / sec. For both types of substrates, dilution of the bottom layer from 6.5 to 4.6 centipoise increased the maximum coating speed by 50 cm / sec.

It is well known that one of the advantages of using the curtain coating method to apply photographic materials over known bead coating methods is that the bottom layer does not need to be diluted more than the other layers. In contrast to the need to use a highly diluted bottom layer in bead coating. In the above example, 40% of the total water is in the bottom layer.
This requirement tends to limit the speed at which continuous coated web supports can be dried. The bottom layer in the bead coating process is typically 10 centipoise or less, preferably 3-5 centipoise, while the outer layer of a stable free-fall curtain is typically 40 centipoise or more.
The benefits of applying high viscosity coating compositions are well known and discussed in Hughes, U.S. Pat. No. 3,508,947, which requires continuous and uniform drying of delicate photographic coatings. One advantage is that water addition requirements on the vessel are significantly reduced.

Example II-Curtain Coating Method A multiple slide curtain coating hopper similar to that shown in Figure 1 of U.S. Pat. No. 3,508,947 was used, except that the hopper was set to apply seven layers of the coating composition simultaneously. The viscosities and wet coverages of all layers except the bottom (blue sensitive) layer were the same as those shown in Table I. The bottom (blue sensitive) layer was replaced with a coating composition having a wet coverage of 20 microns and a viscosity of 26 centipoise. The coating speeds obtained with the matte support side and the gloss support side are shown in Table III.

From the data listed in Table III, 3 for bead coating at a maximum speed of 400 cm / sec
It will be shown that the curtain coating method was able to achieve such speeds without the use of polar charge assist, although it was necessary to use a polar charge of 50 volts.

However, the coating requirements for glossy backings have been found to be quite different and therefore unpredictable based on conventional knowledge of bead coating methods. For example, Table III shows that when polar charge assist is not used, with a 26 centipoise bottom layer a maximum coating speed of only 300 cm / sec is achieved for glossy backings. Table II for the bead coating method shows that on glossy surfaces a maximum coating speed of 400 cm / sec could be achieved without the use of polar charges.

When a 700 volt polar charge was applied to the coating surface in front of the coating point, the glossy surface support could be curtain coated at a speed of at least 500 cm / sec. This translates into at least a 67% increase in achievable curtain coating speed for glossy support surfaces compared to that obtained without the use of polar charges. As shown in FIG. 2, when a higher level of polar charge is used gradually, coating at a higher speed is possible.

From these examples it will be shown that the application of photographic materials using the bead coating method which requires a highly diluted (low viscosity) bottom layer has an upper limit of about 400 cm / sec for the application speed.

It has been unexpectedly found that even faster coating speeds are possible when coating photographic materials using the curtain coating method with polar charge assist. When using curtain coating, even if a high concentration (high viscosity) bottom layer is applied, it is only 00 cm if a very high level of polar charge is applied to apply a smooth web support such as a glossy support. It was unexpectedly found that a coating speed of over / sec could be obtained.

Claims (12)

(57) [Claims] 1. A free-fall curtain is advanced in the coating zone along a path through a coating zone and comprising one or more layers of liquid coating composition and extending across the path. In the method of applying a liquid coating composition to an article to be coated, wherein a coating film consisting of one layer or a plurality of layers is formed by colliding with the moving article to be coated, the article to be coated is at least 250 cm / sec. An electrostatic polar charge is applied on the surface of the substrate to be advanced at the speed of and applying the coating, at any point on the surface, of the charge, measured in volts, cm. Applying the liquid coating composition to the article comprising selecting the amount of the charge according to the speed of the article to be coated such that the ratio to the speed measured in / sec is at least 1: 1. how to. 2. The method of claim 1 including applying an electrostatic polar charge that is 200 volts greater than the voltage resulting from the ratio. 3. A photographic element for producing a photographic element, wherein the substrate to be advanced along a path through a coating zone is a web and the liquid coating composition forming a free-fall curtain is a liquid photographic coating composition. A method according to claim 1 or 2 comprising advancing the web at a speed of at least 400 cm / sec. 4. The web passes around a coating roll having a width narrower than the width of the coating roll so as to project from the coating roll, and the remaining portion on the web surface in a region projecting from the coating roll. The method of claim 3 comprising applying a greater amount of electrostatic polar charge on said web than on said web surface. 5. The method according to claim 3 or 4, wherein the web comprises a cellulose acetate film. 6. A method according to claim 3 or 4 wherein said web comprises polyethylene terephthalate. 7. The method of claim 3 or 4 wherein said web comprises polyethylene coated paper. 8. A method according to claim 3 or 4 wherein at least one of said layers comprises a gelatin silver halide photographic emulsion. 9. A means for advancing an article along a path through a coating zone at a speed of at least 250 cm / sec; a free fall comprising one or more layers of liquid coating composition extending across said path. Means for forming a curtain and then impinging on the surface of the moving article to be coated in the coating zone to form a coating comprising one or more layers thereon; and the surface of the object to which the coating is applied Cm / s of the above charge measured by applying electrostatic polar charge on the above
A curtain coating apparatus comprising means for ensuring that the ratio to the speed measured by ec is at least 1: 1. 10. The article according to claim 9, wherein the article to be coated is a web, and the means for advancing the article to be coated is a web transport system which can be operated at a controlled and predetermined speed. Curtain coating equipment. 11. The curtain coating apparatus of claim 9 wherein the electrostatic charge is a bound polar charge and the means for applying the electrostatic polar charge comprises a grounded backing roll and at least one grid-controlled ionizer. . 12. The curtain coating apparatus of claim 9 wherein said means for forming a free-fall curtain comprises a multiple slide hopper.