US 3928678 A
An improved method of coating one or more layers of liquid coating composition onto a substrate from a coating hopper of the type having an inclined slide surface, wherein each coating composition is fed in the form of a layer, and in superposed relation to any other composition layers, upon an upper portion of the inclined slide surface to flow by gravity toward the substrate. According to the invention, the layer(s) is redirected at a lower lip portion of the slide surface to flow in a more horizontal direction before flowing across a generally horizontal gap onto a substrate moving in a generally upward direction.
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Description (OCR text may contain errors)
[ 1 Dec. 23, 1975 METHOD AND APPARATUS FOR COATING A SUBSTRATE  Inventor: Brian W. Jackson, Pinner, England  Assignee: Eastman Kodak Company,
 Filed: Jan. 26, 1973  Appl. No.: 326,621
 References Cited UNITED STATES PATENTS 3,413,143 ll/1968 Cameron et al. 427/402 3,526,528 9/1970 Takahachi et a1. 117/81 3,531,314 9/1970 Kerr et al 117/34 3,607,345 9/1971 Thomas et al. 117/34 3,627,564 12/1971 Mercier 117/34 3,640,752 2/1972 lchiwati et al. 117/34 FOREIGN PATENTS OR APPLICATIONS 1,151,173 7/1963 Germany 117/34 Primary ExaminerMichael Sofocleous Attorney, Agent, or Firm-A. P. Lorenzo  ABSTRACT An improved method of coating one or more layers of liquid coating composition onto a substrate from a coating hopper of the type having an inclined slide surface, wherein each coating composition is fed in the form of a layer, and in superposed relation to any other composition layers, upon an upper portion of the inclined slide surface to flow by gravity toward the substrate. According to the invention, the layer(s) is redirected at a lower lip portion of the slide surface to flow in a more horizontal direction before flowing across a generally horizontal gap onto a substrate moving in a generally upward direction.
8 Claims, 3 Drawing Figures US. Patent Dec. 23, 1975 METHOD AND APPARATUS FOR COATING A SUBSTRATE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improvements in coating methods and apparatus. The invention is particularly useful in the coating of one or more layers of coating compositions onto a substrate, such as, say, a web.
'2. Description of the Prior Art In the prior art, as represented by US. Pat. Nos. 2,761,419, filed in the name of Mercier et al. and 2,761,791 filed in the name of Russell et al. the contents of which patents are herein incorporated by reference-it is known to simultaneously coat a moving substrate, for example, a web of photographic film base with emulsion coating compositions, by the use of a coating hopper which includes one or more inclined slide surfaces, down which one or more of the fluid coating compositions may be made to flow. The hopper may include a plurality of separate exit slots, whereby respective liquid coating compositions may be metered from individual supplies and distributed uniformly across respective inclined slide surfaces. Each of the respective compositions flows by gravity as a layer down its respective inclined surface, whereby the layer becomes smooth and of uniform thickness. The slide surfaces are arranged so that the layers flow on top of one another. At the end of the last slide surface, i.e., the one adjacent to the substrate, the stratified layers flow into a bead or puddle which bridges a small generally horizontal gap between an edge of the coating hopper and the upwardly moving substrate. The substrate, as it is advanced into contact with the bead, simultaneously picks up all the layers, which layers deposit on the substrate as a composite coating of substantially distinct superimposed layers.
While the above apparatus functions satisfactorily, various defects can arise in the coated layers if the bead is disturbed. For example, a particularly noticeable fault is the appearance of longitudinal striations which render the coated web unacceptable as a commercial tageous to create a pressure differential between the exposed surfaces of the bead, such as by the creation of a vacuum on the trailing surface of the bead to eliminate excessive vibration and/or rupture of the bead. The range of vacuum levels which may be used is, of course, limited as, if it is too great, the high pressure differential across the bead may also cause the bead to be disturbed and/or ruptured. At times, it may develop that the amount of vacuum being used is insufficient to eliminate the excessive vibration in the bead, and thus it is desirable to increase the amount of such vacuum. Where an increase in vacuum will itself cause disturbance and/or rupture of the bead, it is sometimes necessary to change coating conditions, such as by lowering web speed, to make satisfactory coatings.
A further problem with the use of the coating hopper described in the aforementioned Mercier et al. patent is that particles in the liquid coating composition may adhere to the hopper at the lip edge thereof and cause 2 undesirable streaks to be formed in the coating. An apparatus and method which reduces or minimizes the deleterious effects of such particles on coatings represents a significant contribution to the state of the art.
SUMMARY OF THE INVENTION It is therefore one object of the invention to provide an improved method and apparatus for coating a moving substrate at higher speeds with minimum bead disturbance.
It is a further object of the invention to provide an improved method and apparatus for coating a moving substrate which minimizes the undesirable effects of particles which adhere to a lip of the coating apparatus.
In the apparatus and method of the invention, a coating hopper is provided with a downwardly inclined slide surface upon which one or more layers of liquid coating composition(s) may be made to flow so the layers become smooth and of uniform thickness. The hopper includes a lip which extends towards the substrate at an obtuse angle relative to the slide surface and the layer(s) of coating composition(s) may flow on this lip before being fed into a bridge of coating solution,
. which bridge spans a generally horizontal gap between the hopper and an upwardly moving substrate to be coated.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partly in section, showing a coating apparatus including a multiple layer slide hopper as known in the prior art; and
FIG. 2 is a close-up side view of an improved coating apparatus made in accordance with the invention; and
FIG. 3 is a view similar to FIG. 2 showing another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Inprior art coating apparatus, such as disclosed in the Mercier et al. patent and shown herein in FIG. 1, a substrate, such as web W, is supported on and advanced past a coating hopper 10 by a driven coating roller 13. The web may be paper, metal or plastic film and may include one or more substantially dry coatings previously applied thereto. For illustrative purposes the hopper 10 is shown as being adapted to coat two layers simultaneously onto the web W although it will be appreciated that slide hoppers are known in the prior art to coat one or more than two layers and the invention is also applicable to such hoppers. Metering pumps, P and P, are coupled to respective sources of fluid coating compositions and feed the compositions 16,23 at desired rates into respective cavities 11,19 formed in the interior of hopper 10. The compositions 16,23 are each then forced to flow generally vertically as a ribbon through narrow discharge slots 14,21 respectively, each of which 'slots at one end extends into the interior of the hopper to a respective cavity 11,19 and at the other end exits onto the upper end of a downwardly inclined planar slide surface 15,22 respectively. The compositions 16,23 are each thus extruded in the form of a liquid layer onto their respective slide surface. The slide surfaces 15,22 in the prior art multiple layer hopper shown in FIG. 1 are substantially coplanar; however, they may be substantially parallel with slide surface 22 raised slightly above slide surface 15 a distance of the order of the thickness of layer 23. The discharge slots 14,21 are each defined by a pair of opposed spaced-apart parallel planar surfaces 14',l4"; 21, 21" respectively and are arranged so that an exit of the lowermost slot 14 is spaced above an edge 27 of the coating hopper 10. At the exit of the lowermost slot 14, the upper layer 23 flows upon the lower layer 16, whereupon the two layers flow together along the surface 15 in stratified relationship into a bead at 17. The bead 17 spans a generallyhorizontal gap between the edge 27 and the upwardly moving web W. As used herein a web or substrate is generally upwardly moving at any point where at such point its vertical component of velocity is greater in magnitude than its horizontal component of velocity.
As the web is advanced rapidly past the bead, a surface thereof picks up the layers and the layers deposit onto the substrate as a composite coating of substantially distinct superimposed layers. To stabilize the bead, suction or a vacuum may be employed on the trailing surface 24 of the head to establish a pressure differential between the exposed surfaces of the bead. The suction may be provided by a chamber 25 which is coupled to a vacuum pump 26 to exhaust air from the chamber 25.
In FIG. 2, a portion of an improved slide hopper 29 made in accordance with the present invention is shown. For purposes of clarity the suction chamber has been deleted from FIGS. 2 and 3 but such pressure differential establishing means is advantageous when used in conjunction with the embodiments shown in FIGS. 2 and 3. The slide hopper 29 is similar in most respects to the hopper shown in FIG. 1, but differs in that it includes an upturned lip 31 which extends at an obtuse angle from the lower end of the lowermost slide surface 32 toward the surface of a generally upwardly moving substrate W, e.g., the web, being coated. The lip 31 is preferably generally planar and of the same transverse width as the planar slide 32 and terminates in a transverse edge 34, defined by the planar land of the lip 31 and side face 39 of the hopper. Edge 34 is spaced generally horizontally from the coating roller 33 a distance slightly greater than the thickness of the web W, to be coated so as to establish in operation of the hopper a bead or puddle 37 of liquid coating compositions. Preferably the gap between edge 34 and the surface of the web being coated is between about 0.005 inches (0.13 mm) to about 0.03 inches (076mm). As may be noted from FIG. 2, the coating solutions flow down the lowermost slide surface 32 with the uppermost layer 35 on top of the lowermost layer 36. The slide surfaces 30,32 are each inclined relative to the horizontal at a desired angle that is preferably greater than ten degrees and less than 45 depending upon the properties of the composition(s) being coated. As the lip 31 is of lesser inclination to the horizontal than that of the slide surfaces 30,32 99 the layers 35,36 flow over the lip with reduced speed but increased thickness (depth). The increased thickness of the layers on the upturned lip 31 is advantageous in that the effects of any particles that happen to lodge at edge 34 are minimized by a dilution of such effects by the thicker layers on the upturned lip, i.e., a particle of a certain size that is lodged on the edge 34 will tend to create greater disturbances in thin (shallow) layers as opposed to thick (deep) layers as the liquid compositions flow past the particle. The lip 31 is directed in a generally horizontal direction, and is preferably horizontal although it may be inclined relative to the horizontal in the direction of fluid flow within the preferred range of :5 degrees. The length of the lip 31 in the direction of liquid flow is advantageously in the range of about 0.025 to 0.080 inches (0.06cm to 0.2lcm), and is preferably about 0.040 inches (010cm). Such dimensions are advantageous in the coating of photographic compositions wherein emulsion coatings might vary between a dry thickness of 0.0001 0.001 inches and protective layers separating such emulsion layers might be as thin as one micron.
With the use of the coating roller 33 to support the web in a smooth condition it is desirable to have the point of application of the bead to the web W be' within a preferred (acute) angular range of application points and such preferred range is from about a point on the web that is located at about 40 below the horizontal radius of the coating roller 33 vertically to about 30 above said horizontal radius. The generally horizontal gap is advantageous in that it allows gravity to act on the suspending bridge of coating solution and facilitates the formation of the bead.
In the prior art apparatus of FIG. 1, one factor which may contribute to the inability of the bead at times to withstand higher pressure differentials is that the bead must, in addition to the pressure differential across it, support itself against the force of the downwardly rushing liquid layers which are continuously feeding into the head.
The introduction of the coating compositions into the bead along the generally horizontal plane of the lip 31 apparently is able to reduce the force of the liquid rushing into the bead or otherwise increase the stability of the bead so that increased suction levels may be used on the bead at high coating speeds without encountering bead disturbance which produces unsatisfactory coatings. Furthermore, even at the same suction levels, increased coating speeds may be obtained using the apparatus and method of the present invention over that shown in the Mercier et al. patent.
To illustrate the advantages of the invention two superimposed layers of gelatin solution may be coated using a coating hopper of the prior art as shown in FIG. 1 and embodying the invention as shown in FIG. 2. The lower gelatin solution may have a concentration of 3.85% by weight and a wet laydown of 1.125 lbs/ square feet. The upper layer may have a concentration of 6.6% by weight and a wet laydown of 0.303 lbs/100 square feet. Suction may be maintained across the head during coating with a gap between the lip and the web of about 0.008 inch. With the apparatus of the present invention it was found that striations due to instability of the bead and which are present with the use of bead coating apparatus of the prior art are absent at high coating speeds.
If desired, the transverse edge 34 may be rounded or chamfered to prevent nicks or other damage to the otherwise sharp edge. A coating hopper with such a chamfered edge 34" is shown in FIG. 3 wherein like numerals refer to parts similar to that shown and described for FIG. 2. Preferably the lip 31 in the embodiment shown in FIG. 3 should be of the same dimension and angular orientation as for that described for FIG. 2.
A coating hopper which includes such a chamfered edge 34" is advantageous in that with the use of such a hopper there is substantially eliminated the formation of streaks in coating, which streaks may be attributed to imperfections in a sharp transverse lip edge.
While the preferred embodiment has been described with respect to multiple layer coating apparatus, the
invention may also be used to advantage in the form of a single layer slide hopper wherein a single layer of liquid coating composition may be made to flow down an inclined slide surface, and then along an upturned lip before being introduced into a bead, which bead bridges a generally horizontal gap between the hopper and a generally upwardly moving web. In such a single layer slide hopper the preferred degree of inclination of the slide surface and that of the lip and the preferred length of the lip are within the ranges referred to for the double layer slide hopper. To demonstrate the advantages of the above-described apparatus of the invention, a layer of a 5.55 percent by weight aqueous solution of gelatin was formed on a slide surface, and was coated onto a web with a wet laydown of 1.8 lbs/I00 square feet. A differential air pressure was maintained across the coating bead as the layer passed over the edge of the lip of the slide surface. The gap between the lip and the moving web was about 0.005 inch. As compared with results obtained under similar coating conditions using a planar slide surface of the prior art which did not have an upturned lip, it was found that, with apparatus of the present invention, longitudinal striations due to instability of the bead are absent or minimized at coating speeds at whichthey would occur with the prior art apparatus. Thus acceptable coatings could be made at speeds substantially higher than that using the planar slide hoppers of; the prior art.
In its broader aspects, the invention may be embodied as a slide hopper having a curved slide surface(s), such as of the type shown in FIG. 3 of U.S. Pat. No. 2,681,294. However, coating hoppers havingplanar slide surfaces are preferred, as they are substantially easier and less expensive to machine. In machining the planar land 31 on the lowermost planar slide surface 32, it is desirable to round, as shown in FIG. 2, the obtuse angle 38 formed between the surfaces 31,32.
The invention is particularly applicable to a multiple layer slide hopper of the extrusion-slide type, as disclosed in U.S. Pat. No. 2,761,417, which may also have a generally upturned lip formed on the slide surface. Such a coating hopper is described and claimed in copending commonly assigned US. application Ser. No. 327,048 simultaneously filed herewith in the name of Jackson et al. and under the title Method and Apparatus For Coating a Multiple Number of Layers onto a Substrate.
The method and apparatus of this invention can be utilized to coat any material or mixture of materials which can be put in liquid form and bead coated, for example, in the form of a solution, a dispersion, or a suspension. In many instances where this method finds application, the coating composition is an aqueous composition but other liquid vehicles of either an organic or inorganic nature, can also be utilized and are fully within the contemplation of this invention. The respective layers can be formed of the same or different liquid coating compositions and these coating compositions can be either miscible or immiscible with one As indicated hereinbefore, the method and apparatus of this invention are especially useful in the photographic art for manufacture of multilayer photographic elements, i.e., elements comprised of a support coated with a plurality of superimposed layers of photographic coating composition. The number of individual layers may range from two to as many as ten or more. In the photographic art, the liquid coating compositionsutilized are of relatively low viscosity, i.e., viscosities from as low as about 2 centipoise to as high as about 150 centipoise, or somewhat higher, and most commonly in the range from about 5 to about 100 centipoise.
The method and apparatus of this invention are suitable for use with any liquid photographic coating composition and can be employed with any type of photographic support and it is, accordingly, intended to include all such coating compositions and supports as are utilized in the photographic art within the scope of these terms, as employed herein and in the appended claims.
The term photographic normally refers to a radiation sensitive material, but not all of the layers presently applied to a support in the manufacture of photographic elements are, in 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 relates also to the application of such layers, and the term photographic coating composition, as employed herein, is intended to include the compositions from which such layers are formed. Moreover, the invention includes within its scope all radiation sensitive materials, including electrophotographic materials and materials sensitive to invisible radiation as well as those sensitive to visible radiation. While, as mentioned hereinbefore, the layers are generally coated from aqueous media, the invention is not so limited since other liquid vehicles are known in the manufacture of photographic elements and the invention is also applicable to and useful in coating from such liquid vehicles.
More specifically, the photographic layers coated according to the method of this invention can contain light-sensitive materials, such as silver halides, zinc oxide, titanium dioxide, diazonium salts, light-sensitive dyes, etc., as well as other ingredients known to the art for use in photographic layers, for example, chemical sensitizers, development modifiers, antifoggants and stabilizers, developing agents, hardeners, plasticizers and lubricants, coating aids, matting agents, antistatic agents, brighteners, spectral sensitizers, absorbing and filter dyes, color materials, etc. Specific examples of these ingredients may be found in the publication entitled Product Licensing Index, Volume 92, December 1971, Publication 9232, pages 107 through 1 10, and in the publications cited therein. The photographic layers can also contain various colloids either alone or in combination as vehicles. Suitable hydrophilic vehicle materials include both naturally-occurring substances, such as proteins, for example, gelatin, gelatin derivatives, etc., and synthetic polymeric substances, such as water soluble polyvinyl compounds. Further examples of suitable vehicles are disclosed in the aforecited publication.
Various types of supports may be used to support the photographic elements. Typical flexible supports include film base, e.g., cellulose acetate and poly(ethylene terephthalate), paper, metal, etc. Other supports which may be used are also disclosed in the aforecited publication.
It will be appreciated from the above that the invention provides an improved apparatus and method which may be used to coat a web at increased speeds. It will be further appreciated that such apparatus embodying the invention claimed herein may be made relatively inexpensively by modifying coating hoppers of the prior art.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
1. A method of coating a liquid coating composition onto a web substrate from a coating hopper having a downwardly inclined slide, which method comprises the steps of:
feeding the liquid coating composition in the form of a layer upon an upperportion of the inclined slide surface such that the liquid flows as a layer by gravity downwardly on the inclined slide surface toward the substrate;
altering the direction of flow of the layer at a lower lip portion of the slide surface to a direction closer to horizontal than the direction of the upper portion of the slide surface so that the liquid composition flows more horizontally as it approaches the substrate;
flowing the liquid composition from the lower lip of the slide surface across a generally horizontal gap to the substrate; and
moving the substrate .in a generally upward direction across and in contact with the coating composition so that the surface of the substrate is coated with the coating composition.
2. The invention according to claim 1 wherein the coating composition flows along said lower lip portion for a distance of at least about 0.025 inches.
3. The invention according to claim 2 wherein the coating composition flows along said lower lip portion in a direction which is in the range of about i-5 relative to the horizontal.
4. The invention according to claim 3 wherein the coating composition is an aqueous hydrophilic colloidal solution.
5. The invention according to claim 4 wherein the coating composition bridging said gap forms a bead.
6. The invention according to claim 5 wherein a pressure differential is established between the exposed surfaces of the bead so that a relatively lower pressure is created adjacent the trailing surface of the bead.
7. The invention according to claim 3 wherein the liquid coating composition is a photographic coating composition.
8. A method of simultaneously coating a plurality of distinct superposed layers of liquid coating compositions onto a web substrate from a coating hopper of the type having a downwardly inclined slide surface, which method comprises the steps of:
simultaneously feeding each liquid coating composition in the form of a layer and in superposed relation to the other composition layers onto the upper portion of the slide surface inclined downwardly at a first predetermined angle with respect to horizontal such that the liquid coating composition layers flow down the surface by gravity;
altering the direction of flow of the layers at a lower portion of the slide surface to a second predetermined angle of inclination less than said first angle of inclination so that the liquid composition layers flow more horizontally;
flowing the liquid layers from the lower portion of the slide surface across a generally horizontal gap to a substrate to be coated; and
moving the substrate in a generally upward direction across and in contact with the liquid composition layers so that the surface of the substrate engages the lowermost layer and simultaneouslypicks up all of said compositions in distinct superposed layers.